Patent Publication Number: US-2022217591-A1

Title: Cell handover method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2020/117890, filed on Sep. 25, 2020, which claims priority to Chinese Patent Application No. 201910920053.8, filed on Sep. 26, 2019. The disclosures of the aforementioned applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     This application relates to the field of wireless communication technologies, and in particular, to a cell handover method and apparatus. 
     BACKGROUND 
     In a satellite communication system, a satellite usually runs along an orbit and moves relative to the ground. As a result, a coverage area of the satellite changes, a coverage area of a cell of the satellite may also change, and cell handover occurs on a terminal. Conditional handover is a handover method proposed to improve reliability when the terminal performs cell handover. In the conditional handover method, an access resource of a new cell is first reserved, and a relatively high threshold for handover triggering is provided in a handover command. Once handover is performed, the terminal can be directly handed over to a target cell. Because a plurality of trigger conditions and a plurality of candidate cells are introduced, the plurality of candidate cells may simultaneously meet the trigger conditions for conditional handover. 
     To select a most suitable target cell for handover/access when the plurality of cells simultaneously meet the trigger conditions for conditional handover, the terminal calculates, based on ephemeris information and location information of the terminal, duration in which each candidate cell can provide a service for the terminal, and selects a cell that can provide a service for the terminal for the longest time as the target cell. However, an actual situation of a cell may change. For example, a payload of a satellite base station of the cell, a payload of the cell, a location of the cell, and a beam angle and a spectrum resource of the cell may change. As a result, a situation of the cell when the trigger condition is generated is different from that when handover actually occurs. Consequently, the target cell determined by the terminal is not a most suitable access cell. 
     SUMMARY 
     Embodiments of this application provide a cell handover method and apparatus, to determine a suitable access cell for a terminal in a cell handover process. 
     According to a first aspect, a cell handover method is provided, and includes: A terminal sends a random access preamble of a determined first cell to a satellite, where the random access preamble is used to determine terminal information, and the terminal information is used for cell handover of the terminal; the satellite receives the random access preamble that is of the first cell and that is sent by the terminal, determines, based on the terminal information and cell attribute information, a second cell most suitable for the terminal to access, and sends access information of the second cell to the terminal; and the terminal receives the access information that is of the second cell and that is sent by the satellite, and accesses the second cell by using the access information of the second cell. 
     Optionally, the satellite may send one or more trigger conditions for conditional handover and the random access preamble to the terminal in advance. In this way, the terminal determines the first cell in one or more cells meeting the trigger conditions for conditional handover. 
     For example, the satellite may obtain the terminal information in the following manner: The satellite determines the terminal information based on a stored correspondence between a random access preamble and terminal information; the satellite determines a terminal identifier based on the random access preamble, and determines the terminal information based on a stored correspondence between a terminal identifier and terminal information; or the satellite determines a terminal identifier based on the random access preamble, and indicates a terminal corresponding to the terminal identifier to report the terminal information. 
     In a cell handover process, the terminal can determine only a most suitable satellite based on location information of the terminal and ephemeris information of the satellite, and the determined first cell may not be a most suitable access cell. Therefore, in the method, a satellite in a satellite communication system determines the most suitable access cell for a second time. As an important network element in the satellite communication system, the satellite can provide a radio access service for the terminal and schedule a radio resource. Therefore, the satellite determines, based on the terminal information and the cell attribute information, a second cell for the terminal to finally access. Because the cell attribute information is considered in the method, compared with the first cell determined by the terminal, the second cell determined based on the terminal information and the cell attribute information is more suitable for the terminal to access, thereby ensuring that the terminal can access a most suitable cell. In addition, in the method, the satellite determines the most suitable access cell, and the satellite also does not need to send a large amount of cell attribute information to the terminal, thereby reducing signaling overheads. 
     In a possible implementation, the terminal information includes at least one of the following content: location information of the terminal, an identifier of a cell meeting a trigger condition for conditional handover, and the trigger condition that the cell meeting the trigger condition for conditional handover meets. 
     In this implementation, the terminal information is used to help the satellite select/assist the satellite in selecting a suitable access cell for the terminal, and the satellite determines, based on the determined terminal information and the cell attribute information, a suitable second cell for the terminal to access, so as to ensure that the terminal can access a more suitable cell. 
     In a possible implementation, before the terminal receives the access information that is of the second cell and that is sent by the satellite, the method further includes: 
     The terminal sends a first message to the satellite, where the first message is used to indicate to enable the satellite to select an access cell for the terminal, or the first message includes first indication information and the first indication information is used to indicate to enable the satellite to select the access cell for the terminal. 
     Before the satellite determines, based on the terminal information and the cell attribute information that is of the satellite, the second cell most suitable for the terminal to access, the method further includes: The satellite receives the first message sent by the terminal. 
     In this implementation, the terminal may indicate the satellite to select the access cell for the terminal. In this way, the satellite may determine, based on an indication of the first message and based on the terminal information and the cell attribute information, the second cell suitable for the terminal to access, so as to ensure that the terminal can access a more suitable cell. 
     In a possible implementation, before the satellite determines, based on the terminal information and the cell attribute information that is of the satellite, the second cell most suitable for the terminal to access, the method further includes: The satellite sends a second message to the terminal, where the second message is used to indicate the terminal to report the terminal information, or the second message includes second indication information and the second indication information is used to indicate the terminal to report the terminal information. 
     Before the terminal receives the access information that is of the second cell and that is sent by the satellite, the method further includes: The terminal receives the second message sent by the satellite. 
     In this implementation, if the satellite does not know the terminal information or the stored terminal information is out of date, the satellite may indicate the terminal to report the terminal information. In this way, the terminal reports, based on an indication of the second message, the terminal information or other information (for example, a random access code) used to determine the terminal information, so that the satellite can determine, based on the terminal information reported by the terminal and the cell attribute information, the second cell most suitable for the terminal to access, thereby ensuring that the terminal can access a more suitable cell. 
     Optionally, the second message includes access information of the first cell. 
     In a possible implementation, the first cell and the second cell belong to a same satellite, or the first cell and the second cell belong to different satellites. 
     In a possible implementation, the access information of the second cell includes at least one of the following content: a TA, a UL grant, a TC-RNTI, and a third message, where the third message is used for downlink synchronization; 
     the UL grant includes at least one of the following information: identification information indicating whether frequency hopping is enabled in the second cell, indication information of an uplink physical uplink shared channel (PUSCH) frequency resource, indication information of an uplink PUSCH time resource, a modulation and coding scheme (MCS), a PUSCH power control command, and a channel state information (CSI) request; and 
     the third message includes at least one of the following information: a frequency, a subcarrier spacing, and a guard interval of the second cell, a time domain location that is of a primary synchronization signal (PSS)/secondary synchronization signal (SSS) of the second cell and that is relative to the first cell, reserved time, and the indication information of the uplink PUSCH time resource of the UL grant. 
     In this implementation, in addition to determining the second cell suitable for the terminal to access, the satellite further determines the access information of the second cell, so as to ensure that the terminal can access a more suitable cell based on the access information of the second cell. 
     In a possible implementation, that the terminal accesses the second cell by using the access information of the second cell includes: 
     The terminal performs downlink synchronization on the second cell by using the third message, and sends uplink information on the uplink PUSCH frequency resource and/or time resource indicated by the UL grant. 
     In this implementation, the terminal performs downlink synchronization on the second cell based on the access information of the second cell that is determined by the satellite and that is suitable for the terminal to access, and sends the uplink information on the indicated resource. In this way, the terminal accesses the second cell, and the terminal accesses a more suitable cell. 
     According to a second aspect, a cell handover apparatus is provided, and the cell handover apparatus has functions of implementing the satellite in the foregoing method embodiment. These functions may be implemented by hardware, or may be implemented by hardware executing corresponding software. The hardware or the software includes one or more function modules corresponding to the foregoing functions. 
     According to a third aspect, a cell handover apparatus is provided, and the cell handover apparatus has functions of implementing the terminal in the foregoing method embodiment. These functions may be implemented by hardware, or may be implemented by hardware executing corresponding software. The hardware or the software includes one or more function modules corresponding to the foregoing functions. 
     According to a fourth aspect, a cell handover apparatus is provided. The cell handover apparatus may be the satellite in the foregoing method embodiment, or may be a chip disposed in the satellite. The cell handover apparatus includes a transceiver and a processor, and optionally, further includes a memory. The memory is configured to store a computer program or instructions. The processor is separately coupled to the memory and the transceiver. When the processor executes the computer program or the instructions, the communication apparatus is enabled to perform the method performed by the satellite in the foregoing method embodiment. 
     According to a fifth aspect, a cell handover apparatus is provided. The cell handover apparatus may be the terminal in the foregoing method embodiment, or may be a chip disposed in the terminal. The cell handover apparatus includes a transceiver and a processor, and optionally, further includes a memory. The memory is configured to store a computer program or instructions. The processor is separately coupled to the memory and the transceiver. When the processor executes the computer program or the instructions, the communication apparatus is enabled to perform the method performed by the terminal in the foregoing method embodiment. 
     According to a sixth aspect, a computer program product is provided. The computer program product includes computer program code; and when the computer program code is run on a computer, the computer is enabled to perform the method performed by the satellite in any one of the first aspect and the possible implementations of the first aspect. 
     According to a seventh aspect, a computer program product is provided. The computer program product includes computer program code; and when the computer program code is run on a computer, the computer is enabled to perform the method performed by the terminal in any one of the first aspect and the possible implementations of the first aspect. 
     According to an eighth aspect, this application provides a chip system. The chip system includes a processor and a memory, and the processor and the memory are electrically coupled. The memory is configured to store computer program instructions. The processor is configured to execute some or all of the computer program instructions in the memory. When the some or all of the computer program instructions are executed, the processor is configured to implement functions of the satellite in the method according to any one of the first aspect and the possible implementations of the first aspect. 
     In a possible design, the chip system further includes a transceiver, and the transceiver is configured to send a signal processed by the processor, or receive a signal to input the signal to the processor. The chip system may include a chip, or may include a chip and another discrete device. 
     According to a ninth aspect, this application provides a chip system. The chip system includes a processor and a memory, and the processor and the memory are electrically coupled. The memory is configured to store computer program instructions. The processor is configured to execute some or all of the computer program instructions in the memory. When the some or all of the computer program instructions are executed, the processor is configured to implement functions of the apparatus in the method according to any one of the first aspect and the possible implementations of the first aspect. 
     In a possible design, the chip system further includes a transceiver, and the transceiver is configured to send a signal processed by the processor, or receive a signal to input the signal to the processor. The chip system may include a chip, or may include a chip and another discrete device. 
     According to a tenth aspect, a computer-readable storage medium is provided. The computer-readable storage medium stores a computer program; and when the computer program is run, the method performed by the satellite in any one of the first aspect and the possible implementations of the first aspect is implemented. 
     According to an eleventh aspect, this application provides a computer-readable storage medium. The computer-readable storage medium stores a computer program; and when the computer program is run, the method performed by the terminal in any one of the first aspect and the possible implementations of the first aspect is implemented. 
     According to a twelfth aspect, a communication system is provided. The system may include the satellite that performs the method in any one of the first aspect and the possible implementations of the first aspect, and the terminal that performs the method in any one of the first aspect and the possible implementations of the first aspect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of cell handover; 
         FIG. 2  is a signaling flowchart of conditional handover; 
         FIG. 3  is a schematic architectural diagram of a satellite communication system; 
         FIG. 4  is a schematic architectural diagram of another satellite communication system; 
         FIG. 5  is a schematic diagram of a non-contention-based random access process; 
         FIG. 6  is a flowchart of cell handover according to an embodiment of this application; 
         FIG. 7  is a flowchart of cell handover according to an embodiment of this application; 
         FIG. 8  is a flowchart of cell handover according to an embodiment of this application; 
         FIG. 9  is a flowchart of cell handover according to an embodiment of this application; 
         FIG. 10  is a cell handover apparatus according to an embodiment of this application; 
         FIG. 11  is a cell handover apparatus according to an embodiment of this application; 
         FIG. 12  is a cell handover apparatus according to an embodiment of this application; and 
         FIG. 13  is a cell handover apparatus according to an embodiment of this application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following further describes in detail embodiments of this application with reference to the accompanying drawings. 
     All aspects, embodiments, or features are presented in this application by describing a system that may include a plurality of devices, components, modules, and the like. It should be appreciated and understood that each system may include another device, component, module, and the like, and/or may not include all devices, components, modules, and the like discussed with reference to the accompanying drawings. In addition, a combination of the solutions may further be used. 
     In addition, the term “for example” in the embodiments of this application is used to represent giving an example, an illustration, or a description. Any embodiment or design described as an “example” in this application should not be explained as being more preferred or having more advantages than another embodiment or design. Exactly, the term “example” is used to present a concept in a specific manner. 
     A network architecture and a service scenario described in the embodiments of this application are intended to describe the technical solutions in the embodiments of this application more clearly, but constitute no limitation on the technical solutions provided in the embodiments of this application. Persons of ordinary skill in the art may learn that the technical solutions provided in the embodiments of this application are also applicable to a similar technical problem as the network architecture evolves and a new service scenario emerges. 
     The following describes some terms in the embodiments of this application, to facilitate understanding of persons skilled in the art. 
     (1) A terminal is also referred to as user equipment (user equipment, UE), a mobile station (mobile station, MS), a mobile terminal (mobile terminal, MT), or the like, and is a device that provides voice and/or data connectivity for a user. For example, the terminal device includes a handheld device, a vehicle-mount device, an Internet of Things device, or the like that has a satellite communication function. Currently, the terminal device may be a mobile phone (mobile phone), a tablet, a notebook computer, a palmtop computer, a mobile Internet device (mobile Internet device, MID), a wearable device, a virtual reality (virtual reality, VR) device, an augmented reality (augmented reality, AR) device, a wireless terminal in industrial control (industrial control), a wireless terminal in self-driving (self-driving), a wireless terminal in a remote medical surgery (remote medical surgery), a wireless terminal in a smart grid (smart grid), a wireless terminal in transportation safety (transportation safety), a wireless terminal in a smart city (smart city), a wireless terminal in a smart home (smart home), a terminal device in a 5G network, a terminal device in a future evolved public land mobile communication network (Public Land Mobile Network, PLMN) or another future communication system, or the like. 
     (2) A network device is located in a communication system integrated with a satellite communication system, can provide a connection service for a satellite, and can connect the satellite to a core network. For example, the network device may be a network device that has a satellite gateway function in a communication system, for example, a gateway (gateway) device, a ground station device, or a non-terrestrial network gateway (Non-terrestrial network Gateway, NTN-Gateway)/satellite gateway. 
     (3) The satellite, also referred to as a satellite base station, is located in the satellite communication system and can provide a radio access service for a terminal that is in coverage corresponding to the satellite. When the satellite is a communication satellite in the satellite communication system, the satellite may be a geostationary earth orbit (geostationary earth orbit, GEO) satellite, a non-GEO satellite, a low-earth orbit (low-earth orbit, LEO) satellite, or the like. Alternatively, satellites may be divided based on functionality, for example, include a transparent payload satellite (transparent payload satellite), a regenerative payload satellite (regenerative payload satellite), and the like. 
     The transparent payload satellite, also known as a “transparent transponder” or a “bent-pipe transponder”, includes a receiver circuit, a frequency conversion circuit, a modulation circuit, an amplification circuit, a transmitter circuit, and the like. An uplink signal is received from the ground on a frequency band, and a downlink signal is transmitted to the ground on another frequency band after frequency conversion, modulation, and amplification are performed on the uplink signal. This implements long-distance communication and broadcasting on the earth, and may also be used for inter-satellite communication. 
     The regenerative payload satellite not only has functions of wireless spectrum filtering, frequency conversion, and amplification, but also has functions of modulation and demodulation, and even switching routes. The regenerative payload satellite is equivalent to having all or some of base station functions. 
     (4) A beam is a shape formed by electromagnetic waves emitted by satellite antennas on the surface of the earth, and is like a specific range of beams of a flashlight. Alternatively, a signal emitted by the satellite is not 360 degrees of radiation, but is a signal wave emitted in a specific direction in a concentrated manner. 
     (5) Coverage of a cell (cell) is all or a part of coverage of a base station/satellite base station. Usually, coverage of one or more beams is understood as coverage of a cell. 
     (6) In ephemeris information, an ephemeris in GPS measurement is a table of a precise location or a trajectory that is of a moving celestial body and that changes with time, and the ephemeris is a function of time. A satellite ephemeris may determine a running status such as time, a location, or a speed of a flight body. 
     (7) Cell handover (Cell handover) is channel switch (Channel Switch) that needs to be performed in a wireless communication system when a mobile station (such as the terminal) moves from one cell to another cell, to keep communication of a mobile user uninterrupted. 
     (8) A random access process refers to a process that starts from a step of sending, by the terminal, a random access preamble to attempt to access a network and that ends before a basic signaling connection is established between the terminal and the network. The process includes contention-based random access and non-contention-based random access. 
     The term “and/or” in this application describes an association relationship for describing associated objects and represents that three relationships may exist. For example, A and/or B may represent the following three cases: Only A exists, both A and B exist, and only B exists. The character “/” usually indicates an “or” relationship between the associated objects. 
     “A plurality of” in this application means two or more. 
     In addition, it should be understood that in descriptions of this application, terms such as “first” and “second” are merely intended for purposes of description, and should not be understood as an indication or implication of relative importance, and cannot be understood as an indication or implication of a sequence. 
     Technical solutions in the embodiments of this application may be applied to a satellite communication system, where the satellite communication system may be integrated with a conventional mobile communication system. For example, the mobile communication system may be a 4th generation (4th Generation, 4G) communication system (for example, a long term evolution (long term evolution, LTE) system), a worldwide interoperability for microwave access (worldwide interoperability for microwave access, WiMAX) communication system, a 5th generation (5th Generation, 5G) communication system (for example, a new radio (new radio, NR) system), and a future mobile communication system. 
     For ease of understanding the embodiments of this application, the following describes an application scenario of this application. The service scenario described in the embodiments of this application is intended to describe the technical solutions in the embodiments of this application more clearly, but constitutes no limitation on the technical solutions provided in the embodiments of this application. Persons of ordinary skill in the art may learn that the technical solutions provided in the embodiments of this application are also applicable to a similar technical problem as a new service scenario emerges. 
     In a cellular system, a mechanism in which a terminal performs cell handover is mainly implemented based on measurement information such as signal strength and quality. Because the terminal randomly moves in a cellular cell, distances between terminals and a base station are different, a terminal farther away from the base station has weaker signal strength and worse signal quality, and a terminal closer to the base station has stronger signal strength and better signal quality. In this case, a phenomenon of communication interruption occurs on a terminal having weak signal strength and poor signal quality, that is, a terminal farther away from the base station. This phenomenon is referred to as a near-far effect. 
     However, in the satellite communication system, because signal strength and signal quality of a beam center point are little different from those of a beam edge point, if the measurement information is used as a decision condition for cell handover, that is, signal strength and quality are used as a decision condition for a cell, a ping-pong effect of the terminal occurs. Signal quality of a satellite SAT  1  and a satellite SAT  2  is shown in  FIG. 1 . Because some events occur, the signal quality, of the SAT  1 , detected by the terminal is excessively poor (for example, lower than a set quality threshold) at time t 1 , and consequently, the terminal switches from the SAT  1  to the SAT  2 . After the events disappear, and after the terminal detects that the signal quality of the SAT  1  is improved, the terminal switches from the SAT  2  back to the SAT  1 . However, during the occurrence of the events, the signal quality of the SAT  1  does not deteriorate, but ping-pong switching is caused by a shadow effect, such as blocking, generated due to impact of the events. This reduces reliability of satellite switching/cell handover. 
     To improve handover reliability, a handover manner of conditional handover is proposed. A signaling procedure of conditional handover is shown in  FIG. 2 , and includes the following steps. 
     Step  1 : A satellite sends a measurement control (Measurement Control) message to a terminal. 
     A cell that the terminal currently accesses may be considered as a source cell (Source cell) in a handover process, and the source cell is a cell covered by the satellite. 
     The measurement control message includes a first threshold, and the first threshold is used as a trigger threshold for cell handover, to ensure sending of a measurement report. 
     Step  2 : The terminal sends a measurement report (Measurement Report) message to the satellite. 
     Step  3 : The satellite performs handover preparation (HO preparation) with a candidate cell (Candidate Cell)  1  and a candidate cell  2 . 
     The candidate cell  1  may be located in coverage of the satellite, or the candidate cell  1  may be not located in the coverage of the satellite. The candidate cell  2  may be located in the coverage of the satellite, or the candidate cell  2  may be not located in the coverage of the satellite. 
     In this step, the satellite may determine a second threshold of a candidate cell. 
     Step  4 : The satellite sends handover assistance information (HO assistance info) to the terminal. 
     The HO assistance info includes the second threshold of the candidate cell, and the second threshold is used as a trigger threshold for conditional handover. For example, the second threshold may include a reference signal received power (Reference Signal Received Power, RSRP) threshold of the candidate cell  1  and an RSRP threshold of the candidate cell  2 . 
     Step  5 : The terminal performs evaluation of the condition for the candidate cells (Evaluation of the condition for the candidate cells), and selects a target cell from the candidate cells. 
     As shown in  FIG. 2 , the target cell selected by the terminal is the candidate cell  1 . 
     Step  6 : The terminal accesses the target cell. 
     As shown in  FIG. 2 , the terminal accesses the candidate cell  1 . 
     To briefly sum up, in a conditional handover process, a relatively low trigger threshold is first configured as the trigger threshold for cell handover to ensure reporting of the measurement report, and then a relatively high trigger threshold is configured as the trigger condition for conditional handover to ensure reliable transmission. In this way, the foregoing ping-pong effect is mitigated or avoided. In addition, an access resource of a new cell is reserved in the conditional handover process. Once the terminal meets the trigger condition (for example, the second threshold in  FIG. 2 ) for conditional handover, the terminal can be directly handed over to the target cell. Usually, the handover of the terminal to the target cell includes synchronization to the target cell and random access to the target cell. 
     The trigger condition for conditional handover is introduced in a satellite communication system. The trigger condition for conditional handover not only includes measurement information of a signal (signal strength and quality), but also includes triggering of locations (including a location of the terminal and a location of the satellite) and triggering of time (for example, a timer). Because a plurality of trigger conditions are introduced and there may be a plurality of candidate cells, the plurality of candidate cells may simultaneously meet the trigger conditions for conditional handover. In this case, the terminal needs to select, from the plurality of candidate cells meeting the trigger conditions for conditional handover, a most suitable cell as the target cell for handover or access. 
     In one manner, the terminal calculates serviceable duration of these candidate cells based on ephemeris information of the satellite and location information of the terminal, and usually selects a cell with longest serviceable duration as the target cell.  FIG. 3  is used as an example for description. Satellites of three cells move along an orbit from right to left (for example, left and right shown in  FIG. 3 ), and a terminal currently accesses the cell A. A trigger condition, for conditional handover, sent by the satellite to the terminal includes a trigger condition, for conditional handover, of the cell B and a trigger condition, for conditional handover, of the cell C. It is assumed that both the cell B and the cell C meet the trigger conditions for conditional handover at a moment. The terminal determines, based on pre-obtained ephemeris information of the satellite and location information of the terminal, that service duration of the cell C is longer than service duration of the cell B. Therefore, the terminal selects the cell C as a target cell for access. 
     However, when the terminal determines, based on the ephemeris information and a terminal location, to access which cell, there is a relatively large time difference between time at which a handover condition is generated and time at which handover actually occurs. Therefore, when handover actually occurs, factors for generating the handover condition may change, for example, a payload of a base station, a payload of the cell, and a beam angle and a spectrum resource of the cell. As a result, when handover actually occurs, a cell selected by the terminal is not a most suitable cell, or is not a cell that can provide longest service duration for the terminal. In addition, the ephemeris information can only help the terminal select a suitable satellite, but cannot help the terminal select a more suitable cell of the satellite. Therefore, the satellite further needs to inform the terminal of a location relationship of each cell of the satellite. A quantity of beams of a single satellite is large. For example, in an extreme case, a single satellite in the VLEO constellation may have thousands of beams. In addition, coverage of a moving cell of the satellite is frequently adjusted, and interaction between the satellite and the terminal usually takes long time. Therefore, if the satellite informs the terminal of the location relationship of each cell of the satellite, the satellite may not notify the terminal of a real-time location of the cell in a timely manner. As a result, the cell selected by the terminal is not the most suitable cell, and high signaling overheads are also generated. To sum up, the cell selected by the terminal based on the ephemeris information and the terminal location is not the most suitable cell. 
     In view of this, to ensure that the terminal can access a suitable cell in a cell handover process, this application provides a cell handover method to determine a suitable access cell for the terminal in the cell handover process. 
     In the method, in a process in which the terminal performs cell handover, the terminal sends a random access preamble of a determined first cell to a satellite. The satellite determines terminal information based on the random access preamble, where the terminal information is used for cell handover of the terminal, determines, based on the terminal information and cell attribute information, a second cell most suitable for the terminal to access, and sends access information of the second cell to the terminal. The terminal accesses the second cell. According to the method, the satellite determines the suitable access cell for the terminal, and the satellite comprehensively determines the suitable access cell based on the terminal information and the cell attribute information. Because the cell attribute information is considered, compared with the first cell determined by the terminal, the second cell determined by the satellite is more suitable for the terminal to access. In this way, it can be ensured that the terminal can access a most suitable cell, the satellite determines the most suitable access cell, and the satellite does not need to send a large amount of cell attribute information to the terminal, thereby reducing signaling overheads. 
     The most suitable access cell mentioned in this embodiment of this application may be a cell that can provide longest service duration for the terminal, or may be a most suitable cell meeting another condition. It may be understood that in this embodiment of this application, the descriptions are provided only by using the cell that can provide the longest service duration for the terminal as an example. For a case of selecting the most suitable cell meeting another condition, the cell handover method provided in this embodiment of this application may be used. 
     This embodiment of this application provides a cell handover method. The method may be applied to a satellite communication system. Specifically, the method may be applied to the satellite communication system shown in  FIG. 3 , where a cell in which a terminal is located before the terminal is handed over and a cell to which the terminal is handed over in  FIG. 3  belong to different satellites. The method may also be applied to the satellite communication system shown in  FIG. 4 , where a cell in which a terminal is located before the terminal is handed over and a cell to which the terminal is handed over in  FIG. 4  belong to a same satellite. In this embodiment of this application, a satellite in the satellite communication system implements a complete base station function, and simultaneously covers one or more cells on the ground. 
     Before a specific process of the cell handover method in this embodiment of this application is described, a non-contention-based random access process is briefly described first. 
     Non-contention-based random access is random access initiated by the terminal on a specified physical random access channel (Physical Random Access Channel, PRACH) channel resource based on an indication of a base station by using a specified random access preamble (Preamble), and is applicable to scenarios such as handover, downlink data arrival but out-of-synchronization, and localization.  FIG. 5  shows a specific process of non-contention-based random access, and steps are as follows. 
     Step  1 : A base station sends RA preamble assignment (RA Preamble assignment) to a terminal. 
     Specifically, the base station sends, to the terminal, the random access preamble assignment via dedicated signaling in downlink (Random Access Preamble assignment via dedicated signaling in DL, RA Preamble assignment). 
     The base station sends, to the terminal, the preamble and the PRACH channel access resource that are required in the non-contention-based random access process. If a preamble resource is insufficient in this case, the base station can only notify the terminal to initiate contention-based random access, and a manner of this is to set all preamble indexes (indexes) in a physical downlink control channel (Physical Downlink Control Channel, PDCCH) format 1a to 0s. After the preamble indexes decoded by the terminal are all 0s, the terminal performs a contention-based random access process. 
     For handover, a non-contention-based preamble is sent to the terminal through a handover command. In other two scenarios, DCI  1 A scrambled by using a radio network temporary identifier (Radio Network Temporary Identifier, RNTI) needs to be sent to the terminal. 
     Step  2 : The terminal sends a preamble (random access preamble on a PRACH in uplink) to the base station. 
     If a plurality of PRACH channel resources are specified, the terminal randomly selects, from three consecutive and available subframes that have PRACH channel resources, a specified PRACH channel resource to carry MSG 1 . A subsequent processing process on a side of the base station is the same as the contention-based random access process. 
     Step  3 : The base station feeds back a random access response (Random Access Response) (for example, Random Access Response on a DL-SCH) to the terminal. 
     A subsequent processing process on the side of the base station is the same as the contention-based random access process. 
     The following describes in detail a specific process of the cell handover method with reference to  FIG. 6 . The process includes the following steps. 
     Step  601 : A terminal sends a random access preamble of a determined first cell to a satellite. 
     The random access preamble is used to determine terminal information, and the terminal information is used for cell handover of the terminal, for example, the terminal information may be used to help the satellite select a most suitable access cell for the terminal. The terminal information includes at least one of the following content: location information of the terminal, an identifier of a cell meeting a trigger condition for conditional handover, and the trigger condition that the cell meeting the trigger condition for conditional handover meets. If there are a plurality of cells meeting a trigger condition for conditional handover, the trigger condition that the cell meeting the trigger condition for conditional handover meets is the trigger condition that the plurality of cells meeting the trigger condition for conditional handover meet. 
     Specifically, the terminal determines one or more candidate cells meeting the trigger condition for conditional handover, determines the first cell in the candidate cells, and sends the random access preamble of the first cell to the satellite. If there is one candidate cell, the terminal determines the candidate cell as the first cell; if there are a plurality of candidate cells, the terminal selects the first cell from the plurality of candidate cells. 
     Optionally, before step  601 , the base station may send one or more trigger conditions for conditional handover and the random access preamble to the terminal, where the one or more trigger conditions for conditional handover are trigger conditions, for conditional handover, of one or more cells, and the random access preamble may be a random access preamble of one or more cells. 
     Step  602 : The satellite receives the random access preamble that is of the first cell and that is sent by the terminal. The satellite may determine the terminal information based on the received random access preamble. 
     In an implementation, the satellite stores a correspondence between a random access preamble and terminal information, and the satellite can query the correspondence based on the received random access preamble, to determine terminal information corresponding to the random access preamble. 
     In another implementation, the satellite stores a first correspondence between a random access preamble and a terminal identifier, and the satellite can query the first correspondence based on the received random access preamble, to determine a terminal identifier corresponding to the random access preamble. The satellite further pre-stores a second correspondence between a terminal identifier and terminal information, and then the satellite queries the second correspondence based on the determined terminal identifier, to determine corresponding terminal information. 
     In still another implementation, the satellite stores a correspondence between a random access preamble and a terminal identifier, and the satellite can query the correspondence based on the received random access preamble, to determine a terminal identifier corresponding to the random access preamble. The satellite then indicates a terminal corresponding to the terminal identifier to report the terminal information. 
     The terminal information is used for cell handover of the terminal, for example, used to help the base station select a most suitable access cell for the terminal. The terminal information may include, but is not limited to, at least one of the following content: the location information of the terminal, the identifier of the cell meeting the trigger condition for conditional handover, and the trigger condition that each cell meeting the trigger condition for conditional handover meets. 
     Step  603 : The satellite determines, based on the terminal information determined by using the random access preamble and current cell attribute information, a second cell most suitable for the terminal to access. The cell attribute information includes a cell payload status and/or cell coverage. 
     The satellite stores the cell attribute information. Optionally, the cell attribute information may include cell attribute information corresponding to each cell covered by the satellite, or may include cell attribute information corresponding to each cell covered by another satellite other than the satellite. In other words, the second cell determined by the satellite may be a cell in coverage of the satellite, or may be a cell in coverage of another satellite. 
     The first cell and the second cell belong to a same satellite, or the first cell and the second cell belong to different satellites, that is, the first cell and the second cell are in coverage of a same satellite, or the first cell and the second cell are in coverage of different satellites. If the first cell and the second cell belong to the same satellite, the first cell and the second cell may be the same cell, or the first cell and the second cell may be different cells. If the first cell and the second cell belong to different satellites, the first cell and the second cell are different cells. 
     Optionally, before step  603 , the satellite may determine, in one or more of the following manners, whether to select a most suitable access cell for the terminal. 
     1. If the satellite receives a first message sent by the terminal, the satellite determines to select a most suitable access cell for the terminal. This may be specifically implemented as follows: The satellite receives a first message sent by the terminal, where the first message is used to indicate to enable the satellite to select an access cell for the terminal, or the first message includes first indication information and the first indication information is used to indicate to enable the satellite to select the access cell for the terminal. 
     The first message may be signaling sent by the terminal to the satellite or a field in the signaling, or the first message may be signaling sent by the terminal to a network and forwarded by the network to the satellite or a field in the signaling. This is not limited herein. 
     2. If the satellite and the terminal agree in advance that the satellite selects an access cell for the terminal, the satellite determines to select a most suitable access cell for the terminal. 
     3. If the satellite determines that cell handover performed by the terminal is triggered by conditional handover, the satellite determines to select a most suitable access cell for the terminal. 
     Because the satellite may not know the terminal information (for example, may not know the location information of the terminal, that is, may not know that the terminal is in coverage of which cells), or the terminal information stored in the satellite is out of date, before step  603 , the satellite may send a second message to the terminal, where the second message is used to indicate the terminal to report the terminal information, or the second message includes second indication information and the second indication information is used to indicate the terminal to report the terminal information. The terminal information may include at least one of the following content: the location information of the terminal, the identifier of the cell meeting the trigger condition for conditional handover, the trigger condition that each cell meeting the trigger condition for conditional handover meets, and the like. The second message may be a random access response message or other RRC signaling. This is not limited herein. Optionally, the second message may further include access information of the first cell. 
     Step  604 : The satellite sends access information of the second cell to the terminal. Optionally, after determining that the first cell and the second cell are different cells, the satellite may send the access information of the second cell to the terminal. 
     The access information of the second cell may include at least one of the following content: a timing advance (timing advance, TA) of the second cell, a UL grant (uplink scheduling grant) of the second cell, a temporary cell radio network temporary identifier (temporary C-RNTI, TC-RNTI) of the second cell, and a third message used for downlink synchronization of the second cell. For example, the access information of the second cell includes the TA of the second cell, the UL grant of the second cell, the TC-RNTI of the second cell, and the third message used for downlink synchronization of the second cell. 
     Specifically, the UL grant may include at least one of the following information: identification information indicating whether frequency hopping is enabled in the second cell, indication information of an uplink physical uplink shared channel (Physical Uplink Shared Channel, PUSCH) frequency resource, indication information of an uplink PUSCH time resource, a modulation and coding scheme (Modulation and Coding Scheme, MCS), a PUSCH power control command, and a channel state information (Channel State Information, CSI) request, where the PUSCH power control command may include a PUSCH transmit power control (Transmit power control, TPC) command. 
     The third message may include at least one of the following information: a frequency, a subcarrier spacing, and a guard interval of the second cell, a time domain location that is of a primary synchronization signal (Primary Synchronization Signal, PSS)/secondary synchronization signal (Secondary Synchronization Signal, SSS) of the second cell and that is relative to the first cell, reserved time, and the indication information of the uplink PUSCH time resource of the UL grant. 
     In an example, the satellite may determine the TA in the following manners. 
     1. If the first cell and the second cell belong to the same satellite, the TA of the second cell and a TA of the first cell are related only to a delay difference caused by a radio frequency. For example, the satellite may determine the TA of the second cell based on a sum of the TA of the first cell and the delay difference caused by the radio frequency. 
     2. The TA of the second cell may be related to a distance regardless of whether the first cell and the second cell belong to the same satellite. For example, the satellite may pre-estimate a delay difference caused by a radio frequency or the distance. 
     In an example, if the first cell and the second cell belong to the same satellite, the first cell and the second cell may be scheduled together or obtain the UL grant during scheduling, so that the satellite allocates a time-frequency resource of the second cell to the terminal. 
     UL grant information is specifically shown in the following table. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 UL grant field 
                 Number of bits 
               
               
                   
                 (UL grant field) 
                 (number of bits) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Frequency hopping flag 
                 1 
               
               
                   
                 (Frequency hopping flag) 
               
               
                   
                 PUSCH frequency resource allocation 
                 14 
               
               
                   
                 (indication information of a 
               
               
                   
                 PUSCH frequency resource) 
               
               
                   
                 PUSCH time resource allocation 
                 4 
               
               
                   
                 (indication information of a 
               
               
                   
                 PUSCH time resource) 
               
               
                   
                 MCS 
                 4 
               
               
                   
                 TPC command for a PUSCH 
                 3 
               
               
                   
                 (TPC command for a PUSCH) 
               
               
                   
                 CSI request 
                 1 
               
               
                   
                 (CSI request) 
               
               
                   
                   
               
            
           
         
       
     
     The TC-RNTI is allocated by the base station to the terminal. 
     The third message may be understood as a newly added downlink synchronization assistance message. Because previously, the terminal performs downlink synchronization only on the first cell, but now the terminal needs to access a most suitable cell (namely, the second cell) determined by the satellite, the downlink synchronization assistance information may be added, so that the terminal quickly performs downlink synchronization on the most suitable cell. For example, the downlink synchronization assistance information includes a frequency, a subcarrier spacing, and a guard interval of the most suitable cell, and the time domain location that is of the PSS/SSS of the second cell and that is relative to the first cell. In addition, because the downlink synchronization on the second cell requires specific time, the reserved time may be indicated in the information, or indicated in an uplink PUSCH time resource indication of the UL grant, to ensure that the terminal can send an uplink message at specified time. 
     Step  605 : The terminal receives the access information that is of the second cell and that is sent by the satellite. 
     Optionally, corresponding to a side of the satellite, before step  605 , the terminal may send a first message to the satellite, where the first message is used to indicate to enable the satellite to select an access cell for the terminal, or the first message includes first indication information and the first indication information is used to indicate to enable the satellite to select the access cell for the terminal. 
     Optionally, corresponding to the side of the satellite, before step  605 , the terminal may receive a second message sent by the satellite, where the second message is used to indicate the terminal to report the terminal information, or the second message includes second indication information and the second indication information is used to indicate the terminal to report the terminal information. If determining to report the terminal information, the terminal sends the terminal information to the satellite. 
     In an implementation, the terminal may determine whether to access the first cell or report the terminal information. For example, if there is only one trigger condition of the terminal, or there is only one triggered target cell for handover, the terminal may determine to directly access the first cell; otherwise, the terminal may determine to report the terminal information. 
     Optionally, the satellite may send a cell number of the second cell to the terminal. 
     In an example, the access information of the second cell may be sent to the terminal by using a random access response message, and the random access response message further includes the cell number of the second cell. 
     Step  606 : The terminal accesses the second cell by using the access information of the second cell. 
     Specifically, the terminal performs downlink synchronization on the second cell by using the third message, and sends uplink information on the uplink PUSCH frequency resource and/or time resource indicated by the UL grant. 
     According to the solution provided in this embodiment of this application, in a satellite communication system, a cell that the terminal finally accesses is determined by the satellite for a second time, instead of being determined by the terminal, and the satellite comprehensively determines, based on the terminal information and the cell attribute information, the cell that the terminal finally accesses. Because the cell attribute information is considered, compared with the first cell determined by the terminal, the second cell determined by the satellite is more suitable for the terminal to access. In this way, it can be ensured that the terminal can access a most suitable cell, the satellite determines the most suitable access cell, and the satellite does not need to send a large amount of cell attribute information to the terminal, thereby reducing signaling overheads. 
     Based on  FIG. 6 , a specific cell handover process is described in detail in this embodiment of this application. 
     Refer to  FIG. 7 . A case in which a satellite directly calculates a most suitable access cell for a terminal includes the following steps. 
     Step  701 : The satellite sends one or more trigger conditions for conditional handover and a random access preamble to the terminal. 
     Step  702 : When determining that the trigger condition for conditional handover is met, the terminal determines a most suitable first cell, and sends a random access preamble of the first cell to the satellite. 
     In an example, if a trigger condition for conditional handover is met, there is a candidate cell, and the candidate cell is determined as the most suitable first cell. 
     In another example, if a plurality of trigger conditions for conditional handover are met, there are a plurality of candidate cells, and the most suitable first cell is selected from the plurality of candidate cells. 
     Step  703 : The satellite receives the random access preamble of the first cell, and determines terminal information for cell handover and the first cell. 
     Step  704 : The satellite determines whether the satellite is enabled to select an access cell for the terminal. If the satellite is enabled to select an access cell for the terminal, step  705  is performed. 
     Step  705 : The satellite determines, based on the terminal information and cell attribute information, a second cell most suitable for the terminal to access. 
     The satellite knows information about all cells (beams) transmitted by the satellite, for example, a payload status and cell coverage, and the satellite may determine, based on location information of the terminal, which cells can be accessed by the terminal (in coverage) and time in which the cells can provide services after the access. The satellite comprehensively determines the most suitable cell based on the information. 
     Step  706 : The satellite calculates access information of the second cell, and the satellite includes the cell number of the second cell and the access information of the second cell in a random access response message and sends the random access response message to the terminal. 
     The access information of the second cell includes a TA, a UL grant, a TC-RNTI, and a third message used for downlink synchronization. 
     Step  707 : The terminal receives the random access response message, performs downlink synchronization on the second cell, and sends an uplink message. 
     Specifically, the terminal performs downlink synchronization by using the third message, and sends the uplink message at a resource location indicated by the UL grant. 
     The satellite does not know the terminal information, for example, the location information of the terminal, or the terminal information stored in the satellite is out of date. Therefore, the satellite cannot accurately determine which cells can be accessed by the terminal (that is, which cells meet an access criterion or which cells meet the trigger condition for conditional handover). In this way, the satellite needs to enable the terminal to report the terminal information. As shown in  FIG. 8 , the following steps are included. 
     Step  801 : A satellite sends one or more trigger conditions for conditional handover and a random access preamble to a terminal. 
     Step  802 : When determining that the trigger condition for conditional handover is met, the terminal determines a most suitable first cell, and sends a random access preamble of the first cell to the satellite. 
     Step  803 : The satellite receives the random access preamble of the first cell, and the satellite sends a random access response message to the terminal, where the random access response message includes access information of the first cell and indication information used to indicate the terminal to report terminal information. 
     After receiving the random access preamble of the terminal, the base station finds that the terminal belongs to conditional handover triggering. However, due to lack of information, the base station cannot select an optimal handover cell for the terminal. Therefore, in addition to including access information of an initial access cell in the random access response message, the base station further includes, in the random access response message, a reporting indication indicating the terminal to feed back information about a trigger condition. 
     Step  804 : The terminal receives the random access response message, and may determine to directly access the first cell or determine to report the terminal information. If determining to report the terminal information, the terminal sends the terminal information to the satellite. 
     For example, if there is only one trigger condition of the terminal, or there is only one triggered target cell for handover, the terminal may determine to directly access the first cell. 
     The terminal information includes location information of the terminal (if the terminal has a localization capability), an identifier of a cell meeting a trigger condition for conditional handover, and a trigger condition that each cell meets. 
     The terminal information reported by the terminal may be further understood as assistance information of an optimal handover base station. 
     Step  805 : If the base station receives the terminal information, the base station selects, based on the terminal information and cell attribute information, a second cell most suitable for the terminal to access. 
     Step  806 : The satellite calculates access information of the second cell, and the satellite includes a cell number of the second cell and the access information of the second cell in the random access response message and sends the random access response message to the terminal. 
     The access information of the second cell includes a TA, a UL grant, a TC-RNTI, and a third message used for downlink synchronization. 
     Step  807 : The terminal receives the random access response message, performs downlink synchronization on the second cell, and sends an uplink message. 
     Specifically, the terminal performs downlink synchronization by using the third message, and sends the uplink message at a resource location indicated by the UL grant. 
     Based on  FIG. 8 , the terminal may determine to directly access the first cell. Refer to  FIG. 9 . The following steps are included. 
     Step  901 : A satellite sends one or more trigger conditions for conditional handover and a random access preamble to a terminal. 
     Step  902 : When determining that the trigger condition for conditional handover is met, the terminal determines a most suitable first cell, and sends a random access preamble of the first cell to the satellite. 
     Step  903 : The satellite receives the random access preamble of the first cell, and the satellite sends a random access response message to the terminal, where the random access response message includes access information of the first cell and indication information used to indicate the terminal to report terminal information. 
     After receiving the random access preamble of the terminal, the base station finds that the terminal belongs to conditional handover triggering. However, due to lack of information, the base station cannot select an optimal handover cell for the terminal. Therefore, in addition to including access information of an initial access cell in the random access response message, the base station further includes, in the random access response message, a reporting indication indicating the terminal to feed back information about a trigger condition. 
     Step  904 : The terminal receives the random access response message, and may determine to directly access the first cell or to report the terminal information. If determining to directly access the first cell, the terminal accesses the first cell based on the access information of the first cell. 
     Based on a same technical concept as the foregoing cell handover method, as shown in  FIG. 10 , a cell handover apparatus  100  is provided. The cell handover apparatus  100  can perform the steps performed by the satellite in the methods in  FIG. 6  to  FIG. 9 . To avoid repetition, details are not described herein again. The cell handover apparatus  100  may be a satellite, or may be a chip used in the satellite. The cell handover apparatus  100  includes a transceiver unit  1010 , and optionally, further includes a processing unit  1020  and a storage unit  1030 . The processing unit  1020  may be separately connected to the storage unit  1030  and the transceiver unit  1010 , and the storage unit  1030  may also be connected to the transceiver unit  1010 . 
     The storage unit  1030  is configured to store a computer program. 
     For example, the transceiver unit  1010  is configured to receive a random access preamble that is of a first cell and that is sent by a terminal, where the random access preamble is used to determine terminal information, and the terminal information is used for cell handover of the terminal. 
     The processing unit  1020  is configured to determine, based on the terminal information and cell attribute information that is of the satellite, a second cell most suitable for the terminal to access. 
     The transceiver unit  1010  is further configured to send access information of the second cell to the terminal. 
     In an implementation, the transceiver unit  1010  is further configured to receive a first message sent by the terminal, where the first message is used to indicate to enable the satellite to select an access cell for the terminal, or the first message includes first indication information and the first indication information is used to indicate to enable the satellite to select the access cell for the terminal. 
     In an implementation, the transceiver unit  1010  is further configured to send, by the satellite, a second message to the terminal, where the second message is used to indicate the terminal to report the terminal information, or the second message includes second indication information and the second indication information is used to indicate the terminal to report the terminal information. 
     In an implementation, the access information of the second cell includes at least one of the following content: a TA, a UL grant, a TC-RNTI, and a third message, where the third message is used for downlink synchronization; 
     the UL grant includes at least one of the following information: identification information indicating whether frequency hopping is enabled in the second cell, indication information of an uplink physical uplink shared channel (PUSCH) frequency resource, indication information of an uplink PUSCH time resource, a modulation and coding scheme (MCS), a PUSCH power control command, and a channel state information (CSI) request; and 
     the third message includes at least one of the following information: a frequency, a subcarrier spacing, and a guard interval of the second cell, a time domain location that is of a primary synchronization signal (PSS)/secondary synchronization signal (SSS) of the second cell and that is relative to the first cell, reserved time, and the indication information of the uplink PUSCH time resource of the UL grant. 
     Based on a same technical concept as the foregoing cell handover method, as shown in  FIG. 11 , a cell handover apparatus  110  is provided. The cell handover apparatus  110  can perform the steps performed by the terminal in the methods in  FIG. 6  to  FIG. 9 . To avoid repetition, details are not described herein again. The cell handover apparatus  110  may be a terminal, or may be a chip used in the terminal. The cell handover apparatus  110  includes a transceiver unit  1110 , and optionally, further includes a processing unit  1120  and a storage unit  1130 . The processing unit  1120  may be separately connected to the storage unit  1130  and the transceiver unit  1110 , and the storage unit  1130  may also be connected to the transceiver unit  1110 . 
     The storage unit  1130  is configured to store a computer program. 
     For example, the transceiver unit  1110  is configured to: send a random access preamble of a determined first cell to a satellite, where the random access preamble is used to determine terminal information, and the terminal information is used for cell handover of the terminal; and receive access information that is of a second cell and that is sent by the satellite. 
     The processing unit  1120  is configured to enable the terminal to access the second cell by using the access information of the second cell. 
     In an implementation, the transceiver unit  1110  is further configured to send a first message to the satellite, where the first message is used to indicate to enable the satellite to select an access cell for the terminal, or the first message includes first indication information and the first indication information is used to indicate to enable the satellite to select the access cell for the terminal. 
     In an implementation, the transceiver unit  1110  is further configured to: receive, by the terminal, a second message sent by the satellite, where the second message is used to indicate the terminal to report the terminal information, or the second message includes second indication information and the second indication information is used to indicate the terminal to report the terminal information. If determining to report the terminal information, the terminal sends the terminal information to the satellite. 
     In an implementation, the access information of the second cell includes at least one of the following content: a TA, a UL grant, a TC-RNTI, and a third message, where the third message is used for downlink synchronization; 
     the UL grant includes at least one of the following information: identification information indicating whether frequency hopping is enabled in the second cell, indication information of an uplink physical uplink shared channel (PUSCH) frequency resource, indication information of an uplink PUSCH time resource, a modulation and coding scheme (MCS), a PUSCH power control command, and a channel state information (CSI) request; and 
     the third message includes at least one of the following information: a frequency, a subcarrier spacing, and a guard interval of the second cell, a time domain location that is of a primary synchronization signal (PSS)/secondary synchronization signal (SSS) of the second cell and that is relative to the first cell, reserved time, and the indication information of the uplink PUSCH time resource of the UL grant. 
     In an implementation, the processing unit  1120  is specifically configured to: perform downlink synchronization on the second cell by using the third message, and send uplink information on the uplink PUSCH frequency resource and/or time resource indicated by the UL grant. 
       FIG. 12  is a schematic block diagram of a cell handover apparatus  1200  according to an embodiment of this application. It should be understood that, the cell handover apparatus  1200  can perform the steps performed by the satellite in the methods in  FIG. 6  to  FIG. 9 . To avoid repetition, details are not described herein again. The cell handover apparatus  1200  includes a processor  1201  and a memory  1203 , where the processor  1201  and the memory  1203  are electrically coupled. 
     The memory  1203  is configured to store computer program instructions. 
     The processor  1201  is configured to execute some or all of computer program instructions in the memory, and when the some or all of the computer program instructions are executed, the apparatus implements method performed by the satellite in the foregoing embodiments. 
     Optionally, a transceiver  1202  is further included, and is configured to communicate with another device, for example, receive a random access preamble that is of a first cell and that is sent by a terminal, and send access information of a second cell to the terminal. 
     It should be understood that the cell handover apparatus  1200  shown in  FIG. 12  may be a chip or a circuit, for example, a chip or a circuit that may be disposed in the satellite. Alternatively, the transceiver  1202  may be a communication interface. The transceiver includes a receiver and a transmitter. Further, the cell handover apparatus  1200  may include a bus system. 
     The processor  1201 , the memory  1203 , and the transceiver  1202  are connected through the bus system. The processor  1201  is configured to execute the instructions stored in the memory  1203 , to control the transceiver to receive a signal and send a signal, to complete the steps performed by the satellite in the cell handover method in this application. The memory  1203  may be integrated into the processor  1201 , or may be disposed separated from the processor  1201 . 
     In an implementation, it may be considered that functions of the transceiver  1202  are implemented by using a transceiver circuit or a dedicated transceiver chip. It may be considered that the processor  1201  is implemented by using a dedicated processing chip, a processing circuit, a processor, or a general-purpose chip. 
       FIG. 13  is a schematic block diagram of a cell handover apparatus  1300  according to an embodiment of this application. It should be understood that, the cell handover apparatus  1300  can perform the steps performed by the terminal in the methods in  FIG. 6  to  FIG. 9 . To avoid repetition, details are not described herein again. The cell handover apparatus  1300  includes a processor  1301  and a memory  1303 , where the processor  1301  and the memory  1303  are electrically coupled. 
     The memory  1303  is configured to store computer program instructions. 
     The processor  1301  is configured to execute some or all of computer program instructions in the memory, and when the some or all of the computer program instructions are executed, the apparatus implements method performed by the terminal in the foregoing embodiments. 
     Optionally, a transceiver  1302  is further included, and is configured to communicate with another device, for example, send a random access preamble of a first cell to a satellite, and receive access information that is of a second cell and that is sent by the satellite. 
     It should be understood that the cell handover apparatus  1300  shown in  FIG. 13  may be a chip or a circuit, for example, a chip or a circuit that may be disposed in the terminal. Alternatively, the transceiver  1302  may be a communication interface. The transceiver includes a receiver and a transmitter. Further, the cell handover apparatus  1300  may include a bus system. 
     The processor  1301 , the memory  1303 , and the transceiver  1302  are connected through the bus system. The processor  1301  is configured to execute the instructions stored in the memory  1303 , to control the transceiver to receive a signal and send a signal, to complete the steps performed by the terminal in the cell handover method in this application. The memory  1303  may be integrated into the processor  1301 , or may be disposed separated from the processor  1301 . 
     In an implementation, it may be considered that functions of the transceiver  1302  are implemented by using a transceiver circuit or a dedicated transceiver chip. It may be considered that the processor  1301  is implemented by using a dedicated processing chip, a processing circuit, a processor, or a general-purpose chip. 
     The processor may be a central processing unit (central processing unit, CPU), a network processor (network processor, NP), or a combination of the CPU and the NP. 
     The processor may further include a hardware chip or another general-purpose processor. The hardware chip may be an application-specific integrated circuit (application-specific integrated circuit, ASIC), a programmable logic device (programmable logic device, PLD), or a combination thereof. The PLD may be a complex programmable logic device (complex programmable logic device, CPLD), a field programmable logic array (field programmable gate array, FPGA), a generic array logic (generic array logic, GAL) and another programmable logic device, a discrete gate or a transistor logic device, a discrete hardware component, or any combination thereof. The general-purpose processor may be a microprocessor, or the processor may be any conventional processor, or the like. 
     It should be further understood that the memory in the embodiments of this application may be a volatile memory or a nonvolatile memory, or may include the volatile memory and the nonvolatile memory. The nonvolatile memory may be a read-only memory (Read-Only Memory, ROM), a programmable read-only memory (Programmable ROM, PROM), an erasable programmable read-only memory (Erasable PROM, EPROM), an electrically erasable programmable read-only memory (Electrically EPROM, EEPROM), or a flash memory. The volatile memory may be a random access memory (Random Access Memory, RAM), used as an external cache. Through example but not limitative description, many forms of RAMs may be used, for example, a static random access memory (Static RAM, SRAM), a dynamic random access memory (Dynamic RAM, DRAM), a synchronous dynamic random access memory (Synchronous DRAM, SDRAM), a double data rate synchronous dynamic random access memory (Double Data Rate SDRAM, DDR SDRAM), an enhanced synchronous dynamic random access memory (Enhanced SDRAM, ESDRAM), a synchlink dynamic random access memory (Synchlink DRAM, SLDRAM), and a direct rambus random access memory (Direct Rambus RAM, DR RAM). It should be noted that the memory described in this application aims to include but is not limited to these memories and any memory of another proper type. 
     An embodiment of this application provides a computer storage medium. The computer storage medium stores a computer program, and the computer program includes instructions used to perform the foregoing cell handover method. 
     An embodiment of this application provides a computer program product including instructions. When the computer program product runs on a computer, the computer is enabled to perform the foregoing cell handover method. 
     Persons of ordinary skill in the art may be aware that units and algorithm steps in the examples described with reference to the embodiments disclosed in this specification may be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether the functions are performed by hardware or software depends on particular applications and design constraint conditions of the technical solutions. Persons skilled in the art may use different methods to implement the described functions of each particular application, but it should not be considered that the implementation goes beyond the scope of this application. 
     It may be clearly understood by persons skilled in the art that, for the purpose of convenient and brief description, for a detailed working process of the foregoing system, apparatus, and unit, refer to a corresponding process in the foregoing method embodiments. Details are not described herein again. 
     In the embodiments provided in this application, it should be understood that the described apparatus embodiments are merely examples. For example, the unit division is merely logical function division and may be other division in actual implementation, for example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual communication connections may be implemented through some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms. 
     In addition, units in the apparatus embodiments of this application may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units may be integrated into one unit. 
     It may be understood that, the processor in the embodiments of this application may be a central processing unit (central processing unit, CPU), or may be another general-purpose processor, a digital signal processor (digital signal processor, DSP), an application-specific integrated circuit (application-specific integrated circuit, ASIC), a field programmable gate array (field programmable gate array, FPGA) or another programmable logic device, a transistor logic device, a hardware component, or any combination thereof. The general-purpose processor may be a microprocessor or any conventional processor. 
     All or some of the methods in the embodiments of this application may be implemented by software, hardware, firmware, or any combination thereof. When software is used to implement the embodiments, the embodiments may be implemented completely or partially in a form of a computer program product. The computer program product includes one or more computer programs or instructions. When the computer programs or instructions are loaded and executed on the computer, the procedure or functions according to the embodiments of this application are all or partially generated. The computer may be a general-purpose computer, a dedicated computer, a computer network, or other programmable apparatuses. The computer programs or instructions may be stored in a computer-readable storage medium, or may be transmitted by using the computer-readable storage medium. The computer-readable storage medium may be any usable medium accessible by a computer, or a data storage device, such as a server, integrating one or more usable media. The usable medium may be a magnetic medium such as a floppy disk, a hard disk, or a magnetic tape, or may be an optical medium such as a CD-ROM or a DVD, or may be a semiconductor medium such as a solid-state drive (solid-state drive, SSD), a random access memory (random access memory, RAM), a read-only memory (read-only memory, ROM), or a register. 
     Persons skilled in the art should understand that the embodiments of this application may be provided as a method, a system, or a computer program product. Therefore, this application may use a form of a hardware-only embodiment, a software-only embodiment, or an embodiment with a combination of software and hardware. Moreover, this application may use a form of a computer program product that is implemented on one or more computer-usable storage media (including but not limited to a disk memory, a CD-ROM, an optical memory, and the like) that include computer usable program code. 
     This application is described with reference to the flowcharts and/or block diagrams of the method, the device (system), and the computer program product according to the embodiments of this application. It should be understood that computer program instructions may be used to implement each process and/or each block in the flowcharts and/or the block diagrams and a combination of a process and/or a block in the flowcharts and/or the block diagrams. The computer program instructions may be provided for a general-purpose computer, a dedicated computer, an embedded processor, or a processor of another programmable data processing device to generate a machine, so that the instructions executed by the computer or the processor of another programmable data processing device generate an apparatus for implementing a specific function in one or more processes in the flowcharts and/or in one or more blocks in the block diagrams. 
     The computer program instructions may be stored in a computer readable memory that can instruct the computer or another programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory generate an artifact that includes an instruction apparatus. The instruction apparatus implements a specific function in one or more processes in the flowcharts and/or in one or more blocks in the block diagrams. 
     The computer program instructions may be loaded onto the computer or another programmable data processing device, so that a series of operations and steps are performed on the computer or the another programmable device, thereby generating computer-implemented processing. Therefore, the instructions executed on the computer or the another programmable device provide steps for implementing a specific function in one or more processes in the flowcharts and/or in one or more blocks in the block diagrams. 
     Although some embodiments of this application are described, persons skilled in the art may make changes and modifications to the embodiments once they learn the basic inventive concept. Therefore, the following claims are intended to be construed to cover the embodiments and all changes and modifications falling within the scope of this application. 
     Certainly, persons skilled in the art may make various modifications and variations to the embodiments of this application without departing from the spirit and scope of the embodiments of this application. This application is also intended to cover these modifications and variations to embodiments of this application provided that the modifications and variations fall within the scope of protection defined by the following claims and their equivalent technologies.