A comparative study of differential and noncoherent direct sequence spread spectrum over underwater acoustic channels with multiuser interference

Spread spectrum communication provides a robust solution for underwater acoustic communication over noisy or otherwise unfavorable channels while allowing multiple users to occupy the same bandwidth at the same time. In this study we compare two variants, differential and noncoherent, of direct sequence code division multiple access (DS-CDMA) side-by-side on their performance over a range of channel conditions. Analysis of experimental data collected from the UNET06 experiment in St. Margarets Bay, Nova Scotia, Canada, reveals a tradeoff in performance between these two methods when the interference level and data rates change. Through further simulations we develop a good picture of the range of channel conditions for which one method will outperform the other. These results depend largely on the channel coherence value and the interference level: specifically, the differential scheme is better suited to coherent channels with low interference levels while the noncoherent scheme achieves better performance in high-interference scenarios and as the channel coherence decreases. Further, we observe that the noncoherent scheme is more robust relative to the differential alternative when the rate increases from 1 bit to 2 bits per symbol transmission.