Turbo TCM Coded OFDM Systems for Non-Gaussian Channels

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I. INTRODUCTION
Orthogonal Frequency Division Multiplexing (OFDM)
modulation has been a promising solution for efficiently
capturing multipath energy in highly dispersive UWB channels
and delivering high data rate transmission. The main
idea behind OFDM is to split the transmitted data sequence
into N parallel sequences of symbols. The transmission is
carried out in parallel on different frequencies which is a
desirable technique for transmission of the digital data through
multipath fading channels. This structure has the particularity
to enable a simple equalization scheme and to resist to
multipath propagation channel.


TTCM Encoder
The TTCM was proposed by Benedetto et al in [4]. Each
of two component encoders has rate b/(b+1)(b is even), but
only b/2 alternative systematic bits are selected to combine
with the corresponding parity-check bit as the outputs for each
constitute encoder. Two bit interleavers are involved in this
TTCM encoder. The first interleaver permutes the bits selected
by the first encoder and the second interleaves those bits
punctured by the first encoder. For M-QAM, there are 21+b/2
levels in both I channel and Q channel, therefore achieve
a throughput of b bits/sec/Hz. Fig. 2 illustrates a 64QAM
modulated standard TTCM encoder.



III. MODIFIED ITERATIVE BIT MAP DECODER
We apply the turbo iterative decoding scheme in [4] [16]
[17], and make certain modifications to match the statistical
characteristics of the channel impulsive noise and fit our
concatenated encoder structure. To match the statistics of the
impulsive noise, the branch metric in the MAP algorithm
needs to be modified according to the PDF of noise. To fit
the simplification of parity-concatenated encoder structure, we
only need one bit MAP decoder instead of two as that in [4]
for iterative decoding, since the outer parity-check decoder
only exchanges extrinsic information between bit streams.


V. CONCLUSION AND FUTURE WORK
The BER performance of Turbo TCM coded OFDM system
under AWGN noise and impulsive noises were presented. The
coding scheme utilizes a punctured parity-concatenated TCM
encoder to function as a Turbo TCM, which may gain a big
advantage in real world implementation. The iterative bit MAP
algorithm was modified to match the channel noise statistical
characteristics and simplified TTCM encoder structure. The
simulation results have shown that the performance of OFDM
system in the impulsive noise environment depends on the
impulsivity of the noise and the decoding algorithm has to
take the noise impulsivity into account for optimal decoding.
In the future, we will continue the performance evaluation of
mixed noise conditions and derive a robust decoding algorithm
for un-predicted impulsive noise type in the channel.