VDE中OFDM信号的同步技术研究

发布时间：2018-03-01 10:26

本文关键词： 正交频分复用同步误差符号定时同步载波频率同步　出处：《天津理工大学》2017年硕士论文　论文类型：学位论文

【摘要】：甚高频数据交换系统(VHF Data Exchange System,VDES)是在船舶自动识别系统(Automatic Identification System,AIS)上发展起来的新一代海事通信系统,可以有效缓解现有AIS的数据通信压力,能够全面提升水上无线数据通信的能力。VDES允许多种编码速率和调制方式,可以在系统规模和业务可用性上提供更多的灵活性。VDES在特定信道内可以采用正交频分复用技术(OFDM)。在VDES中卫星通信的接收信号可能会存在信道传输时延较大、多普勒频偏等问题,这些可能会造成OFDM信号的符号定时偏差和载波频率偏差。本文主要是对OFDM的符号定时同步算法和载波频率同步算法进行研究。文中对经典同步算法进行了仿真分析,针对现有算法的不足之处对OFDM同步技术进行了研究。主要研究内容包括:(1)针对基于循环前缀的最大似然(ML)估计方法在低信噪比和多径信道环境下符号定时不准确和频偏估计的精度低等问题,本文设计了一种由两个相同部分构成的循环前缀结构并引入了一种定时度量函数,去除了符号间干扰(ISI)的影响。该算法和ML算法相比,实现了在低信噪比和多径信道中良好的定时性能,并且提高了频偏估计的均方误差(MSE)性能和估计范围。(2)针对现有OFDM频率同步算法存在的同步精度不高,估计范围较小等问题,本文提出了基于实训练序列的频率同步算法。根据时域训练序列结构的共轭对称性求出小数倍频偏,利用接收端频域训练序列的循环移位估计整数倍频偏。该算法扩大了小数倍频偏估计的范围,小数倍频偏估计在低信噪比下有较好的MSE性能;该算法提高了整数倍频偏的正确检测率。(3)针对现有OFDM同步算法存在的同步精度不高,在低信噪比环境下定时容易出错等问题,本文提出了一种基于训练序列的OFDM时频同步算法。根据时域训练序列结构中存在的共轭特性进行符号定时偏差估计,这种符号定时同步算法去除了循环前缀对定时的影响,实现了准确定时。根据时域训练序列中存在的相位差求出小数倍载波频偏,在频域根据循环移位估计出整数倍频偏。本文频率同步算法的MSE性能良好,整数倍频偏正确检测率较高。
[Abstract]:VHF Data Exchange system VDES is a new generation maritime communication system developed on the automatic Identification system of ship identification, which can effectively relieve the pressure of data communication of the existing AIS. The ability to fully enhance wireless data communications on water. VDES allows for a variety of coding rates and modulation methods. It can provide more flexibility in system size and service availability. VDES can adopt orthogonal frequency division multiplexing (OFDM) technique in a specific channel. In VDES, the received signals of satellite communication may have some problems such as long channel transmission delay, Doppler frequency offset and so on. This paper mainly studies the symbol timing synchronization algorithm and carrier frequency synchronization algorithm of OFDM. Aiming at the shortcomings of the existing algorithms, the OFDM synchronization technology is studied. The main research contents include: 1) aiming at the maximum likelihood (MLL) estimation method based on cyclic prefix, the symbol timing is inaccurate in low SNR and multipath channel environment. Problems such as low accuracy of sum frequency offset estimation, In this paper, a cyclic prefix structure consisting of two identical parts is designed and a timing metric function is introduced to remove the influence of ISI). The algorithm is compared with ML algorithm. Good timing performance in low signal-to-noise ratio and multipath channel is realized, and the mean square error (MSE) performance and estimation range of frequency offset estimation are improved. (2) aiming at the existing OFDM frequency synchronization algorithms, the synchronization accuracy is not high and the estimation range is small, etc. In this paper, a frequency synchronization algorithm based on real training sequence is proposed, and the fractional double frequency offset is obtained according to the conjugate symmetry of the structure of the training sequence in time domain. The integer frequency-doubling offset is estimated by cyclic shift of the training sequence in the frequency domain of the receiving end. The algorithm expands the range of the fractional frequency-doubling offset estimation, and the fractional frequency-doubling offset estimation has better MSE performance under low SNR. This algorithm improves the correct detection rate of integer frequency-doubling offset. Aiming at the existing OFDM synchronization algorithms, the accuracy of synchronization is not high, and the timing is prone to make mistakes in the environment of low signal-to-noise ratio (SNR). In this paper, a OFDM time-frequency synchronization algorithm based on training sequence is proposed. The symbol timing deviation is estimated according to the conjugate characteristics of the time-domain training sequence structure. The symbol timing synchronization algorithm eliminates the influence of cyclic prefix on timing. Accurate timing is achieved. The fractional carrier frequency offset is calculated according to the phase difference in the time domain training sequence, and the integer frequency doubling offset is estimated in the frequency domain according to the cyclic shift. The MSE performance of the frequency synchronization algorithm in this paper is good. The detection rate of integer frequency doubling deviation is high.
【学位授予单位】：天津理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：U675.7;TN929.53

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