通信系统中多普勒频移估计的研究

【作者】 王国平；

【导师】 杨晓波；

【作者基本信息】 电子科技大学 ， 电路与系统， 2008， 硕士

【摘要】 在无线数据传输环境中,随着使用频率的不断上升和收发终端相对径向运动速率的增加,将导致多普勒频移对系统性能产生极大的影响,所以须对多普勒频移进行研究,以从接收信号中估计出以去除其对系统接收性能的影响。传统的锁相环等捕获跟踪环路,跟踪精度较高,但捕获范围有限,当接收信号中残留的频偏较大时,可能将造成捕获时间过长而无法满足实时应用的要求。开环频偏估计应用了信号估计理论,捕获范围大,甚至可以迅速跟上频偏的变化,本文主要针对开环频偏估计算法进行研究和仿真。本文首先研究了多普勒频移对扩频通信系统的同步性能以及OFDM系统的同步性能的影响。然后针对QPSK信号研究了一种多普勒频偏估计方法。接下来针对扩频通信中的码相位差捕获和频偏估计而进行的二维搜索方式进行了研究,并通过联合的匹配滤波相干检测以使可以采用更少端口数的FFT单元,从而降低系统实现的复杂度,文中对这种联合相干检测所导致的幅度输出衰减进行了研究和仿真。最后,文中对一种近似最大似然估计方法,一种数据辅助频偏估计方法,一种加权相位平均估计方法进行了研究和仿真,通过仿真分析可以看到,该近似最大似然估计方法的频偏估计范围和其采用的最大相关长度成反比,这样将从一定程度上限制该方法的应用,而加权相位平均估计器可以在较高的信噪比情况下获得很好的估计性能,并且其实现的复杂度最低,但其具有较高的门限效应,即信噪比低于8dB左右时,估计的性能急剧恶化,文中针对该情况,在该估计方法的基础上,首先采用一种非线性滤波的方法来得到关于非线性变化的相位的估计,然后通过一改进的Kay估计公式以得到最终的频偏估计结果,仿真结果表明,该方法有效地降低了原来的门限效应,从而使其可以应用于更宽的信噪比范围。文中在最后分别针对不同信噪比情况和不同的观测长度给出了各种算法的性能对比结果,为实际应用中估计方法的选择以及观测长度的选择提供有效地参考。更多还原

【Abstract】 There is a serious decease in the performance because of the doppler frequency shift,caused by the relative motion between the transmitter and receiver,under the wireless communication environments,especially with the gradually increase of the applied carrier frequency.Therefore,it is necessary to estimate the doppler frequency shift,and eliminate it from the received sinals.The conventional phase lock loop has a good frequency estimation resolution,however,it has the limited acquisition range,when the residual doppler shift in the received signal exceed the acquisition range of the closed loop,it will be lead to too large acquisition time to applied to the practical application. The frequency offset estimation algorithm of non-closed loop,adopting the modern signal estimation theoretics,has large acquisition range and can rapidly track the change of frequency offset.Some of these typical estimation algorithm will be studied and simulated in this paper.In this paper,firstly,the influence of the Doppler shift on the spread-spectrum communication system and the orthogonal frequency division multiplexing technology have been studied.Then,some of the basic theory of the frequency estimation have been introduced to this paper,and a frequency offset estimation method of the residual doppler shift in the received QPSK signal has been researched.Finally,several estimation algorithm based on the different estimation rule have been researched,include the two-dimensional search algorithm,a approximate maximum likelihood esimation algorithm,the weighted phase average estimator,the data-aided frequency offset estimation algorithm.Furthermore,the phase estimator based on the stochastic nonlinear filter which has been applied to improved the performance of the weighted phase average estimator. In the mean time,computer simulation has applied to the various estimation algorithms.更多还原