A satellite communication system and its time synchronization method

Abstract

An embodiment of the present invention discloses a satellite communication system and its time synchronization method. The satellite communication system includes a gateway, a terminal, and a satellite; the method includes: using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame, the duration of the first X frames being greater than the round-trip time between the terminal and the gateway, the actual transmission time of the time synchronization signal having a first delay transmission time relative to the frame header of the first X frames, N being a positive integer, and X being a positive integer; using the terminal to receive the time synchronization signal sent by the satellite, calculating a second delay transmission time according to the duration of the first X frames, the round-trip time between the terminal and the gateway, and the first delay transmission time, and sending the Nth terminal frame corresponding to the Nth gateway frame at the end moment of the second delay transmission time so that the Nth gateway frame is time-synchronized with the Nth terminal frame.

Description

Technical Field

[0001] The present invention relates to satellite communication technology. More specifically, it relates to a satellite communication system and its time synchronization method. Background Art

[0002] Currently, due to the working mechanism of multiple frequency-time division multiple access (MF-TDMA) adopted by the uplink of the satellite communication system, there are problems that when a terminal accesses the satellite communication system, it is not time-synchronized with the gateway system, the transceiver data of multiple terminals overlap in the time domain and interfere with each other, and cannot be received by the gateway at a predetermined time. Summary of the Invention

[0003] An object of the present invention is to provide a satellite communication system and its time synchronization method to solve at least one of the problems existing in the prior art.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] In a first aspect of the present invention, a time synchronization method for a satellite communication system is provided. The satellite communication system includes a gateway, a terminal, and a satellite. The method includes:

[0006] Using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames before the Nth gateway frame. The duration of the first X frames is greater than the round-trip time from the terminal to the gateway. The actual transmission time of the time synchronization signal has a first delay transmission time relative to the frame header of the first X frames. N is a positive integer, and X is a positive integer;

[0007] Using the terminal to receive the time synchronization signal sent by the satellite, calculating a second delay transmission time according to the duration of the first X frames, the round-trip time from the terminal to the gateway, and the first delay transmission time, and sending the Nth terminal frame corresponding to the Nth gateway frame at the end moment of the second delay transmission time so that the Nth gateway frame is time-synchronized with the Nth terminal frame.

[0008] Optionally, the satellite is a regenerative satellite, and the second delay transmission time is:

[0009]

[0010] In the formula, is the second delay transmission time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the first delay transmission time.

[0011] Optionally, the step of using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame further includes:

[0012] The gateway includes a link layer gateway and a physical layer gateway. The first X frames are N gateway frames including 1 continuous beam. Use the link layer gateway to send the time synchronization signal, the transmission frame number N-X of the physical layer gateway, and n empty symbols at the frame header of the (N-X-1)th gateway frame, where n is a positive integer;

[0013] Use the physical layer gateway to send the time synchronization signal to the satellite at the moment of delaying the n empty symbols at the frame header of the (N-X)th gateway frame;

[0014] Or

[0015] The step of using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame further includes:

[0016] The gateway includes a link layer gateway and a physical layer gateway. The first X frames are N gateway frames including S hopping beams. Use the link layer gateway to start from the frame header of the (N-X-1)th gateway frame and send S time synchronization signals, S transmission frame numbers N-X of the physical layer gateway, and S groups of empty symbols at the starting point and S-1 offset time points relative to the starting point respectively. Each group of empty symbols includes n empty symbols, and S is a positive integer;

[0017] Use the physical layer gateway to send a total of S time synchronization signals to the satellite at the moments of delaying n empty symbols at the starting point and S-1 offset time points relative to the starting point respectively.

[0018] Optionally, the satellite is a transparent transponder satellite, and the second delay transmission time is:

[0019]

[0020] wherein, is the second delay transmission time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, , is the transmission delay time from the gateway to the satellite, is the transmission delay time from the satellite to the terminal, is the transmission delay time from the terminal to the gateway, , is the transmission delay time from the terminal to the satellite, is the transmission delay time from the satellite to the gateway, is the first delay transmission time.

[0021] Optionally, the sending of the Nth terminal frame corresponding to the Nth gateway frame at the end moment of the second delay transmission time to synchronize the time of the Nth gateway frame and the Nth terminal frame further includes:

[0022] Adding a first timestamp to the time synchronization signal by the physical layer of the terminal;

[0023] Parsing the time synchronization signal with the first timestamp added by the link layer of the terminal to obtain the second delay transmission time;

[0024] Calculating the frame header of the Nth terminal frame of the physical layer of the terminal by the link layer of the terminal according to the time of the first timestamp, the second delay transmission time, and the counting interval of the physical layer of the terminal;

[0025] Adding a second timestamp to the time synchronization signal by the link layer of the terminal, and presetting the internal transmission delay time for the time synchronization signal to be transmitted from the physical layer of the terminal to the link layer of the terminal;

[0026] Calculating the frame header of the Nth terminal frame of the terminal by the link layer of the terminal according to the time of the second timestamp, the second delay transmission time, and the internal transmission delay time.

[0027] Optionally, the using the terminal to receive the time synchronization signal sent by the satellite further includes:

[0028] Using the terminal to search for the random access beam sent by the gateway, and when the random access beam covers the area where the terminal is located, the terminal locks the downlink data stream;

[0029] Using the terminal to receive the time synchronization signal, K preset moments for sending random access requests corresponding to K transmission opportunities, and the time-frequency parameters of the uplink access channel in the downlink data stream, where the K preset moments have a third delay transmission time relative to the frame header of the Nth gateway frame, and K is a positive integer;

[0030] After the time synchronization between the Nth gateway frame and the Nth terminal frame, the method further includes:

[0031] Using the gateway to direct the random reception beam to the terminal at the K preset moments;

[0032] Use the terminal to select one or more preset moments to send a random access request according to the K sending opportunities;

[0033] When the first delayed transmission time is equal to the third delayed transmission time, the i preset moment is:

[0034]

[0035] Wherein, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the i th sending opportunity randomly selected by the terminal, i = 1, 2,..., K.

[0036] Optionally, the step of using the terminal to receive the time synchronization signal sent by the satellite and calculating a second delayed transmission time according to the duration of the first X frames, the round-trip time from the terminal to the gateway, and the first delayed transmission time further includes:

[0037] Use the gateway to measure the delayed arrival time or the early arrival time of the frame header of the Nth terminal frame relative to the frame header of the Nth gateway frame when the Nth terminal frame arrives at the gateway, and send the delayed arrival time or the early arrival time to the terminal;

[0038] Use the terminal to calculate the second delayed transmission time according to the delayed arrival time or the early arrival time;

[0039] The second delayed transmission time is:

[0040]

[0041] Wherein, is the second delayed transmission time, is the duration of the first X frames, is the initial value of the transmission delay time from the terminal to the gateway, is the delayed arrival time or the early arrival time, is the first delayed transmission time.

[0042] A second aspect of the present invention provides a satellite communication system, which includes a gateway, a terminal, and a satellite;

[0043] The gateway is used to send a time synchronization signal containing a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame. The duration of the first X frames is greater than the round-trip time from the terminal to the gateway. The actual transmission time of the time synchronization signal has a first delay transmission time relative to the frame header of the first X frames. N is a positive integer, and X is a positive integer;

[0044] The terminal is used to receive the time synchronization signal sent by the satellite, calculate a second delay transmission time according to the duration of the first X frames, the round-trip time from the terminal to the gateway, and the first delay transmission time, and send the Nth terminal frame corresponding to the Nth gateway frame at the end moment of the second delay transmission time so that the Nth gateway frame is time-synchronized with the Nth terminal frame.

[0045] Optionally, the satellite is a regenerative satellite, and the second delay transmission time is:

[0046]

[0047] In the formula, is the second delay transmission time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the first delay transmission time.

[0048] Optionally, the satellite is a transparent transponder satellite, and the second delay transmission time is:

[0049]

[0050] In the formula, is the second delay transmission time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, , is the transmission delay time from the gateway to the satellite, is the transmission delay time from the satellite to the terminal, is the transmission delay time from the terminal to the gateway, , is the transmission delay time from the terminal to the satellite, is the transmission delay time from the satellite to the gateway, is the first delay transmission time.

[0051] The beneficial effects of the present invention are as follows:

[0052] The technical solution of the present invention enables the terminal to perform time synchronization when accessing the satellite communication system and the gateway system, ensuring that the transmitted and received data of multiple terminals do not overlap in the time domain and do not interfere with each other, and can be received by the gateway at a predetermined time. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The following further describes in detail the specific embodiments of the present invention with reference to the accompanying drawings.

[0054] Figure 1 Schematic diagram showing the superframe structure with a 40 ms frame period in the related art.

[0055] Figure 2 Schematic diagram showing the superframe structure with a 20 ms frame period in the related art.

[0056] Figure 3 Schematic diagram showing the frame boundary between the terminal and the gateway in the related art.

[0057] Figure 4 Flowchart showing the time synchronization method provided by the embodiment of the present invention.

[0058] Figure 5 Schematic diagram showing the time synchronization method of the regenerative satellite provided by the embodiment of the present invention.

[0059] Figure 6 Schematic diagram showing the time synchronization method of the transparent transponder satellite provided by the embodiment of the present invention.

[0060] Figure 7 Schematic diagram showing the restart of the downlink data stream with beam hopping provided by the embodiment of the present invention.

[0061] Figure 8 Schematic diagram showing sending one time synchronization signal per frame provided by the embodiment of the present invention.

[0062] Figure 9 Schematic diagram showing sending multiple time synchronization signals per frame provided by the embodiment of the present invention.

[0063] Figure 10 Schematic diagram showing the transmission delay of the time synchronization signal provided by the embodiment of the present invention.

[0064] Figure 11 Schematic diagram showing the terminal time synchronization calculation provided by the embodiment of the present invention.

[0065] Figure 12 Schematic diagram showing the beam control of the terminal accessing the service point provided by the embodiment of the present invention.

[0066] Figure 13 Schematic diagram showing the measurement of the transmission delay provided by the embodiment of the present invention.

[0067] Figure 14 Shows the schematic architecture diagram of the transparent transponder satellite provided by the embodiments of the present invention.

[0068] Figure 15 Shows the schematic architecture diagram of the regenerative satellite provided by the embodiments of the present invention. Detailed implementation manners

[0069] To describe the present invention more clearly, the present invention will be further described below in conjunction with embodiments and drawings. Similar components in the drawings are denoted by the same reference numerals. Those skilled in the art should understand that the content described specifically below is illustrative rather than restrictive, and should not be used to limit the protection scope of the present invention.

[0070] In a satellite communication system, the uplink resources from the terminal to the gateway are organized into multiple uplink available resource groups, and a dynamic competition on-demand allocation mode is adopted within a group. A group includes multiple channels, with different system parameters configured, such as center frequency and symbol rate, and is managed by a link gateway. The resources at each frequency point are organized into consecutive super frames, each super frame contains a fixed number of data frames, each data frame contains a fixed number of time slots, and the number of time slots included in each frame of channels with different symbol rates is different. Different physical bursts are composed of different numbers of time slots, but one time slot cannot be allocated to two physical bursts. Each frame is divided into multiple time slots. For example, each time slot contains 40 symbols, and the number of time slots per frame is determined by the transmission symbol rate and the frame length. Essentially, a time slot is also a time unit, and the granularity of a time slot is 40 symbols. For the uplink transmission side, two consecutive physical frames of the same user can span time slots or not at the transmission moment, but the physical frames of different users must be strictly aligned at the time slot boundary.

[0071] As Figure 1 shown, the frame period is selected to be 40 ms in length. The frame period can also be selected to be 20 ms, 10 ms, or 5 ms, etc. Among them, when the frame period is 40 ms, a super frame contains 8 single frames, and the super frame time length is 8 * 40 = 320 ms. As Figure 2 shown, the frame period can also be selected to be 20 ms in length, that is, the frame period is 20 ms, a super frame contains 16 single frames, and its time length is 16 * 20 = 320 ms. As Figure 3As shown, the gateway is defined as continuous gateway frames on the time axis, such as the Nth gateway frame 501 and the (N + 1)th gateway frame 502, etc. A gateway frame is divided into multiple time slots. According to the corresponding principle, there are also continuous terminal frames on the time axis for the terminal, such as the Nth terminal frame 5101 of the first terminal, the (N + 1)th terminal frame 5102 of the first terminal; the Nth terminal frame 5201 of the second terminal, the (N + 1)th terminal frame 5202 of the second terminal; the Nth terminal frame 5301 of the third terminal, the (N + 1)th terminal frame 5302 of the third terminal. Since the distance between each terminal and the gateway is different, the time delay for the data sent by the terminal to reach the gateway is different. Therefore, it is necessary to adjust the time for each terminal to send terminal frames to different positions and ensure that the arrival time at the gateway after transmission delay is exactly the start time of the gateway frame. Based on the principle of multi-frequency time division multiple access, different terminals are aligned with the gateway on the time axis, but different terminals use different time slots within a frame for data transmission to avoid interference with each other.

[0072] In summary, since the uplink of the satellite communication system adopts the working system of multi-frequency time division multiple access, there are problems that when a terminal accesses the satellite communication system, it is not synchronized with the gateway system time, the data transmission and reception of multiple terminals overlap in the time domain and interfere with each other, and the data cannot be received by the gateway at a predetermined time.

[0073] In view of this, as Figure 4 shown, an embodiment of the present invention provides a time synchronization method for a satellite communication system. The satellite communication system includes a gateway, a terminal, and a satellite. The method includes: using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames before the Nth gateway frame, the duration of the first X frames being greater than the round-trip time between the terminal and the gateway, the actual transmission time of the time synchronization signal having a first delay transmission time relative to the frame header of the first X frames, N being a positive integer, and X being a positive integer; using the terminal to receive the time synchronization signal sent by the satellite, calculating a second delay transmission time according to the duration of the first X frames, the round-trip time between the terminal and the gateway, and the first delay transmission time, and sending the Nth terminal frame corresponding to the Nth gateway frame at the end time of the second delay transmission time so that the Nth gateway frame is time-synchronized with the Nth terminal frame.

[0074] This embodiment can enable the terminal to be time-synchronized when accessing the satellite communication system and the gateway system, ensure that the data transmission and reception of multiple terminals do not overlap in the time domain and do not interfere with each other, and can be received by the gateway at a predetermined time.

[0075] In a possible implementation manner, the satellite is a regenerative satellite, and the second delay transmission time is:

[0076]

[0077] wherein, is the second delayed transmission time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the first delayed transmission time.

[0078] In a specific example, on the gateway side, the downlink physical layer transmitter and the uplink physical layer receiver use the same frame boundary (Frame Marker). On the terminal side, the frame boundary of the terminal physical layer transmitter is adjusted to align with the frame boundary of the uplink receiver. Alignment means that at the frame boundary, the physical layer data frame transmitted by the Nth terminal frame can reach the frame boundary of the Nth gateway frame of the uplink receiver after the delay of air interface transmission. Since the uplink receiver receives from multiple terminals and due to satellite or user terminal movement, the air interface delay of each user terminal is different and may change over time. When the Nth terminal frame of each user is aligned with the Nth gateway frame of the receiver, the data from different user terminals received by the receiver will not collide and can be correctly received. In fact, the Nth terminal frame of the user terminal and the Nth gateway frame of the receiver are not the same in terms of absolute time, and the difference between them is the one-way air interface time of each user terminal. The purpose of time synchronization is to align the Nth terminal frame of the transmitter of the user terminal with the Nth gateway frame of the receiver, even if the one-way air interface delay of each user terminal is time-varying.

[0079] In a specific example, on the terminal time axis, the superframe number or frame number (Frame Number, FN) is defined. FN is the frame number of the superframe or frame. If each superframe has 8 frames, mod(FN, 8)=0, where mod is the remainder function. That is to say, FN is the frame number, and when mod(FN, 8)=0, FN is also the superframe number. The purpose of time synchronization in the satellite communication system is to make the data sent at the frame boundary of the Nth terminal frame of the terminal reach the frame boundary of the Nth gateway frame of the on-board base station. To achieve this purpose, the on-board base station sends the time synchronization information including the Nth gateway frame regularly or irregularly, so that the terminal can achieve time synchronization with the on-board base station. If the relative position relationship between the satellite and the terminal remains stable, the on-board base station sends the time synchronization information of the Nth gateway frame regularly, such as sending the time synchronization information every 8 frames; if the relative position between the satellite and the on-board terminal changes rapidly, the on-board base station sends the time synchronization information of the Nth gateway frame irregularly to adapt to the rapid time change.

[0080] In a specific example, such as Figure 5As shown, for the on-board base station system when the satellite is a regenerative satellite, the basis for time synchronization is that the sending side and the receiving side of the on-board base station have the same time. That is, the receiving moment when the uplink gateway expects to receive the data of the Nth gateway frame and the sending moment when the terminal sends the data of the Nth terminal frame need to satisfy the time relationship in the above formula.

[0081] Furthermore, at a preset time interval in advance of the Nth gateway frame 6060 at the receiving end of the on-board base station, that is, the duration 601 of the previous X frames, at the starting boundary moment 600 of the duration 601 of the previous X frames, the gateway regularly sends a system time synchronization data signaling message, that is, a time synchronization signal (Super Frame Timing Reference, SFTR). The starting boundary of the corresponding frame number of the starting boundary of the super frame or frame carried in the time synchronization signal is set with a default frame period of 40 ms and can be changed through configuration.

[0082] Furthermore, the time synchronization signal cannot guarantee to be sent exactly at the starting boundary moment 600 of the duration 601 of the previous X frames. Depending on the implementation of the physical layer transmitter, it may be advanced or delayed. This advanced or delayed time interval is denoted as the first delayed transmission time 602, with the unit of T0, where T0 is the time unit of the physical layer clock counter; the unit of the first delayed transmission time 602 can also be a symbol or a time slot.

[0083] Furthermore, the time interval generated from the starting boundary moment 600 of the duration 601 of the previous X frames to the actual sending moment 608 of the time synchronization signal is uniformly labeled as the first delayed transmission time 602.

[0084] Furthermore, the time synchronization signal should be sent within one frame after the starting boundary moment 600 of the duration 601 of the previous X frames. The time synchronization signal can be sent multiple times to different terminals or terminal user groups, and the terminals that receive the time synchronization signal all perform time synchronization on the Nth gateway frame of the on-board base station.

[0085] Furthermore, Figure 5 also includes: gateway frame timing, terminal frame timing, terminal time axis, second delayed transmission time 603, transmission delay time 604 from gateway to terminal, transmission delay time 605 from terminal to gateway, pre-calculated first delayed transmission time 602, Nth gateway frame 6060, Nth terminal frame 6061, time synchronization signal 607, sending side 609, and receiving side 610.

[0086] Furthermore, the second delayed sending time 603 is the terminal delayed sending time, that is, the difference between the ideal receiving time when the terminal receives the time synchronization signal and the actual sending time when the data is sent; the duration 601 of the first X frames is the uplink gateway delayed receiving time, that is, the time difference between the ideal time of sending the time synchronization signal and the time when the data of the Nth gateway frame is scheduled to be received. For a geostationary orbit satellite (GEO), the default value is 580ms or 620ms, and for a satellite operating in a low earth orbit (LEO), the default time is 120ms; the first delayed sending time 602 is the sending delay of the time synchronization signal calculated in advance.

[0087] In a specific example, the information that the time synchronization signal needs to carry includes the frame number; the duration of the previous X frames, which is greater than the round-trip time from the terminal to the gateway, in ms; T0 is a configuration parameter, in ns; the first delayed sending time 602, which can be obtained by pre-calculation; the sending interval, in ms, the default value is 8 frames or 320ms, which is configurable.

[0088] In a possible implementation manner, the satellite is a transparent forwarding satellite, and the second delayed sending time is:

[0089]

[0090] In the formula, is the second delayed sending time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, , is the transmission delay time from the gateway to the satellite, is the transmission delay time from the satellite to the terminal, is the transmission delay time from the terminal to the gateway, , is the transmission delay time from the terminal to the satellite, is the transmission delay time from the satellite to the gateway, is the first delayed sending time.

[0091] In a specific example, for a satellite communication system with a base station deployed on the ground and a transparent forwarding satellite deployed on the satellite, the benchmark for time synchronization is that the time of the sending side and the receiving side of the ground base station are consistent. The ground base station sends a time synchronization signal to the terminal, and the terminal adjusts the time of the terminal frame boundary so that the frame number and frame boundary sent by the terminal are consistent with the frame number and frame boundary of the gateway frame at the receiving end of the ground base station when the signal arrives at the ground base station.

[0092] Further, the ground base station sends a time synchronization signal via a satellite. The time synchronization signal is parsed at the terminal, and the terminal synchronizes its time with the ground base station at the frame boundary of the Nth terminal frame according to the time information carried in the time synchronization signal. At this time, the time synchronization reference points for both transmission and reception are the ground base station.

[0093] In a specific example, as Figure 6 shown, Figure 6 includes: gateway frame timing, terminal frame timing, terminal time axis, duration of the first X frames 701, second delayed transmission time 702, transmission delay time from the gateway to the satellite 703, transmission delay time from the satellite to the terminal 704, transmission delay time from the terminal to the satellite 705, transmission delay time from the satellite to the gateway 706, first delayed transmission time 707, the Nth gateway frame 7080, the Nth terminal frame 7081, ideal time 700 of the duration of the first X frames 701, actual transmission time 709 of the time synchronization signal, transmission side 7010 and reception side 7011.

[0094] Further, the duration of the first X frames 701 is the reception time of the ground gateway side receiving channel delay, that is, the time difference between the ideal time 700 for sending the time synchronization signal and the scheduled reception time of the Nth gateway frame. For GEO or LEO, different default constant values are configured. For example, it is 560 ms for GEO and 120 ms for LEO.

[0095] Further, when the satellite or the terminal moves rapidly, the transmission delay time from the gateway to the terminal and the transmission delay time from the terminal to the gateway both change with time and need to be updated in a timely manner. The transmission delay time from the gateway to the terminal and the transmission delay time from the terminal to the gateway respectively include the delays of the feeder link and the user link, while in the on-board base station system, the transmission delay time from the gateway to the terminal and the transmission delay time from the terminal to the gateway only include the delays of the user link.

[0096] In a possible implementation manner, the step of using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame further includes: the gateway includes a link layer gateway and a physical layer gateway. The first X frames are N gateway frames including 1 continuous beam. The link layer gateway is used to send the time synchronization signal, the transmission frame number N-X of the physical layer gateway, and n empty symbols at the frame head of the (N-X-1)th gateway frame, where n is a positive integer; the physical layer gateway is used to send the time synchronization signal to the satellite at the moment of delaying the n empty symbols at the frame head of the (N-X)th gateway frame;

[0097] In a specific example, to support Beam Hopping, the on-board downlink transmitter needs to restart the data stream transmission sequence every once in a while according to the beam hopping schedule, that is, the data stream continuously transmitted every once in a while is interrupted once. The data stream continuously transmitted once is sent to a beam position on the ground. This time can be of equal length or variable length, and the specific duration is defined by the beam hopping schedule. For example, Figure 7 As shown, since data frames 801 and 802 cannot exactly fill the expected transmission time period, random symbols (Padding), such as random symbol 811 and random symbol 821, need to be added at the end of the previous transmission. Compared with the continuous data stream, these random symbols will waste the air interface resources, but this is an inevitable choice to support beam hopping. Every time beam hopping occurs, a time synchronization signal needs to be sent. Relative to the beam hopping position, the satellite communication system can predict and ensure that the time synchronization signal is sent at the specified position. The time at beam position 1, 810 = data frame 801 + random symbol 811; the time at beam position 2, 820 = data frame 802 + random symbol 821.

[0098] In a specific example, as Figure 8 shown, the modems (MODEMs) of the link layer gateway and the physical layer gateway of the on-board base station achieve frame synchronization through a 1PPS time synchronizer. Set the duration 901 of the first X frames, that is, send the time synchronization signal X frames in advance. The time synchronization signal of the Nth gateway frame 9020 of the link layer gateway needs to be sent in the (N - X)th gateway frame. The link layer gateway assembles the time synchronization signal during the duration 903 of the first X + 1 frames before the Nth gateway frame 9020, that is, assembles and encapsulates the time synchronization signal at the frame header of the (N - X - 1)th gateway frame to obtain the encapsulated time synchronization signal 905. Since the time synchronization signal is a special control signaling, the link layer gateway directly packets and sends it to the modem of the physical layer gateway. The physical layer gateway restarts the data stream in the (N - X)th gateway frame before the Nth gateway frame 9021 of the physical layer gateway, sends n dummy symbols 904 and then sends the time synchronization signal 906. Sending n dummy symbols before sending the time synchronization signal 906 is used for receiver fault tolerance. If the terminal receiver can capture a single frame, then n can be configured to 0. If a time synchronization signal is sent for each frame, then N is the frame number, and the corresponding transmission interval is the frame length, for example, 40 ms. If a time synchronization signal is sent for each superframe, then N is the superframe number.

[0099] In another possible implementation, the step of using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame further includes: the gateway includes a link layer gateway and a physical layer gateway, the first X frames are N gateway frames including S hopping beams, and the link layer gateway is used to start from the frame header of the (N - X - 1)th gateway frame, and send S time synchronization signals, the transmission frame numbers N - X of S physical layer gateways, and S groups of empty symbols at the starting point and S - 1 offset time points relative to the starting point respectively, each group of empty symbols includes n empty symbols, and S is a positive integer; the physical layer gateway is used to send a total of S time synchronization signals to the satellite at the starting point and at the moments delayed by n empty symbols at S - 1 offset time points relative to the starting point respectively.

[0100] In a specific example, as Figure 9 shown, in scenarios such as beam hopping or low-earth orbit satellite applications, it is allowed to send multiple time synchronization signals within one frame along with beam hopping, for example, once every 1 ms or 5 ms, that is, the interval time for sending the time synchronization signal is 1 ms or 5 ms. The multiple time synchronization signals sent within one gateway frame 1003 all use the same frame number specified in the message as the reference time point. The expected transmission time points of the multiple time synchronization signals are all defined as the starting point, that is, relative to the duration 1002 of the first X frames of the Nth gateway frame 1001, and each specific time synchronization signal has a time offset point relative to the starting point. The time unit of the time offset can be defined as nanoseconds (ns), symbols, or time slots, and one time slot contains 40 symbols.

[0101] Furthermore, the length of one gateway frame 1003 is 40 ms, and the time for framing the time synchronization signal needs to be advanced by a certain time, such as 3 ms, to ensure that the modem of the gateway receives the time synchronization signal neither too late nor too early, so that the modem of the gateway can send the time synchronization signal from the air interface at the specified moment.

[0102] Furthermore, the first time synchronization signal 1011 is sent after delaying n empty symbols 1010 at the starting point; the second time synchronization signal 1021 is sent after delaying n empty symbols 1010 at the first offset time point 1020 obtained at the first offset time 1022 relative to the starting point;...; the Sth time synchronization signal 1031 is sent after delaying n empty symbols 1010 at the (S - 1)th offset time point 1030 obtained at the (S - 1)th offset time 1032 relative to the starting point.

[0103] In a specific example, as Figure 10As shown, for the link layer gateway framing time synchronization signal, the time synchronization signal is sent equal to or later than the starting point. During the duration 1102 of the first X frames before the Nth gateway frame 1101, the transmission delay of the time synchronization signal consists of two parts. The first part 1103 is the number of symbols of n empty symbols, and the second part 1104 is the offset time.

[0104] In a possible implementation, as Figure 11 shown, the sending of the Nth terminal frame corresponding to the Nth gateway frame at the end of the second delayed sending time to synchronize the Nth gateway frame and the Nth terminal frame further includes: adding a first timestamp 1202 to the time synchronization signal 1201 by the physical layer of the terminal; parsing the time synchronization signal with the first timestamp 1202 added by the link layer of the terminal to obtain the second delayed sending time 1203; calculating the frame header of the Nth terminal frame of the physical layer of the terminal by the link layer of the terminal according to the time of the first timestamp 1202, the second delayed sending time 1203, and the counting interval of the physical layer of the terminal; adding a second timestamp 1204 to the time synchronization signal by the link layer of the terminal, and presetting the internal transmission delay time for the time synchronization signal to be transmitted from the physical layer of the terminal to the link layer of the terminal; calculating the frame header of the Nth terminal frame 1205 of the terminal by the link layer of the terminal according to the time of the second timestamp 1204, the second delayed sending time 1203, and the internal transmission delay time.

[0105] In a specific example, in order to align the data sending and receiving times between the terminal and the upstream gateway, the terminal needs to establish a time reference relying on the received time synchronization signal. Since better time accuracy can be obtained by performing time control at the physical layer of the terminal, the establishment of the time reference and the sending time control of the terminal are mainly responsible by the physical layer, and the link layer plays an auxiliary calculation role.

[0106] Further, the terminal establishes a time reference according to the time synchronization signal. When the relative positions of the gateway and the terminal remain unchanged, the time reference of the upstream gateway time synchronization is adjusted once every time interval between two consecutive time synchronization signals. If the relative positions of the gateway and the terminal change rapidly, the sending interval of the time synchronization signal is correspondingly shortened. If beam hopping occurs, a time synchronization signal needs to be sent every time a hop occurs.

[0107] Further, if the error between the new Nth gateway frame boundary and the previously synchronized Nth gateway frame boundary is very small, the terminal uses the previously synchronized Nth gateway frame boundary; otherwise, the terminal uses the time reference established by the most recent synchronization time signal. When a certain synchronization time signal message is invalid and the terminal cannot update the current time reference, the terminal can reuse the previous time reference to achieve time control for data transmission.

[0108] In a specific example, for a data transmission of the terminal, the following time information needs to be determined: the time reference information established depending on the time synchronization signal, that is, the current relative time origin; the delayed transmission time information of a specific frame, that is, the second delayed transmission time and the transmission frame boundary of the terminal; the time information of the time slot offset of the bandwidth allocation within this frame for a specific physical frame, that is, the in-frame time boundary of the terminal.

[0109] In a specific example, the physical layer of the terminal uses the following method to establish a time reference.

[0110] Further, the physical layer of the terminal stamps a time stamp on each data packet of the locally received time synchronization signal, with the counter interval t0 as the unit, the counter value is recorded as m1, and the data packet is uploaded to the link layer of the terminal;

[0111] Further, the link layer of the terminal parses the data packet. For the time synchronization signal therein, it extracts the frame number and the time stamp information m1 marked by the physical layer of the terminal. The link layer of the terminal parses the time synchronization signal to obtain the duration of the previous X frames, the first delayed transmission time and the frame number, and calculates the second delayed transmission time, where the transmission delay time from the gateway to the terminal and the transmission delay time from the terminal to the gateway are both calculated using the ephemeris. When the terminal does not have ephemeris conditions or the terminal moves too fast and it is not easy to determine the position, the delay d0 from the satellite to the beam center can be used. The delay d0 from the satellite to the beam center is calculated by the gateway and carried by the time synchronization signal, and the transmission delay time from the gateway to the terminal and the transmission delay time from the terminal to the gateway can both be approximated using d0.

[0112] Further, assume that the counter interval of the physical layer of the terminal is t0. The link layer of the terminal helps the physical layer of the terminal directly calculate the frame boundary corresponding to the Nth terminal frame as: m = m1 + round(T UO / t0), where round is the rounding function, and t0 can be equal to T0, T UO is the second delayed transmission time.

[0113] Further, the time when the link layer of the terminal receives the time synchronization signal is also marked with the link layer timestamp m2 of the terminal, with the unit of ns. It is assumed that the link layer reception time of the terminal is later than the physical layer time j of the terminal, with the unit of ns. And the frame boundary of the Nth terminal frame of the link layer of the terminal is calculated according to the second delay transmission time as: M = m2 + T UO - j.

[0114] Further, the link layer of the terminal sends a control command to the physical layer of the terminal, notifying the physical layer of the terminal of the reception time m1 and the corresponding transmission time m corresponding to the Nth terminal frame. The physical layer of the terminal sets the frame boundary of the Nth terminal frame according to the transmission time m.

[0115] Further, the local time of the terminal generates not only the Nth terminal frame, but also the N + 1th terminal frame, the N + 2th terminal frame,..., the N + 8th terminal frame, etc. If the time gap between the frame number carried by the new time synchronization signal and the frame number carried by the previous time synchronization signal is small enough, the time reference of the Nth terminal frame is not updated, otherwise the new Nth terminal frame is used. If the new Nth terminal frame is abnormal, the time reference of the Nth terminal frame is also not updated.

[0116] Further, after the terminal is time synchronized with the gateway, it establishes its own transmission frame boundary and sends uplink data according to the frame boundary. Due to different distances between different terminals and the satellite, the transmission frame boundaries are different, but they are aligned with the gateway frame boundary after the air interface delay. In this way, the transmission time determined by the terminal based on the local reference frame boundary is the arrival time of the terminal data based on the local frame boundary of the base station. The terminal performs planned data transmission according to the allocated bandwidth of the uplink gateway, so that the uplink data of the terminal is received by the uplink gateway at the appropriate position as planned.

[0117] Further, when the physical layer of the terminal has sufficient buffer, the link layer of the terminal can send data to the physical layer of the terminal in advance. After the physical layer of the terminal caches it, it sends it at an appropriate time, which is relatively easy to realize the coordination between the link layer and the physical layer on the terminal side. It should be noted that this uplink time synchronization method has the same principle for beam continuous coverage and beam hopping coverage.

[0118] In a possible implementation manner, the use of the terminal to receive the time synchronization signal sent by the satellite further includes: using the terminal to search for the random access beam sent by the gateway. When the random access beam covers the area where the terminal is located, the terminal locks the downlink data stream; using the terminal to receive the time synchronization signal, the K preset moments corresponding to the K transmission opportunities for sending random access requests one by one, and the time-frequency parameters of the uplink access channel in the downlink data stream. The K preset moments have a third delay transmission time relative to the frame header of the Nth gateway frame, and K is a positive integer;

[0119] After the Nth gateway frame is time-synchronized with the Nth terminal frame, the method further includes: using the gateway to direct a random receiving beam to the terminal at the K preset moments; using the terminal to select one or more preset moments to send a random access request according to the K transmission opportunities; when the first delayed transmission time is equal to the third delayed transmission time, the i Nth preset moment is:

[0120]

[0121] wherein, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the i th transmission opportunity randomly selected by the terminal, i = 1, 2, …, K.

[0122] In a specific example, as Figure 12 shown, when the terminal is powered on, it first adjusts to the random access beam frequency band to start the search program, waits for the random access beam to cover the area where the terminal is located, and locks the random access downlink data stream. When the random access beam covers this area, the terminal locks successfully. The terminal receives the time synchronization signal for accessing the satellite communication system and the time-frequency parameters of the uplink access channel in the downlink data stream, and then sends an uplink access request command in the random beam.

[0123] Further, the coverage ranges of the downlink beam and the uplink beam for the terminal's random access can be slightly different. This is because after the terminal receives the downlink beam, it needs to wait for the second delayed transmission time to complete frame synchronization, and then send an access request or data. Assuming the radius of the downlink beam is R1 and the radius of the uplink beam is R2, then the distance R between the downlink beam and the uplink beam is R = R2 - R1. If the second delayed transmission time is at most 100 ms and the moving speed of the low-earth orbit satellite is 8 km / s, then R needs to be 800 meters.

[0124] Further, when the beam continuously covers, the terminal actively sends a random access request at the time point specified by the network, or sends it at the transmission moment specified by the bandwidth allocation. The uplink gateway receiver receives the data sent by different terminals at different timeshare.

[0125] Further, when beam hopping coverage occurs, it is divided into random access and reception according to bandwidth allocation. Further, if the terminal is in random access, the base station will notify the terminal of the expected arrival time of random access in the time synchronization signal or other messages, and at the same time point the beam to the terminal at the time when the data should arrive, and the terminal sends a random access request at this time.

[0126] Further, assume that the arrival time of random access designated by the base station for terminal 1 is at the Nth gateway frame + t1 moment of the base station receiver. The base station needs to point the beam to terminal 1 at the Nth gateway frame + t1 moment locally. The terminal searches the downlink channel to receive the time synchronization signal to complete frame synchronization, and then sends a random access request at the Nth terminal frame + t1 moment.

[0127] Further, the second delay transmission time can be: , is the delay from the satellite to the center of the transmitting beam.

[0128] Further, the transmission time for the terminal to send a random access request is: , Both and t1 are pre-computed information provided by the base station and can be carried in the time synchronization signal.

[0129] Further, if , then the transmission time for the terminal to send a random access request can be simplified to: .

[0130] Further, after simplification, it can be expressed as that within the duration of the first X frames after the base station sends the time synchronization signal to the terminal or a group of terminals, the receiving beam is pointed to the terminal or a group of terminals, and the terminal sends a random access request or data at the moment of after receiving the time synchronization signal. If is small enough, such as less than 80 ms or 100 ms, then this method can complete terminal access and data transmission without relying on the terminal location and running speed direction.

[0131] Further, for a group of terminals, it is necessary to avoid collisions caused by terminals sending simultaneously. Assume that the duration of the beam is the time window W, and K random access beam transmission opportunities are divided within W, that is, K transmission opportunities. Each terminal randomly selects a transmission timing k(i), i = 1, 2, 3,..., K, then the i-th transmission time of the terminal is after receiving the time synchronization signal. This can support a group of terminals to access or send data, and at the same time effectively reduce the probability of data collision at the receiving end. The base station can set that terminals are allowed to use multiple transmission opportunities, and a single terminal can use all K transmission opportunities at most.

[0132] Further, in the beam hopping scenario, the base station allocates bandwidth according to the terminal's requirements. The terminal transmits at the transmission time specified by the bandwidth allocation. The receiving beam adjusts its pointing according to the terminal's transmission time specified by the bandwidth allocation, and the gateway receiver receives the data sent by different terminals at different timesharing. The difference between the hopping beam and the continuous beam is that the hopping beam points the beam to the terminal simultaneously at the transmission time specified by the terminal.

[0133] Further, after the terminal accesses the system, the satellite base station determines whether to keep the terminal in the random access beam or allocate a service point beam for the terminal. If the terminal has no high-priority data, the reported data (Backlog) is lower than the threshold, or the terminal does not provide its geographical location, then the terminal continues to stay in the random access beam.

[0134] Further, when it is necessary to allocate service point beams, the base station sends the user terminal access information to the beam control module. The beam control module allocates a beam for the user, and the satellite base station schedules the service point beam to cover the current location of the terminal. The terminal performs two-way data transmission in the service point beam. When planning the hopping beam, the newly accessed user terminal can be allocated with high priority.

[0135] In a possible implementation, as Figure 13 shown, the step of using the terminal to receive the time synchronization signal sent by the satellite and calculating the second delay transmission time according to the duration of the first X frames, the round-trip time between the terminal and the gateway, and the first delay transmission time further includes: using the gateway to measure the delay arrival time 1402 or the early arrival time of the frame header 1400 of the Nth terminal frame relative to the frame header 1401 of the Nth gateway frame when the Nth terminal frame arrives at the gateway, and sending the delay arrival time 1402 or the early arrival time to the terminal; using the terminal to calculate the second delay transmission time 1403 according to the delay arrival time 1402 or the early arrival time;

[0136] The second delay transmission time is:

[0137]

[0138] In the formula, is the second delay transmission time, is the duration of the first X frames, is the initial value of the transmission delay time from the terminal to the gateway, is the delay arrival time or the early arrival time, is the first delay transmission time.

[0139] Further, Figure 13It also includes: the duration 1404 of the first X frames, the first delayed transmission time 1405, the initial value 1406 of the transmission delay time from the gateway to the terminal, and the initial value 1407 of the transmission delay time from the terminal to the gateway.

[0140] In a specific example, the time synchronization signal of the Nth gateway frame is expected to be sent during the duration of the first X frames of the Nth gateway frame. Assuming the transmission delay time from the terminal to the gateway is D, in nanoseconds, for different orbiting satellites, D can be a constant, or vary rapidly or slowly. The transmission offset configuration parameter of the time synchronization signal is the first delayed transmission time.

[0141] Furthermore, the method for measuring the satellite-ground or gateway-to-terminal delay is as follows. Assuming the initial value D(0) of the transmission delay time from the terminal to the gateway, the initial value D(0) can be the delay from the satellite to the beam center where the time synchronization signal is located. In fact, the time synchronization signal is sent over the air interface after the first delayed transmission time. After the terminal receives the time synchronization signal, according to Calculate the start time of the Nth terminal frame.

[0142] Furthermore, the terminal takes the end moment of the second delayed transmission time after receiving the time synchronization signal as the transmission start point of the Nth terminal frame, that is, the frame boundary of the Nth terminal frame, to complete the time synchronization of the Nth gateway frame corresponding to the satellite master station of the Nth terminal frame. If the terminal is at the beam center and there is no deviation in the transmission delay, the physical frame sent by the terminal at the frame boundary of the Nth terminal frame locally arrives exactly at the frame boundary moment of the Nth gateway frame at the gateway. If the terminal is not at the beam center, the physical frame sent by the terminal at the frame boundary of the Nth terminal frame locally has a deviation from the frame boundary of the Nth gateway frame when it arrives at the satellite gateway. This deviation can be accurately measured by the demodulator of the satellite gateway, and the measurement accuracy can reach the nanosecond level.

[0143] Furthermore, the terminal starts to send data packets at the frame boundary of the Nth terminal frame, and the gateway marks the time of the received data packets. The delay arrival time or early arrival time ΔD of the data at the frame boundary of the Nth gateway frame, and ΔD can be negative, indicating early arrival. From this, the transmission delay time from the terminal to the gateway can be calculated as: 。

[0144] Furthermore, the gateway feeds back the measured delay deviation to the terminal, and the terminal uses the new delay parameter when calculating the second delayed transmission time, then the second delayed transmission time is: 。This means that the transmission time of the terminal is advanced or delayed by ΔD time, so that the arrival time at the gateway is just at the start point of the frame boundary. By measuring the transmission delay through the gateway, closed-loop time synchronization can be performed to achieve higher-precision time synchronization.

[0145] Further, assume that the terminal sends a data frame at the Nth terminal frame + t1 moment, carries the information of the Nth terminal frame + t1 in the data frame, and the data frame arrives at the Nth gateway frame + t2 moment. Then the error Δt = t2 - t1. The gateway sends the error Δt to the terminal, and the terminal updates the frame boundary of the Nth terminal frame with the error Δt. The terminal continues to send data, obtains a new error Δt, and continues to update the frame boundary of the Nth terminal frame. Therefore, as long as the terminal can establish time synchronization based on the initial value, the terminal can maintain relatively accurate time synchronization through uplink data transmission. Note that the time synchronization here does not depend on the calculation of the satellite-ground delay based on the positions of the terminal and the satellite. If the terminal does not send data, the satellite-ground delay cannot be measured. Then, after a period of time, a new time synchronization signal needs to be received to establish time synchronization.

[0146] Another embodiment of the present invention provides a satellite communication system, which includes a gateway, a terminal, and a satellite; the gateway is configured to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame, the duration of the first X frames is greater than the round-trip time between the terminal and the gateway, the actual sending moment of the time synchronization signal has a first delay sending time relative to the frame header of the first X frames, N is a positive integer, and X is a positive integer; the terminal is configured to receive the time synchronization signal sent by the satellite, calculate a second delay sending time according to the duration of the first X frames, the round-trip time between the terminal and the gateway, and the first delay sending time, and send the Nth terminal frame corresponding to the Nth gateway frame at the end moment of the second delay sending time so that the Nth gateway frame is time-synchronized with the Nth terminal frame.

[0147] In a possible implementation manner, the satellite is a regenerative satellite, and the second delay sending time is:

[0148]

[0149] In the formula, is the second delay sending time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the first delay sending time.

[0150] In a possible implementation manner, the satellite is a transparent relay satellite, and the second delay sending time is:

[0151]

[0152] In the formula, is the second delayed transmission time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, , is the transmission delay time from the gateway to the satellite, is the transmission delay time from the satellite to the terminal, is the transmission delay time from the terminal to the gateway, , is the transmission delay time from the terminal to the satellite, is the transmission delay time from the satellite to the gateway, is the first delayed transmission time.

[0153] In a specific example, the satellite communication system realizes the connection and networking between the terminal and the gateway system through a transparent relay satellite or a regenerative satellite with on-board processing capabilities.

[0154] In a specific example, as Figure 14 shown, the ground gateway system includes a three-layer gateway and a link gateway. The data sent from the external network to the terminal is sent from the three-layer gateway through the link gateway to the transparent relay satellite, and the transparent relay satellite performs transparent relay and is received by the terminal. The data sent from the terminal to the external network is forwarded from the terminal through the transparent relay satellite to the link gateway and then sent to the external network through the three-layer gateway.

[0155] In a specific example, as Figure 15 shown, the satellite is no longer a transparent relay satellite but a regenerative satellite with on-board processing capabilities. In this case, the link gateway needs to be deployed on the regenerative satellite. The data between the terminal and the external network needs to reach the feeder gateway of the ground gateway system through the inter-satellite link from the feeder line, and then be interconnected with the external network through the three-layer gateway. The downlink data and uplink data of the terminal accessing the satellite network are processed by the on-board link gateway.

[0156] This embodiment enables the terminal to perform time synchronization when accessing the satellite communication system and the gateway system, ensures that the data transmission and reception of multiple terminals do not overlap in the time domain and do not interfere with each other, and can be received by the gateway at a predetermined time.

[0157] Obviously, the above embodiments of the present invention are merely examples for clearly explaining the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is impossible to enumerate all the implementation manners here. Any obvious changes or variations derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. A time synchronization method for a satellite communication system, characterized in that: The satellite communication system comprises a gateway, a terminal and a satellite; the method comprises: Using the gateway to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of an Nth gateway frame, the duration of the first X frames is greater than the round-trip time from the terminal to the gateway, and the actual sending time of the time synchronization signal is delayed by a first sending time relative to the frame header of the first X frames, where N is a positive integer and X is a positive integer; Utilizing the terminal to receive the time synchronization signal sent by the satellite, and calculating a second delayed sending time according to the duration of the previous X frames, the round-trip time from the terminal to the gateway, and the first delayed sending time, and sending an Nth terminal frame corresponding to the Nth gateway frame at the end time of the second delayed sending time so that the Nth gateway frame is time-synchronized with the Nth terminal frame; The step of receiving the time synchronization signal sent by the satellite using the terminal further comprises: Using the terminal to search for a random access beam sent by the gateway, when the random access beam covers an area where the terminal is located, the terminal locks a downlink data stream; Using the terminal to receive the time synchronization signal in the downlink data stream, K preset times for sending random access requests corresponding to K sending opportunities, and uplink access channel time-frequency parameters, the K preset times having a third delayed sending time relative to the frame header of the Nth gateway frame, K being a positive integer; After the Nth gateway frame is time synchronized with the Nth terminal frame, the method further includes: Using the gateway to direct a random receiving beam to the terminal at the K preset time moments; Using the terminal to select one or more preset time points according to the K transmission opportunities to send a random access request; When the first delayed sending time is equal to the third delayed sending time, i The preset times are: In the formula, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, The terminal randomly selects i Sending time, i =1,2,…,K.

2. The time synchronization method according to claim 1, characterized in that: The satellite is a regenerative satellite, and the second delayed sending time is: In the formula, is the second delayed sending time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the first delayed sending time.

3. The time synchronization method according to claim 2, characterized in that: The sending of a time synchronization signal including a superframe number or a frame number to the satellite by the gateway in the first X frames of the Nth gateway frame further comprises: The gateway includes a link layer gateway and a physical layer gateway, the first X frames are N gateway frames including 1 continuous beam, and the link layer gateway is used to send the time synchronization signal, the sending frame number NX of the physical layer gateway and n null symbols in the frame header of the NX-1th gateway frame, where n is a positive integer; Using the physical layer gateway to send the time synchronization signal to the satellite at the moment when the frame header of the Nxth gateway frame is delayed by the n null symbols; or The sending of a time synchronization signal including a superframe number or a frame number to the satellite by the gateway in the first X frames of the Nth gateway frame further comprises: The gateway includes a link layer gateway and a physical layer gateway, the first X frames are N gateway frames including S hopping beams, and the link layer gateway uses the frame header of the NX-1th gateway frame as the starting point to send S time synchronization signals, S sending frame numbers NX of the physical layer gateway and S empty symbol groups at the starting point and S-1 offset time points relative to the starting point, respectively, each empty symbol group includes n empty symbols, and S is a positive integer; The physical layer gateway is used to send a total of S time synchronization signals to the satellite at the starting point and S-1 offset time points relative to the starting point at the time when they are delayed by n null symbols respectively.

4. The time synchronization method according to claim 1, characterized in that: The satellite is a transparent forwarding satellite, and the second delayed sending time is: In the formula, is the second delayed sending time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, , is the transmission delay time from the gateway to the satellite, is the transmission delay time from the satellite to the terminal, is the transmission delay time from the terminal to the gateway, , is the transmission delay time from the terminal to the satellite, is the transmission delay time from the satellite to the gateway, is the first delayed sending time.

5. The time synchronization method according to claim 1, characterized in that: The sending of the Nth terminal frame corresponding to the Nth gateway frame at the end time of the second delayed sending time so that the Nth gateway frame is time synchronized with the Nth terminal frame further includes: adding a first timestamp to the time synchronization signal using a physical layer of the terminal; Parsing the time synchronization signal with the first timestamp by using the link layer of the terminal to obtain the second delayed sending time; Calculate the frame header of the Nth terminal frame of the physical layer of the terminal by using the link layer of the terminal according to the time of the first timestamp, the second delayed sending time and the counting interval of the physical layer of the terminal; Using the link layer of the terminal to add a second timestamp to the time synchronization signal, and presetting an internal transmission delay time of the time synchronization signal from the physical layer of the terminal to the link layer of the terminal; The link layer of the terminal is used to calculate the frame header of the Nth terminal frame of the terminal according to the time of the second timestamp, the second delayed sending time and the internal transmission delay time.

6. The time synchronization method according to claim 1, characterized in that: The step of receiving the time synchronization signal sent by the satellite using the terminal and calculating the second delayed sending time according to the duration of the previous X frames, the round-trip time from the terminal to the gateway and the first delayed sending time further comprises: Measuring, by the gateway, a delayed arrival time or an advanced arrival time of a frame header of the Nth terminal frame relative to a frame header of the Nth gateway frame when the Nth terminal frame arrives at the gateway, and sending the delayed arrival time or the advanced arrival time to the terminal; Calculating the second delayed sending time according to the delayed arrival time or the early arrival time by using the terminal; The second delayed sending time is: In the formula, is the second delayed sending time, is the duration of the first X frames, is the initial value of the transmission delay time from the terminal to the gateway, is the delayed arrival time or the early arrival time, is the first delayed sending time.

7. A satellite communication system, characterized in that: Satellite communication systems include gateways, terminals and satellites; The gateway is used to send a time synchronization signal including a superframe number or a frame number to the satellite within the first X frames of the Nth gateway frame, the duration of the first X frames is greater than the round-trip time from the terminal to the gateway, and the actual sending time of the time synchronization signal has a first delayed sending time relative to the frame header of the first X frames, N is a positive integer, and X is a positive integer; The terminal is used to receive the time synchronization signal sent by the satellite, and calculate a second delayed sending time according to the duration of the previous X frames, the round-trip time from the terminal to the gateway and the first delayed sending time, and send an Nth terminal frame corresponding to the Nth gateway frame at the end time of the second delayed sending time so that the Nth gateway frame is synchronized with the Nth terminal frame in time; The step of receiving the time synchronization signal sent by the satellite using the terminal further comprises: Using the terminal to search for a random access beam sent by the gateway, when the random access beam covers an area where the terminal is located, the terminal locks a downlink data stream; Using the terminal to receive the time synchronization signal in the downlink data stream, K preset times for sending random access requests corresponding to K sending opportunities, and uplink access channel time-frequency parameters, the K preset times having a third delayed sending time relative to the frame header of the Nth gateway frame, K being a positive integer; After the Nth gateway frame is time synchronized with the Nth terminal frame, the method further includes: Using the gateway to direct a random receiving beam to the terminal at the K preset time moments; Using the terminal to select one or more preset time points according to the K transmission opportunities to send a random access request; When the first delayed sending time is equal to the third delayed sending time, i The preset times are: In the formula, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, The terminal randomly selects i Sending time, i =1,2,…,K.

8. The satellite communication system according to claim 7, characterized in that: The satellite is a regenerative satellite, and the second delayed sending time is: In the formula, is the second delayed sending time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, is the transmission delay time from the terminal to the gateway, is the first delayed sending time.

9. The satellite communication system according to claim 7, characterized in that: The satellite is a transparent forwarding satellite, and the second delayed sending time is: In the formula, is the second delayed sending time, is the duration of the first X frames, is the transmission delay time from the gateway to the terminal, , is the transmission delay time from the gateway to the satellite, is the transmission delay time from the satellite to the terminal, is the transmission delay time from the terminal to the gateway, , is the transmission delay time from the terminal to the satellite, is the transmission delay time from the satellite to the gateway, is the first delayed sending time.

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