【作者】 张文娟；

【导师】 陈庆春；

【作者基本信息】 西南交通大学 ， 通信与信息系统， 2010， 硕士

【摘要】 在数字通信系统中,由于接收机振荡器的不稳定性和多普勒频移等因素,使得接收信号的载波与发送信号载波之间存在频率偏差,经过相干解调下变频后得到的基带信号中通常含有载波频率偏移。如何在移动通信环境下实现高效、准确的频偏估计,是现代移动通信研究中的一个热点问题,也是论文研究的主要目标。频偏的估计在突发模式传输通信系统的相干检测中起着关键作用,估计的准确性直接影响到接收机的性能,对频偏的补偿校正有利于系统性能的提升。本文首先介绍了频偏的概念和产生频偏的两个原因、频偏估计的原理以及频偏估计的重要性,接着结合TD-SCDMA标准和LTE标准介绍了现代通信中用于进行频偏估计和信道估计的两种标准无线帧的导频结构,同时讨论了频偏估计的性能评价指标并推导衡量频偏估计准确度的克拉美罗界(CRB)。论文对比几种应用较广泛的有数据辅助的经典频偏估计方法,理论分析和仿真比较这些估计方法的适用性,包括估计的准确度和估计范围、计算量等,并给出了估计中参数的最佳值选取原则。在此基础上,分析了盲频偏估计原理,针对MPSK调制的特点,可以采用M次幂的方法去掉调制信息,结合Kim频偏估计方法和常用的频偏估计方法,提出一种可以降低信噪比门限值又不失估计准确度的盲频偏估计方法,这种估计方法计算量小,可以应用于中低信噪比的通信环境中。最后论文讨论了衰落信道条件下的频偏估计问题,包括平坦衰落信道和频率选择性衰落信道。对于平坦衰落情况,由于存在乘性的时变衰落影响,使得可以应用在AW GN信道的频偏估计方法不再适用。有关KF和MKF算法、uMMV和NLS估计算法以及Simplified NLS算法等估计算法的分析和比较表明,这些估计方法的性能都不太理想,与平坦衰落信道的CRB有很大的差距。本文提出一种基于信道估计即消除衰落的频偏估计方法,该方法首先利用SimplifiedNLS估计方法对频偏进行初始估计,并用估计值求出时变衰落,进而补偿平坦衰落,将衰落信道下的频偏估计问题转化在AW GN信道下完成。这种方法可以得到一个突破平坦衰落信道下CRB的频偏估计,仿真表明,这种方法的均方误差可以接近AW GN下的CRB。更多还原

【Abstract】 Frequency offset between the received carrier and local reference at the receiver may occur due to Doppler shifts and/or oscillator instabilities after down conversion in digital communication system. The primary goal of this paper is to find efficient and precise methods to estimate the frequency offset, which is also a pre-requisite problem in wireless communication research. The frequency offset estimation is one of the key synchronization technologies in coherent detection in burst mode transmission. Since the precision of frequency offset estimation will directly affect the performance of receivers, it is important to investigate the frequency offset scheme with improved accuracy. And this is exactly the research motivation of this thesis.Firstly, the fundamental knowledge of frequency offset and those possible sources resulting in the frequency offset are reviewed. Meanwhile, the principle and the significance of estimation frequency offset are presented. Then the details of two kinds of pilot format in TD-SCDMA and LTE standards are introduced to highlight the pilot signal design utilized for both frequency offset and channel estimation in practical burst mode wireless communication system. Also, the performance metric as well as the theorectical Cramer-Rao low bound are presented for the comparision of different frequency offset estimation schemes. Some classic algorithms for frequency offset estimation over AWGN channel are presented and compared through simulations to show their applicabilities in terms of estimation range, estimation accuracy and calculation complexity. At the same time, the pilot signal design and its influence on the achieved frequency offset estimation performance are addressed and it is disclosed that the pilot placement scheme should be taken into consideration to achieve the optimal estimation performance. After a brief survey of the existing blind frequency offset estimation scheme, the blind frequency offset estimation problems for the MPSK signals are investigated. Basically, the M-power based nonlinear transformation could be utilized to eliminate the effect of the unknown data symbols on the frequency offset estimate. By combining Kim’s algorithm with the existing estimation methods over AWGN channel, a blind frequency offset estimation scheme is proposed in this thesis. It is shown that the proposed blind frequency offset estimation scheme is able to achieve the reasonable accuracy close to the Cramer-Rao bound for a SNR as low as 0 dB. The improved estimation accuracy, a lower SNR threshold, together with the less computational complexity involved make the proposed scheme appealing for medium/low SNR communication systems.Finally, the frequency offset estimation problem over the flat-fading and the frequency-selective fading channel are discussed. Different from the frequency offset estimation problem over the AWGN channel, now the received signals become time varying. Basically, the algorithms suitable for the AWGN channel are no longer suitable for the estimate problem over the fading channel. Some of the mostly commonly utilized frequency offset estimation algorithm for the flat fading channel, such as the KF, MKF, uMMV, NLS and Simplified NLS algorithms are reviewed and compared through simulations as well. And It is show that the achieved estimation performance for all of these algorithms are not good enough to approach the CRB over the flat-fading channel. An estimation scheme based on channel estimation and compensation over flat-fading channel is proposed to enable the following two step estimation scheme. Firstly, using the Simplified NLS method to estimate the initial frequency offset. Secondly, the fading distortion is evaluated and compensated to transform the frequency estimation issue over the AWGN. Simulation results show that the proposed scheme can approach the CRB for AWGN channel.更多还原