MULTIPLE ACCESS TECHNIQUES

DEFFINITION : The ability of the satellite 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.

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

MULTIPLEXING VsMULTIPLE ACCESS

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 :

FDMA :f 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.

Continuous Transmission: Transmit simultaneously and continuously. For continuous transmission, fewer bits are needed for overhead purposes (such as synchronization and framing bits) as compared to TDMA.

Narrowband system : FDMA is usually implemented in narrowband systems.

FDMA uses duplexers since both TX and RX operate at the same time.

Features of FDMA (Cont.)

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

There is no equalization required

Since the transmission is continuous less no. 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 :(C/N)O = 1 [ 1/(C/N)up + 1/(C/N)dn + 1/(C/N)IM ]Where(C/N)up Up link Carrier to noise ratio(C/N)dn Down link Carrier to noise ratio(C/N)im Inter - modulation Carrier to noise ratio

Number Of Channels Supported By FDMA SystemBcBgBgBt Total bandwidth of the System

Example **Find the no. of channels for a satellite system that has a bandwidth of 12.5 MHZ with a channel bandwidth of 30 KHZ and guard band of 10 KHZ

Time Division Multiple Access (TDMA)

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 wide band communication system.

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

PowerTime

Time-frequency characteristics of TDMA

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

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.

Share a single carrier frequency with several users.

No duplexers is required since users employ different time slots for transmission and reception.

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

Features of TDMA (Cont.)

TDMA frame structure

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

Different TDMA standards have different TDMA frame structures.

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

The preamble and trail bits represent overhead.

The smaller the overhead, the more efficient the TDMA system.

Number of channels in TDMA System m --- maximum no. of TDMA usersB tot ---- total frame lengthB guard --- guard timeBc channel length

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

bOH includes all overhead bits such as preamble, guard bits, etc.Efficiency of TDMA

Frame efficiency parameters

Frame efficiency parameters (cont.)

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

efficient power utilization

flexible data rates

more no. 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

Example 1:The GSM System uses a TDMA frame structure where each frame consist of 8 time slots, and each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel. Find: 1. Time duration of a bit2.Time duration of a slot3.Time duration of a frame Time duration of a bit

Time duration of a slot

Time duration of frame

Sol:

Example 2:If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame efficiency

Sol: Time slots have 6 + 8.25 + 26 + 2(58) = 156.25 bits.A frame has 8 * 156.25 = 1250 bits / frame.

The number of overhead bits per frame is given by

bOH = 8(6) + 8(8.25) + 8(26) = 322 bitsFrame efficiency = (1250 322 ) / 1250 = 74.24 %

Code Division Multiple Access (CDMA)

CDMAIn 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 no. of users.

CDMA used for military applications

CDMA :

CDMA Principle:

CDMA transmission & reception

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

Multipath fading may be substantially reduced because of large signal bandwidth

more secure as compared to FDMA & TDMA

CDMA is a digital technique.

capacity is more as compared to FDMA & TDMA

requires less frequency planning

Types of CDMADirect Sequence Spread Spectrum (DS-SS)Frequency Hopping Spread Spectrum (FH-SS)

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.

SPREAD SPECTRUM TRANSMISSION

SPREAD SPECTRUM RECEPTION

Processing Gain of CDMAGp: processing gain

Bspread: PN code rate

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

BspreadRGP =

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.

BMMMMpr(M)At a receiver, the signals may come from various multiple sources.

The strongest signal usually captures the modulator. The other signals are considered as noise

Each source may have different distances to the base stationNear Far Problem