LTM

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9.2.3.5    L1/L2-TriggeredMobility

9.2.3.5.1    General

LTM is a procedure in which a gNB receives L1 measurement report(s) from a UE, and on their basisthe gNB changes UE serving cell by a cell switchcommand signalled via a MAC CE. The cell switch command indicates an LTMcandidate configuration that the gNB previously prepared and provided to the UEthrough RRC signalling. Then the UE switches to the target configurationaccording to the cell switch command. The LTM procedure can be used to reduce the mobility latency as described in AnnexG.

目的：降低移动性延迟，参见附录G

怎么降低延迟的：上下行同步提前，RACH-based，提前下行同步。Rach-lessLTM提前上下行同步。

两次测量上报：

层三测量上报，配置LTM

层一测量上报，通过MAC CE触发LTM。MAC CE（cell switch command）内容？层一测量上报内容？

When configured by the network, it is possible to activate TCI states of one ormultiple cells that are different from the current serving cell. Forinstance, the TCI states of the LTM candidate cells can be activated in advancebefore any of those cells become the serving cell. This allows the UE to be DLsynchronized with those cells, thereby facilitating a faster cell switch to oneof those cells when cell switch is triggered.

候选小区的TCI状态可以在成为服务小区前提前被激活，取决于网络配置，网络如何配置的？即允许UE与这些候选小区提前进行下行同步。依次还是同时？是否与服务小区的传输冲突？UE能力是否可以同时下行同步？

When configured by the network, it ispossible to initiate UL TA acquisition (called early TA) procedure of one ormultiple cells that are different from the current serving cells. If the cell has the same NTA as the current serving cells or NTA=0, early TAacquisition procedure is not required. [91] The network may request the UE to perform early TAacquisition of a candidate cell before a cell switch. The early TAacquisition procedure is triggered by PDCCH order as specified in clause 9.2.6or realized through UE-based TA measurement as configuredby RRC. In the former case, the gNB towhich the candidate cell belongs calculates the TA value and sends it to thegNB to which the serving cell belongs. The serving cell sends the TA value inthe LTM cell switch command MAC CE when triggering LTM cell switch. In thelatter case, the UE performs TA measurement for the candidate cells after beingconfigured by RRC but the exact time the UE performs TA measurement is up to UEimplementation. The UE applies the TA value measured by itself and performsRACH-less LTM upon receiving the cell switch command. The network may also senda TA value in the LTM cell switch command MAC CE without early TA acquisition.

若网络配置，UE可以触发候选小区的上行TA获取，当NTA与服务小区一致或NTA=0时，不需要early TA获取

TA获取在cell-switch之前，由网络配置。如何配置？PDCCH order或RRC配置的基于UE的TA测量。

在前一种情况下，候选小区所属的 gNB 计算 TA 值并将其发送给服务小区所属的 gNB。在触发 LTM 小区切换时，服务小区在 LTM 小区切换命令 MAC CE中发送 TA 值。在后一种情况下，UE 在 RRC 配置后对候选小区实行 TA 测量，但 UE 实行 TA 测量的具体时间取决于 UE 的实现。UE在收到小区切换命令后，应用自己测量的 TA 值并实行RACH less LTM。网络也可在 LTM 小区切换命令 MAC CE 中发送 TA 值，而不提前获取TA 值。

Depending on the availability of a valid TA value,the UE performs either a RACH-less LTM or RACH-basedLTM cell switch. If the TA value is provided in the cell switch command,the UE applies the TA value as instructed by the network. In the case where UE-based TA measurement is configured, but no TA valueis provided in the cell switch command, the UE appliesthe TA value by itself if available. Meanwhile, the UE performsRACH-less LTM cell switch upon receiving the cell switch command. If no validTA value is available, the UE performs RACH-based LTM cell switch.

TA值的有效与否决定rach-less LTM还是rach-basedLTM

cell switch command中提供TA的情况，UE直接使用

基于UE的TA测量，但cell switch command中未提供TA的情况，如果可用，UE自行使用TA值。如果不可用，UE实行rach-based LTM

Regardless of whether the UE is configured forUE-based TA measurement for a certain candidate cell, it will still follow thePDCCH order, which includes requesting a random access procedure towards thecandidate cells. This also applies to the candidate cells for which the UE iscapable of deriving TA values by itself. Additionally, regardlessof whether the UE has already performed a random access procedure towards thecandidate cells, it will still follow the UE-based measurement configuration ifconfigured by the network.

不论是否配置了基于UE的TA测量，PDCCH order仍然需要遵循，包含了向候选小区请求RACH的过程

不论是否向候选小区实行了RACH，如果网络配置了基于UE的TA测量，也仍然需要遵循

For RACH-less LTM, the UE accesses the target cellusing either a configuredgrant or a dynamic grant. The configured grantis provided in the LTM candidate configuration, and the UE selects the configured grant occasion associated with thebeam indicated in the cell switch command. Upon initiation of LTM cellswitch to the target cell, the UE starts to monitor PDCCH on the target cellfor dynamic scheduling. Before RACH-less LTM procedure completion, the UE shallnot trigger random access procedure if it does not have a valid PUCCH resourcefor triggered SRs.

The following principles apply to LTM:

-   Security key is maintained upon an LTM cell switch;

-    Subsequent LTM issupported.

RACH-less TM，CG和DG都可用。CG在LTM候选配置里配置，UE选择cell switch command指示的beam关联的CG occasion。

一旦切到目标小区，UE开始监听目标小区的动态调度PDCCH

Rach-less LTM完成之前，如果没有SR的PUCCH资源，UE不能触发RACH

LTM supports both intra-gNB-DU and intra-gNB-CUinter-gNB-DU mobility. LTM supports both intra-frequency and inter-frequencymobility, including mobility to inter-frequency cell that is not a currentserving cell. LTM is supported only for licensed spectrum. The followingscenarios are supported:

-    PCellchange in non-CA scenario and non-DC scenario;

-    PCelland SCell(s) change in CA scenario;

-    Dualconnectivity scenario, PCell and MCG SCell(s) change and intra-SN PSCell andSCG SCell(s) change without MN involvement. LTM for simultaneous PCell andPSCell change is not supported.

While the UE has stored LTM candidateconfigurations the UE can also execute any L3 handover command sent by thenetwork.

9.2.3.5.2    C-Plane Handling

Cell switch command is conveyed in a MAC CE, whichcontains the necessary information to perform the LTM cell switch.

The overall procedure for LTM is shown in Figure9.2.3.5.2-1 below. Subsequent LTM is done by repeating the earlysynchronization, LTM cell switch execution, and LTM cell switch completionsteps without releasing other LTM candidateconfigurations after each LTM cell switch completion. The generalprocedure over the air interface is applicable to SCG LTM. Further details ofSCG LTM can be found in TS 37.340 [21].

LTM cell switch完成后，不释放其他LTM配置。什么时候释放？

Figure 9.2.3.5.2-1. Signallingprocedure for LTM

The procedure for LTM is as follows:

1.    The UEsends a MeasurementReport message to thegNB. The gNB decides to configure LTM and initiates LTM preparation.

2.    ThegNB transmits an RRCReconfigurationmessage to the UE including the LTM candidate configurations.

3.    The UEstores the LTM candidate configurations and transmits an RRCReconfigurationComplete message to the gNB.

4a.    TheUE performs DL synchronization with the candidate cell(s) before receiving thecell switch command.

4b.    WhenUE-based TA measurement is configured, UE acquires theTA value(s) of the candidate cell(s) by measurement. UE performs early TA acquisition with the candidate cell(s)as requested by the network before receiving the cell switch command asspecified in clause 9.2.6. This is done via CFRA triggered by a PDCCHorder from the source cell, following which the UE sends preamble towards theindicated candidate cell. Inorder to minimize the data interruption of the source cell due to CFRA towardsthe candidate cell(s), the UE does not receive random access response from thenetwork for the purpose of TA value acquisition and the TA value of thecandidate cell is indicated in the cell switch command. The UE does notmaintain the TA timer for the candidate cell and relies on networkimplementation to guarantee the TA validity.

当基于UE的TA测量配置了，UE通过测量获取TA，在 cell switch command之前，通过原小区的基于非竞争的PDCCH order来实现的，UE向指示的候选小区发送preamble。为了最小化延迟时间，UE不接收RAR，TA在cell switch command中发送给UE。UE不维护候选小区的TA定时器，完全依赖网络的实现来保证TA的有效

5.    The UEperforms L1 measurements on the configured candidate cell(s) and transmits L1measurement reports to the gNB. L1 measurement shouldbe performed as long as RRC reconfiguration (step 2) is applicable.

6.    ThegNB decides to execute cell switch to a target cell and transmits a MAC CEtriggering cell switch by including the candidate configuration index of thetarget cell. The UE switches to the target cell and applies the configurationindicated by candidate configuration index.

7.    The UEperforms the random access procedure towards the target cell, if UE does not have valid TA of the target cell as specifiedin clause 6.1.3.xy of TS 38.321[6].

8.    The UEcompletes the LTM cell switch procedure by sendingRRCReconfigurationComplete message to target cell. If the UE hasperformed a RA procedure in step 7 the UE considers that LTM cell switchexecution is successfully completed when the random access procedure issuccessfully completed. For RACH-less LTM, the UE considers that LTM cellswitch execution is successfully completed when the UE determines that thenetwork has successfully received its first UL data.

The steps 4-8 can be performed multiple times for subsequent LTM using the LTMcandidate configuration(s) provided in step 2.

The procedure over the air interface described inFigure x is applicable to both intra-gNB-DU LTM and inter-gNB-DU LTM. Theoverall LTM procedures over F1-C interface are captured in TS 38.401[4].

9.2.3.5.3    U-Plane Handling

After receiving an LTM cell switch command MAC CE,the UE performs MAC reset. Whether the UE performs RLC re-establishment andPDCP data recovery during cell switch is explicitly controlled by the networkthrough RRC signalling.