【作者】 严春林；

【导师】 李少谦；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2004， 博士

【摘要】 OFDM由于具有抗频率选择性衰落、频谱效率高和抗窄带干扰优点,在现代通信中越来越受到重视。MIMO技术能增大系统容量,提高数据速率,提高数据传输质量,是无线通信中一种关键技术。OFDM技术和MIMO技术是新一代移动通信(Beyond 3G或4G)技术核心。MIMO和OFDM的结合可得到MIMO OFDM系统,它能使提供更大的覆盖范围,更好的传输质量,更大的数据速率和频谱效率。但OFDM系统和MIMO OFDM系统对时间和频率同步误差很敏感,特别是频率同步误差。准确的时间同步和精确的频率同步是OFDM系统和MIMO OFDM系统正常工作的前提。本文研究了OFDM系统和MIMO OFDM系统的同步技术,主要内容如下: 同步中要用到伪随机序列。现代通信对安全性要求越来越高,传统的m序列和Gold序列已不能满足这一要求。级联GMW序列和增强型相控序列有很大的线性复杂度和良好的自(互)相关特性,可分别用于单天线系统同步和多天线系统同步。文中提出了级联GMW序列的一种简洁生成算法和增强性相控序列的生成方法; 系统的总结了时间同步误差和频率同步误差对OFDM系统性能影响;针对PN序列的同步方法,提出了自适应门限设置方法及搜索窗设置,并将此方法与常规时间同步方法性能进行了比较;OFDM技术非常适合在多径衰落现代下应用,文中提出了对多径信道中第一径信号捕获,这可提高系统性能,并给出了捕获算法; 频率同步对OFDM系统而言非常关键。本文提出了多种频率同步方法:迭代机制的频率同步方法;重复序列加循环前缀的频率同步方法;一种将接收训练序列与本地训练序列相乘以消除训练序列影响的大范围高精度的频率同步方法;利用CAZAC序列的频率同步方法;分析了多径信道中利用重复序列进行频率偏移估计时重复序列周期选取准则。这些方法丰富了已有的频率同步方法。文中还得出了利用循环前缀作频率偏移估计精度的计算公式和精度更高的频率偏移估计方差。 提出了MIMO OFDM时间同步方法。这些方法可以认为是将单天线下Tufvesson的同步方法的扩展。此方法可以使用一个OFDM符号实现时间同步,而现有一些方法需多个OFDM符号作时间同步估计;可以实现各发射天线到达接收天线时间延迟不同情形下的时间同步,而现有时间同步方法只考虑各发射天线到达接收天线时间延迟相同情形;更多还原

【Abstract】 OFDM is a promising technology for its high spectral efficiency and robustness to selective fading and narrow band interference. As MIMO technology can be adopted to increase system capacity, data rate, and make the transmission more reliable, it has now become a key technology in wireless communications. OFDM and MIMO are widely believed to be the candidate technologies for next generation wireless communications (Beyond 3G or 4G) systems. A MIMO-OFDM system is a seamless merger of MIMO and OFDM, which can potentially provide larger coverage range, more reliable transmission, higher date rate and spectrum efficiency.However OFDM and MIMO OFDM systems are very sensitive to time and frequency synchronization errors, especially the frequency synchronization error. This dissertation investigates the topic of time and frequency synchronization for OFDM and MIMO-OFDM systems. The content of the dissertation is shown as following.It is well known that PN sequence can be used for OFDM synchronization, while the security is an essential requirement for modern communication systems. Unfortunately, m sequences and Gold sequences are not able to meet the security requirement in many cases. Cascaded GMW sequences and enhanced phase control sequences have large linear span, good autocorrelation and cross correlation properties, therefore can be used for synchronization in SISO-OFDM and MIMO-OFDM systems separately. In this dissertation, a simple but efficient method, which can be used to construct cascaded GMW sequence, is proposed. Another method to generate enhanced phase control sequence is proposed as well.The effect of time and frequency synchronization error on the performance of OFDM system is systemically summarized in this dissertation. An adaptive threshold and new synchronization strategy are proposed for the synchronization using PN sequences. Moreover, the performance of the proposed synchronization method is compared with that of conventional method. Furthermore, a new method capturing the first path of the multi path channel is also proposed for OFDM systems, which has been shown to have better performance. Along with this new method, a new capturing algorithm is proposed.Frequency synchronization is very important for OFDM systems too. In this dissertation, I present some new methods such as a new iterative frequency synchronization method, a new frequency synchronization method using repeated sequences and cyclic prefix, a high performance frequency synchronization with large estimation range by correlating the received data with local training sequences to eliminate the influence of training sequences, and a new method using CAZAC sequences. The criterion on how to determine the period of repeated sequences in multi-path channel is also analyzed. These methods enrich the already existing frequency synchronization methods. The estimation variance of frequency synchronization methods using cyclic prefix proposed by van de Beek and a more precise estimation variance than that of Schmidl’s are got.Novel synchronization methods for MIMO-OFDM systems are proposed, which can be considered as the extension of Tufvesson’s method for SISO-OFDM systems.更多还原