The Research On Doppler Frequency Shift And Angle Of Arrival Measurement System Based On Microwave Photonics

Doppler frequency shift(DFS)is the difference between the frequency of the transmitted signal emitted by the radar and the frequency of the echo signal received by the radar.DFS is produced by the relative motion of the measured object and the radar,and it carries parameters such as the speed and movement direction of the measured object,DFS is one of the important parameters in the radar measurement system.With the advent and development of the information age,how to obtain information quickly and accurately has become a top priority,and DFS measurement has become a research hotspot in recent years.By measuring the Doppler frequency shift,the speed and direction of the moving object can be quickly obtained.Therefore,the Doppler frequency shift measurement has a very important application in many fields such as medicine,intelligent driving,electronic warfare and communication systems.For example,the blood flow rate can be obtained by measuring Doppler frequency shift,and DFS measurement can be used for early warning and reconnaissance in electronic warfare.In addition,the methods of determining the orientation of the object to be measured by determining the direction of the echo signal to determine the orientation has also become the key.By measuring the angle of arrival(AOA)of the echo signal,the direction of the object to be measured can be accurately identified.However,the traditional electronic measurement systems are unable to meet the demand due to the electronic bottlenecks constraints of small measurement bandwidth,weak antielectromagnetic interference,large equipment volume,and high cost.Subsequently,microwave photonics which is an interdisciplinary subject has emerged and developed.Compared with traditional electrical systems,it has the advantages of wide frequency band,strong anti-electromagnetic interference,small size and low cost.Microwave photonics opens up new avenues for Doppler frequency shift and angle of arrival measurement.This paper firstly introduces the significance of measuring Doppler frequency shift and angle of arrival,the research background and current situation.And then the proposed measurement schemes are classified and compared.In addition,the modulatiors and photodetectors applied in this paper are introduced and analyzed in detail,which lays a good theoretical foundation for the next proposed measurement system.In this paper,two Doppler frequency shift measurement schemes based on microwave photonics and a Doppler frequency shift and angle of arrival measurement scheme based on microwave photonics are proposed.Firstly,a DFS measurement scheme based on cascaded phase modulators is proposed.In order to measure the direction of DFS conveniently,we set the frequency of the synthetic signal to be the sum of the frequency of the transmitted signal and the frequency of the reference signal.The synthesized signal is input into the first phase modulator for modulation,and the output optical signal is input into the second phase modulator as an optical carrier to be modulated by the echo signal.The output optical signal is input into the low-frequency photodetector for beating frequency and passed through a low-pass filter.The DFS is obtained by subtracting the reference frequency from the resulting peak frequency,where the symbol represents the direction.Then the principle analysis and formula derivation of the scheme are carried out,and the feasibility of the system is verified by Optisystem simulation.The system has the advantages of low cost,simple structure and easy integration.Due to the use of cascaded phase modulation measurement,the optical signal output by the second phase modulator has four sets of positive and negative firstorder sidebands,which need to be filtered and observed with a low-pass filter.A measurement scheme using dual parallel Mach-Zehnder modulators(DPMZM)was then proposed.The synthesized signal and the echo signal are simultaneously input into the two radio frequency input terminals of the DPMZM for modulation,and the optical signal output by the DPMZM is input into the photodetector for beating frequency.The DFS is obtained by subtracting the reference signal frequency from the observed peak frequency by a spectrum analyzer,where the symbol represents the direction.Formula derivation and experimental verification of the scheme are carried out,and the real-time measurement of DFS is realized when the frequency of the transmitted signal is between 1GHz and 25 GHz,and the measurement error is within±0.8Hz.After error analysis,it was found that time,frequency of the transmitted signal and frequency of the reference signal do not affect the measurement error.The system has low cost,simple structure,high measurement accuracy,easy integration and good stability.Finally,a microwave photonics-based DFS and AOA measurement system is proposed,which uses a Mach-Zehnder modulator(MZM)and a dual-driven MachZehnder modulator(DDMZM).Since the echo signal has an angle of arrival,the echo signals received by two parallel radars have a frequency difference,and the angle of arrival can be calculated by measuring the frequency difference of the two echo signals.The two echo signals are input into the two radio frequency input terminals of the DDMZM for modulation,and the synthesized signal is input into the MZM for modulation.After the optical signals output by the two modulators being combined,they are beat frequency by the photodetector.After inputting the obtained electrical signal into a spectrum analyzer for observation,the DFS can be obtained by subtracting the reference frequency from the observed peak frequency,and the AOA can be obtained by calculating the peak power.Then we carried out the formula calculation and simulation verification of the system,realized the simultaneous measurement of DFS and AOA,and drew the normalized curve of peak power and AOA.The system can measure DFS and AOA at the same time,and has the advantages of large measurement bandwidth and easy integration.