wireless and cellular communications-unit five

8/6/2019 Wireless and Cellular Communications-unit Five

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UNIT V

SYSTEM EXAMPLES AND DESIGN ISSSUES


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5.1 Introduction To Multiple Access

Techniques Multiple access schemes are used to allow

many mobile users to share simultaneously a

finite amount of radio spectrum

The sharing of spectrum is required to achievehigh capacity by simultaneously allocating the

available bandwidth (or the available amount

of channels) to multiple users

It is often desirable to allow the subscriber to

send simultaneously information to the base

station while receiving information from the

base station(For example to listen and talk)


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This effect ( to send and receive information

simultaneously) is called duplexing Duplexing may be done using frequency or

time domain techniques

F

requency Division Duplexing (FDD) providestwo distinct bands of frequencies for every

user

The forward band provides traffic from the

base station to the mobile, and the reverse

band provides traffic from the mobile to the

base station


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In FDD, any duplex channel actually consists of

two simplex channels, and a device called a

duplexer is used inside each subscriber unit

and base station to allow simultaneous radio

transmission and reception on the duplex

channel pair Time Division Duplexing (TDD) uses time

instead of frequency to provide both a

forward and reverse link If the time split between the forward and

reverse time slot is small, then the

transmission and reception of data appears

simultaneous to the user


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TDD allows communication on a single

channel (as opposed to requiring two simplex

or dedicated channels) and simplifies the

subscriber equipment since a duplexer is not

required

The three major access techniques that areused to share the available bandwidth in a

wireless communication system are Frequency

Division Multiple Access (FDMA), TimeDivision Multiple Access (TDMA), and Code

Division Multiple Access (CDMA)

TDMA/FDMA are Orthogonal Systems and

CDMA is Non-orthogonal System


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These techniques can be grouped as

narrowband and wideband systems,depending upon how the available bandwidth

is allocated to the users

Narrowband Systems: The term narrowband

is used to relate the bandwidth of a single

channel to the expected coherence bandwidth

of the channel

The available radio spectrum is divided into alarge number of narrowband channel


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To minimize interference between forward

and reverse links on each channel, thefrequency split (FDD) is made as great as

possible within the frequency spectrum, while

still allowing inexpensive duplexers and a

common transceiver antenna to be used in

each subscriber unit

In narrowband FDMA, a user is assigned a

particular channel which is not shared byother users in the vicinity, and if FDD is used

then the system is called FDMA/FDD


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Narrowband TDMA, on the other hand, allows

users to share the same channel but allocatesa unique time slot to each user in a cyclical

fashion on the channel, thus separating a

small number of users in time on a single

channel

For narrowband TDMA, if a large number of

channels allocated using TDD, Such systems

are called TDMA/TDD access systems Wideband systems: here the transmission

bandwidth of a single channel is much larger

than the coherence bandwidth of the channel


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Multipath fading does not greatly affect the

received signal within a wideband channel,

and frequency selective fades occur in only a

small fraction of the signal bandwidth

The users are allowed to transmit in a large

part of the spectrum TDMA allocates time slots to the many

transmitters on the same channel and allows

only one transmitter to access the channel atany instant of time

Spread spectrum CDMA allows all of the

transmitters to access the channel at the same


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5.2Frequency Division Multiple Access

Frequency division multiple access (FDMA)

assigns individual channels to individual user

During the period of the call, no other user

can share the same frequency band

In FDD systems, the users are assigned a

channel as a pair of frequencies; one

frequency is used for the forward channel,while the other frequency is used for the

reverse channel


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FDMA


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The features ofFDMA

o

The FDMA channel carries only one phonecircuit at a time

o If an FDMA channel is not in use, then it sits

idle and cannot be used by other users to

increase or share capacity

o After the assignment of a voice channel, the

base station and the mobile transmit

simultaneously and continuously

o The bandwidths of FDMA channels are

relatively narrow (30 kHz) [usually

implemented in narrowband systems]


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o The FDMA mobile unit uses duplexers since

both the transmitter and receiver operate atthe same time( increases the cost of

subscriber units and base stations)

o

FDMA requires tight RF filtering to minimizeadjacent channel interference

As many channels share the same antenna at

the base station and the power amplifiers or

the power combiners are non-linear, The

nonlinearities cause signal spreading in the

frequency domain and generate

intermodulation (IM) frequencies


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IM is undesired RF radiation which can

interfere with other channels Spreading of the spectrum results in adjacent

channel interference

Intermodulation is the generation of

undesirable harmonics that is generated

outside the mobile radio band cause

interference to adjacent service

Intermodulation distortion products occur at

frequencies mf1 + nf2 for all integer values of

m and n if a base station transmits two carrier

frequencies where


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The Advanced Mobile Phone System (AMPS),

is based on FDMA/FDD A single user occupies a single channel while

the call is in progress, and the single channel is

actually two simplex channels which are

frequency duplexed with a 45 MHz split

In AMPS, analog narrowband frequency

modulation (NBFM) is used to modulate the

carrier


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The number of channels that can be

simultaneously supported in a FDMA system isgiven by

where Bt is total spectrum allocation , Bguard isthe guard band allocated at the edge of the

allocated spectrum, and Bc is the channel

bandwidth

If Bt is 12.5 MHz , Bguard is 10 kHz, Bc 30 kHz,

the number of channels available in an FDMA

system is 416


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5.3 Time Division Multiple Access

Time division multiple access (TDMA) systemsdivide the radio spectrum into time slots, and

in each slot only one user is allowed to either

transmit or receive TDMA systems transmit data in a buffer-and-

burst method, thus the transmission for any

user is non-continuous, digital data and digital

modulation must be used

A channel may be thought of as particular

time slot that reoccurs every frame, where N

time slots comprise a frame


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Each frame is made up of a preamble, an

information message, and trail bits

In a TDMA frame, the preamble contains theaddress and synchronization information that

both the base station and the subscribers use

to identify each other

Guard times are utilized to allow

synchronization of the receivers between

different slots and frames

In TDMA/TDD, half of the time slots in the

frame information message would be used for

the forward link channels and half would be

used for reverse link channels


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In TDMA/FDD systems, an identical or similar

frame structure would be used solely for

either forward or reverse transmission, butthe carrier frequencies would be different for

the forward and reverse links

TDMA


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Features ofTDMA

o TDMA shares a single carrier frequency withseveral users, where each user makes use of

non-overlapping time slots

o Data transmission for users of a TDMA systemis not continuous, but occurs in bursts

(transmitter can be turned off when not in

use) [low battery consumption]

o The handoff process is much simpler for a

subscriber unit(Because of discontinuous

transmissions)


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o Bandwidth can be supplied on demand to

different users by concatenating or reassigningtime slots based on priority

TDMA frame structure


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Efficiency ofTDMA: The efficiency of a TDMA

system is a measure of the percentage of

transmitted data that contains information as

opposed to providing overhead for the access

scheme

The frame efficiency, f is the percentage of

bits per frame which contain transmitted data

The transmitted data may include source and

channel coding bits, so the raw end-userefficiency of a system is generally less than f


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The number of overhead bits per frame is

where, Nr is the number of reference bursts

per frame, Nt is the number of traffic bursts

per frame, br is the number of overhead bitsper reference burst, bp is the number of

overhead bits per preamble in each slot, and

bg is the number of equivalent bits in each

guard time interval

The total number of bits per frame, bT

where Tf is the frame duration, and R is the

channel bit rate


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The frame efficiency f is thus given as

Number of channels in TDMA system: The

number of TDMA channel slots that can be

provided in a TDMA system is found bymultiplying the number of TDMA slots per

channel by the number of channels available

and is given by

where m is the maximum number of TDMA

users supported on each radio channel


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Example

Consider Global System for Mobile, which is aTDMA/FDD system that uses 25 MHz for the

forward link, which is broken into radio

channels of 200 kHz. If 8 speech channels are

supported on a single radio channel, and if no

guard band is assumed, find the number of

simultaneous users that can be

accommodated in GSM


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Example:

If GSM uses a frame structure where eachframe consists of 8 time slots, and each time

slot contains 156.25 bits, and data is

transmitted at 270.833 kbps in the channel,

find (a) the time duration of a bit, (b) the time

duration of a slot, (c) the time duration o f a

frame, and (d) how long must a user

occupying a single time slot must waitbetween two simultaneous transmissions


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Example

If a normal GSM time slot consists of 6 trailingbits, 8.25 guard bits, 26 training bits, and 2

traffic bursts of 58 bits of data, find the frame

efficiency( There are 8 time slots normaly)


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5.4 Spread Spectrum Multiple Access

Spread spectrum multiple access (SSMA) usessignals which have a transmission bandwidth

that is several orders of magnitude greater

than the minimum required RF bandwidth A pseudo-noise (PN) sequence converts a

narrowband signal to a wideband noise, like

signal before transmission

SSMA also provides immunity to multipath

interference and robust multiple access

capability


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Since many users can share the same spread

spectrum bandwidth without interfering with

one another, spread spectrum systems

become bandwidth efficient in a mutiple user

environment

There are two main types of spread spectrum

multiple access techniques; Frequency

Hopped Multiple Access (FHMS) and Direct

Sequence Multiple Access (DSMA) Direct Sequence is also called Code Division

Multiple Access (CDMA)


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Frequency hopped multiple access is a digitalmultiple access system in which the carrier

frequencies of the individual users are varied

in a pseudorandom fashion within a wideband

channel

The digital data is broken into uniform sized

bursts which are transmitted on different

carrier frequencies The pseudorandom change of the carrier

frequencies of the user randomizes the

occupancy of a specific channel at any given

5.4.1Frequency Hopped Multiple Access


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In the FH receiver, a locally generated PN code

is used to synchronize the receivers

instantaneous frequency with that of the

transmitter

If the rate of change of the carrier frequency is

greater than the symbol rate then the system

is referred to as a fast frequency hopping

system

If the channel changes at a rate less than orequal to the symbol rate, it is called slow

frequency hopping


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A frequency hopped system provides a level of

security, especially when a large number of

channels are used

The FH signal is immune to fading, since error

control coding and interleaving can be used to

protect the frequency hopped signal against

deep fades

Error control coding and interleaving can also

be combined to guard against Erasures whichcan occur when two or more users transmit

on the same channel at the same time


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In CDMA systems, the narrowband message

signal is multiplied by a very large bandwidthsignal called the spreading signal

The spreading signal is a pseudo-noise code

sequence that has a chip rate which is ordersof magnitudes greater than the data rate of

the message

All users use the same carrier frequency and

may transmit simultaneously

Each user has its own pseudorandom

codeword which is approximately orthogonal

to all other codewords

5.4.2Code Division Multiple Access


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CDMA


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The receiver performs a time correlation

operation to detect only the specific desired

codeword

All other codewords appear as noise due to

decorrelation

For detection of the message signal, the

receiver needs to know the codeword used by

the transmitter

Each user operates independently with outthe knowledge of the other users


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Features ofCDMA

o Many users of a CDMA system share the same

frequency(TDD or FDD )

o Increasing the number of users raises the

noise floor in a linear manner (no absolute

limit of number of users)

o Multipath fading may be substantially reduced

because the signal is spread over a large

spectrumo Channel data rates are very high, the symbol

(chip) duration is very short and usually much

less than the channel delay spread


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o A RAKE receiver can be used to improve

reception by collecting time delayed versions

of the required signal

o Since CDMA uses co-channel cells, it can use

macroscopic spatial diversity to provide soft

handoff which provided by MSC

o Self-jamming is a problem in CDMA system.

Self-jamming arises from the fact that the

spreading sequences of different users are notexactly orthogonal


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5.4.3 Hybrid Spread Spectrum Techniques

Hybrid Spread Spectrum Techniques are

hybrid combinations of multiple access

schemes

Hybrid FDMA/CDMA (FCDMA) are technique

can be used as an alternative to the DS-CDMA

techniques

The available wideband spectrum is divided

into a number of subspectras with smaller

bandwidths


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Each of these smaller sub channels becomes a

narrowband CDMA system having processinggain lower than the original CDMA system

This hybrid system has an advantage in that

the required bandwidth need not be

contiguous and different users can be allotted

different sub spectrum bandwidths depending

on their requirements

The capacity of this FDMA/CDMA technique iscalculated as the sum of the capacities of a

system operating in the subspectra


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Hybrid Direct Sequence/Frequency Hopped

Multiple Access (DS/FHMA):This technique

consists of a direct sequence modulated signalwhose center frequency is made to hop

periodically in a pseudorandom fashion

Direct sequence, frequency hopped systemshave an advantage in that they avoid the near-

far effect

But frequency hopped CDMA systems are notadaptable to the soft handoff process since it

is difficult to synchronize the frequency

hopped base station receiver to the multiple

hopped signals


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Time Division CDMA (TCDMA):In a TCDMA

(also called TDMA/CDMA) system, different

spreading codes are assigned to different cells Within each cell, only one user per cell is

allotted a particular time slot, thus at any

time, only one CDMA user is transmitting ineach cell

When a handoff takes place, the spreading

code of the user is changed to that of the new

cell

It has an advantage in that it avoids the near-

far effect since only one user transmits at a

time within a cell


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Time Division Frequency Hopping (TDFH):

This technique has an advantage in severe

multipath or when severe co-channelinterference occur

The subscriber can hop to a new frequency at

the start of a new TDMA frame, thus avoidinga severe fade or erasure event on a particular

channel

This technique has been adopted for the GSM

standard, where the hopping sequence is

predefined and the subscriber is allowed to

hop only on certain frequencies which are

assigned to a cell


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5.5 Space Division Multiple Access

Space Division Multiple Access (SDMA)controls the radiated energy for each user in

space

SDMA serves different users by using spot

beam antennas

These different areas covered by the antenna

beam may be served by the same frequency

(in a TDMA or CDMA system) or differentfrequencies (in an FDMA system)

Sectorized antennas may be thought of as a

primitive application of SDMA


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A spatially filtered base station antenna

serving different users by using spot beams

SDMA


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Space division multiple access uses direction

(angle) as another dimension in signal space,

which can be channelized and assigned todifferent users, this is generally done with

directional antennas

Orthogonal channels can be assigned only ifthe angular separation between users exceeds

the angular resolution of the directional

antenna

For mobile users, SDMA must adapt as user

angles change; then a user must be handed

off to a new sector when it moves out of its

original sector


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5.6 Wireless Networking

For several decades, people have been usingcomputer networks to interconnect personal

computers and servers in companies, colleges,

and cities

An evolution has been taking place, however,

toward using networks wirelessly. In fact,

today wireless interfaces are available to

utilize network services that allow us to use e-

mail and access applications, and browse the

Internet from just about anywhere


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These wireless applications are enabling

people to extend their workplace in a way that

results in significant benefits

Business travellers, for example, are able to

respond to e-mails while waiting for a flight at

an airport A homeowner can easily share a common

Internet connection among multiple PCs and

laptops without running cabling A wireless network enables people to

communicate and access applications and

information without wires


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This provides freedom of movement and the

ability to extend applications to different partsof a building, city, or nearly anywhere in the

world

For example, people at home researching on

the Internet can do so in a quiet area away

from noisy children or in front of the television

with the entire family nearby

Wireless networks allow people to interactwith e-mail or browse the Internet from a

location that they prefer


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Wireless networks have been around for many

years. In fact, early forms of wireless

communications include Native Americanswaving buffalo skins over a fire to send smoke

signals to others over great distances

Many types of wireless communicationsystems exist, but a distinguishing attribute of

a wireless network is that communication

takes place between computer devices(PDAs,

laptops, PCs, servers, and printers are

examples)

Having processors, memory, and a means of

interfacing with a particular type of network.


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Traditional cell phones don't fall within the

definition of a computer device; however,

newer phones and even audio headsets are

beginning to incorporate computing power

and network adapters. Eventually, most

electronics will offer wireless networkconnections

"Radio Frequency and Light Signal: The

Invisible Medium," wireless networks useeither radio waves or infrared light as a

medium for communication between users,

servers, and databases


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This type of communication is invisible to the

human eye. In addition, the actual medium

(air) is transparent to the user

Most manufacturers are now integrating the

wireless network interface card (NIC; also

referred to as an adapter) and antenna intocomputing devices and out of view from the

user, this makes wireless computing devices

mobile and easy to use


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Wireless networks fall into several categories,

depending on the size of the physical area that

they are capable of covering. The following

types of wireless networks satisfy diverse user

requirements:

Wireless Personal-Area Network (PAN)

Wireless Local-Area Network (LAN)

Wireless Metropolitan-Area Network (MAN)

Wireless Wide-Area Network (WAN)


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wireless and cellular communications-unit five

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