【作者】 尼俊红；

【导师】 刘泽民；

【作者基本信息】 北京邮电大学 ， 电路与系统， 2009， 博士

【摘要】 由于OFDM系统具有较高的频带利用率和良好的抗多径衰落性能,可以很好的解决频率选择性信道上的高速数据通信,因而广泛应用于宽带无线通信领域。符号时间的增加,使得OFDM系统具有良好的抗多径性能,但同时也使OFDM系统对于无线移动信道的时变性更加敏感。特别是,多普勒扩展会破坏子载波间的正交性,造成子载波间的功率泄漏,从而产生子载波间干扰(ICI), ICI在严重影响整个系统性能的同时,也给信道估计带来了困难。本文的研究工作主要是围绕时变衰落信道上OFDM系统的信道估计和干扰消除技术展开的,在借鉴前人研究成果的基础上,提出一些新的思路和一些新的算法,并在滤波器组理论框架下对OFDM系统进行了分析。主要的研究成果如下：(1)提出两种适用于近似线性信道(即信道在一个OFDM符号时间内近似线性变化)的OFDM信道和ICI系数估计算法。一种是频域估计方法,另一种是时域估计方法。相比较而言,时域方法需要估计的参数比频域方法的要少,仿真结果表明,当多普勒频移增加时,时域方法具有更好的鲁棒性。(2)对ICI进行了比较详细的分析,推导了Jakes信道模型下ICI功率的上下界,并通过仿真分析了不同信道条件下,ICI的功率、分布特性及对OFDM系统性能的影响。(3)在ICI分析的基础上,对时变信道上的MMSE准则进行了修正,基于修正MMSE准则提出一种并行迭代均衡算法消除ICI。仿真结果表明该算法的性能较普通的PIC算法有明显提高,但收敛速度较慢。(4)提出一种基于LMMSE准则的ICI直接消除算法,该算法对慢速和快速时变信道都适用。文中推导了在时变信道下解的一般形式,以及在Jakes谱瑞利衰落信道上的结果。仿真结果表明这种方法对信道的变化速度有较好的鲁棒性,不足之处是当信道失配时,性能会有比较明显的降低。(5)提出一种基于非最大抽取FIR滤波器组预编码器的盲信道估计和均衡的方法,给出了均衡器的最优解及输出信噪比公式,该方法的最大优点是不受信道零点位置的限制。(6)在多率滤波器组的理论框架下对OFDM系统进行分析,给出OFDM系统在单用户和多用户情况下,基于滤波器组传送多路复用器结构的统一模型。更多还原

【Abstract】 OFDM system is widely applied in wireless broadband communications as it has high frequency efficiency and good performance over frequency-selective channels.The increasing of symbol duration makes OFDM more robust against multipath transmission, whereas, it makes OFDM more sensitive to the time varying of wireless mobile channels. Doppler frequency expansion due to the channel variation destroys the orthogonality among different subcarriers, causes power leakage and introduces intercarrier interference (ICI). The ICI degrades the performance of OFDM systems and makes channel estimation more challenging as well.The objective of this thesis is to propose new solutions and algorithms related on OFDM channel estimation and interference cancellation techniques over time-varying channels based on the literatures, and analyze OFDM system based on the framework of filterbank theory.The contributions of this thesis are as follows:(1) Propose two algorithms for channel and ICI coefficients estimation in OFDM system over approximately linear channel (channel characteristics in time-domain vary in an approximately linear way during one OFDM symbol time), one is in frequency-domain, the other is in time-domain. The parameters need to estimate in time-domain method are less than those in frequency-domain method. The simulation results show that the time-domain method is more robust when Doppler frequency is high.(2) Study the ICI in detail. Derive the upper and lower bounds of ICI power on Jakes channel model, analyze the effects of the ICI power distribution on the performance of OFDM system under different channel conditions with MATLAB simulations.(3) With the ICI analysis, Modify the MMSE criterion over time-varying channel and propose a novel parallel iterative equalization method to mitigate the ICI. Simulation results demonstrate that the proposed method outperforms the traditional parallel iterative cancellation method in BER performance, but has relatively slower convergence speed.(4) Propose a direct ICI mitigation solution based on the LMMSE criterion which is both applicable over the channels varying in slow speed and in high speed. We derive the general formulation on time-varying channel, and the special case on Jakes channel as well. Simulation results illustrate that this solution has robustness against the variation speed. But if there is a mismatch of channel model, the performance will degrade.(5) Propose a blind and equalization method based on a nonmaximally decimated filterbank precoder and give the expressions of the optimal equalization solution and the SNR on the output end of the channel. The great strength of this method is that it can estimate unknown FIR channels without limitations of the location of zeros.(6) Analyze the OFDM system under the framework of filterbank theory. Present a general system model for OFDM in both single user and multiuser scenarios based on the filterbank-constructed transmultipexer structures.更多还原