【作者】 史春光；

【导师】 郑林华； 魏急波；

【作者基本信息】 国防科学技术大学 ， 信息与通信工程， 2013， 博士

【摘要】 无线ad hoc网络由于无需固定基础设施支撑、组网快速灵活而受到业界的广泛关注和研究。然而，随着无线宽带业务的普及，传统无线ad hoc网络的性能已经很难满足人们日益增长的需求。为了缓解这个矛盾，亟需研究新的技术来推动无线ad hoc网络的发展和应用。近年来协同通信技术和多信道技术的出现为无线ad hoc网络的发展提供了新的机遇，同时为其性能提升带来更大的潜能。鉴于此，本文将协同通信技术和多信道技术融入无线ad hoc网络展开了相关研究。论文首先深入研究分析了无线ad hoc网络“在什么时机下协同”、“与谁协同”、“如何协同”等协同MAC协议设计关键问题，针对协同中继节点随机分布与无线信道时变导致源节点和中继节点、源节点和目的节点以及中继节点和目的节点间信道状态的差异，提出了一种适用于IEEE802.11无线ad hoc网络的基于即时信道状态信息的跨层自适应协同MAC协议(Cross-layer Adaptive Cooperative MAC,CAC-MAC)。其主要特点包括：1)根据信道状态条件自适应调整数据发送速率；2)根据MAC层数据报文长度自适应选择传输模式：为了减少网络开销，短报文采用IEEE802.11基本接入模式直接传输；长报文采用IEEE802.11RTS/CTS接入模式进行传输。进一步，对于长报文根据信道状态条件自适应选择由源节点到目的节点的直传或者借助中继节点协同传输模式。其中，协同传输模式又分为“源-中继-目的节点”中继传输模式或者目的节点分集合并接收传输模式；3)基于即时信道状态信息判定是否采用协同传输模式以及选择协同中继节点，克服了以往基于协同列表的协同MAC协议时效性低等不足。分析和模拟实验表明，相对于IEEE802.11MAC协议而言，CAC-MAC协议可大幅提高网络吞吐量、减少分组时延。大量研究已表明，协同通信时的网络性能取决于协同中继节点的选择。目前关于协同中继节点的选择大都是从物理层角度出发，即仅考虑信道状态条件、节点能量等因素。事实上，当网络中存在多条数据流时，候选中继节点与网络中的其它源节点、目的节点之间的相互干扰会大大影响网络的整体性能，因而中继节点选择不当会导致网络性能下降甚至低于直传模式下的网络性能。为此，针对IEEE802.11无线ad hoc网络存在多条数据流的场景，论文从全网角度出发提出了一种基于干扰感知的分布式协同中继节点选择算法(Distributed Interference-aware RelaySelectionAlgorithm，DIRSA)。在DIRSA中，协同中继节点的选择兼顾物理层特征（如信道状态条件）和候选中继节点周围的干扰情况（以信道空闲率表示）。该算法通过合理选择协同中继节点减少了网络节点之间的相互干扰，从而显著降低了报文碰撞概率，进一步提高了网络吞吐量。单信道无线网络将整个无线频谱作为唯一的信道资源，当网络中所有节点竞争这一信道时会引发碰撞和不公平竞争，多信道技术是克服这一问题的一个重要研究方向。多信道接入技术使相邻的节点可以采用不同的信道同时发送数据，增加了网络空间复用度，提高了网络的整体性能，可显著缓解单信道接入中存在的高碰撞率和不公平接入等问题。目前提出的多信道MAC协议主要分为两类：第一类是带宽固定的静态多信道MAC协议，此类协议带宽固定因而不能适应无线adhoc网络中节点业务需求的动态变化；第二类是带宽可变的动态多信道MAC协议，此类协议根据网络中节点业务的需求自适应选择不同带宽的信道，但目前此类协议主要适用于具有固定基础设施的无线网络，而且仅有几个固定的信道带宽可供选择（如5MHz,10MHz,20MHz和40MHz等），无法适应无线ad hoc网络中不同节点业务需求的差异性。论文针对每个节点只配备一套收发器的IEEE802.11无线ad hoc网络，提出了一种基于业务感知的多信道MAC协议(Traffic-awareChannelization MAC, TAC-MAC)。其特点包括：1)分布式实现多信道MAC协议。采用基于窗口的协同传输模式，每个传输调度周期分为协商窗口和数据传输窗口；由第一个竞争到信道的节点发起一个传输调度，不依赖于固定的中心节点进行子信道划分以及信道带宽的分配；2)在协商阶段利用整个频谱资源，克服了以往基于窗口的多信道MAC协议存在的频谱资源浪费的不足，而且协商窗口的大小可以根据网络中竞争节点数目进行适时调整；3)在数据传输阶段，采用正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)技术根据节点业务需求提出了一种基于子载波的子信道划分策略以提高频谱利用率和网络性能。分析和模拟实验表明，TAC-MAC协议较固定带宽的多信道MAC协议以及传统的IEEE802.11单信道MAC协议可大幅提高网络吞吐量和分组时延性能，而且网络节点竞争越激烈，网络性能提高优势越明显。更多还原

【Abstract】 Wireless ad hoc network requires no infrastructure and is characterized of agility,quick organization, and easy extention. Therefore, wireless ad hoc network has gainedwidespread attention and study by industry. However, with the popularity of wirelessbroadband services, traditional wireless ad hoc network has difficulty to meet thegrowing needs of network users. In order to alleviate this contradiction, newtechnologies are needed to promote the development and application of wireless ad hocnetworks. Recently proposed cooperative communication technology and multi-channeltechnology provide new potentials to enhance the performance of wireless ad hocnetworks. From this point of view, this dissertation focuses on introducing both thecooperative communication technology and the multi-channel technology into wirelessad hoc network to improve the system performance.This dissertation first analyzes the key aspects of cooperative MAC design, namely,when to cooperate, whom to cooperate with and how to cooperate. In order to reap morebenefit of cooperation, a cross-layer adaptive cooperative MAC (CAC-MAC) protocolfor IEEE802.11DCF based wireless ad hoc networks is proposed, which involvesinteraction between MAC layer and physical layer. The key features of our proposal areas follows. First, each node adjusts its transmission rate according to the instantaneouswireless channel conditions. Second, only when a data frame at the MAC layer is longerthan a specified length, CAC-MAC initiates a RTS/CTS handshake, which brings downthe overhead of network. And for long data frames, RTS/CTS direct transmission orproperly cooperative transmission will be selected according to the wireless channelconditions. Moreover, the cooperative transmission is divided into either“source-relay-destination” transmission scheme or receiver maximal ratio combiningscheme according to the channel conditions among source, relay and destination. Third,the best relay node for a given source-destination pair is selected based on instantaneouswireless channel measurements instead of a relay table that used in traditional ways,which can cut the cost to create and maintain the raly table. Analysis and simulationresults show that the CAC-MAC protocol can significantly improve network throughputand reduce packet delay compared with legacy IEEE802.11MAC protocol.It is proved that the performance improvement by cooperative communicationheavily depends on the selection of relay nodes. Most of previous work on relayselection only considers the factor of physical layer, for example, the channel quality ornode energy. However, in practice when there are multiple data flows in the network,the inter-node interference significantly affects network performance. Inappropriaterelay selection without considering inter-node interference may degrade networkperformance instead of improving it. To mitigate the impact of inter-node interference, this dissertation proposes a distributed interference-aware relay selection algorithm(DIRSA) from the view of the whole network that selects a relay node withconsideration of both inter-node interference and channel conditions. The algorithmreduces the inter-node interference in the network by effectively selecting relay nodes,thereby significantly reduces the collision probability and further improves the networkthroughput.In a single channel wireless network, the available spectrum is usually managed asone only channel resource, and when all nodes in the network contending the onlychannel will cause collisions and possible unfair competition. Multi-channel technologyis an important research direction to overcome this problem. Multiple channel accesstechnology allows neighbor nodes to transmit data simultaneously in different channels,which can increase the spatial reuse, improve the network performance and significantlyalleviate the problem of high collision probability and unfair access existing in singlechannel access networks. Existing multi-channel MAC protocols are mainly dividedinto two categories: the first is the fixed-width static multi-channel MAC protocol.Among these protocols, the channelization structure is pre-configured that the entireavailable spectrum is divided into sub-channels with equal channel width, which isdifficult to adapt to temporal disparity in the traffic demands of nodes. The second is avariable-width dynamic multi-channel MAC protocol. Unfortunately, most of theseprotocols are designed for the infrastructure wireless networks where a centralcontroller is responsible for variable-width channel allocations, which are not suitablefor the distributed ad hoc networks. Meanwhile, the sub-channel division is stillcoarse-grained that the channel width is chosen from several fixed values, e.g.5,10,20or40MHz. This dissertation presents a traffic-aware channelization MAC (TAC-MAC)protocol for the case that each node is equipped with a single half duplex transceiver.Specifically, the key features of TAC-MAC protocol are as follows. First, TAC-MACworks in a distributed manner. It adopts a window-based approach that each scheduleconsists of a negotiation window and a data window. And the node who firstly wins thecontention for access to the channel initiates a transmission schedule. There is no needof central controller for sub-channel division and channel width allocation. Second,TAC-MAC utilizes the whole available spectrum during negotiation windows, whichavoids the spectrum wastage generally existing in previous window-basedmulti-channel MAC. Furthermore, the size of the negotiation window can adapt to thenumber of contending nodes. Third, during data windows, beyond fixed-width orcoarse-grained channelization structure, TAC-MAC adopts a fine-grainedvariable-width channelization strategy based on orthogonal frequency divisionmultiplexing technique that can flexibly commensurate with the traffic demands of eachnode. Analysis and simulation results show that the TAC-MAC can significantlyimprove network throughput and reduce packet delay compared with both the fixed-width multi-channel MAC and the single channel802.11MAC protocols. And themore competitive the network condition is, the more advantage TAC-MAC will bring.更多还原