Nonlinear multiuser precoding for downlink DS-CDMA systems over multipath fading channels

We propose a transmitter (Tx)-based nonlinear decorrelating interference cancellation method, Tomlinson-Harashima precoding (THP), to combat multiple access interference (MAI) for the downlink of direct sequence code division multiple access (DS-CDMA) systems in multipath fading channels. Since diversity combining and MAI cancellation are employed in the transmitter, the mobile user receivers remain as simple as in single-path single user channels. Two THP designs, preRake THP and multipath decorrelating THP (MDTHP), are derived and compared with previously investigated decorrelating techniques. It is shown that the proposed methods improve upon linear precoding and detection techniques and on the decision-feedback detectors for multipath CDMA channels, while retaining low complexity. Moreover, the MDTHP precoder is attractive for rapidly varying mobile radio channels since its filters do not need to be updated as fading coefficients vary.