Using spatial coherence to improve the performance of underwater acoustic communications

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Phase-shifted keying modulation schemes offer an efficient use of signal power for a given bandwidth in a communication channel. Successful communication often requires multichannel data (spatial diversity) to combat the signal fading due to random fluctuations in the channel. For each channel, a signal-to-noise ratio (SNR) of >10 dB is often required to estimate the channel impulse response for proper placement of the tap coefficients. Array beamforming can be used to improve the SNR, but when to use spatial diversity or beamforming is an issue long debated but not yet satisfactorily answered. It is shown with at-sea data that an effective use of beamforming versus spatial diversity depends on the phone spacing relative to the signal coherence. Beamforming requires the signals to be correlated between the sensors, whereas spatial diversity requires the opposite. Neither can fulfill the job of the other. By choosing phones according to spatial coherence and combining beamforming with diversity, bit error rates of the order of ∼0.001 were demonstrated using Advent 99 data over a period of ∼ 9 hours with an SNR as low as ∼1 dB at the phone level. [This work is supported by the Office of Naval Research and SACLANTCEN.]