A residual carrier direct spread waveform design and acquisition method for measuring radars

Abstract

The application discloses a residual carrier direct spread waveform design and acquisition method for measuring radar, and comprises the following steps: step one, receiving a signal, performing frequency down-conversion and fast Fourier transform to obtain a Doppler frequency coarse estimation; step two, taking the Doppler frequency coarse estimation as an initial value of a frequency-locked loop, and obtaining a Doppler frequency accurate estimation through frequency-locked loop tracking; and step three, correcting distance migration caused by long-time accumulation of the receiving signal by using the Doppler frequency accurate estimation, and completing spread spectrum pseudo-code phase acquisition; the direct spread spectrum sequence is designed in the application, so that the spread spectrum signal itself has obvious carrier components, and the carrier Doppler frequency can be directly obtained by using fast Fourier transform without performing pseudo-code correlation operation, and further, the pseudo-code phase acquisition is completed, so that the acquisition time is reduced without increasing the occupation of operation resources.

Description

Technical Field

[0001] This invention belongs to the field of radar detection technology, specifically relating to a design and acquisition method for residual carrier direct-sequence waveforms used in radar measurement. Background Technology

[0002] Direct sequence spread spectrum signals have good anti-interference performance, anti-multipath and anti-interception performance, as well as high measurement accuracy, and have been widely used in the fields of communication and radar measurement and control.

[0003] Traditional direct sequence spread spectrum signal acquisition methods require two-dimensional searches of pseudocode phase and Doppler frequency. To improve acquisition efficiency, methods such as parallel Doppler acquisition, serial pseudocode phase acquisition, parallel pseudocode phase acquisition, serial Doppler acquisition, and segmented pseudocode correlation-fast Fourier transform (PMF-FFT) are generally used. These methods can effectively reduce the search time in one dimension, but they come at the cost of a large amount of computational resources, which is not conducive to applications on resource-constrained platforms such as satellites, rockets, and missiles.

[0004] To address the above issues, in the article "Long-Period Pseudocode Acquisition Combined with Spectrum Correction in High Dynamic Environments" published in Volume 30, Issue 8 of *Computer Applications* in 2010, Pang Tong et al. proposed a method to improve the pseudocode acquisition probability using spectrum correction. However, this method suffers from the picket fence effect during Fast Fourier Transform (FFT), resulting in significant spectral estimation errors. In 2012, Yang Weijun et al. from Zhejiang University proposed a method in their invention patent "A Fully Digital Direct Sequence Spread Spectrum Communication System and Its Fast Pseudocode Acquisition Method" (patent application number CN201210032239.8). This method first eliminates the carrier Doppler frequency through differential multiplication, then obtains the pseudocode phase difference through parallel code phase search, and finally uses the pseudocode phase difference for despreading and obtains the Doppler frequency through FFT. This method performs parallel acquisition of both the pseudocode phase and the Doppler frequency, greatly reducing the acquisition time. However, due to the differential multiplication operation, multiplicative noise is introduced, causing a sharp deterioration in the signal-to-noise ratio (SNR), making it unsuitable for use in low SNR environments.

[0005] Traditional direct sequence spread spectrum signal acquisition methods require significant computational resources to transform two-dimensional search into one-dimensional search. Methods proposed in recent years may have large frequency estimation errors or be unsuitable for spreading spectrum signal acquisition in low signal-to-noise ratio environments. Summary of the Invention

[0006] In view of this, the present invention provides a residual carrier direct-sequence waveform design and acquisition method for measurement radar, which can effectively acquire high dynamic weak signals and resolve the contradiction between computing resources and acquisition time.

[0007] The technical solution for implementing the present invention is as follows:

[0008] A method for designing and acquiring residual carrier direct-sequence waveforms for measuring radar includes the following steps:

[0009] Step 1: The received signal undergoes down-conversion and fast Fourier transform to obtain a rough estimate of the Doppler frequency;

[0010] Step 2: Use the rough estimate of the Doppler frequency as the initial value of the frequency-locking loop, and obtain the accurate estimate of the Doppler frequency through frequency-locking loop tracking;

[0011] Step 3: Use the accurate Doppler frequency estimate to correct the distance migration caused by long-term accumulation of the received signal and complete the spread spectrum pseudocode phase acquisition.

[0012] Furthermore, in step two, the phase detector of the frequency-locked loop is a four-quadrant arctangent phase detector, and the frequency identification is achieved by using the dot product and cross product four-quadrant arctangent algorithm.

[0013] Furthermore, in step three, a motion compensation algorithm is used to correct the distance migration. Based on the pseudo-code phase migration term, the pseudo-code acquisition circuit compensates for the pseudo-code phase migration based on the Doppler frequency estimation result of the tracking circuit.

[0014] Beneficial effects:

[0015] 1. This invention addresses the problem of high dynamic weak signals by designing a direct spread spectrum sequence to have a distinct carrier component. This waveform design method is simple, and a coarse estimate of the carrier Doppler frequency can be directly obtained using fast Fourier transform. The two-dimensional search of Doppler frequency and pseudo-code phase is transformed into a one-dimensional search of pseudo-code phase, which reduces the acquisition time and effectively solves the contradiction between computing resources and acquisition time.

[0016] 2. After the coarse Doppler frequency estimate of the present invention is placed into the frequency-locked loop, the fine Doppler frequency estimate obtained by tracking can be used for pseudo-code phase migration compensation, and the signal energy is effectively accumulated in the low signal-to-noise ratio environment, realizing the rapid acquisition of high dynamic weak signals.

[0017] 3. The waveform design method proposed in this invention is simple and effective. Its acquisition method does not require two-dimensional search of Doppler frequency and pseudocode phase, which effectively solves the contradiction between computing resources and acquisition time. It can be well applied on small and resource-constrained platforms to achieve rapid acquisition of weak maneuvering targets. Attached Figure Description

[0018] Figure 1 This is a comparison of the waveforms of the unbalanced pseudocode after insertion at every 2 points and the original pseudocode.

[0019] Figure 2 This is a flowchart illustrating the overall process of the method of the present invention. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] This invention proposes a direct sequence spread spectrum signal waveform design and a fast acquisition method with residual carriers. Under the condition of low resource utilization, it overcomes the influence of code phase migration caused by high dynamics and achieves fast acquisition of weak maneuvering targets.

[0022] To construct a received signal with a residual carrier, the direct spread spectrum sequence is designed as follows: The m-sequence in the transmitted signal is processed every N... inter Insert a 1(N) at the point inter (For the set value), because the number of 0s and 1s in the constructed sequence is not equal, a pseudo-code sequence with DC quantity is obtained, such as Figure 1 As shown.

[0023] The radar receives a signal containing a residual carrier wave. After digital down-conversion, low-pass filtering, decimation, and fast Fourier transform, the signal is passed through a square detector and subjected to N-wave processing. nc After a non-coherent accumulation, peak-to-average power ratio (PAPR) threshold detection and N-to-M decision are performed to obtain a coarse estimate of the Doppler frequency. This coarse estimate is used as an initial value for the frequency-locked loop (LLL) circuit, and the stable tracking result of the LLL circuit is the precise Doppler estimate. Migration correction is then performed using the precise Doppler estimate to achieve rapid pseudocode acquisition. The implementation process of this invention is as follows: Figure 2 As shown, the specific implementation steps are as follows:

[0024] Step 1: The received signal is down-converted and then subjected to a fast Fourier transform to obtain a rough estimate of the Doppler frequency.

[0025] The radar received signal can be expressed as the sum of the pseudocode modulation term and the residual carrier term, i.e.

[0026]

[0027] In the formula, t k =k·t s , t s The sampling time interval is A1, the amplitude of the pseudocode modulation term signal is A2, and the amplitude of the residual carrier term signal is PN(t). k +τ) is a pseudocode with a delay τ, ω I For the intermediate frequency, ω d For carrier Doppler frequency, To receive the carrier phase, n(t) k () represents received noise.

[0028] Since the pseudocode modulation term has undergone spread spectrum processing, its spectrum approximates narrowband Gaussian noise. Therefore, performing a Fast Fourier Transform on it cannot accumulate energy, and its influence can be ignored when the residual carrier signal-to-noise ratio is constant. Thus, the signal after orthogonal downconversion can be expressed as follows:

[0029]

[0030] Where n'(t) k The noise is the result of orthogonal downconversion.

[0031] After low-pass filtering, high-frequency components are removed, and the time-domain form of the residual carrier in the received signal is:

[0032]

[0033] Where n"(t k () represents the noise after low-pass filtering.

[0034] After the Fast Fourier Transform, T was completed. ca The coherent accumulation of ms, taking N nc The squared moduli of the coherent accumulation results are summed to obtain the noncoherent accumulation result, which is then subjected to peak-to-average power ratio (PAPR) threshold detection and N-to-M selection. The signal frequency domain result F(k) is then obtained in ω. d A peak value appears at a certain point, from which a rough estimate of the Doppler frequency can be obtained.

[0035] Step 2: Use the rough estimate of the Doppler frequency as the initial value of the frequency-locked loop, and obtain the accurate estimate of the Doppler frequency through frequency-locked loop tracking.

[0036] The purpose of a frequency-locked loop (LLL) is to ensure that the local carrier and the received carrier maintain the same frequency, but not necessarily the same phase. Since the Doppler frequency shift introduced by the relative motion between the transmitter and receiver is generally random, the LLL first uses a frequency discriminator to detect the frequency difference between the local carrier and the received carrier. Then, it uses the filtered discriminator result to adjust the frequency of the local carrier generated by the carrier numerically controlled oscillator. After multiple feedback adjustments, the frequency of the locally regenerated carrier is made consistent with the frequency of the received carrier.

[0037] The phase detector of this frequency-locked loop is a four-quadrant arctangent phase detector, and the frequency discrimination is achieved by using the dot product and cross product four-quadrant arctangent algorithm.

[0038]

[0039] In the formula Δf d For the Doppler frequency estimation error, T s NT represents the sampling time interval, N is the number of sampling points to be cleared during integration. s This is the time interval for clearing the integral.

[0040] Frequency-locked loops (LLLs) are better suited to dynamic stresses than phase-locked loops (PLLs). A LLL is implemented using a first-order filter and a second-order loop. Its optimal filter is:

[0041]

[0042] Where ω n ξ is the characteristic frequency, ξ is the damping coefficient, and K0 is the loop gain.

[0043] From the mapping relationship of digital rectangular integrals, we know that: In the formula, T represents the output period of the loop filter.

[0044] Its discrete transfer function expression is:

[0045]

[0046] Step 3: Use the accurate Doppler frequency estimate to correct the distance migration caused by long-term accumulation of the received signal and complete the spread spectrum pseudocode phase acquisition.

[0047] The rate of receiving pseudocode signals can be expressed as

[0048]

[0049] Where R cr For the received pseudocode rate, R c For the rate of sending pseudocode, f up f is the radio frequency. d For Doppler frequency shift, it can be expressed as

[0050]

[0051] The received pseudocode rate can then be expressed as:

[0052]

[0053] Because of the significant velocity and acceleration between the radar and transponder under high dynamic conditions, the received pseudo-code rate changes. This rate deviation causes pseudo-code phase migration, which increases with time, leading to misalignment of code phases in different periods within the coherent accumulation time, resulting in accumulation loss. The received signal is...

[0054]

[0055] In the formula, t k =k·t s (k = 0, 1, ..., K-1), t s Where K is the sampling time interval, K is the total number of sampling points, and P is the sampling point interval. s For the received signal power, d(t) k() is the modulated data, For the existence of a delay τ and a phase migration term The pseudocode, ω I For the intermediate frequency, ω d n(t) is the carrier Doppler frequency. k () represents noise.

[0056] This invention employs a motion compensation algorithm to correct distance migration. Based on the specific pseudocode phase migration term in the above formula, the pseudocode acquisition circuit compensates for the pseudocode phase migration based on the Doppler frequency estimation result of the tracking circuit.

[0057] When the acquisition circuit generates local pseudocode, it compensates for the local pseudocode frequency word based on the Doppler frequency estimated by the tracking circuit, thus obtaining the pseudocode rate containing the code Doppler:

[0058]

[0059] Among them, R c f is the pseudocode rate. d To preset the Doppler frequency for the tracking circuit, f up This refers to the radio frequency. The code Doppler frequency term that needs to be compensated is...

[0060]

[0061] Because the Doppler frequency here is a precise estimate given by the tracking circuit, compensation can be completed well, and the energy of weak signals can be effectively accumulated.

[0062] After compensation, the intermediate frequency signal is digitally orthogonally down-converted, L-point integrated and cleared, and circularly correlated to obtain the N-point I and Q signals:

[0063]

[0064]

[0065] In the formula, For local pseudocode latency, To estimate the Doppler frequency, L is the number of integration clearing points, i (i = 0, 1, ..., N-1) is the intermediate accumulation sequence number in the correlation process, n is the circular correlation result sequence number, N is the total number of circular correlation result points, and N = K / L.

[0066] Combining I(n) and Q(n) into complex form is:

[0067]

[0068] In the formula, ω d In order to receive the Doppler frequency, For the estimated received Doppler frequency, The signal after integration and clearing. For the pseudocode after integral clearing, t L =L·t s This is the time for clearing points.

[0069] Given the relevant theorems for circles:

[0070]

[0071] In the formula, X(K) and Y(K) are the Fourier transform results of x(i) and y(i) respectively, and * represents the conjugate operation. Comparing equation (15) and equation (16), we can... Seen as With PN(it) L ) is related to the circumference.

[0072] If the effects of modulation data d(k) and noise n(k) are ignored, the results of quadrature downconversion will be affected. Perform an FFT and compare it with the local pseudocode PN(it) L Performing conjugate multiplication on the FFT of the given data, followed by IFFT, yields the fast circumferential correlation results. exist and Traversing within the search space, when and hour, The modulus of can achieve the maximum value, (·) N This represents a cyclic operation with a period of N. Therefore, it can be detected... The peak value, after being processed by the detection and decision logic, yields the pseudocode phase estimate.

[0073]

[0074] Where, n max for The index n value corresponding to the peak value.

[0075] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for designing and acquiring residual carrier direct-sequence waveforms for measurement radar, characterized in that, Includes the following steps: The residual carrier direct-sequence waveform is designed using the following method: For each m-sequence in the transmitted signal... Insert a 1, The set value is used to make the number of 0s and 1s in the constructed sequence unequal, resulting in a pseudocode sequence with DC quantity; Step 1: The received signal undergoes down-conversion and Fast Fourier Transform to obtain a rough estimate of the Doppler frequency: After digital down-conversion, low-pass filtering, decimation, and Fast Fourier Transform, the signal is passed through a square detector and subjected to... After a non-coherent accumulation, peak-to-average ratio threshold detection and N-to-M decision are performed to obtain a rough estimate of the Doppler frequency. Step 2: Use the coarse estimate of the Doppler frequency as the initial value of the frequency-locking loop, and obtain the accurate estimate of the Doppler frequency through frequency-locking loop tracking; Step 3: Correct the range migration caused by long-term accumulation of received signal using accurate Doppler frequency estimates and complete the spread spectrum pseudocode phase acquisition; specifically, a motion compensation algorithm is used to correct the range migration, and the pseudocode acquisition circuit compensates for the pseudocode phase migration based on the Doppler frequency estimation results of the tracking circuit according to the pseudocode phase migration term.

2. The residual carrier direct-sequence waveform design and acquisition method for measuring radar as described in claim 1, characterized in that, In step two, the phase detector of the frequency-locked loop is a four-quadrant arctangent phase detector, and the frequency identification is achieved by using the dot product and cross product four-quadrant arctangent algorithm.

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