Apparatus, method, and computer program

Abstract

A method comprising: entering, by a user equipment, UE, a first radio resource control, RRC mode; selecting, by the user equipment, a first radio access technology, RAT cell configured to provide preferred services; determining, by the UE, to perform cell re-selection; re-selecting, by the UE, a first RAT cell not configured to provide preferred services; and performing, by the UE, measurements for a list of second RAT cells configured to provide preferred services. The selecting of the first RAT cell configured to provide preferred services may comprise: selecting, by the UE, a first RAT cell configured to serve a first RAT network slice for preferred services amongst a list of first RAT network slices for preferred services; wherein re-selecting of a first RAT cell not configured to provide preferred services may comprise re-selecting a first RAT cell not configured to serve a first RAT network slice for preferred services amongst the list of first RAT network slices for preferred services; wherein performing the measurements may comprise: performing, by the UE, measurements for a list of second RAT cells configured to serve a list of second network slices for preferred services.

Description

Technical Field Various embodiments of this disclosure relate to a method, an apparatus and a computer program for a communication system, and in particular - but not exclusively - a method, an apparatus, and a computer program for managing a user equipment in a multi-radio access technology communication system. Background A communication system can be seen as a facility that enables communication sessions between two or more entities, such as communication devices, base stations and / or other nodes by providing carriers between the various entities involved in the communications path. The communication system and associated entities typically operate in accordance with a given standard(s) or specification(s) which set forth what the associated entities are permitted to do and how that should be achieved. Communication protocols and / or parameters which shall be used for the connection are also typically defined. Examples of standard(s) include the so-called 5G standards. Summary Some embodiments of this disclosure will be described with respect to certain aspects. These aspects are not intended to indicate key or essential features of the various embodiments of this disclosure, nor are they intended to be used to limit the scope of thereof. Other features, aspects, and elements will be readily apparent to a person skilled in the art in view of this disclosure. For example, it should be appreciated that further aspects may be provided by the combination of any two or more of the various aspects described herein. According to an aspect, there is provided a method comprising: entering, by a user equipment, a first radio resource control mode; selecting, by the user equipment, a first radio access technology cell configured to provide preferred services; determining, by the user equipment, to perform cell re-selection; re-selecting, by the user equipment, a first radio access technology cell not configured to provide preferred services; and performing, by the user equipment, measurements for a list of second radio access technology cells configured to provide preferred services. The method may comprise: determining, by the user equipment, that a first radio access technology cell configured to provide preferred services is not available for cell re-selection. Selecting, by the user equipment, the first radio access technology cell configured to provide preferred services may comprise: selecting, by the user equipment, a first radio access technology cell configured to serve a first radio access technology network slice for preferred services amongst a list of first radio access technology network slices for preferred services; re-selecting, by the user equipment, a first radio access technology cell not configured to provide preferred services may comprise: re-selecting, by the user equipment, a first radio access technology cell not configured to serve a first radio access technology network slice for preferred services amongst the list of first radio access technology network slices for preferred services; performing, by the user equipment, measurements fora list of second radio access technology cells configured to provide preferred services may comprise: performing, by the user equipment, measurements for a list of second radio access technology cells configured to serve a list of second network slices for preferred services. Each network slice may have a priority. Each cell configured to serve a network slice may have a priority. Different cells configured to serve a same network slice may have a same priority (e.g., the priorities of the cells configured to serve the same network slice may be the priority of the network slice). Alternatively, different cells configured to serve a same network slice may have different priorities (e.g., the priorities of the cells configured to serve the same network slice may be different from the priority of the network slice). The priorities of the network slices and / or the cells may be used to perform cell re-selection. The list of first radio access technology network slices for preferred services may be associated with a list of first radio access technology single network slice selection assistance information; and the list of second radio access technology network slices for preferred services may be associated with a list of second radio access technology single network slice selection assistance information. The first radio access technology may comprise 6G and the second radio access technology may comprise 5G. The first radio resource control mode may comprise a radio resource control idle or inactive mode. The method may comprise: sending, by the user equipment to a common first radio access technology and second radio access technology core network element, a common request to register to a first radio access technology core network and a second radio access technology core network including a preference for preferred services; and receiving, by the user equipment from the common first radio access technology and second radio access technology core network element, a response including the list of first radio access technology network slices for preferred services and the list of second radio access technology network slices for preferred services. The common first radio access technology and second radio access technology core network element may comprise a common access and mobility management function. The common request to register to the first radio access technology core network and the second radio access technology core network may be sent via a first radio access technology base station or a second radio access technology base station over a first radio access technology radio resource control connection or a second radio access technology radio resource control connection. The method may comprise: sending, by the user equipment to separate first radio access technology and second radio access technology core network elements, separate requests to register to a first radio access technology core network and a second radio access technology core network including a preference for preferred services; and receiving, by the user equipment from the separate first radio access technology and second radio access technology core network elements, a response including the list of first radio access technology network slices for preferred services and the list of second radio access technology network slices for preferred services. The separate first radio access technology and second radio access technology core network elements may comprise separate access and mobility management functions. The separate requests to register to the first radio access technology core network and the second radio access technology core network may be sent via a first radio access technology base station and a second radio access technology base station over separate first radio access technology and second radio access technology radio resource control connections. The response may include a first radio access technology identifier and a second radio access technology identifier. The method may comprise: receiving, by the user equipment from a first radio access technology base station, the list of second radio access technology cells configured to serve the list of second radio access technology network slices for preferred services. The list of second radio access technology cells configured to serve the list of second radio access technology network slices for preferred services may be received within a first radio access technology system information block or a first radio access technology radio resource control release message. Determining, by the user equipment, to perform cell re-selection may comprise: determining, by the user equipment, that the user equipment has moved outside a coverage area of the selected first radio access technology cell configured to provide preferred services; or determining, by the user equipment, that the first radio access technology cell not configured to provide preferred services has at least one of a higher priority, a higher strength or a higher quality than the selected first radio access technology cell configured to provide preferred services. The method may comprise: transitioning, by the user equipment, from the first radio resource control mode to a second radio resource control mode; and re-selecting, by the user equipment, a second radio access technology cell configured to provide preferred services based on results of the measurements. The method may comprise: determining, by the user equipment, that a first radio access technology cell configured to provide preferred services is not available for cell re-selection. The second radio resource control mode may comprise a radio resource control connected mode. Performing, by the user equipment, measurements for a list of second radio access technology cells may comprise: performing, by the user equipment, measurements of at least one of a strength or quality for a list of second radio access technology cells configured to provide preferred services; and re-selecting, by the user equipment, a second radio access technology cell configured to provide preferred services based on the measurements may comprise: reselecting, by the user equipment, a second radio access technology cell configured to provide preferred services with at least one of a highest priority, a highest strength or a highest quality based on the measurements. The method may comprise transitioning, by the user equipment, from the first radio resource control mode to the second radio resource control mode upon: receiving, by the user equipment from a first radio access technology base station configured to provide the selected first radio access technology cell not configured to provide preferred services, a paging message for preferred services; or initiating, by the user equipment, preferred services. The method may comprise: sending, by the user equipment to a second radio access technology base station configured to provide the re-selected second radio access technology cell configured to provide preferred services, a second radio access technology identifier and a first radio access technology identifier. The second radio access technology identifier and the first radio access technology identifier may comprise a second radio access technology global unique temporary identifier and a first radio access technology global unique temporary identifier. The second radio access technology identifier and the first radio access technology identifier may be sent in a radio resource control setup procedure message. The method may comprise: transitioning, by the user equipment, from the second radio resource control mode to a first radio resource control mode; and re-selecting, by the user equipment, a first radio access technology cell not configured to provide preferred services. The method may comprise: determining, by the user equipment, that a first radio access technology cell configured to provide preferred services is not available for cell re-selection. The method may comprise: transitioning, by the user equipment, from the second radio resource control mode to a first radio resource control mode upon terminating the preferred services. The preferred services may comprise voice services. The voice services may comprise at least one of an emergency call, voice call or a video call. According to an aspect, there is provided a user equipment comprising: means for entering a first radio resource control mode; means for selecting a first radio access technology cell configured to provide preferred services; means for determining to perform cell re-selection; means for re-selecting a first radio access technology cell not configured to provide preferred services; and means for performing measurements for a list of second radio access technology cells configured to provide preferred services. The user equipment may comprise: means for determining that a first radio access technology cell configured to provide preferred services is not available for cell re-selection. Means for selecting the first radio access technology cell configured to provide preferred services may comprise: means for selecting a first radio access technology cell configured to serve a first radio access technology network slice for preferred services amongst a list of first radio access technology network slices for preferred services; means for re-selecting a first radio access technology cell not configured to provide preferred services may comprise: means for re-selecting a first radio access technology cell not configured to serve a first radio access technology network slice for preferred services amongst the list of first radio access technology network slices for preferred services; means for performing measurements of a list of second radio access technology cells configured to provide preferred services may comprise: means for performing measurements for a list of second radio access technology cells configured to serve a list of second network slices for preferred services. Each network slice may have a priority. Each cell configured to serve a network slice may have a priority. Different cells configured to serve a same network slice may have a same priority (e.g., the priorities of the cells configured to serve the same network slice may be the priority of the network slice). Alternatively, different cells configured to serve a same network slice may have different priorities (e.g., the priorities of the cells configured to serve the same network slice may be different from the priority of the network slice). The priorities of the network slices and / or the cells may be used to perform cell re-selection. The list of first radio access technology network slices for preferred services may be associated with a list of first radio access technology single network slice selection assistance information; and the list of second radio access technology network slices for preferred services may be associated with a list of second radio access technology single network slice selection assistance information. The first radio access technology may comprise 6G and the second radio access technology may comprise 5G. The first radio resource control mode may comprise a radio resource control idle or inactive mode. The user equipment may comprise: means for sending, to a common first radio access technology and second radio access technology core network element, a common request to register to a first radio access technology core network and a second radio access technology core network including a preference for preferred services; and means for receiving, from the common first radio access technology and second radio access technology core network element, a response including the list of first radio access technology network slices for preferred services and the list of second radio access technology network slices for preferred services. The common first radio access technology and second radio access technology core network element may comprise a common access and mobility management function. The common request to register to the first radio access technology core network and the second radio access technology core network may be sent via a first radio access technology base station or a second radio access technology base station over a first radio access technology radio resource control connection or a second radio access technology radio resource control connection. The user equipment may comprise: means for sending, to separate first radio access technology and second radio access technology core network elements, separate requests to register to a first radio access technology core network and a second radio access technology core network including a preference for preferred services; and means for receiving, from the separate first radio access technology and second radio access technology core network elements, a response including the list of first radio access technology network slices for preferred services and the list of second radio access technology network slices for preferred services. The separate first radio access technology and second radio access technology core network elements may comprise separate access and mobility management functions. The separate requests to register to the first radio access technology core network and the second radio access technology core network may be sent via a first radio access technology base station and a second radio access technology base station over separate first radio access technology and second radio access technology radio resource control connections. The response may include a first radio access technology identifier and a second radio access technology identifier. The user equipment may comprise: means for receiving, from a first radio access technology base station, the list of second radio access technology cells configured to serve the list of second radio access technology network slices for preferred services. The list of second radio access technology cells configured to serve the list of second radio access technology network slices for preferred services may be received within a first radio access technology system information block or a first radio access technology radio resource control release message. Means for determining to perform cell re-selection may comprise: means for determining that the user equipment has moved outside a coverage area of the selected first radio access technology cell configured to provide preferred services; or means for determining that the first radio access technology cell not configured to provide preferred services has at least one of a higher priority, a higher strength or a higher quality than the selected first radio access technology cell configured to provide preferred services. The user equipment may comprise: means for transitioning from the first radio resource control mode to a second radio resource control mode; and means for re-selecting a second radio access technology cell configured to provide preferred services based on results of the measurements. The user equipment may comprise: means for determining that a first radio access technology cell configured to provide preferred services is not available for cell re-selection. The second radio resource control mode may comprise a radio resource control connected mode. Means for performing, by the user equipment, measurements for a list of second radio access technology cells may comprise: means for performing measurements of at least one of a strength or quality for a list of second radio access technology cells configured to provide preferred services; and means for re-selecting a second radio access technology cell configured to provide preferred services based on the measurements may comprise: means for re-selecting a second radio access technology cell configured to provide preferred services with at least one of a highest priority, a highest strength or a highest quality based on the measurements. The user equipment may comprise means for transitioning from the first radio resource control mode to the second radio resource control mode upon: receiving, from a first radio access technology base station configured to provide the selected first radio access technology cell not configured to provide preferred services, a paging message for preferred services; or initiating preferred services. The user equipment may comprise: means for sending, to a second radio access technology base station configured to provide the re-selected second radio access technology cell configured to provide preferred services, a second radio access technology identifier and a first radio access technology identifier. The second radio access technology identifier and the first radio access technology identifier may comprise a second radio access technology global unique temporary identifier and a first radio access technology global unique temporary identifier. The second radio access technology identifier and the first radio access technology identifier may be sent in a radio resource control setup procedure message. The user equipment may comprise: means for transitioning from the second radio resource control mode to a first radio resource control mode; and means for re-selecting a first radio access technology cell not configured to provide preferred services. The user equipment may comprise: means for determining that a first radio access technology cell configured to provide preferred services is not available for cell re-selection. The user equipment may comprise: means for transitioning from the second radio resource control mode to a first radio resource control mode upon terminating the preferred services. The preferred services may comprise voice services. The voice services may comprise at least one of an emergency call, voice call or a video call. According to an aspect, there is provided a user equipment comprising circuitry configured to perform: entering a first radio resource control mode; selecting a first radio access technology cell configured to provide preferred services; determining to perform cell re-selection; reselecting a first radio access technology cell not configured to provide preferred services; and performing measurements for a list of second radio access technology cells configured to provide preferred services. According to an aspect, there is provided a user equipment comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to perform: entering a first radio resource control mode; selecting a first radio access technology cell configured to provide preferred services; determining to perform cell re-selection; re-selecting a first radio access technology cell not configured to provide preferred services; and performing measurements for a list of second radio access technology cells configured to provide preferred services. According to an aspect, there is provided a computer program comprising computer executable code which when run on at least one processor is configured to perform: entering, by a user equipment, a first radio resource control mode; selecting, by the user equipment, a first radio access technology cell configured to provide preferred services; determining, by the user equipment, to perform cell re-selection; re-selecting, by the user equipment, a first radio access technology cell not configured to provide preferred services; and performing, by the user equipment, measurements for a list of second radio access technology cells configured to provide preferred services. According to an aspect, there is provided a method comprising: entering, by a user equipment, a radio resource control idle or inactive mode; selecting, by the user equipment, a 6G cell configured to serve a 6G network slice for voice services amongst a list of 6G network slices for voice services; determining, by the user equipment, to perform cell re-selection; reselecting, by the user equipment, a 6G cell not configured to serve a 6G network slice for voice services; and performing, by the user equipment, measurements for a list of 5G cells configured to serve a list of 5G network slices for voice services. Each network slice may have a priority. Each cell configured to serve a network slice may have a priority. Different cells configured to serve a same network slice may have a same priority (e.g., the priorities of the cells configured to serve the same network slice may be the priority of the network slice). Alternatively, different cells configured to serve a same network slice may have different priorities (e.g., the priorities of the cells configured to serve the same network slice may be different from the priority of the network slice). The priorities of the network slices and / or the cells may be used to perform cell re-selection. The method may comprise: determining, by the user equipment, that a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services is not available for cell re-selection. The list of 5G network slices for voice services may be associated with a list of 5G single network slice selection assistance information; and the list of 6G network slices for voice services may be associated with a list of 6G single network slice selection assistance information. The method may comprise: sending, by the user equipment to a common 5G and 6G core network element, a common request to register to the 5G core network and the 6G core network including a preference for voice services; and receiving, by the user equipment from the common 5G and 6G core network element, a response including the list of 5G network slices for voice services and the list of 6G network slices for voice services. The common 5G and 6G core network element may comprise a common access and mobility management function. The common request to register to the 5G core network and the 6G core network may be sent via a 5G node B or a 6G node B over a 5G radio resource control connection or a 6G radio resource control connection. The method may comprise: sending, by the user equipment to separate 5G core and 6G core network elements, separate requests to register to the 5G core network and the 6G core network including a preference for voice services; and receiving, by the user equipment from the separate 5G and 6G core network elements, a response including the list of 5G network slices for voice services and the list of 6G network slices for voice services. The separate 5G core network element and 6G core network element may comprise separate access and mobility management functions. The separate requests to register to the 5G core network and the 6G core network may be sent via a 5G node B and a 6G node B over separate 5G radio resource control connection and 6G radio resource control connection. The response may include a 5G global unique temporary identifier and a 6G global unique temporary identifier. The method may comprise: receiving, by the user equipment from a 6G node B configured to provide the selected 6G cell configured to serve the network slice for voice services amongst the list of 6G network slices for voice services, the list of 5G cells configured to serve the list of 5G network slices for voice services. The list of 5G cells configured to serve the list of 5G network slices for voice services may be received within a 6G system information block or a 6G radio resource control release message. Determining, by the user equipment, to perform cell re-selection may comprise: determining, by the user equipment, that the user equipment has moved outside the coverage area of the selected 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slice for voice services; or determining, by the user equipment, that the 6G cell not configured to serve a network slice for voice services has at least one of a higher priority, a higher strength or a higher quality than the selected 6G cell configured to serve a network slice for voice services. The method may comprise: transitioning, by the user equipment, from the radio resource control idle or inactive mode to a radio resource control connected mode; and re-selecting, by the user equipment, a 5G cell configured to serve a 5G network slice for voice services based on the measurements. The method may comprise: determining, by the user equipment, that a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services is not available for cell re-selection. Performing, by the user equipment, measurements for a list of 5G cells configured to serve the list of 5G network slices for voice services may comprise: performing, by the user equipment, measurements of a strength or quality for a list of 5G cells configured to serve the list of 5G network slices for voice services; and re-selecting, by the user equipment, a 5G cell configured to serve a 5G network slice for voice services based on the measurements may comprise: re-selecting, by the user equipment, a 5G cell configured to serve a 5G network slice for voice services with at least one of a highest priority, a highest strength or a highest quality based on the measurements. The method may comprise transitioning, by the user equipment, from the radio resource control idle or inactive mode to the radio resource control connected mode upon: receiving, by the user equipment from a 6G node B configured to provide the selected 6G node B cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services, a paging message for voice services; or initiating, by the user equipment, voice services. The method may comprise: sending, by the user equipment to a 5G node B configured to provide the re-selected 5G cell configured to serve a 5G network slice for voice services, a 5G global unique temporary identifier and a 6G global unique temporary identifier. The 5G global unique temporary identifier and the 6G global unique temporary identifier may be sent in a radio resource control setup procedure message. The method may comprise: transitioning, by the user equipment, from the radio resource control connected mode to a radio resource control idle or inactive mode; and re-selecting, by the user equipment, a 6G cell not configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services. The method may comprise: determining, by the user equipment, that a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slice for voice services is not available for cell re-selection. The method may comprise: transitioning, by the user equipment, from the radio resource control connected mode to a radio resource control idle or inactive mode upon terminating the voice services. The voice services may comprise at least one of an emergency call, voice call or a video call. According to an aspect, there is provided a user equipment comprising: means for entering a radio resource control idle or inactive mode; means for selecting a 6G cell configured to serve a 6G network slice for voice services amongst a list of 6G network slices for voice services; means for determining to perform cell re-selection; means for re-selecting a 6G cell not configured to serve a 6G network slice for voice services; and means for performing measurements for a list of 5G cells configured to serve a list of 5G network slices for voice services. Each network slice may have a priority. Each cell configured to serve a network slice may have a priority. Different cells configured to serve a same network slice may have a same priority (e.g., the priorities of the cells configured to serve the same network slice may be the priority of the network slice). Alternatively, different cells configured to serve a same network slice may have different priorities (e.g., the priorities of the cells configured to serve the same network slice may be different from the priority of the network slice). The priorities of the network slices and / or the cells may be used to perform cell re-selection. The user equipment may comprise: means for determining that a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services is not available for cell re-selection. The list of 5G network slices for voice services may be associated with a list of 5G single network slice selection assistance information; and the list of 6G network slices for voice services may be associated with a list of 6G single network slice selection assistance information. The user equipment may comprise: means for sending, to a common 5G and 6G core network element, a common request to register to the 5G core network and the 6G core network including a preference for voice services; and means for receiving, from the common 5G and 6G core network element, a response including the list of 5G network slices for voice services and the list of 6G network slices for voice services. The common 5G and 6G core network element may comprise a common access and mobility management function. The common request to register to the 5G core network and the 6G core network may be sent via a 5G node B or a 6G node B over a 5G radio resource control connection or a 6G radio resource control connection. The user equipment may comprise: means for sending, to separate 5G core and 6G core network elements, separate requests to register to the 5G core network and the 6G core network including a preference for voice services; and means for receiving, from the separate 5G and 6G core network elements, a response including the list of 5G network slices for voice services and the list of 6G network slices for voice services. The separate 5G core network element and 6G core network element may comprise separate access and mobility management functions. The separate requests to register to the 5G core network and the 6G core network may be sent via a 5G node B and a 6G node B over separate 5G radio resource control connection and 6G radio resource control connection. The response may include a 5G global unique temporary identifier and a 6G global unique temporary identifier. The user equipment may comprise: means for receiving, from a 6G node B configured to provide the selected 6G cell configured to serve the network slice for voice services amongst the list of 6G network slices for voice services, the list of 5G cells configured to serve the list of 5G network slices for voice services. The list of 5G cells configured to serve the list of 5G network slices for voice services may be received within a 6G system information block or a 6G radio resource control release message. Means for determining to perform cell re-selection may comprise: means for determining that the user equipment has moved outside the coverage area of the selected 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slice for voice services; or means for determining that the 6G cell not configured to serve a network slice for voice services has at least one of a higher priority, a higher strength or a higher quality than the selected 6G cell configured to serve a network slice for voice services. The user equipment may comprise: means for transitioning from the radio resource control idle or inactive mode to a radio resource control connected mode; and means for re-selecting a 5G cell configured to serve a 5G network slice for voice services based on the measurements. The user equipment may comprise: means for determining that a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services is not available for cell re-selection. Means for performing measurements for a list of 5G cells configured to serve the list of 5G network slices for voice services may comprise: means for performing measurements of at least one of a strength or quality for a list of 5G cells configured to serve the list of 5G network slices for voice services; and means for re-selecting a 5G cell configured to serve a 5G network slice for voice services based on the measurements may comprise: means for re-selecting a 5G cell configured to serve a 5G network slice for voice services with at least one of a highest priority, a highest strength or a highest quality based on the measurements. The user equipment comprising means for transitioning from the radio resource control idle or inactive mode to the radio resource control connected mode upon: receiving, from a 6G node B configured to provide the selected 6G node B cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services, a paging message for voice services; or initiating voice services. The user equipment may comprise: means for sending, to a 5G node B configured to provide the re-selected 5G cell configured to serve a 5G network slice for voice services, a 5G global unique temporary identifier and a 6G global unique temporary identifier. The 5G global unique temporary identifier and the 6G global unique temporary identifier may be sent in a radio resource control setup procedure message. The user equipment may comprise: means for transitioning from the radio resource control connected mode to a radio resource control idle or inactive mode; and means for re-selecting a 6G cell not configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services. The user equipment may comprise: means for determining that a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slice for voice services is not available for cell re-selection. The user equipment may comprise: means for transitioning from the radio resource control connected mode to a radio resource control idle or inactive mode upon terminating the voice services. The voice services may comprise at least one of an emergency call, voice call or a video call. According to an aspect, there is provided a user equipment comprising circuitry configured to perform: entering a radio resource control idle or inactive mode; selecting a6G cell configured to serve a 6G network slice for voice services amongst a list of 6G network slices for voice services; determining to perform cell re-selection; re-selecting a 6G cell not configured to serve a 6G network slice for voice services; and performing measurements for a list of 5G cells configured to serve a list of 5G network slices for voice services. According to an aspect, there is provided a user equipment comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to perform: entering a radio resource control idle or inactive mode; selecting a 6G cell configured to serve a 6G network slice for voice services amongst a list of 6G network slices for voice services; determining to perform cell re-selection; re-selecting a 6G cell not configured to serve a 6G network slice for voice services; and performing measurements for a list of 5G cells configured to serve a list of 5G network slices for voice services. According to an aspect, there is provided a computer program comprising computer executable code which when run on at least one processor is configured to perform: entering, by a user equipment, a radio resource control idle or inactive mode; selecting, by the user equipment, a 6G cell configured to serve a 6G network slice for voice services amongst a list of 6G network slices for voice services; determining, by the user equipment, to perform cell re-selection; re-selecting a 6G cell not configured to serve a 6G network slice for voice services; and performing, by the user equipment, measurements for a list of 5G cells configured to serve a list of 5G network slices for voice services. According to an aspect, there is provided a computer readable medium comprising program instructions stored thereon for performing at least one of the above methods. According to an aspect, there is provided a non-transitory computer readable medium comprising program instructions stored thereon for performing at least one of the above methods. According to an aspect, there is provided a non-volatile tangible memory medium comprising program instructions stored thereon for performing at least one of the above methods. In the above, many different aspects have been described. As previously noted, it should be appreciated that further aspects may be provided by the combination of any two or more of the aspects described above (or otherwise in this disclosure). Various other aspects are also described in the following detailed description and in the claims. List of Abbreviations AF: Application Function AMF: Access and Mobility Management Function AUSF: Authentication Server Function BS: Base Station CU: Centralized Unit DU: Distributed Unit gNB: gNodeB GUTI: Global Unique Temporary Identifier LTE: Long Term Evolution MS: Mobile Station MTC: Machine Type Communication NEF: Network Exposure Function NR: New radio NRF: Network Repository Function RAM: Random Access Memory RAN: Radio Access Network RAT: Radio Access Technology ROM: Read Only Memory RRC: Radio Resource Control RSRQ: Reference Signal Received Quality RSRP: Reference Signal Received Power SIB: System Information Block S-NSSAI: Single-Network Slice Selection Assistance Information TS: Technical Specification UDM: Unified Data Management UDR: Unified Data Repository UDSF: Unstructured Data Storage Function UE: User Equipment UPF: User Plane Function 5G: 5th Generation 5G Core Network 5GS: 5G System 6G: 6th Generation 6GC: 6G Core Network 6GS: 6G System Brief Description of the Figures Some embodiments will now be described, by way of non-limiting and illustrative example only, with reference to the Figures in which: Fig. 1 shows a schematic representation of multi-radio access technology communication system; Fig. 2 shows a schematic representation of a control apparatus; Fig. 3 shows a schematic representation of a user equipment; Fig. 4a and Fig. 4b show a signalling diagram of a process for managing a user equipment in a multi-radio access technology communication system; Fig. 5 shows a flow chart of a process for managing a user equipment in a multi-radio access technology communication system; Fig. 6 shows a block diagram of a method for managing a user equipment in a multi-radio access technology communication system; Fig. 7 shows a block diagram of another method for managing a user equipment in a multiradio access technology communication system; and Fig. 8 shows a schematic representation of a non-volatile memory medium storing instructions which when executed by a processor enable, allow, or otherwise facilitate a processor to perform one or more of the steps of the methods of Fig. 6 and Fig. 7. Detailed Description Before explaining in detail the embodiments, certain principles of a wireless communication system, access systems thereof, and communication devices are briefly explained with reference to Fig. 1, Fig. 2 and Fig. 3 to assist in understanding the technology underlying the described examples. In the following description, the terms communication device and user equipment may be interchanged. Fig. 1 shows a schematic representation of a multi-radio access technology (RAT) communication system. The multi-RAT communication system may comprise one or more user equipment (UE), a 5G communication system (5GS), a 6G communication system (6GS), one or more application functions (AF) and one or more data networks (DN). The 5GS may comprise multiple public land mobile networks (PLMNs). Each PLMN may comprise a 5G radio access network (5G RAN) and a 5G core network (5GC). The 5G RAN may comprise one or more base stations. A base station may be a 5G gNodeB (gNB). A 5G gNB may comprise distributed units connected to one or more centralized units. The 5GC may comprise one or more of a unified data repository (UDR), a network exposure function (NEF), a network repository function (NRF), a user data management (UDM), an access and mobility management function (AMF), an authentication server function (AUSF), a session management function (SMF) and a user plane function (UPF). Likewise, the 6GS may comprise multiple PLMNs. Each PLMN may comprise a 6G radio access network (6G RAN) and a 6G core network (6GC). The 6G RAN may comprise one or more 6G base stations. A base station may be a 6G gNB. A 6G gNB may comprise distributed units connected to one or more centralized units. The 6GC may comprise one or more of a UDR, a NEF, a NRF, a UDM, an AMF, an AUSF, a SMF and a UPF. It will be understood that core network functions of the 5GC and the 6GC may be a common (e.g., shared) core network element. Alternatively, core network functions of the 5GC and the 6GC may be separate core network elements. Core network functions of the 5GC and the 6GC may be communicate with one another, for example using 5G based procedures. Fig. 2 illustrates an example of a control apparatus 200 for controlling a function of the RAN illustrated in Fig. 1. The control apparatus 200 may comprise at least one random access memory (RAM) 211a, at least on read only memory (ROM) 211b, at least one processor 212, 213 and an input / output interface 214. The at least one processor 212, 213 may be coupled to the RAM 211a and the ROM 211b. The at least one processor 212, 213 may be configured to execute an appropriate software code215. The software code215 may, for example, enable, allow, or otherwise facilitate to perform one or more steps to perform one or more of the embodiments (or aspects thereof) described in this disclosure. The software code 215 may be stored in the ROM 211b. The control apparatus 200 may be interconnected with another control apparatus 200 controlling another function of the 5G RAN or the 5GC. In some embodiments, each RAN comprises a control apparatus 200. In alternative embodiments, two or more RANs may share a control apparatus. Fig. 3 illustrates an example of a UE 300, such as the UE illustrated on Fig. 1. The UE 300 is a communication device that is capable of (or configured for) sending and receiving radio signals. Non-limiting and illustrative examples comprise a mobile station (MS) or mobile device, such as a mobile phone or what is known as a ’smart phone’, a computer provided with a wireless interface card or other wireless interface facility (e.g., USB dongle), a personal data assistant (PDA) or a tablet provided with wireless communication capabilities, a machinetype communications (MTC) device, a Cellular Internet of Things (CloT) device or any combinations of these or the like. The UE 300 may provide, for example, communication of data for carrying communications. The communications may be one or more of voice, electronic mail (email), text message, multimedia, data, machine data and so on. The UE 300 may receive signals over an air or radio interface 307 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Fig. 3, transceiver apparatus is designated schematically by block 306. The transceiver apparatus 306 may be provided, for example, by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally and / or externally to the mobile device. The UE 300 may comprise at least one processor 301, at least one memory ROM 302a, at least one RAM 302b and other possible components 303 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The at least one processor 301 is coupled to the RAM 302b and the ROM 302a. The at least one processor 301 may be configured to execute an appropriate software code 308. The software code 308 may, for example, enable, allow, or otherwise facilitate to perform one or more of the embodiments (or aspects thereof) described in this disclosure. The software code 308 may be stored in the ROM 302a. The processor, storage and other relevant control apparatus can be provided on an appropriate circuit board and / or in chipsets. This feature is denoted by reference 304. The device may optionally have a user interface, such as keypad 305, touch sensitive screen or pad, combinations thereof or the like. Optionally, one or more of a display, a speaker and / or a microphone may be provided depending on the type of the device. As previously noted, various embodiments of this disclosure relate to a method, an apparatus and a computer program for a communication system, and in particular - but not exclusively - a method, an apparatus, and a computer program for managing a UE in a multi-RAT communication system. It will be understood that, although in the following the multi-RAT communication system comprises a 5GS and a 6GS, one or more embodiments (or aspects thereof) of this disclosure may apply to a multi-RAT communication system comprising different cellular systems (e.g., 3GPP communications system(s) and / or non-3GPP communications system(s)). It will be understood that, although in the following the multi-RAT communication system uses network slicing, one or more embodiments (or aspects thereof) of this disclosure may apply to a multi-RAT communication system that does not use network slicing. It will be understood that, although in the following preferred services comprise voice services, one or more embodiments (or aspects thereof) of this disclosure may apply to other preferred services (e.g., extended reality services or streaming services) Fig. 4a and Fig. 4b shows a signalling diagram of a process for managing a UE in a multi-RAT communication system. At step 1 and step 2, a UE may register to a 5GC and a 6GC. In some embodiments, the UE may send, to a common 5G and 6G AMR a common request to register to the 5GC and the 6GC. The request may include a preference for voice services. The request may be sent over a 5G radio resource control (RRC) connection via a 5G gNB or over a 6G RRC connection via a 6G gNB. The request may or may not include an indication to receive a list of (standby) 5G network slices, and a list of 6G network slices. The list of 5G network slices may be associated with (e.g., identified by) a list of 5G single network slice selection assistance information (S-NSSAIs) for (i.e., supporting) voice services. The list of 6G network slices may be associated with (e.g., identified by) a list of 6G S-NSSAIs for (i.e., supporting) voice services. It will be understood that each network slice may have a priority. Each cell configured to serve a network slice may have a priority. Different cells configured to serve a same network slice may have a same priority (e.g., the priorities of the cells configured to serve the same network slice may be the priority of the network slice). Alternatively, different cells configured to serve a same network slice may have different priorities (e.g., the priorities of the cells configured to serve the same network slice may be different from the priority of the network slice). The priorities of the network slices and / or the cells may be used to perform (or otherwise facilitate) cell re-selection. It will be understood that a S-NSSAI may refer to an identifier of a network slice. An S-NSSAI may comprise a slice or service type and an optional slice differentiator. The UE may receive, from the common 5G and 6G AMF, a response including a list of 5G network slices for voice services and a list of 6G network slices for voice services. The response may include a 5G identifier identifying the UE in the 5Gs and a 6G identifier identifying the UE in the 6GS. The 5G identifier may comprise a 5G global unique temporary identifier (GUTI) and a 6G GUTI. It will be understood that the UE may not necessarily receive, from the common 5G and 6G AMF, the list of 5G network slices for voice services and the list of 6G network slices for voice services. The UE may receive, from a common 5G and 6G UDM, the list of 5G S-NSSAIs for voice services and the list of 6G network slices for voice services. The UE may receive, from a common 5G and 6G PCF, network slice selection policies for handling and prioritizing the list of 5G network slices for voice services and the list of 6G S-NSSAIs for voice services. The UE may receive, from a common 5G and 6G PCF, network slice selection policies for handling and prioritizing 5G network slices for other services and 6G S-NSSAIs for other services. In other embodiments, the UE may send, to separate 5G and 6G AMFs, separate requests to register to the 5GC and the 6GC. The separate requests may include a preference for voice services. The separate requests may be sent over a 5G RRC connection via a 5G gNB and over a 6G RRC connection via a 6G gNB. The separate requests may or may not include an indication to receive a list of (standby) 5G network slices for (i.e., supporting) voice services and a list of 6G network slices for (i.e., supporting) voice services. The UE may receive, from the separate 5G and 6G AMFs, a response including a list of 5G network slices for (i.e., supporting) voice services and a list of 6G network slices for (i.e., supporting) voice services. The response may include a 5G identifier and a 6G identifier. The 5G identifier may comprise a 5G GUTI and a 6G GUTI. It will be understood that the UE may not necessarily receive, from the separate 5G and 6G AMFs, the list of 5G network slices for voice services and the list of 6G network slices for voice services. The UE may receive, from separate 5G UDM and 6G UDM, the list of 5G S-NSSAIs for voice services and the list of 6G network slices for voice services. The UE may receive, from separate 5G PCF and 6G PCF, network slice selection policies for handling and prioritizing the list of 5G network slices for voice services and the list of 6G S-NSSAIs for voice services. The UE may receive, from separate 5G PCF and 6G PCF, network slice selection policies for handling and prioritizing 5G network slices for other services and 6G S-NSSAIs for other services. It will be understood that the expressions “voice services” and “voice centric services” may be used interchangeably. The expressions “voice services” and “voice centric services” may comprise at least one of an emergency call, voice call or a video call. It will be understood that network slice selection policies may be based on at least one of priorities of network slices. Network slice selection policies may be based on at least one of priorities, strengths or qualities of cells configured to serve network slices. At step 3, the UE may enter a first RRC mode (i.e., idle or inactive). The UE may select a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services. The selection may be based on the network slice selection policies for handling and prioritizing the list of 6G network slices for voice services. The UE may camp on the selected 6G cell configured to serve the 6G network slice for voice service amongst the list of 6G network slices for voice services. At step 4, the UE may receive, from a 6G gNB configured to provide the selected 6G cell, at least one of a list of carrier frequencies and a list of 5G cells configured to serve the list of 5G network slices for voice services and operating on the carrier frequencies and / or a list of 5G cells not configured to serve the list of 5G network slices for voice services and operating on the carrier frequencies. In some embodiments, the list of carrier frequencies and the list of 5G cells configured to serve the list of 5G network slices for voice services and operating on the carrier frequencies may be received within a 6G system information block (SIB). The 6G SIB may be similar to 5G SIB 16 and may further include the list of carrier frequencies and the list of 5G cells configured to serve the list of 5G network slices for voice services and operating on the carrier frequencies. In other embodiments, the list of carrier frequencies and the list of 5G cells configured to serve the list of 5G network slices for voice services and operating on the carrier frequencies may be received within a 6G RRC release message. Alternatively, at step 3, the UE may select a 5G cell configured to serve a 5G network slice for voice services amongst the list of 5G network slices for voice services. The selection may be based on the network slice selection policies for handling and prioritizing the list of 5G network slices for voice services. The UE may camp on the selected 5G cell configured to serve the 5G network slice for voice service amongst the list of 5G network slices for voice services. At step 4, the UE may receive, from a 5G gNB configured to provide the selected 5G cell, at least one of a list of carrier frequencies and a list of 6G cells configured to serve the list of 6G network slices for voice services and operating on the carrier frequencies and / or a list of 6G cells not configured to serve the list of 5G network slices for voice services and operating on the carrier frequencies. In some embodiments, the list of carrier frequencies and the list of 6G cells configured to serve the list of 6G network slices for voice services and operating on the carrier frequencies may be received within a 5G SIB. The 5G SIB may be a 5G SIB 16 and may further include the list of carrier frequencies and the list of 6G cells configured to serve the list of 6G network slices for voice services and operating on the carrier frequencies. In other embodiments, the list of carrier frequencies and the list of 6G cells configured to serve the list of 6G network slices for voice services and operating on the carrier frequencies may be received within a 5G RRC release message. At step 5, the UE may determine to perform cell re-selection. For example, the UE may move outside the coverage area of the selected 6G cell configured to serve the 6G network slice for voice services amongst the list of 6G network slices for voice services. Alternatively, the UE may determine that a 6G cell not configured to serve the 6G network slice for voice services has at least one of a higher priority, a higher strength or a higher quality than the selected 6G cell configured to serve the 6G network slice for voice services. The UE may determine that no 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services is available for cell reselection. The UE may re-select a 6G cell not configured to serve a 6G network slice for voice services (i.e., a 6G cell configured to serve a 6G S-NSSAI for other services). The re-selection may be based on the network slice selection policies for handling and prioritizing a 6G network slice for other services. The UE may camp on the re-selected 6G cell not configured to serve a 6G network slice for voice services. At step 6, since the UE camps on the re-selected 6G cell not configured to serve a 6G network slice for voice services, the UE may perform measurements for the list of 5G cells configured to serve the list of 5G network slices for voice services received at step 3. The UE may perform measurements of at least one of a reference signal received quality (RSRQ) or a reference signal received power (RSRP) for the list of 5G cells configured to serve the list of 5G network slices for voice services. At step 7, the UE may store the identity of the N best 5G cells configured to serve the list of 5G network slices for voice services based on the measurements. N may be an integer equal to or greater than 1. For example, the UE may store the identity of the 5G cells configured to serve the list of 5G network slices for voice services with at least one of the highest priority, the highest strength or the highest quality based on the measurements. It will be understood that step 6 and step 7 may be performed a single time or multiple times (e.g., periodically) as long as the UE camps on the re-selected 6G cell not configured to serve a 6G network slice for voice services. In this manner, cell re-selection may be accelerated when (e.g., based on) the UE transitions from the first RRC mode (e.g., idle or inactive mode) to a second RRC mode (e.g., connected mode) to provide voice services to the UE. At step 8 and step 9, the 6G gNB configured to provide the re-selected 6G cell not configured to serve a 6G network slice for voice services may receive, from the common 5G and 6G AMF or from the separate 6G AMF, a paging message to initiate voice services (e.g., call terminating at the UE). The UE may receive, from the 6G gNB configured to provide the re-selected 6G cell not configured to serve a 6G network slice for voice services, the paging message to initiate the voice services. Alternatively, at step 10, the UE may initiate voice services (e.g., call originating at the UE). The UE may determine to transition from the first RRC mode (e.g., idle or inactive mode) to the second RRC mode (e.g., connected mode). At step 11, since the UE camps on the re-selected 6G cell not configured to serve a 6G network slice for voice services, the UE may attempt to re-select a 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services. The UE may determine that no 6G cell configured to serve a 6G network slice for voice services amongst the list of 6G network slices for voice services is available for cell re-selection. At step 12, the UE may re-select a 5G cell amongst the N best 5G cells configured to serve the list of 5G network slices for voice services stored by the UE. At step 13, the UE may send, to a 5G gNode B configured to provide the re-selected 5G cell amongst the N best 5G cells configured to serve the list of 5G network slices for voice services, the 5G identifier and the 6G identifier. The 5G identifier and the 6G identifier may be sent as part of a RRC setup procedure (e.g., within RRC setup complete message). At step 14, the 5G gNode B configured to provide the re-selected 5G cell amongst the N best 5G cells configured to serve the list of 5G network slices for voice services may send, to the common 5G and 6GAMF or the separate 5GAMF, the 5G identifier and the 6G identifier. The 5G identifier and the 6G identifier may be sent within an initial context setup message. At step 15, the common 5G and 6G AMF or the separate 5G AMF may update a registration state for the UE from 6G to 5G. The common 5G and 6G AMF or the separate 5G AMF may be aware of how to reach the UE and hence may not require the UE to perform a registration area update procedure. At step 16, the common 5G and 6G AMF or the separate 5GAMF may send, to the 5G gNode B configured to provide the re-selected 5G cell amongst the N best 5G cells configured to serve the list of 5G network slices for voice services, an initial context setup response. At step 17, the UE, the 5G gNode B configured to provide the re-selected 5G cell amongst the N best 5G cells configured to serve the list of 5G network slices for voice services and the common 5G and 6G AMF or the separate 5G AMF may handle the voice services. At step 18, the UE, the 5G gNode B configured to provide the re-selected 5G cell amongst the N best 5G cells configured to serve the list of 5G network slices for voice services and the common 5G and 6G AMF or the separate 5G AMF may release the voice services. At step 19, the UE may transition back from the second RRC mode (e.g., connected mode) to the first RRC mode (e.g., idle or inactive mode). During the release, the UE may be provided with a list of prioritized 6G cells supporting network slices for voice services. At step 20, the UE may re-select a 6G cell not configured to serve a 6G network slice for voice services (i.e., a 6G cell configured to serve a 6G network slice for other services). The reselection may be based on the network slice selection policies for handling and prioritizing 6G network slices for other services. The UE may camp on the re-selected 6G cell not configured to serve a 6G network slice for voice services. At step 21, the UE, the 6G gNB configured to provide the 6G cell not configured to serve a 6G network slice for voice services and the common 5G and 6G AMF (or separate 6G AMF) may perform a registration area update procedure so that the AMF is aware of how to reach the UE. Fig. 5 shows a flow chart of a process for managing a UE in a multi-RAT communication system. At step 10, a UE may receive, from a common 5Gand6GAMF (or separate 5Gand 6GAMFs), a list of 5G network slices for (i.e., supporting) voice services and a list of 6G network slices for (i.e., supporting) voice services. The UE may receive, from the common 5G and 6G AMF or the separate 5G and 6G AMFs, a 5G identifier and a 6G identifier. The method proceeds to step 20. At step 20, the UE may receive network slice selection policies for handling and prioritizing 5G network slices for voice services and 6G network slices for voice services. The UE may receive network slice selection policies for handling and prioritizing 5G S-NSSAIs for other services and 6G network slices for other services. The method proceeds to step 30. At step 30, the UE may enter a first RRC mode (e.g., idle or inactive mode). The method proceeds to step 70. At step 70, the UE may determine whether to transition from the first RRC mode (e.g., idle or inactive mode) to a second RRC mode (e.g., connected mode). If the UE may determine to not transition from the first RRC mode (e.g., idle or inactive mode) to the second RRC mode, the method proceeds to step 40. If the UE may determine to transition from the first RRC mode (e.g., idle or inactive mode) to the second RRC mode, the method proceeds to step 90. At step 40, the UE may select a 6G cell. The 6G cell may or may not be a 6G cell configured to serve a network slice for voice services among the list of network slices for voice services. The selection may be based on the network slice selection policies for handling and prioritizing 6G network slices for voice services or network slice selection policies for handling and prioritizing 6G network slices for other services. The method proceeds to step 50. At step 50, the UE may determine whether the selected 6G cell is configured to serve a network slice for voice services among the list of network slices for voice services. If the UE determines that the selected 6G cell is configured to serve a network slice for voice services among the list of network slices for voice services, the method goes to step 70. If the UE determines that the selected 6G cell is not configured to serve a network slice for voice services among the list of network slices for voice services, the method proceeds to step 60. At step 60, the UE may perform measurements a list of 5G cell configured to serve the list of network slices for voice services. The UE may store the identity of the N best 5G cells configured to serve the list of network slices for voice services. The method proceeds to step 70. At step 90, the UE may determine whether the selected 6G cell is configured to serve a network slice for voice services among the list of network slices for voice services. If the UE determines that the selected 6G cell serves a S-NSSAI for voice services among the list of network slices for voice services, the method goes to step 100. If the UE determines that the selected 6G cell does not serve a network slice for voice services among the list of network slices for voice services, the method proceeds to step 110. At step 100, the UE may transition from the first RRC mode (e.g., idle or inactive mode) to the second RRC mode (e.g., connected mode). At step 110, the UE may re-select a 5G cell amongst the N best 5G cells configured to serve the list of network slices for voice services. The method goes to step 120. At step 120, the UE may transition from the first RRC mode (e.g., idle or inactive mode) to the second RRC mode (e.g., connected mode). The UE may send, to the 5G gNB configured to provide the re-selected a 5G cell amongst the N best 5G cells configured to serve the list of network slices for voice service, the 5G identifier and the 6G identifier. Fig. 6 shows a block diagram of a method for managing a UE in a multi-RAT communication system. At 600, a UE may enter a first RRC mode. At 602, the UE may select a first RAT cell configured to provide preferred services. At 604, the UE may determine to perform cell re-selection. At 606, the UE may re-select a first RRC cell not configured to provide preferred services. At 608, the UE may perform measurements fora list of second RAT cells configured to provide preferred services. Fig. 7 shows a block diagram of another method for managing a UE in a multi-RAT communication system. At 700, a UE may enter a RRC idle or inactive mode. At 702, the UE may select a 6G cell configured to serve a 6G network slice for voice services amongst a list of 6G network slices for voice services. At 704, the UE may determine to perform cell re-selection. At 706, the UE may re-select a 6G cell not configured to serve a 6G network slice for voice services. At 708, the UE may perform measurements for a list of 5G cells configured to serve a list of 5G network slices for voice services. Fig. 8 shows a schematic representation of non-volatile memory media 1100 storing instructions which when executed by at least one processor allow the at least one processor to perform one or more of the steps of the methods of Fig. 6 and Fig. 7. It is noted that while various embodiments are described in this disclosure, there are several variations and modifications which may be made to herein without departing from the scope of this disclosure. The embodiments described herein may thus vary within the scope of the claims. In general, some embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some embodiments (or aspects thereof) may be implemented in hardware, while other embodiments (or aspects thereof) may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the various embodiments are not limited thereto. While the various embodiments described herein may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting and illustrative examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. The various embodiments of this disclosure may be implemented by computer software stored in a memory and executable by at least one data processor of the involved entities or by hardware, or by a combination of software and hardware. Further in this regard, it should be noted that any procedures, e.g., as in Fig. 6 and Fig. 7, may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media, such as hard disk or floppy disks, and optical media, such as, for example, DVD and the data variants thereof, CD. The term “non-transitory”, as used herein, is a limitation of the medium itself (i.e. tangible, not a signal) as opposed to a limitation on data storage persistency (e.g. RAM vs ROM). The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), gate level circuits and processors based on multi-core processor architecture, as non-limiting and illustrative examples. As used herein, “at least one of the following:” and “at least one of: ” and similar wording, where the list of two or more elements are joined by “and”, or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all of the elements. As used herein, the expression “and / or” also includes any and all combinations of the listed terms, including at least any one of the elements, or at least any two or more of the elements, or at least all of the elements. As used herein, the term “or” refers to a non-exclusive “or” unless otherwise indicated (e.g., use of “or else” or “or in the alternative”). As used herein, unless stated explicitly, performing a respective feature, step, or functionality “in response to A” does not indicate that the respective feature, step, or functionality is performed immediately after “A” occurs as one or more intervening features, steps, or functionalities may be performed (at least in part) between an occurrence of the respective feature, step, or function and “A”. Analogously, performing a respective feature, step, or functionality “based on A” does not indicate that the respective feature, step, or functionality is performed solely based on “A” as the respective feature, step, or functionality may be further based on one or more other features, steps, or functionalities in addition to “A”. As used herein, the term “means for”, or “means configured to perform” (or similar) may be any means that are suitable for performing the feature. The “means” may be configured to perform one or more of the functions and / or method steps previously described. For example, the “means” may include one or more of: at least one processor, at least one memory, transceiver circuitry, antenna circuitry, etc. It should be understood that these are provided as non-limiting and illustrative examples. Alternatively or additionally, some embodiments of this disclosure may be implemented using circuitry. The circuitry may be configured to perform one or more of the functions and / or method steps described herein. The circuitry may be provided in a base station and / or in a user equipment. As used herein, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuits (such as only analogue and / or digital circuits); (b) combinations of hardware circuits and software, such as: (i) a combination of analogue and / or digital hardware circuit(s) with software / firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as the communications device or base station to perform the various functions previously described; and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation. This definition of circuitry applies to all uses of this term “means” herein, including in any claims. As a further example, as used herein, the term circuitry also covers an embodiment of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example, integrated device(s). The foregoing description has provided, by way of non-limiting and illustrative examples, a full and informative description of the various embodiments of this disclosure. However, various modifications and adaptations may become apparent to those skilled in the relevant art(s) in view of the foregoing description, when read in conjunction with the drawings and the claims. However, all such and similar modifications of the teachings herein are intended to still fall within the scope of this disclosure. For example, it should be appreciated that further 5 embodiments may be provided by the combination of any two or more of the various embodiments described herein.

Claims

1. A method comprising:entering, by a user equipment, a first radio resource control mode;selecting, by the user equipment, a first radio access technology cell configured to provide preferred services;determining, by the user equipment, to perform cell re-selection;re-selecting, by the user equipment, a first radio access technology cell not configured to provide preferred services; andperforming, by the user equipment, measurements for a list of second radio access technology cells configured to provide preferred services.

2. The method of claims 1, comprising:determining, by the user equipment, that a first radio access technology cell configured to provide preferred services is not available for cell re-selection.

3. The method of claim 1 or claim 2, wherein selecting, by the user equipment, the first radio access technology cell configured to provide preferred services comprises:selecting, by the user equipment, a first radio access technology cell configured to serve a first radio access technology network slice for preferred services amongst a list of first radio access technology network slices for preferred services;wherein re-selecting, by the user equipment, a first radio access technology cell not configured to provide preferred services comprises:re-selecting, by the user equipment, a first radio access technology cell not configured to serve a first radio access technology network slice for preferred services amongst the list of first radio access technology network slices for preferred services;wherein performing, by the user equipment, measurements for a list of second radio access technology cells configured to provide preferred services comprises:performing, by the user equipment, measurements for a list of second radio access technology cells configured to serve a list of second network slices for preferred services.

4. The method of claim 3, wherein the list of first radio access technology network slices for preferred services is associated with a list of first radio access technology single network slice selection assistance information; andwherein the list of second radio access technology network slices for preferred services is associated with a list of second radio access technology single network slice selection assistance information.

5. The method of any of claims 1 to 4, wherein the first radio access technology comprises6G and the second radio access technology comprises 5G.

6. The method of any of claims 1 to 5, wherein the first radio resource control mode comprises a radio resource control idle or inactive mode.

7. The method of any of claims 3 to 6, comprising:sending, by the user equipment to a common first radio access technology and second radio access technology core network element, a common request to register to a first radio access technology core network and a second radio access technology core network including a preference for preferred services; andreceiving, by the user equipment from the common first radio access technology and second radio access technology core network element, a response including the list of first radio access technology network slices for preferred services and the list of second radio access technology network slices for preferred services.

8. The method of claim 7, wherein the common first radio access technology and second radio access technology core network element comprise a common access and mobility management function.

9. The method of claim 7 or claim 8, wherein the common request to register to the first radio access technology core network and the second radio access technology core network is sent via a first radio access technology base station or a second radio access technology base station over a first radio access technology radio resource control connection or a second radio access technology radio resource control connection.

10. The method of any of claims 3 to 6, comprising:sending, by the user equipment to separate first radio access technology and second radio access technology core network elements, separate requests to register to a first radio access technology core network and a second radio access technology core network including a preference for preferred services; andreceiving, by the user equipment from the separate first radio access technology and second radio access technology core network elements, a response including the list of first radio access technology network slices for preferred services and the list of second radio access technology network slices for preferred services.

11. The method of claim 10, wherein the separate first radio access technology and second radio access technology core network elements comprise separate access and mobility management functions.

12. The method of claim 10 or claim 11, wherein the separate requests to register to the first radio access technology core network and the second radio access technology core network are sent via a first radio access technology base station and a second radio access technology base station over separate first radio access technology and second radio access technology radio resource control connections.

13. The method of any of claims 7 to 12, wherein the response includes a first radio access technology identifier and a second radio access technology identifier.

14. The method of any of claims 3 to 13, comprising:receiving, by the user equipment, from a first radio access technology base station, the list of second radio access technology cells configured to serve the list of second radio access technology network slices for preferred services.

15. The method of claim 14, wherein the list of second radio access technology cells configured to serve the list of second radio access technology network slices for preferred services is received within a first radio access technology system information block or a first radio access technology radio resource control release message.

16. The method of any of claims 1 to 15, wherein determining, by the user equipment, to perform cell re-selection comprises:determining, by the user equipment, that the user equipment has moved outside a coverage area of the selected first radio access technology cell configured to provide preferred services; ordetermining, by the user equipment, that the first radio access technology cell not configured to provide preferred services has at least one of a higher priority, a higher strengthor a higher quality than the selected first radio access technology cell configured to provide preferred services.

17. The method of any of claims 1 to 16, comprising:transitioning, by the user equipment, from the first radio resource control mode to a second radio resource control mode; andre-selecting, by the user equipment, a second radio access technology cell configured to provide preferred services based on results of the measuring.

18. The method of claim 17, comprising:determining, by the user equipment, that a first radio access technology cell configured to provide preferred services is not available for cell re-selection.

19. The method of claim 18, wherein the second radio resource control mode comprises a radio resource control connected mode.

20. The method of any of claims 17 to 19, wherein performing, by the user equipment, measurements for a list of second radio access technology cells comprises:performing, by the user equipment, measurements of at least one of a strength or a quality for a list of second radio access technology cells configured to provide preferred services; andwherein re-selecting, by the user equipment, a second radio access technology cell configured to provide preferred services based on the measurements comprises:re-selecting, by the user equipment, a second radio access technology cell configured to provide preferred services with at least one of a highest priority, a highest strength or a highest quality based on the measurements.

21. The method of claim 19 or claim 20, comprising transitioning, by the user equipment, from the first radio resource control mode to the second radio resource control mode upon:receiving, by the user equipment from a first radio access technology base station configured to provide the selected first radio access technology cell not configured to provide preferred services, a paging message for preferred services; orinitiating, by the user equipment, preferred services.

22. The method of any of claims 1 to 21, comprising:sending, by the user equipment to a second radio access technology base station configured to provide the re-selected second radio access technology cell configured to provide preferred services, a second radio access technology identifier and a first radio access technology identifier.

23. The method of claim 22, wherein the second radio access technology identifier and the first radio access technology identifier comprise a second radio access technology global unique temporary identifier and a first radio access technology global unique temporary identifier24. The method of claim 22 or claim 23, wherein the second radio access technology identifier and the first radio access technology identifier are sent in a radio resource control setup procedure message.

25. The method of any of claims 17 to 24, comprising:transitioning, by the user equipment, from the second radio resource control mode to a first radio resource control mode; andre-selecting, by the user equipment, a first radio access technology cell not configured to provide preferred services.

26. The method of claim 25, comprising:determining, by the user equipment, that a first radio access technology cell configured to provide preferred services is not available for cell re-selection.

27. The method of claim 25, comprising:transitioning, by the user equipment, from the second radio resource control mode to a first radio resource control mode upon terminating the preferred services.

28. The method of any of claims 1 to 27, wherein the preferred services comprise voice services.

29. The method of claim 28, wherein the voice services comprise at least one of an emergency call, voice call or a video call.means for entering a first radio resource control mode;means for selecting a first radio access technology cell configured to provide preferred services;means for determining to perform cell re-selection;5 means for re-selecting a first radio access technology cell not configured to providepreferred services; andmeans for performing measurements for a list of second radio access technology cells configured to provide preferred services.10 31. A computer program comprising computer executable instructions which when run onone or more processors perform the steps of the method of any of claims 1 to 29.43

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