  The integration of local device-to-device (D2D) communications and cellular
connections has been intensively studied to satisfy co-existing D2D and
cellular communication demand. In future cellular networks, there will be
numerous standby users possessing D2D communication capabilities in close
proximity to each other. Considering that these standby users do not
necessarily request D2D communications all the time, in this paper we propose a
hybrid D2D-cellular scheme to make use of these standby users and to improve
the rate performance for cellular users. More specifically, through D2D links,
a virtual antenna array can be formed by sharing antennas across different
terminals to realize the diversity gain of MIMO channels. This paper considers
the use of millimeter wave (mmWave) links to enable high data rate D2D
communications. We then design an orthogonal D2D multiple access protocol and
formulate the optimization problem of joint cellular and D2D resource
allocation for downlink transmissions using the proposed scheme. We obtain a
closed-form solution for D2D resource allocation, which reveals useful insights
for practical system design. Numerical results from extensive system-level
simulations demonstrate that the rate performance of cellular users is
significantly improved.
