Iterative channel estimation and decoding for a turbo-coded OFDM system via the EM algorithm

This paper proposes an OFDM receiver that iteratively estimates the channel during the iterative decoding process of a serially-concatenated code. To track a slowly time-varying channel, the receiver can update the channel estimate using decision-direction based on soft symbols to mitigate error propagation. When the channel changes so quickly that decision-directed updates cannot keep up, the receiver re-estimates the channel during the decoding process using the EM algorithm based on the transmitted symbol probabilities provided by the turbo decoder. The EM algorithm is used to approximate a maximum likelihood solution given the distribution of the transmitted symbols. With this technique, the iterative process of the EM algorithm is naturally combined with the decoding iteration. Improved coded performance is observed for both static and fast time-varying channels.