Narrowband waveform design for near-far resistant asynchronous CDMA communications
A technique to design narrowband signature waveforms for asynchronous code-division multiple-access (CDMA) systems is proposed. These waveforms are specially tailored for a class of near-far resistant multiuser detectors. They are strictly time-limited, satisfy a root mean square (RMS) bandwidth constraint, and provide a prescribed worst-case near-far resistance. The optimization criterion is either the maximum worst-case pairwise near-far resistance (NFR), or the maximum worst-case pairwise asymptotic multiuser efficiency (AME) of L-shot decorrelating detector over all relative delays of users. These signal set are shown to be immune to synchronization errors as well by using slope constraints of partial cross-correlations over relative delays. Complex signature waveforms are also considered. It is shown that 'chipless' waveforms provide a substantial increase in performance in near-far situations over their chip-based counterparts using the same transmission bandwidth.