Field

This disclosure relates to communications, and more particularly to a paging scheme for a wireless communication system. Background

A communication system can be seen as a facility that enables communication between two or more devices such as user terminals, machine-like terminals, base stations and/or other nodes by providing communication channels for carrying information between the communicating devices. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communication may comprise, for example, communication of data for carrying data for voice, electronic mail (email), text message, multimedia and/or content data communications and so on. Non-limiting examples of services provided include two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.

In a wireless system at least a part of communications occurs over wireless interfaces. Examples of wireless systems include public land mobile networks (PLMN), satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). A local area wireless networking technology allowing devices to connect to a data network is known by the tradename WiFi (or Wi-Fi). WiFi is often used synonymously with WLAN. The wireless systems can be divided into cells, and are therefore often referred to as cellular systems. A base station provides at least one cell.

A user can access a communication system by means of an appropriate communication device or terminal capable of communicating with a base station. Hence nodes like base stations are often referred to as access points. A communication device of a user is often referred to as user equipment (UE). A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications, for example enabling communications with the base station and/or communications directly with other user devices. The communication device can communicate on appropriate channels, e.g. listen to a channel on which a station, for example a base station of a cell, transmits.

A communication system and associated devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system 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. Non- limiting examples of standardised radio access technologies include GSM (Global System for Mobile), EDGE (Enhanced Data for GSM Evolution) Radio Access Networks (GERAN), Universal Terrestrial Radio Access Networks (UTRAN) and evolved UTRAN (E-UTRAN). An example communication system architecture is the long-term evolution (LTE) of the Universal Mobile Telecommunications System (U MTS) radio-access technology. The LTE is standardized by the 3rd Generation Partnership Project (3GPP). The LTE employs the Evolved U niversal Terrestrial Radio Access Network (E-UTRAN) access and a further development thereof which is sometimes referred to as LTE Advanced (LTE-A). Since introduction of fourth generation (4G) services increasing interest has been paid to the next, or fifth generation (5G) standard.

Paging is a system access functionality that is triggered by the network to locate a UE. It is initiated mainly when there is a downlink transmission (DL data) waiting for the UE. The paging can be initiated from the radio access network (RAN) (e.g. to reach a UE in one of UMTS connected states) or from the core network (CN) (e.g. to reach a UE in UMTS Idle mode or LTE's RRC IDLE state). The paging mechanism chosen may impact the latency to reach a UE and therefore the latency to transmit a first packet to the UE.

Statement of invention

I n a first aspect there is provided a method comprising: determining a paging scheme for paging a communication device, the paging scheme for use when the communication device is in a low activity state; the determining a paging scheme comprising determining at least one condition, and determining whether to use a radio access network based paging scheme or a core network based paging scheme, based on the determined at least one condition. According to some embodiments, the at least one condition comprises one or more of: a number of cells in which the communication device is being tracked; a mobility state of the communication device; a deployment scenario; interface loading; whether load balancing is required; service requirements of the communication device. According to some embodiments, the determining a paging scheme is carried out during a transition of the communication device to the low activity state, or during a location update procedure, or when the communication device is already in the low activity state.

According to some embodiments, the method comprises sending paging configuration information to the communication device, so that the communication device can operate in accordance with the determined paging scheme.

According to some embodiments the sending paging configuration information to the communication device comprises informing the communication device which paging scheme configuration to use. According to some embodiments, the paging configuration information is comprised in a radio resource control suspend message.

According to some embodiments, the determining a paging scheme is carried out by a base station last serving the communication device.

According to some embodiments, the last-serving base station determines that the core network based paging scheme is to be used, the last-serving base station informs the core network to page the communication device when paging is required.

According to some embodiments, the method comprises paging the communication device in accordance with the determined paging scheme.

According to some embodiments the base station comprises a 5G base station. According to some embodiments, the base station comprises a 4G base station.

According to some embodiments the low activity state comprises a RRC CONNECTED INACTIVE state. According to some embodiments, when in the low activity state an SI interface between the core network and the radio access network is maintained.

In a second aspect there is provided a computer program comprising program code means adapted to perform the steps of the first aspect when the program is run on a data processing apparatus.

In a third aspect there is provided a method comprising: receiving, at a communication device a message instructing the communication device to adopt a radio access network based paging scheme or a core network based paging scheme for use when the communication device is in a low activity state; and adopting the particular paging scheme at the communication device in accordance with the instruction.

According to some embodiments, the received message comprises a radio resource control suspend message.

According to some embodiments, the method is carried out during a transition of the communication device to the low activity state, or during a location update procedure, or when the communication device is already in the low activity state.

According to some embodiments, the method comprises receiving one or more paging messages from a last-serving base station, when the paging scheme is adopted.

According to some embodiments, the method comprises receiving the message instructing the communication device to adopt a radio access network based paging scheme from a last-serving base station.

According to some embodiments, the method comprises receiving the message instructing the communication device to adopt a core network based paging scheme from a mobile management entity.

According to some embodiments the low activity state comprises a RRC CONNECTED INACTIVE state.

In a fourth aspect there is provided a computer program comprising program code means adapted to perform the steps of the third aspect when the program is run on a data processing apparatus. In a fifth aspect there is provided an apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to: determine a paging scheme for paging a communication device, the paging scheme for use when the communication device is in a low activity state; determine at least one condition as part of the determining a paging scheme, and determine whether to use a radio access network based paging scheme or a core network based paging scheme, based on the determined at least one condition. According to some embodiments, the at least one condition comprises one or more of: a number of cells in which the communication device is being tracked; a mobility state of the communication device; a deployment scenario; interface loading; whether load balancing is required; service requirements of the communication device.

According to some embodiments, the apparatus is configured to carry out the determine a paging scheme during a transition of the communication device to the low activity state, or during a location update procedure, or when the communication device is already in the low activity state.

According to some embodiments, the apparatus is configured to send paging configuration information to the communication device, so that the communication device can operate in accordance with the determined paging scheme.

According to some embodiments the sending paging configuration information to the communication device comprises informing the communication device which paging scheme configuration to use.

According to some embodiments, the apparatus is configured to comprise the paging configuration information in a radio resource control suspend message.

According to some embodiments, the apparatus comprises a base station last serving the communication device. According to some embodiments, when the apparatus determines that the core network based paging scheme is to be used, the apparatus is configured to inform the core network to page the communication device when paging is required.

According to some embodiments, the apparatus is configured to page the communication device in accordance with the determined paging scheme.

According to some embodiments the apparatus comprises a 5G base station.

According to some embodiments, the apparatus comprises a 4G base station.

According to some embodiments the low activity state comprises a RRC CONNECTED INACTIVE state.

According to some embodiments, when in the low activity state an SI interface between the core network and the radio access network is maintained.

In a sixth aspect there is provided an apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to: receive, at the apparatus, a message instructing the apparatus to adopt a radio access network based paging scheme or a core network based paging scheme for when the apparatus is in a low activity state; and adopt the particular paging scheme at the apparatus in accordance with the instruction. According to some embodiments, the received message comprises a radio resource control suspend message.

According to some embodiments, the apparatus is configured to adopt the particular paging scheme during a transition of the apparatus to the low activity state, or during a location update procedure, or when the apparatus is already in the low activity state. According to some embodiments, the apparatus is configured to receive one or more paging messages from a last-serving base station, when the paging scheme is adopted. According to some embodiments, the apparatus is configured to receive the message instructing the apparatus to adopt a radio access network based paging scheme from a last-serving base station.

According to some embodiments, the apparatus is configured to receive the message instructing the apparatus to adopt a core network based paging scheme from a mobile management entity.

According to some embodiments the low activity state comprises a RRC CONNECTED INACTIVE state. In a seventh aspect there is provided an apparatus comprising means for determining a paging scheme for paging a communication device, the paging scheme for use when the communication device is in a low activity state; means for determining at least one condition as part of the means for determining a paging scheme, and means for determining whether to use a radio access network based paging scheme or a core network based paging scheme, based on the determined at least one condition.

According to some embodiments, the at least one condition comprises one or more of: a number of cells in which the communication device is being tracked; a mobility state of the communication device; a deployment scenario; interface loading; whether load balancing is required; service requirements of the communication device. According to some embodiments, the apparatus comprises means for carrying out the determining a paging scheme during a transition of the communication device to the low activity state, or during a location update procedure, or when the communication device is already in the low activity state.

According to some embodiments, the apparatus comprises means for sending paging configuration information to the communication device, so that the communication device can operate in accordance with the determined paging scheme. According to some embodiments the means for sending paging configuration information to the communication device comprises means for informing the communication device which paging scheme configuration to use.

According to some embodiments, the apparatus comprises means for comprising the paging configuration information in a radio resource control suspend message.

According to some embodiments, the apparatus comprises a base station last serving the communication device.

According to some embodiments, when the apparatus determines that the core network based paging scheme is to be used, the apparatus is configured to inform the core network to page the communication device when paging is required.

According to some embodiments, the apparatus comprises means for paging the communication device in accordance with the determined paging scheme.

According to some embodiments the apparatus comprises a 5G base station.

According to some embodiments, the apparatus comprises a 4G base station. According to some embodiments the low activity state comprises a RRC CONNECTED

INACTIVE state.

According to some embodiments, when in the low activity state an SI interface between the core network and the radio access network is maintained.

In an eighth aspect there is provided an apparatus comprising means for receiving, at the apparatus, a message instructing the apparatus to adopt a radio access network based paging scheme or a core network based paging scheme for when the apparatus is in a low activity state; and means for adopting the particular paging scheme at the apparatus in accordance with the instruction.

According to some embodiments, the received message comprises a radio resource control suspend message.

According to some embodiments, the apparatus comprises means for adopting the particular paging scheme during a transition of the apparatus to the low activity state, or during a location update procedure, or when the apparatus is already in the low activity state.

According to some embodiments, the apparatus comprises means for receiving one or more paging messages from a last-serving base station, when the paging scheme is adopted.

According to some embodiments, the apparatus comprises means for receiving the message instructing the apparatus to adopt a radio access network based paging scheme from a last-serving base station.

Brief description of Figures

The invention will now be described in further detail, by way of example only, with reference to the following examples and accompanying drawings, in which:

Figure 1 shows a schematic example of a wireless communication system where the invention may be implemented;

Figure 2 shows an example of a communication device;

Figure 3 shows an example of control apparatus;

Figure 4 shows a signalling diagram for a RAN paging scheme;

Figure 5 shows a signalling diagram for a CN paging scheme; Figure 6 shows a flow chart of a method according to an embodiment;

Figure 7 shows a flow chart of a method according to an embodiment;

Figure 8 shows a flow chart of a method according to an embodiment;

Figure 9 shows a flow chart of a method according to an embodiment.

Detailed description In the following certain exemplifying embodiments are explained with reference to a wireless communication system serving communication devices adapted for wireless communication. Certain general principles of wireless systems are first briefly explained with reference to Figures 1 to 3. A communication device 20, 21 can be used for accessing various services and/or applications provided via cells 4, 5, 6 of a cellular system. In a wireless communication system the access can be provided via wireless access interfaces between wireless communication devices and one or more base stations of a radio access network 1. Each mobile device and base station may have one or more radio channels open at the same time and may receive signals from more than one source.

The mobile communication devices can move from a cell to another, as illustrated by arrows on top of the devices 20 and 21 in Figure 1. The process of handling the moving from a cell to the other is called handover. Handovers can be provided, for example, in wireless environment comprising one or more fifth generation (5G) radio access networks (RAN). A part of handover procedure is known as cell reselection.

A base station site can provide at least one cell. In the highly schematic Figure 1 example, a base station site 10 comprising a controller 13 and base station apparatus 12 and 14 is shown to provide a plurality of cells 4 and 5, respectively. In the Figure cell 4 is provided by antenna apparatus of station 12 in one location, and at least one further cell is provided by a remote radio head 14. It is noted that this exemplifying arrangement is only shown for illustrative purposes, and that e.g. antenna apparatus 12 can provide more than one cell. The relevance in view of certain examples described below is that the controller 13 of the base station site 10 can control access and devices accessing the radio access network 1 in a number of cells. In addition to the base station site 12, at least one other cell can also be provided by means of another base station or stations. This possibility is denoted by base station 11 in Figure 1. Signalling between base stations, and controllers thereof, can be provided via an appropriate interface, for example an X2 interface or an evolution of X2 interface which will be referred to as X2* interface. The X2* interface may be used in 5G, and may comprise enhancements over the X2 interface. This is denoted by the dashed line between the control entities 13 and 11.

A wireless system is typically divided between a radio access system 1, typically called radio access network (RAN) and a core network (CN) 2. The division is denoted by line 3. The core network can comprise elements such as mobile management entity (MME) 18, home subscriber server (HSS) 19 and so forth. Connection between base station sites of the radio access network (RAN) and core network (CN) element can be provided via appropriate interfaces 15, 16. The connection between the RAN and the CN can be, for example, via a SI interface or an evolution of the SI interface which will be referred to as SI* interface. The SI* interface may be used in 5G, and may comprise enhancements over the SI interface.

A communication device can access a communication system based on various access techniques, for example those based on the third Generation Partnership Project (3GPP) specifications. A non-limiting example of mobile architectures is known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). The architecture may of course alternatively comprise a future equivalent to E-UTRAN, for example the architecture of the "Next Gen" or 5G network. A non-limiting example of a base station of a cellular system is what is termed as a NodeB or E-UTRAN NodeB (eNB / ENodeB) in the vocabulary of the 3GPP specifications. The eNBs may provide E-UTRAN features such as user plane Radio Link Control/Medium Access Control/Physical Layer Protocol (RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards mobile communication devices. At least some of the stations may be arranged to operate on the unlicensed radio spectrum.

Figure 2 shows a schematic, partially sectioned view of a communication device 20 that a user can use for communications. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples include a mobile station (MS) such as a mobile phone or what is known as a 'smart phone', a portable computer provided with a wireless interface card or other wireless interface facility, personal data assistant (PDA) provided with wireless communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia, positioning data, other data, and so on. Users may thus be offered and provided numerous services via their communication devices. Non-limiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet.

A mobile device is typically provided with at least one data processing entity 23, at least one memory 24 and other possible components 29 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with base stations and/or other user terminals. The tasks can include operation related to mobility management such as handling handovers and cell reselections. Further, the tasks can also relate to security aspects of the communications. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This apparatus is denoted by reference 26.

A user may control the operation of the device 20 by means of a suitable user interface such as key pad, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 25, a speaker and a microphone are also typically provided. Furthermore, a mobile communication device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

The device 20 may receive and transmit signals 28 via appropriate apparatus for receiving and transmitting signals. In Figure 2 transceiver apparatus is designated schematically by block 27. The transceiver may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device. A wireless communication device can be provided with a Multiple Input / Multiple Output (MIMO) antenna system.

Figure 3 shows an example of a control apparatus 30 for a station, for example to be coupled to and/or for controlling one of the stations 11, 12 and 14 of Figure 1. The control apparatus 30 can be arranged to provide control on configurations used by the communications devices accessing the station, information processing and/or communication operations. A control apparatus can be configured to provide control functions in association with generation, communications, and interpretation of control information. The control apparatus 30 comprises at least one memory 31, at least one data processing unit 32, 33 and an input/output interface 34. Via the interface the control apparatus can be coupled to the relevant node. The control apparatus 30 can be configured to execute an appropriate software code to provide the control functions.

Work for LTE release 14 and 5G New Radio (NR) has already begun. One of the topics that is under discussion is UE state handling and paging with location tracking from RAN. Based on 3GPP contributions on the topic, there seems to exist a consensus that (in addition to CONNECTED and IDLE states of a UE) a third state is required, which is referred to herein as '"RRC CONNECTED INACTIVE". This may be referred to as a low activity state. It may also be referred to as a connected but inactive radio resource control condition. One of the characteristics of RRC CONNECTED INACTIVE is that the interface between RAN and CN is kept alive and active for UEs in this state. Another characteristic is that the UE context in RAN is not released. With these characteristics, the signalling transition to RRC Connected is expected to be light and not costly in terms of signalling overhead when compared to LTE's RRC IDLE to RRC CONNECTED transitions. However, it has been identified by the present inventors that there are still questions on the impact of the new state handling mechanism to existing functionalities, for example UE reachability.

In 5G, the paging area size can depend on the requested service type and the location of the UE that is tracked. The tracking area can vary from a single cell to hundreds of cells, or any number of cells in between. The tracking area is determined taking into account a trade- off between paging load and the tracking area update (TAU) load. This application deals with paging schemes for UEs in the aforementioned RRC CONNECTED INACTIVE state. In other words this application deals with paging schemes for LTE UEs in LIGHT CONNECTED (an ongoing working item in 3GPP), and new radio (NR) UEs in RRC CONNECTED INACTIVE state. Therefore, the application is related to both LTE and NR. As identified by the present inventors, in 5G one of the paging scheme questions is whether the paging should be initiated from the RAN or the CN. As the RAN-CN interface is kept alive and active for communication devices (e.g. UEs) in RRC CONNECTED INACTIVE, it may be natural to consider that the RAN also initiates the paging. The motivation behind this thinking is that RAN based paging may reduce the signalling overhead compared to the CN based paging. However, for the purposes of explanation let us first compare the performance of RAN based and CN based paging schemes in a distributed deployment scenario (herein called "scenario A"), in terms of signalling overhead. Let M denote the number of cells and N denote the number of 4G(5G)-NBs, per tracking area. Figure 4 shows the RAN initiated paging scheme for UEs in RRC CONNECTED INACTIVE. Figure 5 indicates one possible implementation for CN initiated paging.

Referring to Figure 4, this shows communication between a UE 402, a "last serving" 5G-NB 404 (i.e. the NB most recently serving the UE 402, terminating the RAN-CN interface for UE 402 and stores for example UE 402 context information, for example RAN context information and/or Access Stratum (AS) context information), another 5G-NB 406 which is also in the tracking area, and a serving gateway node (S-GW) 408.

At step SI a Mobile Terminating (MT) packet arrives at the S-GW 408. This is a packet destined for UE 402. At step S2 this packet is forwarded from the S-GW 408 to the last serving 5G-NB 404. Then, at step S3 a paging message is sent from 5G-NB 404 to all 5G-NBs in the TRA. At step S4 the last serving 5G-NB 404 pages the UE 402.

There are then disclosed two different cases, case 1 designated in general by arrow A, and case 2 designated in general by arrow B.

In case 1 the UE is camping in the last serving 5G-NB. In this case, at step S5 a random access preamble is sent from the UE 402 to the last serving 5G-NB 404. At step S6 a random access response is sent from the 5G-NB 404 to the UE 402. Then, at step S7 an RRC resume request is sent from the UE 402 to the last serving 5G-NB 404. At step S8 an RRC resume complete message is sent from last serving 5G-NB 404 to the UE 402. Then, at step S9 the last serving 5G-NB sends the UDP - IP packet to the UE 402.

Case 2 is where the UE is camping in a 5G-NB other than the last serving 5G-NB. In this case, at step S5' the 5G-NB 406 pages UE 402. Then, at step S6' the UE 402 sends a random access preamble to 5G-NB 406, and at step S7' a random access response is sent from the 5G-NB 406 to the UE 402. At step S8' an RRC resume request is sent from UE 402 to 5G-NB 406. There is then communication between the 5G-NB 406 and the 5G-NB 404. More particularly, at step S9' an RRC resume request is sent from 5G-NB 406 to 5G-NB 404. Then, at step S10' an RRC resume complete message is sent from 5G-NB 404 to 5G-NB 406. At step Sll', an RRC resume complete message is sent from 5G-NB 406 to UE 402. At step S12' the 5G-NB 406 sends the UDP/IP packet to UE 402.

Figure 5 shows the case for CN initiated paging. This Figure shows communication between UE 502, last serving 5G-NB 504, any other 5G-NB in the tracking area 506, mobile management entity (MME) 507, and S-GW 508. At step SI inactivity is detected, and at step S2 the MT packet arrives. At step S3 the S-GW 508 sends the UDP - IP packet to the last serving 5G-NB 504. Also, at step S4 an MT packet arrival notification is sent from S-GW 508 and MME 507. At step S5 an SI paging message is sent to all 5G -NBs in the tracking area, including last serving 5G-NB 504. There are then two different use cases, case 1 designated by arrow A, and case 2 designated by arrow B.

Case 1 is where the UE is camping in the last serving 5G-NB. In case 1, at step S6 a random access preamble message is sent from UE 502 to last serving 5G-NB 504, and at step S7 a random access response is sent from last serving 5G-NB 504 to UE 502. At step S8 an RRC resume request message is sent from UE 502 to last serving 5G-NB 504, and an RRC resume complete message is sent at step S9 from 5G-NB 504 to UE 502. At step S10 an SI paging acknowledgement is sent from last serving 5G-NB 504 to MME 507. Then, at step Sll the UDP/IP packet is sent from last serving 5G-NB 504 to UE 502.

Case 2 is where the UE is camping in a 5G-NB other than the last serving 5G-NB. At step S6' a paging message is sent from 5G-NB 506 to UE 502. Then, at step S7' a random access preamble message is sent from UE 502 to 5G-NB 506, and at step S8' a random access response message is sent from 5G-NB 506 to UE 502. At step S9' an RRC resume request message is sent from UE 502 to 5G-NB 506. At step S10' an RRC resume request message is sent from 5G-NB 506 to 5G-NB 504, and at step Sll' an RRC resume complete message is sent back from 5G-NB 504 to 5G-NB 506. As shown at step S12' an RRC resume complete message is then sent from 5G-NB 506 to UE 502, and at step S13' an SI paging acknowledgement message is sent from 5G-NB 506 to MME 507. At step S14' the UDP/IP packet is sent from 5G-NB 506 to UE 502.

It is worthwhile to note that the RRC Resume procedure may be implemented as a three-message procedure. In this case, the UE sends RRC Resume Request, the network replies with RRC Resume Response and the UE replies with RRC Resume Complete.

Similarly, in the CN initiated paging scheme, the UE, upon receiving the paging message, may initiate Service Request procedure in the same way as how an RRC Idle mode UE replies in LTE to paging messages.

In the CN implementation of Figure 5 the S-GW 508 informs the MME 507 about the arrival of packets, for example when an inactivity timer has expired or the MME 507 becomes aware that a CN paging is required using other means. In Figure 5 the CN is assumed to know about the state of the UE either through an implicit approach such as detecting inactivity or explicit signalling from 5G-NB to CN, for example the last 5G-NB has already informed the MME that the UE needs to be paged from the CN when paging is required. The paging load in the RAN initiated paging scheme (i.e. the scheme illustrated in

Figure 4) is M*messages (over radio) + (N-l)*messages (over X2*).

The paging load in the CN initiated paging scheme (i.e. the scheme illustrated in Figure 5) is M*messages (over radio) + (N+l)*messages (over SI*) + l*messages (over Sll*).

Thus, if M = 1 and N = 1 (i.e. the paging area is restricted to a single cell), using the above formula the paging load in the RAN initiated paging is 1 message, whereas the paging load in CN initiated paging is 4 messages. That is the paging load for RAN initiated paging is 75% less than the paging load in the MME (CN) initiated paging.

However, if let's say M = 3*19 (i.e. 57) and N = 19, using the above formula the paging load in the RAN initiated paging is 75 messages, whereas the paging load in CN initiated paging is 78 messages. That is in this case the paging load for RAN initiated paging is only 4% less than the paging load in the MME (CN) initiated paging.

Therefore based on the signalling analysis, RAN based paging carried out on a single cell or a small number of cells may indeed lead to a significant paging signalling reduction when compared to the CN initiated paging. However, as the number of paged cells increases (e.g. if a larger tracking area is defined for UEs with high mobility) the gains (in terms of number of signalling messages) becomes smaller and smaller, even insignificant for a large number of cells. This result holds if we consider a distributed deployment scenario where there is X2*-connection between 5G-NBs. Although the paging load is similar in RAN and CN based paging when the paging area is large, the paging load affects different interfaces. For example for RAN paging, the X2* interface may be affected, whereas for CN paging the SI* and Sll* interfaces may be affected. The radio interface is affected equally in both cases.

Therefore, choosing either a RAN or CN initiated paging approach might not be optimal based just on the UE RRC state. Instead, and as has been appreciated by the present inventors, it may be beneficial to be able to selectively use the RAN or CN approach depending on factors including, for example, the mobility state of the UE, the signalling congestion in the network interfaces, etc. Therefore, an approach is discussed herein enabling adaptive use of the RAN and CN initiated paging approaches using the knowledge of UE mobility state for reaching UEs in RRC CONNECTED INACTIVE state.

In known paging schemes the paging functionality is highly coupled with the RRC state of a UE. More specifically, the paging for UEs that have the same RRC state is initiated from the same entity that is located either in the RAN or in the CN. For example, the paging to UEs in LTE RRC IDLE is always initiated from MME, a CN entity. In UMTS, the paging to UEs in URA_PCH or other connected mode RRC states is initiated from the RAN. Similarly, it has been proposed to use RAN initiated paging for UEs in LTE LIGHT CONNECTED state to reduce the paging signalling overhead compared to the CN based paging.

However, coupling the RRC state and the entity that the paging is initiated from may not necessarily be the optimal solution. For example, from the performance analysis of the CN and RAN based paging approaches discussed above with respect to Figure 4 and Figure 5, it is indicated that the paging signalling reduction when RAN based paging is used instead of CN based paging for UEs in RRC CONNECTED INACTIVE is only significant when the number of paged cells is small. When the number of paged cells is large, the paging load is almost the same in both cases. The RAN based paging specifically affects the X2* interface while the CN-based paging affect the SI* and Sll* interfaces. However, the trend in new radio (NR) is that most if not all UEs will always be kept connected (either in RRC CONNECTED or RRC CONNECTED INACTIVE). The "*" suffix is used to indicate that the interface (e.g. X2*, SI* etc.) might be different in 5G to that used in LTE. However, generally speaking, it will be understood that X2 and/or X2* and SI and/or SI* (or other interfaces) etc could imply those interfaces in LTE or its potential evolution in 5G (i.e. NR).

This implies that in NR the SI* interface will be much less congested than in LTE, while the X2* interface will be much more congested than in LTE (due to RAN controlled mobility, multi-connectivity, etc). Therefore, balancing the signalling load on the X2* and SI* interfaces may be required. However, balancing the paging load between these interfaces may be difficult if either RAN initiated or CN initiated paging approach is used to reach UEs in RRC CONNECTED INACTIVE, depending on the situation.

Accordingly there is proposed herein a hybrid paging scheme when a UE is in the RRC CONNECTED INACTIVE state. According to this scheme the UE in the RRC CONNECTED INACTIVE state is paged using either a RAN or CN initiated paging approach. The decision or determination as to which paging approach to use can depend upon one or more factors. For example, the determination as to which paging approach to use can be based upon one or more of:

• How the location of the UE is tracked e.g. single cell (or small number of cells), or large number of cells

• Mobility state estimation of the UE, a measure that indicates how fast the UE is reselecting cells. This may include the priori knowledge of stationary UEs in mMTC (massive machine type communications) scenario, for example.

• Deployment scenario. For example whether it is a distributed or centralised architecture

• Load on different interfaces, and/or if load balancing is required

• Requirement of services provided to the UE. For example battery life, latency etc. Based on one or more, and indeed any combination, of these factors, the UE is configured for RAN based paging or CN based paging. According to some embodiments it is the last-serving 5G-NB or other RAN entity that is serving as a mobility anchor for the UE, terminating the UE's RAN-CN interface and has UE context information, which makes this decision of whether to configure the UE for RAN or CN based paging.

Based upon the decision, the UE is instructed whether to operate according to a RAN based or a CN based paging scheme. UE operation may vary for the different schemes. In RAN based paging, for example, a RAN identifier (e.g. Resume ID) may be included in the paging message to identify the UE. In CN based paging, a NAS identifier, S-TMSI or IMSI, may be included in the paging message to identify the UE. The discontinuous transmission (DRX) configuration for RAN may be different from the DRX configuration for CN, for example from an implementation perspective. Updating the DRX configuration by the RAN may be less costly than updating the DRX configuration by the CN. The possible values that can be used for the DRX configuration (i.e. DRX length cycle, etc) might be different in both cases. How the paging occasion (PO) is calculated may also differ between the paging schemes used. At least in CN based paging, the PO may be calculated based on the IMSI of the UE. In RAN based paging, this is yet to be standardized, although it may be based on the Resume ID (or other RAN identifier) or still based on the IMSI.

Accordingly, in at least some embodiments the UE is made aware of which paging scheme is used.

In one embodiment, the UE is configured for the determined paging scheme during the transition to RRC CONNNECTED INACTIVE state. For example this may be transition of the UE from RRC CONNECTED to RRC CONNECTED INACTIVE. Additionally or alternatively the UE is configured for the determined paging scheme during a location update procedure. For example the location update procedure may be when the UE is moving out of a predefined area for RAN based paging that potentially consists of a single cell or few cells.

To some extent it may therefore be considered that the reachability functionality of a UE is decoupled from its RRC state. In other words the paging scheme used for a UE is not based just on its RRC state. This may enhance the flexibility of the RRC CONNECTED INACTIVE STATE. For example, consider a plurality of UEs operating in the RRC CONNECTED INACTIVE state. At least some of the plurality of UEs may have different characteristics from each other. If a UE is static or has low mobility, then it might be beneficial to use a small tracking area and to serve the UE using RAN based paging, for example in terms of signalling reduction. On the other hand, if the UE has high mobility and a large tracking area for example, then it might be beneficial to serve the UE using CN based paging, for example to balance the signalling load between the CN and the RAN. Therefore, as discussed above, it is proposed to decouple the paging procedure from the UE state. The proposal may also consider factors impacting the signalling load when determining which paging scheme to use.

Figure 6 is a flow chart showing a method according to an embodiment for determining a paging scheme for paging a communication device, for use when the communication device is in a low activity state. The flow chart of Figure 6 is viewed from a perspective of a network entity. For example, the network entity may be a base station. More particularly the network entity may be a last-serving base station of the communication device.

At step SI the determining a paging scheme comprises determining at least one condition. The at least one condition may be, for example, one or more of: a number of cells in which the communication device is being tracked; a mobility state of the communication device; a deployment scenario; interface loading; whether load balancing is required; service requirements of the communication device.

At step S2, a determination is made as to whether to use a radio access network based paging scheme or a core network based paging scheme, based on the determined at least one condition. Figure 7 is a flow chart showing a method according to an embodiment for adopting a paging scheme at a communication device, for use when the communication device is in a low activity state. The flow chart of Figure 7 is viewed from a perspective of a communication device. For example, the communication device may be a user equipment. At step SI, a message or configuration is received instructing the communication device to adopt a radio access network based paging scheme or a core network based paging scheme for use when the communication device is in a low activity state.

At step S2 the particular paging scheme is adopted at the communication device in accordance with the instruction.

The methods are described in more detail in the following.

In more detail, in one example implementation, a UE entering the RRC CONNECTED INACTIVE state is configured either for RAN based paging or for CN based paging as follows, and described with respect to Figure 8. At step SI, the UE enters the RRC CONNECTED INACTIVE state.

Before the RRC Suspend [for example as per. R2-163441] procedure is initiated, the last serving 5G-NB decides whether the UE should be configured for RAN based or CN based paging. This is shown at step S2. As discussed above, the decision may depend on how the location of the UE is tracked (e.g. single cell versus large number of cells), the deployment scenario, how the network prefers to balance the load on different interfaces, the service requirement of the UE, the mobility state of the UE, etc.

If the last serving 5G-NB decides at step S2 that the UE should be configured for RAN based paging, it sends the required configuration to the UE. This is shown at step S3. In some embodiments this instruction is sent in an RRC Suspend message. The configuration may include one or more of:

• The RAN-level UE identity that will be used during paging. For example the Resume ID might be used for this purpose

• The DRX configuration for monitoring the RAN based paging

• The paging area may include a single cell, the cells of the last serving 5G-NB, or potentially include also the cells of the neighbouring 5G-NBs to avoid "ping-pong". In the paging area, the UE can freely move without the need to update its location to the network.

If, on the other hand, the last serving 5G-NB decides at step S2 that the UE should be configured for CN based paging, it informs the MME that the UE is entering the RRC CONNECTED INACTIVE state and needs CN initiated paging. This is shown at step S4. The UE then receives a paging configuration message from the MME, as shown at step S5. The paging configuration may include:

• a Non-Access Stratum (NAS) level UE identity, e.g. S-TMSI (SAE-Temporary Mobile Subscriber Identity), that will be used during the paging

• the tracking area where the UE can freely move without updating its location to the network, and which defines the paging area for the UE. The last 5G-NB may also send a recommended tracking area list

• configuration for discontinuous monitoring of the paging channel (DRX period, etc). The paging configuration may then be sent to the UE by including it in the RRC Suspend message.

In some embodiments, the MME then informs the S-GW about the UE state and the need for CN initiated paging. Then, when a downlink packet arrives, the S-GW can inform the MME about the packet arrival.

In some embodiments where it is determined that a CN paging scheme is to be used, the last serving cell, or other RAN entity, that determines which paging scheme (RAN or CN based paging schemes) to use, informs the CN that the UE is to be paged using CN based paging. This is because although a UE enters the RRC_lnactive_Connected state, the UE looks to the CN like it is in RRC_Connected state from the CN perspective, and thus the CN network may not be aware that CN based paging is required unless the RAN explicitly informs this to the CN.

In another example implementation, the paging configuration of a UE in RRC CONNECTED INACTIVE is changed while the UE is in RRC CONNECTED INACTIVE - whether from RAN based to CN based or vice-versa. For example, if the mobility state estimation (MSE) of the UE is changed from low mobility to medium or high mobility, the last 5G-NB may decide to change the RAN based paging configuration to CN based paging configuration. MSE roughly measures how fast the UE is reselecting cells. It may have, for example, three values: high, medium or low. There may also be a case where the UE is stationary or substantially stationary based on MSE. The UE may stay in the same cell or reselect to a neighbouring cell while still stationary from MSE perspective. An embodiment is shown for example in the flow chart of Figure 9. At step SI, the MSE of a UE is changed. For example this may be a change from low mobility to medium or high mobility.

At step S2, the UE reselects a cell. For example it may reselect to a cell outside its defined RAN paging area.

At step S3, the UE sends a location update request to the last serving 5G-NB.

In this example, at step S4 the last serving 5G-NB decides to retain the SI* connection of the UE, but change the RAN based paging to CN based paging. If the last serving 5G-NB decides to relocate the SI connection, the new 5G-NB that terminates the SI connection may decide on whether to use RAN or CN based paging.

At step S5, the last serving 5G-NB informs the MME that the UE is in RRC CONNECTED

INACTIVE state. The last serving 5G-NB may also request CN based paging and tracking area configuration. The last 5G-NB may also send a recommendation for the tracking area list.

As shown at step S6 the MME then sends a CN based paging configuration to the last 5G-NB. This may include configurations listed above. The MME may also inform the S-GW about the state of the UE. This enables the S-GW to send the MME a packet arrival notification when a packet arrives.

Then, as shown at step S7 the last serving 5G-NB sends the CN based paging configuration to the UE. In some embodiments the CN based paging configuration is sent in the location update response message.

When downlink data arrives at the S-GW which is targeted to a UE that is configured for

RAN based paging, the S-GW may simply forward the data to the last serving 5G-NB. The last serving 5G-NB may then page the UE in the cells that the UE is known to be camping. The paging area could be a single cell, all the cells in the last serving 5G-NB or all the cells in the last serving 5G-NB and other 5G-NBs such as neighbouring 5G-NBs. When the UE receives the paging message, it follows the standard way of resuming the RRC connection as summarized in Figure 4, for example.

When downlink data arrives at the S-GW for a UE that is configured for CN based paging, the S-GW may send a data arrival notification message to the MME. After receiving the data arrival notification, the MME may then initiate the paging procedure by paging the UE through the cells in the tracking area of the UE. The S-GW may additionally forward the received data to the last serving 5G-NB in order to minimize the user plane latency. The UE may then proceed with the standard approach of resuming the radio bearer as shown for example in Figure 5. Alternatively, the S-GW may keep the data until the UE location is known. If the UE is not located in the last serving 5G-NB, the SI* relocation may be first initiated and the data may be forwarded to the current serving 5G-NB using the new SI* connection. Or the data may simply be forwarded to the last serving 5G-NB.

Therefore according to embodiments the invention enables decoupling of the reachability functionality and the RRC state of a UE. This enhances the flexibility of the RRC CONNECTED INACTIVE state, since paging may be optimised for the RRC CONNECTED INACTIVE state based on factors as described above. Flexible paging configuration may enable the network to balance the load on different interfaces e.g. X2*, SI*, Sll*, and entities, for example 5G-NBs, MME and S-GW. Therefore embodiments have the flexibility to adopt a paging scheme to suit local varying conditions. For example, in a situation where the RAN based paging is inefficient, for example because the tracking is large enough that the last serving 5G-NB cannot reach all the 5G-NBs in the tracking area through the X2* interface, the network will have the opportunity to configure the UE to be reached using CN based paging.

Appropriate means can be provided for implementing the above described functions. The means can be software and/or hardware based, and are not limited by the examples given in this specification.

The required data processing apparatus and functions may be provided by means of one or more data processors. The described functions may be provided by separate processors or by an integrated processor. 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 examples. The data processing may be distributed across several data processing modules. A data processor may be provided by means of, for example, at least one chip. Appropriate memory capacity can be provided in the relevant devices. The memory or memories 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.

An appropriately adapted computer program code product or products may be used for implementing the embodiments, when loaded or otherwise provided on an appropriate data processing apparatus, for example for causing use of specific security credential and/or updates thereof, control operation of the devices and communication of information between the various devices. The program code product for providing the operation may be stored on, provided and embodied by means of an appropriate carrier medium. An appropriate computer program can be embodied on a computer readable record medium. A possibility is to download the program code product via a data network. In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Embodiments of the inventions may thus be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

It is noted that whilst certain embodiments have been described above in relation to certain architectures, this disclosure is not limited by this. Therefore, although certain embodiments were described above by way of example with reference to certain exemplifying architectures for wireless networks, technologies and standards, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein. It is also noted that different combinations of different embodiments are possible. It is also noted herein that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the spirit and scope of the present invention.