A new joint transmit and receive optimization scheme for OFDM-based MIMO systems

To approach the potential MIMO capacity while optimizing the system performance, a joint Tx/Rx MMSE design has been proposed. It is an optimal linear scheme for spatial multiplexing MIMO systems assuming a fixed number of parallel data streams as well as fixed modulation and coding across these streams. The use of OFDM enables a low-complexity implementation of this design for frequency-selective MIMO channels. In such a MIMO/OFDM set-up, the state-of-the-art joint Tx/Rx MMSE approach would arbitrarily choose and fix the number of space-frequency data streams p to be transmitted, which may lead to an inefficient power allocation strategy and a non-minimum bit-error-rate (BER). Therefore, in this paper, we propose to relax the fixed number of streams p constraint and address the issue of optimizing p for this design under fixed average total transmit power P_T and fixed global rate R constraints. In typical frequency-selective indoor channels, the resulting space-frequency optimized joint Tx/Rx MMSE design is shown to lead to a 10 dB SNR gain over the full spatial multiplexing conventional joint Tx/Rx MMSE design for a (2,2) MIMO/OFDM set-up at BER = 10^-3 and an average spectral efficiency of 4 bits/carrier. Such a dramatic improvement is due to the better exploitation of the system´s frequency and space diversity enabled by the proposed optimization.