摘要
在无线通信系统中,正交频分复用(OFDM)技术已被广为应用,使得系统的信号传输性能有了显著提高。OFDM技术中的一个重要部分就是信道估计。信道估计的目的就是得到一系列的信道参数。自适应调制、分集接收与合并、相干检测和解调等环节都需要清楚的知道这些参数。同时,由于多径信道不可避免的产生码间干扰和符号间干扰,所以接收端的干扰抵消算法也起到相当重要的作用。
本文主要研究OFDM系统中的信道估计和干扰抵消算法。在时不变系统中,通过对系统的模拟与分析,给出信道估计的一般方法。为克服循环前缀引起的信息传递效率的降低,本文提出一种新的无循环前缀(CP)OFDM系统在时不变信道下的信道估计与干扰抵消迭代算法。仿真表明,本算法在提高系统的频谱效率的同时,能够达到较高的估计精度。
在时变系统中,CP虽然可以消除符号间干扰,但已无法消除由于多径效应所带来的码间干扰。本文在时变信道下,提出一种基于最小均方误差(MMSE)估计和迭代算法相结合的均衡方法。为了消除一般MMSE均衡因子中将ICI当作加性高斯白噪声(AWGN)所带来的误差,本文先使用MMSE方法对传输信号进行初值估计,再进行联合干扰抵消和迭代检测。多次仿真表明,在时变多径衰落信道中使用本算法能够有效的消除ICI,使系统性能得到很好的改善。
本文分为5章。第一章主要讨论了宽带系统中OFDM技术的发展以及系统构架,介绍了OFDM技术的基本概念、发展历史、特点及优点。第二章主要讨论了多径信道模型。第三章详细分析了OFDM系统的关键技术。第四章着重讨论OFDM系统在时不变系统中的信道估计与干扰抵消技术。第五章讨论OFDM系统在时变信道下的联合信道估计和干扰抵消问题。
OFDM technique is widely used in wireless communication systems, improving the communication system capability.The channel estimation technique makes a key role in the OFDM techniques.The channel information is needed in signal processing procedures.Meanwhile,the interference cancellation algorithm is adapted to cancel the inter-carrier interference and inter-symbol interference.
This thesis mainly proposes some combined algorithms of channel estimation and interference cancellation.In time invariant system,a combination of channel estimation and ISI cancellation method is proposed for OFDM systems without cyclic prefix(CP).Channel vector of temporal domain,which can be converted to the channel characteristic of frequency domain after FFT,is estimated with the least square(LS) algorithm. Combining detection with ISI cancellation,iterative interference cancellation can be implemented in this paper.Simulation results show that the performance of proposed channel estimation algorithm is more accurate and its BER is close to the OFDM system with CP.
In OFDM systems,a time-varying multi-path fading channel leads to the loss of sub-carrier orthogonality which in turn causes the inter-carrier interference and increases inaccuracies in signal detection.In this paper, an iterative ICI cancellation and MMSE estimation algorithm is proposed. Avoiding the inaccurate assumption in conventional one-tap ICI cancellation that ICI is modeled as AWGN,MMSE principle is used to estimate the initial value of signal,followed by the iterative ICI cancellation and MMSE equalization.Extensive simulations show that a small number of iterations can cancel the ICI effectively,improving the system performance.
引文
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