Coherent Multicarrier Receiver for Mobile Acoustic Channels

High data rate coherent communication in acoustic channels are difficult due to the compound effects of motion-induced Doppler and long multipath. To account for these effects, we consider a coherent receiver based on orthogonal frequency division multiplexing (OFDM) which performs an iterative frequency offset compensation and sparse channel estimation. In mobile acoustic systems, Doppler effect can be severe enough that the OFDM signals experiences non-negligible frequency offsets even after initial resampling. To compensate for these offsets, a practical method based on stochastic gradient is employed. The method relies on differential encoding which keeps the receiver complexity at a minimum and requires only a small pilot overhead. Differential encoding is applied across carriers, promoting the use of a large number of closely spaced carriers within a given bandwidth. This approach simultaneously supports frequency domain coherence and efficient use of bandwidth for achieving high bit rates. After compensating for the frequency offset, a sparse channel estimation method based on a physical model of multipath propagation is used to obtain channel state information for coherent detection. The channel estimation method, referred to as path identification (PI), targets the physical propagation paths in a continuous-delay domain, and focuses on explicit estimation of delays and complex amplitudes of the channel paths in an iterative fashion. Using the experimental data transmitted over a 3–7 km shallow water channel in the 10.5-15.5 kHz acoustic band, we study the receiver performance in terms of data detection mean squared error (MSE) and bit error rate (BER), and show that the proposed receiver provides excellent performance at low computational cost.