# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 3 Green Radio-Frequency Identification
### Passive RFID Systems  
Passive RFID tags lack an inbuilt energy source; they are powered by the electromagnetic energy emitted from an RFID reader instead. Passive RFID tags are popular in several applications, such as access control, file tracking, race timing, supply chain management and smart labels. The cheap price of RFID tags allows their wide deployment across a wide spectrum of the industry. RFIDs have already been promoted as the most cost-effective solution with satisfactory achievements in smart parking systems [98]. RFID in smart parking systems have mainly been used for check-ins and check-outs of passing vehicles in parking areas; such vehicles are equipped with e-pass cards with active RFID tags [99]. Each vehicle that enters a parking area has a unique identification number that can be paired with its license plate. The barriers only open if the vehicle is recognised as being registered. Moreover, the authors in [100] combined RFID technology with wireless sensors to collect information on the occupancy state of parking spaces and direct drivers to the nearest vacant parking lot. Other for vehicle validation at the entrance and exit of a parking area, RFID technology has been utilised for parked vehicle localisation by exploiting the time difference of arrival and the received signal strength measurements [101]. Furthermore, the authors in [102] presented an analysis and evaluation of a passive RFID-based framework in the context of smart parking systems. The main novelty of the proposed framework is the adoption of RFID tag-to-tag communication, which can support a more energy-efficient collection of information than the conventional direct type of communication. The overall framework is considered and examined within a smart parking system use case scenario, in which RFID technology is adopted for: (a) vehicle validation when entering and exiting the parking area; (b) identification and localisation of parked vehicles; and (c) smart parking space management. The superiority of the proposed framework over conventional direct RFID reader tag communication is demonstrated in terms of: (i) the reduction of RFID readers' transmission power to the required minimum to guarantee connectivity, and (ii) the expansion of RFID readers' coverage areas to communicate with many distant tags. A summary of the passive RFID system is given in Figure 3.  
Electromagnetic energy transfer techniques can be classified into three main techniques: (i) RF energy harvesting, (ii) inductive coupling, and (iii) magnetic resonance coupling (Table 5). Passive RFID system energy transfer techniques are NFC, either inductive coupling or backscatter. The RFID reader emits a sinusoidal signal. The tag antenna is tuned to receive the signal from the reader. The internal IC of the passive tag contains a rectifier circuit that converts the power into DC, enabling the tag circuitry to work. The circuitry modulates the signal to an extent and then returns it to the reader. During this process, the tag does not create a separate signal; it merely modulates the signal received from the reader. An explanation of the backscattering principle is given in detail in [96]. The Passive RFID tag feature is low power consumption, making it a suitable candidate in wireless sensing applications. However, operating coverage distance is a notable challenge. In best case scenario, the maximum RF coverage distance of passive tags are up to 7-15 m. The reason for this is that the system is powered using electromagnetic induction, which is itself relatively weak. Additionally, path loss, which is considered to be one of the most important parameters in any wireless communication, must be included. Accordingly, path loss is a crucial design parameter in RFID transmission. Path loss is due to many effects, such as free-space loss, refraction, scattering and diffraction, reflection, the height of antenna, the surrounding environment and weather (dry or moist air), the distance between the transmitter and the receiver, the height and location of antennas [95].