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This paper introduces a new weight-optimizing scheme of partial parallel interference cancellation (PPIC) for CDMA systems over multipath fading channels. The effects of receiving BER of last stage, strength of each path signal and channel estimation error are analyzed. An expression determining the optimal interference cancellation weight (OICW) for PPIC over fading channels is derived, and then a weight-optimizing scheme for PPIC is proposed by processing information from channel estimators. The simulation results in single-rate and multi-rate CDMA systems show that the proposed scheme makes bit error rate (BER) at the following stage less than that at the previous stage, validating our reasonable assumption that stage by stage, the proposed scheme can make the variance of residual interference less and less. Therefore, PPIC using the proposed scheme does not have a "ping-pong" effect and performs better than CW-PPIC. Moreover, at a BER of 0.01, the performance loss of the proposed PPIC stage 2 is only 1.5 dB, compared to PPIC with channel-known interference of one stage.