A receiver structure for coherent reception of M-ary orthogonal spread spectrum acoustic communications at very low SNR in shallow water environments

A receiver structure is proposed for M-ary orthogonal spread spectrum signaling that is effective at very low signal to noise ratio and without the aid of pilot symbols. The receiver models the coherent multi-path arrival structure with a motion dilation compensation scheme that takes advantage of the estimated coherent acoustic response function. To accommodate the rapid time varying behavior of the acoustic channel that is induced by platform motion and boundary interactions a hierarchical Gaussian mixture model based adaptive filtering scheme is presented. The mixture assignment over delay-Doppler-beam is used to accurately capture the sparse coherent features of the arrival structure. The correlated path dilation processes attendant with at-depth motion through a waveguide with small angle spreads are estimated from the coherent acoustic response. The mixture model based estimator replaces conventional time recursive Kalman-like schemes for channel estimation and takes advantage of the entire data set to make efficient estimation of the channel response at each symboling instant. This estimate of the response serves to ameliorate the coherence degrading effects of bulk platform motion replacing conventional phase locked loops for symbol timing and time dilation compensation. This allows the receiver to perform coherent symbol decisions against the natural coherence of the acoustic waveguide. This receiver was tested in shallow water ocean environments in Buzzards Bay MA. Empirical bit error rates are demonstrated at very low SNRs for coherent and non-coherent symbold decisions. Tests were conducted at various spreading gains and bandwidths to achieve rates up to 130 bps at 2km with a demonstrated probability of bit error of less than 10^-4 with 3 element combining at SNRs less than -20 dB. [This work is funded by the Naval Innovative Science and Engineering Basic and Applied Research Program at SSC-Pacific as well as the Office of Naval Research].