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. © 2009 Elsevier GmbH. All rights reserved.
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