  Though cooperative relaying is believed to be a promising technology to
improve the energy efficiency of cellular networks, the relays' static power
consumption might worsen the energy efficiency therefore can not be neglected.
In this paper, we focus on whether and how the energy efficiency of cellular
networks can be improved via relays. Based on the spatial Poisson point
process, an analytical model is proposed to evaluate the energy efficiency of
relay-assisted cellular networks. With the aid of the technical tools of
stochastic geometry, we derive the distributions of
signal-to-interference-plus-noise ratios (SINRs) and mean achievable rates of
both non-cooperative users and cooperative users. The energy efficiency
measured by "bps/Hz/W" is expressed subsequently. These established expressions
are amenable to numerical evaluation and corroborated by simulation results.
