# Capturing Features and Performing Human Detection from Human Gaits Using RFID
## Materials and Methods
### Preliminaries
#### 2.2.1 RFID Principle  
A typical RFID system consists of three parts: reader, antenna and tag. Moreover, in RFID technology, as a medium to perceive the target environment, the tag can be divided into three kinds: active, semi-active and passive. In a passive RFID system, tags can work without batteries. The reader sends electromagnetic waves of a certain frequency through the antenna. When the tag is within the working range of the transmitting antenna, it receives electromagnetic waves from the antenna to activate its own circuit and sends out backscattering signals. The antenna transmits the received backscattered signal from the tag to the reader, which demodulates and decodes the signal and then sends it to the server for further processing. The signal received by the reader, namely the backscattered signal \(S(t)\), can be expressed as:  
\[S(t)=a(t)e^{-j\theta(t)} \tag{1}\]  
\[\theta(t)=\left(\frac{2d}{\lambda}\times 2\pi+\mu\right)mod\ 2\pi \tag{2}\]  
where \(a(t)\) and \(\theta(t)\) are the amplitude and phase of the backscattered signal, \(j\) is an imaginary unit and \(d\) is the distance between the antenna and the tag. Since the tag will generate backscatter signal after receiving the signal from the antenna, the actual propagation distance should be \(2d\). \(\mu\) is the initial offset caused by the device, including the phase shift caused by the reader, tag and antenna. \(\lambda\) is the wavelength of the RF signal.  
When the user passes through the detection area, the signal will be propagated in three directions, as shown in Figure 1. This includes direct paths, obstacle reflection paths and dynamic reflection paths as the user moves. The backscattering signal \(S(t)\) is the superposition of dynamic and static reflection signals, the static reflection signal is the composite signal composed of the direct path and the obstacle reflection path, and the dynamic reflection signal is the reflection signal when the user moves. When the user moves between the antenna and the tag, assuming that there is a total of n reflection paths of the user, the received signal \(S(t)\) can be expressed as:  
\[S(t)=S_{s}(t)+S_{d}(t) \tag{3}\]  
\[S_{d}(t)=\sum_{n}a_{n}e^{-j(\frac{2\pi}{\lambda}\int d_{n}(t)dt+\mu)mod\ 2\pi} \tag{4}\]  
\[S_{s}(t)=a_{s}e^{-j\theta_{s}} \tag{5}\]  
where \(S_{s}(t)\) is the static reflected signal, \(S_{d}(t)\) is the dynamic reflected signal when the user moves, \(a_{n}\) is the amplitude of the user in the \(n\)\({}^{th}\) path, and \(d_{n}(t)\) is the propagation distance at time \(t\) in path \(n\).  
When the user moves the antenna-tag directly, the dynamic reflected signal will fluctuate accordingly. As shown in Figure 2, in the process of moving, \(S_{d}\) will rotate relative to \(S_{s}\), and the phase and amplitude of the corresponding synthesized signal \(S\) will also change accordingly. This means that the phase and amplitude information of the synthesized signal can be used for user detection. Specifically, on the one hand, we use phase and amplitude to obtain whether the user exists or not. On the other hand, we set multiple labels to enhance the user's detection ability and reduce the detection blind area.