Study For Equalization Of Underwater Acoustic Channel Based On Orthogonal Frequency Division Multiplexing Technrology

In the traditional underwater acoustic communication system, the distortion of the channel as well as the inter-symbol interference (ISI) caused by frequency selective fading is a major factor to affect the communication quality. Usually, equalization techniques are used to compensate for the distortion of the channel as much as possible and restore the original signals transmitted at the receiving. However, with the development of marine science, how to enhance the rate and the stability of the underwater acoustic communication has become an urgent problem for people to solve. Because of the symbol width of the information flow in a single-carrier system relatively short, complex equalizer and complex equalization algorithm in order to achieve a good equalization effect are taken into consideration, which hinders the development of underwater acoustic communication in some extent. Thus, searching for a new way of underwater acoustic communication has become a subject of many scholars. So, as a representative of multi-carrier transmission scheme, orthogonal frequency division multiplexing (OFDM) technology which can provide higher transmission rate and has stronger anti-multipath interference ability has gradually become one of the most promising options in the underwater acoustic communication.Due to the excellent characteristics of OFDM technology, it has gotten attention of scholars. On the basis of previous researches, this paper illustrates the basic structure of OFDM underwater acoustic communication system, and then studies how to further optimize the equalization effects and improve communication quality by combining OFDM underwater acoustic communication system with traditional equalization technologies such as optimized pilot structure, channel estimation, the fractional sampling theory (FST) model, blind equalization technologies and so on. Till now, the following points have been completed:(1) Due to the defects of using too many pilot sequences in the conventional OFDM communication system, least square equalization algorithm for fractionally spaced OFDM communication system jointing with modifying pilot sequences (MPS-FSOFDM-LSEA) is proposed. By combining fractionally spaced theory with OFDM and optimize structure of pilot sequence, simulation experiments of underwater acoustic channel show that the performance of this algorithm is superior to the traditional OFDM underwater acoustic communication system.(2) In order to solve the problem of complex structure of the existing equalizer and less efficiency of constant modulus algorithm (CMA) in traditional single-carrier underwater acoustic communication system, CMA is combined with OFDM technology and phase-locked loop is used to overcome the signals phase rotation. So the single tap constant modulus blind equalization algorithm for OFDM underwater acoustic communication system combining with digital phase-locked loop (ST-CMA+DPLL) can be proposed. Simulation experiments of underwater acoustic channel show that this algorithm can better recover signals with lower steady state error and faster convergence speed.(3) Because of the feature of correcting phase offset to a certain extent of MMA, it is used instead of CMA. Considering that decision directed (DD) algorithm can make full use of the modulus value of the signals, decision directed blind equalization algorithm with multi-modulus algorithm based on OFDM is proposed. Simulation experiments of underwater acoustic channel show that this algorithm has a lower steady state error.(4) Considering that the performance of the traditional MMA will decline a certain degree when QAM signals whose order is higher is dealt with, A weighted multi-modulus blind equalization algorithm of OFDM underwater acoustic system is proposed where weighted factor is to optimize the statistical modulus. Simulation experiments of underwater acoustic channel show that this algorithm can reduce the steady state error and speed up the convergence speed.