Modified unbiased EM-based channel estimation for MIMO turbo receivers

We consider the channel estimation issue in a multiple inputs multiple outputs (MIMO) system using bit-interleaved coded modulation (BICM) scheme at transmitter and iterative detection based on maximum a posteriori (MAP) at receiver. Channel coefficients are updated in each iteration, using a semiblind estimation technique based on the expectation maximization (EM) algorithm. We first consider a previously proposed approach based on EM and show its disadvantages. We then propose a modification to it that provides an unbiased channel estimate and permits a better convergence of the iterative receiver. The proposed modification is of special interest for small ratios of pilot-to-data sequences length.