Achieving the optimal diversity-vs-multiplexing tradeoff for MIMO flat channels with QAM space-time spreading and DFE equalization

The use of multiple transmit (Tx) and receive (Rx) antennas allows to transmit multiple signal streams in parallel and hence to increase communication capacity. We have previously introduced simple convolutive linear precoding schemes that spread transmitted symbols in time and space, involving spatial spreading, delay diversity and possibly temporal spreading. In this paper we show that the use of the classical MIMO Decision Feedback Equalizer (DFE) (but with joint detection) for this system allows to achieve the optimal diversity versus multiplexing tradeoff introduced in [1], when a Minimum Mean Squared Error (MMSE) design is used. One of the major contributions of this work is the diversity analysis of a MMSE equalizer without the Gaussian approximation. Furthermore, the tradeoff is discussed for an arbitrary number of transmit and receive antennas. We also show the tradeoff obtained for a MMSE Zero Forcing (ZF) design. So, another originality of this paper is to show that the MIMO optimal tradeoff can be attained with a suboptimal receiver, in this case a DFE, as opposed to optimal Maximum Likelihood Sequence Estimation (MLSE).