TDMA, FDMA, and CDMA - Lorenzo Mucchi Time Division Multiple Access (TDMA) Each user is allowed to transmit only within specified time intervals (Time Slots). Different users transmit

TDMA, FDMA, and CDMATDMA, FDMA, and CDMA

Telecomunicazioni

Undergraduate course in Electrical Engineering

University of Rome La Sapienza

Rome, Italy

2007-2008

NB --> I "case study" non sono parte delle domande d'esame.

2

TTime ime DDivision ivision MMultiple ultiple AAccess (TDMA)ccess (TDMA)

Each user is allowed to transmit only within specified time

intervals (Time Slots). Different users transmit in differents Time

Slots.

When users transmit, they occupy the whole frequency bandwidth

(separation among users is performed in the time domain).

3

TDMA : TDMA : FrameFrame StructureStructure

TDMA requires a centralized control node, whose primary function is

to transmit a periodic referencereference burstburst that defines a frame and

forces a measure of synchronization of all the users.

The frame so-defined is divided into time slots, and each user is

assigned a Time Slot in which to transmit its information.

TF

TS

Frame

Time Slot

Ref

eren

ceB

urs

t

4

TDMA : Frame StructureTDMA : Frame Structure

User 1 User 2 User 3

5

TDMA : TDMA : guardguard timestimes

Since there are significant delays between users, each user receives

the reference burst with a different phase, and its traffic burst is

transmitted with a correspondingly different phase within the time slot.

There is therefore a need for guardguard timestimes to take account of this

uncertainty.

Each Time Slot is therefore longer than the period needed for the

actual traffic burst, thereby avoiding the overlap of traffic burst even in

the presence of these propagation delays.

misalignment misalignment

with guard time without guard time

6

TDMA : TDMA : preamblepreamble

Since each traffic burst is transmitted independently with an uncertain

phase relaive to the reference burst, there is the need for a preamblepreamble

at the beginning of each traffic burst.

The preamble allows the receiver to acquire on top of the coarse

synchronization provided by the reference burst a fine estimate of

timing and carrier phase.

preamble information

7

TDMA: TDMA: referencereference transmittertransmitter schemescheme

S STXSLOW

IN

FAST OUT TDMA

coder

PulseShaper

Mod

Codegenerator

Digitalsignal

BUFFER

Carriergenerator

fP

8

TDMA: TDMA: a case a case studystudy

0 1 2 3 4 5 6 7 8 9 10

x 10-3

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1( )( ) ( ) ( )=k

jk

j kTtats

Digital signal of user j

Sequence of equally spacedbinary antipodal symbols

ak(j) : k-th binary antipodal

symbol generated by user j

T : time period between symbols

Userj

s(j)(t)

9

TDMA: a case studyTDMA: a case study

( )( )ts j

SLOW IN

FAST OUT

BUFFER

( )( )ts jC

Compressed signal

The symbols of the original signalare organized in groups of Nbps

symbols. Each group is transmittedin a single Time Slot of duration TS.Time Slots are organized in framesof duration TF.

0 1 2 3 4 5 6 7 8 9 10

x 10-3

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

10

TDMA: a case studyTDMA: a case study

0 1 2 3 4 5 6 7 8 9 10

x 10-3

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

( )( ) ( ) ( )=

+=m

N

1kFC

jmNk

jC

bps

bpsmTkTtats

( )( ) ( ) ( )=k

jk

j kTtats

TC : time interval between symbolsafter compression

11

TDMA: a case TDMA: a case studystudy

TDMAcoder

Codegenerator

TDMA Coded Signal

The position in time of each group ismodified according to the TDMA code,which is assigned to the user.

In other words, the TDMA code indicateswhich slot inside each frame must beoccupied by the user.

( ) ( )ts jTDMA

0 1 2 3 4 5 6 7 8 9 10

x 10-3

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

( )( )ts jC

from the

buffer

12

TDMA: a case TDMA: a case studystudy

0 1 2 3 4 5 6 7 8 9 10

x 10-3

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

( ) ( ) ( ) ( )( )=

+=m

N

1kFS

jmC

jmNk

jTDMA

bps

bpsmTTckTtats

cm(j) : TDMA code assigned to

user j for the m-th frame

( )( ) ( ) ( )=

+=m

N

1kFC

jmNk

jC

bps

bpsmTkTtats

13

S(j)TX(t)

PulseShaper

Mod

Carriergenerator fP

TDMA: a case studyTDMA: a case study

Transmitted signalat RadioFrequencies

All users adopt the samecarrier frequency fp formodulating the base-band signal

0 1 2 3 4 5 6 7 8 9 10

x 10-3

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

-100

-50

0

50

100

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

-100

-50

0

50

100

from

the

TDM

A coder( ) ( )ts jTDMA

( )( )ts jbb

Base-bandsignal

14

TDMA: a case TDMA: a case studystudy

( ) ( ) ( ) ( )( )=

+=m

N

1kFS

jmC

jmNk

jTDMA

bps

bpsmTTckTtats

( ) ( ) ( ) ( )( ) ( )( )jP0

jTDMATX

jTX tf2sin)t(gtsP2ts +=

g0(t) : energy-normalizedimpulse response of thePulse Shaper. It hasunitary energy.

PTX : transmitted power

fP : carrier frequency(j) : istantaneous phase

For the sake of simplifying the notation,let us consider the simple case of BPSK(in phase carrier modulation)

15

TDMA: a case TDMA: a case studystudy

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

-100

-50

0

50

100

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

-15

-10

-5

0

5

10

15

Time [s]

Amplitude [V]

( ) ( )ts j

RX

( ) ( )ts jTX

Received signal afterpropagation over atwo-paths channel

BEWARE!

At risk for multi userinterference!

16

TDMA: a case TDMA: a case studystudy

004 0.006 0.008 0.01 0.012 0.014 0.016Time [s]

1 2 3 4 5 6 7 8 9 10

x 10

Receivedwaveform

Front-end filteringFront-end filtering

DemodulationDemodulation

SamplingSampling

Threshold detectionThreshold detection

Receivedbinaryantipodalsignal

17

FFrequency requency DDivision ivision MMultiple ultiple AAccess (FDMA)ccess (FDMA)

Each user transmits with no limitations in time, but using only a

portion of the whole available frequency bandwidth.

Different users are separated in the frequency domain.

18

FDMA vs. TDMAFDMA vs. TDMA

Frequency division is very simple: all transmitters sharing the

medium have output power spectra in non-overlapping bands.

Many of the problems experienced in TDMA due to different

propagation delays are eliminated in FDMA.

The major disadvantage of FDMA is the relatively expensive and

complicated bandpass filters required.

TDMA is realized primarily with much cheaper logic functions.

Another disadvantage of FDMA is the rather strict linearity

requirement of the medium.

19

FDMA: FDMA: referencereference schemescheme

S STXPulse

ShaperMod

Codegenerator

Digitalsignal

Carriergenerator

20

FDMA: a case FDMA: a case studystudy

0 5 10

x 10-3

-1

-0.5

0

0.5

1

Generated bit stream for each user

0 0.005 0.01 0.015

-60

-40

-20

0

20

40

60

Signal after Pulse Shaping

0 0.005 0.01 0.015-60

-40

-20

0

20

40

60

Signal after FDMA coding

( )( )ts j ( )( )ts jbb

( ) ( )ts jFDMA

Digital binary signal Base-band signal FDMA-coded signal

21

FDMA: a case FDMA: a case studystudy

( )( ) ( ) ( )=k

jk

j kTtats

( )( ) ( )( ) ( )tgtsts 0jj

bb =

( ) ( ) ( ) ( )( )( ) ( )( )jjP

jbbTX

jFDMA tftcf2sin)t(sP2ts ++=

Digital binary signal

Base-band signal

FDMA-coded signal

f : frequency spacing between adjacent users

c(j) : FDMA code assigned to user jSTX

(j)(t)

22

FDMA: a case FDMA: a case studystudy

PropagationPropagation

DemodulationDemodulation

((DecodingDecoding))

SamplingSampling

ThresholdThreshold

detectiondetection

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018-60

-40

-20

0

20

40

Amplitude [V]

Received Signal after Demodulation (Decoding)

TransmittedReceived

-4 -2 0 2 4 6 8 10 12 14 16

-6

-4

-2

0

2

4

x 10-3 Samples of the received waveform

0 0.005 0.01 0.015 0.02

-8

-6

-4

-2

0

2

4

6

8

Time [s]

Amplitude [V]

-4 -2 0 2 4 6 8 10 12 14 16-0.5

0

0.5

1

1.5

Transmittedsignal at RF

Receivedbase-bandwaveform

Samples atthe receiveroutput

Receivedbinarystream

23

TDMA + FDMATDMA + FDMA

FDMA TDMA + FDMA

24

TDMA + FDMA in GSM900 standardTDMA + FDMA in GSM900 standard

25

CCode ode DDivision ivision MMultiple ultiple AAccess (CDMA)ccess (CDMA)

26

CDMA: basic CDMA: basic principlesprinciples

In CDMA each user is assigned a unique code sequence (spreading

code), which it uses to encode its data signal.

The receiver, knowing the code sequence of the user, decodes the

received signal and recovers the original data.

The bandwidth of the coded data signal is chosen to be much larger

than the bandwidth of the original data signal, that is, the encoding

process enlarges (spreads) the spectrum of the data signal.

CDMA is based on spread-spectrum modulation.

If multiple users transmit a spread-spectrum signal at the same time,

the receiver will still be able to distinguish between users, provided

that each user has a unique code that has a sufficiently low cross-

correlation with the other codes.

27

CDMA CDMA schemesschemes

Direct Sequence CDMA (DS-CDMA)The original data signal is multiplied directly by the high chip rate

spreading code.

Frequency Hopping CDMA (FH-CDMA)The carrier frequency at which the original data signal is transmitted is

rapidly changed according to the spreading code.

Time Hopping CDMA (TH-CDMA)The original data signal is not transmitted continuously. Instead, the

signal is transmitted in short bursts where the times of the bursts are

decided by the spreading code.

28

x(t) s(t)CODING

Cx

frequency

Band of the original

signal

band of the coded signal

frequency

Direct SequenceDirect Sequence Spread SpectrumSpread Spectrum

29

Direct SequenceDirect Sequence Spread SpectrumSpread Spectrum

Original signal

(band related to the bit rate)

Spreading sequence composed by

chips, with chip rate >> bit rate

Coded signal

(band related to the chip rate)

30

Sig

na

l 1

Sig

na

l 2

Coded signal

1

Coded signal

2

Sum of coded

signals 1 and 2

Direct SequenceDirect Sequence Spread SpectrumSpread Spectrum

31

Received signal

code used for signal 1

multiplier

signal 1 decoded signal

Direct SequenceDirect Sequence Spread SpectrumSpread Spectrum

32

In FH-SS, the transmitter spreads the spectrum by continuouslyjumping from one frequency channel to another

A larger number of intervals leads to a better spreading

Each user selectees the next frequency hop according to a code

(FH code)

Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

33

Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

Time-frequency occupation for a FH-SS signal

f0

f1

f2

f3

f4

f5

f6

f7

f8

f9

f

t

Dwell time

FH code period

34

Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

FH-SS signal robustness to a interferers at constant frequency

f0

f1

f2

f3

f4

f5

f6

f7

f8

f9

f

t

Interference limited

a un dwell time

Interferer at

constant frequency

35

Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

Coexistence of different FH-SS signals

f0

f1

f2

f3

f4

f5

f6

f7

f8

f9

f

t

Signal 2

Signal 1

If codes are well chosen (orthogonal) No interference!!

36

CDMA : the CDMA : the partial correlation problempartial correlation problem

PartialPartial correlationscorrelations among encoded signals arise when no attempt

is made to synchronize the transmitters sharing the channel, or

when propagation delays cause misalignment even when

transmitters are synchronized.

Partial correlations impede the receiver to totally cancel the

contributions of other users even in the presence of spreading

codes having low cross-correlation.

In presence of partial correlations, the received signal is therefore

affected by Multi User Interference.

The partial correlations can be reduced by proper choice of the

spreading codes, but cannot be totally eliminated.

CDMA system CDMA system capacitycapacity isis thusthus tipicallytipically limitedlimited byby the the

interferenceinterference fromfrom otherother usersusers, , ratherrather thanthan byby thermalthermal noisenoise.

37

CDMA : the CDMA : the near-far problemnear-far problem

If all the users transmit at the same power level, then the received

power is higher for transmitters closer to the receiving antenna.

Thus, transmitters that are far from the receiving antenna are at a

disadvantage with respect to interference from other users.

This inequity can be compensated by using power controlpower control.

Each transmitter can accept central control of its transmitted power,

such that the power arriving at the common receiving antenna is the

same for all transmitters.

In other words, the nearby transmitters are assigned a lower

transmit power level than the far away transmitters.

Power control can be easily achieved in centralized access

schemes (e.g. third generation cellular networks), but is a

challenging issue in distributed systems.

38

DS-CDMA: DS-CDMA: reference schemereference scheme

S STX

CDMA coder(multiplier)

PulseShaper

Mod

Codegenerator

Digitalsignal

Carriergenerator

fP

Transmitter

39

DS-CDMA: DS-CDMA: referencereference schemescheme

Front-Endfilter and

demodulatorMultiplier Integrator

Codegenerator

Receivedsignal

Receiver

SRXto th

e

decisor

40

0 5 10

x 10-3

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Generated bit stream for each user

0 2 4 6 8

-1

-0.5

0

0.5

1

Assigned Codeword

0 0.005 0.01

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Binary signal after coding for each user

DS-CDMA: a case DS-CDMA: a case studystudy

( )( )ts j ( )[ ]kc j ( ) ( )ts jDSCDMA

binary data signal Codeword DS-CDMA-coded signal

41

DS-CDMA: a case DS-CDMA: a case studystudy

Digital binary signal

( )( ) ( ) ( )=k

jk

j kTtats

( ) ( ) ( ) ( )[ ] ( )=

=k

N

1mC

jjk

jDSCDMA

DS

kTmTtmcats

DS-CDMA-coded signal

NDS : length of the codewordTC : chip time

( ) ( ) ( ) ( )( ) ( )( )jP0

jDSCDMATX

jTX tf2sin)t(gtsP2ts +=

( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( )( )=

==L

1l

jl

jTX

jl

jjTX

j tsthtstsRX

Transmittedsignal

Signal afterpropagation over amultipath channel

Spreading Signal

42

DS-CDMA: a case DS-CDMA: a case studystudy

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

-1

0

1

x 10-4 Received Signal after Demodulation

Amplitude [V

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

-1

0

1

x 10-4 Received Signal after Code Multiplication

Amplitude [V]

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

-5

0

5

x 10-4 Received Signal after Integration

Amplitude [V]

Received signal afterFront-End filtering anddemodulation

Signal obtained by directmultiplication of the base-band signal with thespreading signal

Received sequenceafter integration of theabove samples