Research On Timing And Frequency Synchronization Technology For OFDM Underwater Communication System

Ocean channel is a complex environment that experiences continual varieties in time, space, frequency. Moreover inter-symbol interference and frequency selective fading make high speed reliable communication in underwater acoustic (UWA) channel even more difficult. Possessing the ability to cope with frequency selective fading and high bandwidth utilization efficiency, orthogonal frequency division multiplexing (OFDM) has become an outstanding solution in UWA communication. By using a large number of orthogonal sub-carriers, OFDM improves the robustness and reliability. However, OFDM system is so sensitive to time and frequency offset that worsen the BER performance of system. In other words, frequency offset destroys the orthogonality among the sub-carriers and thus bring severe inter-carrier interference. Meantime, error of synchronization leads to inter-symbol interference. Therefore, it seems extremely critical to OFDM system to estimate and compensate timing and frequency offset. In this case, this paper will focus on synchronization technology of OFDM in UWA communication and be organized as following:Firstly, this paper will introduce characteristics of UWA channel and principles of OFDM system, followed by OFDM UWA communication system model and corresponding parameters. The advantages and disadvantages of OFDM will be discussed respectively. Secondly, the paper analyzes how does synchronization error influence OFDM system, and bring forward combined time and frequency synchronization algorithm based on cyclic prefix, pilot symbol and preamble sequence techniques. In addition, an improved algorithm is introduced. Thirdly, this paper will present the simulation results according to related algorithms under both AWGN channel and multipaths fading channel, compare the synchronization performance and discuss timing synchronization parameter selection. In order to solve the problem of timing synchronization when the first path is not the strongest path, the paper analyses the method to detect the first path. And then introduces a method about residual frequency compensation. At last, the results of experiments carried out in lab testing pool and sea will be presented. As it can be seen, the algorithm adopted in this series of experiments can achieve good synchronization performance in actual UWA channel.