Signals, Instruments, and Systems – W9Signals, Instruments, and Systems W9

An Introduction to Communication Systems and

Wireless Sensor NetworksWireless Sensor Networks

Outline

• Wired Communication– Communication model– Modulation, demodulation

• Wireless Communication– Sharing the mediumSharing the medium– Throughput limits (Shannon-Hartley)

Link budget free space path loss– Link budget, free space path loss• Wireless sensor networks

A li i– Applications– The MicaZ – a wireless sensor node

Wired Communication

Communication Model

Transmitterchannel

Receiver

Communication Model

Noise

Transmitterchannel

Receiver

DistortionFiltering

ModulationCoding

DemodulationDecodingFiltering

Frequency shift...

Coding(Compression)

Decoding(Decompression)

RS-232 (serial port)

• Hardware:– 3 wires: TxD, RxD, Ground

RxD TxDRxD

Transceiver1

Transceiver2

TxD

Ground

TxD

RxD

GroundGround Ground

Transceiver = Transmitter + Receiver

RS-232 (serial port)

• Signal: between RxD/TxD and Ground

RxD TxDRxD

Transceiver1

Transceiver2

TxD

Ground

TxD

RxD

GroundGround Ground

RS-232 Modulation

RS-232 Demodulation

RS-232 Delay

• Packet-based– 1 byte / packety p– 8 data bits + 2 control bits = 10 bits

• Transmission speedTransmission speed– max. 115'200 bits/s (bps)

• Propagation speed:• Propagation speed:– approx. c (speed of light)

S 232 lRS-232 Delay

• Transmission delay– 10 bits / 115'200 bps = 86.8 us

• Signal propagation delay (2 m cable) – 2 m / c = 6.6712819 ns

• Processing delay:– ~ 1 us (modulation, demodulation, processing)us ( odu o , de odu o , p ocess g)

• Total: ~ 90 us = 0.09 ms

Wireless Communication

Communication Model

Noise

Transmitterchannel =

Receiver

ElectroMagneticwaves in air

ReflectionsFadingInterferenceOth EMOther EM sources...

Communication Model

Noise

Transmitterchannel =

Receiver

ElectroMagneticwaves in air

ReflectionsFadingInterferenceOth EM

Channel estimationAdvanced modulation typesCoding and error correctionOther EM sources

...

Coding and error correction

Sharing the Medium

2

1

3

Sharing the Medium

• TDMA– Time-Division Multiple Accessp– “You shut up while I talk“– Time allocation

• Fixed, synchronizede.g. mobile phones (GSM) D i ( h k if h l i f )• Dynamic (check if channel is free)e.g. Wireless LAN (802.11b/g/n)

1 2 3 2 31 2 3 2 3 time

Sharing the Medium

• FDMA– Frequency-Division MAq y– e.g. FM radio channels– Frequency regulationq y g

• BAKOM (CH)

bandwidth

1 2 3

fll d b BAKOM

bandwidth

frequencyallocated by BAKOM

B d id hBandwidth

• FM station broadcasting at 106,4 MHz→ actually occupies 106,3 MHz – 106,5 MHz→ Bandwidth = 200 kHz→ Bandwidth = 200 kHz

• Mobile phone (GSM): 200 kHz (around 900 MHz)• WLAN/WiFi: 5 MHz (around 2 4 GHz)• WLAN/WiFi: 5 MHz (around 2,4 GHz)• Analog TV station: 6 MHz (around 180 MHz)

What does the bandwidth depend on?Bandwidth [Hz] ↑ → Data rate (Throughput) [bits/s] ↑Bandwidth [Hz] ↑ → Data rate (Throughput) [bits/s] ↑

B d id hBandwidth

Sharing the Medium

• CDMA (spread spectrum) – Code-Division MA– Using different transmission codes– e.g. GPS, Wifi, 3G cell phonesg , , p– Interesting properties

• Wide channels (fading) • Concurrent communication

– More complex demodulation

Throughput (bits/s)

• TDMA, FDMA, CDMA can be combined• Total throughput is sharedg p

TDMA

CDMA

FDMA

Shannon-Hartley Limit

• Hard theoretical limit on throughput– More bandwidth = higher throughputg g p– More power (SNR) = higher throughput

C: capacity (throughput)p y ( g p )B: bandwidthS: signal power (W) N: noise power (W)

Power

• Increased power– higher throughputg g p– higher range– mobile systems: shorter battery lifey y– increased health risk (?)

• RegulationRegulation– CH: BAKOM– e g WLAN: 100 mWe.g. WLAN: 100 mW

Power

• Unit: W (Watt) – Often written in dBm (decibels mW)( )

• Gain / loss: factorsOft itt i dB (d ib l )– Often written in dB (decibels)

PdBm⎟⎞

⎜⎛= log10 W

dBmPP )log()log()*log( yxyx +=

• 1mW → 10 log(1mW/1mW) → 10 log(1) = 10*0 = 0dBm

⎟⎠

⎜⎝

=mW1

log10dBmP )log()log()log( yxyx +=

• 10mW → 10 log(10mW/1mW) → 10 log(10) = 10*1=10dBm• 2mW → 10 log(2mW/1mW) → 10 log(2) ≈10*0.3=3dBm

100 W ?• 100 mW ?• 23 dBm ?• 10 dBm ?• -10 dBm ?• 100 dBm ?

Link BudgetT i l WLAN li k b d (100 di l )TX powerTX losses

100 mW*0 5

20 dBm-3 dB

Typical WLAN link budget (100 m, dipole antennas):

TX lossesTX antenna gainFree space path lossRX antenna gain

0.5*1.6*1.0106*10-8

*1 6

3 dB+2 dB-80 dB+2 dBRX antenna gain

RX losses

RX power

*1.6*0.5

+2 dB-3 dB

62 dBm0 00000064 mWRX power

RX sensitivity -85 dBm

-62 dBm

M i 23 dB200

0.00000064 mW

0.000000003 mW

Margin 23 dB200

Free Space Path Loss

• Signal power decay in air:

• Proportional to the square of the distance d• Proportional to the square of the frequency f

– high frequency = high loss– low frequency = low bandwidth

Summary

• Communication: wired or wireless • Bandwidth is scarce• Sharing of the medium: TDMA, FDMA, CDMA• Range*2 → Power2

• Bandwidth ↑ → Frequency ↑• Frequency*2 → √Range• Know your dBs

Wi l S Wireless Sensor NetworksNetworks

M i iMotivation

What if we could monitor events which …

– have a large spatial and temporal distribution– require in-situ measurements– take place in hard to access places– generate data which need to be available in

lreal-time

M i iMotivation

What would we need for that?A device which …

– is cheap – so we can distribute many of it p y– is reliable – so we can measure for a long time– uses little power – battery/solar cell poweredp y p– has a radio – so it can communicate– can move – so it can relocate

Motivating Applications

A li i 1 PApplication 1 - Permasense

• What is measured:– rock temperature– rock resistivity– crack width– earth pressure– water pressure

Pictures: courtesy of Permasense

A li i 1 PApplication 1 - Permasense

• Why:“[…] gathering of

environmental data that helps to understand theunderstand the processes that connect climate h d k f llchange and rock fall

in permafrost areas”

Pictures: courtesy of Permasense

A li i 1 PApplication 1 - Permasense

– spatial distribution?– temporal distribution?– in-situ measurements?– take place in hard to

l ?access places?– generate data which

need to be available inneed to be available in real-time?

Pictures: courtesy of Permasense

A li i 2 GITEWSApplication 2 - GITEWSGerman Indonesian Tsunami Early Warning System

• What is measured:– seismic events– water pressure

Pictures: courtesy of Deutsches GeoForschungsZentrum (GFZ)

A li i 2 GITEWSApplication 2 - GITEWS

• Why:To detect seismic events which could cause a Tsunami. Detect a Tsunami andDetect a Tsunami and predict its propagation.

Pictures: courtesy of Deutsches GeoForschungsZentrum (GFZ)

A li i 2 GITEWSApplication 2 - GITEWS

– spatial distribution?– temporal distribution?– in-situ measurements?– take place in hard to

l ?access places?– generate data which

need to be available inneed to be available in real-time?

Pictures: courtesy of Deutsches GeoForschungsZentrum (GFZ)

A li i 3 SApplication 3 - Sensorscope

• What is measured:– temperature– humidity– precipitation– wind speed/direction– solar radiation– soil moisture

Pictures: courtesy of SwissExperiment

A li i 3 SApplication 3 - Sensorscope

• Why:Capture meteorological p gevents with high spatial density.

Pictures: courtesy of SwissExperiment

A li i 3 SApplication 3 - Sensorscope

– spatial distribution?– temporal distribution?– in-situ measurements?– take place in hard to

l ?access places?– generate data which

need to be available inneed to be available in real-time?

Pictures: courtesy of SwissExperiment

MicaZ – a WSN node

MICA f ilMICA mote family

• designed in EECS at UCBerkeley• manufactured/marketed by Crossbowy

– several thousand produced– used by several hundred research groups– about CHF 250/piece

• variety of available sensorsy

MICAMICAz

• Atmel ATmega128L– 8 bit microprocessor, ~8MHz– 128kB program memory, 4kB SRAM– 512kB external flash (data logger)

• Chipcon CC2420• Chipcon CC2420– 802.15.4 (Zigbee)

• 2 AA batteries• 2 AA batteries– about 5 days active (15-20 mA)– about 20 years sleeping (15-20 µA)y p g ( µ )

• TinyOS

S b dSensor board

• MTS 300 CA– Light (Clairex CL94L)– Temp (Panasonic ERT-J1VR103J)– Acoustic (WM-62A Microphone)– Sounder (4 kHz Resonator)

802 15 4 / Zi b802.15.4 / Zigbee

• Emerging standard for low-power wireless monitoring and control

2 4 GH ISM b d (84 h l ) 250 kb d t t– 2.4 GHz ISM band (84 channels), 250 kbps data rate• Chipcon/Ember CC2420: Single-chip transceiver

1 8V supply– 1.8V supply• 19.7 mA receiving• 17.4 mA transmitting

i d i– Easy to integrate: Open source drivers– O-QPSK modulation; “plays nice”

with 802.11 and Bluetooth

Comparison to other standardsp

Operating systemAn operating system (OS) is an interface between hardware and user applications.It is responsible for the management andIt is responsible for the management and coordination of tasks and the sharing of the limited resources of the computer system.A typical OS can be decomposed into the following entities:

Scheduler, which is responsible for the sharing of the processing unit (microprocessor or microcontroller)D i d i hi h l l l th tDevice drivers, which are low-level programs that manage the various devices (sensors, actuators, secondary memory storage devices, etc.). Memory management unit, which is responsible for the y g psharing of the memory (virtual memory).Optional: Graphical User Interface, File System, Security, etc.

Most “OS” for embedded systems include these two

entities only!

Ti OS d i iTinyOS: description

• Minimal OS designed for Sensor Networks• Event driven execution• Programming language: nesC (C-like syntax

but supports TinyOS concurrency model)pp y y )• Widespread usage on motes

– MICA (ATmega128L)( g )– TELOS (TI MSP430)

• Provided simulator: TosSim

Conclusion

SSummary

• Basics in wired/wireless communication– Terms: Bandwidth, TDMA, FDMA, CDMA ..– Constraints for wireless communication

• Wireless sensor networks– Potential– Typical applications– MicaZ

Addi i l Li W k 9Additional Literature – Week 9• Permasense http://www.permasense.ch• GITWES – the German Indonesian Tsunami Early Warning System

http://www.gitews.de ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/sustainable-growth/workshops/workshop-ftp://ftp.cordis.europa.eu/pub/fp7/ict/docs/sustainable growth/workshops/workshop20070531-jwachter_en.pdf

• Sensorscope http://www.sensorscope.ch/