MULTIPLE ACCESS TECHNIQUES

DEFFINITION : The ability of a Communication system to carry many signals at the same time

Multiple access schemes are used to allow many users to share simultaneously a finite amount of radio spectrum.

DEFFINITION :Contd..

MULTIPLEXING VsMULTIPLE ACCESS

Multiplexing : Sharing of resources on links inside the network i.e., core networkMultiple Access : Sharing of resources on the access part of the network

ADVANTAGES : Capacity is maximized Bandwidth is used efficiently Flexibility is maintained Cost to the user is minimized

Multiple Access Techniques Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)

Frequency Division Multiple Access (FDMA)In FDMA, each user is allocated a unique frequency band or channel. During the period of the call, no other user can share the same frequency band.Each transmitter is allocated a channel with a particular bandwidth.All transmitters are able to transmit simultaneously.

FDMA :

FDMA : Guard bands are used between each channel to avoid the interference

Features of FDMA

If an FDMA channel is not in use, then it sits idle and cant be used by other users.

Simultaneous and continuous transmission.

Narrowband system: FDMA is usually implemented in narrowband systems.

Advantages of FDMA : Reducing the information bit rate can increase the capacity

There is no equalization required

Since the transmission is continuous less number of bits are needed for synchronization

Simple to implement

Fairly efficient with a small traffic

Disadvantages of FDMA : Network & spectrum planning are intensive and time consuming

Spectrum inefficiency due to idle channels

The guard bands between two users result in wastage of capacity

The C/N reduction due to inter-modulation products

The C/N ratio with Inter-modulation :

Number Of Channels Supported by FDMA systemBcBgBg

TDMA systems divide the whole transmission time into time slots.

Unlike FDMA, only digital data and digital modulation must be used.

It is generally considered as a wideband communication system.

Time Division Multiple Access (TDMA)

TDMA : Allocation of time slot in TDMA

TDMA : A guard time between the two time slots must be allowed in order to avoid interference

Time-frequency characteristics of TDMA

Features of TDMA

TDMA systems divide the radio spectrum into time slots.

Each user occupies a cyclically repeating time slot.

Data transmission is not continuous.

Flexible data rates by assigning multiple time slots to different users based on their demand.

Efficient power utilization: FDMA systems require a 3- to 6-dB power back off in order to compensate for inter-modulation effects.

Synchronous TDMA:

access to the channel is restricted to regular.

Asynchronous TDMA:

a station may transmit at any time that the channel is free.Types of TDMA

Number of channels in TDMA System m --- maximum no. of TDMA users supported on each radio channel B tot ---- total frame lengthB guard --- guard timeBc channel length

PreambleInformation MessageTrail BitsSlot 1Slot 2Slot 3Slot NTrailBitsSync BitsGuardBitsInformation DataOne TDMA FrameOne TDMA Slot A Frame repeats in timeTDMA frame structure

TDMA frame structure (cont.) In TDMA, the preamble contains the address, synchronization and other essential information.

The trail bits contains error detection bits.

The preamble and trail bits represent overhead.

Different TDMA standards have different TDMA frame structures.

In GEO satellite system frame lengths of 125us up to 20ms have been used.

The smaller the overhead, the more efficient the TDMAsystem.

Efficiency of TDMA is a measure of the percentage of bits per frame which contain transmitted data.

Frame Efficiency of TDMA

Frame efficiency parametersbOH includes all overhead bits such as preamble, guard bits, etc.

Frame efficiency parameters (cont.)Nr --- Number of reference bursts/time slots per framebr --- Number of overhead bits per reference burst/time slotNt --- Number of traffic bursts per framebp --- Number of overhead bits per preamble in each slotbg --- Number of equivalent bits in each guard time interval

Advantages of TDMA : More efficient use of spectrum as compared to FDMA

Efficient power utilization

Flexible data rates

More number of channels, compared to FDMA

Carrier-to-noise ratio is high, because of no inter-modulation

Disadvantages of TDMA : large no. of overhead bits for synchronization and framing are required.

high inter-symbol-interference , due to higher symbol rate

data transmission is not continuous

In CDMA all users transmit on the same frequency and at the same time.

CDMA is also called as spread spectrum multiple access.

Spreading occurs by combining the transmitter signal with a spreading sequence (PN code).

CDMA channels can handle an unspecified number of users.

CDMA used for military applicationsCode Division Multiple Access (CDMA)

CDMA :

CDMA Features: All users use the same carrier frequency and may transmit simultaneously

There is no absolute limit on the number of users in CDMA

More secure as compared to FDMA & TDMA

CDMA is a digital technique.

Capacity is more as compared to FDMA & TDMA

Requires less frequency planning

CDMA Principle:

SPREADING

DESPREADING

CDMA transmission & reception

data Akey Asignal Adata keykey sequence AAdAkAsReal systems use much longer keys resulting in a larger distance between single code words in code space. Spreading Example

Direct Sequence Spread Spectrum (DSSS)

A carrier is modulated by a digital code in which the code bit rate is much larger than the information signal bit rate. These systems are also called pseudo-noise systems.

A short code system uses a PN code length equal to a data symbol.

A long system uses a PN code length that is much longer than a data symbol.

DS-SS Transmitter

DS-SS Receiver

Capacity of CDMACapacity of FDMA and TDMA system is bandwidth limited.

Capacity of CDMA system is interference limited.

The link performance of CDMA increases as the number of users decreases.

Processing Gain of CDMAGp: processing gain

Wspread: PN code rate

R: Data rateMain parameter of CDMA is the processing gain that is defined as:

WspreadRGP =

Number of possible users in CDMA h is the background thermal noise S is the average user power W is the total RF bandwidth R is the information bit rate

Advantages of CDMA :Many users of CDMA use the same frequency. Either TDD or FDD may be used.

Multipath fading may be substantially reduced because of large signal bandwidth.

There is no absolute limit on the number of users in CDMA.

Great spectrum efficiency since no guard bands

CDMA improves call quality since less interference

CDMA systems provide more privacy than TDMA & FDMA

Disadvantages of CDMA :Self-jamming is a problem in a CDMA system. Self-jamming arise because the PN sequence are not exactly orthogonal, non-zero contributions from other users in the system arise

The near- far problem occurs at a CDMA receiver if an undesired user has high detected power as compared to the desired user.

CDMA base station is expensive.

The system performance gradually degrades for all users as the number of users is increased.