Nonlinear Optimization for the Multiuser Downlink

We investigate nonlinear pre-equalization techniques for the downlink of multiuser systems from a transmitter equipped with multiple antennas to noncooperative single antenna mobile receivers. Besides linear Multiuser Transmission (MUT), known as Transmit Zero Forcing or Transmit Wiener Filters, recently nonlinear Minimum Bit Error Rate (BER) MUT was proposed. The latter computes the transmit signal by minimizing the mean BER at the detectors by a constrained numerical optimization method. This paper presents a new framework for an unconstrained optimization for systems with a fixed transmit power constraint. Furthermore, the Minimum BER MUT is extended to higher-order modulation. Simulation results reveal a superior performance of the proposed scheme over linear MUT while the computational complexity is significantly reduced compared to the constrained Minimum BER MUT approach.