摘要
通过无线网络节点的互相协作,无线协同通信能够充分利用多入多出(MIMO, Multiple Input Multiple Output)的技术优势,有效解决传统蜂窝的容量瓶颈和小区覆盖问题。无线协同通信已成为学术研究重点之一,并且已经获得了产业的广泛关注,将成为未来无线通信不可或缺的技术。本论文主要对无线协同中继通信的分集理论进行深入探讨,旨在完善相应的理论研究,在此基础上,对无线协同中继通信中的资源分配和传输策略进行研究以进一步优化系统性能。论文首先对衰落信道下单天线无线协同中继的容量特性展开研究,紧接着分析了无线协同中继系统多天线分集性能,然后对无线协同中继网络多用户分集理论进行探讨；基于这些理论研究,论文随后研究了无线协同中继功率分配策略,最后对多业务协同中继功率和带宽调度进行了探讨。论文主要工作和创新在于：
1.衰落信道下单天线无线协同中继容量的研究
单天线多中继系统为无线协同中继通信的经典结构。论文首先针对通信的基本问题——系统容量开展了研究,通过理论推导研究了衰落信道下单天线无线协同中继的遍历容量和中断容量。在此基础上,进一步研究了系统容量与中继数目和转发方式的关系,研究表明无线协同中继系统的遍历和中断容量均随中继数目的增加而降低。针对两种不同的转发方式,在低信噪比下,放大转发具备更高的系统容量,在高信噪比下,解码转发则可获取更高的系统容量。
2.无线协同中继多天线分集的研究
多天线技术是未来无线通信的关键技术之一。针对MIMO协同中继网络的分集相关理论进行了研究,特别针对空时块码在MIMO协同中继中的性能进行了分析。通过推导卡方随机变量调和平均的统计特性和相关概率论知识,研究了系统在不同转发方式下以及是否存在直接链路时的系统关键性能指标(包括符号错误概率、中断概率、系统容量等),分析了系统转发方式、天线数目等因素对协同中继系统分集性能的影响。研究表明,采用多天线技术可有效提高无线协同中继网络的分集度,空时块码可获得分集度上限,协同分集和多天线分集可有效融合。
3.无线协同中继多用户分集的研究
针对无线网络多用户特征,建立了协同中继网络的多用户分集理论分析框架,给出了在无线协同中继网络中的多用户分集性能分析,通过理论推导得出了在各种协作方式和转发方式下的多用户系统中断概率和符号错误概率的高信噪比渐进表达式。研究表明,多用户分集可以和协同分集有效融合,共同提高多用户协同中继网络的分集度增益,提升系统总体性能。
4.不同优化目标下的无线协同中继功率分配研究
灵活高效的无线资源管理算法是提升和优化无线网络性能的关键所在,也是需重点关注和研究的问题。本章主要通过功率资源的优化分配来进一步提升无线协同中继网络的性能,对多天线协同空时块码的功率分配问题进行了研究。本章针对源节点获知信道信息程度的不同,分别提出了以容量最大化为目标和以中断概率最小化为目标的功率分配策略。研究表明,提出的功率分配策略在各种信道条件和天线配置下均能依据信道状况优化分配功率,其性能优于传统平均分配策略,且增益随天线数目的变化和信道条件的变化各异。
5.多业务场景下的无线协同中继功率和带宽联合调度研究
正交频分复用(OFDM, Orthogonal Frequency Division Multiplexing)技术被广泛认为是未来无线通信的核心技术之一。针对无线网络中多业务共存的特点,对OFDM无线协同中继系统中的多业务协同传输涉及的无线资源管理与调度问题进行了研究。利用优化论得出了单用户场景下多业务协同传输的资源分配策略,在此基础上提出了一种线性复杂度的次优分配算法。进一步对多用户场景下的业务传输资源分配问题进行了研究,得出了最优功率和带宽分配方案,提出了两种新型的线性复杂度多用户多业务资源分配策略。研究表明,提出的资源调度策略均能有效保障业务的QoS (Quality of Service),多用户场景下的资源分配策略能有效利用存在于多用户协同中继网络中的多用户分集增益提升系统性能。
以上研究成果,分别以学术论文的形式被IEEE的期刊和会议以及国内期刊所录用。
With the cooperation between wireless network nodes, wireless cooperative communication can fully utilize the advantages of MIMO (Multiple Input Multiple Output) techniques without requiring multiple antennas at each node. It can efficiently cope with the capacity bottleneck and coverage issues encountered by traditional cellular networks. The concept of wireless cooperative communication has become one of the major research topics, and also has gained widely interest in the telecommunication industry, which is believed to be one of key techniques in next generation wireless communication systems.
This dissertation focuses on the diversity theory in the wireless cooperative relay communication system, arming at perfecting the related theoretical framework. Based on this, the dissertation further presents a comprehensive study on the resource allocation and transmission strategy design with the goal of optimizing the system performance. First, the capacity study on the sing-antenna cooperative relay system in fading channels is presented, followed by the diversity performance analysis in multi-antenna cooperative relay networks. Then, the multiuser diversity theory is discussed in cooperative relay systems. Based on these theoretical analysis, the dissertation further presents various power allocation strategies with different optimization objectives under different channel state information (CSI) assumptions; finally, joint power and bandwidth allocation strategies aiming at providing heterogeneous services in orthogonal frequency division multiplexing (OFDM) cooperative relay systems are investigated. The major work and contributions of this dissertation consist in:
1. Capacity study of single antenna cooperative relay systems in fading channels
Cooperative relay system with multiple single-antenna relay nodes is the classical model for wireless cooperative communications. The dissertation first investigates the capacity issue of this classical model, which is the basic issue for any communication system. We theoretically analyze the ergodic capacity and outage capacity performance, based on which we further study the impact on the system capacity regarding the number of relay and the cooperation protocol. It shows that the capacity of cooperative relay systems decreases with the increasing of relay. Regarding two classical protocols, amplify-and-forward performs better than decode-and-forward at low SNR regions, while decode-and-forward performs better in high SNR regions.
2. Diversity performance of multi-antenna wireless cooperative relay systems
MIMO is one of key techniques in the next generation wireless communication. We present the theoretical diversity performance study in MIMO cooperative relay networks, with specially focus on the space-time coding strategy in obtaining the spatial diversity gain. By deriving the statistics of the harmonic mean of Chi-square distributed random variables and using the related theory in Probability Theory, we present closed form expressions regarding key system performance metrics, including symbol error probability, outage probability and system capacity, by which we further study the impact of various system configurations on the system performance, including cooperation protocols, antenna configurations etc. It shows that by adopting multiple antennas, the system can achieve higher diversity order, while the space-time coding discussed in this dissertation can obtain the full diversity gain. The cooperative diversity and antenna diversity can be efficiently combined.
3. Multiuser diversity in multiuser wireless cooperative relay networks
Multiuser diversity is an inherent diversity in multiuser wireless systems. A theoretical framework to analyze the multiuser diversity in cooperative relay networks is presented, based on which the performance of multiuser diversity in cooperative multi-relay networks is presented. We first derive asymptotic expressions of outage probability and symbol error probability for various cooperation protocols with joint multiuser and cooperative diversity. Both the theoretical analysis and simulations demonstrate that the multiuser diversity can be readily combined with the cooperative diversity, while the multiuser diversity dependent scheduling can efficiently exploit such gain to improve the system performance.
4. Power allocation in the cooperative relay system with different optimization objectives
Efficient and adaptive radio resource management (RRM) is very important for wireless systems. This dissertation discusses the optimizing power allocation in wireless cooperative relay systems. We specifically study power allocation problems in MIMO cooperative relay networks adopting space-time coding. With different CSI assumptions at the transmitter, both optimal power allocation strategies aiming at maximizing the system capacity and at minimizing the system outage probability are proposed. The study shows that the proposed schemes can allocate power adaptively according to channel conditions. All schemes perform far better than the traditional methods, while the gain depends on antenna configurations and channel conditions.
5. Joint power and bandwidth allocation in wireless cooperative relay networks with heterogeneous services
OFDM has been recognized as one of the key techniques for the next generation wireless communications. We discuss RRM strategies in OFDM cooperative relay networks with heterogeneous services. The optimal power and subcarrier allocation scheme for point-to-point transmission of heterogeneous sservices is derived, based on which a sub-optimal algorithm with linear complexity is proposed for the practical implementation. Then, the idea is extended to multiuser scenario. Optimal solution is provided, and two fastest-power-descending based power and subcarrier allocation algorithms are proposed. Simulation result shows that the proposed schemes improve the spectral efficiency while guaranteeing the QoS of each service. And it also demonstrates that the proposed algorithms in multiuser scenario can efficiently exploit multiuser diversity gain to improve the system performance.
The research outputs related to this dissertation have been accepted as papers in IEEE journals and conferences.
引文
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[1]. J. N. Laneman, D. N. C. Tse, and G. W.Wornell, "Cooperative diversity in wireless networks: effcient protocols and outage behavior," IEEE Trans. Inf. Theory, vol.50, no.12, pp.3062-3080, Dec.2004.
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[5]. Y. Zhao, R. Adve, and T. J. Lim, "Improving amplify-and-forward relay networks: optimal power allocation versus selection," IEEE Trans. Wireless Commun., vol.6, no.8, pp.3114-3123, Aug.2007.
[6]. G. Farhadi, and N. C. Beaulieu, "Ergodic capacity analysis of wireless relaying systems in Rayleigh fading," in Proc. of ICC’08, Beijing, May 2008, pp.3730-3735.
[7]. Shuping Chen, Wenbo Wang, Xing Zhang and Dong Zhao, "Ergodic and outage capacity analysis of cooperative diversity systems under rayleigh fading channels," in Proc. of IEEE ICC 2009 workshops, Dresden, Germany, Jun.2009.
[8]. Fundamental Research on Multi-domain Collaboration for Broadband Wireless Communications, National Basic Research Program of China (973 Program in China), http://www.973.gov.cn/English/Index.aspx.
[9]. L. Xie and P. R. Kumar, "A network information theory for wireless communication:scaling laws and optimal operation," IEEE Trans. Inf. Theory, vol.50, no.5, pp.748-767, May 2004.
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