Space-time signal processing of OFDM signals in fast-varying underwater acoustic channel

High data rate communications over underwater acoustic channel is challenging due to time-varying multipath propagation. The orthogonal frequency-division multiplexing (OFDM) is a promising technique for increasing the transmission data rate. However, this requires accurate channel estimation; this can be achieved by using pilot signals superimposed with the data signals. Space-time processing of signals received by an antenna array allows reducing the intersymbol interference (ISI) due to multipath propagation. Several space-time processing techniques are investigated in application to OFDM signals transmitted by a fast moving omnidirectional transducer. A linear vertical antenna array of omnidirectional elements is used for receiving the signals. Three direction-of-arrival (DoA) estimation techniques are considered. The techniques use signal interpolation, Capon beamforming, and the superimposed pilot signal. The DoA estimates are used for angle-separation of signals by applying a modified Capon beamforming. After time synchronisation, Doppler compensation, and channel estimation, a frequency-domain linear equaliser is applied to signals selected according to the DoA information. The equalised OFDM signals from different directions are linearly combined, and an adaptive Doppler filter (ADF) is used in the frequency domain for removing a residual ICI. An experimental trial has been carried out in the Pacific Ocean at a distance of 30 km. The transmission data rates were 0.5 bit/s/Hz and 1 bit/s/Hz; the results obtained show that for these scenarios, the uncoded OFDM transmission can provide the bit-error-rate of 10-3 and 10-2, respectively.