To Transmit Now or Not to Transmit Now

Given an unreliable communication link, this paper studies how to build, in an energy-efficient manner, a reliable communication service that is synchronous with high probability. We consider a Partially Observable Markov Decision Process (POMDP) setting in which a communication link's transmission quality: (i) changes according to a classic Markovian model and (ii) can be only partially observed, through feedback relative to previous transmissions. We perform a thorough analysis under several variations of Ack/Nack feedback mechanisms. Despite the general intractability of POMDPs, we prove that our communication service, under reliable feedback, can be inexpensively implemented. We obtain closed form solutions specifying when to transmit over the link, which allows to derive an energy-optimal implementation. We also analyse the impact of lossy feedback on implementing our communication service. Considering multiple lossy feedback mechanisms, we show that an easily implementable structure for our communication service can also be obtained, depending on the feedback mechanism itself.