## 1 Introduction  
Recently, the number of cellular subscribers and the traffic demand have been increased significantly due to the popularity of wireless Internet services, such as world wide web (WWW), instant messaging services, peer-to-peer (P2P) services, streaming services, _etc_. Due to the explosive growth of the data traffic for wireless Internet services using smart devices, such as smart phones and smart pads, the energy consumption of mobile stations (MSs) and base stations (BSs) is rapidly increasing, too [1; 2; 3; 4]. Furthermore, the needs of the Internet of Things (IoT), which is based on wireless sensor networks, have increased significantly [5], and works on reducing the energy consumption of sensor nodes in wireless sensor networks have been carried out extensively, too [6; 7]. Since the energy consumption of MSs and BSs produces large amounts of CO\({}_{2}\) emissions, works on reducing the energy consumption of MSs and BSs are being carried out actively in order to achieve green cellular networks [8; 9; 10; 11; 12; 13]. Works on reducing the energy consumption of MSs have tried to extend the battery lifetime of MSs and, thus, reduce CO\({}_{2}\) emissions [8; 9; 10]. In these works, MSs stay in power saving mode or sleep mode, by turning off the radio interface when there is no data transmission with BSs. Then, MSs wake up periodically to check whether there is any incoming data packet to them and stay awake if there is any incoming data packet. Otherwise, MSs return to power saving mode or sleep mode to save energy. Works on lowering the energy required by BSs have been carried out actively, too, in order to achieve green cellular networks [11; 12; 13]. In these works, the main idea of achieving green cellular networks is to sleep or turn off unused BSs to save energy, while supporting the coverage area of the sleep or turned off BSs by neighbor BSs. To achieve this goal, several schemes have been proposed, such as cell switching off [11], cell zooming [12] and cell wilting and blossoming [13]. In the cell switching off scheme [11], BSs are switched off when they are not necessary, since the traffic load is very low late at night, and the service area covered by the switched-off BSs is taken care of by the active neighbor BSs in order to guarantee service availability. In the cell zooming scheme [12], BSs adapt the cell size based on the varying traffic load. When the traffic load of a BS is congested, the cell size of the BS is reduced to escape from the traffic congestion. On the other hand, if the traffic load of a BS is very low, the cell size of the BS zooms out and the neighboring cells can zoom in. In the cell wilting and blossoming scheme [13], BSs go to the sleep state gradually by preventing any interruption of active data sessions (cell wilting), and MSs served by the wilting BS should be handed over to neighboring BSs. If the traffic load of any BS is higher than a threshold value, it alerts sleeping neighbor BSs, and then, the neighbor BSs wake up to cover the traffic load (cell blossoming). In these schemes, however, it is not easy to guarantee full service coverage all of the time, and thus, coverage hole problems may happen, which should be avoided for providing service availability to MSs. In heterogeneous networks with macrocells and femtocells, the traffic load of macrocells is offloaded to femtocells, and the coverage hole problem can be avoided efficiently, since the full service coverage can be provided by overlay macrocells. In heterogeneous networks, macrocells and femtocells can use separate frequencies or share the same frequencies [14; 15; 16; 17].