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In this paper, we describe direct-sequence code-division multiple-access (CDMA) systems with M-ary modulation in which channel estimation, coherent demodulation, and decoding are iteratively performed without the use of any training or pilot symbols. An expectation-maximization channel-estimation algorithm for the fading amplitude and interference-plus-noise power spectral density (PSD) is proposed for CDMA systems with low-density parity-check codes. The elimination of pilot symbols simplifies the system design and allows either an enhancement in information throughput or greater spectral efficiency by increasing the information-symbol duration. After initial estimates of the fading amplitude and noise PSD are obtained from the received symbols, subsequent values of these parameters are iteratively updated by using the soft feedback from the channel decoder. The updated estimates are combined with the received symbols and iteratively passed to the decoder. Although the bit error rates of the proposed systems may be slightly larger than those of comparable systems with pilot-assisted channel estimation, the throughputs are higher.
Date of Conference: 16-19 Nov. 2008