Energy-Efficient Opportunistic Forwarding In Delay Tolerant Mobile Networks

With the rapid development and wide application of communication technology, more and more challenged communication environments which may not be well served by the current TCP/IP protocols have come out. To provide communication services in the highly challenged wireless networks where there only exists intermittent connectivity, delay tolerant networks (DTNs) were proposed and have attracted more and more attention of researchers. In DTNs, there is not any end-to-end path between the communication sources and destinations due to unpredictable node mobility. As delay tolerant mobile networks (DTMNs) suffer from high latency and intermittent connectivity, traditional mobile ad hoc routing protocols fail to work. Besides, nodes in DTMNs are usually powered by the battery, which is limited and cannot be replaced easily. Therefore, it is very necessary to design the energy-efficient opportunistic forwarding algorithms in DTMNs. In this dissertation, we investigate the distance-based energy-efficient opportunistic forwarding and the opportunistic forwarding combined with energy harvesting in DTMNs.First, the distance-based energy-efficient opportunistic forwarding (DEEOF) in DTMNs is s-tudied. For the existing opportunistic forwarding that relies on different probability metrics, once the relays move into the maximum transmission radius of the source, the source forwards the pack-et copies immediately with the maximum transmission power, without considering whether the relays will continues to move closer. However, due to node mobility, the transmission power to reach the relays varies in time and is related to the distance between the source and relays. The source waits for a transmission opportunity with lower energy consumption at the expense of re-ducing the probability of reaching the destination before the tolerant delay. In order to quantify the impact of mobility on the energy efficiency, we define the concept of the forwarding equivalent energy-efficiency distance. The forwarding equivalent energy-efficiency distance gives a compar-ison metric on the energy efficiency when the node distance, the delay time, or the number of the relays in the source’s transmission radius is different, which helps the design of energy-efficient opportunistic forwarding. Exploiting node mobility, we propose two DEEOF algorithms which significantly improve the energy efficiency and, meanwhile, achieve a high packet delivery ratio. Taking different transmission mechanisms into account, we discuss the DEEOF algorithms for both the unicast and broadcast cases.Second, the opportunistic forwarding combined with energy harvesting in DTMNs is studied. We investigate how energy harvesting, a promising technique of enabling sustainable commu-nications, can be exploited to improve the performance of opportunistic forwarding in DTMNs. Specifically, we formulate the problem using a Markov Decision Process (MDP) framework in which each source should strike a balance between exploitation, by forwarding the packet to the relay currently in contact, and exploration, by waiting for possible better relays in the future, given the harvested energy constraint. As the formulated MDP having exponential complexity, we devise a heuristic relay-assisted opportunistic forwarding algorithm to alleviate the computation complex-ity. Taking into account both energy harvesting and node mobility, the proposed low-complexity algorithm can use the harvested energy more efficiently to maximize the packet delivery ratio.