## 2 The Green C-IoT for MTC  
The IoT is not a fresh concept, but has gained more popularity and momentum recently. It is widely applied in the deployment of smart cities [29; 30], such as smart homes [31], smart grids [32], smart monitoring [33], smart transportation [34], eHealth [35], etc., which provides us a platform to improve the work efficiency and bring much convenience to our daily lives. However, everything has its cost. With the IoT devices' expansion, the network also faces many challenges [36]. Due to the pervasive number of MTC devices, the huge power consumption and carbon dioxide emissions pose great pressure on environmental conservation. This severe situation pushes us to consider the green IoT deployment, which is the general trend of development toward green wireless communications for the future wireless networks' design [37]. Green IoT aims at reducing power consumption and minimizing operational costs with the aid of energy efficient, sustainable, and environmentally-aware technologies [38; 39]. In this regard, green IoT typically implies two meanings. On the one hand, it refers to the schemes adopted by IoT devices bringing a benefit in terms of saving on energy consumption [40]. On the other hand, it means that the IoT architecture, especially the tremendous amount of IoT devices, such as smartphones, notebooks, household electronics, UAVs, etc., is operationally energy-saving. In practical implementations, both aspects are supposed to be considered when realizing the green IoT deployment [41]. By adopting energy efficient and energy-aware schemes, green IoT is able to reduce the greenhouse effect and achieve a sustainable IoT system [42; 43]. Owing to the proliferation of IoT devices, MTC will be widely applied to multiple applications, where IoT devices are able to gain the opportunities to communicate with remote automatic devices for monitoring and information exchange without or with minimal manual control [44]. In MTC, the requirements of these applications vary in many aspects [33; 45], including the data rate, time latency, coverage area, reliability, security, power consumption, etc. For applications in the residential zone, MTC can be employed in smart homes to provide some intelligent services by automatically controlling the electronic appliances. In farms and factories, MTC is capable of improving the working efficiency of industrial operations. Apart from the aforementioned scenarios, MTC can also be deployed in many other environments, as presented in Figure 1, which has attracted considerable attention from both academia and industry. In particular, some standard work towards the new MTC 3 GPP network architecture was published in Release 12 and Release 13 [46], where the overload problem was solved. Moreover, the researchers from the industrial field focus on implementing more practical applications so as to make adequate preparations for smart cities in the coming years (Though the SDN is not specifically studied in this paper, the SDN architecture can be easily extended to the proposed three-tier C-IoT. Specifically, the control plane in the SDN can be implemented in the cloud computing plane with centralized controllers, and the user plane belonging to the SDN corresponds to the device plane and the access and edge computing plane. With this framework, which combines the SDN with the C-IoT, the functions of network management and control can be realized in our future work.). Due to the high density of MTC devices, the trend of massive MTC is on its way [47], which is more applicable and realistic in IoT environments. In this context, from the aspects of both infrastructure and  
Figure 1: The specific application scenarios for MTC. control signaling, the cellular network naturally provides the platform for ubiquitous and seamless connectivity [23]. Different from the traditional wireless sensor network (WSN), the appearance of multiple sensor nodes makes the C-IoT more complicated, and many existing methods cannot be simply extended to the C-IoT deployment [27].