Chapter 9 Muli-carrier CDMA.pdf

CCU Wireless Comm. Lab.

Chapter 9 Multi-carrier CDMA

CCU Wireless Comm. Lab.2

Chapter 9

) 9 Multi-carrier CDMAA9.1 Multiple Access A9.2 OFDM FDMAA9.3 OFDM TDMAA9.4 OFDM CDMA / Multi-carrier CDMA Systems9.4.1 MC-CDMA System9.4.2 Multi-carrier DS-CDMA System9.4.3 Multi-tone CDMA System9.4.4 System features comparison

A9.5 Differences between OFDM and MC-CDMA


9.1 Multiple Access)Multiple Access AEnable many mobile users to share simultaneously radio

spectrum.

AProvide for the sharing of channel capacity between a number of transmitters at different locations.

AAim to share a channel between two or more signals in such way that each signal can be received without interference from another.

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9.1.1 Multiple Access 2/6


9.1.1 Multiple Access)Multiple Access SchemesAFrequency Division Multiple Access (FDMA)ATime Division Multiple Access (TDMA)ACode Division Multiple Access (CDMA)

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9.1.1 Multiple Access)Frequency Division Multiple Access (FDMA)AEach transmitter is allocated a channel with a particular bandwidth.AAll transmitters are able to transmit simultaneously.AAllocation of separate channels to FDMA signals

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9.1.1 Multiple Access)Time Division Multiple Access (TDMA)ATransmitter share a common channel.AOnly one transmitter is allowed to transmit at a time. AAllocation of time slot in TDMA

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9.1.1 Multiple Access)Code Division Multiple Access (CDMA)A Transmitter may transmit at the same time, in the same channel.A Each signal is modified by spreading it over a large bandwidth.This spreading occurs by combining the transmitter signal with a

spreading sequence.

AAllocation of code in CDMA

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9.2 OFDM FDMA)OFDM FDMAAIn FDMA-OFDM system,the base station assigns each user

a different group of subchannels.AFor instance, if each user assigned equal number of

subcarriers P and the total number of subcarriers is M, then the maximum number of active user in such system is limited to M/P.

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9.2 OFDM FDMA)OFDM FDMA

signalMapper

IFFT

????????????????????????

Channel

?????????????????????????

S/P

1userdata

#

??????????????????????????

??????????????????????????

x bits

????????????

??????????????

signalMapper

2userdata

#

x bits

signalMapper

userMdata

#

x bits##

S/P

S/P

P/S S/P Freqency-selection P/S

signalMapper

FFT

user idata

##

##

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9.3 OFDM TDMA)OFDM TDMAAIn TDMA-OFDM system, the base station assigns each

user different time slots, and each user occupied the whole bandwidth, i.e., all subcarriers.

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9.3 OFDM TDMA)OFDM TDMA

signalM

apper IFFT

Guard Interval

Rem

oval

Channel

Guard Interval

Insertion

User1Data

A / D

& Low

-pass Filter

D / A

& Low

-pass Filter

Up-converter

Dow

n-converter

User2Data

MUser Data

P/S S/P

De-interleaving

P/S

signal Mapper

FFT

user idata

MUX

interleaving S/P

DEMUX

##

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9.4 OFDM CDMA / Multi-carrier CDMA Systems

)OFDM CDMAACombination of CDMA and OFDM/ multi-carrier

modulation techniquesMC-CDMA System / OFDM CDMAMulti-carrier DS-CDMA SystemMulti-tone CDMA System

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) Frequency domain spreading + multi-carrier modulationAMC-CDMA scheme

)Time domain spreading + multi-carrier modulationAMulti-carrier DS-CDMA schemeAMT-CDMA scheme

9.4 OFDM CDMA / Multi-carrier CDMA Systems

13

2

5

6

7

8

9

10

f1f10

f9f8

f7f6

f5f4

f3f2

f

t

a1

a1a1a1a1a1a1a1a1

a1

4

Frequency domain and time domain spreading

f

f1

f10f9f8f7f6f5f4f3f2

t

1 32 4

5

6

7

8

9

10

t

1 32 4

5

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7

8

9

10

t

1 32 4

5

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7

8

9

10

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9.4.1 MC-CDMA System)MC-CDMA SystemA Frequency domain spreadingA The resulting spectrum of each subcarrier can satisfy the

orthogonality condition with the minimum frequency separation.A In a (synchronous) down-link mobile radio communication channel,

we can use Hadamard Walsh codes as an optimum orthogonal set.A It can be implemented via OFDM technique.A Its a potential candidate for the 4th wireless communication system.

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9.4.1 MC-CDMA System)MC-CDMA System

(b) Power spectrum of transmitted signal

1f 2f 3f Nf frequency1Nf

( ) ( )( ) ( )( )

1

0 cos 2

1

MCGj j jMc m

i m

s s

s

s t b i c

p t iT f m f t

fT

= ==

+=

+

+

(a) Transmitter1cos(2 )f t

2cos(2 )f t

( )MC

js tCopier

1c

2c

tt

Input DataStream

cos(2 )Nf tNc

( )jb i

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9.4.1 MC-CDMA System)MC-CDMA System

(c) Receiver1cos(2 )f t

2cos(2 )f t

1jq

2jq

ReceivedSignal

cos(2 )Nf tjNq

LPF

LPF

LPF

jD

my

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9.4.2 Multi-carrier DS-CDMA System)Multi-carrier DS-CDMA systemA Time domain spreadingA The resulting spectrum of each subcarrier can satisfy the

orthogonality condition with the minimum frequency separation.A This scheme can lower the data rate in each subcarrier so that a large

chip time make it easier to synchronize the spreading sequences.A The multicarrier DS-CDMA scheme is originally proposed for a up-

link communication channel, because this characteristic is effective for establishment of a (quasi-) synchronous channel.

AAWhen setting the number of subcarriers to be one, Multi-carrier DS-

CDMA becomes equivalent to a normal DS-CDMA scheme.

c MDN G=

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1cos(2 )f t

2cos(2 )f t

( )MDs t

Serial toParallel

Converter

( )jc t

t

(a) Transmitter

cos(2 )Nf t

t

Multi-carrier DS-CDMA Systemt

Input DataStream

( )jb i( )jc t

( )jc t

( )jc t

(b) Power spectrum of transmitted signal

1f 2f 3f Nf frequency1Nf

9.4.2 Multi-carrier DS-CDMA System)Multi-carrier DS-CDMA system

' 1

C

fT

=

( ) ( )( )( )( ){ }

1 1

0

1

cos 2

c MDN Gj j jMD k m

i k m

c c s

s t b i c

p t m T iT

f k f t

= = ==

+

+

C SC

DS

N TTG

=

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9.4.2 Multi-carrier DS-CDMA System)Multi-carrier DS-CDMA system

Parallel-to-SerialConveter

(c) Receiver1cos(2 )f t

2cos(2 )f t

ReceivedSignal

cos(2 )Nf t

LPF

LPF

LPFND

1D

2D

( )y t

( )jc t

( )jc t

( )jc t

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9.4.3 Multi-Tone CDMA System)Multi-tone CDMA (MT-CDMA) systemA Time domain spreadingA The resulting spectrum of each subcarrier no longer satisfies the

orthogonality condition.A The MT-CDMA scheme uses longer spreading codes in proportion to

the number of subcarriers, as compared with a normal DS-CDMA scheme.

A The MT-CDMA system can accommodate more users than the DS-CDMA system.

A c MTN G

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Chapter 9 Muli-carrier CDMA.pdf