D2D-U: Device-to-Device Communications in Unlicensed Bands

Device-to-Device (D2D) communication, which enables direct communication between nearby mobile devices, is an attractive add-on component to improve spectrum efficiency and user experience by reusing licensed cellular spectrum. Nowadays, LTE-unlicensed (LTE-U) emerges to extend the cellular network to the unlicensed spectrum to alleviate the spectrum scarcity issue. In this paper, we propose to enable D2D communication in unlicensed spectrum (D2D-U) as an underlay of the uplink cellular network for further booming the network capacity. A sensing-based protocol is designed to support the unlicensed channel access for both LTE users and D2D pairs, based on which we investigate the subchannel allocation problem to maximize the sum rate of LTE users and D2D pairs while taking into account their interference to the existing Wi-Fi systems. Specifically, we formulate the subchannel allocation as a many-to-many matching problem with externalities, and develop an iterative usersubchannel swap algorithm. Analytical and simulation results show that the proposed D2D-U scheme can significantly improve the network capacity.