  Energy efficiency becomes increasingly important due to the limited battery
capacity in wireless devices while at the same time user throughput
requirements are relentlessly increasing. In this paper, we study an energy
efficient cooperation scheme which employs network coding to enhance the energy
efficiency for mobile devices. Herein we propose that the mobile devices are
clustered into mobile small cells with one of the mobile devices acting as a
group head with basic transceiver, coding and relaying functionalities. Group
heads coordinate the transmissions from the mobile devices in the mobile small
cell to the network's base stations. The objective function of the cooperative
scheme is to minimize mobile devices' energy consumption subject to a certain
bit error probability. The proposed network-coding based scheme has been
evaluated by means of numerical simulations and compared to both a conventional
direct transmit scheme, with no cooperation groups, and a cooperative relaying
scheme. Results show that, with network-coded cooperation, energy efficiency
may significantly increase provided the density of base stations and mobile
devices is below a certain value. Above this value none of the compared
cooperation schemes may improve energy efficiency, but rather power consumption
is reduced only when mobile devices transmit via base stations in their close
proximity.
