Method for tracking signal with large Doppler frequency change rate by using FFT (Fast Fourier Transform)

Abstract

The invention discloses a method for tracking a signal with a large Doppler frequency change rate by using FFT (Fast Fourier Transform), which comprises the following steps of: S1, acquiring an input signal, and performing preliminary frequency offset compensation on the acquired input signal; s2, de-spreading the signal after the initial frequency offset, and performing phase correction on the de-spread signal; s3, performing FFT calculation on the signal after phase correction, and obtaining a residual frequency offset value; and S4, according to the residual frequency offset value, returning to S1 to circularly carry out frequency offset compensation on the input signal until the signal tracking is finished. According to the invention, an open-loop tracking mode is adopted, and the problem that a traditional three-order phase-locked loop cannot realize carrier tracking of a large Doppler frequency change rate of a short-time burst signal is solved.

Description

Technical Field

[0001] The invention relates to the technical field of digital communication, in particular to a method for tracking a Doppler frequency change rate signal by using FFT. Background Art

[0002] As an important part of modern communication technology, carrier tracking is mainly used for carrier recovery at the communication receiving end. Due to various interference factors in the signal transmission process, the receiving end must use carrier tracking technology to eliminate frequency deviation. In the current high-dynamic receiver solution, the Doppler change rate caused by the high-speed flight of the satellite is often achieved by adding a third-order phase-locked loop to the carrier tracking loop. However, the third-order phase-locked loop takes a long time to achieve locking, and requires a long frame header overhead to ensure the initial lock of the carrier tracking loop. Therefore, for short-term burst frames, the third-order phase-locked loop will reduce the efficiency of wireless communication; in addition, after the carrier tracking loop loses lock due to short-term interference, it needs to re-capture the lock, which takes a long time and causes the short-term burst frame to be discarded.

[0003] Based on this, the present invention proposes a method for tracking a Doppler frequency change rate signal using FFT to achieve carrier tracking of the Doppler frequency change rate of a short-time burst signal. Summary of the invention

[0004] The purpose of the present invention is to provide a method for tracking a Doppler frequency change rate signal using FFT, which adopts an open-loop tracking form to overcome the problem that a traditional third-order phase-locked loop cannot realize carrier tracking of a Doppler frequency change rate of a short-time burst signal.

[0005] To achieve the above object, the present invention provides a method for tracking a Doppler frequency change rate signal using FFT, comprising:

[0006] S1, collecting input signals and performing preliminary frequency offset compensation on the collected input signals;

[0007] S2, despreading the signal after the preliminary frequency deviation, and performing phase correction on the despread signal;

[0008] S3, performing FFT calculation on the phase-corrected signal and obtaining a residual frequency deviation value;

[0009] S4, returning to S1 to loop and perform frequency offset compensation on the input signal according to the residual frequency offset value until the signal tracking is completed.

[0010] Optionally, step S1 includes:

[0011] S1.1, receiving an input signal collected by an analog-to-digital converter through a signal receiver;

[0012] S1.2, performing complex frequency conversion on the received input signal to obtain a first AD complex signal;

[0013] S1.3, performing complex multiplication calculation on the digital complex sinusoidal signal output by the frequency digitally controlled oscillator and the first AD complex signal to obtain a second AD complex signal after frequency offset compensation.

[0014] Optionally, the input signal collected by the analog-to-digital converter is an intermediate frequency signal; the first AD complex signal includes a real signal I and an imaginary signal Q.

[0015] Optionally, the digital complex sinusoidal signal output by the frequency digitally controlled oscillator comes from a captured frequency deviation value.

[0016] Optionally, step S2 comprises:

[0017] S2.1, despreading the second AD complex signal, and obtaining a despread real signal I and an imaginary signal Q by a despreading integral method;

[0018] S2.2, transmitting the real signal I and the imaginary signal Q obtained after despreading through two signal transmission paths, and transmitting them to the first FIFO buffer and the second FIFO buffer respectively;

[0019] S2.3, performing phase correction on the real signal I and the imaginary signal Q after despreading;

[0020] The number of points of the first FIFO buffer and the second FIFO buffer are both set to N points.

[0021] Optionally, the two signal transmission paths are:

[0022] The first path directly transmits the real signal I and the imaginary signal Q output by the despreading to the first FIFO buffer;

[0023] In the second path, the real signal I and the imaginary signal Q output by the despreading are differentially demodulated, and the signal data after the differential demodulation are transmitted to the second FIFO buffer.

[0024] Optionally, in step S2.3, phase correction is performed on the real signal I and the imaginary signal Q cached in the first FIFO buffer using the phase compensation sequence formed by the differential demodulation.

[0025] Optionally, step S3 includes:

[0026] S3.1, performing FFT transformation on the real signal I and the imaginary signal Q after phase correction, and obtaining the output result of FFT with N cache points;

[0027] S3.2, calculating the residual frequency deviation value according to the FFT output results of the N cache points;

[0028] The FFT output result includes: FFT complex value, FFT index value and FFT frequency resolution.

[0029] Optionally, the step S3.2 comprises:

[0030] According to the complex value of FFT of each cache point number, the modulus of the complex value of FFT of the real signal I and the imaginary signal Q corresponding to each cache point number is calculated respectively, and the square value of the modulus is calculated;

[0031] By comparing the square values ​​of N modules, the maximum value among the N square values ​​is obtained;

[0032] The residual frequency offset value used for compensation is calculated according to the FFT index value and the FFT frequency resolution in the FFT output result corresponding to the maximum value.

[0033] Optionally, step S4 includes:

[0034] S4.1, converting the calculated residual frequency deviation value into a complex sinusoidal signal frequency control word of a frequency digitally controlled oscillator to correct the digital complex sinusoidal signal output by the frequency digitally controlled oscillator;

[0035] S4.2, return to execute S1.3, that is, perform complex multiplication calculation again on the corrected digital complex sine signal output by the frequency digital controlled oscillator and the input first AD complex signal to perform frequency deviation compensation on the input first AD complex signal again until the signal tracking is completed.

[0036] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. The method provided by the present invention uses FFT to track a large Doppler frequency change rate signal. Compared with the traditional third-order phase-locked loop, its implementation is more convenient; and its structure is simple and easy to implement with FPGA hardware.

[0038] 2. The method provided by the present invention for tracking a large Doppler frequency change rate signal by using FFT is suitable for incoherent differential demodulation and can use a longer data frequency demodulation to improve the output signal-to-noise ratio of the frequency demodulator. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The present invention is a principle block diagram of a method for tracking a Doppler frequency change rate signal using FFT. DETAILED DESCRIPTION

[0040] The following will be combined with the attached Figure 1, the technical content, structural features, objectives achieved and effects of the present invention are described in detail through preferred embodiments.

[0041] It should be noted that the drawings are in a very simplified form and use non-precise proportions. They are only used to conveniently and clearly assist in explaining the embodiments of the present invention, and are not used to limit the conditions for the implementation of the present invention. Therefore, they have no substantive technical significance. Any structural modification, change in proportional relationship or adjustment of size, without affecting the effects and purposes that can be achieved by the present invention, should still fall within the scope of the technical content disclosed by the present invention.

[0042] In the description of the present invention, it should be noted that the terms "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0043] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0044] Due to the relative motion between the transmitter and the receiver, the frequency of the signal received by the receiver changes relative to the transmission frequency. If the signal source sent by the transmitter approaches the receiver, the received signal frequency will increase; on the contrary, if the signal source sent by the transmitter moves away from the receiver, the received signal frequency will decrease. This phenomenon is called the Doppler frequency change of the carrier frequency. When the acceleration of the relative motion between the transmitter and the receiver is large, the Doppler frequency change rate is also large.

[0045] The present invention provides a method for tracking a Doppler frequency change rate signal using FFT, which is used to track the Doppler frequency change generated by the collected intermediate frequency signal. Figure 1 As shown, the method for tracking the Doppler frequency change rate signal includes:

[0046] S1, input signal acquisition, and preliminary frequency offset compensation is performed on the acquired input signal.

[0047] S1.1, receiving an intermediate frequency signal collected by an analog-to-digital converter (ADC) through a signal receiver.

[0048] S1.2, performing complex frequency conversion on the received intermediate frequency signal, and obtaining a first AD complex signal, including: a real signal I of the first AD complex signal and an imaginary signal Q of the first AD complex signal.

[0049] The real signal I and the imaginary signal Q after complex frequency conversion are both zero intermediate frequency signals.

[0050] S1.3, perform complex multiplication calculation on the digital complex sinusoidal signal output by the frequency numerically controlled oscillator (NCO) and the first AD complex signal (including the real signal I and the imaginary signal Q) to obtain a second AD complex signal after frequency offset compensation.

[0051] Wherein, the digital complex sinusoidal signal output by the frequency numerically controlled oscillator (NCO) comes from the captured frequency deviation value.

[0052] Wherein, the complex multiplication calculation is implemented by a first complex multiplier.

[0053] S2, despreading the signal after preliminary frequency offset compensation, and performing phase correction on the despread signal.

[0054] S2.1, despread the second AD complex signal, and obtain the despread real signal I and imaginary signal Q by despreading integration.

[0055] The despreading integration accumulates the despread signal within a certain period of time, thereby enhancing the amplitude of the useful signal in the real signal I and the imaginary signal Q, while suppressing the noise.

[0056] S2.2, the real signal I and the imaginary signal Q obtained after despreading are transmitted through two signal transmission paths respectively and sent to the FIFO buffer.

[0057] The FIFO (first-in-first-out) cache is an efficient cache method that can be used to temporarily store signal data packets to ensure that signal data packets are transmitted in order to avoid disorder and loss; the FIFO cache has a cache point number set to N points, and the FIFO cache includes: a first FIFO cache and a second FIFO cache.

[0058] The number of cache points N of the FIFO cache affects the frequency calculation accuracy. Moreover, the larger the number of cache points N, the higher the frequency calculation accuracy, and the smaller the Doppler change rate that can be tracked; as the number of cache points N further increases, the phase error of differential demodulation formed becomes larger, and differential demodulation becomes more likely to fail.

[0059] S2.2.1, first path, directly transmits the real signal I and imaginary signal Q output by the despreading to the first FIFO buffer.

[0060] The number of points of the first FIFO buffer is set to N points.

[0061] S2.2.2, the second path, differentially demodulates the real signal I and the imaginary signal Q output by the despreading, and transmits the differentially demodulated signal data to the second FIFO buffer.

[0062] The number of points of the second FIFO buffer is also set to N points.

[0063] S2.3, performing phase correction on the real signal I and the imaginary signal Q after despreading.

[0064] The signal data in the first FIFO buffer and the second FIFO buffer are read through the FIFO buffer full flag, and the demodulated signal obtained by differential demodulation in step S2.2.2 is compared with the phase differential modulation constellation diagram to generate a phase compensation sequence.

[0065] Through the second complex multiplier, the phase compensation sequence performs phase correction on the real signal I and the imaginary signal Q buffered in the first FIFO buffer.

[0066] The real signal I and the imaginary signal Q after phase correction are linear frequency modulation signals, and the linear frequency modulation signal is approximately a single-tone signal when N takes a smaller value.

[0067] S3, performing FFT calculation on the phase-corrected signal to obtain a residual frequency deviation value.

[0068] S3.1, perform FFT transformation (Fast Fourier Transform) on the real signal I and the imaginary signal Q after phase correction, and obtain the FFT output results of N cache points.

[0069] In a specific embodiment of the present invention, FFT (Fast Fourier Transform) is performed on N cache points respectively to obtain an output result of the FFT corresponding to each cache point.

[0070] The FFT output result includes: FFT complex value, FFT index value and FFT frequency resolution.

[0071] S3.2, calculating the residual frequency deviation value according to the FFT output results of the N cache points.

[0072] S3.2.1, based on the complex value of FFT of each cache point, calculate the modulus of the FFT complex value of the real signal I and the imaginary signal Q corresponding to each cache point, and calculate the square value of the modulus.

[0073] S3.2.2, by comparing the square values ​​of N modules, obtain the maximum value among the N square values.

[0074] S3.2.3, based on the FFT index value and FFT frequency resolution in the FFT output result corresponding to the maximum value, the residual frequency deviation value used for compensation is calculated using the existing N-point complex FFT calculation formula.

[0075] S4, returning to S1 and cyclically performing frequency offset compensation on the input signal according to the residual frequency offset value until the signal tracking is completed.

[0076] S4.1, converting the calculated residual frequency deviation value into a complex sinusoidal signal frequency control word of a frequency numerically controlled oscillator (NCO) to correct the digital complex sinusoidal signal output by the frequency numerically controlled oscillator (NCO);

[0077] S4.2, returns to execute S1.3, that is, the modified digital complex sine signal output by the frequency numerically controlled oscillator (NCO) and the input first AD complex signal are again complex multiplied to compensate the frequency deviation of the input first AD complex signal again until the signal tracking is completed.

[0078] In summary, the present invention provides a method for tracking a Doppler frequency change rate signal using FFT, which performs phase correction on the despread data through a phase compensation sequence obtained by differential demodulation of N cache points, and then performs frequency deviation correction on the input signal through a frequency numerically controlled oscillator (NCO), and finally realizes the tracking of the carrier Doppler change rate. In addition, the present invention provides a method for tracking a Doppler frequency change rate signal using FFT, which realizes the carrier tracking of the Doppler frequency change rate of a short-term burst signal.

[0079] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the invention. After reading the above content, it will be obvious to those skilled in the art for various modifications and substitutions of the present invention. Therefore, the protection scope of the present invention should be limited by the attached claims.

Claims

1. A method for tracking a Doppler frequency change rate signal using FFT, characterized in that: Include: S1, collecting input signals and performing preliminary frequency offset compensation on the collected input signals; S2, despreading the signal after the preliminary frequency deviation, and performing phase correction on the despread signal; S3, performing FFT calculation on the phase-corrected signal and obtaining a residual frequency deviation value; S4, returning to S1 to loop and perform frequency offset compensation on the input signal according to the residual frequency offset value until the signal tracking is completed.

2. The method for tracking a Doppler frequency change rate signal according to claim 1, characterized in that: The step S1 comprises: S1.1, receiving an input signal collected by an analog-to-digital converter through a signal receiver; S1.2, performing complex frequency conversion on the received input signal to obtain a first AD complex signal; S1.3, performing complex multiplication calculation on the digital complex sinusoidal signal output by the frequency digitally controlled oscillator and the first AD complex signal to obtain a second AD complex signal after frequency offset compensation.

3. The method for tracking a Doppler frequency change rate signal as claimed in claim 2, characterized in that: The input signal collected by the analog-to-digital converter is an intermediate frequency signal; the first AD complex signal includes a real signal I and an imaginary signal Q.

4. The method for tracking a Doppler frequency change rate signal as claimed in claim 2, characterized in that: The digital complex sinusoidal signal output by the frequency digital controlled oscillator comes from the captured frequency deviation value.

5. The method for tracking a Doppler frequency change rate signal as claimed in claim 2, characterized in that: The step S2 comprises: S2.1, despreading the second AD complex signal, and obtaining a despread real signal I and an imaginary signal Q by a despreading integral method; S2.2, transmitting the real signal I and the imaginary signal Q obtained after despreading through two signal transmission paths, and transmitting them to the first FIFO buffer and the second FIFO buffer respectively; S2.3, performing phase correction on the real signal I and the imaginary signal Q after despreading; The number of points of the first FIFO buffer and the second FIFO buffer are both set to N points.

6. The method for tracking a Doppler frequency change rate signal according to claim 5, characterized in that: The two signal transmission paths are respectively: The first path directly transmits the real signal I and the imaginary signal Q output by the despreading to the first FIFO buffer; In the second path, the real signal I and the imaginary signal Q output by the despreading are differentially demodulated, and the signal data after the differential demodulation are transmitted to the second FIFO buffer.

7. The method for tracking a Doppler frequency change rate signal according to claim 6, characterized in that: In the step S2.3, phase correction is performed on the real signal I and the imaginary signal Q buffered in the first FIFO buffer using the phase compensation sequence formed by the differential demodulation.

8. The method for tracking a Doppler frequency change rate signal according to claim 7, characterized in that: The step S3 comprises: S3.1, performing FFT transformation on the real signal I and the imaginary signal Q after phase correction, and obtaining the output result of FFT with N cache points; S3.2, calculating the residual frequency deviation value according to the FFT output results of the N cache points; The FFT output result includes: FFT complex value, FFT index value and FFT frequency resolution.

9. The method for tracking a Doppler frequency change rate signal according to claim 8, characterized in that: The step S3.2 comprises: According to the complex value of FFT of each cache point number, the modulus of the complex value of FFT of the real signal I and the imaginary signal Q corresponding to each cache point number is calculated respectively, and the square value of the modulus is calculated; By comparing the square values ​​of N modules, the maximum value among the N square values ​​is obtained; The residual frequency offset value used for compensation is calculated according to the FFT index value and the FFT frequency resolution in the FFT output result corresponding to the maximum value.

10. The method for tracking a Doppler frequency change rate signal according to claim 9, characterized in that: The step S4 comprises: S4.1, convert the calculated residual frequency deviation value into the complex sinusoidal signal frequency control word of the frequency digitally controlled oscillator to correct the digital complex sinusoidal signal output by the frequency digitally controlled oscillator; S4.2, return to execute S1.3, that is, perform complex multiplication calculation again on the corrected digital complex sinusoidal signal output by the frequency digitally controlled oscillator and the input first AD complex signal to perform frequency deviation compensation for the input first AD complex signal again until the signal tracking is completed.

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