Design Of Vital Sign Monitoring System Based On FMCW Radar

With the rapid development of science and technology,radar is more and more used in the civil field.Compared with other radars,Frequency Modulated Continuous Wave（FMCW）radar has the advantages of high measuring precision,simple signal processing circuit,small measuring blind area,etc.Its main applications are automobile radar,smart home,medical monitoring,etc.The radar-based non-contact medical monitoring can solve the discomfort caused by wearing electrodes or sensor equipment and meet the monitoring needs of burn patients,patients with mental illness and patients with infectious diseases.Therefore,FMCW radar has great potential in the field of medical monitoring.In this thesis,a vital signs monitoring system based on FMCW radar is designed.The system is composed of transceiver link,transceiver antenna and signal acquisition and processing module.The system works at 5-6 GHz.The vital signs signal represented by the human chest displacement is detected and processed by MATLAB on the computer to obtain the vital signs information.The main tasks include:（1）A wideband high-isolation dual-circularly polarized antenna array is proposed.The antenna element adopts quasi-cross slots to realize coupling feeding,which improves the polarization purity of the antenna element and achieves wide working frequency band.By using the sequential rotation technique,the antenna array can realize the dual-circularly polarization.The phase cancellation technique and spatial decoupling structure are adopted to realize the high dual port isolation in wide band.In the frequency range of 5-6 GHz,S₁₁ and S₂₂ are less than-10 d B,the gain is more than 9 d B,the axial ratio is less than 3 d B,the port isolation is more than 30 d B,and the port isolation can reach 39.4 d B at the center frequency 5.5 GHz.The test results show the antenna array has good circular polarization performance in and high port isolation in wideband.（2）A broadband high isolation single balance mixer is proposed.The mixer adopts the microstrip line-slot line Balun combined with the diplexer.The Balun equivalent circuit is given by odd-even mode analysis method and the electric field distribution of the coupled slot line is analyzed.The local oscillator signal achieves differential output by Balun,thereby improving the isolation of the LO-RF and LO-IF.The diplex adopts a high-pass filter based on CSRR combined with an improved high-low impedance line filter.The diplexer is used to avoid the interference between the RF signal and IF signal.The frequency range of RF signal and LO signal is within 5-6 GHz,if the output IF signal is 100 MHz and 500 MHz,conversion loss is less than 11 d B,and the isolation between any two ports is greater than 35 d B,the return loss of RF port and LO port is greater than 10 d B in 5-6 GHz.（3）The other modules of the FMCW radar system are realized.The signal processing software based MATLAB is used to improve the simplicity of radar operation.The vital signs are monitored through the FMCW radar system,and the collected signals are sent to the upper computer through the serial port,and the upper computer processes the data through MATLAB.It contains the frequency information and phase information of the extraction of the difference frequency signal.The fuzzy problem of phase measurement is solved by the phase difference of DFT spectrum.The radar system determines the target position and the human chest fluctuation through the frequency and phase information.The result of signal processing is displayed in real time in the operating interface.Through the experiment of monitoring the vital signs of human,it is proved that the radar system can monitor the respiratory signal effectively.