On spatial-mode selection for the joint transmit and receive MMSE design

To approach the potential MIMO capacity while optimizing the system Bit-Error Rate (BER) performance, the joint Transmit and Receive Minimum Mean Squared Error (MMSE) design has been proposed. It is the optimal linear scheme for spatial multiplexing MIMO systems, assuming a fixed number of spatial streams p as well as a fixed modulation and coding across these spatial streams. However, state-of-the-art designs arbitrarily choose and fix the value of the number of spatial streams p, which may lead to an inefficient power allocation strategy and a poor BER performance. We have previously proposed to relax the constraint of fixed number of streams p and to optimize this value under the constraints of fixed average total transmit power and fixed spectral efficiency, what we referred to as spatial-mode selection. Our previous selection criterion was the minimization of the system sum MMSE. In the present contribution, we introduce a new and better spatial-mode selection criterion that targets the minimization of the system BER. We also provide a detailed performance analysis, over flat-fading channels, that confirms that our proposed spatial-mode selection significantly outperforms the state-of-the-art joint Tx/Rx MMSE design both for uncoded and coded systems, thanks to its better exploitation of the MIMO spatial diversity and more efficient power allocation.