# RF Energy Harvesting Wireless Communications:
## 1 Introduction
### Contributions of This Article  
In SWIPT, the power transfer and information reception occur simultaneously. The receivers split the RF signal and use the collected energy to decode the information. Therefore, SWIPT is typically used for co-located energy/information access points (APs) to send data to one or more end users (i.e., downlink communications) [22]. In this article, however, we consider the scenario where the wireless devices harvest energy from ambient RF environment or dedicated energy sources and store the energy in a battery (e.g., supercapacitor). The harvested energy can be later utilized for uplink or ad hoc communications. Different from the simultaneous power transfer and data transmission in SWIPT, data transmissions in RF-EHWC take place after the devices have harvested a sufficient amount of energy.  
Enabling energy harvesting brings new challenges for the design of wireless communication systems. First of all, both the energy harvesting efficiency and energy utilization are critical to RF-EHWC. This is because the RF energy suffers from a high spreading loss in the free space. As a result, the energy reaching the RF-EHWC device is very limited in a far-field RF environment [23]. Hence, the hardware (e.g., the energy harvesting circuit and the antenna) should be well designed to improve the energy harvesting efficiency; meanwhile, the software (e.g., the power management policy and the transmission strategy) should also be carefully developed to increase the efficiency of energy utilization. Second, in addition to collecting energy from the ambient RF, the RF-EHWC device can also actively request energy from associated base stations and access points in some applications. In this case, the interaction between the energy flow and the data flow is intricate since the energy transmission can interfere with the information decoding while the information transmission may interrupt the energy reception. Therefore, how to balance the power transmission and the communication throughput is a challenging issue in the RF-EHWC design [24]. Third, the energy harvesting capabilities add a new dimension to resource management in RF-EHWC networks. The new management policy not only needs to properly arrange data flows, but also needs to organize energy flows so that each RF-EHWC device can receive sufficient energy to power the system, and the network can also have a high throughput and a low power consumption [21].  
Despite the growing interests from various research communities, there are still many unresolved problems to make the RF-EHWC realistic. In the past few years, most of research efforts have focused on hardware implementation and transmission management at the physical layer [25]. However, practical issues, such as the thin RF power density in the air, the nonlinear battery charging, intermittent connections in the RF-EHWC network, and security issues are still being overlooked. These under-explored topics inspire us to conduct more in-depth research on the development of practical RF-EHWC networks.  
Following this direction, in this article, we survey the latest research progress achieved in the practical design of RF-EHWC over the past few years. The remainder of the article is organized as follows: In Section 2, we investigate the ambient RF environments and discuss the challenges on ambient energy harvesting. Section 3 introduces the hardware architecture of RF-EHWC systems and design principles of the RF-EHWC hardware circuit. Section 4 introduces the power management problem and discusses practical issues such as the nonlinear battery charging in an efficient power management design. A unique challenge, called the intermittent connection problem, to the resilient routing design in RF-EHWC networks is discussed in Section 5. The secure information transmission in RF-EHWC networks is presented in Section 6. Finally, we introduce several promising applications of the RF-EHWC technique in Section 7 and conclude this article in Section 8.