【作者】 李永波；

【导师】 刘晓明；

【作者基本信息】 重庆大学 ， 电路与系统， 2007， 硕士

【摘要】 随着对通信速率要求的不断提高,频谱资源有限这一矛盾的更加突出,正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)逐渐成为了第四代(4G)移动通信的核心技术。它不仅仅能对抗频率选择性衰落,同时又提高了频谱利用率,而且能更好地满足多媒体通信要求,将语音、数据、图像等大量信息的多媒体业务通过宽频信道高品质、低成本地传输。然而,OFDM技术也存在不足,其对系统的同步非常敏感。符号定时偏差、载波频率偏移以及采样时钟的不同步都将会破坏各子载波间的正交性,从而引入子载波间干扰ICI(Internal Channel Interference)和符号间干扰ISI(Internal Symbol Interference),降低系统的整体性能。因此,在设计OFDM接收机时,需要对符号偏差、频率和采样时钟的偏移进行有效的估计和补偿,尽可能消除不同步因素对系统性能的影响。本文首先简要介绍了无线接入系统的发展历程和OFDM技术的基本原理及其特点,并从理论上分析和研究了同步误差对系统性能的影响。然后根据IEEE802.11a信号帧结构,重点介绍了依据前导训练序列以及导频信号的结构特点,进行同步方案的设计以及算法性能的仿真与分析。主要包括以下几项内容:·比较了帧检测算法,采用基于接收信号能量与延时分组相关相结合的帧检测算法·提出了短训练序列本地相关和归一化能量峰值平原区结合的定时算法,给出长训练序列以及后续OFDM数据符号的FFT窗口位置,为信道估计以及OFDM解调和基带信号处理提供了条件·频率偏移的估计及其纠正,采用粗细级联的频偏估计与校正策略,在保证载波频偏的捕获范围的前提下同时提高了估计精度·相位跟踪算法与实现,在FFT之后利用频域导频采用共轭方法对残留频偏导致的相位误差进行跟踪与补偿最后,在对算法分析的基础上,采用ALTERA公司的FPGA硬件平台对同步方案予以实现。由理论仿真与实际测试表明,该同步方案实现了对IEEE802.11a标准的信号帧进行捕获与时频率同步,满足快速同步的要求,为后续的基带处理提供了必要的同步条件。更多还原

【Abstract】 Along with the contradiction between the requirement about much larger communication capability and the limited spectrum resource becoming much more austere,OFDM(Orthogonal Frequency Division Multiplexing) gradually becomes the kernel technique in the 4G mobile communication.It not noly can confront the frequency selectivity degradation,but also satisfy the require of multimedia communication.It could provide high quality and low cost transfer multimedia-service that contains the abundant information(for example:voice,data and image etc)through wide frequency channel.But the OFDM technique also has shortage,it is very sensitive to the system synchronization capability.All of the offsets with symbol timing、carrier wave frequency and sampling frequency can destroy the orthogonality of every subcarrier.It is not noly introduce the ICI(Internal Channel Interference) among subcarriers and ISI(Internal Symbol Interference) among symbols,and also reduce the whole system capability.So,it needs effectively estimation and compensates the offsets with symbol timing,carrier wave frequency and sampling frequency,in order to eliminate all kinds of asynchronization influence to the system capability.Firstly,this paper simply introduces the developing course of the wireless access and the principle of OFDM technique,then analyses and investigates the asynchronization offsets influence to the system capability.Then,this paper emphasizely introduces the simulation and design scheme for asynchronization by long training sequences,short training sequences and pilot freqency signals,according to characteristics of the IEEE802.11a frame structuer.Following issues were emphatically illuminated in this paper:·The algorithm and realization of frame detecting ,bring forward the combining of power detecting and delaying group,much more declining the probalility of false synchronization and missed synchronization.·The realization of coarse and precise symbol timing,present the long training sequence and the signal’s FFT window positions,supplying the qualifications for channel estimation.·The estimation and correction of frequency offset,adopting coarse and precise two stages estimation,enlarging the rang of estimation and enhancing the precision of estimation.·The algorithm and realization of the phase tracking, using frequency domain pilot subcarriers to track and compensate phase error caused by residual carrier frequency offset after FFT operateFinally, the realization source code is given by VHDL. It has been synthesized in the chip of Altera and simulated. From the simulation and testing result, we know that its performance is good and satisfy the need of the system.更多还原