## 1 Introduction  
The explosive progress of information technology enables the Internet of Things (IoT) to support billions of devices on a global scale [1], which has built a bridge to connect the virtual cyber world to the real physical world [2]. With tremendous heterogeneous IoT devices and various applications, the world is truly transformed into a global village [3], providing us a platform to enjoy the smart environment and deeply changing the manner in which we live, work, and study [4]. In the traditional IoT deployment, sensor nodes are the main components; however, currently, IoT is also composed of other kinds of IoT devices, such as actuators [5], unmanned aerial vehicles (UAVs) [6, 7], robots [8], etc. Furthermore, in the near future, more wearable devices will appear to execute the functionality of health monitoring [9]. Surrounded by various heterogeneous IoT devices, multiple IoT applications are emerging, which advocate diverse latency and energy efficiency (EE) requirements [10, 11]. In this context, machine-type communication (MTC) gains momentum and is now a common trend when deploying the large-scale IoT [12]. Compared to the conventional human-to-human (H2H) communications, MTC shows its unique features, which include massive connectivity and transmission, extra low power consumption level,and high EE and security/privacy requirements [13]. There are several wireless access solutions for realizing MTC in IoT, i.e., ZigBee, WiFi, and Bluetooth [14]. A detailed description of these techniques is given in Table 1. They all work in the 2.4 G spectrum band, and thus, the co-channel interference is severe, bringing a negative effect to the coverage range and transmission distance. Additionally, working in the unlicensed spectrum makes the security issue more challenging to guarantee. As for the wired access solutions, though they are reliable and can provide high data rates, they are not practical in large-scale IoT implementations due to the cost ineffectiveness and lack of scalability and mobility. Therefore, with the trend of massive IoT connectivity, their disadvantages restrict them from going further into practical IoT implementations, and more advanced wireless solutions are advocated [15]. To this end, the cellular network, especially the Fifth-Generation (5G) communication network, exhibits its unique superiority for supporting pervasive connectivity and has been put forward as the most promising wireless solution for massive IoT connectivity [16]. Combining the cellular network with IoT, namely the C-IoT, ubiquitous connectivity and large coverage can be realized [17]. Additionally, the security and quality of service (QoS) requirement can also be guaranteed (Currently, the software-defined network (SDN) is regarded as a crucial technique for IoT deployment. There are two main advantages that can be brought by combining IoT with SDN. Firstly, SDN-based IoT enables the network operator to perform centralized, flexible, and programmable management of numerous IoT devices [18]. Secondly, SDN-based IoT can deal with the challenges arising from heterogeneous access technologies and device infrastructures [19; 20]. In this paper, we review the C-IoT architecture, which aims to provide ubiquitous and seamless connectivity to MTC devices. Under the C-IoT architecture, we concentrate on the energy efficient and harvesting designs to implement green C-IoT. The network management and control, however, are not the main focus of this paper, which will be left for our further investigation). Note that the C-IoT is not a major generator of greenhouse emission and global warming, but still has negative effects on environmental conservation owing to the pervasive and ubiquitous connectivity of MTC devices [21]. As for the MTC devices themselves, distributed everywhere and every time to support the functions of sensing, transferring, and communicating, they are typically battery-powered, and it is impossible to recharge them before replacement.