  Energy consumption is a major limitation of low power and mobile devices.
Efficient transmission protocols are required to minimize an energy consumption
of the mobile devices for ubiquitous connectivity in the next generation
wireless networks. Opportunistic schemes select a single relay using the
criteria of the best channel and achieve a near-optimal diversity performance
in a cooperative wireless system. In this paper, we study the energy efficiency
of the opportunistic schemes for device-to-device communication. In the
opportunistic approach, an energy consumed by devices is minimized by selecting
a single neighboring device as a relay using the criteria of minimum consumed
energy in each transmission in the uplink of a wireless network. We derive
analytical bounds and scaling laws on the expected energy consumption when the
devices experience log-normal shadowing with respect to a base station
considering both the transmission as well as circuit energy consumptions. We
show that the protocol improves the energy efficiency of the network comparing
to the direct transmission even if only a few devices are considered for
relaying. We also demonstrate the effectiveness of the protocol by means of
simulations in realistic scenarios of the wireless network.
