  Device-to-device (D2D) communication is a promising technology for the future
wireless systems. It allows direct communication between devices, which
provides improvements in terms of delay, throughput and energy consumption.
Therefore, it can contribute to achieving the ambitious requirements of future
5G wireless system. In this sense, energy efficiency has become a key
requirement in the design of 5G technology. In this paper we analyze the
energy-efficiency improvement provided by D2D communications in an overlaying
scenario, in the context of a realistic wireless network system. This analysis
takes into account the two D2D phases, discovery and communication. A
centralized architecture is considered to manage discovery, which is a key
phase on D2D communications. Numerical evaluation shows improvement in terms of
energy-efficiency, reachable throughput and outage probability.
