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Multiple Access Techniques

MULTIPLE ACCESS TECHNIQUES

FOR WIRELESS

COMMUNICATION

A key

issue therefore regards

ways in

which this communication channel ispartitioned among the various users suchthat each receiver is able to accuratelyrecover the signal the

sender

intended to

transmit.

There is a need for techniques that

allow the multiple users to share the samefrequency especially as wirelesstelecommunication continues to increasein popularity.

Currently three types of schemes are used:•

Frequency division Multiple Access(FDMA)

Time division Multiple Access ( TDMA)

Code division Multiple Access ( CDMA)

Time division Multiple Access(TDMA)

Time Division Multiple Access is a channel accessmethod for shared medium

networks. It allows

several users to share the

same

frequency

channel by

dividing the signals into different

time slots. The users transmit in rapidsuccession, one after the others,

each

using his

own time slots. this allows multiple station toshare the same transmission medium whileusing only the part of its bandwidth theyrequire.

In other words in TDMA system

,each user uses the

whole channel

bandwidth for a fraction of time.

TDMA is a type of time division

multiplexing, with the special point thatinstead of having one transmitterconnected to one receiver, there aremultiple transmitters.

TDMA Features•

Digital data and modulation must be used.

Shares signal carrier frequency withmultiple user but only 1 user allowed toeither transmit or receiver in a slot.

Non-continuous transmission makes handoffsimpler.

Slots assigned cyclically can be assigned ondemand in dynamic TDMA.

Less stringent power control than CDMA due toreduced intra cell interference.

Higher synchronization overhead than CDMA.

Cell breathing is more complicated.

complex frequency/slot allocation.

Pulsating power envelop; Interference withother devices

The operation of the TDMA requires an

out link control information. This out linkcarrier also had a frame structure thatprovides accurate timing information forall the remote sites. The teleport hubequipment computer tells each VSAT sitewhat particular time slot to use in theTDMA frame and this time planinformation is broadcast to all sitesperiodically.

Advantages•

Shares single carrier frequency withmultiple users.

TDMA permits a flexible bit rate

Non continuous transmission makeshandoff simpler

slots can be assigned on demand.

Bandwidth supplied on demand.

Less stringent power control due toreduced inter-user interference.

Disadvantages•

Higher synchronization overhead.

Equalization necessary for high datarates.

Frequency/slot allocation complexity.

if synchronization lost, the channel maycollide.

Pulsating power envelop.

Applications

TDMA is used in the digital 2g cellular

system such as GSM , GPRS, EDGE,accesses, PDC and iDEN and in the DigitalEnhanced Cordless Telecommunication(DECT) standard for portable phones.

Frequency Division MultipleAccess

FDMA is the oldest of these

schemes and

the least efficient. It started in the 1930;The telephone companies began to useFDM to combine multiple analog voicesignals over the one line to maximizeefficiency of their long distance trunks.With FDMA , only one transmission ispropagated over the

each channel at a

time.