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Inter-Session Network Coding Schemes for 1-to-2 Downlink Access-Point Networks With Sequential Hard Deadline Constraints

Next generation wireless networks will carry traffic from a wide range of applications, and many of them may require packets to be delivered before their respective deadlines. In this paper, we investigate using inter-session network coding to send packets wirelessly for two deadline-constrained unicast sessions. In particular, each unicast session aims to transmit a file, whose packets have hard sequential deadline constraints. We first characterize the corresponding deadline-constrained capacity region under heterogeneous channel conditions and heterogeneous deadline constraints. We show that this deadline-constrained capacity region can be achieved asymptotically by modifying the existing generation-based (G-B) schemes. However, despite its asymptotic optimality, the G-B scheme has very poor performance for small and medium file sizes. To address these problems, we develop a new immediately-decodable network coding (IDNC) scheme that empirically demonstrates much better performance for short file sizes, and we prove analytically its asymptotic optimality when used to send large files. Our analysis uses a novel version of drift analysis, which could also be of independent interest to other IDNC schemes.