WO2022012734A1

WO2022012734A1 - Conditional handover for a multi-ue device

Info

Publication number: WO2022012734A1
Application number: PCT/EP2020/069735
Authority: WO
Inventors: Ahmad AWADA; Ingo Viering; Rakash SIVASIVA GANESAN
Original assignee: Nokia Technologies Oy
Priority date: 2020-07-13
Filing date: 2020-07-13
Publication date: 2022-01-20

Abstract

The present subject matter relates to user equipment for a multi-user equipment device, the user equipment comprising means configured for: receiving, from a source base station, a message for a conditional handover of the user equipment. The message comprises first information indicative of a condition for the conditional handover to a target base station. The message comprises second information indicative of handover periods of the user equipment. In case the condition is satisfied at a time instant of a current handover period of the handover periods, a message may be sent to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods.

Description

DESCRIPTION

CONDITIONAL HANDOVER FOR A MULTI-UE DEVICE

Technical Field

Various example embodiments relate to telecommunication systems, and more particularly to conditional handover for multi-UE devices.

Background

5G refers to a new generation of radio systems and network architecture. 5G is expected to provide higher bitrates and coverage than the current long term evolution (LTE) systems. However, current standard and product development for 5G new radio ultra-reliable and low-latency communications (5G NR URLLC), and specifically for factory automation use cases, require a low latency and low jitter to the URLLC services.

Summary

Example embodiments provide a user equipment for a multi-user equipment (multi- UE) device. The user equipment comprises means configured for: receiving, from a source base station, a message for a conditional handover of the user equipment. The message comprises first information indicative of a condition for the conditional handover to at least one target base station. The message comprises second information indicative of handover periods of the user equipment. The means is configured for: determining whether the condition is satisfied for a base station of the at least one base station, and in case the condition is satisfied at a time instant of a current handover period of the handover periods, sending a message to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods. According to further example embodiments, a method used in a user equipment is provided. The method comprises: receiving, from a source base station, a message for a conditional handover of the user equipment. The message comprises first information indicative of a condition for a conditional handover to a target base station. The message comprises second information indicative of handover periods of the user equipment. The method comprises: determining whether the condition is satisfied, and in case the condition is satisfied during a current handover period of the handover periods, sending a message to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods.

According to further example embodiments, a computer program is provided. The computer program comprises instructions for causing the user equipment for performing at least the following: receiving, from a source base station, a message for a conditional handover of the user equipment. The message comprises first information indicative of a condition for a conditional handover to a target base station. The message comprises second information indicative of handover periods of the user equipment. The user equipment is further caused to perform: determining whether the condition is satisfied, and in case the condition is satisfied during a current handover period of the handover periods, sending a message to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods.

According to further example embodiments, a base station is provided. The base station comprises means configured for: sending to a user equipment of a multi-user equipment device a message for a conditional handover of the user equipment. The message comprising first information indicative of a condition for a conditional handover to at least one target base station. The message comprises second information indicative of handover periods assigned to the user equipment.

According to further example embodiments, a method used in a base station is provided. The method comprises sending to a user equipment of a multi-user equipment device a message for a conditional handover of the user equipment. The message comprises first information indicative of a condition for a conditional handover to at least one target base station. The message comprises second information indicative of handover periods assigned to the user equipment.

According to further example embodiments, a computer program is provided. The computer program comprises instructions for causing the base station for performing at least the following: sending to a user equipment of a multi-user equipment device a message for a conditional handover of the user equipment. The message comprises first information indicative of a condition for a conditional handover to at least one target base station. The message comprises second information indicative of handover periods assigned to the user equipment.

According to further example embodiments, a multi-user equipment device is provided. The multi-user equipment device comprises at least a first user equipment and a second user equipment. The first user equipment comprises means configured for: receiving, from a first source base station, a message for a conditional handover of the first user equipment, the message comprising first information indicative of a condition for the conditional handover to a first target base station, the message comprising second information indicative of handover periods of the first user equipment, determining whether the condition is satisfied, and in case the condition is satisfied at a time instant of a current handover period of the handover periods, sending a message to the first target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods. The second user equipment comprises means configured for: receiving, from a second source base station, a message for a conditional handover of the second user equipment, the message comprising first information indicative of a condition for the conditional handover to a second target base station, the message comprising second information indicative of handover periods of the second user equipment, determining whether the condition is satisfied, and in case the condition is satisfied at a time instant of a current handover period of the handover periods of the second user equipment, sending a message to the second target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods of the second user equipment. The first user equipment, the first source base station and the first target base station belong to a first reliability group. The second user equipment, the second source base station and the second target base station belong to a second reliability group.

The terms “first” and “second” as used herein, are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical) unless explicitly defined as such.

Brief Description of the Drawings

The accompanying figures are included to provide a further understanding of examples, and are incorporated in and constitute part of this specification. In the figures:

FIG.1 illustrates a part of an exemplifying radio access network;

FIG. 2 is a schematic illustration of a wireless communication system;

FIG. 3 is a flowchart of a method used in a user equipment according to an example of the present subject matter;

FIG. 4A illustrates an example of messages exchanged between user equipments of a multi-user equipment device and the base stations for a conditional handover of the user equipments according to an example of the present subject matter;

FIG. 4B is a diagram of handover periods assigned to user equipments of a multi-user equipment device according to an example of the present subject matter;

FIG. 4C is a diagram of handover periods assigned to user equipments of a multi-user equipment device according to an example of the present subject matter;

FIG. 5 is a flowchart of a method used in a user equipment (UE) according to an example of the present subject matter; FIG. 6 is a flow diagram of a signaling method for a conditional handover in accordance with an example of the present subject matter;

FIG. 7 is a flow diagram of a signaling method for a conditional handover in accordance with an example of the present subject matter;

FIG. 8 is a block diagram showing an example of an apparatus according to an example of the present subject matter.

Detailed Description

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc., in order to provide a thorough understanding of the examples. However, it will be apparent to those skilled in the art that the disclosed subject matter may be practiced in other illustrative examples that depart from these specific details. In some instances, detailed descriptions of well-known devices and/or methods are omitted so as not to obscure the description with unnecessary detail.

A communication system may be provided. The communication system comprises base stations, wherein each base station of the base stations serves user equipments located within the base station’s geographical area of service or a cell. The base station and its coverage area may collectively be referred to as a cell. The communication system may support one or more radio access technologies (RATs). A radio access technology of the radio access technologies may, for example, be evolved universal terrestrial radio access (E-UTRA) or 5G new radio (NR), but it is not limited to, as a person skilled in the art may apply the present subject matter to other communication systems provided with necessary properties.

The communication system may support a redundant transmission between devices and base stations of the communication system. In order to ensure the redundant transmission, a device may need to be served by two or more base stations of the communication system. For that, the present multi-user equipment device is provided with multiple user equipments. The multi-user equipment device may be equipped with N user equipments UE₁, UE₂ ... UE_N, where N ≥ 2. The multi-user equipment device may use the user equipments UE₁, UE₂ ... UE_N to carry traffic going between the multi- user equipment device and the base stations. The user equipments of the multi-user equipment device may be connected to multiple base stations of the communication system operating at different carrier frequencies. For example, the multi-user equipment device (with N = 2) may connect to one or more cells of the communication system using user equipment UE₁ of the multi-user equipment device and connect to one or more cells of the communication system using another user equipment UE₂ of the multi-user equipment device. The user equipments of the multi-user equipment device may independently setup protocol data unit (PDU) sessions to user plane functions (UPFs) with orthogonal failure modes. The user equipments of the multi-user equipment device may thus provide redundant communication links and in addition offer robustness against single point of failure.

The user equipments of the multi-user equipment device and the base stations of the communication system may be grouped as different reliability groups. Each reliability group of the reliability groups may include one user equipment of the multi-user equipment device and a distinct set of base stations of the communication system. A reliability group GRP_i, where i varies between 1 and N, may comprise a user equipment UE_i of the multi-user equipment device and a set of base stations , where j varies

between 1 and n₁, n₁ being the number of base stations of the reliability group GRP_i . The reliability groups may be pre-configured/static

groups or dynamically determined groups. Preconfigured grouping may assign the user equipments of the multi-user equipment device to different reliability groups. A dynamic determination or definition of the reliability groups may, for example, be enabled by a coordinated management of the user equipments. The user equipments connect with or are handed over to base stations having the same reliability group membership.

Each user equipment UE_i of the multi-user equipment device may be configured to perform cell measurement e.g. in connected mode, inactive mode or idle mode, of a cell associated with the user equipment UE_i. For example, the user equipment UE₁

may be configured to perform cell measurements of cells served by the set of base stations of the reliability group GRP₁. The user equipment UE₂ may be

configured to perform cell measurements of cells served by the set of base stations of the reliability group GRP₂ and so on. Measuring a cell may, for

example, be performed by measuring (e.g. a power) of at least one beam of the cell. The beam measurement results may be combined (e.g. averaged) to derive a signal quality or cell quality of the cell. The signal quality may, for example, be a reference signal received power (RSRP) or reference signal received quality (RSRQ) measurement.

The cell measurements may advantageously be used in accordance with the present subject matter in order to perform a conditional handover (CHO) procedure. For that, each user equipment of the multi-user equipment device may be configured to check whether an initial handover criterion is fulfilled using the cell measurements of a cell associated with the user equipment. The initial handover criterion may, for example, comprise conditions of a radio resource management (RRM) event A3. The fulfillment of the initial handover condition may trigger the user equipment to send a measurement report to a source base station that is a serving base station of the user equipment, wherein the source base station and the user equipment have the same reliability group membership. Based on the received measurement report, the source base station may prepare one or more target base stations for the conditional handover of the user equipment to one of the one or more target base stations. The one or more target base stations belong to the reliability group of the user equipment. In another example, the source base station may automatically prepare the one or more target base stations for the conditional handover of the user equipment to one of the one or more target base stations e.g. without relying on the measurement report. The preparation of the one or more target base stations may, for example, comprise an identification by the source base station of the one or more target base stations. The identified target base stations may, for example, be candidates targets of the user equipment. For example, the source base station that is serving the user

equipment UE₁ may prepare one or more target base stations of the base stations

of the reliability group GRP₁.

After preparing the one or more target base stations, the source base station may send a message (named herein as a CHO message) to the user equipment for the conditional handover to one target base station of the one or more target base stations. The CHO message may be provided by means of dedicated signalling e.g. using the radio resource control (RRC) Reconfiguration. The CHO message comprises information indicative of a condition (named herein as a CHO condition) for the conditional handover to each target base station of the one or more target base stations and the configuration (CHO command) that the user equipment shall apply for the target base station when its corresponding CHO condition is fulfilled. However, the source base station may not know about the time instant the CHO condition is fulfilled and thus may not know the time at which the synchronization procedure to the target base station may start at the user equipment. The synchronization procedure may, for example, be a random access procedure (PRACH) of LTE or 5G NR system. This may result in an uncolorable handover of the user equipments of the multi-user equipment device. The present subject matter may solve this issue. The present subject matter may coordinate the handover execution of the user equipments on the same device. For that, the source base station provides further configuration data to the user equipment. Specifically, the CHO message further comprises second information indicative of handover periods of the user equipment. The second information may, for example, comprise a handover period configuration. The handover period configuration may enable the user equipment to configure the start of its handover period, its length and periodicity. After sending the CHO message, the source base station may perform data forwarding to the target base station. The data forwarding comprises delivering buffered packets and in transit packets to the target base station. Moreover, the source base station sends a sequence number (SN) status transfer message to the target base station indicating the next missing packets in downlink and uplink. The in-transit packets may be packets that should be sent to the target base station but sent to the source base station for transit after handover before the path switch is completed. The data forwarding may be an early data forwarding or late data forwarding. The early data forwarding may refer to the data forwarding after the source base station configures the CHO and before the synchronization procedure starts. The late data forwarding may refer to the data forwarding after the user equipment has successfully accessed to the target base station and the source base station has received Handover Success message from the target base station.

A handover period of the user equipment is a period during which a handover execution phase may be performed. The handover execution phase may comprise the synchronization procedure. That is, the handover execution phase may enable the user equipment to detach from the source base station and synchronize with the target base station. In case of the late data forwarding, the handover execution phase may further comprise the data forwarding to the target base station. That is, the handover execution phase may further enable the source base station to perform data forwarding to the target base station and submission of the SN status transfer message.

During the handover execution phase, the user equipment may not receive user data from the target base station and may have outage. The outage for the multi-user equipment device may be prevented if the handover execution phase of one user equipment on the device completes within the handover period. For example, during the handover period, the user equipment may execute the synchronization procedure in order to access the target cell. The handover period may be configured long enough such that the execution of the synchronization procedure and possibly the time needed by the target base station to fetch the downlink data forwarded from the source base station, may be completed within the handover period if it is triggered early enough. The handover periods may be uniquely assigned to the user equipment. For example, each user equipment UE_i of the multi-user equipment device may be assigned by the respective source base station a distinct set HP_i of handover periods HP_i =

in order to perform the conditional handover. Assuming, for example, that the multi-user equipment device comprises two user equipments (i.e. N = 2) UE₁ and UE₂, the source base station of the reliability group GRP_i may assign the set of handover periods

of the user equipment UE₁ and the source base station of the reliability group GRP₂ may assign the set of handover periods HP₂ = of the user equipment UE₂. The set of handover periods HP1 and HP₂

are disjoint e.g. the handover periods may be provided in the following order followed by followed by

followed by ... Each handover period of the set of handover periods HP₁ is not overlapping with any handover period of the set of handover periods HP₁. Each handover period of the set of handover periods HP₂ is not overlapping with any handover period of the set of handover periods HP₂. The source base stations of different reliability groups may coordinate, e.g. using Xn interface, the configuration of the handover periods HP₁ and HP₂ of their corresponding user equipment on the same device. For example, the serving base station of one reliability group may share its handover period configuration to another serving base station of a different reliability group which in turn may share back as a response its handover period configuration.

Upon receiving the CHO message, the user equipment may determine whether the CHO condition is satisfied. The CHO condition may require that a first value indicative of a signal received from the target base station exceeds a second value indicative of a signal received from the source base station by at least an offset value for a time-to- trigger (TTT) interval. Each value of the first and second values may be a RSRP or RSRQ. The CHO condition may be checked separately, by the user equipment, for each target base station of the one or more target base stations. And in case the CHO condition is satisfied for a given target base station of the one or more target base stations at a time instant t_CHO and that time instant falls in a current handover period of the handover periods of the user equipment, the user equipment may stop exchange of data with the source base station and send a message (named synchronization message) to the target base station for the conditional handover. The synchronization message may be sent by the user equipment during a certain handover period of the handover periods of the user equipment. Thus, in contrast to a conventional handover procedure, the user equipment may not immediately initiate the synchronization procedure with a target base station upon receiving the CHO message because it has to check the CHO condition. Instead, the user equipment may initiate the synchronization procedure once the CHO condition is fulfilled.

For example, the user equipment UE₁ may receive the CHO message from the source base station of the same reliability group GRP₁. The CHO message may indicate, for example, the CHO condition for the conditional handover to target base stations and and may further indicate the set of handover periods .

Upon receiving the CHO message, the user equipment UE₁ may separately check the CHO condition for the target base station

and for the target base station

. For example, the user equipment UE₁ may check whether the RSRP of the target base station exceeds the RSRP of the source base station

by at least the offset value for the TTT interval. The user equipment UE₁ may further check whether the RSRP of the target base station

exceeds the RSRP of the source base station

by at least the offset value for the TTT interval. If the CHO condition is fulfilled for one of the two target base stations e.g.

, at a time instant t_CHO, the user equipment UE₁ may check whether the time instant t_CHO falls in a current handover period of the set of handover periods

. Assuming, for example, that the duration of each handover period of the set of handover periods HP₁ is 200 milliseconds, that the set of handover periods are spaced by 300 milliseconds and that the start time of the set of handover periods is t₀. Each handover period of the set of handover periods may be defined by the interval

. The start time of each handover period

may be and the end time of the handover period

is

. The user equipment UE₁ may check whether the time instant t_CHO falls in one of the intervals .

In case the time instant t_CHO falls, for example, in the handover period

, the user equipment UE₁ may send the synchronization message to the target base station

in a certain handover period . The user equipment UE₁ may immediately send the synchronization message in the current time period i.e.

or in the subsequent time period

i.e.

depending on additional configuration information sent by the source base station

. The synchronization message may be sent at the start of subsequent handover period or any time instant such that the synchronization procedure may be still completed within subsequent handover period.

The present subject matter may be advantageous for the following reasons. The timing coordination between the user equipments of the same device may ensure a lossless handover. The coordination of CHO periods may be performed among user equipments on a same device without the need for an interface between them. Even though user equipments decide autonomously the exact time of handover execution, simultaneous outage in the reliability groups (and thereby overall outage) may be avoided. In contrast to a conventional handover procedure, the CHO may provide additional mobility robustness by minimizing the number of radio link failures and handover failures, and thus preventing outages that may be relevant for internet of things (loT) applications. The present subject matter may, thus, enable an uninterrupted availability and high reliability of communications between devices and the base stations of the communication system. This may fulfil URLLC requirement during UE handover procedure.

Hence, the communication system may advantageously be used, for example, to implement a distributed automation application. The distributed automation application may, for example, enable a factory automation e.g. for motion control or mobile robots. In another example, the distributed automation application may enable a process automation e.g. for process monitoring.

According to one example, the second information is further indicative of a guard period TP_Gp, wherein each handover period of the handover periods of the user equipment is extended with the guard period. In this example, the certain handover period is the non-extended current handover period. The user equipment is configured to not send the synchronization message to the target base station during the guard period TP_GP. Following the above example, and in case the time instant t_CHO falls, for example, in the handover period , the user equipment UE₁ may send the

synchronization message to the target base station immediately in the current

handover period . That is, the non-extended current handover period is and

the extended current handover period is . This is because, the time between

the time instant t_CHO and the end of the guard period TP_GP may be sufficient to process the synchronization procedure and thus enable the user equipment UE₁ to access the target base station .

According to one example, the second information is further indicative of a time duration T. The user equipment may be configured to: in response to determining that the condition is satisfied during the current handover period, determine whether a difference Δ_t between the end of the current handover period and the time instant t_CHO is longer than the time duration T. The user equipment may send the message to the target base station in the current handover period in case the difference is longer than the time duration T. The user equipment may send the message to the target base station in a handover period of the handover periods that is subsequent to the current handover period in case the difference is shorter than the time duration T. In other words, the user equipment postpones or defers the submission of the message and thus defers the initiation of the synchronization procedure. This may result in postponing or deferring the whole CHO execution phase. Following the above example, the difference may be defined as follows , where

is the end

time of the current handover period . In case , the user

equipment UE₁ may immediately send the synchronization message in the current handover period . That is, the user equipment UE₁ may not postpone the initiation

of the synchronization procedure. In case

T, the user equipment UE₁ may send the synchronization message in the handover period , which is

subsequent to the current handover period . This may enable to postpone the

initiation of the synchronization procedure. According to an example, the user equipment is configured to report a time delay value to the target base station. The time delay value may be a difference between the time instant at which the condition is satisfied and a time instant at which the subsequent handover period starts.

The following is another example to avoid the postponing of the initiation of the synchronization procedure. The user equipment may modify the CHO condition such that the modified condition requires that the first value indicative of a signal received from the target base station exceeds the second value indicative of a signal received from the source base station by at least the offset value for a time interval smaller than the TTT interval and such that the difference Δ_t is longer than the time duration T, wherein the determining that the condition is satisfied comprises determining that the modified condition is satisfied. In other terms, the user equipment may prepone the initiation of the synchronization procedure by terminating earlier the TTT of the CHO condition. For instance, the CHO may be executed by the UE if the CHO condition has been fulfilled for at least percentage X of TTT and the remaining time till the end of the handover period is no longer enough to complete successful HO without terminating early TTT. The conditions on when to terminate TTT early (e.g. value of X) may be provided as well by the serving cell using RRC Reconfiguration containing the CHO message. This method may avoid that the UE defers the CHO execution to its next handover period at the expense of terminating early the TTT. The CHO execution as used herein comprises the execution of the synchronization procedure. In this example, the user equipment may further be configured to determine a time delay value as the difference between the time instant at which the (unmodified) CHO condition would be satisfied and a time instant at which the synchronization message is sent to the target base station.

According to an example, the means of the user equipment is configured to report to the target base station or to another apparatus of the communication system whether the user equipment has deferred or preponed the CHO execution phase. For example, the report may indicate the difference between the time instant at which the modified condition is satisfied and the time instant at which the (unmodified) CHO condition would be satisfied. For example, the user equipment may check both conditions in order to compute said difference.

According to an example, in case the CHO condition is satisfied at a time instant outside the handover periods of the user equipment, the synchronization message may be sent to the target base station during the handover period subsequent to said time instant.

According to an example, the user equipment is configured to report a time delay value to the target base station. The time delay value is a difference between the time instant at which the condition is satisfied or would be satisfied and a time instant at which the synchronization message is sent to the target base station. For example, the user equipment may log and report to the network whether it has preponed or postponed the execution of conditional handover and the amount of time that it has preponed or postponed the execution. The network may fetch this information from the user equipment at any time, or reported by the user equipment immediately to the target cell during random access or after establishing the connection. This information may be used by the network for e.g. optimizing the configuration of future handover periods or other mobility related parameters by a kind of self-organizing networks (SON) or machine-learning optimization methods.

According to one example, an apparatus (such as a base station) of the communication system may be configured to receive the time delay values from multiple user equipments of the communication system. The apparatus may be configured to process the received time delay values in order to determine the handover periods that can be assigned to user equipments of a same device. The processing of the time delay values may for example comprise training a machine learning algorithm using a training set, wherein the training set comprises the time delay values. The machine learning algorithm may be trained to predict the handover period configuration or the parameters of the CHO execution conditions for multiple user equipments of a same device. The trained machine learning algorithm may, for example, receive as input cell measurements and predict based on the received input the handover period configuration or the parameters of the CHO execution conditions for multiple user equipments of a same device.

According to an example, the message sent by the user equipment to the target base station may be a synchronization message. The synchronization message may, for example, be the Physical RACH preamble used to synchronize with the target base station.

According to an example, the source base station is configured for coordinating configuration of handover periods of user equipments of the multi-user equipment device with one or more source base stations serving other one or more other user equipments of the multi-user equipment device respectively, and for determining the handover periods assigned to the user equipment based on the coordination. The coordination may, for example, be performed such that the handover periods assigned to the user equipments of the multi-user equipment device are disjoint and not overlapping.

FIG.1 depicts examples of simplified system architectures only showing some elements and functional entities, all being logical units, whose implementation may differ from what is shown. The connections shown in FIG.1 are logical connections; the actual physical connections may be different. It is apparent to a person skilled in the art that the system typically comprises also other functions and structures than those shown in FIG.1 .

The embodiments are not, however, restricted to the system given as an example but a person skilled in the art may apply the solution to other communication systems provided with necessary properties.

The example of FIG.1 shows a part of an exemplifying radio access network.

FIG.1 shows devices 10 and 12. The devices 10 and 12 may, for example, be user devices. The devices 10 and 12 are configured to be in a wireless connection on one or more communication channels with a node 14. The node 14 is further connected to a core network 20. In one example, the node 14 may be an access node (such as (e/g)NodeB) 14 providing or serving devices in a cell. In one example, the node 14 may be a non-3GPP access node. The physical link from a device to a (e/g)NodeB is called uplink or reverse link and the physical link from the (e/g)NodeB to the device is called downlink 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.

A communications 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 devices. The antenna unit may comprise a plurality of antennas or antenna elements. The (e/g)NodeB is further connected to the core network 20 (CN or next generation core NGC). For example, the (e/g)NodeB may connect to an access and mobility management function (AMF) and user plane function (UPF) in the control plane and user plane, respectively. 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 devices (UEs) to external packet data networks, or mobile management entity (MME), etc.

The device (also called user device, UE, user equipment, user terminal, terminal device, 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 device may be implemented with a corresponding apparatus, such as a relay node. An example of such a relay node is a layer 3 relay (self-backhauling relay) towards the base station.

The device typically refers to a device (e.g. a portable or non-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 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 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, e.g. to be used in smart power grids and connected vehicles. The device may also utilize cloud. In some applications, a device may comprise a user portable device with radio parts (such as a watch, earphones or eyeglasses) and the computation is carried out in the cloud. The device (or in some embodiments a layer 3 relay node) is configured to perform one or more of user equipment functionalities. The 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. Various techniques described herein may also be applied to a cyber-physical 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.

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. 5G enables using multiple input - multiple output (MIMO) antennas, many more base stations or nodes than an existing LTE system (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. 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 being integrable 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- Rl operability (inter-radio interface operability, such as below 6GHz - cmWave, 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. 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).

The communication system is also able to communicate with other networks, such as a public switched telephone network or the Internet as illustrated by the component referenced by reference numeral 22, 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” 24). 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.

The technology of Edge cloud may be brought into a radio access network (RAN) by utilizing network function virtualization (NVF) and software defined networking (SDN). Using the technology of 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 (in a distributed unit, DU 14) and non-real time functions being carried out in a centralized manner (in a centralized unit, CU 18).

It should also be understood that the distribution of labour 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 is 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.

5G may also utilize satellite communication to enhance or complement the coverage of 5G service, for example by providing backhauling. Possible use cases are providing service continuity for machine-to-machine (M2M) or Internet of Things (loT) devices or for passengers on board of vehicles, or ensuring service availability for critical communications, and future railway/maritime/aeronautical communications. 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 16 in the mega- constellation may cover several satellite-enabled network entities that create on- ground cells. The on-ground cells may be created via an on-ground relay node 14 or by a gNB located on-ground or in a satellite.

It is understandable for a person skilled in the art 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 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. One of the (e/g)NodeBs or may be a Home(e/g)nodeB. 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. 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.

For fulfilling the need for improving the deployment and performance of communication systems, the concept of “plug-and-play” (e/g)NodeBs has been introduced. Typically, a network which is able to use “plug-and-play” (e/g)Node Bs, includes, in addition to Flome (e/g)NodeBs (FI(e/g)nodeBs), a home node B gateway, or FINB-GW (not shown in FIG.1 ). A HNB Gateway (HNB-GW), which is typically installed within an operator’s network may aggregate traffic from a large number of HNBs back to a core network.

FIG. 2 is a schematic illustration of a wireless communication system 200. The communication system 200 may be configured to use a time division duplex (TDD) technique for data transmission.

For simplicity, communication system 200 is shown to include four base stations BS 1, BS2, BS 3 and BS4 and a multi-user equipment device 201. The multi-user equipment device 201 may, for example, be an automatic guided vehicle (AGV). The base stations BS 1, BS2, BS 3 and BS4 may, for example, be eNodeBs and/or gNBs e.g. as described with reference to FIG. 1 . That is, the communication system 200 may support a same RAT or different RATs.

Each of the base stations BS 1, BS2, BS 3 and BS4 may serve UEs within a respective geographical coverage area of service. The base station and its coverage area may collectively be referred to as a “cell”. The cells of the communication system 200 are labeled 202a, 202b, 202c and 202d.

In this particular example, the multi-user equipment device 201 is moving from the left to the right. The multi-user equipment device 201 comprises two user equipments UE1 and UE2. For example, no interface can be assumed among UE1 and UE2. For simplification of the description, the multi-user equipment device 201 is equipped with only two UEs but it is not limited to. Deploying the multi-user equipment device 201 with multiple UEs may enable to achieve a high reliability (e.g. of 99.999%) in industrial applications e.g. closed loop motion control.

Initially, the user equipment UE1 is associated with the cell 202a and the user equipment UE2 is associated with the cell 202b. The cell 202a may be a serving or source cell of the user equipment UE1. Accordingly, the base station BS 1 may be a serving or source base station of the user equipment UE1. And the base station BS2 may be a serving or source base station of the user equipment UE2.

In FIG. 2, the user equipment UE1, its source base station BS 1 and the base station BS 3 belong to a first reliability group GRP 1 and user equipment UE2, its source base station BS2 and the base station BS4 belong to a second reliability group GRP2. UEs of a specific reliability group can connect to base stations of that reliability group.

Each of the neighbor cells 202b through 202d may be on a same carrier or different carrier of the serving cell 202a.

While the multi-user equipment device 201 is moving from the left to the right, each of the two user equipments UE1 and UE2 is configured to perform neighbor cell measurements of neighbor cells and to perform at least part of the present subject matter.

FIG. 3 is a flowchart of a method used in a user equipment according to an example of the present subject matter. The user equipment may, for example, be one of multiple user equipments of a multi-user equipment device. For the purpose of explanation, the method described in FIG 3 may be implemented in the system illustrated in FIG. 2, but is not limited to this implementation.

The method starts at step 301 , where the user equipment of the multi-user equipment device 201 receives a message. In this example, the user equipment may be the user equipment UE1 of FIG. 2. The message may be named a CFIO message. The CFIO message is received from the source base station BS 1 of the user equipment UE1. In one first example, the CHO message may automatically be received by the user equipment UE1 from the source base station BS1.

In a second example, the CHO message may be received by the user equipment UE1 in response to sending by the user equipmentUE1 a measurement report to the source base station BS 1. The submission of the measurement report may be performed if an initial handover criterion is satisfied. The initial handover criterion may, for example, comprise conditions of a radio resource management (RRM) event A3.

The received CHO message may be a message for a conditional handover of the user equipment UE1. The CHO message comprises first information indicative of a CHO condition for the conditional handover to the target base station BS 3. The CHO message further comprises second information indicative of a handover configuration of the user equipment UE1. The handover configuration may indicate the start of handover period, its length and periodicity.

The source base station BS 1 may determine the handover configuration of the of the user equipment UE1 based on a handover configuration of the user equipment UE2. The handover configuration of the user equipment UE2 may be received by the source base station BS 1 from the source base station BS2. In another example, the source base station BS 1 may autonomously determine (e.g. using a trained machine learning model) the handover configuration of the user equipment UE1 e.g. if it does not receive the handover configuration of the user equipment UE2. The source base station BS1 may send the handover configuration of the user equipment UE1 to the source base station BS2.

For simplification purpose, the following illustrative example may be used to describe the method of FIG. 3: the handover period of the user equipment UE1 starts at System Frame Number (SFN) 0, has a duration of 200 ms, and a periodicity of 400 ms. That is, the handover periods of the user equipment UE1 may be the first 200 ms of every400 ms. The user equipment UE1 may be configured to perform the synchronization procedure during the handover period. Upon receiving the CHO message, the method then proceeds to step 303, where the user equipment UE1 may determine whether the CHO condition is satisfied. For that, the user equipment UE1 may compare a reference signal of the source cell 202a with a reference signal of the target cell 202c. For example, the user equipment UE1 may check in step 303 whether the RSRP of the reference signal received from the target base station BS 3 exceeds the RSRP of the reference signal received from the source base station BS 1 by at least the offset value for the TTT interval.

The check of the CHO condition may repeatedly be performed in step 303 until the CHO condition is fulfilled.

In case the CHO condition is satisfied, the method then proceeds to step 305, where the user equipment UE1 may determine whether the time instant t_CHO at which the CHO condition is satisfied falls in a current handover period of the handover periods of the user equipment UE1. Following the above example, a time instant t_CHO of (start time instant of SFN 0 + 300 ms) is not part of a current handover period of the user equipment UE1 because the upcoming handover period corresponds to (start time instant of SFN 0 + 400 ms). However, if the time instant t_CHO is (start time instant of SFN 0+ 500 ms), it is part of a current handover period of the user equipment UE1 because it falls in the second handover period of the user equipment UE1.

In case the time instant t_CHO falls in a current handover period of the handover periods of the user equipment UE1, the method proceeds to step 307, where the user equipment UE1 sends a synchronization message to the target base station BS 3 for the conditional handover. The synchronization message may, for example, be the Physical RACH preamble used to synchronize with the target base station BS 3. The user equipment UE1 sends the synchronization message in the current handover period or in a handover period subsequent to the current handover period e.g. depending on whether the user equipment UE1 can complete the synchronization procedure before a handover period of the other user equipment UE2 starts. FIG. 5 provides further details of step 307. The synchronization may, for example, enable the user equipment to exchange user data with the target base station and thus enable access to the target cell by the user equipment. A successful execution of the synchronization procedure may enable a successful conditional handover of the user equipment to the target base station.

In case the time instant t_CHO does not fall in a current handover period of the handover periods of the user equipment UE1, the method proceeds to step 309, where the user equipment UE1 sends the synchronization message to the target base station BS 3 for the conditional handover in a handover period subsequent to the current handover period. Following the above example, if the time instant t_CHO is (start time instant of SFN 0 + 300 ms), the user equipment UE1 may send the synchronization message in the subsequent handover period that corresponds to the interval (start time instant of SFN 0 + 400ms).

FIG. 4A illustrates an example of messages exchanged between user equipments of a multi-user equipment device and the base stations for a conditional handover of the user equipments. For the purpose of explanation, the method described in FIG 4A may be implemented in the system illustrated in FIG. 2, but is not limited to this implementation.

The source base stationBS 1 sends (401 a) a first CFIO message to the user equipment UE1 for a conditional handover to the target base station BS 3. The first CFIO message indicates a first CFIO condition and a first handover configuration of the user equipment UE1. The first handover configuration may indicate the start of a handover period, its length and periodicity. The source base station BS2 sends (401 b) a second CFIO message to the user equipment UE2 for a conditional handover to the target base station BS4. The second CFIO message indicates a second CFIO condition and a second handover configuration of the user equipment UE2. The second handover configuration may, for example, indicate the start of a handover period, its length and periodicity.

In a first handover example, of FIG. 4B, handover periods

... may be assigned to the user equipment UE1. The user equipment UE1 may be configured to perform the synchronization procedure during the handover periods ... FIG. 4B further shows an example of handover periods ... assigned to

the user equipment UE2. The user equipment UE2 may be configured to perform the synchronization procedure during the handover periods .... The

handover periods of the two user equipments UE1 and UE 2 are disjoint. In the first handover example, the first and second handover configuration may each further indicate a time duration T that may be enough to complete the synchronization procedure.

In a second handover example, of FIG. 4C, the handover periods of the two user equipments UE1 and UE 2 are disjoint and separated by a guard period TP_GP. In the second handover example, the first and second handover configuration may each further indicate the guard period.

Upon receiving the first CHO message, the user equipment UE1 may check the first CHO condition and may determine (402a) that the first CHO condition is fulfilled at time instant · For that, the user equipment UE1 may compare a reference signal received from the source base station BS 1 with a reference signal of the target base station BS 3. For example, the user equipment UE1 may check whether the RSRP of the reference signal received from the target base station BS 3 exceeds the RSRP of the reference signal received from the source base station BS 1 by at least the offset value for the TTT interval.

Upon receiving the second CHO message, the user equipment UE 2 may check the second CHO condition and may determine (402b) that the second CHO condition is fulfilled at time instant . For that, the user equipment UE 2 may compare a reference

signal received from the source base station BS2 with a reference signal of the target base station BS4. For example, the user equipment UE 2 may check whether the RSRP of the reference signal received from the target base station BS4 exceeds the RSRP of the reference signal received from the source base station BS2 by at least the offset value for the TTT interval.

In case the time instant

falls out of the handover period assigned to the user equipment UE1, the user equipment UE1 may defer (405a) the initiation of the synchronization procedure to a next handover period of the user equipment UE1 which may result in deferring the whole synchronization procedure. Deferring the synchronization procedure may result in deferring the data forwarding, in case of the late data forwarding.

In case the time instant

falls out of the handover period assigned to the user equipment UE 2, the user equipment UE 2 may defer (405b) the initiation of the synchronization procedure to a next handover period of the user equipment UE2.

However, if (407a) the time instant

falls in the handover period assigned to the user equipment UE1, the user equipment UE1 may decide when to initiate the synchronization procedure depending on whether the difference between the time instant and the start of an upcoming handover period of the other user equipment UE 2 is enough for completing the synchronization procedure of the user equipment UE1. For example, if the time instant

falls in the handover period , the user

equipment UE1 may decide when to initiate the synchronization procedure depending on whether the difference between the time instant

and the start of the upcoming handover period

of the other user equipment UE 2 is enough for completing the synchronization procedure of the user equipment UE1. That is, depending on the handover configuration sent by the source base station BS 1, the user equipment UE1 may decide when to initiate the synchronization procedure. Following the first handover example, and if the difference between the time instant

and the end of the handover period

is longer than the time duration T, the user equipment UE1 may immediately send (409a) a synchronization message in the handover period . If the

difference between the time instant

and the end of the handover period

is shorter than the time duration T, the user equipment UE1 may send (409a) the synchronization message in the subsequent handover period

of the handover period . Following the second handover example, the user equipment UE1 may immediately send (409a) the synchronization message in the handover period

because the guard period provides enough time for completing the synchronization procedure regardless of when it starts in the handover period

. Similarly, if (407b) the time instant falls in the handover period assigned to the

user equipment UE 2, the user equipment UE 2 may decide when to initiate the synchronization procedure depending on whether the difference between the time instant and the start of an upcoming handover period of the other user equipment UE1 is enough for completing the synchronization procedure of the user equipment UE1. For example, if the time instant

falls in the handover period

the user equipment UE 2 may decide when to initiate the synchronization procedure depending on whether the difference between the time instant

and the start of the upcoming handover period

of the other user equipment UE1 is enough for completing the synchronization procedure of the user equipment UE2. That is, depending on the handover configuration sent by the source base station BS2 the user equipment UE 2 may decide when to initiate the synchronization procedure. Following the first handover example, and if the difference between the time instant

and the end of the handover period

is longer than the time duration T, the user equipment UE 2 may immediately send (409b) a synchronization message in the handover period .

If the difference between the time instant

and the end of the handover period

is shorter than the time duration T, the user equipment UE 2 may send (409b) the synchronization message in the subsequent handover period

of the handover period . Following the second handover example, the user equipment UE 2 may immediately send (409b) the synchronization message in the handover period

because the guard period provides enough time for competing the synchronization procedure regardless of when it starts in the handover period .

Steps 401a, 402a, 405a, 407a and 409a related to the user equipment UE1 may be performed concurrently, before or after the execution of the steps 401b, 402b, 405b, 407b and 409b related to the user equipment UE2.

The source base stations BS 1 and BS2 may coordinate using Xn interface the configuration of the handover periods. That is, the determination of the first handover configuration and the second handover configuration may be coordinated between source base stations BS 1 and BS2 in order to avoid or reduce outage of the multi-user equipment device 201 . The outage would occur for a device if all the UEs are in outage. This may happen in the following situations:

- Both UEs are executing handover which is associated with some interruption for accessing the target cell of handover. Herein, the handover execution periods of the UEs may be overlapping or partially overlapping.

- One UE is executing handover and the other one is detecting or recovering from a Radio Link Failure (RLF). An RLF occurs if the radio link quality of the serving cell falls below a certain threshold for some time (without triggering a handover).

- One UE is executing handover and the other one is detecting or recovering from a Handover Failure (HOF). A HOF occurs when a UE fails to access a target cell of handover during T304 timer duration.

- Both UEs are in RLFs or HOF, or one UE is in RLF while the other is in HOF. The present subject matter may reduce the outage that is caused by handover, radio link failures and/or handover failures for devices with multi-UEs as described with the above situations.

FIG. 5 is a flowchart of a method used in a user equipment according to an example of the present subject matter. For the purpose of explanation, the method described in FIG 3 may be implemented in the system illustrated in FIG. 2, but is not limited to this implementation.

Steps 501 , 503, 505 and 511 are steps 301 , 303, 305 and 309 respectively.

In case (505) the time instant t_CHO falls in a current handover period of the handover periods of the user equipment UE1, the method proceeds to step 507, where the user equipment UE1 determines if the difference between the time instant t_CHO and the end of the current handover period is longer than a time duration T. If the difference between the time instant t_CHO and the end of the current handover period is longer than the time duration T, the user equipment UE1 may immediately send in step 509 a synchronization message in the current handover period. If the difference between the time instant t_CHO and the end of the current handover period is longer than the time duration T step 511 may be performed. The time duration T may, for example, be a preconfigured parameter of the user equipment UE1 or may be received at the user equipment UE1 from the source base station (e.g. in step 501 ).

FIG. 6 is a flow diagram of a signaling method for a conditional handover in accordance with an example of the present subject matter.

A user equipment 601 may send (step 1 ) a measurement report to a source node 602. Upon receiving the measurement report, the source node 602 may decide (step 2), based on the measurement report, a conditional handover of the UE 601 to target nodes 603 and 604. After deciding the conditional handover, source node 602 may send (steps 3 and 4) a CHO request to each of the target nodes 603 and 604 passing necessary information to prepare the CHO at the target nodes 603 and 604. Upon receiving the CHO request, each of the target nodes 603 and 604 may perform an admission control (steps 5 and 6) and then send (steps 7 and 8) a CHO request acknowledgement to the source node 602. Upon receiving the CHO request acknowledgements, the source node 602 may send (step 9) an RRC reconfiguration message to the UE 601. The RRC configuration message may indicate, a CHO condition, handover periods of the UE 601 and a time duration T. Upon receiving the RRC reconfiguration message, the UE 601 may evaluate (step 10) the CHO condition for each of the target nodes 603 and 604. The UE 601 may determine (step 11 ) that the CHO condition is fulfilled for the target node 603 at time instant t1 . If the CHO condition is fulfilled (step 12) for TTT outside the handover period of the UE 601 (i.e. t1 falls outside the handover period of the UE 601 ), then the UE defers the handover execution to its next handover period. The CHO execution can be deferred either to the start of next handover period or any time instant in next handover period such that the handover execution can be still completed within next handover period, i.e., at least T time duration before the end of the handover period, where T is configured by the network to a value which is long enough to execute CHO. If t1 falls (step 13) in the handover period of the UE 601 , the UE may assess first the difference between the end of the current handover period and the time instant t1 the CHO execution is fulfilled. If the difference is longer than time duration T, then UE executes the CHO immediately in the current handover period. Otherwise, the UE does not stop the TX/RX to the serving cell and defers the handover execution to its next handover period. The CHO execution starts by a submission (step 14) of a PRACH preamble by the UE 601 to the target node 603. Upon receiving the PRACH preamble, the target node 603 may send (step 15) a RACH response to the UE 601. Upon receiving the RACH response, the UE 601 may send (step 16) an RRC reconfiguration complete message to the target node 603. Upon receiving the RRC reconfiguration complete message, the target node 603 may send (step 17) a handover success message to the source node 602. Upon receiving the handover success message, the source node 602 may stop (step 18) the data transmission to/from the UE 601 and may start data forwarding. For example, the source node 602 may perform SN status transfer (step 19) and data forwarding (step 20) to the target node 603. The source node 602 may send (step 21 ) a release CHO preparation message to the other target node 604 such that the target node 604 may delete data or release reserved resources related to the present CHO. A path switch procedure (step 22) may be coordinated between the source node 602, the target node 603 and a serving gateway/UPF 605 and an MME/AMF and 606.

The synchronization procedure may, for example, comprise steps 14, 15 and 16 of FIG. 6. The handover execution phase may, for example, comprise steps 14 to 20 of FIG. 6.

FIG. 7 is a flow diagram of a signaling method for a conditional handover in accordance with an example of the present subject matter.

A user equipment 701 may send (step 1) a measurement report to a source node 702. Upon receiving the measurement report, the source node 702 may decide (step 2), based on the measurement report, a conditional handover of the UE 701 to target nodes 703 and 704. After deciding the conditional handover, source node 702 may send (steps 3 and 4) a CHO request to each of the target nodes 703 and 704 passing necessary information to prepare the CHO at the target nodes 703 and 704. Upon receiving the CHO request, each of the target nodes 703 and 704 may perform an admission control (steps 5 and 6) and then send (steps 7 and 8) a CHO request acknowledgement to the source node 702. Upon receiving the CHO request acknowledgements, the source node 702 may send (step 9) an RRC reconfiguration message to the UE 701. The RRC configuration message may indicate, a CHO condition, handover periods of the UE 701 and a guard period. Upon receiving the RRC reconfiguration message, the UE 701 may evaluate (step 10) the CHO condition for each of the target nodes 703 and 704. The UE 701 may determine (step 11 ) that the CHO condition is fulfilled for the target node 703 at time instant t1 .

If the CHO condition is fulfilled outside a current handover period of the UE 701 , the UE 701 does not stop TX/RX to the serving cell and defers (step 12) the CHO execution to its next handover period. In this method, the UE does not need to compare the difference between the time instant the CHO condition is fulfilled and the end of the handover period against pre-configured parameter T. In this method, no UEs on the multi-UE device can start the execution of the conditional handover in the guard period. If t1 falls (step 13) in the handover period of the UE 701 , then the UE executes the CHO immediately in the current handover period.

The CHO execution starts by a submission (step 14) of a PRACH preamble by the UE 701 to the target node 703. Upon receiving the PRACH preamble, the target node 703 may send (step 15) a RACH response to the UE 701. Upon receiving the RACH response, the UE 701 may send (step 16) an RRC reconfiguration complete message to the target node 703. Upon receiving the RRC reconfiguration complete message, the target node 703 may send (step 17) a handover success message to the source node 702. Upon receiving the handover success message, the source node 702 may stop (step 18) the data transmission/reception to/from the UE 701 and may start data forwarding. For example, the source node 702 may perform SN status transfer (step 19) and data forwarding (step 20) to the target node 703. The source node 702 may send (step 21 ) a release CHO preparation message to the other target node 704 such that the target node 704 may delete data and release reserved resources related to the present CHO. A path switch procedure (step 22) may be coordinated between the source node 702, the target node 703 and a serving gateway/UPF 705 and an MME/AMF and 706. The synchronization procedure may, for example, comprise steps 14, 15 and 16 of FIG. 7. The handover execution phase may, for example, comprise steps 14 to 20 of FIG. 7.

In FIG. 8, a block circuit diagram illustrating a configuration of an apparatus 1070 is shown, which is configured to implement at least part of the present subject matter. It is to be noted that the apparatus 1070 shown in FIG. 8 may comprise several further elements or functions besides those described herein below, which are omitted herein for the sake of simplicity as they are not essential for the understanding. Furthermore, the apparatus may be also another device having a similar function, such as a chipset, a chip, a module etc., which can also be part of an apparatus or attached as a separate element to the apparatus 1070, or the like. The apparatus 1070 may comprise a processing function or processor 1071 , such as a central processing unit (CPU) or the like, which executes instructions given by programs or the like related to a flow control mechanism. The processor 1071 may comprise one or more processing portions dedicated to specific processing as described below, or the processing may be run in a single processor. Portions for executing such specific processing may be also provided as discrete elements or within one or more further processors or processing portions, such as in one physical processor like a CPU or in several physical entities, for example. Reference sign 1072 denotes transceiver or input/output (I/O) units (interfaces) connected to the processor 1071. The I/O units 1072 may be used for communicating with one or more other network elements, entities, terminals or the like. The I/O units 1072 may be a combined unit comprising communication equipment towards several network elements or may comprise a distributed structure with a plurality of different interfaces for different network elements. Reference sign 1073 denotes a memory usable, for example, for storing data and programs to be executed by the processor 1071 and/or as a working storage of the processor 1071 .

The processor 1071 is configured to execute processing related to the above described subject matter. In particular, the apparatus 1070 may be configured to perform at least part of the method as described in connection with FIG 3, 5, 6 or 7. For example, the processor 1071 is configured to: receive, from a source base station, a message for a conditional handover of the user equipment. The message comprises first information indicative of a condition for the conditional handover to a target base station and second information indicative of handover periods of the user equipment. The processor 1071 is further configured to determine whether the condition is satisfied, and in case the condition is satisfied at a time instant of a current handover period of the handover periods, send a message to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods.

In another example, the processor 1071 is configured to: send to a user equipment of a multi-user equipment device a message for a conditional handover of the user equipment, wherein the message comprises first information indicative of a condition for a conditional handover to at least one target base station and second information indicative of handover periods assigned to the user equipment.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as an apparatus, method, computer program or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer executable code embodied thereon. A computer program comprises the computer executable code or "program instructions".

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable storage medium. A ‘computer-readable storage medium’ as used herein encompasses any tangible storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. The computer-readable storage medium may also be referred to as a tangible computer readable medium. In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device.

‘Computer memory’ or ‘memory’ is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. ‘Computer storage’ or ‘storage’ is a further example of a computer-readable storage medium. Computer storage is any non-volatile computer-readable storage medium. In some embodiments computer storage may also be computer memory or vice versa.

A ‘processor’ as used herein encompasses an electronic component which is able to execute a program or machine executable instruction or computer executable code. References to the computing device comprising “a processor” should be interpreted as possibly containing more than one processor or processing core. The processor may for instance be a multi-core processor. A processor may also refer to a collection of processors within a single computer system or distributed amongst multiple computer systems. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor or processors. The computer executable code may be executed by multiple processors that may be within the same computing device or which may even be distributed across multiple computing devices.

Computer executable code may comprise machine executable instructions or a program which causes a processor to perform an aspect of the present invention. Computer executable code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the "C" programming language or similar programming languages and compiled into machine executable instructions. In some instances the computer executable code may be in the form of a high level language or in a pre-compiled form and be used in conjunction with an interpreter which generates the machine executable instructions on the fly. Generally, the program instructions can be executed on one processor or on several processors. In the case of multiple processors, they can be distributed over several different entities. Each processor could execute a portion of the instructions intended for that entity. Thus, when referring to a system or process involving multiple entities, the computer program or program instructions are understood to be adapted to be executed by a processor associated or related to the respective entity.

Claims

1. A user equipment for a multi-user equipment device, the user equipment comprising means configured for: receiving, from a source base station, a message for a conditional handover of the user equipment, the message comprising first information indicative of a condition for the conditional handover to a target base station, the message comprising second information indicative of handover periods of the user equipment; determining whether the condition is satisfied; and in case the condition is satisfied at a time instant of a current handover period of the handover periods, sending a message to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods.

2. The user equipment of claim 1 , the second information being further indicative of a guard period, wherein each handover period of the handover periods is extended with the guard period, the certain handover period being the current handover period excluding the guard period.

3. The user equipment of claim 1 , the second information being further indicative of a time duration, the means being configured for: in response to determining that the condition is satisfied during the current handover period, determining whether a difference between the end of the current handover period and the time instant is longer than the time duration; performing the sending of the message to the target base station in the current handover period in case the difference is longer than the time duration, the certain handover period being the current handover period; performing the sending of the message to the target base station in a handover period of the handover periods that is subsequent to the current handover period in case the difference is shorter than the time duration, the certain handover period being the subsequent handover period.

4. The user equipment of claim 3, the condition requiring that a first value indicative of a signal received from the target base station exceeds a second value indicative of a signal received from the source base station by at least an offset value for a time-to-trigger (TTT) interval, the means being further configured for: modifying the condition such that the modified condition requires that the first value exceeds the second value by at least the offset value for a time interval smaller than the TTT interval and such that the difference is longer than the time duration, wherein the determining that the condition is satisfied comprises determining that the modified condition is satisfied.

5. The user equipment of claim 4, the time interval being smaller than the TTT interval by a factor, wherein the second information is indicative of the factor.

6. The user equipment of any of the preceding claims, the handover periods of the user equipment and handover periods of another user equipment of the multi- user equipment device being disjoint.

7. The user equipment of claim 6, the user equipment, the source base station and the target base station belonging to a same first reliability group which is different from a reliability group of the other user equipment.

8. The user equipment of any of the preceding claims, the means being configured for: in case the condition is satisfied at a time instant outside the handover periods of the user equipment, sending the message to the target base station during the handover period subsequent to said time instant.

9. The user equipment of any of the preceding claims 4-8, wherein the sending of the message in the subsequent handover period is a deferring of the submission of the message, wherein in case the condition is modified, the sending of the message in the current handover period is a preponing of the submission of the message, the means being configured to report information to the target base station indicating that the preponing or the deferring is performed.

10. The user equipment of claim 9, the reported information indicating a difference between a time instant at which the modified condition is satisfied and a time instant at which the condition would be satisfied.

11. The user equipment of any of the preceding claims, the means being configured to report a time delay value to the target base station, the time delay value being a difference between the time instant at which the condition is satisfied and a time instant at which a subsequent handover period of the current handover period starts.

12. The user equipment of any of the preceding claims, wherein the means comprises at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processor, cause the performance of the user equipment.

13. A multi-user equipment device comprising multiple ones of the user equipment of any of the preceding claims 1 -12.

14. A method used in a user equipment, comprising: receiving, from a source base station, a message for a conditional handover of the user equipment, the message comprising first information indicative of a condition for the conditional handover to a target base station, the message comprising second information indicative of handover periods of the user equipment; determining whether the condition is satisfied; and in case the condition is satisfied at a time instant of a current handover period of the handover periods, sending a message to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods.

15. A computer program comprising instructions for causing a user equipment for performing at least the following: receiving, from a source base station, a message for a conditional handover of the user equipment, the message comprising first information indicative of a condition for the conditional handover to a target base station, the message comprising second information indicative of handover periods of the user equipment; determining whether the condition is satisfied; and in case the condition is satisfied at a time instant of a current handover period of the handover periods, sending a message to the target base station for the conditional handover, wherein the sending is performed during a certain handover period of the handover periods.

16. A base station comprising means configured for: sending to a user equipment of a multi-user equipment device a message for a conditional handover of the user equipment, the message comprising first information indicative of a condition for a conditional handover to at least one target base station, the message comprising second information indicative of handover periods assigned to the user equipment.

17. The base station of claim 16, the second information being further indicative of a guard period, wherein each handover period of the user equipment is separated with the guard period from a subsequent handover period of another user equipment of the multi-user equipment device.

18. The base station of claim 16, the second information being further indicative of a time duration for completion of the conditional handover.

19. The base station of any of the preceding claims 16-18, the means being configured for coordinating configuration of handover periods of user equipments of the multi-user equipment device with one or more source base stations serving other one or more other user equipments of the multi-user equipment device respectively, and for determining the handover periods assigned to the user equipment based on the coordination.

20. The base station of any of the preceding claims 16-19, wherein the means comprises at least one processor; and at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processor, cause the performance of the base station.

21. A method used in a base station, comprising: sending to a user equipment of a multi-user equipment device a message for a conditional handover of the user equipment, the message comprising first information indicative of a condition for a conditional handover to at least one target base station, the message comprising second information indicative of handover periods assigned to the user equipment.