Is rate adaptation beneficial for inter-session network coding?

Abstract

This paper considers the interplay between rate adaptation and inter-session network coding gains in wireless ad hoc or mesh networks. Inter-session network coding opportunities at relay nodes depend on packets being overheard by surrounding nodes - the more packets nodes overhear, the more opportunities relays have to combine packets, resulting in a potential increase in network throughput. Thus, by adapting its transmission rate, a node can increase the range over which its packets are overheard, enabling additional opportunities for coding and increased overall throughput. This paper considers inter-session coding, restricted to a single relay (bottleneck) node, or star topology. Even for such simple topologies the optimal joint rate adaptation and network coding policy is known to be NP-hard, so we consider an optimal pairwise coding policy which can be formulated as a linear programming and provide a simple heuristic for rate adaptation for network coding.We evaluate the averaged throughput in two different scenarios, in which relays have different access opportunities, giving some intuition on the impact of rate adaption in lightly and heavily loaded systems. The gains of joint rate adaptation and network coding are marginal when relay has higher access opportunity than other nodes, or when MAC operates ideally. The gain ranges from 9%to 19% compared to a network without network coding and is around 4% over a network using regular network coding.While, when the relay has equal access opportunity as other source nodes, which is more typical of todays MAC protocols under heavy loads, the gain ranges from 40% to 62% as compared to standard relaying case and is upto around 20% as compared to a network with regular pairwise network coding. And we further increase this gain from 40% to 120% by replacing pairwise coding with sub-optimal general network coding scheme.