  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
in 5G system. In this paper, we propose to enable D2D communication in
unlicensed spectrum (D2D-U) as an underlay of the uplink LTE network for
further booming the network capacity. A sensing-based protocol is designed to
support the unlicensed channel access for both LTE and D2D users. We further
investigate the subchannel allocation problem to maximize the sum rate of LTE
and D2D users 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
user-subchannel swap algorithm. Analytical and simulation results show that the
proposed D2D-U scheme can significantly improve the system sum-rate.
