Multi Carrier Systems

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Wireless Information Transmission System Lab.

National Sun Yat-sen University Institute of Communications Engineering

Multi-Carrier Systems

2006/3/9

王森弘



2006/3/9 Multi-Carrier System 2

OutlineMulti-Carrier Systems Overview

Multi-Carrier System Models

MC-CDMA

MC-DS-CDMA

MT-CDMA

OFDMA

Combining

Equal Gain Combining (EGC)Maximum Ratio Combining (MRC)

Orthogonality Restoring Combining (ORC)




Multi-Carrier Systems OverviewRecently, CDMA technique has been considered to be a

candidate to support multimedia services in mobile radio

communication.

On the other hand, the multicarrier modulation scheme,

often called orthogonal frequency-division multiplexing

(OFDM), has drawn a lot of attention in the field of

radio communications.




Multi-Carrier Systems OverviewIt was in 1993, an epoch of CDMA application, that

three types of new multiple access schemes based on a

combination of code division and OFDM techniqueswere proposed, such as

Multi-carrier (MC-) CDMA

Multi-carrier (MC-) DS-CDMA

Multi-tone (MT-) CDMA

The new multiple access scheme based on a combination

of frequency division and OFDM techniques were

proposed, such as

Orthogonal Frequency-Division Multiple Access (OFDMA)




Multi-Carrier Systems OverviewOFDM scheme is robust to frequency selective fading,

however, it has sever disadvantages such as sensitivity

to frequency offset.

Therefore, The multicarrier CDMA schemes inevitably

have the same drawbacks.

However, the combination of OFDM signaling and

CDMA scheme has one major advantage that it can

lower the symbol duration makes it easier to quasi-

synchronize the transmissions.




Multi-Carrier Systems OverviewThe multicarrier CDMA schemes are categorized mainly

into two groups.

One spreads the original data stream using a given spreading

code, and then modulates a different subcarrier with each chip

(in a sense, the spreading operation in the frequency domain),

and other spreads the serial-to-parallel (S/P) converted datastreams using a given spreading code, and then modulates a

different subcarrier with each of the data stream (the spreading

operation in the time domain), similar to a normal DS-CDMA

scheme.




Multi-Carrier System ModelsMC-CDMA

MC-DS-CDMA

MT-CDMA

OFDMA




MC-CDMA Transmitter

1 j c

2 j c

3 j

c

j N c F

r e q u e n c

y

1

jc

j N c

2

j

c

( ) j MC

t s

j

a

ja

sT

sT




MC-CDMA Receiver

Serial to

Parallel

Converter

Received

SignalFFT j D

1

jc

j N c

2 jc

1

jq

j N q

2 jq

∑




MC-CDMAThe MC-CDMA transmitter spreads the original datastream over different subcarriers using a given spreading

code in the frequency domain.We can use the Hadamard Walsh codes as the givenspreading code.

The capability of suppressing multiuser interference isdetermined by the cross-correlation characteristic of thespreading codes.

Therefore, the capability of distinguishing onecomponent from other components in the compositereceived signal is determined by the auto-correlationcharacteristic of the spreading codes.




Hadamard CodeHadamard code is obtained by selecting the rows of a

Hadamard matrix.

A Hadamard matrix M n is an n x n matrix that any row

differs from any other row in exactly n/2 positions.




Hadamard CodeHadamard code of length 8





MC-DS-CDMA

MT-CDMA

OFDMA




MC-DS-CDMA transmitter

Serial to

Parallel

Converter

Data Stream

( ) jc t

IFFT

Parallel to

Serial

Converter

( ) j MD t s

ja

Time

ja

Time

1

jc

2

j

c

3

jc

j

M c

Time

( ) jc t

( ) jc t

sT 0

0 s NT

0

s NT




MC-DS-CDMA Receiver

Serial to

ParallelConverter

Received

Signal FFT

Parallel to

SerialConverter ( )

j

D t

( ) jc t

( ) jc t

( ) jc t




MC-DS-CDMAThe Multicarrier DS-CDMA transmitter spreads the

Serial-to-Parallel converted data streams using a given

spreading code in the time domain so that the resultingspectrum of each subcarrier can satisfy the orthogonality

condition with the minimum frequency separation.

This scheme is originally proposed for a uplink communication channel, because the introduction of

OFDM signaling into DS-CDMA scheme is effective for

the establishment of a quasi-synchronous channel.





MC-DS-CDMA

MT-CDMA

OFDMA




MT-CDMA Transmitter

Serial to

Parallel

Converter

Data Stream ( ) j MT s t

ja

Time

ja

Time

1

jc

2

j

c

3

jc

j

M c

Time

( ) jc t

( ) jc t

( ) jc t

( )1cos 2 f t π

( )2cos 2 f t π

( )cos 2 N f t π

∑




MT-CDMA Receiver

ReceivedSignal

Parallel to

SerialConverter

( ) j D t

( )1cos 2 f t π

( )2cos 2 f t π

( )cos 2 N f t π

RakeCombiner 1

Rake

Combiner 2

RakeCombiner N




ConclusionSystem features comparison

( N C –1+2G MT )

/( N C T S )

( N C +1)/ N C ．

G MD /T S

( N C +1)/ N C ．

G DS /T S

G DS /T S Nyquist

filter with roll-

off factor = 0

Required band-

width (main

lobe)

1/ N C T S G MD/ N C T S G MC / N C T S Subcarrier separation

N C T S / G MT N C T S / G MDT S / G DS Chip duration

G MT = N C G DS G MD = G DS G MC ≈ G DS G DS Processing gain

N C N C N C 1The number of

subcarriers

N C T S N C T S N C T S /G MC T S

Symbol duration

at subcarrier

MT-CDMAMulticarrier DS-

CDMAMC-CDMADS-CDMA





MC-DS-CDMA

MT-CDMA

OFDMA




OFDMA Transmitter

User 1's

Data

Subcarriers

PlacementIDFT

Add

Guard

Interval

DAC RF

User K's

Data

Subcarriers

PlacementIDFT

Add

Guard

Interval

DAC RF

Remove

Guard

Interval

ADCRFr

1i X

K i X

1n x

K n x

1nh

K

n

h




OFDMA

In OFDMA, closely spaced and overlapped subcarriers

are divided into groups and assigned to multiple users

for simultaneous transmissions.

The N subcarriers are divided into K groups and

assigned to K users. Since one subcarrier is only

allocated to one user, each user has a group of subcarriers.




Combining

Equal Gain Combining (EGC)

Maximum Ratio Combining (MRC)





Combining Overview

In an MC-CDMA receiver the received signal iscombined, in a sense, in the frequency domain, therefore,

the receiver can always employ all the received signalscattered in the frequency domain.

Through a frequency selective fading channel, all the

subcarriers have different amplitude level and different phase shift.

In this section, we will introduce three differentcombining techniques.







MC-CDMA Receiver

Serial toParallel

Converter

Received

SignalFFT j D

1 jc

j N c

2 jc

1 jq

j N q

2 jq

∑




Combining


The equal gain combining only compensates the channel phase

shift.The gain for the EGC is given by


The maximum ratio combining compensates the channel phaseshift and given different weights to each subcarrier.

The gain for the MRC is given by

In the case of one user, the maximum ratio combining methodcan maximum the SNR.

.m j j

mq e

θ −=

.m j j

m mq A eθ −

=




Combining


The orthogonality restoring combining compensates the

channel phase shift and the channel amplitude fading.

The gain for the ORC is given by

However, low level subcarriers tend to be multiplied by highgains, and the noise components are amplified at weaker

subcarriers.

The noise amplification effect degrades the BER performance.

1.m j j

m

m

q e A

θ −=




Reference

S. Hara and R. Prasad, “Overview of Multicarrier

CDMA,” IEEE Communications Magazine, pp. 126-133,

December 1997.S. Hara and R. Prasad, “DS-CDMA, MC-CDMA and

MT-CDMA for Mobile Multi-Media Communications,”

Proc. of IEEE VTC ’96 , Atlanta, USA, April 1996, pp.1106-1110.

Documents

Multi Carrier Systems