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In this paper, we present a system model for multiuser multicode code-division multiple access (CDMA) in a multipath fading channel and propose a hybrid receiver scheme for the system. Conventional receivers for CDMA systems utilized interference cancellation (IC) schemes, with the RAKE receiver as an initial data estimator. However, the RAKE receiver generates inaccurate data estimates due the interference that is caused by the cross correlation between the spreading codes. This inaccurate data estimate leads to significant error propagation to the subsequent stages and thus degrades the bit error rate (BER) performance. Our proposed scheme overcomes the problem by using a more accurate initial data estimator. It consists of an equalizer for obtaining initial data estimation, followed by several stages of parallel IC (PIC). Computer simulation results show that the BER curves of our proposed scheme do not encounter an error floor under all channel conditions; in contrast, the BER curves of the conventional RAKE-PIC receiver are constrained by an irreducible error floor. In addition, in an uplink four-path channel and at a BER of 10-3, our simulation results show that a one-stage PIC achieves a 2.5-dB gain over the first stage equalization receiver.
Date of Publication: Sept. 2007