应用累积系数确认的网络编码机会路由协议

在无线mesh网络中，机会路由通过高效使用无线传输的广播特性显著地提高了无线网络的吞吐量.引入网络编码，使得机会路由协议可以避免复杂的调度，更加易于实现.然而，网络编码的引入给机会路由协议带来新的问题：转发节点应该发送多少编码包?MORE等协议依据平均链路状况信息来预计节点转发编码包数目的方法，无法准确判定发送的冗余.以CCACK为代表的研究采用逐跳反馈的方式来减少编码包的冗余发送.首先，针对采用正交向量确认的CCACK机制进行分析，说明了CCACK尽管可以减少确认开销，减少误判，但却带来了“信息空间已覆盖而无法正交”的漏判问题.在此基础上，提出了一种基于累积编码系数反馈确认的网络编码机会路由协议CFACK.该确认机制中转发节点通过侦听下游节点的编码系数向量，并与来自上游节点的编码系数向量进行相关性分析，从而获知下游节点信息是否覆盖自身信息.证明了在无差错网络环境下该确认机制不存在误判和漏判的可能，同时，在有差错网络环境下对该确认机制的有效性进行了分析.结果表明：在一般节点分布情况下，利用额外的一次携带确认，可以确保90%以上的准确性.仿真测试结果表明：CFACK相比CCACK，显著提高了网络的吞吐量，平均提高率为72.2%，同时在编码计算、存储和包头开销上都少于CCACK.

Network Coding Based Opportunistic Routing Using Cumulative Coding Coefficient Feedback Acknowledgments

Opportunistic routing (OR) significantly improves transmission reliability and network throughput in wireless mesh networks by taking advantage of the broadcast nature of the wireless medium. With network coding (NC), OR can be implemented in a simple and practical way without resorting to a complicated scheduling. With the introduction of NC, how to reduce redundant transmission of coded packets becomes a very important problem in OR protocol. MORE, et al. protocols estimate the expected number of transmissions for each forwarder based on periodic measurements of the average link loss rates. However, these approaches may suffer severe performance degradation in dynamic wireless environments. Recently, some studies, known as CCACK, employ orthogonal vector as feedback to reduce redundant transmission of coded packets. The analysis of CCACK scheme indicates that the false-positive probability is reduced at the cost of increasing the false-negative probability, which results in unnecessary packets transmission. This paper proposes a NC-based OR protocol, named CFACK, based on cumulative coding coefficient feedback acknowledgement. In this scheme, the coding vectors piggybacked in coded packets are used as feedback information, and each forwarder overhears coding vectors sent by downstream nodes. Through correlation analysis between coding vectors from upstream nodes and downstream ones each forwarder knows whether its knowledge space is covered by its downstream nodes. This paper proves that CFACK is completely free from any false-positive and false-negative problem in reliable network. The efficiency of CFACK in unreliable network is also analyzed, and the result shows that in random topologies embedding an extra ACK vector in each packet can guarantee 90% accuracy. Evaluation shows that, compared with CCACK, CFACK significantly improves throughput by reducing unnecessary packet transmission, with average improvements of 72.2%. Furthermore, the overheads of encoding computation, storage, and header of CFACK are less than that of CCACK.