## 1 Introduction  
Wireless power transfer (WPT) technology can provide energy without physical contact. Due to its convenience, safety and waterproof characteristics, this technology has been widely used in many fields, such as mobile phones [1], electric vehicles [2; 3], smart grids [4; 5], and medical devices [6]. In most applications, the load resistance and coupling inductance of the WPT systems are uncertain; therefore, a real-time information exchange is essential for primary and/or secondary side controller to maintain the stability of power transmission. Recently, simultaneous wireless power and information transmission (SWPIT) technology has attracted considerable attentions due to its high integration feature. According to the direction of information transmission, it can be divided into forward, backward and bidirectional communication. For forward communication, the information of primary side is sent to adjust equivalent impedance [7] and track maximum efficiency [8] by the secondary side controller. For backward communication, the information of secondary side is sent to regulate power capacity [9] and realize zero phase angle (ZPA) [10] by the primary side controller. Compared with single-side communication, bidirectional communication can exchange more information for controllers, and it is easy to realize intelligent wireless power transmission [11]. For the implementations of bidirectional communication, there are four basic methods, as shown in Figure 1, where \"TX\", \"RX\" and \"COM\" represent the primary power converter, the secondary power converter and the compensation circuit, respectively. For the first method, as shown in Figure 1a, radio frequency (RF) link technologies, such as WiFi, Zigbee and Bluetooth, are used to exchange information between two sides. Since the power module and communication module are designed independently, the interaction between power and information transmission can be ignored. However, the RF link is subject to EMC problems [12] and may not work properly, especially in the high-power applications. For the second method, a specially designed communication coil placed around power coil is used to exchange information between the two sides, as shown in Figure 1b. To meet strict space constraints, the communication coil can be arranged on coaxial plane [13; 14] or orthogonal plane [15]. Since the frequency of data carrier is not limited by the frequency of power carrier, this method can achieve high data rate. Unfortunately, the data signal is easily affected by the power loop, resulting in low signal-to-noise ratio (SNR) [16]. Moreover, the volume and cost of WPT system are increased significantly. For the third method, information transmission is realized by coupling with compensation circuit, where the data signal is injected from one side and extracted from other side [17], as shown in Figure 1c. The data channel and power channel are shared in this method, and it is promising to realize high integrated SWPIT system. However, the data signal is much smaller than the power signal, and the transmitted data may not be detected from demodulator [18; 19]. To improve the data signal quality, the power level of communication system should be increased. Thus, the power consumption of communication circuit can not be neglected in this method, especially in the low power scenarios [20]. Compared with the previous three methods, the fourth method based on modulating with power converter, as shown in Figure 1d, has minimal hardware consumption and is compatible with a wide range of applications [21; 22; 23; 24; 25]. In [22], a full-duplex bidirectional communication is achieved by adjusting duty cycle of the flyback converter and calculating the times of resonances. In [24], the operating frequency and duty cycle of power converters on both sides vary with the transmitted information.