Adapted demodulation for spread-spectrum receivers which employ transform-domain interference excision

It has been shown previously that the performance of a direct-sequence spread-spectrum (DS/SS) receiver which incorporates transform-domain excision of narrowband interference is improved by the use of time-weighting when the interference-to-signal power ratio (ISR) is relatively large, but is degraded by its use otherwise. The demodulator employed in this receiver is a filter which is matched to the chip sequence. While the matched filter gives minimum probability of error for additive white Gaussian noise (AWGN) in the absence of time-weighting and excision processing, the matched filter is suboptimal in their presence. Alternative demodulation rules which are adapted to time-weighting and excision processing are developed in the present paper. It is shown that, for the weighting functions considered, the matched filter requires as much as 3 dB more signal power than an adapted demodulator, to obtain a given probability of error. Furthermore, the performance with an adapted demodulator is generally superior to that for a receiver which does not use time-weighting when the ISR is at least moderately large, and is comparable otherwise. The potential benefit that may be derived from the use of an adapted demodulator in a CDMA network overlay is discussed