# ANTSpin: Efficient Absolute Localization Method of RFID Tags via Spinning Antenna
## 3 RFID Signal Characteristic  
This section focuses on the basic principles of RFID and signal characteristics. A complete RFID system consists of a reader, an antenna, and RFID tags. The reader antenna emits a specific frequency of radio wave energy and forms a magnetic field in space. After the tag enters the magnetic field, it receives the RF signal from the reader antenna, sends out the information (passive tag) that is stored in the chip by the energy obtained, or actively transmits a signal of a certain frequency (active tag) by the tag. After the reader antenna reads the information that will be decoded, it will be sent to the central information system for data processing. In terms of communication and energy sensing between the RFID reader antenna and tags, it can be roughly divided into two types: inductive coupling and backscatter coupling. Generally, the low-frequency RFID mostly adopts the first type and the higher frequency mostly adopts the second one. \(\bullet\) RSSI  
RSSI is an important part of RFID data used to determine the quality of links. The values are all negative and the maximum value of 0 can only be obtained under ideal conditions (such as in an anechoic chamber). This means that the tag receives all the energy emitted by the reader antenna. Equation (1) shows the indoor path loss of RSSI [12; 21]. \[PL(d)(dB)=PL(d_{0})+10n\lg\left(\frac{d}{d_{0}}\right)+X_{o} \tag{1}\]where \(PL(d)\) is the path loss when the distance between transceivers is d and PL(\(d_{0}\)) is the reference path loss, which is obtained by actual test. \(n\) is the path loss factor, which is determined by the experimental environment. \(X_{\sigma}\) is a normal random variable with a standard deviation of \(\sigma\). \(\bullet\) Phase  
Phase is one of the basic properties of RFID signal and it describes the extent to which a received signal is offset from the transmitted signal. The phase measurement of the reader output is a periodic function and it can be expressed as [18; 23; 30]:  
\[\theta=\left(2\pi\frac{2d}{\lambda}+\theta_{T}\right)\text{mod}2\pi \tag{2}\]  
where \(d\) is the distance between the reader antenna and the tag. The distance that the signal is transmitted from the transmission to the accepted round-trip each time is \(2d\). \(\lambda\) is the wavelength and \(\theta_{T}\) is the phase shift that is introduced by the error of the system itself and the experimental environment. \(\bullet\) Doppler Frequency  
The antenna continuously emits waves of the same frequency. According to the Doppler effect [32], when the antenna and the tag are close to each other, the time interval between two consecutive peaks reaching the tag is continuously reduced, which results in the frequency of the wave received by the tag being higher than the frequency of the antenna. Conversely, the frequency of the wave received by the tag is lower than the frequency of the antenna when the antenna and the antenna are far apart from each other. The relationship between the frequency of the wave received by the tag and the frequency of the wave emitted by the antenna can be expressed as [33]:  
\[f_{R}=\left(\frac{c+v_{T}}{c+v_{A}}\right)\times f_{0} \tag{3}\]  
where \(f_{R}\) is the frequency of the wave received by the antenna, \(f_{0}\) is the frequency emitted by the antenna, and \(c\) is the speed of light. \(V_{T}\) is the moving speed of the tag relative to the antenna and \(V_{A}\) is the moving speed of the antenna relative to the tag. The Doppler frequency in RFID is the frequency difference that is received by the antenna. We assume that the tags to be located are all stationary, and then the frequency difference\(f_{D}\) received by the antenna can be expressed as:  
\[f_{D}=\frac{v_{A}}{c}\times f_{0} \tag{4}\]  
As the experimental scene is not ideal, external RF signals and other electromagnetic signals, affect the indoor environment and there is no electromagnetic wave absorbing material in the room. It is difficult for the RF signal to conform to the law of free propagation.