Performance limitations of joint adaptive channel equalizer and phase locking loop in random oceans: initial test with data

Performance of phase coherent communications depends on the temporal and spatial variability of the underwater acoustic channel as well as the noise environment. Temporal fluctuations of broadband signals need to be characterized from ocean data and appropriate measures of the fluctuation statistics need to be identified. Lacking such measurements, a simulation model was developed previously to evaluate the performance of a joint adaptive channel equalizer and the phase locking loop algorithm in a realistic model that includes the effects of surface waves, bottom roughness, internal waves and some form of turbulence on acoustic communications in littoral environments. Quantitative limits were established for the signal coherence and phase fluctuation spectral density for a typical shallow water environment, beyond which the algorithm is expected to fail to adequately remove the symbol interference resulting in a large bit error rate. This paper reports initial analysis results of at-sea data that lend support to the performance predictions. It is shown that a significant reduction in bit error rate can be achieved by removing the phase fluctuation due to the source Doppler in a pre-processor prior to the equalizer. Without the pre-processor, multiple (2-4) channels are required to equalize the data; even then the equalizer fails after 5 sec. With the pre-processor, a single channel equalizer recovers the original message for the entire 10 seconds of data transmitted