# Backscatter Communications: Inception of the Battery-Free Era--A Comprehensive Survey
## 6 Efficient Power and Data Transfer in BackCom
### Power Sources in BackCom  
BackCom utilizes multiple power sources for the operation of passive tags, which can be dedicated or non-dedicated. Non-dedicated ambient sources include WiFi transceivers, TV broadcast transmitters, Bluetooth chipset, cellular base stations, AM/FM radio transmitters, or other RF power beacons. Figure 29 shows the connectivity of tags and readers through power beacons. An overview of RF-based wireless energy transfer technologies and their utilization in wireless communications have been discussed to highlight key design challenges and solutions, with opportunities in BackCom [79; 121]. Usually, in traditional wireless transceivers, the power consumed by Bluetooth ranges from 20 to 60 mW, while the power consumption of WiFi is more than 200 mW [122; 123]. Interestingly, the power consumption of devices in BackCom is in micro-watts [8; 32]. Figure 30 exhibits the power consumption of various devices operating with active radio and BackCom technologies. Ambient signals can fulfill this tiny power requirement. Moreover, the ambient RF sources with additional peripheral components (e.g., boost converter) could overcome the need of a battery in passive devices [124]. [PERSON] et al. introduced Amb-BackCom to power passive tags. This technique harvests power from surrounding RF waves and modulates the information on backscattered signals [12]. Subsequently, as there is no requirement of dedicated power sources, Amb-BackCom leverages the complete power from the surrounding radio signals. Mobile base transceiver station (BTS) and TV broadcasting towers as power sources in Amb-BackCom were studied in [125]. These power sources are preferred in urban environments, due to their easy availability and high transmitting power. Furthermore, this work considers energy harvesting topology, with dual ambient RF sources, for entirely practical sensor nodes in real-world scenarios. This topology achieves the lowest available input RF power levels and improves operational capabilities of the harvester in urban and semi-urban environments. [PERSON] et al. showed the efficiency of harvesters up to 40% in urban or semi-urban areas, while operating on a single frequency band  
Figure 29: Wirelessly powered Backcom. [126]. This circuit requires low power levels of about \(-25\) dBm for its operation. A hardware prototype was built for BackCom devices, and it could draw complete power for its circuitry through energy harvesting from TV signals and solar energy [32]. This design enabled tags to communicate through walls at a distance of tens of meters. Backscatter fading channel principles and characteristics, such as beam-forming, were considered for BackCom energy transfer [127]. Subsequently, the latest trends in small cells, 5G, mm-waves, and massive MIMO encourage implementation of wireless power transfer, along with information exchange [128]. Likewise, a simple model for energy harvesting, by a tag, was proposed to show the trade-off between link reliability and the duty cycle of the tag for only downlink tags [74]. A model consisting of the WiFi source to power-up RF-powered devices was suggested [37]. The proposed WiFi BackCom model connects RF-powered tags to the Internet. Moreover, a hardware-based model demonstrated a communication link between WiFi devices and RF-powered tags [37]. The passive WiFi chipset that works on BackCom consumes 3-4 times less power in magnitude, compared to the available, common WiFi generating \(802.11b\) transmissions [35,129]. PoWiFi was proposed to utilize existing WiFi chipsets to power up passive tags [130]. The multi-channel harvester in PoWiFi can harvest energy from various 2.4 GHz WiFi channels. Furthermore, the design of WiFi chipset-powered camera sensors and temperature sensors were suggested. Authors also demonstrated the recharging utility of batteries made up of \(NiMH\), \(Li-ion\), and coin-cell materials that can be charged by WiFi signals. Bradio was proposed to consume little power and ranges from 16 to 129 micro-watts [131].