Mitigating the impact of asynchronous ACKs on the performance of opportunistic network coding

Existing opportunistic network coding architectures relies on pseudobroadcast to deliver a coded packet to multiple receivers in a single transmission. In such situation, there is only one primary receiver who acknowledges the reception by synchronous MAC-layer acknowledgements (ACKs) and the other receivers receive it by overhearing and acknowledge the reception by asynchronous ACKs, which are usually piggybacked in outgoing data packets. This may cause a large amount of unnecessary retransmissions if asynchronous ACKs are dropped due to packet losses or they arrive late. Moreover, a large number of redundant retransmissions in IP layer easily cause congestion losses, especially under heavy traffic and thus compromise the throughput gain brought by network coding. In this paper, we propose a framework of joint optimal rate control and code selection (ORC) to mitigate the impact brought by asynchronous ACKs on opportunistic network coding in lossy wireless networks. The operation of ORC consists of two phases. In the first phase, we try to select a suitable transmission rate for the transmission of a coded packet. We formulate this rate control process as a Finite Horizon Markov Decision Process (FH-MDP). We define a metric, called coded packet transmission efficiency (CPTE), to measure the expected cumulative rewards and search the optimal rate control policy. In the second phase, based on the CPTE for a given coded packet, we propose a code selection policy to optimize the performance of opportunistic network coding. We demonstrate the effectiveness and advantages of the proposed ORC framework by computer simulations.