【作者】 刘明珠；

【导师】 杨莘元；

【作者基本信息】 哈尔滨工程大学 ， 通信与信息系统， 2009， 博士

【摘要】 多载波技术已经成为当代通信技术发展的重要一环,它在提高通信系统传输速率、改善频带利用效率和抗多径干扰能力等方面发挥了重要的作用。在多载波调制技术中,正交频分复用技术(Orthogonal Frequency Division Multiplexing, OFDM)更是因其优越的数字化技术、良好的抗频率选择性衰落性能以及高信息传输速率,成为未来移动通信的主流技术,并已经成功地应用于无线本地环路(Wireless Local Loop,WLL)、数字音频广播(Digital Audio Broadcasting, DAB)、无线局域网(Wireless Local Area Network, WLAN)等系统中。另外,OFDM技术还易于结合空时编码、分集、干扰抑制以及智能天线等技术,最大程度地提高物理层信息传输的可靠性。虽然OFDM技术有很多优点,但它的一个最主要的瓶颈问题就是其高峰均功率比(Peak-to-Average Power Ratio, PAPR)问题。由于PAPR较高,使得信号经过非线性信道后,会出现严重的信号畸变,导致系统性能的衰退。同时,由于高峰值功率的存在,使得发送端对高功率放大器的线性度要求也相应提高,不但增加系统成本,还会导致低的功放效率。因此,本论文意在研究OFDM系统的峰均功率比特性,并结合其特性重点研究高PAPR的抑制方法和优化算法,达到有效控制OFDM系统峰值功率的目的。本文首先介绍了正交频分复用调制的基本原理和峰均功率比的定义,并给出PAPR的统计特性和度量方法,研究了与PAPR值有关的参量,并提出通过利用、控制和改善这些参量,可以达到对峰值功率控制的目的。同时论文列举了几种常用的峰值功率控制算法及其优、缺点,并着重研究了克服这些算法不足之处的优化处理算法。论文研究内容包括：改进的相位加扰算法、基于信道编码的峰值功率控制算法和改进的预畸变算法等。相位加扰算法是通过破坏子载波相位的一致性,使OFDM信号出现高PAPR的概率显著降低。但由于相位加扰算法的巨大计算复杂度,使得这种算法的应用受到限制。本文提出一种相位控制编码算法(Phase Control Coding,PCC),该算法可以明显降低传统相位加扰算法的计算复杂度。PCC算法是在传统部分传输序列(Partial Transmit Sequence, PTS)算法的基础上,通过将输入信号的数据部分进行2N→2l映射,再进行IFFT运算得到的。其余n-l个状态用来表示相位旋转因子索引,即相位控制码序列,经IFFT运算后再与数据符号相加。再在备选信号中选择PAPR最低的一路进行传输。另外,本文还提出了一种基于m序列映射的优化分割方案,通过理论证明,该种分割方法对功率峰值的改善性能仅次于随机分割方式,比交织分割和相邻分割方式性能优越。编码法是一种较新的无失真减小PAPR的方法,论文分析了Davis法构造Golay互补序列(Golay Complementary Sequences, GCS)的具体方法,及其能够有效抑制PAPR的理论依据。为提高Golay互补序列法的信息速率,本文研究了Golay码与编码调制相结合的改进GCS算法(Improved GCS, IGCS)原理和构造方法,并分析了结合编码调制时,OFDM系统的PAPR性能。虽然少量增加了系统的PAPR,但IGCS算法比GCS算法有更高的信息速率。当子载波数较大时,畸变法抑制PAPR是一种简单、易于实现的方法,但会产生限幅噪声和非线性失真。本文在Ochiai限幅滤波算法的基础上,提出一种基于MMSE准则的限幅失真重构迭代算法(Clipping Distortion Reconstruction Iteration, CDRI)。并利用LDPC信道编码技术、LLR-BP译码算法和MMSE迭代接收技术,实现对系统PAPR的有效抑制,同时有效减小系统的限幅噪声和非线性失真。最后,本文还研究了STFC-OFDM系统的载波干涉功率控制优化算法,通过理论分析和仿真证明,该算法在不同的分群方式下,具有不同的PAPR抑制能力,同时还会改善系统BER性能。更多还原

【Abstract】 Multicarrier technology has been one of the remarkable parts in the territory of today’s communication research. It can have an impact upon several aspects such as enhancing transmission rate, improving frequency efficiency, overcoming the muilt-path interference and so on. This revitalization of the technique, also known as orthogonal frequency division multiplexing (OFDM), is mainly due to the advancing capabilities of digital signal processors, capability of resisting fading and high transmission rate. OFDM technology has been used in Wireless Local Loop (WLL), Digital Audio Broadcasting (DAB), Wireless Local Area Network (WLAN) etc.. OFDM technology is also easy to combine with space-time code, diversity technique, interference restrain and smart antenna for the greatest extent to enhance physical layer transmission reliability.OFDM technique has many benefits, however, high peak-to-average power ratio (PAPR) has become the vital shortcoming for use. The high PAPR signals will be distorted seriously and decline the system performance when passing through nonlinear channels. On the other hand, because of the high PAPR, the severe linearization for HPA is needed. It will increase the cost of hardware system, or decline the HPA’s efficiency. The main purpose of this dissertation is to study the methods and optimum algorithms to reduce the PAPRs of OFDM system and MIMO-OFDM system, and to obtain better PAPR performance and control OFDM system’s peak power as a consequence.At first, the base principles of OFDM modem and its PAPR definition have been introduced in this dissertation. At the same time, the statistical characters, measure means and relative parameters of PAPR have been studied carefully. It can be realized to reduce PAPR by using, controlling and modifying these parameters. Continuously, universal PAPR reducing methods have been introduced and their advantages and disadvantages inclusive. And the researches on optimum algorithms for overcoming these disadvantages have become the essential problem in this dissertation. The main research aspects contain improved phase disturbance algorithm, channel coding algorithm and improved pre-aberrance methods.Phase disturbance algorithm can make the PAPR value reduced by destroying the phase coherence; it can reduce the probability of high PAPR. Due to its complexity to compute, the use of phase disturbance algorithm has been restricted. So phase control coding (PCC) is proposed to either reduce PAPR or lessen the compute complexity. PCC method bases on classical partial transmit sequence (PTS) algorithm and then makes 2N→2(?) mapping for OFDM symbols, and do IFFT operation at last. Phase rotating index is represented by the other n-l states, it also calls phase control code. After IFFT, the result data are added to original data symbols. From the final results, the group having the lowest PAPR value will be sent. In addition, an new group partition method has been proposed, which bases on the m-sequence mapping technique. By theoretical demonstration, the new group partition method is excellent than interweave and adjacent partition methods, but is not a patch on the random partition method.Coding algorithm is a novel method to reduce PAPR and it can’t bring distortion in the system. In the dissertation, I analyzed the constitution process of Davis’s Golay complementary sequences (GCS), and approached the theory foundations of reducing PAPR. To increase the information rate of GCS construction, an improved GCS (IGCS) algorithm is proposed which is constituted by combining Golay code and coding modem. Successively, the PAPR performance with new algorithm has been analyzed. By simulation, IGCS method is able to reach higher information rate than GCS method at the cost of increase PAPR value. Clipping methods are usually used in the condition of large numbers of subcarriers. These methods can be realized easily, but they can bring clipping noise as well as nonlinear distortion. Based on discussing Ochiai clipping and filter algorithm and MMSE receiver construction, I proposed clipping distortion reconstruction iteration algorithm (CDRI). CDRI algorithm utilizes LDPC coding technique, LLR-BP decoding algorithm and MMSE iteration reception to not only reduce PAPR values but also lessen the clipping noise and nonlinear distortion.At the end of this dissertation, the proposed algorithm has different PAPR capability with various kinds of partition methods, and at the same time, new algorithm will amend BER performance.更多还原