Finally, the energy efficiency of wireless backhaul networks with respect to the path loss
coefficient considering different small cell radii is shown in Fig. 5. When the radius of small
cells is less than or equal to 50 m, the energy efficiency of wireless backhaul networks increases
with the increase of the path loss coefficient. When the radius of small cells is larger than 50 m,
the energy efficiency of wireless backhaul networks decreases with the increase of the path loss
coefficient. The reason for this result is that, based on the Shannon capacity theory, the increase
of path loss coefficients have a little attenuation effect on the wireless capacity when the radius
of small cells is less than or equal to 50 m. In contrast, the increasing path loss coefficients have
obviously an attenuation effect on the wireless capacity when the radius of small cells is larger
than 50 m. When the system energy consumption is fixed, the energy efficiency is proportional
to the wireless capacity in wireless backhaul networks. Compared with central and distribution
solutions in Fig. 5(a) and Fig. 5(b), the energy efficiency of the central solution is obviously
less than the energy efficiency of the distribution solution under the same radius of small cells
and the path loss coefficient.