Introduction to Telecommunications by Gokhale CHAPTER 5 WIRELESS COMMUNICATIONS.

Introduction to Telecommunications by Gokhale

CHAPTER 5

WIRELESS COMMUNICATIONS

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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

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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

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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

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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

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Hierarchical Cell Structure

• Key features of PCS– Variable cell size– Hierarchical cell

structure (picocell, microcell, macrocell, supermacrocell)

Cells

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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

• Not optimized for data

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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.

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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

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 two-way 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 packet-based 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.