## 2 Passive Systems
### Backscatter Communication
The backscatter circuit was tested at different locations with the signal power varying from 24 dBm to \(-8\) dBm, the maximum distance achieved between the tag and the TV tower was approximately 10.4 km [26]. Instead of using the RF signal from a TV tower, the work presented in [23] benefits from the Frequency Modulation (FM) radio to backscattering sensor information. The use of Wi-Fi as a source for ambient backscatter was explored in [27], achieving distances of 5 m between Wi-Fi source and modulation device, and transmission data rates of 60 kbps. Besides ambient backscatter, the use of commercial readers can also create a new set of opportunities for backscatter solutions. Before going further on this topic, it is essential to note that the considered reader architecture is the one presented in the bistatic approach, by other words, the reader only needs to receive and demodulate information. The challenge goes to the tags that need to modulate information according to the reader's protocol. In [28], [PERSON] et al. present a Bluetooth Low Energy (BLE) backscatter prototype that performs band-pass FSK modulation on a CW carrier source. Readers such as Apple iPad Mini or Nordic Semiconductor nRF51822 could receive the signal at a distance of 13 m. The work presented in [29] also explores the BLE backscatter. Focused  
Figure 4: Modulation Schemes. (**a**) On-Off Keying. (**b**) Frequency Shift Keying. (**c**) Phase Shift Keying. (**d**) Quadrature Amplitude Modulation. on allowing backscatter in LPWAN, [PERSON] et al. [30], proposed chirp based backscatter modulator, which allows the reflection of LoRa preamble symbols with a Spreading Factor (SF) of 7 and 125 kHz of bandwidth. Also using LoRa, in [13], [PERSON] presents a low-cost LoRa backscatter Integrated Circuit (IC) whit a power consumption of 9.25 \(\upmu\)W. Figure 5 illustrates the schematics for ambient backscatter and proprietary reader. Another important aspect of devices based on backscatter communication is their capabilities for use in sensing applications. Temperature, humidity, and other environmental parameters can influence dielectric properties and therefore change the natural response of a determined device. If characterized, this alteration can be understood as an indirect measurement [31]. By placing a RFID tag with two sets of parallel and series stubs on the side of a water tank, [PERSON] et al. were able to create a water temperature sensor. In the same work, and using similar technologies, the authors also created a set of on-off tags to measure the water levels. To do that, the tags were placed inside a water tank. When submerged into water, due to the high permittivity, the tags did not respond to interrogation; when the water level dropped and the tags were exposed to air, they started responding, thereby creating the on-off sensor [20]. Taking advantage of indirect measurement, the work presented in [32] shows an adhesive sensor capable of monitoring sweat electrolytes. Besides the IC, the work uses a sensor that changes its impedance according to sweat levels, and the technology was created integrating both in the same device. Another example of combining backscatter techniques with sensing applications is provided in [33]. The authors used two RFID IC connected to two mercury switches with an antiparallel configuration. Using this architecture, [PERSON] et al. created a one-bit accelerometer: when the mercury is at one end of its container, one switch is closed and the other is open, which means that just one IC responds when interrogated; when the mercury is at the other end, the process is inverted creating the one-bit accelerometer. Figure 5: Backscatter systems without dedicated interrogators or readers. (**a**) Ambient backscatter (**b**) Proprietary reader.