【作者】 杨林；

【导师】 李磊；

【作者基本信息】 国防科学技术大学 ， 通信与信息系统， 2010， 博士

【摘要】 网络编码(network coding)技术是近年来网络通信研究领域的一项重大突破,它允许网络节点在传统数据转发的基础上参与数据处理,已成为提高网络吞吐量、鲁棒性和可靠性的有效方法。无线链路的广播传输特性非常适合网络编码技术的应用。目前,无线网络与网络编码技术的结合已引起国内外研究者的广泛关注。本论文以网络编码技术在无线网络中的应用为主旨,以改善无线链路的传输可靠性和提高无线网络的吞吐性能为目标,对结合网络编码的无线局域网多播通信、无线多跳环境下的路由优化和无线广播系统的重传性能改进等问题进行了深入研究,提出了相应的协议设计方案和理论分析模型。本论文的主要内容和创新性成果如下:第一,对网络编码的理论和应用研究现状进行了综述,对网络编码的概念和基本原理作了说明,对网络编码的构造方法和优化措施进行了归类和分析,并指出网络编码在无线网络应用中的现状、关键技术和发展趋势。第二,针对现有IEEE 802.11 MAC协议无法提供可靠多播服务的缺陷,提出一种基于网络编码的无线局域网多播MAC协议MPNC (Multicast Protocol based on Network Coding)。该协议采用网络编码组传输模型发送多播数据。对于多播源节点,采用随机线性码对多播数据帧进行编码组合发送,对于多播接收节点,在接收的编码帧累积到一定数量后通过解码操作恢复出所需的原始数据。根据802.11DCF指数退避机制,建立了多播和单播节点信道竞争的二维Markov分析模型,推导了差错信道和饱和负载条件下MPNC协议的吞吐量理论表达式。模拟实验验证了理论模型的正确性,结果表明MPNC协议可有效减少多播数据帧的发送次数,提高无线带宽的利用效率。第三,针对已有的无线局域网多播MAC协议使用固定物理层发送速率而不能快速适应信道变化的不足,基于交叉层设计思想,提出一种基于网络编码的速率自适应多播MAC协议RAMPNC (Rate Adaptive Multicast Protocol based on Network Coding)。该协议的关键特征包括两部分:一方面,采用网络编码组传输模型发送多播数据,利用网络编码的信息融合特性减少多播帧的发送次数,进而提高无线带宽的利用效率;另一方面,协议利用RTS-CTS握手信号实现信道状态信息交换,使多播源节点能够根据接收节点反馈的信道信噪比估值动态调整物理层发送速率。模拟测试实验表明RAMPNC协议能快速地响应信道变化,在多播吞吐量,平均帧传输延时和帧投递率等方面获得比已有方案更优的性能。第四,研究了无线多跳环境下多单播信息流的网络编码传输方案。结合网络编码和机会路由两种技术提出一种具有网络编码意识的机会路由协议NCAOR(Network Coding-Aware Opportunistic Routing)。选取期望传输次数ETX(Expected Transmission Count)作为机会路由的路由度量,协议允许每一跳节点选取多个符合特定条件的邻居构成机会节点集参与数据转发。通过定义综合路径距离和编码收益的效能函数,使各机会节点分布计算接收报文的转发效能,并根据相应结果智能地设置报文响应时间。以数据报文的平均传输次数作为性能指标,分析了两跳中继双向流模型和多跳中继双向流模型下NCAOR协议的传输性能,并讨论了链路投递率、机会节点数目、转发路径长度等因素对协议性能的影响。模拟实验表明,NCAOR与采用最短路径路由的网络编码传输方案COPE (Complete Opportunity Encoding)相比,在网络吞吐量、能耗和节点编码机会等性能指标上均有更优的表现。第五,研究利用网络编码技术改善无线广播系统的重传性能。提出一种基于随机网络编码的无线广播重传方法RNC-ARQ(Random Network Coding- Automatic Repeat Request)。该方法采用批处理方式发送广播数据包,并以单个批次长度为单位进行逐批重传操作。根据单个批次数据包接收反馈矩阵,源节点对该批次内的所有丢失包进行随机编码组合并重传,使各用户节点通过解线性方程组的方式恢复出丢失的数据包。针对Gilbert-Elliott模型表征的突发差错信道,建立了信道状态和节点接收处理流程合并的多状态马尔可夫模型,并以此为基础推导了随机编码重传方法的吞吐量闭合解。模拟实验验证了所提模型的正确性,结果表明同等信道条件下,基于随机网络编码的重传机制相对传统的选择重传机制和异或编码重传机制能获得更高的广播吞吐性能。更多还原

【Abstract】 Recently, Network coding has been proved to be the breakthrough in the research field of network and communication. It is an effective technique to increase network capacity and improve the robustness and reliability of networks. It allows intermediate nodes to perform processing operations on the incoming packets, in addition to just forwarding them. The broadcast character of wireless link is very appropriate for the application of network coding technique. Currently, the combination of wireless network and network coding has gained significant interest domestic and overseas. This thesis investigates the application of network coding in wireless network. In order to improve the reliability and the throughput of wireless network, we do extensive research on network coding incorporated with the multicast communication in WLAN, the routing optimization in multi-hop network and the retransmission efficiency for wireless broadcast system, and focus on the protocol design and the theory for the performance analysis. The main inventive works and conclusions of the thesis are listed as follows.Firstly,we review the recent development in theory and application of network coding technique. The conception and basic theory of the network coding are introduced. The construction and optimization of network coding schemes are classified and analyzed in detail. Furthermore, we summarize the state of arts in application of network coding in wireless networks and propose the key technologies, the open issues and challenges of network coding referring to both theory and application in near future.Secondly, we propose a multicast MAC protocol called MPNC (Multicast Protocol based on Network Coding) based on network coding to solve the problem of the unreliable multicast service provided by IEEE 802.11 for the wireless local area network (WLAN). MPNC takes use of the network coding group to transmit multicast data frames. In multicast source node, the data frames are combined with random linear codes for transmission. In receiving nodes, the data frames are decoded after receiving enough combined frames. According to the 802.11 DCF exponential backoff mechanisms, we build the 2-dim Markov chain model for the channel contention of the multicast terminal and multiple unicast terminals. Analytic solutions are derived for the saturation throughput of MPNC under the error-prone channels. Simulation result validates the conclusion of the theory analysis, and shows that the MPNC scheme can effectively reduce the number of transmission for multicast frames and improve the bandwidth efficiency. Thirdly, we propose a rate adaptive multicast MAC protocol based on network coding called RAMPNC (Rate Adaptive Multicast Protocol based on Network Coding) to solve the problem that the proposed multicast MAC protocols for WLAN can not adapt the quick channel variations. The proposed protocol has two key characters. In on aspect, RAMPNC takes use of the network coding group to transmit multicast data frames. The transmission times are reduced and the bandwidth efficiency is improved because of the information mixing character of the network coding. In the other aspect, the channel state information is obtained through RTS-CTS signal exchange. The RAMPNC can adjust the physical sending rate in accordance with the estimated SNR from multicast nodes. Simulation shows that RAMPNC can respond to channel variations quickly and achieve better performance than existing protocols in terms of multicast throughput, average frame transmission delay and frame delivery rate, etc.Fourthly, we investigate the network coding transmission scheme for multiple unicast flows in wireless multihop networks. By integrating the advantage of both network coding and opportunistic routing, we propose a network coding-aware opportunistic routing protocol called NCAOR (Network Coding-Aware Opportunistic Routing). The expected transmission count (ETX) is selected as the opportunistic routing metric. For each hop, the node chooses multiple neighbors to form an opportunistic node set for forwarding, which is composed of nodes satisfied special conditions. And each opportunistic node computes the gain of the forwarding packet by the utility function, which considered both the distance to destination and the network coding gain. According to the computation result, each opportunistic node can set the forwarding time for receipted packet intelligently. By choosing the average transmission time of data packet as the metric, we analyze the transmission performance of the NCAOR for the scenarios of both one-hop relay with bi-directional unicast flows and multi-hop relays with bi-directional unicast flows. We discuss the influence of different parameters, such as the delivery probability of the wireless link, the number of opportunistic nodes and the average path length. Simulation results show that the proposed NCAOR can get better improvement on performance such as throughput, energy and network coding chances, when compared to the COPE(Complete Opportunity Encoding) using the traditional routing protocol.Fifthly, we do some research on the improving the retransmission performance for wireless broadcast system with network coding. A broadcast retransmission scheme called RNC-ARQ (Random Network Coding-Automatic Repeat Request) based on the random network coding is put forward for the wireless broadcast system. The scheme transmits broadcast data packets in a batch processing mode and retransmits lost packets in a per-batch way. In accordance with the packet reception feedback matrix, the source combines all lost packets in a batch with random linear codes for retransmission. And all receiver nodes can recover the lost data packets by solving the linear equation system. Based on the Gilbert-Elliott channel model, a multi-state Markov model is generated by uniting the channel state with the reception mode of received nodes. Based on this model, the closed-form expressions are derived for the throughput of RNC-ARQ. Simulation result validates the theory analysis, and shows that the proposed RNC-ARQ scheme is better than the SR-ARQ(Selective Repeat-Automatic Repeat Request) and XOR-ARQ (XOR network coding-Automatic Repeat Request) scheme in terms of the throughput under the same channel conditions.更多还原