TDMA, FDMA, and CDMA - uniroma1.itacts.ing.uniroma1.it/courses/comelet/Slides/20071217_TEL_lecture_2.pdf · 2 Time Division Multiple Access (TDMA) Each user is allowed to transmit

TDMA, FDMA, and CDMATDMA, FDMA, and CDMA Telecomunicazioni

Undergraduate course in Electrical EngineeringUniversity of Rome La Sapienza

Rome, Italy2007-2008

2

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

Each user is allowed to transmit only within specified timeintervals (Time Slots). Different users transmit in differents TimeSlots.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 isto transmit a periodic referencereference burstburst that defines a frame andforces a measure of synchronization of all the users.The frame so-defined is divided into time slots, and each user isassigned a Time Slot in which to transmit its information.

TF

TS

Frame

Time Slot

Ref

eren

ceB

urst

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 receivesthe reference burst with a different phase, and its traffic burst istransmitted with a correspondingly different phase within the time slot.There is therefore a need for guardguard timestimes to take account of thisuncertainty.Each Time Slot is therefore longer than the period needed for theactual traffic burst, thereby avoiding the overlap of traffic burst even inthe 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 uncertainphase relaive to the reference burst, there is the need for a preamblepreambleat the beginning of each traffic burst.The preamble allows the receiver to acquire on top of the coarsesynchronization provided by the reference burst a fine estimate oftiming and carrier phase.

preamble information

7

TDMA: TDMA: referencereference transmittertransmitter schemescheme

S STXSLOW IN

FAST OUT TDMAcoder

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

TIME

A

M

P

L

I

T

U

D

E

( )( ) ( ) ( )! "#=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 jSLOW IN

FAST OUT

BUFFER

( )( )ts jC

Compressed signal

The symbols of the original signalare organized in groups of Nbpssymbols. 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

TIME

A

M

P

L

I

T

U

D

E

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

Signal after compression

TIME [s]

AMPLITUDE [V]

10


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

TIME

A

M

P

L

I

T

U

D

E

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


TIME [s]

AMPLITUDE [V]

( )( ) ( ) ( )! !=

+ ""#=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


TIME [s]

AMPLITUDE [V]

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

TIME [s]

AMPLITUDE [V]

( )( )ts jCfrom the

buffer

12


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


TIME [s]

AMPLITUDE [V]

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

TIME [s]

AMPLITUDE [V]

( ) ( ) ( ) ( )( )! !=

+ """#=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


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

TIME [s]

AMPLITUDE [V]

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

-100

-50

0

50

100

TIME [s]

AMPLITUDE [V]

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

-100

-50

0

50

100

TIME [s]

AMPLITUDE [V]

from

the

TDMA co

der( ) ( )ts jTDMA

( )( )ts jbbBase-bandsignal

14


( ) ( ) ( ) ( )( )! !=

+ """#=m

N

1kFS

jmC

jmNk

jTDMA

bps

bpsmTTckTtats

( ) ( ) ( ) ( )( ) ( )( )jP0jTDMATX

jTX tf2sin)t(gtsP2ts !+"#=

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

PTX : transmitted powerfP : carrier frequency

ϕ(j) : istantaneous phase

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

15


0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

-100

-50

0

50

100

TIME [s]

AMPLITUDE [V]

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 jRX

( ) ( )ts jTX

Received signal afterpropagation over atwo-paths channel

BEWARE!

At risk for multi userinterference!

16


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]

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

TIME [s]

AMPLITUDE [V]

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 aportion 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 themedium have output power spectra in non-overlapping bands.

Many of the problems experienced in TDMA due to differentpropagation delays are eliminated in FDMA.

The major disadvantage of FDMA is the relatively expensive andcomplicated bandpass filters required.

TDMA is realized primarily with much cheaper logic functions.

Another disadvantage of FDMA is the rather strict linearityrequirement 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


( )( ) ( ) ( )! "#=k

jk

j kTtats

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

bb !=

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

jbbTX

jFDMA tftcf2sin)t(sP2ts !+"+#=

Digital binary signal

Base-band signal

FDMA-coded signal

Δf : frequency spacing between adjacent usersc(j) : FDMA code assigned to user j

STX(j)(t)

22


PropagationPropagation

DemodulationDemodulation((DecodingDecoding))

SamplingSampling

ThresholdThresholddetectiondetection

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)

Transmitted

Received

-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 (spreadingcode), which it uses to encode its data signal.The receiver, knowing the code sequence of the user, decodes thereceived signal and recovers the original data.The bandwidth of the coded data signal is chosen to be much largerthan the bandwidth of the original data signal, that is, the encodingprocess 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, providedthat 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 ratespreading code.

Frequency Hopping CDMA (FH-CDMA)The carrier frequency at which the original data signal is transmitted israpidly changed according to the spreading code.

Time Hopping CDMA (TH-CDMA)The original data signal is not transmitted continuously. Instead, thesignal is transmitted in short bursts where the times of the bursts aredecided by the spreading code.

28

x(t) s(t)CODING

Cx

frequencyBand of the original

signalband of the coded signal

frequency

Direct SequenceDirect Sequence Spread SpectrumSpread Spectrum

29


Original signal(band related to the bit rate)

Spreading sequence composed bychips, with chip rate >> bit rate

Coded signal(band related to the chip rate)

30

Sign

al 1

Sign

al 2

Coded signal1

Coded signal2

Sum of codedsignals 1 and 2


31

Received signal

code used for signal 1

multiplier

signal 1 decoded signal


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 spreadingEach user selectees the next frequency hop according to a code(FH code)

Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

33


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


FH-SS signal robustness to a interferers at constant frequency

f0

f1

f2

f3

f4

f5

f6

f7

f8

f9

f

t

Interference limiteda un dwell time

Interferer atconstant frequency

35


Coexistence of different FH-SS signals

f0

f1

f2

f3

f4

f5

f6

f7

f8

f9

f

t

Signal 2Signal 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 attemptis made to synchronize the transmitters sharing the channel, orwhen propagation delays cause misalignment even whentransmitters are synchronized.Partial correlations impede the receiver to totally cancel thecontributions of other users even in the presence of spreadingcodes having low cross-correlation.In presence of partial correlations, the received signal is thereforeaffected by Multi User Interference.The partial correlations can be reduced by proper choice of thespreading codes, but cannot be totally eliminated.CDMA system CDMA system capacitycapacity isis thusthus tipicallytipically limitedlimited byby the theinterferenceinterference 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 receivedpower is higher for transmitters closer to the receiving antenna.Thus, transmitters that are far from the receiving antenna are at adisadvantage 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 thesame for all transmitters.In other words, the nearby transmitters are assigned a lowertransmit power level than the far away transmitters.Power control can be easily achieved in centralized accessschemes (e.g. third generation cellular networks), but is achallenging 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

SRX to the

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


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

( ) ( ) ( ) ( )( ) ( )( )jP0jDSCDMATX

jTX tf2sin)t(gtsP2ts !+"#=

( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( )( )!=

"#$=%=L

1l

jl

jTX

jl

jjTX

j tsthtstsRX

Transmittedsignal

Signal afterpropagation over amultipath channel

Spreading Signal

42


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

Documents

TDMA, FDMA, and CDMA - uniroma1.itacts.ing.uniroma1.it/courses/comelet/Slides/20071217_TEL_lecture_2.pdf · 2 Time Division Multiple Access (TDMA) Each user is allowed to transmit