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Spread spectrum, CDMA, OVSF, MC-CDMA

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EC744 Wireless Communications Spring 2007 Mohamed Essam Khedr Department of Electronics and Communications Spread spectrum, CDMA, OVSF, MC-CDMA
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Page 1: Spread spectrum, CDMA, OVSF, MC-CDMA

EC744 Wireless Communications Spring 2007

Mohamed Essam Khedr

Department of Electronics and Communications

Spread spectrum, CDMA, OVSF, MC-CDMA

Page 2: Spread spectrum, CDMA, OVSF, MC-CDMA

2

Spread Spectrum Communications - Agenda� Basic principles and block diagrams of spread spectrum

communication systems� Characterizing concepts� Types of SS modulation: principles and circuits

– direct sequence (DS)– frequency hopping (FH)

� Error rates� Spreading code sequences; generation and properties

– Maximal Length (a linear, cyclic code)– Gold– Walsh

� Orthogonal Variable spreading factor � Multicarrier CDMA

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3

Hedy Lamarr (November 9, 1913 – January 19, 2000) was an Austrian/American actress and communications technology innovator. Though known primarily for her great beauty, she also co-invented the first form of spread spectrum, a key to modern wireless communication

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4

Page 5: Spread spectrum, CDMA, OVSF, MC-CDMA

5

Direct Sequence Spread Spectrum (DS-SS)

� This figure shows BPSK-DS transmitter and receiver (multiplication can be realized by RF-mixers)

DSCDMA is used in WCDMA, cdma2000 and IS95 systems

2

22av av

AP A P= � =

spreading

Page 6: Spread spectrum, CDMA, OVSF, MC-CDMA

6

Characteristics of Spread Spectrum � Bandwidth of the transmitted signal W is much greater than the original

message bandwidth (or the signaling rate R)� Transmission bandwidth is independent of the message. Applied code is

known both to the transmitter and receiver

� Interference and noise immunity of SS system is larger, the larger the processing gain

� Multiple SS systems can co-exist in the same band (=CDMA). Increased user independence (decreased interference) for (1) higher processing gain and higher (2) code orthogonality

� Spreading sequence can be very long -> enables low transmitted PSD-> low probability of interception (especially in military communications)

Narrow band signal (data)

Wideband signal(transmitted SS signal)

/ /c b cL W R T T= =

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Direct Sequence Spread Spectrum Using BPSK Example

Page 8: Spread spectrum, CDMA, OVSF, MC-CDMA

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ApproximateSpectrum of DSSS Signal

Page 9: Spread spectrum, CDMA, OVSF, MC-CDMA

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Characteristics of Spread Spectrum (cont.)

� Processing gain, in general

– Large Lc improves noise immunity, but requires a larger transmission bandwidth

– Note that DS-spread spectrum is a repetition FEC-coded systems

� Jamming margin

– Tells the magnitude of additional interference and noise that can be injected to the channel without hazarding system operation. Example:

, 10/ (1/ ) /(1/ ) / , 10 log ( )c c b b c c dB cL W R T T T T L L= = = =

[ ( ) ]J c sys despM L L SNR= − +

30dB,available processing gain

2dB,margin for system losses

10dB,required SNR after despreading (at the RX)

18dB,additional interference and noise can deteriorate

receive

c

sys

desp

j

L

L

SNR

M

==

=

� =

d SNR by this amount

Page 10: Spread spectrum, CDMA, OVSF, MC-CDMA

10

Characteristics of Spread Spectrum (cont.)� Spectral efficiency Eeff: Describes how compactly TX signal fits into the

transmission band. For instance for BPSK with some pre-filtering:

� Energy efficiency (reception sensitivity): The value of to obtain a specified error rate (often 10-9). For BPSK the error rate is

� QPSK-modulation can fit twice the data rate of BPSK in the same bandwidth. Therefore it is more energy efficient than BPSK.

/ /b T b Fff Re R B RE B= =

,

2 2log log1/RF filt c c

RFb

B T LB

k M T M≈ ≈ =

/ / 1/c b c c b cL T T L T T= � =

1beff

RF b

RE

B T� = ≈ bT 2 2

log log

c c

M ML L

=

0/b bE Nγ =

2/

1( 2 ), ( ) exp( 2)

2e b

kp Q Q k dγ λ λ

π

∞= = −�

( )22 logkM k M= � =

, : bandwidth for polar mod.

: number of levels: number of bits

RF filtB

M

k

�����

Page 11: Spread spectrum, CDMA, OVSF, MC-CDMA

11

A QPSK-DS Modulator

� After serial-parallel conversion (S/P) data modulates the orthogonal carriers

� Modulation on orthogonal carriers spreaded by codes c1 and c2

� Spreading codes c1 and c2 may or may not be orthogonal (System performance is independent of their orthogonality, why?)

� What kind of circuit can make the demodulation (despreading)?

2 cos oP tω

2 sin oP tω

1( )c t

2 ( )c t/S P( )s t( )d t

i

q

Constellationdiagram

QPSK-modulator

2 cos( ) and 2 sin( )o oP t P tω ω

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12

DS-CDMA (BPSK) Spectra (Tone Jamming)� Assume DS - BPSK transmission, with a single tone jamming (jamming

power J [W] ). The received signal is

� The respective PSD of the received chip-rate signal is

� At the receiver r(t) is multiplied with the local code c(t) (=despreading)

� The received signal and the local code are phase-aligned:

( ) ( )1 0 0( ) 2 ( )cos 2 cos( ) 'd dr t Pc t T t tt J ωω θ ϕ− + + +=

( ) ( )

{ }

2 2

0 0

0 0

1 1( ) sinc si

1( ) ( )

2

nc2 2c cr c cT TS f PT f f PT f f

J f f f fδ δ

= − + +� � � �

− + +

+

( )( )

1 0

0

( ) 2 ( ) cos ( )

2 cos '

ˆ( )

ˆ( )

d dd

d

d t Pc t T tc t T

c t T

t

J t

ω θ

ω ϕ

−− +

+ +−

=

− − = �1ˆ( ) ( ) 1d dc t T c t T ( ) ( )

( ) ( )

( ){ }

2 20 0

0

2 20 0

ˆ2 ( )cos '

1 1( ) sinc s

1 1sinc sinc

inc2

2 2

2d b b b b

d

c c c c

Jc t T t

S f PT f f T PT f

JT f f T

f

J

T

T f f T

ω ϕ− +

− + +� � � �

= − + +� � � �

+ ���������������������

F

data

Data spectraafter phase modulator

Spreading of jammer power

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Tone Jamming (cont.)� Despreading spreads the jammer power and despreads the signal power:

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Tone Jamming (cont.)

� Filtering (at the BW of the phase modulator) after despreading suppresses the jammer power:

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Error Rate of BPSK-DS System*� DS system is a form of coding, therefore number chips, eg code weight

determines, from its own part, error rate (code gain)� Assuming that the chips are uncorrelated, prob. of code word error for a

binary-block coded BPSK-DS system with code weight w is therefore

� This can be expressed in terms of processing gain Lc by denoting the average signal and noise power by , respectively, yielding

� Note that the symbol error rate is upper bounded due to repetition code nature of the DS by

where t denotes the number of erroneous bits that can be corrected in the coded word

0

2, / ( code rate)b

e c m c

EP Q R w R k n

N

� �= = = �

� �

0,b av b av cE P T N N T= = �

,av avP N

2 2av b ave c m c c m

av c av

P T PP Q R w Q L R w

N T N

� � � �= = � �

� � � �

min1

1(1 ) , ( 1)2n

m n mes

m t

nP p p t d

m−

= +

� � � �≤ − = − � � ��

*For further background, see J.G.Proakis: Digital Communications (IV Ed), Section 13.2

Page 16: Spread spectrum, CDMA, OVSF, MC-CDMA

16

Example: Error Rate of Uncoded Binary BPSK-DS

� For uncoded DS w=n, thus and

� We note that and yielding

� Therefore, we note that increasing system processing gain W/R, error rate can be improved

(1/ ) 1cR w n n= =

0 0

2 2b be c m

E EP Q R w Q

N N

� � � �= = � �

� � � �

/b av b av bE P T P R= = 0 /avJ J W=

0

/ // /

b av

av av av

E P R W RJ J W J P

= =

2 //e

av av

W RP Q

J P

� �� = �

� �

Page 17: Spread spectrum, CDMA, OVSF, MC-CDMA

17

Some Cyclic Block Codes � (n,1) Repetition codes. High coding gain, but low rate� (n,k) Hamming codes. Minimum distance always 3. Thus can detect 2

errors and correct one error. n=2m-1, k = n - m, � Maximum-length codes. For every integer there exists a

maximum length code (n,k) with n = 2k - 1,dmin = 2k-1. Hamming codes are dual of of maximal codes.

� BCH-codes. For every integer there exist a code with n = 2m-1, and where t is the error correction capability

� (n,k) Reed-Solomon (RS) codes. Works with k symbols that consist of m bits that are encoded to yield code words of n symbols. For these codes and

� Nowadays BCH and RS are very popular due to large dmin, large number of codes, and easy generation

� For further code references have a look on self-study material!

3k ≥

3≥m≥ −k n mt min

2 1≥ +d t

2 1,number of check symbols 2= − − =mn n k tmin

2 1= +d t

3m ≥

Page 18: Spread spectrum, CDMA, OVSF, MC-CDMA

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Maximal Length Codes

autocorrelation

power spectral density

Page 19: Spread spectrum, CDMA, OVSF, MC-CDMA

19

Maximal Length Codes (cont.)� Have very good autocorrelation but cross correlation not granted� Are linear,cyclic block codes - generated by feedbacked shift registers� Number of available codes* depends on the number of shift register

stages:� Code generator design based on tables showing tap feedbacks:

5 stages->6 codes, 10 stages ->60 codes, 25 stages ->1.3x106 codes

*For the formula see: Peterson, Ziemer: “Introduction to Spread Spectrum Communication”, p. 121

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Design of Maximal Length Generators by a Table Entry

� Feedback connections can be written directly from the table:

Page 21: Spread spectrum, CDMA, OVSF, MC-CDMA

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Other Spreading Codes� Walsh codes: Orthogonal, used in synchronous systems, also in

WCDMA downlink� Generation recursively:

� All rows and columns of the matrix are orthogonal:

� Gold codes: Generated by summing preferred pairs of maximal length codes. Have a guarantee 3-level crosscorrelation:

� For N-length code there exists N + 2 codes in a code family and

� Walsh and Gold codes are used especially in multiple access systems� Gold codes are used in asynchronous communications because their

crosscorrelation is quite good as formulated above

=0 [0]H − −

−−

� �= � �

1 1

11

n nn

nn

H HH

H H � �� �� �=� �� �

2

0 0 0 00 1 0 10 0 1 10 1 1 0

H

� − − + − + − + ⋅ =( 1)( 1) ( 1)1 1( 1) 1 1 0

{ }− −( ) / ,1/ ,( ( ) 2) /t n N N t n N

+

+

� += �

+�

( 1) / 2

( 2) / 2

1 2 ,for odd( )

1 2 ,for even

n

n

nt n

n= −2 1 andnN

Page 22: Spread spectrum, CDMA, OVSF, MC-CDMA

22

Frequency Hopping Spread Spectrum System (Transmitter)

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Frequency Hopping Spread Spectrum System (Receiver)

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Slow and Fast FHSS

� Frequency shifted every Tc seconds� Duration of signal element is Ts seconds� Slow FHSS has Tc ≥ Ts

� Fast FHSS has Tc < Ts

� Generally fast FHSS gives improved performance in noise (or jamming)

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Slow Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

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Fast Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

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Error Rate in Frequency Hopping� If there are multiple hops/symbol we have a fast-hopping system. If

there is a single hop/symbol (or below), we have a slow-hopping system.

� For slow-hopping non-coherent FSK-system, binary error rate is

� A fast-hopping FSK system is a diversity-gain system. Assuming non-coherent, square-law combining of respective output signals from matched filters yields the binary error rate (with L hops/symbol)

(For further details, see J.G.Proakis: Digital Communications (IV Ed), Section 13.3)

( ) 01 exp / 2 , /2e b b bP E Nγ γ= − =

( ) ( )1

2 1 0

1

0

1exp / 2 / 2 ,

22 11

!

L i

e b i b b cL i

L i

i r

P K L

LK

ri

γ γ γ γ−

− =

− −

=

= − =

−� �= �� �

Page 28: Spread spectrum, CDMA, OVSF, MC-CDMA

28

Multiple access: FDMA, TDMA and CDMA

•FDMA, TDMA and CDMA yield conceptually the same capacity

• However, in wireless communicationsCDMA has improved capacity due to

• statistical multiplexing• graceful degradation

•Performance can still be improved byadaptive antennas, multiuser detection,FEC, and multi-rate encoding

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Example of DS multiple access waveforms

channel->

detecting A ... ->

polar sig.->

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FDMA, TDMA and CDMA compared (cont.)� TDMA and FDMA principle:

– TDMA allocates a time instant for a user– FDMA allocates a frequency band for a user– CDMA allocates a code for user

� CDMA-system can be synchronous or asynchronous:– Synchronous CDMA can not be used in multipath channels that

destroy code orthogonality– Therefore, in wireless CDMA-systems as in IS-95,cdma2000,

WCDMA and IEEE 802.11 user are asynchronous

� Code classification:– Orthogonal, as Walsh-codes for orthogonal or

near-orthogonal systems – Near-orthogonal and non-orthogonal codes:

� Gold-codes, for asynchronous systems � Maximal length codes for asynchronous systems

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Variable Data Rate Services

� Two ways to provide higher and variable data rate services:– Multi-code CDMA

� Multiple Orthogonal Constant Spreading Factor (OCSF) codes– OVSF-CDMA

� Single Orthogonal Variable Spreading Factor (OVSF) code

Page 32: Spread spectrum, CDMA, OVSF, MC-CDMA

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Spreading Operation� Spreading means increasing the signal bandwidth� Strictly speaking, spreading includes two operations:

(1) Channelization (increases signal bandwidth) - using orthogonal codes(2) Scrambling (does not affect the signal bandwidth)- using pseudo noise codes

Page 33: Spread spectrum, CDMA, OVSF, MC-CDMA

33

Page 34: Spread spectrum, CDMA, OVSF, MC-CDMA

34

WCDMA Parameters

4 to 512Downlink SF

4 to 256Uplink SF

2560chips (Max. 2560 bits)No. of chips/slot

15No. of slots/frame

10 ms (38400 chips)Frame Length

3.84 McpsChip Rate

Direct SpreadForward RF Channel Structure

5 MHzChannel B.W

Page 35: Spread spectrum, CDMA, OVSF, MC-CDMA

35

Codes

NOYESIncrease B.W?

Long 10ms code: Gold code

Orthogonal Variable Spreading FactorCode Family

UL: Several millionDL: 512

No. of codes under one scrambling code= SF

No. of codes

38400 chipsUL:4-256 chipsDL:4-512 chips

Length

UL: Separation of terminalsDL: Separation ofcells/sectors

UL: Separation of physical dataand control channels from same UEDL: Separation of different userswithin one cell

Usage

Scrambling CodeChannellization Code

Page 36: Spread spectrum, CDMA, OVSF, MC-CDMA

36

OVSF

� WCDMA is used as the radio access technology in UMTS� In WCDMA, OVSF codes are assigned to users’ calls to preserve

the orthogonal between users’ physical channels.� The data rate supported by an OVSF code depends on its

spreading factor (SF).� the rearrangeable scheme can reduce the call blocking

probability.� The drawback of the rearrangeable scheme increases the

computation overhead of the system� The nonrearrangeable schemes are considered simple and with

low system overhead for the OVSF code assignment

Page 37: Spread spectrum, CDMA, OVSF, MC-CDMA

37

3G w

ireless systems: U

MTS

ICTP - MiramareCDMA – Technology review

Ortogonal Variable Spreading FactorCodes

Recursive rule

( )( )

1

2

2

(0) 1

(2 1) ( ), ( )( )

(2 ) ( ), ( )N N N

NN N N

C

C i C i C iC i

C i C i C i

=

− =��→� = −��

1

11

1-1

1111

11-1-1

1-1-11

1-11-1

C1(1)

C2(1)

C2(2)

C4(1)

C4(2)

C4(3)

C4(4)

Page 38: Spread spectrum, CDMA, OVSF, MC-CDMA

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Criteria for OVSF code assignment(3_1)

Page 39: Spread spectrum, CDMA, OVSF, MC-CDMA

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Criteria for OVSF code assignment(3_2)

� OVSF code have three statuses: free�busy�assignable.– Free: the code has not been assigned to a user.– Busy: the code is assigned to a user.– Assignable: if and only if the code and all of its ancestor

and descendant OVSF codes in the tree are free.

Page 40: Spread spectrum, CDMA, OVSF, MC-CDMA

40

Orthogonal Multiple Access� requires synchronization among the users, since the

waveforms are orthogonal only if they are aligned in time.

���������������

������������� ���������������� ������� �

����������������

Page 41: Spread spectrum, CDMA, OVSF, MC-CDMA

41

Orthogonal Multiple Access (2)

Page 42: Spread spectrum, CDMA, OVSF, MC-CDMA

42

Orthogonal Multiple Access (3)

� Disadvantage of Walsh-Hadamard code:1. It have more than one autocorrelation peak , therefore need an

external synchronization scheme. 2. Cross-correlation will affect it, so it is only used in synchronized

CDMA3. Spreading is not over the whole bandwidth, instead over a number of

discrete frequency-component.

Page 43: Spread spectrum, CDMA, OVSF, MC-CDMA

43

Why Orthogonal?

C1,1

C2,1

SF=1

SF=2

SF=4

SF=8

C4,2 C4,3 C4,4C4,1

C8,1 C8,5 C8,6 C8,7 C8,8C8,2 C8,3 C8,4

C2,2

����������������� ���� ���� ���������� ��

Page 44: Spread spectrum, CDMA, OVSF, MC-CDMA

44

Layers in OVSF Code Tree

C1,1

C2,1

SF=1

SF=2

SF=4

SF=8

C4,2 C4,3 C4,4C4,1

C8,1 C8,5 C8,6 C8,7 C8,8C8,2 C8,3 C8,4

C2,2

������� ����������� ����� ���������� ���������������� ��� ������������������� ������

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45

When non-orthogonal codes happens?

C1,1

C2,1

SF=1

SF=2

SF=4

SF=8

C4,2 C4,3 C4,4C4,1

C8,1 C8,5 C8,6 C8,7 C8,8C8,2 C8,3 C8,4

C2,2

������� ����������� ����� ������������� ���������� ��� ������������������� ������

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46

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51

Multi-rate Assignment Issue

C1,1

C2,1

SF=1

SF=2

SF=4

SF=8

C4,2 C4,3 C4,4C4,1

C8,1 C8,5 C8,6 C8,7 C8,8C8,2 C8,3 C8,4

C2,2

��������� ������������� ��������� ���� ��� ����

Page 52: Spread spectrum, CDMA, OVSF, MC-CDMA

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Tree Locations and Code Assignment

C1,1

C2,1

SF=1

SF=2

SF=4

SF=8

C4,2 C4,3 C4,4C4,1

C8,1 C8,5 C8,6 C8,7 C8,8C8,2 C8,3 C8,4

C2,2

����� ���� ��������� ���� ��� ����

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Code Blocking

C1,1

C2,1

SF=1

SF=2

SF=4

SF=8

C4,2 C4,3 C4,4C4,1

C8,1 C8,5 C8,6 C8,7 C8,8C8,2 C8,3 C8,4

C2,2

! �������������� ���� ��������� � ��������� ������� ���������"������#���������$�%

Page 54: Spread spectrum, CDMA, OVSF, MC-CDMA

54

Multi-code assignment

� There are three dimensions when considering the code allocation:– Ordering of assignment

� Increasing or decreasing– Co-location of codes

� United or separated– Assignment of individual codes

� Four code assignment schemes are proposed

Page 55: Spread spectrum, CDMA, OVSF, MC-CDMA

55

Individual code placement schemes

� Random– If there is one or more than one code in the code tree, Randomly pick

anyone.

� Leftmost– If there is one or more than one code in the code tree, pick the leftmost one.

� Crowded-first-space– If there is one or more than one code in the code tree, pick the one whose

ancestor code has the least free capacity.

� Crowded-first-code– If there is one or more than one code in the code tree, pick the one whose

ancestor code is occupied by most codes.

Page 56: Spread spectrum, CDMA, OVSF, MC-CDMA

56

Code assignment example (one code)

� Random– Any of the candidates can be

picked

� Leftmost– C16,1 will be picked.

� ��������������� ��� ��&��'���������$���

� ������������������ ��&�����������$���

����� ������� �����

� �� ���������(���&������&�)����&�&����&�*����&�+����&��'�

Page 57: Spread spectrum, CDMA, OVSF, MC-CDMA

57

Code assignment example (multi-code)

� Leftmost – {C8,1 , C16,3}

� ��������������� ��

�,�+�+����&���-

� �����������������

�,�+�'����&�*-

! ����������������� ���� ��������� � ��������� ����&�� ������#�������������� ������� ��������%�

���� ������������� ��������

Page 58: Spread spectrum, CDMA, OVSF, MC-CDMA

58

Code reassignment

� When a code tree is used for long, it is sometimes inevitable that the code tree may become fragmented .

� To solve this problem, code reassignment can be conducted to move current codes around so as to squeeze a large-enough space for the request.

Page 59: Spread spectrum, CDMA, OVSF, MC-CDMA

59

Code reassignment example

! ����������������� ���� ��������� � ����

����� ����&�� ������

Page 60: Spread spectrum, CDMA, OVSF, MC-CDMA

60

Multicarrier CDMA

� Multicarrier CDMA combines OFDM and CDMA

� Idea is to use DSSS to spread a narrowband signal and then send each chip over a different subcarrier– DSSS time operations converted to frequency domain

� Greatly reduces complexity of SS system– FFT/IFFT replace synchronization and despreading

� More spectrally efficient than CDMA due to the overlapped subcarriers in OFDM

� Multiple users assigned different spreading codes– Similar interference properties as in CDMA

Page 61: Spread spectrum, CDMA, OVSF, MC-CDMA

61

Outline

1 Problem Statement and Solution Considered

2 CDMA Multiple Access Schemes

– SC DS-CDMA, MC-CDMA, and MC DS-CDMA

3 Limitations of Broadband SC DS-CDMA and MC-CDMA

4 MC DS-CDMA for Ubiquitous Broadband Communication

– Advantages in diverse broadband channels

– Improvement through TF-domain spreading

5 Conclusions

Page 62: Spread spectrum, CDMA, OVSF, MC-CDMA

62

CDMA-based schemes� Can combine concepts of CDMA and OFDM� Reap the benefits of both techniques� In 1993, three slightly different schemes were independently proposed:

– MC-CDMA (Yee, Linnartz, Fettweis, and others)*– Multicarrier DS-CDMA (DaSilva and Sousa)*– MT-CDMA (Vandendorpe)

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Page 64: Spread spectrum, CDMA, OVSF, MC-CDMA

64

Multicarrier DS-CDMA� The data is serial-to-parallel converted.� Symbols on each branch spread in time.� Spread signals transmitted via OFDM� Get spreading in both time and frequency

c(t)

IFFT

P/S convert

.. .S/P converts(t)

c(t)

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2.1 Single-Carrier Direct-Sequence CDMA

�Transmitted signal (BPSK modulated):

�Spreading code in the time-domain.

�Processing gain is ratio of bit to chip duration.

�Number of users N dependents upon cross-correlation of

codes.

�In frequency-selective fading channels, autocorrelation

properties also limit number of users.

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2.2 SC DS-CDMA Illustration

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2.3 Multicarrier CDMA

�Transmitted signal:

�Serial-to-parallel conversion generates lower-rate substreams.

�Substreams modulate orthogonal carriers at maximum spacing.

�Spreading code applied across flat-fading subchannels.

�Number of users N depends on processing gain and cross-

correlation, but not on autocorrelation code characteristics.

�Alternatively, efficient IFFT/FFT implementation possible.

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Page 73: Spread spectrum, CDMA, OVSF, MC-CDMA

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2.4 MC-CDMA Illustration

Page 74: Spread spectrum, CDMA, OVSF, MC-CDMA

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2.5 Multicarrier Direct-Sequence CDMA

�Transmitted signal:

�Hybrid scheme: joint spreading across time and frequency.

�Subcarriers not necessarily orthogonal.

�Processing gain usually product of T- and F-spreading codes.

�Number of users N depends on T- and F-domain spreading factors, cross-correlation, and autocorrelation code properties.

�Conventional MC DS-CDMA: only F-domain repetition.

�Unified family of generalized MC DS-CDMA schemes:

SC DS-CDMA and MC-CDMA represent the marginals.

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Page 75: Spread spectrum, CDMA, OVSF, MC-CDMA

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2.6 MC DS-CDMA Illustration

Page 76: Spread spectrum, CDMA, OVSF, MC-CDMA

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2.7 Flexibility Comparison

�Flexibility (degrees of freedom) of multiple access scheme

impact performance in diverse communication environments.

�Utilize during design phase or reconfiguration during operation.

�Assumptions: fixed system bandwidth, same chip waveform

and BPSK modulation, common bit rate.

�NOT fixed: bit-error-rate, processing gain or number of users.

�SC DS-CDMA:

� no degrees of freedom; system fully specified.

Page 77: Spread spectrum, CDMA, OVSF, MC-CDMA

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2.8 Flexibility Comparison Continued

�MC-CDMA

� Number of parallel bit streams (symbol bit depth U).

�Determines symbol duration and number of subcarriers.

�MC DS-CDMA

� Chip duration influences number of subcarriers.

� Number of parallel bit streams U determines F-code length.

� Spacing between adjacent subcarriers: 1/Ts to 2/Tc.

�Chosen to optimize BER or transmitted signal’s spectrum.

� Parameters allow for trade-offs between spectral efficiency,

BER, number of users, and degree of T- and/or F-domain

spreading.

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3.1 Example: Ubiquitous Communication

�Broadband system:

� 20 MHz bandwidth to support range of services/rates.

� Support bit rate of 1 Mbps per user.

� Maximum number of users not specified.

�Channel properties:

� Delay spread between 0.1 and 3 µµµµs.

�Time-varying frequency-selective fading (ISI) channel.

� Different Doppler shift for lowest and highest frequencies.

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3.2 Deficiencies of SC DS-CDMA

�Severe ISI for delay spread greater 1 µµµµs.

�Remedy: exploit frequency diversity using RAKE receiver.

� But optimal number of fingers depends on delay spread.

� Complex solution: adaptive MRC scheme.

� ISI destroys orthogonality of spreading codes.

�Remedy: Multiuser detection.

� But complexity increases at least linear with number of users.

� Requires signal processing at chip rate.

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3.3 Deficiencies of MC-CDMA

�Remove ISI by choosing many subchannels.

� Problem: increased peak-to-average power ratio.

� Problem: varying correlation between adjacent subcarriers.

�Diversity order changes with coherence bandwidth.

�Different subchannel gains destroy orthogonality in F-domain.

�Remedy: Multiuser Detection (MUD).

� But complexity increases at least linear with number of users.

�Remedy: Zero-Forcing Equalizer could restore orthogonality.

� Suffers from noise enhancement.

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4.1 MC DS-CDMA System Design

�Use system parameters to adjust to propagation environment.

�Goes beyond trade-off between SC DS-CDMA and MC-CDMA.

�System design for diverse channels:

1 Choose chip duration > highest delay spread.

� Ensures flat-fading subcarriers.

2 Select associated subcarrier spacing > coherence bandwidth.

� Enables maximum frequency diversity order.

�Result: system that avoids or mitigates problems encountered

in SC DS-CDMA and MC-CDMA over wide range of delay

spreads.

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4.2 Advantages & Disadvantages

�Advantages:

� Independent fading on diversity-combined subcarriers.

� Lower peak-to-average power ratio.

� T-domain spreading codes remain orthogonal.

� Downlink at near single-user performance without MUD.

� Frequency diversity order remains a constant value.

�Disadvantages:

� Difference in Doppler shift destroys subcarrier orthogonality.

� Assertion: negligible at moderate traveling speeds.

� Diversity order may be insufficient for BER target.

� Countermeasure: increase via transmit diversity.

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�MU-MC-CDMA�RAKE RX�OVSF DCA-SCHEDULING�NOVSF


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