GB2560756A

GB2560756A - Data transmission in RRC inactive state

Info

Publication number: GB2560756A
Application number: GB1704705.1A
Authority: GB
Inventors: Jactat Caroline
Original assignee: TCL Communication Ltd
Current assignee: TCL Communication Ltd
Priority date: 2017-03-24
Filing date: 2017-03-24
Publication date: 2018-09-26
Anticipated expiration: 2037-03-24
Also published as: CN110583043A; CN110583043B; GB201704705D0; GB2560756B; WO2018171527A1

Abstract

A cellular communication system is provided with at least one UE in wireless communication with first and second base stations. The UE has a current security context with the first base station. The UE transmits a user plane message to the second base station. The message comprises data and a UE ID. Using the information in this message the second base station can retrieve the current security context from the first base station. The UE may receive a new security context from the second base station, the UE then generating new security context parameters based on the information received in the second user plane message sent from the second base station. The user plane protocol may be MAC or PDCP. The UE ID may be the UE AS Context ID. The first and second base stations may be gNBs. Further aspects of the invention provide the second base station transmitting a message to the first base station requesting the current security context for the received UE ID. The second user plane message may comprise a starting PDCP SN for use with the new security context. The UE may be in an RRC Inactive state.

Description

(71) Applicant(s):

TCL Communication Limited

1910-12A, Tower 3, 33 Canton Road, Tsim Sha Tsui,

Kowloon, Hong Kong, China (72) Inventor(s):

Caroline Jactat (74) Agent and/or Address for Service:

Simmons & Simmons LLP

CityPoint, One Ropemaker Street, London, EC2Y 9SS, United Kingdom (56) Documents Cited:

EP 2637444 A2 EP 2309698 A1

WO 2015/018074 A1 (58) Field of Search:

INT CL H04W

Other: EPODOC, WPI, XP3GPP (54) Title of the Invention: Data transmission in RRC inactive state

Abstract Title: Fetching security context of UE between base stations (57) Acellular communication system is provided with at least one UE in wireless communication with first and second base stations. The UE has a current security context with the first base station. The UE transmits a user plane message to the second base station. The message comprises data and a UE ID. Using the information in this message the second base station can retrieve the current security context from the first base station.

The UE may receive a new security context from the second base station, the UE then generating new security context parameters based on the information received in the second user plane message sent from the second base station.

The user plane protocol may be MAC or PDCP.

The UE ID may be the UE AS Context ID.

The first and second base stations may be gNBs.

Further aspects of the invention provide the second base station transmitting a message to the first base station requesting the current security context for the received UE ID.

The second user plane message may comprise a starting PDCP SN for use with the new security context.

The UE may be in an RRC Inactive state.

Source gNB | Target gNB

1. UE capability_

I (Support of Data transmission in RRC INACTIVE)

1a. Upon data transmission in RRC INACTIVE, the UE includes the UE AS context identifier and the data, integrity protected with the current (PDCP) security context.

1. Uplink data tran: (UE AS context ID, ifoer

Data)

2a. Target eNB does not have the context of UE. It can request the UE context from the source gNB from some information included in the UE context ID.

--. [3. UE Context Request \

3a. In case Target eNB would like to relocate the UE context, target eNB can change for example the UE security context.

3b. Target eNB embeds in a specific MAC control element, the new security configuration (RRC parameters so as the UE can generate new AS keys) and PDCP SN onwards of the data using the new security context. Target eNB includes UE AS Context ID, i integrity protected with the current or new security context.

4.Specific MAC Control Element (RRC security parameters, PDCP SN lusing the new security context, UE AS Context ID)

4a. Upon receipt of the information, the UE generates the new security context.)

UE would consider that data including a PDCP SN older than the received ι

PDCP SN, would be using the previous security context. In case a new security context, the UE includes UE AS Context ID integrity protected with the i new security context.

5. Specific MAC Control Element

306 (UE AS Context ID) | | )

5a. In case of new security context. Target eNB performs UE verification based on received UE AS Context ID. Data transmission can continue based on the | new security context.

6. Downlink data transfer I (Data inclu|de PDCP S|sl onwards) [

6a. Upon receipt of the data including a PDCP SN more recent than that received on Step 4a, the UE would use the new security context generated at Step 4a to

J_decipher, perform integrity verification based upon.

Figure 3

1/3

gNB “Λ > NGC > NG-RAN

Figure 1

2/3

Figure 2

3/3

03 18

Figure 3

Data Transmission In RRC Inactive State

Technical Field [0001] The present disclosure relates to the update of context information in a cellular communication network, and in particular to the update of such context information using user plane protocols. The present disclosure also relates to data transmission in the context of update of such context information.

Background [0002] Wireless communication systems, such as the third-generation (3G) of mobile telephone standards and technology are well known. Such 3G standards and technology have been developed by the Third Generation Partnership Project (3GPP). The 3^rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Communication systems and networks have developed towards a broadband and mobile system. The 3rd Generation Partnership Project has developed the so-called Long Term Evolution (LTE) system, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network where one or more macrocells are supported by a base station known as an eNodeB or eNB (evolved NodeB). More recently, LTE is evolving further towards the so-called 5G or NR (New Radio).

[0003] 5G or NR proposes a new radio link standard for the UE to base station link. The NR configuration and protocols utilise many LTE features as a starting point, but add a wide range of additional features and operation modes very different to LTE.

[0004] As shown in Figure 1 the NR RAN consists of gNBs, providing the NG-RA user plane (new AS sublayer/PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The gNBs are interconnected with each other by means of the Xn interface. The gNBs are also connected by means of the NG interface to the Next Generation Core (NGC), more specifically to the AMF (Access and Mobility Management Function) by means of the N2 interface and to the UPF (User Plane Function) by means of the N3 interface (see 3GPP TS 23.501).

[0005] The User plane protocol stack for NR (as defined in 3GPP TR 38.804) is shown in Figure 2. PDCP, RLC and MAC sublayers (terminated in gNB on the network side) perform similar functions as LTE. Additional functions exist in NR to fulfil those specific requirements not existing in LTE.

[0006] The RRC layer utilises a set of states to define the status of an RRC link between a gNB and a UE. A set of issues exist relating to data transmission in the RRC INACTIVE state, and in particular regarding data transfer during handover or relocation processes which should allow re-use of security context data or use of new security context data.

[0007] For New Radio systems, two methods have been discussed for data transfer in the RRC INACTIVE state, but both have significant disadvantages:a. An RRC signalling based procedure ahead of data transmission to allow for contention resolution between multiple UEs in case the network uses a grant free data access procedure for multiple UEs. The drawback of this method is, in addition to RRC signalling overhead, the data reset (by RLC protocol layer suspension below PDCP) and data interruption until the completion of the RRC reconfiguration procedure.

b. An RRC signalling free based procedure where data transmission is performed. The drawback of this method is that in case the security context needs updating, then an incoming RRC message is necessary, thereby implying the same drawback as the previous method like the data reset and data interruption.

[0008] There is therefore a requirement for a system to allow data transmission in the RRC INACTIVE state which addresses drawbacks with previous systems by avoiding RRC connection set-up, RRC signalling messages exchange thereof and connected mode mobility handling by the network.

Summary [0009] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0010] There is provided a method of transmitting and updating UE Context Information in a cellular communication system, wherein the cellular communication system comprises at least one in wireless communication with a first base station and a second base station, wherein the UE has a current security context with the first base station, the method comprising the steps of transmitting a user plane message from the UE to the second base station, wherein the message comprises data and a UE ID with which the current security context can be retrieved from the first base station; transmitting a message from the second base station to the first base station requesting the current security context for the received UE ID; and receiving at the second base station the current security context from the first base station.

[0011] The method may further comprise the step of subsequently generating a new security context and transmitting that security context in a second user plane message to the UE.

[0012] The second user plane message comprising the security context may also comprise a starting PDCP SN for use with the new security context.

[0013] The UE may generate required parameters for the new security context based on the information received in the second user plane message.

[0014] The UE may subsequently transmit an ID to the network to allow the network to perform UE verification.

[0015] The network may perform UE verification after receiving the current security context at the second base station.

[0016] The user plane protocol may be the MAC or PDCP protocol.

[0017] The UE ID may be the UE AS Context ID.

[0018] There is also provided a method of transmitting and updating UE Context Information in a cellular communication system, wherein the cellular communication system comprises at least one in wireless communication with a first base station and a second base station, wherein the UE has a current security context with the first base station, the method being performed at a UE and comprising the steps of transmitting a user plane message from the UE to the second base station, wherein the message comprises data and a UE ID with which the current security context can be retrieved from the first base station.

[0019] The UE may receive a new security context from the second base station and generates required parameters for the new security context based on the information received in the second user plane message.

[0020] The UE may subsequently transmit an ID to the network to allow the network to perform UE verification.

[0021] There is also provided a UE configured to perform the methods described herein.

[0022] There is also provided a method of receiving and updating UE Context Information in a cellular communication system, wherein the cellular communication system comprises at least one in wireless communication with a first base station and a second base station, wherein the UE has a current security context with the first base station, the method being performed at the second base station and comprising the steps of receiving a user plane message at the second base station from the UE, wherein the message comprises data and a UE ID with which the current security context can be retrieved from the first base station; transmitting a message from the second base station to the first base station requesting the current security context for the received UE ID; and receiving at the second base station the current security context from the first base station.

[0023] The method may further comprise the step of subsequently generating a new security context and transmitting that security context in a second user plane message to the UE.

[0024] The second user plane message comprising the security context may also comprise a starting PDCP SN for use with the new security context.

[0025] The UE may generate required parameters for the new security context based on the information received in the second user plane message.

[0026] The user plane protocol may be the MAC or PDCP protocol.

[0027] The UE ID may be the UE AS Context ID.

[0028] The first base station may be a source base station and the second base station is a target base station.

[0029] The first and second base stations may be first and second gNBs.

[0030] In all the methods above, the first and second gNBs can be the same base station. From the UE viewpoint, the UE has no means to infer whether any information would be transmitted to a first or second gNB. There is actually no gNB identifier for the UE. The UE may behave the same way in terms of information transmitted to the gNB. If the information is received by a second gNB, the gNB may perform UE context update using the information already available from the UE. As a consequence, a significant advantage of the above methods is to allow UE context update even within the same gNB by avoiding RRC connection set-up, RRC signalling messages exchange thereof and connected mode mobility handling by the network.

[0031] According to an aspect of the invention, there is provided a non-transitory computer readable medium having computer readable instructions stored thereon for execution by a processor to perform the method according to the first aspect.

[0032] The non-transitory computer readable medium may comprise at least one from a group consisting of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, EPROM, an Electrically Erasable Programmable Read Only Memory and a Flash memory.

Brief description of the drawings [0033] Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Like reference numerals have been included in the respective drawings to ease understanding.

Figure 1 shows a schematic diagram of the principle components of a network according to the proposed NR standard;

Figure 2 shows a proposed protocol stack; and

Figure 3 shows a message flow chart between a UE and a network component.

Detailed description of the preferred embodiments [0034] Those skilled in the art will recognise and appreciate that the specifics of the examples described are merely illustrative of some embodiments and that the teachings set forth herein are applicable in a variety of alternative settings.

[0035] Figure 3 shows a message chart of a method for providing data transfer in the RRC INACTIVE state which is intended to address the drawbacks of prior systems. Figure 1 shows the flow of messages between a UE, a source gNB, and a target gNB. As will be appreciated the UE communicates with the gNBs via a radio link over which the protocols shown in Figure 2 enable communication.

[0036] At step 300, the UE notifies the network via the source eNB (with which it is currently communicating) that it is capable of receiving UE context update information upon data transfer. This indication allows the network to configure appropriate resources and processes for subsequent actions with this UE. The indication in step 300 may be performed at any appropriate time and is not necessarily performed in conjunction or at the same time as the other steps shown in Figure 1. Furthermore, other methods to indicate a UE's capabilities may be utilised. [0037] As indicated at 301 the UE's connection is in the RRC INACTIVE state.

[0038] At step 302 the UE transmits data to the target gNB (while remaining in the RRC INACTIVE state) and includes the UE AS context ID. The message is integrity protected with the current PDCP security context and is transmitted using a user plane protocol. For example, the transmission may use a PDCP or MAC message.

[0039] The target gNB, receiving the transmission from the UE, does not have the required security context (for example, because the UE has performed mobility from the source gNB) which resides with the source gNB. At step 303 the target gNB requests the UE security context from the source gNB based on the UE AS context ID transmitted from the UE, and receives the requested information in response. The network can also perform UE verification using the data which is integrity protected with the UE AS context ID of the current security context.

[0040] Integrity protection provided by PDCP can be applied to UE AS context ID. The integrity protection of the UE AS context ID is used by the gNB to verify the UE’s identity when for example integrity protection is not provided in AS by the transmitted data. In case the message size allows to transmit the data in addition to UE AS Context ID, the ID and the data can be concatenated and applied the same integrity protection scheme. Alternatively, the ID and the data can be applied the integrity protection scheme separately i.e. two Message Authentication Codes are included upon transmission. The inclusion of two integrity protection information allows the gNB to have two means to verify the UE’s identity. For example, if verification is failed based on data but passed based on UE Context ID, the gNB can consider UE verification has passed though.

[0041] If relocation of the UE to the Target gNB is desired, the target gNB can update the security context to a new one defined for the target gNB.

[0042] At step 304 the target gNB transmits the new security context to the UE using an RRC control message which is transmitted in a MAC control PDU. The message includes a PDCP SN for subsequent data transmission using the new security context and the UE AS Context ID which is integrity protected with the new security context. A different identifier, which also allows network verification, may be transmitted in place of the UE AS Context ID. The context update information may be integrity protected by PDCP or any other upper layer protocol as desired.

[0043] The SN, UE AS Context ID, context update information are integrity protected using the new security context, which security context can include updated security algorithms, next chain count to generate the new security key. In case the radio resource allocation does not allow all the information to fit into one MAC control PDU message, then a follow-on indicator can be transmitted so the UE infers it has to wait to receive all the information before proceeding with them. Such follow-on indicator can also apply for MAC data PDU where data is conveyed and all the data cannot fit into a MAC PDU message. The advantage of the follow-on indicator is to keep the UE in RRCJNACTIVE state i.e. there is no RRC connection with the gNB while the UE can receive and transmit limited size of data in multiple goes.

[0044] At step 305 the UE receives the MAC control PDU with the RRC control message and extracts the information. If the message includes a new security context the UE generates the required parameters, for example new security keys to be used for subsequent transmission to the target gNB. The UE receives the starting PDCP SN for use with the new security context and hence can identify which PDCP PDUs will utilise the old security context, and which will use the new security context.

[0045] The identifier received by the UE, for example the UE AS Context ID, allows the UE to perform network verification. This is achieved by transmitting, at step 306, a specific MAC control element which is integrity protected with the new security context (using an incremented PDCP SN). The UE AS Context ID can be integrity protected by PDCP or any other upper layers protocol. Similarly to the PDCP SN indication from the gNB related to the subsequent gNB transmissions in step 304, the UE can transmit a PDCP SN for the subsequent UE transmissions to identify which PDCP PDUs will utilise the old security context, and which will use the new security context.

[0046] At step 307 the network performs UE verification and at step 308 data transmission can continue using the new security context.

[0047] At step 308, the UE begins to receive data. For PDCP PDUs with an SN greater than that received in step 304 the UE will utilise the new security context. Should data be received with a PDCP SN lower than the provided value, the old security context can be utilised.

[0048] The proposed method for data transfer addresses issues identified with previous methods, and provides the following features which are expected to be advantageous:• The UE AS context identifier used for uplink data transmission in RRCJNACTIVE is the same as the one used in state transition from RRCJNACTIVE to RRC_CONNECTED.

• The UE AS context is located and identified in the network via an “AS Context ID” which is allocated by the network and stored in the UE (and the network) when the UE goes to RRCJNACTIVE and is used to locate the AS context when the UE either tries to transmit small data and/or to perform a transition to RRC_CONNECTED.

• The UE AS Context can be stored in an “anchor’Vsource gNB and may be fetched by a new serving gNB when needed upon the triggering of small data transmission with the new gNB and/or transition from RRCJNACTIVE to RRC_CONNECTED.

• The network has the ability to perform a context update when the UE sends small data in RRCJNACTIVE. That update should uses RRC signalling and is done in a control response message triggered by small data transmission.

• Small data transmission can both operate with 2-step or 4-step RACH procedure. The 2-step RACH procedure consists in small data transmission in a pre-granted radio resource for the UE. The 4-step RACH procedure consists in additional step for the UE to be granted the radio resource for small data transmission.

• Small data transmission uses the AS Context ID transmitted in the “first” message for contention resolution (at least when RACH is used).

• It is beneficial to send small downlink data to the UE with the network response message (e.g. Msg4) if user plane data are available, provided that the user plane design supports it.

[0049] As will be appreciated by the Skilled Person, the steps of the methods described herein are performed by processing entities at the UE and network sides of a cellular wireless connection. The entities may be software running on a computer system, or any other appropriate implementation to provide the functionality required.

[0050] The term base station is used in this document to describe a component that provides the function of terminating a wireless link to a UE and provides a connection to the network. For example an eNB may be considered a base station in the LTE system, and a gNB may be considered a base station in the proposed New Radio system.

[0051] Aspects of the disclosure may be performed by a computing system, for example forming part of the UE or gNB. The computing system can include a main memory, such as random access memory (RAM) or other dynamic memory, for storing information and instructions to be executed by a processor. Such a main memory also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor. The computing system may likewise include a read only memory (ROM) or other static storage device for storing static information and instructions for a processor.

[0052] The computing system may also include an information storage system which may include, for example, a media drive and a removable storage interface. The media drive may include a drive or other mechanism to support fixed or removable storage media, such as a hard disk drive, a floppy disk drive, a magnetic tape drive, an optical disk drive, a compact disc (CD) or digital video drive (DVD) read or write drive (R or RW), or other removable or fixed media drive. Storage media may include, for example, a hard disk, floppy disk, magnetic tape, optical disk, CD or

DVD, or other fixed or removable medium that is read by and written to by media drive. The storage media may include a computer-readable storage medium having particular computer software or data stored therein.

[0053] In alternative embodiments, an information storage system may include other similar components for allowing computer programs or other instructions or data to be loaded into the computing system. Such components may include, for example, a removable storage unit and an interface , such as a program cartridge and cartridge interface, a removable memory (for example, a flash memory or other removable memory module) and memory slot, and other removable storage units and interfaces that allow software and data to be transferred from the removable storage unit to computing system.

[0054] The computing system can also include a communications interface. Such a communications interface can be used to allow software and data to be transferred between a computing system and external devices. Examples of communications interfaces can include a modem, a network interface (such as an Ethernet or other NIC card), a communications port (such as for example, a universal serial bus (USB) port), a PCMCIA slot and card, etc. Software and data transferred via a communications interface are in the form of signals which can be electronic, electromagnetic, and optical or other signals capable of being received by a communications interface medium.

[0055] In this document, the terms ‘computer program product’, ‘computer-readable medium’ and the like may be used generally to refer to tangible media such as, for example, a memory, storage device, or storage unit. These and other forms of computer-readable media may store one or more instructions for use by the processor comprising the computer system to cause the processor to perform specified operations. Such instructions, generally referred to as ‘computer program code’ (which may be grouped in the form of computer programs or other groupings), when executed, enable the computing system to perform functions of embodiments of the present invention. Note that the code may directly cause a processor to perform specified operations, be compiled to do so, and/or be combined with other software, hardware, and/or firmware elements (e.g., libraries for performing standard functions) to do so.

[0056] In an embodiment where the elements are implemented using software, the software may be stored in a computer-readable medium and loaded into computing system using, for example, removable storage drive. A control module (in this example, software instructions or executable computer program code), when executed by the processor in the computer system, causes a processor to perform the functions of the invention as described herein.

[0057] Furthermore, the inventive concept can be applied to any circuit for performing signal processing functionality within a network element. It is further envisaged that, for example, a semiconductor manufacturer may employ the inventive concept in a design of a stand-alone device, such as a microcontroller of a digital signal processor (DSP), or application-specific integrated circuit (ASIC) and/or any other sub-system element.

[0058] It will be appreciated that, for clarity purposes, the above description has described embodiments of the invention with reference to a single processing logic. However, the inventive concept may equally be implemented by way of a plurality of different functional units and processors to provide the signal processing functionality. Thus, references to specific functional units are only to be seen as references to suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure or organisation.

[0059] Aspects of the invention may be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented, at least partly, as computer software running on one or more data processors and/or digital signal processors or configurable module components such as FPGA devices. Thus, the elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units.

[0060] Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term ‘comprising’ does not exclude the presence of other elements or steps.

[0061] Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by, for example, a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate.

[0062] Furthermore, the order of features in the claims does not imply any specific order in which the features must be performed and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus, references to ‘a’, ‘an’, ‘first’, ‘second’, etc. do not preclude a plurality.

[0063] Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term ‘comprising’ or “including” does not exclude the presence of other elements.

Claims

1. A method of transmitting and updating UE Context Information in a cellular communication system, wherein the cellular communication system comprises at least one in wireless communication with a first base station and a second base station, wherein the UE has a current security context with the first base station, the method comprising the steps of transmitting a user plane message from the UE to the second base station, wherein the message comprises data and a UE ID with which the current security context can be retrieved from the first base station;

transmitting a message from the second base station to the first base station requesting the current security context for the received UE ID; and receiving at the second base station the current security context from the first base station.

2. A method according to claim 1, further comprising the step of subsequently generating a new security context and transmitting that security context in a second user plane message to the UE.

3. A method according to claim 2, wherein the second user plane message comprising the security context also comprises a starting PDCP SN for use with the new security context.

4. A method according to claim 3, wherein the UE generates required parameters for the new security context based on the information received in the second user plane message.

5. A method according to claim 4, wherein the UE subsequently transmits an ID to the network to allow the network to perform UE verification.

6. A method according to any preceding claim wherein the network performs UE verification after receiving the current security context at the second base station.

7. A method according to any preceding claim, wherein the user plane protocol is the MAC or PDCP protocol.

8. A method according to any preceding claim, wherein the UE ID is the UE AS Context ID.

9. A method according to any preceding claim, wherein the first base station is a source base station and the second base station is a target base station.

10. A method according to any preceding claim wherein the first and second base stations are first and second gNBs.

11. A method of transmitting and updating UE Context Information in a cellular communication system, wherein the cellular communication system comprises at least one in wireless communication with a first base station and a second base station, wherein the UE has a current security context with the first base station, the method being performed at a UE and comprising the steps of transmitting a user plane message from the UE to the second base station, wherein the message comprises data and a UE ID with which the current security context can be retrieved from the first base station.

12. A method according to claim 1, wherein the UE receives a new security context from the second base station and generates required parameters for the new security context based on the information received in the second user plane message.

13. A method according to claim 12, wherein the UE subsequently transmits an ID to the network to allow the network to perform UE verification.

14. A method according to any of claims 11 to 13, wherein the user plane protocol is the MAC or PDCP protocol.

15. A method according to any of claims 11 to 14, wherein the UE ID is the UE AS Context ID.

16. A method according to any of claims 11 to 15, wherein the first base station is a source base station and the second base station is a target base station.

17. A method according to any of claims 11 to 16 wherein the first and second base stations are first and second gNBs.

18. A UE configured to perform the method of any of claims 11 to 17.

19. A method of receiving and updating UE Context Information in a cellular communication system, wherein the cellular communication system comprises at least one in wireless communication with a first base station and a second base station, wherein the UE has a current security context with the first base station, the method being performed at the second base station and comprising the steps of receiving a user plane message at the second base station from the UE, wherein the message comprises data and a UE ID with which the current security context can be retrieved from the first base station;

5

20. A method according to claim 19, further comprising the step of subsequently generating a new security context and transmitting that security context in a second user plane message to the UE.

21. A method according to claim 20, wherein the second user plane message

10 comprising the security context also comprises a starting PDCP SN for use with the new security context.

22. A method according to claim 21, wherein the UE generates required parameters for the new security context based on the information received in the

15 second user plane message.

23. A method according to any of claimsl 9 to 22, wherein the user plane protocol is the MAC or PDCP protocol.

20

24. A method according to any of claims 19 to 23, wherein the UE ID is the UE

AS Context ID.

25. A method according to any of claims 19 to 24, wherein the first base station is a source base station and the second base station is a target base station.

26. A method according to any of claims 19 to 25 wherein the first and second

25 base stations are first and second gNBs.

Intellectual

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Application No: GB1704705.1