Suitable combination of channel coding and space–time schemes for moderate-to-high spectral efficiency MIMO systems

In order to propose practical solutions for future communication systems employing a combination of multiple-input multiple-output (MIMO) structures and orthogonal frequency division multiplexing (OFDM), we analyse and compare the performances obtained by orthogonal and non-orthogonal space–time coding schemes. A simple iterative detector based on soft parallel interference cancellation (Soft-PIC) is used in the receiver in order to cope with the interference terms when using non-orthogonal schemes. First, we propose a modification of the log-likelihood ratio (LLR) calculation part in the Soft-PIC detector that provides an improvement of the receiver performance compared to previous implementations of this detector. Then, the performances of different space–time schemes proposed for two, three or four antennas at the transmitter and two antennas at the receiver are compared. In the case of perfect channel estimation, a substantial gain is obtained by using the appropriate non-orthogonal schemes for moderate-to-high spectral efficiency MIMO systems, which justifies the increased complexity of the receiver. This conclusion is conditioned to the presence of sufficient frequency diversity that is the case in a practical system. We also study the case of imperfect channel estimation at the receiver and discuss the choice of the appropriate space–time scheme. The gain offered by the non-orthogonal schemes is confirmed in that case, specifically for high spectral efficiency systems.