FDMA : Frequency division Multiple Access TDMA : Time division Multiple AccessCDMA : Code division Multiple AccessSDMA : Space division Multiple AccessPDMA : Polarization division Multiple Access

IntroductionMultiple access schemes are used to allow many

users to share simultaneously a finite amount of radio spectrum

Sharing of spectrum is required to increase capacity

For high quality communication this sharing of spectrum should not degrade performance of the system

high performanceduplexing generally requiredfrequency domaintime domain

DuplexingWhat is Duplexing?

to talk and listen simultaneously is called duplexingClassification of communication systems

according to their connectivity

Simplex

Half-duplex

Duplex

A B

A B

A B

Duplexing Contd…Duplexing may be done using

frequency domain techniquetime domain technique

Frequency division duplexing (FDD)two bands of frequencies for every user

forward band ( for traffic from Base station to mobile unit)

reverse band (for traffic from mobile unit to Base station)

duplexer neededfrequency separation between forward

band and reverse band is constant throughout the system

frequency separation/split

reverse channel forward channel

f

Time division duplexing (TDD)

uses different time slots for forward and reverse linkforward time slotreverse time slot

no duplexer is required (a simple switch can be used)

Communication is not full-duplex

time separation/splitt

forward channelreverse channel

Trade-offs b/w FDD and TDD

FDDProvides individual radio frequencies to each

user hence, transceiver should work on two frequency bands

Frequency allocation must be carefully coordinated with Out-of-band users

Duplexer neededTDD

Single frequency hence simple transceiverDuplexer not needed, a switch can do the job There is time latency, communication is not

full-duplex

Multiple Access Techniques in Wireless Communication System

Frequency division multiple access (FDMA)Time division multiple access (TDMA)Code division multiple access (CDMA)Space division multiple access (SDMA)grouped as:

narrowband systemswideband systems

Narrowband systemsBandwidth of the signal is narrow

compared with the coherence bandwidth of the channel

In NB systems available radio spectrum is divided into large number of narrowband channels usually FDD (large frequency split)Narrowband FDMANarrowband TDMA

Narrowband systemsNarrowband FDMA

a user is assigned a particular channel which is not shared by other users

if FDD is used then each channel has a forward and reverse link (called FDMA/FDD)

Narrowband TDMAAllows users to share the same channel but allocates

a unique time slot to each userFDMA/FDDFDMA/TDDTDMA/FDDTDMA/TDD

Narrowband systems

FDMA/FDDFDMA/TDDTDMA/FDDTDMA/TDD

Logical separation FDMA/FDD

f

t

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...

Logical separation FDMA/TDD

f

t

user 1

user n

forward channel reverse channel

forward channel reverse channel

...

Logical separation TDMA/FDD

f

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...

Logical separation TDMA/TDD

f

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...

Wideband systemsThe transmission BW of a single channel is

much larger than the coherence bandwidth of the channel

users are allowed to transmit in a large part of the spectrum

large number of transmitters on one channel

TDMA techniques allocates time slots to different transmitters

CMA techniques allows the transmitters to access the channel at the same time

Wideband systemsFDD or TDD multiplexing techniquesTDMA/FDDTDMA/TDDCDMA/FDDCDMA/TDD

TDMA

time

power

frequen

cy

CDMA

time

power

frequen

cy

FDMA

time

power

frequen

cy

Multiple Access

Technique

Advanced Mobile Phone System (AMPS) FDMA/FDD

Global System for Mobile (GSM) TDMA/FDD

US Digital Cellular (USDC) TDMA/FDD

Digital European Cordless Telephone (DECT) FDMA/TDD

US Narrowband Spread Spectrum (IS-95) CDMA/FDD

Cellular System

Frequency division multiple access FDMAone phone circuit per channelidle time causes wasting of resourcessimultaneously and continuously transmittingusually implemented in narrowband systemsComplexity of FDMA mobile systems is lower

compared to TDMAFDMA uses duplexersfor example: AMPS is a FDMA system with

bandwidth of 30 kHz

FDMA compared to TDMAfewer bits for synchronization fewer bits for framinghigher costs for duplexer used in base station

and subscriber unitsFDMA requires RF filtering to minimize

adjacent channel interference

Nonlinear Effects in FDMAmany channels - same antennafor maximum power efficiency operate near

saturationnear saturation power amplifiers are

nonlinearnonlinearities causes signal spreading intermodulation frequencies

Nonlinear Effects in FDMAIM are undesired harmonicsinterference with other channels in the

FDMA systeminterference outside the mobile radio band:

adjacent-channel interferenceRF filters needed - higher costs

Number of channels in a FDMA system

N … number of channels Bt … total spectrum allocationBguard … guard bandBc … channel bandwidth

N=Bt - 2Bguard

Bc

Example: Advanced Mobile Phone SystemAMPSFDMA/FDDanalog cellular system12.5 MHz per simplex band - BtBguard = 10 kHz ; Bc = 30 kHz

N=12.5E6 - 2*(10E3)

30E3= 416 channels

Time Division Multiple AccessTime slotsone user per slotBuffer and burst methodNon-continuous transmissionAdvantage:Total bandwidth is utilizedDisadvantage:Strict Burst Timing is required at the earth

station

Slot 1 Slot 2 Slot 3 … Slot N

Repeating Frame Structure

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.

Features of TDMAa single carrier frequency for several userstransmission in burstshandoff process much simpler (can listen

when idle)Low battery consumptionBandwidth can be supplied on demandFDD : switch instead of duplexerhigh synchronization overhead

Number of channels in a TDMA system

N … number of TDMA channel slotsm … number of TDMA users per radio channelBtot … total spectrum allocationBguard … Guard BandBc … channel bandwidth

N=m*(Btot - 2*Bguard)

Bc

Example: Global System for Mobile (GSM)

TDMA/FDDforward link at Btot = 25 MHz radio channels of Bc = 200 kHzif m = 8 speech channels supported, andif no guard band is assumed :

N= 8*25E6200E3

= 1000 simultaneous users

Efficiency of TDMApercentage of transmitted data that contain

informationframe efficiency fusually end user efficiency < f ,because of source and channel codingHow get f ?

Slot 1 Slot 2 Slot 3 … Slot N

Repeating Frame Structure

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.

Efficiency of TDMA

bOH … number of overhead bitsNr … number of reference bursts per framebr … reference bits per reference burstNt … number of traffic bursts per framebp … overhead bits per preamble in each slotbg … equivalent bits in each guard time

interval

bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg

Efficiency of TDMA

bT … total number of bits per frameTf … frame durationR … channel bit rate

bT = Tf * R

Efficiency of TDMA

f … frame efficiencybOH … number of overhead bits per framebT … total number of bits per frame

f = (1-bOH/bT)*100%

Spread Spectrum Multiple Access (SSMA)SSMA uses Signals have transmission BW

that is several orders of magnitude greater than the minimum required BW

A Pseudo-noise sequence converts a narrow band signal to a wideband noise-like signal before transmission

SSMA not BW efficient when used by a single user

Many users can share the same BW without interfering with one another

Type of SSMA techniques: frequency hoped multiple access (FHMA)Direct sequence multiple access (DS) or Code

division multiple access (CDMA)

Pseudo-noise (PN) sequencePN code sets can be generated from linear

feedback shift registersExample:

Modulo 2 adder

Stage 1 Stage 2 Stage 3RegisterOutput

Shift Register

Clock

Correlation matrix of 31-bit PN sequence

5 10 15 20 25 30

5

10

15

20

25

30 -10

-5

0

5

10

15

20

25

30

Frequency Hopped Multiple Access (FHMA)It is digital multiple access systemCarrier frequencies of individual users varied in

pseudorandom fashion with in a wideband channelDigital data broken into uniform sized bursts which

are transmitted on different carriersInstantaneous BW of any one transmission burst is

much smaller than the total spread BWLocally generated PN code is used to synchronize the

receiver frequency with that of transmitter.Provides a High level of security

Fast frequency hopping systemSlow frequency hopping

FHMAErasures can occur when two or more users

transmit on the same channel at the same time

FH(Frequency hopped) signal is immune to fading so error control coding can be combined to guard against erasures .

CDMANarrowband signals is multiplied by a very large

bandwidth signal called the spreading signal.The spreading signal is pseudo noise code

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

All users use the same carrier frequency and transmit simultaneously

Each user has its own pseudo random code word which is approximately orthogonal to other codewords

CDMAReceiver performs time co-relationAll other codewords appear as noise Receiver needs to know the code word used

by transmitterMany users same frequency, TDD or FDDSoft capacity(capacity increases linearly)Self-jammingNear far problem

Packet RadioMany subscribers attempt to access a single

channel in an uncoordinated or minimally coordinated manner

Collisions from simultaneous transmissions are detected at the BS

ACK and NACK is broadcasted by BS (its like perfect feedback)In case of NACK the signal retransmitted

PR easy to implementInduces delays

Packet Radio ProtocolsVulnerable period Vp: time interval during

which the packets are susceptible to collisions with transmission from other usersIf packet length in time is then a packet will suffer collision if other

terminals transmit packets during the period t1 to t1+2

Packet B

Packet A

t1 t1+2

Packet Radio contd …

Subscribers use contention techniquesALOHA protocol example of contention

techniquesAdvantage: ability to serve many users without

any overhead

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Pure ALOHAUncoordinated stations are contesting for the

use of a single shared channelAlgorithm

Stations transmit whenever they have something to send

Colliding frames will be destroyedIf destroyed, the sender waits a random amount of time and sends it again

Maximum channel utilization is 18.4%81.6% of the total available bandwidth is essentially

wasted due to losses from packet collisions

49

In pure ALOHA, frames are transmitted at completely arbitrary times

50

Slotted ALOHA A method for doubling the capacity of pure

ALOHA systems Divide up time into discrete interval slots

Stations share the same time by synchronizing with a master Master periodically broadcasts a synchronization

pulse (clock tick packet) to all stations Stations are only allowed to send their packets

immediately after receiving a clock tick Each interval corresponding to one frame time

Maximum channel utilization is 36.8%

51

52

ALOHA ProtocolsAdvantages

Simple (due to distributed control)Flexible to fluctuations in the number of hostsFair

DisadvantagesLow channel efficiency with a large number of

hostsNot good for real-time traffic (e.g., voice)Cannot support priority traffic