【作者】 梁婷；

【导师】 王玲；

【作者基本信息】 湖南大学 ， 电路与系统， 2007， 硕士

【摘要】 未来无线移动通信系统的目标是实现随时无缝的高质量、高速率的移动多媒体和数据业务传输。OFDM能有效地抗多径衰落,具有非常高的频带利用率,适于宽带移动通信,是下一代移动通信系统中实现高速移动通信的最有前途的技术。但OFDM系统中的信道具有频率选择性和时变特性,要在接收端及时的获得信道状态信息,实现实时而准确的相干解调,必须对信道进行动态的估计,本论文主要对OFDM系统中的信道估计进行研究。首先,论文描述了一般无线信道的多径时变统计特性,分析了广义平稳非相关散射(WSSUS)假设下的信道模型,针对OFDM无线传输系统,分析了信道特性尤其是多径和多普勒频移对系统性能的影响。并建立了无线OFDM系统仿真中常用的多径移动衰落信道的模型。其次,阐述了OFDM系统基本原理,并着重研究了OFDM信道估计技术。给出了在该系统中已经采用的最小二乘(LS,Least Square)估计和最小均方误差(LMMSE,Minimum Mean Square Error)估计及其改进算法。介绍了基于导频的信道估计中采用的各种插值算法,并分别进行了计算机仿真和性能的比较与分析。第三,论文还重点研究和讨论时变信道下的信道估计,利用卡尔曼滤波完成对时变信道的跟踪。在OFDM系统中,信道的时变性使系统子载波之间的正交性遭到破坏,导致子信道间干扰(ICI)。对此,论文提出了基于梳状导频的卡尔曼滤波信道估计算法和基于判决反馈的卡尔曼滤波信道估计算法,对时变信道进行估计和跟踪。其中,对基于梳状导频的卡尔曼滤波估计算法,还将利用载波间的信道频域相关函数和卡尔曼滤波的估计误差的协方差矩阵,对滤波结果在频域进行最小均方误差(MMSE)意义上的估计,使估计精度得到进一步提高。仿真结果表明:在多普勒频移较大的时变信道下,这两种方法能够克服通常算法在高信噪比下出现的地板效应,更好的对时变信道进行估计和跟踪。更多还原

【Abstract】 The future wireless mobile communication systems are expected to provide ubiquitous, high-quality, and high-rate mobile multimedia transmission. Orthogonal Frequency Division Multiplexing (OFDM) is the most promising technology in mobile communication in 4G because of its excellent bandwidth efficiency and robustness against multipath fading and impulse noise. The radio channel is frequency selective and time-varying in OFDM system. In order to obtain the real-time and precise channel state information to complete accurate coherent demodulation, a dynamic estimation of channel is necessary. The dissertation will focus on channel estimation of OFDM system.Firstly, in this dissertation, the statistic properties of multipath and time-varying in general wireless channel is described, and the channel model based on WSSUS (Wide-sense stationary uncorrelated scattering) hypothesis is analyzed. Channel characteristics of OFDM wireless transmission, especially the effect of multipath and Doppler frequency to system performance, are also analyzed. In the mean time, a general wireless channel model with both effects in simulations of OFDM system is constructed.Secondly, in the dissertation, the basic theory of OFDM system is described and the channel estimation technology is researched. Some classical estimation methods, such as Least square and Minimum mean square error, are introduced and then some improvements are proposed. The interpolation methods based on comb-type pilot arrangement are also described. All of these methods are simulated in MATLAB. And the performances are analyzed and compared.Thirdly, this dissertation researches channel estimation in time-varying channel and complete channel estimation in time and frequency-selective fading environment using kalman filter. In OFDM systems, time-varying channels destroy the orthogonality between subchannels and cause interchannel interference (ICI). In order to get right estimation result, a kalman filter based on comb-type pilot signal and a kalman filter based on decision-direct method are proposed for channel estimation. Meanwhile, in the method of kalman filter based on comb-type pilot signal the result is refined using frequency-domain correlation and covariance matrix of estimation error in minimum mean-squared error sense. The simulation results show that both channel estimation methods can achieve good performance.更多还原