【作者】 王荆宁；

【导师】 张中兆；

【作者基本信息】 哈尔滨工业大学 ， 信息与通信工程， 2010， 博士

【摘要】 随着无线通信技术的发展,人们对无线数据服务的需求越来越大。无线信道的衰落特性是阻碍信道容量增加和服务质量改善的主要原因之一,而多天线分集技术是应对衰落的有效方法。多天线的分集要求通信设备具有多个天线,但是很多系统的节点受到体积和功耗等的限制无法安装多个不相关的天线,协作分集通信技术就是在这种背景下应运而生的。协作分集通信的概念是建立在中继信道模型的基础上的,它是一种新的空间分集技术。它利用了无线电波的广播特性,每一个节点都可以接收到其它节点发送的信号并经过一定的处理后转发到目的节点,这样不同的用户就可以共享彼此的天线而形成空间分集。协作分集可以提高无线通信系统的容量,减少通信的中断概率,扩大无线网络的覆盖范围和连通性,节省传输数据的能量等,从而提高无线通信系统的有效性和可靠性。协作分集技术不仅可以应用在蜂窝移动通信网中,而且在传感器网络、认知无线电等无线通信系统中都可以得到很好的应用。协作分集技术的出现也促进了多输入多输出(MIMO)技术的实用化,因而成为未来无线通信技术的重要研究领域。本文着眼于协作分集系统中存在的问题,围绕着什么时候进行协作,与谁协作和怎么协作三个研究方向,重点对协作的时机,协作伙伴的选择,协作方法的改进等协作分集的关键技术进行了研究。第一,协作通信的原理分析与协议设计。首先介绍了协作分集的信道模型和基本原理,给出了协作分集理论计算的增益。然后介绍了解码转发(DF)和放大转发(AF)两种基本的协作方法,并给出了几种协作方法的性能比较。接着介绍了协作通信的流程,考虑了协作通信的特殊性,并将其结合现有的通信系统,设计了带有协作通信的帧结构和通信流程。该协作流程加入了协作伙伴选择、协作建立等所需的时间和流程。并在此基础上分析了协作通信建立的时机,也就是什么时候需要进行协作通信的问题。第二,针对两种不同的场景分别提出了对应的协作伙伴选择方法。在用户节点密集区域,结合最差链路优先匹配的原则,提出了互惠的和非互惠的“用户间信道优先”的匹配方法,该方法不需要求解方程来计算用户与其它节点相互协作可能得到的能量增益,而只需要知道用户到目的节点的信道增益和用户之间的信道增益就可以直接将节点进行分组。该匹配方法可以在能量增益变化不大的情况下显著降低计算量,减少系统的开销。而对于铁路和高速公路场景下的高速移动的用户节点,则在分析了高速移动节点的特殊性的基础上,根据用户节点的运动方向和速度将其分成不同的组,高速移动的用户节点只从与自己方向相同的高速运动的用户中选择协作伙伴,这样就可以降低协作伙伴之间的相对速度,提高协作持续的时间,减少协作伙伴的切换次数,从而降低系统的开销,增加协作的有效性。第三,研究了协作分集策略的改进和协作代价分析。由于放大转发和解码转发各有优缺点,因此提出了一种结合解码转发和放大转发的协作分集策略。该策略在用户节点间信道较好、协作伙伴可以正确解码的时候就采用DF协作,否则就采取AF协作的方法,这样就可以综合DF和AF的优点,提高频谱的利用率,取得更好的物理层和MAC层的性能。然后分析了在DF协作中应用自适应调制的协作方法的性能,可以提高协作系统的频谱效率。最后提出了一个协作代价表示函数,该函数中综合了能量、时间和频率三种资源,可以全面的计算协作带来的增益或者建立协作的开销。通过该函数的计算就可以判断采取协作可能带来的增益,从而为是否需要建立协作以及是否应该更换协作伙伴提供了依据。第四,分析了宽带信道下的协作分集方法。首先针对具有较低峰均比的单载波频域均衡系统,提出了一种均衡转发的协作方法来达到瞬时功率控制的目的,并且把均衡转发与解码转发两种协作方法相结合,进一步改善系统的性能,然后分析了协作伙伴的不同对系统性能的影响,为该系统下的协作伙伴选择提供支持。最后针对正交频分多址(OFDMA)系统,提出了改进的子载波分配方法,通过增加一些限制条件,可以以牺牲部分公平性换来了系统容量的大幅提高,得到一个公平算法与贪婪算法之间的折中。更多还原

【Abstract】 As the development of wireless communications technology, people’s requirement of wireless data services is also growing. The loss of wireless channel is one of the main obstructions for increasing channel capacity and improving service quality. And multi-antenna diversity technology is an effective measure to deal with the channel loss. However, the multi-antenna diversity requires that the communications equipment has multiple antennas, which is difficult to achieve in nodes of many systems because of the limits of size and power. Cooperative diversity technology appeared in this context.Cooperative diversity communication is based on relay channel model, and it is a new spatical diversity technology. It makes use of the broadcast nature of radio waves, each node can receive information which other nodes transmitted and retransmitted it to the destination node after a certain treatment. So different users can share their antennas and form spatial diversity. Cooperative diversity can improve the capacity of wireless communication systems, reduce the outage probability, expand the wireless network coverage and connectivity, and save the energy for data transmission. So it can improve the effectiveness and reliability of wireless communication systems. Cooperative diversity can not only be used in cellular mobile communication network, and it also can be well applied in sensor networks, cognitive radio and other wireless communication systems. And cooperative diversity have also contributed to the practical use of multiple-input multiple-output (MIMO) technology, and thus become an important technology for future research on wireless communications.This article focuses on problems in cooperative diversity system, and three research questions are consideres: when to cooperate, who cooperates with whom and how to cooperate. The opportunity of cooperation, partner selection and the improvement of cooperative strategies have been studied.Firstly, the principle analysis and protocol design of cooperative diversity are introduced. The basic channel model and protocol of cooperative diversity are introduced first and the theoretical cooperative gain is provided. Then decoded-and-forward (DF) and amplify-and-forward (AF) are considered and performances are compared. The flow of cooperative communication is introduced and the particularity of cooperative system is considered. Based on the frame structure in existing communications systems, communications processes and frame structure which can be applied in cooperative diversity systems are designed. The slot and structure of cooperative partner selection, cooperation created are appended in the the new communications processes. And“when to cooperate”is discussed based it.Secondly, the cooperative partner selection methods are proposed for two different scenarios. In high user-density area, combined with“worst-link-first”matching method, the reciprocit and non-reciprocity“channel between users first”matching methods are proposed. With the methods, we do not require solving the equation to computae the possible energy gain for cooperating with other users. We only need to know the channel gain from users to the destination and channel gain between users, we can group the nodes to cooperate. The matching method can reduce the calculation and system overhead with ensuring the energy gain. For high-speed mobile user nodes on the railway or highway, with the analysis of the particularity of high speed nodes, we can divided the nodes into groups based on the users’moving direction and speed, and high-speed mobile user nodes only select partner from the nodes moving by the same direction with themselves. So it can reduce the relative velocity of partners, and which can increase the duration of cooperation, reduce the partner switching frequency, thereby it can reduce system overhead and increase the effectiveness of the cooperation.Thirdly, we study improving the cooperative diversity strategy and analyze the cost of cooperation. As AF and DF both have advantages and disadvantages, so a cooperative method combined DF and AF is proposed. When the channel between the user nods is better and the partner can decode correctly, DF is used, or else AF is used. So this method synthesizes advantages of AF and DF, and it can improve the spectrum efficiency, get better physical layer and MAC layer performance. Then we analyzed the application of adaptive modulation in DF, which can further enhance the spectral efficiency of cooperative diversity system. Finally, a cooperative cost function is designed, which synthesizes energy, time and frenquency, and it can calculate the cooperative gain and cost in starting cooperation. We can determine if cooperation or partner switch is necessary and how much gain we can earn.Fourthly, cooperative diversity in broadband channel is analyzed. First of all, an“equalization and forward”method is proposed for low PAPR single-carrier frequency domain equalization system. It can provide diversity gain which is similar to instantaneous power control in narrowband system. And it also can be combined with DF to further improve system performance. Then the influence to performance of different partners is analyzed, which can support the partner selection. Finally, for orthogonal frequency division multiple access (OFDMA) system, we proposed a modified channel allocated method. By means of setting several thresholds, we can improve the system capacity greatly with less fairness loss. And a tradeoff between fair method and greedy method can be obtained.更多还原