16.1

Chapter 16

Wireless WANs: Cellular Telephone

and Satellite Networks

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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16-1 CELLULAR TELEPHONY16-1 CELLULAR TELEPHONY

Cellular telephonyCellular telephony is designed to provide is designed to provide communications between two moving units, called communications between two moving units, called mobile stations (MSs), or between one mobile unit and mobile stations (MSs), or between one mobile unit and one stationary unit, often called a land unit. one stationary unit, often called a land unit.

Frequency-Reuse PrincipleTransmittingReceivingRoamingFirst GenerationSecond GenerationThird Generation

Topics discussed in this section:Topics discussed in this section:

Wireless Communications

When? Mobile communications is needed Terrain makes wired communication

difficult Communications must be set up quickly Communications must be installed at low

cost Same information broadcast to many

locations

Wireless Disadvantages

More susceptible to interference, noise, signal loss, and eavesdropping

Generally lower data rate than wired Frequencies interfere in close

proximity Less connection stability

Cellular Network Organization Multiple low power transmitters

100w or less Area divided into cells

Each with own antenna Each with own range of frequencies Served by base station

Transmitter, receiver, control unit Adjacent cells on different frequencies

to avoid crosstalk

Shape of Cells Square

Width d cell has four neighbors at distance d and four at distance d

Better if all adjacent antennas equidistant Simplifies choosing and switching to new antenna

Hexagon Provides equidistant antennas Radius defined as radius of circum-circle

Distance from center to vertex equals length of side Distance between centers of cells radius R is R Not always precise hexagons

Topographical limitations Local signal propagation conditions Location of antennas

2

3

Cellular Geometries

Frequency Reuse

Power of base transceiver controlled Allow communications within cell on given frequency Limit escaping power to adjacent cells Allow re-use of frequencies in nearby cells Use same frequency for multiple conversations 10 – 50 frequencies per cell

E.g. The pattern consists of N cells K total number of frequencies used in systems Each cell has K/N frequencies Advanced Mobile Phone Service (AMPS) K=395, N=7

giving 57 frequencies per cell on average

Characterizing Frequency Reuse D = minimum distance between centers of cells that use

the same band of frequencies (called cochannels) R = radius of a cell d = distance between centers of adjacent cells (d = R) N = number of cells in repetitious pattern

Reuse factor Each cell in pattern uses unique band of frequencies

Hexagonal cell pattern, following values of N possible  N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, …

 Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, … D/R= D/d =

N3N

FrequencyReusePatterns

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Figure 16.2 Frequency reuse patterns

N=7, 32 cells, R=1.6km, in total 336 channels

Operation of Cellular Systems Base station (BS) at center of each cell

Antenna, controller, transceivers Controller handles call process

Number of mobile units may in use at a time BS connected to mobile telecommunications switching

office (MTSO) One MTSO serves multiple BS MTSO to BS link by wire or wireless

MTSO: Connects calls between mobile units and from mobile to fixed

telecommunications network Assigns voice channel Performs handoffs Monitors calls (billing)

Fully automated

Overview of Cellular System

Call Stages

Three Generations 1st Generation

based on analog voice using frequency modulation

2nd Generation digital techniques and time-division (TDMA)

or code-division multiple access (CDMA) 3rd Generation

broadband access for personal communications services (PCS)

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AMPS is an analog cellular phone system using FDMA.

Note

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Figure 16.5 Second-generation cellular phone systems

Advanced Mobile Phone Service

1st Generation most common

mobile phone service since early 80’s

developed byAT&T

AMPS Spectral Allocation

Two 25-MHz bands base to mobile (869-894 MHz) mobile to base (824-849 MHz)

Each split in two to allow competition each operator allocated 12.5 MHz bands

416 channels per operator 395 for calls, 21 for control data

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Figure 16.3 Cellular bands for AMPS

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Figure 16.4 AMPS reverse communication band

AMPS Spatial Allocation Limited channels dictate frequency reuse in

nearby cells Generally 10 to 50 frequencies assigned to cell Pattern of 7 cells smallest allowing sufficient

isolation 57 frequencies per cell 6.5 to 13 km per cell May be split with lower power

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D-AMPS, or IS-136, is a digital cellular phone system using TDMA and FDMA.

Note

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Figure 16.6 D-AMPS

AMPS Components

Mobile Units contains a modem that can switch

between many frequencies 3 identification numbers: electronic serial

number, system ID number, mobile ID number

Base Transceiver full-duplex communication with the mobile

Mobile Switching Center

AMPS Mobile Units

Modem that can switch between frequencies

Power output of unit controlled to match size of cell

Three identification numbers electronic serial number - 32 bits system operator identification number - 15 bits mobile identification number - 34 bits - phone

#

AMPS Logon When mobile becomes operational, it

senses control channels to determine channel and base station received best

Exchanges information via base station Announces its system id # to identify its

home carrier Home carrier contacted for

authorization and to locate mobile for incoming calls

AMPS Handoffs

Roaming operator must move between cells Different cells have different frequencies

and power levels Choice of handoff depends on

received power from base stations and controlled by mobile switching center

Global System for Mobile Comm.

2nd Generation First appeared in 1991 in Europe Similar to working of AMPS Designed to support phone, data,

and image Rates up to 9.6 kbps GSM transmission is encrypted using

secret keys

Global System for Mobile Communication Developed to provide common 2nd-

generation technology for Europe 200 million customers worldwide,

almost 5 million in the North America GSM transmission is encrypted Spectral allocation: 25 MHz for base

transmission (935–960 MHz), 25 MHz for mobile transmission (890–915 MHz)

GSM SIM

Subscriber Identity Module Smart card or plug-in module to activate

unit stores

subscriber’s identification number networks subscriber is authorized to use encryption keys

Can use any unit anywhere with your SIM

Multiple Access

Four ways to divide the spectrum among active users frequency-division multiplexing (FDM) time-division multiplexing (TDM) code-division multiplexing (CDM) space-division multiplexing (SDM)

GSM Access Methods

FDM too wasteful TDMA - time-division multiple access

early lead - more successful experience CDMA - code-division multiple access

theoretical advantages increased range choice for 3rd generation

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Figure 16.7 GSM bands

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Figure 16.8 GSM

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Figure 16.9 Multiframe components

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GSM is a digital cellular phone system using TDMA and FDMA.

Note

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Figure 16.10 IS-95 forward transmission

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Figure 16.11 IS-95 reverse transmission

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IS-95 is a digital cellular phone system using CDMA/DSSS and FDMA.

Note

Choice of Access Methods

FDM, used in 1st generation systems, wastes spectrum

Debate over TDMA vs CDMA for 2nd generation TDMA advocates argue there is more successful

experience with TDMA. CDMA proponents argue that CDMA offers additional

features as well, such as increased range. TDMA systems have achieved an early lead in actual

implementations CDMA seems to be the access method of choice for

third-generation systems

3rd Generation Wireless

Provide high speed wireless for voice, data, video and multimedia

ITU’s view voice quality of wired 144 kbps high-speed roaming / 384 kbps low-speed adaptive interface to internet for asymmetric speed more efficient use of spectrum support wide variety of equipment, services, etc

PCS & PCN

Personal Communications Services (PCS) find person easily use communication system anywhere with

single account Personal Communications Network

(PCN) use terminal in wide variety of

environments to connect to information services

WAP

Wireless Application Protocol universal, open standard - WAP forum provide mobile users access to information

services, including internet and web Works with wireless network technologies Based on existing internet standards such

as TCP, IP, HTTP, HTML, XML Support limited resources in and variety of

mobile devices

WAP Specs

Include programming model Wireless Markup Language (adhering to

XML) Microbrowser Lightweight protocol stack Framework for wireless telephony

applications

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The main goal of third-generation cellular telephony is to provide

universal personal communication.

Note

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Figure 16.12 IMT-2000 radio interfaces