【作者】 宋晓晋；

【导师】 宋铁成；

【作者基本信息】 东南大学 ， 通信与信息系统， 2006， 硕士

【摘要】 正交频分复用(OFDM, Orthogonal Frequency Division Multiplexing)技术具备频谱利用率高、抗干扰能力强和带宽扩展性好等一系列特点,已经广泛地被无线局域网标准采用。在接收端,无论是OFDM的相干解调还是接收信号的合并处理,都需要了解信道的信息,因此对于相应系统中的信道估计技术的研究十分有意义。本学位论文将重点研究OFDM无线局域网(WLAN,Wireless Local Area Network)信道估计方法。论文首先分析了无线衰落信道特性,讨论了多普勒频移对传输信号的影响。在此基础上,介绍了IEEE 802.11a/g的几种调制技术,并对OFDM的系统模型进行了分析,讨论了实际的应用环境下OFDM系统物理层参数的设计方法。论文简要阐述了IEEE 802.11a帧头格式,分析了IEEE 802.11a的同步和信道估计方案,最后研究并提出了一种适用于终端移动的OFDM无线局域网的信道估计方法。采用卡尔曼滤波算法进行信道估计,利用导频进行信道跟踪。将基于训练序列的信道估计结果作为Kalman滤波器的初始值和观测值。用基于导频的信道估计结果来计算滤波器参数,并进一步跟踪信道时间上的变化。仿真结果表明该算法比基于导频的信道估计算法和基于训练序列的信道估计算法效果明显提高。针对B3G(Beyond 3 Generation)室外环境的大时延扩展和时间频率选择性衰落,提出了适用于较大载波数的OFDM系统的一种基于导频辅助的低维KALMAN滤波的信道估计算法。利用优化的导频序列,采用低维的KALMAN滤波方法进行信道估计,其中,时变的滤波器衰减因子采用最小平方(LS, Least Square)算法跟踪。然后,利用基于最小均方误差(MMSE, Minimum Mean Square Error)准则的合并器对KALMAN滤波估计结果进一步修正,从而有效地跟踪信道频域和时域的动态变化。在5.5GHz频段上的仿真表明了这种基于导频插入的修正KALMAN滤波的信道估计算法可以有效提高系统抗噪声性能。更多还原

【Abstract】 Orthogonal Frequency Division Multiplexing (OFDM) transmission over mobile communications channels can alleviate the problem of multipath propagation, and has high bandwidth efficiency. OFDM has been widely used in Wireless Local Area Network (WLAN) and is proposed to be the next generation mobile communication system. However, either the coherent demodulation of OFDM or the combiner at the receiver needs information of the channel. So it’s very important to study the channel estimation methods. The channel estimation methods in OFDM WLAN systems are studied in this thesis.Under analyzing several characteristics of frequency-selective fast fading channels, such as large Doppler spread and multi-path interference, we study the basic principles of WLAN and OFDM matrix representation and the necessary elements for constructing an OFDM system and thus analyze some non-ideal effects on the performance of OFDM digital communications, in order to choose optimal parameter of OFDM systems in pratical environment.The implementation for carrier frequency estimation and synchronization and channel estimation of OFDM WLAN systems are discussed. Then a channel estimation method based on vector KALMAN filter and pilot tracking for OFDM WLAN systems with mobile terminals is proposed. The result of training sequence estimation is used as the initialization and observationof KALMAN filter. Simulation results show that this combined estimation method makes use of the characteristics of IEEE 802.11a frame format and has good Symbol Error Rate (SER) performance. Based on the research and simulation of OFDM system and the existing channel estimation methods, a low-dimensional Kalman filter method based on pilot signals is presented for the channel estimation of OFDM systems with large number of carriers in Beyond 3 Generation (B3G) outdoor environment. For simplicity, a one-order AR process is used to model the time-varying channel and the LS algorithm based on pilot signals is adopted to track the time-varying channel fading factor a. In order to utilize the relationship of fading channel in frequency domain, a Minimum Mean Square Error (MMSE) combiner is used to refine the estimation results. The simulation results show that the proposed method achieves a good SER performance close to the theoretic bound of ideal channel estimation.更多还原