Second, the energy efficiency of wireless backhaul networks with respect to the number of small cells considering different frequency bands is illustrated in Fig. 4. In Fig. 4a, the energy efficiency of wireless backhaul networks logarithmically increases with the increase in the number of small cells in the central solution. In Fig. 4b, the energy efficiency of wireless backhaul networks linearly increases with the increase of the number of small cells in the distribution solution. When the number of small cells is fixed, the energy efficiency of wireless backhaul networks decreases with the increase of frequency bands. However, obviously there are energy efficiency gaps for 5.8 GHz, 28 GHz, and 60 GHz frequency bands in the central solution.