cdma & ofdm operation
TRANSCRIPT
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CDMA & OFDM OPERATIONAND COMPARISON
By
L NARESH
08L61A0453
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Overview
Introduction
CDMA
OFDM Comparison between CDMA & OFDM
Future works on OFDM
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Introduction
MULTIPLEXING is a method by which multiple
analog message signals or digital data streams
are combined into one signal over a shared
medium.
The aim is to share an expensive
resource(spectrum)
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CDMA (Code Division Multiple Access)
unique code assigned to each user
used mostly in wireless broadcast channels (cellular,satellite, etc)
all users share same frequency, but each user has own
chipping sequence (i.e., code) to encode data encoded signal= (original data) X (chipping sequence)
decoding: inner-product of encoded signal and chippingsequence
allows multiple users to coexist and transmitsimultaneously with minimal interference (if codes areorthogonal)
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CDMA Encode/Decode
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CDMA: two-sender interference
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Orthogonal Frequency Division
Multiplexing(OFDM)
It is a special kind of FDM
The spacing between carriers are such that
they are orthogonal to one another Therefore no need of guard band between
carriers.
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Example of OFDM
Lets we have following information bits
1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1,
Just converts the serials bits to parallel bitsC1 C2 C3 C4
1 1 -1 -1
1 1 1 -1
1 -1 -1 -1
-1 1 -1 -1
-1 1 1 -1
-1 -1 1 1
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Example of OFDM cont..
Modulate each column with corresponding sub-carrier using BPSK
Modulated signal for C1 Modulated signal for C2
Modulated signal for C3 Modulated signal for C4
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Example of OFDM cont..
Final OFDM Signal = Sum of all signal
)2sin()()(1
0
nttItVN
nn
T
!
!
Generated OFDM signal, V(t)
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OFDM Tx & Rx system
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Advantages of OFDM over CDMA
OFDM can combat multipath interference withgreater robustness and less complexity.Equalisation can be undertaken on a carrier bycarrier basis.
OFDM can achieve higher spectral efficiency withMIMO than CDMA using a RAKE receiver.
Can be used to provide a single frequencynetwork.
It is relatively easy to aggregate spectrum.
It can be scaled according to the requirementsrelatively easily.
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Advantages of CDMA over OFDM
Not as complicated to implement as OFDM
based systems
The CDMA system showed a substantialadvantage over the OFDM system concerning
the transmit power as well as the power
spectral density which influences the radiated
emissions
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Future Issues of OFDM
Peak-to-average power reduction in OFDM
Timing and Frequency Synchronization
Efficient digital signal processing Implementation
of OFDM
Multiple input/Multiple output (MIMO) OFDM
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CONCLUSION
Specially designated frequency bands for
communications would favour the OFDM
system
where as lower and constant limit for large
parts of the frequency band of interest would
recommend the CDMA system.
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Thank You
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Queries?