WO2024073888A1

WO2024073888A1 - Cell change handling by relay ue in an idle or inactive protocol state

Info

Publication number: WO2024073888A1
Application number: PCT/CN2022/123817
Authority: WO
Inventors: Nathan Edward Tenny; Xuelong Wang; Ming-Yuan Cheng
Original assignee: Mediatek Inc.
Priority date: 2022-10-08
Filing date: 2022-10-08
Publication date: 2024-04-11

Abstract

This disclosure describes methods of operating a path switch procedure in a remote and/or a target relay UE, addressing cases in which the target relay UE performs a cell reselection during the path switch procedure, and directed to preventing the target relay UE from forwarding a handover completion message to a base station different from the target base station and unprepared to receive the handover completion message.

Description

CELL CHANGE HANDLING BY RELAY UE IN AN IDLE OR INACTIVE PROTOCOL STATE

FIELD

This disclosure relates to wireless communications, and specifically to methods of handling cell change during a path switch operation by a remote UE to the service of a target relay UE, wherein the target relay UE is in an idle or inactive state of a protocol.

BACKGROUND

In certain cellular systems, such as 3GPP 5G New Radio (NR) from Rel-17 onward, a first user equipment (UE) may function in a relaying relationship with a second UE. For example, the first UE may be out of direct cellular coverage or in poor coverage, while the second UE is in good coverage, and the second UE may deliver communications between the first UE and the serving cellular network. In this scenario, the first UE may be referred to as a remote UE and the second UE may be referred to as a relay UE. The remote UE may be referred to as being in “indirect service” or having an “indirect path” to the network, while the relay UE may be referred to as being in “direct service” or having a “direct path” to the network. The relay and remote UEs may communicate via a sidelink interface, also called a PC5 interface, in which radio resources are used for direct communication between UEs without an intervening network node.

Due to change of radio conditions, physical mobility of the remote and/or relay UEs, and similar considerations, a remote UE may need to switch its service from one path to another-for example, from a direct path to an indirect path, or from an indirect path via a first relay UE to an indirect path via a second relay UE. (Path switch from an indirect path to a direct path is also possible, but it is outside the scope of this disclosure. ) The remote UE’s serving cell and/or serving base station (e.g., gNB) before such a path switch need not be the same as the remote UE’s serving cell and/or serving base station after the path switch; for example, a remote UE may move from direct service with a cell operated by a first (source) gNB to indirect service via a (target) relay UE with a cell operated by a second (target) gNB, or from indirect service via a first (source) relay UE with a cell of a first (source) gNB to indirect service via a second (target) relay UE with a cell of a second (target) gNB. Such an inter-gNB path switch may be facilitated by a handover procedure, in which a handover command is generated by the target gNB and passed via the source gNB to the remote UE, and the remote UE responds with a handover complete message via the target relay UE to the target gNB.

The target relay UE may be in a variety of protocol states, such as an RRC_CONNECTED state, an RRC_INACTIVE state, or an RRC_IDLE state of a radio resource control (RRC) protocol. In an RRC_CONNECTED protocol state, the target relay UE has an RRC connection with the target gNB, and its mobility is under the control of the target gNB. However, in an RRC_INACTIVE or RRC_IDLE state, the target relay UE controls its own mobility, and it may, for example, reselect to a new cell (potentially operated by a different gNB) while the path switch is in progress. In such a case, problems arise in the management of UE contexts and the processing of the handover complete message, because the new cell may receive an unexpected handover complete message from a remote UE for which it has no UE context. This document is directed to addressing these problems.

SUMMARY

The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

In an aspect of the disclosure, a method of cell reselection handling is provided, operable in a target relay UE, and comprising operating in an idle or inactive protocol state in the service of a target cell; performing a discovery procedure with a remote UE, comprising indicating an identity of the target cell; performing a cell reselection procedure to a new cell; receiving, from the remote UE, subsequent to the cell reselection procedure, a message for forwarding to the first cell; and sending, to the remote UE, a notification message comprising an indication of an inability to forward the message.

In another aspect of the disclosure, a method of path switch control is provided, operable in a remote UE and comprising performing a discover procedure with a target relay UE, comprising receiving an identity of a serving cell of the target relay UE; receiving, from a base station operating a serving cell of the remote UE, a reconfiguration message comprising a handover command; establishing a communication link with the target relay UE; sending, to the target relay UE, a handover completion message; receiving, from the target relay UE, a notification message comprising an indication of an inability of the second UE to forward the handover completion message; and initiating a failure handling procedure.

To the accomplishment of the foregoing and related ends, the one or more aspects comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative features of the one or more aspects. These features are indicative, however, of but a few of the various ways in which the principles of various aspects may be employed, and this description is intended to include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of relay and remote UE operation.

FIG. 2 is a diagram illustrating an example of an inter-gNB path switch from a direct path to an indirect path.

FIG. 3 is a diagram illustrating an example of an inter-gNB path switch from an indirect path with a first relay UE to an indirect path with a second relay UE.

FIG. 4 illustrates a message flow for a direct-to-indirect path switch procedure.

FIG. 5 illustrates a message flow for a path switch procedure in which a target relay UE performs a cell reselection procedure during the path switch procedure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

Several aspects of telecommunication systems will now be presented with reference to various apparatus and methods. These apparatus and methods will be described in the following detailed description and illustrated in the accompanying drawings by various blocks, components, circuits, processes, algorithms, etc. (collectively referred to as “elements” ) . These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system.

Figure 1 shows an example of relay and remote UE operation. A base station of a communication system, such as the gNB in the figure, serves a first (relay) UE over a first direct interface, such as the Uu interface shown in the figure. In turn, the first UE serves a second (remote) UE over a second direct interface, such as the PC5 interface shown in the figure. The PC5 interface may also be referred to as a sidelink interface. In the figure, the remote UE is shown as being out of coverage of a cell operated by the gNB, but it should be appreciated that a relaying relationship may also exist for a remote UE in coverage. (For example, the remote UE may be in poor coverage at the edge of a cell, allowing it to receive better service through the combination of a good PC5 link to the relay UE and the relay UE’s good Uu link to the base station than it could receive through its own poor Uu link directly to the base station. ) Communications to and from the remote UE may be carried through the relay UE from and to the base station, allowing the remote UE to be served by the communication system.

Figure 2 shows a path switch procedure from a first (source) gNB A, where the remote UE is in direct service, to a second (target) gNB B, where the remote UE is in indirect service via a target relay UE. The remote UE is initially in the coverage of gNB A. (Note that the figure suggests that the remote UE may be at the extreme edge of the cell; this situation is plausible as a prelude to mobility, since the remote UE may be handed over due to moving out of coverage of the source gNB, but it is not a necessary condition. In general, the source gNB may trigger a handover at any time based on criteria defined by its own implementation. ) In response to a decision by gNB A to trigger a mobility procedure, the remote UE is transferred to indirect service through the target relay UE, which is served by gNB B.

Figure 3 shows a path switch procedure from a source gNB A, where the remote UE is in indirect service via a source relay UE, to a target gNB B, where the remote UE is in indirect service via a target relay UE. The remote UE may be in the physical coverage of either cell, or out of coverage entirely (as shown in the figure) , but it receives its service through the source and target relay UEs. The remote UE is initially served by gNB A via the source relay UE, and in response to a decision by gNB A to trigger a mobility procedure, the remote UE is transferred to indirect service via the target relay UE, which is served by gNB B.

Figure 4 shows a message flow for a direct-to-indirect path switch procedure, in which the remote UE is transferred from direct service with a source gNB to indirect service via a target relay UE with a target gNB. In step 1 of figure 4, the remote UE sends to the source gNB a measurement report that may, for instance, comprise measurements of the target relay UE. In step 2, the source gNB determines to trigger a handover procedure, in the form of a path switch to the target relay UE. In step 3, the source gNB and the target gNB perform a handover preparation procedure, i.e., an exchange of messages to confirm admission of the remote UE to service in the target gNB and to prepare the target gNB to receive the remote UE in handover. The handover preparation procedure may comprise the delivery, to the source gNB from the target gNB, of a handover command to be forwarded to the remote UE. In step 4, the source gNB sends to the remote UE a reconfiguration message (for example, an RRCReconfiguration message of an RRC protocol) , which may include a forwarded version of the handover command from step 3, and which may further include instructions to “add” (i.e., establish a connection with) the target relay UE and to release a connection with the source gNB. In step 5, the remote UE establishes a link with the target relay UE; this step may, for example, take the form of a PC5 unicast link establishment procedure, which may also establish a PC5-RRC connection. In step 6, the remote UE sends to the target gNB, via the target relay UE, a handover completion message (for example, an RRCReconfigurationComplete message of an RRC protocol) , informing the target gNB that the remote UE is in its service and completing the handover procedure.

It should be appreciated that the flow of figure 4 is also substantially applicable to an indirect-to-indirect path switch procedure, in which the remote UE is transferred from indirect service via a source relay UE (not shown in the figure) with the source gNB to indirect service via the target relay UE with the target gNB. The difference from the figure in this case is that the measurement report of step 1 is relayed to the source gNB via the source relay UE. In addition, the “add target/release source” procedures triggered by step 4 may include a release of a connection or unicast link between the remote UE and the source relay UE.

Figure 5 shows a message flow for a direct-to-indirect path switch procedure in which the target relay UE is initially in an RRC_IDLE or RRC_INACTIVE protocol state, and in which the target relay UE reselects to a cell operated by a new gNB during the path switch, in accordance with one novel aspect. In this example, the target relay UE prevents delivery of the handover complete message to the (unprepared) new gNB by rejecting the remote UE’s attempt to have the handover complete message forwarded. It is noted that figure 5 may also be applied, with minimal modifications, to an indirect-to-indirect path switch. The differences between the illustrated procedure and its analogue in the indirect-to-indirect setting are described below.

In step 1 of figure 5, the remote UE sends to the source gNB a measurement report, which may, for example, comprise measurements of the target relay UE; this step is similar to step 1 of figure 4. (In case of an indirect-to-indirect path switch, step 1 is relayed via a source relay UE, which is not shown in the figure. ) At this stage, the target relay UE is assumed to be in an RRC_IDLE or RRC_INACTIVE state and camped on the target gNB, and as a result, the identity of the target gNB may be indicated by the target relay UE to the remote UE during a discovery procedure. The measurement report may contain an indication of the identity of the target gNB, which can be used by the source gNB to determine which gNB it needs to communicate with.

In step 2 of figure 5, the source gNB takes the decision to trigger path switch of the remote UE to the target relay UE via a handover procedure. This step may be in accordance with any criteria embodied in the source gNB implementation; in some embodiments, it may be conditioned on a measurement event, which may be indicated in the measurement report in step 1. For example, a triggering event for the measurement report may indicate that the target relay UE’s signal is better than a defined threshold; that the target relay UE’s signal is better than a first threshold and the source gNB’s signal is worse than a second threshold; or, in the indirect-to-indirect case (not shown in the figure) , that the target relay UE’s signal is better than a source relay UE’s signal by a threshold.

In step 3 of figure 5, the source gNB sends to the target gNB a handover preparation message. The handover preparation message may identify the remote UE, describe a context and/or configuration of the remote UE, and so on, in accordance with legacy handover procedures. The handover preparation message may identify the target relay UE and thus inform the target gNB that the target relay UE is in its service.

In step 4 of figure 5, the target gNB takes an admission control decision; that is, the target gNB determines whether to admit the remote UE and the target relay UE into service. The target gNB may consider substantially any criteria in its admission control decision; in some embodiments, the target gNB may consider its current load of served UEs and/or served traffic flows and evaluate whether it has the capacity to admit the additional traffic required for the remote UE and/or the target relay UE.

In step 5 of figure 5, having taken the decision to admit the remote UE and the target relay UE into service, the target gNB sends to the source gNB a handover accept message. The handover accept message may include a handover command formulated by the target gNB and intended to be delivered to the remote UE. The handover command may include a configuration for the remote UE to apply for operation on an indirect path via the target relay UE in the target gNB.

In step 6 of figure 5, the target relay UE performs a cell reselection procedure to a cell of the new gNB. Because the target relay UE is still in an RRC_IDLE or RRC_INACTIVE protocol state, its mobility is under the control of the target relay UE itself. The target relay UE’s mobility procedures may, for example, be based on measuring downlink signals from a plurality of cells and reselecting to the “best” cell, i.e., the cell with the strongest or best-quality signal when considered in light of a set of cell reselection parameters. It is noted that the cell reselection event to step 6 may not be detectable by the remote UE, the target gNB, or the new gNB; only the target relay UE itself knows that it has moved to a new serving cell. Although step 6 is shown immediately following step 5 in the diagram, it should be appreciated that step 6 may occur at any time from step 1 to step 8. In other words, the target relay UE can be known to be served by the target gNB when it announces its serving cell ID in a discovery procedure (which may occur at any time before step 1 of the figure) , and at any time after the discovery procedure, the target relay UE may perform a cell reselection, unbeknownst to other entities in figure 5. If the cell reselection occurs before or simultaneously with step 8, the target relay UE will be unable to forward the handover completion message (step 9) to the target gNB for which the handover completion message is intended. Furthermore, the target relay UE’s serving cell will not have changed during the lifetime of the unicast link established in step 8, so the target relay UE may not have a basis to notify the remote UE of the new serving cell. Thus, for a cell reselection occurring at any time up to and including step 8 of the figure, the remote UE may be expected to be unaware of the resulting cell change of the target relay UE.

In step 7 of figure 5, the source gNB sends to the remote UE a reconfiguration message (for example, an RRCReconfiguration message of an RRC protocol) , which may contain the handover command from step 5. This step comprises delivery of the handover command to the remote UE, resulting in the remote UE triggering a mobility procedure. The reconfiguration message may contain a configuration for the remote UE to communicate with the target relay UE. In the indirect-to-indirect case, the reconfiguration message may contain an instruction for the remote UE to release a connection with a source relay UE (not shown in the figure) .

In step 8 of figure 5, the remote UE establishes a communication link (for instance, a PC5 unicast link and/or a PC5-RRC connection) with the target relay UE. This communication link allows data, such as control signalling and/or user data, to be conveyed between the remote UE and the target relay UE. At substantially the same time as step 8, in the indirect-to-indirect case, the remote UE may release a communication link with a source relay UE (not shown in the figure) .

In step 9 of figure 5, the remote UE transmits to the target relay UE a handover completion message, e.g., an RRCReconfigurationComplete message of an RRC protocol. The handover completion message may be intended for forwarding to the target gNB. However, the target relay UE is no longer camped on the target gNB, and the target relay UE may as a result be unable to forward the handover completion message to the target gNB. Furthermore, if the target relay UE were to forward the handover completion message to the new gNB, the new gNB would not be able to apply the message and complete the handover, since the new gNB has not been informed of the context and/or configuration of the remote UE and has not had the opportunity to take an admission control decision for the remote UE and the target relay UE. Thus, the target relay UE may not be able to assist the remote UE in completing the path switch procedure.

In step 10 of figure 5, the target relay UE sends to the remote UE a notification message (for example, a NotificationMessageSidelink message of a PC5-RRC protocol) . The notification message may indicate that the target relay UE’s serving cell has changed between the discovery procedure and the reception of the handover completion message. The notification message may indicate that the handover completion message is rejected by the target relay UE, i.e., that the target relay UE does not forward the handover completion message to any gNB.

In step 11 of figure 5, the remote UE determines, based on the notification message in step 10, that the handover procedure for its path switch operation has failed. This determination may induce various procedures for path switch failure handling, such as relay reselection, cell reselection, connection re-establishment, and so on. The remote UE’s procedures for path switch failure handling may be aligned with the procedures for the expiration of a supervisory timer governing the path switch procedure, such as a timer T420 defined in an RRC protocol.

In step 12 of figure 5, the target gNB experiences a timeout based on its failure to receive the handover completion message from the remote UE. After step 12, both the remote UE and the target gNB are aware that the path switch has failed.

It is noted that existing mechanisms for detecting the failure of a path switch operation may not apply to the case where a target relay UE performs cell reselection during the path switch procedure. As noted above, the target relay UE may not be prompted to notify the remote UE of the cell change, since the cell change did not occur during the lifetime of the unicast link between the remote UE and the target relay UE. Moreover, since the handover completion message is successfully delivered to the target relay UE in step 9 of figure 5, a supervisory timer (for instance, T420) may be stopped with the assumption that the handover completion message has been delivered successfully.

In some cases, the target relay UE may be unable to operate as a relay after the cell change occurs. For example, if permission to operate as a relay UE is based on the serving cell’s downlink signal strength on a Uu interface being within a range (e.g., below a maximum threshold and/or above a minimum threshold) , the target relay UE may find that its downlink signal strength on the Uu interface with the new serving cell is outside the range, meaning that the target relay UE is no longer permitted to operate as a relay UE. As another example, the new serving cell may not support relay operation at all. In such cases, it is clear that the relay UE cannot forward the handover completion message received from the remote UE. Instead, the relay UE may notify the remote UE of its inability to continue relay operation, for instance, in the notification message shown in step 10 of figure 5. As one example, a NotificationMessageSidelink message of a PC5-RRC protocol may contain a cause code or indication type, whose value may indicate cell reselection (in accordance with legacy signalling for indicating a cell reselection, e.g., a value “relayUE-CellReselection” ) and/or an inability to operate as a relay UE (e.g., indicated by a new value of the cause code or indication type) .

It is understood that the specific order or hierarchy of blocks in the processes /flowcharts disclosed is an illustration of exemplary approaches. Based upon design preferences, it is understood that the specific order or hierarchy of blocks in the processes /flowcharts may be rearranged. Further, some blocks may be combined or omitted. The accompanying method claims present elements of the various blocks in a sample order, and are not meant to be limited to the specific order or hierarchy presented.

The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more. ” The word “exemplary” is used herein to mean “serving as an example, instance, or illustration. ” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects. Unless specifically stated otherwise, the term “some” refers to one or more. Combinations such as “at least one of A, B, or C, ” “one or more of A, B, or C, ” “at least one of A, B, and C, ” “one or more of A, B, and C, ” and “A, B, C, or any combination thereof” include any combination of A, B, and/or C, and may include multiples of A, multiples of B, or multiples of C. Specifically, combinations such as “at least one of A, B, or C, ” “one or more of A, B, or C, ” “at least one of A, B, and C, ” “one or more of A, B, and C, ” and “A, B, C, or any combination thereof” may be A only, B only, C only, A and B, A and C, B and C, or A and B and C, where any such combinations may contain one or more member or members of A, B, or C. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. The words “module, ” “mechanism, ” “element, ” “device, ” and the like may not be a substitute for the word “means. ” As such, no claim element is to be construed as a means plus function unless the element is expressly recited using the phrase “means for. ”

Claims

A method of cell reselection handling operable in a first UE, comprising:

operating in an idle or inactive protocol state in the service of a first cell;

performing a discovery procedure with a second UE, the discovery procedure comprising indicating, to the second UE, an identity of the first cell;

performing a cell reselection procedure to a second cell;

receiving, from the second UE, subsequent to the performing a cell reselection procedure, a message for forwarding to the first cell; and

sending, to the second UE, a notification message, the notification message comprising an indication of an inability to forward the message.
The method of claim 1, wherein the first cell is operated by a first base station and the second cell is operated by a second base station, the first base station and the second base station being different.
The method of claim 1, wherein the notification message comprises an indication that the first UE has performed a cell reselection.
The method of claim 1, wherein the notification message comprises an indication that the first UE cannot operate as a relay UE in the second cell.
A method of path switch control operable in a first UE, comprising:

performing a discovery procedure with a second UE, the discovery procedure comprising receiving, from the second UE, an identity of a serving cell of the second UE;

receiving, from a base station operating a serving cell of the first UE, a reconfiguration message, the reconfiguration message comprising a handover command;

establishing a communication link with the second UE;

sending, to the second UE, a handover completion message;

receiving, from the second UE, a notification message, the notification message comprising an indication of an inability of the second UE to forward the handover completion message; and

initiating a failure handling procedure.
The method of claim 5, wherein the receiving the reconfiguration message comprises receiving, from a third UE served by the base station, the reconfiguration message.
The method of claim 5, wherein the notification message comprises an indication that the second UE has performed a cell reselection.
The method of claim 5, wherein the notification message comprises an indication that the second UE cannot operate as a relay UE in the second cell.
The method of claim 5, wherein the failure handling procedure comprises one or more of a relay reselection procedure, a cell reselection procedure, and a connection re-establishment procedure.