Method and apparatus for cell selection / reselection in wireless communication system

Abstract

The disclosure relates to a 5G or 6G communication system for supporting a higher data transmission rate. Embodiments herein relate to methods and systems for cell selection and reselection in a fifth-generation (5G) network, comprising at least one User Equipment (UE), multiple base stations, and a Core Network. The UE is configured to monitor the signal quality of a serving cell and neighboring cells within either a Public Land Mobile Network (PLMN) or a Standalone Non-Public Network (SNPN). When the signal quality of the serving cell falls below a predefined threshold, the UE identifies candidate cells based on signal measurements from the neighboring cells. The system determines the N1 mode status and Network Slice Selection Assistance Information (NSSAI) support for each candidate cell. The UE then reselects a new serving cell based on these criteria, thereby optimizing connectivity and service continuity in the 5G network.

Description

METHOD AND APPARATUS FOR CELL SELECTION / RESELECTION IN WIRELESS COMMUNICATION SYSTEM

[0001] The present disclosure relates generally to wireless communication networks, and more particularly to methods and systems for cell selection / reselection in 5G (Fifth Generation) networks.

[0002] 5G mobile communication technologies define broad frequency bands such that high transmission rates and new services are possible, and can be implemented not only in “Sub 6GHz” bands such as 3.5GHz, but also in “Above 6GHz” bands referred to as mmWave including 28GHz and 39GHz. In addition, it has been considered to implement 6G mobile communication technologies (referred to as Beyond 5G systems) in terahertz bands (for example, 95GHz to 3THz bands) in order to accomplish transmission rates fifty times faster than 5G mobile communication technologies and ultra-low latencies one-tenth of 5G mobile communication technologies.

[0003] At the beginning of the development of 5G mobile communication technologies, in order to support services and to satisfy performance requirements in connection with enhanced Mobile BroadBand (eMBB), Ultra Reliable Low Latency Communications (URLLC), and massive Machine-Type Communications (mMTC), there has been ongoing standardization regarding beamforming and massive MIMO for mitigating radio-wave path loss and increasing radio-wave transmission distances in mmWave, supporting numerologies (for example, operating multiple subcarrier spacings) for efficiently utilizing mmWave resources and dynamic operation of slot formats, initial access technologies for supporting multi-beam transmission and broadbands, definition and operation of BWP (BandWidth Part), new channel coding methods such as a LDPC (Low Density Parity Check) code for large amount of data transmission and a polar code for highly reliable transmission of control information, L2 pre-processing, and network slicing for providing a dedicated network specialized to a specific service.

[0004] Currently, there are ongoing discussions regarding improvement and performance enhancement of initial 5G mobile communication technologies in view of services to be supported by 5G mobile communication technologies, and there has been physical layer standardization regarding technologies such as V2X (Vehicle-to-everything) for aiding driving determination by autonomous vehicles based on information regarding positions and states of vehicles transmitted by the vehicles and for enhancing user convenience, NR-U (New Radio Unlicensed) aimed at system operations conforming to various regulation-related requirements in unlicensed bands, NR UE Power Saving, Non-Terrestrial Network (NTN) which is UE-satellite direct communication for providing coverage in an area in which communication with terrestrial networks is unavailable, and positioning.

[0005] Moreover, there has been ongoing standardization in air interface architecture / protocol regarding technologies such as Industrial Internet of Things (IIoT) for supporting new services through interworking and convergence with other industries, IAB (Integrated Access and Backhaul) for providing a node for network service area expansion by supporting a wireless backhaul link and an access link in an integrated manner, mobility enhancement including conditional handover and DAPS (Dual Active Protocol Stack) handover, and two-step random access for simplifying random access procedures (2-step RACH for NR). There also has been ongoing standardization in system architecture / service regarding a 5G baseline architecture (for example, service based architecture or service based interface) for combining Network Functions Virtualization (NFV) and Software-Defined Networking (SDN) technologies, and Mobile Edge Computing (MEC) for receiving services based on UE positions.

[0006] As 5G mobile communication systems are commercialized, connected devices that have been exponentially increasing will be connected to communication networks, and it is accordingly expected that enhanced functions and performances of 5G mobile communication systems and integrated operations of connected devices will be necessary. To this end, new research is scheduled in connection with eXtended Reality (XR) for efficiently supporting AR (Augmented Reality), VR (Virtual Reality), MR (Mixed Reality) and the like, 5G performance improvement and complexity reduction by utilizing Artificial Intelligence (AI) and Machine Learning (ML), AI service support, metaverse service support, and drone communication.

[0007] Furthermore, such development of 5G mobile communication systems will serve as a basis for developing not only new waveforms for providing coverage in terahertz bands of 6G mobile communication technologies, multi-antenna transmission technologies such as Full Dimensional MIMO (FD-MIMO), array antennas and large-scale antennas, metamaterial-based lenses and antennas for improving coverage of terahertz band signals, high-dimensional space multiplexing technology using OAM (Orbital Angular Momentum), and RIS (Reconfigurable Intelligent Surface), but also full-duplex technology for increasing frequency efficiency of 6G mobile communication technologies and improving system networks, AI-based communication technology for implementing system optimization by utilizing satellites and AI (Artificial Intelligence) from the design stage and internalizing end-to-end AI support functions, and next-generation distributed computing technology for implementing services at levels of complexity exceeding the limit of UE operation capability by utilizing ultra-high-performance communication and computing resources.

[0008] the embodiment herein provides a disclose a method for cell selection / reselection in a fifth generation (5G) network.

[0009] Aspects of the present disclosure provide efficient communication methods in a wireless communication system.

[0010] Aspects of the present disclosure provide efficient communication methods in a wireless communication system.

[0011] Embodiments herein are illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the following illustratory drawings. Embodiments herein are illustrated by way of examples in the accompanying drawings, and in which:

[0012] FIG. 1A and FIG. 1B depict the coding of the Alternative NSSAI information element, according to existing arts;

[0013] FIG. 2 depicts an example scenario, wherein the UE may prioritise wrong cells during Network slice based cell reselection, according to existing arts;

[0014] FIG. 3 depicts an example scenario, wherein the alternative NSSAI is not passed to the lower layers, according to existing arts;

[0015] FIG. 4 depicts an example scenario, wherein an unused slice is prioritized for cell reselection, according to existing arts;

[0016] FIG. 5 depicts a scenario, wherein the UE may be able to select a SNPN (which is not allowed), which can lead to ping pong, according to existing arts;

[0017] FIG. 6 illustrates a system for cell selection / reselection in 5G networks, according to the embodiments herein;

[0018] FIG. 7 illustrates the method steps involved in the cell selection / reselection, according to the embodiments herein;

[0019] FIG. 8 depicts a scenario, wherein the UE enables N1 mode for SNPN, according to the embodiments herein;

[0020] FIG. 9 depicts a scenario, wherein the UE enables N1 mode for SNPN, according to the embodiments herein;

[0021] FIG. 10 depicts a scenario, wherein the UE re-enables N1 mode for SNPN, according to the embodiments herein;

[0022] FIG. 11 depicts a scenario, wherein the UE disables N1 mode for SNPN, according to the embodiments herein;

[0023] FIG. 12 depicts a scenario, wherein the UE enables N1 mode for SNPN, according to the embodiments herein;

[0024] FIG. 13 depicts a scenario, wherein the UE enables N1 mode for SNPN, according to the embodiments herein;

[0025] FIG. 14 depicts an example scenario, wherein the partial slicing supported area is informed, according to the embodiments herein;

[0026] FIG. 15 depicts an example scenario, wherein the partial slicing supported area is informed, according to the embodiments herein;

[0027] FIG. 16 depicts an example scenario, wherein the alternative NSSAI is passed to the lower layers, according to the embodiments herein; and

[0028] FIG. 17 depicts an example scenario, wherein unused slice(s) are removed for cell reselection, according to the embodiments herein.

[0029] FIG. 18 illustrates the configuration of a UE in a wireless communication system according to various embodiments.

[0030] FIG. 19 illustrates the configuration of a base station or a network entity in a wireless communication system according to various embodiments.

[0031] Accordingly, the embodiments herein provide a disclose a method for cell selection / reselection in a fifth generation (5G) network. This method involves monitoring, by at least one UE, the signal quality of a serving cell and one or more neighboring cells, wherein the serving cell and the neighboring cells belong to either a Public Land Mobile Network (PLMN) or a Standalone Non-Public Network (SNPN). The method further includes identifying, by the UE, one or more candidate cells based on signal measurements from the neighboring cells when the signal quality of the serving cell is detected to be below a predefined threshold. Subsequently, the UE determines the status of the N1 mode capability for each of the candidate cells and whether each candidate cell supports the required Network Slice Selection Assistance information (NSSAI). Finally, the UE reselects a new serving cell based on the determined status of the N1 mode and the NSSAI.

[0032] Accordingly, the embodiments herein provide a a method for handling the disabling and re-enabling of an N1 mode. This method includes sending, by at least one UE, a registration request on a camped first network, where the camped first network is one of the PLMN or SNPN. The method continues with the UE receiving either a registration accept or a registration reject with one of 5GMM cause #62 or cause #27, indicating that no network slices are available. Subsequently, the UE determines that the camped first network is to be disabled as there are no S-NSSAIs allowed. The method then involves selecting, by the UE, an available second network and sending a registration request on this second network. Upon receiving a registration accept with the camped first network and a third network in an equivalent network list, the UE determines that the N1 mode capability for the camped first network is disabled to prevent selection / reselection to the camped first network. Finally, the method allows for cell selection / reselection if the UE determines to enable the N1 mode for the camped first network.

[0033] Accordingly, the embodiments herein provide a method for handling Network Slice Selection Assistance Information (NSSAI) lists in a network. This method involves receiving, by at least one UE, an alternative NSSAI (S-NSSAI-A) along with the allowed NSSAI list from the UE Non-Access Stratum (NAS) when the UE NAS receives an alternative NSSAI mapping indicating that the current S-NSSAI is to be replaced with the alternative S-NSSAI. The method further includes performing, by the UE, a network slice-based cell reselection by evaluating information related to the alternative NSSAI, wherein the UE considers the current S-NSSAI as a lower or equal priority compare to alternative S-NSSAI.

[0034] Accordingly, the embodiments herein provide a system for performing cell selection / reselection in a fifth generation (5G) network. The system comprises at least one User Equipment (UE), one or more base stations, and a Core Network, wherein the UE is communicatively coupled to the base stations, and the base stations are communicatively coupled to the Core Network. The UE is configured to monitor the signal quality of a serving cell and one or more neighboring cells, wherein the serving cell and neighboring cells belong to either a Public Land Mobile Network (PLMN) or a Standalone Non-Public Network (SNPN). The system further includes the UE identifying one or more candidate cells based on signal measurements from the neighboring cells when the signal quality of the serving cell is detected to be below a predefined threshold. Additionally, the UE determines the status of the N1 mode for each candidate cell and whether each candidate cell supports the required Network Slice Selection Assistance Information (NSSAI). Based on the determined status of the N1 mode and the NSSAI, the UE reselects a new serving cell.

[0035] Accordingly, the embodiments herein disclose a system for handling the disabling and re-enabling of N1 mode capability. The system comprises at least one User Equipment (UE), one or more base stations, and a Core Network, wherein the UE is communicatively coupled to the base stations, and the base stations are communicatively coupled to the Core Network. The UE is configured to send a registration request on a camped first network, which is either a Public Land Mobile Network (PLMN) or a Standalone Non-Public Network (SNPN). The UE then receives either a registration accept or a registration reject with one of 5GMM cause #62 or cause #27, indicating that no network slices are available. Upon determining that the camped first network is to be disabled due to the lack of S-NSSAIs, the UE selects an available second network and sends a registration request to it. The UE receives a registration accept with the camped first network and a third network listed as equivalent networks. The UE then determines that the N1 mode capability for the camped first network is disabled to prevent reselection to it. Finally, the UE allows cell reselection if it determines to enable the N1 mode capability for the camped first network.

[0036] Accordingly, the embodiments herein disclose a system for handling Network Slice Selection Assistance Information (NSSAI) lists in a network. The system comprises at least one User Equipment (UE), one or more base stations, and a Core Network, wherein the UE is communicatively coupled to the base stations, and the base stations are communicatively coupled to the Core Network. The UE is configured to receive an alternative NSSAI (S-NSSAI-A) along with the allowed NSSAI list from the Non-Access Stratum (NAS) upon receiving an alternative NSSAI mapping indicating that a current S-NSSAI is to be replaced with the alternative NSSAI. The UE then performs a network slice-based cell reselection by evaluating information related to the alternative NSSAI, considering the current S-NSSAI as a lower priority and the alternative S-NSSAI as a higher priority (i.e. UE uses alternative S-NSSAI for the slice based cell reselection not the current S-NSSAI).

[0037] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating at least one embodiment and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

[0038] This application is based on and derives the benefit of Indian Provisional Applications IN202341053970 and IN 202341065334, the contents of which are incorporated herein by reference.

[0039] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

[0040] For the purposes of interpreting this specification, the definitions (as defined herein) will apply and whenever appropriate the terms used in singular will also include the plural and vice versa. It is to be understood that the terminology used herein is for the purposes of describing particular embodiments only and is not intended to be limiting. The terms “comprising”, “having” and “including” are to be construed as open-ended terms unless otherwise noted.

[0041] The words / phrases "exemplary", “example”, “illustration”, “in an instance”, “and the like”, “and so on”, “etc.”, “etcetera”, “e.g.,” , “i.e.,” are merely used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein using the words / phrases "exemplary", “example”, “illustration”, “in an instance”, “and the like”, “and so on”, “etc.”, “etcetera”, “e.g.,” , “i.e.,” is not necessarily to be construed as preferred or advantageous over other embodiments.

[0042] Embodiments herein may be described and illustrated in terms of blocks which carry out a described function or functions. These blocks, which may be referred to herein as managers, units, modules, hardware components or the like, are physically implemented by analog and / or digital circuits such as logic gates, integrated circuits, microprocessors, microcontrollers, memory circuits, passive electronic components, active electronic components, optical components, hardwired circuits and the like, and may optionally be driven by a firmware. The circuits may, for example, be embodied in one or more semiconductor chips, or on substrate supports such as printed circuit boards and the like. The circuits constituting a block may be implemented by dedicated hardware, or by a processor (e.g., one or more programmed microprocessors and associated circuitry), or by a combination of dedicated hardware to perform some functions of the block and a processor to perform other functions of the block. Each block of the embodiments may be physically separated into two or more interacting and discrete blocks without departing from the scope of the disclosure. Likewise, the blocks of the embodiments may be physically combined into more complex blocks without departing from the scope of the disclosure.

[0043] It should be noted that elements in the drawings are illustrated for the purposes of this description and ease of understanding and may not have necessarily been drawn to scale. For example, the flowcharts / sequence diagrams illustrate the method in terms of the steps required for understanding of aspects of the embodiments as disclosed herein. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the present embodiments so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Furthermore, in terms of the system, one or more components / modules which comprise the system may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the present embodiments so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

[0044] The accompanying drawings are used to help easily understand various technical features and it should be understood that the embodiments presented herein are not limited by the accompanying drawings. As such, the present disclosure should be construed to extend to any modifications, equivalents, and substitutes in addition to those which are particularly set out in the accompanying drawings and the corresponding description. Usage of words such as first, second, third etc., to describe components / elements / steps is for the purposes of this description and should not be construed as sequential ordering / placement / occurrence unless specified otherwise.

[0045] The principal object of embodiments herein is to disclose a method for cell reselection in 5G networks by determining a status of an N1 mode for each of the one or more candidate cells and by determining whether each of the one or more of the candidate cells support required Network Slice Selection Assistance information (NSSAI).

[0046] Another object of embodiments herein is to disclose systems and methods for handling NSSAI lists sent to the UE AS.

[0047] Another object of embodiments herein is to disclose systems and methods for handling NSSAI lists sent to the UE AS, wherein a partial slicing supported area is provided.

[0048] Another object of embodiments herein is to disclose systems and methods for handling NSSAI lists sent to the UE AS, wherein an alternative NSSAI is passed to the lower layers.

[0049] Another object of embodiments herein is to disclose systems and methods for handling NSSAI lists sent to the UE AS, wherein unused slices are removed for cell reselection.

[0050] Another object of embodiments herein is to disclose systems and methods for handling NSSAI lists sent to the UE AS, it is provided as to when the allowed NSSAI is to be provided by the UE to the lower layers.

[0051] Another object of embodiments herein is to disclose methods and systems for handling disabling and re-enabling of N1 mode for SNPNs in wireless communication networks.

[0052] Cell selection / reselection is a process in 5G networks that ensures a User Equipment (UE) maintains an optimal connectivity and service quality as it moves through one geographical region to another. Unlike handover, which is a network-controlled process typically used during active sessions, cell selection and reselection is primarily a UE-controlled procedure that occurs when the device is in idle (5GMM-IDLE mode or RRC_IDLE state) or inactive (5GMM-CONNECTED mode with RRC inactive indication or RRC_INACTIVE state) mode / state. When the UE is camped on a cell, the UE shall regularly search for a better cell according to the cell reselection criteria. If a better cell is found, that cell is selected The UE search for belter cell includes continuously monitoring of the signal quality of its current serving cell and neighboring cells, measuring parameters such as Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). When the signal quality of the serving cell falls below a predefined threshold, indicating a potential drop in service quality, the UE initiates the cell reselection process. This involves identifying and evaluating candidate cells, considering factors such as signal strength, network slice support, and the availability of necessary features like N1 mode (or any other radio access technology support / enable). The UE then selects the best candidate cell and re-establishes its connection, ensuring consistent and reliable network performance.

[0053] Network slicing is a key concept in telecommunications, allowing a single physical network to be divided into multiple virtual networks tailored for different services or customer needs. The 3GPP TS 24.501 standard explains the procedures for network slicing, including UE configuration updates, registration processes, and Network Slice Specific Authentication and Authorization (NSSAA). These procedures enable allocation of network resources efficiently and maintain service quality across slices, even when the number of connected UEs is at its maximum. However, there are challenges such as synchronizing backoff timers, prioritizing incorrect cells during slice-based cell reselection, and properly disseminating alternative NSSAI information. These issues are discussed in the following sections:

[0054] 3GPP TS 24.501 Section 5.4.4 Generic UE configuration update procedure:

[0055] If the UE supports extended rejected NSSAI and the AMF determines that maximum number of UEs reached for one or more S-NSSAI(s) in the allowed NSSAI as specified in subclause 4.6.2.5, the AMF shall include the rejected NSSAI containing one or more S-NSSAIs with the rejection cause "S-NSSAI not available due to maximum number of UEs reached" in the Extended rejected NSSAI IE in the CONFIGURATION UPDATE COMMAND message. In addition, the AMF may include a back-off timer value for each S-NSSAI with the rejection cause "S-NSSAI not available due to maximum number of UEs reached" included in the Extended rejected NSSAI IE of the CONFIGURATION UPDATE COMMAND message. To avoid large numbers of UEs simultaneously initiating deferred requests, the network should select the value for the backoff timer for each S-NSSAI for the informed UEs so that timeouts are not synchronised.

[0056] 3GPP TS 24.501 Section 5.5.1 Registration procedure:

[0057] If the UE supports extended rejected NSSAI and the AMF determines that maximum number of UEs reached for one or more S-NSSAI(s) in the requested NSSAI as specified in subclause 4.6.2.5, the AMF shall include the rejected NSSAI containing one or more S-NSSAIs with the rejection cause "S-NSSAI not available due to maximum number of UEs reached" in the Extended rejected NSSAI IE in the REGISTRATION ACCEPT message. In addition, the AMF may include a back-off timer value for each S-NSSAI with the rejection cause "S-NSSAI not available due to maximum number of UEs reached" included in the Extended rejected NSSAI IE of the REGISTRATION ACCEPT message. To avoid the large numbers of UEs simultaneously initiating deferred requests, the network should select the value for the backoff timer for each S-NSSAI for the informed UEs so that timeouts are not synchronised.

[0058] 3GPP TS 24.501 Section 4.6 Network slicing:

[0059] When the network slice-specific authentication and authorization procedure is to be initiated for one or more S-NSSAIs in the requested NSSAI or the network slice-specific authentication and authorization procedure is ongoing for one or more S-NSSAIs, these S-NSSAI(s) will be included in the pending NSSAI. When the network slice-specific authentication and authorization procedure is completed for an NSSAI that has been in the pending NSSAI, the S-NSSAI will be moved to the allowed NSSAI or rejected NSSAI depending on the outcome of the procedure.

[0060] 3GPP TS 24.501:

[0061] If the UE has indicated that the UE supports network slice replacement feature and the AMF determines to provide the mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI to the UE, the network shall provide the UE with the alternative NSSAI. The alternative NSSAI is managed per access type independently, i.e., 3GPP access or non-3GPP access, and is applicable for the registration area.

[0062] If the AMF determines that the S-NSSAI which has been replaced is available, the AMF provides the updated alternative NSSAI excluding the S-NSSAI which has been replaced and the corresponding alternative S-NSSAI to the UE during the UE configuration update procedure or during the registration procedure.

[0063] When a new alternative NSSAI for a given PLMN or SNPN is received and the new alternative NSSAI does not include any mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI, the UE shall delete any stored alternative NSSAI for this PLMN and its equivalent PLMN(s) or this SNPN for the same access type.

[0064] If all the S-NSSAI(s) that were replaced in alternative NSSAI are available, the AMF provides the alternative NSSAI with Length of Alternative NSSAI contents set to 0 in the UE configuration update procedure or registration procedure. The AMF also provides the updated allowed NSSAI and configured NSSAI to the UE.

[0065] If the UE has indicated that the UE supports the partial network slice feature and includes the S-NSSAI(s) in the requested NSSAI, the AMF determines the S-NSSAI(s) to be included in the partially allowed NSSAI or the partially rejected NSSAI. When the AMF provides both the partially allowed NSSAI and the partially rejected NSSAI to the UE, each S-NSSAI shall be either in the partially allowed NSSAI or in the partially rejected NSSAI but not both.

[0066] 3GPP TS 24.501:

[0067] 5.4.4 Generic UE configuration update procedure

[0068] 5.4.4.1 General

[0069] The following parameters are supported by the generic UE configuration update procedure without the need to request the UE to perform the registration procedure for mobility and periodic registration update:

[0070] a) 5G-GUTI;

[0071] b) TAI list;

[0072] c) Service area list;

[0073] d) NITZ information;

[0074] e) LADN information;

[0075] e1) Extended LADN information;

[0076] f) Rejected NSSAI;

[0077] NOTE: A cause value associated with a rejected S-NSSAI can be included in the Rejected NSSAI IE or in the Extended rejected NSSAI IE and a back-off timer value associated with rejected S-NSSAI(s) can be included in the Extended rejected NSSAI IE.

[0078] g) void;

[0079] h) Operator-defined access category definitions;

[0080] i) SMS indication;

[0081] j) "CAG information list";

[0082] k) UE radio capability ID;

[0083] l) 5GS registration result;

[0084] m) Truncated 5G-S-TMSI configuration;

[0085] n) T3447 value;

[0086] o) "list of PLMN(s) to be used in disaster condition";

[0087] p) disaster roaming wait range;

[0088] q) disaster return wait range; and

[0089] r) PEIPS assistance information;

[0090] s) Priority indicator;

[0091] t) NSAG information;

[0092] u) RAN timing synchronization;

[0093] v) Alternative NSSAI;

[0094] w) Discontinuous coverage maximum NAS signalling wait time.

[0095] x) Partially allowed NSSAI; and

[0096] y) Partially rejected NSSAI.

[0097] Support for Network Slices with Network Slice Area of Service not matching deployed Tracking Areas:

[0098] The network support for a Network Slice is defined on a per Tracking Area granularity. It may be beneficial to deploy some Network Slices such that the Network Slice have a limited geographical availability that is not matching existing Tracking Area boundaries. The operator can in this case decide to change the topology of the Tracking Areas so they match the boundaries of the Network Slice, or the operator may configure resources for the Network Slices in the cells of TAs where the Network Slices are to be available, and in areas of the TAs where the network slice is defined to be not available the cells are configured with zero resources. The AMF receives from the OAM the information on availability of a network slice when the granularity is smaller than TA, i.e., if the NS-AoS includes TAs where the network slice is not available in some cells of the TA. In order to optimize the end-to-end behaviour, the AMF can, based on NS-AoS information received from OAM, configure supporting UEs with S-NSSAI location availability information, and the 5GCnetwork may need to monitor the S-NSSAI usage and enforce the NS-AoS, e.g., if the UE does not support the S-NSSAI location availability information. A UE that receives S-NSSAI location availability information applies the information as follows.

[0099] - If the S-NSSAI is rejected in the RA or rejected partially in the RA or rejected with a cause code that allows attempting to register the S-NSSAI again, the UE can request the S-NSSAI only if the S-NSSAI location availability information indicates that the S-NSSAI is available at the cell where the UE is camping.

[0100] - If the S-NSSAI is in the Partially Allowed NSSAI, the UE shall not activate User Plane for any already established PDU Session with that S-NSSAI if the UE is in a cell within the RA but outside the Location information of the S-NSSAI.

[0101] - If the S-NSSAI is in the Partially Allowed NSSAI, and the UE in CM-IDLE mode is moved to a cell outside the Location information of the S-NSSAI, and the UE has an established PDU Session with that S-NSSAI, the PDU Session is kept.

[0102] 3GPP TS 24.501:

[0103] 9.11.3.97 Alternative NSSAI:

[0104] The purpose of the Alternative NSSAI information element (as depicted in FIGs. 1A and 1B) is to identify a list of mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI. The Alternative NSSAI information element is coded as shown in FIGs. 1A and 1B. The Alternative NSSAI is a type 4 information element with minimum length of 6 octets and maximum length of 146 octets. The Alternative NSSAI is a type 4 information element with minimum length of 2 octets.

[0105] FIG. 2 depicts an example scenario, wherein the UE may prioritise wrong cells during Network slice based cell reselection. In step 201, the UE sends UL NAS message; e.g., Registration Request message including S-NSSAI-1 in the requested NSSAI list. In step 202, the Network sends DL NAS message e.g. Registration Accept:

[0106] - by including S-NSSAI-1 in Allowed NSSAI list / Partially allowed NSSAI list / Partially Rejected NSSAI list; or

[0107] - by including S-NSSAI-1 in Pending NSSAI list.

[0108] In an embodiment herein, the network entity may send an Alternative NSSAI by mapping S-NSSAI-1 (S-NSSAI to be replaced) to S-NSSAI-A (Alternative S-NSSAI). In an embodiment herein, the network entity may send S-NSSAI location validity information / S-NSSAI time validity information. In step 203, as per spec 24.501, the UE NAS shall send the allowed NSSAI and the partially allowed NSSAI (if any) or the requested NSSAI to the UE AS for the purpose of network slice-based cell reselection. In step 204, the specification does not consider the below slice information for Network slice based cell reselection:

[0109] -a. If S-NSSAI-1 is in Partially rejected NSSAI list.

[0110] -b. If S-NSSAI-1 is associated with NS-AoS / S-NSSAI location validity information / S-NSSAI time validity information.

[0111] - c.If S-NSSAI-1 is sent as part of Pending NSSAI list / S-NSSAI-1 has been put into back off by network (e.g UE received 5GSM cause value #69 "insufficient resources for specific slice" in Downlink NAS message along with back-off timer T3585); or

[0112] -d. If S-NSSAI-1 is replaced by S-NSSAI-A, by sending Alternative NSSAI by Network.

[0113] If the above information is not considered, the UE may prioritise wrong cells during Network slice based cell reselection. For example, S-NSSAI-1 is replaced with S-NSSAI-A in Alternative NSSAI. Since the UE AS does not have information of Alternative NSSAI, the UE AS may perform Network slice based cell reselection using only S-NSSAI-1.

[0114] FIG. 3 depicts an example scenario, wherein the alternative NSSAI is not passed to the lower layers. In step 301, the UE sends UL NAS message e.g. Registration Request message including S-NSSAI-1 in the requested NSSAI list. In step 302, the Network sends DL NAS message (e.g. Registration Accept) by including S-NSSAI-1 and S-NSSAI-A in allowed NSSAI list and includes Alternative NSSAI mapping of S-NSSAI-1 (Slice to be replaced) to S-NSSAI-A (alternative slice). In step 303, the Alternative NSSAI is not passed to the lower layers. In step 304, the UE NAS shall send allowed NSSAI list / partially allowed NSSAI list / requested NSSAI list / NSAG to the UE AS, so that the UE AS shall perform network slice based cell reselection. In step 305, the UE AS is not aware of Alternative NSSAI information ( i.e. S-NSSAI-1 is replaced by S-NSSAI-A); hence the UE AS shall perform network slice based cell reselection by considering all the slices in allowed and partially allowed NSSAI lists (i.e., S-NSSAI-1 and S-NSSAI-A), even though S-NSSAI-1 will not be used. In an example, suppose UE in the location where S-NSSAI-1 is supported specific cell or frequencies (Say C1), S-NSSAI-A is supported in specific cell or frequencies (say C2) and S-NSSAI-1, S-NSSAI-A is allowed in specific cell or frequencies (say C3). When the NAS layer indicates the S-NSSAI-1 and S-NSSAI-A to the lower layer in allowed / partially allowed list to the lower layer, the lower layer reselects the cell based on S-NSSAI-1 and S-NSSAI-A. Ideally, the AS shall select C3 (as it supports both S-NSSAI-1, S-NSSAI-A ) and C2 which supports S-NSSAI-A. the UE can use S-NSSAI-A for the dedicated PDU session. If the UE selects C1, it will not get any services.

[0115] FIG. 4 depicts an example scenario, wherein an unused slice is prioritized for cell reselection. In step 401, the UE sends the UL NAS message (e.g., Registration Request message including S-NSSAI-1) in the requested NSSAI list. In step 402, the Network sends DL NAS message (e.g., Registration Accept) by including S-NSSAI-1 in allowed NSSAI. In step 403, the UE NAS shall send the allowed NSSAI list (S-NSSAI-1) to the UE AS so that the UE AS shall perform network slice based cell reselection for S-NSSAI-1. In step 404, the UE sends the UL NAS message (e.g., PDU Session establishment request for S-NSSAI-1). In step 405, the Network sends DL NAS message (e.g., PDU session establishment reject for S-NSSAI-)1 with back-off timer or with cause congestion etc (e.g., T3396, T3584 or T3585 (see 3GPP TS 24.501 clause 6.2.7 and clause 6.2.8). In step 406, the UE AS shall consider S-NSSAI-1 for the purpose of network slice based cell reselection, even though S-NSSAI-1 is currently put in back-off list or slice is rejected with congestion cause etc.

[0116] NSAG information to lower layer:

[0117] In the current state of art, NAS provide lower layer with the following information.

[0118] TS 24.501:

[0119] The UE NAS layer shall provide the lower layers with

[0120] a) the most recent NSAG information stored in the UE (see subclause 4.6.2.2);

[0121] b) the allowed NSSAI or the requested NSSAI for the purpose of network slice-based cell reselection (see 3GPP TS 23.501);

[0122] TS 23.501:

[0123] The UE NAS provides to the UE AS the NSAG Information as received from the AMF and the S-NSSAIs in the Allowed NSSAI as input to cell reselection, except when the UE intends to register with a new set of S-NSSAIs with a Requested NSSAI different from the current Allowed NSSAI, in which case the UE NAS provides to the UE AS layer the NSAG Information as received from the AMF and the S-NSSAIs in the Requested NSSAI, and this may trigger a cell reselection, before sending the Registration Request including the new Requested NSSAI.

[0124] But when the UE will provide the allowed NSSAI to the lower layer is not specified.

[0125] Consider a scenario, wherein all the S-NSSAIs in the configured and allowed NSSAI lists are not available in the current TA. For example, if the UE has only S1 and S2 as configured as allowed NSSAIs / partially allowed and both are not allowed in the current TA (i.e., TAI-1). In this scenario, even if the UE selects TAI-1, the UE will not be able to use Network slice services.

[0126] Consider a scenario, wherein all the S S-NSSAIs in the configured and allowed NSSAI lists are not available in a cell(s). For example, if the UE has only S1 and S2 as configured are allowed NSSAIs and both are not allowed in the current cell (i.e., Cell-1). In this scenario, even if the UE selects Cell-1, the UE will not be able to use Network slice services.

[0127] To ensure optimal connectivity and service continuity, the UE engages in cell reselection by evaluating neighboring cells based on signal quality, network slice support, and N1 mode status. When managing network slice information, the UE must consider equivalent SNPNs and the current PLMN subscription to maintain seamless service. The UE maintains equivalent SNPN per entry of the "list of subscriber data" or the selected PLMN subscription. The UE disables N1 mode capabilities when a specific cause is received (for example, 5GMM cause 27 “N1 mode is not allowed”) in any downlink message like registration reject, service reject, or deregistration request (Network initiated).

[0128] #27 (N1 mode not allowed):

[0129] The UE shall set the 5GS update status to 5U3 ROAMING NOT ALLOWED (and shall store it according to subclause 5.1.3.2.2) and shall delete any 5G-GUTI, last visited registered TAI, TAI list and ngKSI. Additionally, the UE shall reset the registration attempt counter and shall enter the state 5GMM-DEREGISTERED.LIMITED-SERVICE. If the message has been successfully integrity checked by the NAS, the UE shall set:

[0130] 1) the PLMN-specific N1 mode attempt counter for 3GPP access and the PLMN-specific N1 mode attempt counter for non-3GPP access for that PLMN in case of PLMN; or

[0131] 2) the SNPN-specific attempt counter for 3GPP access for the current SNPN in case of SNPN and the SNPN-specific attempt counter for non-3GPP access for the current SNPN;

[0132] to the UE implementation-specific maximum value. The UE shall disable the N1 mode capability for the specific access type for which the message was received (see subclause 4.9).

[0133] The UE disables N1 mode capability for the current PLMN / SNPN when it receives the specific cause.

[0134] #62 (No network slices available):

[0135] The UE may perform a PLMN selection or SNPN selection according to 3GPP TS 23.122 and additionally, the UE may disable the N1 mode capability for the current PLMN or SNPN if each S-NSSAI in the default configured NSSAI was rejected with cause "S-NSSAI not available in the current PLMN or SNPN" or "S-NSSAI not available due to the failed or revoked network slice-specific authentication and authorization" as described in subclause 4.9.

[0136] The UE may disable the N1 mode capability for the currently camped PLMN or SNPN over non-3GPP access, if no network slice is available for the camped PLMN or SNPN.

[0137] The UE re-enable N1 mode in my cases (for example, 3247 timer expiry)

[0138] - re-enable the N1 mode capability for 3GPP access and, for each PLMN-specific N1 mode attempt counter for 3GPP access that has a value greater than zero and less than a UE implementation-specific maximum value, remove the respective PLMN from the list of PLMNs where N1 mode is not allowed for 3GPP access (see 3GPP TS 23.122);

[0139] - re-enable the N1 mode capability for non-3GPP access and, for each PLMN-specific N1 mode attempt counter for non-3GPP access that has a value greater than zero and less than a UE implementation-specific maximum value, remove the respective PLMN from the list of PLMNs where N1 mode is not allowed for non-3GPP access;

[0140] - re-enable the N1 mode capability for 3GPP access and, for each SNPN-specific attempt counter for 3GPP access that has a value greater than zero and less than a UE implementation-specific maximum value, remove the respective SNPN from the list of SNPNs for which the N1 mode capability was disabled due to receipt of a reject from the network with 5GMM cause #27 "N1 mode not allowed" (see 3GPP TS 23.122);

[0141] - re-enable the N1 mode capability for non-3GPP access and, for each SNPN-specific attempt counter for non-3GPP access that has a value greater than zero and less than a UE implementation-specific maximum value, remove the respective SNPN from the list of SNPNs for which N1 mode capability was disabled for non-3GPP access due to receipt of a reject from the network with 5GMM cause #27 "N1 mode not allowed" or 5GMM cause #72 "non-3GPP access to 5GCN not allowed"; and

[0142] Above is just an example and there can be many cases where the UE may re-enable N1 mode capabilities for the PLMN and SNPN. In an example, the UE may disable N1 mode capability due to the UE's registration attempt counter reaches 5 on PLMN and SNPN or voice is not available on the SNPN.

[0143] Disabling and re-enabling of UE's N1 mode capability for 3GPP access:

[0144] The UE shall only disable the N1 mode capability for 3GPP access when in 5GMM-IDLE mode. When the UE is disabling the N1 mode capability for 3GPP access for a PLMN not due to redirection to EPC, it should proceed as follows:

[0145] a) select an E-UTRA cell connected to EPC of the registered PLMN or a PLMN from the list of equivalent PLMNs, if the UE supports S1 mode and the UE has not disabled its E-UTRA capability as specified in 3GPP TS 24.301;

[0146] b) if an E-UTRA cell connected to EPC of the registered PLMN or a PLMN from the list of equivalent PLMNs cannot be found, the UE does not support S1 mode or the UE has disabled its E-UTRA capability as specified in 3GPP TS 24.301

[0015] , the UE may select another RAT of the registered PLMN or a PLMN from the list of equivalent PLMNs that the UE supports;

[0147] c) if another RAT of the registered PLMN or a PLMN from the list of equivalent PLMNs cannot be found, then enter the state 5GMM-REGISTERED.PLMN-SEARCH or 5GMM-DEREGISTERED.PLMN-SEARCH, or the UE does not have a registered PLMN, then enter the state 5GMM-DEREGISTERED.PLMN-SEARCH and performs PLMN selection as specified in 3GPP TS 23.122. If disabling of the N1 mode capability for 3GPP access was not due to a UE-initiated de-registration procedure for 5GS services over 3GPP access not due to switch-off, the UE may re-enable the N1 capability for this PLMN selection. As an implementation option, if the UE does not have a registered PLMN, instead of performing PLMN selection, the UE may select another RAT of the selected PLMN if the UE has chosen a PLMN and the RAT is supported by the UE; or

[0148] d) if no other allowed PLMN and RAT combinations are available, then the UE may re-enable the N1 mode capability for 3GPP access and indicate to lower layers to remain camped in NG-RAN of the registered PLMN, and may periodically scan for another PLMN and RAT combination which can provide EPS services or non-EPS services (if the UE supports EPS services or non-EPS services). How this periodic scanning is done, is UE implementation dependent.

[0149] When the UE is disabling the N1 mode capability for 3GPP access for an SNPN, it should proceed as follows:

[0150] a) enter the state 5GMM-REGISTERED.PLMN-SEARCH or 5GMM-DEREGISTERED.PLMN-SEARCH and perform SNPN selection as specified in 3GPP TS 23.122 [5]. If disabling of the N1 mode capability for 3GPP access was not due to a UE-initiated de-registration procedure for 5GS services over 3GPP access not due to switch-off, the UE may re-enable the N1 capability for this SNPN selection; or

[0151] b) if no other SNPN is available, then the UE may re-enable the N1 mode capability for 3GPP access and indicate to lower layers to remain camped in NG-RAN of the registered SNPN.

[0152] When the UE is disabling the N1 mode capability upon receiving cause value #31 "Redirection to EPC required" as specified in subclauses 5.5.1.2.5, 5.5.1.3.5 and 5.6.1.5, it should proceed as follows:

[0153] a) If the UE is in NB-N1 mode:

[0154] 1) if lower layers do not provide an indication that the current E-UTRA cell is connected to EPC or lower layers do not provide an indication that the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, search for a suitable NB-IoT cell connected to EPC according to 3GPP TS 36.304;

[0155] 2) if lower layers provide an indication that the current E-UTRA cell is connected to EPC and the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, perform a core network selection to select EPC as specified in subclause 4.8.4A.1; or

[0156] 3) if lower layers cannot find a suitable NB-IoT cell connected to EPC or there is no suitable NB-IoT cell connected to EPC which supports CIoT EPS optimizations that are supported by the UE, the UE, as an implementation option, may indicate to lower layers to remain camped in E-UTRA cell connected to 5GCN, may then start an implementation-specific timer, and enter the state 5GMM-REGISTERED.LIMITED-SERVICE. The UE may re-enable the N1 mode capability for 3GPP access at expiry of the implementation-specific timer, if the timer had been started, and may then proceed with the appropriate 5GMM procedure.

[0157] b) If the UE is in WB-N1 mode:

[0158] 1) if lower layers do not provide an indication that the current E-UTRA cell is connected to EPC or lower layers do not provide an indication that the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, search for a suitable E-UTRA cell connected to EPC according to 3GPP TS 36.304;

[0159] 2) if lower layers provide an indication that the current E-UTRA cell is connected to EPC and the current E-UTRA cell supports CIoT EPS optimizations that are supported by the UE, then perform a core network selection to select EPC as specified in subclause 4.8.4A.1; or

[0160] 3) if lower layers cannot find a suitable E-UTRA cell connected to EPC or there is no suitable E-UTRA cell connected to EPC which supports CIoT EPS optimizations that are supported by the UE, the UE, as an implementation option, may indicate to lower layers to remain camped in E-UTRA cell connected to 5GCN, may then start an implementation-specific timer, and enter the state 5GMM-REGISTERED.LIMITED-SERVICE. The UE may re-enable the N1 mode capability for 3GPP access at expiry of the implementation-specific timer, if the timer had been started, and may then proceed with the appropriate 5GMM procedure.

[0161] When the UE supporting both N1 mode and S1 mode needs to stay in E-UTRA connected to EPC (e.g., due to the domain selection for UE originating sessions as specified in subclause 4.3.2), in order to prevent unintentional handover or cell reselection from E-UTRA connected to EPC to NG-RAN connected to 5GCN, the UE operating in single-registration mode shall disable the N1 mode capability for 3GPP access and:

[0162] a) shall set the N1mode bit to "N1 mode for 3GPP access not supported" in the UE network capability IE (see 3GPP TS 24.301

[0015] ) of the ATTACH REQUEST message and the TRACKING AREA UPDATE REQUEST message in EPC; and

[0163] b) the UE NAS layer shall indicate the access stratum layer(s) of disabling of the N1 mode capability for 3GPP access.

[0164] If the UE is required to disable the N1 mode capability for 3GPP access and select E-UTRA or another RAT, and the UE is in the 5GMM-CONNECTED mode,

[0165] - if the UE has a persistent PDU session, then the UE waits until the radio bearer associated with the persistent PDU session has been released;

[0166] - otherwise, the UE shall locally release the established NAS signalling connection;

[0167] and enter the 5GMM-IDLE mode before selecting E-UTRA or another RAT.

[0168] If the UE is disabling its N1 mode capability for 3GPP access before selecting E-UTRA or another RAT, the UE shall not perform the UE-initiated de-registration procedure of subclause 5.5.2.2. The UE shall re-enable the N1 mode capability for 3GPP access when the UE performs PLMN selection, SNPN selection or SNPN selection for onboarding services over 3GPP access, unless

[0169] - disabling of the N1 mode capability for 3GPP access was due to a UE-initiated de-registration procedure for 5GS services over 3GPP access not due to switch-off; or

[0170] - the UE has already re-enabled the N1 mode capability for 3GPP access when performing items c) or d) above ; or

[0171] - the UE disables the N1 mode capability for 3GPP access for cases described in subclauses 5.5.1.2.7 and 5.5.1.3.7.

[0172] If the disabling of N1 mode capability for 3GPP access was due to IMS voice is not available over 3GPP access and the UE's usage setting is "voice centric", the UE shall re-enable the N1 mode capability for 3GPP access when the UE's usage setting is changed from "voice centric" to "data centric", as specified in subclauses 4.3.3.

[0173] The UE should memorize the identity of the PLMN or SNPN where N1 mode capability for 3GPP access was disabled and should use that stored information in subsequent PLMN or SNPN selections as specified in 3GPP TS 23.122.

[0174] If the disabling of N1 mode capability for 3GPP access was due to successful completion of an emergency services fallback, the criteria to enable the N1 mode capability again are UE implementation specific.

[0175] The UE shall disable the N1 mode capability for 3GPP access if requested by the upper layers (e.g., see subclause U.2.2.6.4 in 3GPP TS 24.229

[0014] ). If the UE disabled the N1 mode capability for 3GPP access based on the request from the upper layers (e.g., see subclause U.2.2.6.4 in 3GPP TS 24.229), the criteria to re-enable the N1 mode capability for 3GPP access after the completion of an emergency service are UE implementation specific.

[0176] If the N1 mode capability for 3GPP access was disabled due to the UE initiated de-registration procedure for 3GPP access or for 3GPP access and non-3GPP access and the UE is operating in single-registration mode (see subclause 5.5.2.2.3), upon request of the upper layers to re-register for 5GS services over 3GPP access or the UE needs to come out of unavailability period and resume normal services, the UE shall enable the N1 mode capability for 3GPP access again.

[0177] As an implementation option, the UE may start a timer for enabling the N1 mode capability for 3GPP access when the UE's registration attempt counter reaches 5 and the UE disables the N1 mode capability for 3GPP access for cases described in subclauses 5.5.1.2.7 and 5.5.1.3.7. The UE should memorize the identity of the PLMNs or SNPNs where N1 mode capability for 3GPP access was disabled. On expiry of this timer:

[0178] - if the UE is in Iu mode or A / Gb mode and is in idle mode as specified in 3GPP TS 24.008

[0013] on expiry of the timer, the UE should enable the N1 mode capability for 3GPP access;

[0179] - if the UE is in Iu mode and a PS signalling connection exists, but no RR connection exists, the UE may abort the PS signalling connection before enabling the N1 mode capability for 3GPP access;

[0180] - if the UE is in S1 mode and is in EMM-IDLE mode as specified in 3GPP TS 24.301

[0015] , on expiry of the timer, the UE should enable the N1 mode capability for 3GPP access; and

[0181] - if the UE is in N1 mode, on expiry of the timer, the UE should remove the PLMN or SNPN from the memorized identity of the PLMNs or SNPNs where N1 mode capability for 3GPP access was disabled.

[0182] If the UE is in Iu mode or A / Gb mode and an RR connection exists, the UE should delay enabling the N1 mode capability for 3GPP access until the RR connection is released. If the UE is in S1 mode and is in EMM-CONNECTED mode as specified in 3GPP TS 24.301, the UE should delay enabling the N1 mode capability for 3GPP access until the NAS signalling connection in S1 mode is released.

[0183] The UE may disable the N1 mode capability for currently camped PLMN or SNPN over 3GPP access (see 3GPP TS 23.122) if no network slice is available for the camped PLMN or SNPN. If the UE receives ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message provided with S-NSSAI and the PLMN ID in the Protocol configuration options IE or Extended protocol configuration options IE (see subclause 6.5.1.3 of 3GPP TS 24.301), the UE may re-enable N1 mode capability for the corresponding PLMN.

[0184] If the UE attempts to establish an emergency PDU session in a PLMN where N1 mode capability was disabled due to the UE's registration attempt counter have reached 5, the UE may enable N1 mode capability for that PLMN memorized by the UE.

[0185] NOTE: If N1 mode capability is disabled due to the UE's registration attempt counter reaches 5, the value of the timer for re-enabling N1 mode capability is recommended to be the same as the value of T3502 which follows the handling specified in subclause 5.3.8. If the value of T3502 is indicated as zero by the network, an implementation specific non-zero value can be used for the timer for re-enabling N1 mode capability.

[0186] FIG. 5 depicts a scenario, wherein the UE may be able to select a SNPN (which is not allowed), which can lead to ping pong. In step 1, the UE sends a registration request on the camped SNPN-1. In step 2, the UE receives a registration accept / reject with 5GMM cause 62. In step 3, the UE determines that the current SNPN is to be disabled as there is not S-NSSAI allowed in the current SNPN. In step 4, the UE selects the available / allowed SNPN-2. In step 5, the UE sends a registration request on SNPN-2. In step 6, the UE receives the registration accept with SNPN-1 and SNPN-3 in E-SNPN list (Equivalent SNPN). In step 7, the UE may be able to select SNPN-1 which is not allowed. This will lead to ping pong. A similar problem can arise in PLMN too.

[0187] If the UE supports access to an SNPN using credentials from a credential holder, equivalent SNPNs or both then how to select E-SNPN for selected entry or disabling N1 mode for the selected entry is not known. For example, Entry 1: SNPN1, E-SNPN (SNPN2), and Entry 2: SNPN1. The UE is registered on SNPN-1, gets the reject, UE disabled N1 mode capability for the SNPN-1, UE move to SNPN-2 and received SNPN-1 as E-SNPN. The UE selects entry-2, as the N1 mode capability of SNPN-1 is disabled, and the UE may not be able to select SNPN-1.

[0188] Consider that the UE disables N1 mode capability for the SNPN (on a specific condition being met in the UE (e.g., receive specific reject cause like 5GMM cause 27 or 62 is received) or an attempt counter reaches 5. In this case, the UE disables N1 mode capability for the SNPN. If the UE has a single SNPN and the capability is disabled, then the UE will not be able to get services.

[0189] As discussed above, NSAG (Network Slice Assistance Group) information is provided to the UE NAS layer, which then passes it to the AS layer for usage in cell selection and RACH (Random Access Channel) handling. The primary purpose of NSAG information is to utilize the S-NSSAI (Single-Network Slice Selection Assistance Information) for these tasks. Currently, suppose the UE has S1 as the allowed S-NSSAI, but S1 is congested. In this scenario, the AMF (Access and Mobility Management Function) may determine that the S-NSSAI SA can be an alternative to S-NSSAI S1 and inform the UE accordingly. However, problems can arise: firstly, it is possible that SA is not yet included in the NSAG information, causing the UE to continue using S1 for cell selection / cell reselection and potentially move to a UE to the cell where S1 is available but congested and thus unusable. Secondly, the UE might still prioritize S1 based on the NSAG information if no priority is given for SA, leading to suboptimal cell selection / cell reselection decisions.

[0190] Further, with respect to the disabling and re-enabling of N1 mode for SNPN, as discussed above, the problem begins when the UE selects SNPN-1 and receives a reject message (e.g., 5GMM cause 62), leading to the disabling of N1 mode. Subsequently, the UE moves to SNPN-2, initiates registration, and receives SNPN-1 and SNPN-3 as equivalent SNPNs. This information is passed from the NAS layer to the AS layer. However, if the cell reselection criteria are met for SNPN-1, the UE might reselect SNPN-1, despite its N1 mode capability being disabled, which is not allowed. This results in a ping-pong effect where the UE repeatedly reselects SNPN-1, fails due to the disabled N1 mode, and then searches for another SNPN where N1 mode is enabled, potentially returning to SNPN-2. This causes unnecessary network coverage issues and service disruptions, as the NAS layer will not take corrective action when the UE attempts to use SNPN-1 with N1 mode disabled, leading to an out-of-service condition.

[0191] Hence, there is a need in the art for solutions which will overcome the above mentioned drawback(s), among others.

[0192] According to an aspect of an exemplary embodiment, there is provided a communication method in a wireless communication. Specifically, the embodiment herein provides a disclose a method for cell selection / reselection in a fifth generation (5G) network.

[0193] The embodiments herein disclose a method for cell reselection in 5G networks by determining a status of an N1 mode for each of the one or more candidate cells and by determining whether each of the one or more of the candidate cells support required Network Slice Selection Assistance information (NSSAI). Referring now to the drawings, and more particularly to FIGs. 6 through 17, where similar reference characters denote corresponding features consistently throughout the figures, there are shown embodiments.

[0194] FIG. 6 illustrates a system 600 for cell selection / reselection in 5G networks, according to the embodiments herein. The system 600 comprises a User Equipment (UE) 602, a gNodeB 604, and a Core Network 606. For illustrative purposes, a single UE is shown. It is understood that the system can comprise a plurality of UEs, gNodeBs, and core networks.

[0195] The User Equipment (UE) 602 comprises a measurement module 608, an NAS (Non-Access Stratum) layer 610, and an AS (Access Stratum) layer 612. The NAS (Non-Access Stratum) Layer 610 within the UE 602 manages signaling and control messages related to mobility and session management. The NAS layer 610 is responsible for receiving Network Slice Assistance Group (NSAG) information and passing it to the AS (Access Stratum) layer 612, which handles radio communication with the network. The AS layer 612 executes the procedures for cell reselection and handover, ensuring seamless connectivity as the UE 602 moves between cells. The measurement module 608 continuously monitors the signal quality of the serving cell and neighboring cells using parameters such as Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and Signal-to-Interference-plus-Noise Ratio (SINR). The measurement module 608 ensures that the UE 602 can detect when the signal quality drops below a predefined threshold, prompting the need for cell reselection.

[0196] The gNodeB 604 serves as the intermediary between the UE 602 and the core Network 606. It comprises radio transmitters 614 and receivers 616 that facilitate the radio interface for communication with UEs 602. The gNodeB 604 also includes a signal processing unit that manages incoming and outgoing signals and a control unit 617 for controlling the overall operation of the base station, including resource allocation and handover procedures.

[0197] The core network 608 comprises several key functions that manage the processes essential for cell reselection and overall network performance. The Access and Mobility Management Function (AMF) 618 handles mobility management tasks, including the registration, authentication, and tracking of UEs. The User Plane Function (UPF) 620 manages data traffic between the UE 602 and external data networks, ensuring efficient data routing and transmission. The Network Slice Selection Function (NSSF) 622 is responsible for managing network slice selection and assignment, which is critical for supporting diverse services and applications in a 5G environment. The Unified Data Management (UDM) function 624 stores subscription data and policy information for UEs, enabling personalized and efficient service delivery.

[0198] The system 600 is configured for performing cell selection / reselection in 5G networks, according to the embodiments herein. FIG. 7 illustrates the method steps 700 involved in the cell selection / reselection, according to the embodiments herein. At step 702, the UE monitors a signal quality of a serving cell and one or more neighbouring cells, the serving cell and the one or more neighbouring cells belonging to one of Public Land Mobile Network (PLMN) and Standalone Non-Public Network (SNPN). At step 704, the UE identifies one or more candidate cells based on signal measurements from the one or more neighbouring cells, on detecting that a signal quality of the serving cell is below a predefined threshold. At step 706, the UE determines a status of an N1 mode capability for each of the one or more candidate cells. At step 708, the UE determines whether each of the one or more of the candidate cells support required Network Slice Selection Assistance information (NSSAI). At step 710, the UE reselects a new serving cell based on the determined status of the N1 mode and the determined NSSAI. Each of the above steps is explained in detail below.

[0199] According to the embodiments herein, the UE determines a status of an N1 mode capability for each of the one or more candidate cells. FIG. 8 depicts a scenario, wherein the UE enables N1 mode for SNPN. In step 801, the UE sends a registration request on the camped SNPN-1. In step 802, the UE receives a registration accept / reject with 5GMM cause 62. In step 803, the UE determines that the current SNPN is to be disabled as the S-NSSAI is not allowed in the current SNPN. In step 804, the UE selects an available / allowed SNPN-2. In step 805, the UE sends a registration request on SNPN-2. In step 806, the UE receives the registration accept with SNPN-1 and SNPN-3 in E-SNPN list (Equivalent SNPN). In step 807, the UE / NAS / AS removes SNPN-1 from the E-ESNPN before passing it to lower layer (AS layer); i.e., only SNPN-3 is passed to lower layer in E-SNPN. The UE memorizes SNPN-1 and SNPN-3 as E-SNPN. In step 808, if the UE determines to enable N1 mode for the SNPN-1 (e.g., UE Implementation dependent timer or TJ timer expiry or T3402 or any over reason) then, in step 809, the UE indicates E-SNPN to the lower layer including SNPN-1 (i.e., SNPN-1 and SNPN-3 are indicated).

[0200] FIG. 9 depicts a scenario, wherein the UE enables N1 mode for SNPN, according to the embodiments herein. In step 901, the UE sends a registration request on the camped SNPN-1. In step 902, the UE receives a registration accept / reject with 5GMM cause 62. In step 903, the UE determines that the current SNPN is to be disabled as there is not S-NSSAI allowed in the current SNPN. In step 904, the UE selects an available / allowed SNPN-2. In step 905, the UE sends a registration request on SNPN-2. In step 906, the UE receives a registration accept with SNPN-1 and SNPN-3 in E-SNPN list (Equivalent SNPN). In step 907, the UE / NAS / AS maintains that N1 mode capability for SNPN-1 is disabled so it does not allow reselection / HO / SNPN selection on SNPN-1. If N1 mode capability is disabled for a SNPN that is part of the list of "equivalent SNPN", the UE implementation ensures that registration to a different SNPN within the list of "equivalent SNPNs" does not result in SNPN selection / reselection to the SNPN which is equivalent SNPN whose N1 mode capability is disabled. In step 908, if the UE determines to enable N1 mode for the SNPN-1 (e.g., UE Implementation dependent timer or TJ timer expiry or T3402 or any over reason). The UE implementation allows for the reselection / SNPN selection etc. In step 909, the UE / NAS / AS allows reselection / HO / SNPN selection on SNPN-1.

[0201] FIG. 10 depicts a scenario, wherein the UE re-enables N1 mode for SNPN, according to the embodiments herein. In step 1001, the UE selects entry-1 on "list of subscriber data" UE send registration request on the camped SNPN-1. In step 1002, the UE receives a registration accept / reject with 5GMM cause 62. In step 1003, the UE determines that the current SNPN is to be disabled as there is not S-NSSAI allowed in the current SNPN. In step 1004, the UE selects an available / allowed SNPN-2. In step 1005, the UE sends a registration request on SNPN-2. In step 1006, the UE receives the registration accept with SNPN-1 and SNPN-3 in E-SNPN list (Equivalent SNPN). In step 1007, the UE / NAS / AS removes SNPN-1 from E-ESNPN before passing it to lower layer (AS layer); i.e., only SNPN-3 is passed to the lower layer in E-SNPN. The UE memorize SNPN-1 and SNPN-3 as E-SNPN. In step 1008, if UE determines to select other entry in "list of subscriber data" (e.g., entry-2) then, in step 1009, the UE shall re-enable N1 mode capability for SNPN-1. Additionally, the UE maintains the disabled N1 mode capability for SNPN per entry in the "list of subscriber data". When another entry is selected, the UE considers the disabled N1 mode capability for SNPN for the selected entry in the "list of subscriber data". Additionally, the UE enables N1 mode capability for the SNPN when select entry is changed in "list of subscriber data".

[0202] FIG. 11 depicts a scenario, wherein the UE disables N1 mode for SNPN. In step 1101, the UE sends a registration request on the camped SNPN-1. In step 1102, the UE receives a registration accept / reject with 5GMM cause 62. In step 1103, the UE determines that the current SNPN is to be disabled as there is not S-NSSAI allowed in the current SNPN. In step 1104, the UE selects an available / allowed SNPN-2. In step 1105, the UE sends a registration request on SNPN-2. In step 1106, the receives the registration accept with SNPN-1 and SNPN-3 in E-SNPN list (Equivalent SNPN). In step 1107, the UE / NAS / AS maintains that the N1 mode capability for SNPN-1 is disabled, so it does not allow reselection / HO / SNPN selection on SNPN-1. If N1 mode capability is disabled for a SNPN that is part of the list of "equivalent SNPN", the UE implementation ensures that registration to a different SNPN within the list of "equivalent SNPNs" does not result in SNPN selection / reselection to the SNPN which is equivalent SNPN whose N1 mode capability is disabled. In step 1108, if the UE determines to select other entry in the “list of subscriber data”, then in step 1109, the UE / NAS / AS allows reselection / HO / SNPN selection on SNPN-1. The UE implementation allows for the reselection / SNPN selection etc.

[0203] FIG. 12 depicts a scenario, wherein the UE enables N1 mode for SNPN. In step 1201, the UE sends a registration request on the camped PLMN-1. In step 1202, the UE receives a registration accept / reject with 5GMM cause 62. In step 1203, the UE determines that the current PLMN is to be disabled as there is not S-NSSAI allowed in the current PLMN. In step 1204, the UE selects an available / allowed PLMN-2. In step 1205, the UE sends a registration request on PLMN-2. In step 1206, the UE receives a registration accept with SNPN-1 and SNPN-3 in E-PLMN list (Equivalent PLMN). In step 1207, the UE / NAS / AS removes PLMN-1 from E-PLMN before passing it to lower layer (AS layer); i.e., only PLMN-3 is passed to lower layer in E-PLMN. UE memorize PLMN-1 and PLMN-3 as E-PLMN. In step 1208, if the UE determines to enable N1 mode for the PLMN-1, then in step 1209, the UE indicates E-PLMN to the lower layer including PLMN-1 (i.e., PLMN-1 and PLMN-3 are indicated).

[0204] Consider that the UE disables N1 mode capability for the SNPN (on a specific condition being met in the UE (e.g., receive specific reject cause like 5GMM cause 27 or 62 is received) or an attempt counter reaches 5. If N1 mode capability of SNPN is disabled and there is no other SNPN available, the UE can exist SNPN access mode.

[0205] FIG. 13 depicts a scenario, wherein the UE enables N1 mode for SNPN. In step 1301, the UE sends a registration request on the camped SNPN-1 from enrtry-1 of the “list of subscriber data”. In step 1302, the UE receives a registration accept / reject with 5GMM cause 62 or 27 or the registration attempt count reaches 5. In step 1303, the UE determines that the current SNPN is to be disabled as there is not S-NSSAI allowed in the current SNPN. In embodiment herein, if the MS does not support access to an SNPN using credentials from a credentials holder and does not support equivalent SNPNs, the MS should maintain a list of SNPNs where the N1 mode capability was disabled because received 5GMM reject cause 27 or cause 62 or registration attempt count reaches 5 or any other reason. If the MS supports access to an SNPN using credentials from a credentials holder, equivalent SNPNs or both the MS should maintain one or more lists of SNPNs where the N1 mode capability was disabled because received 5GMM reject cause 27 or cause 62 or registration attempt count reaches 5, each associated with an entry of the "list of subscriber data". In step 1304, the UE selects an available / allowed SNPN-2. In step 1305, the UE sends a registration request on SNPN-2. In step 1306, the UE receives a registration accept with SNPN-1 and SNPN-3 in E-SNPN list (Equivalent SNPN). In step 1307, the UE / NAS / AS maintains that N1 mode capability for SNPN-1 is disabled, so it does not allow for reselection / HO / SNPN selection on SNPN-1. In step 1308, if the UE determines to select other entry in the “list of subscriber data”, then in step 1309, the UE / NAS / AS allows reselection / HO / SNPN selection on SNPN-1. In an embodiment herein, if the UE re-enables N1 mode capability for the SNPN, the UE should remove the SNPN identity from the selected entry of the “list of subscriber data” for which the UE intended to re-enable SNPN. In an embodiment herein, when the UE selects a different entry of the “list of subscriber data”, then it may delete the SNPNs for which the N1 mode capability was disabled for all the entries in the “list of subscriber data”.

[0206] In an embodiment herein, if the MS does not support access to an SNPN using credentials from a credentials holder and does not support equivalent SNPNs, the MS should maintain a list of SNPNs for which the N1 mode capability was disabled due to receipt of a reject from the network with 5GMM cause #27 "N1 mode not allowed". When the MS disables its N1 mode capability due to receipt of a reject from an SNPN with 5GMM cause #27 "N1 mode not allowed":

[0207] - the MS should add the SNPN identity of the SNPN which sent a reject with 5GMM cause #27 "N1 mode not allowed" to the list of SNPNs for which the N1 mode capability was disabled and should start timer TJ if timer TJ is not already running. The number of SNPNs for which the N1 mode capability was disabled that the MS can store is implementation specific, but it shall be at least one. The value of timer TJ is MS implementation specific.

[0208] - in automatic SNPN selection, the MS shall not select an SNPN for which the N1 mode capability was disabled as SNPN selection candidates, unless no other SNPN is available.

[0209] - if the MS is not configured to use timer T3245, the MS maintains a list of SNPN-specific attempt counters for 3GPP access as specified in 3GPP TS 24.501

[0064] , and T3247 expires, then the MS removes for each SNPN-specific attempt counter for 3GPP access that has a value greater than zero and less than the MS implementation-specific maximum value the respective SNPN from the list of SNPNs for which the N1 mode capability was disabled, as specified in clause 5.3.20.3 in 3GPP TS 24.501; and

[0210] - the MS shall delete stored information on SNPNs for which the N1 mode capability was disabled when the MS is switched off, the USIM is removed, the entries of the "list of subscriber data" for the SNPNs are updated, or timer TJ expires.

[0211] In an embodiment herein, if the MS supports access to an SNPN using credentials from a credentials holder, equivalent SNPNs or both, the MS should maintain a list of SNPNs for which the N1 mode capability was disabled due to receipt of a reject from the network with 5GMM cause #27 "N1 mode not allowed“each associated with an entry of the "list of subscriber data". When the MS disables its N1 mode capability due to receipt of a reject from an SNPN with 5GMM cause #27 "N1 mode not allowed":

[0212] - the MS should add the SNPN identity of the SNPN which sent a reject with 5GMM cause #27 "N1 mode not allowed" to the list of SNPNs for which the N1 mode capability was disabled each associated with an entry of the "list of subscriber data“ and should start timer TJ if timer TJ is not already running. The number of SNPNs for which the N1 mode capability was disabled that the MS can store is implementation specific, but it shall be at least one. The value of timer TJ is MS implementation specific;

[0213] - in automatic SNPN selection, the MS shall not select an SNPN associated with an entry of the "list of subscriber data" for which the N1 mode capability was disabled as SNPN selection candidates , unless no other SNPN is available;

[0214] - if the MS is not configured to use timer T3245, the MS maintains a list of SNPN-specific attempt counters for 3GPP access as specified in 3GPP TS 24.501

[0064] , and T3247 expires, then the MS removes for each SNPN-specific attempt counter for 3GPP access that has a value greater than zero and less than the MS implementation-specific maximum value the respective SNPN from the list of SNPNs for which the N1 mode capability was disabled, as specified in clause 5.3.20.3 in 3GPP TS 24.501; and

[0215] - the MS shall delete stored information on SNPNs for which the N1 mode capability was disabled when the MS timer TJ expires when the MS is switched off, the USIM is removed, the entries of the "list of subscriber data" for the SNPNs are updated, or timer TJ expires.

[0216] In an embodiment herein, the TJ timer can be maintained for the each entry of the “list of subscriber data”.

[0217] In an embodiment herein, the UE can delete the SNPNs for which the N1 mode capability was disabled associated with an entry of the "list of subscriber data“ for which the timer TJ has expired.

[0218] In an embodiment herein, an additional TJ timer is maintained (for all entries of the "list of subscriber data") and delete the SNPNs for which the N1 mode capability was disabled associated with each entry of the "list of subscriber data“ on TJ timer expiry.

[0219] In an embodiment herein, if the MS does not support access to an SNPN using credentials from a credentials holder and does not support equivalent SNPNs, the MS should maintain a list of SNPNs where the N1 mode capability was disabled because reason “x”. If the MS supports access to an SNPN using credentials from a credentials holder, equivalent SNPNs or both the MS should maintain one or more lists of SNPNs where the N1 mode capability was disabled because of reason “X”, each associated with an entry of the "list of subscriber data". If the MS supports access to an SNPN using credentials from a credentials holder, the MS should maintain a list of SNPNs where the N1 mode capability was disabled because of reason “X”, associated with the PLMN subscription. If the MS supports access to an SNPN using credentials from a credentials holder, equivalent SNPNs or both, the MS shall use the lists associated with the selected entry of the "list of subscriber data" or the selected PLMN subscription. When the MS disables its N1 mode capability due to reason “X”:

[0220] - the MS should add the SNPN identity of the SNPN to the list of SNPNs where N1 mode is disabled due to reason “X” and should start timer YY if timer YY is not already running. The number of SNPNs that the MS can store where N1 mode is disabled is implementation specific, but it shall be at least one. The value of timer YY is MS implementation specific;

[0221] - in automatic SNPN selection the MS shall not consider SNPNs where N1 mode is disabled in N1 mode as SNPN selection candidates, unless no other SNPN is available; and

[0222] - the MS shall delete stored information on SNPNs where N1 mode is disabled when the MS is switched off, the USIM is removed, the entries of the "list of subscriber data" for the SNPNs are updated, or timer YY expires.

[0223] In this embodiment, reason “X” can be UE receiving 5GMM cause 27 in NAS message (details of NAS message where reject cause 27 can come is defined in TS 24.501) or registration attempt counter reaches 5 or 5GMM cause 62 is received in NAS message which cause UE to disable N1 mode capability.

[0224] In an embodiment herein, the MS can provide the list of equivalent SNPNs associated with the selected entry of "list of subscriber data" or the selected PLMN subscription to the lower layer by removing the SNPN(s) where N1 mode is disabled (e.g. UE memorizes the E-SNPN(s) associated with the entry in the “list of subscriber data” or the selected PLMN subscription where N1 mode capability is disabled).

[0225] In an embodiment heren, when N1 mode is enabled for an SNPN and the MS can provide the list of equivalent SNPNs associated with the selected entry of "list of subscriber data" or the selected PLMN subscription to the lower layers including the SNPN(s) where N1 mode previously in the selected entry of "list of subscriber data" or the selected PLMN subscription to the lower layers (include the SNPN(s) from UE memorized E-SNPN(s) associated with the selected entry in the “list of subscriber data” or the selected PLMN subscription) i.e. suppose an UE is on selected entry (say entry-1) of the “list of subscriber data” and for the SNPN-1, N1 mode capability is disabled, UE is on SNPN-2 and for the SNPN-2, SNPN-1,SNPN-3 is equivalent SNPN on the selected entry (entry-1) of the subscriber data, the MS / NAS can provide lower layer by removing SNPN-1 from E-SNPN list (i.e., only SNPN-3 is sent to lower layer). When N1 mode is enabled for the SNPN-1, MS / NAS layer provide the SNPN-1, SNPN-3 in the E-SNPN list to the lower layer The UE can enable N1 mode for SNPN upon the UE Implementation dependent timer expiry or TJ timer expiry or T3402 timer expiry or any over reason

[0226] Embodiments disclosed herein are applicable for the case where N1 mode capability is disabled due to voice was not available and the MS's usage setting was "voice centric“ or 5GMM reject cause #62 , 5GMM reject cause #27 or registration attempt counter reaches 5 etc.

[0227] In an embodiment, one of the way the UE implementation ensure not to reselect on E-SNPN for which N1 mode capability is disabled through maintaining N1 mode capability for each SNPN per entry of "list of subscriber data" or the PLMN subscription, if the MS supports access to an SNPN using credentials from a credentials holder (e.g entry 1:- SNPN-1, N1 mode capability disabled, SNPN-2: N1 mode capability enabled, SNPN-3: N1 mode capability unknown. Entry 2:- SNPN-1, N1 mode capability disabled: SNPN-2, N1 mode capability enabled). UE consider reselection on equivalent SNPN (e.g SNPN-1) only if the N1 mode capability for the SNPN in selected entry or PLMN subscription is enabled. For an example, UE selected entry is entry#1 received the SNPN-1 and SNPN-3 in the equivalent SNPN when registered on SNPN-2. UE will not reselect to the SNPN-1 as SNPN-1 N1 mode capability is disabled in selected entry#1.

[0228] The following paragraphs explain how the UE determines whether each of the one or more of the candidate cells support required Network Slice Selection Assistance information (NSSAI).

[0229] The following definitions and abbreviations have been referred to herein:

[0230] AF: Application Function.

[0231] AMF: Access and Mobility Management Function

[0232] AOI: Area Of interest

[0233] CIoT: Cellular IoT

[0234] DL: Downlink

[0235] DNN: Data Network Name

[0236] IoT: Internet of Things

[0237] MA PDU: Multi Access-PDU

[0238] NAS: Non-Access Stratum

[0239] NS-AoS: Network Slice Area of Service

[0240] NSSAI: Network Slice Selection Assistance Information

[0241] NTN: Non-Terrestrial Networks

[0242] N3GPPA: Non 3GPP Access

[0243] PDU: Packet Data Unit

[0244] PCF: Policy Control Function

[0245] RA: Registration Area

[0246] RAT: Radio Access Technology

[0247] SM: Session Management

[0248] SMF: Session Management Function

[0249] S-NSSAI: Single - Network Slice Selection Assistance Information

[0250] SR: Service Request

[0251] TA: Tracking Area

[0252] TAI: Tracking Area Identity

[0253] TAC: Tracking Area Code

[0254] UL: Uplink

[0255] UP: User Plane

[0256] UPF: User Plane Function

[0257] 3GPP: 3rd generation partnership project

[0258] 3GPPA: 3GPP Access

[0259] NS-AoS: Network Slice Area of Service

[0260] NSAG: Network slice AS group

[0261] NSSAA: Network slice-specific authentication and authorization

[0262] NSSRG: Network Slice Simultaneous Registration Group

[0263] In embodiments disclosed herein, the term EMM sublayer states can be at least one of the below:

[0264] 1) EMM-NULL

[0265] 2) EMM-DEREGISTERED

[0266] a) EMM-DEREGISTERED.NORMAL-SERVICE

[0267] b) EMM-DEREGISTERED.LIMITED-SERVICE

[0268] c) EMM-DEREGISTERED.ATTEMPTING-TO-ATTACH

[0269] d) EMM-DEREGISTERED.PLMN-SEARCH

[0270] e) EMM-DEREGISTERED.NO-IMSI

[0271] f) EMM-DEREGISTERED.ATTACH-NEEDED

[0272] g) EMM-DEREGISTERED.NO-CELL-AVAILABLE

[0273] h) EMM-DEREGISTERED.eCALL-INACTIVE

[0274] 3) EMM-REGISTERED-INITIATED

[0275] 4) EMM-REGISTERED

[0276] a) EMM-REGISTERED.NORMAL-SERVICE

[0277] b) EMM-REGISTERED.ATTEMPTING-TO-UPDATE

[0278] c) EMM-REGISTERED.LIMITED-SERVICE

[0279] d) EMM-REGISTERED.PLMN-SEARCH

[0280] e) EMM-REGISTERED.UPDATE-NEEDED

[0281] f) EMM-REGISTERED.NO-CELL-AVAILABLE

[0282] g) EMM-REGISTERED.ATTEMPTING-TO-UPDATE-MM

[0283] h) EMM-REGISTERED.IMSI-DETACH-INITIATED

[0284] 5) EMM-DEREGISTERED-INITIATED

[0285] 6) EMM-TRACKING-AREA-UPDATING-INITIATED

[0286] 7) EMM-SERVICE-REQUEST-INITIATED

[0287] The term 5GMM sublayer states as referred to herein are at least one of the below:

[0288] 1) 5GMM-NULL

[0289] 2) 5GMM-DEREGISTERED

[0290] a) 5GMM-DEREGISTERED.NORMAL-SERVICE

[0291] b) 5GMM-DEREGISTERED.LIMITED-SERVICE

[0292] c) 5GMM-DEREGISTERED.ATTEMPTING-REGISTRATION

[0293] d) 5GMM-DEREGISTERED.PLMN-SEARCH

[0294] e) 5GMM-DEREGISTERED.NO-SUPI

[0295] f) 5GMM-DEREGISTERED.NO-CELL-AVAILABLE

[0296] g) 5GMM-DEREGISTERED.eCALL-INACTIVE

[0297] h) 5GMM-DEREGISTERED.INITIAL-REGISTRATION-NEEDED

[0298] 3) 5GMM-REGISTERED-INITIATED

[0299] 4) 5GMM-REGISTERED

[0300] a) 5GMM-REGISTERED.NORMAL-SERVICE

[0301] b) 5GMM-REGISTERED.NON-ALLOWED-SERVICE

[0302] c) 5GMM-REGISTERED.ATTEMPTING-REGISTRATION-UPDATE

[0303] d) 5GMM-REGISTERED.LIMITED-SERVICE

[0304] e) 5GMM-REGISTERED.PLMN-SEARCH

[0305] f) 5GMM-REGISTERED.NO-CELL-AVAILABLE

[0306] g) 5GMM-REGISTERED.UPDATE-NEEDED

[0307] 5) 5GMM-DEREGISTERED-INITIATED

[0308] 6) 5GMM-SERVICE-REQUEST-INITIATED

[0309] 5GMM-IDLE mode: In this specification, if the term is used standalone, a UE in 5GMM-IDLE mode means the UE can be either in 5GMM-IDLE mode over 3GPP access or in 5GMM-IDLE mode over non-3GPP access.

[0310] 5GMM-CONNECTED mode: In this specification, if the term is used standalone, a UE in 5GMM-CONNECTED mode means the UE can be either in 5GMM-CONNECTED mode over 3GPP access or in 5GMM-CONNECTED mode over non-3GPP access.

[0311] 5GMM-IDLE mode over 3GPP access: A UE is in 5GMM-IDLE mode over 3GPP access when no N1 NAS signalling connection between the UE and Network (i.e., Network function e.g., AMF / SMF / UPF) over 3GPP access exists. The term 5GMM-IDLE mode over 3GPP access used in the present document corresponds to the term CM-IDLE state for 3GPP access used in 3GPP TS 23.501.

[0312] 5GMM-CONNECTED mode over 3GPP access: A UE is in 5GMM-CONNECTED mode over 3GPP access when an N1 NAS signalling connection between the UE and Network (i.e., Network function e.g., AMF / SMF / UPF) over 3GPP access exists. The term 5GMM-CONNECTED mode over 3GPP access used in the present document corresponds to the term CM-CONNECTED state for 3GPP access used in 3GPP TS 23.501.

[0313] Partially Allowed NSSAI: Indicating the S-NSSAIs values the UE could use in the Serving PLMN or SNPN in some of the TAs in the current Registration Area. Each S-NSSAI in the Partially Allowed NSSAI is associated with a list of TAs where the S-NSSAI is supported.

[0314] Partially rejected NSSAI: Indicating the S-NSSAI(s) is rejected by the network in some TA(s) but not all TAs of the registration area. Each S-NSSAI in the partially rejected NSSAI is associated with a list of TAs where the S-NSSAI is rejected.

[0315] Network Slice Area of Service: The area where a UE can access and get service of a particular network slice as more than zero resources are allocated to the network slice in the NG-RAN cells.

[0316] S-NSSAI location availability information: The S-NSSAI location availability information sent to the UE includes, for each applicable S-NSSAI (optionally of the Configured NSSAI), Location information indicating the cells of TAs in the RA where the related S-NSSAI is available(or supported) if the S-NSSAI is not available(or not supported) in all the cells of the TA. This information can also include the cells where the related / respective S-NSSAI is not supported. The cell can be identified using a cell ID or any other identifier which can identify a given cell or group of cells.

[0317] Always-on PDU session: A PDU session for which user-plane resources have to be established during every transition from 5GMM-IDLE mode to 5GMM-CONNECTED mode. A UE requests a PDU session to be established as an always-on PDU session based on indication from upper layers and the network decides whether a PDU session is established as an always-on PDU session.

[0318] Alternative NSSAI: A list of mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI.

[0319] Pending NSSAI: NSSAI provided by the Serving PLMN during a Registration procedure, indicating the S-NSSAI(s) for which the network slice-specific authentication and authorization procedure is pending.

[0320] Network Slice AS Group (NSAG): identifies a slice or a set of slice(s). An NSAG is defined within a TA, used for slice specific cell reselection and / or slice specific RACH configuration.

[0321] In the embodiments herein, the term “Network slice based cell reselection” is used for illustration purpose and all the claims apply to Network slice based random access as well.

[0322] In the embodiment, the term “required Network Slice Selection Assistance Information” is the list of S-NSSAI(s) required by the UE.

[0323] In the embodiment, the term “equivalent network list” is the list of equivalent PLMN list when UE is registered on the PLMN and equivalent SNPN when UE is registered on SNPN

[0324] FIG. 14 depicts an example scenario, wherein the partial slicing supported area is informed, according to the embodiments herein. In step 1401, the UE sends UL NAS message (e.g., Registration Request message including S-NSSAI-1) in the requested NSSAI list. In step 1402, the Network sends DL NAS message; e.g., Registration Accept By including S-NSSAI-1 in Partially Rejected NSSAI list with TAI list set to TAI-1 in a Registration Area consisting of TAI-1 and TAI-2, i.e., S-NSSAI-1 is not allowed in TAI-1, but allowed in TAI-2. In step 1403, consider that the UE is located in TAI-1. In step 1404, the UE NAS shall remove S-NSSAI-1 from allowed NSSAI / partially allowed NSSAI / requested NSSAI list or any other list given to the UE AS so that the UE AS avoids cell reselection based on S-NSSAI-1. In step 1405, the UE moves to TAI-2. In step 1406, the UE NAS shall NOT remove S-NSSAI-1 from allowed NSSAI / partially allowed NSSAI / requested NSSAI list or any other list given to the UE AS so that the UE AS shall perform cell reselection based on S-NSSAI-1. In step 1407, additionally, if the S-NSSAI-1 was removed earlier from the allowed NSSAI / partially allowed NSSAI / requested NSSAI list or any other list given to the UE AS, then the UE NAS shall include the S-NSSAI-1 in the corresponding list and send to the UE AS, so that the UE AS shall perform cell reselection based on S-NSSAI-1. In yet another embodiment, if the S-NSSAI-1 is associated with NS-AoS / S-NSSAI location validity information / S-NSSAI time validity information and the UE is outside the boundary of S-NSSAI-1 supported area or time, then the UE NAS shall remove S-NSSAI-1 from allowed NSSAI / partially allowed NSSAI / requested NSSAI list or any other list given to the UE AS, so that the UE AS avoids cell reselection based on S-NSSAI-1. Additionally, if the S-NSSAI-1 was removed earlier from the allowed NSSAI / partially allowed NSSAI / requested NSSAI list or any other list given to the UE AS because the UE was outside the boundary of S-NSSAI-1 supported area or time, then the UE NAS shall include S-NSSAI-1 in the corresponding list and send to the UE AS, so that the UE AS shall perform cell reselection based on S-NSSAI-1.

[0325] FIG. 15 depicts an example scenario, wherein the partial slicing supported area is informed, according to the embodiments herein. If NSAG information has the Tracking area code, the NAS layer shall remove those TA(s) for the where S-NSSAI is not allowed from the NSAG before passing information to lower / AS layer.

[0326] In an example, consider that the UE receives NSAG information as S-NSSAI-1, TAI- TAI-1, TAI-2. The UE also receives the partially allowed / partially rejected NSSAI and the TAI-1 is not allowed for the S-NSSAI-1. Ideally, the UE shall remove TAI-1 from NSAG and forward it to the lower layer; i.e., the UE send the NSAG information to the lower / AS layer as S-NSSAI-1, TAI-2 only. In yet another embodiment, the AMF does not include TAI-1 in the NSAG information corresponding to the S-NSSAI-1, when TAI-1 is not allowed for the S-NSSAI-1.

[0327] In step 1501, the UE sends UL NAS message e.g. Registration Request message including S-NSSAI-1 in requested NSSAI list. In step 1502, the network sends DL NAS message e.g. Registration Accept By including S-NSSAI-1 in Partially Rejected NSSAI list with TAI list set to TAI-1 in a Registration Area consisting of TAI-1 and TAI-2, i.e. S-NSSAI-1 is not allowed in TAI-1, but allowed in TAI-2 or network indicated S-NSSAI-1 location validity information(NS-AOS) / S-NSSAI time validity information. In step 1503, the UE NAS layer shall pass Alternate S-NSSAI, Partially allowed / Partially rejected NSSAI with associated tracking area identifier information, NS-aos information to the lower layer (AS layer). Lower layer ignores the TA or cell for the cell selection / reselection where TA for the S-NSSAI-1 is not allowed or cell for the S-NSSAI-1 is not allowed. NAS layer shall also indicate to the AS about the alternative_S-NSSAI and its mapping with allowed / partially allowed S-NSSAI so that AS layer make decision based on alternative S-NSSAI for cell reselection.

[0328] FIG. 16 depicts an example scenario, wherein the alternative NSSAI is passed to the lower layers, according to the embodiments herein. In step 1601, the UE sends UL NAS message (e.g., Registration Request message including S-NSSAI-1) in the requested NSSAI list. In step 1602, the Network sends the DL NAS message (e.g., Registration Accept) by including S-NSSAI-1 and S-NSSAI-A in the allowed NSSAI list and includes the alternative NSSAI mapping of the S-NSSAI-1 (Slice to be replaced) to the S-NSSAI-A (alternative slice). In step 1603, the UE NAS shall send the alternative NSSAI (S-NSSAI-1 replaced with S-NSSAI-A along with allowed NSSAI list / partially allowed NSSAI list / requested NSSAI list / NSAG information to UE AS, so that the UE AS shall perform network slice based cell reselection. In step 1604, the UE AS shall evaluate the alternative NSSAI info and consider the S-NSSAI-1 (slice to be replaced) as a lower priority and considers the S-NSSAI-A (alternative slice) as a higher priority for network slice based cell reselection. In step 1605, alternatively, in NSAG information, the S-NSSAI-A (Alternative Slice) should be given a higher priority than S-NSSAI-1 (slice to be replaced). Accordingly, the Network should update the priority in NSAG information, or the UE shall locally change the priority of NSAG list. In step 1606, in yet another embodiment, the NSAG information (containing S-NSSAI-1 and S-NSSAI-A) should be given priority. Accordingly, the Network should update the priority in NSAG information, or the UE shall locally change the priority of the NSAG list. In yet another embodiment, the AMF sends NSAG information (containing S-NSSAI-1 and S-NSSAI-A) if the NSAG information for the S-NSSAI-A is not yet configured (i.e S-NSSAI-A is not part of UE Configured NSSAI) and the UE uses the NSAG information for the S-NSSAI-1 and S-NSSAI-A and its priority for the cell reselection. In yet another embodiment, UE consider priority of the alternative S-NSSAI for the slice based cell reselection not the slice to be replaced.

[0329] FIG. 17 depicts an example scenario, wherein unused slice(s) are removed for cell reselection. In step 1701, the UE sends a UL NAS message (e.g., Registration Request message) including S-NSSAI-1 in requested NSSAI list. In step 1702, the Network sends a DL NAS message (e.g., Registration Accept) by including S-NSSAI-1 in allowed NSSAI. In step 1703, the NAS shall send the allowed NSSAI list (S-NSSAI-1) to the UE AS so that the UE AS shall perform network slice based cell reselection for S-NSSAI-1. In step 1704, the UE sends a UL NAS message (e.g., PDU Session establishment request for S-NSSAI-1). In step 1705, the Network sends DL NAS message; e.g., PDU session establishment reject for S-NSSAI-1 with back-off timer or with cause congestion etc. In step 1706, the UE NAS shall remove the back-off slice S-NSSAI-1 from the allowed NSSAI list / partially allowed / requested NSSAI or any other lists sent to the UE AS for the purpose of Network slice based cell reselection. Alternatively, if the UE does not intend to use any of the allowed slices (e.g., S-NSSAI-1) for any other reasons, it shall be removed from the allowed NSSAI list / partially allowed NSSAI / requested NSSAI or any other lists sent to the UE AS for the purpose of Network slice based cell reselection or Network Slice AS Group based (NSAG) cell-reselection / RACH procedure In the embodiment, S-NSSAI-1 removed from allowed or requested NSSAI means that S-NSSAI-1 is not send to AS / Lower in the allowed NSSAI or requested NSSAI and even S1-SNNAI-1 is in the allowed NSSAI / Partially allowed NSSAI or the UE wants to request S-NSSAI-1 as it in the configured list. The S-NSSAI-1 is not deleted at the NAS layer. In step 1707, the UE AS does not prioritise the S-NSSAI-1 for cell reselection. In step 1708, the UE sends a UL NAS message (e.g., PDU Session establishment request) for S-NSSAI-1, on the Back-off timer expiring. In step 1709, the Network sends the DL NAS message; e.g., PDU session establishment Accept for S-NSSAI-1. In step 1710, the UE NAS shall include S-NSSAI-1 in the allowed NSSAI list / partially allowed or any other lists sent to the UE AS for the purpose of Network slice based cell reselection / Network Slice AS Group based slice reselection or NSAG based RACH, if it was previously removed due to back-off or congestion reject etc. Alternatively, if the UE intends to use any of the allowed slices which was previously removed (e.g., S-NSSAI-1) from the allowed NSSAI list / partially allowed or any other lists sent to the UE AS for the purpose of Network slice based cell reselection, Network Slice AS Group based reselection or NSAG based RACH, the slice shall be included back in the corresponding NSSAI list and the UE NAS shall report the updated list to the UE AS. In step 1711, the UE AS prioritises S-NSSAI-1 for cell reselection. In yet other embodiment, UE uses the S-NSSAIs where PDU session is active or being activated for the slice based cell reselection instead of allowed / partially allowed NSSAI (if PDU session is active or being activated on S-NSSAI-1 and S-NSSAI-2 but allowed NSSAI is S-NSSAI-1, S-NSSAI-2, S-NSSAI-3 then UE uses S-NSSAI-1 and S-NSSAI-2 for the slice based cell resection), The UE may not have activated PDU session on S-NSSAI-3 or back-off timer is running on S-NSSAI-3.

[0330] In an embodiment herein, the UE shall send the allowed NSSAI to the lower layer, on detecting a change in the allowed NSSAI and if not sending requested NSSAI. For an example,

[0331] - The UE receives a new allowed NSSAI through Configuration update command / registration accept.

[0332] - The UE deletes the S-NSSAI from allowed NSSAI after registration timer expiry for the on-demand slice.

[0333] If UE has sent the requested S-NSSAI to the lower layer, the UE shall send the allowed NSSAI to the lower layer once the current procedure with the requested NSSAI is completed (e.g., registration procedure) and optionally, the UE moves to idle mode.

[0334] Consider a scenario, wherein all the S-NSSAIs in the configured and allowed NSSAI lists are not available in the current TA. For example, consider that the UE has only S1 and S2 as configured as allowed NSSAIs and both are not allowed in the current TA (i.e., TAI-1). Here, the UE shall store TAI-1 in “5GS forbidden tracking areas for roaming” and also memorize that current TAI (i.e. TAI-1) is stored in the “5GS forbidden tracking areas for roaming” list, due to No Allowed S-NSSAIs in the TA. In an embodiment herein, the UE shall remove the memorized TAI (i.e., TAI-1) from the “5GS forbidden tracking areas for roaming” due to No Allowed S-NSSAIs in the current TA, after receiving subscription change indication (E.g. Network slicing indication IE with the Network slicing subscription change indication set to "Network slicing subscription changed“, or contains a Configured NSSAI IE with a new configured NSSAI for the current PLMN or SNPN). In an embodiment herein, the UE shall remove the memorized TAI (i.e., TAI-1) from the “5GS forbidden tracking areas for roaming” due to No Allowed S-NSSAIs in the current TA using a UE implementation specific timer expiry. In an embodiment herein, the UE shall remove the memorized TAI (i.e., TAI-1) from the “5GS forbidden tracking areas for roaming” due to No Allowed S-NSSAIs in the current TA, on receiving a new partially allowed / configured / allowed NSSAI.

[0335] Consider a scenario, wherein all the S S-NSSAIs in the configured and allowed NSSAI lists are not available in a cell(s). For example, if the UE has only S1 and S2 as configured are allowed NSSAIs and both are not allowed in the current cell (i.e., Cell-1). Here, the UE shall consider the Cell-1 as barred and memorize that Cell-1 was barred due to No Allowed S-NSSAIs in the cell. In an embodiment herein, the UE shall unbar Cell-1, on receiving subscription change indication (e.g., Network slicing indication IE with the Network slicing subscription change indication set to "Network slicing subscription changed", or contains a Configured NSSAI IE with a new configured NSSAI for the current PLMN or SNPN). In an embodiment herein, the UE shall unbar Cell-1, using UE implementation specific timer expiry. In an embodiment herein, the UE shall unbar Cell-1, on receiving a new NS-AoS / configured / allowed NSSAI.

[0336] FIG 18 is a block diagram of an internal configuration of a UE, according to an embodiment.

[0337] As shown in FIG 18, the UE according to an embodiment may include a transceiver 1810, a memory 1820, and a processor 1830. The transceiver 1810, the memory 1820, and the processor 1830 of the UE may operate according to a communication method of the UE described above. However, the components of the UE are not limited thereto. For example, the UE may include more or fewer components than those described above. In addition, the processor 1830, the transceiver 1810, and the memory 1820 may be implemented as a single chip. Also, the processor 1830 may include at least one processor.

[0338] Furthermore, the UE of FIG 18 corresponds to the UE 602 of FIG 6.

[0339] The transceiver 1810 collectively refers to a UE receiver and a UE transmitter, and may transmit / receive a signal to / from a base station or a network entity. The signal transmitted or received to or from the base station or a network entity may include control information and data. The transceiver 1810 may include a RF transmitter for up-converting and amplifying a frequency of a transmitted signal, and a RF receiver for amplifying low-noise and down-converting a frequency of a received signal. However, this is only an example of the transceiver 1810 and components of the transceiver 1810 are not limited to the RF transmitter and the RF receiver.

[0340] Also, the transceiver 1810 may receive and output, to the processor 1830, a signal through a wireless channel, and transmit a signal output from the processor 1830 through the wireless channel.

[0341] The memory 1820 may store a program and data required for operations of the UE. Also, the memory 1820 may store control information or data included in a signal obtained by the UE. The memory 1820 may be a storage medium, such as read-only memory (ROM), random access memory (RAM), a hard disk, a CD-ROM, and a DVD, or a combination of storage media.

[0342] The processor 1830 may control a series of processes such that the UE operates as described above. For example, the transceiver 1810 may receive a data signal including a control signal transmitted by the base station or the network entity, and the processor 1830 may determine a result of receiving the control signal and the data signal transmitted by the base station or the network entity.

[0343] FIG 19 is a block diagram of an internal configuration of a base station or a network entity, according to an embodiment.

[0344] As shown in FIG 19, the base station or the network entity according to an embodiment may include a transceiver 1910, a memory 1920, and a processor 1930. The transceiver 1910, the memory 1920, and the processor 1930 of the base station or the network entity may operate according to a communication method of the base station or the network entity described above. However, the components of the base station or the network entity are not limited thereto. For example, the base station or the network entity may include more or fewer components than those described above. In addition, the processor 1930, the transceiver 1910, and the memory 1920 may be implemented as a single chip. Also, the processor 1930 may include at least one processor.

[0345] Furthermore, the network entity of the FIG 19 corresponds to the gNodeB 604 of FIG 6 or core network entity of FIG 6.

[0346] The transceiver 1910 collectively refers to the base station(or the network entity receiver) and a base station(or the network entity) transmitter, and may transmit / receive a signal to / from a terminal or a network entity or a base station. The signal transmitted or received to or from the terminal or a network entity or the base station may include control information and data. The transceiver 1910 may include a RF transmitter for up-converting and amplifying a frequency of a transmitted signal, and a RF receiver for amplifying low-noise and down-converting a frequency of a received signal. However, this is only an example of the transceiver 1910 and components of the transceiver 1910 are not limited to the RF transmitter and the RF receiver.

[0347] Also, the transceiver 1910 may receive and output, to the processor 1930, a signal through a wireless channel, and transmit a signal output from the processor 1930 through the wireless channel.

[0348] The memory 1920 may store a program and data required for operations of the base station or the network entity. Also, the memory 1920 may store control information or data included in a signal obtained by the base station or the network entity. The memory 1920 may be a storage medium, such as read-only memory (ROM), random access memory (RAM), a hard disk, a CD-ROM, and a DVD, or a combination of storage media.

[0349] The processor 1930 may control a series of processes such that the base station or the network entity operates as described above. For example, the transceiver 1910 may receive a data signal including a control signal transmitted by the terminal or the network entity or the base station, and the processor 1930 may determine a result of receiving the control signal and the data signal transmitted by the terminal or the network entity or the base station.

[0350] The embodiments disclosed herein can be implemented through at least one software program running on at least one hardware device and performing network management functions to control the network elements. The elements include blocks which can be at least one of a hardware device, or a combination of hardware device and software module.

[0351] The embodiments disclosed herein describe methods and systems for cell selection / reselection in 5G networks by determining a status of an N1 mode for each of the one or more candidate cells and by determining whether each of the one or more of the candidate cells support required Network Slice Selection Assistance information (NSSAI). Therefore, it is understood that the scope of the protection is extended to such a program and in addition to a computer readable means having a message therein, such computer readable storage means contain program code means for implementation of one or more steps of the method, when the program runs on a server or mobile device or any suitable programmable device. The method is implemented in at least one embodiment through or together with a software program written in e.g., Very high speed integrated circuit Hardware Description Language (VHDL) another programming language, or implemented by one or more VHDL or several software modules being executed on at least one hardware device. The hardware device can be any kind of portable device that can be programmed. The device may also include means which could be e.g., hardware means like e.g., an ASIC, or a combination of hardware and software means, e.g., an ASIC and an FPGA, or at least one microprocessor and at least one memory with software modules located therein. The method embodiments described herein could be implemented partly in hardware and partly in software. Alternatively, the invention may be implemented on different hardware devices, e.g., using a plurality of CPUs.

[0352] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and / or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of embodiments and examples, those skilled in the art will recognize that the embodiments and examples disclosed herein can be practised with modification within the scope of the embodiments as described herein.

Claims

1.A method, performed by at least one user equipment (UE), for cell selection / reselection in a fifth generation (5G) network, comprising:monitoring a signal quality of a serving cell and one or more neighbouring cells, wherein the serving cell and the one or more neighbouring cells belong to one of Public Land Mobile Network (PLMN) and Standalone Non-Public Network (SNPN);identifying one or more candidate cells based on signal measurements from the one or more neighbouring cells, on detecting that a signal quality of the serving cell is below a predefined threshold;determining a status of an N1 mode capability for each of the one or more candidate cells;determining whether each of the one or more of the candidate cells support required Network Slice Selection Assistance information (NSSAI); andreselecting a new serving cell based on the determined status of the N1 mode and the determined NSSAI.2.The method of claim 1, wherein the signal quality comprises one or more of Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and Signal-to-Interference-plus-Noise Ratio (SINR).3.The method of claim 1, wherein the method comprises handling disabling and re-enabling the N1 mode based on the determined status of the N1 mode.4.The method of claim 3, wherein handling the disabling and re-enabling of the N1 mode comprises:sending a registration request on a camped first network, wherein the camped first network is one of the PLMN and SNPN;receiving one of a registration accept and registration reject with one of 5GMM cause #62 and cause #27, indicating that no network slices are available;determining that the camped first network is to be disabled as there are no S-NSSAIs allowed or “N1 mode not allowed”;selecting an available second network;sending a registration request on the available second network;receiving a registration accept with the camped first network and a third network in an equivalent network list;determining that a N1 mode capability for the camped first network is disabled for preventing reselection to the camped first network; andallowing the cell reselection if the at least one UE determines to enable the N1 mode for the camped first network.5.The method of claim 1, wherein determining whether each of the one or more of the candidate cells support required Network Slice Selection Assistance information (NSSAI) comprises:receiving an alternative S-NSSAI (S-NSSAI-A) along with the allowed NSSAI list from at least one UE Non Access Stratum (NAS), on the at least one UE NAS receiving an alternative NSSAI mapping indicating that a current S-NSSAI is to be replaced with the alternative S-NSSAI; andperforming a network slice-based cell reselection by evaluating information related to the alternative NSSAI, wherein the at least one UE considers the current S-NSSAI as a lower priority and the alternative S-NSSAI as a higher priority.6.The method of claim 5, wherein the method comprises:transmitting an uplink Non-Access Stratum (UL-NAS) message in a requested NSSAI list, wherein the UL-NAS message includes the current S-NSSAI; andreceiving a downlink (DL) NAS message including the current S-NSSAI and the alternative S-NSSAI in the allowed NSSAI list and alternative NSSAI, wherein the alternative NSSAI indicates the mapping of current S-NSSAI is to be replaced with the alternative S-NSSAI.7.The method of claim 5, wherein the method comprises updating the priority of Network Slice Assistance Group (NSAG) information on performing the network slice-based cell reselection, wherein the alternative S-NSSAI is given a higher priority than the current S-NSSAI, wherein the AMF (Access and Mobility Management Function) updates the priority.8.The method of claim 5, wherein the at least one UE locally changes the priority of the NSAG information.9.The method of claim 5, wherein, the method includes receiving, by the at least one UE NAS (non-access stratum), the updated NSAG information from the AMF (Access and Mobility Management Function), on replacing the current S-NSSAI with the alternative S-NSSAI, wherein the updated NSAG information includes the alternative S-NSSAI and NSAG priority, and the alternative S-NSSAI was not part of at least one UE configured NSSAI and is added to the at least one UE configured NSSAI.10.A method, performed by at least one user equipment (UE), for handling disabling and re-enabling of a N1 mode, comprising:sending a registration request on a camped first network, wherein the camped first network is one of the PLMN and SNPN;receiving one of a registration accept and registration reject with one of 5GMM cause #62 and cause #27, indicating that no network slices are available;determining that the camped first network is to be disabled as there are no S-NSSAIs allowed;selecting an available second network;sending a registration request on the available second network;receiving a registration accept with the camped first network and a third network in an equivalent network list;determining that a N1 mode capability for the camped first network is disabled for preventing reselection to the camped first network; andallowing the cell reselection if the at least one UE determines to enable the N1 mode for the camped first network.11.A method, performed by at least one user equipment (UE), for handling Network Slice Selection Assistance Information (NSSAI) lists in a network, comprising:receiving an alternative NSSAI (S-NSSAI-A) along with the allowed NSSAI list from at least one UE Non Access Stratum (NAS), on the at least one UE NAS receiving an alternative NSSAI mapping indicating that a current S-NSSAI is to be replaced with the alternative S-NSSAI; andperforming a network slice-based cell reselection by evaluating information related to the alternative NSSAI, wherein the at least one UE considers the current S-NSSAI as a lower priority and the alternative S-NSSAI as a higher priority.12.The method as claimed in claim 11, wherein the method comprises:transmitting an uplink Non-Access Stratum (UL-NAS) message in a requested NSSAI list, wherein the UL-NAS message includes the current S-NSSAI; andreceiving a downlink (DL) NAS message including the current S-NSSAI and the alternative S-NSSAI in the allowed NSSAI list and alternative NSSAI, wherein the alternative NSSAI indicates the mapping of current S-NSSAI is to be replaced with the alternative S-NSSAI.13.The method as claimed in claim 11, wherein the method comprises updating the priority of Network Slice Assistance Group (NSAG) information on performing the network slice-based cell reselection, wherein the alternative S-NSSAI is given a higher priority than the current S-NSSAI.14.The method as claimed in claim 11, wherein the at least one UE locally changes the priority of the NSAG information.15.The method as claimed in claim 11, wherein, the method includes receiving, by the at least one UE NAS, the updated NSAG information from the AMF, on replacing the current S-NSSAI with the alternative S-NSSAI, wherein the updated NSAG information includes the alternative S-NSSAI, and the alternative S-NSSAI was not part of at least one UE configured NSSAI and is added to the at least one UE configured NSSAI.

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