CN116015411B

CN116015411B - High dynamic terminal and switching method for satellite mobile communication system

Info

Publication number: CN116015411B
Application number: CN202211621396.2A
Authority: CN
Inventors: 丁亚南; 江丹; 鲍峰; 孙亮亮; 肖跃; 刘剑锋
Original assignee: Nanjing Panda Handa Technology Co Ltd
Current assignee: Nanjing Panda Handa Technology Co Ltd
Priority date: 2022-12-16
Filing date: 2022-12-16
Publication date: 2024-06-18
Anticipated expiration: 2042-12-16
Also published as: CN116015411A

Abstract

The invention discloses a high dynamic terminal of a satellite mobile communication system and an access method, wherein the terminal comprises an antenna radio frequency module, a high crystal oscillator stabilizing module, a baseband module and a protocol processing module. The method comprises the following steps: adopting parallel computing and ephemeris data auxiliary self-adaptive quick capture technology to perform initial synchronization under the high dynamic link condition; the gateway station detects the timing or frequency offset of an uplink service channel of the satellite terminal, sends a ranging burst PAB3 to the gateway station, accurately adjusts the uplink timing and frequency offset of the terminal according to the PAB3, and sends the uplink timing and frequency offset to the satellite terminal; and the satellite terminal sends switching access signaling to the gateway station by using PAB3 burst, and is used for measuring uplink time difference and frequency difference, sharing the switching access signaling by a circuit channel and a packet channel, and performing terminal cross-satellite switching and terminal cross-beam switching. The invention realizes the high dynamic terminal access by using the satellite mobile communication system under the condition of not adding a new communication mode, reduces the network operation complexity and improves the utilization rate of system resources.

Description

High dynamic terminal and switching method for satellite mobile communication system

Technical Field

The invention relates to the technical field of satellite mobile communication terminals, in particular to a high-dynamic terminal and a switching method of a satellite mobile communication system.

Background

The satellite mobile communication system is used as extension and supplement of ground cellular mobile communication, is mainly used for communication and emergency communication in remote areas, and well solves the problem that the coverage capability of the ground cellular mobile communication system in remote areas and sea areas is limited. In general, a typical satellite mobile communication system is composed of GEO/LEO satellites, satellite terminals/mobile earth stations, gateway stations GW, and operation control systems, and can provide services such as telephone, short message, internet surfing, fax, etc. for users, and through interconnection with PSTN, PLMN, internet, the global service interconnection is realized. Figure 1 shows a block diagram of the elements of a satellite mobile communication system based on the GMR-1 standard.

In order to acquire service of a satellite mobile communication system, a user terminal firstly blindly searches an FCCH signal broadcasted by the system, selects an optimal beam to reside according to the size of a receiving level to acquire network information in a broadcast channel BCCH of the system, and then, the user obtains a special control channel resource DACCH through a random access channel RACH, and completes a network access authentication flow on the channel. After network access is completed, the user terminal can apply for a circuit traffic channel (such as NT 3) or a packet traffic channel (such as PNB (10, 3)) to the network, and complete corresponding traffic data transmission through different traffic channels.

The L or S frequency band satellite terminal is favored by high-dynamic platforms such as low-orbit spacecrafts and hypersonic aircrafts due to the advantages of miniaturization and low power consumption, and the L or S frequency band satellite terminal is utilized to realize tasks such as track monitoring, flight control, maneuvering orbit change, remote measurement/remote control and the like, however, the satellite mobile communication system mainly provides medium-low speed service access service for satellite terminals such as ground, sea, air and the like with lower movement rate, and the system supports the maximum timing change rate of 0.32us/S and the maximum frequency change rate of 24.6Hz/S. However, the high dynamic platform has high moving speed and fast speed change, which results in that the satellite terminal exceeds the timing change rate and frequency change rate threshold supported by the mobile communication system.

The current satellite mobile communication system is mainly designed aiming at the access of low-dynamic terminals such as ground, offshore, air and the like, and cannot support the satellite terminals carried on high-dynamic platforms such as low-orbit spacecrafts, hypersonic aircrafts and the like, and the common practice is to develop an independent system to support the access of the high-dynamic satellite terminals, however, the system operation is complex and the resource utilization rate is low.

Disclosure of Invention

The invention aims to provide a high dynamic satellite terminal and an access method for supporting the high dynamic satellite terminal access of a satellite mobile communication system, which have low system operation complexity and high system resource utilization rate.

The technical solution for realizing the purpose of the invention is as follows: a high dynamic terminal of a satellite mobile communication system comprises an antenna radio frequency module, a high crystal oscillator module, a baseband module and a protocol processing module;

The antenna radio frequency module adopts a multi-plane array antenna design, and realizes the receiving and transmitting of mobile communication satellite signals under different communication elevation angles by disposing a plurality of antenna array elements on array planes in different directions; all array surfaces are provided with independent low-noise amplifiers, and share one power amplifier through a high-speed transmission electronic change-over switch; the radio frequency channel comprises 2 independent down-conversion channels and 1 up-conversion channel, which are respectively connected with 2 ADCs and 1 DAC; the down-conversion channel is connected with the low-noise amplifier through a high-speed receiving electronic change-over switch; the ADC samples signals in the whole communication frequency band to form a broadband digital intermediate frequency signal;

The high-stability crystal oscillator module adopts high-stability crystal oscillator to provide clocks for the frequency converter, the ADC/DAC and the FPGA module, and can take an external clock as a reference signal;

The baseband module is operated on the FPGA, and a multipath parallel acquisition module, a high-dynamic dedicated channel processing module and a link precompensation mechanism module are added on the basis of mobile communication baseband processing; the multi-channel parallel capturing module is used for mixing the broadband digital intermediate frequency signal and the Doppler precompensated digital local oscillation signal, extracting the signals through the multi-stage filter to form a multi-channel broadcast channel baseband data buffer zone, and finally carrying out parallel blind capturing and fine synchronization on the data of the baseband data buffer zone to generate resident frequency point and time frequency offset information; the high dynamic special channel processing module is used for the functions of modulation and demodulation, encoding and decoding and power control of the channel; the link precompensation mechanism module is used for calculating the relative distance and speed between the terminal and the satellite according to the local time, the local position and satellite ephemeris information, so as to estimate time difference and frequency difference information and adjust the system timing;

The protocol processing module is operated on the general CPU, and a link layer time-frequency adjusting module, a system switching control module, a measuring module and a radio frequency switching control module are added on the basis of a mobile communication high-layer protocol; the link layer time-frequency adjustment module adjusts the sending timing and frequency offset of the uplink signal in real time according to the time-frequency adjustment command sent by the gateway station, and sends the PAB3 ranging message when SyncStatus=0, so as to accelerate the rapid convergence of the system timing; the system switching control module and the gateway station complete a star-crossing beam switching process, so that time-frequency resource reconfiguration and data retransmission in the switching process are realized; the measurement module periodically performs the measurement of the adjacent beam of the current service beam, reports the receiving level of the adjacent beam, and reports the position information of the current terminal after the movement distance of the terminal exceeds a certain threshold; the radio frequency switching control module calculates the direction from the terminal to the communication satellite in real time according to the local time position and the satellite ephemeris information, so that the service channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving and transmitting, and the measurement channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving according to the requirement of the measurement module.

Furthermore, the highly dynamic terminal also needs to provide the satellite terminal with clock signals synchronized with GNSS, high-precision position and time information for system synchronization, handover control and link precompensation.

The access method of the high-dynamic terminal of the satellite mobile communication system is based on the high-dynamic terminal of the satellite mobile communication system and comprises an initial synchronization flow under the condition of a high-dynamic link, a synchronization maintaining flow under the condition of the high-dynamic link, a terminal cross-satellite switching flow and a terminal cross-beam switching flow.

Compared with the prior art, the invention has the remarkable advantages that: the invention realizes the access of the high-dynamic satellite terminal in the infrastructure of the existing satellite mobile communication system by the mode of sharing the common control channel and expanding the service channel, reduces the complexity of system operation and improves the utilization rate of system resources.

Drawings

Fig. 1 is a schematic diagram of a typical satellite communication system network.

Fig. 2 is a schematic structural diagram of a high dynamic terminal of a satellite mobile communication system according to the present invention.

Fig. 3 is a schematic flow chart of a terminal cross-star switching in the present invention.

Fig. 4 is a schematic flow chart of cross-beam handover of a terminal in the present invention.

Figure 5 is a graph of doppler frequency offset received by a low-orbit satellite based terminal within a certain visible arc in an embodiment.

Figure 6 is a graph of the rate of change of the received doppler frequency offset for a low-orbit satellite based terminal within a certain visible arc in an embodiment.

Fig. 7 is a graph of a low-orbit satellite-borne terminal and a mobile communication satellite time delay change rate in a certain visible arc segment in an embodiment.

Detailed Description

Referring to fig. 2, a high dynamic terminal of a satellite mobile communication system includes an antenna radio frequency module, a high crystal oscillator module, a baseband module and a protocol processing module;

the protocol processing module is operated on the general CPU, and a link layer time-frequency adjusting module, a system switching control module, a measuring module and a radio frequency switching control module are added on the basis of a mobile communication high-layer protocol; the link layer time-frequency adjustment module adjusts the sending timing and frequency offset of the uplink signal in real time according to the time-frequency adjustment command sent by the gateway station, and sends the PAB3 ranging message when the link synchronization fails, so as to accelerate the rapid convergence of the system timing; the system switching control module and the gateway station complete a star-crossing beam switching process, so that time-frequency resource reconfiguration and data retransmission in the switching process are realized; the measurement module periodically performs the measurement of the adjacent beam of the current service beam, reports the receiving level of the adjacent beam, and reports the position information of the current terminal after the movement distance of the terminal exceeds a certain threshold; the radio frequency switching control module calculates the direction from the terminal to the communication satellite in real time according to the local time position and the satellite ephemeris information, so that the service channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving and transmitting, and the measurement channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving according to the requirement of the measurement module.

The invention also provides an access method of the high-dynamic terminal of the satellite mobile communication system, which comprises an initial synchronization flow under the high-dynamic link condition, a synchronization maintaining flow under the high-dynamic link condition, a terminal cross-star switching flow and a terminal cross-beam switching flow. Fig. 3 is a schematic flow chart of terminal cross-star switching in the present invention, and fig. 4 is a schematic flow chart of terminal cross-beam switching in the present invention.

As a specific example, the initial synchronization procedure under the high dynamic link condition includes the following steps:

Step 1.1, writing information such as a broadcast channel frequency point list, ephemeris data, beam pointing data and the like into nonvolatile storage equipment of a high-dynamic satellite terminal, or uploading updated data such as the broadcast frequency point list, ephemeris, beam pointing and the like to the satellite terminal through a measurement and control channel in an in-orbit operation stage;

Step 1.2, the satellite terminal estimates the time T reaching the coverage area of the mobile communication satellite according to the information such as the local time and the position information, the ephemeris data, the beam pointing data and the like, sets a wake-up timer, and enters a dormant state to wait for wake-up;

step 1.3, after the satellite terminal wakes up, estimating broadcast channels of all beams at the current position according to the local time and position information, ephemeris data and beam pointing data, and forming a candidate broadcast frequency point set to perform frequency search so as to reduce related calculation of the useless broadcast channels;

Step 1.4, if the ephemeris data are not expired, extrapolating the position and the relative speed of the mobile communication satellite according to the local time, the position information and the ephemeris data, calculating Doppler frequency shift by combining a broadcasting channel center frequency point in a candidate channel set, pre-compensating the Doppler frequency shift, accelerating the synchronization process through multi-channel parallel calculation, performing sliding search on each path by utilizing a matched filter, and selecting the path with the largest amplitude as an access channel;

Step 1.5, if the ephemeris data are expired, the satellite terminal enters a parallel searching stage, and parallel capturing calculation is carried out for each central frequency point in the candidate broadcast channel frequency point set;

Step 1.6, once the frequency point is selected, the satellite terminal continuously receives the broadcast signal and obtains the latest ephemeris information, if the ephemeris data is not expired, the position and the relative speed of the mobile communication satellite are extrapolated according to the local time, the position information, the ephemeris data and the like, the Doppler frequency shift and the round trip delay are calculated by combining the central frequency point of the broadcast channel in the candidate channel set, and the uplink random access channel is pre-compensated;

Step 1.7, after receiving the random access message sent by the satellite terminal, the gateway station measures the random access burst, calculates the time difference frequency offset, and immediately feeds back the time frequency difference to the terminal through the AGCH channel and distributes a special service channel for transmission.

As a specific example, if the ephemeris data in step 1.4 is not expired, extrapolating the position and the relative speed of the mobile communication satellite according to the local time, the position information and the ephemeris data, calculating the doppler shift by combining the central frequency point of the broadcast channel in the candidate channel set, pre-compensating the doppler shift, accelerating the synchronization process by multiple parallel calculation, performing sliding search on each path by using a matched filter, and selecting the path with the largest amplitude as the access channel, which is specifically as follows:

step 1.4.1, sampling signals in the whole communication frequency band through an ADC (analog to digital converter), forming a broadband digital intermediate frequency signal, mixing the digital intermediate frequency signal with a Doppler precompensated digital local oscillation signal, and then extracting through a multi-stage filter to form a multi-channel broadcast channel baseband data buffer zone;

step 1.4.2, performing blind detection on the data in each baseband data buffer zone, performing sliding correlation processing s (t) on the baseband data and a locally generated dual Chirp signal, finding the maximum energy position of a correlation peak, adjusting the pointer position of the buffer zone to the maximum energy point, and performing fine synchronization;

Wherein f is the sweep frequency of the chirp signal, and T is the burst interval;

step 1.4.3, correlating s _up (t) generated locally with data in a baseband buffer area, performing FFT operation, searching for a peak value of amplitude, calculating a signal-to-noise ratio (SNR), judging that a broadcast signal is not detected if the current SNR is smaller than a set threshold SNR, and terminating searching; otherwise, if the SNR is higher than the set threshold SNR, judging that the signal arrives, recording the FFT peak value f _up at the moment, and stopping searching;

the calculation formula of s _up (t) is as follows:

Step 1.4.4, after all paths are searched, selecting a broadcasting channel with the maximum SNR as a resident channel, carrying out FFT operation after the value s _dn (t) is related to the data in the baseband buffer area, recording the FFT peak value f _dn at the moment, and calculating the frequency shift and time delay estimation at the moment;

The calculation formula of s _dn (t) is as follows:

f_d＝0.5·(f_dn+f_up)

Where f _s is the symbol rate of the mobile communication system.

As a specific example, if the ephemeris data in step 1.5 has expired, the satellite terminal enters a parallel search phase, and performs parallel acquisition calculation for each center frequency point in the candidate broadcast channel frequency point set, which is specifically as follows:

Step 1.5.1, calculating the number of parallel paths Num _path according to the central frequency point of the broadcast channel, the search window size Win _search and the maximum Doppler frequency Max _doppler information:

the center frequency point of the nth search path is f _n＝f±n*Win_search;

Step 1.5.2, the search window size Win _search is determined by the frequency correction range of the broadcasting channel FCCH, the value is + -7.5 KHz, and when the maximum Doppler frequency shift is 47.88KHz, one frequency point needs to be configured with 15 parallel search paths; selecting the strongest signal as the final signal;

Step 1.5.3, sampling the appointed communication frequency band signal through an ADC to form a broadband digital intermediate frequency signal, mixing the digital intermediate frequency signal with the Doppler precompensated digital local oscillation signal, and then extracting through a multi-stage filter to form a multi-channel broadcast channel baseband data buffer area;

Step 1.5.4, performing blind detection and accurate synchronization on all the search paths according to a normal search mode, after all the paths are searched, if the blind detection cannot obtain a broadcast signal, indicating that no broadcast signal exists on a relevant frequency point, otherwise, selecting a path with the maximum SNR, and calculating frequency shift and time delay estimation at the moment;

And 1.5.5, compensating other candidate broadcast channels in advance by using the obtained Doppler frequency offset and delay parameters, measuring the SNR of all broadcast signals, and selecting the broadcast channel with the largest SNR as a resident cell.

As a specific example, the synchronization maintaining process under the high dynamic link condition includes the following steps:

Step 2.1, the gateway station sends time-frequency synchronous signaling to the satellite terminal, and sets Sync status=1;

Step 2.2, after receiving the time-frequency synchronous signaling, the satellite terminal sends a normal service data packet when the Sync status=0; when Sync status=1, the satellite terminal transmits a ranging instruction PAB3 to the gateway station;

Step 2.3, the gateway station accurately measures uplink offset and frequency offset according to PRACH3, and sends the uplink offset and frequency offset to the satellite terminal through time-frequency synchronous signaling, and if the uplink offset or frequency offset exceeds a synchronous window, the Sync status=1 is set; otherwise, set Sync status=0;

and 2.4, repeating the steps 2.2 to 2.3 until the terminal detects that the Sync Status is 0, and stopping transmitting the PRACH3 channel.

As a specific example, a terminal cross-star switching process and a terminal cross-beam switching process adopt a MAC layer switching control process, and based on MAC layer signaling, position report signaling, neighbor cell measurement report signaling and switching control signaling are extended;

The position report signaling is sent to the gateway station by the satellite terminal, and when the satellite terminal finds that the movement distance exceeds the threshold, the position report signaling is used for reporting the position information of the satellite terminal and judging whether the terminal needs to switch between satellites or wave beams; in the circuit type channel, the position report signaling does not need to increase the MAC head, and occupies 40 bits before the user service information or signaling information; in the packet-type channel, inheriting the MAC layer PACKET MEASUREMENT REPORT signaling in the satellite mobile communication system;

The neighbor cell measurement report signaling is sent to the gateway station by the satellite terminal, and the satellite terminal reports the receiving level condition of the neighbor wave beam to the network according to a fixed period and is used for judging whether the terminal needs to switch between satellites or wave beams by the gateway station; in a circuit type channel, the measurement report signaling does not need to increase an MAC header, and occupies 40 bits before user service information or signaling information; in the packet-type channel, inheriting the MAC layer PACKET MEASUREMENT REPORT signaling in the satellite mobile communication system;

The switching command signaling is sent to the satellite terminal by the gateway station, and the gateway station distributes switched time-frequency resources for the user according to neighbor cell measurement information, position information and resource conditions of the satellite terminal; in the circuit-type channel, the measurement report signaling does not need to add a MAC header, and occupies 40 bits before the user service information or signaling information.

As a specific example, the terminal cross-star switching process includes the following steps:

Step 3.1, the terminal periodically measures the broadcasting signal intensity of the adjacent satellite by adjusting the frequency point corresponding to the measuring channel and periodically broadcasting the signal intensity of the adjacent beam under the condition of not influencing service communication, and periodically measures the broadcasting signal intensity of the adjacent satellite by controlling the receiving electronic change-over switch to adjust the antenna array surface corresponding to the measuring channel to point to the adjacent satellite, or reports the receiving level and the terminal position information of the measured adjacent beam to the gateway station through the source satellite after the distance exceeds a threshold;

step 3.2, the gateway station makes a switching decision according to the terminal measurement report and the position information, and provides switching command information to the user, wherein the switching command information comprises a target satellite, beam information, time-frequency resources and power; the gateway station simultaneously maintains two receiving links on the source satellite and the target satellite and a transmitting link on the source satellite;

Step 3.3, the terminal adjusts the measuring channel to aim at the target satellite according to the target ephemeris, the local position and the time information, pre-compensates the receiving timing and Doppler frequency offset information, synchronizes the downlink service channel of the target satellite, and simultaneously keeps the communication link between the service channel and the source satellite uninterrupted;

step 3.4, after the terminal completes the synchronization of the downlink service channel, controlling the electronic switching switch to adjust the antenna array surface corresponding to the service transmission channel to the target satellite, and transmitting switching access signaling on the uplink service channel of the target satellite through the PAB3 channel;

Step 3.5, the gateway station monitors an uplink service channel on the target satellite, measures the PAB3 immediately after receiving the PAB3 signaling, calculates the time difference and the frequency difference, and immediately informs the terminal of uplink time-frequency adjustment through a time-frequency synchronous command;

Step 3.6, after receiving the time-frequency synchronous command, the terminal checks a SyncStatus mark, if the mark is 0, the synchronization is finished, the adjustment is carried out according to the time frequency offset given by the time-frequency synchronous command, the service data is normally sent, and meanwhile, the service channel on the source satellite is released; if the mark is 1, the synchronization is not completed, and the terminal adjusts the time frequency offset according to the time frequency synchronization command and sends PAB3;

and 3.7, continuously executing the steps 3.5-3.6 until the gateway station judges that the uplink time-frequency adjustment of the target satellite is successful, transmitting and adjusting downlink service data to the target satellite, and simultaneously releasing a service channel on the source satellite.

As a specific example, the terminal cross-beam switching process includes the following steps:

step 4.1, the terminal periodically measures the broadcasting signal intensity of the adjacent beam by adjusting the frequency point corresponding to the measuring channel, or reports the receiving level of the adjacent beam and the position information of the terminal to the gateway station by the source beam after the distance exceeds the threshold;

Step 4.2, the gateway station makes a switching decision according to the terminal measurement report and the position information, and provides switching command information to the user, wherein the switching command information comprises target beam information, time-frequency resources and power; the gateway station simultaneously maintains two receiving links on the source beam and the target beam and a transmitting link on the source satellite;

Step 4.3, the terminal adjusts the frequency point corresponding to the antenna receiving channel to receive on the target beam according to the target ephemeris, the local position and the time information, does not need to pre-compensate the receiving timing and Doppler frequency offset information, synchronizes the downlink service channel of the target beam, and simultaneously keeps the communication link with the original satellite uninterrupted;

Step 4.4, after the terminal completes the synchronization of the downlink service channel of the target beam, adjusting the frequency point corresponding to the service transmission channel, and transmitting a switching access signaling on the uplink service channel of the target satellite through the PAB3 channel;

step 4.5, the gateway station monitors an uplink service channel on the target beam, measures the PAB3 immediately after receiving the PAB3 signaling, calculates the time difference and the frequency difference, and immediately informs the terminal of uplink time-frequency adjustment through a time-frequency synchronous command;

Step 4.6, after receiving the time-frequency synchronous command, the terminal checks a SyncStatus mark, if the mark is 0, the synchronization is finished, the adjustment is carried out according to the time frequency offset given by the time-frequency synchronous command, the service data is normally sent, and meanwhile, the service channel on the source beam is released; if the mark is 1, the synchronization is not completed, and the terminal adjusts the time frequency offset according to the time frequency synchronization command and sends PAB3;

and 4.7, continuously executing the steps 4.5-4.6, successfully adjusting the uplink time frequency of the target beam, adjusting the downlink service data transmission to the target beam, and simultaneously releasing the service channel on the source beam.

The invention will be described in further detail with reference to the drawings and the specific examples.

Examples

1. High dynamic satellite communication link analysis

In order to realize the access of the high dynamic satellite terminal, firstly, the condition of a communication link between the terminal and a satellite is analyzed, the condition of frequency change is analyzed by taking a low orbit spacecraft as an example, the low orbit spacecraft is assumed to have a 380km orbit, the orbit inclination angle is 32 degrees, the lowest communication elevation angle of the low orbit satellite terminal is 8.5 degrees, and the working frequency band is 2GHz. The doppler frequency deviation and the doppler frequency deviation change rate in a certain visible arc section are shown in fig. 5 and 6.

As can be seen from fig. 5 and fig. 6, the low-orbit satellite can establish communication connection with the satellite in 3 time periods, and fig. 1 shows that the maximum doppler shift of the downlink caused by the motion of the low-orbit satellite is 47.88kHz, which is far beyond the frequency correction range and the effective communication bandwidth of the satellite mobile communication system, that is, the satellite-borne terminal directly adopts a general frequency correction mode, so that the acquisition synchronization of the spot beam broadcast signal cannot be completed. In fig. 2, the rate of change of doppler shift relative to a mobile communication satellite due to rapid movement of a low-orbit satellite is at most 62.32Hz/s,

The frequency change condition is analyzed by a supersonic aircraft contour maneuvering platform, and the speed and the acceleration of the high maneuvering platform relative to the satellite motion are main parameters affecting the two indexes. Assuming that the movement speed of the supersonic aircraft is Mach 20, the acceleration is 20g for example, the working frequency band is 2GHz, the maximum Doppler frequency shift is about 45.3KHz, and the maximum Doppler change rate is about 1.3KHz/s.

From the above analysis of doppler shift and the rate of change of doppler shift, it is known that: under the condition of a low orbit spacecraft, the Doppler frequency shift correction capability of the satellite terminal is not less than +/-47.88 kHz, and the Doppler change rate correction capability is not less than 62.32Hz/s; under the condition of a supersonic aircraft, the Doppler frequency shift correction capability of the satellite terminal is not less than +/-45.3 KHz, and the Doppler change rate correction capability is not less than 1.3KHz/s. And the maximum frequency adjustment rate of the mobile communication system is 24.6Hz/s, the mobile communication system cannot maintain the frequency synchronization during the communication between the terminal and the satellite.

Taking a low orbit spacecraft as an example to analyze the time change condition, assuming that the orbit of the low orbit spacecraft is 380km, the orbit inclination angle is 32 degrees, the lowest communication elevation angle of a low orbit satellite terminal is 8.5 degrees, and the time change rate in a certain visible arc section is shown in figure 7.

As can be seen from FIG. 7, the maximum rate of timing change due to the relative motion of the low-orbit satellite and the mobile communication satellite is about 24us/s. The maximum timing rate of change is about 22.67us/s for supersonic aircraft contour platform maximum motion speed 20 mach. And the maximum frequency adjustment rate supported by the mobile communication system is 0.64us/s, the mobile communication system cannot maintain time synchronization in the communication process between the terminal and the satellite. In a satellite mobile communications system, the system can tolerate a timing offset of + -2.5 symbols, and if closed loop timing adjustment is not performed, the transmit timing will drift out of the synchronization window after about 4.5 s.

2. Initial synchronization mechanism under high dynamic link condition

The maximum Doppler frequency shift of the downlink caused by the motion of the high dynamic platform is +/-47.88 kHz, which is more than 2 times of the effective communication bandwidth, and under the condition of no assistance of ephemeris information, a matched filter is directly adopted to carry out sliding search on the frequency points of a broadcast channel list, so that the satellite terminal cannot obtain initial synchronization. In addition, serial searching in the doppler frequency offset range of the broadcast frequency point is not feasible, and in general, the synchronization processing of the satellite terminal with low motion speed to the single broadcast frequency point takes about 650ms at most, if the system configures 200 broadcast frequency points, the longest searching time is about 2.2 minutes, which exceeds the longest residence time of the satellite terminal in one beam.

The invention adopts parallel computing and ephemeris data auxiliary self-adaptive quick capture technology to carry out initial synchronization under the condition of high dynamic link, and comprises the following steps:

step 1.4, if the ephemeris data are not expired, extrapolating the position and the relative speed of the mobile communication satellite according to the local time, the position information and the ephemeris data, calculating Doppler frequency shift by combining the central frequency point of the broadcasting channel in the candidate channel set, pre-compensating the Doppler frequency shift, accelerating the synchronization process through multi-channel parallel calculation, carrying out sliding search on each path by utilizing a matched filter, and selecting the path with the largest amplitude as an access channel, wherein the method comprises the following steps of:

the calculation formula of s _up (t) is as follows:

The calculation formula of s _dn (t) is as follows:

f_d＝0.5·(f_dn+f_up)

wherein f _s is the symbol rate of the mobile communication system;

step 1.5, if the ephemeris data has expired, the satellite terminal enters a parallel search stage, and parallel capturing calculation is performed for each center frequency point in the candidate broadcast channel frequency point set, specifically as follows:

the center frequency point of the nth search path is f _n＝f±n*Win_search;

Step 1.5.5, compensating other candidate broadcast channels in advance by using the obtained Doppler frequency offset and delay parameters, measuring the SNR of all broadcast signals, and selecting the broadcast channel with the largest SNR as a resident cell;

3. Synchronization maintenance mechanism

In a satellite mobile communication system, the tolerable frequency deviation of a physical channel is +/-65 Hz, the frame length is 40ms, the frequency deviation of each frame frequency of a low-orbit spacecraft is about +/-2.5 Hz, a terminal dynamically tracks the downlink frequency change through downlink receiving the physical channel, the uplink needs to deviate from a frequency synchronization window by about 1s, the shortest time of closed loop adjustment of the system is about 560ms when the round trip time from the terminal to a gateway station is about, and the frequency synchronization of the terminal physical channel of the low-orbit spacecraft cannot be ensured through closed loop adjustment; the deviation of the frequency of each frame of the supersonic aircraft is about +/-78.4 Hz, the tolerable frequency deviation threshold of the downlink physical channel is exceeded, and the synchronization of the frequency of the physical channel of the supersonic aircraft terminal cannot be ensured through closed-loop adjustment. From the aspect of time change rate, the guard interval of the uplink physical channel is 1/23400 x 2.5= 106.84us, and the high dynamic terminal can at least maintain timing synchronization of about 4s, so that the communication can be ensured not to be interrupted through periodical uplink closed-loop timing adjustment; the downlink wideband physical channel is 10 times of the downlink narrowband physical channel bandwidth, the guard interval is about 10.68us, the timing synchronization of about 0.4s can be maintained, and the terminal dynamically tracks the downlink frequency change through the downlink receiving physical channel. Therefore, in order to realize normal access of the high dynamic platform terminal, all traffic channels except the FCCH must be enhanced and designed, so as to improve the capability of the traffic channels for resisting frequency offset.

In order to ensure that a satellite terminal correctly demodulates a downlink common control channel under a large Doppler condition and rapidly acquires message contents such as system broadcasting, access permission, paging and the like, a broadcasting channel is continuously broadcasted every frame by using an idle time slot of a carrier wave where the broadcasting channel is located, wherein the broadcasting mode is 'common control channel + CCCH + FCCH + CCCH', and the modulation mode of the CCCH channel is replaced by pi/4-DQPSK. The random access control channel sent by the satellite terminal in the uplink direction is pre-compensated for frequency difference based on satellite ephemeris, and the frequency deviation is smaller.

In order to facilitate satellite terminal service application under high dynamic state, two special low-speed narrow-band service channels and high-speed broadband service channels are designed respectively, in order to ensure that satellite terminals correctly transmit and receive service channels under the condition of large frequency offset change rate, the burst of the low-speed narrow-band service channels is NT4, and on the basis of NT3, a double chirp signal of one time slot is added to resist the influence of +/-1.0 KHz frequency offset; the high-speed broadband service channel is PNB3 (10, 4), on the basis of PNB3 (10, 3), a double chirp signal of one time slot is added to resist +/-1.0 KHz frequency offset, the system can allocate a plurality of parallel channels of NT4 or PNB3 (10, 4) for service transmission to a user so as to improve the service rate, wherein the low-speed narrowband service rate is 2.0 Kbps-12 Kbps, and the high-speed broadband service rate is 384 Kbps-2048 Kbps.

In a satellite mobile communication system, in order to realize that a communication link between a gateway station and a satellite terminal is not interrupted, the gateway station periodically adjusts the time frequency offset of an uplink of the terminal through a dedicated control channel FACCH, however, the motion characteristic of a high dynamic satellite terminal brings a larger time frequency offset change rate, in order to adjust the time frequency offset of the communication link, frequency adjustment or timing adjustment is carried out at least once every 1/4s, the frequency of the time frequency offset adjustment is far higher than that of a low-speed satellite terminal, the FACCH is realized by preempting service channel resources, and obviously, the resource utilization rate is greatly reduced through FACCH adjustment. For this purpose, control signaling is extended in the RLC/MAC layer, and is mainly used for time-frequency synchronization in the communication process, and the format of the time-frequency synchronization command is shown in table 1.

Table 1 time-frequency synchronization signaling format

The time-frequency synchronous signaling is sent to the satellite terminal by the gateway station, the gateway station continuously measures time difference and frequency difference information in the communication process and feeds back the time difference and the frequency difference information to the satellite terminal through the signaling, and the time-frequency synchronous signaling occupies 32 bits before the MAC head is not required to be added to the user service information in the circuit type channel; in the packet channel, the time-frequency synchronous signaling needs to add a MAC control header, and is placed at any position according to the requirement, wherein MESSAGE TYPE fields are expanded on the basis of the existing MAC control signaling, and the type of the measurement report signaling is 0x110101.

In order to ensure reliable and stable communication between the satellite terminal and the gateway station, when the gateway station detects that the timing or frequency offset of an uplink service channel of the satellite terminal exceeds a synchronous window, the gateway station immediately sends a ranging burst PAB3 to the gateway station, accurately adjusts the uplink timing and frequency offset of the terminal according to the PAB3, and sends the ranging burst PAB3 to the satellite terminal through a time-frequency synchronous signaling. The specific process is as follows:

4. Cross-beam cross-star handoff design

The simulation finds that the longest time of the low orbit satellite crossing one satellite mobile communication spot beam is about 170s, the shortest time is about 45s, and in this scene, the cross-beam switching is frequently performed between the satellite terminal mounted on the high dynamic platform and the mobile communication satellite. In a satellite mobile communication system, switching control is completed by an RRC layer between a gateway station and a satellite terminal, related switching and measurement signaling occupy a service burst, the switching frequency of the satellite terminal with low movement speed is low, the switching process of the RRC control can meet the application requirement, however, the high-dynamic satellite terminal is frequently switched, and the utilization rate of wireless resources of the system is reduced. Therefore, the scheme adopts a MAC layer switching control process. Based on the MAC layer signaling, signaling such as location reporting, measurement reporting, handover control, etc. is extended, as shown in table 2.

Table 2 position report signaling format definition

The position report signaling is sent to the gateway station by the satellite terminal, when the satellite terminal finds that the movement distance exceeds the threshold, the position information of the satellite terminal is reported through the position report signaling, and is used for judging whether the terminal needs to switch and switch the wave beam or not by the gateway station, and in a circuit type channel, the position report signaling occupies 40 bits before the MAC head is not required to be added to be placed in the user service information or the signaling information; in the packet-type channel, MAC layer PACKET MEASUREMENT REPORT signaling in the satellite mobile communication system is inherited. The measurement report signaling format definition is shown in table 3:

table 3 measurement report signaling format definition

The neighbor cell measurement report signaling is sent to the gateway station by the satellite terminal, the satellite terminal reports the receiving level condition of the neighbor wave beam to the network according to a fixed period, and is used for the gateway station to judge whether the terminal needs to switch and switch the wave beam, and in a circuit type channel, the measurement report signaling occupies 40 bits before the MAC head is not required to be added to be placed in the user service information or the signaling information; in the packet-type channel, MAC layer PACKET MEASUREMENT REPORT signaling in the satellite mobile communication system is inherited. The handover command format definitions 1, 2 are shown in tables 4, 5:

Table 4 switch command format definition 1

Table 5 handover command format definition 2

The switching command signaling is sent to the satellite terminal by the gateway station, the gateway station allocates the switched time-frequency resources for the user according to the measurement information, the position information, the resource condition and the like of the satellite terminal, and in the circuit type channel, the measurement report signaling occupies 40 bits before the user service information or the signaling information without adding the MAC header. The packet switch command format definition is shown in table 6:

Table 6 packet switch command format definition

In the PACKET-type channel, modifications are made on the basis of the MAC layer PACKET Downlink ASSIGNMENT TYPE and PACKET Downlink ASSIGNMENT TYPE in the satellite mobile communication system. The handover access signaling format definition is shown in table 7:

table 7 handover access signaling format definition

Parameters (parameters)	Number of bits	Function of	Remarks
				RetryTimer	2
G-RNTI	32	Terminal access network communication identifier
				Reserved	6	Reserved field

The switching access signaling is sent to the gateway station by the satellite terminal, and the satellite terminal sends the switching access signaling to the gateway station by using PAB3 burst, and is used for measuring uplink time difference and frequency difference, and the circuit channel and the packet channel are used for the common switching access signaling.

Referring to fig. 3, the terminal cross-star switching process includes the following steps:

Referring to fig. 4, the cross-beam switching process of the terminal includes the following steps:

And 4.7, continuously executing the steps 4.5 to 4.6 until the gateway station judges that the uplink time-frequency adjustment of the target beam is successful, transmitting and adjusting downlink service data to the target beam, and simultaneously releasing a service channel on the source beam.

5. High dynamic terminal design

The high dynamic terminal is mainly composed of antenna radio frequency, clock source, baseband, protocol processing and other modules, as shown in fig. 2. In a high dynamic scene, in order to realize uninterrupted communication between a satellite terminal and a mobile communication satellite, a design mode of 'two receipts and one transmission' is adopted, wherein the design mode comprises a service receiving channel, a signal measuring channel and a service transmitting channel, the service receiving and transmitting channel is mainly used for normal transmission of service data, and the measuring channel is mainly used for measurement of periodic adjacent wave beams of the terminal or broadcast signals of adjacent satellites and can also be used for downlink synchronization of target wave beams or satellites in a switching process.

The antenna adopts a multi-plane array antenna design, a plurality of antenna elements are deployed on the planes in different directions, so that the receiving and transmitting of mobile communication satellite signals under different communication elevation angles are realized, all planes are provided with independent low-noise amplifiers, and a power amplifier is shared by high-speed transmission electronic change-over switches. The radio frequency channel comprises 2 independent down-conversion channels and an up-conversion channel which are respectively connected with 2 ADCs and a DAC, the down-conversion channels are interconnected with the low-noise amplifier through a high-speed receiving electronic change-over switch, and the ADC samples signals in the whole communication frequency band to form a broadband digital intermediate frequency signal.

High stable crystal oscillator: in order to reduce the influence of unstable crystal oscillator on frequency offset, the system adopts high-stability crystal oscillator to provide clocks for the frequency converter, the ADC/DAC, the FPGA and other modules, and can take an external clock as a reference signal.

And a baseband module: the method comprises the steps of operating on an FPGA, adding a plurality of channels of parallel capturing, high-dynamic dedicated channel processing, link precompensation mechanism and other modules on the basis of mobile communication baseband processing, wherein the plurality of channels of parallel capturing comprises the steps of mixing a broadband digital intermediate frequency signal with a digital local oscillator signal after Doppler precompensation, extracting the mixed signals through a multi-stage filter to form a multi-channel broadcast channel baseband data buffer area, carrying out parallel blind capturing and fine synchronization on data of the baseband data buffer area, generating resident frequency point and time frequency offset information, and the high-dynamic dedicated channel mainly realizes the functions of modulation and demodulation, encoding and decoding, power control and the like of the channel; the link precompensation is used for calculating the relative distance and speed between the terminal and the satellite according to the local time and position, satellite ephemeris and other information, so as to estimate the time difference and frequency difference information and adjust the system timing.

The protocol processing module: the method comprises the steps that the method runs on a general CPU, on the basis of a mobile communication high-layer protocol, a link layer time-frequency adjustment module, a system switching control module, a measurement module and the like are added, the link layer time-frequency adjustment module adjusts the sending timing and frequency offset of an uplink signal in real time according to a time-frequency adjustment command sent by a gateway station, and PAB3 ranging information is sent when SyncStatus=0, so that the rapid convergence of system timing is quickened; the system switching control module and the gateway station complete a star-crossing beam switching process, so as to realize time-frequency resource reconfiguration, data retransmission and the like in the switching process; the measurement module periodically executes the measurement of the adjacent beam of the current service beam, reports the receiving level of the adjacent beam, and reports the position information of the current terminal after the movement distance of the terminal exceeds a certain threshold; the radio frequency switching control module calculates the direction from the terminal to the communication satellite in real time according to the local time position and the satellite ephemeris information, so that the service channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving and transmitting, and the measuring channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving according to the requirement of the measuring module. In addition, the high dynamic platform needs to provide the satellite terminal with clock signals synchronized with the GNSS, high-precision position and time information for system synchronization, handover control and link pre-compensation.

The invention is based on a satellite mobile communication system, on the basis of high dynamic satellite communication link analysis, adopts modes such as a shared common control channel, an extended service channel and the like, provides a parallel multipath signal capturing algorithm, designs a common control channel and a special service channel for a high dynamic platform terminal, realizes link synchronization maintenance under a high dynamic condition, designs a cross-satellite and cross-beam switching scheme based on a link layer, and provides a framework of the high dynamic satellite terminal on the basis. The invention expands and supports the access of the high dynamic satellite terminal on the basis of the original satellite mobile communication system, reduces the complexity of system operation and improves the utilization rate of system resources.

Claims

1. The high dynamic terminal of the satellite mobile communication system is characterized by comprising an antenna radio frequency module, a high crystal oscillator stabilizing module, a baseband module and a protocol processing module;

the high-stability crystal oscillator module adopts high-stability crystal oscillator to provide clocks for the frequency converter, the ADC/DAC and the FPGA module, and takes an external clock as a reference signal;

The protocol processing module is operated on the general CPU, and a link layer time-frequency adjusting module, a system switching control module, a measuring module and a radio frequency switching control module are added on the basis of a mobile communication high-layer protocol; the link layer time-frequency adjustment module adjusts the sending timing and frequency offset of the uplink signal in real time according to a time-frequency adjustment command sent by the gateway station, and sends a PAB3 ranging message when the link synchronization fails; the system switching control module and the gateway station complete a star-crossing beam switching process, so that time-frequency resource reconfiguration and data retransmission in the switching process are realized; the measurement module periodically performs the measurement of the adjacent beam of the current service beam, reports the receiving level of the adjacent beam, and reports the position information of the current terminal after the movement distance of the terminal exceeds a set threshold; the radio frequency switching control module calculates the direction from the terminal to the communication satellite in real time according to the local time position and the satellite ephemeris information, so that the service channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving and transmitting, and the measurement channel is switched to the corresponding antenna array surface and the matched radio frequency link for signal receiving according to the requirement of the measurement module.

2. The satellite mobile communication system high dynamic terminal according to claim 1, wherein the high dynamic terminal provides the satellite terminal with clock signal, position and time information synchronized with GNSS for assisting system synchronization, handover control and link precompensation.

3. The method for accessing the high-dynamic terminal of the satellite mobile communication system is characterized in that the method is based on the high-dynamic terminal of the satellite mobile communication system according to claim 1 or 2, and comprises an initial synchronization flow under a high dynamic link condition, a synchronization maintaining flow under the high dynamic link condition, a terminal cross-satellite switching flow and a terminal cross-beam switching flow.

4. The method for accessing a highly dynamic terminal of a satellite mobile communication system according to claim 3, wherein the initial synchronization procedure under the highly dynamic link condition comprises the steps of:

step 1.1, writing the information of the broadcasting channel frequency point list, the ephemeris data and the beam pointing data into nonvolatile storage equipment of a high-dynamic satellite terminal, or uploading the broadcasting frequency point list, the ephemeris and the beam pointing update data to the satellite terminal through a measurement and control channel in an in-orbit operation stage;

step 1.2, the satellite terminal estimates the time T reaching the coverage area of the mobile communication satellite according to the local time and position information, ephemeris data and beam pointing data information, sets a wake-up timer, and enters a dormant state to wait for wake-up;

Step 1.3, after the satellite terminal wakes up, estimating broadcast channels of all beams at the current position according to the local time and position information, ephemeris data and beam pointing data, and forming a candidate broadcast frequency point set to perform frequency search so as to reduce the related calculation of the useless synchronization channel;

Step 1.6, once the frequency point is selected, the satellite terminal continuously receives the broadcast signal and obtains the latest ephemeris information, if the ephemeris data is not expired, the position and the relative speed of the communication satellite are extrapolated according to the local time, the position information and the ephemeris data, doppler frequency shift and round trip delay are calculated by combining the central frequency point of the broadcast channel in the candidate channel set, and time-frequency precompensation is carried out on the uplink random access channel;

5. The method for accessing a high dynamic terminal in a satellite mobile communication system according to claim 4, wherein if the ephemeris data is not expired in step 1.4, extrapolating the position and the relative speed of the mobile communication satellite according to the local time, the position information and the ephemeris data, calculating the doppler shift by combining the center frequency point of the broadcasting channel in the candidate channel set, precompensating the doppler shift, accelerating the synchronization process by multiple parallel calculation, performing sliding search by using a matched filter on each path, and selecting the path with the largest amplitude as the access channel, wherein the method comprises the following steps:

the calculation formula of s _up (t) is as follows:

The calculation formula of s _dn (t) is as follows:

f_d＝0.5·(f_dn+f_up)

Where f _s is the symbol rate of the mobile communication system.

6. The method for accessing a high dynamic terminal in a satellite mobile communication system according to claim 4, wherein if the ephemeris data has expired in step 1.5, the satellite terminal enters a parallel search phase, and performs parallel acquisition calculation for each center frequency point in the candidate broadcast channel frequency point set, specifically as follows:

the center frequency point of the nth search path is f _n＝f±n*Win_search;

Step 1.5.4, performing blind detection and accurate synchronization on all the search paths according to the search mode of step 1.4, after all the paths are searched, if the blind detection cannot obtain a broadcast signal, indicating that no broadcast signal exists on the relevant frequency point, otherwise, selecting the path with the maximum SNR, and calculating the frequency shift and time delay estimation at the moment;

7. The method for accessing a high dynamic terminal in a satellite mobile communication system according to claim 3, wherein the synchronization maintaining process under the high dynamic link condition comprises the steps of:

8. The method for accessing a high dynamic terminal in a satellite mobile communication system according to claim 3, wherein a cross-satellite switching process and a cross-beam switching process of the terminal are performed by using a MAC layer switching control process, and based on a MAC layer signaling, a location report signaling, a neighbor cell measurement report signaling and a switching control signaling are extended;

The position report signaling is sent to the gateway station by the satellite terminal, and when the satellite terminal finds that the movement distance exceeds the threshold, the position report signaling is used for reporting the position information of the satellite terminal and judging whether the terminal needs to switch between satellites or wave beams; in a circuit channel, the position report signaling does not need to increase an MAC head, and occupies 40 bits before user service information or signaling information; in the packet-type channel, inheriting the MAC layer PACKET MEASUREMENT REPORT signaling in the satellite mobile communication system;

The switching control signaling is sent to the satellite terminal by the gateway station, and the gateway station distributes switched time-frequency resources for the user according to neighbor cell measurement information and position information and resource conditions of the satellite terminal; in the circuit-type channel, the measurement report signaling does not need to add a MAC header, and occupies 40 bits before the user service information or signaling information.

9. The method for accessing a highly dynamic terminal of a satellite mobile communication system according to claim 3, wherein the terminal cross-star switching process comprises the steps of:

Step 3.2, the gateway station makes a switching decision according to the terminal measurement report and the position information, and provides switching command information to the user, wherein the switching command information comprises a target satellite, beam information, time-frequency resources and power resources; the gateway station simultaneously maintains two receiving links on the source satellite and the target satellite and a transmitting link on the source satellite;

10. The method for accessing a high dynamic terminal in a satellite mobile communication system according to claim 3, wherein the terminal cross-beam switching process comprises the steps of:

step 4.4, after the terminal completes the synchronization of the downlink service channel of the target beam, the frequency point corresponding to the service transmission channel is adjusted, and the switching access signaling is transmitted on the uplink service channel of the target satellite through the PAB3 channel;