Post on 18-Sep-2015
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
CCU Wireless Comm. Lab.3
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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CCU Wireless Comm. Lab.4
9.1.1 Multiple Access 2/6
CCU Wireless Comm. Lab.5
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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CCU Wireless Comm. Lab.6
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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CCU Wireless Comm. Lab.7
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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CCU Wireless Comm. Lab.8
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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CCU Wireless Comm. Lab.9
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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CCU Wireless Comm. Lab.10
9.2 OFDM FDMA)OFDM FDMA
signalMapper
IFFT
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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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CCU Wireless Comm. Lab.11
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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CCU Wireless Comm. Lab.12
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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CCU Wireless Comm. Lab.13
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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CCU Wireless Comm. Lab.14
) 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
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5
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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
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10
t
1 32 4
5
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t
1 32 4
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CCU Wireless Comm. Lab.15
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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CCU Wireless Comm. Lab.16
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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CCU Wireless Comm. Lab.17
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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CCU Wireless Comm. Lab.18
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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CCU Wireless Comm. Lab.19
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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CCU Wireless Comm. Lab.20
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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CCU Wireless Comm. Lab.21
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