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 introdu ced. 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 s preading and despreading waveform for different users, instead of the despr eading code only. The idea is to take the advantages of exploitation, rathe r than mitigation, of the multipath propagation effects. In TDD-CDMA system s, the channel parameter of the downlink can be estimated from the uplink. We can calculate the spreading and despreading code by using the instantane ous channel parameters, so they can accommodate to the characteristic of th e multipath channel. The maximum receive signal to interference plus noise ration (SINR) criterion is used to optimize the spreading and despreading c ode. An iteration algorithm is given to create the optimal spreading and de spreading code. Numerical results show that our scheme can greatly decrease the multiple access interference (MAI), hence greatly increase the SINR, c ompared with the conventional spreading code such as Gold code. Numerical r esults of the proposed scheme assuming different number of active users, Do ppler frequency and coefficient estimation error are also given.