循环后缀在水声时反正交频分复用系统中的应用研究

受声速梯度分布、收发节点相对位置等因素影响,水声信道中存在能量最强的多途信号不是最先到达接收端的情况,此情况下对于正交频分复用(OFDM)通信若单一添加循环前缀作保护间隔,在以接收功率最强信号作定时同步时基时,当前符号内信息会受到下一符号信息的污染,进行快速傅里叶变换(FFT)的数据除了本符号数据外还包括了下一符号的循环前缀数据,从而破坏了子载波间正交性,也引入了符号间干扰,导致通信性能下降。为此提出在原有的OFDM数据结构中添加循环后缀,以保证FFT操作的数据为当前符号内信息,即使定时不精确产生k个采样点的偏差,也只是相位偏转2πnk/N,差分编解码技术即可完成相位补偿和校正,保护了子载波间的正交性;同时提出将循环后缀与时间反转镜技术相结合应用于水声OFDM通信系统中,抑制时反信道主峰两侧旁瓣引起的等效多途影响,使得时间反转镜技术适用于OFDM系统,提高系统性能。理论分析和仿真研究验证了循环后缀应用于水声OFDM通信系统的可行性和有效性。

Application of Cyclic Postfix in the Time Reversed-OFDM-based Underwater Communication

A signal with the strongest energy may arrive at the receiver later than other signals do in the underwater acoustic channel on account of the sound velocity gradient distribution and the relative location between the receiving and sending nodes, etc. If the time-tracking is inexact, using a cyclic prefix only as guard interval may make the data of the current orthogonal frequency division multiplexing (OFDM) symbol for fast Fourier transform (FFT) be polluted by the data from the next symbol, thus destroying the orthogonality between subcarriers. The cyclic postfix is used in the OFDM underwater acoustic communication system to solve the above problems. Meanwhile, the cyclic postfix combines with the time reversal mirror technology for application in the OFDM underwater acoustic communication system to suppress the impact of sidelobe before the main peak of the time reversal channel and improve the channel equalization and the system performance. Theoretical analysis and simulation results show that the use of cyclic postfix in the OFDM underwater acoustic communication system is feasible and effective.