Satellite communication system and time synchronization method thereof

Abstract

The embodiment of the invention discloses a satellite communication system and a time synchronization method thereof. The satellite communication system comprises a gateway, a terminal and a satellite. The method comprises the steps that a gateway is used for sending a time synchronization signal containing a super frame number or a frame number to a satellite in first X frames of an Nth gateway frame, the duration time of the first X frames is larger than the round-trip time from a terminal to the gateway, the actual sending moment of the time synchronization signal has first delay sending time relative to frame headers of the first X frames, N is a positive integer, and the first delay sending time is larger than the first delay sending time. X is a positive integer; receiving the time synchronization signal sent by the satellite by using the terminal, and calculating a second delay sending time according to the duration of the first X frames, the round-trip time from the terminal to the gateway and the first delay sending time, and the Nth terminal frame corresponding to the Nth gateway frame is sent at the end moment of the second delay sending time, so that the Nth gateway frame and the Nth terminal frame are synchronized in time.

Description

Technical Field

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

[0002] At present, since the uplink of the satellite communication system adopts the Multiple Frequency-Time Division Multiple Access (MF-TDMA) working system, there is a problem that when the terminal accesses the satellite communication system, the time is not synchronized with the gateway system time, the sent and received data of multiple terminals overlap and interfere with each other in the time domain, and cannot be received by the gateway at the scheduled time. Summary of the invention

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

[0004] In order to achieve the above object, the present invention adopts the following technical solutions: A first aspect of the present invention provides a time synchronization method for a satellite communication system, the satellite communication system comprising a gateway, a terminal and a satellite; the method comprising: 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; The terminal is used to receive the time synchronization signal sent by the satellite, and a second delayed sending time is calculated 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 an Nth terminal frame corresponding to the Nth gateway frame is sent at the end of the second delayed sending time to synchronize the time of the Nth gateway frame with the Nth terminal frame.

[0005] Optionally, the satellite is a regenerative satellite, and the second delayed sending time is:

[0006] 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.

[0007] Optionally, 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 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.

[0008] Optionally, the satellite is a transparent forwarding satellite, and the second delayed sending time is:

[0009] 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.

[0010] Optionally, sending 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.

[0011] Optionally, the receiving, by the terminal, the time synchronization signal sent by the satellite 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:

[0012] 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.

[0013] Optionally, the using the terminal to receive the time synchronization signal sent by the satellite, 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 includes: 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:

[0014] 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.

[0015] A second aspect of the present invention provides a satellite communication system, the satellite communication system comprising a gateway, a terminal and a satellite; 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 of the second delayed sending time so that the Nth gateway frame is synchronized with the Nth terminal frame time.

[0016] Optionally, the satellite is a regenerative satellite, and the second delayed sending time is:

[0017] 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.

[0018] Optionally, the satellite is a transparent forwarding satellite, and the second delayed sending time is:

[0019] 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.

[0020] The beneficial effects of the present invention are as follows: The technical solution of the present invention can achieve time synchronization when a terminal accesses a satellite communication system and a gateway system, ensuring that the data sent and received by 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

[0021] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings.

[0022] Figure 1 A schematic diagram of a superframe structure with a 40ms frame period in the related art is shown.

[0023] Figure 2 A schematic diagram of a superframe structure with a 20ms frame period in the related art is shown.

[0024] Figure 3 A schematic diagram of the frame boundary between a terminal and a gateway in the related art is shown.

[0025] Figure 4 A flow chart of a time synchronization method provided by an embodiment of the present invention is shown.

[0026] Figure 5 A schematic diagram of a time synchronization method for a regenerative satellite provided in an embodiment of the present invention is shown.

[0027] Figure 6 A schematic diagram of a time synchronization method for a transparent forwarding satellite provided in an embodiment of the present invention is shown.

[0028] Figure 7 A schematic diagram of restarting downlink data flow by beam hopping provided by an embodiment of the present invention is shown.

[0029] Figure 8 A schematic diagram showing a time synchronization signal sent in one frame provided by an embodiment of the present invention is shown.

[0030] Fig. 9 A schematic diagram showing sending multiple time synchronization signals in one frame provided by an embodiment of the present invention is shown.

[0031] Fig.10 A schematic diagram showing the transmission delay of a time synchronization signal provided by an embodiment of the present invention is shown.

[0032] Fig.11 A schematic diagram of terminal time synchronization calculation provided by an embodiment of the present invention is shown.

[0033] Fig.12 A schematic diagram of beam control for terminal access service points provided by an embodiment of the present invention is shown.

[0034] Fig.13 A schematic diagram of transmission delay measurement provided by an embodiment of the present invention is shown.

[0035] Fig.14 A schematic diagram of the architecture of a transparent forwarding satellite provided by an embodiment of the present invention is shown.

[0036] Fig.15 A schematic diagram of the architecture of a regenerative satellite provided by an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0037] In order to explain the present invention more clearly, the present invention is further described below in conjunction with embodiments and drawings. Similar components in the drawings are represented by the same reference numerals. It should be understood by those skilled in the art that the content specifically described below is illustrative rather than restrictive, and the protection scope of the present invention should not be limited thereto.

[0038] In satellite communication systems, 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 (Channel), configured with different system parameters, such as center frequency and symbol rate, and managed by a link gateway. The resources on each frequency point are organized into continuous superframes (Super Frame), each superframe contains a fixed data frame (Frame), each data frame contains a fixed time slot (Slot), and each frame of channels with different symbol rates (Symbol Rate) contains a different number of time slots. Different physical frames (Burst) are composed of different numbers of time slots, but a time slot cannot be allocated to two physical frames. 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 frame length. In essence, the time slot is also a time unit, and the granularity of the time slot is 40 symbols. For the uplink transmission side, two consecutive physical frames of the same user may or may not span time slots when transmitting, but the physical frames of different users must be strictly aligned according to the time slot boundary.

[0039] like Figure 1 As shown, the frame period is selected as 40ms, and the frame period can also be selected as 20ms, 10ms or 5ms, etc., where the frame period is 40ms, a superframe contains 8 single frames, and the superframe time length is 8*40=320ms. Figure 2 As shown, the frame period can also be selected to have a time length of 20ms, that is, the frame period is 20ms, and a superframe contains 16 single frames, and its time length is 16*20=320ms. Figure 3 As shown, the gateway is defined as a continuous gateway frame on the time axis, such as the Nth gateway frame 501 and the N+1th gateway frame 502, etc., and a gateway frame is divided into multiple time slots. Correspondingly, the terminal also has continuous terminal frames on the time axis, such as the Nth terminal frame 5101 of the first terminal, the N+1th terminal frame 5102 of the first terminal; the Nth terminal frame 5201 of the second terminal, the N+1th terminal frame 5202 of the second terminal; the Nth terminal frame 5301 of the third terminal, the N+1th terminal frame 5302 of the third terminal. Since the distance between each terminal and the gateway is different, the delay of the data sent by the terminal to reach the gateway is different. Therefore, it is necessary to adjust the time when each terminal sends the terminal frame to different positions, and realize that the time when it arrives at the gateway after the transmission delay is exactly the starting 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 the terminals will use different time slots within a frame for data transmission to avoid mutual interference.

[0040] To sum up, since the uplink of the satellite communication system adopts a multi-frequency time division multiple access working system, there is a problem that when the terminal accesses the satellite communication system, the time is not synchronized with the gateway system time, the sent and received data of multiple terminals overlap and interfere with each other in the time domain, and cannot be received by the gateway at the scheduled time.

[0041] In view of this, if Figure 4 As shown, an embodiment of the present invention provides a time synchronization method for a satellite communication system, the satellite communication system comprising a gateway, a terminal and a satellite; the method comprising: 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 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; using 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 first 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 of the second delayed sending time so that the Nth gateway frame is time synchronized with the Nth terminal frame.

[0042] This embodiment enables time synchronization when terminals access the satellite communication system and the gateway system, ensuring that the data sent and received by 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.

[0043] In a possible implementation manner, the satellite is a regenerative satellite, and the second delayed sending time is:

[0044] 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.

[0045] 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 sent by the Nth terminal frame can arrive at the frame boundary of the Nth gateway frame of the uplink receiver after the delay of air interface transmission. Since the uplink receiver is a one-to-many reception, and because the satellite or user terminal moves, 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 received by different user terminals will not collide and can be received correctly. In fact, the Nth terminal frame of the user terminal and the Nth gateway frame of the receiver are different in absolute time. The difference between the two 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.

[0046] In a specific example, on the terminal time axis, a 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, and mod is a remainder function. That is, FN is the frame number. When mod (FN, 8) = 0, FN is also the superframe number. The purpose of time synchronization in a satellite communication system is to allow the data sent by the terminal at the frame boundary of the Nth terminal frame to arrive at the frame boundary of the Nth gateway frame of the satellite base station. To achieve this goal, the satellite base station periodically or irregularly sends time synchronization information containing the Nth gateway frame so that the terminal can achieve time synchronization with the satellite base station. If the relative position relationship between the satellite and the terminal is stable, the satellite base station periodically sends the time synchronization information of the Nth gateway frame, for example, every 8 frames; if the relative position of the satellite and the satellite terminal changes rapidly, the satellite base station irregularly sends the time synchronization information of the Nth gateway frame to adapt to the rapid time change.

[0047] In a specific example, Figure 5 As shown, for the satellite-borne base station system when the satellite is a regenerative satellite, the time synchronization benchmark is that the sending side and the receiving side of the satellite-borne base station are consistent. That is, the receiving time when the uplink gateway expects to receive the data of the Nth gateway frame and the sending time when the terminal sends the data of the Nth terminal frame need to satisfy the time relationship in the above formula.

[0048] Furthermore, the gateway periodically sends a system time synchronization data signaling message, i.e., a time synchronization signal (Super Frame Timing Reference, SFTR), at a preset time interval in advance of the Nth gateway frame 6060 at the receiving end of the satellite base station, i.e., the duration 601 of the previous X frames, at the starting boundary time 600 of the duration 601 of the previous X frames. The starting boundary of the super frame or frame carried in the time synchronization signal corresponds to the starting boundary of the frame number. The default value of the frame period is set to 40ms and can be changed through configuration.

[0049] Furthermore, the time synchronization signal cannot be guaranteed to be sent exactly at the starting boundary time 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. The time interval of this advance or delay is recorded as the first delayed sending time 602, and the unit is T0. T0 is the time unit of the physical layer clock counter; the unit of the first delayed sending time 602 can also be a symbol or a time slot.

[0050] Furthermore, the time interval generated from the starting boundary time 600 of the duration 601 of the previous X frames to the actual sending time 608 of the time synchronization signal is uniformly marked as the first delayed sending time 602 .

[0051] Furthermore, the time synchronization signal should be sent within a frame after the starting boundary time 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 satellite base station.

[0052] Furthermore, Figure 5 It also includes: gateway frame timing, terminal frame timing, terminal time axis, second delayed sending time 603, gateway to terminal transmission delay time 604, terminal to gateway transmission delay time 605, first delayed sending time 602 calculated in advance, Nth gateway frame 6060, Nth terminal frame 6061, time synchronization signal 607, sending side 609 and receiving side 610.

[0053] 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.

[0054] 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.

[0055] In a possible implementation manner, the satellite is a transparent forwarding satellite, and the second delayed sending time is:

[0056] 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.

[0057] 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.

[0058] Furthermore, the ground base station sends a time synchronization signal via a satellite, the time synchronization signal is parsed by the terminal, and the terminal implements time synchronization between the terminal and the ground base station at the frame boundary of the Nth terminal frame according to the time information carried by the time synchronization signal. At this time, the time synchronization reference point for sending and receiving is the ground base station.

[0059] In a specific example, Figure 6 As shown, Figure 6It includes: gateway frame timing, terminal frame timing, terminal time axis, duration of previous X frames 701, second delayed sending time 702, transmission delay time from gateway to satellite 703, transmission delay time from satellite to terminal 704, transmission delay time from terminal to satellite 705, transmission delay time from satellite to gateway 706, first delayed sending time 707, Nth gateway frame 7080, Nth terminal frame 7081, ideal time 700 of duration of previous X frames 701, actual sending time 709 of time synchronization signal, sending side 7010 and receiving side 7011.

[0060] Furthermore, the duration 701 of the first X frames is the delayed reception time of the receiving channel on the ground gateway side, that is, the time difference between the ideal time 700 for sending the time synchronization signal and the scheduled time for receiving the Nth gateway frame. For GEO or LEO, different default constant values ​​are configured, such as 560ms for GEO and 120ms for LEO.

[0061] Furthermore, when the satellite or terminal moves quickly, the transmission delay time from the gateway to the terminal and the transmission delay time from the terminal to the gateway change with time and need to be updated in time. The transmission delay time from the gateway to the terminal and the transmission delay time from the terminal to the gateway include the delay of the feeder link and the user link respectively, while in the satellite 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 delay of the user link.

[0062] In a 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 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; and the physical layer gateway is used to send the time synchronization signal to the satellite at the moment of delaying the n null symbols in the frame header of the NXth gateway frame; In a specific example, to support beam hopping, the satellite downlink transmitter needs to restart the data stream transmission sequence at regular intervals according to the beam hopping planning table, that is, the continuously transmitted data stream is interrupted once at regular intervals. The continuously transmitted data stream is sent to a beam position on the ground for a certain period of time, which can be of equal or variable length, and the specific duration is defined by the beam hopping planning table. Figure 7As shown, since data frame 801 and data frame 802 cannot just fill the expected transmission time period, random symbols (Padding) are required at the end of the previous transmission, such as random symbols 811 and random symbols 821. Compared with continuous data streams, these random symbols will waste air interface resources, but this is an inevitable choice to support beam hopping. Each time a beam hop is performed, a time synchronization signal needs to be sent. Relative to the beam hop position, the satellite communication system can predict and ensure that the time synchronization signal is sent at the specified position. Wave position 1 time 810 = data frame 801 + random symbol 811; wave position 2 time 820 = data frame 802 + random symbol 821.

[0063] In a specific example, Figure 8 As shown, the link layer gateway of the satellite base station and the modem (MODEM) of the physical layer gateway realize frame synchronization through the 1PPS time synchronizer. The duration 901 of the first X frames is set, that is, the time synchronization signal is sent 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 NXth gateway frame. The link layer gateway assembles the time synchronization signal in the duration 903 of the first X+1 frames of the Nth gateway frame 9020, that is, the time synchronization signal is assembled in the frame header of the NX-1th gateway frame and encapsulated to obtain the encapsulated time synchronization signal 905. Since the time synchronization signal is a special control signaling, the link layer gateway directly assembles the package and sends it to the modem of the physical layer gateway. The physical layer gateway restarts the data flow in the NXth gateway frame before the Nth gateway frame 9021 of the physical layer gateway, and sends the time synchronization signal 906 after sending n empty symbols (Dummy) 904. Sending n empty symbols before sending the time synchronization signal 906 is used for receiver fault tolerance. If the terminal receiver can capture a single frame, n can be configured as 0. If a time synchronization signal is sent per frame, N is the frame number, and the corresponding sending interval is the frame length, for example, 40ms. If a time synchronization signal is sent per superframe, N is the superframe number.

[0064] In another possible implementation, the using of 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, the link layer gateway uses the frame header of the NX-1th gateway frame as the starting point, and respectively sends 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, each empty symbol group includes n empty symbols, and S is a positive integer; and the physical layer gateway uses the physical layer gateway 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, respectively delayed by n empty symbols.

[0065] In a specific example, Fig. 9 As shown, in scenarios such as beam hopping or low-orbit satellite applications, it is allowed to send multiple time synchronization signals with beam hopping in one frame, such as once every 1ms or 5ms, that is, the interval time for sending time synchronization signals is 1ms or 5ms. Multiple time synchronization signals sent in a gateway frame 1003 all use the same frame number specified in the message as the reference time point. The expected sending time points of multiple time synchronization signals are all defined as the starting point, that is, the duration 1002 of the previous X frames relative to the Nth gateway frame 1001, and each specific time synchronization signal has a time offset point with respect to the starting point. The time unit of the time offset can be defined as nanoseconds (ns), symbols or time slots, and a time slot contains 40 symbols.

[0066] Furthermore, the length of a gateway frame 1003 is 40ms, and the framing time synchronization signal needs to be advanced by a certain time, such as 3ms, to ensure that the gateway modem receives the time synchronization signal neither too late nor too early, so that the gateway modem can send the time synchronization signal from the air interface at the specified time.

[0067] Further, the first time synchronization signal 1011 is sent after being delayed by n empty symbols 1010 from the starting point; the second time synchronization signal 1021 is sent after being delayed by n empty symbols 1010 from 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 being delayed by n empty symbols 1010 from the S-1th offset time point 1030 obtained at the S-1th offset time 1032 relative to the starting point.

[0068] In a specific example, Fig.10 As shown, for the link layer gateway framing time synchronization signal, the time synchronization signal is sent equal to or later than the starting point. In the duration 1102 of the first X frames of the Nth gateway frame 1101, the time synchronization signal sending delay consists of two parts, the first part 1103 is the number of n empty symbols, and the second part 1104 is the offset time.

[0069] In one possible implementation, Fig.11As shown, sending the Nth terminal frame corresponding to the Nth gateway frame at the end moment of the second delayed sending time so that the Nth gateway frame is time synchronized with the Nth terminal frame further includes: using the physical layer of the terminal to add a first timestamp 1202 to the time synchronization signal 1201; using the link layer of the terminal to parse the time synchronization signal with the first timestamp 1202 to obtain the second delayed sending time 1203; using the link layer of the terminal to calculate the frame header of the Nth terminal frame of the physical 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; using the link layer of the terminal to add a second timestamp 1204 to the time synchronization signal, and presetting the internal transmission delay time of the time synchronization signal from the physical layer of the terminal to the link layer of the terminal; using the link layer of the terminal to calculate the frame header of the Nth terminal frame 1205 of the terminal according to the time of the second timestamp 1204, the second delayed sending time 1203 and the internal transmission delay time.

[0070] In a specific example, in order to achieve the data transmission and reception time alignment between the terminal and the upstream gateway, the terminal needs to establish a time reference based on the received time synchronization signal. Since time control at the physical layer of the terminal can obtain better time accuracy, the physical layer is mainly responsible for the establishment of the terminal's time reference and the control of the transmission time, and the link layer plays an auxiliary calculation role.

[0071] Furthermore, the terminal establishes a time reference based on the time synchronization signal. When the relative position of the gateway and the terminal remains unchanged, the uplink gateway time synchronization adjusts the time reference once every two consecutive time synchronization signals. If the relative position of the gateway and the terminal changes rapidly, the interval between sending the time synchronization signal is shortened accordingly. If the beam jumps, the time synchronization signal must be sent for each jump.

[0072] Furthermore, if the error between the new Nth gateway frame boundary and the Nth gateway frame boundary derived by the previous synchronization is small, the terminal uses the Nth gateway frame boundary generated by the previous synchronization; otherwise, the terminal uses the time reference established by the most recent synchronization time signal. When a synchronization time signal message is invalid and the terminal cannot update the current time reference, the last time reference can be reused to achieve time control of data transmission.

[0073] In a specific example, a terminal needs to determine the following time information for a data transmission: time reference information established based on the time synchronization signal, that is, the relative time origin currently used; delayed transmission time information of a specific frame, that is, the second delayed transmission time, and the terminal's transmission frame boundary; and time information of the time slot offset of the bandwidth allocation of a specific physical frame within this frame, that is, the intra-frame time boundary of the terminal.

[0074] In a specific example, the physical layer of the terminal uses the following method to establish a time reference.

[0075] Further, the physical layer of the terminal timestamps each data packet of the time synchronization signal received locally, with the counter interval t0 as the unit, the counter value is recorded as m1, and uploads the data packet to the link layer of the terminal; Furthermore, the link layer of the terminal parses the data packet, and extracts the frame number and the timestamp information m1 marked by the physical layer of the terminal for the time synchronization signal therein. The link layer of the terminal parses the time synchronization signal to obtain the duration of the previous X frames, the first delayed sending time and the frame number, and calculates the second delayed sending time, wherein 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 ephemeris. When the terminal does not have ephemeris conditions or the terminal moves too fast to determine the position, the delay d0 from the satellite to the center of the beam can be used. The delay d0 from the satellite to the center of the beam 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.

[0076] Furthermore, it is assumed 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 to directly calculate the frame boundary corresponding to the Nth terminal frame: m=m1+round(T UO / t0), round is the rounding function, t0 can be equal to T0, T UO The second delayed sending time.

[0077] Furthermore, the moment when the link layer of the terminal receives the time synchronization signal is also stamped with the link layer timestamp m2 of the terminal, in ns, and the link layer reception time of the terminal is set to be later than the physical layer time j of the terminal, in ns, and the frame boundary of the Nth terminal frame of the link layer of the terminal is calculated according to the second delayed sending time: M=m2+T UO -j.

[0078] 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 receiving time m1 and the corresponding sending 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 sending time m.

[0079] Furthermore, the terminal local time 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 difference 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 not updated.

[0080] Furthermore, after the terminal is synchronized with the gateway, it establishes its own transmission frame boundary and sends uplink data according to the frame boundary. Different terminals have different transmission frame boundaries due to their different distances from the satellite, but they are aligned with the gateway frame boundary after air interface delay. In this way, the transmission time determined by the terminal based on the local reference frame boundary is the terminal data arrival time 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 terminal uplink data can be received by the uplink gateway at the appropriate location as planned.

[0081] Furthermore, when the physical layer of the terminal has enough buffer, the link layer of the terminal can send data to the physical layer of the terminal in advance, and the physical layer of the terminal sends the data at an appropriate time after buffering, which makes it easier to achieve coordination between the link layer and the physical layer on the terminal side. It should be noted that the principle of this uplink time synchronization method is the same for beam continuous coverage and beam hopping coverage.

[0082] In a possible implementation, the receiving the time synchronization signal sent by the satellite using the terminal further includes: searching the random access beam sent by the gateway using the terminal, and when the random access beam covers the area where the terminal is located, the terminal locks the downlink data stream; receiving the time synchronization signal, K preset time points for sending random access requests corresponding to K transmission opportunities, and uplink access channel time-frequency parameters in the downlink data stream using the terminal, the K preset time points having a third delayed transmission time relative to the frame header of the Nth gateway frame, and K is a positive integer; After the Nth gateway frame is time-synchronized with the Nth terminal frame, the method further includes: using the gateway to point the 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, i The preset times are:

[0083] 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.

[0084] In a specific example, Fig.12 As shown, when the terminal is turned on, it first adjusts to the random access beam frequency band to start the search procedure, 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 is successfully locked. The terminal receives the time synchronization signal of the access satellite communication system and the time and frequency parameters of the uplink access channel in the downlink data stream, and then sends an uplink access request instruction in the random beam.

[0085] Furthermore, the coverage of the downlink beam and uplink beam randomly accessed by the terminal may be slightly different. This is because after receiving the downlink beam, the terminal needs to wait for the second delayed transmission time to complete frame synchronization and then send an access request or data. Assuming that the radius of the downlink beam is R1 and the radius of the uplink beam is R2, the distance between the downlink beam and the uplink beam is R=R2-R1. If the second delayed transmission time is a maximum of 100ms and the low-orbit satellite moves at a speed of 8km / s, then R needs to be 800 meters.

[0086] Furthermore, when the beam covers continuously, the terminal actively sends a random access request at a time point specified by the network, or sends it at a sending time specified by bandwidth allocation, and the uplink gateway receiver receives data sent by different terminals in a time-sharing manner.

[0087] Furthermore, when the beam hopping coverage is changed, it is divided into random access and reception according to bandwidth allocation. Furthermore, if the terminal is random access, the base station notifies the terminal of the expected arrival time of random access in a time synchronization signal or other message, and directs the beam to the terminal at the time when the data should arrive, and the terminal sends a random access request at this time.

[0088] Furthermore, it is assumed that the Nth gateway frame + t1 of the base station receiver is the arrival time of the random access specified by the base station for terminal 1. The base station needs to point the beam to terminal 1 at the local Nth gateway frame + t1. 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.

[0089] Furthermore, the second delayed sending time may be: , is the delay from the satellite to the center of the transmit beam.

[0090] Furthermore, the sending time of the terminal sending the random access request is: , and t1 are pre-calculated information provided by the base station and can be carried in the time synchronization signal.

[0091] Furthermore, if , then the sending time of the terminal sending the random access request can be simplified to: .

[0092] Further, it can be simplified as the duration of the first X frames after the base station sends a time synchronization signal to a terminal or a group of terminals, directs the receiving beam to the terminal or a group of terminals, and the terminal receives the time synchronization signal. Send a random access request or data at the time. If the time is small enough, such as less than 80ms or 100ms, this method can complete terminal access and data transmission independently of the terminal position and running speed direction.

[0093] Furthermore, for a group of terminals, it is necessary to avoid collisions caused by simultaneous transmissions. Assume that the duration of the beam is a time window W, and divide K random access beam transmission opportunities within W, that is, K transmission opportunities. Each terminal randomly selects a transmission opportunity k(i), i=1, 2, 3, ..., K, then the i-th transmission time of the terminal is the time after receiving the time synchronization signal. This can support a group of terminals to access or send data, while effectively reducing the probability of data collision at the receiving end. The base station can be set to allow terminals to use multiple transmission opportunities, and a single terminal can use a maximum of all K transmission opportunities.

[0094] Furthermore, in the beam hopping scenario, the base station allocates bandwidth according to the terminal demand, the terminal sends according to the sending time specified by the bandwidth allocation, the receiving beam adjusts the beam direction according to the terminal sending time specified by the bandwidth allocation, and the gateway receiver receives the data sent by different terminals in time division. The difference between the hopping beam and the continuous beam is that the hopping beam points the beam to the terminal at the same time at the sending time specified by the terminal.

[0095] Furthermore, 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 spot beam to the terminal. If the terminal does not have high-priority data, the reported data (Backlog) is lower than the threshold, or the terminal does not provide a geographic location, the terminal continues to stay in the random access beam.

[0096] Furthermore, when it is necessary to deploy a service spot beam, the base station sends the user terminal access information to the beam control module. The beam control module allocates a beam to the user, and the satellite base station schedules the service spot beam to cover the current location of the terminal. The terminal performs two-way data transmission in the service spot beam. When planning beam hopping, the newly connected user terminal can be allocated with high priority.

[0097] In one possible implementation, Fig.13 As shown, the use of the terminal to receive the time synchronization signal sent by the satellite, 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 includes: using the gateway to measure the delayed 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 delayed arrival time 1402 or the early arrival time to the terminal; using the terminal to calculate the second delayed sending time 1403 according to the delayed arrival time 1402 or the early arrival time; The second delayed sending time is:

[0098] 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.

[0099] Furthermore, Fig.13 The data packet also includes: a duration of the first X frames 1404, a first delayed sending time 1405, an initial value 1406 of the transmission delay time from the gateway to the terminal, and an initial value 1407 of the transmission delay time from the terminal to the gateway.

[0100] In a specific example, the time synchronization signal of the Nth gateway frame is expected to be sent at a duration of X frames before the Nth gateway frame. Assume that the transmission delay time from the terminal to the gateway is D, in ns. For satellites in different orbits, D can be a constant, fast or slow change. The sending offset configuration parameter of the time synchronization signal is the first delayed sending time.

[0101] Furthermore, the method for measuring the delay from the satellite to the ground or from the gateway to the terminal is as follows. Assuming that the initial value of the transmission delay time from the terminal to the gateway is D(0), the initial value D(0) can be the delay from the satellite to the center of the beam where the time synchronization signal is located. In fact, the time synchronization signal is sent over the air interface after the first delay transmission time. After the terminal receives the time synchronization signal, according to Calculate the start time of the Nth terminal frame.

[0102] Furthermore, the terminal uses the end time of the second delayed transmission time after receiving the time synchronization signal as the transmission starting point of the Nth terminal frame, that is, the frame boundary of the Nth terminal frame, to complete the time synchronization of the Nth terminal frame corresponding to the Nth gateway frame of the satellite master station. If the terminal is at the center of the beam and there is no deviation in the transmission delay, the physical frame sent by the terminal at the frame boundary of the local Nth terminal frame arrives at the frame boundary of the gateway at the time of the Nth gateway frame. If the terminal is not at the center of the beam, the physical frame sent by the terminal at the frame boundary of the local Nth terminal frame will have 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 with a measurement accuracy of nanoseconds.

[0103] Furthermore, the terminal starts to send data packets at the frame boundary of the Nth terminal frame, and the gateway time-marks the received data packets. The delayed arrival time or early arrival time of the data at the frame boundary of the Nth gateway frame is ΔD, where ΔD can be a negative number, indicating early arrival. From this, the transmission delay time from the terminal to the gateway can be calculated as: .

[0104] Further, the gateway feeds back the measured delay deviation to the terminal, and the terminal uses the new delay parameter when calculating the second delayed sending time, and the second delayed sending time is: This means that the terminal's transmission time is advanced or delayed by ΔD time, so that the time it reaches the gateway is just at the starting point of the frame boundary. By measuring the transmission delay through the gateway, closed-loop time synchronization can be performed to achieve more accurate time synchronization.

[0105] Furthermore, assuming that the terminal sends a data frame at the Nth terminal frame + t1, and the data frame carries the frame Nth terminal frame + t1 information, and the data frame arrives at the Nth gateway frame + t2, then the error Δt = t2-t1. The gateway sends the error Δt to the terminal, and the terminal uses the error Δt to update the frame boundary of the Nth terminal frame. 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 according to the initial value, the terminal can maintain relatively accurate time synchronization through uplink data transmission. Note that the time synchronization here does not rely on the position of the terminal and the satellite to calculate the satellite-to-ground delay. If the terminal does not send data, the satellite-to-ground delay cannot be measured, so after a period of time, it is necessary to receive a new time synchronization signal to establish time synchronization.

[0106] Another embodiment of the present invention provides a satellite communication system, which includes a gateway, a terminal and a satellite; 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 first 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 of the second delayed sending time so that the Nth gateway frame is synchronized with the Nth terminal frame.

[0107] In a possible implementation manner, the satellite is a regenerative satellite, and the second delayed sending time is:

[0108] 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.

[0109] In a possible implementation manner, the satellite is a transparent forwarding satellite, and the second delayed sending time is:

[0110] 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.

[0111] In a specific example, the satellite communication system realizes the connection and networking between the terminal and the gateway system through a transparent forwarding satellite or a regeneration satellite with on-board processing capability.

[0112] In a specific example, Fig.14 As shown in FIG. 1 , 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 to the transparent forwarding satellite via the link gateway, and the transparent forwarding satellite performs transparent forwarding and is received by the terminal. The data sent from the terminal to the external network is forwarded from the terminal to the link gateway via the transparent forwarding satellite, and then sent to the external network via the three-layer gateway.

[0113] In a specific example, Fig.15 As shown in the figure, the satellite is no longer a transparent forwarding satellite, but a regenerative satellite with onboard processing capabilities, so the link gateway needs to be deployed on the regenerative satellite. The data between the terminal and the external network needs to pass through the intersatellite link and reach the feeder gateway of the ground gateway system through the feeder, and then interconnect with the external network through the three-layer gateway. The downlink and uplink data of the terminal accessing the satellite network are processed by the onboard link gateway.

[0114] This embodiment enables time synchronization when terminals access the satellite communication system and the gateway system, ensuring that the data sent and received by 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.

[0115] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the embodiments here. All obvious changes or modifications derived from the technical solution of the present invention are still 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; The terminal is used to receive the time synchronization signal sent by the satellite, and a second delayed sending time is calculated 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 an Nth terminal frame corresponding to the Nth gateway frame is sent at the end of the second delayed sending time to synchronize the time of the Nth gateway frame with the Nth terminal frame.

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 within 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 within 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 at S-1 offset time points relative to the starting point at a time 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 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.

7. 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.

8. 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 of the second delayed sending time so that the Nth gateway frame is synchronized with the Nth terminal frame time.

9. The satellite communication system according to claim 8, 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.

10. The satellite communication system according to claim 8, 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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