Transmit nonlinear zero forcing: energy efficient receiver oriented transmission in MIMO CDMA mobile radio downlinks

For the downlink of MIMO TDD/CDMA mobile radio systems, transmission schemes were recently proposed which can be classified as receiver (Rx) oriented. A main asset of these schemes is their low receiver complexity. In addition, they do without sacrificing downlink transmission resources to training signals for channel estimation, which is beneficial for capacity. First investigations concerned linear transmitter (Tx) algorithms and nonlinear Tx algorithms based on the principle of Tomlinson-Harashima precoding; both of these, especially in scenarios with high system loads, show an unsatisfactory performance. Optimum nonlinear Tx algorithms studied later by Peel et al. circumvent this performance drawback, but have been shown to be far from feasible in today´s mobile radio systems. We try to fill the gap between the feasible Tx algorithms, with their performance drawbacks, and the optimum Tx algorithm by a scalable nonlinear transmitter algorithm termed transmit nonlinear zero forcing (TxNZF). The crux of this algorithm consists in designing the transmitted signal at the access point (AP) groupwise by a nonlinear approach, and in using multiply connected decision regions in the detectors of the mobile terminals (MT). TxNZF achieves almost optimum performance.