04-04-2011, 02:15 PM
Presented by:
KALPANA.B,
KANNAN. M,
MURALI. B.
[attachment=11676]
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.
KALPANA.B,
KANNAN. M,
MURALI. B.
[attachment=11676]
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.