## 2 The Green C-IoT for MTC
In addition, different from cellular network, such as the LTE-A network, the C-IoT is confronted with a severer situation, which is caused by heterogeneous MTC devices, various delay requirements, wide area coverage, and massive access to the evolved NodeB (eNB). The C-IoT architecture is shown in Figure 2. Additionally, the proposed architecture contains three planes, whose structure is presented in Figure 3. More details are presented as follows. * Device plane: This plane consists of various heterogeneous MTC devices. These devices are capable of accessing the cellular network and can enjoy the ubiquitous connectivities of the C-IoT. * Access and edge computing plane: This plane provides users with a platform for wireless access. Moreover, the fog nodes are equipped with some storage, computation, and processing capabilities. With these nodes, some data traffic for MTC devices can be cached and computed locally, and the traffic burden of the central data center can be alleviated. * Cloud computing plane: In this plane, the cloud server with cloud storage is placed, which are used to execute the centralized computation and processing tasks. Based on the proposed three-plane architecture, the challenges when deploying green C-IoT are summarized and given as follows. * Massive access: With plentiful heterogeneous MTC devices deployed in the practical scenario, they make diverse attempts to realize access and modify, release, or handover processes [48; 49]. Therefore, the random access scheme should be carefully designed so as to provide the high QoS requirement and a high successful access probability, as well as avoid attempt collisions. * Load imbalance: There are many kinds of heterogeneous MTC devices that execute diverse functions and that are distributed in different geographical positions. For some MTC devices providing real-time applications, they often stay in active mode and transmit the sensed data, leading to more power consumption [33]. Moreover, it is obvious that urban areas generally generate more data traffic than rural areas. Therefore, the traffic load is severe in some areas, while in some places, there is light traffic, which will result in the load imbalance problem [50]. * Coverage holes and interference: Though the C-IoT has the ability to provide wide area coverage with ubiquitous and seamless connectivity, there still remain some coverage holes due to the improper cell planning and the mobility scheduling of the MTC devices [51]. In addition, the considerable amount co-existing MTC devices brings interference [52], and their interference may be aggravated when the number of devices increases rapidly [53]. * High EE requirement: In general, for some sensor nodes, their lifespan of is supposed to be more than 15 years [48]. Though they work at a low power level, it is still necessary and challenging to make them operate in an energy efficient manner [54], since they cannot be re-charged again. Besides, large-scale energy management is a tough task in C-IoT scenario [55]. * Unavailability of servers: In some remote and traffic overloaded areas, the sensed data are difficult to transmit to the server, which is power consuming and has negative effects on real-time and delay-sensitive applications [33]. In summary, the deployment of green C-IoT is confronted with huge challenges in terms of the architectural and technical aspects [22]. In the following sections, we intend to handle these severe challenges through energy efficient and energy harvesting schemes. Figure 3: The three-plane C-IoT architecture. Figure 2: The cellular IoT (C-IoT) architecture.