Spreading-Hopping Tradeoff in Wideband Ad-hoc Communications

We explore the tradeoff between hopping and spreading in a wideband spread spectrum transmitter-receiver link in an ad-hoc network with multi-user interference. The receiver is assumed to track the user's channel, spreading signature and the overall SINR. Using expressions for the ergodic capacity of the link, we determine the amount of spreading required that maximizes the link capacity. As a comparison we also discuss optimal spreads in centralized architectures where the receiver has interference knowledge. While we find that in general a combination of spreading and hopping may achieve higher throughputs than either pure spreading or pure hopping, we prove that pure hopping and pure spreading are optimal strategies for ad-hoc networks at low SINR and for centralized networks at all SINR, respectively. Numerical results suggest that in an ad-hoc network, spreading is beneficial at high SNR.