## 5 Experiment Results and Discussion  
In the experiment the ESP8266 modules are emulated as communication modules in the DSRC technology and are used to simulate the signal transmission that conforms to the IEEE 802.11p protocol standard. By configuring the corresponding parameters, eight ESP8266 modules of the same model are set to client mode or AP mode to simulate Vehicle-to-Vehicle, Vehicle-to-Infrastructure, Vehicle-to-Network, and other various modes. Afterwards, the transmitted wireless signals are subsequently received by USRP. The USRP model used in the experiment is USRP N210 produced by Ettus Research company. The USRP is connected to the computer through a Gigabit Ethernet port for the acquisition of IEEE 802.11p signals, and the computer is equipped with the UHD software driver for USRP, the open-source software defined platform GNU Radio, the compilation environment, etc. Then, the received wireless signals are preprocessed and the extraction and identification of RFF features are performed by MATLAB software. The experiment was carried out in a laboratory of 10 m \(\times\) 9 m with a corridor outside it, and there is a wall between the laboratory and the corridor. The schematic diagram of the laboratory is shown in Figure 3. A number of tables, chairs and partitions are placed in the laboratory as obstacles, and the experiment modules can be moved inside and outside the laboratory, so that the wireless signal may be blocked from the process of transmitting to receiving by obstacles or walls, which can simulate the LOS and NLOS situations, as well as moving multipath scenarios. During the experiment, the USRP is placed in a fixed position on the table shown in the figure, the ESP8266 module set to AP or client mode is placed near it, and another module that communicates with the module in the fixed position is placed on the mobile trolley. The experiment allows one or more modules to simultaneously communicate with the module in the fixed position on the table, and the USRP can collect the signals of several modules  
Figure 3: Schematic diagram of laboratory. at the same time. In this experiment, the signals of the ESP8266 module are collected in three scenes of static state, indoor LOS moving state and partition NLOS moving state to test the RFF features extraction and recognition performance. The environment of the three experimental scenes and the placement or moving route of the communication module are described in detail as follows:  
1. Static state: The modules are placed at the four fixed static positions p1 to p4 marked in Figure 3. The distances between these four positions and the USRP are different, hence the received signal strength and the signal-to-noise ratios in each position are also different. At some points, there are obstacles in the path between the USRP and the transmitting modules with rich channel multipath effect, which is closer to the real communication environment. The reference signals for classifier training can be obtained in this state and can be also used for comparison with the moving state signals. 2. Indoor LOS moving state: The to-be-identified communication modules are placed on the mobile trolley and the experimenter pushes the mobile trolley to walk arbitrarily in the laboratory shown in Figure 3. The average moving speed is 1 m/s, and the moving route and moving direction are random and irregular. As a result, the uncertainty of movement makes the wireless channel time-varying when the USRP receives the signal. In addition, another experimenter is allowed to walk around the laboratory freely during the movement process to simulate the disturbances by external movement. Since there is no wall obstruction in the room, it belongs to the moving multipath scene under the LOS situation. 3.