Optimal spectrum partition and mode selection in device-to-device overlaid cellular networks

In this paper we jointly address two fundamental issues in device-to-device (D2D) overlaid cellular networks: 1) how to partition the spectrum assuming orthogonal cellular and D2D transmissions, and 2) what is the optimal threshold for distance-based D2D mode selection? To this end, we first derive rate expressions for both cellular and D2D users. Three design criteria - (weighted) max-sum, max-min and proportional fairness - are considered and optimal spectrum partition rules are derived. Finally we validate our model and give rules of thumb for optimizing the spectrum partition and mode selection.