【作者】 姜华；

【导师】 都思丹；

【作者基本信息】 南京大学 ， 电路与系统， 2013， 博士

【摘要】 近几年来,为了应对用户对更高的数据速率、更好的服务质量以及更大的网络容量需求,人们在多天线多输入多输出(Multiple-Input Multiple-Output, MIMO)的基础上引入了一种新的无线通信模式,即协作通信(Cooperative Communication)。架设一个或多个中继节点,在通信终端之间的直接信道中建立辅助信道,可以对抗无线信道的衰落,有效地扩大无线传输的覆盖半径,增加传输速率,提高传输可靠性。作为一种能够获得空间分集增益的虚拟MIMO技术,协作通信技术广泛应用于蜂窝网络、传感器网络、和Ad Hoc网络,在国内外学术界和工业界已经成为无线通信中主流研究热点。本文对于国内外在无线协作通信技术领域的研究动态进行了系统性的阐述与分析,并对分布式协作通信和面向多用户的MIMO协作通信的关键技术进行研究,属于第四代无线移动通信系统以及未来无线移动通信系统的研究范畴。本文在物理层层面上的创新性研究内容包括全双工模式下提出的一种新的分布式空时编码；基于正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)技术的异步协作系统下的空时编码方案；面向多用户的MMO中继协作通信的联合预编码设计等方面。协作通信在全双工(Full-Duplex)模式下相比于半双工(Half-Duplex)模式具有更高的频谱效率。从全双工模式下的协作通信场景出发,本文提出了一种新的分布式分圆准正交空时分组码(Quasi Space-Time Block Code, QOSTBC)。它的基本思想主要是通过在多个中继节点对信号作简单的处理,使接收端接收到的信号具有空时分组码特有的结构,从而对接收信号矢量进行联合解码,并能获得较高的协作分集增益。此外,相比于传统的分布式准正交空时分组码,本文提出的分圆准正交空时分组码不仅需要更少的解码运算量,在全双工模式下还具有更低的终端到终端延迟时间的特点。本文通过蒙特卡罗仿真实验证实了方案的优越性和理论分析的正确性。在理想研究情况下,多个中继节点之间是完全同步的,即假设多个中继节点同时发送,并且发射载频以及不同中继节点到接收端的传输延迟也是相同的。而在实际应用中,由于受到中继节点分布性特点以及不同晶振等客观条件的限制,多中继协作系统存在着其内禀的时间异步特性,因此,研究协作通信系统中的时间同步问题具有重要意义。本文研究了异步协作通信的空时编码实现方案,将异步下的双中继Alamouti玛传输方案扩展到使用分圆准正交空时分组码的四中继场景中。利用了正交频分复用技术的特点,对时域信号添加足够长的循环前缀以克服异步条件下引起的时延偏差,并在中继节点采用了时序逆转等高级信号处理方法,从而解决传统空时码字无法在异步环境下直接使用的缺点,并在全双工异步协作下获得了分集增益。MIMO协作通信系统在面对多用户的条件下设计时,往往存在用户间的数据流串扰问题。预编码技术是通过在发射端对发射信号增加某种预扰动来针对性地对抗无线通信中存在的用户间干扰等各种不良效应,从而有效改善系统性能并简化接收端的设计。本文首次提出了一种基于格基约减的Tomlinson-Harashima Precoding (THP)基站,中继联合预编码的技术,在发射端通过反馈技术获得信道状态信息的情况下,利用预编码(或者预均衡)等方法,将移动台(MS)端的复杂信号处理部分转移到基站(BS)和中继是可以实现的。格基约减技术是一种将信道矩阵转变成近似正交的等效信道矩阵的技术。从基站到用户发送数据需要受到基站到中继以及中继到用户的两个衰落信道和噪声的影响,格基约减对信道矩阵的等效变换能够在信号检测时减少这种不利影响,提高检测的准确性。在信道矩阵约减的基础下,THP预编码技术通过判决反馈加强对变换星座边界的控制,提高判决的准确性。仿真结果表明,所提出的预编码方法能够获得满分集增益,显著提高系统误码率性能。本文对于新一代无线协作通信系统中的分布式空时编码和MIMO多用户预编码技术的总结、研究和创新丰富了协作通信的理论研究成果,对实际系统中的分布式空时编码和MIMO多用户协作预编码技术的应用也具有重要的指导意义。更多还原

【Abstract】 Recently, since the fast growing demands for high-data-rate, high-quality com-munications and high-network-capacity over wireless channels make the design and implementation of wireless communication systems more and more challenging, co-operative communications, which is based on multiple-input multiple-output (MIMO) technique, attracts much attention and extensive research. While one or more relays are equipped with, auxiliary channels can be built between the communication terminals. This technology can combat the wireless fading channels, expand the network cover-age and increase the network capacity. As a special form of distributed MIMO system, cooperative communication technique can be applied to cell networks, sensor network-s and ad hoc networks. Therefore, cooperative communication has already become a research hotshot in both academic and industrial society.In this thesis, recent research activities in multi-point cooperative communications are comprehensively addressed and systematically analyzed. This dissertation studies the key technologies of distributed cooperative communications and multiuser MIMO cooperative communications, the scopes of which cover the enabling technologies of the fourth and future generations of wireless mobile communication systems. The main contributions of the dissertation in physical layer lie in a new distributed space-time block codes in full-duplex cooperation, Orthogonal Frequency Division Multiplexing (OFDM) based STBC transmission schemes in asynchronous cooperative systems, and joint base station/relay station precoder design for multiuser MIMO relay systems.Compared to half-duplex mode, full-duplex mode can provide higher spectral effi-ciency in cooperative systems. In this dissertation, cyclotomic quasi-orthogonal space time block codes (QOSTBC) are proposed for full-duplex cooperative communication scenarios. With simple signal processing at the relays, the received signals at the des-tination node have the STBC structure. Then, the decoding can be performed by a joint method, and full cooperative diversity can be achieved. Besides, compared to that of conventional QOSTBC, the proposed codes achieve a lower decoding com-plexity without any performance degradation, and induces lower end-to-end delay in full-duplex mode. The proposed transmission scheme is evaluated and compared in terms of performance by monte carlo simulation.In ideal case, the different relay nodes are assumed to be synchronous. That is, these relay nodes simultaneously transmit the signals. Besides, not only transmit car-rier frequency but also the timing delay between relay nodes and destination node are same. However, in actual applications, since different relay nodes have different oscil-lators and different locations, multi-relay cooperative communication system is inher-ently time asynchronous. Consequently, it is very important to do the research on the time synchronization for these systems. This dissertation investigates Alamouti space-time transmission scheme for asynchronous cooperative systems, and extends it to the four-relay case by using cyclotomic QOSTBC. The adoption of OFDM technique has been fully taken advantage of, then it is possible to combat timing delay by preceding cyclic prefix with enough length. In addition, the relay nodes adopt advanced signal processing methods such as time-reversion. These methods solve the problem that the conventional STBC schemes are not effective in the asynchronous cooperative scenar-ios.In MIMO multi-user cooperative communications, the base station (BS) can trans-mit the data to the multiple users via relay station (RS) simultaneously. However, there exists inter-stream interference among different users. Then, precoding technique is adopted to make some kind of pre-disturbance to the transmitted signals so that it could combat with the adverse effects, and thus effectively improves system perfor-mance as well as simplifies the design of the receivers. This paper propose a joint BS/RS lattice-reduction-aided Tomlinson-Harashima Precoding (THP). As BS know the perfect channel state information by feedback technique, it is possible to transfer the complicated signal processing from user terminals to transmitters. Lattice reduc-tion technique is capable of transforming channel matrix to nearly orthogonal one. The fading channel and noises matrices between BS to RS and RS to users may cause the data transmission conditions deteriorate. Lattice reduction can reduce this influence, and improve detection performance. THP precoding is able to enhance the control of constellation boundary by decision feedback and improve the accuracy of decision. Simulation results show that the proposed scheme significantly improves system per-formance and achieves full diversity.The summary, research and innovation of the distributed STBC scheme and pre-coding scheme in the new generation of wireless communication systems enrich the theoretical results of cooperative communication, providing insights to the practical system design.更多还原