Self-Interference Signal Cancellation Method for Continuous-Wave Microwave Photonic Radar

Abstract

The method for eliminating self-interference signals of a continuous-wave microwave photon radar belongs to the field of microwave photon technology. The system used in this method includes a laser, a Mach-Zehnder modulator, an arbitrary waveform generator, an optical coupler, an electric power amplifier, a transmitting antenna, a receiving antenna, a first low-noise amplifier, a dual-drive Mach-Zehnder modulator, a first photodetector and a second photodetector, a low-pass filter, and a signal acquisition and processing module. By using the present invention, the disadvantages of traditional electronics methods in terms of bandwidth, weight, volume, electromagnetic interference, etc. are overcome, and the integration of a multifunctional microwave photon system is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of microwave photonics technology. More specifically, it is a microwave photon radar ranging system, which is a device that simultaneously realizes ranging and self-interference cancellation of radio frequency signals. Background Art

[0002] Radar technology plays an important role in measuring distance, speed, atmospheric parameters, and vibration. By emitting electromagnetic waves, it can detect information such as the distance, speed, azimuth, altitude, and shape of an object. Modern radar has developed rapidly, continuously breaking through and innovating in aspects such as integration, modularization, and integration, which also puts forward new requirements for modern radar technology, and continuously innovating and breaking through towards technical goals such as large bandwidth, multi-band, reconfigurability, and miniaturization.

[0003] The basis for realizing these characteristics is the high-speed manipulation of broadband microwave signals. However, limited by the "electronic bottleneck", the generation, control, and processing of broadband signals are extremely complex or even impossible to complete in traditional electronics. Microwave photonics, as an interdisciplinary subject of microwave technology and photon technology, can make full use of its advantages to control and process microwave signals in the optical domain, with advantages such as large bandwidth, low loss, and strong anti-electromagnetic interference ability, breaking through the traditional electronics bottleneck. The topics involved in microwave photonics generally mainly include five types: photonic generation of microwave and millimeter-wave signals, photonic processing of microwave and millimeter-wave signals, optically controlled phased array antennas, radio over fiber systems, and photonic analog-to-digital conversion. Compared with traditional electrical systems, microwave photonics has the advantages of large transmission bandwidth, wide operating frequency band, flat frequency response, immunity to electromagnetic interference, low transmission loss, small volume and weight, low power consumption, multi-dimensional multiplexing, and fast analog signal processing, making it widely used in many fields.

[0004] As one of the commonly used radars, the continuous-wave radar operates in a continuous transmit-receive mode, with equal average power and peak power. It has the technical characteristics of low intercept probability, high clutter rejection ability, and no velocity blind spot. Moreover, its architecture is lightweight, making it easier to miniaturize and integrate. It is commonly used in reconnaissance radars, intercept radars, and the detection of low, slow, and small targets. However, during the development of continuous-wave radars, some challenges have emerged. Since the system transmits and receives simultaneously, and the isolation between the transmitting antenna and the receiving antenna is not high enough, the transmitted signal may radiate into the receiving antenna, interfering with the received echo signal and even potentially causing the annihilation of weak echo signals, resulting in the self-interference problem of the radar. The self-interference problem not only leads to the failure of extracting target information but also reduces the sensitivity of the receiver, causes non-linear effects in the receiver, and significantly degrades the performance of the receiver. However, currently, few researchers have focused on the self-interference cancellation problem of microwave photon radars, and there are relatively few relevant research reports internationally. Since the self-interference signal and the echo signal are of the same frequency, it is difficult to filter out the self-interference signal using an electrical filter. Therefore, self-interference cancellation technology is crucial for continuous-wave microwave photon radars. Summary of the Invention

[0005] In view of this, the main objective of the present invention is to propose a dual-functional system that simultaneously realizes radar ranging and radio frequency self-interference cancellation technology based on microwave photon technology modulation. Applying microwave photon radars overcomes the disadvantages of traditional electronics in terms of bandwidth, volume, weight, and electromagnetic interference, and integrates the two functions of ranging and radio frequency self-interference cancellation technology, overcoming the self-interference problem of continuous-wave radars.

[0006] The specific technical solution of the present invention is as follows:

[0007] A method for simultaneously realizing radio frequency self-interference cancellation and ranging based on microwave photon technology. The specific implementation link of the method includes a laser, a Mach-Zehnder modulator MZM, a dual-drive Mach-Zehnder modulator DDMZM, an arbitrary waveform generator, an optical coupler OC, a first photodetector, an electric power amplifier, a transmitting antenna, a receiving antenna, a first low-noise amplifier, a second photodetector, a low-pass filter, and a signal acquisition and processing module. Among them, the DD-MZM includes an upper-arm sub-phase modulator PM and a lower-arm sub-PM. The method includes the following steps:

[0008] Step 1: Generate a detection signal with a large instantaneous bandwidth, specifically as follows:

[0009] The optical signal generated by the laser is injected into the Mach-Zehnder modulator MZM as an optical carrier. The radio frequency drive signal of the Mach-Zehnder modulator is an intermediate frequency linear frequency modulation signal generated by an arbitrary waveform generator. The DC bias voltage is tuned to make the MZM operate at the minimum bias point to suppress the carrier and even-order sidebands;

[0010] Next, use a 50:50 coupler OC to split the output signal of the MZM into two paths. One path is injected into the first photodetector for photoelectric conversion to obtain a second-harmonic chirp signal.

[0011] Finally, first inject this signal into a broadband power amplifier for amplification, and then use a transmitting antenna to transmit it into free space as a radar detection signal.

[0012] Step 2: Distance detection and self-interference signal suppression

[0013] Use the other path split from the output signal of the MZM by the 50:50 coupler OC as a reference signal and inject it into a dual-drive Mach-Zehnder modulator DD-MZM as its carrier wave. By tuning the bias voltage of the DD-MZM, both the upper and lower sub-PMs of the DD-MZM operate in the minimum bias point state. The DD-MZM has two drive signals. The first drive signal is the microwave signal received through the receiving antenna and the first low-noise amplifier, and the second drive signal is the modulated transmitted signal. After the output of the DD-MZM passes through the second photodetection and low-pass filter, it is collected by the signal acquisition and processing module to obtain a low-frequency signal related to the target distance after eliminating the self-interference signal. Next, solve this signal to obtain the distance information of the target.

[0014] In the transmitting end of the present invention, use a Mach-Zehnder modulator MZM to perform second-harmonic operation on the intermediate-frequency chirp signal generated by an arbitrary waveform generator to generate a chirp (LFM) signal with a large instantaneous bandwidth, and transmit it into free space as a detection signal. Use the transmitted signal after delay phase modulation and the received echo signal as the drive signals of the dual-drive Mach-Zehnder modulator DD-MZM. Finally, by adjusting the DC bias voltage of the dual-drive Mach-Zehnder modulator (DDMZM), invert the signals modulated by the upper and lower sub-PMs to cancel each other out, solve the low-frequency signal related to the target distance after eliminating the self-interference signal, and obtain the distance information of the target.

[0015] Advantages of the present invention:

[0016] The present invention proposes a ranging and cancellation dual-functional system based on microwave photonics technology, which combines the microwave photon cancellation method and the microwave photon radar detection method. Using a single radar detection signal, it can achieve the target distance while achieving RF self-interference cancellation. The detection link built by the method of the present invention has a smaller volume and lighter weight, which is more conducive to applications in military and civilian environments, and solves the self-interference problem existing in transceiving integrated radars, especially for short-distance high-precision transceiving integrated radars. Description of the drawings

[0017] Figure 1Schematic diagram of the link structure of a dual - function system based on microwave - photon technology modulation for radar ranging and radio - frequency self - interference cancellation technology.

[0018] Figure 2 (a) Figure 1 Spectrum schematic diagram at point a in

[0019] Figure 2 (b) Figure 1 Spectrum schematic diagram at point b in

[0020] Figure 2 (c) Figure 1 Spectrum schematic diagram at point c in Detailed implementation manner

[0021] In order to solve the problem of self - interference existing in the existing transceiver - integrated radar, especially for the short - distance and high - precision transceiver - integrated radar, which affects the detection effect, and the detection link built based on the existing method has a larger volume and weight, which is not conducive to applications in military and civilian environments. The present invention proposes a dual - function system based on microwave - photon technology modulation for radar ranging and radio - frequency self - interference cancellation technology, which uses the radar detection signal to detect the distance of the target simultaneously, and finally realizes radar ranging and radio - frequency self - interference cancellation.

[0022] The detection method of the dual - function system based on microwave - photon technology modulation for radar ranging and radio - frequency self - interference cancellation technology of the present invention is as follows:

[0023] The optical signal generated by the narrow - line - width laser enters the Mach - Zehnder modulator MZM and serves as its optical carrier. The drive signal of the MZM is generated by an arbitrary waveform generator. By tuning the DC bias voltage, the MZM works in the minimum working state, thereby realizing the suppression of the carrier and the positive and negative first - order sidebands. The output signal of the MZM is injected into a 50:50 coupler and divided into two paths. One path enters the first photodetector to obtain a doubled - frequency linear frequency - modulated signal through beat - frequency. After the doubled - frequency linear frequency - modulated signal is amplified by a broadband electrical amplifier, it is transmitted into free space through a transmitting antenna as the radar detection signal. The other path is used as a reference signal and injected into the dual - drive Mach - Zehnder modulator DD - MZM as its carrier; the two drive signals of the DD - MZM are respectively the microwave signal received by the receiving antenna and the transmitted signal after modulation. After the output of the DD - MZM passes through photoelectric detection and low - pass filtering, it is collected by the signal acquisition and processing module, and a low - frequency signal related to the direction and distance of the non - cooperative target can be obtained; next, by solving this signal, the distance of the target can be obtained simultaneously.

[0024] For the convenience of public understanding, the present invention is further described below with reference to the drawings and mathematical derivations:

[0025] Figure 1Schematic diagram of the link structure of the non-cooperative target spatial position detection method based on microwave photon technology of the present invention, including a laser, a Mach-Zehnder modulator MZM, a dual-drive Mach-Zehnder modulator DDMZM, an arbitrary waveform generator, an optical coupler OC, a first photodetector, an electric power amplifier, a transmitting antenna, a receiving antenna, a first low-noise amplifier, a second photodetector, a low-pass filter, and a signal acquisition and processing module.

[0026] Using Figure 1 the link structure shown, the process of the non-cooperative target spatial position detection method based on microwave photon technology is completed as follows:

[0027] Step 1: Generate a detection signal with a large instantaneous bandwidth;

[0028] The optical signal generated by the continuous wave laser is expressed as E(t) = E 0 exp(jω m t), where E 0 and ω m represent the amplitude and the central angular frequency of this optical signal respectively, and j in the formula represents the imaginary unit; inject the optical signal into the Mach-Zehnder modulator MZM, this optical signal serves as the modulated optical carrier, and the intermediate frequency chirp signal generated by the arbitrary waveform generator serves as the radio frequency drive signal of the Mach-Zehnder modulator, which can be expressed as:

[0029]

[0030] In the formula, V LFM , f 0 , T, and k represent the amplitude, initial frequency, duration, and chirp slope of the intermediate frequency chirp signal respectively; and among them is the rectangular function, representing the envelope information of the signal here. Adjust the DC bias voltage to make the Mach-Zehnder modulator MZM work at the minimum bias point, so as to suppress the carrier and even sidebands and obtain odd sidebands. Therefore, the output signal of MZM can be expressed as

[0031]

[0032] where J n (m 0 ) is the Bessel function of the first kind. Here, the value of n represents the order of the sideband. For example: J 0 (m 0 ) represents the carrier amplitude, J 1 (m 0 ) represents the positive first-order sideband amplitude, J -1 (m 0 ) represents the negative first-order sideband amplitude. Here m 0 is the modulation coefficient of MZM, that is, m0 = πV LFM / V π1 , where V π1 is the half-wave voltage of the Mach-Zehnder modulator MZM.

[0033] Next, use a 50:50 coupler to split the obtained output signal into two paths, inject one path into the first photodetector for optoelectronic conversion and beat frequency, and a second-harmonic chirp signal can be obtained, which can be expressed as

[0034]

[0035] The instantaneous frequency of the chirp signal after frequency doubling is f LFM (t) = 2(f 0 + kt). Finally, first inject this signal into a power amplifier for amplification, and use a transmitting antenna to transmit it into free space as a radar detection signal.

[0036] Step 2: Distance detection and self-interference signal suppression

[0037] Inject the other optical signal output by the OC into the dual-drive Mach-Zehnder modulator DD-MZM at the receiving end as a reference signal; after this signal enters the DD-MZM, it is equally divided in power and enters the upper and lower sub-PMs.

[0038] At the receiving end, the echo signal and interference signal received through the receiving antenna, the echo signal S(t) and the self-interference signal I(t), can be respectively expressed as

[0039] S(t) = S 1 cos(2π(2f 0 + 2k·t + 2k·Δτ)·t (4)

[0040] I(t) = I 1 cos(2π(2f 0 + 2k·t + 2k·t 1 )·t) (5)

[0041] In the formula, S 1 and I 1 respectively represent the amplitudes of the echo signal and the interference signal, Δτ and t 1 respectively represent the delays experienced by the echo signal and the self-interference signal. Here, the echo signal S(t) and the self-interference signal I(t) are input into the DD-MZM as a path of modulation signal for modulation. By splitting a path of the modulated second-harmonic signal, a reference signal R(t) can be obtained after delay phase modulation, and this signal is input into the other PM as a modulation signal. Here, the reference signal R(t) can be expressed as

[0042] R(t) = R1 cos(2π(2f 0 +2k·t+2k·t 2 )·t) (6)

[0043] Where R 1 represents the amplitude of the reference signal, and t 2 represents the delay experienced by the reference signal. Adjust the DC bias voltage of the dual-drive Mach-Zehnder modulator to the half-wave voltage, then the output optical signal of the upper sub-PM can be expressed as

[0044]

[0045] Here, m 1 is the modulation coefficient of the upper sub-PM for the echo signal S(t), that is, m 1 =πS 1 / V π2 , and here, m 2 is the modulation coefficient of the upper sub-PM for the self-interference signal I(t), that is, m 2 =πI 1 / V π2 , where V π2 is the half-wave voltage of the upper sub-PM of this modulator, and f 0 is the center frequency of the intermediate-frequency signal.

[0046] Since the DC bias voltage of the dual-drive Mach-Zehnder modulator is the half-wave voltage at this time, then the output optical signal of the lower sub-PM can be expressed as

[0047]

[0048] Here, m 3 is the modulation coefficient of the lower sub-PM for the reference signal R(t), that is, m 3 =πR 1 / V π3 , where V π3 is the half-wave voltage of the lower sub-PM of this modulator.

[0049] Therefore, the output of the dual-drive Mach-Zehnder modulator can be obtained and expressed as

[0050]

[0051] Then, when adjusting the amplitude and delay of the reference signal to satisfy the condition

[0052]

[0053] Then, the output of the dual-drive Mach-Zehnder modulator can be obtained as

[0054]

[0055] Substitute

[0056]

[0057] into Equation (11), then we can obtain

[0058]

[0059] The frequency obtained after the signal selected by the filter is beat by the optical detector PD2 is

[0060] Δf = 2k·Δτ (14)

[0061] Therefore, after optical mixing and de-chirping, a low-frequency microwave signal can be obtained, which not only reduces the high-speed requirements for signal processing such as the radar backend receiver, sampling, and analog-to-digital conversion, but also further calculates the delay Δτ of the echo signal according to the known chirp rate of the linear frequency modulation signal. Then, the distance of the target to be measured from the radar antenna can be calculated by the following formula

[0062]

[0063] where c represents the speed of light; B = kt is the bandwidth of the signal.

[0064] The present invention uses a continuous-wave radar and can achieve detection and imaging through a mature radar imaging algorithm.

[0065] In summary, the present invention proposes a ranging and cancellation dual-functional system based on microwave photon technology, which combines the microwave photon cancellation method and the microwave photon radar detection method, and uses a single radar detection signal to achieve both target distance measurement and radio frequency self-interference cancellation. The self-interference problem existing in the transceiver integrated radar, especially in the short-distance high-precision transceiver integrated radar, is solved.

Claims

1. A self-interference signal cancellation system for a continuous-wave microwave photon radar, characterized in that, the system includes: a laser (1) for providing continuous detection light for a Mach-Zehnder modulator A (2); the Mach-Zehnder modulator A (2) for modulating the input light from the laser (1) with a loaded chirp signal and outputting an odd-order modulated optical signal; a first photodetector (3) and a 50:50 coupler, the 50:50 coupler splitting the output signal of the Mach-Zehnder modulator A (2) into two paths, one of which is injected into the first photodetector (3) for photoelectric conversion to obtain a second-harmonic signal; a transmitting antenna (4) for transmitting the second-harmonic signal; a receiving antenna (8) for receiving an echo signal and a self-interference signal; a dual-drive Mach-Zehnder modulator DD-MZM (5), the other optical signal output by the coupler being injected into the DD-MZM as a first reference signal; after the first reference signal enters the DD-MZM, it is equally divided in power and enters the upper and lower sub-phase modulators; the echo signal and the self-interference signal are input into the DD-MZM as a modulation signal of one path of the DD-MZM for modulation, a second reference signal R(t) is obtained by delaying and phase-modulating one path of the second-harmonic signal, the second reference signal R(t) is input into the other sub-phase modulator of the DD-MZM as a modulation signal, and the self-interference signal is cancelled by adjusting the amplitude and delay of the second reference signal; a filter (6) connected to the output end of the dual-drive Mach-Zehnder modulator (5) for selecting a desired frequency signal; a second photodetector (7) for converting the optical signal obtained by the filter (6) into an electrical signal to obtain the time information of the echo.

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