Integration of network coding, spatial diversity and opportunistic routing/forwarding in wireless mesh networks

Wireless Mesh Network is an answer to the last mile problem. It offers easy deployment and provides coverage over large area with fewer wires. Nevertheless, its limited throughput is inadequate for next generation applications. Motivated by its features and advantages, we propose a solution to mitigate this problem of limited throughput by leveraging the broadcast nature of the wireless medium. In particular, network coding, spatial diversity and opportunistic routing/forwarding capitalize on the broadcast nature of the wireless links to improve the network performance. These techniques target different network conditions and usually are considered in separation. In this thesis a cross-layer based integration of the mentioned three techniques is presented to accumulate their potential gains using the same network protocol stack in wireless mesh networks. The proposed integration approach is based on a new CDARM metric (Coding opportunity and Data rate Aware Routing Metric) used for the route selection and a method for creating relay links at the MAC layer. In particular to leverage on the broadcast nature we developed a cooperative protocol, based on link creation at the MAC layer that introduces opportunism into the cooperative protocol. Based on this cooperative protocol and the routing metric, we integrate the network coding mechanism. Then we introduce cooperation between the network and MAC layers. The numerical study, based on the system level simulation results, shows significant improvement of the integrated protocol performance in terms of network throughput and reliability over the individual mechanisms. To the best of our knowledge this dissertation is the first attempt to integrate network coding, spatial diversity and opportunistic routing/forwarding mechanisms in the same protocol stack. The integrated protocol requires modifications into the network protocol stack that can be easily incorporated in future generation devices

Network coding: performance analysis and robust design in multi-hop wireless mesh networks

Network coding is an innovative idea to boost the capacity of wireless networks. However, there are not enough analytical studies on throughput and end-to-end delay of network coding in multi-hop wireless mesh network that incorporates the specifications of IEEE 802.11 Distributed Coordination Function. In this dissertation, we utilize queuing theory to propose an analytical framework for bidirectional unicast flows in multi-hop wireless mesh networks. We study the throughput and end-to-end delay of inter-flow network coding under the IEEE 802.11 standard with CSMA/CA random access and exponential back-o↵ time considering clock freezing and virtual carrier sensing, and formulate several parameters such as the probability of successful transmission in terms of bit error rate and collision probability, waiting time of packets at nodes, and retransmission mechanism. Our model uses a multi-class queuing network with stable queues, where coded packets have a non-preemptive higher priority over native packets, and forwarding of native packets is not delayed if no coding opportunities are available. The accuracy of our analytical model is verified using computer simulations. Furthermore, while inter-flow network coding is proposed to help wireless networks approach the maximum capacity, the majority of research conducted in this area is yet to fully utilize the broadcast nature of wireless networks, and to perform e↵ectively under poor channel quality. This vulnerability is mostly caused by assuming fixed route between the source and destination that every packet should travel through. This assumption not only limits coding opportunities, but can also cause bu↵er overflow at some specific intermediate nodes. Although some studies considered scattering of the flows dynamically in the network, they still face some limitations. This dissertation explains pros and cons of some prominent research in network coding and proposes a Flexible and Opportunistic Network Coding scheme (FlexONC) as a solution to such issues. Moreover, this research discovers that the conditions used in previous studies to combine packets of di↵erent flows are overly optimistic and would a↵ect the network performance adversarially. Therefore, we provide a more accurate set of rules for packet encoding. The experimental results show that FlexONC outperforms previous methods especially in networks with high bit error rates, by better utilizing redundant packets permeating the network, and benefiting from precise coding conditions