Unit V Multi-User Radio Communication ADVANCED MOBILE …

Srividya college of engineering & Technology, Virudhunagar Course material ( Lecture Notes)

CS6304 - Analog & Digital Communication ,Unit 5 Page 1

Unit V

Multi-User Radio Communication

ADVANCED MOBILE PONE SERVICE (AMPS)

• 1906: 1st radio transmission of Human voice.

– What’s the medium?

– Used an RC circuit to modulate a carrier frequency that radiated up and down an

antenna.

– Receiver had a matched RC circuit with an antenna

• 1910: Lars Ericsson in Sweden invents the first car phone.

• However, Morse Code continues to be the primary method of sending information without

a wire.

• 1934: Federal Communications Commission is founded by Roosevelt’s New Deal.

Charged to allocate the radio spectrum with the public interest in mind.

• The FCC was corrupt until the mid 60’s, it propped up AM radio for years to keep out

newer FM stations.

• The FCC gave priority in terms of broadcast channels to emergency and government units.

• WW2: Many innovations, including RADAR.

• Also the first mobile FM Transmitter/Receiver.

– Weighed over 30 lbs.

– More like a Walkie Talkie.

– Developed by Motorola.

• 1946: In St. Louis, AT&T and Southwestern Bell introduced the first mobile telephone

service.

– There were 6 channels in the 150 MHz band with 60 KHz allocated to each

channel.

– A very powerful antennae sat atop a centrally located building. All calls were

routed through here.

– Not full duplex; it was like a walkie/talkie.

– Operators routed all calls

• 1954/1958: Silicon transistor and integrated circuits are invented at Texas Instruments.

– Walkie/Talkies were now the size of a large shoe.

• 1960: Bell employees informally outline a cellular plan and request 75 MHz of bandwidth

around the 800 MHz band.

– Everything that was needed to have mobile communications was invented at this

point except the microprocessor (1971 by Intel).

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• 1973: Martin Cooper from Motorola files for a patent on the first handheld mobile phone.

– He didn’t invent cellular phones, however,

– Bell had a working system on trains 4 years earlier, but it wasn’t handheld.

– They were both cellular with frequency reuse.

1974: The FCC releases all the requested bandwidth

• 1975: Bell receives permission to start a commercial cellular network in Chicago.

– They order 135 phones at a cost of over $500,000

• 1979: Lucent makes the first DSP on one chip.

• October 12, 1983: Bell rolls out the first full-scale cellular network in Chicago.

– Covers 2100 square miles with 12 cellular sites.

– Uses a system called AMPS

– Operated in the 800 MHz band that had been allocated by the FCC.

– Telephones were expensive suit case type phones

• 1983 are cell phone.

• AMPS: Advanced Mobile Phone System

• Known as First Generation Wireless

• Analog channels of 30 KHz.

• Uses Frequency Division Multiple Access

• Uses frequency reuse – people in other cells can use your frequency without interference.

• Very susceptible to static.

• Very easy to ease drop.

• Introduced in 1983.

– Must be supported by every wireless carrier until February, 2008.

• OnStar still uses AMPS.

– In 2005, 15% of Alltel’s customers were still using AMPS.

– Replaced by TDMA and now CDMA (all digital) technologies.

• First generation of cell phones

• 3 basic devices

– mobile

– base transceiver

– mobile telephone switching office (MTSO)

• Voice and Control Channels

• Outgoing from mobile

– input phone number and press send





– mobile links to base xcvr via control channel

– base to MTSO to POTS

– MTSO routes connection back to mobile via voice channel

– mobile shifts from control to voice

• Incoming to mobile

– call goes from POTS to MTSO

– on control channel, MTSO searches for mobile by PAGING every active mobile

– if found, MTSO rings it and establishes voice channel connection

– uses xcvr with strongest signal from mobile

The only system available in the United States until about 1997

The first system used for cellular telephony- is analog

Uses the 800 MHz frequency band of the spectrum

Is still being used widely - The number of subscribers began to decrease in

1999 due to migration to digital

Utilizes FDMA (Frequency division multiple access) to separate users

In FDMA, users are separated in frequency. i.e. mobile phones communicate

at different frequencies than the others within each cell. The radio spectrum is

shared among users

GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM)

• A version of time division multiple access (TDMA) technology, because it divides

frequency bands into channels and assigns signals time slots within each channel.

• Makes more efficient use of limited bandwidth than the IS-136 TDMA standard common

in the United States.

• Makes use of silences in a phone call to increase its signal compression, leaving more open

time slots in the channel.

• More than 800 million end users in 190 countries and representing over 70% of today's

digital wireless market.

• source: GSM Association

• Israel

• Orange uses GSM

Pelephone and Cellcom are about to use GSM

GSM Services

• GSM was designed to do 3 things:

– 1. Bearer data services: Faxes, text messages, web pages.

• Basic GSM had a basic data rate that is limited to 9.6 kbps





– Extended by GMRS and EDGE to around 384 Kbps

– 2. Tele Services:

• Voice traffic But, at a lower quality than analog.

– 3. Supplementary services

• Call forwarding, caller id, etc…

– Meaning, we need to connect to the SS7 network

• GSM functional architecture

System architecture

• As with all systems in the telecommunication area, GSM comes with a hierarchical,

complex system architecture comprising many entities, interfaces, and acronyms. A GSM

system consists of three subsystems, the radio sub system (RSS), the network and

switching subsystem (NSS), and the operation subsystem (OSS). Each subsystem will be

discussed in more detail in the following sections. Generally, a GSM customer only

notices a very small fraction of the whole network - the mobile stations (MS) and some

antenna masts of the base transceiver stations (BTS)

Radio subsystem

• Base station subsystem (BSS): A GSM network comprises many BSSs, each controlled

by a base station controller (BSC). The BSS performs all functions necessary to maintain

radio connections to an MS, coding/decoding of voice, and rate adaptation to/from the

wireless network part. Besides a BSC, the BSS contains several BTSs.

Base transceiver station (BTS): A BTS comprises all radio equipment, i.e., antennas,

signal processing, amplifiers necessary for radio transmission. A BTS can form a radio





cell or, using sectorized antennas, several cells, and is connected to MS via the Um

interface (ISDN U interface for mobile use), and to the BSC via the Abis interface. The

Um interface contains all the mechanisms necessary for wireless transmission (TDMA,

FDMA etc.) and will be discussed in more detail below. The Abis interface consists of 16

or 64 kbit/s connections. A GSM cell can measure between some 100 m and 35 km

depending on the environment (buildings, open space, mountains etc.) but also expected

traffic.

• Base station controller (BSC): The BSC basically manages the BTSs. It reserves radio

frequencies, handles the handover from one BTS to another within the BSS, and performs

paging of the MS. The BSC also multiplexes the radio channels onto the fixed network

connections at the A interface.

• Mobile station (MS): The MS comprises all user equipment and software needed for

communication with a GSM network. An MS consists of user independent hard- and

software and of the subscriber identity module (SIM), which stores all user-specific data

that is relevant to GSM.3 While an MS can be identified via the international mobile

equipment identity (IMEI), a user can personalize any MS using his or her SIM, i.e., user-

specific mechanisms like charging and authentication are based on the SIM, not on the

device itself. Device-specific mechanisms, e.g., theft protection, use the device specific

IMEI. Without the SIM, only emergency calls are possible. The SIM card contains many

identifiers and identity number (PIN), a PIN unblocking key (PUK), an authentication

key Ki, and the inter-national mobile subscriber identity tables, such as card-type, serial

number, a list of subscribed services, a personal (IMSI) The PIN is used to unlock the

MS. Using the wrong PIN three times will lock the SIM. In such cases, the PUK is

needed to unlock the SIM. The MS stores dynamic information while logged onto the

GSM system, such as, e.g., the cipher key Kc and he location information consisting of a

temporary mobile subscriber identity (TMSI) and the location area identification (LAI).

Typical MSs for GSM900 have a transmit power of up to 2 W, whereas for GSM 1800 1

W is enough due to the smaller cell size. Apart from the telephone interface, an MS can

also offer other types of interfaces to users with display, loudspeaker, microphone, and

programmable soft keys. Further interfaces comprise computer modems, IrDA, or

Bluetooth. Typical MSs, e.g., mobile phones, comprise many more vendor-specific

functions and components, such as cameras, fingerprint sensors, calendars, address

books, games, and Internet browsers. Personal digital assistants (PDA) with mobile

phone functions are also available. The reader should be aware that an MS could also be

integrated into a car or be used for location tracking of a container.

Network and switching subsystem

• The “heart” of the GSM system is formed by the network and switching subsystem

(NSS). The NSS connects the wireless network with standard public networks, performs

handovers between different BSSs, comprises functions for worldwide localization of

users and supports charging, accounting, and roaming of users between different





providers in different countries. The NSS consists of the following switches and

databases:

• Mobile services switching center (MSC): MSCs are high-performance digital ISDN

switches. They set up connections to other MSCs and to the BSCs via the A interface,

and form the fixed backbone network of a GSM system. Typically, an MSC manages

several BSCs in a geographical region. A gateway MSC (GMSC) has additional

connections to other fixed networks, such as PSTN and ISDN. Using additional

interworking functions (IWF), an MSC can also connect to public data networks (PDN)

such as X.25. An MSC handles all signaling needed for connection setup, connection

release and handover of connections to other MSCs. The standard signaling system No. 7

(SS7) is used for this purpose. SS7 covers all aspects of control signaling for digital

networks (reliable routing and delivery of control messages, establishing and monitoring

of calls). Features of SS7 are number portability, free phone/toll/collect/credit calls, call

forwarding, three-way calling etc. An MSC also performs all functions needed for

supplementary services such as call forwarding, multi-party calls, reverse charging etc

• Home location register (HLR): The HLR is the most important database in a GSM

system as it stores all user-relevant information. This comprises static information, such

as the mobile subscriber ISDN number (MSISDN), sub-scribed services (e.g., call

forwarding, roaming restrictions, GPRS), international mobile subscriber identity (IMSI).

Dynamic information is also needed, e.g., the current location area (LA) of the MS, the

mobile subscriber roaming number (MSRN), the current VLR and MSC. As soon as an

MS leaves its current LA, the information in the HLR is updated. This information is

necessary to localize a user in the worldwide GSM network. All these user-specific

information elements only exist once for each user in a single HLR, which also supports

charging and accounting. HLRs can manage data for several million customers and

contain highly specialized data bases which must fulfill certain real-time requirements to

answer requests within certain time-bounds.

• Visitor location register (VLR): The VLR associated to each MSC is a dynamic

database which stores all important information needed for the MS users currently in the

LA that is associated to the MSC (e.g., IMSI, MSISDN, HLR address). If a new MS

comes into an LA the VLR is responsible for, it copies all relevant information for this

user from the HLR. This hierarchy of VLR and HLR avoids frequent HLR updates and

long-distance signaling of user information. Some VLRs in existence, are capable of

managing up to one million customers.

Operation subsystem

• The third part of a GSM system, the operation subsystem (OSS), contains the necessary

functions for network operation and maintenance. The OSS possesses network entities of

its own and accesses other entities via SS7 signaling The following entities have been

defined:





• Operation and maintenance center (OMC): The OMC monitors and controls all other

network entities via the O interface (SS7 with X.25). Typical OMC management

functions are traffic monitoring, status reports of network entities, subscriber and security

management, or accounting and billing. OMCs use the concept of telecommunication

management network (TMN) as standardized by the ITU-T.

• Authentication centre (AuC): As the radio interface and mobile stations are particularly

vulnerable, a separate AuC has been defined to protect user identity and data

transmission. The AuC contains the algorithms for authentication as well as the keys for

encryption and generates the values needed for user authentication in the HLR. The AuC

may, in fact, be situated in a special protected part of the HLR.

• Equipment identity register (EIR): The EIR is a database for all IMEIs, i.e., it stores all

device identifications registered for this network. As MSs are mobile, they can be easily

stolen. With a valid SIM, anyone could use the stolen MS. The EIR has a blacklist of

stolen (or locked) devices. In theory an MS is useless as soon as the owner has reported a

theft. Unfortunately, the blacklists of different providers are not usually synchronized and

the illegal use of a device in another operator’s network is possible (the reader may

speculate as to why this is the case). The EIR also contains a list of valid IMEIs (white

list), and a list of malfunctioning devices (gray list).

Code Division Multiple Access (CDMA)

• CDMA

– Third generation system

– Separates users by assigning them digital codes within a broad range of the radio

frequency

– First technology to use soft-handoff

– Employs spread spectrum technique

– Advantages

• Improved capacity, coverage, voice quality, and immunity from

interference

• Each voice signal is digitized and assigned a unique code, and then small components of

the signal are issued over multiple frequencies using the spread spectrum technique.





Cells

§ In a city, there might be one MTSO

§ There will be many xcvrs, each in its own area or CELL

§ Cell is hexagonal, with dia. of 6 miles or less

Channels

§ Every mobile in a cell who is talking must have its own channel

§ Otherwise, there will be interference

§ More channels = more users

§ Rule of thumb - one channel can support 20 users

Cell Sectorization

• Cells can be divided into sectors to provide a smaller coverage area, and therefore, more

frequency reuse.

Cell Area

• Cellular areas aren’t really circular as the area depends on the terrain and the interference

that’s present.

Why Hexagons?





• Using hexagons, as opposed to circles or boxes, allows for a better visualization of the

coverage areas.

• Also, a system of hexagons helps offset cells from linear road boundaries (where cell

phones were envisioned to be used).

Making a Call

• When a mobile is idle, i.e., it is not experiencing the process of a call, then it searches all

the FCCs to determine the one with the highest signal strength. The mobile then monitors

this particular FCC. However, when the signal strength falls below a particular threshold

that is insufficient for a call to take place, the mobile again searches all the FCCs for the

one with the highest signal strength. For a particular country or continent, the control

channels will be the same. So all mobiles in that country or continent will search among

the same set of control channels. However, when mobile moves to a different country or

continent, then the control channels for that particular location will be different and hence

the mobile will not work. Each mobile has a mobile identification number (MIN). When

a user wants to make a call, he sends a call request to the MSC on the reverse control

channel. He also sends the MIN of the person to whom the call has to be made. The MSC

then sends this MIN to all the base stations. The base station transmits this MIN and all

the mobiles within the coverage area of that base station receive the MIN and match it

with their own. If the MIN matches with a particular MS, that mobile sends an

acknowledgment to the BS. The BS then informs the MSC that the mobile is within its

coverage area. The MSC then instructs the base station to access specific unused voice

channel pair. The base station then sends a message to the mobile to move to the

particular channels and it also sends a signal to the mobile for ringing. In order to

maintain the quality of the call, the MSC adjusts the transmitted power of the mobile

which is usually expressed in dB or dBm. When a mobile moves from the coverage area

of one base station to the coverage area of another base station i.e., from one cell to

another cell, then the signal strength of the initial base station may not be sufficient to

continue the call in progress. So the call has to be transferred to the other base station.

This is called handoff. In such cases, in order to maintain the call, the MSC transfers the





call to one of the unused voice channels of the new base station or it transfers the control

of the current voice channels to the new base station.

Spectral Allocation

§ In U.S., we use two 25mHz bands

l one band from mobile to xcvr

l one band from xcvr to mobile

§ Each channel uses 30 kHz

§ So - about 832 channels for a city

§ But - channels split between two companies

§ 21 for control, 395 for voice

§ So - can support about 7900 users

Frequency Reuse

§ Also called Spatial Allocation

§ Cells are small, so signals can be low power

§ Can use same freq. in a cell that is far away, but not one nearby

l at least 7 cells must intervene

§ So - each cell can use about 57 channels

§ So - each cell can support 1140 users

Hand-offs

§ Mobile phones move from cell to cell

§ As signal fades, mobile asks MTSO for a new base xcvr

§ MTSO polls all base xcvrs to see which has strongest signal with mobile

§ Mobile shifts to new channel coming from new base

Roaming

§ Mobile has moved to a different city or subscriber area

§ MTSO will check with your provider, via land-line

§ Then will allow you access

§ Keeps track of billing data

Multiple Access

– First generation system

– Based on FDMA (Frequency Division Multiple Access), where frequency band is

divided into a number of channels. Each channel carries only one voice

conversation at a time.

– AMPS operates on 800 MHz or 1800 MHz





– Advantages:

• Widest coverage

– Limitations:

• Inadequate to satisfy the increasing demand

• Poor security

• Not optimized for data

FDMA

TDMA

– Second generation system

– Enables users to access the whole channel bandwidth for a fraction of the time,

called slot, on a periodic basis

– Has applications in satellite communications

– Advantages

• Improved capacity





CDMA

– Third generation system

– Separates users by assigning them digital codes within a broad range of the radio

frequency

– First technology to use soft-handoff

– Employs spread spectrum technique

Advantages

Improved capacity, coverage, voice quality, and immunity from interference























































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Unit V Multi-User Radio Communication ADVANCED MOBILE …