MAP EQUALIZATION AND DECODING IN WIRELESS MOBILE CODED OFDM
#1

Presented by:
KALPANA.B,
KANNAN. M,
MURALI. B.

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MAP EQUALIZATION AND DECODING IN WIRELESS MOBILE CODED OFDM
OBJECTIVE

 OFDM suffers extra performance degradation in multi path channels due to ICI
 Bit error rate(BER) increases with this ICI
 To reduce the BER suitable equalizer is needed.
 That equalizer should not limit the advantages of OFDM
OFDM
 Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multi carrier modulation technique which is used to generate waveforms that are mutually orthogonal.
 In an OFDM scheme, a large number of orthogonal, overlapping, narrow band sub-carriers are transmitted in parallel. These carriers divide the available transmission bandwidth.
 The separation of the sub-carriers is such that there is a very compact spectral utilization.
TYPICAL SPECTRUM OF OFDM
PROBLEM WITH OFDM

 In a multipath environment, a transmitted symbol takes different times to reach the receiver through different propagation paths.
 From the receiver‘s point of view, the channel introduces time dispersion in which the duration of the received symbol is stretched.
 In ideal case , at the maximum of each sub-carrier spectrum, all the spectra of other subcarriers are zero.
 But due to the multipath fading effect part of the energy of other sub-carriers adds into the sub-carrier which is to be received.
 This is called as Inter Carrier Interference.
 The above problem can be reduced by using a block called ‘Equalizer’.
EXISTING EQUALIZERS
 Iterative equalizers that uses normal trellis structures and uses viterbi decoding.
 Pilot Symbol Method
 Pulse shaping.
 OFDM SYSTEM
PROPOSED EQUALIZER
MAP EQUALIZER:

◦ Maximum a posteriori (MAP) equalizer is derived by constructing a trellis which taking into account of ICI channel structure.
 MAP equalizer employs BCJR forward- backward algorithm to compute the A Posteriori Probability of each coded bit
 In statistical terms, the A Posterior Probability is the probability of event A occurring given that event B has occurred.
 The optimal receiver calls for Maximum Likelihood joint equalization and decoding using a hypertrellis which is constructed from both the outer channel code and the ICI channel trellis structure
MAP ALGORITHM
Step 1: Take the block( of length L) of sample values
Step 2: Count the number of output possibilities from a starting point ( forward probabilities )
Step 3: Count the number of possibilities to get a certain output ( backward probabilities )
Step 4: Calculate the LLR between the forward and backward probabilities.
PERFORMANCE ANALYSIS
ADVANTAGES

 Spectrum is not wasted.
 Bit error rate is much lower than any other equalization or any other techniques.
 Computational complexity is lesser than any other iterative equalizer.
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