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CDMA- Code Division

Multiple Access

Presented ByPresented By

Isha JoshiIsha JoshiECEECE--33rdrdyearyear

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Multiple Access Systems

Multiple Access Systems Wirelesstelecommunications has dramatically increased inpopularity, resulting in the need for technologiesthat allow multiple users to share the same

frequency. These are called "multiple accesssystems." The three types of multiple accesssystem are:

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

These multiple access systems have very differentapproaches to the bandwidth problem.

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Frequency Division Multiple

Access (FDMA)

Each FDMAsubscriber is assigneda specific frequencychannel. No one else

in the same cell or aneighboring cell canuse the frequencychannel while it isassigned to a user.

This reducesinterference, butseverely limits thenumber of users.

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FDMA Cocktail Party

Think of a noisycocktail party where itgets harder to hear aconversation. The

FDMA host solves theproblem by allowingonly two guests to useeach room at a time.The two guests have

continuous access totheir room, but notvery many people cancome to the party.

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Time Division Multiple Access

(TDMA)

TDMA users share acommon frequencychannel, but use thechannel for only a veryshort time. They are each

given a time slot and onlyallowed to transmit duringthat time slot. When allavailable time slots in agiven frequency are used,the next user must beassigned a time slot on

another frequency. Thesetime slices are so smallthat the human ear doesnot perceive the timeslicing.

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TDMA Cocktail Party

TDMA is like a cocktail

party where multiple

pairs of guests have

access to the room,but only one pair can

use the room at a

time. They must then

leave and allow

another couple toenter. Throughout the

evening all the guests

rotate using the room.

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Code Division Multiple Access

CDMA users share a

common frequency

channel. All users are

on the same

frequency at the same

time. However, each

pair of users is

assigned a special

code that reducesinterference while

increasing privacy.

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CDMA Cocktail Party

CDMA is like a cocktailparty where everyone is inthe same room talking atthe same time. However,each pair of guests uses a

different language tocommunicate. Usingdifferent languages makesit very easy for the gueststo tune in to what theperson speaking their

language is saying andtune out everyone else.

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Comparison of multiple access

systems

This table summarizes

some of the technical

aspects of the multiple

access technologies.

The technology used

determines the

channel's capacity.

TDMA triples the

capacity of FDMA, butCDMA capacity can

be up to seven times

that of TDMA.

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Features of CDMA

The following features are unique to

CDMA technology:

Universal frequency reuseFast and accurate power control

Rake receiver

Different types of handoff

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Frequency reuse

The frequencyspectrum is a limitedresource. Therefore,wireless telephony,like radio, must reusefrequencyassignments. Forexample, two radiostations might transmitat 91.3 FM. There is

no interference aslong as the radiostations are farenough apart.

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Cell interference

Cell A and B of aconventional, analogsystem are using thesame frequency. Thearea of overlap, areaC, has a frequencyconflict andinterference. This issimilar to what youexperience when you

are driving betweenthe broadcast zones oftwo radio stationstransmitting at thesame frequency.

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FDMA and TDMA frequency

reuse planning

A frequency (channel) can beused again within an FDMA orTDMA network, but cells usingthe same frequency must beseparated by an appropriatedistance. Adjacent cells must

be assigned a different set offrequencies. For example, acell using frequency A mustnot be adjacent to another cellusing frequency A.

As a result, each cell site inthe site is able to use only 1/7of the possible frequencies.

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CDMA frequency reuse planning

Each BTS in a CDMAnetwork can use allavailable frequencies.

Adjacent cells cantransmit at the samefrequency becauseusers are separatedby code channels, notfrequency channels.This feature of CDMA,

called "frequencyreuse of one,"eliminates the need forfrequency planning.

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Powercontrol

Power control is a CDMAfeature that enablesmobiles to adjust the powerat which they transmit. Thisensures that the base

station receives all signalsat the appropriate power.The CDMA networkindependently controls thepower at which eachmobile transmits.

Both forward and reverselinks use power controltechniques.

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Why powercontrol is needed

If all mobilestransmitted at thesame power level, thebase station wouldreceive unnecessarilystrong signals frommobiles nearby andextremely weaksignals from mobilesthat are far away. This

would reduce thecapacity of thesystem. This problemis called the near-farproblem.

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Rake Receiver

The rake receiver

is a CDMA feature

that turns what is a

problem in othertechnologies into

an advantage for

CDMA.

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The multi-path problem

Signals sent over the aircan take a direct path tothe receiver, or they canbounce off objects andthen travel to the receiver.

These different paths,called multi-paths, canresult in the receivergetting several versions ofthe same signal but atslightly different times.

Multi-paths can cause aloss of signal throughcancellation in othertechnologies.

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How the rake receiverworks

CDMA's rake receiver ismultiple receivers in one.The rake receiver identifiesthe three strongest multi-path signals and combines

them to produce one verystrong signal. The rakereceiver therefore usesmultipath to reduce thepower the transmitter mustsend.

Both the mobile and theBTS use rake receiver

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Handoff in CDMA

Handoff is theprocess oftransferring a call

from one cell toanother. This isnecessary tocontinue the call asthe phone travels.

CDMA is unique inhow it handleshandoff.

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Types of CDMA handoff

CDMA has three

primary types of

handoff:

hard

soft

idle

The type of handoffdepends on the

handoff situation.

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Soft handoff

A soft handoff establishes a connection with thenew BTS prior to breaking the connection with theold one. This is possible because CDMA cells usethe same frequency and because the mobile uses

a rake receiver. The CDMA mobile assists thenetwork in the handoff. The mobile detects a newpilot as it travels to the next coverage area. Thenew base station then establishes a connectionwith the mobile. This new communication link is

established while the mobile maintains the linkwith the old BTS.

Soft handoffs are also called "make-before-break."

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Variations of the soft handoff

There are two variations of soft handoffs involvinghandoffs between sectors within a BTS:

Softer

Soft-softer

The softer handoff occurs between two sectors ofthe same BTS. The BTS decodes and combinesthe voice signal from each sector and forwards thecombined voice frame to the BSC. The soft-softerhandoff is combination handoff involving multiplecells and multiple sectors within one of the cells

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CDMA hard handoff

A hard handoff requires the mobile to break theconnection with the old BTS prior to making theconnection with the new one. CDMA phones use ahard handoff when moving from a CDMA system

to an analog system because soft handoffs are notpossible in analog systems. A Pilot Beacon Unit(PBU) at the analog cell site alerts the phone thatit is reaching the edge of CDMA coverage. Thephone switches from digital to analog mode as

during the hard handoff.Hard handoffs are also called "break-before-make."

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When does CDMA use a hard

handoff?

The CDMA hard handoff may be usedwhen moving from a CDMA network to ananalog one. It may also be used when

moving to a different:RF channel

MTSO

Carrier

MarketAnalog to CDMA handoff is not availabledue to the limitations of analog technology.

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CDMA idle handoff

An idle handoff occurs when thephone is in idle mode. The mobile willdetect a pilot signal that is stronger

than the current pilot. The mobile isalways searching for the pilots fromany neighboring BTS. When it finds astronger signal, the mobile simply

begins attending to the new pilot. Anidle handoff occurs without anyassistance from the BTS.

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TDMA and FDMA handoff

TDMA and FDMA systems use a hard

handoff when the mobile is moving

from one cell site to another. These

technologies do not allow for any type

of make-before-break handoff. A hard

handoff can increase the likelihood of

a dropped call.

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Advantages of CDMA

CDMA technology has numerousadvantages including:

Coverage

Capacity

Clarity

Cost

Compatibility

Customer satisfaction

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Capacity

CDMA capacity is ten totwenty times that of analogsystems, and it's up to fourtimes that of TDMA.Reasons for this include:

CDMA's universalfrequency reuse

CDMA users are separatedby codes, not frequencies

Power control minimizesinterference, resulting inmaximized capacity.

CDMA's soft handoff alsohelps increase capacity.This is because a softhandoff requires lesspower.

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Clarity

Often CDMA systems canachieve "wireline" clarity becauseof CDMA's strong digitalprocessing. Specifically:

The rake receiver reduces errors

The variable rate vocoder reducesthe amount of data transmitted

per person, reducing interference.The soft handoff also reducespower requirements andinterference.

Power control reduces errors bykeeping power at an optimal level.

CDMA's wide band signal reducesfading.

Encoding and interleaving reduceerrors that result from fading.

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Cost

CDMA's better coverageand capacity result in costbenefits:

Increased coverage perBTS means fewer areneeded to cover a givenarea. This reducesinfrastructure costs for theproviders.

Increased capacityincreases the serviceprovider's revenue

potential.CDMA costs per subscriberhas steadily declined since1995 for both cellular andPCS applications.

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Compatibility

CDMA phones are usually dual mode.

This means they can work in both

CDMA systems and analog cellular

systems. Some CDMA phones are

dual band as well as dual mode. They

can work in CDMA mode in the PCS

band, CDMA mode in the cellularband, or analog mode in an analog

cellular network.

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Customersatisfaction

CDMA results ingreater customersatisfaction becauseCDMA provides better:

Voice qualityLonger battery life dueto reduced powerrequirements

No cross-talk becauseof CDMA's uniquecodingPrivacy--again,because of coding.

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