  Network-assisted device-to-device communication is a promising technology for
improving the performance of proximity-based services. This paper demonstrates
how the integration of device-to-device communications and dynamic
time-division duplex can improve the energy efficiency of future cellular
networks, leading to a greener system operation and a prolonged battery
lifetime of mobile devices. We jointly optimize the mode selection,
transmission period and power allocation to minimize the energy consumption
(from both a system and a device perspective) while satisfying a certain rate
requirement. The radio resource management problems are formulated as
mixed-integer nonlinear programming problems. Although they are known to be
NP-hard in general, we exploit the problem structure to design efficient
algorithms that optimally solve several problem cases. For the remaining cases,
a heuristic algorithm that computes near-optimal solutions while respecting
practical constraints on execution times and signaling overhead is also
proposed. Simulation results confirm that the combination of device-to-device
and flexible time-division-duplex technologies can significantly enhance
spectrum and energy-efficiency of next generation cellular systems.
