Research On SDF And CHF Equalization Techniques In Underwater Acoustic DS-CDMA Communication System

Underwater acoustic channel is one of the most challenging channels for its space-time-frequency varying and frequency-selective characters. These features serve as the major obstacles to high-speed and reliable underwater communication. In phase coherent communication systems, which are usually bandwidth effective, it is a leading method to adopt adaptive equalization to resist ISI induced by multipath spread. Coupled with the phase-locked loop (PLL) and Doppler frequency shift compensation techniques, the equalizer can track time-varying channels more effectively. Hence, the receiver's ability to compensate the distortion due to Doppler shift may be improved, which allows an advantage of data transmission between underwater moving platforms.In recent years, due to the demands of remote controlling and measuring of torpedoes and other underwater vehicles, spread spectrum communications, especially direct sequence spread spectrum (DSSS) technique, has been researched a lot in underwater acoustic channels. The key idea is to make sure the bandwidth of transmitted signal is much larger than the information bandwidth in order to obtain the gain on SNR, which makes it possible for reliable data transmission in the case of low SNR. In the DSSS system, because of the adoption of correlative receiver, the ISI could be suppressed effectively as long as the multipath propagation delay is greater than the chip interval. On the other hand, it is a promising scheme to integrate the above adaptive equalizer into the DSSS receiver, which is called the symbol decision feedback (SDF) receiver. That will be of great advantage to not only suppress ISI but also compensate Doppler shift in frequency-selective channels. Further more, it is of great potential to update the coefficients at the chip rate, which enables the receiver do better in tracking the variation of signal phase in fast time-varying channels. That is named the chip hypothesis feedback (CHF) receiver.In this thesis, applications of both the SDF and CHF equalizers were mainly researched in underwater acoustic DS-CDMA communication systems. The main contents of this thesis are as follow:1. The selection of spreading code type, frame synchronization and Doppler estimation/compensation. In this thesis, the m sequences, Gold sequences and Kasami sequences were respectively considered. Then the Barker code, LFM signal and several other methods were compared, through which the best sync mode was selected. As for the Doppler estimation, the CW pulse algorithm, the LFM block algorithm and the LFM pulse pairing algorithm were analyzed and compared. And the method which did well in the case of low SNR was preferred.2. The performance research of both the SDF and CHF receivers. Firstly, the theories of SDF and CHF were analyzed and discussed. Consequently, the model of underter acoustic DS-CDMA was simulated. At last, the simulation study and comparison between the two types of receivers were completed in several aspects including the influence of spreading code period, the ability of tracking time-varying channels, the effect of multiuser interference, the requirement of input SNR, the selection of receiver parameters, etc.3. The tank experiment. The feasibility of SDF and CHF equalization algorithms for the underwater acoustic DS-CDMA communication system was demonstrated. Besides, the performance comparison between the SDF and CHF receivers was completed.