Iterative maximum-likelihood sequence estimation for space-time coded systems

In recent work on decoding space-time codes, it is either assumed that perfect channel state information (CSI) is present, or a channel estimate is obtained using pilot symbols and then used as if it were perfect to extract symbol estimates, In the latter case, a loss in performance is incurred, since the resulting overall receiver is not optimal. In this letter we look at maximum-likelihood (ML) sequence estimation for space-time coded systems without assuming CSI, The log-likelihood function is presented for both quasi-static and nonstatic fading channels, and an expectation-maximization (EM)-based algorithm is introduced for producing ML data estimates, whose complexity is much smaller than a direct evaluation of the log-likelihood function. Simulation results indicate the EM-based algorithm achieves a performance close to that of a receiver which knows the channel perfectly.