Underwater Acoustic Multipath Channel Characteristics And Anti-multipath Blind Equalization Technology Research

One of the main obstacles to reliable underwater acoustic digital communications is considered to be Inter-Symbol Interference (ISI), which is mainly caused by multipath propagation. Equalization technique is an effective approach to suppress ISI. Underwater acoustic channel is severely band-limited due to high absorption, and conventional adaptive equalizations, which require training sequences transmitted periodically, will reduce the usable bandwidth further. In this case, blind equalization techniques which need not any training sequence are preferred for underwater acoustic communication.The characteristics of the underwater acoustic channel are analyzed in this thesis, with the emphasis on the multipath propagation characteristics. And the underwater acoustic multipath channel model is established on the basis of ray theory. Channel equalization theory is presented in detail, together with adaptive equalization and blind equalization techniques. Then, one of the most important measures on the performance of equalizers is introduced in the thesis.Based on the characteristics of the underwater acoustic channel and channel equalization theory mentioned above, this thesis focuses on the Sato and the CMA blind equalization algorithms, both of which belong to the Bussgang class algorithm. With the work of a lot of computer simulations, the performances of these two algorithms are surveyed from the aspects of equalizer-tap initialization scheme, step-size of the blind algorithms, the order of equalizer, and the signal-to-noise ratio. From simulation results, we get the following main conclusions: 1) the convergence performance can be improved with the Ist-tap initialization strategy for the underwater acoustic channel established in this paper. 2) Both the convergence rate and the steady-state value of the average squared error should be taken into account when choosing the step-size. 3) The lower order equalizer should be selected if the condition of convergence performance can be guaranteed.And the Fractionally Spaced Equalizers (FSE's) are also presented in this thesis through comparing the performances of FSE's and the common Baud spaced equalizers. The computer simulations show that the FSE's are especially suitable to the underwater acoustic channel with low SNRs.