Patent Publication Number: US-2023164649-A1

Title: Method and apparatus for providing access policy in wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0160347, filed on Nov. 19, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     The disclosure relates to a method and apparatus for providing an access and mobility (AM) policy in a wireless communication system. 
     2. Description of Related Art 
     In order to meet the demand for wireless data traffic soaring since the 4 th  generation (4G) communication system came to the market, there are ongoing efforts to develop enhanced 5 th  generation (5G) communication systems or pre-5G communication systems. For the reasons, the 5G communication system or pre-5G communication system is called the beyond 4G network communication system or post LTE system. For higher data transmit rates, 5G communication systems are considered to be implemented on ultra-high frequency bands (mmWave), such as, e.g., 60 GHz. To mitigate pathloss on the ultra-high frequency band and increase the reach of radio waves, the following techniques are taken into account for the 5G communication system, such as beamforming, massive multi-input multi-output (MIMO), full dimensional MIMO (FD-MMO), array antenna, analog beamforming, and large scale antenna. Also being developed are various technologies for the 5G communication system to have an enhanced network, such as evolved or advanced small cell, cloud radio access network (cloud RAN), ultra-dense network, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-point (CoMP), and reception interference cancellation. There are also other various schemes under development for the 5G system including, e.g., hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC), which are advanced coding modulation (ACM) schemes, and filter bank multi-carrier (FBMC), non-orthogonal multiple access (NOMA) and sparse code multiple access (SCMA), which are advanced access schemes. 
     The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology”, “wired/wireless communication and network infrastructure”, “service interface technology”, and “Security technology” have been demanded for IoT implementation, a sensor network, machine-to-machine (M2M) communication, machine type communication (MTC), and so forth have been recently researched. Such an IoT environment may provide intelligent Internet technology (IT) services that create a new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing information technology (IT) and various industrial applications. 
     In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, MTC, and M2M communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud RAN as the above-described big data processing technology may also be considered to be as an example of convergence between the 5G technology and the IoT technology. 
     SUMMARY 
     To enable a network to control access of a user equipment (UE) to the network using 4th generation (4G) and 5th generation (5G) wireless communication systems, an access policy is required. Without a consistent access policy applicable to the UE in the 4G and 5G wireless communication systems, the communication performance and experienced performance of the UE may be significantly reduced, and a service may be interrupted. 
     Provided are a method and apparatus for, when a UE moves between 4G and 5G systems, providing an access policy for the UE in a wireless communication system enabling interworking between 4G and 5G networks. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments of the disclosure. 
     According to an embodiment of the disclosure, a base station (BS) in a wireless communication system may include a transceiver and at least one controller coupled to the transceiver. The at least one controller may be configured to determine handover for a UE based on a communication state of the UE, transmit a handover request message for the UE to an access and management function (AMF), receive a handover command for the UE from the AMF in response to the transmission of the handover request message, and perform a handover procedure with the UE. The handover request message may include information related to a rule used to select a network and a frequency applied to the UE. 
     According to another embodiment of the disclosure, a BS in a wireless communication system may include a transceiver and at least one controller coupled to the transceiver. The at least one controller may be configured to receive a handover request message for a UE from a mobility management entity (MME), transmit a handover response message for the handover request message to the MME, and perform a handover procedure with the UE. The handover request message may include information related to a rule used to select a network and a frequency applied to the UE. 
     According to another embodiment of the disclosure, a method performed by a BS in a wireless communication system may include determining handover for a UE based on a communication state of the UE, transmitting a handover request message for the UE to an AMF, receiving a handover command for the UE from the AMF in response to the transmission of the handover request message, and performing a handover procedure with the UE. The handover request message may include information related to a rule used to select a network and a frequency applied to the UE. 
     According to another embodiment of the disclosure, a method performed by a BS in a wireless communication system may include receiving a handover request message for a UE from an MME, transmitting a handover response message for the handover request message to the MME, and performing a handover procedure with the UE. The handover request message may include information related to a rule used to select a network and a frequency applied to the UE. 
     According to various embodiments of the disclosure, a stable service may be provided to a UE by providing a consistent access and mobility (AM) policy for movement of the UE between 4G and 5G networks in a wireless communication system. 
     Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. 
     Moreover, various functions described below can be implemented or supported by one or more computer programs, each of which is formed from computer readable program code and embodied in a computer readable medium. The terms “application” and “program” refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or a portion thereof adapted for implementation in a suitable computer readable program code. The phrase “computer readable program code” includes any type of computer code, including source code, object code, and executable code. The phrase “computer readable medium” includes any type of medium capable of being accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disc (CD), a digital video disc (DVD), or any other type of memory. A “non-transitory” computer readable medium excludes wired, wireless, optical, or other communication links that transport transitory electrical or other signals. A non-transitory computer readable medium includes media where data can be permanently stored and media where data can be stored and later overwritten, such as a rewritable optical disc or an erasable memory device. 
     Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates a system structure of a 5th generation system (5GS) according to various embodiments of the present disclosure; 
         FIG.  2    illustrates a signal flow for a procedure of providing an access and mobility (AM) policy to a user equipment (UE) when the UE switches from a 5G network to a 4G network according to various embodiments of the present disclosure; 
         FIG.  3    illustrates a signal flow for a procedure of providing an AM policy to a UE when the UE switches from a 4G network to a 5G network according to various embodiments of the present disclosure; 
         FIG.  4    illustrates a signal flow for a handover procedure for providing an AM policy to a UE when the UE switches from a 5G network to a 4G network according to various embodiments of the present disclosure; 
         FIG.  5    illustrates a signal flow for a handover procedure for providing an AM policy to a UE when the UE switches from a 4G network to a 5G network according to various embodiments of the present disclosure; 
         FIG.  6    illustrates an internal structure of a UE according to various embodiments of the present disclosure; 
         FIG.  7    illustrates an internal structure of a radio access network (RAN) according to various embodiments of the present disclosure; 
         FIG.  8    illustrates an internal structure of an access and mobility management function (AMF) according to various embodiments of the present disclosure; 
         FIG.  9    illustrates an internal structure of a mobility management entity (MME) according to various embodiments of the present disclosure; and 
         FIG.  10    illustrates an internal structure of a user data management (UDM) according to various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS.  1  through  10   , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged system or device. 
     Embodiments of the disclosure are described below in detail with reference to the accompanying drawings. A detailed description of a generally known function or structure of the disclosure will be avoided lest it should obscure the subject matter of the disclosure. Although the terms as described later are defined in consideration of functions in the disclosure, the terms may be changed according to the intention of a user or an operator, or customs. Therefore, the definitions should be made, not simply by the actual terms used but by the meanings of each term lying within. 
     The advantages and features of the disclosure, and a method of achieving them will become apparent from reference to embodiments described below in detail in conjunction with the attached drawings. However, the disclosure may be implemented in various manners, not limited to the embodiments set forth herein. Rather, these embodiments are provided such that the disclosure is complete and thorough and its scope is fully conveyed to those skilled in the art, and the disclosure is only defined by the appended claims. The same reference numerals denote the same components throughout the specification. 
     The disclosure relates to a method and apparatus for supporting various services in a wireless communication system. Specifically, the disclosure describes a technique of providing a stable service by managing and applying a policy for a system and a frequency band to be used for communication of a user equipment (UE) using 4th generation (4G) and 5th generation (5G) networks in a wireless communication system. 
     Terminology identifying an access node, terminology signifying network entities or network functions (NFs), terminology signifying messages, terminology signifying interfaces between network entities, and terminology signifying various types of identification information as used in the following description are given for convenience of description. Accordingly, the disclosure is not limited to the terms described below, and the terms may be replaced by other equivalent terms in technical meanings. 
     For convenience of description, the disclosure uses terms and names defined in the 3rd generation partnership project long term evolution (3GPP LTE) and 5G standards. However, the disclosure may be equally applied to systems conforming to other standards, not limited by the above terms and names. 
     For convenience of description, the names of NFs (e.g., access and mobility management function (AMF), session management function (SMF), network slicing selection function (NSSF), and so on) are used to indicate entities for exchanging information for access control and state management. However, embodiments of the disclosure are equally applicable even when NFs are actually implemented as instances (AMF instance, SMF instance, NSSF instance, and so on). 
       FIG.  1    illustrates a system structure of a 5G system (5GS) according to various embodiments of the present disclosure. The 5GS may include a 5G core network, a radio access network (RAN), and a UE. The 5G core network may include an AMF  120 , an SMF  135 , a user plane function (UPF)  130 , a policy control function (PCF)  140 , a user data management (UDM)  145 , an NSSF  160 , a network data analytics function (NWDAF)  165 , a non-3GPP function (N3F) and so on. 
     A UE  100  may access the 5G core network through a RAN BS (hereinafter, a RAN)  110 . The RAN  110  may support a 3GPP access network type (e.g., new radio (NR), evolved UMTS terrestrial radio access (E-UTRA), and so on) or a non-3GPP access network type (e.g., wireless fidelity (Wi-Fi)). The UE  100  may be connected to the AMF  120  via an N2 interface and to the UPF  130  via an N3 interface, through the RAN  110 . The RAN  110  may also be referred to as “access point (AP),” “eNodeB or eNB,” “5G node,” “gNodeB or gNB,” or other terms having equivalent technical meanings, as well as base station (BS). The N3F is an NF that operates as an N2 interface and N3 interface termination for the UE  100  that has accessed through a non-3GPP access network (e.g., Wi-Fi or the like) not defined by the 3GPP. The N3F may process N2 control-plane signaling and N3 user-plane packets. 
     The AMF  120  is an NF that manages wireless network access and mobility for a UE. The SMF  135  is an NF that manages a session for a UE, and session information includes quality of service (QoS) information, charging information, and packet processing information, The UPF  130  is an NF that processes user-plane traffic, and is controlled by the SMF  135 . The PCF  140  is an NF that manages an operator policy for providing a service in the wireless communication system. The UDM  145  is an NF that stores and manages UE subscription information. A UDR is an NF that stores and manages data. The UDR may store UE subscription information and provide the subscription information to the UDM. 
     In addition, the UDR may store operator policy information and provide the operator policy information to the PCF. The NWDAF  165  is an NF that provides analysis information to operate the 5G system. The NWDAF may collect data from another NF or an operations, administration, and maintenance (OAM) included in the 5GS, analyze the collected data, and provide analysis results to another NF. A network slice admission control function (NSACF)  180  is an NF that monitors and controls the number of registered UEs and the number of sessions of a network slice subject to network slice admission control (NSAC). The NSACF stores configuration information about a maximum number of registered UEs and a maximum number of sessions for each network slice. 
     For convenience of description, entities for exchanging information for access control and state management will be generically described as NFs (or a core network). However, embodiments of the disclosure are equally applicable even when NFs are actually implemented as instances (e.g., AMF instance, SMF instance, NSSF instance, and so on, respectively). 
     In the present disclosure, an instance may refer to a state in which a specific NF exists in the form of software code, and is executable by physical and/or logical resource allocation for executing a function of the NF from a physical computing system, for example, a specific computing system existing on a core network. Therefore, an AMF instance, an SMF instance, and an NSSF instance may mean that physical and/or logical resources are allocated and usable for AMF, SMF, and NSSF operations from a specific computing system existing on the core network, respectively. As a result, physical AMF, SW, and NSSF devices may perform the same operations as AMF, SMF, and NSSF instances using physical and/or logical resources allocated for AMF, SMF, and NSSF operations by a specific computing system existing on the network. Therefore, in an embodiment of the disclosure, the term NF (AMF, SMF, UPF, NSSF, NRF, SCP, and so on) may be replaced with NF instance, or vice versa. Likewise, the term network slice may be replaced with network slice instance, or vice versa in an embodiment of the disclosure. 
     According to an embodiment of the disclosure, in the 5G system defined by the 3GPP, one network slice may be referred to as single-network slice selection assistance information (S-NSSAI). The S-NSSAI may include a slice/service type (SST) and a slice differentiator (SD). The SST may indicate the features (e.g., enhanced mobile broadband (eMBB), Internet of things (IoT), ultra-reliable low-latency communication (URLLC), vehicle to everything (V2X), or the like) of a service supported by the slice. The SD may be a value used as an additional identifier (ID) for the specific service indicated by the SST. 
     An NSSAI may include one or more S-NSSAIs. Examples of the NSSAI may include, but not limited to, a configured NSSAI stored in a UE, a requested NSSAI requested by a UE, an allowed NSSAI that is allowed to be used by a UE, as determined by an NF (e.g., the AMF  120 , the NSSF  160 , or the like) of the 5G core network, and a subscribed NSSAI to which a UE has subscribed. 
     The UE  100  may be simultaneously connected to the RAN  110  and registered in the 5G system. Specifically, the UE  100  may access the RAN  110  and perform a UE registration procedure with the AMF  120 . During the registration procedure, the AMF  120  may determine an allowed slice (allowed NSSAI) available to the UE  100  that has accessed the RAN  110 , and allocate the allowed NSSAI to the UE  100 . The UE may select a specific slice establish a protocol data unit (PDU) session for communication with an actual application server. One PDU session may include one or more QoS flows, and each QoS flow may provide a different transmission performance required for each application service by configuring a different QoS parameter. While the UE is performing the registration procedure, the PCF may establish an access and mobility (AM) policy to be applied to the UE based on subscription information stored in the UDR and transmit the AM policy to the AMF. The AM policy may include a network/radio access technology (RAT) to be used by the UE and a RAT/frequency selection priority (RFSP) rule that prioritizes frequency bands. 
       FIG.  2    illustrates a signal flow for a procedure of providing an AM policy to a UE when the UE switches from a 5G network to a 4G network according to various embodiments of the present disclosure. Referring to  FIG.  2   , as a PCF according to an embodiment of the disclosure provides an AMF with an AM policy to be applied to a UE, and the AMF transmits an RFSP rule to a RAN and a UDM, a network and a frequency band to be used from between 4G and 5G networks for communication by the UE may be controlled consistently. 
     A mobility management entity (MME)  206  of the 4G network and a UDR  212  of the 5G network may store a 4G default RFSP rule and a 5G default RFSP rule to be applied to each UE as part of UE subscription information (subscription data), respectively by an operator or the like (S 205  and S 210 ). The default RFSP rules may be predetermined by the operator and stored. 
     The UDR  212  may notify a UDM  210  that the UE subscription information has changed, and transmit the changed UE subscription information to the UDM  210  (S 215 ). 
     The UDR  212  may notify a PCF  208  that the UE subscription information has changed, and transmit the changed UE subscription information to the PCF  208  (S 220 ). 
     A UE  200  may transmit a registration request message to an AMF  204  to use a communication service (S 225 ). For example, the UE  200  may transmit the registration request message to the AMF  204  through a RAN  202  to establish a connection with the 5G network. 
     The AMF  204  may request the UDM  210  to provide the UE subscription information about the UE  200  in order to process the registration request of the UE  200 . When the UE  200  connected to the 4G network has moved to the 5G network, the AMF  204  may request RFSP information used for communication in the 4G network by the UE  200  from the MME  206  by transmitting a handover message or a context transfer request message, apart from requesting the UE subscription information from the UDM  210  (S 230 ). The UE subscription information received from the UDM  210  by the AMF  204  may include information about the 5G default RFSP rule set in the UDR  212  by the operator in operation S 210 . The RFSP rule may include information specifying RATs and/or frequency bands to be used by the UE  200 , and may be provided in the form of a list sequentially arranged according to priorities. 
     For example, the RFSP rule may designate a 20-MHz frequency band in a 2.5 GHz band in which 4G LTE is used as a highest priority, and a 100-MHz frequency band in a 3.5 GHz band in which 5G NR is used as a second-highest priority, for communication of the UE  200 . In another embodiment, in the case of a network system without the PCF  208 , the AMF  204  may apply the received default RFSP rule as part of an AM policy for the UE. 
     The AMF  204  may request an AM policy to be applied to the UE  200  from the PCF  208  (S 235 ). The PCF  208  may establish an AM policy including an RFSP rule to be applied to the UE  200  in consideration of information about the default RFSP rule received from the UDM  210 , an operator policy, a network congestion state, and so on, and transmit the established AM policy to the AMF  204 . 
     The AMF  204  may complete a series of procedures for registration of the UE  200  (S 240 ). 
     Operations S 245  and S 250  may be performed in parallel with operation S 240  or simultaneously in the process of performing operation S 240 , and various combinations may be possible depending on implementation. The AMF  204  may transmit the RFSP rule included in the AM policy received from the PCF  208  in operation S 235  to the RAN  202  (S 245 ). According to implementation and an operator policy, the AMF  204  may additionally change the RFSP rule received from the PCF  208  by applying a congestion state for each frequency, a user class, and so on before transmitting the information to the RAN  202 . The RAN  202  may determine a frequency band to be used by the UE  200  for communication by applying priorities included in the received RFSP rule, and allocate radio resources to the UE  200 . The RAN  202  may transmit, to the UE  200 , information about the priorities of networks/RATs available to the UE  200  for selection, so that the UE  200  may select the 4G or 5G network with priority by applying the specified priorities of the networks/RATS in a network/RAT selection or reselection process. For example, the RAN  202  may transmit the information about the priorities of the networks/RATs to the UE  200  by broadcasting system information. Further, the RAN  202  may transmit the RFSP rule to the UE  200 . 
     The AMF  204  may transmit the same information as the RFSP rule transmitted to the RAN  202  to the UDM  210  (S 245 ), and the UDM  210  may store the RFSP rule in case the UE  200  moves to the 4G network in the future (S 250 ). Operations S 245  and S 250  may be repeatedly performed whenever information about the RFSP rule is changed or updated during communication of the UE  200  in the 5G network. 
     The UE  200  may move to the 4G network out of 5G coverage or move to the 4G network to use a specific application such as a voice service, and perform an access procedure with the MIME  206  (S 255 ). 
     The MME  206  may request and receive the UE subscription information about the UE  200  from the UDM  210  in the process of processing an access request of the UE  200 . The UE subscription information may include information about the RFSP rule applied to the UE  200  in the 5G network, stored by the UDM  210  in operation S 250  (S 260 ). 
     The MME  206  may transmit, to the RAN  202 , information about the default RFSP rule to be applied in the 4G network set by the operator in operation S 205  and information about the RFSP rule used in the 5G network received from the UDM  210  in operation S 260  (S 265 ). 
     In another embodiment, the MME  206  may transmit the information about the RFSP rule used in the 5G network received from the UDM  210  to the RAN  202  as it is without interpreting the information about the RFSP rule, or may combine the 4G default RFSP rule and the received 5G RFSP rule into a new RFSP rule and transmit the new RFSP rule to the RAN  202 . When the RAN  202  receives both the 4G default RFSP rule and the 5G RFSP rule, the RAN  202  may apply the 5G RFSP rule with priority over the 4G default RFSP rule in order to control the network/RAT and frequency band use of the UE  200 . This process may enable application of a consistent AM policy, when the UE  200  switches networks in the wireless communication system including the 4G network and the 5G network. 
       FIG.  3    illustrates a signal flow for a procedure of providing an AM policy to a UE when the UE switches from a 4G network to a 5G network according to various embodiments of the present disclosure. 
     According to an embodiment, an MME  306  of the 4G network and a UDR  312  of the 5G network may store a 4G default RFSP rule and a 5G default RFSP rule to be applied to each UE  300  as part of UE subscription information (subscription data), respectively by an operator or the like (S 305  and S 310 ). 
     The  300  may perform an access procedure with the MME  306  in order to use a 4G communication service (S 315 ). For example, the UE  300  may transmit an attach message to the MMF  306  through a RAN  302 . 
     The MME  306  may transmit information about the RFSP rule stored in operation S 305  to the RAN  302  (S 320 ). 
     The MME  306  may transmit the same information about the RFSP rule as transmitted to the RAN  302  in operation S 320  to a home subscriber server (HSS)+UDM  310 , and the HSS+UDM  310  may store the received information about the RFSP rule in case the UE  300  may move to the 5G network in the future (S 325 ). 
     The UE  300  may perform a registration procedure with an AMF  304  in order to use a 5G communication service (S 330 ). For example, the UE  300  may transmit a registration message to the AMF  304  through the RAN  302  to establish a connection with the 5G network (S 330 ). 
     The AMF  304  may request the HSS+UDM  310  to provide UE subscription information about the UE  300  in order to process the registration request of the UE  300  (S 335 ). When the UE  300  connected to the 4G network has moved to the 5G network, the AMF  304  may request RFSP information used for communication in the 4G network by the UE  300  from the MME  306  by transmitting a handover message or a context transfer request message, apart from requesting the UE subscription information from the HSS+UDM  310 . While not shown, the UDR  312  may preliminarily notify the HSS+UDM  310  that the UE subscription information has changed and transmit the changed UE subscription information to the HSS+UDM  310 . 
     Further, the UDR  312  may preliminarily notify a PCF  308  that the UE subscription information has changed and transmit the changed UE subscription information to the PCF  308 . The UE subscription information that the AMF  304  has received from the HSS+UDM  310  may include information about the 5G default RFSP rule set in the UDR  312  by the operator in operation S 310  and/or information about the 4G default RFSP rule stored in the HSS+UDM  310  by the MMS  306  in operation S 325 , The RFSP rule may include information specifying RATs and/or frequency bands to be used by the UE  300 , and may be provided in the form of a list sequentially arranged according to priorities. 
     For example, the RFSP rule may designate a 20-MHz frequency band in the 2.5GHz band in which 4G LTE is used as a highest priority, and a 100-MHz frequency band in the 3.5GHz band in which 5G NR is used as a second-highest priority, for communication of the UE  200 . In another embodiment, in the case of a network system without the PCF  308 , the AMF  304  may apply the received default RFSP rule as part of an AM policy for the UE. 
     The AMF  304  may request an AM policy to be applied to the UE  300  from the PCF  308  (S 340 ). The PCF  308  may establish an AM policy including an RFSP rule to be applied to the UE  300  in consideration of the information about the RFSP rule received from the UDR  312  (or the HSS+UDM  310 ), an operator policy, a network congestion state, and so on, and transmit the established AM policy to the AMF  304 . 
     The AMF  304  may transmit the RFSP rule included in the AM policy received from the PCF  208  in operation S 340  to the RAN  302 . Further, the AMF  304  may transmit the 4G default RFSP rule stored in the UDM to the RAN  302  (S 345 ). Depending on implementation and an operator policy, the AMF  304  may additionally change the RFSP rule received from the PCF  308  by applying a congestion state for each frequency, a user class, and so on before transmitting the information to the RAN  302 . The AMF  304  may transmit the information about the RFSP rule used in the 5G network received from the HSS+UDM  310  to the RAN  302  as it is without interpreting the information about the RFSP rule, or may combine the 4G default RFSP rule and the received 5G RFSP rule into a new RFSP rule and transmit the new RFSP rule to the RAN  302 . 
     The RAN  202 . may determine a frequency band to be used by the UE  300  for communication by applying priorities included in the received RFSP rule, and allocate radio resources to the UE  300 . The RAN  302  may transmit, to the UE  300 , information about the priorities of networks/RATs available to the UE  300  for selection, so that the UE  300  may select the 4G or 5G network with priority by applying the specified priorities of the networks/RATs in a network/RAT selection or reselection process. 
     For example, the RAN  302  may transmit the information about the priorities of the networks/RATs to the UE  300  by broadcasting system information. Further, the RAN  302  may transmit the RFSP rule to the UE  300 . When the RAN  302  has received both the 4G default RFSP rule and the 5G RFSP rule, the RAN  302  may apply the 5G RFSP rule with priority over the 4G default RFSP rule in order to control the network/RAT and frequency band use of the UE  300 . 
     The AMF  304  may transmit the same information as the RFSP rule transmitted to the RAN  302  to the HSS+UDM  310 , and the HSS+UDM  310  may store the RFSP rule in case the UE  300  may move to the 4G network again in the future (S 350 ). Operations S 345  and S 350  may be repeatedly performed whenever information about the RFSP rule is changed or updated during communication of the UE  300  in the 5G network. 
     The AMF  304  may complete a series of procedures for registration of the UE  300  (S 355 ). This process may enable application of a consistent AM policy, when the UE  300  switches networks in the wireless communication system including the 4G network and the 5G network. 
       FIG.  4    illustrates a signal flow for a handover procedure for providing an AM policy to a UE when the UE switches from a 5G network to a 4G network according to various embodiments of the present disclosure. 
     Referring to  FIG.  4   , a PCF  408  according to an embodiment of the disclosure may provide an AMF  404  with an AM policy to be applied to a UE, and the AMF  404  may transmit an RFSP rule to a RAN  402  so that the RFSP rule is used. Further, the AMF  404  may transmit the RFSP rule to a RAN of a new network to which the UE has moved by a handover signal message, thereby controlling the UE to consistently use a network and a frequency band for communication during movement to the 5G network or the 4G network. 
     A UE  400  may perform a registration procedure with the 5G network (S 405 ). The registration procedure may include the processes of operations S 210  to S 245  described above in the embodiment of  FIG.  2   , which are equally applied in this embodiment, and thus are not described redundantly in detail. 
     A 5G RAN  402  may determine handover to the 4G network in consideration of the radio quality of the UE  400  (S 410 ). 
     The 5G RAN  402  may transmit a handover request message requesting handover to a 4G RAN  414  to the AMF  404  (S 415 ). The handover request message may include at least one of information related to a 5G RFSP rule applied to the UE  400  by the 5G RAN  402 , UE identification information (a UE identifier (ID)), and 4G RAN identification information (a 4G RAN ID). 
     The AMF  404  may transmit a relocation request message to the MME  406  of the 4G network to forward the handover request received from the 5G RAN  402  (S 420 ). The relocation request message may include at least one of the information related to the 5G RFSP rule, the UE identification information, or the 4G RAN identification information included in the handover request message by the 5G RAN  402  in operation S 415 . According to another embodiment, the AMF  404  may directly include the information related to the 5G RFSP rule in the relocation request message without receiving the information related to the 5G RFSP rule from the 5G RAN  402  in operation S 420 , rather than the 5G RAN  402  includes the information related to the 5G RFSP rule in the handover request message in operation S 415 . 
     The MME  406  may transmit handover request information received from the AMF  404  to the 4G RAN  414  through another handover request message (S 425 ). The other handover request message may include the information related to the 5G RFSP rule transmitted by the 5G RAN  402  in operation S 415  (or the information related to the 5G RFSP rule included by the AMF  404  in operation S 420 ). In addition, the other handover request message may include information related to a 4G default RFSP rule to be applied to the 4G network, preconfigured for the MME  406  by the operator. According to another embodiment, the MME  406  may determine a new RFSP rule by combining the RFSP rule used in the 5G network received from the AMF  404  and the 4G default RFSP rule, and transmit the new RFSP rule. 
     The 4G RAN  414  may transmit a handover request acknowledgment (Ack) message to the MME  406  in response to the handover request (S 430 ). 
     The MME  406  may transmit a relocation response message to the AMF  404  to notify the AMF  404  that the handover request has been accepted (S 435 ). 
     Upon receipt of the relocation response message, the AMF  404  may transmit, to the 5G RAN  402 , a handover command message instructing the handover of the UE  40  requested by the 5G RAN  402  to be performed (S 440 ). 
     The UE  400  may switch to the 4G network according to a command from the 5G RAN  402 . and access the 4G RAN  414  to perform a handover procedure (S 445 ). 
     When the 4G RAN  414  has received both the information related to the 4G default RFSP rule and the information related to the 5G RFSP rule in the handover process, the 4G RAN may apply the 5G MP rule with priority to control the network/RAT and frequency band use of the UE  400  (S 450 ). For example, when the 4G RAN  414  has received both the 4G RFSP rule and the 5G RFSP rule, the 4G RAN  414  may assign a higher application priority to the 5G RFSP rule. This process may enable application of a consistent AM policy, when the UE  400  moves in the wireless communication system including the 4G network and the 5G network. 
       FIG.  5    illustrates a signal flow for a handover procedure for providing an AM policy to a UE when the UE switches from a 4G network to a 5G network according to various embodiments of the present disclosure. 
     Referring to  FIG.  5   , a PCF  508  according to an embodiment of the disclosure may provide an AMF  504  with an AM policy to be applied to a UE, and the AMF  504  may transmit an RFSP rule to a RAN  502  so that the RFSP rule is used. Further, the AMF  504  may transmit the RFSP rule to a RAN of a new network to which the UE has moved by a handover signal message, thereby controlling the UE to consistently use a network and a frequency band for communication during movement to the 5G network or the 4G network. 
     A UE  500  may perform a registration procedure with the 4G network (S 505 ). The registration procedure may include the process of operation S 315  described above in the embodiment of  FIG.  3   , which is equally applied in this embodiment, and thus is not described redundantly in detail. 
     A 4G RAN  514  may determine handover to the 5G network in consideration of the radio quality of the UE  500  (S 510 ). 
     The 4G RAN  514  may transmit a handover request message requesting handover to the 5G RAN  502  to an MME  506  (S 515 ). The handover request message may include at least one of information related to a 4G RFSP rule applied to the UE  500  by the 4G RAN  514 , UE identification information (a UE ID), and 5G RAN identification information (a 5G RAN ID). 
     The MME  506  may transmit a relocation request message to the AMF  504  of the 5G network to forward the handover request received from the 4G RAN  514  (S 520 ). The relocation request message may include at least one of the information related to the 4G RFSP rule, the UE identification information, or the 5G RAN identification information included in the handover request message by the 4G RAN  514  in operation S 515 . According to another embodiment, the MME  506  may directly include the information related to the 4G RFSP rule in the relocation request message without receiving the information related to the 4G RFSP rule from the 4G RAN  514  in operation S 520 , rather than the 4G RAN  514  includes the information related to the 4G RFSP rule in the handover request message in operation S 515 . 
     The AMF  504  may transmit handover request information received from the MME  506  to the 5G RAN  502  through another handover request message (S 525 ). The other handover request message may include the information related to the 4G RFSP rule transmitted by the 4G RAN  514  in operation S 515  (or the information related to the 4G RFSP rule included by the MME  506  in operation S 520 ). In addition, the other handover request message may include information related to a 5G default RFSP rule to be applied to the 5G network, preconfigured for a UDR  512  by an operator and transmitted to an HSS+UDM  510  and the AMF  504 , or information related to a 5G RFSP rule generated by reflecting a network congestion state, an operator policy, and so on based on the 5G default RFSP rule by the PCF  508 . The UDR  512  may transmit the information related to the 5G default RFSP rule to the AMF in the manner illustrated in  FIG.  2    or  FIG.  3   . According to another embodiment, the AMF  504  may determine a new RFSP rule by combining the RFSP rule used in the 4G network received from the MME  506  and the 5G RFSP rule, and transmit the new RFSP rule. 
     The 5G RAN  502  may transmit a handover request Ack message to the AMF  504  in response to the handover request (S 530 ). 
     The AMF  504  may transmit a relocation response message to the MME  506  to notify the MMF  506  that the handover request has been accepted (S 535 ). 
     Upon receipt of the relocation response message, the MME  506  may transmit, to the 4G RAN  514 , a handover command message instructing the handover of the UE  500  requested by the 4G RAN  514  to be performed (S 540 ). 
     The UE  500  may switch to the 5G network according to a command from the 4G RAN  514  and access the 5G RAN  502  to perform a handover procedure (S 545 ). 
     The UE  500  may perform a registration procedure with the 5G network through the 5G RAN  502  (S 550 ). When the RFSP rule is updated as in operations S 230  and S 235  of  FIG.  2   , the AMF  504  may update information related to a changed RFSP rule and transmit the updated information to the 5G RAN  502 . 
     When the 5G RAN  502  has received both the information related to the 5G RFSP rule and the information related to the 4G RFSP rule in the handover process, the 5G RAN  502  may apply the 5G RFSP rule with priority to control the network/RAT and frequency band use of the UE  500  (S 555 ). For example, when the 5G RAN  502  has received both the 4G RFSP rule and the 5G RFSP rule, the 5G RAN  502  may assign a higher application priority to the 5G RFSP rule. This process may enable application of a consistent AM policy, when the UE  500  moves in the wireless communication system including the 4G network and the 5G network. 
       FIG.  6    illustrates a structure of a UE according to various embodiments of the present disclosure. As illustrated in  FIG.  6   , a UE  600  of the disclosure may include at least one controller (or processor)  610  and a transceiver  620  including a receiver and a transmitter. The UE  600  may include memory (not shown). The transceiver  620  and the memory may be coupled to the at least one controller  610  to operate under the control of the at least one controller  610 . 
     The at least one controller  610  may control to perform the operations of the UE described in  FIGS.  2  to  5    of the disclosure. The transceiver  620  may transmit/receive signals to and from a RAN  700 , an AMF  800 , and an MME  900 . 
       FIG.  7    illustrates a structure of a RAN according to various embodiments of the present disclosure. As illustrated in  FIG.  7   , the RAN  700  of the disclosure may include at least one controller (or processor)  710  and a transceiver  720  including a receiver and a transmitter. The RAN  700  may include memory (not shown). The transceiver  720  and the memory may be coupled to the at least one controller  710  to operate under the control of the at least one controller  710 . 
     The at least one controller  710  may control to perform the operations of a RAN described in  FIGS.  2  and  3    of the disclosure and the operations of a 4G RAN and a 5G RAN described in  FIGS.  4  and  5   . The transceiver  720  may transmit and receive signals to and from the UE  600 , the AMF  800 , and the MME  900 . 
       FIG.  8    illustrates a structure of an AMF according to various embodiments of the present disclosure. As illustrated in  FIG.  8   , the AMF  800  of the disclosure may include at least one controller (or processor)  810  and a transceiver  820  including a receiver and a transmitter. The AMF  800  may include memory (not shown). The transceiver  820  and the memory may be coupled to the at least one controller  810  to operate under the control of the at least one controller  810 . 
     The at least one controller  810  may control to perform the operations of an AMF described in  FIGS.  2  to  5    of the disclosure. The transceiver  820  may transmit and receive signals to and from the UE  600 , the RAN  700 , the MME  900 , a UDM  1000 , a PCF, and a UDR. 
       FIG.  9    illustrates a structure of an MME according to various embodiments of the present disclosure. As illustrated in  FIG.  9   , the MME  900  of the disclosure may include at least one controller (or processor)  910  and a transceiver  920  including a receiver and a transmitter. The MME  900  may include memory (not shown). The transceiver  920  and the memory may be coupled to the at least one controller  910  to operate under the control of the at least one controller  910 . 
     The at least one controller  910  may control to perform the operations of an MME described in  FIGS.  2  to  5    of the disclosure. The transceiver  920  may transmit and receive signals to and from the UE  600 , the RAN  700 , the AMF  800 , the UDM  1000 , the PCF, and the UDR. 
       FIG.  10    illustrates a structure of a UDM according to various embodiments of the present disclosure. As illustrated in  FIG.  10   , the UDM  1000  of the disclosure may include at least one controller (or processor)  1010  and a transceiver  1020  including a receiver and a transmitter. The UDM  1000  may include memory (not shown). The transceiver  1020  and the memory may be coupled to the at least one controller  1010  to operate under the control of the at least one controller  1010 . The UDM of  FIG.  10    may be configured as a combo node together with an HSS in a system providing interworking. In this case, the UDM may be expressed as HSS+UDM, and HSS+UDM may perform the operations of both the UDM and the HSS and communicate with both the AMF and the MME. 
     The at least one controller  1010  may control to perform the operations of a UDM described in  FIGS.  2  to  5   . The transceiver  1020  may transmit and receive signals to and from the UE  600 , the RAN  700 , the AMF  800 , the MME  900 , the PCF, and the UDR. 
     The structures of the devices illustrated in  FIGS.  6  to  10    are not limited to the above-described ones, and components may be added according to design. In addition, the devices illustrated in  FIGS.  7  to  10    may be configured to be included together in one device as a network, or configured as separate entities. 
     Although the present disclosure has been described with various embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.