In underwater acoustic (UWA) communications, an emerging research area is the high data rate and robust transmission using multi-carrier modulation, such as orthogonal frequency-division multiplexing (OFDM). However, difficulties in the OFDM communications include Doppler estimation/compensation, beamforming, and channel estimation/equalization. In this thesis, to overcome these difficulties, advanced low complexity OFDM receivers of high performance are developed. A novel low complexity Doppler estimation method based on computing multi-channel autocorrelation is proposed, which provides accurate Doppler estimates. In simulations and sea trials with guard-free OFDM signal transmission, this method outperforms conventional single-channel autocorrelation method, and shows a less complexity than the method based on computing the cross-ambiguity function between the received and pilot signals with a comparable performance. Space-time clustering in UWA channels is investigated, and a low complexity multi-antenna receiver including a beamformer that exploits this channel property is proposed. Various space-time processing techniques are investigated and compared, and the results show that the space-time clustering demonstrates the best performance. Direction of arrival (DOA) fluctuations in time-varying UWA channels are investigated, and a further developed beamforming technique with DOA tracking is proposed. In simulation and sea trials, this beamforming is compared with the beamforming without DOA tracking. The results show that the tracking beamforming demonstrates a better performance. For the channel estimation, two low complexity sparse recursive least squares adaptive filters, based on diagonal loading and homotopy, are presented. In two different UWA communication systems, the two filters are investigated and compared with various existing adaptive filters, and demonstrate better performance. For the simulations, the Waymark baseband UWA channel model is used, to simulate the virtual signal transmission in time-varying UWA channels. This model is modified from the previous computationally efficient Waymark passband model, improving the computational efficiency further.
