Temporal coherence: A critical function for phase coherent underwater acoustic communications

Abstract

The temporal evolution of an acoustic channel can be characterized by its temporal correlation function from which temporal coherence (the value) and temporal correlation time (at coherence equal to, say, 0.5) can be deduced. Temporal coherence and correlation time have been experimentally measured in the past when the coherence time is much longer than the pulse duration. This is the intra-packet coherence. As frequency increases to tens of kHz or above, the coherence time can be shorter than the packet length. Under such circumstances, the channel coherence can impede the equalizer performance. In the extreme case when the coherence time is shorter than the multipath delay time, the later arrivals may travel through a channel that is different from the earlier arrivals. This paper presents the simulation of communications signals through a time-evolving channel including random media due to internal and turbulence for which temporal correlation time can be predicted for stationary source/receivers. For moving source/receivers, the temporal correlation time (the inverse of the Doppler spread) can be significantly shorter. The performance of the equalizer is found limited by the coherence value. Some at-sea data are presented that support the simulation predictions. [Work supported by ONR.]