An EM-based channel estimation algorithm for space-time and space-frequency block coded OFDM

The combination of multiple-antenna and orthogonal frequency division multiplexing (OFDM) provides reliable communications over frequency selective fading channels. We investigate this approach and focus on the application of space-time block codes (STBC) and space-frequency block codes (SFBC) in OFDM systems. We compare the performance of maximum likelihood (ML), zero forcing (ZF) and conventional detection algorithms. We show that ZF provides a good trade-off between computational complexity and performance. The problem of channel estimation in STBC-OFDM and SFBC-OFDM systems is also studied, including the derivation of the Cramer-Rao lower bound (CRLB). Since knowledge of the channel is required to coherently decode STBC-OFDM and SFBC-OFDM, we propose an iterative channel estimation algorithm based on the EM algorithm that requires very few pilot symbols. The CRLB can be achieved by the channel estimation algorithm.