# Modulation Recognition System of Electromagnetic Interference Signal Based on SDR
## 4 Signal Modulation Recognition System Based on SDR  
HackRF One is used as the hardware platform for receiving electromagnetic interference modulation signals, and the frequency and symbol rate of the signals are estimated by the GNU Radio software to complete the clock synchronization and matching filtering; real-time performance needs to be taken into account in the actual engineering applications, so the modulation recognition algorithm based on the higher-order cumulative quantity is used to achieve modulation recognition and complete the phase correction of the signals, and finally the signal constellation diagram is combined with the system. The recognition result of the system is discriminated.  
The HackRF One hardware structure is shown in Figure 4. The received signal passes through the antenna into the radio frequency circuit, through the programming settings, which determine whether the signal is amplified by a 14 dB amplifier, through the RFFC5072 chip to complete the signal mixing process, in which it will be changed into an intermediate frequency signal, and then through the MAX2837 chip, where it will be changed into an intermediate frequency signal mixing baseband IQ signals, then through the MAX5864 chip to the IQ signal by the sampling, where it will become a baseband digital signal (MAX5864 is responsible for the signal for ADC/DAC sampling process, sampling frequency of 22 MHz, sampling accuracy of 8 bit); and then by the CPLD controller, which is responsible for the baseband signal. Then, the CPLD controller, responsible for the baseband signal timing control, behind the MAX5864, plays a role in buffering the data and controls the role of the transmission channel switch. Finally, the signal reaches the LPC4320/4330 processor, where the signal will be converted to data and through the USB interface into the PC and the GNU Radio software for communication.  
Figure 3: Simulation results of characteristic parameter \(f_{4}\) after order reduction processing.  
The received signal enters the RF circuit through the antenna, and it is set whether to amplify the signal through programming. Then, the signal is down converted twice through the rfc5072 chip and max2837 chip to convert it into an analog baseband signal. Based on the GNU radio platform, the GRC flow diagram of electromagnetic interference signal modulation identification is designed and built, as shown in Figure 5. The functions of each module are as follows.  
Osmocom source: Receiving module. Connects the hardware of HackRF One with GNU radio software platform, and sends the electromagnetic interference modulation signal received by HackRF One to the software platform for processing.  
Frequency correction: Frequency correction module. Estimates the frequency and symbol rate of the received signal and completes the frequency correction of the signal. At the same time, the number of samples for each symbol is obtained according to the estimated symbol rate, and it is passed to the next module as a stream label.  
Figure 4: HackRF One hardware structure diagram.  
Figure 5: GRC flow chart of interference recognition system.  
Polyphase clock sync: Clock synchronization module. Modifies the clock synchronization module provided by GNU Radio. The number of samples per symbol marked in the stream label of the previous module is used.  
Modulation classifier: Classification and identification module. Carries out feature extraction on the signal, and completes classification and recognition.  
QT GUI time sink: Visualization module. Displays the time domain waveform of the signal.  
QT GUI constellation sink: Visualization module. Displays signal constellation.