# Green IoT: A Review and Future Research Directions
## 6 Energy-Efficient Radio-Frequency Identification
### Passive RFID Systems  
RFID passive systems use tags that are powered externally by electromagnetic energy from an RFID reader rather than having an internal power source. Passive RFID tags are commonly used in a wide range of applications such as inventory tracking, access control, timing events, smart labels, and supply chain management. They are also a cost-effective solution for many industries, as they have a lower cost per tag. Figure 12 illustrates the basic operation of a passive RFID system.  
Figure 11: Classifications of RFID tags.  
Figure 10: Operating principle of RFID system.  
As discussed in Section 5.3.3, there are three main techniques for transferring electromagnetic energy in RFID systems: the RF energy harvesting scheme, magnetic resonance coupling, and inductive coupling, which are presented in (Table 5). NFC is commonly used in passive RFID systems, and it can be either backscatter or inductive coupling. The RFID reader sends out a signal in the form of sinusoidal wave, which the tag antenna receives and converts into power using a rectifier circuit. The internal IC of the tag is powered by this energy, and it modulates the signal received and delivers it back to the reader. The tag solely modulates the signal that the reader sends it, not producing any signals of its own throughout this process. Detailed information on the backscattering fundamentals can be found in [107].  
Passive RFID tags have low power consumption and are well-suited for wireless sensing applications. Notably, their operating range is limited, with the finest passive tags able to function at a distance of 7-15 m. This is mainly due to the weak electromagnetic induction-based communication principle. Additionally, path loss, which is a significant parameter in any wireless communication, also plays a pivotal role in limiting the operating range of RFID transmission in passive tags. Path loss is caused by a variety of factors such as refraction, free-space loss, diffraction, scattering, antenna height and location, reflection, weather conditions and surrounding environment. On the other hand, the inter-site distance between the transmitter and receiver and the height and position of antennas also play a vital role in path loss [108]. Therefore, selecting an appropriate path loss calculation model is crucial. In [109], the author extensively compared various channel path loss prediction models, for example, Hata, Friis, CCIR, etc., under different scenarios in various environments. However, such models are highly dependent on the system's operating frequency and application.  
According to [PERSON] et al. [110], RFID readers and WISP-Motes, passive RFID wake-up radios, are the main components of the network. The energy generated by the reader transmitter wakes up these passive RFID wake-up radios, allowing them to wake up the node. However, equipping all sensors with RFID readers is not practical due to their high power consumption, which limits their use to single-hop environments. Simulation results revealed that WISP-Motes can save a considerable amount of energy, but this comes at the cost of additional hardware and slower data transmission by increasing latency in data delivery. The advantages of this strategy have been illustrated by the authors in the context of a sparse delay-tolerant network with mobile elements that include RFID readers.