  Offloading data traffic from Infrastructure-to-Device (I2D) to
Device-to-Device (D2D) communications is a powerful tool for reducing
congestion, energy consumption, and spectrum usage of mobile cellular networks.
Prior network-level studies on D2D data offloading focus on high level
performance metrics as the offloading efficiency, and take into account the
radio propagation aspects by using simplistic wireless channel models. We
consider a D2D data offloading protocol tailored to highly dynamic scenarios as
vehicular environments, and evaluate its performance focusing on physical layer
aspects, like energy consumption and spectral efficiency. We do this by taking
into account more realistic models of the wireless channel, with respect to the
simplistic ones generally used in the previous studies. Our objective is
twofold: first, to quantify the performance gain of the considered D2D
offloading protocol with respect to a classic cellular network, based on I2D
communications, in terms of energy consumption and spectral efficiency. Second,
to show that using simplistic channel models may prevent to accurately evaluate
the performance gain. Additionally, the use of more elaborated models allows to
obtain insightful information on relevant system-level parameters settings,
which would not be possible to obtain by using simple models. The considered
channel models have been proposed and validated, in the recent years, through
large-scale measurements campaigns.
  Our results show that the considered protocol is able to achieve a reduction
in the energy consumption of up to 35%, and an increase in the system spectral
efficiency of 50%, with respect to the benchmark cellular system. The use of
different channel models in evaluating these metrics may result, in the worst
case, in a sixfold underestimation of the achieved improvement.
