Chapter 16
Wireless WANs:
Cellular Telephone
and Satellite Networks
16.1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
16-1 CELLULAR TELEPHONY
Cellular
telephony
is
designed
to
provide
communications between two moving units, called
mobile stations (MSs), or between one mobile unit and
one stationary unit, often called a land unit.
Topics discussed in this section:
Frequency-Reuse Principle
Transmitting
Receiving
Roaming
First Generation
Second Generation
Third Generation
16.2
Wireless Communications

When?

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
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

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
Each with own antenna
Each with own range of frequencies
Served by base station

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Transmitter, receiver, control unit
Adjacent cells on different frequencies to
avoid crosstalk
Shape of Cells

Square

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Width d cell has four neighbors at distance d and four at
distance 2 d
Better if all adjacent antennas equidistant

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Simplifies choosing and switching to new antenna
Hexagon

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Provides equidistant antennas
Radius defined as radius of circum-circle

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Distance from center to vertex equals length of side
Distance between centers of cells radius R is
Not always precise hexagons

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Topographical limitations
Local signal propagation conditions
Location of antennas
3R
Cellular Geometries
Frequency Reuse

Power of base transceiver controlled

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
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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


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Hexagonal cell pattern, following values of N possible




Reuse factor
Each cell in pattern uses unique band of frequencies
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= 3N
D/d = N
Frequency
Reuse
Patterns
Figure 16.2 Frequency reuse patterns
16.11
N=7, 32 cells, R=1.6km, in total 336
channels
Operation of Cellular Systems

Base station (BS) at center of each cell

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Controller handles call process
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One MTSO serves multiple BS
MTSO to BS link by wire or wireless
MTSO:
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Number of mobile units may in use at a time
BS connected to mobile telecommunications switching
office (MTSO)
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Antenna, controller, transceivers
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
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2nd Generation

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based on analog voice using frequency
modulation
digital techniques and time-division (TDMA) or
code-division multiple access (CDMA)
3rd Generation

broadband access for personal
communications services (PCS)
Note
AMPS is an analog cellular phone
system using FDMA.
16.25
Figure 16.5 Second-generation cellular phone systems
16.26
Advanced Mobile Phone Service



1st Generation
most common
mobile phone
service since
early 80’s
developed by
AT&T
AMPS Spectral Allocation

Two 25-MHz bands



Each split in two to allow competition


base to mobile (869-894 MHz)
mobile to base (824-849 MHz)
each operator allocated 12.5 MHz bands
416 channels per operator

395 for calls, 21 for control data
Figure 16.3 Cellular bands for AMPS
16.29
Figure 16.4 AMPS reverse communication band
16.30
AMPS Spatial Allocation
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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
Note
D-AMPS, or IS-136, is a digital cellular
phone system using TDMA and FDMA.
16.32
Figure 16.6 D-AMPS
16.33
AMPS Components

Mobile Units



Base Transceiver


contains a modem that can switch between
many frequencies
3 identification numbers: electronic serial
number, system ID number, mobile ID
number
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

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

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

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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.
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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

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Developed to provide common 2ndgeneration 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

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Subscriber Identity Module
Smart card or plug-in module to activate unit
stores

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
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

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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
Figure 16.7 GSM bands
16.45
Figure 16.8 GSM
16.46
Figure 16.9 Multiframe components
16.47
Note
GSM is a digital cellular phone system
using TDMA and FDMA.
16.48
Figure 16.10 IS-95 forward transmission
16.49
Figure 16.11 IS-95 reverse transmission
16.50
Note
IS-95 is a digital cellular phone system
using CDMA/DSSS and FDMA.
16.51
Choice of Access Methods

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FDM, used in 1st generation systems, wastes spectrum
Debate over TDMA vs CDMA for 2nd generation

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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 thirdgeneration systems
3rd Generation Wireless

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Provide high speed wireless for voice, data,
video and multimedia
ITU’s view

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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)


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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


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
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

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programming model
Wireless Markup Language (adhering to XML)
Microbrowser
Lightweight protocol stack
Framework for wireless telephony applications
Note
The main goal of third-generation
cellular telephony is to provide
universal personal communication.
16.57
Figure 16.12 IMT-2000 radio interfaces
16.58