Method for background search of RedCap cell by RedCap terminal under LTE (Long Term Evolution)

Abstract

The invention discloses a method for a RedCap terminal to search a RedCap cell under the background of LTE (Long Term Evolution). The method comprises the following steps: firstly, pre-initializing RedCapList, and configuring parameters such as a timer; then, the terminal judges the SIB24 broadcast state of the LTE network and the RedCap compatibility of the associated 5G cell in the LTE idle state; and if the SIB24 is not broadcasted or the SIB24 is broadcasted but the associated 5G cell does not support the RedCap, respectively triggering a corresponding search process according to the attribution of the current LTE cell, and dynamically updating the list and adjusting the failure times according to the search result until the 5G RedCap cell is successfully reselected or the current LTE cell is judged to have no available 5G RedCap network. According to the invention, the method does not need to depend on LTE network SIB24 configuration, can autonomously and efficiently search the 5G RedCap cell, improves the 5G access efficiency, and guarantees the service continuity.

Description

Technical Field

[0001] This invention relates to the field of wireless communication technology, specifically a method for a RedCap terminal to perform background search for RedCap cells under LTE. Background Technology

[0002] 5G RedCap (Reduced Capability) terminals are lightweight 5G terminals defined by the 3GPP Rel-17 “Reduced Capability” specification. Their speed and hardware complexity are between NB-IoT (Narrowband Internet of Things) and eMBB (Enhanced Mobile Broadband) terminals. They are primarily positioned for “medium-speed transmission, medium cost, and low power consumption” scenarios and can be widely adapted to terminal devices such as smart meters, industrial sensors, and video surveillance in the Internet of Things field.

[0003] RedCap terminals must strictly comply with 3GPP TS 38.300, 38.304 and 38.331 protocol specifications for cell access. Whether a terminal can camp on a particular 5G cell depends on the specific parameter configuration in the cell's system information block SIB1:

[0004] (1) Access permission determination: The cellBarredRedCap1Rx and cellBarredRedCap2Rx parameters indicate whether the cell allows RedCap terminals with a single receiving antenna (1Rx) or dual receiving antennas (2Rx) to access the cell. If the intraFreqReselectionRedCap parameter is missing in SIB1, the protocol will forcibly determine that the cell prohibits RedCap terminals from accessing the cell.

[0005] (2) Standard compatibility determination: For 5G cells with frequency division duplex (FDD) standard, if the RedCap terminal only supports half duplex mode, the halfDuplexRedCapAllowed parameter must indicate "allow half duplex access" in order to meet the camping conditions.

[0006] Existing RedCap terminal 5G RedCap cell reselection schemes in LTE idle state (RRC_Idle), such as Figure 1 As shown, System Information Block 24 (SIB24) relies entirely on LTE network broadcasts: the terminal needs to periodically wake up and decode SIB24 to obtain configuration information such as the frequency point, reselection priority, and signal threshold of the 5G NR neighboring cells. Based on this information, it then measures the signal quality of the target 5G cell. InterRAT reselection is only triggered when the 5G cell SIB1 supports RedCap access and the signal quality meets the standards. This scheme has two major flaws that affect the 5G access experience of RedCap terminals, including:

[0007] (1) The "blindness" problem when there is no SIB24: If the LTE cell where the terminal is currently stationed is not configured or broadcasting SIB24, that is, it does not provide any 5G neighbor cell information, even if there are available 5G RedCap cells in the vicinity, the terminal cannot initiate a search on its own and can only stay on the 4G LTE network for a long time, missing the high bandwidth and low latency advantages of the 5G network.

[0008] (2) The problem of “misjudgment” when SIB24 configuration is invalid: If the LTE cell broadcasts SIB24, but the 5G neighbor cells carried in it have problems such as the protocol version not being upgraded to Rel-17 or the SIB1 parameter prohibiting RedCap access, the terminal will be unable to discover other available 5G RedCap cells in the vicinity because it only relies on the 5G cell information in SIB24, and will also fall into the dilemma of “having 5G but being unable to access it”.

[0009] In summary, during the transitional phase before 5G RedCap networks have achieved full coverage and adaptation, the existing reselection schemes that rely on SIB24 can no longer meet the 5G access requirements of RedCap multi-mode terminals. A technical solution is needed that can autonomously and efficiently search for 5G RedCap cells when the LTE network has no SIB24 or the SIB24 configuration is invalid. Summary of the Invention

[0010] To address the aforementioned shortcomings of existing technologies, this invention solves the problems of 5G access blind spots and efficiency bottlenecks caused by existing RedCap terminals relying on SIB24 in LTE network environments through a "list management + timer scheduling" mechanism.

[0011] This invention proposes a method for a RedCap terminal to perform background search for RedCap cells under LTE, comprising the following steps:

[0012] S1: The RedCap terminal enters the initial working state; the RedCap terminal is a multi-mode terminal that simultaneously supports 5G RedCap cell access and 4G LTE network access. After completing LTE network registration and entering the RRC_Idle idle state, it loads the network configuration; the loading of the network configuration includes setting core parameters and pre-initializing the RedCap background search cell list RedCapList; the RedCapList includes RedCapAvailableList and RedCapNonAvailableList;

[0013] S2: The RedCap terminal periodically listens to the LTE cell system information broadcast, determines the broadcast status of the LTE network SIB24 and the RedCap compatibility of the associated 5G cell. If it is determined that the LTE network is not broadcasting SIB24, or if it is determined that the LTE network is broadcasting SIB24 but the 5G cell carried by SIB24 does not support RedCap terminal access, then according to the current LTE cell's affiliation in the RedCapList, the corresponding background search process is executed, and the RedCapList and the number of background search failures for the current LTE cell are dynamically updated based on the search results until the process termination condition is met.

[0014] S3: Terminate the current RedCap background search process when the preset process termination conditions are met.

[0015] Preferably, the pre-initialized RedCap background search cell list RedCapList includes:

[0016] (1) In RedCapAvailableList, record the LTE cell information that “has found an available 5G RedCap cell during the period of camping in this LTE cell” in the historical search. The LTE cell information includes the frequency point, physical cell identifier and global cell identifier of the LTE cell.

[0017] (2) In RedCapNonAvailableList, record the LTE cell information that "did not find an available 5G RedCap cell multiple times during the period of camping in the LTE cell" in the historical search.

[0018] Preferably, the core parameters include:

[0019] (1) ShortBgSearchTimer: Short background search timer with a default duration of 30 seconds. It can be dynamically adjusted according to the current network signal strength, 5G coverage density and other network environments. It is specifically used for high-frequency supplementary searches in RedCapAvailableList related scenarios.

[0020] (2) LongBgSearchTimer: Long background search timer with a default duration of 180 seconds. It is specifically used for low-frequency retries in RedCapNonAvailableList related scenarios to reduce the terminal's invalid power consumption.

[0021] (3)HPLMN_IRAT_SearchTimer: HPLMN high priority selection timer, with a default duration of 60 seconds, used for initial detection when the current LTE cell is not in the RedCapList scenario, and prioritizes the network selection priority of the Home Public Land Mobile Network (HPLMN);

[0022] (4) max_redcap_failures: The maximum number of background search failures, with a default configuration of 3 times, which serves as the threshold for triggering "stop autonomous search for the current LTE cell" or "switching the status of sublists in RedCapList".

[0023] Preferably, the criterion for determining that "the 5G cell carried by SIB24 does not support RedCap terminal access" is as follows: If the terminal parses the 5G neighbor cell frequency point corresponding to 5G cell SIB1 in SIB24, and any of the following conditions exist, then the 5G cell is determined to not support RedCap access:

[0024] (1) The 5G base station has not been upgraded to 3GPP Rel-17 or higher protocol version that supports RedCap function;

[0025] (2) The intraFreqReselectionRedCap parameter is missing in SIB1.

[0026] (3) The cellBarredRedCap1Rx or cellBarredRedCap2Rx parameter in SIB1 indicates "Access Denied";

[0027] (4) When the 5G cell is a frequency division duplex (FDD) standard and the RedCap terminal only supports half-duplex mode, the halfDuplexRedCapAllowed parameter in SIB1 does not indicate "allow half-duplex access".

[0028] Preferably, in the scenario where "it is determined that the LTE network broadcasts SIB24, but the 5G cell carried by SIB24 does not support RedCap terminal access", the RedCap terminal performs the following procedure:

[0029] S2.2.1 Confirm that none of the 5G cells carried by SIB24 support RedCap access;

[0030] S2.2.2 Query the RedCapList affiliation of the current LTE cell by its frequency point, physical cell identifier, and global cell identifier;

[0031] S2.2.3 Execute the corresponding background search process based on the current LTE cell's affiliation in the RedCapList.

[0032] Preferably, the step of "executing the corresponding background search process based on the current LTE cell's affiliation in the RedCapList" specifically includes the following three sub-scenarios:

[0033] S2.1.1 Sub-scenario 1: The current LTE cell is in RedCapAvailableList.

[0034] (1) The terminal immediately initiates the first RedCap background search, scans the preset 5G RedCap typical frequency bands and analyzes the target 5G cell SIB1 to determine whether RedCap terminal access is supported;

[0035] (2) If an available 5G RedCap cell is found: the terminal triggers InterRAT reselection and camps on the cell, and at the same time updates the "last successful search time" recorded in RedCapAvailableList for the LTE cell;

[0036] (3) If no available 5G RedCap cell is found: The terminal starts ShortBgSearchTimer, and the search is restarted after the timer expires; for each failure, the background search failure count for the current LTE cell is incremented by 1; when the failure count > max_redcap_failures, the LTE cell is removed from RedCapAvailableList and added to RedCapNonAvailableList;

[0037] S2.1.2 Sub-scenario 2: The current LTE cell is in RedCapNonAvailableList.

[0038] (1) The terminal directly starts LongBgSearchTimer. After the timer expires, it initiates RedCap background search and parses 5G cell SIB1.

[0039] (2) If an available 5G RedCap cell is found: the terminal triggers InterRAT reselection and camps on the cell, and at the same time removes the LTE cell from RedCapNonAvailableList and adds it to RedCapAvailableList;

[0040] (3) If no available 5G RedCap cell is found: the terminal will increment the background search failure count for the current LTE cell by 1; if the failure count is less than or equal to max_redcap_failures, restart LongBgSearchTimer and repeat the search; if the failure count is greater than max_redcap_failures, it is determined that there is no available RedCap network around the LTE cell for a long time, and the autonomous search for the current LTE cell is stopped.

[0041] S2.1.3 Sub-scenario 3: The current LTE cell is not in the RedCapList.

[0042] (1) The terminal starts HPLMN_IRAT_SearchTimer, and initiates the first RedCap background search after the timer expires;

[0043] (2) If an available 5G RedCap cell is found: the terminal triggers InterRAT reselection and camps on the cell, and at the same time adds the frequency point, physical cell identifier (PCI) and global cell identifier (GCI) of the LTE cell to the RedCapAvailableList;

[0044] (3) If no available 5G RedCap cell is found: the terminal will increment the background search failure count for the current LTE cell by 1; if the failure count is less than or equal to max_redcap_failures, restart HPLMN_IRAT_SearchTimer and repeat the search; if the failure count is greater than max_redcap_failures, add the LTE cell to RedCapNonAvailableList.

[0045] Preferably, the "process termination condition" includes any of the following:

[0046] (1) The RedCap terminal successfully reselected to the 5G RedCap cell and completed 5G network registration;

[0047] (2) The RedCap terminal reselects from the current LTE cell to another LTE cell;

[0048] (3) The RedCap terminal enters the RRC_Connected state from the RRC_Idle idle state and initiates LTE service. The background search is paused and can be resumed after the terminal enters the LTE idle state again.

[0049] (4) If the number of RedCap background search failures for the current LTE cell by the RedCap terminal is greater than max_redcap_failures, it is determined that there is no available RedCap network around the current LTE cell for a long time.

[0050] The present invention has the following beneficial effects:

[0051] (1) Solving the "blind spot" and "misjudgment" problems in scenarios without SIB24. This invention enables terminals to actively detect 5G RedCap frequency bands, parse the SIB1 parameters of target cells, and determine access permissions without relying on SIB24 by pre-setting RedCapList and autonomous frequency band scanning logic, thus eliminating the access blind spot of "having 5G but not being able to see it". This invention verifies the RedCap compatibility of SIB24 associated with 5G cells. If it is determined to be invalid, it automatically reuses the "list + timer" autonomous search process to ensure that the terminal can break through the limitation of invalid SIB24 and discover other available 5G RedCap cells in the vicinity.

[0052] (2) Improve 5G RedCap access efficiency and achieve fast reconnection through high-frequency supplementary searches: For LTE cells in the RedCapAvailableList that have been previously found to be available 5G RedCap cells, this invention designs an "immediate first search + short timer retry" mechanism. Compared with the fixed period of the existing technology, this improves the 5G reconnection speed. When camping on an unknown LTE cell for the first time, this invention prioritizes starting HPLMN_IRAT_SearchTimer to trigger the initial probe through the timer, avoiding blind searches and wasting resources, and ensuring that the terminal efficiently discovers the 5G RedCap network within the compliance framework.

[0053] (3) Accurately balance terminal power consumption to adapt to low-power IoT scenarios. High-frequency retry is used for RedCapAvailableList cells, prioritizing efficiency, while low-frequency retries are used for RedCapNonAvailableList cells, prioritizing power consumption. Set the max_redcap_failures threshold to avoid meaningless frequency band scanning and parameter parsing.

[0054] The present invention provides a method for a RedCap terminal to perform background search for RedCap cells under LTE without relying on the LTE network SIB24 configuration, thereby improving 5G access efficiency and ensuring service continuity. Attached Figure Description

[0055] Figure 1 This is a solution for reselecting to RedCap in the LTE idle state in existing technologies;

[0056] Figure 2 This is the background search process when the LTE cell in this embodiment exists in the RedCapAvailableList;

[0057] Figure 3 This is the background search process when the LTE cell in this embodiment exists in RedCapNonAvailableList;

[0058] Figure 4This describes the background search process when the LTE cell in this embodiment is not in the RedCapList. Detailed Implementation

[0059] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] Example 1

[0061] This invention provides a method for a RedCap terminal to search for RedCap cells in the background under LTE. It is applicable to RedCap multi-mode terminals that simultaneously support 5G RedCap cell access and 4G LTE network access, addressing the autonomous 5G RedCap cell search problem in two typical scenarios during LTE idle mode: first, the LTE network does not broadcast SIB24, and the 5G neighbor cell configuration information is unknown; second, LTE broadcasts SIB24, but the 5G cells it carries do not support RedCap terminal access. The following are the specific implementation steps, each step and sub-step revolving around the logic of "list management + timer scheduling" to balance network access efficiency and terminal power consumption.

[0062] S1: The RedCap terminal enters its initial working state.

[0063] The RedCap terminal completes LTE network registration, enters RRC_Idle (idle state), and loads network configuration:

[0064] (1) Pre-initialize RedCapList (RedCap background search cell list), which contains two sublists:

[0065] (a) RedCapAvailableList: Records information on LTE cells that have been found to be available 5G RedCap cells under the current LTE cell in the history of the search. Each record must include the frequency point, physical cell identifier (PCI), and global cell identifier (GCI) of the LTE cell.

[0066] (b) RedCapNonAvailableList: Records LTE cell information where no available 5G RedCap cell was found multiple times during the historical search for the current LTE cell. The recorded content is consistent with the available list.

[0067] (2) Pre-configured core parameters:

[0068] (a) ShortBgSearchTimer: The default duration is 30 seconds, which can be dynamically adjusted according to the network environment. It is used for high-frequency supplementary searches in the scenario of "available RedCap cells were previously found".

[0069] (b) LongBgSearchTimer: The default duration is 180 seconds, used for low-frequency retries in the scenario of "multiple unsuccessful search for available RedCap cells";

[0070] (c) HPLMN_IRAT_SearchTimer (HPLMN high priority selection timer): The default duration is 60 seconds. It is used in the "first judgment (LTE cell is not in RedCapList)" scenario and prioritizes the network priority of the home public land mobile network (HPLMN).

[0071] (d) max_redcap_failures (maximum number of background search failures): The default configuration is 3 times, which is the threshold for triggering "stop autonomous search" or "list state switching".

[0072] S2: The RedCap terminal determines the broadcast status of the LTE network SIB24 and the compatibility of 5G cells.

[0073] In LTE idle mode, the RedCap terminal periodically listens to the system information broadcast of the LTE cell. First, it determines the broadcast status of SIB24 and the RedCap support capability of the associated 5G cell, and performs a background search process in the following two scenarios:

[0074] S2.1: The LTE network did not broadcast SIB24.

[0075] If the terminal does not resolve the SIB24 broadcast by the LTE cell, i.e., it does not have 5G neighbor cell frequency points, reselection priority, or other configurations, the terminal queries the RedCapList for the affiliation of the currently camped LTE cell (matched by frequency point + PCI + GCI) and performs a background search according to the following three sub-scenarios:

[0076] S2.1.1 Sub-scenario 1: The current LTE cell is in RedCapAvailableList.

[0077] like Figure 2 As shown, this scenario means that "a usable RedCap cell has been found under this LTE cell in the past", and the terminal needs to prioritize high-frequency search to ensure a fast reconnection to 5G.

[0078] (1) Initiate the first RedCap background search immediately. The terminal starts the 5G RedCap cell search process, scans the preset 5G RedCap frequency bands, such as NR n48 / n78, parses the SIB1 of the target 5G cell, and determines whether RedCap access is supported. The following conditions must be met simultaneously: cellBarredRedCap1Rx or cellBarredRedCap2Rx (corresponding to the number of receiving antennas of the terminal) indicates "access allowed", and the intraFreqReselectionRedCap parameter exists (the protocol requires that access is prohibited if this parameter is missing); if it is an FDD standard, halfDuplexRedCapAllowed must also indicate "support half-duplex".

[0079] (2) Initial search result processing. If an available RedCap cell is found: the terminal immediately triggers InterRAT reselection, camps on the 5G RedCap cell, and maintains the current LTE cell's record in the RedCapAvailableList (updating the last success time). If no available RedCap cell is found: the terminal starts ShortBgSearchTimer and enters a waiting state.

[0080] (3) Retry and failure handling after short timer timeout. When ShortBgSearchTimer times out, the terminal initiates RedCap background search again. If no available cell is found this time: the terminal increments the number of RedCap background search failures (initially 0). If the number of failures > max_redcap_failures (e.g., 3 times): the terminal determines that "there is no available RedCap network under the current LTE cell", removes the LTE cell from RedCapAvailableList, and adds it to RedCapNonAvailableList, and then executes the procedure in "S2.1.2" afterwards.

[0081] S2.1.2 Sub-scenario 2: The current LTE cell is in RedCapNonAvailableList.

[0082] like Figure 3 As shown, this scenario means that "historically, no available RedCap cells have been found multiple times under this LTE cell," and the terminal needs to reduce the search frequency to reduce power consumption.

[0083] (1) Start a long timer to wait. The terminal directly starts LongBgSearchTimer without immediately initiating a search to avoid unnecessary power consumption.

[0084] (2) Search is initiated after the long timer expires. After the LongBgSearchTimer expires, the terminal initiates a RedCap background search and parses the 5G SIB1 to determine access permissions.

[0085] (3) Search result processing. If an available RedCap cell is found: the terminal reselects the 5G cell and updates the list, removing the current LTE cell from RedCapNonAvailableList and adding it to RedCapAvailableList. If no available cell is found: the terminal increments the failure count by 1 and checks if it exceeds max_redcap_failures: if it does not exceed, restart LongBgSearchTimer and repeat the search process; if it exceeds, the terminal determines that "there is no available RedCap network around this LTE cell for a long time" and stops the autonomous RedCap background search under the current LTE cell until the terminal reselects another LTE cell.

[0086] S2.1.3 Sub-scenario 3: The current LTE cell is not in the RedCapList.

[0087] like Figure 4 As shown, this scenario is "the terminal camps on this LTE cell for the first time", and initial detection needs to be performed according to HPLMN priority.

[0088] (1) Start the high-priority retrieval timer. The terminal starts HPLMN_IRAT_SearchTimer to prioritize access to the HPLMN network (in accordance with the priority definition of network selection in the 3GPP communication protocol).

[0089] (2) The first probe is initiated after the timer expires. After HPLMN_IRAT_SearchTimer expires, the terminal initiates a RedCap background search.

[0090] (3) Detection result processing. If an available RedCap cell is found: the terminal reselects the 5G cell and adds the current LTE cell (frequency point + PCI + GCI) to RedCapAvailableList. If no available cell is found, the terminal increments the failure count by 1 and checks if it exceeds max_redcap_failures. If it does not exceed, the HPLMN_IRAT_SearchTimer is restarted to repeat the RedCap background search; if it exceeds, the terminal determines that "there is no available RedCap network under this LTE cell", adds it to RedCapNonAvailableList, and then proceeds according to the process in sub-scenario 2.

[0091] S2.2: LTE network broadcasts SIB24, but the 5G cell it carries does not support RedCap.

[0092] If the terminal resolves to SIB24 broadcast by the LTE cell, but further verification reveals that the 5G cell carried by SIB24 does not meet the RedCap access conditions, such as the 5G base station not being upgraded to 3GPP R17, the cellBarredRedCap1Rx in SIB1 indicating "access prohibited", or the intraFreqReselectionRedCap parameter being missing, the terminal executes the following procedure:

[0093] S2.2.1 Confirm that the 5G cell does not support RedCap. The terminal parses the 5G neighboring cell frequency points carried in SIB24, and for the 5G cell corresponding to each frequency point, reads the RedCap access parameters (cellBarredRedCap1Rx / 2Rx, intraFreqReselectionRedCap, halfDuplexRedCapAllowed) in its SIB1 to confirm that all associated 5G cells do not support RedCap access.

[0094] S2.2.2 Query the current LTE cell's affiliation in the RedCapList. The terminal uses the current LTE cell's frequency point + PCI + GCI to match the two sub-lists of the RedCapList to determine its affiliation (either in the RedCapAvailableList, RedCapNonAvailableList, or not in the RedCapList).

[0095] S2.2.3 Reuse the background search process of S2.1. Based on the attribution judgment result of "S2.2.2", reuse the complete process of "S2.1.1", "S2.1.2" and "S2.1.3" respectively to perform autonomous RedCap background search, ensuring that regardless of whether SIB24 exists, as long as the target 5G cell does not support RedCap, the terminal can achieve autonomous search through a unified "list + timer" logic.

[0096] S3: Process Termination Condition. The RedCap background search process will terminate when any of the following conditions are met:

[0097] (1) The RedCap terminal successfully reselected to the 5G RedCap cell and completed 5G network registration;

[0098] (2) The RedCap terminal reselects from the current LTE cell to another LTE cell;

[0099] (3) The RedCap terminal is woken up, enters the connected state from the idle state, initiates LTE service, the background search is suspended, and resumes after entering the idle state again;

[0100] (4) The terminal determines that "there is no available RedCap network around the current LTE cell for a long time", that is, the number of failures exceeds max_redcap_failures, and stops autonomous background search.

[0101] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for a RedCap terminal to perform background search for RedCap cells under LTE, characterized in that, Includes the following steps: S1: The RedCap terminal enters the initial working state; the RedCap terminal is a multi-mode terminal that simultaneously supports 5G RedCap cell access and 4G LTE network access. After completing LTE network registration and entering the RRC_Idle idle state, it loads the network configuration; the loading of the network configuration includes setting core parameters and pre-initializing the RedCap background search cell list RedCapList; the RedCapList includes RedCapAvailableList and RedCapNonAvailableList; S2: The RedCap terminal periodically listens to the LTE cell system information broadcast, determines the broadcast status of the LTE network SIB24 and the RedCap compatibility of the associated 5G cell. If it is determined that the LTE network is not broadcasting SIB24, or if it is determined that the LTE network is broadcasting SIB24 but the 5G cell carried by SIB24 does not support RedCap terminal access, then according to the current LTE cell's affiliation in the RedCapList, the corresponding background search process is executed, and the RedCapList and the number of background search failures for the current LTE cell are dynamically updated based on the search results until the process termination condition is met. S3: Terminate the current RedCap background search process when the preset process termination conditions are met.

2. The method according to claim 1, characterized in that, The pre-initialized RedCap background search cell list, RedCapList, includes: (1) In RedCapAvailableList, record the LTE cell information that "available 5GRedCap cells were found during the period of camping in this LTE cell" in the historical search. The LTE cell information includes the frequency point, physical cell identifier and global cell identifier of the LTE cell; (2) In RedCapNonAvailableList, record the LTE cell information that "did not find any available 5G RedCap cells multiple times during the period of camping in this LTE cell" in the historical search.

3. The method according to claim 1, characterized in that, The core parameters include: (1) ShortBgSearchTimer: Short background search timer with a default duration of 30 seconds. It can be dynamically adjusted according to the current network signal strength, 5G coverage density and other network environments. It is specifically used for high-frequency supplementary searches in RedCapAvailableList related scenarios. (2) LongBgSearchTimer: Long background search timer with a default duration of 180 seconds. It is specifically used for low-frequency retries in RedCapNonAvailableList related scenarios to reduce the terminal's invalid power consumption. (3)HPLMN_IRAT_SearchTimer: HPLMN high priority selection timer, with a default duration of 60 seconds, used for initial detection when the current LTE cell is not in the RedCapList scenario, and prioritizes the network selection priority of the Home Public Land Mobile Network (HPLMN); (4) max_redcap_failures: The maximum number of background search failures, with a default configuration of 3 times, which serves as the threshold for triggering "stop autonomous search for the current LTE cell" or "switching the status of sublists in RedCapList".

4. The method according to claim 1, characterized in that, The criterion for determining that "the 5G cell carried by SIB24 does not support RedCap terminal access" is as follows: If the terminal resolves the 5G neighbor frequency point corresponding to the 5G cell SIB1 in SIB24, and any of the following conditions exist, then the 5G cell is determined to not support RedCap access: (1) The 5G base station has not been upgraded to 3GPP Rel-17 or higher protocol version that supports RedCap function; (2) The intraFreqReselectionRedCap parameter is missing in SIB1. (3) The cellBarredRedCap1Rx or cellBarredRedCap2Rx parameter in SIB1 indicates "Access Denied"; (4) When the 5G cell is a frequency division duplex (FDD) standard and the RedCap terminal only supports half-duplex mode, the halfDuplexRedCapAllowed parameter in SIB1 does not indicate "allow half-duplex access".

5. The method according to claim 3, characterized in that, In the scenario described above, where "it is determined that the LTE network is broadcasting SIB24, but the 5G cell carried by SIB24 does not support RedCap terminal access," the RedCap terminal executes the following procedure: S2.2.1 Confirm that none of the 5G cells carried by SIB24 support RedCap access; S2.2.2 Query the RedCapList affiliation of the current LTE cell by its frequency point, physical cell identifier, and global cell identifier; S2.2.3 Execute the corresponding background search process based on the current LTE cell's affiliation in the RedCapList.

6. The method according to claim 5, characterized in that, The phrase "execute the corresponding background search process based on the current LTE cell's affiliation in the RedCapList" specifically includes the following three sub-scenarios: S2.1.1 Sub-scenario 1: The current LTE cell is in RedCapAvailableList. (1) The terminal immediately initiates the first RedCap background search, scans the preset 5G RedCap typical frequency bands and analyzes the target 5G cell SIB1 to determine whether RedCap terminal access is supported; (2) If an available 5G RedCap cell is found: the terminal triggers InterRAT reselection and camps on the cell, and at the same time updates the "last successful search time" recorded in RedCapAvailableList for the LTE cell; (3) If no available 5G RedCap cell is found: The terminal starts ShortBgSearchTimer, and the search is restarted after the timer expires; for each failure, the background search failure count for the current LTE cell is incremented by 1; when the failure count > max_redcap_failures, the LTE cell is removed from RedCapAvailableList and added to RedCapNonAvailableList; S2.1.2 Sub-scenario 2: The current LTE cell is in RedCapNonAvailableList. (1) The terminal directly starts LongBgSearchTimer. After the timer expires, it initiates RedCap background search and parses 5G cell SIB1. (2) If an available 5G RedCap cell is found: the terminal triggers InterRAT reselection and camps on the cell, and at the same time removes the LTE cell from RedCapNonAvailableList and adds it to RedCapAvailableList; (3) If no available 5G RedCap cell is found: the terminal will increment the background search failure count for the current LTE cell by 1; if the failure count is less than or equal to max_redcap_failures, restart LongBgSearchTimer and repeat the search; if the failure count is greater than max_redcap_failures, it is determined that there is no available RedCap network around the LTE cell for a long time, and the autonomous search for the current LTE cell is stopped. S2.1.3 Sub-scenario 3: The current LTE cell is not in the RedCapList. (1) The terminal starts HPLMN_IRAT_SearchTimer, and initiates the first RedCap background search after the timer expires; (2) If an available 5G RedCap cell is found: the terminal triggers InterRAT reselection and camps on the cell, and at the same time adds the frequency point, physical cell identifier (PCI) and global cell identifier (GCI) of the LTE cell to the RedCapAvailableList; (3) If no available 5G RedCap cell is found: the terminal will increment the background search failure count for the current LTE cell by 1; if the failure count is less than or equal to max_redcap_failures, restart HPLMN_IRAT_SearchTimer and repeat the search; if the failure count is greater than max_redcap_failures, add the LTE cell to RedCapNonAvailableList.

7. The method according to claim 3, characterized in that, The "process termination condition" includes any of the following: (1) The RedCap terminal successfully reselected to the 5G RedCap cell and completed 5G network registration; (2) The RedCap terminal reselects from the current LTE cell to another LTE cell; (3) The RedCap terminal enters the RRC_Connected state from the RRC_Idle idle state and initiates LTE service. The background search is paused and can be resumed after the terminal enters the LTE idle state again. (4) If the number of RedCap background search failures for the current LTE cell by the RedCap terminal is greater than max_redcap_failures, it is determined that there is no available RedCap network around the current LTE cell for a long time.

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