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Symbol aided and pilot aided demodulation and its performance improvement using iterative decoding for fading channels

We model and analyze the performance of both symbol-aided coherent demodulation (SAD) and pilot-aided coherent demodulation (PAD) schemes for multipath fading channels. An iterative algorithm for joint channel estimation and decoding is proposed. Both schemes are promising alternatives to differentially coherent demodulation for scenarios characterized with uncertainties in the carrier phase that make coherent demodulation infeasible. Multipath fading, the Doppler phenomenon due to user mobility and/or satellite movement, and different temperature and ventilation conditions at the sites of the various local oscillators that generate the transmitted signals cause the carrier phase uncertainty. Coherent demodulation requires the extraction of a reliable phase reference from the received signal. The traditional solution to these problems is differential demodulation that uses the phase of the previous symbol as reference but requires almost 3 dB (for M-ary PSK modulation in AWGN channels, it is more than that for fast fading channels) of additional signal-to-noise ratio (E/sup b//N/sub 0/) in order to achieve the same bit error rate (BER) as coherent demodulation. This problem is more severe in DS/CDMA systems which are limited by other-user interference. We propose two coded systems that use either PAD or SAD schemes for the reverse link. In both schemes, the estimate of the multipath channel is extracted by filtering either the known symbols in the SAD scheme or the pilot in the PAD scheme. The optimum pilot power in PAD and known symbols insertion rate in SAD are obtained.