Method and device for range resolution of laser coherent radar based on composite modulation coding of linear frequency modulation and barker code

Abstract

A laser coherent radar ranging and deblurring method based on linear frequency modulation and barker code composite modulation coding. The device comprises a linear frequency modulation signal and barker code composite modulation module, a laser light source, a laser emission system, a photoelectric detection module and a coherent mixing processing module. The invention utilizes the complementary advantages of linear frequency modulation signal and barker code signal modulation transmission, and the local oscillator signal and the echo delay signal are processed twice for pulse compression, and through the design of signal parameters, the invention can be used for long-distance laser coherent radar ranging, has strong range resolution, and has stronger Doppler shift tolerance.

Description

Technical Field

[0001] This invention relates to the field of lidar ranging technology, and in particular to a method and device for unambiguous laser coherent radar ranging based on linear frequency modulation and Barker code composite modulation coding. By utilizing the complementary advantages of the two modulation signals, long-range, high-resolution laser coherent ranging is achieved through composite modulation signals. Background Technology

[0002] Laser ranging, as a photoelectric detection technique, is a valuable supplement to traditional microwave radar ranging methods. Generally, laser ranging sources use single-photon emission, allowing for ranging distances of up to several thousand kilometers. However, it is highly susceptible to background noise such as sunlight. Furthermore, since its ranging power depends on the number of returning single photons, achieving long-range photoelectric detection requires significant laser reflection power, resulting in high demands on the size and power consumption of the light source terminal, making it unsuitable for mobile deployment environments. Coherent laser ranging offers the advantage of being unaffected by background noise such as visible light, but it requires good wavefront coherence of the target echo for coherent demodulation of the pre-local signal. However, in long-range detection scenarios, the target echo coherence deteriorates significantly, making ranging impossible. Consequently, coherent laser ranging is primarily used in close-range or indoor scenarios such as autonomous driving and robotic patrols. For long-distance photoelectric detection applications, in order to achieve long-distance laser ranging by taking advantage of the fact that coherent lidar ranging is insensitive to sunlight noise, in addition to matching the appropriate laser emission power, it is also necessary to meet the coherence of the echo and improve the signal-to-noise ratio of the echo signal. This goal can be achieved by designing coding modulation methods.

[0003] Typically, laser coherent ranging uses a single-signal modulation and coding method for its transmitted signal, such as linear frequency modulation (LFM) or binary phase coding. LFM and Barker code binary phase coding are two commonly used high-resolution pulse compression signals. LFM obtains a large signal bandwidth by continuously and linearly modulating the pulse through a pulse-continuous device according to a certain pattern. Barker code binary phase coding achieves high range resolution by randomly coding the inter-pulse phase and performing correlation processing. LFM, as a typical pulse compression signal, has high tolerance to Doppler shift in the echo signal and easily achieves high range resolution, but it suffers from the disadvantages of coupling between range and Doppler shift and high sidelobes in the matched filter output. Phase coding, on the other hand, uses code agility technology, has strong anti-interference capabilities, and is more sensitive to Doppler shift, but it has high sidelobes after pulse compression, requiring compensation when the Doppler frequency varies over a large range. Summary of the Invention

[0004] To address the limitations of single-signal modulation modes in laser ranging and to overcome the limitations of coherent laser ranging, enabling long-range laser photoelectric detection, this invention proposes a laser coherent radar ranging device and deambiguity method based on linear frequency modulation and Barker code composite modulation coding. The transmitted ranging signal is modulated using composite coding, and the target echo is subjected to secondary pulse compression with the locally coded signal to calculate the distance. Composite coding improves the signal-to-noise ratio of the calculated signal, and when combined with a photodetector approaching the quantum limit, the ranging distance can be significantly increased, enhancing the power of laser ranging and enabling long-range photoelectric detection.

[0005] The technical solution of the present invention is as follows:

[0006] On the one hand, the present invention provides a laser coherent radar ranging and deambiguity method based on linear frequency modulation and Barker code composite modulation coding, characterized in that the method includes the following steps:

[0007] Step 1: Divide the laser beam into two parts, with the majority serving as the optical carrier and the minority serving as the local oscillator signal;

[0008] Step 2: Modulate the composite modulation signal of the linear frequency modulation signal and the Barker code onto the optical carrier to generate a local linear frequency modulation complex conjugate signal;

[0009] Step 3: Use the composite modulation signal as a ranging laser signal and emit it toward the object to be measured;

[0010] Step 4: Shift the local oscillator signal to a frequency band consistent with the echo signal frequency band, and mix it with the echo signal to obtain delay information containing the echo signal;

[0011] Step 5: Compare the phase of the mixing signal with the complex conjugate signal of the local linear frequency modulation to extract the phase delay information of the pulse;

[0012] Step 6: Perform a first pulse compression process on the linear frequency modulation portion within the mixed signal pulse to obtain a narrow pulse signal and a pulse compression coefficient.

[0013] Step 7: After performing a second pulse compression process of inter-pulse phase modulation on the signal processed in Step 6, add it to the pulse phase delay information obtained in Step 5 to obtain the unambiguous distance information of the target under test. The complex conjugate signal of the local linear frequency modulation is formed by linear frequency modulation within the pulse and Barker code binary phase encoding between pulses.

[0014] The first pulse compression process specifically involves multiplying the echo signal with the complex conjugate signal of the local linear frequency modulation (LFM) signal and then performing matched filtering. By analyzing a symbol, the LFM pulse compression coefficient and the narrow pulse signal are obtained, thus completing the first pulse compression process of the echo signal.

[0015] The second pulse compression process specifically involves using a local barker code to perform a delay search, generating a binary coded pulse compression coefficient and delay information.

[0016] On the other hand, the present invention also provides a laser coherent radar ranging device based on linear frequency modulation and Barker code composite modulation coding, characterized in that it includes...

[0017] The fiber optic beam splitter module is used to split the laser beam into two parts, with the majority serving as the optical carrier and the minority serving as the local oscillator signal.

[0018] The composite modulation module is used to modulate the composite modulation signal of linear frequency modulation signal and Barker code onto the optical carrier to generate a local linear frequency modulation complex conjugate signal;

[0019] The transmitting module is used to transmit the local linear frequency modulated complex conjugate signal generated by the composite modulation module as a ranging laser signal to the object under test;

[0020] The receiving module is used to receive the echo signal reflected by the object under test;

[0021] The photoelectric frequency shifting and detection module is used to shift the local oscillator signal to a frequency band consistent with the echo signal frequency band, and mix it with the echo signal to obtain delay information containing the echo signal;

[0022] The phase detection module is used to perform phase detection processing on the complex conjugate signal of the local linear frequency modulation and the local oscillator signal;

[0023] The signal acquisition and processing module is used to perform secondary pulse compression processing on the linear frequency modulation part within the mixed signal pulse, and then add it with the pulse phase delay information to obtain the unambiguous distance information of the target under test.

[0024] Furthermore, it also includes a ranging signal output module, which is used to output the unambiguous distance information of the target under test.

[0025] A linear frequency modulated (LFM) pulse signal is a signal whose frequency is continuously and linearly modulated within the pulse duration. The complex form of a LFM pulse signal is:

[0026]

[0027] Its complex envelope is:

[0028]

[0029] Where μ = B / T L B represents the slope of a linear frequency modulated pulse (LFM) signal, and B is the bandwidth.

[0030] The complex form of the Barker code is:

[0031]

[0032] Where f0 is the carrier frequency of the phase-coded signal. Let be the phase modulation function. The complex envelope is rectangular, represented as:

[0033]

[0034] Among them, c k T is the binary sequence of the Barker code. B P is the subpulse width of the Barker code, P is the code length of the Barker code, and PT is the subpulse width of the Barker code. B The duration of the Barker code signal.

[0035] The complex envelope of this invention is formed by a combination of linear frequency modulation and 13-bit Barker code modulation, and its complex envelope form is expressed as follows:

[0036] u(t)=u LFM (t)*u PCM (t)

[0037] Right now:

[0038]

[0039] When T L <T B At that time, the composite signal is obtained by converting each sub-pulse of the Barker code-encoded signal into a linear frequency modulated signal and multiplying it by the corresponding coding coefficients, resulting in:

[0040]

[0041] in,

[0042] The delayed echo signal reflected from the target is represented as:

[0043]

[0044] Where τ=2R / c is the time delay of the echo, fd=2v / λ is the Doppler frequency shift caused by the target motion, Ai is the gain after reflection from the target (proportional to the target RCS), N is the number of scattering centers of the target, R is the distance between the target and the radar, v is the target velocity, and λ is the wavelength.

[0045] Since the composite signal is a combination of two single modulation methods, the echo signal undergoes two pulse compression processes. First, the linear frequency modulation (LFM) portion within the pulse is compressed by multiplying the echo signal with the complex conjugate signal of the local LFM signal and then performing matched filtering. The LFM pulse compression coefficient is obtained by analyzing one symbol and used to process the composite signal. Then, the processed result is used to generate a binary phase-coded pulse compression coefficient, completing the second pulse compression process for the composite signal. Specifically, a time delay search is performed using local Barker codes to obtain the final pulse compression result for the composite signal. Because the intensity of the echo signal gradually decreases with increasing distance, the signal becomes weaker with greater distance, and the signal-to-noise ratio (SNR) gradually decreases to 6 dB or even lower relative to the detector shot noise. Under these conditions, the FPGA hardware algorithm cannot effectively identify the target and the detection noise. However, the composite signal, due to Barker code modulation, can improve the SNR gain. Even with increased detection distance, the SNR of the detected echo signal can still be recognized by the FPGA hardware. This technical advantage effectively increases the ranging distance and enhances the ranging capability of photoelectric detection. When deambiguously processing the echo signal, the transformation cycle of the local Barker code is used to match the echo delay signal. The fractional delay estimated before matching is combined with the integer delay estimated by the Barker code traversal to obtain the actual delay of the real target, thereby effectively deambiguously processing the target distance beyond the theoretical unambiguous distance of the system.

[0046] The technical effects of this invention are as follows:

[0047] This invention overcomes the background noise from sunlight and other sources encountered in traditional single-photon laser ranging, enabling all-weather operation. It employs a composite modulation signal design using linear frequency modulation (LFM) and Barker code binary phase-coded modulation (BPM). These two modulated signals complement each other, are easy to implement, exhibit strong anti-interference capabilities, and high Doppler tolerance. Furthermore, when combined with a quantum-limited photodetector, it effectively improves the signal-to-noise ratio of the coherent received mixed signal, significantly increasing the detection range and enhancing photoelectric detection power. Simultaneously, within the increased detection range, a Barker code ergonomic cyclic matching algorithm is used to de-blur the signal, calculating the true target distance.

[0048] Composite coding improves the signal-to-noise ratio of the decoded signal. When combined with a photodetector that approaches the quantum limit, it can significantly increase the ranging distance, enhance the power of laser ranging, and realize long-distance photoelectric detection.

[0049] By using secondary pulse compression processing, the radar's range resolution accuracy and range discrimination capability for targets are improved. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of Embodiment 1 of the laser coherent radar ranging device based on linear frequency modulation and Barker code composite modulation coding of the present invention.

[0051] Figure 2.1 Simulation diagram of composite modulation signal transmission signal

[0052] Figure 2.2 Partial simulation diagram of composite modulated signal transmission signal

[0053] Figure 3.1 Simulation diagram of the target reflected echo signal

[0054] Figure 3.2 Partial simulation image of the target reflected echo signal

[0055] Figure 4.1 The result of the first pulse compression processing of the target echo signal

[0056] Figure 4.2 Results of the second pulse compression processing of the target echo signal

[0057] Figure 5 This is a schematic diagram of distance defuzzification. Detailed Implementation

[0058] The technical solution of the present invention will be further described in conjunction with the accompanying drawings and embodiments, but this should not be construed as limiting the scope of protection of the present invention.

[0059] Please refer to Figure 1 , Figure 1 This is a schematic diagram of Embodiment 1 of the laser coherent radar ranging device based on linear frequency modulation and Barker code composite modulation coding of the present invention. As shown in the figure, the laser coherent radar ranging device based on linear frequency modulation and Barker code composite modulation coding of the present invention consists of a narrow linewidth stable frequency seed source 1, an optical fiber beam splitter 2, a composite modulation module 3, an optical fiber amplifier 4, an optical fiber collimator 5, a transceiver optical head 6, a relay beam expander optical path 7, an optical antenna 8, a heterodyne receiving optical path 9, a balanced receiver 10, an AD conversion module 11, a signal acquisition and processing module 12, a ranging signal output module 13, a broadband frequency shifter 14, and a phase detector 15. The connection relationship of the above components is as follows:

[0060] The output of the narrow-linewidth stable seed light source 1 is connected to the fiber optic beam splitter 2, which splits the input laser into a 1:9 ratio. 90% of this split is fed into the composite modulation module 3. The output of the composite modulation module 3 is connected to the input of the fiber optic amplifier 4. The output of the fiber optic amplifier 4 is split into a ranging laser signal and a locally linearly frequency-modulated complex conjugate signal. (See...) Figure 2.1 Simulation diagram of composite modulation signal transmission. Figure 2.2 (Partial enlarged view of the simulation of the composite modulated signal transmission)

[0061] The ranging laser signal is sequentially transmitted to the target via the fiber collimator 5, transceiver optical head 6, relay beam expander optical path 7, and optical antenna 8. The echo signal reflected by the target (see...) Figure 3.1 Simulation diagram of the echo signal reflected from the target. Figure 3.2 The simulated enlarged view of the echo signal reflected from the target is sequentially input into the heterodyne receiving optical path 9 via the optical antenna 8, the relay beam expander optical path 7, and the transceiver optical head 6; the complex conjugate signal of the local linear frequency modulation is input into the signal acquisition and processing module 12 via the phase detector 15;

[0062] The 10% energy is used as a local oscillator signal and input into the heterodyne receiving optical path 9 via the broadband frequency shifter 14. The local oscillator signal and the echo signal are mixed in the heterodyne receiving optical path 9 to form a mixed signal output, which is then detected by the balanced receiver 10. The detection signal output by the balanced receiver 10 is input into the signal acquisition and processing module 12 via the AD conversion module 11.

[0063] In the signal acquisition and processing module 12, the mixed signal and the complex conjugate signal of the local linear frequency modulation are processed and then output through the ranging signal output module 13.

[0064] In the above embodiments, those skilled in the art can also replace the combination of fiber collimator 5, transceiver optical head 6, relay beam expander optical path 7 and optical antenna 8 with a commercial optical telescope, which requires a high damage threshold and high transmittance coating treatment.

[0065] The combination of heterodyne receiving optical path 9 and balanced receiver 10 can be replaced with commercially available heterodyne balanced receiver devices.

[0066] A method for deblurring a laser coherent radar ranging device includes the following steps:

[0067] 1) The composite modulation module 3 is modulated and driven by a composite modulation signal of linear frequency modulation signal and Barker code. It adopts FPGA design and encoding form. After radio frequency amplification, the encoded information is modulated onto the phase of the input optical carrier to form a local linear frequency modulation complex conjugate signal. The local linear frequency modulation complex conjugate signal is formed by modulating the linear frequency modulation signal within the pulse to form a linear frequency modulation pulse signal, and by using Barker code binary encoding between pulses. The composite modulation module 3 is designed as an FPGA board, and the composite modulation signal can be generated by writing the built-in hardware language VHDL or Verilog.

[0068] 2) The echo signal undergoes two pulse compression processes: First, the signal acquisition and processing module 12 performs pulse compression on the linear frequency modulation (LFM) portion of the pulse. This is done by multiplying the echo signal with the complex conjugate signal of the local LFM signal and then performing matched filtering. The LFM pulse compression coefficient is obtained by analyzing one symbol to process the composite signal (see [link]). Figure 4.1 The result of the first pulse compression processing of the target echo signal is used; then, the processed result is used to generate the binary phase-coded pulse compression coefficients to complete the second pulse compression processing of the composite signal (see...). Figure 4.2 The second pulse compression processing result of the target echo signal is obtained by using the local barker code to perform time delay search, thus obtaining the final pulse compression result of the composite signal.

[0069] 3) When defuzzifying the echo signal, the transformation cycle of the local Barker code is used to match the echo delay signal. The fractional delay estimated before matching is combined with the integer delay estimated by the Barker code traversal to obtain the actual delay of the real target, thereby effectively defuzzifying the target distance beyond the theoretical unfuzzy distance of the system.

[0070] Example

[0071] To analyze the signal structure and operating characteristics, a composite modulation signal waveform is designed with the following specific parameters:

[0072] The target characteristics are set at a range resolution of 0.3m, a maximum detection range of 200km, and a target length of 20m. Based on the design formula mentioned earlier, the radar signal bandwidth B is designed to be 500MHz; the pulse width is selected to consider both target coverage and return time, and is designed to be 0.4ms, with a linear frequency modulation time of 0.1ms and a duty cycle of 1:4. To reduce echo signal noise, the Barker code length is designed to be 13 bits.

[0073] Three target distances were selected as 20km, 90km, and 110km. Since the ambiguity range limit is 60km, without range ambiguity resolution, the three echo distance values ​​would be 20km, 90-60=30km, and 110-60=50km, respectively. Figure 5 As shown by the black line in the middle.

[0074] After distance deblurring, the distance values ​​for the three targets are 20km, 90km, and 110km, respectively. Figure 5 The first black line (20km) and the two blue lines (representing 90km and 110km respectively) are shown in the diagram.

[0075] Experiments show that this invention can overcome the background noise from sunlight and other sources encountered in traditional single-photon laser ranging, enabling all-weather operation. The radar signal waveform is designed using a composite modulation signal of linear frequency modulation and Barker code binary phase-coded modulation. These two modulated signals complement each other, are easy to implement, have strong anti-interference capabilities, and high Doppler tolerance. Furthermore, when combined with a quantum-limited photodetector, the signal-to-noise ratio of the coherent received mixed signal can be effectively improved, significantly increasing the detection range and enhancing the photoelectric detection power. Simultaneously, within the improved detection range, a Barker code ergonomic cyclic matching algorithm is used to de-blur the signal, calculating the true target distance.

[0076] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for unambiguous ranging in laser coherent radar based on linear frequency modulation and Barker code composite modulation coding, characterized in that, The method includes the following steps: Step 1: Divide the laser beam into two parts, with the majority serving as the optical carrier and the minority serving as the local oscillator signal; Step 2: Modulate the composite modulation signal of the linear frequency modulation signal and the Barker code onto the optical carrier to generate a local linear frequency modulation complex conjugate signal; Step 3: Use the composite modulation signal as a ranging laser signal and emit it toward the object to be measured; Step 4: Shift the local oscillator signal to a frequency band consistent with the echo signal frequency band, and mix it with the echo signal to obtain a mixed signal containing the delay information of the echo signal. Step 5: Compare the phase of the mixing signal with the complex conjugate signal of the local linear frequency modulation to extract the phase delay information of the pulse; Step 6: Perform a first pulse compression process on the linear frequency modulation portion within the mixed signal pulse to obtain a narrow pulse signal and a pulse compression coefficient. Step 7: After performing a second pulse compression process of inter-pulse phase modulation on the signal processed in step 6, add it to the phase delay information of the pulse obtained in step 5 to obtain the unambiguous distance information of the target to be measured.

2. The laser coherent radar ranging and deambiguity method based on linear frequency modulation and Barker code composite modulation coding according to claim 1, characterized in that, The complex conjugate signal of the local linear frequency modulation is formed by linear frequency modulation within the pulse and binary phase encoding using Barker code between pulses.

3. The laser coherent radar ranging and deambiguity method based on linear frequency modulation and Barker code composite modulation coding according to claim 1, characterized in that, The first pulse compression process specifically involves multiplying the echo signal with the complex conjugate signal of the local linear frequency modulation (LFM) signal and then performing matched filtering. By analyzing a symbol, the LFM pulse compression coefficient and the narrow pulse signal are obtained, thus completing the first pulse compression process of the echo signal.

4. The laser coherent radar ranging and deambiguity method based on linear frequency modulation and Barker code composite modulation coding according to claim 1, characterized in that, The second pulse compression process specifically involves using a local barker code to perform a delay search, generating a binary coded pulse compression coefficient and delay information.

5. A laser coherent radar ranging device based on linear frequency modulation and Barker code composite modulation coding, characterized in that, include The fiber optic beam splitter module is used to split the laser beam into two parts, with the majority serving as the optical carrier and the minority serving as the local oscillator signal. The composite modulation module is used to modulate the composite modulation signal of linear frequency modulation signal and Barker code onto the optical carrier to generate a local linear frequency modulation complex conjugate signal; The transmitting module is used to transmit the local linear frequency modulated complex conjugate signal generated by the composite modulation module as a ranging laser signal to the object under test; The receiving module is used to receive the echo signal reflected by the object under test; The photoelectric frequency shifting and detection module is used to shift the local oscillator signal to a frequency band consistent with the echo signal frequency band, and mix it with the echo signal to obtain a mixed signal containing the delay information of the echo signal; The phase detection module is used to compare the phase of the mixed signal with the complex conjugate signal of the local linear frequency modulation and extract the pulse phase delay information. The signal acquisition and processing module is used to perform secondary pulse compression processing on the linear frequency modulation part within the mixed signal pulse, and then add it with the pulse phase delay information to obtain the unambiguous distance information of the target under test.

6. The laser coherent radar ranging device based on linear frequency modulation and Barker code composite modulation coding according to claim 5, characterized in that; It also includes a ranging signal output module, which is used to output the unambiguous distance information of the target to be measured.

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