Design of minimum bit error rate precoders for multiuser OFDM systems fitted with MMSE receivers

In this paper, we consider a multiuser downlink OFDM system having a single transmitting antenna and a single receiving antenna for which the channel state information (CSI) are known to both the transmitter and the receivers. For such a system, we design optimal precoders that minimize the average bit error rate (BER) subject to a total power constraint when minimum mean square error (MMSE) receivers are employed. This problem is solved by a two-stage optimization procedure in which a lower bound on the BER is first minimized, followed by showing that this lower bound is actually achieved by the solution obtained in the first stage. In the first stage, we can transform the formulation of the original non-convex optimization problem into a convex one by optimally allocating the subcarriers based on the largest subchannel gain. Furthermore, an alternative efficient power loading method is proposed here in order to reduce the computation complexity. Simulation results show that for moderate to high SNR, our design achieves a gain of several dBs over several other design methods, including currently available precoder design based on MMSE.