Patent Description:
When moving between radio cells, an apparatus such as a user equipment (UE) may be subject to handovers between base stations managing the radio cells, in order to keep the apparatus connected to a radio network.

3GPP TDoc R2-<NUM> and 3GPP TDoc R2-<NUM> discuss conditional handover. They both describe introducing an exit condition to release network resources corresponding to a target cell, in case the target cell is no longer suitable for conditional handover.

To assist understanding of the present disclosure and to show how some embodiments may be put into effect, reference is made by way of example only to the accompanying drawings in which:.

The invention is disclosed in the embodiment corresponding to <FIG>. Other embodiments are not encompassed by the wording of the claims but are considered as useful for understanding the invention. Some examples may be provided in the context of cell handovers.

Failed handovers negatively impact the service quality provided to a User Equipment (UE) in a cellular system. In Long Term Evolution (LTE), failed handovers can be caused by failure of handover preparation signalling. For example, failed handovers can be caused by failure of a measurement report or a handover command.

Handover preparation failures could be mitigated, or avoided, by an earlier preparation of target cells. A target cell can be considered to be a cell which a UE attempts to handover to from a source cell. In 3GPP (<NUM> Partnership Project), this technique is known as UE based Handover or Conditional Handover (CHO). In the preparation phase of CHO, a network prepares a set of candidate cells based on a measurement report from the UE, and signals preparation information of the candidate cells to the UE. The preparation may comprise Radio Resource Control (RRC) configurations of the candidate cells and an execution trigger. In the execution phase of CHO, the UE may trigger a random access to a target cell based on the execution trigger configured by the network.

An example of a CHO procedure is shown in <FIG>. In this example, the CHO procedure comprises two phases: a CHO preparation phase shown at S108 in <FIG>, and a CHO execution phase shown at S110 in <FIG>.

At the beginning of S108, a source base station, such as a source gNB <NUM> is connected to the UE. At S112, a measurement control message is sent to UE <NUM>. In response, a CHO Add Event takes place at the UE in S114.

At S116, a measurement report is sent from the UE to the source gNB. Based on the measurement report, the source gNB can prepare a set of candidate cells for handover. The set of candidate cells may comprise, for example, target gNB <NUM>.

At s118, source gNB <NUM> sends a handover preparation request for UE <NUM> to target gNB <NUM>. The handover preparation request may comprise an instruction for target gNB <NUM> to prepare for the handover. In response to the handover preparation request, target gNB <NUM> may send a handover preparation request acknowledgement at S120. At S122, source gNB <NUM> sends a handover command message to UE <NUM>. The handover command message may comprise a list of prepared cells (or cells in the process of preparing for handover). The handover command message may also comprise RRC configurations of prepared cells (or cells in the process of preparing for handover). In examples, the handover command message may also comprise an execution trigger. In response to the handover command message sent at S122, UE <NUM> sends a handover command acknowledgement message at S126.

At S124, target gNB is in a state where it has made preparations for handover. This may comprise, for example, reserving resources for a potential handover of UE <NUM> from source gNB <NUM>.

In examples, the second phase of a CHO handover, shown at S110 of <FIG>, may be triggered by an execution trigger at the UE. The execution trigger may be configured by a network connected to the UE. The execution trigger may be sent from source gNB <NUM> to UE <NUM>. At S128, the UE begins the CHO execution phase. At S130, the UE triggers a random access to target gNB <NUM>. At S132, UE <NUM> sends a handover complete message to target gNB <NUM>. At S134, target gNB <NUM> sends a handover complete acknowledgement message to source gNB <NUM>.

In examples, at the end of the CHO execution phase S110, the old CHO preparations for the UE are no longer valid as shown at S136.

In 3GPP TS <NUM>, a UE state RRC_INACTIVE is specified for NG-RAN (Next Generation Radio Access Network). In this UE state, a UE remains in a CM-CONNECTED (Connection Management-CONNECTED) state and can move within an area configured by an NG-RAN without notifying the NG-RAN. This area is known as a Radio Access Network Notification Area (RNA). In RRC_INACTIVE, the last serving gNB node can keep the UE context and the UE-associated connection with a serving Access and Mobility Management Function (AMF) and a User Plane Function (UPF).

<FIG> shows an example of a UE triggered transition from an inactive state (RRC_INACTIVE) to a connected state (RRC_CONNECTED). It is to be appreciated that certain steps of <FIG> can be performed in an order other than that shown in <FIG>, and that some steps of <FIG> may be optional in some examples.

At S244 of <FIG>, UE <NUM> is in an RRC_INACTIVE/CM-CONNECTED state. At S246, UE <NUM> resumes from RRC-INCATIVE and sends an RRCConnectionResumeRequest message to gNB <NUM>. Then, at S248, gNB <NUM> requests the last serving gNB <NUM> to provide UE context data. In response, the last serving gNB <NUM> provides UE context data to gNB <NUM>, as shown at S250. The RRC connection is then resumed at S252 by gNB <NUM>.

At S254, there is an optional data forwarding address indication sent from gNB <NUM> to the last serving gNB <NUM>. This can be used to prevent loss of downlink user data buffered in the last serving gNB <NUM>.

At S256 and S260 a path switch is performed by gNB <NUM> contacting AMF <NUM> at S256 and receiving a response at S260.

At S262, the UE is in an RRC_CONNECTED/CM-CONNECTED state. At S264 gNB <NUM> triggers the release of the UE resources at the last serving gNB <NUM>.

<FIG> shows a situation where a UE transitions between a connected state (for example, an RRC_CONNECTED state) and an inactive state (for example, an RRC_INACTIVE state).

In the exemplary scenario shown in <FIG>, a UE approaches Cell <NUM> (hosted by gNB <NUM>) from Cell <NUM> (hosted by gNB <NUM>). During the approach, the UE may enter a region of cell <NUM> where candidate target cells are prepared for the UE. This region may be considered a "CHO region" of cell <NUM>. The candidate target cells may be prepared using the CHO procedure described above. A candidate target cell may be cell <NUM>.

At position <NUM> shown in <FIG>, data may arrive at the UE or at gNB1 triggering the UE to transition to a connected state, such as an RRC_CONNECTED state for example. The data may arrive at a buffer of gNB1.

At position <NUM>, as the UE crosses a boundary <NUM> within cell <NUM> into the CHO region of cell <NUM>. The UE is then prepared for CHO. A similar procedure to the exemplary procedure shown in <FIG> may be used. For example, the CHO preparation may comprise the following steps:.

Following the CHO preparation at position <NUM>, there may be a period of time where there is inactivity in uplink and downlink data transmission. This time period may occur when the UE is at position <NUM>, for example. An inactivity timer may expire during this time period. The inactivity timer may, for example, be located at gNB1 or the UE. It should be noted, however, that other inactivity timer locations may be used. When the inactivity timer expires, an RRC connection for the UE is suspended. This may happen when the UE is at position <NUM>, for example.

During the RRC connection suspend procedure, without any information on how to handle prepared cells in the inactive state (such as the RRC_INACTIVE state), the UE will release any cell preparation information. This may happen when the UE is at position <NUM> of <FIG>, for example. In order to release the prepared target cell(s), gNB1 is required to send a CHO Release message to the prepared target cell(s) and the respective gNBs of the prepared target cells then release the reserved resources in their cells and send a CHO Release Acknowledgement message to gNB1. In an example where cell <NUM> is the only prepared target cell, in order to release the prepared target cell the following steps are taken:.

In examples, at position <NUM>, there is uplink or downlink activity for the UE. The RRC connection is then resumed at position <NUM>, for example. At position <NUM>, the cycle of steps for each position <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> described above may be repeated indefinitely.

The steps described for each position <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> may repeat until either: the UE leaves the CHO region of Cell <NUM> (for example, at position <NUM>) triggering a release of the prepared cells; or the UE accesses one of the prepared cells (at position <NUM>). In examples, the UE accesses one of the prepared cells when it crosses a cell boundary <NUM> between cell <NUM> and cell <NUM>.

When the UE transitions repeatedly between a connected state and an inactive state, there may be excessive signalling overhead caused by preparing and releasing CHO target cells. This signalling overhead may be increased when there is a slow moving UE, a short inactivity timer value for suspending and/or a long CHO deconfiguration timer value. In this situation, there may be a high number of preparation/release cycles before a UE enters a target cell or preparations are released following elapse of the CHO deconfiguration timer. As each preparation cycle comprises each of the steps described for positions <NUM> to <NUM> above, there is potential for a large amount of signalling in both UE-gNB interfaces and gNB-gNB interfaces.

Also, during repeated preparation/release cycles, it will take time after the RRC connection is resumed to prepare candidate target cells using the CHO procedure again. This increases the probability of failed handovers. This delay is due to UE measurement (for example, L1 filtering, L3 filtering, Time To Trigger (TTT)) and cell preparation signalling (as described for position <NUM> of <FIG>).

In order to reduce signalling overhead and to decrease the probability of failed handovers, a release condition can be used to control when a prepared cell should be released. Releasing a prepared cell may comprise a UE releasing (i.e. no longer storing) preparation information, such as RRC configurations. Releasing a prepared cell may also comprise a release of reserved resources on the network side (for example, a gNB may release resources reserved for a UE).

<FIG> shows an example where a release condition for a prepared cell is used. In <FIG>, communications between UE <NUM>, gNB1 <NUM>, gNB2 406a and gNB3 406b are shown, however it will be appreciated that further UEs and base stations may also be involved in the communication procedure. In this example, cell <NUM> is managed by gNB1 <NUM>, cell <NUM> is managed by gNB2 406a and cell <NUM> is managed by gNB3 406b. Other configurations are also envisaged.

At <NUM> of <FIG>, UE <NUM> is in an active state (for example RRC_ACTIVE). At <NUM>, cells <NUM> and <NUM> are prepared for a conditional handover based on a measurement report from UE <NUM>. At <NUM>, there is an interruption in the data activity lasting longer than a configured inactivity time. In examples, this inactivity time may be pre-configured at gNB1 <NUM>.

In response to the expiry of the inactivity timer at <NUM>, at <NUM> gNB1 <NUM> sends a RRC Connection Suspend message to UE <NUM>. This message may contain a release condition which controls how long the UE shall keep the prepared cells configuration. In some examples, a prepared cell is released only when a release condition is met. In some examples, the release condition may also be sent to the UE <NUM> separately to the RRC Connection Suspend message.

In examples, the release condition may be related to cell quality of a prepared cell. Possible measures of cell quality which may be used include Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ) and Signal to Interference Noise Ratio (SINR).

In an example, the release condition could be a threshold of cell quality for prepared cells 406a, 406b. When a cell quality drops below the quality threshold, the cell may be released. In another example, when the cell quality of a best prepared cell drops below a configured threshold, all prepared cells may be released. In examples, the best prepared cell may be the cell with the highest cell quality.

In an example, the release condition may be whether a prepared cell drops X dB below the quality of cell C, where X is a predetermined number. Cell C could be any configured cell. Cell C could be the cell where the RRC Connection Suspend took place (in the example of <FIG>, this is gNB1 <NUM>). When the release condition is fulfilled, the cell which has dropped X dB below the quality of cell C may be released. In examples, when the cell quality of the best prepared cell drops X dB below the quality of cell C, all prepared cells are released.

In an example, the release condition may be related to a timer. The timer may be a UE timer. When the timer elapses prepared cell may be released. In examples, the timer is set to a configured value at the connections suspend <NUM>. The timer may be reset at state transitions. In other examples, the timer could be a common timer for both active and connected states i.e. not reset at state transitions.

Advantageously, for the above described release conditions, no additional UE measurements are required. The same measurements used for cell-reselection can be used. Therefore, in examples, an increase in UE power consumption can be avoided.

At <NUM>, UE <NUM> is in an inactive state (e.g. RRC_INACTIVE). At <NUM>, the release condition triggers in UE <NUM> and the prepared cells are released according to the release condition. In the example shown in <FIG>, cells <NUM> and <NUM> managed by gNB2 406a and gNB3 406b are released at S413 , however as described above, in some examples only one cell may be released.

At <NUM>, UE <NUM> sends a Release Indication message to gNB1 <NUM>, containing a list of cells to be released. This message may be transmitted by using an efficient small data transmission method, if specified for RRC_INACTIVE state, to avoid a full transition to an RRC_CONNECTED state. At <NUM>, the network releases prepared cells 406a and 406b according to the request.

In examples where the cell to which the UE <NUM> sent the Release Indication was the Last Serving gNB, the Last Serving gNB sends a Release Request message to all gNBs with cells to be released, receiving ACKs for a response. As a result, all cells can be released in the network side.

In examples where the cell to which the UE <NUM> sent the Release Indication was not the Last Serving gNB, there may be an extra step of contacting the Last Serving gNB, which will then trigger the release procedure.

In another example, the release of the prepared cells may be initiated at the network side instead of at the UE. For example, release may be initiated based on an elapse of a timer at a base station (e.g. gNB1 <NUM>) or based on uplink measurements.

<FIG> shows an example of a method where prepared candidate cells 506a, 506b are retained while a UE <NUM> is in an inactive state. In subsequent data session, the prepared candidate cells can be exploited to carry out a conditional handover to one of the prepared cells 506a, 506b from cell <NUM>.

Steps <NUM> to <NUM> are similar to steps <NUM> to <NUM> in <FIG>.

At step <NUM>, data is transmitted to UE <NUM> or to a network buffer (to gNB1 in <FIG>). At step <NUM>, base station <NUM> sends a message to UE <NUM> comprising instructions for the UE <NUM> to transition from an inactive state to an active state. In examples, at step <NUM> gNB1 <NUM> sends an RRC Connection Resume message to UE <NUM>. In examples, the RRC Connection Resume message contains a retain/discard field that will control the UE's actions with respect to prepared candidate cells gNB2 506a and gNB3 506b. In some examples, the RRC Connection Resume Message comprises an indication of whether the user equipment should retain or discard part or all of the preparation information for at least one cell. For example, the message may comprise an indication to release preparation information for gNB2 506a and not gNB3 506b, or vice versa. This may depend on certain conditions in each of the cells managed by gNB2 506a and gNB3 506b.

In examples where the UE is instructed to retain the prepared cells 506a and 506b (as shown in <FIG>), the connection is resumed and UE <NUM> will start evaluating the CHO execution trigger(s) for the configured target cell(s). This option can be used to avoid unnecessary signalling due to re-preparation of candidate cells 506a and 506b.

In examples where UE <NUM> is instructed to discard the RRC configurations, UE <NUM> will discard all RRC configurations, triggering a removal procedure in the network side. In case UE <NUM> is still in the CHO region of the cell managed by gNB1 <NUM> (which is likely as the cells were not released during RRC_INACTIVE state), a preparation procedure may start after a delay period. This option could be applied e.g. if the RRC configurations of the prepared target cells 506a and 506b are no longer valid.

At <NUM>, UE <NUM> transitions to an active state. At <NUM>, UE <NUM> accesses one of the target cells (cell <NUM> managed by gNB2 506a in <FIG>) by utilizing one of the stored RRC configurations.

In examples, a reduction of the UE-network and network-network signalling overhead due to minimization of unnecessary prepare-release cycles is provided. Significant savings are provided in scenarios involving slow moving UEs, and/or short inactivity timers, and/or long CHO deconfiguration timers.

In examples, a reduction of the handover failures that occur due to a UE resuming connection before any cells are prepared is provided.

An exemplary system of some examples will now be described in more detail with reference to <FIG> showing a schematic representation of a system <NUM>. The exemplary system <NUM> comprises a first apparatus <NUM> and a second apparatus <NUM>. The first apparatus <NUM> may be a UE. The second apparatus <NUM> may be a base station, for example, a gNB.

The first apparatus <NUM> may comprise at least at least one data processing entity <NUM>, at least one memory <NUM>, and other possible components for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with network devices and other communication devices. The at least one memory <NUM> may be in communication with the data processing entity <NUM>, which may be a data processor. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets.

The first apparatus <NUM> may optionally comprise a user interface such as key pad, voice commands, touch sensitive screen or pad, combinations thereof or the like. One or more of a display, a speaker and a microphone may optionally be provided. Furthermore, the first apparatus <NUM> may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto. The first apparatus <NUM> may receive signals over an air or radio interface via appropriate apparatus for receiving, and may transmit signals via appropriate apparatus for transmitting radio signals. In <FIG> a transceiver apparatus is shown schematically at <NUM>. The transceiver apparatus <NUM> may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the wireless device. The transceiver apparatus <NUM> may be controlled by communication unit <NUM>.

The second apparatus <NUM> may receive signals over an air or radio interface via appropriate apparatus for receiving, and may transmit signals via appropriate apparatus for transmitting radio signals. In <FIG> a transceiver apparatus of second apparatus <NUM> is shown schematically at <NUM>. The transceiver apparatus <NUM> may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the wireless device. The transceiver apparatus <NUM> may be controlled by a communication unit <NUM>.

The first apparatus <NUM> may be in contact with a second apparatus <NUM> over an interface. The second apparatus <NUM> may comprise at least at least one data processing entity <NUM>, at least one memory <NUM>, and other possible components for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with user devices and other communication devices. The at least one memory <NUM> may be in communication with the data processing entity <NUM>, which may be a data processor. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets.

<FIG> shows an example method. The method may be performed by a user equipment. The method comprises receiving, while connected to a first cell, preparation information for preparing the apparatus for a conditional handover to at least one other cell at S773. At S775, the method comprises receiving, from the first cell, a message, wherein the message causes the apparatus to transition from a connected state to an inactive state. At S777, the method comprises receiving, from the first cell, information of a release condition. At S779, when it is determined that the release condition is met, sending a release indication and releasing the preparation information.

<FIG> shows an example method. The method may be performed by a base station. The method comprises sending, to a user equipment, preparation information for preparing the user equipment for a conditional handover to at least one cell at S881. At S883, the method comprises sending, to the user equipment, a message, wherein the message causes the user equipment to transition from a connected state to an inactive state. At S885, the method comprises sending, to the user equipment, information of a release condition. At S887, the method comprises receiving, from the user equipment, a release indication for the at least one cell when it is determined that the release condition is met.

In general, the various examples shown may be implemented in hardware or in special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

Some embodiments may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Computer software or program, also called program product, including software routines, applets and/or macros, may be stored in any apparatus-readable data storage medium and they comprise program instructions to perform particular tasks. A computer program product may comprise one or more computer-executable components which, when the program is run, are configured to carry out methods are described in the present disclosure. The one or more computer-executable components may be at least one software code or portions of it.

Examples of the disclosed embodiments may be practiced in various components such as integrated circuit modules.

Claim 1:
A User Equipment UE comprising means for performing:
sending, while connected to a first cell, a measurement report (<NUM>); receiving, in response to sending the measurement report, preparation information, the preparation information including a list of candidate cells for preparing the UE for a conditional handover of the UE to at least one other cell (<NUM>);
receiving, from the first cell, a RRC Connection Suspend message, wherein the message causes the UE to transition from a connected state (<NUM>) to an inactive state (<NUM>) without the UE sending a release indication to the network, a release consisting of the UE no longer storing the preparation information (<NUM>);
receiving, from the first cell, information of a release condition (<NUM>);
when it is determined that the release condition is met when in the inactive state (<NUM>), sending a release indication containing a list of cells to be released and releasing the preparation information (<NUM>).