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 slight 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 obviously have 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 Figs. 5a and 5b, 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.