MMSE transmit optimization with interference pre-compensation

We address the problem of nonlinear transmitter design for a point-to-multipoint wireless downlink, where the transmitter is equipped with multiple antennas and each of the independent receivers has a single antenna. We show that the downlink with perfect interference pre-compensation (IPC) has the same normalize MSE achievable region under a sum power constraint as an equivalent uplink with perfect successive interference cancellation (SlC). Based on this duality, we propose a nonlinear downlink transmit framework for joint MSE optimization. The strategy is equivalent to MMSE-DFE with perfect detection in terms of normalized MSE and SINR. As application examples, we solve the problem of minimizing the total transmit power while maintaining individual normalized MSE targets. For practical use, we propose a transmit scheme based on Tomlinson-Harashima-precoding (THP), which has a very simple structure and can be considered as the downlink counterpart of MMSE-DFE. Furthermore, in order to reduce the preceding loss, a strategy which combines THP with signal constellation shaping is utilized to compensate the power enhancement caused by interference pre-subtraction.