# Modulation Recognition System of Electromagnetic Interference Signal Based on SDR
## 5 Experimental Results and Analysis  
In order to test the performance of the modulation recognition system based on software radio, the modulation recognition system test platform is built in the laboratory, as shown in Figure 6, the distance between the AV1445 signal generator and the receiver. The HackRF One hardware is about 6 m, and the HackRF One hardware is connected to the PC host computer through the USB2.0 interface, and on the PC side, the Ubuntu 18.04 system is equipped with the GNU Radio software platform, the electromagnetic interference signal is sent by the signal source, and the identification system receives the signal for processing and modulation identification.  
The signal source generates six kinds of signals: BPSK, QPSK, 8 PSK, 8 QAM, 16 QAM, and 64 QAM, and runs the GRC flow chart to make the whole system is in a working state. The HackRF One hardware is used as the receiver to receive the signal, the received signal is down-converted and analog-to-digital converted into a baseband digital signal, and then the processed signal is sent to the PC. The signal pre-processing and classification are realized through the module in the GNU Radio software, and finally the recognition results are displayed.  
When the signal generator sends out different modulation signals, the recognition results of the system are displayed through the time-domain waveform and constellation of the signal. The time-domain waveform and constellation of different modulation signals are obtained through the test, as shown in Figure 7. In Figure 7, the recognition result is passed, is marked in the time domain waveform by det-mod, and the identified interference signal type is compared with the constellation mapped by the interference signal.  
When the recognition results are BPSK, QPSK, and 8 PSK, 2, 4, and 8 signal points are equally spaced on the unit circle in the corresponding constellation. When the recognition result is 8 QAM, 8 signal points in the corresponding constellation are distributed in a star shape. The constellation points of these four signals are obviously different, and the constellation map results are relatively clear. When the recognition results are 16 QAM and 64 QAM, 16 and 64 signal points in the corresponding constellation are square distributed, respectively.  
Due to the large number of constellation points of the 16 QAM signal and the 64 QAM signal and the fact that the adjacent constellation points are close, the two signals will be more seriously affected by environmental noise, and the constellation points are easy to overlap. The experimental results show that the recognition results of the six modulation types of interference signals are consistent with the constellation, so the electromagnetic interference detection and recognition system meets the design requirements.  
Figure 6: Structure diagram of system test platform.  
In order to further verify the recognition performance of the system, the signal generator is used to generate six kinds of modulation signals, respectively, test each type of modulation signal for 100 times, and count the recognition results of each modulation signal to obtain the accuracy of the test results of each modulation signal, as shown in Table 3.  
It can be seen from Table 3 that in the test of the actual environment, the recognition results of the system for six modulation signals can reach more than 90%, with high accuracy, which is basically consistent with the simulation results. Therefore, the modulation recognition system based on software radio me  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Modulation Type** & **Accuracy (\%)** \\ \hline BPSK & 100 \\ QPSK & 99 \\
8 PSK & 94 \\
8 QAM & 98 \\
16 QAM & 93 \\
64 QAM & 90 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Phase estimation parameters of each interference signal.  
Figure 7: Time domain waveform and constellation of each interference signal.