Advanced multi-band modulation technology for underwater communication systems

In recent years, underwater communication (UWC) systems have been attractive in the development of civilian and military applications. However, there are many challenging issues surrounding such communication systems due to severe channel conditions such as ambient noise, frequency selectivity, multi-path with significant tap delays and Doppler shifts. In order to conduct effective ocean exploration, solutions for these problems are desirable to ensure efficient image transmission over an underwater acoustic channel. This thesis investigates and develops signal processing techniques for UWC systems to communicate effectively in an underwater channel. Techniques such as provision of efficient image transmission over an underwater acoustic channel, achieving a low bit error rate, increasing bandwidth and saving transmitted energy are covered. Based on the time reversal technique, an orthogonal frequency division multiplexing (OFDM) based communication scheme for use in underwater acoustic channels was developed. The OFDM guard interval to improve bandwidth and energy efficiency with the aim to increase the underwater network lifetime was adopted and carefully customized. Efficient image transmission for an underwater channel can be considered to be the critical task of next generation underwater acoustic communication systems. In this thesis, a novel rate allocation scheme for efficient transmission of image bit streams in an underwater acoustic channel with optimum bit rate is proposed. Optimality is achieved in the sense that the overall peak signal to noise ratio (PSNR) of the image transmission is maximized under channel bit rate and bit error rate constraints. Based on a modified Set Partitioning in Hierarchical Trees (M-SPIHT) image coder, four different groups of bit-streams are generated based on their significance. The significant bits, the sign bits, the set bits and the refinement bits are transmitted in four different groups with different protection levels in order to reduce the total distortion of the reconstructed image. In addition, the Hierarchical Quadrature Amplitude Modulation (HQAM) is used to provide unequal error protection based on the modulation technique. For efficient underwater communication, the multipath issue must be addressed. To reduce the multipath effect, feedback wave focusing techniques are used, leading to a time reversal process. In such a process, at the very beginning of the communication the transmitter transmits a probe symbol sequence that is recorded by the receiver. The received probe sequence, serving as channel information, is time reversed and transmitted back to the transmitter. In time reversal operations, each multipath signal sequence received convolves with the time-reversed version of its corresponding channel estimate based on this probe signal. Thus, signal focusing can be achieved at the transmitter if the channel does not change significantly. This thesis considers a passive time reversal system in the multipath UWA fading channels with significant delays. By combining the time reversal technique with single-input multiple-output OFDM (SIMO-OFDM), a novel transmission scheme is proposed to transmit progressive image data with progressive protection in the presence of inter-block interferences. The proposed new unequal error protection scheme can significantly improve the peak signal-to-noise ratio of the reconstructed image. In addition, by using the time reversal technique, multiple time-dispersive fading channels can be made impulse-like. As such, a moderated guard interval length can be used without introducing much inter-block interference, even in a UWA with a long tap delay. Based on this, a demonstrably robust performance with significant bandwidth efficiency can be achieved. In addition, the guard interval can be designed shorter than the maximum channel tap delay. In UWA communication, the zero padding OFDM (ZP-OFDM) multicarrier system is preferred for less transmission power consumption in the guard interval because the OFDM system requires a long guard interval to avoid inter-block interference problems. In this thesis, streams of new OFDM schemes for better energy efficiency as well as significant bit error rate improvement are proposed. Based on the preamble signal and a training sequence, a receiver design is proposed for a UWA communication system. The Doppler scaling factor and UWA channel feature are estimated by using the preamble and the training sequence that is used to update the estimated parameter to avoid preamble signal retransmission. Due to underwater channel features such as frequency selective and time varying, the proposed schemes are evaluated via simulated channels as well as real channels measured from sea-going experiments. The results of the study show that the proposed systems outperform conventional OFDM systems in energy and spectrum efficiency as well as the received image quality. In this thesis, efficient image transmissions over an underwater acoustic channel are studied by optimal designs in both the transmitter and the receiver. Improvements of inter-carrier interference reduction methods for multicarrier systems used in underwater communication systems have also been achieved. Improvements are demonstrated in both the image transmission system over the underwater channel and the multicarrier modulation used in underwater communication with data image transmission. As a consequence of this research, up until now four peer-reviewed journal papers and three conference papers have been published.