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新型OFDM系统中信道估计与同步技术研究
【作者】 杨刚；
【导师】 李少谦；
【作者基本信息】 电子科技大学 ， 通信与信息系统， 2011， 硕士
【摘要】 虽然基于多载波调制的正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)技术已成为未来陆地无线通信系统(IMT-Advanced)的核心无线传输技术,但困扰OFDM系统的子载波间干扰(Inter-Carrier Interference, ICI)问题,以及为消除符号间干扰(Inter-Symbol Interference,ISI)引入循环前缀(Cyclic Prefix, CP)而导致的频谱效率下降问题,是限制OFDM系统传输可靠性与效率的两个关键因素。针对该问题,本文研究了基于交错正交幅度调制的OFDM系统(OFDM/Offset QAM, OFDM/OQAM)和基于协同通信方式的OFDM系统。通过引入优化设计的成形滤波器函数,并利用其良好的时域和频域聚焦特性,OFDM/OQAM系统不再需要传统OFDM系统中的循环前缀,从而提高了频谱利用率。但是,如何设计OFDM/OQAM系统的导频结构,并实现可靠的信道估计与同步,是其中的关键问题之一。另一方面,协同通信(Cooperative Communications)可使得单天线移动终端在多中继或多用户环境下,通过共享自身天线的方式,构成能获得空间分集增益的虚拟多输入多输出(Multi-Input Multi-Output,MIMO)系统。而应用协同通信于OFDM系统中的一个关键问题则是如何完成信道估计与同步。基于上述原因,本文主要研究了上述两种OFDM系统的信道估计和同步问题。在OFDM/OQAM系统中,本文研究了一种基于干扰消除的信道估计方法和两种高效的同步算法;在协同OFDM系统中,本文则提出了一种基于交织频带映射的新型同步技术。第二章研究了OFDM/OQAM系统结构及特点。首先在概述传统OFDM系统特点的基础上,研究了OFDM/OQAM系统的信号模型和快速实现结构。然后,以扩展高斯函数(Extended Gauss Function, EGF)作为成形原型函数,从原型函数时频聚焦特性的角度,研究OFDM/OQAM系统抵抗时间-频率双选择性衰落的能力。第三章讨论了OFDM/OQAM系统的信道估计和同步技术。首先阐述了无线衰落信道模型及其特点,并描述了传统OFDM系统中基于导频的信道估计方法。其次,在分析OFDM/OQAM系统导频信号干扰来源的基础上,研究了基于干扰消除的导频设计和相应的估计算法。然后,研究了OFDM/OQAM系统的时间和频率同步技术,并提出两种分别基于频域和时域同步导频的高效同步算法。最后通过计算机仿真,分析验证了本章信道估计和同步方案的性能。第四章研究了协同OFDM系统的信道估计和同步技术。首先概述了协同OFDM系统的结构和信号模型。其次对比研究了基于导频的信道估计方法。然后,首次提出了基于交织频带映射的同步导频结构设计和相应的同步算法。最后通过计算机仿真验证,对本章的信道估计及同步方法的性能进行分析研究。本文为上述两种新型OFDM系统提供了有效的信道估计和同步方案,进一步提高了OFDM系统的频谱效率和传输可靠性,可作为IMT- Advanced等下一代无线通信标准的候选技术方案。
【Abstract】 Although multiple-carrier-modulation-based orthogonal frequency division multiplexing (OFDM) has been adopted as one core wireless-transmission technology in next-generation terrestrial wireless communication system (IMT-Advanced), the inter-carrier interference (ICI) and the descended spectrum efficiency resulted from cyclic prefix (CP) aiming to combat inter-symbol interference (ISI), are two crucial factors for limiting the transmission reliability and spectrum efficiency of OFDM systems. As to the above two problems, this dissertation investigates in two novel OFDM systems, the offset-quadrature-amplitude-modulation (OQAM)-based OFDM (OFDM/OQAM) system and cooperative OFDM system.Through introducing optimal prototype function of pulse-shaping filter and utilizing its well time-domain and frequency-domain localization property, the OFDM/OQAM systems can improve the spectrum efficiency, by discarding the cyclic prefix required in conventional OFDM systems. However, how to design the optimal preamble structure to achieve reliable channel estimation and synchronization is a key issue in OFDM/OQAM systems.On the other hand, cooperative communications achieve space-diversity gain, in which mobile terminal deployed with single antenna can form vitual multi-input-multi-output (MIMO) systems by sharing individual antenna in multiple-relay/-user environment. And a crucial problem for cooperative communications utilized in OFDM systems is to achieve accurate channel estimation and synchronization.As a result, this dissertation works on the channel estimation and synchronization problems in the above-two novel OFDM systems. In OFDM/OQAM systems, this dissertation proposes a ICI-cancellation-based channel-estimation scheme and a low-complexity efficient synchronization algorithm. As to the cooperative OFDM systems, this dissertation also proposes a novel interleaving-based synchronization scheme. Chapter two investigates the system structure and characteristic of OFDM/OQAM systems. After summarizing the conventional OFDM systems, the chapter addresses the signal model and fast implementation structure of OFDM/OQAM systems. And then by selecting extended gauss function (EGF) as pulse-shaping prototype function, the ability of combating time-and-frequency double-selective fading is studied.Chapter three discusses new channel estimation and synchronization scheme for OFDM/OQAM systems. Based on the description of the wireless channel model, the pilot-based channel estimation in conventional OFDM systems is introduced. And after analyzing the interference source of pilots in OFDM/OQAM systems, this dissertation gives a ICI-cancellation-based channel-estimation preamble and the corresponding algorithms. Next, two new time-frequency synchronization schemes are proposed effectively, which are based on frequency-domain preamble and time-domain preamble. The above algorithms are finally verified by computer simulation.In cheaper four, the channel estimation and synchronization problems in cooperative OFDM systems are studied. First and foremost, this cheaper summarizes the system and signal model of cooperative OFDM systems, and does research for the preamble-based channel estimation schemes by comparison. Then a novel interleaving-based synchronization scheme is proposed. Simulation is also finally utilized to verify the performance of channel estimation schemes and proposed synchronization technique.The research results in this dissertation provide effective approaches of channel estimation and synchronization in the above-two novel OFDM systems. Utilizing these novel techniques, the spectrum efficiency and transmission reliability are further improved, and the results can be applied in the next generation mobile communication systems, such as IMT- Advanced.