100.

The RLC layer transmits data to the RLC layer on the reception side using the functions of the MAC layer and the PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the base station 200 via a logical channel.

The PDCP layer performs header compression/decompression and encryption/decryption.

A service data adaptation protocol (SDAP) layer may be provided as an upper layer of the PDCP layer. The service data adaptation protocol (SDAP) layer performs mapping between an IP flow that is a unit in which a core network performs quality of service (QoS) control and a radio bearer that is a unit in which an access stratum (AS) performs QoS control.

The RRC layer controls a logical channel, a transport channel, and a physical channel according to establishment, re-establishment, and release of a radio bearer. RRC signaling for various configurations is transmitted between the RRC layer of the UE 100 and the RRC layer of the base station 200. When there is an RRC connection between the RRC of the UE 100 and the RRC of the base station 210, the UE 100 is in an RRC connected state. When there is no RRC connection between the RRC of the UE 100 and the RRC of the base station 200, the UE 100 is in an RRC idle state. When the RRC connection between the RRC of the UE 100 and the RRC of the base station 200 is suspended, the UE 100 is in an RRC inactive state.

A non-access stratum (NAS) layer located above the RRC layer performs session management and mobility management of the UE 100. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of a mobility management apparatus 221.

Note that the UE 100 has an application layer and the like in addition to the protocol of the radio interface.

(Overview of Cell Selection and Cell Reselection by UE)

Next, an overview of cell selection and cell reselection by the UE 100 will be described. The UE 100 can perform cell selection or cell reselection as a selection operation of selecting a camp-on-cell. For example, the UE 100 can perform the selection operation in any of the following cases.

- A case in which a new public land mobile network (PLMN) or a new stand-alone non-public network (SNPN) is selected
- A case in which a universal subscriber identity module (USIM) is inserted into the UE 100 or when a SNPN subscription is added
- A case in which a suitable cell is not found
- A case in which the RRC connected state transitions to the RRC idle state or RRC inactive state
  
  (A) Cell Selection

In cell selection, for example, the UE 100 searches for a frequency band to identify, for each frequency, the strongest cell (for example, a cell having the highest detection level of a cell-defining SS/PBCH block (CD-SSB), a cell having the highest received power (RSRP) of the CD-SSB, or a cell having the highest reception quality (RSRQ) of the CD-SSB). Next, the UE 100 identifies a suitable cell from among the strongest cells. If the UE 100 cannot identify the suitable cell (that is, cannot find the suitable cell), the UE 100 identifies an acceptable cell. If the UE 100 can identify the suitable cell (that is, can find the suitable cell), the UE 100 selects the suitable cell as a camp-on-cell. If the UE 100 can identify only the acceptable cell (that is, can find the acceptable cell), the UE 100 selects the acceptable cell as the camp-on-cell.

The suitable cell is a cell in which the measured communication quality of the cell satisfies a cell selection criterion. The PLMN of the suitable cell is a selected PLMN, registered PLMN, or a PLMN equivalent to these PLMNs. The suitable cell is not a prohibited or reserved cell and is not part of a tracking area included in a list of “prohibited tracking areas for roaming”. The acceptable cell is a cell in which the measured communication quality of the cell satisfies the cell selection criterion, and is not the prohibited cell.

For example, the cell selection criteria are Srxlev>0 and Squal>0. Srxlev represents cell selection received power. Srxlev is calculated by Srxlev=Q_rxlevmeas−(Q_rxlevmin+Q_{rxlevminoffset})−P_compensation−Qoffset_temp. Q_rxlevmeasis the measured received power (reference signal received power (RSRP)) of a cell. Q_rxlevminis the minimum required received power. Q_{rxlevminoffset}is a predetermined offset that is constantly applied. P_compensationis a parameter related to an uplink capability. Qoffset_tempis an offset that is temporarily applied. Squal represents a cell selection quality level. Squal is calculated by Squal=Q_qualmeas−(Q_qualmin+Q_{qualminoffset})−Qoffset_temp. Q_qualmeasis the measured cell quality level (reference signal received quality (RSRQ)). Q_qualminis the minimum required quality level. Q_{qualminoffset}is a predetermined offset that is constantly applied. Qoffset_tempis an offset that is temporarily applied.

(B) Cell Reselection

In the cell reselection, the UE 100 in the RRC idle state or the RRC inactive state measures the communication quality of a serving cell and a neighboring cell. The UE 100 selects a camp-on-cell to be used as the serving cell, for example, based on the following criteria. The UE 100 determines the priority of the cell to be selected according to the priority of frequencies. The UE 100 may select the camp-on-cell based on the following criteria when 1 second or more passes after camping on the current serving cell.

- (B1) The priority of the frequency of the neighboring cell is higher than the priority of the frequency of the current serving cell:

The UE 100 selects a cell that satisfies the relationship of Squal>Thresh_X,HighQover a predetermined duration (for example, Treselection_RAT) or a cell that satisfies the relationship of Srxlev>Thresh_X,HighPover a predetermined duration. Each of Thresh_X,HighPand Thresh_X,HighQis a predetermined threshold value.

- (B2) The priority of the frequency of the neighboring cell is the same as the priority of the frequency of the current serving cell:

The UE 100 calculates a ranking Rs of the current serving cell and a ranking Rn of the neighboring cell. The UE 100 selects a cell having a ranking Rn higher than Rs over a predetermined duration as the camp-on-cell.

For example, Rs is calculated by Rs=Q_meas,s+Q_hyst−Qoffset_temp. Rn is calculated by Rn=Q_meas,n−Qoffset−Qoffset_temp. Q_meas,sis received power (RSRP) of the current serving cell. Q_meas,nis received power (RSRP) of the neighboring cell. Q_hystis a hysteresis value used to facilitate the reselection of the current serving cell. Qoffset_tempis an offset that is temporarily applied.

- (B3) The priority of the frequency of the neighboring cell is lower than the priority of the frequency of the current serving cell:

For example, it is assumed that the serving cell satisfies the relationship of Squal<Thresh_Serving,LowQor Srxlev<Thresh_Serving,LowPover a predetermined duration. Under this assumption, the UE 100 selects, for example, a cell satisfying the relationship of Squal>Thresh_X,LowQover a predetermined duration or a cell satisfying the relationship of Srxlev>Thresh_X,LowPover a predetermined duration. Each of Thresh_X,LowPand Thresh_X,LowQis a predetermined threshold value.

Various parameters used for cell selection are included in system information (system information block (SIB)) broadcasted from the base station 200. The various parameters include, for example, the priority of the frequency (for example, cellReselectionPriority, cellReselectionSubPriority), a predetermined duration (Treselection_RAT), various offsets (Q_{qualminoffset}, Q_{rxlevminoffset}, Q_offsettemp, Q_hyst, Q_offset), and/or various threshold values (Thresh_X,HighQ, Thresh_X,HighP, Threshs_Serving,LowQ, Thresh_Serving,LowP, Thresh_X,LowP, Thresh_X,LowQ). The cellReselectionPriority indicates the priority of the frequency in eight stages, and the cellReselectionSubPriority indicates the sub-priority of the frequency in four stages. As a result, the base station 200 can configure a maximum of 32 priority levels for the frequency in the UE 100. The priority of the frequency may be sent to the UE 100 as an RRC release message.

(System Operation)

Next, an operation of the mobile communication system 1 according to the present embodiment will be described.

(1) Example of Operation Environment

An example of an operation environment of the mobile communication system 1 according to the present embodiment will be described with reference to FIG. 5.

In FIG. 5, cells C1 to C7 are illustrated as cells of the mobile communication system 1. The cell C1 is a cell belonging to a frequency F1, and a physical cell identifier (PCI) thereof is PCI #a. The cell C1 supports the specific UE 100B. The cell C2 is a cell belonging to the frequency F1, and the PCI thereof is PCI #b. The cell C2 does not support the specific UE 100B. The cell C3 is a cell belonging to a frequency F2, and the PCI thereof is PCI #c. The cell C3 supports the specific UE 100B. The cell C4 is a cell belonging to a frequency F3, and the PCI thereof is PCI #d. The cell C4 does not support the specific UE 100B. The cell C5 is a cell belonging to a frequency F4, and the PCI thereof is PCI #e. The cell C5 supports the specific UE 100B. The cell C6 is a cell belonging to the frequency F 1, the PCI thereof is PCI #f. The cell C6 supports the specific UE 100B. The cell C7 is a cell belonging to the frequency F2, and the PCI thereof is PCI #g. The cell C7 supports the specific UE 100B. It is noted that the UE 100 receives a synchronization signal and/or a reference signal transmitted from each cell, for example, a synchronization signal (SS)/PBCH block (SSB). As a result, the UE 100 can detect the cell and recognize the PCI of the cell.

As described above, in the mobile communication system 1, cells having various frequencies may be mixed, and the cells supporting the specific UE 100B and the cells not supporting the specific UE 100B may be mixed. It is noted that each cell may be managed by a different base station 200.

FIG. 5 illustrates an example in which the UE 100 in the RRC idle state or the RRC inactive state selects the cell C1 as a camp-on-cell. When the cell C1 is used as a reference, the cells C2 to C7 correspond to neighboring cells. Further, when the cell C1 is used as a reference, the cells C2 and C6 correspond to intra-frequency neighboring cells, and the other cells correspond to inter-frequency neighboring cells.

Here, it is assumed that the UE 100 is the specific UE 100B. The UE 100 in the RRC idle state or the RRC inactive state periodically monitors paging from the base station 200 that manages the cell C1. Here, for example, the UE 100 changes the camp-on-cell of its own UE from the cell C1 to a neighboring cell by performing cell reselection along with the movement of its own UE. However, since the cell C2 and the cell C4 among the neighboring cells do not support the specific UE 100B, the specific UE 100B cannot camp on the cell C2 and the cell C4.

In the present embodiment, the base station 200 that manages the cell C1 broadcasts, in the cell C1, system information used to allow the UE 100 existing in the cell C1 to perform cell reselection using a neighboring cell different from the cell C1 as a candidate. Specifically, the base station 200 that manages the cell C1 broadcasts system information including a second neighboring cell list different from a first neighboring cell list for the general UE 100A as a neighboring cell list indicating a neighboring cell to be excluded from a cell reselection candidate or a neighboring cell to be selected as a cell reselection candidate. The second neighboring cell list is a neighboring cell list for the specific UE 100B. As a result, the specific UE 100B existing in the cell C1 can determine whether a neighboring cell can be set as a cell reselection candidate based on the second neighboring cell list without acquiring and decoding the system information (SIB1) from the corresponding neighboring cell. Therefore, the specific UE 100B existing in the cell C1 can perform efficient cell reselection.

(2) Example of Operation Flow

An example of an operation flow of the mobile communication system 1 according to the present embodiment will be described with reference to FIG. 6. Here, it is assumed that the general UE 100A and the specific UE 100B exist in the cell C1.

In step S1, the general UE 100A is in the RRC idle state or the RRC inactive state in the cell C1.

In step S2, the specific UE 100B is in the RRC idle state or the RRC inactive state in the cell C1.

In steps S3 and S4, the base station 200 (the communicator 210) broadcasts system information in the cell C1. The system information is used to allow the UE 100 (the general UE 100A and the specific UE 100B) existing in the cell C1 to perform cell reselection using a neighboring cell different from the cell C1 as a candidate. Each of the general UE 100A (the communicator 110) and the specific UE 100B (the communicator 110) receives the system information.

The base station 200 (the communicator 210) broadcasts the system information by transmitting the system information using a broadcast channel. The system information is information capable of being received (acquired) even by the UE 100 in the RRC idle state or the RRC inactive state. The base station 200 (the communicator 210) may periodically broadcast such system information.

Specifically, in step S3, the base station 200 (the communicator 210) broadcasts a first system information block for cell reselection with respect to an intra-frequency neighboring cell(s), which is a neighboring cell(s) belonging to the same frequency as the frequency of the cell C1. For example, the first system information block is a system information block type 3 (SIB3). The SIB3 includes neighboring cell related information for intra-frequency cell reselection.

In step S4, the base station 200 (the communicator 210) broadcasts a second system information block for cell reselection with respect to an inter-frequency neighboring cell(s), which is a neighboring cell(s) belonging to a frequency different from the frequency of the cell C1. For example, the second system information block is a system information block type 4 (SIB4). The SIB4 includes information relevant for inter-frequency cell reselection (that is, information on neighboring NR frequencies and inter-frequency neighboring cells relevant for cell reselection). It is noted that the order of steps S3 and S4 may be reversed, or steps S3 and S4 may be performed simultaneously.

In step S5, the general UE 100A (the controller 120) controls the cell reselection based on a first neighboring cell list included in the system information received from the gNB 200 (the cell C1). Specifically, the general UE 100A (the controller 120) performs intra-frequency cell reselection control based on the first neighboring cell list included in the first system information block and inter-frequency cell reselection control based on the first neighboring cell list included in the second system information block.

In step S6, the specific UE 100B (the controller 120) controls the cell reselection based on a second neighboring cell list included in the system information received from the gNB 200 (the cell C1). Specifically, the specific UE 100B (the controller 120) performs intra-frequency cell reselection control based on the second neighboring cell list included in the first system information block and inter-frequency cell reselection control based on the first neighboring cell list included in the second system information block.

As described above, when its own UE is the specific UE 100B, the controller 120 of the UE 100 controls the cell reselection based on the second neighboring cell list included in the system information. As a result, the specific UE 100B (the controller 120) can use the second neighboring cell list optimized for the specific UE 100B. Therefore, the specific UE 100B (the controller 120) can appropriately determine, based on the second neighboring cell list, a neighboring cell to be excluded from a cell reselection candidate or a neighboring cell to be selected as a cell reselection candidate.

Specifically, the specific UE 100B (the controller 120) controls the cell reselection based on the second neighboring cell list included in each of the first system information block and the second system information block. As a result, the specific UE 100B (the controller 120) can appropriately determine, based on the second neighboring cell list included in the first system information block, a neighboring cell(s) to be excluded from a candidate for intra-cell reselection or a neighboring cell(s) to be selected as a candidate for intra-frequency cell reselection. In addition, the specific UE 100B (the controller 120) can appropriately determine, based on the second neighboring cell list included in the second system information block, a neighboring cell(s) to be excluded from a candidate for inter-frequency cell reselection or a neighboring cell(s) to be selected as a candidate for inter-frequency cell reselection.

(3) Example of First System Information Block

An example of the first system information block according to the present embodiment will be described with reference to FIG. 7.

As described above, the first system information block is a system information block for intra-frequency cell reselection with respect to an intra-frequency neighboring cell(s), and is, for example, the SIB3.

The SIB3 includes the first neighboring cell list indicating a neighboring cell(s) to be excluded, by the general UE 100A, from a candidate for intra-frequency cell reselection or a neighboring cell(s) to be selected, by the general UE 100A, as a candidate for intra-frequency cell reselection. The first neighboring cell list includes at least one of “IntraFreqBlackCellList” illustrated in A1 of FIG. 7 and “IntraFreqWhiteCellList-r16” illustrated in A2 of FIG. 7. The “IntraFreqBlackCellList” is an information element indicating a neighboring cell(s) to be excluded, by the general UE 100A, from a candidate for intra-frequency cell reselection by one or a plurality of PCI ranges (SEQUENCE (SIZE (1 . . . maxCellBlack)) OF PCI-Range) (refer to A4 in FIG. 7). “IntraFreqWhiteCellList-r16” is an information element indicating a neighboring cell(s) to be selected, by the general UE 100A, as a candidate for intra-frequency cell reselection by one or a plurality of PCI ranges (SEQUENCE (SIZE (1 . . . maxCellWhite)) OF PCI-Range) (refer to A4 in FIG. 7).

In addition, the SIB3 includes the second neighboring cell list indicating a neighboring cell(s) to be excluded, by the specific UE 100B, from a candidate for intra-frequency cell reselection or a neighboring cell(s) to be selected, by the specific UE 100B, as a candidate for intra-frequency cell reselection.

As illustrated in A3 in FIG. 7, the second neighboring cell list included in the SIB3 may be at least one of the following (a1) to (a3).

- (a1) “IntraFreqExcludedCellListRedCap-r17” which is a first excluded cell list indicating an intra-frequency neighboring cell(s) to be excluded, by the specific UE 100B, from a candidate
- (a2) “IntraFreqExcludedCellListSingleRx-r17” which is a second excluded cell list indicating an intra-frequency neighboring cell(s) to be excluded, by the UE 100 (the specific UE 100B) equipped with only one receiver 111, from a candidate
- (a3) “IntraFreqExcludedCellListDualRx-r17” which is a third excluded cell list indicating an intra-frequency neighboring cell(s) to be excluded, by the UE 100 (the specific UE 100B) equipped with only two receivers 111, from a candidate

Each of “IntraFreqExcludedCellListRedCap-r17”, “IntraFreqExcludedCellListSingleRx-r17”, and “IntraFreqExcludedCellListDualRx-r17” is an information element indicating a neighboring cell(s) to be excluded from a candidate for intra-frequency cell reselection by one or a plurality of PCI ranges (SEQUENCE (SIZE (1 . . . maxCellBlack)) OF PCI-Range) (refer to A5 in