Patent Description:
In order to meet wireless data traffic demands that have increased after 4th Generation (<NUM>) communication system commercialization, efforts to develop an improved <NUM> communication system or a pre-<NUM> communication system have been made. For this reason, the <NUM> communication system or the pre-<NUM> communication system is called a beyond <NUM> network communication system or a post LTE system.

In order to achieve a high data transmission rate, implantation of the <NUM> communication system in an mmWave band has been considered. In the <NUM> communication system, technologies such as beamforming, massive MIMO, Full Dimensional MIMO (FD-MIMO), an array antenna, analog beam-forming, and a large scale antenna are discussed to mitigate a propagation path loss in the mmWave band and increase a propagation transmission distance.

Further, technologies such as an evolved small cell, an advanced small cell, a cloud Radio Access Network (cloud RAN), an ultra-dense network, Device to Device communication (D2D), a wireless backhaul, a moving network, cooperative communication, Coordinated Multi-Points (CoMP), and interference cancellation have been developed to improve the system network in the <NUM> communication system.

In addition, the <NUM> system has developed Advanced Coding Modulation (ACM) schemes such as Hybrid FSK and QAM Modulation (FQAM) and Sliding Window Superposition Coding (SWSC), and advanced access technologies such as Filter Bank Multi Carrier (FBMC), Non Orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA).

<NPL>) discloses further consideration on CN type selection in LTE connectivity to <NUM>-CN based on SA2 reply LS (S2-<NUM> - `LS on LTE connectivity to <NUM>-CN (R2-<NUM>/S2-<NUM>)').

Further aspects of the invention are outlined in the dependent claims. When the term "embodiment" is used for describing an unclaimed combination of features, it has to be understood as referring to examples useful for understanding the present invention.

According to the disclosure, a UE which can be connected to a <NUM> CN (NR core network) or an EPC (LTE network) can determine that a cell of a target eNB connected to the EPC or the <NUM> CN is a source eNB for handover as necessary in a next-generation mobile communication system.

According to the disclosure, it is possible to specify a core network selection or reselection process, and move to an EPC so as to receive functions or services which are not supported by the <NUM> CN or move to a <NUM> CN so as to receive functions or services which are not supported by the EPC.

Hereinafter, the operating principle of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description of the present disclosure, a detailed description of known configurations or functions incorporated herein will be omitted when it is determined that the detailed description may make the subject matter of the present disclosure unclear. The terms as described below are defined in consideration of the functions in the embodiments, and the meaning of the terms may vary according to the intention of a user or operator, convention, or the like. Therefore, the definitions of the terms should be made based on the contents throughout the specification.

In the following description, terms for identifying an access node, terms referring to network entities, terms referring to messages, terms referring to interfaces between network entities, and terms referring to various pieces of identification information are used for convenience of description. Accordingly, the disclosure is not limited to the following terms and other terms having the same technical meaning may be used.

For convenience of description, the disclosure uses terms and names defined in a 3rd-Generation Partnership Project, Long-Term Evolution (3GPP LTE) standard or terms and names changed on the basis thereof. However, the disclosure may not be limited by the terms and names, and may be equally applied to a system that is based on another standard. Particularly, the disclosure may be applied to 3GPP New Radio (NR, <NUM>th-generation mobile communication standard).

<FIG> illustrates a structure of a next-generation mobile communication system.

Referring to <FIG>, a radio access network of a next-generation mobile communication system (New Radio: NR) includes a new radio node B <NUM> (hereinafter, referred to as an NR NB, an NR gNB, or a gNB) and a new radio core network <NUM> (hereinafter, referred to as a NG CN, a Next Generation Core Network (NG CN), or an AMF). A new radio user equipment <NUM> (hereinafter, referred to an NR UE, a UE, or a terminal) accesses an external network through the gNB <NUM> and the AMF <NUM>.

The gNB corresponds to an evolved Node B (hereinafter, referred to as an eNB) of the conventional LTE system. The gNB may be connected to the NR UE through a radio channel and may provide better service than the conventional node B as indicated by reference numeral <NUM>.

In the next-generation mobile communication system, all user traffic is served through a shared channel. Accordingly, a device for collecting and scheduling status information such as buffer statuses, available transmission power statuses, and channel statuses of UEs is needed, which is served by the gNB <NUM>. One gNB generally controls a plurality of cells. The next-generation mobile communication system may have a maximum bandwidth wider than or equal to the conventional maximum bandwidth in order to implement super-high data transmission compared to conventional LTE.

The next-generation mobile communication system uses an Orthogonal Frequency Division Multiplexing (OFDM) as radio access technology and may additionally use a beamforming technique. Further, the next-generation mobile communication system applies a modulation scheme and an Adaptive Modulation and Coding (hereinafter, referred to as an AMC) scheme of determining a channel coding rate in correspondence to a channel status of the UE.

The AMF <NUM> performs a function of supporting mobility, establishing a bearer, and configuring Quality of Service (QoS). The AMF is a device for performing a function of managing mobility of the UE and various control functions and is connected to a plurality of eNBs.

Further, the next-generation mobile communication system may interwork with the conventional LTE system, and the AMF is connected to a Mobility Management Entity (MME) <NUM> through a network interface. The MME is connected to the eNB <NUM>, which is a conventional eNB. The UE supporting LTE-NT dual connectivity may transmit and receive data while maintaining the connection not only to the gNB but also to the eNB as indicated by reference numeral <NUM>.

<FIG> is a block diagram illustrating the UE according to the disclosure.

Referring to <FIG>, the UE may include a transceiver <NUM> and a controller <NUM> and may further include a storage unit <NUM>. The transceiver <NUM> may include a Radio Frequency (RF) processor <NUM> and a baseband processor <NUM>.

The RF processor <NUM> performs a function for transmitting and receiving a signal through a wireless channel, such as band conversion and amplification of a signal. That is, the RF processor <NUM> up-converts a baseband signal provided from the baseband processor <NUM> into an RF band signal, transmits the RF band signal through an antenna, and then down-converts the RF band signal received through the antenna into a baseband signal.

For example, the RF processor <NUM> may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a digital-to-analog convertor (DAC), an analog-to-digital convertor (ADC), and the like. Although only one antenna is illustrated in <FIG>, the UE may include a plurality of antennas.

The RF processor <NUM> may include a plurality of RF chains. Moreover, the RF processor <NUM> may perform beamforming. For the beamforming, the RF processor <NUM> may control a phase and a size of each signal transmitted/received through a plurality of antennas or antenna elements. The RF processor may perform MIMO and receive a plurality of layers when performing the MIMO operation.

The baseband processor <NUM> performs a function for a conversion between a baseband signal and a bitstream according to a physical layer standard of the system. For example, when data is transmitted, the baseband processor <NUM> generates complex symbols by encoding and modulating a transmission bitstream. Further, when data is received, the baseband processor <NUM> reconstructs a reception bitstream by demodulating and decoding a baseband signal provided from the RF processor <NUM>. In an OFDM (orthogonal frequency division multiplexing) scheme, when data is transmitted, the baseband processor <NUM> generates complex symbols by encoding and modulating a transmission bitstream, mapping the complex symbols to subcarriers, and then configures OFDM symbols through an IFFT (inverse fast Fourier transform) operation and a CP (cyclic prefix) insertion. When data is received, the baseband processor <NUM> divides the baseband signal provided from the processor <NUM> in the unit of OFDM symbols, reconstructs the signals mapped to the subcarriers through an FFT(fast Fourier transform) operation, and then reconstructs a reception bitstream through demodulation and decoding.

The baseband processor720 and the RF processor710 transmit and receive signals as described above. Accordingly, the baseband processor <NUM> and the RF processor <NUM> may be referred to as a transmitter, a receiver, a transceiver, or a communication unit.

At least one of the baseband processor <NUM> and the RF processor <NUM> may include a plurality of communication modules to support a plurality of different radio access technologies. In addition, at least one of the baseband processor <NUM> and the RF processor <NUM> may include different communication modules to process signals of different frequency bands. The different radio access technologies may include a WLAN (for example, IEEE <NUM>) and a cellular network (for example, LTE). The different frequency bands may include a super-high-frequency (SHF) (for example, <NUM> NRHz, NRhz) band and a millimeter (mm) wave (for example, <NUM>) band.

The storage unit <NUM> stores data such as a basic program, an application, and setting information for the operation of the UE. The storage unit <NUM> may store information related to a second access node for performing wireless communication through second radio access technology. The storage unit <NUM> provides the stored data according to a request from the controller <NUM>.

The controller <NUM> controls the overall operation of the UE. The controller <NUM> transmits and receives signals through the baseband processor <NUM> and the RF processor <NUM>. The controller <NUM> records data in the storage unit <NUM> and reads the data. To this end, the controller <NUM> may include at least one processor. For example, the controller <NUM> may include a CP (communication processor) that performs a control for communication, and an AP (application processor) that controls a higher layer such as an application program.

The controller <NUM> includes a multi-connection processor <NUM> for processing the operation in a multi-connection mode. For example, the controller <NUM> may control the UE to perform a procedure of the operation of the UE illustrated in <FIG>.

According to an embodiment of the disclosure, the UE may receive measurement configuration information from the eNB, perform cell measurement on the basis of the measurement configuration information, and report result information of the performed measurement to the eNB, and the result information may include a core network type of the cell.

<FIG> is a block diagram illustrating a master eNB in a wireless communication system according to an embodiment of the disclosure.

Referring to <FIG>, the eNB may include a transceiver <NUM> and a controller <NUM> and may further include a storage unit <NUM>. The eNB may further include a backhaul communication unit <NUM> unlike the UE. The transceiver <NUM> includes an RF processor <NUM> and a baseband processor <NUM>.

The RF processor <NUM> performs a function for transmitting and receiving a signal through a wireless channel, such as band conversion and amplification of a signal. That is, the RF processor <NUM> up-converts a baseband signal provided from the baseband processor <NUM> into an RF band signal and then transmits the converted signal through an antenna, and down-converts an RF band signal received through the antenna into a baseband signal. For example, the RF processor <NUM> may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, and an ADC.

Although only one antenna is illustrated in <FIG>, the first access node may include a plurality of antennas. The RF processor <NUM> may include a plurality of RF chains. The RF processor <NUM> may perform beamforming. For the beamforming, the RF processor <NUM> may control a phase and a size of each of the signals transmitted and received through a plurality of antennas or antenna elements. The RF processor may perform a downlink MIMO operation by transmitting one or more layers.

The baseband processor <NUM> performs a function of performing conversion between a baseband signal and a bitstream according to a physical layer standard of the first radio access technology. For example, when data is transmitted, the baseband processor <NUM> generates complex symbols by encoding and modulating a transmission bitstream. Further, when data is received, the baseband processor <NUM> reconstructs a reception bitstream by demodulating and decoding a baseband signal provided from the RF processor <NUM>.

In an OFDM scheme, when data is transmitted, the baseband processor <NUM> may generate complex symbols by encoding and modulating the transmission bitstream, map the complex symbols to subcarriers, and then configure OFDM symbols through an IFFT operation and CP insertion. In addition, when data is received, the baseband processor <NUM> divides a baseband signal provided from the RF processor <NUM> in units of OFDM symbols, recovers signals mapped with sub-carriers through an FFT operation, and then recovers a reception bit string through demodulation and decoding.

The baseband processor <NUM> and the RF processor <NUM> transmit and receive signals as described above. Accordingly, the baseband processor <NUM> and the RF processor <NUM> may be referred to as a transmitter, a receiver, a transceiver, or a communication unit.

The backhaul communication unit <NUM> provides an interface for communicating with other nodes within the network. The backhaul communication unit <NUM> converts a bitstream transmitted from the master eNB to another node, for example, a secondary eNB or a core network into a physical signal and a physical signal received from the other node into a bitstream.

The storage unit <NUM> stores data such as a basic program, an application, and setting information for the operation of the master eNB. The storage unit <NUM> may store information on a bearer allocated to the accessed UE and the measurement result reported from the accessed UE. Further, the storage unit <NUM> may provide multiple connections to the UE and store information on a reference for determining whether to stop the multiple connections. In addition, the storage unit <NUM> provides the stored data according to a request from the controller <NUM>.

The controller <NUM> controls the overall operation of the master eNB. The controller <NUM> transmits and receives signals through the baseband processor <NUM> and the RF processor <NUM> or through the backhaul communication unit <NUM>. The controller <NUM> may record data in the storage unit <NUM> and read the data. To this end, the controller <NUM> may include at least one processor.

According to an embodiment of the disclosure, the eNB may transmit measurement configuration information to the UE and receive result information of cell measurement performed on the basis of the measurement configuration information from the UE, and the result information may include a core network type of the cell.

<FIG> illustrates a structure of the LTE system.

Referring to <FIG>, the wireless communication system may include a plurality of evolved Node B (eNBs) (or base stations) <NUM>, <NUM>, <NUM>, and <NUM>, a Mobility Management Entity (MME) <NUM>, and a Serving-Gateway (S-GW) <NUM>. A User Equipment (UE) (or a terminal) <NUM> may access an external network through the eNBs and the S-GW <NUM>.

The eNBs <NUM>, <NUM>, <NUM>, and <NUM> provide radio access to UEs which access the network as access nodes of the cellular network. That is, in order to serve traffic of users, the eNBs <NUM>, <NUM>, <NUM>, and <NUM> collect and schedule status information such as buffer statuses, available transmission power statuses, and channel statuses of UEs and support connection between the UEs and a Core Network (CN).

The MME <NUM> is a device performing a function of managing mobility of the UE and various control functions and is connected to a plurality of eNBs, and the S-GW <NUM> is a device providing a data bearer. The MME <NUM> and the S-GW <NUM> may further perform authentication of the UE accessing the network and management of the bearer, and processes packets received from the eNB or packets transmitted from the eNB.

<FIG> illustrates a structure of a wireless protocol in the LTE system. A structure of a wireless protocol of NR may be partially different from the structure of the wireless protocol of <FIG>.

Referring to <FIG>, the UE and the eNB includes PDCPs (Packet Data Convergence Protocols) <NUM> and <NUM>, RLCs (Radio Link Controls) <NUM> and <NUM>, Medium Access Controls (MACs) <NUM> and <NUM>, respectively, in the wireless protocol of the LTE system.

The PDCPs <NUM> and <NUM> perform an operation such as compressing/decompressing an IP header. The RLCs <NUM> and <NUM> reconfigure a PDCP Packet Data Unit (PDU) to be a proper size. The MACs <NUM> and <NUM> are connected with various RLC layer devices configured in one UE, and performs an operation of multiplexing RLC PDUs to the MAC PDU and demultiplexing the RLC PDUs from the MAC PDU.

The PHY layers <NUM> and <NUM> perform an operation for channel-coding and modulating higher layer data to generate an OFDM symbol and transmitting the OFDM symbol through a radio channel or demodulating and channel-decoding the OFDM symbol received through the radio channel and transmitting the demodulated and channel-decoded OFDM symbol to the higher layer.

Further, the PHY layer uses Hybrid ARQ (HARQ) to correct an additional error, and a receiving side transmits <NUM>-bit information to indicate whether a packet transmitted by a transmitting side is received. The <NUM>-bit information is referred to as HARQ ACK/NACK information.

Downlink HARQ ACK/NACK information for uplink transmission is transmitted through a Physical Hybrid-ARQ Indicator CHannel (PHICH) physical channel. Uplink HARQ ACK/HARQ information for downlink transmission is transmitted through a Physical Uplink Control CHannel (PUCCH) or a Physical Uplink Shared CHannel (PUSCH) physical channel. The PUCCH is used when the UE transmits not only the HARQ ACK/NACK information but also downlink Channel Status Information (CSI) and a Scheduling Request (SR) to the eNB.

The SR is <NUM>-bit information. When the eNB transmits the SR to resources within the PUCCH configured by the eNB, the eNB recognizes that the UE has data to be transmitted through uplink and thus allocates uplink resources. The UE may transmit a detailed Buffer Status Report (BSR) message through the uplink resources. The eNB may allocate a plurality of SR resources to one UE.

Meanwhile, the PHY layer may include one or a plurality of frequencies/subcarriers, and technology in which one eNB simultaneously configures and uses a plurality of frequencies is referred to as carrier aggregation (CA).

CA significantly increases the amount of transmission by the number of subcarriers by additionally using a primary carrier and one or a plurality of subcarriers, which is beyond the conventional technology, in which only one subcarrier is used for communication between the UE and the E-UTRAN NodeB (eNB).

Meanwhile, in LTE, a cell within the eNB using a primary carrier is referred to as a Primary Cell (PCell) and a secondary carrier is referred to as a Secondary Cell (SCell). The technology obtained by expanding the CA function to two eNBs is referred to as dual connectivity (DC).

In the DC, the UE is simultaneously connected to and uses a master E-UTRAN NodeB (MeNB) and a secondary E-UTRAN NodeB (SeNB), and cells belonging to the MeNB are referred to as a master cell group (MCG) and cells belonging to the SeNB are referred to as a secondary cell group (SCG).

Each cell group has a representative cell. A representative cell of the primary cell group is referred to as a Primary Cell (PCell) and a representative cell of the secondary cell group is referred to as a Primary Secondary Cell (PSCell). When the NR is used, as the MCG uses LTE technology and the SCG uses NR, the UE may simultaneously use LTE and NR.

Although not illustrated, there is a radio resource control (RRC) layer above the PDCP layer of each of the UE and the eNB, and the RRC layer may transmit and receive an access- and measurement-related configuration control message to control radio resources. The eNB may indicate measurement to the UE through a message of the RRC layer and the UE may report the measurement result to the eNB through the message of the RRC layer.

<FIG> illustrates message flow between the UE and the eNB when a method of transmitting a scheduling request is used.

Referring to <FIG>, a UE <NUM> in an idle mode (RRC_IDLE) accesses the eNB for the reason of generation of data to be transmitted. In the idle mode, the UE is not connected to the network to save power of the UE, so the UE cannot transmit data. In order to transmit data, the UE is required to switch to a connected mode (RRC_CONNECTED).

When the UE successfully accesses the eNB <NUM>, the UE switches to the connected mode (RRC_CONNECTED) and the connected-mode UE can transmit and receive data to and from the eNB through security activation and bearer configuration for data.

Thereafter, the eNB establishes a bearer (Data Radio Bearer (DRB)) which serves as a logical passage for data transmission to the UE and transmits SR resources for an uplink resource request and relevant configuration information in step <NUM>. The eNB may configure a plurality of periodic SR configuration information in the UE according to the purpose. For example, the UE may configure two pieces of SR configuration information. SR configuration information <NUM> may be configuration information to be used when data is generated in logical channels a and b and SR configuration information <NUM> may be configuration information to be used when data is generated in logical channels c and d. Each piece of the SR configuration information may include information on a logical channel related to the SR configuration information and also information on a plurality of periodic SR resources.

Meanwhile, in the next-generation mobile communication system, the eNB may have a significantly wide bandwidth and accordingly, the eNB may configure only a portion of the corresponding bandwidth in the UE even through the eNB uses the broadband. Such a concept is referred to as a Bandwidth Part (BWP).

With respect to one cell, one or a plurality of BWPs may be configured in one UE depending on an operation scenario, and numerology/Transmission Time Interval (TTI) used by each BWP may be configured differently.

The eNB may configure periodic SR resources according to each BWP in one UE through each piece of SR configuration information. For example, when the UE is configured to use two cells having different frequencies and each cell has three BWPs, the UE has a total of six BWPs, and the eNB may configure one periodic SR resource according to each of the six BWPs for SR configuration information <NUM> and one periodic SR resource according to each of the six BWPs for SR configuration information2. Accordingly, when only one BWP is activated for each of the two different cells (that is, when two of the six BWPs are activated), the UE still has the SR configured for each BWP and thus make a request for resources to the eNB.

Further, within each piece of the SR configuration information, an SR-prohibiting timer for preventing frequent SR transmission and a maximum number of SR transmissions may be configured independently from each other.

Meanwhile, the SR-prohibiting timer may be expressed by a unit of a SR period. For example, when the SR period is set to <NUM> and a value of the SR-prohibiting timer is set to <NUM>, a time duration of the SR-prohibiting timer corresponds to <NUM>. When there are six BWPs and respective SR resources have different periods as described in the above example, the UE may use the shortest configuration information period within the corresponding SR configuration information regardless of activation. Alternatively, the UE may use the shortest configuration information period among SR resource periods of the currently activated BWP within the corresponding SR configuration information. Alternatively, the UE may use not only the corresponding SR configuration information but also the shortest configuration information period among all the configured SR resource periods.

Accordingly, the following information may be configured according to each piece of SR configuration information.

The eNB may transmit various configurations to the UE through an RRCConnectionReconfiguration message of the RRC layer. Thereafter, the UE transmits an acknowledgement message of the configuration indication, in which case the UE may use an RRCConnectionReconfigurationComplete message of the RRC layer in step S1115.

Meanwhile, the UE may trigger a current buffer status report (BSR) of the UE according to various conditions below and the BSR is divided into three types according to a condition of triggering transmission. A first type is a regular BSR, a second type is a periodic BSR, and a third type is a padding BSR.

When the regular BSR is triggered by generation of traffic of any logical channel according to the condition in step <NUM>, the UE selects particular SR configuration information (for example, SR configuration information <NUM> or <NUM>) to which the corresponding logical channel is mapped in step <NUM>, and when there is the corresponding configuration information, triggers the SR as the selected SR configuration information in step <NUM>.

Accordingly, the UE transmits an SR signal to the eNB through the earliest SR resources among one or a plurality of SR resources configured in the corresponding SR configuration information or SR resources according to the sequence in the SR configuration information in step <NUM>.

After transmitting the SR signal to the eNB, the UE drives the SR-prohibiting timer configured in the corresponding SR configuration information, and when the timer is driven according thereto, does not transmit the SR for the corresponding SR configuration information. Further, after transmitting the SR signal to the eNB, the UE increases a count of the number of SR transmissions and determines whether the counter reaches the corresponding maximum number of transmissions configured. When the count of the SR transmission reaches the corresponding maximum number of SR transmissions configured, the UE performs random access to the eNB, transmits the BSR to the eNB, and reports a current buffer status of the UE.

<FIG> is a flowchart illustrating the operation of the UE when a method of transmitting a scheduling request is used.

Referring to <FIG>, it is assumed that the UE is connected to the LTE eNB and is thus in a connected mode (RRC CONNECTED). Thereafter, the UE receives configuration of a DRB from the eNB, receives configuration of SR resources and relevant configuration information for an uplink resource request, and transmits an acknowledgement message thereof in step <NUM>.

The SR resources and the relevant configuration information for the uplink resource request may include a plurality of pieces of periodic SR configuration information. For example, the UE may configure two pieces of SR configuration information. SR configuration information <NUM> may be configuration information to be used when data is generated in logical channels a and b and SR configuration information <NUM> may be configuration information to be used when data is generated in logical channels c and d. Each piece of the SR configuration information may include information on a logical channel related to the SR configuration information and also information on one or a plurality of periodic SR resources. Within each piece of the SR configuration information, the UE may receive configuration of periodic SR resources according to each BWP.

When the UE is configured to use two cells having different frequencies and each cell has three BWPs, the UE has a total of six BWPs, and the eNB may configure one periodic SR resource according to each of the six BWPs for SR configuration information <NUM> and one periodic SR resource according to each of the six BWPs for SR configuration information2. Accordingly, when only one BWP is activated for each of the two different cells (that is, when two of the six BWPs are activated), the UE still has the SR configured for each BWP and thus make a request for resources to the eNB.

After transmitting the SR signal to the eNB, the UE drives the SR-prohibiting timer configured in the corresponding SR configuration information, and when the timer is driven according thereto, does not transmit the SR for the corresponding SR configuration information. If the UE receives resources for the BSR from the eNB for a predetermined time, the UE transmits the BSR to the eNB in step <NUM>. If the UE does not receive resources for the BSR from the eNB for a predetermined time in step <NUM> and the corresponding number of SR transmissions does not reach the configured maximum number in step <NUM>, the UE may retransmit the corresponding SR in step <NUM>.

Since there are a lot of UEs, the eNB may not afford to provide uplink resources to the corresponding UE or the eNB may not properly receive the SR. If the number of SR transmissions reaches the configured maximum number of transmissions, the UE may perform a random access procedure to the eNB, makes a request for uplink resources, and transmit the BSR through the corresponding resources in step <NUM>.

<FIG> illustrates a structure of the LTE system which is referred to for description of the disclosure.

Referring to <FIG>, a radio access network of the LTE system includes next-generation evolved Node Bs (hereinafter, referred to as eNBs, Node Bs, or base stations) <NUM>, <NUM>, <NUM> and <NUM>, a Mobility Management Entity (MME) <NUM>, and a Serving-Gateway (S-GW) <NUM>. A User Equipment (UE) <NUM> (or a terminal) may access an external network through the eNBs <NUM> to <NUM> and the S-GW <NUM>.

The eNBs <NUM> to <NUM> correspond to nodeBs of a UMTS system. The eNB is connected with the UE <NUM> through a wireless channel, and plays a more complicated role than the node B. In the LTE system, since all user traffic including a real time service such as a VoIP (Voice over IP) through an Internet protocol are serviced through a shared channel, an apparatus for collecting and scheduling status information on buffer statuses of UEs, available transmission power status, and channel statuses is required, and the ENBs <NUM> to <NUM> serve as this apparatus.

One eNB generally controls a plurality of cells. For example, in order to implement a transmission rate of <NUM> Mbps, the LTE system uses Orthogonal Frequency Division Multiplexing (OFDM) as radio access technology in a bandwidth of <NUM>. Further, an AMC (Adaptive Modulation and Coding) scheme of determining a modulation scheme and a channel coding rate is applied according to a channel status of the UE. The S-GW <NUM> is a device for providing a data bearer, and generates or removes the data bearer under a control of the MME <NUM>. The MME is a device for performing not only a function of managing mobility of the UE but also various control functions and is connected to a plurality of eNBs.

<FIG> illustrates the structure of the wireless protocol in the LTE system which is referred to for description of the disclosure.

The PDCPs <NUM> and <NUM> perform an operation such as compressing/decompressing an IP header. Main functions of the PDCP are described below.

The MAC <NUM> and <NUM> are connected with various RLC layer devices configured in one UE, and perform a function of multiplexing RLC PDUs to the MAC PDU and demultiplexing the RLC PDUs from the MAC PDU. Main functions of the MAC are described below.

<FIG> illustrates a structure of a wireless protocol of the next-generation mobile communication system to which the disclosure can be applied.

Referring to <FIG>, the UE and the NR NB include NR PDCPs <NUM> and <NUM>, NR RLCs <NUM> and <NUM>, and NR MACs <NUM> and <NUM>, respectively, in the wireless protocol of the next-generation mobile communication system.

Main functions of the NR PDCPs <NUM> and <NUM> may include some of the following functions.

The reordering function of the NR PDCP device is a function of sequentially reordering PDCP PDUs received from a lower layer on the basis of a PDCP Sequence Number (SN) and may include a function of sequentially transferring the reordered data to a higher layer, a function of recording PDCP PDUs lost due to the reordering, a function of reporting statuses of the lost PDCP PDUs to a transmitting side, and a function of making a request for retransmitting the lost PDCP PDUs.

Main functions of the NR RLCs <NUM> and <NUM> may include some of the following functions.

The sequential delivery function (In-sequence delivery) of the NR RLC device is a function of sequentially transferring PDCP PDUs received from a lower layer to a higher layer) and may include, when one original RLC SDU is divided into a plurality of RLC SDUs and then received, a function of reassembling and transmitting the RLC SDUs, a function of reordering the received RLC PDUs on the basis of an RLC Sequence Number (SN) or a PDCP SN, a function of recording PDCP PDUs lost due to the reordering, a function of reporting statuses of the lost PDCP PDUs to a transmitting side, a function of making a request for retransmitting the lost PDCP PDUs, if there is a lost RLC SDU, a function of sequentially transferring only RLC SDUs before the lost RLC SDU to the higher layer, if a predetermined timer expires even through there is a lost RLC SDU, a function of sequentially transferring all RLC SDUs received before the timer starts to the higher layer, or if a predetermined timer expires even through there is a lost RLC SDU, a function of sequentially transferring all RLC SDUs received up to now to the higher layer.

Further, the NR RLC device may process the RLC PDUs sequentially according to a reception order thereof (according to an arrival order regardless of a serial number or a sequence number) and transfer the RLC PDUs to the PDCP device regardless of sequences thereof (out of sequence delivery). In the case of segments, the NR RLC device may receive segments which are stored in the buffer or will be received in the future, reconfigure the segments to be one RLC PDU, process the RLC PDU, and transmit the same to the PDCP device. The NR RLC layer may not include a concatenation function, and the function may be performed by the NR MAC layer or may be replaced with a multiplexing function of the NR MAC layer.

The non-sequential function (Out-of-sequence delivery) of the NR RLC device is a function of transferring RLC SDUs received from a lower layer directly to a higher layer regardless of sequences of the RLC SDUs and may include, when one original RLC SDU is divided into a plurality of RLC SDUs and then received, a function of reassembling and transmitting the RLC PDUs and a function of storing RLC SNs or PDCP SNs of the received RLC PDUs, reordering the RLC PDUs, and recording lost RLC PDUs.

The NR MACs <NUM> and <NUM> may be connected to a plurality ofNR RLC layer devices configured in one UE and main functions of the NR MAC may include some of the following functions.

<FIG> illustrates a procedure in which the UE in the idle state measures and reselects a cell in the LTE system.

Referring to <FIG>, cell reselection is a procedure in which the UE determines a cell on which the UE camps when QoS with a serving cell becomes lower than QoS with a neighboring cell due to movement of the UE in the idle state.

While handover is determined by a network (MME or source eNB), cell reselection is determined on the a measurement value by the UE. The cell which the UE reselects during movement may be a cell having the same LTE frequency (intra-frequency) as the serving cell, a cell using a different LTE frequency (inter-frequency), or a cell having different RAT (inter-RAT).

The UE in the idle state performs a series of operations while camping on in the serving cell in step <NUM>. First, the UE receives system information (System Information Block (SIB) broadcasted by the eNB of the serving cell in step <NUM>. For reference, an MIB, SIB <NUM>, and SIB <NUM> are system information applied to all UE in common, and SIB <NUM> to SIB <NUM> include information required when the UE in the idle state reselects a cell.

Information related to intra-LTE frequency measurement is transmitted through SIB <NUM> and information related to inter-frequency measurement is transmitted through SIB <NUM>. The system information includes a threshold value used when it is determined whether to measure a neighboring cell signal and a parameter used for calculating ranks of the serving cell and neighboring cells. Further, for intra-frequency measurement, a carrier frequency is the same as that of the current serving cell, so that SIB <NUM> specifies carrier frequency information of neighboring cells required to be measured even through the carrier frequency information is not separately signaled through SIB <NUM>.

The UE in the idle state wakes up every Discontinuous Reception (DRX) period and measures an absolute signal strength (for example, Reference Signal Received Power (RSRP) Qrxlevmeas and relative signal quality (Reference Signal Received Quality (RSRQ) Qqualmeas) of the serving cell in step <NUM>. The UE calculates a reception level (Srxlev) and a reception quality (Squal) of the serving cell on the basis of the measurement values and parameters received from the eNB and compares the values with threshold values to determine whether to perform cell reselection. The reception level (Srxlev) and the reception quality (Squal) of the serving cell are calculated through equations below. <MAT> <MAT>.

Definition of the parameters used herein refers to the 3GPP standard document "<NUM>: User Equipment (UE) procedures in idle mode".

When the signal strength and quality of the serving cell calculated from the measurement values are smaller than threshold values (Srxlev < SIntraScarchP or Squal < SIntraSearchQ), cell reselection is triggered in step <NUM>. If the condition is not satisfied, the UE continuously camps on the corresponding serving cell without cell reselection in step <NUM>. If the condition is satisfied and thus cell reselection is triggered, the UE measures neighboring cells on the basis of the priority in step <NUM>. With respect to inter-frequency inter-RAT cells having a high priority, the UE starts measurement of neighboring cells regardless of quality of the serving cell. Further, with respect to inter-frequency cells having a priority which is the same as or lower than the serving cell, the UE starts measurement of neighboring cells if the signal strength and quality of the serving cell are smaller than threshold values received as system information, that is, if Srxlev < SnonIntraSearchP or Squal < SnonIntraScarchQ, starts measurement neighboring cells.

When measurement of the neighboring cells end in step <NUM>, the UE performs cell reselection based on the priority in step <NUM>. First, when reselecting the inter-frequency inter-RAT cell having a high priority, if the signal quality of the corresponding cell is higher than a threshold value ThreshX, HighQ for a particular time TreselectionRAT (Squal > ThreshX, HighQ), the UE performs reselection of the corresponding cell. Second, for reselection of the inter-frequency cell having a low priority, the UE first determines whether a condition in which the signal quality of the serving cell is lower than a threshold value ThreshServing, LowQ is satisfied (Squal < ThreshServing, LowQ), and if the condition is satisfied and the signal quality of the inter-frequency cell is higher than the threshold value ThreshServing, LowQ for a particular time TreselectionRAT (Squal > ThreshX, LowQ), reselects the corresponding cell. Third, for reselection of intra-frequency/inter-frequency cells having the same priority, the UE acquires a rank of each cell on the basis of measurement values (for example, RSRP) from neighboring cells. The ranks of the serving and the neighboring cells are calculated through equations below. <MAT> <MAT>.

Qmeas , s denotes a measurement value of RSRP of the serving cell, Qmeas , n denotes a measurement value of RSRP of a neighboring cell, QHyst denotes a hysteresis value of the serving cell, Qoffset denotes an offset between the serving cell and the neighboring cell, and Qoffsettemp denotes an offset temporarily applied to the cell. When the ranks of the neighboring cells acquired through the equation are higher than the rank of the serving cell (Rn > Rs), the UE camps on an optimal cell among the neighboring cells.

When the cell reselection is determined in the process, the UE receives system information from the corresponding cell and checks suitability indicating whether a service can be received from a new serving cell in step <NUM>. If a Tracking Area Identity (TAI) is not in a TAI list of the UE, the UE performs a Tracking Area Update (TAU) procedure and, when the corresponding cell is determined as a new cell, performs an operation for the serving cell (acquiring system information, paging monitoring, and measuring serving cell signal).

<FIG> illustrates a channel measurement and report procedure of the UE in the connected state in the LTE system.

Referring to <FIG>, a UE in an idle mode (RRC_IDLE) searches for a suitable cell and camps on the corresponding eNB in step <NUM>, and then accesses the eNB for the reason of generation of data to be transmitted in step <NUM>. In the idle mode, the UE is not connected to the network to save power of the UE, so the UE cannot transmit data. In order to transmit data, the UE is required to switch to a connected mode (RRC CONNECTED). Camping means that the UE receives a paging message in order to determine whether data is received through downlink while staying in the corresponding cell. When the UE successfully perform the procedure of access to the eNB, the UE switches to the connected mode (RRC_CONNECTED) and the UE in the connected mode can transmit and receive data to and from the eNB.

According to movement of the connected-mode UE to the inside or the outside of the cell, the UE may be required to instruct movement for transmission and reception from and to another cell/eNB. To this end, the eNB configures indication of measurement (L3 measurement) for another cell through an RRC message in step <NUM>. The measurement indication may include an object, a condition, and parameters for the measurement result which the UE reports to the eNB. The UE receiving the configuration information transmits an acknowledgement message indicating successful reception of configuration information to the eNB in step <NUM>. For the acknowledgement message, an RRCConnectionReconfigurationComplete message may be used.

The UE may transmit and receive data to and from the eNB in step <NUM> and measure strength of signals of the serving cell and a downlink cell of measurement objects <NUM>, <NUM>, and <NUM> in step <NUM>. In the above step, the UE identifies a measurement result of a cell level and determines a report condition configured by the eNB. The configuration condition may be configured differently according to the intra-frequency or the inter-frequency. Particularly, in the case of inter-frequency channel measurement configuration, carrier frequency information indicating the corresponding frequency is needed. The UE may report the measurement result to the eNB through an RRC message according to the configured measurement value report condition in step <NUM>, and the eNB may perform a handover procedure on the basis of the measurement value received from the UE in step <NUM>.

Definition of inter-frequency/intra-frequency measurement may be differently applied to the next-generation mobile communication system (NR) unlike the conventional LTE system. In NR, Radio Resource Measurement (RRM) is performed on the basis of Synchronization Signal Block (SSB). While a subcarrier spacing (SCS) applied to one frequency is constant in LTE, various subcarrier spacings may be used for the same frequency band in NR. That is, if channel measurement for neighboring cell eNBs is instructed in NR, an SSB in a particular cell is measured and it should be additionally determined whether subcarrier spacing of the SSB is constant in order to make definition of intra-frequency/inter-frequency measurement. Intra-frequency/inter-frequency measurement is defined below.

Definition of the SSB-based measurement may be made on the assumption that the same cell transmits only one SSB. That is, intra-frequency/inter-frequency measurement may be determined on the basis of the central frequency and SCS of neighboring cells. Particularly, for intra-frequency measurement of the idle UE, system information may include measurement configuration for neighboring intra-frequency cells (SIB <NUM> in LTE) and measurement configuration for neighboring inter-frequency cells (SIB <NUM> in LTE) like in LTE. The number and identification of the corresponding system information may be equally used in NR. SIB <NUM> and SIB <NUM> may be transmitted through Other System Information (OSI). System information in NR may be largely divided into two pieces of information such as Master System Information (MSI) which all UEs requires in common and OSI which may be provided according to an on-demand request from the UE.

<FIG> illustrates the overall operation of the UE according to an embodiment of the disclosure.

Referring to <FIG>, the UE in the idle state performs a series of operations while camping on the serving cell in step <NUM>. First, the UE receives system information (System Information Block (SIB) broadcasted by the eNB of the serving cell in step <NUM>. For reference, an MIB, SIB1, and SIB <NUM> are system information applied to all UEs in common, and are defined as MSI in NR and broadcasted to all UE in common by the eNB. On the other hand, SIB3 to SIB <NUM> include information required when the UE in the idle state reselects a cell, and may be defined as OSI in NR and broadcasted by the eNB according to a request from the UE or directly transmitted through RRC signaling. Particularly, information related to intra-frequency measurement in NR is transmitted through SIB <NUM> and information related to inter-frequency measurement is transmitted through SIB <NUM>.

SIB <NUM> may include a threshold value (threshold <NUM>) used for determining whether to perform neighboring cell signal measurement and parameters (cell identifier or offsets of respective cells) which can be used for calculating ranks by providing priorities to neighboring intra-frequency cells according to each cell list.

On the other hand, SIB <NUM> includes carrier frequency information of neighboring cells required to be measured and information on subcarrier intervals. The carrier frequency information may be omitted in SIB <NUM>. If SIB <NUM> does not include the carrier frequency information, it is determined that the carrier frequency information is the same as carrier frequency information of the serving cell. That is, measurement of the inter-frequency having the same central frequency as a central frequency of SSB of the current serving cell and having a different subcarrier interval is performed. Multi-set configuration for a plurality of cells can be performed in SIB <NUM>, and each set has a configuration value below.

SIB <NUM> may also include a threshold value (threshold <NUM>) used for determining whether to perform neighboring cell signal measurement and parameters (cell identifier or offsets of respective cells) which can be used for calculating ranks by providing priorities to neighboring inter-frequency cells according to each cell list.

In step <NUM>, the UE measures signal strength (for example, RSRP) of the reference SSB for the serving cell and compares the signal strength of the reference SSB of the corresponding serving cell with threshold values received through SIB <NUM> and SIB <NUM> to determine the neighboring cell measurement operation. If the RSRP value of the SSB of the serving cell is smaller than threshold <NUM> but larger than threshold <NUM> (threshold <NUM> <= RSRP of serving cell < threshold <NUM>), the UE measures the reference SSB for neighboring cells indicated by SIB <NUM> in step <NUM> (perform intra-frequency measurement). The SSB central frequency of the neighboring cell to be measured is the same as information on the SSB central frequency of the current serving cell. Information on the SSB subcarrier interval of the neighboring cell to be measured is the same as information of the SSB subcarrier interval of the current serving cell. That is, the corresponding central frequency information and the subcarrier information may be omitted in SIB <NUM>.

If the RSRP value of the SSB of the serving cell is smaller than threshold <NUM> (RSRP of serving cell < threshold <NUM>), the UE measures the reference SSB for neighboring cells indicated by SIB <NUM> and the SSB for different RAT indicated by another SIB in step <NUM> (perform inter-frequency measurement).

The SSB central frequency of the neighboring cell to be measured is specified as ARFCN in the SIB, and SSB subcarrier interval information of the neighboring cell to be measured is specified in SIB <NUM>. The ARFCN information or the subcarrier interval information may be omitted in SIB <NUM>. If the ARFCN information or the subcarrier interval information is omitted, it may mean that the ARFCN information or the subcarrier interval information is the same as the SSB central frequency information or the subcarrier interval information of the serving cell. However, the central frequency information of the reference SSB and the subcarrier interval information cannot be simultaneously omitted. This is because definition of inter-frequency measurement means the case in which only one of the central frequency information and the subcarrier interval information in the SSB of the neighboring cell is different. In the case of measurement for different RAT, SCS information and PCI range are omitted, and only ARFCN information may be included.

Referring to <FIG>, as movement of the connected-mode UE to the inside or the outside of the cell, the UE may be required to instruct movement form transmission and reception from and to another cell/eNB. To this end, the eNB configures indication of measurement (L3 measurement) for another cell through an RRC message in step <NUM>. The measurement indication may include an object, a condition, and parameters for the measurement result which the UE reports to the eNB. Particularly, the measurement configuration value may include the following configuration value according to an object to be measured (Radio Access Technology (RAT)).

In step <NUM>, the UE measures a signal strength of a serving cell and a downlink cell for a measurement object configured in step <NUM>. In step <NUM>, the UE identifies a measurement result of a cell level and determines a report condition configured by the eNB. The configuration condition may be configured differently according to the intra-frequency and the inter-frequency. Particularly, in the case of inter-frequency channel measurement configuration, carrier frequency information and subcarrier interval information indicating the corresponding frequency are needed. The UE may report the measurement result to the eNB through an RRC message according to the configured measurement value report condition in step <NUM>, and the eNB may perform a handover procedure on the basis of the measurement value received from the UE in step <NUM>. As described above, the measurement object may be not only NR but also different RAT (E-UTRA).

A table below shows summary of the content of the specification.

According to the disclosure, when the NR core network (<NUM> or NR NG core) can be connected to both the eNB using LTE RAT and the eNB using the NR RAT in the next-generation mobile communication system, the UE should be simultaneously connected to the <NUM> CN (NR core network) and the EPC (LTE core network). The eNB using the LTE RAT can be connected to the LTE core network and the eNB using the NR RAT can be connected to the NR core network.

The UE should be able to use an Evolved Packet Core (EPC) and a <NUM> Core Network (CN) Non Access Stratum (NAS). This is because the UE may be connected to both the eNB using the LTE RAT and the eNB using the NR RAT and each eNB may be connected to both the LTE core network and the NR (<NUM>) core network as described above. For example, when the UE which can be connected to the <NUM> CN is connected to the network, the UE can always select the <NUM> CN NAS. However, the <NUM> CN may not support a particular function (for example, an MBMS) supported by the EPC of LTE. On the other hand, when the UE which can be connected to the EPC is connected to the network, the UE may always select the EPC. However, the EPC may not support QoS or a slice (RAN slice or network slice) service supported by the <NUM> CN. Further, the UE registered in the EPC and the UE registered in the <NUM> CN may receive different services even though the UEs are the same UE. Accordingly, the UE registered in the <NUM> CN may be required to reestablish the EPC as necessary. The disclosure proposes a method by which, when the UE performs handover from a source eNB to a target eNB, the source eNB selects a cell of the target eNB connected to a particular core network (EPC or <NUM> CN) and hands over the UE.

<FIG> illustrates a method by which the UE is connected to the EPC (LTE core network) and the <NUM> CN (<NUM> core network or NR core network) in the next-generation mobile communication system according to the disclosure.

As illustrated in <FIG>, in the next-generation mobile communication system, a <NUM> core network <NUM> can be connected to an eNB <NUM> using LTE RAT and a gNB <NUM> using NR RAT, and UEs <NUM> and <NUM>, which can be connected to the <NUM> CN should be able to be connected to each of the <NUM> CN <NUM> and the EPC <NUM>. That is, the UEs should be able to be connected to both the EPC and the Non Access Stratum (NAS) of the <NUM> CN.

When the UE which can be connected to the <NUM> CN is connected to the network, the UE can always select the <NUM> CN NAS. However, the <NUM> CN may not support a particular function (for example, MBMS) supported by the EPC of LTE (inversely, the EPC of LTE may not support a function such as a slice (network slice or RAN slice) provided by the <NUM> CN). The slice function may be a service for specializing any service and providing a dedicated network, radio access transmission resources, or a dedicated data link to satisfy QoS or requirements suitable for the service, and a plurality of slices may be configured in a core network (Non-Access Stratum (NAS) or radio access technology (Access Stratum (AS)). Further, the UE registered in the EPC and the UE registered in the <NUM> CN may receive different services even though the UEs are the same UE. Accordingly, even the UE registered in the <NUM> CN may be required to reestablish the EPC as necessary, and inversely even the UE registered in the EPC may be required to reestablish the <NUM> CN as necessary. Further, an LTE UE <NUM> having only EPC-connectable UE capability may receive a service only through the connection to the EPC.

For the connection of the eNB to both the <NUM> CN and the EPC, a new <NUM> eNB (gNB) should be used or the eNB, which is the conventional LTE eNB (for example, eLTE eNB or enhanced LTE eNB), should be upgraded for the connection to the <NUM> CN.

The CN according to the disclosure may include a CN supporting <NUM> and a hybrid CN supporting different RATs such as <NUM> and LTE.

When the UE can access both the EPC and the <NUM> CN when performing handover in the state illustrated in <FIG> according to the disclosure, a process in which the UE or the source eNB selects the EPC or the <NUM> CN, a process in which the UE registered in the <NUM> CN reestablishes the EPC as necessary, or a process in which the UE registered in the EPC establishes the <NUM> CN as necessary, that is, a core network selection/reselection process is specified. A detailed operation will be described in more detail in the following embodiment.

<FIG> illustrates a first embodiment of a method of selecting a PLMN by the UE proposed by the disclosure, in which the UE selects a preferred CN type or slice type and indicates the result thereof so as to establish a core network (<NUM> CN or EPC).

The method of searching for and determining a PLMN according to the first embodiment of disclosure is automatically performed by an Access Stratum (AS) or triggered through and manually performed by the NAS. In general, the UE operates on a Home PLMN (HPLMN) or an Equivalent Home PLMN (EHPLMN), but a VPLMN may be selected. Basically, the AS layer reports all pieces of PLMN-related information including a list of connectable PLMNs to the NAS and performs an additional PLMN selection operation on the basis of priority information.

For PLMN selection, the UE scans E-UTRA bands through all RF channels suitable for capability, searches for valid PLMNs, reads system information in a cell having the highest signal strength, and performs a PLMN selection process according to a PLMN list provided by the cell.

A basic PLMN selection process in a manual mode is described. When the UE is turned on in step <NUM>, the UE identifies whether there is a Registered PLMN (RPLMN) around the UE in step <NUM>. If the turned-on UE does not have a Subscriber Identity Module (SIM) or the SIM is not valid, the UE maintains the state before the SIM becomes valid in step <NUM>. When the UE found the RPLMN and selects the PLMN in step <NUM>, the UE attempts access to the corresponding PLMN in step <NUM>. When the UE completely succeeds in registration and connection, the UE indicates the connected PLMN in step <NUM> and performs a service in the corresponding PLMN in step <NUM>. However, when the registration and connection are failed in step <NUM>, the UE cannot be connected to the corresponding PLMN in step <NUM> and attempts access to the PLMN, which is selected on the basis of the priority in step <NUM>, in step <NUM>.

The PLMN selection process based on the priority follows the following priority.

<FIG> illustrates a method on efficient core network (EPC or <NUM> CN) selection/reselection method of the initial accessed UE according to a first embodiment proposed by the disclosure.

Referring to <FIG>, a UE <NUM> has UE capability to access both the <NUM> CN and the EPC. The UE <NUM> performs an initial cell search, camps on the cell, and receives system information (for example, SIB <NUM>) in step <NUM>, and identifies whether the corresponding cell is an HPLMN. The cell <NUM> may be a <NUM> eNB (gNB), an LTE eNB, or an upgraded eLTE eNB having capability to access the <NUM> CN. The system information for example, SIB <NUM>) includes Radio Access Technology (RAT) information (for example, <NUM> RAT or LTE RAT) indicating which RAT is used, a PLMN list (for example, a PLMN list corresponding to the RATs), and a connectable CN type (for example, a CN type which can be applied to each PLMN, that is, a <NUM> CN or an EPC). The system information may include a slice type (for example, a slice type provided by the CN type) information. As the RAT information, the PLMN list, and the CN type information are provided through the system information, RAT information, the PLMN list, the CN type, or the slice information, which the initial accessed UE can access, may be detected and preferred RAT information, PLMN list, CN type, or slice information suitable for the service which the UE desires to currently receive may be selected. In the next-generation mobile communication system, even the UE registered in the <NUM> CN may be required to reestablish the EPC as necessary (or inversely, the UE registered in the EPC may be required to reestablish the <NUM> CN as necessary), so that it is possible to provide a CN type or a slice type according to each radio access and each PLMN for a core network (CN) reestablishment function. When the system information is received, the UE selects a PLMN, camps on the selected PLMN, and receives the remaining system information. The PLMN determination method may be determined on the basis of a first embodiment of a PLMN selection method of the UE illustrated in <FIG>.

The PLMN selection method of the UE may be determined on the basis of a second embodiment of the PLMN selection method of the UE proposed below rather than the first embodiment of the PLMN selection method of UE according to the disclosure.

The method of searching for and determining the PLMN according to the second embodiment of the PLMN selection method of the UE according to the disclosure may automatically performed by an Access Stratum (AS) or triggered through and manually performed by the NAS. In general, the UE operates on a Home PLMN (HPLMN) or an Equivalent Home PLMN (EHPLMN), but a VPLMN may be selected. Basically, the AS layer reports all pieces of PLMN-related information including RAT information (for example, <NUM> RAT or LTE RAT) indicating which RAT is used, a list of connectable PLMNs (for example, a list of PLMNs corresponding to the RATs), connectable CN type (for example, CN type which can be applied to each PLMN, that is, the <NUM> CN or the EPC) or slice type (for example, slice type provided by the CN type information to the NAS and performs an additional PLMN selection operation on the basis of priority information. That is, for the PLMN selection, the UE scans E-UTRA bands through all RF channels suitable for capability, searches for valid PLMNs, reads system information in a cell having the highest signal strength, and performs a PLMN selection process according to a PLMN list provided by the cell.

The UE may perform a procedure similar to the fist embodiment of the PLMN selection method of the UE. However, the UE may select the PLMN on the basis of the following priority and attempt access.

In the disclosure, the PLMN selection process based on the priority according to the second embodiment of the PLMN selection method of the UE follows the following priority.

In the disclosure, the PLMN selection process based on the priority according to a third embodiment of the PLMN selection method of the UE follows the following priority.

According to the second embodiment and the third embodiment of the PLMN selection method of the UE, the UE may select preferred RAT, PLMN list, CN type, and slice type suitable for the service which the UE desires to receive in consideration of information such as RAT, PLMN list, CN type, and slice type. For example, if there is a preferred CN type, the UE may select a PLMN and RAT supporting the preferred CN type, and if there is a preferred slice type, may select a CN type, a PLMN, and RAT supporting the preferred slice type.

If data to be transmitted is generated by a UE of which connection is not currently configured (hereinafter, referred to as an idle-mode UE), the UE performs an RRC connection establishment process with the eNB. The UE establishes backward transmission synchronization with the eNB through a random access process and transmits an RRCConnectionRequest message to the eNB in step <NUM>. The message includes a reason (establishmentCause) to establish the connection with an identifier of the UE.

The eNB transmits an RRCConnectionSetup message to allow the UE to establish the RRC connection in step <NUM>. The message includes RRC connection configuration information. The RRC connection is also referred to as a Signaling Radio Bearer (SRB), and is used for transmitting and receiving an RRC message which is a control message between the UE and the eNB.

The UE establishing the RRC connection transmits an RRCConnectionSetupComplete message to the eNB in S2220. The message may include a control message corresponding to a service request by which the UE makes a request for establishing a bearer for a predetermined service to the MME. The UE may insert the PLMN which the UE prefers, CN type information, and slice information into the message.

In the disclosure, the service request control message may include both an indicator indicating the selected PLMN and CN type information or slice type information in the corresponding PLMN. The eNB transmits a service request message <NUM> or a CN re-direction request control message <NUM> including the PLMN indicator or CN type information, which is included in the RRCConnectionSetupComplete message to the currently connected MME (in the embodiment, the connection to the <NUM> CN is assumed. In the case of the connection to the EPC, all the following processes may be performed on the basis of the EPC rather than the <NUM> CN).

The CN re-direction request control message <NUM> may be transmitted while including the same content as that of the service request message <NUM>, and the CN receiving the control message selects a proper CN according to a predetermined method in step <NUM>. The selection may be determined according to a predetermined priority or may be determined according to a UE type and establishmentCause, that is, a service type.

The initially configured CN determines whether to maintain the current CN connection or receive a change to another CN, inserts the result thereof into the CN re-direction control message, and transmits the CN re-direction control message to the eNB in step <NUM>. In such a process, the eNB may identify preference of the UE and determine configuration of/connection to the <NUM> CN or the EPC or reconfiguration of/reconnection to the <NUM> CN or the EPC. Alternatively, in the process, the MME may identify preference of the UE and determine configuration of/connection to the <NUM> CN or the EPC or reconfiguration of/reconnection to the <NUM> CN or the EPC. Alternatively, in the process, the core network may identify preference of the UE and determine configuration of/connection to the <NUM> CN or the EPC or reconfiguration of/reconnection to the <NUM> CN or the EPC.

The CN re-direction control message <NUM> may be transmitted while including only information on the determined CN or being included in an initial context setup message <NUM>, or may be transmitted while including information which should be included in the initial context setup message <NUM>. If the CN should be changed, the eNB transmits, to the CN, which should be changed (the EPC in this example), a control message <NUM> corresponding to a service request for making a request for establishing a bearer for a predetermined service of the UE to the MME, and the MME determines whether to provide the service requested by the UE. If it is determined to provide the service requested by the UE on the basis of the determination result, the MME transmits an initial context setup request message <NUM> to the eNB. The message includes Quality of Service (QoS) information to be applied to Data Radio Bearer (DRB) configuration and security-related information to be applied to the DRB (for example, a security key and a security algorithm).

The eNB exchanges a SecurityModeCommand message <NUM> and a SecurityModeComplete message <NUM> with the UE in order to set security. When security has been completely set, the eNB transmits an RRCConnectionReconfiguration message to the UE in step <NUM>. The message includes configuration information of the DRB for processing user data, and the UE configures the DRB by applying the information and transmits an RRCConnectionReconfigurationComplete message to the eNB in S2275.

The eNB completely establishing the DRB with the UE transmits an initial context setup complete message to the MME in step <NUM>, and the MME receiving the initial context setup complete message exchanges an S1 bearer setup message and an S1 bearer setup response message in order to establish an S1 bearer with the S-GW. The S1 bearer is a connection for data transmission established between the S-GW and the eNB and corresponds to the DRB in one-to-one correspondence in step <NUM> or <NUM>. When the process is completed, the UE transmits and receives data to and from the eNB through the S-GW in step <NUM>. As described above, the general data transmission process largely consists of three steps such as RRC connection setup, security setup, and DRB setup. Further, the eNB may transmit an RRCConnectionReconfiguration message in order to provide new configuration to the UE or add or change the configuration for a predetermined reason in step <NUM>.

<FIG> and <FIG> illustrate a second embodiment of the PLMN selection method of the UE proposed by the disclosure. The lower part of <FIG> is connected to the upper part of <FIG>. Hereinafter, <FIG> and <FIG> are collectively referred to as <FIG>.

<FIG> corresponds to the second embodiment proposed by the disclosure and illustrates a method by which the UE selects a preferred CN type or slice type and indicates the result thereof so as to establish a core network (<NUM> CN or EPC).

In <FIG>, a UE <NUM> is a UE having capability for the connection to NR, and receives SIB <NUM> for an initial cell search in step <NUM> and identifies whether the corresponding cell is an HPLMN. The cell <NUM> may be an NR eNB (gNB), an LTE eNB, or an upgraded eLTE eNB having capability to access the <NUM> CN.

The system information (for example, SIB <NUM>) may include a PLMN list and a CN type or a slice type which can be applied to each PLMN. That is, the system information includes Radio Access Technology (RAT) information (for example, RAT may be <NUM> RAT or LTE RAT, but is not limited thereto) indicating which RAT is used, a PLMN list (for example, a PLMN list corresponding to the RATs), and a connectable CN type (for example, a CN type which can be applied to each PLMN, that is, a <NUM> CN or an EPC). Further, the system information may include slice type (for example, slice type provided by the CN type) information.

As the RAT information, the PLMN list, and the CN type information are provided through the system information, RAT information, the PLMN list, the CN type, or the slice information, which the initial accessed UE can access, may be detected and preferred RAT information, PLMN list, CN type, or slice information suitable for the service which the UE desires to currently receive may be selected.

In the next-generation mobile communication system, even the UE registered in the <NUM> CN may be required to reestablish the EPC as necessary (or inversely, the UE registered in the EPC may be required to reestablish the <NUM> CN as necessary), so that it is possible to provide a CN type or a slice type according to each RAT and each PLMN for a core network (CN) reestablishment function.

When the UE receives the system information in step <NUM>, the UE selects a PLMN, camps on the selected PLMN, and receives the remaining system information. The PLMN determination method may be determined on the basis of a priority according to the first embodiment, the second embodiment, or the third embodiment of the PLMN selection method of the UE proposed above. Thereafter, the UE may determine a CN value in the corresponding PLMN according to the CN priority recorded in the SIM or CN priority information according to each PLMN. Alternatively, the priority information may be received through a NAS message and a value thereof may be managed as a black list by the UE. That is, the UE may determine and store the priority of the PLMN and the CN through previous access and reception of the NAS message. Further, in the step, the UE may simultaneously select the PLMN and the CN. A condition for the selection may be variously implemented.

If data to be transmitted is generated by a UE of which connection is not currently configured (hereinafter, referred to as an idle-mode UE), the UE performs an RRC connection establishment process with the eNB. The UE establishes backward transmission synchronization with the eNB through a random access process and transmits an RRCConnectionRequest message to the eNB in step <NUM>. The message includes a reason (establishmentCause) to establish the connection with an identifier of the UE. The eNB transmits an RRCConnectionSetup message to allow the UE to establish the RRC connection in step <NUM>. The message includes RRC connection configuration information. The RRC connection is also referred to as a Signaling Radio Bearer (SRB), and is used for transmitting and receiving an RRC message which is a control message between the UE and the eNB.

The UE establishing the RRC connection transmits an RRCConnectionSetupComplete message to the eNB in S2320. The message may include a control message corresponding to a service request by which the UE makes a request for establishing a bearer for a predetermined service to the MME. In the disclosure, the service request control message includes indicator indicating selected radio access or PLMN and CN type or slice type.

The eNB transmits a service request message <NUM> or <NUM> included in the RRCConnectionSetupComplete message to the currently connected MME (in the embodiment, the connection to the <NUM> CN is assumed. In the case of the connection to the EPC, all the following processes may be performed on the basis of the EPC rather than the <NUM> CN). The service request control message <NUM> or <NUM> may be selected according to a preferred CN type or a preferred slice type determined by the UE and transmitted to the corresponding CN (<NUM> CN or EPC). The service request control message <NUM> or <NUM> includes a request from the UE for establishing a bearer for a predetermined service to the MME, and the MME determines whether to provide the service requested by the UE.

If it is determined to provide the service requested by the UE on the basis of the determination result, the MME transmits an initial context setup request message <NUM> or <NUM> to the eNB. The message includes Quality of Service (QoS) information to be applied to Data Radio Bearer (DRB) configuration and security-related information to be applied to the DRB (for example, a security key and a security algorithm).

The eNB exchanges a SecurityModeCommand message <NUM> and a SecurityModeComplete message <NUM> with the UE in order to set security. When security has been completely set, the eNB transmits an RRC connection reconfiguration message to the UE in step <NUM>. The message includes configuration information of the DRB for processing user data, and the UE configures the DRB by applying the information and transmits an RRC connection reconfiguration complete message to the eNB in S2365.

The eNB completely establishing the DRB with the UE transmits an initial context setup complete message to the MME in step <NUM>, and the MME receiving the initial context setup complete message exchanges an S1 bearer setup message and an S1 bearer setup response message in order to establish an S1 bearer with the S-GW. The S1 bearer is a connection for data transmission established between the S-GW and the eNB and corresponds to the DRB in one-to-one correspondence in step <NUM> or <NUM>.

When the process is completed, the UE transmits and receives data to and from the eNB through the S-GW in step <NUM>. As described above, the general data transmission process largely consists of three steps such as RRC connection setup, security setup, and DRB setup. For a particular reason, the current CN may make a request for a CN change to the UE. At this time, a condition for the CN change is that the currently connected CN cannot support a particular service requested by the UE or to provide a better service. For the reason, the MME may transmit a UE CN re-selection control message <NUM> to the UE or transmit a UE CN re-selection control message <NUM> to the eNB, and then the eNB may transmit a CN re-selection priority through an RRC connection reconfiguration message in step <NUM>.

The UE receiving the NAS control message or the RRC control message performs a procedure for reselecting the CN again on the basis of the CN re-selection priority in step <NUM>. That is, instead of the RRC messages of steps <NUM> to <NUM>, a new RRC message or an RRC connection reconfiguration complete message which is a response message to the previously received RRC connection reconfiguration message may include CN re-selection information and then may be transmitted to the eNB in step <NUM>. Thereafter, the procedure for CN reselection may be performed, which may include all procedures for CN connection setup and data transmission/reception and may be mapped to steps <NUM> to <NUM> in step <NUM>.

The disclosure proposes the procedure for allowing the initial accessed UE (RRC idle-mode UE) to access the eNB or the cell connected to a CN type (<NUM> CN or EPC) preferred to supported by the UE when the UE accesses the network.

Thereafter, the disclosure proposes a procedure in which, when the RRC-connected mode UE performs handover from a source eNB (source cell) to a target eNB (target cell) in the network, the UE selects a target cell connected to a CN type (<NUM> CN or EPC) preferred by the UE, suitable for a service, or preferred by the source eNB to perform the handover.

<FIG> illustrates a first embodiment in which the RRC-connected mode UE performs handover from a source eNB to a target eNB according to the disclosure.

Referring to <FIG>, when a periodic or a particular event is satisfied in a current source eNB <NUM>, a connected-mode UE <NUM> performs cell measurement on the conventionally accessed cell or other cells and reports cell measurement information (measurement report) including identifier of the measured cells and the measurement result (for example, absolute or relative signal strength) to the source eNB in step <NUM>.

The source eNB determines whether to hand over the UE to an adjacent cell on the basis of the measurement information. The handover is technology for switching a source eNB, which provides a service to the connected-mode UE, to another eNB.

When the source eNB determines to perform the handover, the source eNB transmits a handover (HO) request message to a new eNB, that is, a target eNB to provide a service to the UE and make a request for the handover in step <NUM>.

The handover request message may include a target cell identifier indicating a cell of the target eNB to which the UE is handed over and asks the target eNB about if the handover is possible while making a request for preparing the handover to the target cell.

When the target eNB accepts the handover request, the target eNB transmits a handover request acknowledgement message (HO request ack message) to the source eNB <NUM>. If the target eNB cannot prepare the handover to the requested target cell in the handover request message for a predetermined reason (for example, lack of transmission resources), the target eNB may transmit a handover preparation failure message to the source eNB to reject the handover request. The source eNB receiving the handover request acknowledgement (ack) message transmits a HO command message to the UE in step <NUM>.

The HO command message is transmitted from the source eNB to the UE through an RRC connection reconfiguration message and may indicate a target cell to which the UE is handed over through a target cell identifier in step <NUM>. Upon receiving the message, the UE stops data transmission and reception to and from the source eNB and starts a timer T304. When the handover of the UE to the target eNB is not successful for a predetermined time, T304 returns the UE to the original configuration and switches the UE to the RRC idle state.

The source eNB transmits a PDCP sequence number (or count value) status (Sequence Number (SN) status) of uplink/downlink data, and if there is downlink data, transmits the downlink data to the target eNB in steps <NUM> and <NUM>. The UE attempts random access to the target cell instructed by the source eNB in step <NUM>.

The random access is to inform the target cell that the UE moves through the handover and also to synchronize uplink. For the random access, the UE transmits a preamble corresponding to a preamble ID received from the source eNB or a randomly selected preamble ID to the target cell.

After transmitting the preamble, the UE monitors whether a Random Access Response (RAR) is transmitted from the target cell after a predetermined number of sub frames. A time window during which monitoring is performed is referred to as a Random Access Response (RAR) window. When the RAR is received for a particular time in step <NUM>, the UE transmits a handover (HO) complete message, which is an RCConnectionReconfigurationComplete message, to the target eNB in step <NUM>.

As described above, upon successfully receiving the RAR from the target eNB, the UE terminates the timer T304 in step <NUM>. When the UE successfully receives the RAR before the timer expires, the UE may perform fallback and perform the random access procedure again after a predetermined time from the RRC idle mode.

In order to switch a path of bearers established in the source eNB, the target eNB makes a request for switching the path in steps <NUM> and <NUM>, and notifies the source eNB of deletion of UE context in step <NUM>. Accordingly, the UE attempts data reception from the target eNB at a RAR window start time point, and after RAR reception, starts data transmission to the target eNB while transmitting an RRCConnectionReconfigurationComplete message.

<FIG> illustrates a second embodiment of the disclosure in which, when the RRC-connected mode UE performs handover from a source eNB (source cell) to a target eNB (target cell) in the network, the UE selects a target cell connected to a CN type (<NUM> CN or EPC) preferred by the UE, suitable for a service of the UE, or preferred by the source eNB to perform the handover.

Referring to <FIG>, when a periodic or a particular event is satisfied in a current source eNB <NUM>, a connected-mode UE <NUM> may perform cell measurement on the conventionally accessed cell or other cells, read system information of the corresponding cell, and identify a CN type (<NUM> CN or EPC) to which the corresponding cell is connected in step <NUM>.

System information of each cell indicates a CN type (<NUM> CN or EPC) to which the current cell is connected. The UE reports cell measurement information (measurement report) including an identifier of the cell of which the CN type is identified in the system information and the measurement result (for example, absolute or relative signal strength) to the source eNB in step <NUM>.

Through the cell measurement report, the UE may provide a cell measurement report only on cells connected (corresponding) to a CN type preferred by the UE. The source eNB determines whether to hand over the UE to an adjacent cell on the basis of the measurement information. The handover is technology for switching a source eNB, which provides a service to the connected-mode UE, to another eNB. When the source eNB determines to perform the handover, the source eNB transmits a handover (HO) request message to a new eNB, that is, a target eNB <NUM> to provide a service to the UE and make a request for the handover in step <NUM>.

When indicating the target cell in the message, the source eNB reflects the CN type preferred by the UE in initial access of the UE as illustrated in <FIG> and <NUM>, and may identify cells and CN types reported by the UE in step <NUM> and select and indicate a target cell suitable for the preferred CN type.

When the target eNB accepts the handover request, the target eNB transmits a handover request acknowledgement message (HO request Ack message) to the source eNB <NUM>. If the target eNB cannot prepare the handover to the requested target cell in the handover request message for a predetermined reason (for example, lack of transmission resources), the target eNB may transmit a handover preparation failure message to the source eNB to reject the handover request.

The source eNB receiving the handover request acknowledgement (ack) message transmits a HO command message to the UE in step <NUM>. The HO command message is transmitted from the source eNB to the UE through an RRC connection reconfiguration message and may indicate a target cell to which the UE is handed over through a target cell identifier in step <NUM>.

Upon receiving the message, the UE stops data transmission and reception to and from the source eNB and starts a timer T304. When the handover of the UE to the target eNB is not successful for a predetermined time, T304 returns the UE to the original configuration and switches the UE to the RRC idle state. The source eNB transmits a PDCP sequence number (or count value) status (Sequence Number (SN) status) of uplink/downlink data, and if there is downlink data, transmits the downlink data to the target eNB in steps <NUM> and <NUM>.

The UE attempts random access to the target cell instructed by the source eNB in step <NUM>. The random access is to inform the target cell that the UE moves through the handover and also to synchronize uplink. For the random access, the UE transmits a preamble corresponding to a preamble ID received from the source eNB or a randomly selected preamble ID to the target cell.

After transmitting the preamble, the UE monitors whether a Random Access Response (RAR) is transmitted from the target cell after a predetermined number of subframes. A time window during which monitoring is performed is referred to as a Random Access Response (RAR) window. When the RAR is received for a particular time in step <NUM>, the UE transmits a handover (HO) complete message, which is an RCConnectionReconfigurationComplete message, to the target eNB in step <NUM>. As described above, upon successfully receiving the RAR from the target eNB, the UE terminates the timer T304 in step <NUM>.

When the UE successfully receives the RAR before the timer expires, the UE may perform fallback and perform the random access procedure again after a predetermined time from the RRC idle mode. In order to switch a path of bearers established in the source eNB, the target eNB makes a request for switching the path in steps <NUM> and <NUM>, and notifies the source eNB of deletion of UE context in step <NUM>. Accordingly, the UE attempts data reception from the target eNB at a RAR window start time point, and after RAR reception, starts data transmission to the target eNB while transmitting an RRCConnectionReconfigurationComplete message.

<FIG> illustrates a third embodiment of the disclosure in which, when the RRC-connected mode UE performs handover from a source eNB (source cell) to a target eNB (target cell) in the network, the UE selects a target cell connected to a CN type (<NUM> CN or EPC) preferred by the UE, suitable for a service of the UE, or preferred by the source eNB to perform the handover.

Referring to <FIG>, when a source eNB <NUM> and a target eNB <NUM> configures and establishes an X2 or an Xn interface, which is a connection interface therebetween, the source eNB <NUM> and the target eNB <NUM> may share information on cells supported by each eNB (cell identifiers, connected CN types, time/frequency information of cells) in step <NUM>.

When a periodic or a particular event is satisfied in the current source eNB <NUM>, a connected-mode UE <NUM> performs cell measurement on the conventionally accessed cell or other cells and reports cell measurement information (measurement report) including identifier of the measured cells and the measurement result (for example, absolute or relative signal strength) to the source eNB in step <NUM>.

The source eNB determines whether to hand over the UE to an adjacent cell on the basis of the measurement information. The handover is technology for switching a source eNB, which provides a service to the connected-mode UE, to another eNB. When the source eNB determines to perform the handover, the source eNB transmits a handover (HO) request message to a new eNB, that is, a target eNB to provide a service to the UE and make a request for the handover in step <NUM>.

The handover request message may include a target cell identifier indicating a cell of the target eNB to which the UE is handed over and asks the target eNB about if the handover is possible while making a request for preparing the handover to the target cell. The source eNB has already known information on cells supported by the target eNB (cell identifiers, connected CN types, and time/frequency information of cells) by sharing the information in step <NUM>.

In the disclosure, since the source eNB can be aware of the CN type preferred by the UE like in <NUM> or <NUM>, the source eNB may reflect the CN type to select and indicate a target cell connected (corresponding) to the preferred CN type (CN type preferred by the UE or suitable for the service) in the handover request message and ask the target eNB if the handover is possible.

When the target eNB accepts the handover request, the target eNB transmits a handover request acknowledgement message (HO request Ack message) to the source eNB <NUM>. If the target eNB cannot prepare the handover to the requested target cell in the handover request message for a predetermined reason (for example, lack of transmission resources), the target eNB may transmit a handover preparation failure message to the source eNB to reject the handover request. The source eNB receiving the handover request acknowledgement (ack) message transmits a HO command message to the UE in step <NUM>.

The source eNB transmits a PDCP sequence number (or count value) status (Sequence Number (SN) status) of uplink/downlink data, and if there is downlink data, transmits the downlink data to the target eNB in steps <NUM> and <NUM>. The UE attempts random access to the target cell instructed by the source eNB in step <NUM>. The random access is to inform the target cell that the UE moves through the handover and also to synchronize uplink. For the random access, the UE transmits a preamble corresponding to a preamble ID received from the source eNB or a randomly selected preamble ID to the target cell.

After transmitting the preamble, the UE monitors whether a Random Access Response (RAR) is transmitted from the target cell after a predetermined number of subframes. A time window during which monitoring is performed is referred to as a Random Access Response (RAR) window. When the RAR is received for a particular time in step <NUM>, the UE transmits a handover (HO) complete message, which is an RCConnectionReconfigurationComplete message, to the target eNB in step <NUM>.

In order to switch a path of bearers established in the source eNB, the target eNB makes a request for switching the path in steps <NUM> and <NUM>, and notifies the source eNB of deletion of UE context in step <NUM>. Accordingly, the UE attempts data reception from the UE at a RAR window start time point, and after RAR reception, starts data transmission to the target eNB while transmitting an RRCConnectionReconfigurationComplete message.

<FIG> illustrates a fourth embodiment of the disclosure in which, when the RRC-connected mode UE performs handover from a source eNB (source cell) to a target eNB (target cell) in the network, the UE selects a target cell connected to a CN type (<NUM> CN or EPC) preferred by the UE, suitable for a service of the UE, or preferred by the source eNB to perform the handover.

The handover request message may include a target cell identifier indicating a cell of the target eNB to which the UE is handed over and asks the target eNB about if the handover is possible while making a request for preparing the handover to the target cell. In the disclosure, since the source eNB can be aware of the CN type preferred by the UE like in <NUM> or <NUM>, the source eNB may reflect the CN type and transmit the handover request message including the preferred CN type (CN type preferred by the UE or suitable for the service). Further, the source eNB may ask the target eNB if the handover to the selected target cell is possible.

Uplink receiving the handover request message, the target eNB may identify whether the handover to the target cell instructed by the message is possible, identify the CN type instructed by the message, and transmit a handover response message including target cells to which the UE can be handed over in step <NUM>. That is, the target eNB may transmit the handover response message including information on whether the handover to the target cell selected by the source eNB is possible and cells to which the UE can be handed over among cells connected to the CN type preferred by the source eNB (or UE).

Upon receiving the handover response message, the source eNB may select a target cell suitable for the preferred CN type from among the target cells and indicate the selected target cell to the UE through a handover command message. If the target eNB cannot prepare the handover to the requested target cell in the handover request message for a predetermined reason (for example, lack of transmission resources), the target eNB may transmit a handover preparation failure message to the source eNB to reject the handover request.

After transmitting the preamble, the UE monitors whether a Random Access Response (RAR) is transmitted from the target cell after a predetermined number of sub frames. A time window during which monitoring is performed is referred to as a Random Access Response (RAR) window. When the RAR is received for a particular time in step <NUM>, the UE transmits a handover (HO) complete message, which is an RRC reconfiguration complete message, to the target eNB in step <NUM>.

<FIG> illustrate the UE operation and the eNB operation according to the second embodiment of the disclosure in which, when the RRC-connected mode UE performs handover from a source eNB (source cell) to a target eNB (target cell) in the network, the UE selects a target cell connected to a CN type (<NUM> CN or EPC) preferred by the UE, suitable for a service of the UE, or preferred by the source eNB to perform the handover.

Referring to <FIG>, when a predetermined event or condition is satisfied, the UE performs cell measurement on neighboring cells, reads system information of the corresponding cell, and identifies a CN type in step <NUM>. When providing the cell measurement report, the UE provides the cell measurement report including cell measurement information, cell identifiers, and CN types to the source eNB in step <NUM>. Upon receiving a handover command message in step <NUM>, the UE establishes the connection to the target cell instructed by the message in step <NUM> and transmits and receives data in step <NUM>.

Referring to <FIG>, when receiving a cell measurement result report message from the UE in step <NUM>, the source eNB identifies information such as cell measurement information, cell identifiers, and CN types in the message and selects a target cell corresponding to a CN type preferred in initial UE access or suitable for the current UE service in step <NUM>, transmits a handover request message to the target eNB in step <NUM>, and when receiving a handover ack message in step <NUM>, instructs the UE to perform handover in step <NUM>.

<FIG> illustrates the eNB operation according to the third embodiment of the disclosure in which, when the RRC-connected mode UE performs handover from a source eNB (source cell) to a target eNB (target cell) in the network, the UE selects a target cell connected to a CN type (<NUM> CN or EPC) preferred by the UE, suitable for a service of the UE, or preferred by the source eNB to perform the handover.

Referring to <FIG>, when receiving a cell measurement result report message from the UE in step <NUM>, the source eNB identifies information on target cells (cell identifier, and CN type) shared with the target eNB in X2/Xn interface configuration and establishment, selects a target cell corresponding to a CN type preferred in initial UE access or suitable for the current UE service in step <NUM>, transmits a handover request message to the target eNB in step <NUM>, and when receiving a handover ack message in step <NUM>, instructs the UE to perform handover in step <NUM>.

Methods according to embodiments stated in claims and/or specifications of the present disclosure may be implemented in hardware, software, or a combination of hardware and software.

The at least one program may include instructions that cause the electronic device to perform the methods according to various embodiments of the present disclosure as defined by the appended claims and/or disclosed herein.

The programs (software modules or software) may be stored in non-volatile memories including a random access memory and a flash memory, a Read Only Memory (ROM), an Electrically Erasable Programmable Read Only Memory (EEPROM), a magnetic disc storage device, a Compact Disc-ROM (CD-ROM), Digital Versatile Discs (DVDs), or other type optical storage devices, or a magnetic cassette. Alternatively, any combination of some or all of the may form a memory in which the program is stored.

Claim 1:
A method performed by a terminal (<NUM>, <NUM>) which is capable of connecting to evolved packet core, EPC, and <NUM> core, 5GC, in a wireless communication system, the method comprising:
receiving (<NUM>, <NUM>), from a base station (<NUM>, <NUM>), system information block, SIB, including information on a core network associated with a cell on which the SIB is received, a public land mobile network, PLMN, list and RAT information, wherein the core network is 5GC, and wherein the information on the core network includes a core network type;
performing a measurement of a synchronization signal block, SSB, based on measurement configuration information received from the base station;
generating a measurement report message for reporting a measurement result, the measurement report message including first information on a cell identifier of the cell, second information on the measurement result, and third information on the core network type; and
transmitting (<NUM>), to the base station, the measurement report message.