Patent Description:
As will be described in more detail herein, a BS may be referred to as an eNode B, a gNB, an access point (AP), a radio head, a transmit receive point (TRP), a New Radio (NR) BS, a <NUM> Node B, and/or the like.

A handover technique may include a multi-phase handover where two protocol stacks are used to hand over the UE from a source BS to a target BS (e.g., a first protocol stack is used for continued communications with the source BS while the second protocol stack is used for handover of the UE from the source BS to the target BS). While this technique may facilitate satisfaction of design targets related to reliability and/or latency, this technique may experience a threshold handover failure rate, a threshold amount of ping-pong handover signaling, and/or the like.

Some techniques and apparatuses described herein provide for handover failure during an enhanced MBB handover (e.g., a two protocol stack handover, a low latency or zero latency handover, and/or the like). For example, some techniques and apparatuses described herein provide for dynamically controlling when a UE declares a radio link failure (RLF) and provide for other signaling that the UE can use to notify a source BS that the UE has experienced a handover failure. This facilitates performance of actions by the UE, the source BS, and/or a target BS to rectify the handover failure without a loss of a source connection between the UE and the source BS. This reduces or eliminates a loss of communications for the UE during a handover failure, thereby improving communications of the UE. In addition, this reduces or eliminates ping-pong signaling that would otherwise occur as a result of a handover failure, thereby conserving processing resources of devices that send and/or receive the ping-pong signaling, thereby conserving network resources (e.g., bandwidth) that would otherwise be consumed communicating the ping-pong signaling, and/or the like. Further, this improves a reliability of a handover when a handover failure occurs, such as by reducing or eliminating loss of communications for the UE as a result of the handover failure.

ZTE CORPORATION: "RLF analysis on make before break solution", 3GPP DRAFT; R2-<NUM> relates to issues related to the radio link failure of the make-before-break handover. <CIT> relates to systems and methods for reducing interupptions in data transmissions due to handover operations. <CIT> relates to baton handover in TD-SCDMA communications.

A BS is an entity that communicates with user equipments (UEs) and may also be referred to as a base station, a NR BS, a Node B, a gNB, a <NUM> node B (NB), an access point, a transmit receive point (TRP), and/or the like.

UEs <NUM> (e.g., 120a, 120b, 120c, 120d, 120e) may be dispersed throughout wireless network <NUM>, and each UE may be stationary or mobile.

At base station <NUM>, a transmit processor <NUM> may receive data from a data source <NUM> for one or more UEs, select one or more modulation and coding schemes (MCS) for each UE based at least in part on channel quality indicators (CQIs) received from the UE, process (e.g., encode and modulate) the data for each UE based at least in part on the MCS selected for the UE, and provide data symbols for all UEs.

Controller/processor <NUM> of base station <NUM>, controller/processor <NUM> of UE <NUM>, and/or any other component(s) of <FIG> may perform one or more techniques associated with enhanced make-before-break (MBB) handover failure, as described in more detail elsewhere herein. For example, controller/processor <NUM> of base station <NUM>, controller/processor <NUM> of UE <NUM>, and/or any other component(s) of <FIG> may perform or direct operations of, for example, process <NUM> of <FIG>, process <NUM> of <FIG>, process <NUM> of <FIG>, and/or other processes as described herein. Memories <NUM> and <NUM> may store data and program codes for base station <NUM> and UE <NUM>, respectively.

In some aspects, UE <NUM> may include means for detecting that a timer has expired prior to completing a handover of the UE <NUM> from a source BS to a target BS, means for selectively declaring, based at least in part on detecting that the timer has expired, a radio link failure, and/or the like. In some aspects, such means may include one or more components of UE <NUM> described in connection with <FIG>.

In some aspects, base station <NUM> may include means for determining to release a handover resource associated with a handover of a UE from a source BS <NUM> to a target BS <NUM> based at least in part on an indication that the handover has experienced a failure without declaration of a radio link failure, means for releasing the handover resource associated with the handover of the UE based at least in part on determining to release the handover resource, and/or the like. In some aspects, such means may include one or more components of base station <NUM> described in connection with <FIG>.

In some aspects, base station <NUM> may include means for detecting that a connection between a UE and a source BS <NUM> is associated with a threshold connection condition, means for selectively maintaining, during a handover of the UE from the source BS <NUM> to a target BS <NUM>, a UE context associated with the UE based at least in part on detecting that the connection between the UE and the source BS <NUM> is associated with the threshold connection condition, and/or the like. In some aspects, such means may include one or more components of base station <NUM> described in connection with <FIG>.

A source BS (e.g., a serving BS, etc.) may perform a handover of a UE to a target BS. For example, a UE in motion may be handed over from one BS to another BS as the UE moves through cells provided by different BSs.

Mobility enhancements for UE handover may have certain design targets, such as approximately zero millisecond handover interruption latency (e.g., for real-time gaming applications and other ultra-reliable low latency communications (URLLC) applications), a threshold handover reliability, and/or the like. To satisfy these design targets, an enhanced make-before-break (MBB) handover technique may be used to hand over a UE from a source BS to a target BS. For example, an enhanced MBB handover may include a two protocol stack handover, a low latency or zero latency handover, and/or the like.

Continuing with the previous example, this technique may include dual Rx/dual Tx, dual Rx/single Tx, single Rx/single Tx, dual PHY, dual L2 stack (e.g., medium access control (MAC), radio link control (RLC), packet data convergence protocol (PDCP), and/or the like) during handover, security key switch during handover, PDCP sequence number continuity after handover, support for both intra-frequency and inter-frequency BS handover, support for synchronous and asynchronous BS handover, random access channel (RACH)-based handover, and/or the like.

<FIG> is a diagram illustrating an example <NUM> of enhanced make-before-break (MBB) handover failure, in accordance with various aspects of the present disclosure. As shown in <FIG>, example <NUM> includes a UE (e.g., UE <NUM>), a source BS (e.g., BS <NUM>) (e.g., a BS to which the UE is connected via a source connection), and a target BS (e.g., BS <NUM>) (e.g., a BS to which the UE is being handed over and is to be connected via a target connection).

As shown in <FIG>, and by reference number <NUM>, the UE and the source BS may be connected via the source connection. In the example shown in <FIG>, the UE is being handed over from the source BS to the target BS. As shown by reference number <NUM>, the UE may be attempting to establish the target connection with the target BS. For example, the UE, the source BS, and the target BS may be performing an enhanced MBB handover to hand over the UE from the source BS to the target BS (e.g., where two protocol stacks are used to maintain the source connection with the source BS and to establish the target connection with the target BS).

As shown by reference number <NUM>, the UE may detect that a timer has expired prior to completing a handover of the UE from the source BS to the target BS. For example, the timer may include a T304 timer that the UE initiates based at least in part on receiving a radio resource control (RRC) connection reconfiguration message (e.g., that includes mobility control information) from the source BS. In some aspects, the UE may use the timer to detect a handover failure (e.g., expiration of the timer may indicate a handover failure with regard to a handover of the UE from the source BS to the target BS). For example, the UE may detect a timeout a handover of the UE if the handover is not completed by expiration of the timer.

As shown by reference number <NUM>, the UE may selectively declare a radio link failure (RLF). For example, the UE may declare the RLF, or may not declare the RLF, based at least in part on detecting that the timer has expired.

In some aspects, the UE may determine whether the source connection between the UE and the source BS is associated with a threshold connection condition. For example, a threshold connection condition may be based at least in part on a threshold reference signal received power (RSRP) associated with the source connection, a threshold reference signal received quality (RSRQ) associated with the source connection, a threshold block error rate (BLER) associated with the source connection, and/or the like.

In some aspects, if the UE determines that the source connection is associated with a threshold connection condition, then the UE may not declare the RLF. In this way, the UE can maintain communications with the source BS while attempting to establish the target connection with the target BS. This provides a way for the UE to handle a handover failure while reducing or eliminating ping-pong handover signaling as a result of the handover failure, to improve reliability of the handover during a handover failure, and/or the like.

In some aspects, if the UE determines not to declare the RLF (e.g., when the source connection is associated with a threshold connection condition), then the UE may transmit, to the source BS, an RRC message. For example, the RRC message may be associated with indicating to the source BS, or may indicate to the source BS, that a handover failure has occurred with respect to the handover of the UE from the source BS to the target BS. In some aspects, the RRC message may be associated with causing the source BS to notify the target BS of the handover failure (e.g., the source BS may notify the target BS of the handover failure based at least in part on receiving the RRC message from the UE).

In some aspects, the RRC message may cause the source BS to continue with data scheduling, data transfer, and/or the like for the UE during the handover failure. This reduces or eliminates interruptions to communications of the UE despite occurrence of a handover failure. In some aspects, when the UE does not declare the RLF during a handover failure, the UE may continue to use a signaling radio bearer (SRB) associated with the source BS or a data radio bearer (DRB) associated with the source BS during the handover failure. This facilitates continued communications between the UE and the source BS during a handover failure.

In some aspects, if the UE detects that the source connection between the UE and the source BS is not associated with a threshold connection condition, then the UE may declare the RLF. For example, the UE may declare the RLF based at least in part on detecting that the source connection is not associated with the threshold connection condition. For example, the UE may need a threshold connection condition to maintain communications between the UE and the source BS, and may declare the RLF based at least in part on being incapable of maintaining communications with the source BS. In some aspects, and based at least in part on declaring the RLF, the UE may use an RLF recovery mechanism, such as to recover the source connection. For example, the UE may return to using a configuration associated with a source cell associated with the source BS. This facilitates recovery of a source connection when the source connection is not associated with a threshold connection condition and when the UE experiences a handover failure during a handover of the UE from the source UE to the target UE.

In some aspects, when the UE detects the RLF (e.g., on the source BS while performing a random access channel procedure on the target BS, or after successfully connecting to the target BS), the UE may continue the random access channel procedure on the target BS without triggering a radio resource control connection reestablishment. In some aspects, the UE may release a source stack and a source connection, and may forgo releasing a source connection that does not exist in connection with receiving a radio resource control connection reconfiguration message. In some aspects, the UE may transmit a radio link failure information message (e.g., identifying the RLF on the source BS and identifying a source cell identifier for the source BS) to the target BS. In this case, the target BS may cause a release of UE resources and/or a UE context at the source BS. In some aspects, if a target BS setup procedure fails (e.g., based at least in part on a T304 timer) after the source BS is released, the UE may trigger a radio resource control connection reestablishment (e.g., based at least in part on the source BS connection being released).

<FIG> is a diagram illustrating an example <NUM> of release of a handover resource by a target BS, in accordance with various aspects of the present disclosure. As shown in <FIG>, example <NUM> includes a UE (e.g., UE <NUM>), a source BS (e.g., BS <NUM>) (e.g., a BS to which the UE is connected via a source connection), and a target BS (e.g., BS <NUM>) (e.g., a BS to which the UE is being handed over and to which the UE is to be connected via a target connection).

In example <NUM>, the UE has experienced a handover failure during a handover of the UE from the source BS to the target BS, as described in more detail above in connection with <FIG>. For example, a timer (e.g., a T304 timer) initiated by the UE may have expired without the UE being successfully handed over to the target BS.

As shown by reference number <NUM>, the UE may transmit, and the source BS may receive, a handover failure indication. For example, the UE may transmit the handover failure indication based at least in part on determining that the timer has expired and/or determining that a source connection between the UE and the source BS is associated with a threshold connection condition, in a manner similar to that described elsewhere herein. In some aspects, the UE may transmit the handover failure indication based at least in part on not declaring an RLF, in a manner similar to that described elsewhere herein with respect to reference number <NUM> (e.g., the handover failure indication may be included in the RRC message that the UE transmits).

As shown by reference number <NUM>, the source BS may transmit, and the target BS may receive, the handover failure indication. For example, the source BS may transmit the handover failure indication based at least in part on receiving the handover failure indication from the UE. In some aspects, the source BS may transmit the handover failure indication to the target BS based at least in part on the UE having experienced the handover failure. In this way, the target BS may be notified of the handover failure despite the target BS lacking the target connection between the UE and the target BS as a result of the handover of the UE experiencing a failure.

As shown by reference number <NUM>, the target BS may determine to release a handover resource. For example, the target BS may determine to release the handover resource based at least in part on receiving the handover failure indication from the source BS (e.g., that indicates that the handover has experienced a failure) without the UE declaring an RLF. In some aspects, the handover resource may be associated with a handover of the UE from the source BS to the target BS. In some aspects, a handover resource may include a time frequency resource allocated for the handover of the UE from the source BS to the target BS.

In some aspects, the target BS may initiate a timer related to releasing the handover resource. For example, the target BS may initiate a timer related to releasing the handover resource based at least in part on receiving the handover failure indication from the source BS. Additionally, or alternatively, and as another example, the target BS may initiate a timer related to releasing the handover resource based at least in part on transmitting, to the source BS, a handover request acknowledgement (ACK). In some aspects, the target BS may determine to release the handover resource based at least in part on expiration of the timer relating to releasing the handover resource.

As shown by reference number <NUM>, the target BS may release the handover resource. For example, the target BS may release the handover resource based at least in part on determining to release the handover resource (e.g., based at least in part on receiving the handover failure indication, expiration of the timer after initiating the timer, and/or the like). Thus, time resources of the target BS and the UE are conserved and can be used for other purposes.

<FIG> is a diagram illustrating an example <NUM> of release of a UE context by a source BS, in accordance with various aspects of the present disclosure. As shown in <FIG>, example <NUM> includes a UE (e.g., UE <NUM>), a source BS (e.g., BS <NUM>) (e.g., a BS to which the UE is connected via a source connection), and a target BS (e.g., BS <NUM>) (e.g., a BS to which the UE is to be handed over and to which the UE is to be connected via a target connection).

As shown in <FIG>, and by reference number <NUM>, the UE and the source BS may be connected via the source connection. In the example shown in <FIG>, the UE is being handed over from the source BS to the target BS. As shown by reference number <NUM>, the UE may be attempting to establish the target connection with the target BS. For example, the UE, the source BS, and the target BS may be performing an enhanced MBB handover of the UE from the source BS to the target BS (e.g., where two protocol stacks are used to maintain the source connection with the source BS, and to establish the target connection with the target BS, respectively).

As shown by reference number <NUM>, the source BS may detect that the source connection is associated with a threshold connection condition (e.g., a threshold connection condition may facilitate a data transfer between the UE and the source BS). For example, the source BS may detect that the source connection is associated with a threshold connection condition in a manner that is similar to that described elsewhere herein. In some aspects, the source BS may detect that the source connection is associated with the threshold connection condition based at least in part on receiving a handover failure indication, similar to that described elsewhere herein, from the UE.

As shown by reference number <NUM>, the source BS may maintain a UE context associated with the UE during a handover of the UE to the target BS. For example, the source BS may maintain the UE context based at least in part on detecting that the source connection between the UE and the source BS is associated with the threshold connection condition. The UE context may identify information associated with handing over the UE from the source BS to the target BS, such as security information, a UE identifier, and/or other information used to perform handover of the UE.

In some aspects, the source BS may maintain the UE context based at least in part on failing to receive, from the target BS, an indication to transmit an X2 data forward indication to the UE. For example, as a result of the handover experiencing a handover failure, the target BS may fail to transmit, to the source BS, the indication to transmit the X2 data forward indication to the UE, and the source BS may maintain the UE context based at least in part on failing to receive the indication.

Additionally, or alternatively, the source BS may maintain the UE context based at least in part on failing to receive a UE context release indication from the target BS. For example, as a result of the handover experiencing the handover failure, the target BS may fail to transmit a UE context release indication to the source BS, and the source BS may maintain the UE context based at least in part on this.

Additionally, or alternatively, the source BS may maintain the UE context based at least in part on detecting expiration of a timer associated with the handover of the UE from the source BS to the target BS. For example, the source BS may initiate a timer based at least in part on initiation of the handover of the UE from the source BS to the target BS, and may maintain the UE context based at least in part on detecting expiration of the timer without receiving an indication that the UE has been successfully handed over from the source BS to the target BS.

<FIG> is a diagram illustrating an example process <NUM> performed, for example, by a UE, in accordance with various aspects of the present disclosure. Example process <NUM> is an example where a UE (e.g., UE <NUM> and/or the like) performs operations associated with enhanced make-before-break (MBB) handover failure.

As shown in <FIG>, in some aspects, process <NUM> may include detecting that a timer has expired prior to completing a handover of the UE from a source BS to a target BS (block <NUM>). For example, the UE (e.g., using receive processor <NUM>, transmit processor <NUM>, controller/processor <NUM>, memory <NUM>, and/or the like) may detect that a timer has expired prior to completing a handover of the UE from a source BS to a target BS, as described above.

As further shown in <FIG>, in some aspects, process <NUM> may include selectively declaring, based at least in part on detecting that the timer has expired, a radio link failure (block <NUM>). For example, the UE (e.g., using receive processor <NUM>, transmit processor <NUM>, controller/processor <NUM>, memory <NUM>, and/or the like) may selectively declare, based at least in part on detecting that the timer has expired, a radio link failure, as described above.

In a first aspect, the timer is a T304 timer.

In a second aspect, alone or in combination with the first aspect, the UE does not declare the radio link failure, based at least in part on detecting that a connection between the UE and the source BS is associated with a threshold connection condition.

In a third aspect, alone or in combination with one or more of the first and second aspects, the threshold connection condition is based at least in part on at least one of: a threshold reference signal received power (RSRP) associated with the connection, a threshold reference signal received quality (RSRQ) associated with the connection, or a threshold block error rate (BLER) associated with the connection.

In a fourth aspect, alone or in combination with one or more of the first through third aspects, process <NUM> includes transmitting, to the source BS, a radio resource control (RRC) message based at least in part on detecting that the connection between the UE and the source BS is associated with the threshold connection condition, where the RRC message is associated with indicating to the source BS that a handover failure has occurred between the UE and the target BS, and where the RRC message is associated with maintaining data scheduling by the source BS for the UE.

In a fifth aspect, alone or in combination with one or more of the first through fourth aspects, process <NUM> includes using, based at least in part on transmitting the RRC message, a signaling radio bearer (SRB) associated with the source BS or a data radio bearer (DRB) associated with the source BS to maintain communications between the UE and the source BS during the handover failure.

In a sixth aspect, alone or in combination with one or more of the first through fifth aspects, process <NUM> includes detecting that a connection between the UE and the source BS is not associated with a threshold connection condition, and declaring the radio link failure based at least in part on detecting that the connection is not associated with the threshold connection condition.

In a seventh aspect, alone or in combination with one or more of the first through sixth aspects, process <NUM> includes continuing to perform a random access channel procedure on the target BS without triggering a radio resource control connection reestablishment based at least in part on selectively declaring the radio link failure.

In an eighth aspect, alone or in combination with one or more of the first through seventh aspects, process <NUM> includes releasing a source stack and a source connection based at least in part on declaring the radio link failure.

In a ninth aspect, alone or in combination with one or more of the first through eighth aspects, process <NUM> includes transmitting an indication of the radio link failure to the target BS based at least in part on detecting the radio link failure.

In a tenth aspect, alone or in combination with one or more of the first through ninth aspects, a target BS setup procedure fails in connection with the timer; and process <NUM> includes triggering a radio resource control connection reestablishment after a release of the source BS.

In an eleventh aspect, alone or in combination with one or more of the first through tenth aspects, process <NUM> includes using a radio link failure recovery mechanism based at least in part on declaring the radio link failure to establish a source connection between the source BS and the UE, or to attempt to establish a target connection between the target BS and the UE.

<FIG> is a diagram illustrating an example process <NUM> performed, for example, by a target BS, in accordance with various aspects of the present disclosure. Example process <NUM> is an example where a target BS (e.g., target BS <NUM> and/or the like) performs operations associated with release of a handover resource.

As shown in <FIG>, in some aspects, process <NUM> may include determining to release a handover resource associated with a handover of a UE from a source BS to the target BS based at least in part on an indication that the handover has experienced a failure without declaration of a radio link failure (block <NUM>). For example, the BS (e.g., using transmit processor <NUM>, receive processor <NUM>, controller/processor <NUM>, memory <NUM>, and/or the like) may determine to release a handover resource associated with a handover of a UE from a source BS to the target BS based at least in part on an indication that the handover has experienced a failure without declaration of a radio link failure, as described above.

As further shown in <FIG>, in some aspects, process <NUM> may include releasing the handover resource associated with the handover of the UE based at least in part on determining to release the handover resource (block <NUM>). For example, the BS (e.g., using transmit processor <NUM>, receive processor <NUM>, controller/processor <NUM>, memory <NUM>, and/or the like) may release the handover resource associated with the handover of the UE based at least in part on determining to release the handover resource, as described above.

In a first aspect, process <NUM> includes receiving, from the source BS, the indication that the handover has experienced the failure without declaration of the radio link failure, and determining to release the handover resource based at least in part on receiving the indication from the source BS.

In a second aspect, alone or in combination with the first aspect, process <NUM> includes initiating a timer based at least in part on at least one of receiving the indication from the source BS, or transmitting, to the source BS, a handover request acknowledgement (ACK), and releasing the handover resource includes releasing the handover resource based at least in part on expiration of the timer after initiating the timer.

In a third aspect, alone or in combination with one or more of the first and second aspects, process <NUM> includes receiving a radio link failure indication from the UE, and causing a release of a UE resource or a UE context on the source BS based at least in part on receiving a radio link failure indication.

<FIG> is a diagram illustrating an example process <NUM> performed, for example, by a source BS, in accordance with various aspects of the present disclosure. Example process <NUM> is an example where a source BS (e.g., BS <NUM> and/or the like) performs operations associated with release of a UE context.

As shown in <FIG>, in some aspects, process <NUM> may include detecting that a connection between a UE and the source BS is associated with a threshold connection condition (block <NUM>). For example, the BS (e.g., using transmit processor <NUM>, receive processor <NUM>, controller/processor <NUM>, memory <NUM>, and/or the like) may detect that a connection between a UE and the source BS is associated with a threshold connection condition, as described above.

As further shown in <FIG>, in some aspects, process <NUM> may include selectively maintaining, during a handover of the UE from the source BS to a target BS, a UE context associated with the UE based at least in part on detecting that the connection between the UE and the source BS is associated with the threshold connection condition (block <NUM>). For example, the BS (e.g., using transmit processor <NUM>, receive processor <NUM>, controller/processor <NUM>, memory <NUM>, and/or the like) may selectively maintain, during a handover of the UE from the source BS to a target BS, a UE context associated with the UE based at least in part on detecting that the connection between the UE and the source BS is associated with the threshold connection condition, as described above.

In a first aspect, maintaining the UE context includes maintaining the UE context based at least in part on at least one of failing to receive, from the target BS, an indication to transmit an X2 data forward indication to the UE, failing to receive a UE context release indication from the target BS, or detecting expiration of a timer associated with the handover of the UE from the source BS to the target BS.

In a second aspect, alone or in combination with the first aspect, selectively maintaining the UE context includes causing a release of a UE resource or the UE context on the source BS based at least in part on receiving the radio link failure indication.

Claim 1:
A method of wireless communication performed by a user equipment, UE (<NUM>), comprising:
detecting that a handover failure timer has expired prior to completing a handover of the UE (<NUM>) from a source base station, BS (<NUM>), to a target BS (<NUM>);
detecting that a connection between the UE (<NUM>) and the source BS (<NUM>) is not associated with a threshold connection condition;
selectively declaring, based at least in part on detecting that the timer has expired, a radio link failure, unless the UE also detects that a connection between the UE (<NUM>) and the source BS (<NUM>) satisfies a threshold connection condition, wherein selectively declaring the radio link failure comprises:
declaring the radio link failure based at least in part on detecting that the connection is not associated with the threshold connection condition;
transmitting, to the source BS (<NUM>), a radio resource control, RRC, message based at least in part on detecting that the connection between the UE (<NUM>) and the source BS (<NUM>) is associated with the threshold connection condition, wherein the RRC message is configured to indicate to the source BS (<NUM>) that a handover failure has occurred between the UE (<NUM>) and the target BS (<NUM>);
using, based at least in part on transmitting the RRC message, a signaling radio bearer, SRB, associated with the source BS (<NUM>) or a data radio bearer, DRB, associated with the source BS (<NUM>) to maintain communications between the UE (<NUM>) and the source BS (<NUM>) during the handover failure; and
continuing to perform a random access channel procedure on the target BS (<NUM>) without triggering a radio resource control connection reestablishment based at least in part on selectively declaring the radio link failure.