Space-frequency coded OFDM for underwater acoustic communications

This work proposes an adaptive receiver for the space-frequency block coding scheme, in which Alamouti coding is applied over the carriers of an OFDM system with the aim of obtaining transmit diversity in an underwater acoustic channel. The receiver relies on the assumption that the channel transfer function does not change much over two consecutive carriers, and that the channel is slowly time-varying. Building on these assumptions we propose an adaptive channel estimation method based on Doppler prediction and time-smoothing, which effectively reduces the pilot overhead. System performance is demonstrated using real data transmitted over a shallow water channel in the 10-15 kHz acoustic band from a vehicle moving at 0.5-2 m/s, using QPSK and a varying number of carriers ranging from 64 to 1024. Results demonstrate a uniform gain over the single-transmitter case.