Transmission Mode Selection and Resource Allocation for D2D Unicast Communications

In this paper, we propose solutions to the transmission mode selection and resource allocation problems for D2D unicast communications. We first formulate a joint mode selection and resource allocation problem, which we further decompose into separate mode selection and resource allocation problems, which are run at different timescales. Our mode selection algorithm determines the optimal transmission mode for all D2D transmitter-receiver user equipment (UE) pairs. Our proposed algorithm involves computing a switching metric that takes into account the spectral efficiency in the two modes, possibility of resource reuse, and the traffic load in the two modes. We allow a D2D UE pair to switch the transmission mode only if it improves the overall system performance. Our resource allocation scheme schedules the cellular and D2D users based on the selected transmission mode such that it maximizes the sum proportional fairness (PF) metric across all users. Because an exhaustive search over all UEs will be prohibitive when there are a large number of active UEs, we develop a greedy heuristic that incrementally adds UEs such that co-scheduling of every additional UE results in improvement of the sum PF metric. Our numerical results show that we are able to achieve near-optimal solution with our proposed greedy heuristic. We also show numerical results that quantify the gains from our mode selection and resource allocation algorithms for different scenarios.