A Low-Earth Orbit Satellite QPSK Signal Demodulation Method Based on Data Buffering

Abstract

This invention discloses a low-Earth orbit satellite QPSK signal demodulation method based on data buffering, relating to the field of satellite navigation technology. In this invention, the radio frequency signal is down-converted and sampled by an analog-to-digital converter (AD), then filtered by a digital filter, downsampled, acquired, and buffered in a ping-pong RAM within an FPGA for post-processing. Data in the RAM buffer is read by the system clock and sequentially played back to multiple tracking channels. Within each tracking channel, the intermediate frequency (IF) signal is down-converted and despread. Preamble data is used to rapidly reduce the frequency deviation between the local carrier and the received signal via a frequency-locked loop (FLL), while simultaneously maintaining pseudocode phase synchronization. Differential demodulation is used for message data. This invention, on the one hand, extends the coherent integration time during signal acquisition, thereby improving acquisition sensitivity and frequency estimation accuracy; on the other hand, it utilizes preamble data and a FLL for tracking to quickly stabilize the loop, accelerating message data demodulation.

Description

Technical Field

[0001] This invention relates to the field of satellite navigation technology, and in particular to a method for demodulating QPSK signals from low-Earth orbit satellites based on data buffering. Background Technology

[0002] As an evolution and enhancement of traditional GNSS satellite navigation systems, low-Earth orbit (LEO) satellite navigation systems have unique advantages in system construction, signal system, and service capabilities. However, LEO satellite signal processing also faces many new challenges. This invention provides a LEO satellite QPSK signal demodulation method based on data buffering, which can improve the acquisition sensitivity and message parsing speed of LEO satellite signals in high-dynamic environments.

[0003] Low-Earth orbit (LEO) satellites move at high speeds relative to ground receiving equipment, resulting in significant Doppler frequency offset and its rate of change. Therefore, the signal processing at the receiver must be fully considered during signal system design. To improve the receiver's signal acquisition sensitivity, the length of the preamble data (typically a synchronization header) needs to be increased, but this increases overhead. Traditional signal reception processes involve real-time signal processing: first, signal acquisition; after successful acquisition, tracking; and only after stable tracking can reliable message data be demodulated. Generally, it takes tens of milliseconds or even longer from initiating signal acquisition to achieving stable tracking. For signal frame structures with short preamble data, both acquisition sensitivity and message demodulation speed are limited, especially for bursty signal frames, which require rapid signal acquisition and demodulation of the original message. The aforementioned processing flow is no longer applicable in these cases. Summary of the Invention

[0004] The present invention addresses the shortcomings of existing technologies by providing a low-Earth orbit satellite QPSK signal demodulation method based on data buffering. This method overcomes the problems of low acquisition sensitivity, long tracking loop locking time, and slow message parsing speed in traditional navigation signal reception processes, and is helpful for receiving low-Earth orbit satellite signals in high-dynamic environments.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for demodulating low-Earth orbit satellite QPSK signals based on data buffering includes the following steps:

[0007] Step 1: The antenna receives the signal transmitted by the low-orbit satellite, performs down-conversion processing through the radio frequency front-end, and then completes sampling through analog-to-digital conversion to obtain the digital intermediate frequency signal;

[0008] Step 2: The obtained digital intermediate frequency signal is preprocessed and downsampled using digital preprocessing techniques;

[0009] Step 3: Capture the downsampled intermediate frequency signal to complete the initialization and activation settings of each tracking channel; at the same time, write the downsampled intermediate frequency signal into the digital buffer in the FPGA for buffering.

[0010] Step 4: Read the intermediate frequency data from the digital buffer according to the system clock and play it back to each open tracking channel. After the data playback is completed, the FPGA issues an interrupt flag.

[0011] Step 5: Each tracking channel generates its own local carrier signal with orthogonal phase, and performs a mixing operation with the playback intermediate frequency signal to achieve orthogonal downconversion;

[0012] Step 6: Perform correlation operations on the baseband signal after orthogonal downconversion and the local pseudocode sequence, extract the mutually orthogonal I and Q integral values, and complete the despreading of the original transmitted signal;

[0013] Step 7: Based on the obtained I and Q integral values, a frequency error estimate is calculated using a frequency-locked loop and feedback adjustment is performed to quickly converge the local carrier frequency to the received signal frequency; in addition, based on the obtained I and Q integral values, the local pseudocode is kept in phase with the received signal pseudocode.

[0014] Step 8: Use differential demodulation to perform data recovery processing on the received baseband signal to complete information demodulation.

[0015] Furthermore, the preprocessing in step 2 includes filtering the obtained digital intermediate frequency signal using a digital filter to achieve out-of-band noise suppression.

[0016] Furthermore, the downsampling in step 2 involves downsampling the filtered digital intermediate frequency signal by summing up multiple consecutive sampling points to reduce the signal rate.

[0017] Furthermore, the digital buffer in step 3 is implemented using ping-pong RAM, that is, two RAM spaces of the same size are opened, and the writing and reading operations of the two RAMs are alternated to eliminate read and write conflicts.

[0018] Furthermore, the storage depth of the digital buffer should be greater than the length of the preamble data after downsampling.

[0019] Furthermore, in step 4, the system clock frequency is greater than or equal to the digital buffer write clock frequency multiplied by the number of tracking channels.

[0020] Furthermore, in step 7, the frequency-locked loop uses a four-quadrant arctangent frequency discriminator to achieve frequency error estimation.

[0021] Furthermore, the frequency-locking loop described in step 7 employs a second-order loop filter.

[0022] Furthermore, in step 8, the differential demodulation method extracts information bits by comparing the phase changes of adjacent symbols.

[0023] Due to the adoption of the above technical solution, the present invention has the following specific beneficial effects compared to the prior art:

[0024] This invention employs a post-processing method for received intermediate frequency (IF) data, allowing buffered data to be reused for acquisition and tracking. This extends the coherent integration time during signal acquisition, improving acquisition sensitivity and frequency estimation accuracy. Furthermore, using preamble data for tracking enables rapid loop stabilization, accelerating message data demodulation. Simultaneously, frequency-locked loops and differential demodulation methods are used to further speed up QPSK signal message data demodulation. This method is highly effective for receiving high-dynamic burst signals with short preamble sequences. Attached Figure Description

[0025] Figure 1 This is a flowchart illustrating a low-Earth orbit satellite QPSK signal demodulation method based on data caching, as described in an embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of the symbol decision principle of the differential decomposition method in an embodiment of the present invention. Detailed Implementation

[0027] The invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0028] A method for demodulating low-Earth orbit satellite QPSK signals based on data buffering, such as... Figure 1 As shown:

[0029] (1) The antenna receives high-dynamic navigation signals transmitted by low-orbit satellites, performs down-conversion processing through the radio frequency front-end, and then completes sampling via analog-to-digital (AD) conversion to obtain digital intermediate frequency signals; in this embodiment, the data sampling rate is taken as .

[0030] (2) After filtering the obtained digital intermediate frequency signal, downsampling is performed by summing multiple consecutive sampling points. The N consecutive sampling points output by the digital filter are { , , ... }, summing them up yields the new sampling points. The sequence after downsampling is denoted as { , , … …}, the rate drops to In this example, N is set to 30, meaning the signal rate after downsampling is... .

[0031] The acquisition module uses the downsampled sequence from step (2) to acquire different satellite signals sequentially. Upon successful acquisition, it provides the Doppler frequency offset of the current satellite signal. The pseudo-code phase is then transmitted to the corresponding tracking channel. Doppler frequency offset. The frequency of the local carrier of the tracking channel is initialized, and the pseudo-code phase is used to calculate the start time of the tracking channel. After the acquisition module completes the initialization of the corresponding tracking channel, it moves on to the acquisition of the next satellite. Once all satellites have been acquired, the acquisition module stops working.

[0032] (3) Instantiate two single-port RAM memories of the same size in the FPGA to cache the downsampled data. The depth of each RAM memory is 32768, ensuring that the data depth that can be cached is greater than the duration of the preceding data by 6ms.

[0033] (4) Read the data cached in the ping-pong RAM in step (3) using the system clock at a rate of 75MHz, and send it to each tracking channel in sequence; when the data playback of each data segment is completed, the FPGA gives an interrupt flag, and the CPU software processes the tracking and demodulation of the data segment.

[0034] (5) A local carrier signal with orthogonal phase is generated inside the tracking channel and mixed with the input intermediate frequency signal to achieve orthogonal downconversion;

[0035] Intermediate frequency input signal The local carrier is Multiplying the two together, we get:

[0036] .

[0037] Each tracking channel corresponds to one satellite, and each tracking channel corresponds to a local carrier. Based on the Doppler frequency offset value obtained after successful satellite acquisition Calculations show that , for The random value.

[0038] (6) Baseband signal after quadrature downconversion Correlation operations are performed with the locally generated pseudocode sequence to obtain two mutually orthogonal integral outputs, I (in-phase) and Q (orthogonal), completing the despreading of the original transmitted signal. Three pseudocode sequences, denoted as E, P, and L, are generated locally, with phase differences of half a chip, and are respectively correlated with... After performing the correlation calculation, three correlation values ​​are obtained, denoted as... , , ,in Used in a carrier tracking loop to keep the local carrier frequency synchronized with the received signal carrier frequency; and Used in a pseudocode tracking loop to keep the local pseudocode and the received signal pseudocode in phase synchronization. In this embodiment, the symbol rate of the QPSK signal is... The integration time for the related operations is 0.5ms.

[0039] (7) After receiving the interrupt flag in step (4), the CPU software reads the relevant value validity flag and the corresponding value in the tracking channel. Correlation value; calculate the frequency difference between the local carrier and the received signal using preamble data and update the loop frequency control word. Take the integral value corresponding to two adjacent symbols and define...

[0040]

[0041]

[0042] Frequency error In the formula To find the arctangent function in the four quadrants, a lookup table method is used. This refers to the integration time of the relevant operations, in this embodiment. The loop control word uses edge-up updating; Update the local carrier within the tracking channel. The value is used to complete the subsequent orthogonal downconversion.

[0043] (8) Based on the calculation in step (7), the following is obtained: The information bits are obtained using differential demodulation. Each symbol of a QPSK modulated signal contains two bits of information, such as... Figure 2 As shown: The following method was used for analysis, which also solved the problem of 90° phase ambiguity in QPSK signal demodulation. Among them, "-" indicates the inversion of "0" or "1".

[0044]

[0045] This invention discloses a low-Earth orbit satellite QPSK signal demodulation method based on data buffering. By buffering and processing the intermediate frequency data, the coherent integration time during the signal acquisition stage is extended, thereby improving acquisition sensitivity and frequency estimation accuracy. Simultaneously, a frequency-locked loop and differential demodulation method are employed to accelerate the demodulation speed of the QPSK signal message data. This invention is highly effective for receiving high-dynamic burst signals with short preamble sequences.

[0046] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of this application and not to limit it. Although this application has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the spirit and scope of the technical solutions of this application, and all such modifications and substitutions should be covered within the scope of the claims of this application.

Claims

1. A method for demodulating low-Earth orbit satellite QPSK signals based on data buffering, characterized in that, Includes the following steps: Step 1: The antenna receives the signal transmitted by the low-orbit satellite, performs down-conversion processing through the radio frequency front-end, and then completes sampling through analog-to-digital conversion to obtain the digital intermediate frequency signal; Step 2: The obtained digital intermediate frequency signal is preprocessed and downsampled using digital preprocessing techniques; Step 3: Capture the downsampled intermediate frequency signal to complete the initialization and activation settings of each tracking channel; at the same time, write the downsampled intermediate frequency signal into the digital buffer in the FPGA for buffering. Step 4: Read the intermediate frequency data from the digital buffer according to the system clock and play it back to each open tracking channel. After the data playback is completed, the FPGA issues an interrupt flag. Step 5: Each tracking channel generates its own local carrier signal with orthogonal phase, and performs a mixing operation with the playback intermediate frequency signal to achieve orthogonal downconversion; Step 6: Perform correlation operations on the baseband signal after orthogonal downconversion and the local pseudocode sequence, extract the mutually orthogonal I and Q integral values, and complete the despreading of the original transmitted signal; Step 7: Based on the obtained I and Q integral values, a frequency error estimate is calculated using a frequency-locked loop and feedback adjustment is performed to quickly converge the local carrier frequency to the received signal frequency; in addition, based on the obtained I and Q integral values, the local pseudocode is kept in phase with the received signal pseudocode. Step 8: Use differential demodulation to perform data recovery processing on the received baseband signal to complete information demodulation.

2. The method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, The preprocessing in step 2 includes filtering the obtained digital intermediate frequency signal using a digital filter to achieve out-of-band noise suppression.

3. The method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, Step 2 downsampling involves performing downsampling on the filtered digital intermediate frequency signal by summing up multiple consecutive sampling points, thereby reducing the signal rate.

4. The method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, The digital buffer in step 3 is implemented using ping-pong RAM, which means opening two RAM spaces of the same size and alternating between writing and reading operations on the two RAMs to eliminate read-write conflicts.

5. The method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, The storage depth of the digital buffer should be greater than the length of the preamble data after downsampling.

6. The method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, In step 4, the system clock frequency is greater than or equal to the digital buffer write clock frequency multiplied by the number of tracking channels.

7. The method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, In step 7, the frequency-locked loop uses a four-quadrant arctangent frequency discriminator to estimate the frequency error.

8. The method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, The frequency-locking loop mentioned in step 7 uses a second-order loop filter.

9. A method for demodulating low-Earth orbit satellite QPSK signals based on data buffering according to claim 1, characterized in that, In step 8, the differential demodulation method extracts information bits by comparing the phase changes of adjacent symbols.

Images