Power delay profiles and temporal vs. spatial degrees of freedom: Predicting and optimizing performance in underwater acoustic communications systems

The performance of underwater acoustic communications systems is often characterized as a function of source to receiver range or the received in-band SNR. However, these measures are incomplete with respect to predicting performance. There is ample field data for which performance improved with increasing range or for which data sets with similar SNRs exhibited markedly different performance. Using field and simulation data, the Primary to Intersymbol Interference (ISI) and Noise Ratio (PINR) is shown to be a more complete predictor of performance than SNR. The results also demonstrate the significant adverse effect of ISI on system performance. With this in mind, the optimal configuration in terms of mitigating the impact of ISI is considered for the receive array and processor structure of equalizers. The role of total degrees of freedom (DOFs), equalizer filter adaptation averaging interval, and the relative stability of the channel's spatial and temporal structures is evaluated. The performance gains attainable with array spatial aperture vs the gains attainable with filter temporal depth is analyzed using closed form expressions as well as simulation and field data.