OFDM downlink scheduling for delay-optimality: Many-channel many-source asymptotics with general arrival processes

We consider the downlink of an OFDM system for supporting a large number of delay-sensitive users. The OFDM scheduling problem can be modeled as a discrete-time multi-source multi-server queuing system with time-varying connectivity. For such a system, the Max-Weight policy is known to be throughput-optimal and the Server-Side Greedy (SSG) policy has been recently shown to achieve small queue lengths for i.i.d. arrival processes. However, there is often significant difference between queue-length optimality and delay optimality, and there exist arrival patterns such that algorithms with small queue backlog can still lead to large delay. In this work, we propose a new OFDM scheduling algorithm that gives preference to packets with large delay. Assuming ON-OFF channels, we show that for a large class of arrival processes, the proposed policy is rate-function delay-optimal. We substantiate the result via both analysis and simulation.