摘要
正交频分复用技术(OFDM)是一种多载波调制技术,可以提供更多的有效带宽,并且能够有效的对抗多径延时。因此,OFDM技术在数字电视广播领域的应用引起人们广泛关注,并被纳入多种数字电视地面传输标准。
OFDM对接收端子载波的同步性要求十分精确,否则就会破坏子载波间正交性,从而引起传输性能的急剧下降。所以我们需要找到有效的同步方法来对载波频偏进行校正。因此在基于OFDM的数字电视地面广播系统中,对接收符号进行载波频偏估计和校正的探讨具有重要的意义。在当前对载波同步算法的研究中,有很多是针对移动通信领域的载波同步算法,对于基于OFDM的数字电视地面广播系统中的载波同步方法,有进一步研究的必要性。
本文将首先回顾OFDM技术的产生背景、基础原理和其中的关键技术,接着简要介绍了传统OFDM系统和TDS-OFDM系统,并例举典型的DVB-T 2K系统、DMB-T系统和基于一种PN时域同步头的系统。然后分析了OFDM系统中的同步误差,尤其是对于接收信号的影响。
本文接着重点阐述了传统OFDM系统和TDS-OFDM系统中的载波频偏同步技术。在传统OFDM系统中,以DVB-T 2K系统为例对多种载波频偏估计技术进行了分析和仿真,并提出DVB-T 2K系统的一种在时域联合平滑和平均模块以提高估计精度的载波频偏同步方案。在TDS-OFDM系统中,以解决DMB-T和基于一种PN时域同步头系统中的载波同步问题为思路,把多种载波频偏估计技术应用于这两个系统并作为这两个系统中载波同步的一部分,进行了分析和仿真验证,而且其中一些载波频偏估计方法可以适用于其它时域同步头的系统,最后构成了DMB-T系统的一种基于两级折算的载波频偏同步方案,提出了基于一种PN时域同步头系统的基于自适应选取模式的载波频偏同步方案。本文所提到的三个系统的载波频偏同步方案,实现复杂度较低,计算机仿真结果显示不仅最终估计精度达到10?3的数量级,而且同步补偿后系统性能良好。
OFDM (Orthogonal Frequency Division Multiplexing) is a kind of multi-carrier modulation technology. It uses the bandwidth more efficiently and can minimize the impact brought about by the multi-path delay. Therefore, the application of the OFDM technologies in digital TV broadcasting area has gained more and more interest and the technologies have been included in several digital TV terrestrial transmission standards.
OFDM requires accurate carrier synchronization. Otherwise the orthogonal character between sub-carriers is destroyed to cause the dramatically drop of the receiver performance. Consequently, we need efficient carrier synchronization methods to compensate the frequency offset. In the OFDM-based Digital TV broadcasting systems, the estimation and compensation of carrier’s frequency offset have important meanings. In the current research of the carrier synchronization algorithms, many of them are focused on the mobile communication systems. It is necessary to further research the carrier synchronization methods in OFDM-based digital TV terrestrial broadcasting systems.
In this paper, the history, the basic theory and the key technology of OFDM systems are reviewed firstly. Then the traditional OFDM systems and the TDS-OFDM system are briefly introduced by raising some typical examples, such as DVB-T 2K system, DMB-T system and a system which uses a PN sequence as the synchronization preamble in the time domain. Afterwards, the effects brought about by the carrier’s frequency offset are analyzed, especially the effects to the receiving signals.
In this paper, we mainly focus on the carrier’s frequency synchronization technology in traditional OFDM systems and TDS-OFDM systems. In the traditional OFDM systems, several kinds of carrier’s frequency offset estimation technologies are explained. The simulation results in the DVB-T 2K system are shown. Finally, a carrier synchronization scheme of DVB-T 2K system is studied by using slicing and average. In the TDS-OFDM systems, two systems are researched. They are DMB-T system and the system which uses a PN sequence as the synchronization preamble in the time domain. In order to solve the carrier’s frequency offset problems in these systems, several carrier’s frequency offset estimation methods are applied and explained. Some of the methods can be used in other systems which have a different frame head. The simulation results are also shown. Finally, a carrier synchronization scheme with two-stage calculation in DMB-T system is explained. A carrier synchronization scheme with self-adapted selecting module in the system which uses a PN sequence as the synchronization preamble is presented. The above mentioned carrier synchronization schemes used in these three systems not only have low complexity (the estimation precision can research the 10?3 grade) but also have good performance.
引文
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