A Low-Delay MAC for IoT Applications: Decentralized Optimal Scheduling of Queues Without Explicit State Information Sharing

For a system of collocated nodes sharing a time-slotted wireless channel, we seek a medium access control that provides low mean delay, has distributed control, and does not require explicit exchange of state information or control signals. We consider a practical information structure where each node has local information and some common information obtained from overhearing. We approach the problem via two steps: 1) we show that it is sufficient for the policy to be "greedy" and "exhaustive"; limiting the policy to this class reduces the problem to obtaining a queue switching policy at queue emptiness instants; and 2) by formulating the delay optimal scheduling as a partially observed Markov decision process, we show that the optimal switching rule is stochastic largest queue. Using this theory as the basis, we develop a practical, tunable, distributed scheduler, QZMAC, which is an extension to the existing ZMAC protocol. We implement QZMAC on standard off-the-shelf TelosB motes and also use simulations to compare QZMAC with the full-knowledge centralized scheduler and with ZMAC. We use our implementation to study the impact of false detection, while overhearing the common information, and the efficiency of QZMAC. Simulation results show that the mean delay with QZMAC is close to that of the full-knowledge centralized scheduler.