A low-orbit satellite communication system time difference estimation method based on burst time slot capture

Abstract

The application discloses a low-orbit satellite communication system time difference estimation method based on burst time slot capture. In the low-orbit satellite communication system, the satellite and the ground are synchronized with the same time reference, but due to the influence of the low-orbit satellite-ground system time precision, the synchronization is not complete, and even after compensation, there is an unknown residual time difference. Therefore, the burst signal sent by the ground terminal in the low-orbit satellite communication system will be ahead of or lag behind the time slot, the demodulator on the low-orbit satellite accurately calculates the time difference value of the burst signal ahead of or lagging behind the current time slot by using the mathematical relationship between the time counting of each time slot and the capture of the burst signal, takes the time difference value as the time difference estimation value, packs the time difference value as part of the demodulation information, and uploads the time difference value to the upper layer of the satellite for distribution. On the basis of the pre-calculated time difference, the ground terminal repeatedly compensates and corrects the time difference estimation value distributed by the satellite, and the accuracy of the satellite-ground closed-loop time difference adjustment is improved.

Description

Technical Field

[0001] This invention discloses a time difference estimation method for low-Earth orbit satellite communication systems based on burst time slot acquisition, belonging to the field of communication signal processing. Background Technology

[0002] In low-Earth orbit (LEO) mobile communication systems, a regenerative forwarding processing mode is used, employing an FDMA / TDMA communication architecture. The system consists of four independent beams, each divided into four time slots, supporting high-speed communication for 16 burst users. Burst communication between satellite and ground is affected by dynamically changing time delays, generally requiring tracking and pre-compensation by the ground terminal. Time difference information is typically calculated using ephemeris time-of-delay (TOD), second pulses, and orbital information. However, due to issues with the accuracy of terminal time difference compensation, even after compensation, a residual time difference still exists between satellite and ground. Excessive time difference can cause the burst signal transmitted by the terminal to be ahead or behind the start of the time slot, potentially leading to failure in satellite acquisition and demodulation of the burst signal. Therefore, in this LEO satellite communication system, the onboard demodulator is required to estimate the residual time difference of the burst signal transmitted by the ground terminal in each time slot, report the result to the upper layer, and finally transmit it back to the ground as a reference for the ground terminal to supplement the time difference again. Summary of the Invention

[0003] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide a time difference estimation method for low-Earth orbit satellite communication systems based on burst time slot acquisition. This solves the problem that excessive residual time difference between satellite and ground in low-Earth orbit communication systems causes burst signals sent by terminals to be ahead or behind the start position of the time slot, which may lead to the failure of satellite acquisition and demodulation of burst signals.

[0004] The technical solution of this invention is: a time difference estimation method for low-Earth orbit satellite communication systems based on burst time slot acquisition, the method comprising the following steps:

[0005] S1. The ground terminal pre-calculates the satellite-to-ground time difference based on the ephemeris and calibrates the ground terminal's clock according to the satellite-to-ground time difference, so that the satellite and ground systems have the same time reference.

[0006] S2. The ground terminal adjusts the uplink burst signal transmission time before the burst time slot arrives, based on the estimated satellite-to-ground delay value and transmits the uplink burst signal, so that the uplink burst signal reaches the low-orbit satellite at the start of the burst time slot.

[0007] S3. The low-orbit satellite captures the uplink burst signal preamble in the corresponding time slot and calculates the satellite-to-ground residual time difference of the uplink burst signal in the current time slot based on the transmission time occupied by the uplink burst signal preamble.

[0008] S4. The low-orbit satellite transmits the residual time difference between the satellite and the ground to the ground terminal.

[0009] S5. The ground terminal uses the residual time difference between the satellite and the ground transmitted by the low-orbit satellite to compensate and correct the pre-calculated time difference between the satellite and the ground, thereby realizing the closed-loop time difference adjustment between the satellite and the ground.

[0010] Preferably, the low-orbit satellite is equipped with a physical layer on-board demodulator, an upper-layer network processor, and a lower-planet on-board modulator;

[0011] The physical layer on-board demodulator captures the uplink burst signal preamble in the corresponding time slot of the low-Earth orbit satellite. Based on the transmission time occupied by the uplink burst signal preamble, it calculates the satellite-to-ground residual time difference of the uplink burst signal in the current time slot, packages the satellite-to-ground residual time difference into the current demodulated data, and uploads it to the upper-layer network processor.

[0012] The upper-layer network processor forwards the demodulated data to the lower-layer modulator;

[0013] The modulator on the lower planet sends downlink signals to transmit the time difference value to the ground.

[0014] Preferably, the period length of the burst time slot is 20ms. The process by which the physical layer on-board demodulator calculates the satellite-to-ground residual time difference of the uplink burst signal in the current time slot is as follows:

[0015] At the beginning of each time slot, the time count restarts and is recorded as the time slot time count value.

[0016] The uplink burst signal leader is captured and demodulated, the data in the demodulated uplink burst signal is stored, and the data storage address is recorded;

[0017] At the synchronization moment of the uplink burst signal preamble, the capture flag is obtained, the time slot count is recorded, and the time T1 of the capture moment relative to the start moment of the burst time slot is calculated based on the time slot count. Based on the known preamble symbol length, the transmission time T2 occupied by the uplink burst signal preamble is calculated.

[0018] The satellite-to-ground residual time difference T is calculated based on the time T1 between the capture time and the start time of the burst time slot and the transmission time T2 occupied by the uplink burst signal preamble.

[0019] Preferably, the time T1 between the capture time and the start time of the burst time slot is calculated using the following formula:

[0020] T1=n·b

[0021] Where n is the number of bits in the preamble (in bits), and b is the preamble bit rate.

[0022] Preferably, the method for calculating the residual time difference T between satellite and ground is as follows:

[0023] If the value of T1 is less than 10ms, calculate the residual time difference between satellite and ground, T = T1 - T2;

[0024] If the T1 value is greater than or equal to 10ms, calculate the residual time difference between the satellite and the ground, T = 20ms - T1 + T2.

[0025] Preferably, when T is positive, the residual time difference between satellite and ground is determined as the lag time difference value, and the uplink burst signal is considered to lag behind the burst time slot;

[0026] When T is negative, the residual time difference between the satellite and the ground is determined to be the lead time difference value, and the uplink burst signal is considered to lead the burst time slot.

[0027] Preferably, the method for the ground terminal to compensate and correct the pre-calculated satellite-to-ground time difference using the residual time difference transmitted by the low-orbit satellite is as follows:

[0028] When the residual time difference between satellite and ground is a lagging time difference value, send an uplink burst signal in advance;

[0029] When the residual time difference between satellite and ground is greater than the lead time difference, the uplink burst signal is sent with a delay.

[0030] The advantages of this invention compared to the prior art are:

[0031] (1) This invention uses the mathematical reasoning relationship between each time slot time and the capture of the burst signal to accurately calculate the time when the burst signal is ahead or behind the current time slot, which has theoretical and practical verification.

[0032] (2) The on-board demodulator of this invention packages the time difference value as part of the demodulation information and uploads it to the upper layer of the satellite for transmission. Based on the pre-calculated time difference, the ground terminal repeatedly compensates and corrects the estimated time difference value transmitted from the satellite to the ground, thereby improving the accuracy of the satellite-ground closed-loop time difference adjustment. Attached Figure Description

[0033] Figure 1 A block diagram of the satellite-to-ground time difference correction process for low-Earth orbit communication systems;

[0034] Figure 2 Block diagram of physical frame format for high-speed communication in low-Earth orbit satellite user links;

[0035] Figure 3 Block diagram for transmitting normal time-slot signals to ground terminals;

[0036] Figure 4 Block diagram for transmitting lead or lag time slot signals to ground terminals;

[0037] Figure 5 The time difference value is a positive block diagram representing the burst signal lagging behind the time slot.

[0038] Figure 6The diagram shows a burst signal that precedes the time slot, with a negative time difference value. Detailed Implementation

[0039] The invention will now be further described with reference to the accompanying drawings.

[0040] This invention provides a time difference estimation method for low-Earth orbit (LEO) satellite communication systems based on burst time slot acquisition. This method achieves time synchronization under a unified ephemeris and time synchronization between the LEO satellite and ground systems. Simultaneously, the onboard demodulator uses ephemeris TOD and second pulse information periodically to generate burst time slot initiation pulses. Then, by utilizing the acquisition information of the burst leader in the uplink burst signal and the mathematical relationship between acquisition time and time slot count, the residual time difference between the satellite and ground systems in the current time slot is calculated. This time difference value is packaged as part of the demodulation information and uploaded to the upper satellite layer for transmission. The ground terminal, based on the pre-calculated time difference, repeatedly compensates and corrects the estimated time difference values ​​transmitted from the satellite to the ground, ensuring the accuracy of the satellite-ground closed-loop time difference adjustment.

[0041] like Figure 1 As shown, the satellite-to-ground time difference correction process for a low-Earth orbit satellite communication system:

[0042] S1. The ground terminal pre-calculates the satellite-to-ground time difference based on the ephemeris and calibrates the ground terminal's clock according to the satellite-to-ground time difference, so that the satellite and ground systems have the same time reference.

[0043] S2. The ground terminal adjusts the uplink burst signal transmission time before the burst time slot arrives, based on the estimated satellite-to-ground delay value and transmits the uplink burst signal, so that the uplink burst signal reaches the low-orbit satellite at the start of the burst time slot.

[0044] S3. The low-orbit satellite captures the uplink burst signal preamble in the corresponding time slot and calculates the satellite-to-ground residual time difference of the uplink burst signal in the current time slot based on the transmission time occupied by the uplink burst signal preamble.

[0045] S4. The low-orbit satellite transmits the residual time difference between the satellite and the ground to the ground terminal.

[0046] S5. The ground terminal uses the residual time difference between the satellite and the ground transmitted by the low-orbit satellite to compensate and correct the pre-calculated time difference between the satellite and the ground, thereby realizing the closed-loop time difference adjustment between the satellite and the ground.

[0047] Repeat the above process, making continuous corrections.

[0048] The low-orbit satellite is equipped with a physical layer on-board demodulator, an upper-layer network processor, and a lower-planet on-board modulator.

[0049] The physical layer on-board demodulator captures the uplink burst signal preamble in the corresponding time slot of the low-Earth orbit satellite. Based on the transmission time occupied by the uplink burst signal preamble, it calculates the satellite-to-ground residual time difference of the uplink burst signal in the current time slot, packages the satellite-to-ground residual time difference into the current demodulated data, and uploads it to the upper-layer network processor.

[0050] The upper-layer network processor forwards the demodulated data to the lower-layer modulator;

[0051] The modulator on the lower planet sends downlink signals to transmit the time difference value to the ground.

[0052] During initial access, the ground terminal estimates the transmission time based on ephemeris information. After receiving the information from the ground terminal, the onboard demodulator estimates the residual time difference and uploads it as part of the demodulation information to the upper-layer onboard network processor. The network processor attaches this information to the next satellite downlink signal and sends it to the ground terminal via the downlink modulator. The ground terminal then corrects the compensation value again before starting the transmission of the uplink signal in the next time slot. Throughout this process, the residual time difference is a slowly varying process; through repeated estimation and correction, the normal operation of the satellite-to-ground communication link is ensured.

[0053] like Figure 2 As shown, the physical frame structure of the low Earth orbit system's satellite-to-ground uplink burst signal is such that the ground terminal sends burst signals according to the access-allocated time slot group physical frames.

[0054] like Figure 3 As shown, under normal circumstances, each ground terminal sends an uplink burst signal physical frame at the start of the requested time slot. This ensures that the time slot and the burst signal are basically aligned when they reach the onboard demodulator. If the burst signal falls into another time slot, it will affect the normal communication of other time slot terminals.

[0055] like Figure 4 As shown, due to the time accuracy limitations of low-Earth orbit (LEO) satellite-to-ground systems, they are not perfectly synchronized. Even after compensation, there will be an unknown residual time difference, requiring real-time estimation and calibration compensation. Of course, the residual time difference should not exceed half a time slot. Therefore, burst signals transmitted by ground terminals often fall into two categories: either the burst signal is ahead of the current time slot, or it is behind the current time slot. At the onboard demodulator, the burst signal has actually fallen into other adjacent time slots. If not corrected in time, the time difference accumulates over multiple frames, ultimately leading to onboard demodulation failure and affecting user communication functions.

[0056] A time difference estimation method for low-Earth orbit (LEO) satellite communication systems based on burst time slot acquisition. For situations where burst signals in LEO satellite communication systems lead or lag behind time slots, the method includes the following processing steps:

[0057] At the beginning of each time slot, the time count restarts and is recorded as the time slot time count value.

[0058] The uplink burst signal leader is captured and demodulated, the data in the demodulated uplink burst signal is stored, and the data storage address is recorded;

[0059] At the synchronization moment of the uplink burst signal preamble, the capture flag is obtained, the time slot count is recorded, and the time T1 of the capture moment relative to the start moment of the burst time slot is calculated based on the time slot count. Based on the known preamble symbol length, the transmission time T2 occupied by the uplink burst signal preamble is calculated.

[0060] The satellite-to-ground residual time difference T is calculated based on the time T1 between the capture time and the start time of the burst time slot and the transmission time T2 occupied by the uplink burst signal preamble.

[0061] The time T1 between the capture time and the start time of the burst time slot is calculated using the following formula:

[0062] T1=n·b

[0063] Where n is the number of bits in the preamble (in bits), and b is the preamble bit rate.

[0064] The method for calculating the residual time difference T between satellite and ground is as follows:

[0065] If the value of T1 is less than 10ms, calculate the residual time difference between satellite and ground, T = T1 - T2;

[0066] If the T1 value is greater than or equal to 10ms, calculate the residual time difference between the satellite and the ground, T = 20ms - T1 + T2.

[0067] When T is positive, the residual time difference between satellite and ground is determined to be the lag time difference value, and the uplink burst signal is considered to lag behind the burst time slot; for example... Figure 5 As shown.

[0068] When T is negative, the residual time difference between satellite and ground is determined as the lead time difference value, and the uplink burst signal is considered to lead the burst time slot. For example... Figure 6 As shown.

[0069] The method for ground terminals to compensate and correct pre-calculated satellite-to-ground time differences using the residual time difference transmitted by low-Earth orbit satellites is as follows:

[0070] When the residual time difference between satellite and ground is a lagging time difference value, send an uplink burst signal in advance;

[0071] When the residual time difference between satellite and ground is greater than the lead time difference, the uplink burst signal is sent with a delay.

[0072] In summary, the method of this invention utilizes the mathematical relationship between time counting in each time slot and the acquisition of burst signals to accurately calculate the time difference between the burst signal leading or lagging behind the current time slot, which is used as a time difference estimate. This time difference value is then packaged as part of the demodulation information and uploaded to the upper layer of the satellite for transmission. Based on the pre-calculated time difference, the ground terminal repeatedly compensates and corrects the time difference estimate transmitted from the satellite, thereby improving the accuracy of the satellite-ground closed-loop time difference adjustment. After on-orbit testing and verification, the performance meets the requirements.

[0073] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A time difference estimation method for low-Earth orbit satellite communication systems based on burst time slot acquisition, characterized in that... Includes the following steps: The ground terminal pre-calculates the satellite-to-ground time difference based on the ephemeris, and calibrates the ground terminal's clock according to the satellite-to-ground time difference, so that the satellite and ground systems have the same time reference; Based on the estimated satellite-to-ground delay according to the time difference value, the ground terminal adjusts the uplink burst signal transmission time before the burst time slot arrives and transmits the uplink burst signal so that the uplink burst signal reaches the low-orbit satellite at the start of the burst time slot. The low-orbit satellite captures the uplink burst signal preamble in the corresponding time slot, and calculates the satellite-to-ground residual time difference of the uplink burst signal in the current time slot based on the transmission time occupied by the uplink burst signal preamble. Low-Earth orbit satellites transmit the residual time difference between the satellite and the ground to ground terminals; The ground terminal uses the residual time difference between satellite and ground transmitted by the low-orbit satellite to compensate and correct the pre-calculated satellite-to-ground time difference, thereby achieving closed-loop time difference adjustment between satellite and ground. The period length of the burst time slot is 20ms. The process by which the physical layer on-board demodulator calculates the residual time difference between the uplink burst signal and the ground in the current time slot is as follows: At the beginning of each time slot, the time count restarts and is recorded as the time slot time count value. The uplink burst signal leader is captured and demodulated, the data in the demodulated uplink burst signal is stored, and the data storage address is recorded; At the synchronization moment of the uplink burst signal preamble, the capture flag is obtained, the time slot count is recorded, and the time T1 of the capture moment relative to the start moment of the burst time slot is calculated based on the time slot count. Based on the known preamble symbol length, the transmission time T2 occupied by the uplink burst signal preamble is calculated. Calculate the satellite-to-ground residual time difference T based on the time T1 between the acquisition time and the start time of the burst time slot and the transmission time T2 occupied by the uplink burst signal preamble; The method for calculating the residual time difference T between satellite and ground is as follows: If the T1 value is less than 10ms, calculate the residual time difference between satellite and ground, T = T1 - T2; If the T1 value is greater than or equal to 10ms, calculate the residual time difference between the satellite and the ground, T = 20ms - T1 + T2.

2. The time difference estimation method for a low-Earth orbit satellite communication system based on burst time slot acquisition according to claim 1, characterized in that... The low-orbit satellite is equipped with a physical layer on-board demodulator, an upper-layer network processor, and a lower-planet on-board modulator. The physical layer on-board demodulator captures the uplink burst signal preamble in the corresponding time slot of the low-Earth orbit satellite. Based on the transmission time occupied by the uplink burst signal preamble, it calculates the satellite-to-ground residual time difference of the uplink burst signal in the current time slot, packages the satellite-to-ground residual time difference into the current demodulated data, and uploads it to the upper-layer network processor. The upper-layer network processor forwards the demodulated data to the lower-layer modulator; The modulator on the lower planet sends downlink signals to transmit the time difference value to the ground.

3. The time difference estimation method for a low-Earth orbit satellite communication system based on burst time slot acquisition according to claim 1, characterized in that... The time T1 between the capture time and the start time of the burst time slot is calculated using the following formula: =n·b in, The number of bits in the header data, in bits. This is the preamble bit rate.

4. The time difference estimation method for a low-Earth orbit satellite communication system based on burst time slot acquisition according to claim 1, characterized in that... : When T is positive, the residual time difference between satellite and ground is determined to be the lag time difference value, and the uplink burst signal is considered to lag behind the burst time slot; When T is negative, the residual time difference between the satellite and the ground is determined to be the lead time difference value, and the uplink burst signal is considered to lead the burst time slot.

5. The time difference estimation method for a low-Earth orbit satellite communication system based on burst time slot acquisition according to claim 4, characterized in that... The method for ground terminals to compensate and correct pre-calculated satellite-to-ground time differences using the residual time difference transmitted by low-Earth orbit satellites is as follows: When the residual time difference between satellite and ground is a lagging time difference value, send an uplink burst signal in advance; When the residual time difference between satellite and ground is greater than the lead time difference, the uplink burst signal is sent with a delay.

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