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CHAPTER 5
WIRELESS
COMMUNICATIONS
Introduction to Telecommunications
by Gokhale
Introduction
• Wireless
– Communications system in which
electromagnetic waves carry a signal through
atmospheric space rather than along a wire
– Most systems use radio frequency (RF, which
ranges from 3 kHz to 300 GHz) or infrared (IR,
which ranges from 3 THz to 430 THz) waves
– IR products do not require any form of licensing
by the FCC
2
Timeline of Major Developments
• Mobile Telephone System (MTS)
– Introduced in 1946
– Simplex (one-way transmission) and manual operation
• Improved Mobile Telephone System (IMTS)
– Introduced in 1969 using a 450 MHz band
• Advanced Mobile Phone Service (AMPS)
– Introduced in 1983
– First system to employ a “cellular” concept
3
Cellular Topology
• Cellular network:
– Series of overlapping hexagonal cells in a honeycomb
pattern
• Cellular network components
– Base Station:Transmitter, Receiver, Controller, Antenna
– Cell: Base station’s span of coverage
– Mobile Switching Center: Contains all of the control and
switching elements to connect the caller to the receiver,
even as the receiver moves from one cell to another
4
Personal Communications Systems
(PCS)
• PCS is also called Personal Communications
Networks (PCN)
• Goal of PCS is to provide integrated voice, data
and video communications
• Three categories of PCS:
– Broadband: cellular and cordless handsets
– Narrowband: enhanced paging functions
– Unlicensed: allows short distance operation
5
Hierarchical Cell Structure
• Key features of PCS
– Variable cell size
– Hierarchical cell
structure (picocell,
microcell, macrocell,
supermacrocell)
6
Cells
Analog Access
• Analog Cellular Systems
– 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
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• Not optimized for data
FDMA
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Digital Access
• D-AMPS (Digital-AMPS)
• TDMA (Time Division Multiple Access)
• CDMA (Code Division Multiple Access)
Digital wireless technologies provide
greater system capacity.
10
TDMA
• 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
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TDMA
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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
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An Overview of Cellular Technologies
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Spread Spectrum Technique: FHSS
• Frequency Hopping Spread Spectrum (FHSS)
– Resists interference by jumping rapidly from
frequency to frequency in a pseudo-random way
– Advantage
• Increases the total amount of available bandwidth
through the assignment of multiple hopping sequences
within the same physical area
• More flexible than DSSS
– Application
• In large facilities especially with multiple floors
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Spread Spectrum Technique: DSSS
• Direct Sequence Spread Spectrum (DSSS)
– Resists interference by mixing in a series of
pseudo-random bits with the actual data
– Advantage
• If bits are damaged in transmission, the original data can
be recovered as opposed to having to be retransmitted
– Application
• Is substituted for point-to-point or multi-point
connectivity to bridge LAN segments
– Limitation
• Roaming capabilities are less robust
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Spread Spectrum Technique: CDPD
• Cellular Digital Packet Data
– Allows for a packet of information to be
transmitted in between voice telephone calls
– Enables data specific technology to be tacked
onto existing cellular telephone infrastructure
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Wireless Applications
• Cellular Phone
– High mobility and narrow bandwidth (20 to 30 kHz)
• Cordless Phone
– Low mobility and narrow bandwidth (20 to 30 kHz)
• Wireless LAN
– Low mobility and high bandwidth (typically 10 Mbps)
– Wireless Application Protocol (WAP) is a standard for
wireless data delivery, loading web pages, and navigation
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The Wireless Spectrum
Narrowband, Broadband, and Spread
Spectrum Signals
• Narrowband - a transmitter concentrates the signal
energy at a single frequency or in a very small range of
frequencies.
• Broadband - a type of signaling that uses a relatively
wide band of the wireless spectrum.
• Spread spectrum - the use of multiple frequencies to
transmit a signal.
Cellular Communications
• Mobile telephone service - a system for providing
telephone services to multiple, mobile receivers using twoway radio communication over a limited number of
frequencies.
• Mobile wireless evolution:
– First generation
– Second generation
– Third generation
Cellular Call Completion
• Components of a signal:
– Mobile Identification Number (MIN) - an enclosed
representation of the mobile telephone’s 10-digit
telephone number.
– Electronic Serial Number (ESN) - a fixed number
assigned to the telephone by the manufacturer.
– System Identification Number (SID) - a number
assigned to the particular wireless carrier to which the
telephone’s user has subscribed.
Cellular Call Completion
Call Completion
Advanced Mobile Pone Service
(AMPS)
• A first generation
cellular technology
that encodes and
transmits speech as
analog signals.
Time Division Multiple Access
(TDMA)
Code Division Multiple Access
(CDMA)
•
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.
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.
Emerging Third Generation (3G)
Technologies
The promise of these technologies is that a user can
access all her telecommunication services from one
mobile phone.
• CDMA2000 - a packet switched version of CDMA.
• Wideband CDMA (W-CDMA) - based on
technology developed by Ericson, is also packetbased and its maximum throughput is also 2.4 Mbps.
Wireless Local Loop (WLL)
• A generic term that describes a wireless link used in the
PSTN to connect LEC central offices with subscribers.
• Acts the same as a copper local loop.
• Used to transmit both voice and data signals.
Local Multipoint Distribution Service
(LMDS)
• A point-to-multipoint, fixed wireless technology that was
conceived to supply wireless local loop service in densely
populated urban areas and later on a trial basis to issue
television signals.
• A disadvantage is that its use of very high frequencies
limits its signal’s transmission distance to no more than
4km between antennas.
Multipoint Multichannel Distribution
System (MMDS)
• Uses microwaves with frequencies in the 2.1 to 2.7 GHz
range of the wireless spectrum.
• One advantage is that because of its lower frequency
range, MMDS is less susceptible to interference.
• MMDS does not require a line-of-sight path between the
transmitter and receiver.

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