宽带无线OFDM系统同步算法的研究
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摘要
下一代移动通信的目标是利用有限的频谱资源提供可靠、高速的宽带数据业务。正交频分复用(OFDM)系统能够有效的对抗多径衰落并具有较高的频谱利用率;多输入多输出(MIMO)系统能够大幅度提高系统的吞吐量,因此OFDM技术和MIMO技术成为下一代移动通信的关键技术。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。本文主要针对OFDM系统及MIMO-OFDM系统中的同步问题进行了较全面而深入的研究,并将所提出的算法和当前国际上的一些经典算法进行了比较。本文的主要研究成果如下:
一、同步误差对OFDM系统性能的联合影响。论文系统地研究了AWGN信道和多径衰落信道下定时偏差和载波频偏对OFDM系统解调信号及均值信干比的联合影响,并通过仿真对分析结果进行了验证。较之以往的同步误差影响分析,本文的分析结果具有更一般的意义。此分析结果为后续章节的同步算法设计提供了理论基础。
二、OFDM系统数据辅助类同步算法的研究。论文针对传统同步算法存在的“峰值平台”效应带来的定时模糊问题,提出了一套完整的同步解决方案。新算法利用新定义的前导结构和相关特性优异的训练序列,采用接收信号与本地训练序列直接相关的方法捕获定时位置,定时同步只需一步完成,同时利用整数频偏对训练序列相关特性的影响,可以结合定时位置在时域直接估计整数频偏,从而省去了FFT运算,降低了系统同步的运算量,并且算法能够准确检测到信号到达的第一径。仿真结果表明,新算法在较低的信噪比条件下和多径衰落信道下都具有很好的性能。
三、OFDM系统CP类同步算法的研究。论文针对基于CP的同步算法在多径衰落信道下性能较差的情况,提出了改进算法,改进算法联合利用接收信号的集相关特性和离散导频构造新的定时度量函数,并在分析接收信号集相关特性的基础上给出了离散导频的设计要求。新算法与传统算法相比,能够抗多径干扰,同时可以加快定时捕获的速度。
四、MIMO-OFDM系统同步算法研究。论文针对集中式MIMO信道环境下的MIMO-OFDM系统,利用各个发射天线同时发送的移位正交训练序列,设计了新的同步解决方案,包括符号同步、整数频偏估计和小数频偏估计。该算法利用两个OFDM符号构成的前导结构,对已有的定时和频偏估计算法加以改进,相比传统的集中式MIMO-OFDM同步算法,所提算法具有更高的定时估计精度和频偏估计精度,更适用于多径衰落信道。
The next generation mobile communications systems is aiming at achieving high quality and data rate transmission in finite wireless resource. Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology for its robustness to selective fading and high spectral efficiency. Multi-Input Multi-Output (MIMO) technology can be adopted to achieve high throughput. So OFDM and MIMO have now become key technologies for the next generation mobile communication systems. However OFDM and MIMO-OFDM systems are very sensitive to time and frequency synchronization errors, especially the frequency synchronization errors. Correct time and precise frequency synchronization are necessary for OFDM and MIMO-OFDM systems.
In this thesis, synchronization theories and algorithms for OFDM and MIMO-OFDM systems are deeply analyzed and researched. Several new synchronization algorithms are proposed and compared with key conventional algorithms. The major achievements are outlined as follows:
Firstly, the influence of synchronization errors to OFDM system performance is analyzed. The joint influences of symbol timing offset and carrier frequency offset on OFDM demodulated signals and mean signal to interference energy ratio in the receiver are investigated comprehensively and confirmed by computer simulations, that provides theoretical foundations for synchronization algorithm designs in the following chapters. Compared with previous analysis, the joint influence analyzed here is more common.
Secondly the research on the data-aided synchronization algorithms has been analyzed. An integrated synchronization solution is proposed to avoid the timing metric plateau inherent in the conventional data-aided synchronization methods which causes large variance of the timing estimate. A new preamble is designed in the proposed algorithm and timing instant is obtained by correlating the received signals with the known training sequence which has the best correlation properties. Meanwhile, with the help of the integral frequency offset's influence upon the training sequence relativity, the integral frequency offset can be obtained in time domain without FFT transform, that will reduce the computation complexity for the synchronization in OFDM systems. Furthermore the proposed algorithm can obtain the first path of the arrival signal. The valid of the new algorithm is verified in low SNR and multipath fading channel.
Thirdly the cyclic prefix (CP) based synchronization algorithms have been investigated. After the analysis of the bad performance of CP based algorithms in multipath fading channel, an improved synchronization algorithms based on CP is proposed. The proposed method uses the ensemble correlation in CP and available pilot symbols used for channel estimation to construct new timing metric and the demands on pilot symbols design are investigated based on the analysis of the ensemble correlation. Compared with conventional CP based synchronization algorithms, the proposed algorithm is robustness to multipath interference and the time for estimating timing is more shoter.
Finally the synchronization problem in MIMO-OFDM systems is investigated. A new synchronization algorithm is proposed in localized MIMO channel environment including estimation of symbol timing, integral frequency offset and fractional frequency offset. The new synchronization is achieved using shift-orthogonal training sequence which is simultaneously transmitted from all transmit antennas in the same OFDM time instant. Compared with conventional algorithms, the analysis method gives out better detection properties in terms of higher correct timing detection probability and accurate fractional frequency synchronization even in multipath fading channel.
一、同步误差对OFDM系统性能的联合影响。论文系统地研究了AWGN信道和多径衰落信道下定时偏差和载波频偏对OFDM系统解调信号及均值信干比的联合影响,并通过仿真对分析结果进行了验证。较之以往的同步误差影响分析,本文的分析结果具有更一般的意义。此分析结果为后续章节的同步算法设计提供了理论基础。
二、OFDM系统数据辅助类同步算法的研究。论文针对传统同步算法存在的“峰值平台”效应带来的定时模糊问题,提出了一套完整的同步解决方案。新算法利用新定义的前导结构和相关特性优异的训练序列,采用接收信号与本地训练序列直接相关的方法捕获定时位置,定时同步只需一步完成,同时利用整数频偏对训练序列相关特性的影响,可以结合定时位置在时域直接估计整数频偏,从而省去了FFT运算,降低了系统同步的运算量,并且算法能够准确检测到信号到达的第一径。仿真结果表明,新算法在较低的信噪比条件下和多径衰落信道下都具有很好的性能。
三、OFDM系统CP类同步算法的研究。论文针对基于CP的同步算法在多径衰落信道下性能较差的情况,提出了改进算法,改进算法联合利用接收信号的集相关特性和离散导频构造新的定时度量函数,并在分析接收信号集相关特性的基础上给出了离散导频的设计要求。新算法与传统算法相比,能够抗多径干扰,同时可以加快定时捕获的速度。
四、MIMO-OFDM系统同步算法研究。论文针对集中式MIMO信道环境下的MIMO-OFDM系统,利用各个发射天线同时发送的移位正交训练序列,设计了新的同步解决方案,包括符号同步、整数频偏估计和小数频偏估计。该算法利用两个OFDM符号构成的前导结构,对已有的定时和频偏估计算法加以改进,相比传统的集中式MIMO-OFDM同步算法,所提算法具有更高的定时估计精度和频偏估计精度,更适用于多径衰落信道。
The next generation mobile communications systems is aiming at achieving high quality and data rate transmission in finite wireless resource. Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology for its robustness to selective fading and high spectral efficiency. Multi-Input Multi-Output (MIMO) technology can be adopted to achieve high throughput. So OFDM and MIMO have now become key technologies for the next generation mobile communication systems. However OFDM and MIMO-OFDM systems are very sensitive to time and frequency synchronization errors, especially the frequency synchronization errors. Correct time and precise frequency synchronization are necessary for OFDM and MIMO-OFDM systems.
In this thesis, synchronization theories and algorithms for OFDM and MIMO-OFDM systems are deeply analyzed and researched. Several new synchronization algorithms are proposed and compared with key conventional algorithms. The major achievements are outlined as follows:
Firstly, the influence of synchronization errors to OFDM system performance is analyzed. The joint influences of symbol timing offset and carrier frequency offset on OFDM demodulated signals and mean signal to interference energy ratio in the receiver are investigated comprehensively and confirmed by computer simulations, that provides theoretical foundations for synchronization algorithm designs in the following chapters. Compared with previous analysis, the joint influence analyzed here is more common.
Secondly the research on the data-aided synchronization algorithms has been analyzed. An integrated synchronization solution is proposed to avoid the timing metric plateau inherent in the conventional data-aided synchronization methods which causes large variance of the timing estimate. A new preamble is designed in the proposed algorithm and timing instant is obtained by correlating the received signals with the known training sequence which has the best correlation properties. Meanwhile, with the help of the integral frequency offset's influence upon the training sequence relativity, the integral frequency offset can be obtained in time domain without FFT transform, that will reduce the computation complexity for the synchronization in OFDM systems. Furthermore the proposed algorithm can obtain the first path of the arrival signal. The valid of the new algorithm is verified in low SNR and multipath fading channel.
Thirdly the cyclic prefix (CP) based synchronization algorithms have been investigated. After the analysis of the bad performance of CP based algorithms in multipath fading channel, an improved synchronization algorithms based on CP is proposed. The proposed method uses the ensemble correlation in CP and available pilot symbols used for channel estimation to construct new timing metric and the demands on pilot symbols design are investigated based on the analysis of the ensemble correlation. Compared with conventional CP based synchronization algorithms, the proposed algorithm is robustness to multipath interference and the time for estimating timing is more shoter.
Finally the synchronization problem in MIMO-OFDM systems is investigated. A new synchronization algorithm is proposed in localized MIMO channel environment including estimation of symbol timing, integral frequency offset and fractional frequency offset. The new synchronization is achieved using shift-orthogonal training sequence which is simultaneously transmitted from all transmit antennas in the same OFDM time instant. Compared with conventional algorithms, the analysis method gives out better detection properties in terms of higher correct timing detection probability and accurate fractional frequency synchronization even in multipath fading channel.
引文
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