In TDD-CDMA systems, the downlink multipath channel parameter can be estimated from the uplink. In this paper, we use the instantaneous channel parameter to calculate the spreading spectrum code and despreading code for different users. We present the optimal criterion of the generation of the spreading code and despreading code, so that the multiple access interference can be greatly decreased. Numerical results show that our scheme can greatly increase the signal to interference and noise ratio, compared with conventional CDMA systems which use a Gold sequence as the spreading code.
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