To Continue Transmission or to Explore Relays: Millimeter Wave D2D Communication in Presence of Dynamic Obstacles

Millimeter wave (mmWave) device to device (D2D) communication is highly susceptible to obstacles due to severe penetration losses. Dynamic obstacles may cause unpredictable fluctuations to D2D channel quality and hence a D2D relay initially chosen by base station (BS) might undergo failed transmissions resulting in severe packet loss and delay. This local information regarding link quality deterioration needs to be informed to the BS by the user equipments (UEs) which may result in some delay. Also, exploring a new relay on mmWave channel results in significant delay due to directional search. Hence the following optimal sequential decision must be made when packet loss occurs: whether to explore for a new relay link considering exploration cost, or to continue communication via the existing relay. We model this sequential decision problem locally at each UE as partially observable Markov decision process to capture uncertainty in D2D links while minimizing delay. We derive an optimal threshold policy for both positively and negatively correlated links. We also provide a simplified and easy to implement policy based on successive acknowledgment failures for positively correlated links. Through simulation, we validate theoretical findings and demonstrate that our approach outperforms existing state of the art algorithms.