Chip waveform design for DS/SSMA systems with aperiodic random spreading sequences
In direct-sequence spread-spectrum multiple-access (DS/SSMA) systems with matched filter receivers, the chip waveform affects not only the system bandwidth efficiency, but also the error performance. This is because the statistical property of the multiple-access interference (MAI) depends on the actual shape of the chip waveform as well as its energy. In this paper, we propose an approach to design waveforms that may result in interchip interference (ICI) and derive a criterion for design based on the conditional Gaussian approximation (CGA) of the MAI for systems with aperiodic random spreading sequences. A novel performance metric including the effect of ICI is derived. Based on this metric, a closed form solution for optimal band-limited waveforms is obtained for excess bandwidth less than or equal to one. Numerical results, based on an analytical method as well as Monte-Carlo simulations, are provided to evaluate the performance of the proposed waveforms in systems with conventional matched filter receivers.