# RF Energy Harvesting Wireless Communications:
## 1 Introduction
### Wireless Information and Power Transfer (WIPT)  
The initial developments of wireless power transfer systems (WPT) and wireless information transfer (WIT) systems are independent. The former has a huge size in order to achieve efficient energy transfer at high-power levels [6], while the latter is often intended to build portable devices with low power consumption. Until 1948, [PERSON] introduced a new concept, called the communication by means of reflected power [7]. His work is the prototype of the radio frequency identification (RFID), the first attempt to combine WIT with WPT.  
Three decades later, a short-range RFID system was implemented based on the modulated backscatter [8]. In this system, the RF reader sent a 1 GHz continuous wave (CW) to an RF tag, which then changed the load on its rectifier to modulate the amplitude of backscatter waves for the data transmission. Since a portion of incident energy was rectified to power the RF tag for wireless communications, the passive RFID can be considered as the first WIPT system in the history. Nowadays, RFID and its derivatives, near-field communication (NFC) [9], have been widely used in different places for item classification, electronic payment and tool management [10; 11].  
In 1990s, the concept of smart dust was proposed. As demonstrated in Figure 1, the smart dust is a miniature wireless mode that integrates multiple tiny microelectromechanical systems (MEMS) for environment sensing and wireless communications [12]. Different from the passive RFID that obtains energy from a dedicated RF reader, the smart dust can harvest energy from surrounding environment, such as solar, vibrations, or even ambient RF waves [13]. Owing to the features of miniaturization and sustainability, the smart dust is gradually penetrating into our daily life [14; 15].  
Simultaneous wireless information and power transfer (SWIPT) has caught considerable research attention for its efficient spectrum usage [17; 18; 19; 20]. The SWIPT seeks to deliver wireless information and controllable energy concurrently in the same RF signal. Since information processing and energy harvesting cannot be simultaneously performed by the same circuit, the received signal has to be split into two distinct parts, one for energy harvesting and one for information decoding, in the time switching (TS) or power splitting (PS) manner. In the time switching (TS) structure, the RF signal is divided into two time slots, one for transferring power and the other for transmitting data. The power splitting (PS) structure splits the received signal into two streams, one with power ratio \(\rho\) for energy harvest and the other stream with power ratio \(1-\rho\) for information decoding. By varying the length of data/energy slots in the TS scheme or adjusting the value of power ratio in the PS scheme, a balance can be achieved between the performance of information decoding and energy transfer. Extensive research has been conducted to investigate the rate-energy tradeoff for efficient design of SWIPT system [21].  
Figure 1: Smart dust developed by University of Michigan, sitting on a penny [16].