Overloaded TDD-CDMA cells with multiuser transmission

The number of supportable users in the TDD-CDMA downlink is usually limited by multiple-access interference and by the limited number of available orthogonal spreading codes. By using multiuser transmission (MUT) methods in the base station in conjunction with smart antennas, the interference can be mitigated and the downlink cell capacity can be substantially increased. The number of supported users and the necessary transmit power in the CDMA downlink with linear MUT methods, like joint transmission and nonlinear methods like minimum bit error rate multiuser transmission is investigated. By co-locating different scrambling code groups in one base station, the number of supported users is roughly proportional to the number of applied antennas, but the spatio-temporal dimension can also be exceeded in certain circumstances. Furthermore, a simplified phase-only minimum bit error rate multiuser transmission is proposed, including the first and second order analytic BER derivatives. The simulation environment is similar to 3GPP TDD CDMA or the Chinese TD-SCDMA.