The effect of Doppler shift/spread on the performance of the phased locked loop for phase coherent underwater acoustic communications

Abstract

Phase coherent acoustic communication systems, which employ joint adaptive channel equalization and phase-locking synchronization, have been shown to yield a much higher symbol rate (e.g., 1–2 kb/s at 2–5 kHz) by successfully tracking and updating time varying acoustic channels dominated by multipath components. While the decision feedback equalizer (DSE) can in principle adapt to the change of both the signal amplitude and phase, the adaptation rate (requiring typically 20–30 symbols) is in practice too slow compared with the phase rate, hence an additional phase locked loop (PLL) is required. However, the PLL has its limitations. It is found in data analysis that the failure of the PLL is responsible for the high bit error rate (BER) for many situations. This paper illustrates the effect of the phase shift/fluctuations (emulating the Doppler shift/spread) on the performance of PLL and BER, keeping the channel impulse response fixed in time. It is shown with at-sea data that the BER is significantly reduced by removing the phase wander after precise Doppler estimation. The residue phase is apparently below the tolerance threshold as determined by simulations. [Work supported by ONR.]