【作者】 李勇；

【导师】 沙学军； 郑福春；

【作者基本信息】 哈尔滨工业大学 ， 信息与通信工程， 2014， 博士

【摘要】 现代信息社会迅速发展,通信设备的快速移动性以及通信信号的高速率传输对通信行业的要求也越来越高。而现在通信环境也越来越复杂,诸如在LTE-A、WIMAX、水声(underwater acoustic,UWA)通信以及数字视频广播(digitalvideo broadcasting,DVB)通信等这样的通信环境中,传输信号不仅会经历由于高速传输而造成的多普勒频移的影响,进而产生信道时间选择性衰落,而且还会经历由于多个复杂传播路径而造成的多径干扰影响,进而产生频率选择性衰落,在这样的快速时变多径(或者称为双选择性衰落)信道下,通信信号的传输将会受到载波间干扰(inter-carrier interference,ICI),符号间干扰(inter-symbolinterference,ISI)以及载波频率偏移(carrier frequency offset,CFO)干扰的影响,而这些会严重影响通信质量,因此研究快速时变多径信道下的信号传输和干扰抑制问题具有重要意义。传统的快速时变多径信道下干扰抑制方法主要是基于OFDM系统。但是,由于信道的强时变性引起的多普勒频移的影响,破坏了子载波之间的正交性,从而产生严重的ICI。为了抑制ICI的影响,一些文献提出基于单载波系统的频域均衡算法。然而,由于多径延迟的干扰,单载波系统很容易受到ISI的影响,这也会影响到无线通信系统的性能。为此提出一种基于加权分数傅里叶变换的混合载波系统,来解决快速时变多径信道下ICI/ISI干扰抑制问题。特别地,混合载波系统可以看成是单载波系统和OFDM系统的一种融合,能够表现出相比较于单载波和OFDM在干扰抑制上更大的优势。另外一方面,从解调体制上分析,传统的OFDM系统利用了傅里叶变换进行解调,由于傅里叶变换无法分析局部干扰的影响,因此迫切希望找到一种简单的能够进行局部干扰分析的工具。幸运地是,一种新的解调工具,部分FFT解调提出,在局部干扰分析方面能够优于传统的傅里叶变换解调。本文的目的是利用混合载波和部分FFT解调的优势,提出一种新型的混合载波-部分FFT解调系统,对快速时变多径信道下的ICI以及CFO干扰,进行进一步地抑制。并且提出一种基于这种新型系统的低复杂度均衡算法,本文主要的研究工作如下:本文首先发展了多项加权分数傅里叶变换(multiple weighted fractional Fouriertransform,m-WFRFT)的理论。针对不同多项加权分数傅里叶变换之间关系不明朗的问题,本文基于加权分数傅里叶变换矩阵,建立了不同的多项加权分数傅里叶变换的调制阶数的关系。基于多项加权分数傅里叶变换之间调制阶数的关系,本文提出了一种广义的混合载波系统,这可以看成是基于4-WFRFT的混合载波系统的广义形式。最后分析了其在无线通信中的潜在应用,为后文提供了重要的理论基础。针对快速时变信道下载波间的干扰问题,本文提出了基于混合载波-部分FFT解调系统的ICI抑制算法,并且设计了该算法的实现结构。首先,混合载波系统在双选信道下干扰抑制方面相比较于单载波和OFDM系统具有明显的优势。另外,部分FFT解调能够将有用信号上的干扰分配到很多无用的信号点上,这样在有用信号点上的干扰就会相应的减少,更有利于符号判决,而这是传统的傅里叶变换所不能实现的。优化的分块数根据快速时变信道下残余干扰的变化得到。进一步地,优化的调制阶数可以根据归一化多普勒设计得到。从数值仿真中可以得到,在典型的快速时变信道下,结合ICI抑制算法的混合载波-部分FFT解调系统,相比较于传统单载波以及OFDM系统,在相同的分块数下,具有更优异的误码率性能。针对快速时变信道下的载波频率偏移干扰问题,本文提出了基于混合载波-部分FFT解调的载波频率偏移抑制算法。分析了载波频率偏移的产生机理,得出在快速时变信道下,多径延迟扩展的时变性是产生载波频率偏移的主要原因。进一步地,本文将所提出的CFO抑制算法应用到了典型的快速时变多径信道中,即DVB通信信道以及水声通信信道。最后,通过数值仿真揭示了基于混合载波-部分FFT解调系统的CFO抑制算法相比较于单载波以及OFDM系统在相同的分块数下的优势。考虑到基于部分FFT解调的传统干扰抑制算法的计算复杂度高的问题,本文利用频域信道的带状性质,提出了基于混合载波-部分FFT解调的带状MMSE均衡算法。经过研究发现频域信道矩阵,经过部分FFT解调后,仍然能够保持频域信道的带状性质,进而能够降低干扰抑制算法的计算复杂度。因此结合带状MMSE均衡,提出了一种低复杂度的均衡算法。进一步地,经过数值仿真验证,本文提出的基于混合载波-部分FFT解调系统的低复杂度均衡算法,能够均衡考虑系统的计算复杂度和性能。更多还原

【Abstract】 Under the rapidly development of modern information age, the high mobility ofcommunication equipment and the fast transmission of communication signal are moreand more challenging for the whole communication industry. However, the communi-cation environment is more complexity than before. And, there will be not only pro-duce high Doppler interference due to the high mobility, but also generate multiple pathinterference due to the complex transmission paths, such as the long term evolution ad-vanced(LTE-A), worldwide interoperability for microwave access(Wi MAX), underwateracoustic(UWA) communication and digital video broadcasting(DVB) communication.These rapidly time-varying multipath(or doubly selective) fading channels will inducethe inter-carrier interference(ICI), inter symbol interference(ISI) and carrier frequencyoffset(CFO), which will severely impair the communication quality. Therefore, the re-search technologies, on the suppression of ICI, ISI and CFO under rapidly time-varyingmultipath fading channels, are important for wireless communications.The traditional methods, to overcome the interference from the rapidly time-varyingmultipath channels, are based on the orthogonal frequency division multiplexing(OFDM)system. However, the orthogonality between different subcarriers has been destroyed dueto the significant time variations under rapidly time-varying multipath channels, whichwill induce significant ICI. Some other works, proposed to mitigate the ICI, are based onthe single carrier modulation(SCM) system with frequency domain equalization(FDE).Unfortunately, SCM system will be easily plagued by the inter-symbol interference(ISI)caused by the large time-delay spread under the rapidly time-varying fading channels,which will also deteriorate the performance of wireless communications. To this end,a weighted type fractional Fourier transform(WFRFT) based hybrid carrier modulation(HCM) system has been exploited to suppress the ISI and ICI under rapidly time-varyingmultipath channels. Specially, HCM system can be as a convergence of SCM and OFDMsystems according to its constitution. Thus, it can be expected to inherit the advantage ofSCM and OFDM system, and to achieve a better performance than both SCM and OFDMsystems under rapidly time-varying multipath channels. Besides, from the demodulationmode, the traditional OFDM system employ the traditional Fourier transform that cannotanalysis the local characteristics of interference. It is a crucial to find a local analysis toolto mitigate the ICI/ISI and CFO under doubly selective channels with the high Dopplerspread. Fortunately, a novel demodulation mode–partial FFT demodulation will outper-form the traditional Fourier transform at the local interference analysis. The aims to thisdissertation, utilizing the novel HCM with partial FFT demodulation system, are to fur-ther suppress the ICI and CFO under typical rapidly time-varying multipath channels.Furthermore, a low complexity equalization algorithm based on HCM with partial FFTdemodulation system will be derived in the dissertation. The main research work can beexpressed as follows:This dissertation firstly develops the theorem of multiple weighted type fractionalFourier transform(m-WFRFT). To the vague relationship of different m-WFRFT, this dis-sertation builds the relationship of modulation order between different m-WFRFT basedon the weight fractional Fourier transform matrix. Furthermore, the dissertation providesits implementation structure and explains its physical meaning. Based on the modulationorder relationship between different m-WFRFTs, this dissertation proposes the general-ized hybrid carrier system, which can be the general form of hybrid carrier system basedon 4-WFRFT. At last, the dissertation also analysis its applications on wireless commu-nications, which can provide the significant theorem basis of other chapters.To the inter-carrier interference(ICI) suppression under rapidly time-varying multi-path channels, this dissertation also proposes the hybrid carrier with partial FFT demod-ulation system based ICI mitigation algorithm, and designs the implement structure ofthis algorithm. The research interest stems from the superiority of the WFRFT basedHCM system over the traditional SCM and OFDM systems as previously explained. Be-sides, the partial FFT demodulation can redistribute the interference on the useful signalinto many other useless signal points. Therefore, the power of ICI over the useful signalpoints will be less than before. However, this cannot be implemented by the traditionalFourier transform. The optimal division number can be derived by the average power ofinterference under the rapidly time-varying multipath channels. Moreover, the optimalmodulation order can be selected by previously design a simple dictionary according tonormalized Doppler frequency. It can be demonstrated from the numerical simulation thatthe superiority of HCM with partial FFT demodulation system involving ICI suppressionalgorithm over doubly selective channels.To the carrier frequency offset(CFO) mitigation under rapidly time-varying multi-path channels, this dissertation has proposed the CFO suppression algorithm based on theHCM with partial FFT demodulation system previously mentioned. Also the disserta-tion has analyzed the generating mechanism of CFO, and obtained that the time-varyingof multipath delay spread is the primary reason to produce CFO under the rapidly time-varying multipath channels. Moreover, the dissertation finally applies the proposed al-gorithm into the typical rapidly time-varying multipath channels, DVB communicationchannels and UWA channels. Furthermore, numerical simulation has been presented, un-der the typical rapidly time-varying multipath channels, to demonstrated the superiorityof HCM system with the CFO suppression algorithm over SCM and OFDM system at thesame division number.Considering the high computational complexity of the partial FFT demodulationbased interference suppression algorithm, the dissertation, employing the banded charac-teristic of frequency channel matrix, has proposed that the HCM with partial FFT demod-ulation system based banded MMSE equalization algorithm. The interests stem from thebanded characteristic of the frequency channel matrix. Also, the dissertation finds thatthe frequency channel matrix after partial FFT demodulation, will also keep that char-acteristic. Therefore, associated with the banded MMSE equalization, a low complexityequalization algorithm has been proposed. Moreover, it achieves a good trade-off betweencomputational complexity and performance.更多还原