CN110446254B - Uplink timing advance terminal prediction method for satellite communication system - Google Patents
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Abstract
The invention discloses an uplink timing advance terminal prediction method for a satellite communication system, and belongs to the field of satellite communication. The method is suitable for a scene that the ground terminal communicates with the ground gateway station through the satellite, and the uplink synchronous tracking among a plurality of terminals in a beam coverage range of the satellite is realized. The terminal obtains the initial uplink timing advance through random access and transmits an uplink signal by adopting a timing advance mode. The terminal predicts the uplink transmission delay variation by using the method of the invention to obtain the uplink timing advance correction value, and updates the timing advance according to a certain frequency, thereby obtaining a more accurate open-loop uplink timing tracking mechanism. The method utilizes the satellite ephemeris to predict the change condition of the uplink timing advance of the terminal by predicting the satellite position, thereby ensuring the uplink synchronization performance; the terminal can automatically maintain uplink timing synchronization without gateway station control, can effectively reduce system load and is easy to realize.
Description
Technical Field
The invention belongs to the field of satellite communication, and particularly relates to a satellite communication system uplink timing advance terminal prediction method.
Technical Field
Satellite communication is an important achievement of combining aerospace technology and modern communication technology, is widely applied to the fields of broadcast television, mobile communication, broadband internet and the like, and is one of indispensable communication modes at present. Compared with the ground communication, the satellite communication has the advantages of large coverage area, long communication distance and irrelevant communication cost and communication distance. Meanwhile, the establishment of the satellite communication system is not limited by geographical conditions, and the satellite communication system has wide-range mobility.
Satellites include space vehicles in Low Earth Orbit (LEO), Medium Earth Orbit (MEO), geostationary orbit (GEO), or High Elliptic Orbit (HEO). Unlike a mobile communication system based on a ground infrastructure, a satellite is far from the ground, and transmission delay is large. The LEO satellite runs on an orbit 500 km-1500 km away from the ground, while the orbit of the GEO satellite is up to 36000km away from the ground, and the distance between the terminal and the satellite is far beyond the value when the satellite antenna is declined for a small angle. The Round-Trip delay of a satellite transmission is generally expressed in terms of an absolute RTT (Round-Trip Time), which refers to the Round-Trip Time required for a signal to travel between the satellite and the terminal, and a relative RTT, which refers to the difference between the maximum Round-Trip Time and the minimum Round-Trip Time within the coverage of the satellite. As shown in fig. 1, a terminal closest to a satellite has the smallest transmission delay in a beam range. For LEO satellites, a typical absolute RTT can be up to about 40ms, while a relative RTT can be up to about 4 ms. Therefore, since the area covered by a single beam of the satellite is far larger than a cell range in the terrestrial cellular communication system, the satellite mobile communication system has a characteristic of large transmission delay difference compared with the terrestrial mobile communication system.
In satellite communication systems, the main problem is to maintain the orthogonality of the uplink of the terminals, i.e. the frequency and time synchronization of the transmitted signals. In accordance with terrestrial communication systems, time alignment of uplink transmissions is achieved in satellite communication systems by using a timing advance at the terminal. Its main role is to offset the propagation delay differences between different terminals.
In a ground communication system, after a terminal receiver establishes downlink transmission synchronization with a base station for the first time, a random access channel (PRACH) process is used for setting initial time advance. Before and after the timing advance is set for each terminal for the first time, the updating is continuously needed to offset the time change of the uplink signal reaching the base station. These changes include movement of the terminal, changes in the propagation path, crystal frequency shift, etc. The timing advance update procedure to remove these effects is implemented by a closed loop mechanism, where the base station measures the received uplink timing and issues a timing advance update command to instruct the terminal to adjust its new transmission timing relative to the previous transmission timing.
This process is applied to the field of satellite communications and faces the following problems: one is that the path transmission delay in satellite communication is very large, and when the terminal obtains new transmission timing, the new transmission timing may not be applicable in the current uplink transmission window after a period of time of the uplink signal measured by the gateway station; secondly, in the satellite communication system, the terminal and the satellite may be in high-speed movement, and the time change is too fast, so that the time advance update command needs to be updated frequently. The prior technical scheme has no method for accurately tracking the open-loop uplink timing.
In a satellite communication system, a satellite and a terminal may both be in high-speed motion, the motion speed is up to several kilometers per second, the transmission distance of a link is relatively long, and the one-way delay is up to tens of milliseconds. The ground gateway station measures the time when the uplink transmission delay calculates the update amount of the uplink time, and a larger time difference exists between the time when the terminal adopts the timing advance to send data, so that the position of the satellite is changed, and the timing advance is overdue.
In view of the above problems, the present invention provides a method for predicting uplink timing advance, in which a terminal predicts uplink transmission delay deviation caused by satellite motion according to a gateway station position, an ephemeris and a terminal position, and obtains a timing advance corresponding to a data transmission time of the terminal.
Disclosure of Invention
The invention provides a satellite communication system uplink timing advance terminal prediction method, aiming at solving the problems of large propagation delay of a satellite communication link and uplink synchronous tracking caused by high-speed movement of a satellite.
The invention provides a satellite communication system uplink timing advance terminal prediction method, which is suitable for a scene that a ground terminal communicates with a ground gateway station through a satellite and realizes uplink synchronous tracking among a plurality of terminals in a beam coverage range of the satellite. The terminal obtains the initial uplink timing advance through random access and transmits an uplink signal by adopting a timing advance mode. The terminal predicts the uplink transmission delay variation by using the method of the invention to obtain the uplink timing advance correction value, and updates the timing advance according to a certain frequency, thereby obtaining a more accurate open-loop uplink timing tracking mechanism. As shown in fig. 1, the present invention is implemented by the following technical solutions, and a method for predicting an uplink timing advance terminal for a satellite communication system includes:
step 1: the terminal acquires the position information of the gateway station through the broadcast signal of the gateway station and estimates the satellite positions at different moments based on ephemeris;
step 2: the terminal estimates the uplink transmission time delay by combining the position of the gateway station, the position of the terminal and the position of the satellite; first, the existing timing advance TA is calculatedINTUplink transmission delay d of corresponding time0Then, calculating the uplink transmission time delay d of the next time of updating the timing advance time1;
And step 3: terminal calculating timing advance correction value TAUP=2(d1-d0) And updating TA for timing advanceNEW=TAINT+TAUPAnd transmitting the uplink data by the new timing advance.
Compared with the prior art, the method for predicting the uplink timing advance terminal of the satellite communication system has the following beneficial effects that:
(1) the invention utilizes the satellite ephemeris to predict the change condition of the uplink timing advance of the terminal by predicting the satellite position, thereby ensuring the uplink synchronization performance.
(2) The terminal can automatically maintain the uplink timing synchronization without the control of the gateway station, can effectively reduce the system load and is easy to realize.
Drawings
Fig. 1 is a flowchart of a method for predicting an uplink timing advance terminal of a satellite communication system according to the present invention.
Fig. 2 is a timing diagram of an uplink transmission of a satellite communication system.
FIG. 3 is a schematic diagram of uplink synchronization tracking process of satellite communication system
Fig. 4 is a diagram illustrating uplink transmission timing based on timing advance.
FIG. 5 is a diagram of an embodiment of the present invention
Detailed Description
The invention is further described with reference to the following figures and examples.
The invention provides a satellite communication system uplink timing advance terminal prediction method, which is suitable for a scene that a ground terminal communicates with a ground gateway station through a satellite and realizes uplink synchronous tracking among a plurality of terminals in a beam coverage range of the satellite.
The timing of the uplink transmission of the satellite communication system is shown in fig. 2.
T0: the terminal according to the existing timing advance TAINTTime of transmitting the uplink signal.
T01: terminal T0The time at which the transmitted signal arrives at the satellite.
T1: terminal T0The time at which the transmitted signal arrives at the gateway station.
T2: and the terminal updates the local timing advance and sends the time of the uplink signal by the updated timing advance.
T23: terminal T2The time at which the transmitted signal arrives at the satellite.
T3: terminal T2The time at which the transmitted signal arrives at the gateway station.
T4: and the terminal updates the local timing advance and sends the time of the uplink signal by the updated timing advance.
T45: terminal T4The time at which the transmitted signal arrives at the satellite.
T5: terminal T4The time at which the transmitted signal arrives at the gateway station.
Specific examples are as follows: consider a satellite communication system deployed in our country covering asia with the ground at different sites, such as beijing, guangzhou, rassa, deploying three gateway stations as ground control nodes for the satellites. Suppose a satellite communication system covering asia is implemented with 60 low earth polar satellites launched, an orbital altitude of 1175km, and a satellite radial velocity of 6.1 km/s. The present invention will be described by taking the cell covered by the beam of the beijing gateway station G1, the connected satellite S1 as an example.
The scenario of the satellite communication system in the above example is shown in fig. 5, and includes: a ground gateway station G1, a satellite in space orbit S1, and terminals 1 and 2 within satellite beam coverage;
as shown in fig. 3, the terminal obtains an initial uplink timing advance by random access, and transmits an uplink signal by a timing advance method. The terminal predicts the uplink transmission delay variation by using the method of the invention to obtain the uplink timing advance correction value, and updates the timing advance according to a certain frequency, thereby obtaining a more accurate open-loop uplink timing tracking mechanism.
The implementation steps and methods of the patent are illustrated below by the above implementation examples:
step 1: the gateway station establishes a connection with the satellite S1, and the satellite S1 informs the gateway station G1 of the satellite number, orbit information, and beam coverage information (there may be multiple beams for the satellite S1, and users under each beam are divided into the same cell). Consider here the cell C1 in which terminal 1, terminal 2 are located.
Step 2: the terminal 1 and the terminal 2 respectively receive the downlink synchronization frame, complete downlink time synchronization, and acquire cell system information including uplink random access information.
And step 3: the terminal 1 and the terminal 2, after completing the downlink synchronization, send the uplink random access signal to the satellite S1 in the first random access window, and the satellite S1 forwards the uplink random access signal to the gateway station G1. The transmission time is T0.
And 4, step 4: the gateway station G1 calculates uplink transmission delays of the terminal 1 and the terminal 2 through the uplink random access signal, and calculates initial timing advance TA1 and TA2 according to the transmission delays. Assuming TA1 is 32.2ms and TA2 is 33.6ms, the corresponding feeder link distance is 3600km and the corresponding subscriber link distance is 1230km and 1440km, respectively.
And 5: the gateway station G1 feeds back the initial timing advance TA1 and TA2 to the terminal 1 and the terminal 2 in the random access response signal RAR.
Step 6: after receiving initial uplink timing advance TA1 and TA2, terminal 1 and terminal 2 adjust local time, respectively.
And 7: suppose that the next uplink transmission time is T2, and T2-T0 is 40 ms.
And 8: the terminal 1 and the terminal 2 calculate the variation of their own uplink delays from ephemeris, and first calculate the delay variation of the feeder link to obtain Dcom 3600.202 km. The distances of the user links are D1-1230.065 km respectively; d2 ═ 1440.127 km. The corresponding new uplink transmission delays are 16.10089ms and 16.80109ms, respectively.
And step 9: and the terminal 1 and the terminal 2 calculate the corresponding timing advance to be 32.20178ms and 33.60218ms according to the uplink transmission delay at the time T4. Therefore, the update amounts of the timing advance can be calculated to be TAupd1 ═ 1.78 us; TAupd2 ═ 2.18 us;
step 10: and the time of the terminal 1 and the time of the terminal 2, namely T4, are taken as the reference to update the local time, and the synchronous transmission of the uplink signals is completed.
As shown in fig. 4, when the uplink signal is transmitted with the uplink timing advance, the timing advance of the uplink signal is equal to 2 times of the link delay between the terminal and the gateway station. Taking terminal 1 as an example, the propagation delay of the link between the terminal and the gateway station is TP1Then its timing advance of response is 2TP1. When the uplink signal is transmitted in a timing advance manner, propagation delays of paths are different due to different distances from the satellite for different terminals, so that timing advances of each terminal are different. But the time of arrival of the uplink signal at the gateway station for each terminal is substantially aligned. In the invention, the terminal predicts the time delay error caused by the satellite motion by using the ephemeris information and corrects the uplink timing advance at different sending moments.
The above detailed description of the embodiments of the present invention, and the detailed description of the embodiments of the present invention used herein, is merely intended to facilitate the understanding of the methods and apparatuses of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, there may be variations in the specific embodiments and the application scope, and in summary, the content of the present specification should not be construed as a limitation to the present invention.
Claims (1)
1. An uplink timing advance terminal prediction method for a satellite communication system, the method comprising:
step 1: the gateway station establishes a connection with satellite S1, and satellite S1 informs the gateway station G1 of the satellite number, orbit information, and beam coverage information, considering here the cell C1 in which terminal 1, terminal 2 are located
Step 2: the terminal 1 and the terminal 2 respectively receive the downlink synchronous frame, complete downlink time synchronization and acquire cell system information including uplink random access information;
and step 3: the terminal 1 and the terminal 2 send uplink random access signals to the satellite S1 in a first random access window after downlink synchronization is completed, the satellite S1 forwards the uplink random access signals to the gateway station G1, and the sending time is T0;
and 4, step 4: the gateway station G1 calculates uplink transmission delays of the terminal 1 and the terminal 2 through the uplink random access signal, and calculates initial timing advance TA1 and TA2 according to the transmission delays; assuming that TA1 is 32.2ms, TA2 is 33.6ms, the corresponding feeder link distance is 3600km, and the corresponding user link distances are 1230km and 1440km, respectively;
and 5: the gateway station G1 feeds back the initial timing advance TA1 and TA2 to the terminal 1 and the terminal 2 in the random access response signal RAR;
step 6: after receiving the initial uplink timing advance TA1 and TA2, terminal 1 and terminal 2 respectively adjust the local time;
and 7: assuming that the next uplink transmission time is T2, and T2-T0 is 40 ms;
and 8: the terminal 1 and the terminal 2 respectively calculate the variation of the uplink time delay of the terminal according to ephemeris, and firstly calculate the time delay variation of a feed link to obtain Dcom which is 3600.202 km; the distances of the user links are D1-1230.065 km respectively; d2 ═ 1440.127 km; the corresponding new uplink transmission time delay is 16.10089ms and 16.80109ms respectively;
and step 9: the terminal 1 and the terminal 2 calculate the corresponding timing advance to be 32.20178ms and 33.60218ms according to the uplink transmission delay at the time of T4; therefore, the update amounts of the timing advance can be calculated to be TAupd1 ═ 1.78 us; TAupd2 ═ 2.18 us;
step 10: and the time of the terminal 1 and the time of the terminal 2, namely T4, are taken as the reference to update the local time, and the synchronous transmission of the uplink signals is completed.
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