兰州交通大学毕业设计（论文）
OFDM频率偏移估计算法分析
摘要
作为一种特殊的多载波调制技术，正交频分复用（OFDM，Orthogonal Frequency Division Multiplexing）因其高频谱利用率、高数据传输速率以及良好的抗多径干扰性能，广泛地应用于数字音视频广播、无线局域网等高速数据传输系统中。OFDM通信系统具备所有这些优势的前提是收发两端子载波均要保持良好的正交性，然而，在实际应用中，晶振的非理想因素以及移动通信中多径信道产生的多普勒频移将会造成OFDM系统发射机与接收机载波中心频率的偏移（CFO，Carrier Frequency Offset），而这将严重破坏子载波之间的正交性，因此OFDM系统接收机必须对载波频偏加以估计并对接收信号进行相应补偿以保证解调数据的准确性。通常，将这一操作称为载波频率同步，也可简称为频偏估计。由于OFDM系统对CFO非常敏感，微小的CFO就能造成系统误码性能的大幅下降，因此，频率同步技术是OFDM 系统的关键技术之一。
本论文首先回顾了OFDM技术发展的历史，然后从基本的OFDM系统的原理出发，阐述了OFDM系统中的同步问题。接着详细阐述了定时同步偏差和载波频率偏差对系统性能的影响。最后，对现有的频率同步技术（即，盲同步算法和非盲同步算法）进行了介绍且重点介绍了三种具有代表性的载波频偏估计算法：子载波间干扰（ICI，Intercarrier interference）自消除方法，高阶子载波间干扰（ICI）自消除方法和频率偏移盲估计方法，并通过仿真比较分析了它们在加性高斯白噪声信道和频率选择性信道下的估计性能。
关键词：正交频分复用；载波频率偏移；子载波间干扰；盲载波频偏估计；自消除
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兰州交通大学毕业设计（论文）
ABSTRACT
As a special multicarrier modulation technique OFDM（Orthogonal Frequency Division Multiplexing）has been widely used in data transmission systems such as Digital Video/Audio Broadcasting systems and wireless LAN systems, due to its high spectral efficiency, high data rate, and excellent performance to combat with multi-path interference. It is the orthogonality between the sub-carriers in OFDM systems that guarantees all those strengths mentioned above. However，the carrier frequency offset, caused by the non-ideal factors of the oscillators and the Doppler Shift introduced by the multipath channel, will destroy the orthogonality between the subcarriers. In order to get a better performance, the receiver should estimate the carrier frequency offset and then compensate the received signal, which is usually called as carrier frequency synchronization or frequency offset estimation for short. OFDM systems are very sensitive to carrier frequency offset, that is to say, a small frequency shift could degrade the system performance dramatically. Therefore, frequency synchronization technique has become one of the key techniques of OFDM systems.
Firstly, this dissertation reviews the history of the development of OFDM technique. According to the basic OFDM theory, this dissertation describes synchronization of OFDM systems. Then, describes the effect of the timing offset and frequency offset to the system in （i.e. the blind algorithm detail. Finally, the frequency synchronization of existing technologies
and the non-blind algorithm）is introduced and this dissertation focus on the three typical carrier frequency offset estimation algorithm: Intercarrier Interference Self-Cancellation, High-order Intercarrier Interference Self-Cancellation and Blind Deterministic Frequency Offset Estimation Method. Performances under AWGN channel and frequency-selective channel are analyzed and compared by simulation.
Keywords: Orthogonal Frequency Division Multiplexing（OFDM）; Carrier Frequency Offset; Intercarriers Interference; Blind Carrier Frequency Offset Estimation; Self-cancellatio
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兰州交通大学毕业设计（论文）
目录
摘要 ............................................................................................................................................. I ABSTRACT .............................................................................................................................. II 1 绪论 ...................................................................................................................................... 1
1.1 引言 ............................................................................................................................... 1 1.2 OFDM技术的历史及发展现状 ................................................................................... 1 1.3 OFDM技术的优缺点 ................................................................................................... 2
1.3.1 OFDM技术的优点 ............................................................................................. 2 1.3.2 OFDM技术的缺点 ............................................................................................. 3 1.4 论文的主要工作和章节安排 ....................................................................................... 4 2 OFDM系统基础.................................................................................................................. 5
2.1 无线信道特征 ............................................................................................................... 5
2.1.1 多径信道模型和信道参数 ................................................................................ 6 2.1.2 多径时延扩展衰落效应 .................................................................................... 7 2.1.3 Doppler扩展衰落效应 ....................................................................................... 9 2.2 正交频分复用（OFDM）技术与频分复用（FDM）技术的比较 .......................... 9 2.3 OFDM系统的分类 ..................................................................................................... 10
2.3.1 CP-OFDM 系统 ............................................................................................... 10 2.3.2 ZP-OFDM系统 ................................................................................................. 11 2.4 基于IFFT/FFT的OFDM系统原理 ......................................................................... 11
2.4.1 OFDM的基本原理 ........................................................................................... 11 2.4.2 基于IFFT/FFT的OFDM系统 ...................................................................... 14 2.4.3 串/并变换 ......................................................................................................... 14 2.4.4 信号映射 .......................................................................................................... 15 2.4.5 利用IFFT/FFT实现OFDM符号的调制与解调 .......................................... 15 2.4.6 保护间隔 .......................................................................................................... 16 2.4.7 循环前缀 .......................................................................................................... 17 2.5 OFDM系统的关键技术 ............................................................................................. 18
2.5.1 信道建模 .......................................................................................................... 18 2.5.2 时域和频域同步 .............................................................................................. 19 2.5.3 信道估计 .......................................................................................................... 19
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