Enabling device-to-device communications in LTE-unlicensed spectrum

LTE-Unlicensed (LTE-U) is considered as a groundbreaking technology to address the increasing scarcity of available spectrum by extending cellular communications to unlicensed band. In this paper, we investigate the performance of D2D communications in conjunction with LTE-U, which can alleviate traffic load of cellular networks. However, in the same unlicensed band, the coexistence of D2D and WiFi technologies should be carefully designed to satisfy user's quality of service (QoS) and to avoid severe interferences and contentions among devices using unlicensed spectrum. We model the transmissions in unlicensed band as hard core point processes (HCPPs) and thus the transmission probabilities of D2D and WiFi access points (APs) are obtained via the clear channel assessment (CCA) mechanism. Furthermore, by characterizing the intra-tier and inter-tier interferences in such complex communication networks, the average transmit power for the D2D link is investigated given that the user's QoS can be guaranteed. Moreover, the throughput of a typical WiFi AP in the large scale networks is theoretically analyzed, and the outage probability of a D2D link is characterized which results from insufficient transmit power under a pre-determined QoS requirement. Simulations justify successful D2D communications in the LTE-U operation and validate the accuracy of this analytical approach.