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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
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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)
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2006/3/9 Multi-Carrier System 3
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.
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2006/3/9 Multi-Carrier System 4
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)
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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.
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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.
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Multi-Carrier System ModelsMC-CDMA
MC-DS-CDMA
MT-CDMA
OFDMA
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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
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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
∑
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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.
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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.
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Hadamard CodeHadamard code of length 8
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Multi-Carrier System ModelsMC-CDMA
MC-DS-CDMA
MT-CDMA
OFDMA
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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
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MC-DS-CDMA Receiver
Serial to
ParallelConverter
Received
Signal FFT
Parallel to
SerialConverter ( )
j
D t
( ) jc t
( ) jc t
( ) jc t
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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.
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Multi-Carrier System ModelsMC-CDMA
MC-DS-CDMA
MT-CDMA
OFDMA
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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
∑
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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
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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
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Multi-Carrier System ModelsMC-CDMA
MC-DS-CDMA
MT-CDMA
OFDMA
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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
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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.
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Combining
Equal Gain Combining (EGC)
Maximum Ratio Combining (MRC)
Orthogonality Restoring Combining (ORC)
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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.
Equal Gain Combining (EGC)
Maximum Ratio Combining (MRC)
Orthogonality Restoring Combining (ORC)
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MC-CDMA Receiver
Serial toParallel
Converter
Received
SignalFFT j D
1 jc
j N c
2 jc
1 jq
j N q
2 jq
∑
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Combining
Equal Gain Combining (EGC)
The equal gain combining only compensates the channel phase
shift.The gain for the EGC is given by
Maximum Ratio Combining (MRC)
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 −
=
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Combining
Orthogonality Restoring Combining (ORC)
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 −=
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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.