## 2 Passive Systems
### Backscatter Communication  
Backscatter communication consists of reflecting the electromagnetic wave with a specific modulation, making it possible for the receiver to decode it. This principle relies on Modulated Scattering Technique (MST) to transmit unidirectional information from tag to reader. [19; 20]. A passive system is usually composed of three different elements: an interrogator, a reader, and a tag. The interrogator is responsible for sending a Continuous Wave (CW) to the tag, which by its turn, modulates and reflects it. This reflection is received by the reader that demodulates the tag's data. Backscatter communication can assume two different structures, monostatic and bistatic (collocated or dislocated). In monostatic architectures, the same device performs interrogator and reader functions. This structure allows simple implementation and cost reduction and is mostly used in commercial readers. The main drawback of this topology is that it results in higher path losses meaning shorter distances and less coverage (the round-trip, reader-tag-reader, the path loss is proportional to the 8 th power of the distance). In bistatic, there are independent devices that allow separated locations. It has less path loss and allows the distribution of multiple interrogators or transmitter. It is possible to build a system with a centralized reader and multiple tags illuminated by multiple transmitters, which would allow a higher field coverage [18; 21; 22]. Figure 3 schematizes both topologies. As mentioned, tags are responsible for modulating and reflect the incoming wave. The modulation process consists of changing the load impedance, and different load switching results in different reflection coefficients and thus distinct modulations. Most commercial systems employ the simplest way of the Amplitude Shift Keying (ASK) scheme, the On-Off Keying (OOK). This 1-bit modulation consists in transmitting zeros and ones represented by the existence or absence of reflected wave. The reflection occurs when the circuit connects to a full-reflection load, i.e., open or short, and the absorption happens when it switches to a matching load. This binary scheme can further evolve to more complex ASK modulation based on different levels of reflected power [21]. Instead of using amplitude variations, it is possible to perform the modulation in different frequencies and phase reflections, i.e., Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). Besides modulation that involves one particular component, it is also possible  
Figure 3: Backscatter architectures. (**a**) Monostatic (**b**) Bistatic. to perform backscatter modulation using multiple variations. The classic example is Quadrature Amplitude Modulation (QAM), which encompasses amplitude and phase modulations. Figure 4 illustrates some modulation schemes. The modulation schemes represent the bases from which it is possible to define new scientific advances in backscatter communication. The interrogator replacement and the use of commercial readers are two areas of great interest once they can expand the use of backscatter to other systems. The way to not use a dedicated interrogator is to take advantage of existing Radio Frequency (RF) signals e.g., radio, television, Wi-Fi, which is a technique known as ambient backscatter [23]. When compared with the traditional approach, the use of ambient backscatter has the clear advantage of not requiring a dedicated interrogator. However, it also presents some negative aspects like its signal stability and the limited modulation possibilities. As the interrogator has different purposes, it is not possible to control its stability, strength, or availability. It also needs to take into consideration the modulation (if any) of the source signal, i.e., using ASK modulation in a source signal with amplitude variation can make the backscatter communication unreadable [24; 25]. The first work that introduced the concept of ambient backscatter used a TV tower signal broadcasting in the 536-542 MHZ range.