Flavio Felici Dissertation

The Application of Low-Power Wireless

Networks to Wide-Area Distributed

Audio Systems

A dissertation submitted to The University of Manchester for

the degree of Master of Science in the Faculty of Engineering

and Physical Sciences

2009

Flavio Felici

School of Electrical and Electronic Engineering

“The Application of Low-Power Wireless Networks to Wide-Area Distributed Audio Systems”

Flavio Felici

University of Manchester 1

List of Contents

List of Figures .................................................................................................................... 3

List of Tables ..................................................................................................................... 5

Abstract ............................................................................................................................. 5

Declaration ........................................................................................................................ 6

Copyright Statement .......................................................................................................... 6

Acknowledgment ............................................................................................................... 7

Chapter 1 ........................................................................................................................... 8

Introduction ....................................................................................................................... 8

1.1 Sound Masking Overview and Project Aim .............................................................. 8

1.2 Structure of Dissertation ......................................................................................... 12

1.3 Achievements ......................................................................................................... 13

Chapter 2 ......................................................................................................................... 14

Technical Requirements of a Typical Sound Masking System .......................................... 14

2.1 Frequency of Operation .......................................................................................... 14

2.2 Range and Network layout ..................................................................................... 15

2.3 Data Rate ............................................................................................................... 16

2.4 Wireless Coexistence ............................................................................................. 17

Chapter 3 ......................................................................................................................... 18

Review of Communication in the ISM Band .................................................................... 18

3.1 Wireless Transmission in the Unlicensed Band....................................................... 18

3.2 ISM Band selection ................................................................................................ 20

3.2.1 The 2.4 and 5.8 GHz ISM Bands ..................................................................... 20

3.3 Comparison between wireless standards in the 2.4GHz ISM band .......................... 21

3.3.2 IEEE 802.15.1 – The Bluetooth Standard......................................................... 23

3.3.3 IEEE 802.15.4-2006 – The ZigBee Standard ................................................... 24

3.3.4 Texas Instruments SimpliciTI .......................................................................... 24


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3.4 2.4 GHz ISM Band Survey .................................................................................... 26

3.5 Custom Built Protocol ............................................................................................ 27

3.6 Additional Requirements for FHSS systems in the 2.4 GHz ISM band ................... 27

Chapter 4 ......................................................................................................................... 29

System Design ................................................................................................................. 29

4.1 High Level Design ................................................................................................. 29

4.2 Hardware Component Selection ............................................................................. 31

4.2.1. Microcontroller ............................................................................................... 31

4.2.2 Transceiver ...................................................................................................... 33

4.3 Hardware Assembly and Testing ............................................................................ 35

4.4 Software Design ..................................................................................................... 44

4.4.1 Software Environment ..................................................................................... 44

4.4.2 SPI Interface and Transceiver Configuration.................................................... 46

Chapter 5 ......................................................................................................................... 52

Development of a FHSS MAC Layer Protocol ................................................................. 52

5.1 Basic Functions ...................................................................................................... 52

5.2 Output Functions .................................................................................................... 61

5.3 Configuring the Transceiver ................................................................................... 63

5.4 Fixed Frequency Transmission ............................................................................... 68

5.5 Frequency Hopping System .................................................................................... 71

5.6 Adaptive Frequency Hopping System ..................................................................... 75

5.7 Variable Base Adaptive Frequency Hopping System .............................................. 77

5.8 Network Setup ....................................................................................................... 80

5.9 Variable Base Adaptive Fast Frequency Hopping System ....................................... 81

Chapter 6 ......................................................................................................................... 85

System Testing and Performance ..................................................................................... 85

6.1 Range Test ............................................................................................................. 85

6.2 Variable Base Adaptive FHSS System Test ............................................................ 87


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Chapter 7 ......................................................................................................................... 94

Conclusions and Further Work ......................................................................................... 94

References ....................................................................................................................... 96

Appendix A ................................................................................................................... 102

Source Code .................................................................................................................. 102

SPIc ........................................................................................................................... 102

Fast Hopping RF.c ..................................................................................................... 111

Appendix B.................................................................................................................... 120

Feasibility Study ............................................................................................................ 120

Appendix C.................................................................................................................... 144

Datasheets Front Sheets ................................................................................................. 144

List of Figures

Figure 1.1 Sound Masking System installed under the office ceiling tile1............................8

Figure 1.2 Wireless transmission system applied to a network node....................................9

Figure 1.3 Frequency Hopping terminology4 ......................................................................11

FIGURE 2.1:NETWORK SETUP6..............................................................................................14

Figure 2.2: Traffic patterns in a typical sound masking system7.........................................15

Figure 2.2: Traffic patterns in a typical sound masking system7 ........................................16

Figure 3.1: Wi-Fi Channels in the 2.4 GHz ISM band29

......................................................23

Figure 3.2: Spectrum Survey of the 2.4 GHz ISM Band.....................................................27

Figure 4.1 Wireless board and its main components...........................................................31

Figure 4.2 PIC32MX360F512L main features42

.................................................................34

Figure 4.3 Overview of External Components....................................................................37

Figure 4.4 Microchip Explorer 16 Development Board47

...................................................38

Figure 4.5 Quasar CC2500 module48

...................................................................................39

Figure 4.6 Texas Instruments 2.4 GHz Inverted F Antenna78

..............................................40

Figure 4.7 Pictail connector pinout49

....................................................................................41


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Figure 4.8 Quasar CC2500 module pinout50

.......................................................................43

Figure 4.9 (left) Soldering the glued female connector ......................................................44

Figure 4.10 (right) Plugging the CC2500 module in the glued female connector.............44

Figure 4.11 The breadboard inserted into the Pictail connector .........................................44

Figure 4.12 PIC32 Starter Kit52

(bottom view)....................................................................45

Figure 4.13 I/O Expansion Board with the Pictail connector53

...........................................................45

Figure 4.14 All the hardware assembled and tested, ready to be programmed..................46

Figure 4.15 4-Wire SPI Interface55

......................................................................................48

Figure 4.16 CC2500 Address Header56

...............................................................................48

Figure 4.17 Single Byte Access: register writing (top) and reading (bottom)57

............... 49

Figure 4.18 SPI Clock Phase and Polarity55

.........................................................................49

Figure 4.19 Configuration registers overview59

...................................................................50

Figure 4.20 Complete Radio Control State Diagram60

.........................................................52

Figure 5.1 Command Strobes list61

......................................................................................56

Figure 5.2 SPI Module Block Diagram68

.............................................................................60

Figure 5.3 Read and Write SPI operation sampled with a logic analyzer...........................62

Figure 5.4 State transition timing70

......................................................................................63

Figure 5.5 Hyperterminal screenshot while calling different output functions..................66

Figure 5.6 Packet Size options71

..........................................................................................68

Figure 5.7 Packet Format72

..................................................................................................69

Figure 5.8 PATABLE Schematic view75

.............................................................................74

Figure 5.9 Available output power levels75

..........................................................................74

Figure 5.10 Variable Base Adaptive Frequency Hopping System program flow...............82

Figure 5.11 Variable Base Adaptive Frequency Hopping System program flow...............87


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List of Tables

Table 3.1 Unlicensed Bands in the UK21

.............................................................................20

Table 3.2: Network protocols characteristics40

....................................................................26

Table 4.1Product comparison guide for the 2.4 GHz ISM band44

.......................................36

Table 4.2 PIN connection list...............................................................................................43

Table 6.1 Test Results....................................................................................................... ...88

Table 6.2 Test Results..........................................................................................................89






Abstract

The 2.4-2.4835 GHz ISM band has become very popular for home, office, and industrial

wireless systems. This band is shared by a vast variety of different applications and

protocols, making coexistence and interference a key issue.

The purpose of this paper is to design and develop a low-cost wireless MAC layer capable

of operating in the 2.4 GHz ISM band, coexisting with other common wireless standards.

To avoid interference and not to collide with other wireless transmissions, the Frequency

Hopping Spread Spectrum technique is used in this custom built network protocol.

The developed wireless protocol is especially designed to interconnect a network of nodes,

which are part of a sound masking system.


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At the end of the project period a FHSS system was developed, able to avoid interference

and coexist with the major wireless standards in the 2.4 GHz ISM band. This paper shows

the key challenges and all the steps involved in developing such system.

Declaration

The writer declares that no portion of the work referred to in the dissertation has been

submitted in support of an application for another degree or qualification of this or any

other university or other institute of learning.

Copyright Statement

Copyright in text of this dissertation rests with the author. Copies (by any process) either in

full, or of extracts, may be made only in accordance with instructions given by the author.

Details may be obtained from the appropriate Graduate Office. This page must form part of

any such copies made. Further copies (by any process) of copies made in accordance with

such instructions may not be made without the permission (in writing) of the author.

The ownership of any intellectual property rights which may be described in this

dissertation is vested in the University of Manchester, subject to any prior agreement to the

contrary, and may not be made available for use by third parties without the written

permission of the University, which will prescribe the terms and conditions of any such

agreement.

Further information on the conditions under which disclosures and exploitation may take

place is available from the Head of the School of Electrical and Electronic Engineering.


Flavio Felici


Acknowledgment

I would like to thank Mr. R. Green for giving me the opportunity to take part in this

project; his highly competent teaching skills have made these past few months a very steep

learning curve, introducing me to the aspects involved in developing a wireless system.

I also take this opportunity to thank Dr. R. Brassington for his technical advices during

project development and Anand for sharing his practical experience with me.

I am indebted to my family for always being there and for giving me emotional and

financial support.


Flavio Felici


Chapter 1

Introduction

1.1 Sound Masking Overview and Project Aim

Sound masking is the process of artificially creating a sound in order to smother an

unwanted noise. This technique, especially effective in enclosed spaces, is achieved by

arranging a network of sensors and loudspeakers that constantly survey the environment

for any unwanted sound. If a noise source is detected, the network of loudspeakers

dynamically emit a smothering sound wave that covers up the noise, reducing the noise

impact on the people nearby.

If for example an intermittent noise is present in a room, the occupants of that room will

constantly perceive that noise and will repeatedly be disturbed. If instead a sound masking

system is installed in that same room, the system sensors will detect the noise source and

compensate it by constantly emitting (from its loudspeakers array) an especially

engineered sound wave, effectively reducing the noise awareness by the room occupants.

Such a system increases people‟s concentration and attentiveness by reducing noise

consciousness and distractions from the nearby environment.

For this reason, sound masking technology is often used in places like open plan offices or

meeting halls, since fitting an enclosed space with this kind of system improves

productivity and efficiency. Furthermore, the system loudspeakers can be used to broadcast

messages in the area or can be used for paging. Most of the time these systems installed

above the ceiling tile, making them invisible to the people beneath them.

Figure 1.1 shows a typical sound masking system installed in a working environment.


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Figure 1.1 Sound Masking System installed under the office ceiling tile1

Sound masking is not to be confused with active noise control: in fact, sound masking

systems reduce noise awareness by covering pre-existing noises up with special sound

patterns, while active noise control systems tend to cancel noises by re emitting the noise

sound wave with an opposite phase, ideally deleting the original noise. The latter technique

is more effective in reducing noise perception when the noise pattern is unique and both

the noise source and the listener are in a constant and known position- for example, active

noise control is an effective way to remove the rotor noise from a helicopter pilot‟s

headphones. Consequently to improve productivity, concentration and speech privacy in

work and public environments sound masking systems represent the best choice and are

widely used.

Embedded System Projects2 is an audio systems company based in Manchester, England. It

develops and produces a range of products focused on acoustics and digital signal

processing. The company also produces a sound masking system which consists of a

network of digital signal processing boards each coupled with a sound sensor and a

loudspeaker, all forming an independent network node.


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In order to achieve a coherent and effective sound masking effect each network node has

to be connected and be able to communicate with the rest of the network. This ensures that

the information about the environment is shared among all the network nodes and enables

the digital signal processing boards to generate an efficient smothering sound, where and

when needed.

The company present-day sound masking system implements a wired connection between

the network nodes. This ensures a safe and reliable connection, however such a wired link

makes the system installation costs even higher than the system itself and limits the

flexibility of arranging the nodes where most needed. In fact, to ensure the best system

performances, each node has to be strategically placed in the room -this is not always

possible if a wired connection is used, especially if the installation takes place in historical

buildings. Moreover, the average system requires several dozens of nodes, each few meters

apart and the wire connecting all of them can become very expensive and time consuming.

As a result, the aim of this project is to develop a board that can be attached to existing

sound masking systems, adding wireless communication capability. Each signal processing

board will be connected with a wireless board, forming a wireless network. Consequently,

adding a wireless device to each node will eliminate the need for expensive and bulky

wired connection.

Using a wireless link to connect each network node will improve the overall sound

masking performance, allowing a more flexible and more efficient, node positioning; and

above all, it will greatly reduce

installation costs.

Such a wireless transmission system is

meant to provide the same link quality

and reliability offered by the wire

connection, ensuring a constant linkage

between all the system units.

To accomplish the drastic installation cost

reduction, the wireless system has to

operate within a frequency range where no

license is required to transmit; To achieve this, it will have to operate within a license free

Figure 1.2 Wireless transmission system applied to a

network node


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frequency range -therefore coexist and not interfere with a wide variety of different devices

and protocols that crowd the license free spectrum.

One of the main challenges of this project is to develop a low cost transmission system

able to establish a wireless link using frequencies shared by a vast number of popular

standards such as local area networks, cordless phones, personal area networks, alarm

systems and so on.

To achieve this result, a special communication technique called Frequency Hopping

Spread Spectrum3 (also known as FHSS) is used. This method, initially developed for

military applications, allows wireless communication even in presence of strong

electromagnetic interference and avoids jammed frequencies.

In particular, with this technique the sender transmits information over a particular

frequency for a certain amount of time, then hops to another carrier and start transmitting

on that frequency until it hops to another one, and so on. The time spent transmitting on

each frequency is called dwell time, while the time that lapses between hops is called blank

time. The shorter the dwell time is, the faster the hopping rate will be; and the shorter the

blank time, the more efficient the system is, as less time is spent without data transmission.

Often the carrier frequency pattern used is pseudo-random, which makes very hard for an

unauthorized person to intercept data, as the frequency of transmission is unknown.

However, one of the main challenges for a FHSS system is synchronization: in fact, to

ensure constant communication between sender and receiver, both must be tuned in the

same frequency at the exact same time –which is particularly hard when the frequency

pattern used by the transmitter is random.


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Figure 1.3 Frequency Hopping terminology4

Such a method, even if developed by the military to avoid data interception, is ideal for

civilian application where the need to coexist with other wireless standards in the area

rises. As matter of fact, as FHSS can constantly use different carrier frequencies, it avoids

busy channels, thus permitting a reliable wireless link even in those electromagnetically

crowded bands.

1.2 Structure of Dissertation

This paper is divided into chapters that reflect the project development timeline. Firstly, in

chapter 2, the project technical requirements dictated by the requesting company are listed.

These requirements impose specific choices for both the hardware component selection

and software structure. Chapter 3 analyzes what is probably the main property of a wireless

system: the frequency of operation. Various aspects, including the need of a license free

transmission and antenna dimensions, determine the frequency choice. This choice, in its

turn, has consequent repercussions in all the project development.

Such aspects are discussed in chapter 4, where the whole system in analyzed in its main

functionalities, from a system high-level point of view. Chapter 4 also focuses on the

hardware components that make up the transmission system, with some aspects of the

hardware testing that was involved after the hardware assembly.


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Moreover, this chapter exposes the high-level software design, explaining what the

software main tasks will be.

A more detailed explanation of the software implementation is given in chapter 5, in which

all the steps involved in building the system software are analyzed in deep.

In fact, given the complexity of the project, to achieve the requested aim and requirements

the main task was divided in subsequent phases. Each phase represent a key step toward

the main task accomplishment.

The developed system is tested over chapter 6, where all different system parameters are

discussed to achieve better performances.

Lastly, chapter 7 discusses the conclusions and gives propositions for further work on the

whole system.

1.3 Achievements

The results achieved during the development of this project are:

Peer to peer FHSS link with error detection system

Scan of signal strength in the entire 2.4 GHz ISM spectrum

Adaptive FHSS transmission system with network addresses

These results comply with the system requirements given at the start of the project.


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Chapter 2

Technical Requirements of a Typical Sound

Masking System

Key aspects for every wireless transmission system are frequency of operation, range,

throughput and link reliability. Embedded System Projects has produced a list of

requirements that the transmission system has to fulfil in order to make the sound masking

system work efficiently and reducing both production and installation costs.

2.1 Frequency of Operation

The wireless transmission system must be able to operate within an unlicensed spectrum

common in the UK, Europe, USA and Japan. In addition, the transmission system must use

transmission power, a modulation scheme and a data rate compatible with the above-

mentioned country regulations. In these countries, many companies are interested in

increasing efficiency and production in offices and work environments, hence those

nations represent the main market for sound masking technology.

Operating within an unlicensed spectrum increases flexibility and reduces costs because

transmitting in those frequencies does not require obtaining a license. However, such

frequencies tend to get always more and more electromagnetically crowded making

interference a major issue.

A requirement such as this deeply affects the hardware components selection, especially

the transceiver choice. As a matter of fact, it is the transceiver that will synthesize the

operation frequency, regulate the output power and apply a modulation scheme on the

carrier.


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2.2 Range and Network layout

The range for each network node must be adequate for effectively reaching every element

of the sub network that node is in. In fact, the whole sound masking network will be

divided into smaller sub networks, each positioned in an environment with particular

acoustic properties. The system works if every element in each sub network is able to

communicate with all the rest of the nodes making up that sub network. In addition, a

„master node‟, which is shared by two or more sub networks, grants communication

between different sub networks. In a typical sound masking system, each sub network is

formed by around 50 nodes5; each about 10 meters apart. Figure 2.1 shows a typical

network layout.

The requirements imposed for the communication range also have an effect on the

hardware components selection. As a matter of fact, both the transmission power and the

antenna type must comply with those requirements.

Figure 2.1: Network Setup6


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2.3 Data Rate

To ensure an effective and successful sound masking action the nodes in each sub network

need to be in constant communication, but not necessarily using a real time protocol. In

actual fact, each node transmits data at predetermined intervals to its neighbouring nodes,

sharing information about the environment noise pattern and level. This communication is

vital for the whole system to work properly and therefore has to be error proof.

For this reason, the transmission system must be able to detect when a transmission error

has occurred, in order not to share and process incorrect data. This implies that the

transceiver which is chosen for the wireless system has to be able to perform an error

detection technique.

Since the communication is not on real time if a faulty packet is received, the transmission

can either be repeated until the data is successfully received or either delayed until the

channel is good for data transmission. Such choices are to be taken when implementing the

upper layer of the communication protocol and will be made by the company, depending

on the required system performances.

However, it is fundamental that the hardware components in the wireless transmission

system are able to detect a faulty packet reception when it occurs.

The average traffic pattern of data shared between nodes in an typical system is shown on

figure 2.2.


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Figure 2.2: Traffic patterns in a typical sound masking system7

2.4 Wireless Coexistence

Another requirement is the coexistence between the sound masking transmission system

and other wireless standards that might be present in the same operational area.

In the places where sound masking technology is installed it is often easy to find other

wireless protocols –such as IEEE 802.118 (also known as Wi-Fi) or IEEE 802.15.1

9 (also

known as Bluetooth) or other short range devices like cordless phones, wireless security

cameras, pagers, alarm systems and wireless temperature sensors, to name a few. Such

systems nowadays are very common in working environments or public places, and all of

them use an unlicensed frequency range to operate.

Consequently, it is very likely to find a fully engaged unlicensed spectrum, making

coexistence and interference a major issue.

The requirement for the sound masking transmission system to be able to successfully

operate without jamming or interfering with other systems that could be transmitting in the

same range of frequencies at the same time is particularly demanding and represents the

main challenge for the project.

The sound masking wireless transmission system property to coexist with other wireless

standards present in the area is vital and represent a key feature for the system; in fact, if

this requirement is not completely fulfilled the whole product would be useless and

unsuccessful in the market.


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Chapter 3

Review of Communication in the ISM Band

3.1 Wireless Transmission in the Unlicensed Band

As discussed in the previous section the transmission system has to operate within an

unlicensed frequency range common to various countries; the International

Telecommunication Union10

is a worldwide organization which defines frequencies

allocation and wireless standards for every part of the globe. In particular, the ITU has

divided the world into three main regions: Region1 includes Europe, Africa, the Middle

East, the former Soviet Union area and Mongolia; Region2 comprises the Americas,

Greenland and some eastern Pacific Islands; while Region3 covers most of non former

Soviet Union countries in Asia, Iran and most of Oceania11

.

ITU determines common regulations used as a guideline for each region .Such regulations

are then adjusted by each singular country to suit their own needs, maintaining the basic

properties defined by the international organization.

The ITU regulations for each frequency range contain, among other things, information on

the purpose of the transmission, the maximum transmission power, modulation scheme and

license required.

In addition other standardization bodies that regulate the use of radio equipment for more

specific areas, for instance in Europe the standards are set by The European Conference of

Postal and Telecommunications Administrations12

(also known as CEPT) and also by the

European Telecommunications Standards Institute13

(or ETSI). While in the United States

the governing body concerning wireless systems is the Federal Communications

Commission14

(or FCC). In Japan instead, there is the Association of Radio Industries and

Business15

(or ARIB).


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Such bodies regulate the requirements to a higher-level in respect to the general guidelines

given by the ITU. To be noted that in the case of the European Union the final regulations

are set by each country governing body: in the UK by the Office of Communications16

(or

Ofcom), in Germany by the Federal Network Agency for Electricity, Gas,

Telecommunications, Post and Railway17

, in Italy by the Ministero delle Communicationi18

and in France by the Ministere de l'Econonie des Finances et des L'Industrie19

, to name a

few. However, most of the frequency allocations and regulations tend to be very similar

throughout Europe, especially for those bands used by the public.

Some specific frequency ranges, allocated in different positions of the spectrum, are called

Industrial, Scientific and Medical band or ISM20

. In these ranges, one could operate a

wireless system without obtaining a license first.

Each nation could make use of some, all, or even more ISM bands than what assigned as a

guideline from the ITU. For instance table 3.1 shows all the ISM bands available in the

UK, the ranges and purposes are defined by the Ofcom; nevertheless, those bands are

common for most of the countries laying in the ITU Region1.

Generic Frequency Band

Application

9 kHz to 30 MHz Short Range Inductive Applications 27 MHz Telemetry, Telecomm and Model Control 40 MHz Telemetry, Telecomm and Model Control 49 MHz General Purpose Low Power Devices

173 MHz Alarms, Telemetry, Telecomm and Medical Applications 405 MHz Ultra Low Power Medical Implants Devices 418 MHz General Purpose Telemetry and Telecomm and Applications 458 MHz Alarms, Telemetry, Telecomm and Medical Applications 864 MHz Cordless Audio Applications 868 MHz Alarms, Telemetry and Telecomm and Applications

2400 MHz General Purpose Short Range Applications, including CCTV and RFID. Also used for WLANs including Bluetooth Applications.

5.8 GHz Hyper LANs, General Purpose Short Range Applications, including Road Traffic and Transport Telematics

10.5 GHz Movement Detection 24 GHz Movement Detection 63 GHz 2nd Phase Road Traffic and Transport Telematics 76 GHz Vehicle Radar Systems

Table 3.1 Unlicensed Bands in the UK21


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Although transmissions in those frequencies don‟t require one to obtain a license first,

unlicensed band does not mean unregulated –maximum transmission power, modulation

scheme, channel spacing and duty cycle rules are still imposed for system working in those

ranges. Often in these regulations when referring to power, all the measurements are done

using the Effective Isotropic Radiated Power parameter (also known as EIRP); this is by

definition the transmitted power radiating equally in all direction in the form of a spherical

wave. Therefore, a wireless system has to be approved by the respective governing body of

each country where it is going to be used before it could be sold in the public marked. The

regulations set for operating within an ISM band have safety reasons and regulate

transmissions between different protocols. In fact, those bands tend always to be busier,

with various types of protocols and modulations used at the same time; therefore

coexistence has to be kept.

3.2 ISM Band selection

To choose what ISM band is most suitable for this application different parameters have to

be taken into account: antenna dimension, components availability, path loss, scattering

and band usage status are among them. As the transmission system will be installed indoor

(often above the ceiling tile) and as the standard isotropic antenna length is related with the

frequency wavelength, ISM bands in the megahertz order and below are discarded (those

would require antennae several meters long). Another factor is path loss, in this case the

lowest the carrier frequency the lower atmosphere attenuation is encountered; however, the

relatively short distances between nodes make this parameter minor. Instead, the scattering

produced by the electromagnetic waves being reflected by objects increases with the

frequency, offering to the receiver a better multipath reception at higher frequencies -

particularly in small indoor environments where many objects could reflect the incident

wave. Given these considerations, the most suitable ISM bands for this kind of

transmission system are the 2.4 and 5.8 GHz, both offering small antenna dimension and

good scattering.

3.2.1 The 2.4 and 5.8 GHz ISM Bands

Both the 2.4 and 5.8 GHz ISM bands are very common worldwide and both offer good

physical characteristics in terms of scattering and multipath reception, short antenna

dimensions and certainly a high data rate. Yet, nowadays the 2.4 GHz band is more

popular as it is widely used by protocols such as IEEE 802.118, IEEE 802.15.1

9 and IEEE

802.15.4-20061122

(also popularly known as Zigbee) to name a few. This means that this


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frequency range is already likely to be electromagnetically crowded in actual public places

and working environments (even microwave ovens share this very same band).

On the other hand, this entails that manufacturing companies around the world have

already produced quite a large variety of components specially designed to work in this

frequency range. On the contrary, the 5 GHz ISM band is, at the moment this paper is

written, far less used for public products such as wireless local area networks or

communication devices. Consequently, this band is to be expected having far less data

traffic, hence interference. In contrast, as it is fairly new ISM band the choice of

components produced by manufacturers is fairly limited, which makes hard to find

hardware components that fulfil the system requirements.

There are also differences in the two bands in terms of maximum transmission power –for

example in Europe the European Conference of Postal and Telecommunications

Administrations has set the maximum EIRP power to 10mW for short-range devices23

.

Instead, in the 5.150-5.350 GHz ISM range (also called Band A, for indoor use only) the

maximum EIRP limit is set to 200mW with a power density of 10 mW/MHz24

; while in the

5.470-5.725 GHz ISM range (also called Band B, indoor and outdoor use allowed) the

maximum EIRP power is set to 1W with a power density of 50mW/MHz24

. This

difference in the maximum allowed EIRP output power in those two bands is due to the

fact that the path loss increases with the frequency, meaning that to achieve the same range

a 5GHz system would require higher output power.

Given the above considerations and the project technical requirements, the advantage of

having a wider choice in the transceiver selection makes the 2.4 GHz ISM band the

frequency range of choice for this project. This band in fact, even if more

electromagnetically crowded than the 5.4GHz ISM band, offers a vast variety of hardware

components that can accomplish in full the system performance required.

3.3 Comparison between wireless standards in the 2.4GHz ISM band

Since the 2.4GHz ISM band has been chosen as the project frequency range, here below is

listed a selection of the main protocols worldwide used in this frequency range. One of

these protocols could be chosen to be used in the sound masking transmission system; but

above all, these protocols are likely to be found in the area where the transmission system

will be operating. Hence, a good understanding of these standards and their properties


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(such as bandwidth and modulation) makes easier to develop a system that will coexist

with them, avoiding interference.

3.3.1 IEEE 802.11 – The Wi-Fi Standard

Wireless fidelity (or Wi-Fi), is the popular name for the IEEE 802.11 standard.

With its many variants, has become the standard for public and private local area networks.

This standards allows to build up wireless local area networks in which users can access

the internet and share data at high speeds, reaching 600 Mbps with its latest standard

802.11n25

(which uses MIMO technology), with a reasonably wide operative range of

several tens of meters (although the maximum output power limit is set at 10dBm EIRP). It

is capabilities allow interconnecting up to 200726

users at the same time, far beyond what

needed in the case of sound masking systems.

On the other hand, these powerful performances come with a price of complexity; to

implement and to run the software with network protocol requires a relatively large amount

of computational power, which is not compatible with a low-cost system.

The Wi-Fi standards uses the whole 2.4 ISM spectrum, however the frequency range is

divided into 13 channels (in Europe), each of them 22MHz wide (the standards has been

adapted to different variants in the ISM band depending on the country, hence uses 11

channels in USA and 14 in Japan27

).

Each channel is masked with an attenuation of 30dB on the channel edges and since each

channel is 22MHz wide28

a maximum of four Wi-Fi channels can be used at the same time

without channel overlapping. In particular, to ideally achieve zero interference four LANs

could be set up in the same area using channel 1, 5, 9 and 13. Figure 3.1 shows a

representation of the Wi-Fi channels in the 2.4 ISM band.

Figure 3.1: Wi-Fi Channels in the 2.4 GHz ISM band29


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Thus, this protocol is most suited to share large quantities of data among a large number of

users, reaching rather high data rates. As this standard is very popular, it is reasonable to

expect some Wi-Fi channels present in the transmission system operating environment.

However, the standard property to use the ISM range dividing it into channels makes it

easier to avoid mutual interference between different systems.

3.3.2 IEEE 802.15.1 – The Bluetooth Standard

Another popular standard that uses the 2.4 ISM bad is the IEEE 802.15.1 Bluetooth

standard. It is particularly used for wireless personal area networks (WPAN) and short

range devices such as wireless keyboard or mouse, wireless microphones and wireless

printers. The standard development has been focused to achieve very low power

consumption and a transmission range of few meters (depending on the transceiver class),

with a modest data rate –making it suitable to interconnect the above mentioned devices.

The Bluetooth network is established between a device serving as a master and one or

more slave devices, this is also called Bluetooth piconet network. Unlike the Wi-Fi

standard, Bluetooth does not divide the ISM band into wide portions; instead, FHSS is

used, with the 2.4GHz ISM band divided in 79 channels of 1 MHz each30

. The fist channel

used has a frequency of 2402 MHz30

, while the other 78 channels are found incrementing

the frequency with a 1 MHz step (channel 2 at 2403MHz, channel 3 at 2404, and so on..).

For Bluetooth the dwell time is 625μs30

(corresponding to a hop rate of 1600 hops/second).

In the Piconet network the hopping pattern is pseudo-randomly chosen by the master,

which depending on the surrounding environment makes the selection on what frequencies

to use; the slave device instead keeps following the frequency pattern described by the

master24

. Designers used FHSS for this popular standard to avoid interference as the

2.4GHz ISM band is notoriously a crowded band. This standard both fits the requirements

of coexistence with other protocols (as it constantly moves along the spectrum it does not

jam other systems) and low cost.

However to accomplish the sound masking transmission system requirements a Bluetooth

class 1 or 2 would have to be used (slightly more expensive) and above all, the 802.15.1

standard allows a maximum number of active connection equal to eight30

, which is

incompatible with the requirements set by the sound masking system company. However,

as the example of this standard shows, the FHSS technique is an ideal method for wireless

coexistence.


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3.3.3 IEEE 802.15.4-2006 – The ZigBee Standard

ZigBee instead, is an industrial standard specially developed for low cost, low data rate

applications with a typical operating range of around 10 meters31

. As the bit rate that it can

support is quite limited, the Zigbee protocol is ideal for applications such as infrared

sensor, smoke detectors, meters readings or light switches. Most of all, this standard is

designed around an extremely low power consumption, in fact a transmission system to be

certified with the Zigbee standard must be battery powered, with at least a 2 year operating

lifetime. However the number of nodes in a Zigbee network can be greater than 65,00031

.

The modulation technique used in this standard is Direct Sequence Spread Spectrum,

which is particularly useful for fitting many users in a limited frequency range, but does

not offer the same anti-interference capabilities as FHSS.

3.3.4 Texas Instruments SimpliciTI

SimpliciTI is another network protocol commonly used within ISM bands. It has been

developed by Texas Instruments to help creating networks with its produces transceivers. It

is very similar to Zigbee for both data rate and range, making it ideal for application like

smoke detectors, alarms or automatic meter readings32

. Nonetheless, it is an open protocol

and does not require to be implemented into battery-powered systems. The maximum

number of nodes that can be set up into a network using the SimpliciTI standard is defined

as 232

, well beyond what required by the sound masking transmission system. On the other

hand, in order to use the SimpliciTI protocol Texas Instruments components have to be

used and the protocol used is not customizable, reducing the overall efficiency –as most of

its features will not be used in the transmission system. But above all, using this protocol

does not allow flexibility –every system aspect, including data rate, coexistence, bit-error-

rate will be strictly related to Texas Instruments original design, making it hard to modify

it to the sound masking transmission system needs.

Table 3.2 reassumes all the different network protocols characteristics


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PROTOCOL PURPOSE ADVANTAGES DISADVANTAGES FREQUENCY

RANGE

802.11 a/b/g/n Medium range

wireless LAN

Worldwide standard

high data rates

can implement

encrypted

communications

Sensible cost

Not readily

available

Requires

elevated

computational

load and

software to be

implemented

2.4 GHz,

5 GHz

Bluetooth Short range

wireless personal

area networks

Worldwide standard

Wide choice of low

cost components

Good interoperability

Short range

Non suited for

crowded

environments

(limited

number of

channels)

2.4 GHz

Zigbee

Short range, very

low power

wireless

communication

link

Worldwide standard

Low cost components

and wide choice

Very low power

consumption

Low system

requirements

Battery

powered

systems only

Strict power

consumption

specifications

Low data rate

2.4 GHz

TI SimpliciTI

Medium range

low power

wireless

communication between devices

3

Low cost components

and wide choice

Low power

consumption and duty

cycle;

Low system

requirements

TI devices to

be used

Low data rate

Non

customizable

protocol

Sub 1 GHz,

2.4 GHz

Custom

Purpose built:

bidirectional

peer-to-peer

communication link

Very low cost

hardware

Highly efficient data

rate

Low power

consumption

Robust to channel

interferences

Able to coexist with

other wireless

transmission

Wide choice of low

cost components

Time to

develop

Network not

interoperable

with other

wireless

standards

Any ISM BAND

Table 3.2: Network protocols characteristics40


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3.4 2.4 GHz ISM Band Survey

Figure 3.2 shows a 2.4 GHz ISM band survey taken with a spectrum analyzer in a public

metropolitan area of Manchester, UK. The measurements were taken in the frequency

range spanning from 2.4 to 2.485 GHz using a resolution bandwidth of 1 kHz. In the graph,

it is possible to distinguish three different Wi-Fi channel used and Bluetooth activity over

different frequencies –seen as narrow channels 1MHz wide along all the spectrum. This

graph is a practical confirmation that the Wi-Fi standard uses portion of the spectrum well

defined in width and position, while Bluetooth uses a FHSS technique probing the

spectrum over different frequencies.

As the bit rate required by sound masking system does not need a wide bandwidth, the

transmission system could still operate in such crowded ISM band, avoiding those

frequencies already being used by the other standards.

It is then vital for the system to be able to first perform a scan of the band in order to

operate in those unused regions of the spectrum.

Figure 3.2: Spectrum Survey of the 2.4 GHz ISM Band

-140

-120

-100

-80

-60

-40

-20

0

2400

.000

001

2402

.472

728

2404

.945

455

2407

.418

182

2409

.890

91

2412

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637

2414

.836

364

2417

.309

091

2419

.781

819

2422

.254

546

2424

.727

273

2427

.2

2429

.672

728

2432

.145

455

2434

.618

182

2437

.090

91

2439

.563

637

2442

.036

364

2444

.509

091

2446

.981

819

2449

.454

546

2451

.927

273

2454

.4

2456

.872

728

2459

.345

455

2461

.818

182

2464

.290

91

2466

.763

637

2469

.236

364

2471

.709

091

2474

.181

819

2476

.654

546

2479

.127

273

2481

.600

001

2484

.072

728


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3.5 Custom Built Protocol

The previously mentioned protocols represent a common and reliable solution used in great

deal of applications all over the world; yet, every standard protocol has its own benefits

and drawbacks and its characteristics are designed for a specific purpose.

An alternative of using a standard protocol would be developing a custom protocol from

ground up, a protocol especially designed for the sound masking transmission system.

A custom-made network protocol could be designed just for this wireless system,

improving efficiency and reducing the computational power required, hence the hardware

cost. Such custom build protocol can be fitted around the components selected and deal

with more flexibility issues like coexistence and interference.

In addition, if a non-standard protocol is implemented in the transmission, any possible

maintenance operation or future upgrade to be done on the system could be exclusively

performed by the manufacturing company, leaving no space for competitors.

Such protocol could make use of FHSS technology, which is one of the best methods to

improve standards coexistence.

It is then believed, that the best solution to meet the project requirements with the lowest

cost, is to develop a custom protocol especially designed for this application.

3.6 Additional Requirements for FHSS systems in the 2.4 GHz ISM

band

For wireless systems operating in Europe, The European Conference of Postal and

Telecommunications Administrations classifies, in its recommendation 70-0333

, the non-

specific short range devices in the 2.4GHz ISM band as class 1l23

. For this class of devices

the maximum transmission power is set at 10mW EIRP23

.

Furthermore, if such devices use FHSS technique they must also comply with the

European Telecommunications Standards Institute EN 300 44034

directive.


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This directive defines, among other specifications, that all systems belonging to class 1l

and using FHSS must

–define the occupied bandwidth where the spectral density is greater than -74.8dBm/Hz35

(or -30 dBm if measuring the power in a 30 KHz bandwidth);

-the transmitter maximum output power must not exceed the limit set in the ERC 70 03

directive35

;

-such measurements must be made using a hopping sequence with both the highest and

lowest hop frequency 35

(or each frequency in two different measurements);

-FHSS must make use of at least 20 channels separated by a channel bandwidth measured

with a drop of 20dB below the peak level; -the dwell time for each frequency must not

exceed 0.4 s36

;

-each channel must be used at least once in a period of time calculated with the formula:

𝑡𝑚𝑖𝑛 = 4 × 𝑑𝑤𝑒𝑙𝑙𝑇𝑖𝑚𝑒 × 𝑡𝑜𝑡𝑎𝑙𝐶ℎ𝑎𝑛𝑛𝑒𝑙𝑠𝑁𝑢𝑚𝑏𝑒𝑟

-during the blanking time the transmit power must drop below 20nW36

(-47 dBm);

To be noted that as an eventual system malfunction would not involve any risk or danger to

a person, the ETSI does not require any selectivity or reliability requirement.

The above requirements are mandatory for every FHSS operating in Europe and

appropriate measurements must be made before the final system is sold in the market.

These regulations are harmonized throughout all the European countries, while in the USA

and Japan similar rules apply, however those are less restrictive.

To be noted that American FCC regulations measure the maximum transmission power in

terms of field strength instead of effective isotropic radiated power.

The transmission system development will then start implementing a custom FHSS

network protocol especially designed for the sound masking application. This protocol,

while fulfilling the company requirements, will also have to comply with the

aforementioned regulations.


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Chapter 4

System Design

4.1 High Level Design

In this section, the project system level design and main components will be outlined,

focusing on the reasons for each component to be included in the system.

In order to form a wireless network each node of the sound masking system has to be

connected with a wireless board. Each wireless board must consist, to perform the required

task, of several hardware components and software (which contains the network protocol).

As discussed in chapter 3.5, such software will be a custom-made network protocol built

from ground up; this can be designed to make each network node process, manage and

share data coming from the respective sound masking digital signal processing board. The

software must perform all different kind of operations required to transmit and receive the

information from one node to another. Such software can be written in a programming

language and then, once converted by compiler into machine language, stored into a non-

volatile memory inside each wireless board.

The main hardware components for each network node of the transmission system are: a

processing unit (to acquire, process and mange data), a transceiver (to convert the

information from binary code to modulated radio frequencies), a non-volatile memory

(large enough to contain all the network protocol) and an antenna (to transmit and receive

information using radio frequencies). Depending on the transceiver of choice could also be

needed (in the RF front-end) a power amplifier or low noise amplifier. Respectively in case

the output power has to be increased or the received signal is too weak to be successfully

decoded.

Figure 4.1 shows the schematic view of the main components for each network node.


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Figure 4.1 Wireless board and its main components connected to a sound masking node

The processing power needed for promptly executing the network protocol can be

addressed using a modern microcontroller. This is a unit which represents a good trade-off

between processing power and unit cost. Furthermore, high-end modern microcontrollers

are fitted with non-volatile memory on board big enough to store the whole network

protocol. Using this component, would remove the need to install for a separate non-

volatile memory, reducing the size of each node and facilitating the board layout.

In line with the system requirements, the transceiver main properties are: the possibility to

span the entire 2.4 GHz ISM band; to have FHSS capabilities; to transmit/receive with an

adequate data rate; and have the ability to detect when a faulty data reception occurs. Such

capabilities can be found in modern low-cost transceivers, which are especially made to

interoperate with microcontroller units. However those chips have often limited

transmission power, so it could become necessary to boost the output power with a power

amplifier to reach the require range (this always respecting the regulations regarding the

maximum EIRP output power). Moreover, low-cost transceivers sensitivity could be

enhanced adding a low-noise amplifier to increase the power lever of weak received

signals. Such extra components to be installed between the transceiver chip and the

antenna will increase the system capabilities, but will also increase the price and design

complexity.

The antenna is another vital system component, it can be isotropic (irradiating power

equally in all directions) or directional. Theoretically, to achieve best performances its

length must be equal to half of the carrier wavelength. This means, for frequencies in the

2.4 GHz ISM band, its length would be around 12.5 cm.

On the other hand, for short-range communications it is also possible to use a printed

circuited antenna (also called PCB antenna): this type of antenna often consists in a


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microstrip track printed on the surface of the PCB board. Over the years, RF engineers

have designed advanced layouts for printed circuit antennae; probably the most successful

type of PCB antenna is what is commonly called the F antenna. This printed circuit

antenna is f-shaped and its dimensions are much shorter than conventional antennae: in

fact, f-antennae are usually not longer than few centimeters for the 2.4 GHz ISM band

frequencies. In addition, since it is only made of a microstrip track (with no need for a

balun) both production cost and layout complexity are reduced significantly. However, f-

antennae are designed for relatively short-range communications and its performances are

inferior to standard isotropic or directional antennae.

4.2 Hardware Component Selection

In this section, topics discussed comprise the selection for all hardware components to be

installed in each wireless board; such components will be chosen using parameters like the

requirements fulfilment, cost and ease of assembly as the main guideline.

In addition, the main aspects of hardware assembly and testing are discussed.

4.2.1. Microcontroller

As discussed in the previous section, a microcontroller unit is the ideal choice for giving

the wireless board enough processing power with both low cost and power consumption.

Even though low power consumption is not a requirement for this project, such feature

allows smaller packaging and eliminates the need for cooling fans. This makes the whole

board smaller, simpler to design and build, and above all, cheaper to produce.

Since the microcontroller will handle all the network protocol, the unit of choice has to be

a high-end model. Furthermore, as mentioned in the previous section, to simplify the board

layout and reduce the mounted components the microcontroller unit must be equipped with

enough onboard non-volatile memory to contain the whole network protocol software.

The major producers of these devices are Texas Instruments37

, Motorola38

and

Microchip39

. All these companies produce low-power, low-cost microcontrollers.

Currently, the most popular and widely used models are the MSP430 and PIC from Texas

Instruments and Microchip respectively. Using one of these devices would guarantee good

processing power, excellent reliability and low-cost. However, the company Embedded

Systems Projects have used Microchip components in the past and has some expertise in

programming such units. This would enable to company to provide product assistance and


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upgrades in the future. Hence, the PIC microprocessor is the microprocessor type of

choice.

The PIC family is divided in three main categories, depending on the word length: 8-bit,

16-bit and 32-bit. The 32-bit PIC microcontroller is most performing in the PIC family; it

provides both relatively high processing power and built-in memory space.

However, PIC32 models are in respect to the 8 and 16-bit types generally more expensive,

have bigger packaging dimensions and higher power consumption.

Due to the complexity of the network protocol and as neither power consumption or layout

dimensions are critical in this project, the PIC32 microcontroller is chosen as the

processing unit. This high performance microcontroller is an adequate device to manage

communication with both the sound masking node and the transceiver.

The the PIC32 family contains several device models, each one built with different

characteristics and features designed for specific area of application.

The microcontroller will be interfaced between the digital signal processing board (in the

sound masking system node) and the transceiver, both using an SPI connection to

communicate. Therefore, the model of choice must feature at least two SPI ports.

This essential feature narrows down the list of possible models to be chosen within the

PIC32 family. As mentioned, enough on-board memory is another vital requirement;

which is why, a model with 512 KB of built in non-volatile memory is preferred.

A PIC32 model that contains the required feature is the PIC32MX360F512L41

; in fact, this

high performance microcontroller has a clock frequency of 80 MHz and a word length of

32 bit, giving it fine processing power. It also features 512 KB of on-board flash memory;

it is equipped with 2 SPI ports; and it has a low price unit.

As a result, the PIC32MX360F512L is chosen to be the project microcontroller unit.

Figure 4.2 reassumes the microcontroller main features.


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Figure 4.2 PIC32MX360F512L main features42

4.2.2 Transceiver

The transceiver unit has the purpose to transmit data over RF frequencies to be shared in

the network. Therefore this is a vital component and extreme care has been taken to select

the most suitable chip. As mentioned in chapter 3, the 2.4 GHz ISM band is very used

worldwide therefore there is quite a wide variety of components to choose from. However,

the nature of the project and its requirements –the FHSS implementation above all- require

an accurate component selection. In particular properties such as frequency span,

frequency resolution, maximum output power, data rate, receiver sensitivity and frequency

calibration speed are the main criteria used to select the appropriate component.

As previously said, for this frequency range many companies such as RF Micro Devices 43

,

Texas Instruments and Microchip, however Texas Instruments has developed over the

years a wide variety of transceivers for the most used frequency range, gaining a leading

position in the market. As a matter of fact, table 4.1 shows all the transceiver made

available from Texas Instruments for the 2.4 GHz ISM band.


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Table 4.1Product comparison guide for the 2.4 GHz ISM band44

As table 4.1 shows there is a vast selection of transceivers from Texas Instruments for the

required frequency range, however it is easily noticeable that the CC2500 is the most

suitable for the project. This transceiver has the best frequency resolution in the product

range (427 Hz, which is remarkable for a low-cost chip), a maximum data rate of 500 kbps

(which is adequate for the project requirements), a fairly good sensitivity and, above all,

it‟s capable of FHSS technique –meaning it can change carrier frequency at a high rate.

This transceiver is also capable of Cyclic Redundancy Check (CRC), which is a method

for detecting corrupted data reception. All these features make this chip fulfil the given

requirements.

Furthermore, with a high sensitivity and a maximum output power of 1 dBm, the CC2500

transceiver eliminates the need for a power amplifier and a low noise amplifier to be

mounted between the transceiver and the antenna for increasing the communication range.

If for example the free space propagation model is applied, it is possible to calculate the

attenuation in the RF signal in the typical distance between two network nodes:


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𝐿𝐹.𝑆. 𝑑𝐵 = −10 log 𝜆2

4𝜋𝑑 2 = −10 log((

𝑐

2.44×109 )2

(4𝜋×10)2 ) ≅ 60.18 dB equation 4.145

Where λ is the wavelength of the carrier in the middle of the ISM band, c is the speed of

light in vacuum and d the typical node distance (10 meters). Hence, the RF signal

travelling from one node to another is expected to suffer an attenuation of about 60.18 dB.

This large attenuation in the signal strength is due to the propagation of the wave – in fact,

as an electromagnetic wave travels it expands in a spherical order; therefore as the wave

travels in space its energy density decreases as the inverse of the squared distance.

The calculated loss in the signal strength is only an approximation, assuming no objects or

reflections occur. However, for relatively short distances this path loss approximation

could still be accurate.

If two isotropic antennae are used, the received signal strength can be calculated as:

𝑃𝑅 = 𝑃𝑇 + 𝐺𝑇 + 𝐺𝑅 − 𝐿𝐹.𝑆. = −29 𝑑𝐵 − 60.18 𝑑𝐵 = −89.18 𝑑𝐵 = −59.18 𝑑𝐵𝑚

equation 4.245

Where PT is the transmission power in dB; while GT and GR are respectively the transmitter

and the receiver antenna gains, set to 0 dB as both antennae are passive devices ideally

radiating equally in every direction. Such power budget calculation will be repeated once

the antenna model is chosen; resulting in a more precise approximation.

As table 4.1 shows, a received signal strength of -59.18 dBm is well above the sensitivity

limit for the CC2500 transceiver (-99 dBm at 10kbps44

), even if the transmission uses with

the maximum data rate.

Consequently, due to its features, the Texas Instruments CC2500 is selected as the

transceiver of choice for this project.

4.3 Hardware Assembly and Testing

At this stage of the project, the two main components for the wireless transmission system

have been selected. The PIC32MX360F512L microcontroller will host on board and

execute the network protocol, sending and receiving data with both the sound masking

node and the CC2500 transceiver. The Texas Instruments chip instead will send and

receive data over RF frequencies, establishing a wireless link with all network nodes in its

sub network.


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Each wireless board will contain these two main components and an antenna.

However, the previously mentioned chips require the addition of several surface-mounted

components in order to work. These components can be mainly classified in resistors (to

adjust the voltage levels), capacitors (to filter frequencies or to accumulate charge for

current consumption peaks) and oscillators (to synthesize frequencies). For instance, figure

4.3 shows the external components that need to be connected to the CC2500 transceiver.

Figure 4.3 Overview of External Components to be connected to the CC2500 transceiver46

The wireless board layout has to take account of those external components as well as the

SPI connection tracks and the antenna matching circuit. The layout design will be made for

mass production, optimizing size and cost for each board; however, the creation of such

layout is not part of this project.

In this project instead of using the boards with the custom layout, general-purpose

development boards will be used. Manufacturers equip these boards with all the surface

mounted components needed, plus some additional hardware (such as LCD screens,

communication ports, LEDs and sensors), to provide developers a ready to use platform for

a vast variety of applications. Development boards are in fact the best solution to reduce

the time required for hardware setup, as they come already fitted with all the parts a

developer needs for whole variety of different projects. On the other hand, these boards

have generally a higher unit price and bigger dimensions with respect to single purpose

boards, but are usually bought in small quantities just for prototyping.

Microchip produces several development boards; Explorer 1647

(shown in figure 4.4) is

among them and it is designed for the PIC microcontroller family. It supports different

types of microcontrollers, including the PIC32MX360F512L. This board includes many


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different features with the most significant being a LCD display, serial connection,

potentiometer, USB port and a Pictail connector slot. Using such a board during the project

development stage would allow the use of all the PIC32 features without spending time for

the wireless board layout.

Figure 4.4 Microchip Explorer 16 Development Board47

To furthermore reduce the time spent on hardware setup during the developing stage, a

pre-fabricated module could also be used for the transceiver. As a matter of fact, Texas

Instruments produces the CC2500EM module, which is a PCB board with installed the

CC2500 transceiver, all the external components required for a correct functioning and a

balun circuit for plugging an external antenna. This module represents a very effective and

rapid solution for using the transceiver; nevertheless, the unit price is fairly high and is not

a convenient choice for the prototyping stage.

Quasar UK48

, a company that produces RF modules, also manufactures a board with the

CC2500 chip installed with all the surface mounted components. This module also

includes an F antenna in the same PCB, which eliminates the need of purchasing an

external isotropic antenna. In addition, above all, the unit price for each module is

relatively very low.

On the other hand, the quality of the components used in this module –such as the

oscillator or the F antenna- will not offer the same transmission quality offered by the

Texas Instruments module or by a custom PCB layout with first quality components. Yet

this reduction in quality is acceptable for the prototype board, and this module was


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approved to be part of the project; figure 4.5shows an image of the Quasar module, in

which can be seen: the CC2500 chip; its external surface mounted components; the SPI

connector; and the F antenna made by Microstrip tracks.

Figure 4.5 Quasar CC2500 module48

Since an F antenna is mounted in the prototyping board, a reduction in the link range is

likely to occur; Quasar does not include in the module datasheet the antenna radiation

pattern, however an estimation can be made using the radiation pattern from a known

similar type of F antenna. The Texas Instruments 2.4 GHz Inverted F antenna has almost

the same layout characteristics, is therefore a good performance reference point for the

Quasar module F antenna. In figure 4.6 is shown the radiation pattern for the antenna XY

axis (the plane parallel to the PCB board).


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Figure 4.6 Texas Instruments 2.4 GHz Inverted F Antenna78

From the XY radiation pattern shown on figure 4.6, it is possible to notice that the antenna

gain drops consistently in those regions around 10º and 190 º, which correspond to the

ends of the antenna longitudinal axis. The typical F antenna radiation patter is far from

being isotropic. However, as an approximation, the average gain along the XY plane for

this type of antenna is 1.1 dB79

. Therefore. the received signal strength (equation 4.2) can

be re-calculated as (with both transmitter and receiver gains set as 1.1dB):

𝑃𝑅 = 𝑃𝑇 + 𝐺𝑇 + 𝐺𝑅 − 𝐿𝐹.𝑆. = −29 𝑑𝐵 + 1.1 𝑑𝐵 + 1.1𝑑𝐵 − 60.18 𝑑𝐵 =

−86.98 𝑑𝐵 = −56.98 𝑑𝐵𝑚 equation 4.3

As a result, using the free space loss approximation over a 10 meters distance, the received

signal strength of at the receiver is -56.98 dBm; such power level is well above the

transceiver sensitivity (-99 dBm at 10kbps44

), making the Quasar CC2500 module an

adequate choice for the project requirements.


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To connect the transceiver module with the microcontroller SPI interface, the Explorer 16

Pictail connector can be used. Hence, a breadboard is plugged in the Pictail socket -each

contact in the breadboard matches to a pin in the PIC32. At this point a female connector

(which matches the male connector of the Quasar CC2500 module) is glued on one end of

the breadboard, creating a socket in which the transceiver module could be plugged in.

Finally, each pin of the female connector is soldered with the correspondent contact on the

breadboard, effectively putting in contact the CC2500 module with the PIC32 pins.

More in detail the connection between the CC2500 and the microcontroller was established

following the SPI protocol pinout. In fact, each SPI interface is made of 4 pins: serial data

in, serial data out, clock, and cable select. A description of the SPI protocol will be made in

the next chapter, however a correct hardware connection between the transceiver and the

microcontroller is vital for the system functioning.

Figure 4.7 shows the pinout on the Pictail connector on the Explorer 16 board (where the

SPI interface, power and ground PINs can be seen); while figure 4.8 shows the Quasar

CC2500 module.

Figure 4.7 Pictail connector pinout49


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Figure 4.8 Quasar CC2500 module pinout50

Hence the connection order followed was:

CC2500 module Explorer 16 board

PIN 1 (Vcc 3.3 V) ↔ PIN 21 (Vcc 3.3 V)

PIN 2 (DATA IN) ↔ PIN 7 (DATA OUT)

PIN 3 (SPI CLOCK) ↔ PIN 3 (SPI CLOCK)

PIN 4 (DATA OUT) ↔ PIN 5 (DATA IN)

PIN 5 (NOT CONNECTED)

PIN 6 (GROUND) ↔ PIN 9 (GROUND)

PIN 7 (GDO0) ↔ PIN 101 (I/O PIN)

PIN 8 (CABLE SELECT) ↔ PIN 1 (CABLE SELECT)

Table 4.2 PIN connection list

Table 4.2 shows the SPI pinout configuration: the microcontroller data output pin is

connected with the data input pin on the CC2500, and similarly the microcontroller data

input is connected with the data output pin on the CC2500. These pins are used to transmit

data on the SPI interface. Instead, the clock pin is used to synchronize the SPI data

transfer; in fact, data will be sampled only when a transition occurs on this pin. The cable

select pin is used to enable or disable the communication with the transceiver. As a matter

of fact, communication with the transceiver can occur only if the cable select pin is at a

low logic level (0 V), instead if this pin is at a high logic level (3.3 V) the CC2500 will

ignore every communication.


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In order to avoid a „cold soldered joint‟ each connection was tested after the soldering. A

multimeter was used to check if electrical connection was established between the

microcontroller and the transceiver pins. Figures 4.9, 4.10 and 4.11 show the main steps

followed for connecting the microcontroller with the CC2500 module.

Figure 4.9 (left) Soldering the glued female connector with the corresponding contacts. Figure 4.10 (right)

Plugging the CC2500 module in the glued female connector.

Figure 4.11 The breadboard inserted into the Pictail connector of the Explorer 16 board.


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Once all the connections were verified and the breadboard (fitted with the CC2500

module) inserted into the Pictail connector, full communication was established between

the microcontroller and the CC2500 transceiver –effectively creating the prototype

wireless board. This board, in addition, is equipped with a serial connection which will

become very handy for debugging and testing during the network protocol development

phase.

However, in order to establish a wireless link at least two wireless boards are needed; so

another development board and transceiver module are required. Microchip produces a

cheaper development board in respect to the Explorer 16, this smaller and simpler board is

named PIC32 Starter Kit51

. This development board comes fitted with a

PIC32MX360F512L microcontroller, a USB connection for programming, three LEDs and

a general-purpose expansion slot. This board has a lower price compared to other

development board; however, it does not include a serial connection or a Pictail connector.

For this reason, to connect the Starter Kit board to the breadboard fitted with the CC2500

module, a Microchip „I/O Expansion Card‟ is first plugged on top of the Starter Kit

expansion slot. Since such expansion card is equipped with a Pictail connector, a

breadboard could then be inserted, finally connecting the microcontroller in the Starter Kit

with the CC2500 module. Figure 4.12 and 4.13 show the afore mentioned components.

Figure 4.12 PIC32 Starter Kit52

(bottom view). Figure 4.13 I/O Expansion Board with the Pictail connector53


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As the Pictail connector in Microchip I/O Expansion card has the exact same pin layout as

the Pictail connector, the pin connection list used is the same as shown in table 4.2 in the

case of the Explore 16 board. For this board also the all series of connection tests were

performed, assuring a reliable link between the CC2500 and microcontroller pins.

Figure 4.14 All the hardware assembled and tested, ready to be programmed

At this point all the hardware which is needed to perform a peer to peer wireless link is

assembled and tested. Setting up a link with two boards will allow to develop a network

protocol that can be extended to a network of nodes. As all the hardware required is all set,

the next project phase is the software development.

4.4 Software Design

Both development boards are equipped with the same microcontroller unit, the well

mentioned PIC32MX360F512L. To make the microcontrollers manage their respective

transceiver unit a firmware must first be programmed into the onboard flash memory. Such

firmware, (also simply called microcontroller software) will contain all the instructions and

modus operandi to run the wireless link, so such element represents the key project aspect.

This protocol will have to make the most of the hardware installed on each board.

4.4.1 Software Environment

Microchip has developed a software environment called MPLAB54

specifically designed

for programming Microchip microcontrollers and digital signal processors. These units are

general-purpose processors and need a firmware to be installed in their on board flash

memory to operate. The device function is only dictated by the firmware installed. The


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MPLAB PC program allows developers to write the firmware in C language, compile it,

and then transfer it into the device.

The first step for programming a device is selecting from a drop down menu, then

choosing what type of device is going to be used, the PIC32MX360F512L for this project.

Once the device is selected, a C program can be written in one or more source files, saved

as .c source file. The syntax and instructions set used in the MPLAB environment is very

similar to the standard ANSI C, however some specific functions are also implemented

(such as those for code optimization or device specific ones). After the C code is

completed, the MPLAB C compiler will first check the syntax of the code (looking for

errors or inconsistencies); then, if the check is successful, it will compile the C code.

Compiling the code transforms the human oriented C instructions into binary code, the also

called machine language. This series of ones and zeros contains all the instructions that the

device will run. Such code can vary from device to device: that is the reason why, prior to

compiling, the exact device type must be selected.

The next step is to transfer the just created code into the device, hence the device board

must be connected to the PC. Such operation can be done in several ways: by using a USB

cable, by using a ICD2 or a REAL ICE programmer. The method by which a device is

programmed depends on the connection used between the PC and the microcontroller

board; the main factor that changes using different transfer method is the transfer speed.

Furthermore, the REAL ICE programmer has the lowest program time and can be used for

debugging. In fact, the REAL ICE programmer can in real time pause the execution of a

device program, allowing developers to check the program flow and variables content. The

Explorer 16 developing board is compatible with a REAL ICE programmer while the

Starter Kit will be programmed through a USB cable.

After the device is programmed the code can be ran and paused from the MPLAB

environment, permitting developers to verify the program tasks. Figure 4.12 shows the

MPLAB environment with the development boards connected ready to be programmed.


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4.4.2 SPI Interface and Transceiver Configuration

While the microcontroller unit offers a

general purpose processing unit, that will

execute the instructions saved in its built it

memory, the CC2500 transceiver is

programmed through the 4 wired SPI

interface (figure 4.15). Moreover, the same

interface will be used to establish

communication between the wireless board

and sound masking node, however such

communication will not be discussed as it it not part of the project. The developemnt will

in fact focus on the network protocol (installed in the microcontroller) and in the

transceiver configuration.

More specifically, the CC2500 transceiver has 47 one byte long configuration registers,

that can be either read or written through the SPI protocol, configuring the transceiver; to

each register is assigned one or more specific chip feature. Hence, to modify the CC2500

behaviour and performance the microcontroller unit must access and modify several

transceiver registers. Each register is given a unique hexadecimal address, consequently for

accessing each register, the microcontroller unit must initially send the register address

(specifying if that register is going to be read or written) and then perform the read or write

operation. Such commands are sent using the SPI serial data out pin from the

microcontroller (MOSI); and received in the serial data in pin in the CC2500 (SI). Figure

4.16 shows the address header that must be sent by the microcontroller for accessing a

transceiver register. It is possible to notice that the most significant bit (MSB) specifies if

that register is going to be read or written (if zero it is a write operation, if one it is a read

operation). The second bit instead is the burst bit: if this bit is set, the transceiver will

expect multiple register read/write operations, not requiring the transmission of an address

header (an internal counter will automatically increase the address by one for every data

byte sent by the microcontroller). Finally, the last 6 bits represent the register address that

is going to be read or written.

Figure 4.15 4-Wire SPI Interface55


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Figure 4.16 CC2500 Address Header56

The all sequence for reading and writing a register in the CC2500 is well shown by

figure4.17, found in the DN503 Chipcon design note.

Figure 4.17 Single Byte Access: register writing (top) and reading (bottom)57

It is possible to notice that for establishing communication with the transceiver, the

microcontroller unit must first set the CS pin low, and then send the address header byte

-specifying in the most significant bit whether is a reading or writing operation. In case of

a write operation, after the header byte the microcontroller sends the byte to be written in

that register. Instead, in case of a read operation, after the header byte the microcontroller

just toggles the clock pin to receive the register content over its MISO pin. In fact, as the

SPI protocol specifies, every data transfer is done only when the CLK pin is toggled. In

case of the CC2500 the data is sampled on the clock transition from a low to a high level,

as shown by figure 4.18.

Figure 4.18 SPI Clock Phase and Polarity55

Each configuration register has to be programmed with the specific value corresponding to

the desired functionality. In fact, it is by changing some specific configuration registers


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that characteristics like modulation scheme, data rate, packet length or output power are

applied. Due to the big number of configuration registers and the high number of

transceiver functionalities, it can be a complex and time consuming operation to find, for

each configuration register, the exact value that produces the desired chip behaviour.

To help developers finding the correct registers values to program into the chip, a PC

software can be used. This software, produced by Texas Instruments, is called Studio RF58

.

Such software allows to selectively enable all the different options and transceiver

parameters, converting such choices into configuration registers values. It is also possible

to export such values with syntax compatible with the C programming language, making

easy to implement such code into the microcontroller software. However, the current

version of the program (v.6.11.5.0), is best suitable for programming transceivers installed

on Texas Instruments development boards. This is the reason why, for some specific

registers, the configuration values were calculated by hand. Figure 4.19 shows the list of

all the CC2550 configuration registers: the whole transceiver programming is dictated by

those registers values.

Figure 4.19 Configuration registers overview59


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The CC2500 transceiver it is quite a complex chip, however it is possible to send direct

command in order to change it is behaviour (or state). Such commands are also called

command strobes. A command strobe takes the shape of a one-byte long instruction, that

once received in the CC2500 serial data input pin, makes the chip „jump‟ in the requested

state. For instance, it is possible to send command strobes to make the chip go in reception

mode, transmission mode, idle mode or flush the data FIFOs (First Input First Output are

two special registers that contain data to be sent or just received. Both the transmit and

receive FIFO are for the CC2500 transceiver 64 bytes long.

In addition, every time the CC2500 receives a command strobe it also automatically

outputs on its serial output pin a status byte. Such status byte contains the main information

about the system status; such as the state in which the transceiver is or the number of bytes

in the FIFOs. Figure 4.19 shows the complete radio control state diagram of the CC2500

transceiver.


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Figure 4.20 Complete Radio Control State Diagram60


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As it is possible to notice from figure 4.20, sending command strobes allows to move

through the CC2500 states.

Therefore, the software managing the communication between the microcontroller and the

transceiver can be built on few basic functions:

A function to read a specific register;

A function to write a specific register;

A function to send command strobes;

A function to read the status byte;

All the tasks performed by the network protocol are made using those above basics

functions. For instance, to transmit data one only needs to fill the transmit FIFO (using the

write register function) and then enter in the transmit mode by sending the transmit strobe.

Similarly to receive data the procedure is to first enter in receive mode by sending the

receive strobe, then ensure a full packet is received and finally read the receive FIFO

(using the read function). Even changing carrier frequency involves, in this order, writing

into a register, sending a command strobe and reading a register.

Hence, the whole firmware is divided into four main source files:

main.c (which contains the high level instructions);

MicroSetup.c (which mainly contains a function used to setup the microcontroller

clock and serial connection. These functions are part of the Microchip library);

SPI.c (which contains the mentioned basic functions);

and RF.c (which contains those functions used to manage data transfer);

In the next chapter, all the functions used in the network protocol will be analyzed in

detail.


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Chapter 5

Development of a FHSS MAC Layer Protocol

Developing a network protocol that uses FHSS technique is not a straightforward task,

since many aspects and details need to be taken into account. The approach adopted in this

project is to develop the MAC layer of the network protocol step-by-step, first starting with

a simple peer-to-peer link over a fixed carrier frequency. Once such result is achieved the

development will gradually evolve toward an adaptive FHSS network protocol. Hence, the

final network protocol will still contain most of the code used for a fixed frequency link;

with the addition of the more advanced functions.

The following functions are given in pseudo code, and describe all the operations needed

to control the CC2500. As the pseudo code is not device specific, such functions could

easily be ported to be ran by different microcontrollers. Furthermore, the device specific

functions, such as the ones used to configure the microcontroller clock speed and serial

communication are not explained. In fact, those are part of the standard code library and

have not been developed in this project. Such functions are standard for every PIC32

device and are all only called in the file MicroSetup.c. All the source code explained in this

chapter is found in full in the appendix at the end of this paper.

5.1 Basic Functions

As previously mentioned, the functions that perform basic transceiver operations (such as

read or write a register) are vital for the whole network protocol. These functions are

contained in the source file SPI.c. This file contains in its first lines some useful

definitions, which will make the code more readable and compact; the functions are here

explained in the same order as they appear in the SPI.c source file.

As the CC2500 has both address header and registers one byte long, it is useful to define a

custom variable type of the same size. This variable type has been called „BYTE‟ and is

defined as an unsigned char, meaning its content can be a number ranging from 0 to 255

(exactly as 28, the byte definition).

The pseudo code instruction is: define type unsigned char as BYTE


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It is also useful to define the command strobes names: in fact, a hexadecimal number

represents each command strobe that can be sent to the transceiver. It is then easier to

define a name for each command strobe, in order to use just a name throughout the code

instead of its number. Table 5.1 shows the list of all command strobes that can be sent to

the CC2500 transceiver.

Figure 5.1 Command Strobes list61

Therefore, for each command strobe a name was defined (the names are the same used in

Figure 5.1).

The pseudo code instruction is: define command strobe 0x30 as SRES

As showed by figure 4.17 for enabling communication with the CC2500 transceiver the

microcontroller must fist pull the cable select pin (CS) low. This is can be programmed

into the microcontroller using a PIC32 library function called PORTClearBits(PORT,

NUMBER)62

. Such function takes two parameters as input: the port number and the pin

number. In fact, in the PIC microcontroller family, all the pins are divided into ports

(ranging from A to G in the PIC32), where each port contains pin numbers (ranging from 0

to 15 in the PIC32). Hence, for referring to a specific microcontroller pin both the port and

the number must be specified. Therefore, a simple function named CSlow is created, which

by calling the PORTClearBits, allows to set the CS pin to a low logic level of 0 V,

enabling communication with the transceiver. As showed by figure 4.4, the cable select pin


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is connected with the pin at port B, number 2 at the microcontroller interface.

Consequently, the pseudo code is:

function CSlow{

PORTClearBits(B,2)

}

Similarly, to interrupt communication with the transceiver the cable select pin must be set

at a high logic level of 3.3 V. This can be done using the PIC32 Library function

PORTSetBits(PORT, NUMBER)63

; which has an analogue behaviour as the previous

mentioned PORTClearBits. Therefore, the function created to set the CS high is:

Function CSHigh{

PORTSetBITS(B,2)

}

However, as mentioned in the transceiver datasheet, the CC2500 might take some time

before being operative after the microcontroller pulls the CS pin low. The ready state for

the transceiver is signalled by its data out pin, which goes low once the chip is fully

operative. Hence, a function called CCready is created; this function contains a while cycle

which continuously checks the state of the CC2500 output pin (connected with the pin F7

at the microcontroller, as figure 4.4 shows). Only when the transceiver data output pin goes

low, the program exits the while cycle and the code execution can continue. This function

effectively prevents the microcontroller from communicating with a non-ready transceiver.

The pseudo code is:

Function CCready{

while(F7 is high) stays here

}

As most of the PIC32 pins are capable of different options, part of the code must enable

those pins used the SPI protocol (as showed by figure 4.4). The PIC32 Library contains a

function called SpiChnOpen64

, which enables the SPI module in the PIC32. By selecting

different parameters, it is possible to set, among other things, the communication speed,

the SPI channel, the master device, the clock polarity, in what clock transition data should

be sampled, and the transfer size word. Given the parameters, this function automatically

finds the correct values to put in the SPI1CON register (the register that defines the

parameters for the SPI channel 1). The transceiver datasheet specifies that the CC2500 is


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always to be selected as the slave device in the SPI communication, the communication

speed must not exceed the 10 MHz, the clock idle state is low and the data sampling

happens on the clock rising edge (such information is also contained in the Chipcon design

note DN503). Hence, all those mentioned parameters are included in the function

OpenSPI, which calls the SpiChnOpen with all the required options. Furthermore, this

function also enables the pin D8 as an input pin (using the PIC32 Library function

PORTSetPinsDigitalIn65

). Such pin will be used in the protocol to sample the value of the

transceiver GDO0 pin, as explained later in the document.

Function OpenSPI{

SpiChnOpen(1, SPICON_ON | SPICON_MSTEN | SPICON_CKE , 400)

PORTSetPinsDigitalIn(IOPORT_D, BIT_8);

}

The parameters used in the function SpiChnOpen fulfil the requirements for

communicating with the transceiver, this using the SPI channel 1, a transfer word of 8 bit,

and a communication speed of 200Kbaud. A transfer word of 8 bit has been chosen since

both the address headers and the registers, in the CC2500, have a 8 bit size.

To be noted that the communication speed has been set finding the value for fpbDiv,

following the formula:

𝑓𝑝𝑏𝐷𝑖𝑣 = 𝑓𝐶𝐿𝐾

𝐵𝑎𝑢𝑑𝑅𝑎𝑡𝑒=

80𝑥106

200𝑥103 = 400 equation 5.164

By selecting a fpbDiv parameter equal to 400 for the SPI channel 1, the SpiChnOpen

function will automatically put in the SPIBRG register (the register where the SPI

communication speed is set) a value of:

𝑆𝑃𝐼𝐵𝑅𝐺 = 𝑓𝐶𝐿𝐾 −2×𝐵𝑎𝑢𝑑𝑅𝑎𝑡𝑒

2 ×𝐵𝑎𝑢𝑑𝑅𝑎𝑡𝑒=

80 × 106− 2 ×200×103

2 ×200 × 103 = 199 equation 5.264

A communication speed of 200 Kbaud between the microcontroller and the transceiver is

believed to be sufficient for the required application.

Instead, to close a previously opened SPI connection the function CloseSPI() is created.

This function simply contains a call to the PIC32Library function SpiChnClose66

(where

the channel closed is channel 1).

At this point, the microcontroller can open the SPI connection by calling the OpenSPI

function, and the communication with the transceiver can start by calling the function

CSlow and waiting for a „ready acknowledgement‟ from the CCready code.


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As a result, one of the protocol key functions can be written: readRegister is a function that

returns the register content for a given address. As illustrated by figure 4.14 to read the

content of a single CC2500 register first an address header as to be sent: such header will

contain the most significant bit set in read option (set to 1) and the successive bit, the burst

bit set to zero (resulting in a single read). The rest of the header must contain the address of

the register to be read for the remaining bits (see figure 4.13). Once such address header is

sent, the CC2500 will output the register content on its serial output pin. The register

content is outputted each time the SPI clock pin is toggled, outputting the most significant

bit first. To make the SPI clock pin toggle at connection speed of 200kbaud, a „dummy

byte‟ is sent by the microcontroller. This byte will not be read by the CC2500, but instead

will just make the clock pin toggle 8 times making the transceiver outputting the requested

register content. The pseudo code for the readRegister function is:

Function readRegister(register given as input parameter){

enable communication

when CC2500 is ready

send one byte with (requested register + read bit set to 1)

wait for address header to be sent

read the status byte sent by default by the CC2500

send one byte of dummy data (just to make the SPI clock pin toggle 8 times)

wait for dummy byte to be sent

read the register content outputted by the transceiver

return such value

close communication

}

These are the steps required for successfully reading a transceiver register. To send a byte

over the SPI interface, the PIC32 Library function putcSPI167

has been used. This function,

when the 8 bit transfer word is enabled, sends a byte of information from the

microcontroller SPI output pin, starting with the byte most significant bit first. A bit is sent

each time the SPI clock pin toggles; which happens, at 200kbaud, every 5μs. Successfully

reading a transceiver register is, as previously said, a vital point in the program

development. However, during the code development many difficulties were encountered

before accomplishing this task. In fact, reading a register is not as straight forward as

sending the address header, a „dummy byte‟, and reading the content. Every time the

transceiver receives data on its input pin it automatically outputs, on the CC2500 output


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pin, a status byte. As figure 5.2 shows, on the SPI module of the PIC32 both the SPI

transmitter and SPI receiver register share the same address, named SPI1BUF (in the case

of SPI channel 1). Hence, after each byte is sent to the CC2500 transceiver, the content of

the SPI1BUF register has to be read out. This in order to clear the microcontroller

transmission/reception SPI1BUF register from the received transceiver status byte. This

procedure is a peculiarity of the PIC32 microcontroller and has to be strictly followed each

time a byte is sent to the CC2500. The content of the received status byte can either be read

and processed or just read a discarded.

Figure 5.2 SPI Module Block Diagram68

Additionally, in the early stages of the project a function from the PIC32 Library named

readSPI169

was used to read data over the SPI protocol, yet with no results. The function

description in the PIC32 Library Guide states “This function will read single byte/half

word/word from SPI receive register”69

. As known, the SPI protocol requires the SPI clock

pin to toggle for each bit transfer in both send and read operations. Instead, after accurate

testing performed with a logic analyzer, this function was proven not to make the SPI clock


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pin toggle, therefore not reading any data. This is the reason why, in the final code, this

library function is not used. The solution found for reading data from the transceiver

consist in: manually making the clock pin toggle by sending a „dummy byte‟ and afterward

reading out the content of the SPI1BUF register.

Having implemented this code allows the PIC32 microcontroller to access and read every

register in the transceiver. To use such a function one only needs to specify the register

address as all the other steps are automatically performed.

Another key function for the whole firmware is the one that allows modifying a transceiver

register value. It is by changing the CC2500 configuration registers values that the

transceiver can be programmed. Hence, the function writeRegister is implemented. The

pseudo code for this function is:

Function writeRegister(register address, value to be written){



send one byte with (register address + write bit set to 0)



send the value to be written in the register

wait for data to be sent


close communication

}

This function is closely related to the previous one; the main difference is that the PIC32

doesn‟t read a value after sending the address header, but instead sends the data to be

stored on the specified register. Therefore, the function allows change the content of any

transceiver configuration register. Figure 5.3 shows a screenshot of the logic analyzer

output. The logic analyzer is attached to the SPI interface wires while a read a write

operation are performed. In this figure, it is possible to notice that the cable select line is

pulled low and for every bit sent by the microcontroller on the MOSI pin the transceiver

outputs a bit on the MISO line (either part of its status byte or a register content value).

Moreover, it is possible to notice that every communication is made one byte at the time;

the clock pin toggles 8 times for every bit, and data is sampled on its rising edge. With the


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aid of the marker was also possible to verify the communication speed: in fact, every clock

cycle takes 5 us, equivalent to the communication speed of 200 kbaud.

Figure 5.3 Read and Write SPI operation sampled with a logic analyzer

As mentioned in chapter 4.4.2 for changing the transceiver state command strobes can be

used. A simple function sendStrobe is then written allowing the microcontroller to move

through the CC2500 states showed on figure 4.17. The pseudo used is:

Function sendStrobe(desidered next state){



send one byte the next state name



close communication

}

As a result, is now possible to dynamically change the transceiver states, entering for

example into receive, transmission or idle mode; one only needs to call the function

sendStrobe specifying the strobe name (as such names where previously defined). To be

noted that the path to follow when moving from one state to another is still the one

depicted on figure 4.17.

Moving from one state to another, however, is not instantaneous. In fact, the CC2500 takes

a specified number of its clock cycles to change its state. Figure 5.4 shows such transitions

timing requirements.


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Figure 5.4 State transition timing70

Consequently, the microcontroller has to take account of the time needed by the transceiver

to change state. With a clock frequency of 80 MHz, each microcontroller clock cycle takes

about 12.5 ns;

Hence,

to change state from RX to IDDLE (with no calibration involved) the

microcontroller has to wait a number of cycles equal to:

𝑃𝐼𝐶32 𝐶𝐿𝐾 𝐶𝑌𝐶𝐿𝐸𝑆 = 𝑅𝑋 𝑡𝑜 𝐼𝐷𝐿𝐸 𝑛𝑜 𝑐𝑎𝑙𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛 𝑇𝐼𝑀𝐸

𝑇𝐶𝐿𝐾 =

0.1 ×10−6

12.5 ×10−9 = 8 𝑐𝑦𝑐𝑙𝑒𝑠

to change state from IDLE to RX (with no calibration involved) the

microcontroller has to wait a number of cycles equal to:

𝑃𝐼𝐶32 𝐶𝐿𝐾 𝐶𝑌𝐶𝐿𝐸𝑆 = 𝐼𝐷𝐿𝐸 𝑡𝑜 𝑅𝑋 𝑛𝑜 𝑐𝑎𝑙𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛 𝑇𝐼𝑀𝐸

𝑇𝐶𝐿𝐾 =

88.4 ×10−6

12.5 ×10−9 = 7072 𝑐𝑦𝑐𝑙𝑒𝑠

to perform a frequency calibration the microcontroller has to wait a number of

cycles equal to:

𝑃𝐼𝐶32 𝐶𝐿𝐾 𝐶𝑌𝐶𝐿𝐸𝑆 = 𝐶𝐴𝐿𝐼𝐵𝑅𝐴𝑇𝐼𝑂𝑁 𝑇𝐼𝑀𝐸

𝑇𝐶𝐿𝐾 =

721 × 10−6

12.5 × 10−9= 57,680 𝑐𝑦𝑐𝑙𝑒𝑠


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Therefore three functions called delayRX_TXtoIDLE, delayIDLEtoRX_TX,

delayCALIBRATION are written; each functions contains a while loop of 15, 7500 and

60000 cycles respectively (the cycles number are slightly bigger than previously calculated

to compensate possible variation in the PIC32 clock frequency). Such functions guarantee

a sufficient delay when switching between reception, transmission and idle states (those

are the main states used in the network protocol).

It is now possible to efficiently change the CC2500 state by just sending a command strobe

and calling the adequate delay function.

5.2 Output Functions

Since the Explorer 16 development board has a serial connection built in, it is possible by

connecting such board to a PC, to output data from the microcontroller unit to the PC

screen. In the source file MicroSetup.c are listed the functions used to configure the serial

module (done with Microchip PIC32 Library functions). Several functions have been

written in the source file SPI.c to output information about the transceiver state and

configuration registers values.

Here below is a list of all the functions used to output different information about the

transceiver status on a PC screen (data displayed with the Hyperterminal PC program):

noBytesInTXFIFO; this function outputs the number of data bytes currently

written in the TX FIFO. This done by reading the transceiver register 0xFA.

noFreeBytesInTXFIFO; this function outputs the number of bytes currently still

available to be written in the TX FIFO. This done by interpreting the CC2500

status byte.

noBytesInRXFIFO; this function outputs the number of data bytes currently

written in the RX FIFO. This done by reading the transceiver register 0xFB.

noAvaBytesInRXFIFO; this function outputs the number of data bytes currently

still available in the RX FIFO. This done by interpreting the CC2500 status byte.

displayAllregisters; this function displays on the PC screen the name and the

content of each transceiver configuration register (addresses ranging from 0x00 to

0x2E).

getState; this function outputs the name of the current CC2500 state by reading and

interpreting the value stored in the MARCSTATE register (address 0xF5).


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getStatusByte; this function has the same purpose as the previous getState, but the

information in this case is recovered by getting and interpreting the CC2500 status

byte.

These functions have been linked to keyboard keys, helping development and debugging.

In fact, the final wireless board is unlikely to have installed a serial connection.

Consequently, such output functions are only useful during the development stage.

The keys layout used is:

L key: calls and outputs the function readRegister with the channel number register

set as address header. Therefore pressing the L key outputs the current channel

number used by transceiver.

V key: sends a strobe to flush the RX FIFO.

R key: sends a strobe to reset the transceiver.

D key: calls the function displayAllRegisters.

I key: sends a strobe to change the state into idle.

S key: writes a test value into the channel number register.

P key: outputs the bytes in the RX FIFO by calling the noBytesInRXFIFO function.

A key: outputs the available bytes in the RX FIFO by calling the

noAvaBytesInRXFIFO function.

G key: outputs the current transceiver state by calling the getState function.

K key: outputs the current transceiver state by calling the getStatusByte function.

Z key: outputs the bytes in the TX FIFO by calling the noBytesInTXFIFO function.

X key: outputs the bytes available in the TX FIFO by calling the

noFreeBytesInTXFIFO function.

T key: sends a strobe to enter in the transmit state.

F key: sends a strobe to enter in the receive state.

Figure 5.5 shows a screenshot of the Hyperterminal window while communication with the

transceiver is established. In this picture it possible to notice the output of the functions

getStatusByte, sendStrobe(IDLE), noBytesInRXFIFO, noAvaBytesInRXFIFO and

noBytesInTXFIFO.


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Figure 5.5 Hyperterminal screenshot while calling different output functions

5.3 Configuring the Transceiver

Configuring the transceiver is a matter of writing the desired values into the configuration

registers. As mentioned, most of the registers values have been found using the Studio RF

PC program, which calculated most suitable values for the specified parameters. The

configuration registers to be set, range from the address 0x00 to 0x2E; however not all of

them have to been changed from their default values. In fact, after every power on or reset,

these registers return to a default state. Most of the registers values have been found using

the Studio RF program, except the most critical ones. Here is a list of those registers that

have been manually set to suit the project needs:

GDO0 pin configuration (IOCFG0);

packet length (PKTLEN);

data packet configuration (PKTCTRL0, PKTCTRL1);

device address (ADDR);

channel spacing registers (MDMCFG0, MDMCFG1);

channel number register (CHANNR);


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main radio control state machine configuration (MCSM1, MCSM0);

The GDO0 output pin configuration register (IOCFG0, found at the 0x02 address) sets the

configuration for the transceiver general purpose GDO0 pin. While its default value is 63

(or 0x3F in hexadecimal), it can be changed to suit a vast variety of settings. However, as

advised in the Chipcon design note DN500, the register value can be changed for applying

an interrupt driven solution for packet handling. In fact, setting the register IOCFG0 to a

value equal to 0x06, makes the GDO0 pin (while the chip is in transmit mode) assert when

a packet is starting to be transmitted and de-assert when the packet has been fully

transmitted. Instead, while the chip in receive mode, it makes the GDO0 pin assert when

the beginning of a packet has been received and de-assert when the packet has been fully

received. As showed by the table 4.2 the transceiver GDO0 pin is connected with the

microcontroller 101pin. Therefore, by sampling the value of such pin it is possible to

retrieve information about packet transmission/reception (also called interrupt driven

solution for packet handling).

The packet length register (PKTLEN, found at the 0x06 address) specifies the payload

length for each packet. As a matter of fact, the CC2500 is capable of transmitting payloads

of fixed, variable and infinite lengths. If a fixed length payload is selected, the PKTLEN

register specifies the payload length. To be noted that, as figure 5.6 shows, the CC2500

transceiver is capable of many different packet-handling options. It is then possible to add,

for each packet, a variety of information regarding the packet destination address, the

payload length, the signal strength of the received packet and the result of the CRC test. It

is important to underline that the PKTLEN register specifies just the payload length. This

value determines the number of bytes expected in the RX FIFO for every received packet

(or the bytes expected in the TX FIFO when a packet is transmitted). When using a fixed

packet length transmission, every packet with a payload size larger than the PKTLEN

value will be discarded. It is then vital that all the transceivers involved in the

communication share this same register value.


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Figure 5.6 Packet Size options71

The packet automation control registers (PKTCTRL1 and PKTCTRL0, respectively found

at 0x07 and 0x08) determine which packet handling options are enabled. In detail, it is

possible to: -enable the flush of RX FIFO in case of corrupted data reception (CRC check

failed), this would avoid the system to process corrupted data; -to append two status bytes

after the payload to each packet (the first byte containing the signal strength indicator, also

called RSSI and the second one containing information about the link quality and the result

of the CRC check); -to enable the address check (this adds a prefix before the payload,

specifying the destination for each packet), in this way only the transceiver with a

matching address would receive that packet; -to enable data whitening (enables an

algorithm to avoid sending long series of zeros or ones as this decreases transmission

performances); -to enable or disable the CRC check; -and to select between fixed, variable

or infinite packet length mode. Figure 5.7 reassumes the different options for packet

handling. To note that at the beginning of each packet transmission, a minimum number of

preamble bytes are sent (4 by default). Such preamble bytes (which are sent as soon as the

transmit state is entered by the transmitter) are useful to make the low noise amplifier

adjust its gain at the receiver, as the variable gain depends on the strength of the received

signal power. Moreover, after the preamble bytes a sync word is also automatically sent.

This sync word, which is a predefined pattern of bits that can be changed in size and

length, has the purpose of making distinguish the CC2500 packets from other wireless

transmissions.


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Figure 5.7 Packet Format72

These are key registers and their values will be changed to suit the protocol needs as the

project develops.

The device address register (ADDR, found at 0x09 address) specifies the transceiver

network address. If the network address check option in the aforementioned PKTCTRL1

register is enabled, this register can be by the network protocol to deliver data only to

certain recipients. As this register, as all the configuration registers, is one byte long, a

maximum number of 256 addresses are available. Practically, the addresses available are

255 as 0x00 is set as the broadcast address (this if the last two significant bits of the

PKTCTRL1 register are set to 10). However, for the project needs, this number is adequate

and such address register will be implemented in the network protocol for packet sorting.

The modem configuration registers (MDMCFG0 and MDMCFG1, respectively found at

0x14 and 0x13 addresses) are responsible for the channel spacing. The CC2500 can

synthesize carrier frequencies ranging in the whole 2.4 GHz ISM spectrum, however to

make changing frequency easy, the frequency range is divided in 256 channels. The

bandwidth of each channel is determined by the data rate and automatically set by Studio

RF. It is then possible, by changing the values in the modem configuration registers to

vary the spacing between channels. More in detail, the value can be calculated with the

formula:

𝛥𝑓𝐶𝐻𝐴𝑁𝑁𝐸𝐿 = 𝑓𝑋𝑂𝑆𝐶

218 × 256 + 𝐶𝐻𝐴𝑁𝑆𝑃𝐶𝑀 × 2𝐶𝐻𝐴𝑁𝑆𝑃𝐶 _𝐸 equation 5.173

Where fXOSC is the transceiver oscillator frequency (26 MHz in the Quasar module used),

CHANSPCM is the value stored in the MDMCFG0 register and CHANSPC_E are the last

two least significant bits in the MDMCFG1 register.

In order to use the whole 2.4 GHz ISM frequency range, the required channel spacing was

calculated as:


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𝐼𝑆𝑀𝑅𝐴𝑁𝐺𝐸 = 2483.5 × 106 − 2400 × 106 = 83.5 × 106 𝐻𝑧 (spectrum available)

equation 5.2

𝑀𝐴𝑋𝛥𝑓𝐶𝐻𝐴𝑁𝑁𝐸𝐿 =𝐼𝑆𝑀𝑅𝐴𝑁𝐺𝐸

# 𝑐ℎ𝑎𝑛𝑛𝑒𝑙𝑠 =

83.5×106

256 = 326.17 𝐾𝐻𝑧 (maximum allowed channel

spacing) equation 5.3

Given the maximum allowed channel spacing when using a 256 channels system, equation

5.1 is then left with two unknown. The approach followed was to fix CHANSPC_E at a

value of 3, and solve the equation in respect to CHANSPCM, which is the value to put in

the MDMCFG0 register. Hence,

326.17 × 103 = 26 × 106

218 × 256 + 𝐶𝐻𝐴𝑁𝑆𝑃𝐶𝑀 × 23

𝐶𝐻𝐴𝑁𝑆𝑃𝐶_𝑀 = 326.17 × 103 − 793.45 × 256

793.45 = 155 = 0𝑥9𝐵

Therefore, setting the last two significant bits of the MDMCFG1 register (also called

CHANSPC_E) to a value of 3 and setting the MDMCFG0 register (its value called

CHANSPC_M to a value of 155, gives a frequency separation of 326.17 KHz between

channels; making use, with all the 256 channels of the whole 2.4 GHz ISM band.

Due to rounding in the frequency synthesizer, the base frequency (which is set in the

values stored in the registers FREQ2, FREQ1 and FREQ0) has been set at a frequency of

2400.000031x106 Hz, slightly shifting all the channels. However, as most of the RF power

is centred around the carrier frequency, the system still complies with all the frequency

regulations mentioned in chapter 3.

The main advantage of using a channelized system is that once the transceiver is in

operation, changing channel will be just a matter of setting a new value in the channel

number register (CHANNR, found at 0x0A address). To be noted that in order to have a

correct frequency matching between receiver and transmitter, all devices must share the

same register values.

The main radio control state machine configuration register (MCSM0, found at 0x18

address) is used to set the default return state after packet transmission and reception. In

fact, it is possible to specify if the transceiver should remain in transmit state after a packet

is transmitted or return to idle state (dually if to remain in receive mode or to go to idle


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after a packet is received). It was chose to set this register four most significant bits to zero,

making the transceiver return to idle after both packet reception and transmission.

The above mentioned registers are the ones that have a biggest impact on the wireless

performance, consequently during the all development phase different settings and

combination were tested. Thus, a source file that automatically configures the transceiver

with the specified settings is created, such file is called setRegisters.c. This file, created

with a Studio RF custom template and then manually modified, is a list of all the register to

program. Instead of using a burst write operation, only those selected registers are

modified with a single write access (as not all the configuration registers have to be

modified from their default values).

5.4 Fixed Frequency Transmission

The first step toward an adaptive FHSS network is to establish a simple peer-to-peer link

over a fixed frequency. All the functions developed in the source files SPI.c and

MicroSetup.c are an essential basis from which the network MAC layer can be developed.

During development, one board will act as a transmitter and the other one as a receiver;

since the Explorer 16 development board is fitted with a serial connection, such board will

be used as the receiver. This will in fact allow to collect and output statistics from the

received data and check the program functioning.

At this point a new source file, called RF.c is created. This file contains all those functions

related to data transmission and reception. One of those is RFreceive, which is designed to

receive data. This function in fact makes the transceiver: enter into receive state, wait for a

packet to be fully received (by using the GDO0 pin as an interrupt), read the RX FIFO,

verify data integrity with a CRC check and return to idle state. This first transmission uses

a fixed length payload size of 50 bytes, plus the addition of the 2 bytes containing link

information for every received packet (this done by setting the PKTLEN register to 0x32,

and both PKTCTRL1 and PKTCTRL0 to 0x04). Also, statistics are collected by updating 4

variables: faulty packets received (CRC_fail), correct CRC checks (CRC_ok), total packets

received (pktcount) and interferences received (interference). Such variables provide useful

information on the link quality under different environments, and are used for

benchmarking the protocol.


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The pseudo-code for this function is:

Function RFreceive{



enter in receive mode

wait for full packet reception (GDO0 asserts and de-asserts)

if there is data in the RX FIFO

read out the payload from the RX FIFO (with a burst read)

read the signal strength of the received packet

verify the result of the CRC check (and update the statistics count)

}

To be noted that in this case there is no need to call for a delay function after sending the

receive state strobe. In fact, the next instruction after the strobe is a while loop which will

not exit until a valid sync word is found (the successive instruction is then another while

loop that exits when the packet is fully received). The fact that a valid sync word is

detected implies that the receive mode is already entered, making the delayIDLEtoRX_TX

function useless in this portion of the code. Once a packet is fully received an if statement

verifies that some data are effectively been written in the RX FIFO, this prevents from

reading an empty RX FIFO: if such event occurs, the function considers that as

interference, and updates the interference received variable. The RX FIFO might be empty

even after a packet is received due to CRC fail if the CRC auto-flush option is enabled, or

due to address filtering, or in the unfortunate case that a false sync word was produced by

some interferer. Then, if some data is available to be read, the RX FIFO is scanned with a

burst access, for the entire payload length (except for the two appended byte which are

read with two singles accesses) Finally, a counter keeps statistics by updating the pktcount,

CRC_ok, CRC_fail and interference variables.

To be noted that one of the major problems encountered while developing the RFreceive

function was due to a failure due to a Microchip PIC32 Library function while sampling

the value of the GDO0 pin. Initially, to sample the pin value the function

mPORTDReadBits74

was used, yet with no results. After debugging the code and

analyzing the signals with a spectrum analyzer it was seen that that library function was

not respecting the timing imposed by the C code. To solve this problem an alternative

code, which does not make use of the mPORTDReadBits function is instead implemented.


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The reason for such malfunction in the mPORTDReadBits function is still unknown and

was reported to the company website.

As previously mentioned, the Explorer 16 board will act as the receiver so the RFreceive

function will be programmed into such board. Instead, the Starter Kit development board

will act as the transmitter, hence a function nam ed RFsend is programmed into this board.

The function RFsend is designed to fill the TX FIFO with data, send a enter transmit mode

strobe, and then wait until the packet is fully transmitted (by using the GDO0 pin as an

interrupt). In this, and in all the transmissions performed during the project development

the RFsend function will transmit random data. Such random information is only meant to

provide information on the link quality for benchmarking the protocol. Hence, the pseudo

code for the RSsend function is:

Function RFsend{



fill the TX FIFO with random data (50 bytes)

send transmit strobe

wait for full packet transmission (GDO0 asserts and de-asserts)

}

As in the RFreceive function, no delay needs to be implemented after sending the strobe as

the code execution is held by the GDO0 pin interrupt; moreover also for this function the

FIFO is filled by using a burst access for the whole payload length.

Additionally, two more functions were developed for this test: readPower and setPower.

As their name suggest, these two functions read and set the output power level. The output

power is defined by setting the PATABLE, which is a sequence of 8, one byte long

registers. Each table row defines the output power for a different modulation sequence (the

first row sets the output power for frequency shift keying). For accessing the first row of

PATABLE, the address 0x3E must be first called; then every byte sent to the transceiver

will make access the successive table rows. However if the cable select pin is toggled, the

index is reset. Figure 5.8 shows a PATABLE schematic view, while figure 5.9 depicts the

different output power levels available for the CC2500 with the relative current

consumption and register value. For this first test the output power was set at the maximum

value of 1dBm, done by setting the PATABLE at a value of 0xFF.


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Figure 5.8 PATABLE Schematic view75

Figure 5.9 Available output power levels75

This first transmission test was successful as it provided a constant wireless peer-to-peer

link. Both the receiver and transmitter board shared the same transceiver configuration as

they both must use the same transmission frequency (fixed in this text in the arbitrarily

chosen channel 3), same modulation (frequency shift keying), same data rate (10 kbaud),

same packet length and all the other above mentioned configuration registers. The link

statistics collected for this kind of link are showed in the next chapter. The next step will

be furthermore develop the code to establish a frequency hopping transmission system.

5.5 Frequency Hopping System

Even though the previous test provided a good wireless link between transmitter and

receiver, operating over a single frequency is heavily subjected to interference and is not

the aim of this project. Instead, as discussed in chapter 3, a FHSS protocol must be

implemented in the transmission protocol. In this section will be discussed the

development of a FHSS transmission link, again taking place between a transmitting

device (Started Kit) and a receiver (Explorer 16). The channels pattern used in this FHSS is

predetermined and stored in the transmitter device.

In order to make the wireless link constantly operate over a different frequency, several

modifications must be made to the previous program. Above all, the key new features to


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add in the new program are: a function to successfully change and calibrate frequency, and

a method to keep the transmitter and receiver synchronized on the frequency to use.

Changing the carrier frequency, hence the channel, during transmission is done by

following these steps:

go to idle state

put the new channel in the channel register

run calibration

wait for the PPL to be locked

return to receive/transmit mode

These are the steps to be followed for changing channel and calibrating the frequency

synthesizer to the new carrier frequency. Calibrating the new frequency is necessary as the

VCO characteristics vary with the operating frequency, temperature and supply voltage76

.

Such calibration can be performed by calling a calibration strobe and, as figure 5.3 shows,

such operation takes a determined amount of transceiver clock cycles. Therefore, a delay

function that waits for that amount of time can be created (as showed in the paragraph 5.1).

To furthermore ensure that the Phased-Locked Loop (PLL) is „locked‟ with the new carrier

frequency the register FSCAL1 can be read: in fact, if such register has a content value

different than 0x3F means that the PLL is in lock. Consequently to new functions are

created: waitPLLinLock and changeChanell.

The first one consists in a while loop, which is exited only once the content of the FSCAL1

register is different than 0x3F. Effectively putting the program execution on hold until the

PLL is in lock.The latter instead is the complete function to be called when a channel

change is desired (as a matter of fact this function includes the waitPLLinLock code). The

pseudo code for the changeChannel function is:

Function changeChannel{

go to idle

wait to change state into idle

write the new channel value into the channel register

run calibration

wait to perform calibration

verify PLL is in lock

}


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Therefore by only calling the function changeChannel, allows the transmission system to

change its operating frequency.

Nevertheless, each time the changeChannel function is called, data transmission is

interrupted; the time spent changing time is also commonly called „blanking time‟.

The low blanking time is a good figure of merit for a FHSS transceiver. The blanking time

for the changeChannel function can be calculated as:

IDLE STROBE 5x10-6

s +

CHANGE STATE TIME (RX/TX to IDLE) 0.18 x10-6 s +

WRITE NEW CHANNEL IN REGISTER 10 x10-6 s +

SEND CALIBRATION STROBE 5 x10-6 s +

CALIBRATION TIME 750 x10-6 s +

READ OPERATION (PLL in lock check) 10 x10-6 s +

SEND RX STROBE 5x10-6

s +

CHANGE STATE TIME (IDLE to RX/TX) 93.75 x10-6

s =

__________

878.93 x10-6

s

Such value takes into account every step involved in changing channel and also the SPI

communication speed (200 Kbaud). These figures are then believed to be more realistic

than what stated in the CC2500 datasheet (810 us76

), which do not consirer the time spent

to send instructions over the SPI interface (in fact, if the SPI is ignored the blanking time is

found to be 809.5us, but this is not a realistic figure).

A blanking time of 878.93 us allows a maximum frequency hopping rate of about 1137

hops per second (constantly changing channel without sending any data). Yet, even a

much lower hopping rate is more than adequate for the application required and also fulfils

the international and national regulations for FHSS systems. For example if 4 packets are

sent between each hop (transmitting with a data rate of 10 kbaud, a 2-FSK modulation and

a payload size of 52 bytes), the time elapsed between two consecutive hopes (also called

dwell time) would be:

𝑑𝑤𝑒𝑙𝑙𝑡𝑖𝑚𝑒 = 1

10 ×103 × 52 × 4 = 20.8 𝑚𝑠 equation 5.4

A dwell time of 20.8 ms is well smaller than the maximum dwell time dictated by the ETSI

EN 300 440 regulation (which corresponds to a dwellmax=0.4s36

).


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There is also another technique for changing channels more rapidly, such method will be

showed later on in the chapter. However, calibrating the frequency synthesizer for every

new channel, especially for a low-cost module, is the most reliable solution and therefore

offers the best performances.

The other key issue for a FHSS system is the frequency synchronization between

transmitter and receiver. In the early system development synchronization was kept by

making both receiver and transmitter change channel after a predefined amount of packets.

This technique was proven to be quite weak as if the receiver loses just one packet the

whole frequency synchronization is lost.

Hence, to produce a more robust protocol, the RFsend and RFreceive functions were

modified. Instead of just sending 50 bytes of payload, these function have been modified to

transfer 50 bytes of data plus 2 extra bytes. The first of the two extra bytes is named the

„instruction byte‟, while the second one is the „new channel byte‟. With this update, the

transmitter constantly sends the payload, instruction and new channel bytes with every

packet. Both wireless boards start communication in the same channel (channel 0 by

default), then when the transmitter „decides‟ to change channel, it starts sending an change

channel instruction and a new channel along with the payload. The receiver checks the

instruction for every packet received and, once the change channel instruction is detected,

it automatically changes into the channel specified in the „channel byte‟.

The pseudo code for theupgraded RFsend and RFreceive functions is:

Function RFsend{




send instruction byte

send new channel byte



}

Function RFreceive{



enter in receive mode


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wait for full packet reception (GDO0 asserts and de-asserts)

if there is data in the RX FIFO

read out the payload from the RX FIFO (with a burst read)

read the instruction byte

read the new channel byte

read the signal strength of the received packet

verify the result of the CRC check (and update the statistics count)

}

With this technique, the transmitter can constantly control in what channel the receiver

should be positioned; thus, the transmitter acts as a master for the wireless communication.

The frequency at which channel is changed, and the channel number is only determined by

the transmitting board.

This synchronization method proved to be a much more robust one, especially in the

presence of interferer. On the other hand, synchronization could be lost if the packet

containing the „change channel‟ instruction (and the new channel) is corrupted or lost. To

prevent this from happening, the transmitter is programmed to send that instruction in more

than one packet, assuring frequency synchronization.

With such software a FHSS communication link was established, greatly improving

performances in electromagnetically crowded environments. However, as for this protocol

the channels pattern was predetermined, the link is still prone to interference, such as

wireless LANs and Bluetooth devices. To solve such issue, in the next section an adaptive

FHSS protocol will be developed.

5.6 Adaptive Frequency Hopping System

In the previous FHSS link the master, that instructs the receiver on what channel to use,

follows a channel pattern previously stored in its memory. This method, even if benefits

from the advantages of a spread spectrum technique, does not adapt to the surrounding

environment. Consequently, it is not capable of avoiding particularly crowded channels.

To overcome this problem a function called scan is developed and added to the existing

FHSS software. This function, before any transmission is started, senses the signal power


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for every channel, effectively scanning the all ISM spectrum. This is possible by reading

the content of the Received Signal Strength Indication (RSSI) register, found at the 0xF4

address. In fact, every time the transceiver enters in receive mode this register is constantly

updated with a value proportional to the RF power sensed in that channel. Therefore, if all

channels are scanned before any transmission is started, the transmitter can store the RSSI

level and use for transmission only those channels with the lowest sensed RF power.

Consequently, the transmission between the two development board will take place only

on those channels not currently used by other devices, therefore it adapts to the

surrounding environment. Furthermore, the transmission between the device should be

stopped regularly to allow a new scan to take place, allowing to update the environment

status. This methods allow to constantly use just those channels not used by other

protocols. To be noted that at the beginning of transmission both devices are by default set

into channel 0. Once the transmitter has performed the ISM scan, it will start

communicating the receiver the channels to use.

More in detail, the pseudo code for the scan function is:

Function scan{

for every channel

read the RSSI value

convert the value into a dBm number and save it into an array

save into an array the best 50 channels (those with the lowest RSSI value)

}

More in detail, this fairly elaborated function saves a list of the 50 (out of the total 256)

electromagnetically quietest channels, saving them into an array where the first position is

occupied by the best channel, hence the first one to use. If two channels are found having

the same low RSSI readings, both of them are added to the list as both of them are suitable

for transmission. The list of the channels to use is elaborated by the transmitter board,

which will use all of them one by one in crescent order. Will in fact be the transmitter to

instruct the receiver when to change into the next best channel. During development phase

was chosen to use each best channel for the transmission of 4 packets and to use the same

best channel list for 3 times, before executing another environment scan. However these

parameters can easily be changed to suit the final product needs.

The more often the scan function is ran, the safer the transmission is; this due to the fact

that having an updated scan of the surrounding area allows to avoid those systems that


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could have possibly be turned on after the previous scan was performed (for example

someone walking in the room with a Bluetooth device turned on).

On the other hand, as the Texas Instruments Design Note DN505 shows, in order to make a

reliable RSSI reading, the transceiver should spend a minimum amount of time in receive

mode for every channel (at least 376 us to properly read a strong RF signal when the data

rate is set at 10kbaud). In addition, since all the 256 channels have to be scanned, the

changeChannel function is called 255 times, furthermore increasing the time required to

fully execute the band scan. Consequently, calling the scan often sensibly reduces the

overall network throughput.

In the final product a trade off must be made, evaluating the rate of change in the

environment in which the system is installed.

5.7 Variable Base Adaptive Frequency Hopping System

The protocol depicted in the previous section offers an adaptive FHSS wireless system,

however it has one weak point. As a matter of fact, at system start up both devices are by

default set to channel 0 (called the base frequency). It will be in fact from this channel that

the transmitter will start communicating to the receiver the best channels to use. Once the

receiver has received the first packet (over channel 0), it will then follow the channel

pattern dictated by the transmitter. The intrinsic problem with this approach is in the fact

that no communication would be possible if channel 0 is heavily used by another network

protocol. Nevertheless, as most wireless routers have as default setting the Wi-Fi channel

1, the transceiver channel 0 (which corresponds to the very beginning of the 2.4 GHz ISM

band) is likely to be electromagnetically busy. This will not allow any initial link between

the transmitter and receiver, resulting in a total system failure.

To solve this problem two new functions are added to the previous protocol:

scanAndPosition and SearchBase. Respectively added to the receiver and to the

transmitter.

The function scanAndPosition is implemented in the Explorer 16 board, which still acts a

receiver. This function makes the receiver scan the entire ISM band looking for the

channel with the lowest RSSI reading, hence the best one to use. Once the receiver has

found the electromagnetically quietest channel, independently tunes into it. It therefore

changes its default base channel depending on the surrounding environment. The code for

this function is very similar to the transmitter scan function, for the exception that when the


Flavio Felici


receiver finds the very best channel automatically tunes into that frequency. At this point,

the receiver is positioned into a variable base frequency waiting for communication from

the transmitter.

The function SearchBase, which is programmed into the transmitter, is called at the very

beginning of the communication protocol. This function makes the transmitter board send

a packet to every single channel of the ISM, looking for an answer from the receiver. In

fact, as the base frequency is variable, the receiver does not know in which channel the

receiver is awaiting communication. After sending a packet, the receiver awaits (for a fixed

amount of time) a possible answer from the receiver. If no signal is received from the

receiver, the transmitter assumes that the channel currently sampled is not the one chosen

by the receiver as a base frequency and therefore moves on sampling the next one. Once an

answer is found, the transmitter cease the base channel search and acquires the current

channel as the communication base channel. From that moment on, data transfer can start

as showed in the previous FHSS system, with the exception that the base frequency is not

the default channel 0 but the channel chosen by the receiver.

Given the relatively short distance between two sound masking system nodes (5 to 10

meters), it is assumed that the quietest channel found by the receiver board can be likely to

host data transmission. However, if for any reason the transmitter (after scanning all the

256 channels) has not received any base channel acknowledgement from the receiver,

restart the search from channel one. In fact, the transmitter is programmed to run an

indefinite number of scans in order to find the base channel.

Figure 5.10 shows a synthetic view of the program flow.


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This FHSS transmission protocol is believed to be robust against interference, especially

when large parts of the spectrum are occupied by other network protocols (for example

wireless LANs). It is then believed to fulfil the project technical requirements.

Nevertheless, the receiver is programmed to store link statistics (storing the number of

corrupted packets and interference received). In the chapter system testing and

performance this program will be benchmarked under different ranges and environments.

TRANSMITTER RECEIVER

SCAN AND POSITION

WAITING IN BASE

TO SEND

ACKNOLEDGMENT

START RECEIVING DATA

SEARCH BASE

PROBING EVERY

CHANNEL LOOKING

FOR BASE

START SENDING DATA

AND HOPPING

INSTRUCTIONS

PACKET RECEIVED,

ACKNOLEDGMENT SENT

DATA

TRANSFER

ACKNOLEDGMENT RECEIVED,

BASE FOUND

Figure 5.10 Variable Base Adaptive Frequency Hopping System program flow

SCAN FOR 50

BEST CHANNELS

POWER ON POWER ON

TRANSCEIVER CONFIGURATION TRANSCEIVER CONFIGURATION


Flavio Felici


5.8 Network Setup

At this phase of the project a robust adaptive FHSS transmission protocol has been

developed. Such protocol allows peer-to-peer data transmission even in presence of other

wireless devices in the same band. However, one last project requirement needs to be

addressed, the network setup. As specified in the project requirements section, the wireless

transmission system must be able to deliver information to a whole network of nodes, not

just deliver information in a peer-to-peer link.

As only two wireless boards were available during development phase, such network of

nodes could only be simulated. Nevertheless, the lack of physical nodes does not prevent

from design the protocol to support such network. Taking the adaptive peer-to-peer FHSS

transmission protocol as foundation, the feature to send data to different nodes can be

added. As figure 5.7 depicts, the CC2500 supports in the packet format an address byte.

This method makes available a maximum number of 255 addresses, which are enough for

the average sound masking system. Nevertheless, this number could be increased to 65536

by using two bytes as an address field (but this solution has to be implemented in the

protocol as not implemented in the CC2500 packet handling).

Therefore, to setup a network of multiple nodes each transceiver must be assigned with a

unique network address (ranging from 1 to 256, included) and must have the address check

enabled in the PKTCTRL1 register. In this configuration, every time a node sends a packet

it must include, before the actual payload, the destination address for that packet

(information could be either addressed to one single node or broadcasted to the whole

network). The packet, even if received by all the nodes in the communication range, will

only be acquired by the designed recipient. In fact, the CC2550 automatically filters out the

packet payload if the receiver network address does not match with the address field

included in the packet. During the developing stages the transmitter was arbitrarily given

the network address of 0x01 and receiver 0x02.

This technique effectively allows every packet to be delivered only to the intended network

nodes. To include such feature in the existing protocol some upgrades have to be made.

Since one more byte will be included for every packet, the PKTLEN register must be

incremented by one unit; and the two least significant bits of the PKTCTRL1 must be set

to 2 (broadcast delivery enabled).


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As the recipient address field must be included before the payload during transmission, the

RFsend function contains now 3 paramenters (instruction, new channel, recipient address).

The correspondent pseudo code is:

Function RFsend{



send recipient address


send instruction byte

send new channel byte



}

To be noted that the function RFreceive does not need to be modified thanks to the RX

FIFO „if‟ statement included. In fact, even if a packet is fully received (asserts and de-

asserts the GDO0 pin) the RX FIFO will not be read if the payload has been filtered out by

the address check. This means that if a packet is received, and its content fills the RX

FIFO, such packet has a matching address byte and is meant to be received.

By using this version of the transmission protocol, it is possible to sort packets within the

network still using a FHSS transmission. Hence, with this final version, the protocol

complies with the project requirements and therefore it is suitable to be implemented as a

sound masking wireless transmission system.

5.9 Variable Base Adaptive Fast Frequency Hopping System

The updated network protocol showed in the previous section can be seen as the project

final stage as it fulfils the system requirements. Yet, further development was made to the

protocol. That next step has not been requested by the sound masking system company,

and represents only a technical exercise to make use of other options offered by the

CC2500 transceiver. It is in fact possible to greatly reduce the actual system blanking time,

increasing the overall link throughput.

As demonstrated in section 5.5, using the function changeChannel to hop from one

frequency to another allows achieving an adequate frequency-hopping rate. In actual fact,


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even calibrating each new carrier frequency, results in a blanking time inferior to a

millisecond. The blanking time is a delay resulting from the sum of different transceiver

processes; although, when calling the changeChannel function the main delay is the time

needed for the transceiver to calibrate the frequency synthesizer. Every time a frequency is

calibrated, the resulting parameters are saved in the transceiver registers: FSCAL1,

FSCAL2, FSCAL3. These registers are update with new values every time a new channel

is calibrated.

It is then possible before starting the data transmission, to run the calibration and save the

respective results in the microcontroller memory, this for each channel that is going to be

used. Once the calibration results are saved in memory, when a new channel has to be used

the results relative to that frequency could be recalled and substituted into the FSCAL

registers, this would avoid running a new calibration. Therefore if an array is created,

containing the calibration results for each channel that is going to be used, the change

channel operation would only require a writing those results into the FSCAL registers,

cutting the blanking time.

IDLE STROBE 5x10-6

s +

CHANGE STATE TIME (RX/TX to IDLE) 0.18 x10-6 s +

WRITE NEW CHANNEL IN REGISTER 10 x10-6 s +

WRITE IN FSCAL1 10 x10-6 s +



SEND RX STROBE 5x10-6

s +

CHANGE STATE TIME (IDLE to RX/TX) 93.75 x10-6

s =

__________

143.93 x10-6

s

Using the previously stored calibration results reduces the blanking time to only 143.93us;

therefore, by applying fast hopping the overall transmission data rate will increase. On the

other hand, changes in operating temperature and frequency drift tend to degrade the

wireless capabilities; therefore, fast hopping transmissions tend are likely to have a lower

performance in terms of link quality. This will be showed in the next chapter where this

system will be benchmarked and confronted with the standard hopping protocol.


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As for the previous protocol version, the transmitter will choose the 50 best channel to

transmit, while the receiver will decide which channel to use as a the base. To store the

calibration results for each of those 51 channels a new C structure type is defined in the

code. The structure includes three byte type variables, since the calibration results for each

channel are stored in three different transceiver registers. After the transceiver has

acknowledged what base channel to use, and has selected the 50 best frequencies, a loop

calibrates each channel that is going to be used. The calibration results are then saved into

an array of the structure previously defined. To make the receiver pre-calibrate all the

channels that are going to be used, the best channel list is sent from the transmitter to the

receiver over the base channel. Once the receiver has acquired the list of the 50 channels

chosen by the transceiver, it will calibrate each one of them; preparing the results for the

fast hopping. From this point on every time the transmission channel needs to be changed,

a function called changeChannelFast will substitute the previously stored calibration

results (relative to that specific channel number) into the transceiver registers; this avoids

running a new calibration for each new hop and effectively reduces the blanking time.

The fast hopping program flow can be synthetically depicted as figure 5.11 shows on the

next page.


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TRANSMITTER RECEIVER

POWER ON

SCAN AND POSITION

WAIT IN BASE TO

SEND

ACKNOLEDGMENT

POWER ON

SEARCH BASE

PROBE EVERY

CHANNEL LOOKING

FOR BASE

PACKET RECEIVED,

ACKNOLEDGMENT SENT

ACKNOLEDGMENT RECEIVED, BASE

FOUND AND CALIBRATED

Figure 5.11 Variable Base Adaptive Frequency Hopping System program flow

START RECEIVING DATA START SENDING DATA

AND HOPPING

INSTRUCTIONS

DATA

TRANSFER

SCAN FOR 50 BEST

CHANNELS AND

TRANSMIT LIST

WAIT FOR BEST

CHANNEL LIST

CALIBRATE BEST

CHANNELS

CALIBRATE BEST

CHANNELS

LIST RECEIVED

TRANSCEIVER CONFIGURATION TRANSCEIVER CONFIGURATION


Flavio Felici


Chapter 6

System Testing and Performance

In this chapter will be represented the results of various tests conducted to measure the

transmission system performance. The main figures of merit used to characterize the

system are: packet error rate, payload length, communication range, and immunity to

different sources of interference. For all the tests, the transmitter output power is set to

1dBm, the transmission data rate has been fixed to 10kbaud and the modulation used is 2-

FSK (Frequency Shift Keying). These fixed parameters are believed to be the most

adequate solution for the project needs and will not therefore be modified. All the others

main parameters and the test conditions involved, instead, will be mentioned for each test.

6.1 Range Test

This series of tests are aimed to verify how the distance between transmitter and receiver

affects the received signal strength. Non-ideal components and fabrication defects may

give the system different performances than what theoretically calculated. The real

received signal strength is measured through the reading of the RSSI value attached on the

first byte after the payload of each packet. The source code used to perform the tests and to

output the results is provided in this document appendix.


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Link Range

[meters]

TX Antenna Orientation

TX Antenna Orientation

RSSI value

[dBm]

10 XZ

(antenna long axis on X)

XZ

(antenna long axis on X) -68

10 XY


XY


10 XY

(antenna long axis on Y)

XY

(antenna long axis on Y) -78

28 XZ


XZ


28 XY


XY


28 XY

(antenna long axis on Y)

XY

(antenna long axis on Y) -80

Table 6.1 Test Results

This test results are compatible from what expected: the real received signal strength is

lower that what theoretically calculated (equation 4.3), this due to hardware imperfections.

Furthermore, it has been verified that the worst antenna performance occurs when both

transmitter and receiver are oriented with their long axis on the Y-axis.


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6.2 Variable Base Adaptive FHSS System Test

These results have been collected to test the efficiency of the network protocol discussed

on section 5.7. Different payload and ranges have been tested, making the protocol work

under different electromagnetic environments. However all tests have been performed

sending a total number of 1000 packets (containing random data) with a data rate of 10

kbaud and transmitter output power set at 1dBm. The full transceiver configuration can be

seen in the code provided in the document appendix.Table 6.2, 6.3, 6.4, and 6.5 show the

test results of the Variable Base Adaptive Frequency Hopping System.

PAYLOAD

LENGTH [BYTES]

LINK RANGE

[meters] INTERFERANCE

CRC

FAIL

PACKETS

LOST

FALSE PACKET

RECEIVED

52 2, line of sight none 0 0 0

52 2, line of sight

Bluetooth device

(class 1)

transmitting in

between transmitter

and receiver

12 0 0

52 2, line of sight

Bluetooth device

(class 1) 5 meters

apart from both

transmitter and

receiver

9 0 0

52 2, line of sight

Wi-Fi channel 1

(router D-

Link802.11g using

full bandwidth)

2 0 0

52 2, line of sight

Wi-Fi channel-1

(router D-

Link802.11g using

full bandwidth) +

Bluetooth device

(class 1) 5 meters

apart from both

transmitter and

receiver

8 0 0



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PAYLOAD

LENGTH [BYTES]

LINK RANGE


CRC

FAIL

PACKETS

LOST

FALSE PACKET

RECEIVED


5 2, line of sight

Bluetooth device

(class 1)

transmitting in

between transmitter

and receiver

5 0 0

5 2, line of sight

Bluetooth device

(class 1) 5 meters

apart from both

transmitter and

receiver

3 0 0

5 2, line of sight

Wi-Fi channel 1

(router D-

Link802.11g using

full bandwidth)

0 0 0

5 2, line of sight

Wi-Fi channel-1

(router D-

Link802.11g using

full bandwidth) +

Bluetooth device

(class 1) 5 meters

apart from both

transmitter and

receiver

1 0 0



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PAYLOAD

LENGTH [BYTES]

LINK RANGE


CRC

FAIL

PACKETS

LOST

FALSE PACKET

RECEIVED


52 15, line of sight

Bluetooth device

(class 1)

transmitting in

between transmitter

and receiver

41 0 0


Bluetooth device

(class 1) 10 meters

apart from both

transmitter and

receiver

35 0 0


Wi-Fi channel 1

(router D-

Link802.11g using

full bandwidth)

103 0 0


Wi-Fi channel-1

(router D-

Link802.11g using

full bandwidth) +

Bluetooth device

(class 1) 10 meters

apart from both

transmitter and

receiver

101 0 0



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PAYLOAD

LENGTH [BYTES]

LINK RANGE


CRC

FAIL

PACKETS

LOST

FALSE PACKET

RECEIVED


5 15, line of sight

Bluetooth device

(class 1)

transmitting in

between transmitter

and receiver

7 0 0

5 15, line of sight

Bluetooth device

(class 1) 10 meters

apart from both

transmitter and

receiver

6 0 0

5 15, line of sight

Wi-Fi channel 1

(router D-

Link802.11g using

full bandwidth)

26 0 0

5 15, line of sight

Wi-Fi channel-1

(router D-

Link802.11g using

full bandwidth) +

Bluetooth device

(class 1) 10 meters

apart from both

transmitter and

receiver

29 0 0



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Table 6.6 shows the test results of the Variable Base Adaptive Frequency Fast Hopping

System:

PAYLOAD

LENGTH [BYTES]

LINK RANGE


CRC

FAIL

PACKETS

LOST

FALSE PACKET

RECEIVED


52 2, line of sight

Bluetooth device

(class 1)

transmitting in

between transmitter

and receiver

18 0 0

52 2, line of sight

Bluetooth device

(class 1) 5 meters

apart from both

transmitter and

receiver

21 0 0

52 2, line of sight

Wi-Fi channel 1

(router D-

Link802.11g using

full bandwidth)

4 0 0

52 2, line of sight

Wi-Fi channel-1

(router D-

Link802.11g using

full bandwidth) +

Bluetooth device

(class 1) 5 meters

apart from both

transmitter and

receiver

7 0 0



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Table 6.7 shows the test results of the Variable Base Adaptive Frequency Hopping System

when between the transmitter and the receiver boards two walls are present. These walls

are thin plasterboard walls, which could symbolize the type of obstruction represented by

typical office ceiling tiles.

PAYLOAD

LENGTH [BYTES]

LINK RANGE


CRC

FAIL

PACKETS

LOST

FALSE PACKET

RECEIVED

52

5, two walls

between

transmitter and

receiver

none 0 0 0

5

5, two walls

between

transmitter and

receiver

none 0 0 0


The test results show that the developed transmission protocol can achieve acceptable

packet error rates even in presence of strong interferer. In fact, during the tests the Wi-Fi

router was set to transmit with an output power of 20dBm (the maximum allowed for

wireless LANs in the 2.4 GHz34

), which is well above the 1dBm used by the CC2500

transceiver. Nevertheless, the interference produced by wireless LANs is the easiest to

avid, as the portion of the spectrum used by that protocol is fixed. Conversely, the

interference produced by Bluetooth devices was found harder to avoid, as the frequency

used by that protocol continuously change (as previously mentioned Bluetooth uses

FHSS). Since both the Bluetooth start frequency and channel width are known (section

3.3.2), the transmission system has been programmed to use a start frequency and channel

width designed to avoid Bluetooth channels (see setRegisters.c for detail). However, as

both Bluetooth and the transmission system developed use a pseudo random channel

pattern, eventually some packet collision can happen. In fact, if by chance the same

frequency (or the same small portion of the spectrum) is used by both systems at the exact

same time, an error will occur. Those errors can be detected by the CRC check and have to

be taken into account in common wireless systems.


Flavio Felici


The system configuration tested that was less susceptible to interference is the one using

the standard hopping method with a payload of 5 bytes. A smaller payload size reduces the

time necessary to transmit a single packet, hence reduces the time spent in each channel

(the number of packets transmitted per channel is fixed). On the other hand, this reduces

the network throughput, as more each packet contains less data.

For the final product production, is then strongly advised to use the smaller payload size

compatible with the required data rate. Moreover, depending on the quality of the

components and on the type of antenna used, the data rate could be increased up to

500kbps44

. When transmitting real data some CC2500 additional features can be used

(such as data blanking, forward error correction or Manchester encoding).

The configuration provided in the source code file setRegisters.c fulfils the projects

requirements; however, according to the final product needs (packet error rate, data rate,

range), such configuration can be furthermore tuned using the many CC2500 features to

suit specific usage.


Flavio Felici


Chapter 7

Conclusions and Further Work

In this document have been showed the main steps followed to develop the prototype MAC

layer of a wireless transmission system. As the 2.4 GHz ISM band tends to get always

more electromagnetically crowded, coexistence in this band between many different

protocols is a present and growing issue. The technique exploited to solve what can be seen

as a interference related problem is Frequency Hopping Spread Spectrum. As a result

FHSS is a growing technology becoming more and more widely used. Implementing such

method has given the results shown on chapter 6, achieving what is believed to be an

acceptable link even in case of strong interference; in real life situations such interference

are likely to be less intense.

Given the project requirements, the first step taken was selecting the band of operation.

The wireless transmission system was required to operate within an unlicensed frequency

range and the 2.4 GHz ISM band was chosen because it is found worldwide and

transceiver manufacturers produce a vast products selection. The hardware components

were then chosen, selecting only low-cost devices fully compatible with the required

performances. For speeding up the hardware assembly and testing, development boards

and pre-assembled modules were used; this project did not involve any hardware design.

As a matter of fact, the whole focus of the dissertation was the development of a FHSS

network MAC layer. For producing a working protocol, the approach followed was to

achieve protocol key functionalities step by step. The network protocol in fact was built

from ground up following the procedure shown throughout chapter 6. The final protocol,

presented in chapter 5.8, addresses all the project requirements. In fact, the adaptive

channel selection ensures a reliable communication even when different potions of the

spectrum are being used by other protocols; while the communication between multiple

nodes is supported by the implementation of a network destination address for every

packet sent.


Flavio Felici


Further development could be done in designing a time-based synchronization. In the

present protocol, synchronization is kept thanks to the addition of an instruction and a new

channel byte appended to every packet sent. To ensure a robust synchronization every

channel hop is sent multiple times. However, sending this delicate information could be

inconvenient, especially if the channel pattern is intended to be secret (for military

applications). In fact with the present system, the information on the next channel to use is

sent in the open range; making possible, once a packet is intercepted, to completely follow

the transmission channel pattern. If for any reason the communication channel pattern is

wanted to be secret, synchronization could be kept by storing a predefined subset of

channels into all devices, also specifying the moment in time for each hop. With this

method, all the devices involved in the communication would change channel according to

a list stored inside their memory at predetermined moments in time.


Flavio Felici


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http://www.americansoundmasking.com/faq.html [accessed on 19/08/09];

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hopping_spread_spectrum [accessed on 19/08/09];

4: Frequency Hopping Systems Application Note AN014, page: 3 of 7, Chipcon Products

from Texas Instruments;

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http://www.etsi.org/WebSite/homepage.aspx


Flavio Felici


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http://www.broadcom.com/collateral/wp/802_11n-WP100-R.pdf


Flavio Felici


27: List of WLAN channels, http://en.wikipedia.org/wiki/List_of_WLAN_channels


28: IEEE 802.11, http://en.wikipedia.org/wiki/IEEE_802.11 [accessed on 06/05/09];

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30: Behrouz A. Forouzan, “Data Communications and Networking”, 3rd

edition, page 375

of 972, McGraw Hill;


Wireless Protocols: Bluetooth, UWB, Zigbee, and Wi-Fi”, page 2 of 6, Industrial

Technology Research Institute (ITRI) Hsinchu, Taiwan, 2007;

32: SimpliciTI Network Protocol,

http://focus.ti.com/docs/toolsw/folders/print/simpliciti.html, [accessed on 10/05/09];

33: ERC Recommendation 70-03, European Conference of Postal and

Telecommunications Administrations, June 2009;

34: ETSI EN 300 440-2 V1.1.1 (2001-09), European Telecommunications Standards

Institute, reference: REN/ERM-RP08-0406-2;


Application Note AN032, page 11 of 28, SWRA060, Chipcon Products from Texas

Instruments;


Application Note AN032, page 13 of 28, SWRA060, Chipcon Products from Texas

Instruments;

37: © Copyright 1995-2009 Texas Instruments Incorporated;

38: ©2009 Motorola Inc.;

39: ©2009 Microchip Technology Inc.;

40: Feasibility Study Report, page 11 of 18, Flavio Felici, University of Manchester 2009;

http://en.wikipedia.org/wiki/List_of_WLAN_channels

http://en.wikipedia.org/wiki/IEEE_802.11


http://focus.ti.com/docs/toolsw/folders/print/simpliciti.html


Flavio Felici


41: PIC32MX360F512L main features,

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en532441 [accessed

on 20/08/09];

42: PIC32MX360F512L home page,

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en532441 [accessed

on 27/08/09];

43: RF Micro Devices, Inc. (RFMD®);

44: Low-Power RF Guide, page 33 of 43, http://focus.ti.com/lit/sg/slab052b/slab052b.pdf

Texas Instruments Incorporated;

45: “Tutorial for Personal Mobile Communications 2009” page 1 of 4, Dr. D K C So,

University of Manchester 2009;

46: “CC2500 Low-Cost Low-Power 2.4 GHz RF Transceiver” page 19 of 92, Chipcon

Products from Texas Instruments, version of 2007-11-29;

47: Explorer 16 Development Board,

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&d

DocName=en024858 [accessed 04/08/09], ©2009 Microchip Technology Inc;

48: Quasar UK Ltd, http://www.quasaruk.co.uk/acatalog/index.html, [accessed 04/08/09];

49: “Explorer 16 Developing Board Datasheet”, page 36 of 46, ©2009 Microchip

Technology Inc;

50: “CC2500 2.4 GHz Transceiver Module Datasheet”, page 2 of 6, Quasar UK Ltd.;

51: “PIC32 Starter Kit”,


DocName=en532453, [accessed 04/08/09], ©2009 Microchip Technology Inc;

52: PIC32 Starter Kit bottom view image,

http://farm3.static.flickr.com/2046/2510143398_10822ee5f7.jpg?v=0 [accessed 09/08/09];

53: I/O Expansion Card,


DocName=en535444, [accessed 09/08/09], ©2009 Microchip Technology Inc;

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en532441

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en532441

http://focus.ti.com/lit/sg/slab052b/slab052b.pdf

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en024858


http://www.quasaruk.co.uk/acatalog/index.html



http://farm3.static.flickr.com/2046/2510143398_10822ee5f7.jpg?v=0




Flavio Felici


54: MPLAB Integrated Development Environment,


DocName=en019469&part=SW007002, [accessed 09/08/09], ©2009 Microchip

Technology Inc;

55 “SPI Access Design Note DN503”, page 3 of 9, Chipcon Products from Texas

Instruments;

56: “SPI Access Design Note DN503”, page 4 of 9, Chipcon Products from Texas

Instruments;

57: “SPI Access Design Note DN503”, page 5 of 9, Chipcon Products from Texas

Instruments;

58: SmartRF Studio, http://focus.ti.com/docs/toolsw/folders/print/smartrftm-studio.html

[accessed 01/08/09];

59: : “CC2500 Low-Cost Low-Power 2.4 GHz RF Transceiver” page 59 of 92, Chipcon






62: “PIC32 Peripheral Libraries for MPLAB C32 Compiler”, page 148 of 320, ©2007

Microchip Technology Inc.;









http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en019469&part=SW007002

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en019469&part=SW007002

http://focus.ti.com/docs/toolsw/folders/print/smartrftm-studio.html


Flavio Felici


67: PIC32 Peripheral Libraries for MPLAB C32 Compiler”, page 206 of 320, ©2007


68: “PIC32MX Family Reference Manual”, page 23-4, ©2008 Microchip Technology

Inc.;





71: : “Packet Transmission Basics Design NoteDN500”, page 4 of 8, Texas Instruments;

72 : “Packet Transmission Basics Design Note DN500”, page 3 of 8, Texas Instruments;











78: “2.4 GHz Inverted F antenna Design Note DN0007”, page 5 of 4, Texas Instruments;

79: “2.4 GHz Inverted F antenna Design Note DN0007”, page 12 of 4, Texas Instruments;


Flavio Felici


Appendix A

Source Code

Due to the size of the source code, the full project source code is available in the CD-ROM

attached with this document.

However here below is the content of two key source files: SPI.c and RF.c

SPIc

#include <plib.h>

typedef unsigned char BYTE; //defining a 1 byte var

/*----COMMAND STROBES----*/

#define SRES 0x30

#define SFSTXON 0x31

#define SXOFF 0x32

#define SCAL 0x33

#define SRX 0x34

#define STX 0x35

#define SIDLE 0x36

#define SWOR 0x38

#define SPWD 0x39

#define SFRX 0x3A

#define SFTTX 0x3B

#define SWORRST 0x3C

#define SNOP 0x3D

void CShigh(){//THIS FUNCTION WAS GIVING TIMING PROBLEMS SO THE CSn IS KEPT

ALWAYS LOW -CC2500 IS THE ONLY SLAVE ANYWAY

PORTSetBits(IOPORT_B, BIT_2); //puts CS at an high value (CS is active low, so goes low during

transfers)

}

void CSlow(){

PORTClearBits(IOPORT_B, BIT_2); //CS low

}


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BYTE CCready(){

while(PORTReadBits(IOPORT_F, BIT_7) == 1){

; //stays in the cycle until the CC2500 out is high (F7 at the micro)

}

return 1;

}

//Initialize and Set SPI channel 1 as MASTER

void OpenSPI(){

SpiChnOpen(1, SPICON_ON | SPICON_MSTEN | SPICON_CKE , 400); //200 KBaud

PORTSetPinsDigitalOut(IOPORT_B, BIT_2); //makes the CS pin software driven as an output

PORTSetPinsDigitalIn(IOPORT_D, BIT_8); //makes the port D8 an input, in this way we can read

the packet status from the CC

CShigh();

}

//this function closes SPI channel 1

void CloseSPI(){

SpiChnClose(1);

}

/*-------THIS FUNCTION READS THE REGISTER WITH A SINGLE ACCESS--------*/

BYTE readRegister(BYTE registro){

BYTE value, mask;

mask = 0x80; //this mask always adds the 1 for the reading 1000.0000

CSlow();

while(!CCready());

putcSPI1(registro | mask);

while(!SpiChnDataRdy(1));

value = SPI1BUF; //status byte

putcSPI1(0x00);


value = SPI1BUF; //actual read

return value;

CShigh();

}


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/*-------THIS FUNCTION WRITES THE REGISTER WITH A SINGLE ACCESS--------*/

void writeRegister(BYTE registro, BYTE value){

BYTE mask, dvalue; //dvalue is a dummyread to read the buffer

//both the R/W and BURST bit have to be set to 0 (SINGLE WRITE MODE), 00xxxxxx so the mask is

00111111 ---> 3f

mask = 0x3f;

CSlow();

while(!CCready());

putcSPI1(registro & mask); //ADDRESS HEADER: telling we want to write that register with a single access


dvalue = SPI1BUF; //status byte, received to clear the buffer

putcSPI1(value); //sending the register value


dvalue = SPI1BUF; //status byte, received to clear the buffer

CShigh; //CS high, returns high after transfer is done

}

/*-------------THIS FUNCTION SENDS A COMMAND STROBE TO THE CC2500-------*/

void sendStrobe(BYTE strobe) {

BYTE value;

CSlow();

while(!CCready());

putcSPI1(strobe);


value = SPI1BUF; //status byte

CShigh();

}

/*--THIS FUNCTION DISPLAYS ALL THE REGISTERS AND VALUE ON THE HYPERTERMINAL

WINDOW IN THE HEX FORMAT--*/

void displayAllregisters(void){

BYTE i,value;

char string[8];

for(i = 0x00; i <= 0x2e; i++) {

putsUART2("The content of register ");

sprintf(string, "%x", i);

putsUART2(string);


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putsUART2(" is: ");

value = readRegister(i);

sprintf(string, "%x", value);

putsUART2(string);

putsUART2("\r\n");

}

}

/*--------------THIS FUNCTION RETURNS THE NUMBER OF BYTES CURRENTLY IN THE TX FIFO--

------*/

//NOTE: THIS FUNCTION DIFFERS FROM noFreeBytesInTXFIFO BECAUSE THIS ONE GIVES THE

ACTUAL

//NUMBER OF BYTES THAT ARE CURRENTLY IN THE TX FIFO

//THAT VALUE IS HELD IN THE REGISTER 0xFA

BYTE noBytesInTXFIFO(){

BYTE value;

char string[8];

value = readRegister(0xFA); //adress of register TXBYTES

value = (value & 0x3F); //we are interested only in the bits 6:0 so 0011.1111= 0x3F

putsUART2("-from register- The number of bytes in the TXFIFO register is: ");

sprintf(string, "%d", value);

putsUART2(string);

putsUART2("\r\n");

return value;

}

/*--------------THIS FUNCTION RETURNS THE NUMBER OF FREE BYTE IN THE TX FIFO--------*/

//NOTE: THIS FUNCTION READS THE STATUS BYTE AND TELLS HOW MANY BYTES ARE

CURRENTLY AVAILABLE

//TO BE WRITTEN IN THE TX FIFO

//THIS INFO IS OBTAINED FROM THE STATUS BYTE

BYTE noFreeBytesInTXFIFO(){

BYTE value;

char string[8];

CSlow();

while(!CCready());


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putcSPI1(SNOP); //sends the no operation strobe (in Write and Single mode)


value = SPI1BUF; //status byte read

value = (value & 0x0F); //the mask is 00001111 -> 0x0F in order to read just the last 4 bits

putsUART2("-from status byte- The number of free bytes in the TX FIFO is equal or greater than ");


putsUART2(string);

putsUART2("\r\n");

return value;

CShigh();

}

/*-------THIS FUNCTION RETURNS THE NUMER OF BYTES CURRENTLY IN THE RX FIFO----------

*/

//NOTE: THIS FUNCTION DIFFERS FROM noAvaBytesInRXFIFO BECAUSE THIS ONE GIVES THE

ACTUAL

//NUMBER OF BYTES THAT ARE CURRENTLY IN THE RX FIFO

//THAT VALUE IS HELD IN THE REGISTER 0xFB

BYTE noBytesInRXFIFO(){

BYTE value;

char string[8];

value = readRegister(0xFB);

value = (value & 0x3F); //we are interested only in the bits 6:0 so 0111111 = 0x3F

//_____THE SERIAL OUTPUT IS DONE IN THE INTERRUPT IN ORDER TO USE THIS FUNCTION

IN RFreceive

//putsUART2("-from RXFIFO register- The RX FIFO has got bytes: ");

//sprintf(string, "%d", value);

//putsUART2(string);

//putsUART2("\r\n");

return value;

}

/*-------THIS FUNCTION RETURNS THE NUMER OF BYTES AWAILABLE IN THE RX FIFO----------

*/

//THIS INFO IS OBTAINED FROM THE STATUS BYTE

BYTE noAvaBytesInRXFIFO(){

BYTE value;

char string[8];


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CSlow();

while(!CCready());

putcSPI1(0xBD); //sends the No Operation Header in Read mode and Burst=0 --> 1011.1101 =

0xBD


value = SPI1BUF;

value = (value & 0x0F); //the mask is 00001111 -> 0x0F in order to read just the last 4 bits

putsUART2("-from status byte- The number of bytes available in the RX FIFO is: ");


putsUART2(string);

putsUART2("\r\n");

return value;

CShigh();

}

/*----------------------THIS FUNCTION GETS THE CC2500 STATE-------------*/

//THIS FUNCTION READS THE STATE FROM THE MARCSTATE REGISTER

BYTE getState(){

BYTE value;

char string[8];

value = readRegister(0xF5); //0xF5 is 1111.0101 is in Read and Burst mode.. but are not

bursting

value = (value & 0x1F); //this mask is 0001.1111 = 0x1F is to read just the last 5 bits of the register

if(value == 0){

putsUART2("The chip is in the state SLEEP\r\n");

}

if(value == 1){

putsUART2("The chip is in the state IDLE\r\n");

}

if(value == 2){

putsUART2("The chip is in the state XOFF\r\n");

}

if(value == 3){

putsUART2("The chip is in the state VCOON_MC\r\n");

}


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if(value == 4){

putsUART2("The chip is in the state REGON_MC\r\n");

}

if(value == 5){

putsUART2("The chip is in the state MANCAL\r\n");

}

if(value == 6){

putsUART2("The chip is in the state VCOON\r\n");

}

if(value == 7){

putsUART2("The chip is in the state REGON\r\n");

}

if(value == 8){

putsUART2("The chip is in the state STARTCAL\r\n");

}

if(value == 9){

putsUART2("The chip is in the state BWBOOST\r\n");

}

if(value == 10){

putsUART2("The chip is in the state FS_LOCK\r\n");

}

if(value == 11){

putsUART2("The chip is in the state IFADCON\r\n");

}

if(value == 12){

putsUART2("The chip is in the state ENDCAL\r\n");

}

if(value == 13){

putsUART2("The chip is in the state RX\r\n");

}

if(value == 14){

putsUART2("The chip is in the state RX_END\r\n");

}

if(value == 15){


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putsUART2("The chip is in the state RX_RST\r\n");

}

if(value == 16){

putsUART2("The chip is in the state TXRX_SWITCH\r\n");

}

if(value == 17){

putsUART2("The chip is in the state RXFIFO_OVERFLOW\r\n");

}

if(value == 18){

putsUART2("The chip is in the state FSTXON\r\n");

}

if(value == 19){

putsUART2("The chip is in the state TX\r\n");

}

if(value == 20){

putsUART2("The chip is in the state TX_END\r\n");

}

if(value == 21){

putsUART2("The chip is in the state RXTX_SWITCH\r\n");

}

if(value == 22){

putsUART2("The chip is in the state TXFIFO_UNDERFLOW\r\n");

}

return value;

}

/*----THIS FUNCTION READS THE STATUS BIT AND INTERPRETS THE CHIP STATE----------*/

BYTE getStatusByte(){

BYTE value;

char string[8];

CSlow();

while(!CCready());

putcSPI1(SNOP); //sends the Nop in order to receive the status byte


value = SPI1BUF; //reads the status byte


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value = (value & 0x70) >> 4; //the mask is 01110000 -> 0x70 in order to read just the 3 status bit, and then

shift it by 4 positions

if(value == 0){

putsUART2("The chip is in IDLE mode.. it's LAZY, must be Italian!! \r\n");

}

if(value == 1){

putsUART2("The chip is in RX mode\r\n");

}

if(value == 2){

putsUART2("The chip is in TX mode\r\n");

}

if(value == 3){

putsUART2("The chip is in ready for the Fast Transmission mode \r\n");

}

if(value == 4){

putsUART2("The chip Frequency Synthesizer calibration is running\r\n");

}

if(value == 5){

putsUART2("The chip PLL is settling.. give him a brake!\r\n");

}

if(value == 6){

putsUART2("The chip RX FIFO has overflowed.. Read out and then flush the FIFO with the

sendStrobe(SFRX)\r\n");

}

if(value == 7){

putsUART2("The chip TX FIFO has underflowed, acknowledge with sendStrobe(SFTX)\r\n");

}

CShigh();

return value;

}

void delayRX_TXtoIDLE(){

unsigned int i;

//theoretically to go from either RX or TX to IDLE should only take 8 cycles

//(with a 80MHz clk) but let's give it 15 cycles to be safe


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for(i=0; i<15; i++){

Nop();

}

}

void delayIDLEtoRX_TX(){

unsigned int i;

//theoretically to go from IDLE to either RX or TX should only take 7072 cycles


for(i=0; i<7500; i++){

Nop();

}

}

void delayCALIBRATION(){

unsigned int i;

//theoretically to calibrate (SCAL strobe) should only take 57680 cycles


for(i=0; i<60000; i++){

Nop();

}

}

Fast Hopping RF.c

#include <plib.h>

//#include <p32mx360f512l.h>

#include "SPI.h"

extern unsigned int pktCount, CRC_fail, CRC_ok, interferance;

extern BYTE bestChannels[50], base, transmitter;

typedef struct{

BYTE FSCAL1;

BYTE FSCAL2;

BYTE FSCAL3;

}calibrationResults_type;

calibrationResults_type calibrationResults[50]; //this array of struct contains calibration

//info for every best channel


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calibrationResults_type calibrationResults_base;

/*----COMMAND STROBES----*/

#define SRES 0x30

#define SFSTXON 0x31

#define SXOFF 0x32

#define SCAL 0x33

#define SRX 0x34

#define STX 0x35

#define SIDLE 0x36

#define SWOR 0x38

#define SPWD 0x39

#define SFRX 0x3A

#define SFTTX 0x3B

#define SWORRST 0x3C

#define SNOP 0x3D

/*--------THIS FUNCTION SENDS DATA OVER THE AIR-------------*/

void RFsend(BYTE tosend[], BYTE instruction, BYTE newchannel, BYTE address){

BYTE dvalue, i, payload;

CSlow();

while(!CCready());

payload = 50; //(50 bytes payload)

putcSPI1(0x3F); //TX SINGLE ACCESS


dvalue = SPI1BUF;

putcSPI1(address); //transmitter address


dvalue = SPI1BUF;

putcSPI1(0x7F); //TX FIFO address 0x3F plus (OR) the Burst bit 0x40 = 0111.1111


dvalue = SPI1BUF; //dummy read status byte, you might wanna check if there is space in txfifo by

now..

//now sending the payload (50 bytes)

for (i = 0; i < payload; i++) {


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//putcSPI1(tosend[i]); //filling in the TX FIFO with data

putcSPI1((BYTE)(rand() % 10)); //sending random data


dvalue = SPI1BUF; //dummy read

}

putcSPI1(instruction); //the first byte after the payload is the instrucion


dvalue = SPI1BUF;

putcSPI1(newchannel); //the second byte after the payload is the new channel

number


dvalue = SPI1BUF;

PORTSetBits(IOPORT_B, BIT_2); //CS high so the CC2500 knows that burst data has finished

PORTClearBits(IOPORT_B, BIT_2); //CS low again to continue communicating with the CC2500

sendStrobe(STX); //no need to add delay because the code waits anyway for the bit8 to go high

//and it does only when in tx mode

//SINCE WE HAVE SET THE REGISTER IOCFG0 to 0x06, we can get the packet transmission status from GDO0 (port-D bit-8)

while(PORTDbits.RD8 == 0); // Wait for GDO0 to be cleared -> end of packet

// Wait for GDO0 to be set -> sync transmitted

while(PORTDbits.RD8 == 1); //stays in here until D8 goes low

mPORTAToggleBits(BIT_0); //turns on the led to tell it has transmitted

//now in IDLE (TXOFF_MODE=00)

CShigh;

}

/*-------------------THIS FUNCTION IS TO RECEIVE RF PACKETS---------------------*/

void RFreceive(BYTE rxBuffer[], BYTE *instruction, BYTE *newchannel){

BYTE dvalue, i, payload, RSSI, CRC;

unsigned char string[100]; //for outputting the received

CSlow();

while(!CCready());

payload = 50; //(50 bytes payload)

sendStrobe(SRX); //NOW IN RECEIVE MODE (no need to wait because of the while)


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// Wait for GDO0 to be set -> sync received

while(PORTDbits.RD8 == 0); //stays in here until D8 goes high

// Wait for GDO0 to be cleared -> end of packet


if(noBytesInRXFIFO() != 0){

//First read is to read out the destination address

// (if we are at this point means it's the same as this device address.. so it's a dummy read)

putcSPI1(0xBF); //RX FIFO SINGLE ACCESS


dvalue = SPI1BUF;

putcSPI1(0x00); //to clock


dvalue = SPI1BUF; //unuseful

//now sends the Read Burst TXFIFO command

putcSPI1(0xFF); //RX FIFO is 0x3F + (OR) BURST READ MODE = 0x3F | 0xC0 = 0xFF


dvalue = SPI1BUF;

//now reading the RX FIFO

for(i=0; i<payload; i++){



rxBuffer[i] = SPI1BUF; //read

}

//now reading the instruction (byte 51)



*instruction = SPI1BUF; //read the instruction

//now reading the channel (byte 52)



*newchannel = SPI1BUF; //read the new channel

PORTSetBits(IOPORT_B, BIT_2); //CS high so the CC2500 knows that burst data has finished

PORTClearBits(IOPORT_B, BIT_2); //CS low again to continue communicating with the CC2500


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RSSI = readRegister(0xBF);

CRC = readRegister(0xBF);

CRC = (CRC & 0x80) >> 7;

if(CRC == 0){

CRC_fail++; //times that CRC checks gives 0 (failiure)

}else{

CRC_ok++;

}

pktCount++; //pkt received (maybe with errors, but it did fill in the RX FIFO)

}//end if noBytesInRXFIFO check

else{

putsUART2("INTERFERANCE\r\n ");

interferance++;

}

mPORTAToggleBits(BIT_0 | BIT_1 | BIT_2 | BIT_3); //turn the 4 LEDs off at startup

CShigh();

//since RXOFF_MODE=00 after receiving a pkt the state is IDLE

}

/*---THIS FUNCTION CHECKS (READING THE STATUS BYTE) IF THE CHIP IT'S IN THE

RXOVERFLOW CONDITION,

IF SO RETURNS IT SENDS A SFRX STROBE, PUTTING THE CHIP IN THE IDDLE STATE

---*/

void clearPotentialRXoverflow(){

BYTE value;

CSlow();

while(!CCready());

putcSPI1(SNOP); //sends the Nop in order to receive the status byte


value = SPI1BUF; //reads the status byte

value = (value & 0x70) >> 4; //the mask is 01110000 -> 0x70 in order to read just the 3 status bit, and then

shift it by 4 positions

if(value == 6){//if it was in over flow goes in the cycle

sendStrobe(SFRX);

//after issueing the SFRX strobe you are in IDLE mode

sendStrobe(SRX);


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//now in RX mode

}

CShigh();

}

/*------THIS FUNCTION WAITS UNTIL THE PLL LOCKS THE FREQUENCY (CALL IT AFTER

CALIBRATING)----*/

void waitPLLinLock(){

//when the FSCAL1 register is different than 0x3F it means the PPL locked the freq

while(readRegister(0x25) == 0x3F); //stays in there until PPL locks

}

/*---THIS FUNCTION CHANGES THE CHANNEL, CALIBRATES IT AND WAITS UNTIL THE PLL

LOCKS IN THE FREQUENCY---*/

//NOTE: THE CHANGECHANNEL FUNCTION GOES TO RX STATE AT THE END (optimization

purposes)

void changeChannel(BYTE newchannel){

sendStrobe(SIDLE);

delayRX_TXtoIDLE();

writeRegister(0x0A, newchannel);

sendStrobe(SCAL);

delayCALIBRATION();

waitPLLinLock();

//leaves it in IDLE

}

/*---THIS FUNCTION READS THE CRC FLAG FROM THE PKTSTATUS REGISTER---*/

//NOTE: in order to work this function requires PKTCTRL0.CC2400_EN=1

//ANYWAY IT'S BETTER TO READ THE CRC FLAG FROM THE APPEND STATUS

BYTE getCRCflag(){

BYTE value;

value = readRegister(0xF8); //reads the value from the PKTSTATUS register

value = (value & 0x80) >> 7; //masks the register to leave untouched just bit 7 and shifts it to lsb

return value;

void changeChannelFast(BYTE newBestChannel){

writeRegister(0x25, calibrationResults[newBestChannel].FSCAL1); //writing the values previously stored

writeRegister(0x24, calibrationResults[newBestChannel].FSCAL2); //during best channels calibration

writeRegister(0x23, calibrationResults[newBestChannel].FSCAL3);


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writeRegister(0x0A, newBestChannel); //also update the cannel number

//leave it in IDLE

}

/*-----THIS FUNCTION SCANS ALL THE CHANNELS AND CHANGES THE CHANNEL INTO THE

BEST ONE-----*/

void ScanAndPosition(){

BYTE RSSI_dec, offset=69, j;

int RSSI_dBm, allChannels[256], i, s=0;

//get a RSSI value for every channel (256 channels)

//i corresponds to the channel number, and allChannels[] contains the RSSI value for that channel (in dBm)

for(i=0; i<256; i++){

changeChannel((BYTE)i); //goes from channel 0 to 255 (INCLUDED)

//change channel finishes in IDLE

sendStrobe(SRX);

delayIDLEtoRX_TX();

delayCALIBRATION(); //to get a better RSSI reading the chip has to spend some time in RX

MODE (DN 505)

RSSI_dec = readRegister(0xF4);

if (RSSI_dec >= 128){

RSSI_dBm = (int)(((int)( RSSI_dec - 256) / 2) - offset);

}else{

RSSI_dBm = (int)((RSSI_dec / 2) - offset);

}

allChannels[i] =RSSI_dBm;

}

//now all the channels have been scanned, you are now in RX STATE and in channel 255

//search for the smallest RSSI value (best channel)

for(i=0; i<256; i++){

if(allChannels[i] <= s){

s = allChannels[i];

base = i;

}

}//end search best channel

changeChannel(base); //change channel to the best just found (base)

//the base channel can potentially not be part of the bestChannel list found by the TX


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//the base channel has to be calibrated separately

calibrationResults_base.FSCAL1=readRegister(0x25); //FSCAL1



//now the calibration results have been stored

//as now you are in the best channel and in IDLE state

sendStrobe(SRX);

delayIDLEtoRX_TX();

// Wait for GDO0 to be set -> sync received

while(PORTDbits.RD8 == 0); //stays in here until D8 goes high

// Wait for GDO0 to be cleared -> end of packet


//when a pkt is detected send a packet to tell that this is the base freq

RFsend(0x00, 0x00, 0x00, transmitter); //sends a packet to tell the TX that this is base frequency

sendStrobe(SFRX); //because the RX FIFO wasn't read

//now in the base channel and IDLE

}

void changeIntoBaseFast(){

//you must be in idle (you are because RXOFF_MODE=00)

writeRegister(0x25, calibrationResults_base.FSCAL1);



writeRegister(0x0A, base); //also update the cannel number

//leave it in IDLE

}

/*------------------------CALIBRATES THE LIST OF BEST CHANNELS RECEIVED-----------------------*/

void CalibrateBestReceived(){

BYTE i;

//the bestChannels array has been filled

//for each channel we have to run the calibration and save the results in the calibration array

//you are in base now and idle

for(i=0; i<50; i++){

//calibrate each best channel

writeRegister(0x0A, bestChannels[i]);


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sendStrobe(SCAL);

delayCALIBRATION();

waitPLLinLock();

calibrationResults[i].FSCAL1=readRegister(0x25); //FSCAL1



}//end for calibrate best

changeIntoBaseFast(); //goes back to base using the values previously stored for the base freq

}//function end

/*-----------THIS FUNCTION MODIFIES THE PATABLE------------*/

void setPower(BYTE newvalue){

CShigh(); //resets the PATABLE counter

CSlow(); //re-establishes the communication with the chip

while(!CCready()); //waits for the chip to be ready

writeRegister(0x3E, newvalue);




}

/*-----------THIS FUNCTION READS THE PATABLE------------*/

BYTE readPower(){

BYTE value;




value=readRegister(0x3E);




return value;

}


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Appendix B

Feasibility Study

University of Manchester

Faculty of Engineering and Physical Sciences

School of Electrical & Electronic Engineering

The Application of low-power wireless networks

to wide-area distributed audio systems

-Feasibility Study Report-

Flavio Felici

ID: 7317556

Project Supervisor:

Mr. Peter Green

May 2009


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Abstract

In this document is studied the feasibility to design and develop a wireless transmission

system: Such wireless link will be applied to inter connect a network of loudspeakers

forming a noise masking system.

The connection between each loudspeaker unit is required to work under an unlicensed

frequency range and through a protocol which preserves coexistence with other wireless

standards.

All the technical requirements, the basic background information, high level layout and

developing methodology are illustrated in this study.

Table of contents

1. Introduction

1.1 Sound Masking Systems Overview

1.2 Project Aim

1.3 Report Content

2. Transmission System Technical Requirements


2.2 Network Layout

2.3 Traffic Pattern


3. Wireless Transmission in the Unlicensed Band

3.1 ISM Band properties

3.1.1 The 2.4 and 5.8 GHz ISM Band

3.2 Comparison between wireless standards in ISM bands





3.2.5 2.4 GHz ISM Band Survey

3.2.6 Custom Built Protocol


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4. High Level Design

4.1 Transmission System Overview

4.2 Transceiver ISM Band Selection

4.3 Component Selection and Network Protocol

5. Methodology

5.1 Peer-to-Peer Link

5.2 Fixed Frequency Hopped Spread Spectrum Token Ring Network

5.3 Adaptive Frequency Hopped Spread Spectrum Token Ring Network

6. Project Management

7. Conclusions


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1. Introduction

1.1 Sound Masking Systems Overview

The purpose of this document is to verify the feasibility and to describe the design

methodology involved to develop a low-power wireless network applied to wide-area

distributed audio systems. The project serves as the final stage of a postgraduate course

and will last for four months. The wireless network that will be designed and developed

will provide a communication link between different nodes making up a distributed audio

system; in particular the audio system is an arrangement of loud speakers emitting sound

waves designed to eliminate any unwanted noise source in the area. Such method is called

sound masking, which by the emission of specially created sound patterns it allows to

reduce or eliminate the awareness of unwanted noises in the environment. Using this kind

of technique speech privacy is greatly enhanced as well as the awareness of noise sources

nearby, therefore improving concentration and productivity in work places.

Noise masking systems work thanks to a loudspeaker array counterbalancing any

unwanted sound source: this is made possible by each independent loudspeaker node that

acquires, processes and counteracts disturbing sounds. To accomplish good noise masking

performance and a coherent action, each node in the array has to share information on

noise sources, noise level and have synchronization with the other units. Therefore the

need for a communication link between the nodes rises; such a link could be wired or

wireless.


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Figure 1: Typical sound masking loud speakers network -using wired network connection

For early systems such a communication link was wired but high installation costs and the

impossibility to install wiring in historic buildings make the wireless option the better

solution.

1.2 Project Aim

Apple Dynamics1 is a audio system company based in Salford, England; it develops a

variety of products focused on acoustics and digital signal processing. –Ism® 2 is the name

of their sound masking system, which consists in a network of digital signal processing

boards coupled with a loudspeaker, each forming an independent network node.

At the moment the nodes network is connected through wires, which represent a high

expense in terms of both installation costs and installation flexibility. It‟s easy to

understand how complicate and expensive can be interconnecting tens of network nodes

through big spaces such as congress halls or open plan offices

Hence the aim of this project is to replace the wired link with a wireless transmission

system, interconnecting each node of the sound masking system without the need of wires.

Such transmission system will interface with the existing sound processing board,

upgrading it with wireless capability.

To avoid radio license costs and to keep installation flexibility, the wireless link will have


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to operate within an unlicensed ISM3 (Industrial, Scientific and Medical band), operating

under SRD4 (Short Range Devices) regulations. Both the operation frequency range and

power/modulation regulations will be key elements in the design solutions as are imposed

by the national electromagnetic spectrum regulator scheme.

Another fundamental element for the wireless network is coexistence –in fact as the sound

masking system will work within an unlicensed frequency range in work places and

crowded public areas, it is likely to find a busy electromagnetic environment.

The wireless link therefore will have to coexist with other wireless standards in the area

such as IEEE 802.115(also popularly known as Wi-Fi) and IEEE 802.15.1 (also popularly

known as Bluetooth).

1.3 Report Content

In this document the project key aspects are analyzed and discussed, underling the major

issues that the transmission system has to overcome.

In chapter two the technical requirements dictated by the sound masking manufacturing

company are listed, these parameters represent what performances and characteristics the

wireless system must have. In chapter three the major properties and regulations of the

unlicensed ISM band are discussed, the wireless system will have to obey to such set of

regulations imposed by the telecommunication authority. In chapter four the hardware

components properties and the high level design are discussed, where instead in chapter

five the project methodology is showed. This represents a guideline for the project

development. Chapter six briefly gives explanation to the project time scale, referencing

the project planning to a Gantt chart. Finally in chapter seven the feasibility study report

conclusions are discussed.


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2. Transmission System Technical Requirements

The sound masking system is made up by an array of loudspeakers forming a network

where each loudspeaker represent an independent node. Each node senses the sounds in its

surrounding environment to acquire, analyse and counterbalance what is recognized as

disturbing noise. To achieve an effective reduction in noise the counter noise action must

be simultaneous and synchronized among all the loudspeakers nodes in the area of interest,

hence the interconnection between all the network nodes has to effective and reliable. In

the early versions of sound masking systems a wired connection had such properties,

however implementing such type of connection wirelessly needs to take consideration of

several aspects. Nevertheless, being the number of nodes quite high in the average sound

masking network, the transmission system has to be low cost, making the choice of

software and hardware components a key issue.

The requirements imposed by the sound masking manufacturing company for the wireless

transmission system are listed as follows.


The transmission system must be able to work within the unlicensed spectrum of different

countries: As the main markets for such type of sound masking devices are Europe, USA

and Japan, both the frequencies used for data transmission and the maximum transmission

power must comply with the unlicensed spectrum regulation of the country of installation.

Hence the transmission hardware and software must be capable of working in all the

above mentioned countries within an unlicensed frequency range. However European

common laws and international agreements have set common frequency windows,

standard transmission power levels and modulating schemes to address such type of issues,

making easier to find common unlicensed windows. One of these common unlicensed

frequency ranges is defined at 2.4 GHz, it has been asked by the company to take such

range into account when selecting the frequency of operation.


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2.2 Network Layout

To allow a correct sound masking action, the wireless link must be able to provide constant

communication between all the independent nodes, in round-robin. The nodes network can

be made of smaller ring sub-networks in which every node communicates with all the units

in its network ring and where inter sub-networks link is assured through a common node,

shared by two or more sub-networks. This implies that it won‟t be necessary for each node

to be able to communicate with every other node in the system.

Previously installed wired systems have shown that the typical number of nodes for each

sub-network is around 50 with each node within a 10 meters range.

This kind of link is required as each node contains an independent audio processor which

acquires and processes the audio signals in the nearby area, sharing them with all the other

nodes for a synchronized sound masking action. Figure 2 shows a typical network layout.

Figure 2: Ring Networks set up21

2.3 Traffic Pattern

In order for the sound masking system to work the link between nodes has to be reliable

and constant, but not necessarily in real time. This implies that each node will transmit data

at predetermined intervals applying some sort of error detection technique, this to ensure

that the transmission has been successful. As the communication is not in real time, if an

error is detected the transmission can be repeated until the data is successfully received.


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Statistics from previously installed wired systems have shown that the typical traffic bit

rate is around 100 kbps, with a possible peak of 200 kbps, with data packets not exceeding

256 bytes.

Figure 3 shows a table containing values of a typical traffic pattern.

Figure 3: Sound Masking System Typical Traffic Pattern21


Sound masking systems are most useful in working environments –such as offices or

meeting rooms; or in crowded public spaces –such as congress halls;

consequently it is very likely to find a fully engaged unlicensed spectrum. This because

popular devices such as cordless phones, wireless security cameras, local area networks

(like IEEE 802.115) and personal area networks (like IEEE 802.15.1

6) operate within

unlicensed bands, making interference a key issue.

The wireless link will have not only to withstand interference from other devices and

standards but also co-exist with them.

In fact the transmission between the network nodes must successfully take place without

jamming other devices. For this reason has been commissioned that the transmission

system requires a Frequency Hopping Spread Spectrum (FHSS) technique, which allows a

dynamic change of transmission frequency reducing channel losses and jamming other

devices.


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3. Wireless Transmission in the Unlicensed Band

Around the world all wireless transmissions have to obey the rules set by the International

Telecommunication Union7, these worldwide shared set of rules are furthermore modified

and adjusted by each singular country according to its needs and own regulations. More

specifically the ITU divides the world in three large regions: Region1 comprises Europe,

Africa, the Middle East west of the Persian Gulf including Iraq, the former Soviet Union

and Mongolia; Region2 covers the Americas, Greenland and some of the eastern Pacific

Islands; while Region3 is consists of most of non former Soviet Union, Asia, east of and

including Iran, and most of Oceania8.

The ITU regulations define for each particular frequency range, among other things, the

purpose of the transmission, the kind of license required, the maximum allowed

transmitted power and the modulation scheme to be used. However for defined frequency

ranges wireless transmission are allowed without any license or specific purpose –these

ranges are often called Industrial, Scientific and Medical (ISM3) bands.

For instance in the UK, which is lies in the ITU Region1, the unlicensed frequency ranges

are defined by the Office of Communications9 (OFCOM) as showed in table 1.

Generic Frequency Band Application

9 kHz to 30 MHz Short Range Inductive Applications

27 MHz Telemetry, Telecomm and Model Control

40 MHz Telemetry, Telecomm and Model Control

49 MHz General Purpose Low Power Devices

173 MHz Alarms, Telemetry, Telecomm and Medical Applications

405 MHz Ultra Low Power Medical Implants Devices

418 MHz General Purpose Telemetry and Telecomm and Applications

458 MHz Alarms, Telemetry, Telecomm and Medical Applications

864 MHz Cordless Audio Applications

868 MHz Alarms, Telemetry and Telecomm and Applications

2400 MHz General Purpose Short Range Applications, including CCTV and

RFID. Also used for WLANs including Bluetooth Applications.

5.8 GHz Hyper LANs, General Purpose Short Range Applications,

including Road Traffic and Transport Telematics

10.5 GHz Movement Detection

24 GHz Movement Detection


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63 GHz 2nd

Phase Road Traffic and Transport Telematics

76 GHz Vehicle Radar Systems

Table 1 : Unlicensed Bands in the UK10

Unlicensed band doesn‟t mean unregulated, in fact transmissions within those bands

require to obey public health safety regulations, transmission purposes, often

predetermined modulation schemes and anti jamming regulations.

Transmitting in such bands offers a great deal in terms of reducing bureaucracy, cost and

increasing flexibility, on the other hand these frequencies tend to be always more and more

electromagnetically crowded –coexistence and interference are then key issues within these

bands.

3.1 ISM Band properties

As table 1 shows there are quite a few possible ISM bands to choose from. At this point

other parameters have to be taken into account for selecting the operating frequency:

components availability, antenna dimension, path loss, scattering and interference are

among them.

Since the transmission system will be installed indoor (often in small areas such as false

ceiling or suspended on walls) and since the standard isotropic antenna length is related

with the transmitting frequency wavelength, ISM bands below the megahertz order are

discarded (those would require antennae more than meters long). Another factor is path

loss, the higher the transmitting frequency the higher the attenuation caused by the

atmosphere will be; however the transmission system will only be installed indoor with

relatively short distances between nodes, making this parameter negligible. The scattering

produced by the electromagnetic waves being reflected by objects increases with the

frequency, offering to the receiver a good multipath reception -particularly in small indoor

environments. Given these considerations, the most suitable ISM bands for this kind of

transmission system are the 2.4 and 5.8 GHz, both offering small antenna dimension and

good scattering.


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3.1.1 The 2.4 and 5.8 GHz ISM Band

Yet the 2.4 GHz band already hosts various public wireless services, such as IEEE 802.11,

IEEE 802.15.1 and IEEE 802.15.4-200611

(also popularly known as Zigbee) to name a few.

This involves that this frequency range is likely to be very electromagnetically crowded

and full of interferences, especially in urban environments (since cordless phone, cordless

security cameras, microwave ovens share this very same band).

On the other hand this implies that the market has produced quite a large variety of

components specially designed to work at this frequency range. Instead the 5 GHz ISM

band is likely to have less interferences and data traffic but, as it is fairly new ISM band

the choice of components produced by manufacturers is fairly limited.

Another key parameter to be taken into account is the maximum transmission power level

defined for each ISM band: for instance throughout the European Union in the 2.4-2.4835

GHz ISM range the maximum output power out of the antenna is set as 10 mW19

; in the

5.150-5.350 GHz ISM range (also called Band A, indoor use only) the limit is set at 200

mW with a power density of 10 mW/MHz20

; while in the 5.470-5.725 GHz ISM range

(also called Band B, indoor and outdoor use allowed) the maximum irradiated power is set

to be 1 W with a power density of 50 mW/MHz20

.

To be noted that regulations have allowed higher power levels for the 5 GHz ISM band

because of the higher path loss encountered at these higher frequencies. On the other hand,

at this frequency range regulations also impose to use the OFDM modulation scheme.


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3.2 Comparison between wireless standards in ISM bands

With the commercial development of devices working in the 2.4 GHz ISM band over the

years many protocols have been developed for the public market. The most common for

the public use are probably Wi-Fi, Bluetooth while ZigBee and SimpliciTI12

are common

for industrial applications.


Wireless fidelity (Wi-Fi), in its many variants, has become the standard for public and

private local area networks. It allows users to access the internet and share data within the

local network at high speeds, achieving 600 Mbps with its latest standard 802.11n13

(which

uses MIMO technology), with a reasonably large operative range of several tens of meters

(despite the transmit power limit is set at 10dBm EIRP). However this standard is fairly

complex to implement on software and requires a relatively high amount of computational

power to run, raising the cost for each network node. Nevertheless the Wi-Fi standards

allows a great number of network users to be connected, in fact the protocol can handle up

to 2007 users16

, far beyond what is necessary for the sound masking system. Nowadays

most of the Wi-Fi LANs are operating in the 2.4 GHz ISM band, making this frequency

range electromagnetically crowded. However it is possible to set up several LANs in the

same area at the same time: In fact the Wi-Fi standard divides the 2.4 GHz band into 13

channels in Europe (14 in Japan and 11 in USA14

) where each channel is 22 MHz wide5.

As noticeable from figure 4 these channels overlap, giving interference between

neighbouring channels, however each channel is masked as its power is attenuated by 30

dB at ± 11 MHz from the channel central frequency. This means that channels 22 MHz

apart don‟t interfere with each other.

In particular, to ideally achieve zero interference four LANs could be set up operating in

the non overlapping channels 1, 5, 9 and 13.


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Figure 4: Wi-Fi Channels in the 2.4 GHz ISM band15

Therefore the Wi-Fi protocol is ideal to set up local area networks sharing large quantities

of data between users and from the internet, yet this powerful protocol comes with the

price of a complex software implementation and as a result needs powerful hardware to be

ran.


Bluetooth is another commonly applied standard in the 2.4 GHz band, particularly used for

personal area network devices (WPAN) such as mice, mobile phones and printers. The

standard has been developed to achieve very low power consumption and a transmission

range of few meters (depending on the transceiver class), with a modest data rate –making

it ideal choice to interconnect such personal devices. The Bluetooth connection is

established between a device serving as a master and another set as slave. Instead of

dividing the band into channels, as the Wi-Fi standard defines, the Bluetooth link uses a

frequency hopping technique to constantly change the communication frequency between

the master and the slave devices16

. The frequency hopping pattern is defined by the master

devices which dynamically selects what frequency to communicate at, avoiding

interference with other appliances. Yet the maximum number of active connections is

eight16

, which limits its network capabilities.


ZigBee instead, is a standard specially developed for industrial applications and its typical

operating range is around 10 meters6. The bit rate that it can support is quite limited,

making the Zigbee protocol ideal for applications such as infrared sensor, smoke detectors

or light switches. Most of all, this standard is designed around an extremely low power


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consumption, in fact a transmission system to be certified with the Zigbee standard must be

battery powered, with at least a 2 year operating lifetime. However the number of nodes in

a Zigbee network can be greater than 65,00016

.


SimpliciTI is another network protocol commonly used within ISM bands. It has been

developed by Texas Instruments to help creating networks with its produces transceivers. It

is very similar to Zigbee for both data rate and range, making it ideal for application like

smoke detectors, alarms or automatic meter readings17

. Nonetheless it‟s an open protocol

and doesn‟t require to be implemented into battery powered systems. The maximum

number of nodes that can be set up into a network using the SimpliciTI standard is defined

as 232

, well beyond what required by the sound masking transmission system. On the other

hand in order to use the SimpliciTI protocol Texas Instruments components have to be

used and the protocol used is not customizable, reducing the overall efficiency.

3.2.5 2.4 GHz ISM Band Survey

Figure 5 shows a 2.4 GHz ISM band survey taken with a spectrum analyzer. The

measurements were made in the frequency range spanning from 2.4 to 2.485 GHz using a

resolution bandwidth of 1 kHz. In the picture it is possible to distinguish three different

Wi-Fi channel used and Bluetooth activity over different frequencies. In fact it is clear

from this picture that the Wi-Fi standard uses portion of the spectrum well defined in width

and position, while Bluetooth uses a FHSS technique probing the spectrum over different

frequencies.

As the bit rate imposed by sound masking system doesn‟t require a wide bandwidth, the

wireless transmission system can operate in such ISM band, even if it is so

electromagnetically crowded.

Will be fundamental for the transmission system to operate in those unused regions of the

spectrum.


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Figure 5: Spectrum Survey of the 2.4 GHz ISM Band

3.2.6 Custom Built Protocol

The above mentioned protocols represent a common and reliable solution used in great

deal of applications all over the world, however every standard protocol has its own

drawbacks as it has to suit a vast variety of applications.

A custom made network protocol could instead be developed just for this wireless system,

improving efficiency and reducing the computational power required. Such custom build

protocol can be fitted around the components selected and deal with more flexibility issues

like coexistence and interference. Furthermore if a non standard protocol is used, any

possible maintenance operation will have to be performed only by the manufacturing

company, leaving no space for competitors.

Is consequently believed that the best solution to meet the project requirements (in terms of

performances, characteristics and cost) is to develop a custom protocol.

Table 2 reassumes all the different network protocols characteristics.

-150

-100

-50

0

2400

.000

001

2402

.627

273

2405

.254

546

2407

.881

819

2410

.509

091

2413

.136

364

2415

.763

637

2418

.390

91

2421

.018

182

2423

.645

455

2426

.272

728

2428

.9

2431

.527

273

2434

.154

546

2436

.781

819

2439

.409

091

2442

.036

364

2444

.663

637

2447

.290

91

2449

.918

182

2452

.545

455

2455

.172

728

2457

.8

2460

.427

273

2463

.054

546

2465

.681

819

2468

.309

091

2470

.936

364

2473

.563

637

2476

.190

91

2478

.818

182

2481

.445

455

2484

.072

728


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PROTOCO

L PURPOSE ADVANTAGES DISADVANTAGES

FREQUENCY

RANGE

802.11

a/b/g/n

Medium range wireless

LAN

Worldwide

standard

high data rates

can implement

encrypted

communications

Sensible cost

Not readily available

Requires elevated

computational load

and software to be

implemented

2.4 GHz;

5 GHz;

Bluetooth Short range wireless

personal area networks

Worldwide

standard

Wide choice of low

cost components

Good

interoperability

Short range

Non suited for

crowded

environments

(limited number of

channels)

2.4 GHz;

Zigbee

Short range, very low

power wireless

communication link

Worldwide

standard

Low cost

components and

wide choice

Very low power

consumption

Low system

requirements

Battery powered

systems only

Strict power

consumption

specifications

Low data rate

2.4 GHz;

TI

SimpliciTI

Medium range low

power wireless

communication

between devices3

Low cost

components and

wide choice

Low power

consumption and

duty cycle;

Low system

requirements

TI devices to be used

Low data rate

Non customizable

protocol

Sub 1 GHz;

2.4 GHz;

custom Purpose built: Very low cost Time to develop Any ISM BAND


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bidirectional peer-to-

peer communication

link

hardware

Highly efficient

data rate

Low power

consumption

Robust to channel

interferences

Able to coexist

with other wireless

transmission

Wide choice of low

cost components

Network not

interoperable with

other wireless

standards

Table 2: Network protocols characteristics

4. High Level Design

4.1 Transmission System Overview

The transmission system will have to be able to receive data from the digital processing

unit in the sound masking board, process it to apply the network protocol and hand it over

to the transceiver, which will send the bitstream to the antenna for transmission. Vice versa

the signals received from the antenna will have to be converted into logic signals by the

transceiver, processed by the microcontroller to extract the data and then handed over to

the sound masking board.

Therefore the system will require an adequate processing unit, that supplies enough

computational power to handle the bit rate requested and the complexity of the network

protocol.

Those tasks can be adequately handled by a microcontroller unit, which once programmed

with the network protocol will manage all the data received from the sound masking unit.

In particular, the sound masking board will communicate with the microcontroller through

some kind of interface, fast enough to handle the bitstream.

It is requested by Apple Dynamics that the microcontroller is chosen from the PIC-32

family, communicating with the DSP board through the SPI interface.

Both requirements perfectly suit the task as the SPI interface can easily handle the


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maximum bitstream declared in the requirements and the PIC-32 has enough

computational power to acquire and manage the data, applying the custom protocol.

Once the data have been processed, they are sent to transceiver which is transforms the

logic signals into RF signals ready to be sent for transmission. Furthermore the transceiver

will be required to apply the required frequency hopping spread spectrum technique,

changing its transmission frequency at the pace requested by the protocol in the

microcontroller unit. Once the RF signals have been produced are sent from the transceiver

unit to the antenna. Nevertheless the transceiver will also transform the RF signal received

from the antenna into logic signals to be handed the microcontroller. Figure 6 shows the

transmission system schematic view.

Figure 6: Transmission System High Level Design

4.2 Transceiver ISM Band Selection

As mentioned in the technical requirements section the transceiver hardware has to able to

operate within an ISM band common into all ITU regions and, more specifically, the 2.4

GHz and 5 GHz band have been found to suit the application the most due to the smaller

antenna size required. Since the 2.4 GHz band has been used for longer, currently the

choice available for components in this band is far greater compared to the 5 GHz band. In

fact, as this document is produced, is quite hard to find a cheap 5 GHz transceiver capable

of implementing all the requirements imposed by the project. Therefore, due to

components availability, the band of choice is the 2.4 GHz, which is also preferred by the

Apple Dynamics company. Yet, if in the future the range of components in the 5 GHz band

will broaden, the transceiver unit could be updated without drastic changes in the

transmission system protocol. The communication protocol between the transceiver unit

and the microcontroller wasn‟t dictated in the project requirements and various options are

supported by both the microcontroller and the transceiver unit –such as UART, SPI and


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I2C. Though, this was chosen to be the SPI protocol as it represents a reliable and

consistent solution. As the distance between nodes declared in the project specification is

in the order of meters, the transmission system could not need a RF front end –such front

end would be a power amplifier to boost the signals to achieve a greater range (the

transmission power however has to be within the maximum imposed by the ITU and

country regulation for the 2.4 GHz band) and a low noise amplifier to recover and amplify

very weak signal received. The choice of adding in cascade between the transceiver and

the antenna such a RF front end will be evaluated during the final components selection.

The actual signals transmission is relegated to an antenna, which, to achieve maximum

efficiency in both transmission and reception pattern has to be at least 6 cm long for the

operation in the 2.4 GHz range. Due to the ring network configuration, to achieve the best

link with all the nodes in the network the antenna is chosen to be isotropic, which radiates

equally in all directions along it‟s orthogonal axis. Such types of antenna are readily found

in the market at a reasonably price, and since they radiate equally in all direction, don‟t

require special positioning when installed.

4.3 Component Selection and Network Protocol

As mentioned above the microcontroller choice has been fixed in the Microchip PIC-32

family. Such microcontroller delivers a 32-bit performance with a clock frequency of 80

MHz, capable of complex embedded designs18

. It supports various standards connection

protocols such as UART, SPI and I2C. In particular it supports 2 SPI connections, which

allows to use this protocol for the connection with both the DSP board and the transceiver.

The custom built network protocol will be developed with a programming language and,

once compiled, downloaded into the device. This will take control of the device processing

power and drive/read its pins.

The transceiver unit can be chosen from a selection available to the public market, and the

final selection will be made during the project development.

Fundamental requirements for this unit are compliance with the 2.4 GHz band, FHSS

capability, low noise amplification, the possibility to attach an isotropic antenna and the

possibility to reach the maximum transmission level allowed in the ITU regions for the

chosen ISM band.

Such requirements can be found on components like the Texas Instruments-Chipcon

CC2400, CC2420, CC2520, CC2430, CC2431, CC2510, CC2511, CC2591; or in the

Microlinear models ML2724 and 5805. All this devices are compatible with the

application and are relatively low cost.


Flavio Felici


Better performances in terms of longer range and lower noise can be achieved with the

CC2591 and ML5805 which implement a built in RF front end, however these chips have a

slightly higher price.

The protocol will manage the communication with the DSP board and drive the transceiver

through the microcontroller. Furthermore it will manage the transmission frequency and

power levels in the transceiver adjusting its internal registers.

5. Methodology

The project aim to establish a wireless network within an ISM band that interconnects tens

of independent sound processing units requires different aspects to be taken into account.

As showed in the component selection paragraph both the microcontroller and the

transceiver frequency of operation have been decided during the project feasibility stage.

These choices have been made considering several aspects such as system performance,

flexibility and cost. Taking into account that the operation frequency has been selected in

one of the most common and used ISM bands and that the transmission system has to

coexist with other wireless standard present in the area, the project will be developed

around transmission efficiency and focusing on low interference. Once the transceiver

model has been selected and the hardware assembled, the project development will follow

the following steps.

5.1 Peer-to-Peer Link

The first step will be establishing a wireless connection between two nodes at a fixed

transmission frequency. This kind of transmission will ensure that a peer-to-peer link

between two independent nodes, at the required distance range, is established. Making

possible data transfer between two nodes.

Such operation represents the first step because, to be successful, requires several different

aspects to work simultaneously: it requires a correct hardware assemblage, a correct peer-

to-peer protocol to be installed into the microcontroller and a correct transceiver setup. All

these aspects are fundamental for the project development and no further progress could be

made without successfully completing such a step.

To verify the completion of this stage a data comparison between the transmitter and the

receiver node will be made, verifying transmission speed, range, modulation scheme and


Flavio Felici


bit error rate. Additionally a spectrum analyzer can be used to confirm whether the actual

transmission frequency used is accurate with the one set in the protocol.

5.2 Fixed Frequency Hopped Spread Spectrum Token Ring Network

Once a fixed frequency transmission is established between two nodes the project

development shall move toward a frequency hopped transmission. Frequency Hopped

Spread Spectrum technique consists of continuously changing the transmission frequency,

the change in frequency is conceded between transmitter and receiver, which follow the

same frequency pattern. As interference and coexistence are key issues in this project,

implementing such a technique is vital. In fact constantly changing the transmission

frequency allows to use the entire ISM spectrum one little portion at the time, avoiding

possible busy frequencies. The frequency pattern will in effect avoid those frequencies

used by common standards such as Bluetooth or Wi-Fi.

Furthermore if the transmission happens to be at a frequency already occupied by another

wireless protocol (hence the transmission will be corrupted) thanks to the FHSS technique,

the unsuccessful transmission will be repeated at the next frequency in the pattern, until the

data is correctly transmitted.

As a matter of fact at this point in the project some kind of error detection technique will

be developed, verifying the correct data transmission. Furthermore, once the data

transmission between two nodes is successful, other nodes can be added following the

same frequency pattern. This will build up a network of nodes transmitting and receiving

round-robin along a fixed frequency pattern.

Such kind of network already represents a valuable solution to the issues faced in a

electromagnetically busy ISM band, therefore meeting the system requirements.

It is believed that a fixed frequency hopped spread spectrum token ring network meets the

technical requirements imposed by Apple Dynamics and, in the given project time, it is a

feasible solution.

5.3 Adaptive Frequency Hopped Spread Spectrum Token Ring Network

A further development for the transmission system can be the implementation of an

adaptive frequency hopped spread spectrum protocol. The adaptive system differs from the

fixed as the transmission frequency pattern followed by the network is not fixed, but

dynamically selected depending on the surrounding environment. In fact an adaptive


Flavio Felici


system would sense the spectrum to acknowledge what frequencies are being used,

allowing the transmission only in those portions of the spectrum unused by nearby wireless

systems. Such a solution increases the transmission efficiency and decreases the bit error

rate, however requires a more complex network protocol. The feasibility to implement an

adaptive frequency hopped spread spectrum transmission system is not assured in the given

project time, however if the project development would finish ahead of schedule the

adaptive protocol could be implemented.

6. Project Management

As shown by the Gantt chart attached in the appendix, the project development will start

during the second half of may; the first task is going to be the component selection and the

consequent hardware setup and testing. This task has to be completed by the end of the

month. The second step will be establish a peer-to-peer communication between two

boards, at a fixed transmission frequency. As this task is comprehensive of several aspects,

it will require the whole month of June, and possibly even the first week of July. Afterward

a fixed FHSS communication will be developed during the month of July and then a token

ring network will be built up between the end of July and the first week of August. If the

project development will be ahead of this schedule, and adaptive FHSS system will be

developed during the first two weeks of August. Certainly the last two weeks of August

will be left for the dissertation project writing up.

As the deadline for the dissertation submission has been set to September 7th, a week time

is left for possible delays throughout the project time.

7. Conclusions

In this feasibility study report the key aspects involved in designing a wireless transmission

system have been discussed and analyzed. The actual project development will follow the

guidelines showed in this report, following the given timescale.

At the end of the project development the transmission system will have to meet those

requirements imposed by the Apple Dynamics company, both in terms of performances

and coexistence.

These requirements are going to be met following the methodology and planning discussed

in this report.

8. References


Flavio Felici


1: Apple Dynamics -company website, http://www.appledynamics.com/ [accessed on

20/04/09];

2: Apple Dynamics -ism®, http://www.appledynamics.com/toplevel_pages/-ism.html


3: ISM Band, http://en.wikipedia.org/wiki/ISM_band [accessed on 07/05/09];

4: SRD –Low Power Communication Devices, http://en.wikipedia.org/wiki/SRD [accessed

on 08/05/09];

5: IEEE 802.11, http://en.wikipedia.org/wiki/IEEE_802.11 [accessed on 06/05/09];

6:IEEE 802.15.1, http://en.wikipedia.org/wiki/IEEE_802.15.1#Bluetooth_1.1 [accessed on

06/05/09];

7: ITU –Radio communication Sector, http://www.itu.int/ITU-

R/index.asp?category=information&rlink=rhome&lang=en [accessed on 01/05/09];

8: International Telecommunication Union region,

http://en.wikipedia.org/wiki/International_Telecommunication_Union_region [accessed on

05/05/09];

9: Office of Communications -Ofcom , http://www.ofcom.org.uk/ [accessed on 03/05/09];

10: Unlicensed Spectrum | ICT Regulation,

http://www.ictregulationtoolkit.org/en/Section.2843.html [accessed on 08/05/09] ;

11: IEEE 802.15.4-2006, http://en.wikipedia.org/wiki/IEEE_802.15.4-2003 [accessed on

01/05/09];


http://focus.ti.com/docs/toolsw/folders/print/simpliciti.html [accessed on 1/03/09] ;

13: 802.11n: Next-Generation Wireless LAN Technology,

http://www.broadcom.com/collateral/wp/802_11n-WP100-R.pdf, page 4 [accessed on

9/05/09] ;

14: List of WLAN channels, http://en.wikipedia.org/wiki/List_of_WLAN_channels

[accessed on 10/05/09] ;

15: Wireless Networking in the Developing World,

http://upload.wikimedia.org/wikipedia/commons/c/c6/Wireless_Networking_in_the_Developing_World.pdf , page 25 [accessed on 10/05/09];


Wireless Protocols: Bluetooth, UWB, Zigbee, and Wi-Fi”, Industrial Technology Research

Institute (ITRI) Hsinchu, Taiwan, 2007, page 2;


http://focus.ti.com/docs/toolsw/folders/print/simpliciti.html, [accessed on 10/05/09]

18: Product Overview, Microchip,

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2607,

[accessed on 10/05/09]



http://www.appledynamics.com/

http://www.appledynamics.com/toplevel_pages/-ism.html

http://en.wikipedia.org/wiki/ISM_band

http://en.wikipedia.org/wiki/SRD


http://en.wikipedia.org/wiki/IEEE_802.15.1#Bluetooth_1.1

http://www.itu.int/ITU-R/index.asp?category=information&rlink=rhome&lang=en

http://www.itu.int/ITU-R/index.asp?category=information&rlink=rhome&lang=en

http://en.wikipedia.org/wiki/International_Telecommunication_Union_region

http://www.ofcom.org.uk/

http://www.ictregulationtoolkit.org/en/Section.2843.html

http://en.wikipedia.org/wiki/IEEE_802.15.4-2003


http://www.broadcom.com/collateral/wp/802_11n-WP100-R.pdf

http://en.wikipedia.org/wiki/List_of_WLAN_channels

http://upload.wikimedia.org/wikipedia/commons/c/c6/Wireless_Networking_in_the_Developing_World.pdf

http://upload.wikimedia.org/wikipedia/commons/c/c6/Wireless_Networking_in_the_Developing_World.pdf


http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2607



Flavio Felici



1&user=1&documenttype=-1&sortingcolumn=0&submitSearch=Search [accessed on

28/04/09];




1&user=1&documenttype=-1&sortingcolumn=0&submitSearch=Search [accessed on

28/04/09];

21: Apple Dynamics, Project Technical Requirements Paper, Dr. Roger Brassington,

26/03/09;

Appendix C

Datasheets Front Sheets




Documents

Flavio Felici Dissertation