【作者】 但黎琳；

【导师】 李少谦；

【作者基本信息】 电子科技大学 ， 通信与信息系统， 2010， 博士

【摘要】 作为未来移动通信的主要候选技术,正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)的局限在于信号包络不稳定,具有很高的峰值平均功率比(Peak-to-Average Power Ratio, PAPR,简称峰均比)的发送信号降低了发射端放大器的效率,并可能导致严重的非线性失真和带外辐射。针对OFDM系统的峰值抑制问题,本文从多个角度进行了联合优化,力求在峰均比、传输性能、频谱效率、计算复杂度等因素之间达到较佳的平衡。本文首先介绍了OFDM通信系统和峰值抑制技术的发展现状,总结了需要进一步解决的问题,并指出了全文的研究内容。本文的第二部分对OFDM信号的峰值波动问题进行了分析,阐明了峰均比的定义、分布以及与功率放大器之间的联系,为后面的章节提供了理论依据。限幅法(clipping)是一类应用广泛的峰值抑制预失真算法,为了同时抑制峰值和带外辐射,常用迭代限幅滤波对发射信号进行处理。但传统迭代限幅滤波算法会引入很高的计算复杂度,不利于上行传输。而另一类基于峰值抵消的预失真算法虽然能够降低计算复杂度,其缺陷在于带内失真偏高,峰值回升过大。针对传统预失真算法的缺点,本文在第三部分提出了一种改进的峰值抵消预失真算法。与迭代限幅滤波等传统算法相比,该算法可以在计算复杂度、峰值抑制和带内失真之间取得更佳平衡,且计算复杂度非常低,利于上行传输。针对改进峰值抵消算法所引入的非线性失真,本文还研究了相应的接收信号恢复算法以进一步提升系统的传输性能。此外,峰值抵消预失真算法所引起的非线性失真会对导引符号造成干扰,恶化信道估计性能。针对这个问题,本文讨论了一种低复杂度的基于窗函数优化设计的峰值抵消算法,从而高效地消除位于导引位置的非线性失真,提升系统在衰落信道下的传输性能。以部分传输序列(Partial Transmitting Sequence, PTS)为代表的多信号表示法可以无失真地抑制发射信号的峰均比,利于下行传输。但过高的计算复杂度是其实用化的一个障碍。针对这个问题,本文的第四部分研究了一种改进的PTS算法,通过几何内插估计发射信号的最大包络值,从而降低传统PTS法的计算复杂度。PTS的另一个缺点在于发射机需要传输额外的副信息,影响了系统的频谱效最后,本文研究了联合多址OFDM系统的峰值抑制技术。对于OFDM-CDMA (Orthogonal Frequency Division Multiplexing Code Division Multiple Access)系统,研究了一种PTS盲检测的改进算法,同时提升频谱效率和优化传输性能；此外,还提出了一种具有更低计算复杂度和更佳峰值抑制性能的多信号表示算法。针对集中式频分多址(Localized Frequency Division Multiple Access, LFDMA)系统,本文分析了信号的PAPR特性,比较了现有的峰值抑制算法在该系统的适用性,并讨论了适用于该系统的低复杂度峰值抵消算法。更多还原

【Abstract】 As one of the leading candidates for future wireless communication systems, orthogonal frequency division multiplexing (OFDM) suffers from the drawback of high peak-to-average power ratio (PAPR), which would reduce the efficiency of the high power amplifier (HPA) and cause severe in-band nonlinear distortion as well as out-of-band distortion. To tackle PAPR reduction problems in OFDM systems, this dissertation focuses on optimizing total system performance from several perspectives, by seeking a tradeoff among factors like PAPR, transmission performance, spectrum efficiency and computational complexity.The first chapter of this dissertation introduces the features of OFDM systems, reviews the state-of-the-art PAPR reduction techniques, summarizes the problems needed to be addressed, and outlines the main contents of this dissertation.In the second chapter, this dissertation analyzes the PAPR of OFDM signals, and clarifies the definition and distribution of PAPR models and their relations with HPA, serving as the theoretical base of the latter chapters.Clipping methods are a class of widely used peak reduction algorithms. In order to reduce peak values and out-of-band distortion simultaneously, transmitted signals are usually processed by iterative clipping-filtering algorithms. The high computational complexities of the conventional iterative-clipping-filtering algorithms make them unsatisfactory for uplink transmission. While peak cancellation methods have low computational complexities, they would bring problems such as in-band distortion and large peak regrowth. The third part of this dissertation proposes an improved peak cancellation method, which could obtain a better tradeoff among computational complexity, peak reduction and in-band distortion. This method is suitable for uplink transmission for its considerably low computational complexity. Furthermore, a receiver structure with distortion reconstruct scheme is introduced to further improve system performance. The nonlinear distortion introduced by the pre-distortion method would interfer pilots and deteriorate channel estimations. Accordingly, this dissertation presents an improved cancellation algorithm with low complexity, which could effectively eliminate nonlinear distortions on pilots and improve the transmission performance in fading channels.Multiple signal representation (MSR), such as partial transmit sequence (PTS), could reduce PAPR without introducing distortion. Therefore MSR is suitable for downlink transmission. However its high computational complexity is the main obstacle of its utilization. The fourth part of the dissertation proposes a modified PTS algorithm, which could efficiently reduce the computational complexity by estimating the maximal envelope of transmitted signals using geometric interpolation. Another drawback of PTS is the requirement of extra side information. To deal with this problem, a blind PTS algorithm is introduced for pilot-aided OFDM systems.The last chapter of the thesis investigates PAPR reductions of multiple access combined OFDM systems. For Orthogonal Frequency Division Multiplexing Code Division Multiple Access (OFDM-CDMA) systems, an enhanced blind PTS algorithm is proposed to increase spectrum efficiency and transmission performance. A distortionless peak reduction algorithm with low computational complexity is also introduced. For localized frequency division multiple access (LFDMA) systems, a selection among available peak reduction methods has been made and the advantages of the peak cancellation algorithm proposed by this dissertation have been verified.更多还原