分布式MIMO OFDM系统同步技术研究
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摘要
移动通信技术始终向着提供更高数据传输速率、更好传输质量的方向发展。正交频分复用(OFDM)技术可以很好地克服无线信道的频率选择性衰落,具有高效的频谱利用率和良好的抗窄带干扰能力,受到人们的广泛关注。多入多出(MIMO)技术能够显著增加信道容量,提高频谱利用率,也成为当今无线通信研究领域的热点之一。MIMO与OFDM的结合——MIMO-OFDM技术已经成为下一代移动通信系统中的核心技术。
同步是通信系统首先需要解决的问题,同步的好坏直接影响整个系统的性能。OFDM系统对同步误差非常敏感,很小的频率偏移都将导致系统性能的急剧恶化。准确的时间同步和精确的频率同步是OFDM系统正常工作的前提。同时,MIMO系统,特别是分布式MIMO系统中的同步误差会带来严重的天线间干扰。因此,同步技术成为分布式MIMO-OFDM系统中的关键技术。
论文针对OFDM系统、分布式MIMO系统和分布式MIMO-OFDM系统中的同步技术进行了研究,主要包括以下内容:
分析了同步误差对OFDM系统及MIMO-OFDM系统性能的影响;针对基于互相关的时间同步方法,比较了几种时间同步检测门限的设置方法,分析表明,其中利用接收序列能量的方法不仅能够适应信道衰落的变化,而且可以工作在信噪比大范围变化的环境中。
基于对恒模零自相关序列的研究,本文提出两种特殊环境下的OFDM同步方法:一种是存在窄带干扰时的OFDM时间同步方法,该方法在频域同时完成干扰的检测、消除和时间同步估计;另一种是存在大频偏时的时间、频率同步方法,该方法通过特殊的训练序列设计,在不改变互相关时间同步检测方式的前提下,仍能取得互相关尖锐的相关峰,同时互相关的检测方式也保证了对大范围频偏的估计性能。
考虑分布式MIMO系统中存在多时延、多频偏的情况,建立了多参数最大似然(ML)联合估计的模型;并且针对直接计算ML估计复杂度高的问题,提出一种基于空间交替期望最大化算法的迭代计算方法;同时推导得出了多参数联合估计的克拉美-罗限(CRB);仿真表明所提算法的性能能够达到CRB。
针对分布式MIMO-OFDM系统中存在多个时延偏差的情况,提出两种利用频域导频的定时跟踪方法:一种是低复杂度的基于异步相干的方法;另一种是考虑信道衰落影响的ML估计方法,该方法在取得良好定时跟踪性能的同时,也能完成对信道衰落参数的估计。
本文丰富了对传统OFDM系统同步技术的研究,并对分布式MIMO和分布式MIMO-OFDM系统的同步技术进行了探索性的研究,所得到的研究成果具有一定的理论价值和应用价值。
Mobile communications always develop to provide higher data rate and better transmission quality. Orthogonal frequency division multiplexing (OFDM), which has been attracting much attention, can achieve higher frequency spectrum efficiency and overcome frequency-selective fading or narrow-band interference. Multiple input multiple output (MIMO), which has become one of the focuses in the area of wireless communications, can increase channel capacity and frequency spectrum efficiency remarkably. Hence MIMO-OFDM which is the combination of MIMO and OFDM has become the core technology of the next generation of mobile communications.
Synchronization is the primary mission of communication systems, which influences the performance of the whole systems directly. OFDM is sensitive to synchronization errors. The performance of OFDM system will be deteriorated severely only by small carrier frequency offset. Correct time synchronization and precise frequency synchronization are the necessary precondition for OFDM systems. Furthermore, synchronization errors would bring severe inter-antenna interference to distributed MIMO systems, especially to distributed MIMO-OFDM systems. Hence synchronization has become the key technology of distributed MIMO-OFDM systems.
This thesis focuses on the synchronization problems in OFDM, distributed MIMO and distributed MIMO-OFDM systems. The main contributions of this thesis can be described as follows:
Firstly, the influences of timing error and carrier frequency offset on OFDM and MIMO-OFDM systems are mathematically modeled and analyzed. For the time synchronization method using cross-correlation, several different methods of setting detection threshold are compared. The method using the power of the received sequence not only adapts to the change of channel fading, but also works well with large change of signal-to-noise ratio.
Secondly, based on the research on const amplitude zero auto-corelation (CAZAC) sequences, two synchronization methods for OFDM systems are proposed. One is the time synchronization method which works under narrowband interference. In this method, the interference detection and suppression procedure and the timing procedure are both processed in the frequency domain. Another method is the time and frequency synchronization method which works well with large frequency offset. By specially designed training sequences, this method can achieve sharp timing correlation peak without any additional operations in timing detection. And the cross-correlation also ensures the estimation performance for the large frequency offset.
Thirdly, considering the multiple timing and frequency offsets in distributed MIMO systems, a maximum likelihood (ML) estimation model is set up. An iterative space-alternating generalized expectation-maximization (SAGE) based algorithm is proposed to resolve the problem of high computational complexity in ML estimation. Furthermore, the Cramér-Rao bound (CRB) for the multi-parameter estimation is also derived as a benchmark. The simulation results show the performance of the proposed method can reach the CRB.
Finally, considering the multiple timing offsets in distributed MIMO-OFDM systems, two time tracking methods using pilots in the frequency domain are proposed. One is the method based on asynchronous coherence. This method is with simple structure and easy to perform. The other method is a ML estimation method with considering channel fading. This method can achieve better time tracking performance both with channel estimation.
The research in this thesis extends the existing researches on OFDM synchronization, and explores the problem of synchronization in distributed MIMO systems and distributed MIMO-OFDM systems. The achievements have certain theoretical and practical values.
同步是通信系统首先需要解决的问题,同步的好坏直接影响整个系统的性能。OFDM系统对同步误差非常敏感,很小的频率偏移都将导致系统性能的急剧恶化。准确的时间同步和精确的频率同步是OFDM系统正常工作的前提。同时,MIMO系统,特别是分布式MIMO系统中的同步误差会带来严重的天线间干扰。因此,同步技术成为分布式MIMO-OFDM系统中的关键技术。
论文针对OFDM系统、分布式MIMO系统和分布式MIMO-OFDM系统中的同步技术进行了研究,主要包括以下内容:
分析了同步误差对OFDM系统及MIMO-OFDM系统性能的影响;针对基于互相关的时间同步方法,比较了几种时间同步检测门限的设置方法,分析表明,其中利用接收序列能量的方法不仅能够适应信道衰落的变化,而且可以工作在信噪比大范围变化的环境中。
基于对恒模零自相关序列的研究,本文提出两种特殊环境下的OFDM同步方法:一种是存在窄带干扰时的OFDM时间同步方法,该方法在频域同时完成干扰的检测、消除和时间同步估计;另一种是存在大频偏时的时间、频率同步方法,该方法通过特殊的训练序列设计,在不改变互相关时间同步检测方式的前提下,仍能取得互相关尖锐的相关峰,同时互相关的检测方式也保证了对大范围频偏的估计性能。
考虑分布式MIMO系统中存在多时延、多频偏的情况,建立了多参数最大似然(ML)联合估计的模型;并且针对直接计算ML估计复杂度高的问题,提出一种基于空间交替期望最大化算法的迭代计算方法;同时推导得出了多参数联合估计的克拉美-罗限(CRB);仿真表明所提算法的性能能够达到CRB。
针对分布式MIMO-OFDM系统中存在多个时延偏差的情况,提出两种利用频域导频的定时跟踪方法:一种是低复杂度的基于异步相干的方法;另一种是考虑信道衰落影响的ML估计方法,该方法在取得良好定时跟踪性能的同时,也能完成对信道衰落参数的估计。
本文丰富了对传统OFDM系统同步技术的研究,并对分布式MIMO和分布式MIMO-OFDM系统的同步技术进行了探索性的研究,所得到的研究成果具有一定的理论价值和应用价值。
Mobile communications always develop to provide higher data rate and better transmission quality. Orthogonal frequency division multiplexing (OFDM), which has been attracting much attention, can achieve higher frequency spectrum efficiency and overcome frequency-selective fading or narrow-band interference. Multiple input multiple output (MIMO), which has become one of the focuses in the area of wireless communications, can increase channel capacity and frequency spectrum efficiency remarkably. Hence MIMO-OFDM which is the combination of MIMO and OFDM has become the core technology of the next generation of mobile communications.
Synchronization is the primary mission of communication systems, which influences the performance of the whole systems directly. OFDM is sensitive to synchronization errors. The performance of OFDM system will be deteriorated severely only by small carrier frequency offset. Correct time synchronization and precise frequency synchronization are the necessary precondition for OFDM systems. Furthermore, synchronization errors would bring severe inter-antenna interference to distributed MIMO systems, especially to distributed MIMO-OFDM systems. Hence synchronization has become the key technology of distributed MIMO-OFDM systems.
This thesis focuses on the synchronization problems in OFDM, distributed MIMO and distributed MIMO-OFDM systems. The main contributions of this thesis can be described as follows:
Firstly, the influences of timing error and carrier frequency offset on OFDM and MIMO-OFDM systems are mathematically modeled and analyzed. For the time synchronization method using cross-correlation, several different methods of setting detection threshold are compared. The method using the power of the received sequence not only adapts to the change of channel fading, but also works well with large change of signal-to-noise ratio.
Secondly, based on the research on const amplitude zero auto-corelation (CAZAC) sequences, two synchronization methods for OFDM systems are proposed. One is the time synchronization method which works under narrowband interference. In this method, the interference detection and suppression procedure and the timing procedure are both processed in the frequency domain. Another method is the time and frequency synchronization method which works well with large frequency offset. By specially designed training sequences, this method can achieve sharp timing correlation peak without any additional operations in timing detection. And the cross-correlation also ensures the estimation performance for the large frequency offset.
Thirdly, considering the multiple timing and frequency offsets in distributed MIMO systems, a maximum likelihood (ML) estimation model is set up. An iterative space-alternating generalized expectation-maximization (SAGE) based algorithm is proposed to resolve the problem of high computational complexity in ML estimation. Furthermore, the Cramér-Rao bound (CRB) for the multi-parameter estimation is also derived as a benchmark. The simulation results show the performance of the proposed method can reach the CRB.
Finally, considering the multiple timing offsets in distributed MIMO-OFDM systems, two time tracking methods using pilots in the frequency domain are proposed. One is the method based on asynchronous coherence. This method is with simple structure and easy to perform. The other method is a ML estimation method with considering channel fading. This method can achieve better time tracking performance both with channel estimation.
The research in this thesis extends the existing researches on OFDM synchronization, and explores the problem of synchronization in distributed MIMO systems and distributed MIMO-OFDM systems. The achievements have certain theoretical and practical values.
引文
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