Linear interference suppression for band-limited DS/SS communications with spectral overlapping

Two groups of communicators, which utilize direct-sequence spread-spectrum modulation with spectral overlapping, strictly band-limited chip waveforms, and pseudo-random spreading sequences are considered. A class of linear receivers is considered for signal detection in additive white Gaussian noise. The linear receiver, consisting of a linear filter followed by a de-spreader and a threshold device, demodulates binary phase-shift-keyed data symbols with quadriphase-shift-keyed spreading sequences. The decision statistics are analyzed with the cyclostationarity of the interference taken into account. Using a variational technique, a closed-form signal-to-interference ratio is derived in terms of the spectral characteristics of the desired signal and the interferences. Based on the analysis, it is shown that the system performance can be significantly improved by adaptively optimizing the receiver chip waveform to achieve the maximum signal-to-interference ratio (MSIR).