Adaptive Single-Hop and Relay-Selection-Based Double-Hop D2D Transmissions Underlaying Large-Scale Cellular Networks

Device-to-device (D2D) communication enables direct data transmissions between devices, thereby filling in the blind zones of base stations (BSs) and alleviating their load pressures. In this work, we propose an adaptive single-hop and relay-selection-based double-hop D2D transmission scheme underlaying cellular networks with full-frequency reuse. When the communication distance between two D2D users is shorter than a threshold $d_{0}$ , the single-hop D2D link can be established. However, when the distance between two D2D users is longer than $d_{0}$ but shorter than $2d_{0}$ , and there exists at least one relay node in the overlapping coverage areas of both users, the double-hop D2D transmission is performed. Based on the stochastic geometry theory, we properly model the random distributions of D2D users, relay nodes, cellular users, and BSs, as well as the aggregate interference for each type of communication mode. We propose to divide the overlapping area of the coverage regions of two D2D users into small grids, and the relay selection priority is assigned to each grid according to the selection criterion. The griding of overlapping areas can greatly simplify the analysis of successful relaying by considering the possible locations of the selected relay. We also present two benchmark schemes, namely cellular only, cellular with D2D single-hop. Numerical results show that our proposed scheme can significantly improve the area throughput compared with the benchmark schemes.