WO2024170821A1

WO2024170821A1 - Resources for lower layer triggered mobility

Info

Publication number: WO2024170821A1
Application number: PCT/FI2024/050046
Authority: WO
Inventors: Srinivasan Selvaganapathy; Samuli Heikki TURTINEN; Ahmad AWADA; Panagiotis SPAPIS
Original assignee: Nokia Technologies Oy
Current assignee: Nokia Technologies Oy
Priority date: 2023-02-17
Filing date: 2024-02-06
Publication date: 2024-08-22
Anticipated expiration: 2025-08-17
Also published as: MX2025009515A; KR20250151465A; CN120712839A; EP4666696A1

Abstract

There is provided improved performance for lower layer mobility by determining at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell to change the user equipment from the source cell to the target cell, determining, by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, transmitting, by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

Description

RESOURCES FOR LOWER LAYER TRIGGERED MOBILITY

TECHNICAL FIELD

[0001] The present invention relates to lower layer triggered mobility and more particularly controlling resources for radio access at a target cell.

BACKGROUND

[0002] This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.

[0003] One objective of 3GPP Release 18 work item on mobility enhancements includes support for lower layer triggered mobility (LTM) to improve interruption time and execution delay for switching between cells of the same Distributed Unit (DU) or between cells of different DUs but controlled by the same Centralized Unit (CU). [0004] Unlike Layer 3 (L3) based mobility where a handover decision is based on L3 cell quality measurement results, a handover in LTM is triggered by DU based on measurement results below L3, for example Layer 1 (L1 ) beam measurement results.

[0005] Target cells can be prepared for LTM for facilitating dynamically switching of UEs to the target cells by reserving CFRA resources at the target cells. In order to improve interruption time and execution delay for LTM, reservation of the CFRA resources can be increased. However, this limits the number of UEs that can utilize the LTM. In particular, it can happen that one target cell is prepared for LTM by different source nodes for different UEs, whereby the target cell may need to reserve at least one CFRA preamble for each of the UEs. Therefore, the CFRA resource reservation can become high particularly in dense networks, where a number of DUs is high. SUMMARY

[0006] The scope of protection sought for various embodiments of the invention is set out by the independent claims. The embodiments, examples and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

[0007] According to a first aspect there is provided apparatus comprising one or more processors, and a memory storing instructions that, when executed by the one or more processors, the apparatus is caused to:

- determine, at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell;

- determine, by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility;

- transmit, by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0008] According to a second aspect there is provided apparatus comprising one or more processors, and a memory storing instructions that, when executed by the one or more processors, the apparatus is caused to:

- receive, by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a contention free random access, CFRA, resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility; and

- prepare, at the base station distributed unit, a random access channel based on the received information indicating the CFRA resource for changing the user equipment from the source cell to the target cell.

[0009] According to a third aspect there is provided apparatus comprising one or more processors, and a memory storing instructions that, when executed by the one or more processors, the apparatus is caused to:

- determine, by a base station centralized unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility; and

- transmit, by the base station centralized unit to a base station distributed unit of the target cell, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0010] According to a fourth aspect there is provided a method comprising:

- determining, at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell;

- determining, by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility;

- transmitting, by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0011] According to a fifth aspect there is provided a method comprising:

- receiving, by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a contention free random access, CFRA, resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility;

- preparing, at the base station distributed unit, a random access channel based on the received information indicating the CFRA resource for changing the user equipment from the source cell to the target cell.

[0012] According to a sixth aspect there is provided a method comprising:

- determining, by a base station centralized unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility; - transmitting, by the base station centralized unit to a base station distributed unit of the target cell, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0013] According to a seventh aspect there is provided a computer program comprising computer readable program code means adapted to perform at least the following:

[0014] According to an eighth aspect there is provided a computer program comprising computer readable program code means adapted to perform at least the following:

[0015] According to a ninth aspect there is provided a computer program comprising computer readable program code means adapted to perform at least the following:

- determining, by a base station centralized unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility;

- transmitting, by the base station centralized unit to a base station distributed unit of the target cell, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0016] One or more of the above aspects may comprise at least some of the following features of the following list: o receive, at the base station distributed unit, physical layer measurements; o determine, by the base station distributed unit, to change the user equipment from the source cell to the target cell based on the received physical layer measurements; o transmit, by the base station distributed unit, based on the determined change, a request to a base station centralized unit for the CFRA resource for accessing the target cell by the user equipment; o receive, by the base station distributed unit, the pool of CFRA resources reserved for the lower layer triggered mobility from a base station centralized unit; o transmit, by the base station distributed unit to the base station centralized unit, information indicating the determined CFRA resource for accessing the target cell by the user equipment; o receive, at the base station distributed unit, information indicating a completion of a radio access procedure of the user equipment; and o release, at the base station distributed unit, the CFRA resource based on the received information indicating a completion of a radio access procedure of the user equipment; o transmit, by the base station distributed unit, information indicating the release of the CFRA resource to a base station node managing the pool of CFRA resources reserved for the lower layer triggered mobility; o the determined CFRA resource is transmitted included to a cell change command; the information indicating the determined CFRA resource comprises at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; transmit, by the base station distributed unit, based on a completion of a radio access procedure of the user equipment, a message to a for releasing the assigned CFRA resource to a centralized unit; the received information indicating the CFRA resource comprises at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; transmit, by the base station distributed unit to the user equipment, a message, for example Medium Access Control, MAC, Control Element, CE, or downlink control information, DCI, for triggering the change of the user equipment from the source cell to the target cell; receive, by the base station centralized unit, a request for the CFRA resource for accessing the target cell by the user equipment; receive, by the base station centralized unit from one or more base station distributed units, information indicating one or more CFRA resources for lower layer triggered mobility; generate, by the base station centralized unit, the pool of CFRA resources reserved for the lower layer triggered mobility based on the received information; request, by the base station centralized unit from one or more base station distributed units, at configuration of the base station distributed units for the lower layer triggered mobility, information indicating one or more CFRA resources for lower layer triggered mobility transmit, by the base station centralized unit, at configuration of the base station distributed units for the lower layer triggered mobility, the pool of CFRA resources reserved for the lower layer triggered mobility to base station distributed units; transmit, by the base station centralized unit, a message to one or more other base station distributed units for causing the one or more other base station distributed units to refrain from configuring the determined CFRA resource to other user equipment attempting to access the target cell; o receive, at the base station centralized unit, information indicating a completion of a radio access procedure of the user equipment to the target cell; and o release, at the base station centralized unit, the CFRA resource based on the received information indicating a completion of a radio access procedure of the user equipment; o allocate, by the base station centralized unit, one or more physical radio access channel mask indices to each CFRA resource of the pool; o the information indicating the determined CFRA resource comprises at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index.

[0017] At least some embodiments provide improved performance for lower layer mobility.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

[0019] Fig. 1 shows an example of communications system in accordance with at least some embodiments described herein;

[0020] Fig. 2 illustrates an example of a method in accordance with at least some embodiments;

[0021] Fig. 3 illustrates an example of a method in accordance with at least some embodiments;

[0022] Fig. 4 illustrates an example of a method in accordance with at least some embodiments;

[0023] Fig. 5 illustrates an example of a sequence in accordance with at least some embodiments;

[0024] Fig. 6 illustrates an example of a sequence in accordance with at least some embodiments; [0025] Fig. 7 illustrates an example of a sequence in accordance with at least some embodiments;

[0026] Fig. 8 illustrates an example of a sequence in accordance with at least some embodiments; and

[0027] Fig. 9 illustrates an example of an apparatus in accordance with at least some embodiments.

DETAILED DESCRIPTON OF SOME EXAMPLE EMBODIMENTS

[0028] The following embodiments are exemplary. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments.

[0029] Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims and description to modify a described feature does not by itself connote any priority, precedence, or order of one described feature over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one described feature having a certain name from another described feature having a same name (but for use of the ordinal term) to distinguish the described feature.

[0030] As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements.

[0031] There is provided improved performance for lower layer mobility by determining at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell to change the user equipment from the source cell to the target cell, determining, by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, transmitting, by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell. In this way sufficient CFRA resources at the target cell may be provided for improving interruption time and execution delay of cell switching. CFRA resources are selected from the pool that is controlled by the base station centralized unit. Furthermore, on completion of a radio access procedure by the user equipment, a message is transmitted to a base station centralized unit for releasing the CFRA resource. In this way dynamic CFRA resource management controlled by the base station centralized unit is provided. [0032] As used herein lower layer triggered mobility refers to a mobility procedure, where a user equipment is changed, or switched, from a source cell to a target cell of a radio access network based on a base station distributed unit providing the source cell of the user equipment transmitting the user equipment a message, for example a Medium Access Control, MAC, Control Element, CE, or downlink control information, DCL Examples of the mobility procedures comprise at least one of the following: a cell switch; or handover; or cell change. The LTM procedure may be performed between cells of the same radio access node or between cells of different radio access nodes. The lower layer triggered mobility (LTM) may refer to a mobility procedure that is initiated based on processing at one of the lower layers of radio protocol stack of the radio access network. The lower layers of the radio protocol stack comprise at least one of the following: a physical layer, or Layer 1 (L1 ); or base band layer for real-time processing, or Layer 2 (L2). Layers above the lower layers of the radio protocol stack, e.g. Layer 3 (L3), may comprise at least a base band layer for non-real time processing. Accordingly, in LTM, processing performed on at least one of the L1 and L2 may trigger the mobility procedure. An example of the LTM procedure is a cell switch that is triggered based on an L1 measurement report.

[0033] 3GPP TS 38.331 NR; Radio Resource Control (RRC); Protocol specification, version 17.3.0, and 3GPP TS 38.321 , NR; Medium Access Control (MAC) protocol specification, version 17.3.0 may be referred to for examples useful for understanding the examples described herein.

[0034] Fig. 1 shows an example of communications system in accordance with at least some embodiments described herein. [0035] Figure 1 shows user devices 100 and 102 configured to be in a wireless connection on one or more communication channels in a cell with an access node (such as (e/g)NodeB) 104 providing the cell. The physical link from a user device to a (e/g)NodeB is called uplink (UL) or reverse link and the physical link from the (e/g)NodeB to the user device is called downlink (DL) or forward link. It should be appreciated that (e/g)NodeBs or their functionalities may be implemented by using any node, host, server or access point etc. entity suitable for such a usage.

[0036] A communication system typically comprises more than one (e/g)NodeB in which case the (e/g)NodeBs may also be configured to communicate with one another over links, wired or wireless, designed for the purpose. These links may be used for signaling purposes. The (e/g)NodeB is a computing device configured to control the radio resources of communication system it is coupled to. The NodeB may also be referred to as a base station, an access point or any other type of interfacing device including a relay station capable of operating in a wireless environment. The (e/g)NodeB includes or is coupled to transceivers. From the transceivers of the (e/g)NodeB, a connection is provided to an antenna unit that establishes bi-directional radio links to user devices. The antenna unit may comprise a plurality of antennas or antenna elements. The (e/g)NodeB is further connected to core network 110 (CN or next generation core NGC). Depending on the system, the counterpart on the CN side can be a serving gateway (S-GW, routing and forwarding user data packets), packet data network gateway (P-GW), for providing connectivity of user devices (UEs) to external packet data networks, or mobile management entity (MME), etc. The CN may comprise network entities or nodes that may be referred to management entities. Examples of the network entities comprise at least an Access and Mobility Management Function (AMF).

[0037] The user device, also called a user equipment (UE), a user terminal, a terminal device, a wireless device, a mobile station (MS) etc., illustrates one type of an apparatus to which resources on the air interface are allocated and assigned, and thus any feature described herein with a user device may be implemented with a corresponding network apparatus, such as a relay node, an eNB, and an gNB. An example of such a relay node is a layer 3 relay (self-backhauling relay) towards the base station. [0038] The user device typically refers to a portable computing device that includes wireless mobile communication devices operating with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: a mobile station (mobile phone), smartphone, personal digital assistant (PDA), handset, device using a wireless modem (alarm or measurement device, etc.), laptop and/or touch screen computer, tablet, game console, notebook, and multimedia device. It should be appreciated that a user device may also be a nearly exclusive uplink only device, of which an example is a camera or video camera loading images or video clips to a network. A user device may also be a device having capability to operate in Internet of Things (loT) network which is a scenario in which objects are provided with the ability to transfer data over a network without requiring human-to-human or human-to-computer interaction. Accordingly, the user device may be an loT-device. The user device may also utilize cloud. In some applications, a user device may comprise a small portable device with radio parts (such as a watch, earphones or eyeglasses) and the computation is carried out in the cloud. The user device (or in some embodiments a layer 3 relay node) is configured to perform one or more of user equipment functionalities. The user device may also be called a subscriber unit, mobile station, remote terminal, access terminal, user terminal or user equipment (UE) just to mention but a few names or apparatuses.

[0039] Various techniques described herein may also be applied to a cyberphysical system (CPS) (a system of collaborating computational elements controlling physical entities). CPS may enable the implementation and exploitation of massive amounts of interconnected ICT devices (sensors, actuators, processors microcontrollers, etc.) embedded in physical objects at different locations. Mobile cyber physical systems, in which the physical system in question has inherent mobility, are a subcategory of cyber-physical systems. Examples of mobile physical systems include mobile robotics and electronics transported by humans or animals. [0040] Additionally, although the apparatuses have been depicted as single entities, different units, processors and/or memory units (not all shown in Fig. 1 ) may be implemented. [0041] 5G enables using multiple input - multiple output (MIMO) antennas, many more base stations or nodes than the LTE (a so-called small cell concept), including macro sites operating in co-operation with smaller stations and employing a variety of radio technologies depending on service needs, use cases and/or spectrum available. The access nodes of the radio network form transmission/reception (TX/Rx) points (TRPs), and the UEs are expected to access networks of at least partly overlapping multi-TRPs, such as macro-cells, small cells, pico-cells, femtocells, remote radio heads, relay nodes, etc. The access nodes may be provided with Massive MIMO antennas, i.e. very large antenna array consisting of e.g. tens or hundreds of antenna elements, implemented in a single antenna panel or in a plurality of antenna panels, capable of using a plurality of simultaneous radio beams for communication with the UE. The UEs may be provided with MIMO antennas having an antenna array consisting of plurality of antenna elements a.k.a. patches, implemented in a single antenna panel or in a plurality of antenna panels. Thus, the UE may access one TRP using one beam, one TRP using a plurality of beams, a plurality of TRPs using one (common) beam or a plurality of TRPs using a plurality of beams.

[0042] 5G mobile communications supports a wide range of use cases and related applications including video streaming, augmented reality, different ways of data sharing and various forms of machine type applications (such as (massive) machine-type communications (mMTC), including vehicular safety, different sensors and real-time control. 5G is expected to have multiple radio interfaces, namely below 6GHz, cmWave and mmWave, and also capable of being integrated with existing legacy radio access technologies, such as the LTE. Integration with the LTE may be implemented, at least in the early phase, as a system, where macro coverage is provided by the LTE and 5G radio interface access comes from small cells by aggregation to the LTE. In other words, 5G is planned to support both inter- RAT operability (such as LTE-5G) and inter-RI operability (inter-radio interface operability, such as below 6GHz - cmWave - mmWave). One of the concepts considered to be used in 5G networks is network slicing in which multiple independent and dedicated virtual sub-networks (network instances) may be created within the same infrastructure to run services that have different requirements on latency, reliability, throughput and mobility.

[0043] The current architecture in LTE networks is fully distributed in the radio and fully centralized in the core network. The low latency applications and services in 5G require to bring the content close to the radio which leads to local break out and multi-access edge computing (MEC). 5G enables analytics and knowledge generation to occur at the source of the data. This approach requires leveraging resources that may not be continuously connected to a network such as laptops, smartphones, tablets and sensors. MEC provides a distributed computing environment for application and service hosting. It also has the ability to store and process content in close proximity to cellular subscribers for faster response time. Edge computing covers a wide range of technologies such as wireless sensor networks, mobile data acquisition, mobile signature analysis, cooperative distributed peer-to-peer ad hoc networking and processing also classifiable as local cloud/fog computing and grid/mesh computing, dew computing, mobile edge computing, cloudlet, distributed data storage and retrieval, autonomic self-healing networks, remote cloud services, augmented and virtual reality, data caching, Internet of Things (massive connectivity and/or latency critical), critical communications (autonomous vehicles, traffic safety, real-time analytics, time-critical control, healthcare applications).

[0044] The communication system is also able to communicate with other networks, such as a public switched telephone network or the Internet 112, or utilize services provided by them. The communication network may also be able to support the usage of cloud services, for example at least part of core network operations may be carried out as a cloud service (this is depicted in Fig. 1 by “cloud” 114). The communication system may also comprise a central control entity, or a like, providing facilities for networks of different operators to cooperate for example in spectrum sharing.

[0045] Edge cloud may be brought into radio access network (RAN) by utilizing network function virtualization (NFV) and software defined networking (SDN). Using edge cloud may mean access node operations to be carried out, at least partly, in a server, host or node operationally coupled to a remote radio head or base station comprising radio parts. It is also possible that node operations will be distributed among a plurality of servers, nodes or hosts. Application of cloudRAN architecture enables RAN real time functions being carried out at the RAN side (e.g. in a distributed unit, DU) and non-real time functions being carried out in a centralized manner (e.g. in a centralized unit, CU 108).

[0046] It should also be understood that the distribution of labor between core network operations and base station operations may differ from that of the LTE or even be non-existent. Some other technology advancements probably to be used are Big Data and all-IP, which may change the way networks are being constructed and managed. 5G (or new radio, NR) networks are being designed to support multiple hierarchies, where MEC servers can be placed between the core and the base station or NodeB (gNB). It should be appreciated that MEC can be applied in 4G networks as well. The gNB is a next generation Node B (or, new Node B) supporting the 5G network (i.e., the NR).

[0047] 5G may also utilize non-terrestrial nodes 106, e.g. access nodes, to enhance or complement the coverage of 5G service, for example by providing backhauling, wireless access to wireless devices, service continuity for machine-to- machine (M2M) communication, service continuity for Internet of Things (loT) devices, service continuity for passengers on board of vehicles, ensuring service availability for critical communications and/or ensuring service availability for future railway/maritime/ aeronautical communications. The non-terrestrial nodes may have fixed positions with respect to the Earth surface or the non-terrestrial nodes may be mobile non-terrestrial nodes that may move with respect to the Earth surface. The non-terrestrial nodes may comprise satellites and/or High Altitude Platforms Stations (HAPSs). Satellite communication may utilize geostationary earth orbit (GEO) satellite systems, but also low earth orbit (LEO) satellite systems, in particular mega-constellations (systems in which hundreds of (nano)satellites are deployed). Each satellite in the mega-constellation may cover several satellite-enabled network entities that create on-ground cells. The on-ground cells may be created through an on-ground relay node 104 or by a gNB located on-ground or in a satellite.

[0048] A person skilled in the art appreciates that the depicted system is only an example of a part of a radio access system and in practice, the system may comprise a plurality of (e/g)NodeBs, the user device may have an access to a plurality of radio cells and the system may comprise also other apparatuses, such as physical layer relay nodes or other network elements, etc. At least one of the (e/g)NodeBs or may be a Home(e/g)NodeB.

[0049] Furthermore, the (e/g)NodeB or base station may also be split into: a radio unit (RU) comprising a radio transceiver (TRX), i.e. a transmitter (TX) and a receiver (RX); a distributed unit (DU) that may be used for the so-called Layer 1 (L1 ) processing and real-time Layer 2 (L2) processing; and a centralized unit (CU) or a central unit that may be used for non-real-time L2 and Layer 3 (L3) processing. Such a split may enable the centralization of CUs relative to the cell sites and DUs, whereas DUs may be more distributed and may even remain at cell sites. The CU and DU together may also be referred to as baseband or a baseband unit (BBU). The CU and DU may be connected by F1 interface. The RU and DU may also be comprised into a radio access point (RAP). Cloud computing platforms may also be used to run the CU or DU. The CU may run in a cloud computing platform (vCU, virtualized CU). In addition to the vCU, there may also be a virtualized DU (vDU) running in a cloud computing platform. Furthermore, there may also be a combination, where the DU may use so-called bare metal solutions, for example application-specific integrated circuit (ASIC) or customer-specific standard product (CSSP) system-on-a-chip (SoC) solutions. It should also be understood that the distribution of labor between the above-mentioned base station units, or different core network operations and base station operations, may differ.

[0050] Additionally, in a geographical area of a radio communication system a plurality of different kinds of radio cells as well as a plurality of radio cells may be provided. Radio cells may be macro cells (or umbrella cells) which are large cells, usually having a diameter of up to tens of kilometers, or smaller cells such as micro- , femto- or picocells, or so-called small cells. The (e/g)NodeBs of Fig. 1 may provide any kind of these cells. A cellular radio system may be implemented as a multilayer network including several kinds of cells. Typically, in multilayer networks, one access node provides one kind of a cell or cells, and thus a plurality of (e/g)NodeBs are required to provide such a network structure. [0051] Fig. 2 illustrates an example of a method in accordance with at least some embodiments. The method provides improved performance for LTM. The method may be performed by a base station distributed unit hosting a source cell for a lower layer triggered mobility.

[0052] Phase 202 comprises determining, at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell.

[0053] Phase 204 comprises determining, by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility.

[0054] Phase 206 comprises transmitting, by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0055] In an example in accordance with at least some embodiments, phase 202 comprises receiving, at the base station distributed unit, physical layer measurements, and determining, by the base station distributed unit, to change the user equipment from the source cell to the target cell based the on received physical layer measurements.

[0056] In an example in accordance with at least some embodiments, phase 202 comprises transmitting, by the base station distributed unit, based on the determined change, a request to a base station centralized unit for the CFRA resource for accessing the target cell by the user equipment.

[0057] In an example in accordance with at least some embodiments, phase 204 comprises receiving, by the base station distributed unit, the pool of CFRA resources reserved for the lower layer triggered mobility from a base station centralized unit; and transmitting, by the base station distributed unit to the base station centralized unit, information indicating the determined CFRA resource for accessing the target cell by the user equipment. [0058] In an example in accordance with at least some embodiments, phase 206 comprises that the determined CFRA resource is transmitted included to a cell change command.

[0059] In an example in accordance with at least some embodiments, phase 206 comprises receiving, at the base station distributed unit, information indicating a completion of a radio access procedure of the user equipment, and releasing, at the base station distributed unit, the CFRA resource based on the received information indicating a completion of a radio access procedure of the user equipment.

[0060] In an example in accordance with at least some embodiments, phase 206 comprises transmitting, by the base station distributed unit, information indicating the release of the CFRA resource to a base station node managing the pool of CFRA resources reserved for the lower layer triggered mobility.

[0061] In an example in accordance with at least some embodiments, phase 206 comprises that the information indicating the determined CFRA resource comprises at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index. An example, of the CFRA resource is Physical Random Access Channel (PRACH) preamble or a combination of PRACH preamble and PRACH Mask index.

[0062] Fig. 3 illustrates an example of a method in accordance with at least some embodiments. The method provides improved performance for LTM. The method may be performed by a base station distributed unit hosting a target cell for a lower layer triggered mobility.

[0063] Phase 302 comprises receiving, by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a contention free random access, CFRA, resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility.

[0064] Phase 304 comprises preparing, at the base station distributed unit, a random access channel based on the received information indicating the CFRA resource for changing the user equipment from the source cell to the target cell.

[0065] In an example in accordance with at least some embodiments, phase 304 comprises transmitting, by the base station distributed unit, based on a completion of a radio access procedure of the user equipment, a message for releasing the assigned CFRA resource to a base station centralized unit.

[0066] In an example in accordance with at least some embodiments, phase 302 comprises that the received information indicating the CFRA resource comprises at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index. An example, of the CFRA resource is PRACH preamble or a combination of PRACH preamble and PRACH Mask index. [0067] Fig. 4 illustrates an example of a method in accordance with at least some embodiments. The method provides improved performance for LTM. The method may be performed by a base station centralized unit.

[0068] Phase 402 comprises determining, by a base station centralized unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility.

[0069] Phase 404 comprises transmitting, by the base station centralized unit to a base station distributed unit of the target cell, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0070] In an example in accordance with at least some embodiments, phase 404 comprises receiving, by the base station centralized unit, a request for the CFRA resource for accessing the target cell by the user equipment.

[0071] In an example in accordance with at least some embodiments, phase 402 comprises receiving, by the base station centralized unit from one or more base station distributed units, information indicating one or more CFRA resources for lower layer triggered mobility, and generating, by the base station centralized unit, the pool of CFRA resources reserved for the lower layer triggered mobility based on the received information.

[0072] In an example in accordance with at least some embodiments, phase 402 comprises requesting, by the base station centralized unit from one or more base station distributed units, at configuration of the base station distributed units for the lower layer triggered mobility, information indicating one or more CFRA resources for lower layer triggered mobility.

[0073] In an example in accordance with at least some embodiments, phase 402 comprises transmitting, by the base station centralized unit, at configuration of the base station distributed units for the lower layer triggered mobility, the pool of CFRA resources reserved for the lower layer triggered mobility to base station distributed units.

[0074] In an example in accordance with at least some embodiments, phase 402 comprises transmitting, by the base station centralized unit, a message to one or more other base station distributed units for causing the one or more other base station distributed units to refrain from configuring the determined CFRA resource to other user equipment attempting to access the target cell.

[0075] In an example in accordance with at least some embodiments, phase 404 comprises receiving, at the base station centralized unit, information indicating a completion of a radio access procedure of the user equipment to the target cell, and releasing, at the base station centralized unit, the CFRA resource based on the received information indicating a completion of a radio access procedure of the user equipment.

[0076] In an example in accordance with at least some embodiments, phase 402 comprises allocating one or more physical radio access channel mask indices to each CFRA resource of the pool.

[0077] In an example in accordance with at least some embodiments, phase 404 comprises that the information indicating the determined CFRA resource comprises at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index. An example, of the CFRA resource is PRACH preamble or a combination of PRACH preamble and PRACH Mask index.

[0078] Fig. 5 illustrates an example of a sequence in accordance with at least some embodiments. The sequence provides configuring base station distributed units for lower layer triggered mobility. In the sequence, the CU may serve for assigning CFRA resources from a pool of CFRA resources. Accordingly, a base station node managing the pool of CFRA resources is the CU. Then, in connection with a cell switch of user equipment from a source cell to a target cell, a DU hosting the source cell may request from the CU a CFRA resource for the UE to access the target cell and the CU may assign the CFRA resource for the UE for changing the UE from the source cell to the target cell.

[0079] Phases 501 and 502 comprise transmitting an L3 measurement report from a UE to a centralized unit, CU, via a distributed unit, DU1 .

[0080] Phase 503 comprises determining, by the CU, to prepare one or more target cells, in this case a cell at distributed unit DU2, for L1/L2 inter-cell mobility. The determined target cell, cell2, at DU2 may be configured for a lower layer triggered mobility of the user equipment.

[0081] Phase 504 comprises the CU transmitting a UE Context Setup Request to the DU2. The UE Context Setup Request comprises a request for CFRA resources for LTM. In an example, the UE Context Setup Request may comprise a specific number of CFRA resources for LTM. The CU may deduce the required number of CFRA preambles based on current connected mode users enabled for LTM mobility and the estimated number of LTM mobility based on previous mobility history events. For example, the DU2 may allocate a specific number of CFRA preambles along with RACH resource configuration as part of LTM preparation. Additionally or alternatively, the DU2 can allocate one or more different PRACH Mask indices associated with each of the CFRA preambles. The number of CFRA preambles may be provided per cell or per each beam used for LTM. Examples of the beams comprise at least a Synchronization Signal Block beam and Channel Status Information beam. It should be noted that additionally, the CU may provide to the DU1 a source cell.

[0082] Phase 505 comprises the CU transmitting a UE Context Setup Response to the CU. The UE Context Setup Response comprises CFRA preambles for LTM for one or more cells of the DU2. In this case the UE Context Setup Response comprises CFRA preambles for cell2 of the DU2.

[0083] Phase 506 comprises the CU transmitting a UE Context Modification Request to the DU1 . The UE Context Modification Request comprises a request for CFRA resources for LTM. In an example, the UE Context Setup Modification Request may comprise a specific number of CFRA resources for LTM. The CU may deduce the required number of CFRA preambles based on current connected mode users enabled for LTM mobility and the estimated number of LTM mobility based on previous mobility history events. For example, the DU1 may allocate a specific number of CFRA preambles along with RACH resource configuration as part of LTM preparation. Additionally or alternatively, the DU1 can allocate one or more different PRACH Mask indices associated with each of the CFRA preambles. The number of CFRA preambles may be provided per cell or per each beam used for LTM. Examples of the beams comprise at least a Synchronization Signal Block beam and Channel Status Information beam. It should be noted that additionally, the CU may provide to the DU2 a source cell.

[0084] Phase 507 comprises the CU transmitting a UE Context Setup Response to the CU. The UE Context Setup Response comprises CFRA preambles for LTM for one or more cells of the DU1. In this case the UE Context Setup Response comprises CFRA preambles for celU of the DU1 .

[0085] Phase 508 comprises preparing, by the CU, a Radio Resource Control protocol (RRC) configuration.

[0086] Phases 509 comprises transmitting, by the CU, an RRC Reconfiguration message, to the UE via the DU1 . The RRC Reconfiguration message comprises the CFRA resources of celU and cell2 received in phases 505 and 507. Accordingly, the RRC Reconfiguration message provided to UE for LTM mobility includes the CFRA RACH configuration and CFRA-Pool for LTM which may be different from the CFRA pool broadcasted in each target cell.

[0087] It should be noted that, e.g. in phase 508, the base station centralized unit may assign each CFRA resource with index which will be referred to in the LTM switching command by the source cell, in this case celU . Alternatively or additionally, a PRACH Mask index is also assigned and indicated in an LTM switching command to the UE.

[0088] Phase 510 comprises transmitting, by the UE, an RRC Reconfiguration Complete message to the CU via the DU1 .

[0089] Fig. 6 illustrates an example of a sequence in accordance with at least some embodiments. The sequence provides improved performance for LTM. The sequence may be performed after base station distributed units have been configured for lower layer triggered mobility, for example as described with Fig. 5. The sequence is described with reference to the entities described with Fig. 5.

[0090] Phase 611 comprises the UE transmitting an L1 Measurement Report to the DU1.

[0091] Phase 612 comprises the DU1 determining to switch a serving cell of the UE. Accordingly, the DU1 determines to change the UE from the source cell, i.e. celU , at DU1 to a target cell, i.e. cell2, at DU2 based on the received L1 Measurement Report.

[0092] Phase 613 comprises the DU1 transmitting, based on determined switch, a request to the CU for CFRA resources for the UE to access the target cell, in this case cell2 at DU2. The request may comprise identifiers of the UE (UE ID) and the target cell.

[0093] Phase 614 comprises the CU determining a CFRA resource, e.g. Y1, from the pool based on the received request and transmitting the determined CFRA resource, e.g. Y1, for the UE to access the cell2 at DU2. In this way the determined CFRA resource may be assigned to the UE for LTM to cell2. In an example, phase 614 comprises the CU allocating a unique CFRA resource, e.g. a PRACH preamble or a combination of PRACH preamble and PRACH Mask index, in such way that there is no collision between LTM mobility from different source cells to same target cell, i.e., the CFRA resource will be reserved for the serving DU until the UE performs the cell switch.

[0094] Phase 615 comprises the CU transmitting to the DU2 information indicating the UE, e.g. UE ID, attempting a random access using the determined CFRA resource, e.g. Y1. In this way the determined CFRA resource, e.g. Y1, may be reserved at the DU2 for the UE.

[0095] Phase 616 comprises the DU2 transmitting an acknowledgement to the CU in response to the information received at phase 615.

[0096] Phase 617 comprises the DU1 transmitting to the UE an LTM switching command comprising a Medium Access Control (MAC) Control Element (CE) comprising the determined CFRA resource, e.g. Y1. Alternatively or additionally, a PRACH Mask index is also assigned and indicated in the LTM switching command to the UE. It should be note that the LTM switching command may be transmitted by the DU1 based on the DU1 determining that the CU has informed the DU2 that is hosting the target cell of the assigned CFRA resource to the UE. The DU1 may determine that the CU has informed the DU2 about the assigned CFRA resource to the UE based on receiving the determined CFRA in phase 614.

[0097] Phase 618 comprises the UE performing a random access procedure to cell2 based on the determined CFRA resource, e.g. Y1.

[0098] Phase 619 comprises the DU2 activating UE context of the UE at DU2.

[0099] Phase 620 comprises the DU2 transmitting a Random Access Response (RAR) to the UE.

[0100] Phase 621 comprises the UE transmitting an RRC reconfiguration Complete message to the CU via the DU2. The CU may release the CFRA resource assigned to the UE based on the RRC reconfiguration Complete. In this way the assigned CFRA may be released back to the pool, where it can be assigned again for LTM. It should be noted that in phase 621 , the CU may determine a successful completion of a random access procedure of the UE based on the RRC reconfiguration Complete message. Alternatives to the RRC reconfiguration Complete message comprise at least one of the following: the CU allocating an UL grant to UE in response to reception of the CFRA preamble; or the CU receiving transmission by the UE over the allocated UL grant.

[0101] It should be noted that, alternatively or additionally, the target cell, or the DU2 hosting the target cell, may transmit information indicating a successful completion of the random access procedure of the UE to the CU, for example at phase 619.

[0102] Fig. 7 illustrates an example of a sequence in accordance with at least some embodiments. The sequence provides configuring base station distributed units for lower layer triggered mobility. CU may provide each DU a pool of CFRA resources in connection with preparing the DUs for LTM, whereby each DU may serve for assigning CFRA resources from the pool. Accordingly, base station nodes, here the base station distributed units, may manage each their own pool of CFRA resources. Then, in connection with a cell switch of user equipment from a source cell to a target cell, a DU hosting the source cell may assign a CFRA resource, e.g. Y2, for changing the UE from the source cell to the target cell and inform the selected CFRA resource to the CU. The CU may inform other prepared cells about the unavailability, or reservation, of the selected CFRA resource, e.g. Y2, whereby the other prepared cells may refrain from selecting the same selected CFRA resource, e.g. Y2, when attempting to handover another UE to the same target cell.

[0103] Phases 701 and 702 comprise transmitting an L3 measurement report from user equipment to a centralized unit, CU, via a distributed unit, DU1 .

[0104] Phase 703 comprises determining, by the CU, to prepare one or more target cells, in this case cells at distributed units DU1 and DU2, for L1/L2 inter-cell mobility. The determined target cells at DU2 and DU3 may be configured for a lower layer triggered mobility of the user equipment from a source cell to a target cell.

[0105] Phase 704 comprises that the target cell at DU2 may be configured for a lower layer triggered mobility of the user equipment from a source cell to a target cell.

[0106] Phase 704 comprises the CU transmitting a UE Context Setup Request to DU2. The UE Context Modification Request comprises CFRA resources to be used for LTM with respect to each other prepared target cell. The each other prepared target cell comprise cells that are hosted by the same or different DU. For example, the CFRA resources received at the DU2 may comprise CFRA preambles allocated at the prepared target cells for LTM. Additionally or alternatively, the CFRA resources received at the DU2 may comprise one or more different PRACH Mask indices allocated by the CU associated with each of the allocated CFRA preambles. In this way the DU2 may assign the UE a CFRA resource comprising a PRACH preamble and a PRACH Mask index for LTM to a target cell even if the PRACH preamble would be reserved for LTM of another UE.

[0107] Phase 705 comprises the DU2 transmitting a UE Context Setup Response to the CU.

[0108] Phase 706 comprises the CU transmitting a UE Context Modification Request to one or more other DUs. The UE Context Modification Request comprises CFRA resources to be used for LTM with respect to each other prepared target cell. For example, the CFRA resources received at the one or more other DUs may comprise CFRA preambles allocated at the prepared target cells for LTM. Additionally or alternatively, the CFRA resources received at the one or more other Dlls may comprise one or more different PRACH Mask indices allocated associated with each of the allocated CFRA preambles. The PRACH mask indices may be used by the one or more other Dlls as described for DU2 in phase 704.

[0109] Phase 707 comprises the one or more other Dlls transmitting UE Context Modification Responses to the CU.

[0110] Phase 708 comprises the CU transmitting a UE Context Modification Request to the DU1. The UE Context Modification Request comprises CFRA resources to be used for LTM with respect to each other prepared target cell. For example, the CFRA resources received at the DU1 may comprise CFRA preambles allocated at the prepared target cells for LTM. Additionally or alternatively, the CFRA resources received at the DU1 may comprise one or more different PRACH Mask indices allocated by the CU associated with each of the allocated CFRA preambles. In this way the DU1 is provided information for assigning the UE a CFRA resource comprising a PRACH preamble and a PRACH Mask index for LTM to a target cell even if the PRACH preamble would be reserved for LTM of another UE.

[0111] Phase 709 comprises the DU1 transmitting a UE Context Modification Response to the CU.

[0112] Phase 710 comprises preparing, by the CU, a Radio Resource Control protocol (RRC) configuration.

[0113] Phase 711 comprises transmitting, by the CU, an RRC Reconfiguration message, to the UE via the DU1 . The RRC Reconfiguration message comprises the CFRA resources of target cells.

[0114] Phase 712 comprises transmitting, by the UE, an RRC Reconfiguration Complete message to the CU via the DU1 .

[0115] Fig. 8 illustrates an example of a sequence in accordance with at least some embodiments. The sequence provides improved performance for LTM. The sequence may be performed after distributed units have been configured for lower layer triggered mobility, for example as described with Fig. 5

[0116] Phase 813 comprises the DU1 determining to change the UE from the source cell at DU1 to a target cell at DU2 based on the L1 Measurement Report. The DU1 may determine a CFRA resource of cell2 from the pool. [0117] Phase 814 comprises DU1 determining to change the UE from the source cell, i.e. celll , at DU1 to a target cell, i.e. cell2, at DU2 based on the L1 Measurement Report. The DU1 further assigns a CFRA resource, Y2, to the UE.

[0118] Phase 815 comprises the DU1 transmitting to the CU information indicating the UE, e.g. UE ID, attempting a random access using the assigned CFRA resource, Y2, to cel I2. In this way the CFRA resource, Y2, may be reserved for the UE.

[0119] Phase 816 comprises the CU transmitting to the DU2 information indicating the UE, e.g. UE ID, attempting a random access using the assigned CFRA resource. In this way the CFRA resource, Y2, may be reserved at the DU2 for the UE.

[0120] Phase 817 comprises the DU2 transmitting an acknowledgement to the CU. In this way the CU is indicated that the assigned CFRA resource, Y2, is reserved.

[0121] Phase 818 comprises the CU transmitting a message to one or more other DUs for causing the one or more other DUs to refrain from configuring the assigned/determined CFRA resource, Y2, to other UE attempting to switch to cell2. In an example, the CU may transmit the one or more other CUs information indicating the CFRA resource and cell2. In this way use of the assigned CFRA for RACH at cells of the one or more other DUs may be prevented.

[0122] Phase 819 comprises the one or more other DUs transmitting acknowledgements to the CU. In this way the CU is indicated that the one or more other DUs refrain from using the assigned CFRA resource, Y2.

[0123] Phase 820 comprises the CU transmitting an acknowledgement to the DU1. In this way the DU1 is indicated that the assigned CFRA resource, Y2, has been reserved at DU2 for the UE.

[0124] Phase 821 comprises the DU1 transmitting to the UE the LTM switching command comprising a Medium Access Control (MAC) Control Element (CE) comprising the assigned CFRA resource, Y2. Alternatively or additionally, a PRACH Mask index is also assigned and indicated in the LTM switching command to the UE. It should be note that the LTM switching command may be transmitted by the DU1 based on the DU1 determining that the CU has informed the DU2 that is hosting the target cell of the assigned CFRA resource to the UE. The DU1 may determine that the CU has informed the DU2 about the assigned CFRA resource to the UE based on receiving the acknowledgement in phase 820.

[0125] Phase 822 comprises the UE performing a random access procedure to cell2 based on the assigned CFRA resource, Y2.

[0126] Phase 823 comprises the DU2 activating UE context of the UE at DU2.

[0127] Phase 824 comprises the DU2 transmitting a Random Access Response (RAR) to the UE.

[0128] Phase 825 comprises the UE transmitting an RRC Reconfiguration Complete message to the CU via the DU2. It should be noted that in phase 825, the CU may determine a successful completion of a random access procedure of the UE based on the RRC reconfiguration Complete message. Alternatives to the RRC reconfiguration Complete message comprise at least one of the following: the CU allocating an UL grant to UE in response to reception of the CFRA preamble; or the CU receiving transmission by the UE over the allocated UL grant.

[0129] It should be noted that, alternatively or additionally, the target cell, or the DU2 hosting the target cell, may transmit information indicating a successful completion of the random access procedure of the UE to the CU, for example in connection with phase 823.

[0130] Fig. 9 illustrates an example of an apparatus in accordance with at least some embodiments. The apparatus may be a radio access network node, for example base station or base station distributed or base station centralized unit. The apparatus comprises at least one processor 902 and at least one transceiver 904. The processor is operatively connected to the transceiver for controlling the transceiver. The apparatus may comprise at least one memory 906. The memory may be operatively connected to the processor. It should be appreciated that the memory may be a separate memory or included to the processor and/or the transceiver.

[0131] In an example, the apparatus of Fig. 9 comprises a CFRA module 908 connected operatively to the processor. The CFRA module may be configured to cause one or more functionalities described in one or more examples described herein, comprising at least the following: a. determining, at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell; determining, by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility; transmitting, by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell; or b. receiving, by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a contention free random access, CFRA, resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility; preparing, at the base station distributed unit, a random access channel based on the received information indicating the CFRA resource for changing the user equipment from the source cell to the target cell; or c. determining, by a base station centralized unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility; transmitting, by the base station centralized unit to a base station distributed unit of the target cell, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

[0132] An embodiment comprises a computer program comprising computer readable program code means adapted to perform at least the following:

[0133] An embodiment comprises a computer program comprising computer readable program code means adapted to perform at least the following:

- receiving, by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a CFRA resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility;

[0134] An embodiment comprises a computer program comprising computer readable program code means adapted to perform at least the following:

[0135] An embodiment comprises a non-transitory computer readable medium comprising program instructions for causing a network entity to perform at least the following:

- determining, at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell; - determining, by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility;

[0136] An embodiment comprises a non-transitory computer readable medium comprising program instructions for causing a network entity to perform at least the following:

[0137] An embodiment comprises a non-transitory computer readable medium comprising program instructions for causing a network entity to perform at least the following:

[0138] Embodiments may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. The software, application logic and/or hardware may reside on memory, or any computer media. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a “memory” or “computer-readable medium” may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

[0139] Reference to, where relevant, “computer-readable storage medium”, “computer program product”, “tangibly embodied computer program” etc., or a “processor” or “processing circuitry” etc. should be understood to encompass not only computers having differing architectures such as single/multi-processor architectures and sequencers/parallel architectures, but also specialized circuits such as field programmable gate arrays FPGA, application specify circuits ASIC, signal processing devices and other devices. References to computer readable program code means, computer program, computer instructions, program instructions, instructions, computer code etc. should be understood to express software for a programmable processor firmware such as the programmable content of a hardware device as instructions for a processor or configured or configuration settings for a fixed function device, gate array, programmable logic device, etc.

[0140] In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits or any combination thereof. While various aspects of the invention may be illustrated and described as block diagrams or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0141] Embodiments of the inventions may be practiced in various components such as integrated circuit modules, field-programmable gate arrays (FPGA), application specific integrated circuits (ASIC), microcontrollers, microprocessors, a combination of such modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

[0142] Programs, such as those provided by Synopsys, Inc. of Mountain View, California and Cadence Design, of San Jose, California automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or “fab” for fabrication.

[0143] As used in this application, the term “circuitry” may refer to one or more or all of the following:

(a) hardware-only circuit implementations (such as implementations in only analogue and/or digital circuitry) and

(b) combinations of hardware circuits and software, such as (as applicable):

(i) a combination of analogue and/or digital hardware circuit(s) with software/firmware and

(ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and

© hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

[0144] This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its he (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. [0145] The foregoing description has provided by way of exemplary and nonlimiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

List of Acronyms

AMF Access and Mobility Management Function BLER Block Error Rate CE Control Element CFRA Contention Free Random Access CN Core Network CPS Cyber-Physical System CU Centralized Unit DL Downlink DU Distributed unit GEO Geostationary Earth Orbit HAPS High Altitude Platforms Stations loT Internet of Things IP Internet Protocol LEO Low Earth Orbit LTE Long Term Evolution LTM Lower Layer triggered Mobility M2M Machine-to-Machine MAC Medium Access Control MEC Multi-Access Edge Computing MIMO Multiple Input - Multiple output MME Mobile Management Entity mMTP massive Machine-Type Communications MS Mobile Station MTP Machine-Type Communications NFV Network Function Virtualization NGC Next Generation Core NR New Radio PDA Personal Digital assistant PDCP Packet Data Convergence protocol P-GW Packet Data Network Gateway PHY Physical Layer PRACH Physical Random Access Channel RAN Radio Access Network RF Radio Frequency RRC Radio Resource Control RU Radio Unit SDN Software Defined Networking S-GW Serving Gateway SIM Subscriber Identification Module TDD Time Division Duplex TRP Transmission/Reception Point TRX Transmission/Reception UE User Equipment UL Uplink

Claims

1 . An apparatus comprising one or more processors, and a memory storing instructions characterized in that, when executed by the one or more processors, the apparatus is caused to:

- determine (202), at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell;

- determine (204), by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, the CFRA resource comprising at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; and

- transmit (206), by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

2. The apparatus according to claim 1 , caused to:

- receive, at the base station distributed unit, physical layer measurements; and

- determine, by the base station distributed unit, to change the user equipment from the source cell to the target cell based on the received physical layer measurements.

3. The apparatus according to claim 1 or 2, caused to:

- transmit, by the base station distributed unit, based on the determined change, a request to a base station centralized unit for the CFRA resource for accessing the target cell by the user equipment.

4. The apparatus according to claim 1 or 2, caused to: - receive, by the base station distributed unit, the pool of CFRA resources reserved for the lower layer triggered mobility from a base station centralized unit; and

- transmit, by the base station distributed unit to the base station centralized unit, information indicating the determined CFRA resource for accessing the target cell by the user equipment.

5. The apparatus according to any of claims 1 to 4, caused to:

- receive, at the base station distributed unit, information indicating a completion of a radio access procedure of the user equipment; and

- release, at the base station distributed unit, the CFRA resource based on the received information indicating a completion of a radio access procedure of the user equipment.

6. The apparatus according to claim 5, caused to:

- transmit, by the base station distributed unit, information indicating the release of the CFRA resource to a base station node managing the pool of CFRA resources reserved for the lower layer triggered mobility.

7. The apparatus according to any of claims 1 to 6, wherein the determined CFRA resource is transmitted included to a cell change command.

8. The apparatus according to any of claims 1 to 7, caused to:

- transmit, by the base station distributed unit to the user equipment, a message, for example a Medium Access Control, MAC, Control Element, CE, or downlink control information, DCI, for triggering the change of the user equipment from the source cell to the target cell.

9. An apparatus comprising one or more processors, and a memory storing instructions characterized in that, when executed by the one or more processors, the apparatus is caused to: - receive (302), by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a contention free random access, CFRA, resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, the CFRA resource comprising at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; and

- prepare (304), at the base station distributed unit, a random access channel based on the received information indicating the CFRA resource for changing the user equipment from the source cell to the target cell.

10. The apparatus according to claim 9, caused to:

- transmit, by the base station distributed unit, based on a completion of a radio access procedure of the user equipment, a message to a for releasing the assigned CFRA resource to a centralized unit.

11 .An apparatus comprising one or more processors, and a memory storing instructions characterized in that, when executed by the one or more processors, the apparatus is caused to:

- determine (402), by a base station centralized unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, the CFRA resource comprising at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; and

- transmit (404), by the base station centralized unit to a base station distributed unit of the target cell, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

12. The apparatus according to claim 11 caused to:

- receive, by the base station centralized unit, a request for the CFRA resource for accessing the target cell by the user equipment.

13. The apparatus according to claim 11 or 12 caused to:

- receive, by the base station centralized unit from one or more base station distributed units, information indicating one or more CFRA resources for lower layer triggered mobility;

- generate, by the base station centralized unit, the pool of CFRA resources reserved for the lower layer triggered mobility based on the received information.

14. The apparatus according to any of claims 11 to 13, caused to:

- request, by the base station centralized unit from one or more base station distributed units, at configuration of the base station distributed units for the lower layer triggered mobility, information indicating one or more CFRA resources for lower layer triggered mobility.

15. The apparatus according to any of claims 11 to 14, caused to:

- transmit, by the base station centralized unit, at configuration of the base station distributed units for the lower layer triggered mobility, the pool of CFRA resources reserved for the lower layer triggered mobility to base station distributed units.

16. The apparatus according to any of claims 11 to 15, caused to:

- transmit, by the base station centralized unit, a message to one or more other base station distributed units for causing the one or more other base station distributed units to refrain from configuring the determined CFRA resource to other user equipment attempting to access the target cell.

17. The apparatus according to any of claims 11 to 16, caused to:

- receive, at the base station centralized unit, information indicating a completion of a radio access procedure of the user equipment to the target cell; and

- release, at the base station centralized unit, the CFRA resource based on the received information indicating a completion of a radio access procedure of the user equipment.

18. The apparatus according to any of claims 11 to 17, caused to:

- allocate, by the base station centralized unit, one or more physical radio access channel mask indices to each CFRA resource of the pool.

19. The apparatus according to any of claims 11 to 18 wherein the information indicating the determined CFRA resource comprises at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index.

20. A method comprising:

- determining (202), at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell;

- determining (204), by the base station distributed unit, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, the CFRA resource comprising at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; and

- transmitting (206), by the base station distributed unit to the user equipment, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

21 .A method comprising:

- receiving (302), by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a contention free random access, CFRA, resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, the CFRA resource comprising at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; and

- preparing (304), at the base station distributed unit, a random access channel based on the received information indicating the CFRA resource for changing the user equipment from the source cell to the target cell.

22. A method comprising:

- determining (402), by a base station centralized unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, a contention free random access, CFRA, resource for accessing the target cell by the user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, the CFRA resource comprising at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; and

- transmitting (404), by the base station centralized unit to a base station distributed unit of the target cell, information indicating the determined CFRA resource for changing the user equipment from the source cell to the target cell.

23. A computer program comprising computer readable program code means adapted to perform at least the following: - determining (202), at a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, to change the user equipment from the source cell to the target cell;

24. A computer program comprising computer readable program code means adapted to perform at least the following:

- receiving (302), by a base station distributed unit configured for a lower layer triggered mobility of a user equipment from a source cell to a target cell, information indicating a CFRA resource assigned to a user equipment from a pool of CFRA resources reserved for the lower layer triggered mobility, the CFRA resource comprising at least one of the following: a CFRA preamble; or a combination of a CFRA preamble and a physical radio access channel mask index; and

25. A computer program comprising computer readable program code means adapted to perform at least the following:

26. An apparatus comprising one or more processors, and a memory storing instructions characterized in that, when executed by the one or more processors, the apparatus is caused to:

- receive (617), from a source cell, a medium access control, MAC, control element, MAC-CE, indicating a contention free random access, CFRA, resource for accessing a target cell; and

- perform (618) a random access procedure to access the target cell on the indicated CFRA resource.