【作者】 王欣；

【导师】 魏急波；

【作者基本信息】 国防科学技术大学 ， 信息与通信工程， 2007， 博士

【摘要】 多输入多输出(MIMO)技术和正交频分复用(OFDM)相结合大大扩展了MIMO系统的应用环境,使得系统可以有效工作在频率选择性衰落环境中,并同时具有MIMO和OFDM系统的优势。MIMO-OFDM系统可以同时使用空间和频率分集来进一步提高系统的性能,达到系统性能的理论极限,而获得这一极限的一个重要途径就是空频编码(Space-Frequency Coding:SFC)。根据接收端检测方法的不同,目前空频编码MIMO-OFDM系统主要分为三类:相干空频编码系统、非相干空频编码系统和差分空频调制系统。论文首先提出了一个空频码字传输过程的等效模型,它可以将频率选择性衰落信道下空频码字的传输模型转化成平衰落信道下空时码字的传输模型,这样有效克服了空频码字尤其是非相干空频码字性能分析的瓶颈,大大简化了空频码字的性能分析,使得现有空时编码中性能分析的结论可以平滑地扩展到对空频码的性能分析中来。同时通过所提出的等效模型,就可以根据空时码已有的研究结论对空频码字的某些性质做出预测,从而指导空频码字的研究工作,这一推论在论文的后续研究中也得到了证实。在所提出的等效模型基础上,论文提出了一种新的相干空频码字性能分析方法,得到了一个优良的性能分析表达式,从该表达式中可以严格地得到分集增益和编码增益这两个最重要的空频码性能指标定义,它遵循了Tarokh在空时编码研究中所给出的关于这两个编码系统性能指标的概念规范。根据所提出的性能分析表达式,论文提出了相干空频码的编码准则和一种实现结构,在获取指定阶数分集效果的基础上最优化了编码增益,并通过仿真验证了理论分析的结论。非相干空频码具有接收端不需要知道信道信息的特点,而到目前为止,针对非相干空频码的性能分析和如何降低编译码复杂度的问题还没有得到很好的解决,这些问题已成为研究非相干空频码的瓶颈。论文提出了一种具有子块编码结构的非相干空频码,通过减少码块大小有效降低了系统编译码复杂度,并利用所提出的等效模型,给出了具有子块编码结构非相干空频码的性能分析表达式,提出了获取全分集的非相干空频码的编码准则和优化编码增益的方法,并且设计了一种非相干空频码的实现结构,通过理论分析和计算机仿真验证了论文中所提出的结论。目前大部分对空频编码的研究都假设了不同天线对上的信道衰落系数是相互独立的,而在实际系统中,天线之间总是会存在相关性的,论文研究了天线相关情况下空频码的性能。利用所提出的空频码字性能分析方法,论文分析了在天线不相关情况下所设计的空频码字(包含相干和非相干两种情况)在存在天线相关性的实际信道上传输时所能获得的性能,证明了为不相关天线所设计的最优码字在相关天线情况下也能达到最优分集性能的这一结论。在接收端不知道信道状态信息的情况下,差分调制也是一种被广泛采用的技术,论文提出了应用于差分空频调制的非相干序列检测方法,该方法通过积累多个符号上的信道信息来完成发送符号的检测,可以消除由于长的信道延迟扩展带来的误码平层。论文同时还提出了一种数值方法来评估所提出的非相干序列检测的性能。理论分析和仿真结果都验证了这种检测算法对长信道时延的适应能力论文最后还对差分空时调制中的多符号检测算法进行了研究,提出了一种降低差分空时调制系统中多符号差分球形译码算法复杂度的方法,通过改进球形译码中搜索树的排序策略,使得系统在牺牲较少性能的基础上大大降低了算法实现的复杂度。更多还原

【Abstract】 The combination of multiple-input multiple-output (MIMO) technology and orthogonal frequency division multiplexing (OFDM) systems can greatly expend the application area of MIMO, and it can work well in frequency selective fading channels. It owns the both advantages of MIMO and OFDM. MIMO-OFDM system can exploit the space and frequency diversity simultaneously to improve the performance of wireless communication system, and achieves the theoretic margin of system. One of the most important ways to achieve this margin is space-frequency coding (SFC).According to the decoding method using in the receiver, we assort the SFC into three types: coherent SFC, noncoherent SFC, and differential space-frequency modulation.In this dissertation, an equivalent signal model of the transmission of space-frequency codes is proposed in the first place. It can be used to transform the transmission model of space-frequency codes under frequency selective fading channels into the model of space-time codes in flat-fading channels, and this simplified the analysis of the performance of space-frequency codes significantly. With this equivalent model, some important conclusions of space-time codes can be extended smoothly into the description of space-frequency coding. This facilitates the prediction of the property of space-frequency coding and help to guide our research, and these conclusions are also verified by the following research and simulation.Based on the equivalent model, a novel analysis method of coherent space-frequency coding is proposed and an expression with excellent form for the performance is obtained. From this expression, the two important indexes of space-frequency codes, diversity order and coding gain, are inferred strictly, and they follow the concept firstly proposed by Tarokh in the research of space-time coding. With these two indexes, the coding criterion and a realization structure are proposed. Simulation results are provided to verify our conclusion.Up to now, the performance analysis of noncoherent space-frequency coding whose receiver does not know the channel state information and the methods to reduce the encoding-decoding complexity have not been solved, and this has constituted a bottleneck of the research of noncoherent space-frequency coding. Here a sub-block noncoherent space-frequency coding is proposed to reduce the size of codeword and the complexity. Using the equivalent model proposed previously, the performance expression of noncoherent space-frequency coding is deduced, and the coding criterion to achieve full diversity and optimize the coding gain is obtained. It also provides an implementary construction of noncoherent SFCs and uses computer simulation to verify our conclusion.The former researches of space-frequency coding are all assume that channel coefficients on the different antenna pairs are mutually independent, but it is not true in reality. Under our analysis of pairwise error probability, we provide some propositions to describe the performance of SFCs (both coherent and noncoherent) designed under the assumption of independent fading but transmitted in a correlated channel. We also point out that how to encode the system under a correlated channel.Differential modulation is also a widely used method to the case of no knowledge of the channels in the receiver. Here we propose noncoherent sequence detection for differential space-frequency modulation, which can be used to alleviate the error floor that occurs in a multipath channel with long time delay expansion. We also propose a performance evaluation method to our detection algorithm. Simulation results show that our algorithm enforces the adaptability of differential space-frequency modulation to the time delay expansion.Finally, we give our work to reduce the complexity of multiple symbol differential sphere decoding in differential space-time modulation. Through the modification of the enumeration strategy of the search tree in sphere decoding, we significantly reduce the complexity of detection at the expense of little loss on performance.更多还原