【作者】 魏蔚；

【导师】 胡晓毅；

【作者基本信息】 厦门大学 ， 通信与信息系统， 2008， 硕士

【摘要】 海洋水声信道是一个复杂的时、空、频变信道。信道带宽窄,多途干扰严重,高噪声和时延扩展大,使得在海洋中实现高速率、可靠的通信成为挑战性的研究课题。而由信道多径传播引起的码间干扰和频率选择性衰落是水声通信面临的主要困难。正交频分复用技术(OFDM)具有良好的抗频率选择性衰落性能和充分的带宽利用率,成为目前水声通信研究的热点。但它也存在不足,对同步误差敏感是影响系统性能的主要因素之一。OFDM技术的高频谱利用率和传输的可靠性均以子载波间的正交性为基础,载波频偏的存在破坏了子载波间的正交性,导致严重的子载波间干扰;符号定时误差又会引起符号间干扰。因此,研究OFDM的同步技术,对符号定时和频率偏移进行有效的估计和补偿具有非常重要的意义,本文主要研究无线中的OFDM时频联合同步算法,通过对算法的分析改进使之能够应用于OFDM水声通信系统中,主要工作概括如下:首先,在综合分析水声信道特性和OFDM技术原理的基础上,介绍了OFDM水声通信系统模型与系统参数的设计,指出OFDM系统的优点和存在的缺点。接着分析了同步误差对系统的影响。详细介绍了基于循环前缀、导频符号和训练序列的OFDM系统符号定时和载波频率联合同步算法,并提出了改进算法。通过仿真,比较分析算法在高斯白噪信道(AWGN)下和多径衰落信道下的同步性能。介绍了符号定时同步参数的选择,并针对水声信道中第一径不是最强径的问题,阐述了第一径检测方法,接着介绍了残余频偏补偿方法。最后进行水池和海洋试验,验证算法在实际水声信道中的性能。结果表明,本文采用的同步算法能够在试验中达到准确同步。更多还原

【Abstract】 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.更多还原