Iterative receivers for space-time block-coded OFDM systems indispersive fading channels

摘要

We consider the design of iterative receivers for space-timenblock-coded orthogonal frequency-division multiplexing (STBC-OFDM)nsystems in unknown wireless dispersive fading channels, with or withoutnouter channel coding. First, we propose a maximum-likelihood (ML)nreceiver for STBC-OFDM systems based on the expectation-maximizationn(EM) algorithm. By assuming that the fading processes remain constantnover the duration of one STBC code word and by exploiting thenorthogonality property of the STBC as well as the OFDM modulation, wenshow that the EM-based receiver has a very low computational complexitynand that the initialization of the EM receiver is based on the linearnminimum mean square error (MMSE) channel estimate for both the pilot andnthe data transmission. Since the actual fading processes may vary withinnone STBC code word, we also analyze the effect of a modeling mismatch onnthe receiver performance and show both analytically and throughnsimulations that the performance degradation due to such a mismatch isnnegligible for practical Doppler frequencies. We further propose a turbonreceiver based on the maximum a posteriori-EM algorithm for STBC-OFDMnsystems with outer channel coding. Compared with the previousnnoniterative receiver employing a decision-directed linear channelnestimator, the iterative receivers proposed here significantly improventhe receiver performance and can approach the ML performance in typicalnwireless channels with very fast fading, at a reasonable computationalncomplexity well suited for real-time implementations