【作者】 汪涛；

【导师】 李鸥；

【作者基本信息】 解放军信息工程大学 ， 信息与通信工程， 2011， 博士

【摘要】 正交频分复用(OFDM)等多载波数字调制技术采用多路并行的正交子载波来发送/接收高速数据信号。该技术具有高频谱效率、优良的抗频率选择性衰落能力、能根据信道条件灵活地决定子载波调制方式与信号功率、具有天然的与多输入多输出(MIMO)系统相结合的能力,因而在无线通信领域引起了广泛关注。目前,OFDM通信技术的研究主要集中在高信噪比、高速传输速率的应用,而基于OFDM的拓展距离UWB通信、军事自适应速率隐蔽通信以及移动通信系统的小区边缘数据传输则更关注OFDM技术在低信噪比下的通信能力。本文开展了针对低信噪比下OFDM相关关键通信技术的研究,主要的研究内容和创新点如下:1.提出了一种具有良好自相关性能的OFDM前导码的设计方法。该前导码的主要特征是,频域序列具有相等的幅度,时域序列具有良好的相关性能且表现为类似高斯白噪声的统计特性,本文中称该种前导码为“频域恒包络前导码”。频域恒包络前导码产生方法简单,且当频域前导码为实序列时,由其生成的前导训练序列具有中心共轭对称性,这种性质可用于OFDM的自相关同步方法中。2.提出了两种级联方式的OFDM符号定时与载波频偏联合同步方法:差分互相关联合同步方法与自相关联合同步方法。两种联合同步方法的载波频率同步原理相同,主要区别在符号定时同步上。差分互相关定时同步是在互相关同步的基础上引入差分运算,用以消除大的载波频偏对前导码相关性能的影响;自相关联合同步方法充分利用实频域恒包络前导训练序列的中心共轭对称性质,来设计具有尖锐相关特性的自相关定时度量判决函数以实现高精度定时同步。仿真结果表明两种联合同步方法的都具有良好的抗噪声和抗频偏性能,优于当前几种经典联合同步方法。3.提出了基于频域恒包络前导码的OFDM时频二维符号定时与整数倍载波频偏联合同步方法,并且给出了两种实现方法。OFDM时频二维联合同步方法具有定时同步精度高、整数倍载波频偏估计范围大、抗噪声性能好、收敛速度快等优点。如果采用直接实现的方式需要耗费很大的硬件资源,在深入研究OFDM时频二维联合同步方法的基础上,本文提出了两种高效的实现算法:基于并行FFT的时频二维联合同步算法和基于并行Chirp-Z的时频二维联合同步算法。两种并行算法在保证OFDM时频二维联合同步算法优良性能的基础上,大大降低了资源消耗,具有很强的实际应用价值。4.基于频域恒包络前导码的循环相关性,结合频域恒包络前导码的同步阶段获得的多径时延分布信息,提出了基于多径时延信息的OFDM时域信道估计方法。该方法具有以下优点:首先,信道估计所用导频信号与同步阶段所用导频同为一个频域恒包络前导码,因此系统较小;其次,与现有的几种经典OFDM信道估计方法相比,该信道估计方法在低信噪比条件下具有良好的性能;然后,该信道估计方法计算复杂度低,意味着硬件资源的节省和处理时间的缩短。理论分析与仿真结果均表明该基于多径时延信息的OFDM时域信道估计方法是一种低复杂度、高性能的OFDM信道估计方法,适用于很多OFDM系统。更多还原

【Abstract】 Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation technique, which uses several parallel orthogonal sub-carriers to transmit/receive a high data rate signal. OFDM shows many advantages over single-carrier transmissions, such as high frequency efficiency; robustness against frequency-selective fading in a multipath channel; flexibility to determine modulation manner and power of sub-carriers; natural ability to cooperate with Multiple Input Multiple Output (MIMO). At present, the researches on OFDM mainly focuses on the applications of high Signal-to-Noise Ratio(SNR) and high speed. Meanwhile, it shows an increasingly demand of OFDM transmissions under low SNR conditions, such as extending distence Ultra-Wide Band(UWB) systems, low detectable probability adaptive speed military communication systems and data transmissions between the brim and the base station in mobile communications.It is significant to develop the OFDM techniques under low SNR conditions. The major work and contribution of this dissertation are focus on the key techniques of low SNR OFDM systems, it summarized as follows:1. A new OFDM preamble design method is presented. It has been testified that if the OFDM frequency domain preamble has constant amplitude at all sub-carriers, the time domain preamble (IDFT form of frequency domain preamble) shows an ideal correlation performance, this kind of special OFDM preamble is named“Constant Amplitude Frequency Domain Preamble”in this dissertation. It is easy to generate the CAFDP sequence, and for real CAFDP sequence, the time domain form is central conjugate symmetrical.2. Two joint symbol timing and frequency offset synchronization methods are proposed: the differential cross-correlation synchronization method and the auto-correlation synchronization method. They are both multi-stage processes, the only difference between them is at the symbol timing stage. The former introduces differential operation to cross-correlation to eliminate the influence of large frequency offsets, the latter utilizes the conjugate symmetrical structure of CAFDP to get sharp auto-correlation peak. Computer simulations in the AWGN(additive white Gaussian noise) channels and the Rayleigh fading channels show that the proposed synchronization methods achieves superior performance to the existing methods.3. Aimed at obtaining better performance and fast synchronization convergence, a time and frequency two dimension joint synchronization method is present. It has excellent performance: high timing precision, large integer frequency estimation range, fast convergency speed etc. At the same time, the realization of the method needs great hardware resources. Based on the analysis of the mathematic expressions, two promoted algorithms are proposed which can realize the method with the same performance but saving much hardware resources, they are the parallel FFT synchronization algorithm and the parallel Chirp-Z synchronization method.4. Utilzing the cyclic correlation character of the CAFDP and the multipath delay distribution information achived at the timing synchronization stage, a time domain channel estimation method is proposed. This method shows several advantages over the existing channel estimation method: first, it uses the same preamble with the synchronization method, that means less systems consumed cost; second, it shows superior perfomace over existing methods; last, it is a low computing complexity algorithm that leads small hardware resources cost and fast processing speed.更多还原