Optimal sequences to suppress the multiuser interference in the downlink of the TDD-CDMA systems

dConventional multiuser interference canceling is always done in the uplink. In this paper, a novel multiuser interference canceling scheme is introduced. It can be used in the downlink of the TDD-CDMA systems. The multiuser interference canceling is done through the joint optimization of both the spreading and despreading waveform for different users, instead of the despreading code only. The idea is to take the advantages of exploitation, rather than mitigation, of the multipath propagation effects. In TDD-CDMA systems, the channel parameter of the downlink can be estimated from the uplink. We can calculate the spreading and despreading code by using the instantaneous channel parameters, so they can accommodate to the characteristic of the multipath channel. The maximum receive signal to interference plus noise ration (SINR) criterion is used to optimize the spreading and despreading code. An iteration algorithm is given to create the optimal spreading and despreading code. Numerical results show that our scheme can greatly decrease the multiple access interference (MAI), hence greatly increase the SINR, compared with the conventional spreading code such as Gold code. Numerical results of the proposed scheme assuming different number of active users, Doppler frequency and coefficient estimation error are also given.