Robust and optimal opportunistic scheduling for downlink 2-flow inter-session network coding with varying channel quality

This paper considers the downlink traffic from a base station to two different clients. Assuming infinite backlog, it is known that inter-session network coding (INC) can significantly increase the throughput of each flow. However, the corresponding scheduling solution (assuming dynamic arrivals and requiring bounded delay) is still nascent. For the 2-flow downlink scenario, we propose the first opportunistic INC + scheduling solution that is provably optimal for time-varying channels, i.e., the corresponding stability region matches the optimal linear-INC capacity. To that end, we first introduce a new binary INC operation, which is distinctly different from the traditional wisdom of XORing two overheard packets. We then develop a queue-length-based scheduling scheme, which, with the help of the new INC operation, can robustly and optimally adapt to time-varying channel quality. A byproduct of our results is a scheduling scheme for stochastic processing networks (SPNs) with random departure. The new SPN results relax the previous assumption of deterministic departure, a major limitation of the existing SPN model, by considering stochastic packet departure behavior, and could further broaden the applications of SPN scheduling to other real-world scenarios.