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We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a Turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations.Index Terms-Expectation-maximization, fast fading, minimum mean square error, orthogonal frequency-division multiplexing, space-time block codes, turbo principle, wireless dispersive channels.

We consider the performance analysis and the receiver design of low-density parity-check (LDPC) code based space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system over correlated frequency-and time-selective fading channels. I. SYSTEM DESCRIPTIONConsider an STC-OFDM system with K subcarriers, N transmitter antennas and M receiver antennas, signaling through frequency-and time-selective fading channels. Each STC code word spans P adjacent OFDM words (or slots); and each OFDM word consists of (Nh') STC symbols. It is assumed that the fading processes associated with different transmitter-receiver antenna pairs are uncorrelated; and for a particular transmitter-receiver antenna pair, the fading process is correlated in both frequency and time. PERFORMANCE ANALYSIS AND DESIGN PRINCIPLESBy numerically evaluating the outage channel capacity of the STC-OFDM systems, we find that the frequency selectivity and the time selectivity are equivalent in terms of their impact on the outage capacity; and the product of their order, denoted by L, ultimately affects the outage capacity.By analyzing the Chernoff upper bound of the pairwise error probability (PEP) of the STC-OFDM systems, we conclude that the highest possible diversity order the STC-OFDM system can provide is ( N M L ) , i.e., the product of the number of transmitter antennas, the number of receiver antennas and the diversity order of selective-fading channels.Moreover, we argue that systematic coding design is less helpful in the system considered here, due to the assumption that the fading channel profiles (e.g., the delay profile or the Doppler frequency) is known only at the receiver. Instead, we propose that two general principles should be met in choosing STC codes in order to robustly exploit the rich diversity resources in this system, namely, large eflective length [l] and ideal interleaving. LDPC-BASED STC-OFDM SYSTEMSLow-density parity-check (LDPC) codes are the class of linear block codes characterized by "sparse" parity-check matrices [2]. The LDPC codes have the following advantages for the STC-OFDM system considered here: ( I ) The minimum distance of binary LDPC codes increases linearly with the block-length with probability close to 1 [a], which in turn 'increases the effective length of the LDPC-based STC's. (2) No extra interleaver is required because the parity-check matrix of LDPC codes is randomly generated. (3) The decoding complexity of LDPC codes is usually low and is highly parallelizable. (4) For a given block-length and code rate, a "good" LDPC code can be easily constructed. ( 5 )LDPC codes do not typically show an error floor, which is suitable for short-frame applications.At the transmitter of an LDPC-based STC-OFDM system, the information bits are first encoded by an LDPC encoder, and then the binary LDPC coded bits are modulated into STC symbols, which are transmitted from N antennas and over P adjacent OFDM words. IV. TURBO RECEIVERWe consider the receiver design for LDPC-based STC-OFDM systems in unknown ...

The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DS-CDMA system. The receiver performs two successive softoutput decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in Turbo decoding. Given the multipath CDMA channel model, a direct implementation of a sliding-window SISO multiuser detector has a prohibitive computational complexity. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum mean-square error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed low-complexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signal-to-noise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and single-user performance can be approached.