叠加训练序列OFDM的频率同步及信道估计技术研究

【作者】 周键；

【导师】 唐友喜；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2007， 硕士

【摘要】 随着移动通信业务从传统话音业务向以互联网接入为代表的多媒体业务的不断发展,如何在有限的无线频谱资源上提供满足人类物质文化需求的宽带移动多媒体业务,对未来移动通信技术的发展提出了巨大的挑战。叠加训练序列OFDM将用于时间和频率同步的训练序列与OFDM数据符号同时发送,相对于传统的先发送训练序列再发送数据符号的方法,提高了频谱利用率,在未来移动通信系统中有着广阔的应用前景。对于一个实际的无线通信系统而言,收发端的时间与频率同步以及对时变的无线信道的估计是两项关键技术。同步是信号正确解调和恢复的前提,而信道估计性能的好坏直接影响到整个系统性能的优劣。在叠加训练序列OFDM中,由于训练序列算术叠加在OFDM数据符号上,接收端还需要解决训练序列与数据符号的分离问题,并考虑如果在同步和信道估计中利用已知的训练序列,从而进一步提高系统效率。此外,由于无线信道中存在的各类干扰,有必要研究系统的抗干扰性能。本文拟在上述几个方面针对叠加训练序列OFDM展开研究工作:本文针对叠加训练序列OFDM对频率偏移十分敏感的问题,提出了一种利用特殊设计的用于信道估计的导引子载波对应的时域周期性,联合OFDM循环前缀实现接收端频率同步的同步算法,同时提出了一种迭代的频率同步算法,在提高OFDM的系统效率的基础上实现了高性能的数据检测。本文详细描述了两种同步算法的系统模型及具体的同步算法,给出了仿真结果及其分析,仿真结果表明,两种同步算法的性能能够满足下一代移动通信频率同步的需求。本文在瑞利多径衰落信道中针对不插入导引子载波的叠加训练序列OFDM提出了一种基于迭代思想的最小均方误差信道估计算法,实现了接收端高性能的信号检测,进一步提高了系统频谱效率。计算机仿真结果表明,随着迭代次数的增加,系统的误比特率性能越来越好,逐渐逼近理想信道时的系统性能,在误比特率为1.5×10?3时,系统性能比理想信道时的性能相差约1.5dB。本文分析了时频二维扩频系统的抗多音干扰性能,指出了其对下一步的叠加训练序列OFDM抗干扰性能研究的意义,给出了时频二维扩频系统的系统模型,并对该系统在多音干扰环境中的误码率性能进行了理论分析,得到了一种特殊的抗多音干扰参数,最后通过了计算机仿真验证。本文在叠加训练序列OFDM的同步技术、信道估计技术和抗干扰性能方面的研究成果,可应用于如无线本地接入网、下一代移动通信系统、下一代高清晰度数字电视等的空中接口中,给无线通信空中接口的设计带来灵活性,具有重要的理论及经济价值。更多还原

【Abstract】 With the rapid development of mobile communications business evolving from the traditional voice service to those mainly based on Internet access and multimedia technologies, how to use the limited available wireless spectrum resources to provide mobile broadband multimedia services and thus meet the material and cultural needs of human beings, has become a huge challenge to the development of future mobile communications technologies. Compared with the traditional ways, which transmit training sequence first and then send the data symbols, OFDM system using superimposed training sequence transmits the training sequence, which is used for time and frequency synchronization, and OFDM data symbol simultaneously. This new scheme improves the efficiency of the frequency spectrum, and has broad application prospects in the future mobile communications systems.For practical wireless communications systems, time and frequency synchronization between the transmitter and receiver, and the estimation of the time-varying channel are two key technologies. Synchronization is the premise for the correct signal demodulation and restoration, while the performance of channel estimation has a direct impact on the overall performance of the system. Because the training sequence is arithmetically added on the OFDM data symbols, the separation issue of the training sequence and data symbols should be resolved in OFDM system using superimposed training sequence; besides, to further enhance system efficiency, the method of utilizing the known training sequence to do synchronization and channel estimation must be considered. In addition, since there are various types of interference or jamming in wireless channels, it is necessary to study the anti-jamming performance of the system. This article did some researches on the above-mentioned aspects of OFDM system using superimposed training sequence:Aiming at coping with the sensitivity of OFDM system using superimposed training sequence to frequency offset, a frequency synchronization method based on pilot subcarriers and cyclic prefix is proposed in this paper. The time-domain periodic property of the pilot subcarriers, which are used for channel estimation, is exploited for coarse frequency synchronization, and the redundant information of the cyclic prefix is used for fine frequency synchronization. Then, an iterative frequency synchronization algorithm is given to realize high performance data detection and the improvement of system efficiency. This paper describes in detail the system models, specific algorithms, and simulation results and their analysis of these two types of synchronization methods. Simulation results show that the performance of these two algorithms can meet the frequency synchronization requirements of next generation mobile communications.This paper proposes a method of iterative channel estimation, based on minimum mean square error algorithm, for OFDM system using superimposed training sequence with iterative receiver. High performance data detection is realized at the receiver, with further improvement of system efficiency. Computer simulations show that the bit error rate performance of the system becomes better when the number of iteration increases, which approaches the performance under ideal channel state information. There is about 1.5dB gap to the ideal performance when the bit error rate performance of the system is about 1.5×10?3.This paper investigates the performance of the time-frequency two-dimensional spread spectrum system in multi-tone jamming environment and points out the significance of this investigation to the further research on the anti-jamming performance of OFDM system using superimposed training sequence. The system model of the time-frequency two-dimensional spread spectrum system is given, the theoretical bit error rate of the system under multi-tone jamming environment is analyzed, and a kind of special anti-jamming parameter is derived. Computer simulations show agreement between the theoretical analysis and simulation results.The research results of the synchronization, channel estimation and anti-jamming performance of OFDM system using superimposed training sequence in this paper can be applied in the air interface of wireless local access network, next generation mobile communications systems, next generation high resolution digital television, and etc, and bring flexibility to the design of air interface for wireless communications, which are of great theoretical importance and economic value.更多还原