Methods, communications devices, and infrastructure equipment

Abstract

A method of operating a communications device is provided. The method comprises transmitting signals to and / or receiving signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, determining that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and determining whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

Description

[0001] METHODS, COMMUNICATIONS DEVICES, AND INFRASTRUCTURE EQUIPMENT

[0002] BACKGROUND Field of Disclosure

[0003] The present disclosure relates to communications devices and source and target infrastructure equipment of wireless communications networks and methods of operating such communications devices and source and target infrastructure equipment for the more efficient handling of mobility procedures in such wireless communications networks.

[0004] The present application claims the Paris Convention priority from European patent application number EP23184429.1, filed on 10 July 2023, the contents of which are hereby incorporated by reference.

[0005] Description of Related Art

[0006] The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.

[0007] Previous generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture, are able to support a wider range of services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy such networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to continue to increase rapidly.

[0008] Current and future wireless communications networks are expected to routinely and efficiently support communications with an ever-increasing range of devices associated with a wider range of data traffic profiles and types than existing systems are optimised to support. For example, it is expected future wireless communications networks will be expected to efficiently support communications with devices including reduced complexity devices, machine type communication (MTC) devices, high resolution video displays, virtual reality headsets, extended Reality (XR) and so on. Some of these different types of devices may be deployed in very large numbers, for example low complexity devices for supporting the “The Internet of Things”, and may typically be associated with the transmissions of relatively small amounts of data with relatively high latency tolerance. Other types of device, for example supporting high-definition video streaming, may be associated with transmissions of relatively large amounts of data with relatively low latency tolerance. Other types of device, for example used for autonomous vehicle communications and for other critical applications, may be characterised by data that should be transmitted through the network with low latency and high reliability. A single device type might also be associated with different traffic profiles / characteristics depending on the application(s) it is running. For example, different consideration may apply for efficiently supporting data exchange with a smartphone when it is running a video streaming application (high downlink data) as compared to when it is running an Internet browsing application (sporadic uplink and downlink data) or being used for voice communications by an emergency responder in an emergency scenario (data subject to stringent reliability and latency requirements).

[0009] In view of this there is expected to be a desire for current wireless communications networks, for example those which may be referred to as 5G or new radio (NR) systems / new radio access technology (RAT) systems, or indeed future 6G wireless communications, as well as future iterations / releases of existing systems, to efficiently support connectivity for a wide range of devices associated with different applications and different characteristic data traffic profiles and requirements. The desire to support these new use-cases and scenarios gives rise to new challenges for efficiently handling communications in wireless communications systems that need to be addressed.

[0010] SUMMARY OF THE DISCLOSURE

[0011] The present disclosure can help address or mitigate at least some of the issues discussed above.

[0012] Some embodiments of the present technique can provide a method of operating a communications device. The method comprises transmitting signals to and / or receiving signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, determining that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and determining whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0013] Such embodiments of the present technique, which, in addition to methods of operating communications devices, relate to methods of operating source and target infrastructure equipment, to communications devices, source infrastructure equipment, and target infrastructure equipment, to circuitry for communications devices, source infrastructure equipment, and target infrastructure equipment, wireless communications systems, computer programs, and computer-readable storage mediums, can allow for the more efficient handling of mobility procedures in such wireless communications networks.

[0014] Respective aspects and features of the present disclosure are defined in the appended claims.

[0015] It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein like reference numerals designate identical or corresponding parts throughout the several views, and wherein:

[0018] Figure 1 schematically represents some aspects of an LTE-type wireless telecommunication system which may be configured to operate in accordance with certain embodiments of the present disclosure;

[0019] Figure 2 schematically represents some aspects of an NR-type wireless telecommunications system which may be configured to operate in accordance with certain embodiments of the present disclosure; Figure 3 is a schematic block diagram of an example infrastructure equipment and communications device which may be configured to operate in accordance with certain embodiments of the present disclosure;

[0020] Figure 4A is a representation of a gNB distributed unit (DU) and a gNB controlling unit (CU) communicating via an Fl interface;

[0021] Figure 4B is a schematic representation of a protocol stack operated respectively by the gNB DU and gNB CU shown in Figure 4A for communicating control plane data;

[0022] Figure 4C is a schematic representation of a protocol stack operated respectively by the gNB DU and gNB CU shown in Figure 4A for communicating user plane data;

[0023] Figure 5 schematically illustrates a split of gNB functionality between a CU-CP, CU-UP and the DU according to current architecture;

[0024] Figure 6 illustrates the scope of Uayer 1 / Uayer 2 Triggered Mobility (UTM) in current 3GPP specifications;

[0025] Figure 7 is reproduced from [9], and shows an example message sequence for UTM in accordance with current 3 GPP specifications;

[0026] Figure 8 shows a first example message sequence for inter-CU UTM, in accordance with which embodiments of the present technique may be configured to operate;

[0027] Figure 9 shows a part schematic, part message flow diagram representation of a wireless communications system comprising a communications device and an infrastructure equipment in accordance with embodiments of the present technique;

[0028] Figure 10 illustrates an example of the architecture for inter-CU UTM in accordance with embodiments of the present technique;

[0029] Figure 11 shows a second example message sequence for inter-CU UTM in accordance with embodiments of the present technique; and

[0030] Figure 12 shows a flow diagram illustrating an example process of communications in a communications system in accordance with embodiments of the present technique.

[0031] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] Long Term Evolution Advanced Radio Access Technology (4G)

[0033] Figure 1 provides a schematic diagram illustrating some basic functionality of a mobile telecommunications network / system 6 operating generally in accordance with LTE principles, but which may also support other radio access technologies, and which may be adapted to implement embodiments of the disclosure as described herein. Various elements of Figure 1 and certain aspects of their respective modes of operation are well-known and defined in the relevant standards administered by the 3GPP (RTM) body, and also described in many books on the subject, for example, Holma H. and Toskala A [1], It will be appreciated that operational aspects of the telecommunications networks discussed herein which are not specifically described (for example in relation to specific communication protocols and physical channels for communicating between different elements) may be implemented in accordance with any known techniques, for example according to the relevant standards and known proposed modifications and additions to the relevant standards.

[0034] The network 6 includes a plurality of base stations 1 connected to a core network 2, which may be for example an Evolved Packet Core (EPC). Each base station provides a coverage area 3 (i.e., a cell) within which data can be communicated to and from communications devices 4. Although each base station 1 is shown in Figure 1 as a single entity, the skilled person will appreciate that some of the functions of the base station may be carried out by disparate, inter-connected elements, such as antennas (or antennae), remote radio heads, amplifiers, etc. Collectively, one or more base stations may form a radio access network. Data is transmitted from base stations 1 to communications devices 4 within their respective coverage areas 3 via a radio downlink (DL). Data is transmitted from communications devices 4 to the base stations 1 via a radio uplink (UL). The core network 2 routes data to and from the communications devices 4 via the respective base stations 1 and provides functions such as authentication, mobility management, charging and so on. Terminal devices may also be referred to as mobile stations, user equipment (UE), user terminal, mobile radio, communications device, and so forth. Services provided by the core network 2 may include connectivity to the internet or to external telephony services. The core network 2 may further track the location of the communications devices 4 so that it can efficiently contact (i.e., page) the communications devices 4 for transmitting downlink data towards the communications devices 4.

[0035] Base stations, which are an example of network infrastructure equipment, may also be referred to as transceiver stations, nodeBs, e-nodeBs, eNB, g-nodeBs, gNB and so forth. In this regard different terminology is often associated with different generations of wireless telecommunications systems for elements providing broadly comparable functionality. However, certain embodiments of the disclosure may be equally implemented in different generations of wireless telecommunications systems, and for simplicity certain terminology may be used regardless of the underlying network architecture. That is to say, the use of a specific term in relation to certain example implementations is not intended to indicate these implementations are limited to a certain generation of network that may be most associated with that particular terminology.

[0036] New Radio Access Technology (5G)

[0037] An example configuration of a wireless communications network which uses some of the terminology proposed for and used in NR and 5G is shown in Figure 2. In Figure 2 a plurality of transmission and reception points (TRPs) 10 are connected to distributed control units (DUs) 41, 42 by a connection interface represented as a line 16. Each of the TRPs 10 is arranged to transmit and receive signals via a wireless access interface within a radio frequency bandwidth available to the wireless communications network. Thus, within a range for performing radio communications via the wireless access interface, each of the TRPs 10, forms a cell of the wireless communications network as represented by a circle 12. As such, wireless communications devices 14 which are within a radio communications range provided by the cells 12 can transmit and receive signals to and from the TRPs 10 via the wireless access interface. Each of the distributed units 41, 42 are connected to a central unit (CU) 40 (which may be referred to as a controlling node) via an interface 46. The central unit 40 is then connected to the core network 20 (which may be for example referred to as 5GC) which may contain all other functions required to transmit data for communicating to and from the wireless communications devices and the core network 20 may be connected to other networks 30.

[0038] The elements of the wireless access network shown in Figure 2 may operate in a similar way to corresponding elements of an LTE network as described with regard to the example of Figure 1. It will be appreciated that operational aspects of the telecommunications network represented in Figure 2, and of other networks discussed herein in accordance with embodiments of the disclosure, which are not specifically described (for example in relation to specific communication protocols and physical channels for communicating between different elements) may be implemented in accordance with any known techniques, for example according to currently used approaches for implementing such operational aspects of wireless telecommunications systems, e.g. in accordance with the relevant standards.

[0039] The TRPs 10 of Figure 2 may in part have a corresponding functionality to a base station or eNodeB of an LTE network. Similarly, the communications devices 14 may have a functionality corresponding to the UE devices 4 known for operation with an LTE network. It will be appreciated therefore that operational aspects of a new RAT network (for example in relation to specific communication protocols and physical channels for communicating between different elements) may be different to those known from LTE or other known mobile telecommunications standards. However, it will also be appreciated that each of the core network component, base stations and communications devices of a new RAT network will be functionally similar to, respectively, the core network component, base stations and communications devices of an LTE wireless communications network.

[0040] In terms of broad top-level functionality, the core network 20 connected to the new RAT telecommunications system represented in Figure 2 may be broadly considered to correspond with the core network 2 represented in Figure 1, and the respective central units 40 and their associated distributed units / TRPs 10 may be broadly considered to provide functionality corresponding to the base stations 1 of Figure 1. The term network infrastructure equipment / access node may be used to encompass these elements and more conventional base station type elements of wireless telecommunications systems. Depending on the application at hand the responsibility for scheduling transmissions which are scheduled on the radio interface between the respective distributed units and the communications devices may lie with the controlling node / central unit and / or the distributed units / TRPs. A communications device 14 is represented in Figure 2 within the coverage area of the first communication cell 12. This communications device 14 may thus exchange signalling with the first central unit 40 in the first communication cell 12 via one of the distributed units / TRPs 10 associated with the first communication cell 12.

[0041] It will further be appreciated that Figure 2 represents merely one example of a proposed architecture for a new RAT based telecommunications system in which approaches in accordance with the principles described herein may be adopted, and the functionality disclosed herein may also be applied in respect of wireless telecommunications systems having different architectures.

[0042] Thus, certain embodiments of the disclosure as discussed herein may be implemented in wireless telecommunication systems / networks according to various different architectures, such as the example architectures shown in Figures 1 and 2. It will thus be appreciated the specific wireless telecommunications architecture in any given implementation is not of primary significance to the principles described herein. In this regard, certain embodiments of the disclosure may be described generally in the context of communications between network infrastructure equipment / access nodes and a communications device, wherein the specific nature of the network infrastructure equipment / access node and the communications device will depend on the network infrastructure for the implementation at hand. For example, in some scenarios the network infrastructure equipment / access node may comprise a base station, such as an LTE-type base station 1 as shown in Figure 1 which is adapted to provide functionality in accordance with the principles described herein, and in other examples the network infrastructure equipment may comprise a control unit / controlling node 40 and / or a TRP 10 of the kind shown in Figure 2 which is adapted to provide functionality in accordance with the principles described herein.

[0043] A more detailed diagram of some of the components of the network shown in Figure 2 is provided by Figure 3. In Figure 3, a TRP 10 as shown in Figure 2 comprises, as a simplified representation, a wireless transmitter 30, a wireless receiver 32 and a controller or controlling processor 34 which may operate to control the transmitter 30 and the wireless receiver 32 to transmit and receive radio signals to one or more UEs 14 within a cell 12 formed by the TRP 10. As shown in Figure 3, an example UE 14 is shown to include a corresponding transmitter 49, a receiver 48 and a controller 44 which is configured to control the transmitter 49 and the receiver 48 to transmit signals representing uplink data to the wireless communications network via the wireless access interface formed by the TRP 10 and to receive downlink data as signals transmitted by the transmitter 30 and received by the receiver 48 in accordance with the conventional operation.

[0044] The transmitters 30, 49 and the receivers 32, 48 (as well as other transmitters, receivers and transceivers described in relation to examples and embodiments of the present disclosure) may include radio frequency fdters and amplifiers as well as signal processing components and devices in order to transmit and receive radio signals in accordance for example with the 5G / NR standard. The controllers 34, 44 (as well as other controllers described in relation to examples and embodiments of the present disclosure) may be, for example, a microprocessor, a CPU, or a dedicated chipset, etc., configured to carry out instructions which are stored on a computer readable medium, such as a non-volatile memory. The processing steps described herein may be carried out by, for example, a microprocessor in conjunction with a random access memory, operating according to instructions stored on a computer readable medium. The transmitters, the receivers and the controllers are schematically shown in Figure 3 as separate elements for ease of representation. However, it will be appreciated that the functionality of these elements can be provided in various different ways, for example using one or more suitably programmed programmable computer(s), or one or more suitably configured application-specific integrated circuit(s) / circuitry / chip(s) / chipset(s). As will be appreciated the infrastructure equipment / TRP / base station as well as the UE / communications device will in general comprise various other elements associated with its operating functionality.

[0045] As shown in Figure 3, the TRP 10 also includes a network interface 50 which connects to the DU 42 via a physical interface 16. The network interface 50 therefore provides a communication link for data and signalling traffic from the TRP 10 via the DU 42 and the CU 40 to the core network 20.

[0046] The interface 46 between the DU 42 and the CU 40 is known as the F 1 interface which can be a physical or a logical interface. The Fl interface 46 between CU and DU may operate in accordance with 3GPP technical specifications [2] and [3], and may be formed from a fibre optic or other wired or wireless high bandwidth connection. In one example the connection 16 from the TRP 10 to the DU 42 is via fibre optic. The connection between a TRP 10 and the core network 20 can be generally referred to as a backhaul, which comprises the interface 16 from the network interface 50 of the TRP 10 to the DU 42 and the Fl interface 46 from the DU 42 to the CU 40.

[0047] As will be appreciated by those acquainted with 5G architecture, the CU 40 may be a logical node which hosts Radio Resource Control (RRC) protocols, Service Data Adaptation Protocols (SDAP), and Packet Data Convergence Protocols (PDCP) of a gNB. Alternatively, the CU 40 may be a logical node which hosts RRC and PDCP protocols of an en-gNB (which is a gNB that is able to connect with both EPC and eNBs and can be understood as being, for example, a secondary node (SgNB) used in dual connectivity scenarios). The CU 40 partly controls the operation of one or more DUs 40 and terminates the Fl interface 46 for the DUs that it controls. The DU 42 may be a logical node which hosts Radio Link Control (RLC), Medium Access Control (MAC), and Physical (PHY) layers of a gNB or en-gNB. The operation of the DU 42 is partly controlled by the CU 40 for which the DU 42 terminates the Fl interface 46.

[0048] Although not shown in Figures 2 or 3, it will be familiar to those acquainted with 5G architecture that the CU 40 may be further split into a CU-CP which performs the control plane functions of the CU 40 and a CU-UP which performs the user plane functions of the CU 40 (see for example, [4]). In more detail, the CU-CP may be a logical node hosting an RRC protocol and a control plane part of a PDCP protocol of the CU 40 for the gNB or en-gNB. The CU-CP terminates an El interface connected with the CU-UP and an Fl-C interface connected with the DU 42. As will be appreciated, the Fl-C interface carries control plane signalling of the Fl interface 46. The CU-UP may be a logical node which hosts a user plane part of a PDCP protocol of the CU 40 for an en-gNB. Alternatively, the CU-UP may be a logical node which hosts a user plane part of the PDCP protocol and an SDAP protocol of the CU 40 for a gNB. The CU-UP terminates an El interface connected with the CU-CP and an Fl-U interface connected with the DU 42. As will be appreciated, the Fl-U interface carries user plane signalling of the Fl interface 46.

[0049] In order to appreciate example embodiments, a protocol stack for forming a conventional F 1 interface shown in Figure 2 and 3 will be explained with reference to Figures 4A, 4B, and 4C. In respect of a protocol stack, Figures 4A, 4B, and 4C provide an illustration of processing performed by the elements shown in Figures 2 and 3 which form the packet data communications path 46 between the gNB-DU 42 and the gNB-CU 40 via the Fl interface 46. Control plane communications are considered separately to user plane data, although in practice they form the same interface and are processed and transmitted by the same hardware equipment.

[0050] As shown in Figure 4 A, communication is formed between the gNB DU 42 and a gNB CU 40 for the Fl interface 46. However, the control plane protocol stack to form this interface is shown in Figure 4B, and the user plane protocol stack for communicating the user data between the gNB-CU 40 and gNB-DU 42 is shown in Figure 4C. As shown in Figure 4B, at the radio network layer, the control plane is formed by Fl Application Protocols (FlAPs) 301a in the gNB-CU 40 and by Fl APs 301b in the gNB DU 42. As will be understood by those acquainted with the 5G Architecture, communication between a gNB-CU and a gNB DU is by IPv6 or IPv4 Internet protocols as specified in [5], This is shown in Figure 4B as an IP layer 302a in the gNB-CU 40 and an IP layer 302b in the gNB-DU 42, forming an IP communication interface 302c. A Stream Control Transmission Protocol (SCTP) layer of the protocol stack 304a, 304b, 304c controls end to end communication via the IP layer 302 including flow control and quality of service. The IP data is communicated between the gNB DU and gNB CU via logical data link layer 306a, 306b, 306c and the physical layer 308a, 308b, 308c.

[0051] In the user plane, the radio network layer is formed by REC layer 320a, 320b to form the Fl interface for communicating use plane data 46. The protocol stack in the transport layer comprises a General Packet Ratio Service (GPRS) Tunnelling Protocol for user plane data (GTP-U) 322a, 322b, 322c, which controls communication of user plane data for roaming and home subscribers via a UDP layer 324a, 324b, 324c which controls communication of user plane data via an IP layer 326a, 326b, 326c. As with the control plane, the IP data is communicated between the gNB DU and gNB CU via logical data link layer 328a, 328b, 328c and the physical layer 330a, 330b, 330c.

[0052] CU-DU Split Functions

[0053] As indicated above, the CU 40 and DU 42 are configured to execute gNB functionality. The allocation or splitting of gNB functions between the CU 40 and the DU 42 is discussed in [6] . Such gNB functions include:

[0054] • Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);

[0055] • IP and Ethernet header compression, encryption and integrity protection of data;

[0056] • Selection of an Access and Mobility Management Function (AMF) at UE attachment when no routing to an AMF can be determined from the information provided by the UE;

[0057] • Routing of User Plane data towards User Plane Functions (UPFs);

[0058] • Routing of Control Plane information towards AMF; • Connection setup and release;

[0059] • Scheduling and transmission of paging messages;

[0060] • Scheduling and transmission of system broadcast information (originated from the AMF or 0AM);

[0061] • Measurement and measurement reporting configuration for mobility and scheduling;

[0062] • Transport level packet marking in the uplink;

[0063] • Session Management;

[0064] • Support of Network Slicing;

[0065] • QoS Flow management and mapping to data radio bearers;

[0066] • Support of UEs in RRC INACTIVE state;

[0067] • Distribution function for NAS messages;

[0068] • Radio access network sharing;

[0069] • Dual Connectivity;

[0070] • Tight interworking between NR and E-UTRA; and

[0071] • Maintain security and radio configuration for User Plane Cellular Internet of Things (CIoT) 5GS Optimisation, as defined in [7] (ng-eNB only).

[0072] All functions listed above, except for the final function, can be performed by gNBs and ng-eNBs while the final function relating to the maintenance of security and radio configurations is performed by ng- eNBs only. As those skilled in the art would understand, an ng-eNB is an eNB which connects 5G UEs to the 5G core network using 4G LTE air interface. Furthermore, bandwidth reduced low complexity (BL) UEs or UEs in enhanced coverage are only supported by ng-eNBs (see, for example, [8]). Additionally, NB-IoT UE is only supported by ng-eNBs (see, for example, [8]).

[0073] As will be appreciated, 5G networks are currently deployed in millimetre wave band. However, due to the higher communication bandwidths that can be achieved at higher frequencies, there is an increased demand for wireless communications at higher frequencies in 5G networks. The use of higher frequencies in wireless communications leads to increased path loss and therefore smaller cell sizes than if lower frequencies were used. As a result, in order to achieve suitable coverage, a dense deployment of networks may be required.

[0074] A dense deployment of networks creates a number of technical challenges for the current 5G architecture as explained with reference to Figures 2 and 3. For example, as mentioned previously, the DU 42 and the CU 40 may be connected over a wired connection 46 such as fibre optic. Therefore, a dense deployment of networks may increase the number of required wired connections which may not always be possible. Furthermore, the use of wired connections 46 between the CU 40 and the DU may lead to increased latency due to current topology designs. For example, as will be appreciated from Figure 2, two DUs 41, 42 are connected to the CU 40 via wired connections 46 in a “tree topology”. Therefore, if any direct signalling is required between two cells 12 (for example, one cell provided by DU 41 and one cell provided by DU 42) then such signalling must propagate via the CU 40. This can lead to congestion and increased latency, especially when a large number of cells are deployed in close proximity, such as in dense deployment scenarios.

[0075] Furthermore, as will be explained in more detail with reference to Figure 5, a higher cell density creates technical challenges in providing cell mobility and interference management. Figure 5 illustrates a split of gNB functionality between a CU-CP 40a, CU-UP 40b and the DU 42 according to current 5G architectures. The CU-CP 40a and CU-UP 40b are logical nodes which perform the functions of the CU 40 described with reference to Figures 2, 3, and 4. As shown in Figure 5, the CU-CP 40a is configured to perform radio resource management (RRM) functions 402, Radio Resource Control (RRC) functions 403, Packet Data Convergence Protocol Control Plane (PDCP-CP) functions 404, Security functions 406, Non-Access Stratum (NAS) functions 408, user equipment (UE) context functions 410, and Quality of Service (QoS) functions 412. The CU-UP 40b is configured to perform Packet Data Convergent Protocol User Plane (PDCP-UP) functions 414. The DU 42 is configured to perform REC functions 416, MAC functions 418, PHY functions 420. As will be appreciated, a scheduler in the DU 42 may be implemented based on the guidelines as mentioned in section 10 of [6],

[0076] Layer 1 / Layer 2 Triggered Mobility (LTM)

[0077] Release 18 of the 3GPP standards specified intra-CU - both intra-DU and inter-DU - Ll / 2 Triggered Mobility (LTM). LTM may alternatively stand for “Lower-layer Triggered Mobility”. The operation of LTM itself will be described in more detail in the paragraphs below, but in simplistic terms, as would be well understood by those skilled in the art, LTM refers to UE mobility which is controlled entirely by layer 1 (i.e. the physical layer) and layer 2 (i.e. the data link layer) of the 5G protocol stack. Both intra- DU and inter-DU LTM (both intra-CU) can be understood with reference to figure 6. As can be seen in the example of Figure 6, two DUs 62, 63 are connected to a single CU 61. Each DU 62, 63 supports two cells, with DU 62 supporting cells 64 and 65, and DU 63 supporting cells 66 and 67. In intra-CU LTM a UE is handed over between a source and target cell which are both under control (via the same or separate DUs) of the same CU, such as CU 61 of Figure 6. For intra-DU LTM, handover may be triggered and controlled by layers 1 and 2 for a UE between cells 64 and 65 (controlled by the same DU 62) or cells 66 and 67 (controlled by the same DU 63). For inter-DU LTM, handover may be triggered and controlled by layers 1 and 2 for a UE between one of cells 64 and 65 (controlled by DU 62) and one of cells 66 and 67 (controlled by DU 63) - e.g., inter-DU LTM may involve the UE handing over from cell 67 to cell 64. Inter-CU LTM is not yet specified in the 3GPP standards at the priority date of the present disclosure, but is expected to be specified in Release 19 of the 3GPP standards.

[0078] A proposed message sequence for LTM is as shown in Figure 7, which is reproduced in [9], The detailed procedure for LTM, as shown in Figure 7, is described in [9], and such description is reproduced from [9] in the paragraph below for the purposes of better understanding of LTM.

[0079] In step 1, the UE 71 (while in RRC_CONNECTED mode) sends a MeasurementReport message to the gNB 72. The gNB 72 decides to use LTM and initiates candidate cell(s) preparation. In step 2, the gNB 72 transmits an RRCReconfiguration message to the UE 71 including the LTM candidate cell configurations of one or multiple candidate cells. In step 3, the UE 71 stores the LTM candidate cell configurations and transmits a RRCReconfigurationComplete message to the gNB 72. In step 4a, the UE 71 may perform DL synchronisation with candidate cell(s) before receiving the cell switch command, and in step 4b, the UE 71 may perform early timing advance (TA) acquisition with candidate cell(s) before receiving the cell switch command. This is done via Contention Free Random Access (CFRA) triggered by a Physical Downlink Control Channel (PDCCH) order from the source cell, following which the UE

[0080] 71 sends a preamble towards a candidate cell. The information that identifies the allocated CFRA resource can be indicated in the PDCCH order to enable shared preamble resource among multiple UEs 71. In order to minimize the data interruption of the source cell due to CFRA towards the candidate cell(s), the UE 71 either receives a Random Access Response (RAR) from the source cell instead of the candidate cell or doesn’t receive a RAR at all. Whether a RAR needs to be received is configured by the network and specific per each candidate cell. If RAR is not needed, the TA value of the candidate cell is indicated in the cell switch command. In step 5, the UE 71 performs LI measurements on the configured candidate cell(s), and transmits lower-layer measurement reports to the gNB 72. Then, in step 6, the gNB

[0081] 72 decides to execute cell switch to a target cell, and transmits a MAC control element (CE) triggering cell switch by including the candidate configuration index of the target cell. The UE 71 switches to the configuration of the target cell. In step 7, the UE 71 performs a random-access procedure towards the target cell, if the cell switch needs to include the performance of such a random-access procedure. Finally, in step 8, the UE 71 completes the LTM cell switch procedure by sending RRCReconfigurationComplete message to target cell. For RACH-based LTM, the UE 71 considers that LTM execution procedure is successfully completed when the random-access procedure itself is successfully completed. For RACH-less LTM, the UE 71 considers that LTM execution procedure is successfully completed when the UE 71 determines that the network has successfully received its first UL data. The UE 71 can perform steps 4 to 8 as shown in Figure 7 multiple times for subsequent LTM cell switch based on the configuration provided in step 2.

[0082] Some observations on LTM are provided in

[0010] , Specifically, it is noted in

[0010] that the number of handover attempts of LTM is 20 times higher than that of legacy handover if the time-to-trigger (TTT) is set to a typical value, e.g., 160ms with 2dB A3-offset. The data loss rate of LTM reaches 3.9% without applying filtering. However, the data loss rate is still high at 1.7% even if filtering is applied. There is no data loss issue for RLC acknowledged mode (AM) bearers, but hybrid automatic repeat request (HARQ) reset at LTM cell switch will result in frequent ARQ retransmissions and relatively long packet delay for those packets which are subject to ARQ retransmission. For RLC AM bearers, if HARQ buffers are flushed at each LTM cell switch, the lost RLC service data units (SDUs) or RLC segments can be recovered through ARQ retransmission. Therefore, PDCP data recovery procedures are required to avoid data loss due to inter-DU LTM cell switch. At the network side, the PDCP data recovery procedures may be similar to layer 3 handover without PDCP re-establishment, or secondary node (SN) release / addition for split bearers. Therefore, the high data loss rate (1.7% or 3.9%) due to HARQ reset at LTM cell switch can’t meet the QoS requirements for those services.

[0083] Thus, it would be understood by the skilled in the art that, while LTM techniques reduce latency as compared to conventional higher-layer handover techniques (because such higher layers do not need to be involved), the rate of handover itself, as well as the rate of ping-pongs and data loss, are increased. Here, a “ping-pong” can effectively be understood as a handover of a UE between a source and target cell which is then quickly handed back to the source cell. The increase in the rate of such ping-pongs is effectively because the triggers for handover at L1 / L2 are more sensitive, and there are not the same well- defined events at these layers as there are at L3 (which are well defined, but may delay handover). Also, there is measurement filtering performed at L3 in L3 handover procedures, whereas L1 / L2 triggers are generally based on one-shot measurements, which therefore decreases robustness and increases the likelihood of inaccuracy of such measurements. The rate of data loss, as mentioned above, is also increased. This is primarily because of the increased rate of handovers and ping-pongs, but also because LTM (at L1 / L2) requires HARQ reset and the absence of RLC ARQ for RLC unacknowledged mode (UM).

[0084] As noted above, inter-CU LTM is not yet specified in the 3GPP standards at the priority date of the present disclosure. Figure 8 shows an example expected message sequence for inter-CU LTM, where a UE 81 is handed over from a source CU 83 (and its associated DU 82) to a target CU 85 (and its associated DU 84). The example expected message sequence shown in Figure 8 for inter-CU LTM is broadly comparable to the proposed message sequence for intra-CU LTM as shown in Figure 7. Some further messages exchanged between the target CU 85 and both the target DU 84 and source CU 83 are also shown, between the LTM preparation step and RRC reconfiguration message transmission step, but otherwise, the operations are similar to those described above with respect to Figure 7.

[0085] However, there are some open issues arising from this example expected message sequence for inter-CU LTM. One of these is when should the target cell security key be applied, and how? Security key change requires PDCP re-establishment, resulting in data loss as any data maintained at the PDCP is deleted. Because there is likely to be a greater number of handovers using LTM instead of procedures involving higher layers, as well as the likely increased number of ping-pong handovers between source and target cells, as observed in Rel-18, such PDCP re-establishment (as well as RLC and MAC reset or reestablishment) would be required much more frequently, meaning more frequent data loss. The number of ping-pong handovers is not affected by whether the source and target cells are controlled by different CUs or the same CU, but is instead based on radio conditions, as they are generally based on one-shot measurements as described above. As noted above, security key changes will require PDCP / RLC / MAC reset / re-establishment in addition to the signalling of the new keys themselves, and so in addition to the higher data loss rate issue noted above, this will introduce latencies to the procedure, potentially making LTM performance similar to or worse than that of current L3 handover procedures. Therefore, changing security keys every time when such LTM handover takes place (where there is the possibility of a ping- pong handover following it) will result in both a higher amount of data interruption (due to PDCP reestablishments and hence loss of data maintained on PDCP) and higher latencies.

[0086] Embodiments of the present disclosure seek to provide solutions to such problems.

[0087] Inter-CU LTM UE Timers

[0088] Figure 9 shows a part schematic, part message flow diagram representation of a first wireless communications system comprising a source infrastructure equipment (e.g. gNB) 91, a target infrastructure equipment (e.g. gNB) 92, and a communications device 93 (e.g. a UE 14) in accordance with at least some embodiments of the present technique. Here, the source and target infrastructure equipment 91, 92 may be hosted by respective source and target CUs and respective source and target DUs in the manner described above with reference to Figures 4A, 4B, and 4C, where such source and target CUs and DUs are not shown in Figure 9. The source infrastructure equipment 91, target infrastructure equipment 92, and communications device 93 each comprise a transceiver (or transceiver circuitry) 91.1, 92.1, 93.1, and a controller (or controller circuitry) 91.2, 92.2, 93.2. Each of the controllers 91.2, 92.2, 93.2 may be, for example, a microprocessor, a CPU, or a dedicated chipset, etc.

[0089] As shown in the example of Figure 9, the transceiver circuitry 93.1 and the controller circuitry 93.2 of the communications device 91 are configured in combination transmit 94 signals to and / or to receive 94 signals from the source infrastructure equipment 91 while the communications device 93 is operating in a connected state with the source infrastructure equipment 91, to determine 95 that the communications device 93 is to hand over from the source infrastructure equipment 91 to the target infrastructure equipment 92, to perform 96, with the source infrastructure equipment 91 and the target infrastructure equipment 92, a handover procedure to hand the communications device 93 over from the source infrastructure equipment 91 to the target infrastructure equipment 92, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment 91, the target infrastructure equipment 92, and the communications device 93 and / or a data link layer at each of the source infrastructure equipment 91, the target infrastructure equipment 92, and the communications device 93, and to determine 97 whether the communications device 93 is to maintain configuration information associated with the source infrastructure equipment 91 following the handover procedure 96.

[0090] Essentially, such embodiments of the present technique as exemplified by Figure 9 propose that the source cell configuration is kept by a UE for at least a certain period of time after being handed over to a target cell. In at least some arrangements of embodiments of the present technique as exemplified by Figure 9, in addition to keeping the source cell configuration, the UE may also keep the current (source cell) security keys, and not change these to - or, at least, not delete these existing security keys and replace them by - newly derived security keys for the target cell, for at least a certain period of time after being handed over to the target cell.

[0091] Figure 10 illustrates an example of the architecture for inter-CU LTM, in accordance with which certain embodiments of the present technique may be configured to operate. In the example of Figure 10, CU 1

[0092] 101 and DU 1 103 may between them host functions of the source infrastructure equipment 91 (e .g . source gNB) of Figure 9, which controls cell 106, while CU2 102 and DU2 104 may between them host functions of the target infrastructure equipment 92 (e.g. target gNB) of Figure 9, which controls cell 107. In the example architecture of Figure 10, following handover 109 of a UE (such as communications device 93 of Figure 9) from the source infrastructure equipment to the target infrastructure equipment, an Fl-U (GTP tunnel) link 108 may be set up between CUI and DU2 for carrying RRC messages and UE user data, for at least a certain period of time following the handover 109. As shown in Figure 10, DU2

[0093] 104 may be connected (via the newly set up GTP tunnel) to CU 1 101 , in addition to connecting to CU2 102, and may send / receive data traffic directly to CU 1 101. At the same time, RRC signalling from the UE and received via DU2 104 may also be terminated at CUI 101. In other words, there is an Fl-U path 108 set up between DU2 104 and CUI 101 for this UE. Figure 10 also shows an Fl-U path between CU2

[0094] 102 and DU 1 103 - this is to demonstrate that a path corresponding to Fl-U path 108 could be set up for UEs which are handed over 109 in the opposite direction or the same UE is handed back to the previous serving cell. As those skilled in the art would appreciate, this architecture is similar to mesh networks for user plane path. CUI 101 and CU2 102 here may exchange signalling indicating their (and indeed DU2’s 104) respective capabilities to support the additional GTP-U tunnel 108 between DU2 104 and CUI 101. This newly setup GTP tunnel therefore allows the UE, following handover to the target cell, to maintain and use configuration information and / or security keys associated with the source cell for at least a period of time following handover, such that in the event of a ping-pong handover, data interruption is minimised. As noted above, LTM increases the number of handovers compared to L3 handover procedures. If security keys are changed at every inter-CU handover, then every ping-pong handover will result in another security key change (i.e., back and forth), thus resulting in more data interruption due to PDCP re-establishment.

[0095] Therefore if - as proposed by arrangements of embodiments of the present disclosure - a UE keeps the source cell configuration for some time after moving to the target cell, then the UE will have two security keys and will not need to receive an RRCReconfiguration message from that target gNB when performing a second handover from that target gNB to a previous source gNB, thus avoiding the requirement to perform a full reconfiguration procedure (in respect of that first, intermediate, handover). However, the UE and source gNB should not keep the context forever, as this would have impacts both on memory usage at both nodes as well as security related issues. The period of maintenance of the source gNB context can be controlled by a timer, as proposed by arrangements of embodiments of the present technique. Alternatively, or in addition, if there is a timer in the UE for applying new security keys, then it will not be necessary to perform PDCP re-establishment until such a timer expires and the UE applies the new security keys, provided no further handovers or ping-pong handovers occur during the running of that timer. Instead, during the running of this timer, a solution such as that shown in Figure 10, with the newly set-up GTP tunnel 108 between DU2 104 and CUI 101, may be used.

[0096] Here, although in Figures 9, 10, and 11, there are only two (source and target) gNBs / CUs shown those skilled in the art would appreciate that any number of gNBs / CUs may be involved in such handovers and ping-pong handovers described in view of embodiments of the present technique. For example, arrangements of embodiments of the present technique may find application in a case where there are three (or indeed more) gNBs / CUs (e.g. an original source gNB, an original target gNB, and a new target gNB), and where the ping-pong handovers may occur between all three or more of those gNBs / CUs.

[0097] Figure 11 shows a second example message sequence for inter-CU LTM in accordance with embodiments of the present technique. This example message sequence as shown in Figure 11 is similar to that shown by Figure 8, but focuses on the UE’s behaviour during (and following) inter-CU handovers in accordance with embodiments of the present disclosure. Furthermore, in the example message sequence shown by Figure 11, after undergoing handover from the source gNB 112 to the target gNB 113, the UE 111 may then undergo handover (e.g. a ping-pong handover) back from the old target gNB 113 (now as a source gNB) to the original source gNB 112 (now as a target gNB).

[0098] At the time of the first cell switch MAC-CE reception, UE 111 has the gNB cell 112 as its source cell and the gNB cell 113 as its target cell. The UE 111 applies target gNB security keys in this case during the LTM complete procedure at the conclusion of the first cell switch. At the time of the second cell switch MAC-CE reception, the UE 111 has gNB 113 as its source gNB and gNB 112 as its target gNB. Generally speaking, the UE 111 has stored both configurations (for gNB 112 and gNB 113), then all that the UE 111 needs to perform when handing over between these two gNBs is a simple switch between these two stored configurations, and the application by the UE of the (target) security configuration. As noted above, however, one problem with this approach is that PDCP will be needed to be re-established even if the need for the reception of the RRCReconfiguration message is eliminated. In the example of Figure 11, it is therefore proposed that the UE 111 keeps applying the gNB 113 security keys when it hands back over to the gNB 112; i.e., after receipt of the second cell switch MAC-CE.

[0099] Normally, when a UE applies a network configuration (i.e. if the network configures the UE to continue to use the existing security keys) the UE will continue to use this applied network configuration until the time a new security key change procedure is triggered. However, in general, it is beneficial (in LTM given the likelihood of ping-pong handovers) that the UE keeps the source cell configuration for use in the event of a possible fall-back to the source cell, as shown in the example of Figure 11. The UE is not generally aware if the handover is an intra-CU handover or an inter-CU handover, and so if the source cell configuration is kept only for a specific scenario (such as in the case of inter-CU handover only), then an indication for the communications device to keep the source cell configuration is required in, for example, the RRCReconfiguration message (where this new indication to keep the source cell configuration in the RRCReconfiguration message may be sent along with the LTM configuration). In other words, the communications device may be configured to receive, from the source infrastructure equipment during the handover procedure, a first control message comprising an indication that the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure, and to determine, based on the first control message, to maintain the configuration information associated with the source infrastructure equipment following the handover procedure. Here, the source infrastructure equipment may be associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment may be associated with a second CU of the wireless communications network; i.e. the handover is an inter-CU handover. In some arrangements of embodiments of the present technique, the source infrastructure equipment may only be configured to transmit the indication that the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure if the handover procedure is an inter-CU handover procedure. Here, in accordance with at least some arrangements of embodiments of the present technique, the source infrastructure equipment may also indicate to the target infrastructure equipment that the communications device will maintain the configuration information associated with the source infrastructure equipment following the handover procedure. Regarding the duration for which the resources should be kept, it has been agreed within 3GPP that LTM should be executed within a time limit. That is, a timer will be maintained for either an overall duration or for a period after sending / receiving the LTM MAC-CE. The purpose of this timer (which is already disclosed) relates only to the execution of the LTM procedure itself, and so is different to the timer proposed herein in accordance with embodiments of the present technique, which defines the duration during which the UE is to maintain the configuration and / or resources for the serving cell following handover to the target cell. In other words, the first control message may comprise an indication of a time duration for which the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

[0100] In at least some arrangements of embodiments of the present technique, the timer for the duration should be equal to or greater than the observed Time Of Stay (TOS) in a cell for this UE. While this timer is useful for - and may, in accordance with embodiments of the present technique, be applied for - intra / inter-DU (LTM) handovers and intra-CU (LTM) handovers as well, such a timer is more critical for inter-CU (LTM) handovers because, here, resources are reserved across different CUs.

[0101] In at least some arrangements of embodiments of the present technique, this timer may be started when the UE receives the cell switch MAC-CE. In other words, the time duration may start upon the communications device receiving a second control message (e.g. the cell switch MAC-CE) from the source infrastructure equipment during the handover procedure.

[0102] In at least some arrangements of embodiments of the present technique, this timer may be stopped after another cell switch MAC-CE is received by the communications device. In other words, the time duration may end upon the communications device receiving a third control message (e.g. the cell switch MAC-CE) from the target infrastructure equipment during performance of a second handover procedure. Here, this second handover procedure may be an inter-CU, intra-CU, inter-DU, or intra-DU handover, and may involve handing back from the target cell to the source cell, or from the original target cell to a new target cell.

[0103] Alternatively, in at least some arrangements of embodiments of the present technique, this timer may be stopped upon receipt, by the UE, of a network instruction to stop the timer (in the case network wants the UE to keep the source cell configuration for longer). In other words, the time duration may be an indefinite time duration, and the time duration may end upon the communications device receiving an explicit indication that the time duration is to end from the target infrastructure equipment.

[0104] In at least some arrangements of embodiments of the present technique, upon expiry of this timer, the UE may delete the source cell configuration. In other words, the communications device may be configured to discard, upon expiry of the timer, the configuration information associated with the source infrastructure equipment.

[0105] Alternatively, in at least some arrangements of embodiments of the present technique, upon expiry of this timer, the UE may send a deletion request to the network and wait for the network’s confirmation before deleting. In other words, the communications device may be configured to transmit, to the target infrastructure equipment, a request to discard the configuration information associated with the source infrastructure equipment, to receive, from the target infrastructure equipment in response to the request, a confirmation indication indicating that the communications device is to discard the configuration information associated with the source infrastructure equipment, and to discard, in response to receiving the confirmation indication, the configuration information associated with the source infrastructure equipment.

[0106] In at least some of the above-described arrangements of embodiments of the present technique, it is assumed that security keys are changed at every handover involving a new CU (i.e. inter-CU handovers). However, it at least some other arrangements of embodiments of the present technique, a new timer may be configured for the UE, where during the running of this timer, no security keys are changed at the time of handover. Upon expiry of this timer however, the UE may then apply new security keys. In other words, the communications device may be configured to receive, from the source infrastructure equipment during the handover procedure, an indication that the communications device is to derive one or more new security keys) associated with the target infrastructure equipment (where this indication may also comprise indications of one or more parameters for the communications device to use in deriving the new security keys), wherein the one or more new security keys are to be used by the communications device for communicating with the target infrastructure equipment following the handover procedure. Here, the communications device may be configured not to use the one or more new security keys for communicating with the target infrastructure equipment for a predetermined time period following the handover procedure, and, during the predetermined time period, the communications device may be configured to use one or more existing security keys for communicating with the target infrastructure equipment, the one or more existing security keys having been used by the communications device for communicating with the source infrastructure equipment before the handover procedure.

[0107] This means that the UE still performs derivation of the new security keys while receiving the LTM configuration, but continues to use the old security keys until this new timer expires. The benefit of this timer will be that, firstly, security keys are changed only when UE has stayed at the new cell for a sufficient period (for example more than the TOS) indicating that the UE’s radio conditions are stable and that a ping-pong handover is unlikely.

[0108] This new timer may be restarted upon any subsequent (e.g. ping-pong) handover that takes place before it expires. In other words, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period may be restarted upon the second handover procedure being completed. However, in another alternative, the UE may keep this timer running during any ping-pong handover, with the benefit being that subsequent handovers within a certain period will not result in security key change, as the previously derived keys that are changed to may actually be fairly old by the time the ping-pong handover(s) have ended and the UE’s radio conditions have become stable. In other words, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period may not be restarted upon the second handover procedure being completed.

[0109] Figure 12 shows a flow diagram illustrating a first example process of communications in a communications system in accordance with embodiments of the present technique. The process shown by Figure 12 is specifically a method of operating a communications device (e.g. UE).

[0110] The method begins in step S 1. The method comprises, in step S2, transmitting signals to and / or receiving signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment. In step S3, the process comprises determining that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network. Then, in step S4, the method comprises performing, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device. In step S5, the method comprises determining whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure. The process ends in step S6.

[0111] Those skilled in the art would appreciate that the method shown by Figure 12 may be adapted in accordance with embodiments of the present technique. For example, other intermediate steps may be included in such a method, or the steps may be performed in any logical order. Though embodiments of the present technique have been described largely by way of the example communications system shown in Figure 9, it would be clear to those skilled in the art that they could be equally applied to other systems to those described herein.

[0112] Those skilled in the art would further appreciate that such infrastructure equipment and / or communications devices as herein defined may be further defined in accordance with the various arrangements and embodiments discussed in the preceding paragraphs. It would be further appreciated by those skilled in the art that such infrastructure equipment and communications devices as herein defined and described may form part of communications systems other than those defined by the present disclosure.

[0113] The following numbered paragraphs provide further example aspects and features of the present technique:

[0114] Paragraph 1. A method of operating a communications device, the method comprising transmitting signals to and / or receiving signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, determining that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and determining whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0115] Paragraph 2. A method according to Paragraph 1, comprising receiving, from the source infrastructure equipment during the handover procedure, a first control message comprising an indication that the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure, and determining, based on the first control message, to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

[0116] Paragraph 3. A method according to Paragraph 2, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network. Paragraph 4. A method according to Paragraph 2 or Paragraph 3, wherein the first control message comprises an indication of a time duration for which the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

[0117] Paragraph 5. A method according to Paragraph 4, wherein the time duration starts upon the communications device receiving a second control message from the source infrastructure equipment during the handover procedure.

[0118] Paragraph 6. A method according to Paragraph 4 or Paragraph 5, wherein the time duration ends upon the communications device receiving a third control message from the target infrastructure equipment during performance of a second handover procedure.

[0119] Paragraph 7. A method according to any of Paragraphs 4 to 6, wherein the time duration is an indefinite time duration, and wherein the time duration ends upon the communications device receiving an explicit indication that the time duration is to end from the target infrastructure equipment.

[0120] Paragraph 8. A method according to any of Paragraphs 4 to 7, comprising discarding, upon expiry of the timer, the configuration information associated with the source infrastructure equipment.

[0121] Paragraph 9. A method according to any of Paragraphs 4 to 8, comprising transmitting, to the target infrastructure equipment, a request to discard the configuration information associated with the source infrastructure equipment, receiving, from the target infrastructure equipment in response to the request, a confirmation indication indicating that the communications device is to discard the configuration information associated with the source infrastructure equipment, and discarding, in response to receiving the confirmation indication, the configuration information associated with the source infrastructure equipment.

[0122] Paragraph 10. A method according to any of Paragraphs 1 to 9, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network. Paragraph 11. A method according to Paragraph 10, comprising receiving, from the source infrastructure equipment during the handover procedure, an indication that the communications device is to derive one or more new security keys associated with the target infrastructure equipment, wherein the one or more new security keys are to be used by the communications device for communicating with the target infrastructure equipment following the handover procedure.

[0123] Paragraph 12. A method according to Paragraph 11, wherein the communications device is not to use the one or more new security keys for communicating with the target infrastructure equipment for a predetermined time period following the handover procedure, and wherein, during the predetermined time period, the communications device is to use one or more existing security keys for communicating with the target infrastructure equipment, the one or more existing security keys having been used by the communications device for communicating with the source infrastructure equipment before the handover procedure.

[0124] Paragraph 13. A method according to Paragraph 12, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is restarted upon the second handover procedure being completed.

[0125] Paragraph 14. A method according to Paragraph 12 or Paragraph 13, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is not restarted upon the second handover procedure being completed.

[0126] Paragraph 15. A communications device comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to determine whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0127] Paragraph 16. Circuitry for a communications device, the circuitry comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to determine whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0128] Paragraph 17. A method of operating a source infrastructure equipment of a wireless communications network, the method comprising transmitting signals to and / or receiving signals from a communications device while the communications device is operating in a connected state with the source infrastructure equipment, determining that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the communications device and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and determining whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure. Paragraph 18. A method according to Paragraph 17, comprising transmitting, to the communications device during the handover procedure, a first control message comprising an indication that the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure. Paragraph 19. A method according to Paragraph 18, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network. Paragraph 20. A method according to Paragraph 18 or Paragraph 19, wherein the first control message comprises an indication of a time duration for which the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

[0129] Paragraph 21. A method according to Paragraph 20, wherein the time duration starts upon the communications device receiving a second control message transmitted by the source infrastructure equipment during the handover procedure.

[0130] Paragraph 22. A method according to Paragraph 20 or Paragraph 21, wherein the time duration ends upon the communications device receiving a third control message from the target infrastructure equipment during performance of a second handover procedure.

[0131] Paragraph 23. A method according to any of Paragraphs 20 to 22, wherein the time duration is an indefinite time duration, and wherein the time duration ends upon the communications device receiving an explicit indication that the time duration is to end from the target infrastructure equipment.

[0132] Paragraph 24. A method according to any of Paragraphs 20 to 23, comprising determining that the communications device will discard, upon expiry of the timer, the configuration information associated with the source infrastructure equipment.

[0133] Paragraph 25. A method according to any of Paragraphs 17 to 24, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network. Paragraph 26. A method according to Paragraph 25, comprising transmiting, to the communications device during the handover procedure, an indication that the communications device is to derive one or more new security keys associated with the target infrastructure equipment, wherein the one or more new security keys are to be used by the communications device for communicating with the target infrastructure equipment following the handover procedure.

[0134] Paragraph 27. A method according to Paragraph 26, wherein the communications device is not to use the one or more new security keys for communicating with the target infrastructure equipment for a predetermined time period following the handover procedure, and wherein, during the predetermined time period, the communications device is to use one or more existing security keys for communicating with the target infrastructure equipment, the one or more existing security keys having been used by the communications device for communicating with the source infrastructure equipment before the handover procedure.

[0135] Paragraph 28. A method according to Paragraph 27, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is restarted upon the second handover procedure being completed.

[0136] Paragraph 29. A method according to Paragraph 27 or Paragraph 28, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is not restarted upon the second handover procedure being completed.

[0137] Paragraph 30. A source infrastructure equipment of a wireless communications network, the source infrastructure equipment comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a communications device while the communications device is operating in a connected state with the source infrastructure equipment, to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to perform whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0138] Paragraph 31. Circuitry for a source infrastructure equipment of a wireless communications network, the circuitry comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a communications device while the communications device is operating in a connected state with the source infrastructure equipment, to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to perform whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0139] Paragraph 32. A method of operating a target infrastructure equipment of a wireless communications network, the method comprising determining that a communications device is to hand over from a source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the communications device and the source infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and receiving, from the source infrastructure equipment, an indication of whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0140] Paragraph 33. A method according to Paragraph 32, comprising determining, based on the indication received from the source infrastructure equipment, that the communications device is to maintain the configuration information associated with the source infrastructure equipment for a specified time duration following the handover procedure.

[0141] Paragraph 34. A method according to Paragraph 33, comprising transmitting, to the communications device, a control message during performance of a second handover procedure, wherein the specified time duration ends upon the communications device receiving the control message.

[0142] Paragraph 35. A method according to Paragraph 33 or Paragraph 34, wherein the specified time duration is an indefinite time duration, and the method comprises determining that the specified time duration is to end, and transmitting, to the communications device, an explicit indication that the time duration is to end, wherein the time duration ends upon the communications device receiving the explicit indication. Paragraph 36. A method according to any of Paragraphs 33 to 35, comprising determining that the communications device will discard, upon expiry of the timer, the configuration information associated with the source infrastructure equipment.

[0143] Paragraph 37. A method according to any of Paragraphs 33 to 36, comprising receiving, from the communications device, a request to discard the configuration information associated with the source infrastructure equipment, transmitting, to the communications device in response to the request, a confirmation indication indicating that the communications device is to discard the configuration information associated with the source infrastructure equipment, and determining that the communications device will discard, in response to receiving the confirmation indication, the configuration information associated with the source infrastructure equipment.

[0144] Paragraph 38. A method according to any of Paragraphs 32 to 37, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network. Paragraph 39. A target infrastructure equipment of a wireless communications network, the target infrastructure equipment comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to determine that a communications device is to hand over from a source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the source infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to receive, from the source infrastructure equipment, an indication of whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0145] Paragraph 40. Circuitry for a target infrastructure equipment of a wireless communications network, the circuitry comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to determine that a communications device is to hand over from a source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the source infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to receive, from the source infrastructure equipment, an indication of whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

[0146] Paragraph 41. A wireless communications system comprising a communications device according to Paragraph 15, a source infrastructure equipment according to Paragraph 30, and a target infrastructure equipment according to Paragraph 39.

[0147] Paragraph 42. A computer program comprising instructions which, when loaded onto a computer, cause the computer to perform a method according to any of Paragraphs 1 to 14, Paragraphs 17 to 29, or Paragraphs 32 to 38.

[0148] Paragraph 43. A non-transitory computer-readable storage medium storing a computer program according to Paragraph 42.

[0149] It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and / or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and / or processors may be used without detracting from the embodiments.

[0150] Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and / or digital signal processors. The elements and components of any embodiment 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. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and / or processors.

[0151] Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. 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 any manner suitable to implement the technique.

[0152] References

[0153] [1] Holma H. and Toskala A, “LTE for UMTS OFDMA and SC-FDMA based radio access”, John Wiley and Sons, 2009.

[0154] [2] TS 38.470, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NG-RAN; Fl general aspects and principles (Release 17)”, 3GPP, V17.4.0, March 2023.

[0155] [3] TS 38.473, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NG-RAN; Fl application protocol (F1AP) (Release 17)”, 3GPP, V17.4.1, April 2023.

[0156] [4] TS 38.401, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NG-RAN; Architecture description (Release 17)”, 3GPP, V17.4.0, March 2023.

[0157] [5] TS 38 472, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NG-RAN; Fl signalling transport (Release 15)”, 3GPP, V15.2.0, September 2018.

[0158] [6] TS 38.300, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NR; NR and NG-RAN Overall Description; Stage 2 (Release 17)”, 3GPP, V17.4.0, March 2023.

[0159] [7] TS 23.501, “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; System architecture for the 5G System (5GS); Stage 2 (Release 18)”, 3GPP, V18.2.0, June 2023.

[0160] [8] TS 36.300, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 17)”, 3GPP, V17.4.0, March 2023.

[0161] [9] R2 -2305305, “Change Request for 38.300 running CR for introduction of NR further mobility enhancements”, MediaTek Inc., vivo, 3GPP TSG-RAN WG2 Meeting #122, May 2023.

[0162]

[0010] R2-2305305, “Data Loss at LTM Cell Switch”, MediaTek Inc., 3GPP TSG-RAN WG2 Meeting #122, May 2023.

Claims

CLAIMSWhat is claimed is:

1. A method of operating a communications device, the method comprising transmitting signals to and / or receiving signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, determining that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and determining whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

2. A method according to Claim 1, comprising receiving, from the source infrastructure equipment during the handover procedure, a first control message comprising an indication that the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure, and determining, based on the first control message, to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

3. A method according to Claim 2, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network.

4. A method according to Claim 2, wherein the first control message comprises an indication of a time duration for which the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

5. A method according to Claim 4, wherein the time duration starts upon the communications device receiving a second control message from the source infrastructure equipment during the handover procedure.

6. A method according to Claim 4, wherein the time duration ends upon the communications device receiving a third control message from the target infrastructure equipment during performance of a second handover procedure.

7. A method according to Claim 4, wherein the time duration is an indefinite time duration, and wherein the time duration ends upon the communications device receiving an explicit indication that the time duration is to end from the target infrastructure equipment.

8. A method according to Claim 4, comprising discarding, upon expiry of the timer, the configuration information associated with the source infrastructure equipment.

9. A method according to Claim 4, comprising transmitting, to the target infrastructure equipment, a request to discard the configuration information associated with the source infrastructure equipment, receiving, from the target infrastructure equipment in response to the request, a confirmation indication indicating that the communications device is to discard the configuration information associated with the source infrastructure equipment, and discarding, in response to receiving the confirmation indication, the configuration information associated with the source infrastructure equipment.

10. A method according to Claim 1, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network.

11. A method according to Claim 10, comprising receiving, from the source infrastructure equipment during the handover procedure, an indication that the communications device is to derive one or more new security keys associated with the target infrastructure equipment, wherein the one or more new security keys are to be used by the communications device for communicating with the target infrastructure equipment following the handover procedure.

12. A method according to Claim 11, wherein the communications device is not to use the one or more new security keys for communicating with the target infrastructure equipment for a predetermined time period following the handover procedure, and wherein, during the predetermined time period, the communications device is to use one or more existing security keys for communicating with the target infrastructure equipment, the one or more existing security keys having been used by the communications device for communicating with the source infrastructure equipment before the handover procedure.

13. A method according to Claim 12, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is restarted upon the second handover procedure being completed.

14. A method according to Claim 12, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is not restarted upon the second handover procedure being completed.

15. A communications device comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, andto determine whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

16. Circuitry for a communications device, the circuitry comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a source infrastructure equipment of a wireless communications network while the communications device is operating in a connected state with the source infrastructure equipment, to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the source infrastructure equipment and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to determine whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

17. A method of operating a source infrastructure equipment of a wireless communications network, the method comprising transmitting signals to and / or receiving signals from a communications device while the communications device is operating in a connected state with the source infrastructure equipment, determining that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the communications device and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and determining whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

18. A method according to Claim 17, comprising transmitting, to the communications device during the handover procedure, a first control message comprising an indication that the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

19. A method according to Claim 18, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network.

20. A method according to Claim 18, wherein the first control message comprises an indication of a time duration for which the communications device is to maintain the configuration information associated with the source infrastructure equipment following the handover procedure.

21. A method according to Claim 20, wherein the time duration starts upon the communications device receiving a second control message transmitted by the source infrastructure equipment during the handover procedure.

22. A method according to Claim 20, wherein the time duration ends upon the communications device receiving a third control message from the target infrastructure equipment during performance of a second handover procedure.

23. A method according to Claim 20, wherein the time duration is an indefinite time duration, and wherein the time duration ends upon the communications device receiving an explicit indication that the time duration is to end from the target infrastructure equipment.

24. A method according to Claim 20, comprising determining that the communications device will discard, upon expiry of the timer, the configuration information associated with the source infrastructure equipment.

25. A method according to Claim 17, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network.

26. A method according to Claim 25, comprising transmitting, to the communications device during the handover procedure, an indication that the communications device is to derive one or more new security keys associated with the target infrastructure equipment, wherein the one or more new security keys are to be used by the communications device for communicating with the target infrastructure equipment following the handover procedure.

27. A method according to Claim 26, wherein the communications device is not to use the one or more new security keys for communicating with the target infrastructure equipment for a predetermined time period following the handover procedure, and wherein, during the predetermined time period, the communications device is to use one or more existing security keys for communicating with the target infrastructure equipment, the one or more existing security keys having been used by the communications device for communicating with the source infrastructure equipment before the handover procedure.

28. A method according to Claim 27, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is restarted upon the second handover procedure being completed.

29. A method according to Claim 27, wherein, if a second handover procedure is performed before the end of the predetermined time period, the predetermined time period is not restarted upon the second handover procedure being completed.

30. A source infrastructure equipment of a wireless communications network, the source infrastructure equipment comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a communications device while the communications device is operating in a connected state with the source infrastructure equipment,to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to perform whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

31. Circuitry for a source infrastructure equipment of a wireless communications network, the circuitry comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to transmit signals to and / or to receive signals from a communications device while the communications device is operating in a connected state with the source infrastructure equipment, to determine that the communications device is to hand over from the source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the target infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to perform whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

32. A method of operating a target infrastructure equipment of a wireless communications network, the method comprising determining that a communications device is to hand over from a source infrastructure equipment to a target infrastructure equipment of the wireless communications network, performing, with the communications device and the source infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and receiving, from the source infrastructure equipment, an indication of whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

33. A method according to Claim 32, comprising determining, based on the indication received from the source infrastructure equipment, that the communications device is to maintain the configuration information associated with the source infrastructure equipment for a specified time duration following the handover procedure.

34. A method according to Claim 33, comprisingtransmitting, to the communications device, a control message during performance of a second handover procedure, wherein the specified time duration ends upon the communications device receiving the control message.

35. A method according to Claim 33, wherein the specified time duration is an indefinite time duration, and the method comprises determining that the specified time duration is to end, and transmitting, to the communications device, an explicit indication that the time duration is to end, wherein the time duration ends upon the communications device receiving the explicit indication.

36. A method according to Claim 33, comprising determining that the communications device will discard, upon expiry of the timer, the configuration information associated with the source infrastructure equipment.

37. A method according to Claim 33, comprising receiving, from the communications device, a request to discard the configuration information associated with the source infrastructure equipment, transmitting, to the communications device in response to the request, a confirmation indication indicating that the communications device is to discard the configuration information associated with the source infrastructure equipment, and determining that the communications device will discard, in response to receiving the confirmation indication, the configuration information associated with the source infrastructure equipment.

38. A method according to Claim 32, wherein the source infrastructure equipment is associated with a first central unit, CU, of the wireless communications network and the target infrastructure equipment is associated with a second CU of the wireless communications network.

39. A target infrastructure equipment of a wireless communications network, the target infrastructure equipment comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitry to determine that a communications device is to hand over from a source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the source infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to receive, from the source infrastructure equipment, an indication of whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

40. Circuitry for a target infrastructure equipment of a wireless communications network, the circuitry comprising transceiver circuitry configured to transmit signals to and / or to receive signals, and controller circuitry configured in combination with the transceiver circuitryto determine that a communications device is to hand over from a source infrastructure equipment to a target infrastructure equipment of the wireless communications network, to perform, with the communications device and the source infrastructure equipment, a handover procedure to hand the communications device over from the source infrastructure equipment to the target infrastructure equipment, wherein the handover procedure consists of signalling only at a physical layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device and / or a data link layer at each of the source infrastructure equipment, the target infrastructure equipment, and the communications device, and to receive, from the source infrastructure equipment, an indication of whether the communications device is to maintain configuration information associated with the source infrastructure equipment following the handover procedure.

41. A wireless communications system comprising a communications device according to Claim 15, a source infrastructure equipment according to Claim 30, and a target infrastructure equipment according to Claim 39.

42. A computer program comprising instructions which, when loaded onto a computer, cause the computer to perform a method according to Claim 1, Claim 17, or Claim 32.

43. A non-transitory computer-readable storage medium storing a computer program according to Claim 42.

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