Abstract
Considered here is the use of an M-ary orthogonal spread spectrum signal set for coherent symbol detection at very low signal to noise ratios (SNR). We consider symbols that are convolutionally orthogonal over the entire multipath delay spread of the channel and allow for the minimum mean square error (MMSE) estimation of an observed acoustic response function with minimal computational effort at the receiver. We employ these signals with an assumed sparsity prior to implement a joint symbol and broadband channel estimation scheme for coherent detection without the use of intra-packet training symbols of any kind. The approach allows us to compensate for the shared time varying Doppler process of the various coherent arrivals. The approach is demonstrated with at-sea recordings at extremely low received SNR.