Frequency shift compensation method of ground network terminal direct connection satellite and base station

Abstract

The invention discloses a frequency shift compensation method for a ground network terminal directly connected with a satellite. The method comprises the following steps: A1, dividing a cell generated by a single beam of a satellite base station on the ground into a plurality of banded areas; a2, distributing a frequency shift value for each band-shaped region; a3, establishing a time-frequency shift curve of the cell ground network terminal; a4, determining an initial frequency shift value of each ground network terminal in the cell; and A5, performing frequency shift compensation on satellite-ground transmission data according to the time-frequency shift curve and the initial frequency shift value of each ground network terminal.

Description

Technical Field

[0001] The present invention belongs to the technical field of satellite mobile communications, and in particular relates to a frequency shift compensation method and a base station for a ground network terminal directly connected to a satellite. Background Art

[0002] Usually, the access network part of the satellite communication system for mobile phones is composed of two types of equipment: base stations and terminals. The base stations are deployed on the satellite and the terminals work on the ground. There are two main technical routes for the implementation of this access network part. One is the non-terrestrial network (NTN) standardized by 3GPP, which is based on the evolution of the traditional terrestrial network (TN), but the NTN base station is different from the TN base station, and the NTN terminal is also different from the TN terminal. The other technical route is for the existing TN terminal to directly connect to the on-board base station, which requires the communication protocol of the on-board base station to be modified, while the terminal only needs to meet the TN protocol and does not need to be modified.

[0003] In wireless communication, if there is relative motion between the transmitter and the receiver, the Doppler effect causes the signal received by the receiver to have a frequency offset relative to the signal sent by the transmitter. The frequency offset causes the received signal to be distorted relative to the transmitted signal, affecting the demodulation and decoding of the received signal by the receiver. Excessive frequency offset will cause the bit error rate to increase until normal communication cannot be performed. Therefore, wireless communication systems used in scenarios with relative motion, including mobile phone direct satellite communication systems, need to design a compensation mechanism for Doppler frequency shift to ensure normal communication. Summary of the invention

[0004] One of the embodiments of the present disclosure is a method for compensating frequency shift of a ground network terminal directly connected to a satellite base station communication system, the method comprising the following steps:

[0005] A1, divides the cell generated on the ground by a single beam of a satellite base station into several strip-shaped areas;

[0006] A2, assigns a frequency shift value to each strip area;

[0007] A3, establishing a time-frequency shift curve of the ground network terminal of the cell;

[0008] A4, determining an initial frequency shift value of each ground network terminal in the cell;

[0009] A5, performing frequency shift compensation on satellite-to-ground transmission data according to the time-frequency shift curve and the initial frequency shift value of each ground network terminal.

[0010] One of the embodiments of the present disclosure is a base station on a satellite system directly connected to a ground network terminal, and a frequency shift compensation method for the base station serving existing ground network terminals includes:

[0011] The cell formed by a single beam of the satellite base station is divided into several strip-shaped areas, and each area uses a frequency shift value to replace the frequency shift value of the entire area;

[0012] The base station determines the initial frequency shift of each ground network terminal by post-compensating the uplink resource block and pre-compensating the downlink resource block during the initial random access process;

[0013] A time-frequency shift curve for ground network terminals within the cell is established. Based on the curve and the initial frequency shift value of each terminal, frequency shift compensation is performed on the subsequent data transmission process of each terminal, that is, frequency shift pre-compensation is performed on the downlink resources for transmitting data, and frequency shift post-compensation is performed on the uplink resources for transmitting data. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily understood by reading the detailed description below with reference to the accompanying drawings. In the accompanying drawings, several embodiments of the present invention are shown in an exemplary and non-limiting manner, in which:

[0015] Figure 1 Schematic diagram of a cell generated by a single beam of an onboard base station serving ground network terminals in different geographical locations in the prior art.

[0016] Figure 2 An example diagram of downlink Doppler frequency shift distribution and strip area division of a cell generated by a single beam of an onboard base station according to one embodiment of the present invention.

[0017] Figure 3 Schematic diagram of initial random access resource block distribution without adopting the disclosed solution.

[0018] Figure 4 A schematic diagram of pre-compensation and post-compensation for a random access resource block according to one embodiment of the present invention.

[0019] Figure 5 Schematic diagram of resource block distribution during data transmission when the disclosed solution is not applied.

[0020] Figure 6 A schematic diagram of downlink frequency shift pre-compensation and uplink frequency shift post-compensation of resource blocks in a data transmission process according to one embodiment of the present invention.

[0021] Figure 7 An example diagram of a time-downlink Doppler frequency shift curve diagram of a single beamforming cell according to an embodiment of the present invention.

[0022] Figure 8A schematic diagram comparing the time-varying downlink Doppler frequency shift felt by a user terminal when receiving a service from an onboard base station according to one embodiment of the present invention. DETAILED DESCRIPTION

[0023] In both ground network scenarios and satellite-to-ground scenarios, the communication system has an obvious Doppler effect, and frequency shift will significantly affect the communication quality. Among them, the instantaneous value of the downlink frequency shift felt by the terminal depends on the instantaneous relative speed between the terminal and the base station; the dynamic range of the frequency shift felt by the terminal within the coverage area of ​​the base station depends on the absolute value of the difference between the maximum frequency shift and the minimum frequency shift generated by the cell within the coverage area. The frequency shift compensation in the ground network usually adopts the following method: the downlink signal sent by the base station contains a reference signal, and the terminal obtains the downlink frequency shift value by detecting the reference signal, and then compensates the received signal according to this value; the uplink signal sent by the terminal contains a reference signal, and the base station obtains the uplink frequency shift value by detecting the reference signal, and then compensates the received signal according to this value. The high-speed rail scenario in the ground network is a typical scenario that is significantly affected by Doppler frequency shift. The ground network terminal, ground terminal or terminal here includes communication equipment such as mobile phones.

[0024] Compared with the relative movement between the ground base station and the ground terminal, the relative movement speed of the satellite base station deployed on the low-orbit satellite relative to the ground terminal is higher; and the diameter of the cell formed by the satellite base station on the ground is larger than that of the ordinary ground cell. Therefore, the terminal served by the satellite base station feels a larger absolute value of the instantaneous value of the frequency shift; at the same time, the dynamic range of the frequency shift is larger, and the impact on the communication quality is more obvious. The ability of the ground terminal to handle Doppler frequency shift is designed according to the ground base station scenario and cannot directly adapt to the larger Doppler frequency shift dynamic range brought by the satellite base station. Because the ground terminal will not make any modifications to the satellite-to-ground communication scenario, the satellite base station should track the Doppler frequency shift changes of the ground terminal and provide appropriate compensation to ensure communication quality.

[0025] When the satellite base station is able to track the position of the ground terminal, it can calculate the uplink and downlink Doppler frequency shift between the base station and the ground terminal at any time based on its own ephemeris information. However, the position information of the ground terminal needs to be obtained through an additional positioning system and transmitted to the satellite base station. The ground terminal may not have the positioning conditions, and the TN protocol does not include the function of transmitting positioning information to the base station. In this way, the satellite base station cannot track the Doppler frequency shift changes between the satellite base station and the ground terminal. This is the first difficulty encountered by the satellite base station in frequency shift compensation.

[0026] Another difficulty for satellite base stations to perform frequency shift compensation is that it is difficult to provide good compensation for all ground terminals in the cell. Satellite base stations will serve multiple ground terminals at the same time, and these ground terminals are distributed in different geographical locations. At a certain moment, the relative speed of the satellite base station with respect to each ground terminal is different, that is, the Doppler frequency shift between the satellite base station and each ground terminal is different. Taking downlink communication as an example, the currently proposed frequency shift compensation method is to perform unified frequency shift compensation on the complete baseband signal. The satellite base station can compensate the entire baseband signal to have no frequency shift with the ground terminal in a certain geographical location, thereby improving the communication quality. However, the baseband signal contains signals sent to multiple ground terminals. The frequency shift between the satellite base station and the ground terminals distributed in other geographical locations cannot be eliminated, and is likely to be deteriorated, and the communication quality of all ground terminals cannot be improved at the same time. For example, Figure 1 , gives an example of the above situation. The satellite base station forms a cell on the ground and serves three ground network terminals. The arrow shows the approximate direction of the satellite movement. The ground network terminals are distributed in different geographical locations, one is located at the leading edge of the cell's forward direction, one is located at the trailing edge of the cell's forward direction, and the other is roughly located in the center of the cell's forward direction. At the same time, the Doppler frequency shifts felt by the three ground network terminals are different. Taking the downlink Doppler frequency shift as an example, there are a positive frequency shift value, a negative frequency shift value, and a frequency shift value close to 0.

[0027] The 5G TN protocol itself provides more degrees of freedom, and it is possible to solve the above problems within the framework of the 5G TN protocol without modifying the ground terminal protocol. For example, the ground base station periodically sends a synchronization signal block (SSB) group. Each synchronization signal block group contains several synchronization signal blocks. Each synchronization signal block contains a waveform for establishing synchronization and some system information. The ground terminal will try to detect the synchronization signal block and select the synchronization signal block with the best signal quality for subsequent processing to obtain the initial time and frequency synchronization of the downlink communication. After detecting the synchronization signal block, the ground terminal will further parse its content to obtain the time-frequency position of the physical random access channel (PRACH); the base station can configure the time-frequency position of the physical random access channel corresponding to each synchronization signal block, that is, the time-frequency resources of the physical random access channel used by the ground network terminal that detects different synchronization signal blocks and uses the information therein can not overlap with each other, that is, from which physical random access channel the ground network terminal uses, it can be inferred which synchronization signal block the terminal has detected and the information contained therein; the ground terminal sends a preamble on the time-frequency resources of the physical random access channel to complete the initial random access process. The uplink and downlink transmissions of the ground terminal are uniformly scheduled by the base station. The uplink and downlink resources allocated by the base station to the ground terminal are in units of resource blocks (RBs), and each RB is 12 consecutive subcarriers in the time domain and 1 time slot in the time domain.

[0028] In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a frequency shift compensation method and a base station when a satellite base station communicates with a ground terminal. The problem solved in the prior art is how the base station tracks the frequency shift changes of ground terminals distributed in different geographical locations, and how the base station performs frequency shift compensation for different ground terminals respectively.

[0029] According to one or more embodiments, a frequency shift compensation method for a satellite base station serving a ground network terminal includes the following steps:

[0030] The cells generated by the satellite base station using a single beam to illuminate the ground are divided into several strip-shaped areas;

[0031] Assign a frequency shift value to each strip region as an approximation of all frequency shift values ​​within the strip region;

[0032] According to the speed of the cell sweeping across the ground and the division of the strip area, the time-downlink frequency shift curve of the cell and the time-uplink frequency shift curve of the cell are established;

[0033] The satellite base station attempts to determine the initial downlink and uplink frequency shift values ​​of the ground network terminal, and based on the initial frequency shift values ​​of the ground network terminal, roughly determines in which strip area the initial position of the ground network terminal falls;

[0034] The satellite base station determines the downlink frequency shift value and uplink frequency shift value of the ground network terminal at any subsequent time in the cell according to the initial frequency shift value of the ground network terminal and the time-downlink frequency shift curve and time-uplink frequency shift curve of the cell;

[0035] In the subsequent data transmission process, the satellite base station performs frequency shift pre-compensation on the downlink resource block sent to the ground network terminal according to the downlink frequency shift value of the ground network terminal at any subsequent time in the cell, and then sends it to the ground network terminal. The pre-compensation value is equal to the downlink frequency shift value × -1;

[0036] In the subsequent data transmission process, according to the uplink frequency shift value of the ground network terminal at any subsequent time in the cell, the uplink resource block received from the ground network terminal is first frequency shifted and compensated, and then handed over to the baseband algorithm for processing. The post-compensation value is equal to the uplink frequency shift value × -1.

[0037] Furthermore, the method for determining the initial frequency shift value of the ground network terminal by the satellite base station includes:

[0038] The onboard base station periodically sends synchronization signal block groups, each synchronization signal block group consists of a number of synchronization signal blocks, and the number of synchronization signal blocks is equal to the number of cell strip areas;

[0039] Each synchronization signal block in the synchronization signal block group corresponds to a strip area;

[0040] Before each synchronization signal block is sent, it is pre-compensated for frequency shift, and the compensation value is equal to the frequency shift value of the corresponding strip area × -1;

[0041] A synchronization signal block is associated with a physical layer random access channel. The base station first performs frequency shift and compensation on the received physical layer random access channel signal and then passes it to the baseband algorithm for processing, trying to detect whether it contains the preamble code sent by the ground network terminal;

[0042] The post-frequency shift compensation value is obtained by querying the time-uplink frequency shift curve of the cell: first, the strip area corresponding to the synchronization signal block and the time interval between the synchronization signal block and the physical layer random access channel are known, and the frequency shift value corresponding to the above strip area is found on the time-uplink frequency shift curve. The corresponding time is used as the time starting point, and the above time interval is pushed back to obtain a new time. The frequency shift value corresponding to the new time on the curve ×-1 is used as the post-frequency shift compensation value;

[0043] In the above process, if a preamble code is detected in the physical layer random channel n, it can be considered that the ground network terminal that sent the preamble code detected and adopted the synchronization signal block n, and it can be inferred that the terminal falls in the strip area n at the moment when the base station sends the synchronization signal block n. From this, the initial downlink frequency shift value and the initial uplink frequency shift value of the terminal at this moment can be known, which can be used as the starting value for subsequent data transmission frequency shift compensation.

[0044] Here, a downlink resource block refers to a series of downlink (from the base station to the terminal) time-frequency resources, which consists of a certain number of subcarriers that last for a certain period of time. When the satellite base station sends data to the ground network terminal, the downlink resource block carries this data. The satellite base station performs frequency shift pre-compensation on the downlink resource block according to the downlink frequency shift value of the ground network terminal to ensure reliable transmission of downlink data.

[0045] An uplink resource block refers to a series of uplink (from the terminal to the base station) time-frequency resources, consisting of a certain number of subcarriers that last for a certain period of time. When the ground network terminal sends data to the satellite base station, the uplink resource block carries these uplink data. After receiving the uplink resource block, the satellite base station performs post-frequency shift compensation according to the uplink frequency shift value of the ground network terminal to ensure reliable transmission of uplink data.

[0046] A synchronization signal block group is a signal set consisting of several synchronization signal blocks. Each synchronization signal block occupies a series of downlink time-frequency resources, corresponding to a strip area, and frequency shift pre-compensation is performed before transmission. The onboard base station periodically sends a synchronization signal block group, and the ground network terminal synchronizes with the onboard base station by receiving the synchronization signal blocks. Each synchronization signal block in the synchronization signal block group is associated with a physical layer random access channel. The ground network terminal that receives and adopts a synchronization signal block will use the physical layer random access channel corresponding to the synchronization signal block for subsequent random access processes.

[0047] The physical layer random access channel is a series of uplink time-frequency resources that appear periodically and are used by ground network terminals to initiate a random access process to request access to the network. When the ground network terminal detects the synchronization signal block and attempts to access the network, it sends a preamble through the physical layer random access channel. The onboard base station performs frequency shift compensation on the received physical layer random access channel signal and attempts to detect whether it contains the preamble sent by the ground network terminal to determine the initial frequency shift value of the ground network terminal.

[0048] Furthermore, the method for selecting the number N of strip areas in a cell is:

[0049]

[0050] Furthermore, the processing when a cell switching occurs in a ground network terminal includes:

[0051] According to the spatial adjacency relationship between the source cell and the target cell, the adjacency relationship between the strip area of ​​the source cell and the strip area of ​​the target cell can be known. According to the strip area where the ground network terminal is initially located and the total time of receiving service from the source cell, it is inferred from which strip area the ground network terminal leaves the source cell. It is further inferred which strip area of ​​the target cell the ground network terminal will enter, and this information is notified to the target cell through traditional handover signaling. The target base station then learns the starting frequency shift of the terminal in this cell, and compensates for the subsequent data transmission process according to the frequency shift curve of the target cell.

[0052] The above method is implemented by the onboard base station equipment.

[0053] According to one or more embodiments, a method for tracking frequency shift changes of a ground base station and a method for frequency shift compensation for a ground terminal are applied to an onboard base station, including: the onboard base station captures the initial frequency shift of the ground terminal during the initial random access phase of the ground terminal; establishes a relationship between the frequency shift and time of the ground terminal to track its frequency shift changes at any time after capturing its initial frequency shift; and performs frequency shift pre-compensation on resource blocks carrying signals sent to the ground terminal and frequency shift post-compensation on resource blocks carrying signals from the ground terminal based on the relationship between the frequency shift and time of the ground terminal.

[0054] The single beam of the satellite base station illuminates the earth's surface, forming a coverage area in the shape of a perfect circle or ellipse, which is called a cell. The satellite base station moves around the earth along its own orbit, and the cell it generates also sweeps across the earth's surface. Based on the satellite's ephemeris information and the direction and angle information of the beam, the coverage range of the cell at any time can be calculated; the relative speed between any point in the cell and the base station at any time can also be calculated; further, the frequency shift between the ground terminal at any point in the cell and the base station can be calculated.

[0055] The above frequency shift is divided into two categories, one is the downlink frequency shift when the base station is used as the transmitter and the ground network terminal is used as the receiver; the other is the uplink frequency shift when the base station is used as the receiver and the ground network terminal is used as the transmitter. The movement of any fixed point on the ground in the cell is the vector synthesis of the two movements of the earth's rotation and the cell sweeping across the earth's surface. Therefore, as long as the position information of a certain point on the ground at a certain moment is known, the movement trajectory of the point in the cell in the subsequent time, that is, the position of the point in the cell at any subsequent time, can be known. Furthermore, as long as the position of the ground terminal in the cell at a certain moment is known, the Doppler frequency shift value between the terminal and the satellite base station at any subsequent time can be known, and the frequency shift value includes uplink frequency shift and downlink frequency shift.

[0056] According to calculations, no matter the shape of the cell is a perfect circle or an ellipse, its uplink and downlink Doppler frequency shifts show a decreasing trend from the leading edge to the trailing edge in the cell's forward direction, and the decreasing rate is basically fixed. Figure 2 An example of the downlink Doppler frequency shift of a cell generated by a beam from a satellite base station irradiating the ground is given. The center of the beam irradiates the sub-satellite point, and the cell is a perfect circle. The frequency shift value at the leading edge of the cell in the forward direction is the largest, which is 900Hz in this example, and the frequency shift value at the trailing edge in the forward direction of the cell is the smallest, which is -900Hz in this example. The cell can be divided into several strips. Figure 2 In the example, a total of 9 strip areas are divided and numbered from area 0 to area 8 from the forward direction of the cell to the rear edge. The maximum and minimum values ​​of the downlink Doppler frequency shift in this area are recorded, and the average of the maximum and minimum values ​​is recorded as the average value of the downlink Doppler frequency shift in this area. Figure 2 In the figure, the right column gives the average downlink Doppler shift of each strip area. The average downlink Doppler shift of each strip area can be used to roughly replace the downlink Doppler shift value of the area. The more strip areas there are and the narrower the width of each area, the better this method approximates the distribution of the real Doppler shift value. Similarly, after determining the above strip area division, the maximum and minimum values ​​of the uplink Doppler shift in each area can be calculated, and the average value of the maximum and minimum values ​​is also recorded as the average value of the uplink Doppler shift of this area.

[0057] Based on the above, two curves can be established, namely the time-downlink Doppler shift curve and the time-uplink Doppler shift curve. Assume that the initial position of the ground network terminal is at the forefront of the cell, and assume that the time when it is at the initial position is time 0. As time passes, the ground network terminal moves in the cell until it finally moves out of the cell. When the ground network terminal is in a strip area, the average downlink Doppler shift value of the area is used to replace the actual downlink Doppler shift value felt by the ground network terminal, and a time-downlink Doppler shift curve in the form of a broken line is generated; similarly, when the ground network terminal is in a strip area, the average uplink Doppler shift value of the area is used to replace the actual uplink Doppler shift value felt by the base station, and a time-uplink Doppler shift curve in the form of a broken line is generated.

[0058] When the initial position of the ground network terminal is known, its downlink initial Doppler frequency shift value and uplink initial Doppler frequency shift value are known respectively. By querying the previously established time-downlink Doppler frequency shift curve, the time corresponding to the time when the ground network terminal has the initial downlink Doppler frequency shift value can be found as the initial time. If a downlink Doppler frequency shift value corresponds to a period of time, the midpoint of the period of time can be taken as the initial time. Using this initial time as the time starting point, the downlink Doppler frequency shift value of the ground network terminal at any subsequent time can be inferred, and the downlink frequency shift compensation can be performed accordingly. In the same principle, by querying the previously established time-uplink Doppler frequency shift curve, the time corresponding to the time when the ground terminal has the initial uplink Doppler frequency shift value can be found. Using this time as the time starting point, the uplink Doppler frequency shift value of the ground network terminal at any subsequent time can be inferred, and the uplink frequency shift compensation can be performed accordingly.

[0059] The satellite base station determines the initial Doppler frequency shift of the ground terminal during the initial random access process of the ground terminal, and the specific method is as follows. The satellite base station periodically sends a synchronization signal block group, and the number of synchronization signal blocks in each synchronization signal block group is equal to the number of strip areas divided in the cell. Let the number of synchronization signal blocks be N, and the synchronization signal blocks be numbered 0, 1, ..., N-1. The satellite base station arranges N uplink physical layer random access channels in the uplink frequency band with the same period, and the uplink physical layer random access channels are numbered 0, 1, ..., N-1. Let n belong to 0, 1, ..., N-1. The baseband signal corresponding to each synchronization signal block is pre-compensated with a frequency shift before being sent. The pre-compensated frequency shift value of synchronization signal block n is equal to the average downlink Doppler frequency shift of strip area n × -1. Synchronization signal block n corresponds to uplink physical layer random access channel n, that is, the ground network terminal that detects the use of synchronization signal block n and adopts the information therein will send a preamble code on the uplink physical layer random access channel n. After receiving the signal of uplink random access channel n, the on-board base station first performs a frequency shift compensation on it and then sends it to the baseband algorithm for processing.

[0060] The post-compensation value of the frequency shift of the uplink physical layer random access channel n is determined in the following manner: on the time-uplink Doppler shift curve of the cell, first find the time corresponding to the uplink Doppler shift of the strip area n, recorded as time 1; specifically, if the uplink Doppler shift of the strip area n corresponds to a time period, the midpoint of the time period can be taken as time 1. Calculate the time difference between the uplink physical layer random access channel n and the synchronization signal block n, specifically, the time difference between the center time of the uplink physical layer random access channel n in the time domain and the center time of the synchronization signal block n in the time domain can be calculated; add the above time difference to time 1 to obtain time 2, and find the uplink Doppler shift value corresponding to time 2 on the curve chart, then the post-compensation value is equal to the uplink Doppler shift value corresponding to time 2 × -1. If a preamble is detected on the uplink physical layer random access channel, it can be considered that the ground network terminal that sent the preamble is located in the strip area n at time 1, and it is further considered that its uplink and downlink Doppler shift values ​​at time 1 are equal to the uplink and downlink Doppler shift average values ​​of the strip area n. Obviously, according to the uplink and downlink Doppler shift values ​​at time 1 and the time-uplink and downlink Doppler shift curve of this cell, the uplink and downlink Doppler shift values ​​of the ground network terminal at any time in this cell can be inferred. Since each ground network terminal needs to perform an initial random access process once, the onboard base station can obtain the Doppler shift value of each ground network terminal at the initial time, and can track the uplink and downlink Doppler shift values ​​of each ground network terminal in the subsequent time in this cell based on the time-Doppler shift curve of this cell.

[0061] In the subsequent communication process between the onboard base station and the ground network terminal, based on the tracking of the frequency shift value of the ground network terminal, the onboard base station pre-compensates the downlink signal sent to the ground network terminal before sending it, and the onboard base station post-compensates the uplink signal received from the ground network terminal before handing it over to the baseband algorithm for processing.

[0062] For the above-mentioned processing process of the satellite base station, the ground network terminal does not need to make any modification, but only needs to ensure that it complies with the ground network protocol.

[0063] The number of strips in a cell needs to take into account the dynamic range of Doppler frequency shift that the ground network terminal can tolerate when following the ground network protocol. For example, the number of strips can be:

[0064] When the above method is not implemented, the dynamic range of Doppler frequency shift experienced by the ground network terminal in the cell is from the maximum value of the cell Doppler frequency shift to the minimum value of the cell Doppler frequency shift, which is greater than the dynamic range of Doppler frequency shift that the ground network terminal can tolerate when following the ground network protocol. The ground network terminal cannot communicate continuously and normally in the cell.

[0065] After implementing the above method, the dynamic range of Doppler frequency shift experienced by the ground network terminal in the cell becomes (maximum value of the cell Doppler frequency shift - minimum value of the cell Doppler frequency shift) / N, which is smaller than the dynamic range of Doppler frequency shift that the ground network terminal can tolerate when following the ground network protocol. The ground network terminal can communicate continuously and normally in the cell.

[0066] The processing when a ground network terminal switches a cell includes: the source cell and the target cell each divide their own strip area. According to the spatial adjacency relationship between the source cell and the target cell, the adjacency relationship between the strip area of ​​the source cell and the strip area of ​​the target cell can be known, that is, if the ground network terminal leaves the source cell from a certain strip area of ​​the source cell, it can be known which strip area of ​​the target cell it will enter. According to the initial strip area of ​​the ground network terminal and the total time of receiving service from the source cell, it can be inferred from which strip area the ground network terminal left the source cell. It can be further known which strip area of ​​the target cell the ground network terminal will enter, and this information can be notified to the target cell through traditional switching signaling. The target base station then learns the starting frequency shift of the terminal in this cell, and can compensate for the subsequent data transmission process according to the frequency shift curve of the target cell.

[0067] The following is a further description of the accompanying drawings.

[0068] Figure 2 An example is given of the division of the cell strips when this solution is applied. The cell is divided into 9 strips. The signal transmitted by the satellite base station is irradiated to the ground through a beam, forming a coverage area, i.e., the cell. The center point of the beam, i.e., the ground point directly below the satellite, is the location where the Doppler shift effect is least obvious. Figure 2In the figure, the ground coverage area formed by the beam of the satellite base station is a perfect circle. Due to the relative motion of the satellite with respect to the ground, the frequency of the signal received on the ground is different from the frequency of the signal transmitted by the satellite. At the leading edge of the cell in the forward direction (i.e., the direction of satellite motion), the frequency shift value is the largest; at the trailing edge in the forward direction of the cell, the frequency shift value is the smallest. In order to finely manage the Doppler shift, the cell is divided into multiple strip areas. The Doppler shift characteristics of each area can be obtained by calculating the average value of the maximum and minimum frequency shift values ​​of the area. The average value of the maximum and minimum downlink Doppler shift of each strip area is used to approximate the downlink Doppler shift value of the area. Similar to the downlink Doppler shift, there is also a Doppler shift effect in the uplink. The uplink Doppler shift average value of each strip area can be calculated by the same method and used to approximate the uplink Doppler shift value of the area. The more strip areas there are, the narrower the width of each area, and the better the distribution of the real Doppler shift value is approximated by this method, thereby improving the accuracy of frequency shift compensation.

[0069] Figure 3 An example is given, that is, the distribution of the time-frequency resources of SSB and PRACH related to the random access process when this solution is not applied. An SSB group contains N SSBs, namely SSB 0, SSB 1, ..., SSB n, ..., SSB N-1. The SSB group is transmitted by the base station in the downlink frequency band, and the frequency resources occupied by each SSB are measured in f SSB is the frequency center. A PRACH group contains N PRACHs, namely PRACH 0, PRACH 1, ..., PRACH n, ..., PRACH N-1. The correspondence between SSB and PRACH is SSB n corresponds to PRACH n, where n = 0, 1, ..., N-1. PRACH is distributed in the uplink frequency band, and the frequency resources occupied by each PRACH are based on f PRACH is the frequency center. The time difference between the time domain center of SSB n and the time domain center of PRACH n is Δt SPn , where n = 0, 1, …, N-1.

[0070] Figure 4 An example is given in Figure 3 After applying this solution based on the random access process, the time-frequency resource distribution of SSB and PRACH related to the random access process. An SSB group contains N SSBs, namely SSB 0, SSB 1, ..., SSB n, ..., SSBN-1. The center frequency of the resources occupied by SSB n is the same as f SSB The difference is Δf Sn, where n = 0, 1, ..., N-1. A PRACH group contains N PRACHs, namely PRACH 0, PRACH 1, ..., PRACH n, ..., PRACH N-1. The center frequency of the resource occupied by PRACH n is the same as f PRACH The difference is Δf Pn , where n = 0, 1, …, N-1.

[0071] Figure 4 In the downlink frequency band, there are N synchronization signal blocks (SSBs), each of which is transmitted by the base station. The center frequency of each SSB is relative to the reference frequency f SSB There is an offset, denoted as Δf Sn , where n represents the index of the SSB. It contains N physical random access channels (PRACH), which are distributed in the uplink frequency band. The center frequency of each PRACH is relative to the reference frequency f PRACH There is also an offset, denoted as Δf Pn , where n represents the index of PRACH. Figure 4 In the RF transmission, the center frequency of each SSB and PRACH is adjusted from its original reference frequency by adding a frequency offset Δf.

[0072] Figure 5 An example is given, that is, the distribution of downlink resource blocks and uplink resource blocks corresponding to a downlink data transmission and an uplink data transmission when this solution is not applied. The signal that occupies continuous resources in the frequency domain sent by the base station to a certain ground network terminal m is regarded as a downlink resource block, which is represented by f DLm is the center frequency, where m is the number of the ground network terminal served by the cell. The signal that occupies continuous resources in the frequency domain sent by the ground network terminal m to the base station is regarded as an uplink resource block, which is based on f ULm is the center frequency, and similarly, m is the terrestrial network terminal number served by the cell.

[0073] Figure 6 An example is given, that is, Figure 5 When this solution is applied based on the , the distribution of downlink resource blocks and uplink resource blocks corresponding to one downlink data transmission and one uplink data transmission. The downlink resource blocks sent by the base station to the ground network terminal m are originally based on f DLm is the center frequency; after applying this solution, the base station performs a frequency shift pre-compensation on the resource block, and the center frequency of the resource block after compensation is equal to f DLm The difference is Δf DLm In addition, the time from the moment when the ground network terminal m determines the initial frequency shift to the center moment of the downlink resource block is Δt DLm Δf DLmThe value of is determined by the initial downlink frequency shift of the ground network terminal m and Δt DLm The uplink resource block sent by the ground network terminal m to the base station is originally based on f ULm is the center frequency; after applying this solution, the base station performs a frequency shift compensation on the resource block, and the center frequency of the compensated resource block is equal to f ULm The difference is Δf ULm In addition, the time from the moment when the ground network terminal m determines the initial frequency shift to the center moment of the uplink resource block is Δt ULm Δf ULm The value of Δ is determined by the initial uplink frequency shift of the ground network terminal m and Δt ULm It is determined by querying the time-uplink Doppler frequency shift curve of the cell.

[0074] Figure 6 In the downlink, the base station determines the initial downlink frequency shift and time difference Δt according to the ground network terminal m. DLm , by querying the cell's time-downlink Doppler frequency shift curve, calculate the appropriate Δf DLm In the uplink, the base station determines the initial uplink frequency shift and time difference Δt according to the ground network terminal m. ULm , by querying the cell's time-uplink Doppler frequency shift curve, calculate the appropriate Δf ULm Value, post-compensation is performed on the uplink resource block. Δt DLm and Δt ULm They represent the time difference from the time when the ground network terminal m determines the initial frequency shift to the time when the downlink resource block and the uplink resource block are centered. The time-Doppler frequency shift curve of the cell describes the frequency change caused by the Doppler effect at different time points. By querying these curves, the appropriate frequency shift compensation value can be calculated for each terminal. Figure 6 A method for optimizing data transmission quality through pre-compensation and post-compensation is presented by showing the frequency distribution of downlink and uplink resource blocks after applying a frequency shift compensation scheme.

[0075] Figure 7 An example is given, that is, the time-downlink Doppler frequency shift curve of the cell is obtained by applying the solution and dividing the cell into 9 strip areas.

[0076] Figure 8An example is given, that is, the comparison of the downlink Doppler frequency shift felt by the ground network terminal in the cell from the front edge of the cell when the solution is not applied and when the solution is applied. The dotted line is the frequency shift change felt when the solution is not applied, which continuously drops from 900Hz to -900Hz, exceeding the Doppler frequency shift dynamic range that the ground network terminal can tolerate. The solid line is the frequency shift change felt when the solution is applied, which oscillates repeatedly between 100Hz and -100Hz, and remains within the Doppler frequency shift dynamic range that the ground network terminal can tolerate.

[0077] In summary, the method disclosed herein has the following beneficial effects compared to the prior art:

[0078] 1. The existing Doppler frequency shift compensation method for satellite-to-ground communication cannot obtain the initial geographical location of the ground network terminal, that is, it cannot obtain the initial frequency shift value of the ground network terminal; this method can obtain the approximate initial geographical location of the ground network terminal and thus its initial frequency shift by performing frequency shift pre-compensation and post-compensation on the signaling blocks involved in the initial random access process.

[0079] 2. The existing Doppler frequency shift compensation method for satellite-to-ground communication performs fixed frequency shift compensation on the entire baseband signal and cannot track the frequency shift changes of the ground network terminal. This method, based on the above-mentioned beneficial effect point 1, can track the frequency shift changes of the ground network terminal.

[0080] 3. The existing Doppler frequency shift compensation method for satellite-to-ground communication performs unified frequency shift compensation on the entire baseband signal. When multiple ground terminals are distributed in different geographical locations, they cannot obtain good compensation at the same time;

[0081] This method performs independent frequency shift compensation on the resource blocks occupied by each ground network terminal. On the basis of the above-mentioned beneficial effect point 2, it can provide good compensation for multiple ground network terminals distributed in different geographical locations at the same time.

[0082] In summary, compared with the existing solutions, the technical solution disclosed in the present invention can greatly prolong the service time of the ground network terminal in the cell formed by the satellite-borne base station.

[0083] It should be understood that in the embodiments of the present invention, the term "and / or" is only a description of the association relationship of the associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0084] It is worth noting that, although the foregoing content has described the spirit and principle of the invention with reference to several specific embodiments, it should be understood that the invention is not limited to the disclosed specific embodiments, and the division of various aspects does not mean that the features in these aspects cannot be combined, and such division is only for the convenience of expression. The invention is intended to cover various modifications and equivalent arrangements included in the spirit and scope of the appended claims.

Claims

1. A frequency shift compensation method for a ground network terminal directly connected to a satellite, characterized in that: The method comprises the following steps: A1, divides the cell generated on the ground by a single beam of a satellite base station into several strip-shaped areas; A2, assigns a frequency shift value to each strip area; A3, establishing a time-frequency shift curve of the ground network terminal of the cell; A4, determining an initial frequency shift value of each ground network terminal in the cell; A5, performing frequency shift compensation on satellite-to-ground transmission data according to the time-frequency shift curve and the initial frequency shift value of each ground network terminal.

2. The method according to claim 1, characterized in that The step A3, the step of establishing the time-frequency shift curve of the ground network terminal of the cell, includes establishing the time-downlink frequency shift curve and the time-uplink frequency shift curve of the cell.

3. The method according to claim 2, characterized in that In step A4, the base station determines the initial frequency shift value of each ground network terminal by performing post-compensation of uplink resource blocks and pre-compensation of downlink resource blocks in the initial random access process.

4. The method according to claim 3, characterized in that The step A5 includes performing frequency shift pre-compensation on downlink resources for transmitting data and performing frequency shift post-compensation on uplink resources for transmitting data.

5. The method according to claim 4, characterized in that The satellite base station performs frequency shift pre-compensation on the downlink resource block sent to the ground network terminal according to the downlink frequency shift value of the ground network terminal in the cell at any time, and then sends it to the ground network terminal.

6. The method according to claim 4, characterized in that The satellite base station first performs frequency shift and then compensation on the uplink resource blocks received from the ground network terminal according to the uplink frequency shift value of the ground network terminal in the cell at any time, and then performs subsequent processing.

7. A satellite base station, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: The processor runs the computer program to implement the method according to any one of claims 1 to 6.

8. A satellite-to-earth communication network system, characterized in that: The system comprises the satellite base station as claimed in claim 7, and a plurality of ground network terminals, wherein the ground network terminals are directly connected to the satellite base station.

9. A ground network terminal, characterized in that: The terminal is connected to the satellite-to-ground communication network system as claimed in claim 8.

10. A computer program product, comprising a computer program, characterized in that The computer program is executed by a processor to implement the method according to any one of claims 1 to 6.

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