## 1 Introduction  
In recent years, breakthrough progress has been made in the internet of things (IoT) technique which has been widely applied in industrial automation, intelligent medicine, smart grids, etc. [1; 2; 3]. According to estimates, the communication system will support more than 50 billion IoT devices by 2015 [4]. In the meantime, these put forward larger requirements on energy supplies [5]. Therefore it is extremely urgent to design a green sustainable scheme which provides a larger transmission rate without requiring frequent battery replacements [6]. Compared to the traditional battery energy supplies, energy harvesting (EH) technology can extract renewable energy from the surrounding environment, which is not always stable and controllable. Magnetic induction and radio frequency (RF) are two common ways to realize wireless power transfer, which can supply stable energy. However, the power transfer range will be very limited through magnetic induction [7; 8]. With the continuous breakthrough of ultra-low-power semiconductors, RF plays a more important role in long-distance communication [9; 10], which has the ability to carry both information and energy. Therefore, as a technology with broad application prospects, simultaneous wireless information and power transfer (SWIPT) has roused great concern from academia, and has been widely investigated [11; 12; 13; 14; 15]. In the SWIPT enabled IoT system, extending the radio coverage and increasing spectrum utilization are two key points of the design. Cooperative relaying technology has been widely applied by virtue of its ability to extend the coverage of radio waves, reduce fading, and increase system capacity [16; 17; 18; 19]. However, relays require to have battery storage capability or external charging to keep running, which is not conducive to flexible placement of relays. The SWIPT technology can solve the energy limitation problem of relays. Therefore, the introduction of cooperative relaying technology into a SWIPT enabled IoT system is an effective way [20]. The author in [21] investigates the spectrum efficiency optimization problem in a cognitive radio system with SWIPT, where IoT devices serve as relays operating decode-and-forward (DF) to assist the system's information interaction. A joint optimization design for SWIPT-enabled IoT network with a DF relay is studied in [22], where the reliability is improved by combining the advantages of power-splitter (PS) and time-splitter (TS). To achieve the balance between energy efficiency and spectral efficiency, a locally optimal power allocation scheme is designed in [23] for SWIPT amplify-and-forward (AF) relaying networks, which reduces computational complexity and maintains high performance. Efficient utilization of spectrum is another emphasis for IoT design. Due to the high spectrum efficiency of orthogonal frequency division multiplexing (OFDM) and its resistance to multipath fading [24; 25], the design combining OFDM and SWIPT for IoT system becomes a promising way to enable higher data demands and lower energy requirements. In [26], a multiuser scenario was considered in an OFDM-based SWIPT system, where the optimal rate is obtained by iteratively adjusting resource allocation under the condition of meeting the energy harvesting requirement. In [27], two suboptimal algorithms were provided for a downlink SWIPT based orthogonal frequency division multiple access (OFDMA) network with the goal of maximizing the energy harvested, which achieves the balance between reducing complexity and maintaining system performance. In [28], a new receiver architecture was designed which employs SWIPT technology and reduces the energy required for decoding. This scheme expands the rate-energy boundary greatly. Therefore, combining relaying cooperation technology and OFDM technology with SWIPT in IoT systems is a promising scheme which can enable higher data demands effectively while meeting the design requirements of spectrum and radio coverage.