Research Of Network Coding Based Opportunistic Routing Protocol

Wireless mesh networks are used increasingly for providing cheap Internet access everywhere. Wireless mesh networks, however, need to deal with poor link quality caused by urban structures and the many interferers including local WLANs. 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 protocol. However, the introduction of NC incurs the issue of redundant packet transmissions. Existing NC-based OR protocols such as MORE mitigate this issue by controlling the packet transmission based on prior calculated expected transmission count. However, these approaches suffer significant performance degradation in fast fading channel conditions. Recently, some studies employ orthogonal vector to reduce the redundant transmission, not affecting by the fast fading channel. However, our theoretical analysis shows that there is a false-negative problem exists in the above approach, false-negative problem occurs when the knowledge space is covered while coding vectors can’t be orthogonal with orthogonal vectors, which results in unnecessary packets transmission.Furthermore, this paper proposes a NC-based OR protocol based on cumulative Coding Coefficient Acknowledgments (C3ACK). In C3ACK, each forwarder overhears coding vectors sent by downstream nodes, and makes correlation analysis between coding vectors from upstream nodes and downstream ones. Therefore, it can know whether its knowledge space is covered by its downstream nodes. In this paper, we prove that C3ACK has no false-positive and omission judgment in reliable network. The efficiency of C3ACK in unreliable network is also analyzed, and the result shows that in random topologies embedding an extra ACK vector in each packet can guarantee90%accuracy. Our simulation evaluation shows that, compared with MORE and CCACK, by significantly reducing unnecessary packet transmission, C3ACK significantly improves throughput by98.7%and72.2%, respectively. Also, the overheads of encoding computation, storage, and header of C3ACK are less than that of CCACK.In order to verify the performances of the protocol further in real network, we build a wireless mesh testbed based on Linux platform, implentment the MORE、CCACK and C3ACK protocols on the Click modular router, and test the performances of MORE, CCACK and C3ACK. The real performances of the two protocols are compared through deploying multi mesh nodes. In spite of the limitation of the size of the testbed, the benefit of C3ACK over MORE and CCACK in the challenged topologies still demonstrate that C3ACK can significantly improve the throughput in the real wireless mesh network environment.