Patent Publication Number: US-2023164650-A1

Title: Conditional procedure operations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of the filing date of (1) U.S. Provisional Patent Application No. 63/008,610 entitled “Conditional Procedure Operations,” filed on Apr. 10, 2020, and (2) U.S. Provisional Patent Application No. 63/028,294 entitled “Conditional Procedure Operations,” filed on May 21, 2020, the entire disclosures of each of which are hereby expressly incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to wireless communications and, more particularly, to conditional procedures such as conditional handover, conditional primary secondary cell (PSCell) addition or change procedures, and conditional secondary node addition or change procedures (i.e., PSCell addition or change procedures with SN change). 
     BACKGROUND 
     This background description is provided for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     In telecommunication systems, the Packet Data Convergence Protocol (PDCP) sublayer of the radio protocol stack provides services such as transfer of user-plane data, ciphering, integrity protection, etc. For example, the PDCP layer defined for the Evolved Universal Terrestrial Radio Access (EUTRA) radio interface (see 3GPP specification TS 36.323) and New Radio (NR) (see 3GPP specification TS 38.323) provides sequencing of protocol data units (PDUs) in the uplink direction (from a user device, also known as a user equipment (UE), to a base station) as well as in the downlink direction (from the base station to the UE). Further, the PDCP sublayer provides signaling radio bearers (SRBs) and data radio bearers (DRBs) to the Radio Resource Control (RRC) sublayer. Generally speaking, the UE and a base station can use SRBs to exchange RRC messages as well as non-access stratum (NAS) messages, and can use DRBs to transport data on a user plane. 
     UEs can use several types of SRBs and DRBs. When operating in dual connectivity (DC), the cells associated with the base station operating the master node (MN) define a master cell group (MCG), and the cells associated with the base station operating as the secondary node (SN) define the secondary cell group (SCG). So-called SRB1 resources carry RRC messages, which in some cases include NAS messages over the dedicated control channel (DCCH), and SRB2 resources support RRC messages that include logged measurement information or NAS messages, also over the DCCH but with lower priority than SRB1 resources. More generally, SRB1 and SRB2 resources allow the UE and the MN to exchange RRC messages related to the MN and embed RRC messages related to the SN, and also can be referred to as MCG SRBs. SRB3 resources allow the UE and the SN to exchange RRC messages related to the SN, and can be referred to as SCG SRBs. Split SRBs allow the UE to exchange RRC messages directly with the MN via lower layer resources of the MN and the SN. Further, DRBs using the lower-layer resources of only the MN can be referred as MCG DRBs, DRBs using the lower-layer resources of only the SN can be referred as SCG DRBs, and DRBs using the lower-layer resources of both the MCG and the SCG can be referred to as split DRBs. 
     The UE in some scenarios can concurrently utilize resources of multiple RAN nodes (e.g., base stations or components of a distributed base station), interconnected by a backhaul. When these network nodes support different radio access technologies (RATs), this type of connectivity is referred to as Multi-Radio Dual Connectivity (MR-DC). When a UE operates in MR-DC, one base station operates as a master node (MN) that covers a primary cell (PCell), and the other base station operates as a secondary node (SN) that covers a primary secondary cell (PSCell). The UE communicates with the MN (via the PCell) and the SN (via the PSCell). In other scenarios, the UE utilizes resources of one base station at a time. One base station and/or the UE determines that the UE should establish a radio connection with another base station. For example, one base station can determine to hand the UE over to the second base station, and initiate a handover procedure. 
     3GPP technical specifications (TS) 36.300 and 38.300 describe procedures for handover (also called reconfiguration with sync) scenarios. These procedures involve messaging (e.g., RRC signaling and preparation) between RAN nodes that generally causes latency, which in turn increases the probability of failure for handover procedures. Some handover procedures do not involve conditions associated with the UE, and can be referred to as “immediate” handover procedures. Other handover procedures involve conditions associated with the UE, and 3GPP TS 36.331 v16.0.0 and 38.331 v16.0.0 describe conditional handover scenarios. 
     3GPP specification TS 37.340 v16.1.0 describes procedures for a UE to add or change an SN in DC scenarios. These procedures involve messaging (e.g., RRC signaling and preparation) between radio access network (RAN) nodes. This messaging generally causes latency, which in turn increases the probability that the SN addition or SN change procedure will fail. These procedures, which do not involve conditions that are checked at the UE, can be referred to as “immediate” SN addition and SN change procedures. 
     UEs can also perform handover procedures to switch from one cell to another, whether in single connectivity (SC) or DC operation. The UE may handover from a cell of a first base station to a cell of a second base station, or from a cell of a first distributed unit (DU) of a base station to a cell of a second DU of the same base station, depending on the scenario. 3GPP specifications 38.401 v16.0.0, 36.300 v16.0.0 and 38.300 v16.0.0 describe a handover procedure that includes several steps (RRC signaling and preparation) between RAN nodes, which causes latency in the handover procedure and therefore increases the risk of handover failure. This procedure, which does not involve conditions that are checked at the UE, can be referred to as an “immediate” handover procedure. 
     More recently, for both SN or PSCell addition/change, “conditional” procedures have been considered (i.e., conditional SN or PSCell addition/change). Unlike the “immediate” procedures discussed above, these procedures do not add or change the SN or PSCell, or perform the handover, until the UE determines that a condition is satisfied. As used herein, the term “condition” may refer to a single, detectable state or event (e.g., a particular signal quality metric exceeding a threshold), or to a logical combination of such states or events (e.g., “Condition A and Condition B,” or “(Condition A or Condition B) and Condition C”, etc.). 
     To configure a conditional procedure, the RAN provides the condition to the UE, along with a configuration (e.g., one or more random-access preambles, etc.) that will enable the UE to communicate with the appropriate base station, or via the appropriate cell, when the condition is satisfied. For a conditional addition of a base station as an SN or a candidate cell as a PSCell, for example, the RAN provides the UE with a condition to be satisfied before the UE can add that base station as the SN or that candidate cell as the PSCell, and a configuration that enables the UE to communicate with that base station or PSCell after the condition has been satisfied. 
     In an immediate handover procedure, the RAN transmits a handover command including multiple configuration parameters to the UE and the UE attempts to connect to a target PCell configured by the handover command. After the UE successfully connects to the RAN via the target PCell, the UE communicates with the RAN on the single target PCell using the multiple configuration parameters and security key(s) associated to the target PCell and derived from one or more security configuration parameters in the handover command. The RAN also derives security key(s) which are the same as the security key(s) derived by the UE. After the UE successfully connects to the target PCell, the RAN communicates data with the UE using the same security key(s) and the multiple configuration parameters. 
     In a conditional handover procedure, the RAN can transmit a conditional handover command including multiple configuration parameters for a candidate PCell to the UE. If the UE determines that a condition is satisfied, the UE attempts to connect to the candidate PCell. After the UE successfully connects to the RAN via the candidate PCell, the UE communicates with the RAN on the candidate PCell using the multiple configuration parameters and security key(s) associated to the candidate PCell and derived from one or more security configuration parameters in the conditional handover command. The RAN also derives security key(s) which are the same as the security key(s) derived by the UE. After the UE successfully connects to the candidate PCell, the RAN communicates data with the UE using the same security key(s) and the multiple configuration parameters. 
     In an immediate PSCell addition or change procedure, the RAN (i.e., MN or SN) transmits an RRC reconfiguration message including multiple configuration parameters to the UE and the UE attempts to connect to a (target) PSCell configured by the RRC reconfiguration message. After the UE successfully connects to the SN via the PSCell, the UE communicates with the SN on the PSCell using the multiple configuration parameters and security key(s) associated to the PSCell and derived from one or more security configuration parameters in the RRC reconfiguration message. The SN also derives security key(s) which are the same as the security key(s) derived by the UE. After the UE successfully connects to the PSCell, the RAN (i.e., SN) communicates data with the UE using the same security key(s) and the multiple configuration parameters. 
     In a conditional PSCell addition or change procedure, the RAN (i.e., MN or SN) transmits an RRC reconfiguration message including multiple configuration parameters to the UE and the UE attempts to connect to a candidate PSCell configured by the RRC reconfiguration message. After the UE successfully connects to the SN via the candidate PSCell, the UE communicates with the SN on the candidate PSCell using the multiple configuration parameters and security key(s) associated to the candidate PSCell and derived from one or more security configuration parameters in the RRC reconfiguration message. The SN also derives security key(s) which are the same as the security key(s) derived by the UE. After the UE successfully connects to the candidate PSCell, the RAN (i.e., SN) communicates data with the UE using the same security key(s) and the multiple configuration parameters. 
     In each of the conditional procedures, the RAN can prepare multiple candidate cells operated by a candidate base station for the UE. For each prepared candidate cell, the RAN transmits an RRC message including a set of configuration parameters to the UE, and the RAN communicates with the UE in accordance with the configuration parameters. 
     However, scenarios involving a disaggregated base station architecture (e.g., a base station including a distributed unit (DU) and a central unit (CU)) can introduce new challenges to conditional procedure schemes. For example, a DU of a candidate base station can operate multiple candidate cells. To configure a conditional procedure, a CU prepares configuration parameters, which may be different for each of the multiple candidate cells. When the UE connects to the DU via one of the multiple candidate cells, the CU does not know which of the multiple candidate cells the UE connected to. As a result, the CU cannot determine which configuration parameters to use to communicate with the CU via the DU. 
     SUMMARY 
     An example embodiment of these techniques is a method in a central unit (CU) of a distributed base station for configuring a connection with a UE. The method can be executed by processing hardware and includes providing, to the UE, a conditional configuration for a cell of a distributed unit (DU) of the base station, and receiving an identifier of the cell of the DU. The method further includes determining that the UE connects to the cell based on the identifier of the cell, and communicating with the UE in accordance with the conditional configuration for the cell. 
     Yet another example embodiment of these techniques is a method in a distributed unit (DU) of a distributed base station for configuring a connection with a UE. The method can be executed by processing hardware and includes receiving, from a central unit (CU) of the base station, a request message to obtain a conditional configuration for connecting to a cell of the DU, and providing, to the CU, the conditional configuration for the cell. The method further includes performing a random access procedure with a UE to connect the UE to the cell, and providing, to the CU, an identifier of the cell of the DU to indicate to the CU that the UE is connected to the cell corresponding to the conditional configuration. 
     Another example embodiment of these techniques is a base station including processing hardware and configured to execute the methods above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a block diagram of an example system in which a radio access network (RAN) and a user device can implement the techniques of this disclosure for managing conditional procedures related to a secondary node (SN); 
         FIG.  1 B  is a block diagram of an example base station including a centralized unit (CU) and a distributed unit (DU) that can operate in the system of  FIG.  1 A ; 
         FIG.  2    is a block diagram of an example protocol stack according to which the UE of  FIG.  1 A  communicates with base stations; 
         FIG.  3 A  is a messaging diagram of an example scenario in which a UE in dual connectivity (DC) transmits an identity of a candidate primary secondary cell (C-PSCell) to a central unit (CU) of an SN via an MN, and the CU determines to use a particular C-SN configuration based on the identity of the C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  3 B  is a messaging diagram of another example scenario in which a candidate DU (C-DU) sends an identity of a C-PSCell to a CU of an SN upon detecting that a UE connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on the identity of C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  3 C  is a messaging diagram of an example scenario in which a C-DU of an SN sends a DL Data Delivery Status message to a CU of an SN after detecting that a UE connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on tunnel endpoint identifier(s) (TEID(s)) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  3 D  is a messaging diagram of an example scenario in which a C-DU of an SN sends an identity of a C-PSCell to a CU of an SN upon detecting that a UE connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on the identity of C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  3 E  is a messaging diagram of an example scenario in which a C-DU of an SN sends a UL RRC Message Transfer message to a CU of an SN after detecting that a UE connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on UE ID(s) in the UL RRC Message Transfer message, in accordance with the techniques of this disclosure; 
         FIG.  3 F  is a messaging diagram of an example scenario in which a C-DU of an SN sends a DL Data Delivery Status message to a CU of an SN after detecting that a UE connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  4 A  is a messaging diagram of an example scenario in which a UE in single connectivity (SC) or DC sends an identity of a C-PSCell to a CU of an SN via an MN, and the CU determines to use a particular C-SN configuration based on the identity of the C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  4 B  is a messaging diagram of an example scenario in which a C-DU of an SN sends an identity of a C-PSCell to a CU of the SN upon detecting that a UE connected to the C-PSCell, and a CU of the SN determines to use a particular C-SN configuration based on the identity of the C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  4 C  is a messaging diagram of an example scenario in which a C-DU of an SN sends a DL Data Delivery Status message to a CU of an SN after detecting that a UE in SC or DC connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  5 A  is a messaging diagram of an example scenario in which a UE in SC or DC transmits an identity of a C-PSCell to a CU of an SN via an MN, and the CU determines to use a particular C-SN configuration based on the identity of the C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  5 B  is a messaging diagram of an example scenario in which a C-DU sends an identity of a C-PSCell to a CU of an SN upon detecting that a UE in SC or DC connected to the C-PSCell, and the SN determines to use a particular C-SN configuration based on the identity of the C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  5 C  is a messaging diagram of an example scenario in which a C-DU of an SN sends a DL Data Delivery Status message to a CU of an SN after detecting that a UE in SC or DC connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  5 D  is a messaging diagram of an example scenario in which a C-DU of an SN sends an identity of a C-PSCell to a CU of an SN upon detecting that a UE in DC connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on the identity of the C-PSCell, in accordance with the techniques of this disclosure; 
         FIG.  5 E  is a messaging diagram of an example scenario in which a C-DU of an SN sends a UL RRC Message Transfer message to a CU of an SN after detecting that a UE in DC connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on UE ID(s) included in the UL RRC Message Transfer message, in accordance with the techniques of this disclosure; 
         FIG.  5 F  is a messaging diagram of an example scenario in which a C-DU of an SN sends a DL Data Delivery Status message to a CU of an SN after detecting that a UE in DC connected to the C-PSCell, and the CU determines to use a particular C-SN configuration based on TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  6 A  is a messaging diagram of an example scenario in which a C-DU of a target base station sends an identity of a C-PSCell to a CU of the target base station upon detecting that a UE in SC or DC connected to the C-PCell, and the CU determines to use a particular C-SN configuration based on the identity of the C-PCell, in accordance with the techniques of this disclosure; 
         FIG.  6 B  is a messaging diagram of an example scenario in which a C-DU of a target base station sends a DL Data Delivery Status message to a CU of the target base station after detecting that a UE in SC or DC connected to the C-PCell, and the CU determines to use a particular C-SN configuration based on TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  7 A  is a messaging diagram of an example scenario in which a C-DU of a target base station sends an identity of a C-PSCell to a CU of the target base station upon detecting that a UE in SC or DC connected to the C-PCell, and the CU determines to use a particular C-SN configuration based on the identity of the C-PCell, in accordance with the techniques of this disclosure; 
         FIG.  7 B  is a messaging diagram of an example scenario in which a C-DU of a target base station sends a DL Data Delivery Status message to a CU of the target base station after detecting that a UE in SC or DC connected to the C-PCell, and the CU determines to use a particular C-SN configuration based on TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  8    is a messaging diagram of an example scenario for inter-base-station mobility in which a C-DU of a target base station sends a DL Data Delivery Status message to a CU of the target base station after detecting that a UE connected to the C-PCell, and the C-CU determines to use a particular C-MN configuration based on the TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  9    is a messaging diagram of an example scenario for intra-base-station mobility in which a C-DU of a base station sends a DL Data Delivery Status message to a CU of the target base station after detecting that a UE connected to the C-PCell, and the C-CU determines to use a particular C-MN configuration based on the TEID(s) for the DL Data Delivery Status message, in accordance with the techniques of this disclosure; 
         FIG.  10    is a flow diagram of an example method for preparing multiple conditional configurations for a UE and receiving a message including a cell ID from a DU or the UE and determining the particular conditional configuration based on the cell ID, which can be implemented in the base station CU of this disclosure; 
         FIG.  11    is a flow diagram of an example method for preparing multiple conditional configurations for a UE and receiving a message including UE ID(s) from a DU and determining the particular conditional configuration based on at least one of the UE ID(s) and the DU&#39;s IP address, which can be implemented in the base station CU of this disclosure; 
         FIG.  12    is a flow diagram of an example method for preparing multiple conditional configurations for a UE and receiving a User Plane frame/message from a DU and determining the particular conditional configuration based on at least one of the TEID(s) for the User Plane frame/message and the DU&#39;s IP address, which can be implemented in the base station CU of this disclosure; 
         FIG.  13 A  is a flow diagram of an example method for configuring a candidate cell for a UE and refraining from configuring a second candidate cell for the UE to avoid a conditional configuration management issue, which can be implemented in the base station or base station CU of this disclosure; 
         FIG.  13 B  is a flow diagram of an example method for configuring a candidate cell in a first network node for a UE and refraining from configuring a second candidate cell in the first network node for the UE to avoid a conditional configuration management issue, which can be implemented in the base station or base station CU of this disclosure; 
         FIG.  14    is a flow diagram of an example method for initiating a conditional configuration depending on whether the measurement result meets a threshold and refraining from configuring a second candidate cell to a UE from the same candidate network node to avoid a conditional configuration management issue, which can be implemented in the base station or base station CU of this disclosure; 
         FIG.  15    is a flow diagram of an example method for applying a conditional configuration when the condition is satisfied and including the cell identity in the RRC response message to aid conditional configuration management at the candidate base station, which can be implemented in the UE of this disclosure. 
         FIG.  16    is a flow diagram of an example method for transmitting an RRC response message and including the cell identity in the RRC response message to aid conditional configuration management at the candidate base station, which can be implemented in the UE of this disclosure. 
         FIG.  17    is a flow diagram of an example method for transmitting an RRC response message to a candidate secondary base station and determining whether to include the cell identity in the RRC response message based on the SRBs, which can be implemented in the UE of this disclosure. 
         FIG.  18    is a flow diagram of an example method for processing a message for conditional mobility for a UE, which can be implemented in the base station CU of this disclosure. 
         FIG.  19    is a flow diagram of an example method for transmitting a message for conditional mobility for a UE, which can be implemented in the base station DU of this disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In general, the techniques of this disclosure allow a first base station to configure the UE to use multiple conditional configurations related to multiple candidate cells of a second base station (which can be the same or different from the first base station), along with one or more conditions to be satisfied before the UE connects to a particular candidate cell. The techniques also enable the base station to determine which conditional configuration and associated security key(s) to apply to communicate with the UE on the particular candidate cell. The conditional procedure can be, for example, a conditional handover procedure, a conditional SN addition or change procedure, or a conditional PSCell addition or change procedure. In the discussion below, the term “CPAC” is used to refer to conditional PSCell addition or change without SN change. The term “CSAC” is used to refer to conditional SN addition or change. 
       FIG.  1 A  depicts an example wireless communication system  100  in which communication devices can implement these techniques. The wireless communication system  100  includes a UE  102 , a base station  104 , a base station  106 A, a base station  106 B and a core network (CN)  110 . The UE  102  initially connects to the base station  104 . 
     In some scenarios, the base station  104  can perform immediate SN addition to configure the UE  102  to operate in dual connectivity (DC) with the base station  104  and the base station  106 A. The base stations  104  and  106 A operate as an MN and an SN for the UE  102 , respectively. Later on, the MN  104  can perform an immediate SN change to change the SN of the UE  102  from the base station  106 A (source SN, or “S-SN”) to the base station  106 B (target SN, or “T-SN”) while the UE  102  is in DC with the MN  104  and the S-SN  106 A. 
     In other scenarios, the base station  104  can perform a conditional SN Addition procedure to first configure the base station  106 A as a candidate SN (C-SN) for the UE  102 . At this time, the UE  102  can be in single connectivity (SC) with the base station  104  or in DC with the base station  104  and another base station  106 B. In contrast to the immediate SN Addition case discussed above, the UE  102  does not immediate attempt to connect to the C-SN  106 A. In this scenario, the base station  104  again operates as an MN, but the base station  106 A initially operates as a C-SN rather than SN. 
     More particularly, when the UE  102  receives a configuration for the C-SN  106 A, the UE  102  does not connect to the C-SN  106 A until the UE  102  has determined that a certain condition is satisfied (the UE  102  in some cases can consider multiple conditions, but for convenience only the discussion below refers to a single condition). When the UE  102  determines that the condition has been satisfied, the UE  102  connects to the candidate SN  106 A, so that the C-SN  106 A begins to operate as the SN  106 A for the UE  102 . Thus, while the base station  106 A operates as a C-SN rather than an SN, the base station  106 A is not yet connected to the UE  102 , and accordingly is not yet servicing the UE  102 . 
     In some scenarios, the condition associated with conditional SN addition can be signal strength/quality, which the UE  102  detects on a candidate primary secondary cell (PSCell) of the C-SN  106 A, exceeding a certain threshold or otherwise corresponding to an acceptable measurement. For example, when the one or more measurement results the UE  102  obtains on the candidate PSCell (C-PSCell) are above a threshold configured by the MN  104  or above a pre-determined or pre-configured threshold, the UE  102  determines that the condition is satisfied. When the UE  102  determines that the signal strength/quality on C-PSCell of the C-SN  106 A is sufficiently good (again, measured relative to one or more quantitative thresholds or other quantitative metrics), the UE  102  can perform a random access procedure with the C-SN  106 A to connect to the candidate SN  106 A. After the UE  102  successfully completes the random access procedure, the base station  106 A begins to operate as an SN, and the C-PSCell becomes a PSCell for the UE  102 . The SN  106 A then can start communicating data with the UE  102 . 
     In various configurations of the wireless communication system  100 , the base station  104  can be implemented as a master eNB (MeNB) or a master gNB (MgNB), and the base station  106 A or  106 B can be implemented as a secondary gNB (SgNB) or a candidate SgNB (C-SgNB). The UE  102  can communicate with the base station  104  and the base station  106 A or  106 B ( 106 A/B) via the same RAT such as EUTRA or NR, or different RATs. When the base station  104  is an MeNB and the base station  106 A is a SgNB, the UE  102  can be in EUTRA-NR DC (EN-DC) with the MeNB and the SgNB. In this scenario, the MeNB  104  may or may not configure the base station  106 B as a C-SgNB to the UE  102 . When the base station  104  is an MeNB and the base station  106 A is a C-SgNB for the UE  102 , the UE  102  can be in SC with the MeNB. In this scenario, the MeNB  104  may or may not configure the base station  106 B as another C-SgNB to the UE  102 . 
     In some cases, an MeNB, an SeNB or a C-SgNB is implemented as an ng-eNB rather than an eNB. When the base station  104  is a Master ng-eNB (Mng-eNB) and the base station  106 A is a SgNB, the UE  102  can be in next generation (NG) EUTRA-NR DC (NGEN-DC) with the Mng-eNB and the SgNB. In this scenario, the MeNB  104  may or may not configure the base station  106 B as a C-SgNB to the UE  102 . When the base station  104  is an Mng-NB and the base station  106 A is a C-SgNB for the UE  102 , the UE  102  can be in SC with the Mng-NB. In this scenario, the Mng-eNB  104  may or may not configure the base station  106 B as another C-SgNB to the UE  102 . 
     When the base station  104  is an MgNB and the base station  106 A/B is an SgNB, the UE  102  may be in NR-NR DC (NR-DC) with the MgNB and the SgNB. In this scenario, the MeNB  104  may or may not configure the base station  106 B as a C-SgNB to the UE  102 . When the base station  104  is an MgNB and the base station  106 A is a C-SgNB for the UE  102 , the UE  102  may be in SC with the MgNB. In this scenario, the MgNB  104  may or may not configure the base station  106 B as another C-SgNB to the UE  102 . 
     When the base station  104  is an MgNB and the base station  106 A/B is a Secondary ng-eNB (Sng-eNB), the UE  102  may be in NR-EUTRA DC (NE-DC) with the MgNB and the Sng-eNB. In this scenario, the MgNB  104  may or may not configure the base station  106 B as a C-Sng-eNB to the UE  102 . When the base station  104  is an MgNB and the base station  106 A is a candidate Sng-eNB (C-Sng-eNB) for the UE  102 , the UE  102  may be in SC with the MgNB. In this scenario, the MgNB  104  may or may not configure the base station  106 B as another C-Sng-eNB to the UE  102 . 
     In the scenarios where the UE  102  hands over from the base station  104  to the base station  106 A, the base stations  104  and  106 A operate as the source base station (S-BS) and a target base station (T-BS), respectively. When the handover is conditional, the base station operates as a conditional T-BS (C-T-BS) or simply C-BS. The UE  102  can operate in DC with the base station  104  and a base station  106 B for example prior to the handover, and continue to operate in DC with the base station  106 A, and the base station  106 B or another base station (not shown in  FIG.  1 A ), after completing the handover. The base stations  104  and  106 A in this case operate as a source MN (S-MN) and a target MN (T-MN), respectively, provided the handover is immediate. When the handover is conditional, the base station operates as a conditional T-MN (C-T-MN) or simply C-MN. 
     The base stations  104 ,  106 A, and  106 B can connect to the same core network (CN)  110  which can be an evolved packet core (EPC)  111  or a fifth-generation core (5GC)  160 . The base station  104  can be implemented as an eNB supporting an Si interface for communicating with the EPC  111 , an ng-eNB supporting an NG interface for communicating with the 5GC  160 , or as a base station that supports the NR radio interface as well as an NG interface for communicating with the 5GC  160 . The base station  106 A can be implemented as an EN-DC gNB (en-gNB) with an Si interface to the EPC  111 , an en-gNB that does not connect to the EPC  111 , a gNB that supports the NR radio interface as well as an NG interface to the 5GC  160 , or a ng-eNB that supports an EUTRA radio interface as well as an NG interface to the 5GC  160 . To directly exchange messages during the scenarios discussed below, the base stations  104 ,  106 A, and  106 B can support an X2 or Xn interface. 
     Among other components, the EPC  111  can include a Serving Gateway (S-GW)  112  and a Mobility Management Entity (MME)  114 . The S-GW  112  in general is configured to transfer user-plane packets related to audio calls, video calls, Internet traffic, etc., and the MME  114  is configured to manage authentication, registration, paging, and other related functions. The 5GC  160  includes a User Plane Function (UPF)  162  and an Access and Mobility Management (AMF)  164 , and/or Session Management Function (SMF)  166 . Generally speaking, the UPF  162  is configured to transfer user-plane packets related to audio calls, video calls, Internet traffic, etc., the AMF  164  is configured to manage authentication, registration, paging, and other related functions, and the SMF  166  is configured to manage PDU sessions. 
     As illustrated in  FIG.  1 A , the base station  104  supports a cell  124 , the base station  106 A supports a cell  126 A, and the base station  106 B supports a cell  126 B. The cells  124  and  126 A can partially overlap, as can the cells  124  and  126 B, so that the UE  102  can communicate in DC with the base station  104  (operating as an MN) and the base station  106 A (operating as an SN) and, upon completing an SN change, with the base station  104  (operating as MN) and the SN  106 B. The base station  106 A can also support additional cells  125 A and  127 A. More particularly, when the UE  102  is in DC with the base station  104  and the base station  106 A, the base station  104  operates as an MeNB, an Mng-eNB or an MgNB, and the base station  106 A operates as an SgNB or an Sng-eNB. when the UE  102  is in SC with the base station  104 , the base station  104  operates as an MeNB, an Mng-eNB or an MgNB, and the base station  106 A operates as a C-SgNB or a C-Sng-eNB. 
     In general, the wireless communication network  100  can include any suitable number of base stations supporting NR cells and/or EUTRA cells. More particularly, the EPC  111  or the 5GC  160  can be connected to any suitable number of base stations supporting NR cells and/or EUTRA cells. Although the examples below refer specifically to specific CN types (EPC, 5GC) and RAT types (5G NR and EUTRA), in general the techniques of this disclosure also can apply to other suitable radio access and/or core network technologies such as sixth generation (6G) radio access and/or 6G core network or 5G NR-6G DC. 
     With continued reference to  FIG.  1 A , the base station  104  includes processing hardware  130 , which may include one or more general-purpose processors (e.g., central processing units (CPUs)) and a computer-readable memory storing machine-readable instructions executable on the general-purpose processor(s), and/or special-purpose processing units. The processing hardware  130  in the example implementation of  FIG.  1    includes a conditional configuration controller  132  that is configured to manage or control the conditional configuration techniques of this disclosure. For example, the conditional configuration controller  132  may be configured to support RRC messaging associated with immediate and conditional handover procedures, and/or to support the necessary operations when the base station  104  operates as an MN relative to an SN. Moreover, in some implementations and/or scenarios, the conditional configuration controller  132  may be responsible for maintaining (for the UE  102  and a number of other UEs not shown in  FIG.  1   ) current sets of conditional configurations in accordance with various implementations discussed below. 
     The base station  106 A includes processing hardware  140 , which may include one or more general-purpose processors (e.g., CPUs) and a computer-readable memory storing machine-readable instructions executable on the general-purpose processor(s), and/or special-purpose processing units. The processing hardware  140  in the example implementation of  FIG.  1    includes a conditional configuration controller  142  that is configured to manage or control RRC procedures and RRC configurations. For example, the conditional configuration controller  142  may be configured to support RRC messaging associated with immediate and conditional handover procedures, and/or to support the necessary operations when the base station  106 A operates as MN, an SN, a candidate MN (C-MN) and/or candidate SN (C-SN). Moreover, in some implementations and/or scenarios, the conditional configuration controller  142  may be responsible for maintaining (for the UE  102  and a number of other UEs not shown in  FIG.  1   ) current sets of conditional configurations in accordance with various implementations discussed below. The base station  106 B may include processing hardware similar to the processing hardware  140  of the base station  106 A. 
     Although  FIG.  1 A  illustrates the RRC controllers  132  and  142  as operating in an MN and an SN, respectively, a base station generally can operate as an MN, an SN, a candidate MN or a candidate SN in different scenarios. Thus, the MN  104 , the SN  106 A, and the SN  106 B can implement similar sets of functions and support both MN, SN, conditional MN and conditional SN operations. 
     The UE  102  includes processing hardware  150 , which may include one or more general-purpose processors (e.g., CPUs) and a computer-readable memory storing machine-readable instructions executable on the general-purpose processor(s), and/or special-purpose processing units. The processing hardware  150  in the example implementation of  FIG.  1    includes a conditional configuration controller  152  that is configured to manage or control RRC procedures and RRC configurations related to conditional configurations. For example, the conditional configuration controller  152  may be configured to support RRC messaging associated with immediate and conditional handover and/or secondary node addition/modification procedures, and may also be responsible for maintaining a current set of conditional configurations for the UE  102  (e.g., adding, releasing or modifying conditional configurations as needed) in accordance with any of the implementations discussed below. 
     In operation, the UE  102  can use a radio bearer (e.g., a DRB or an SRB) that at different times terminates at the MN  104  or the SN  106 A. The UE  102  can apply one or more security keys when communicating on the radio bearer, in the uplink (from the UE  102  to a base station) and/or downlink (from a base station to the UE  102 ) direction. 
       FIG.  1 B  depicts an example distributed implementation of a base station such as the base station  104 ,  106 A, or  106 B. The base station in this implementation can include a centralized unit (CU)  172  and one or more distributed units (DUs)  174 . The CU  172  is equipped with processing hardware that can include one or more general-purpose processors such as CPUs and non-transitory computer-readable memory storing machine-readable instructions executable on the one or more general-purpose processors, and/or special-purpose processing units. In one example, the CU  172  is equipped with the processing hardware  130 . In another example, the CU  172  is equipped with the processing hardware  140 . The processing hardware  140  in an example implementation includes an (C-)SN RRC controller  142  configured to manage or control one or more RRC configurations and/or RRC procedures when the base station  106 A operates as an SN or a candidate SN (C-SN). The base station  106 B can have hardware same as or similar to the base station  106 A. The DU  174  is also equipped with processing hardware that can include one or more general-purpose processors such as CPUs and non-transitory computer-readable memory storing machine-readable instructions executable on the one or more general-purpose processors, and/or special-purpose processing units. In some examples, the processing hardware in an example implementation includes a medium access control (MAC) controller configured to manage or control one or more MAC operations or procedures (e.g., a random access procedure) and a radio link control (RLC) controller configured to manage or control one or more RLC operations or procedures when the base station  106 A operates as an MN, an SN or a candidate SN (C-SN). The process hardware may include further a physical layer controller configured to manage or control one or more physical layer operations or procedures. 
     Next,  FIG.  2    illustrates in a simplified manner a radio protocol stack according to which the UE  102  can communicate with an eNB/ng-eNB or a gNB. Each of the base stations  104 ,  106 A, or  106 B can be the eNB/ng-eNB or the gNB. 
     The physical layer (PHY)  202 A of EUTRA provides transport channels to the EUTRA Medium Access Control (MAC) sublayer  204 A, which in turn provides logical channels to the EUTRA Radio Link Control (RLC) sublayer  206 A, and the EUTRA RLC sublayer in turn provides RLC channels to the EUTRA PDCP sublayer  208  and, in some cases, NR PDCP sublayer  210 . Similarly, the PHY  202 B of NR provides transport channels to the NR MAC sublayer  204 B, which in turn provides logical channels to the NR RLC sublayer  206 B, and the NR RLC sublayer  206 B in turn provides RLC channels to the NR PDCP sublayer  210 . The UE  102  in some implementations supports both the EUTRA and the NR stack, to support handover between EUTRA and NR base stations and/or DC over EUTRA and NR interfaces. Further, as illustrated in  FIG.  2 A , the UE  102  can support layering of NR PDCP  210  over EUTRA RLC  206 A. 
     The EUTRA PDCP sublayer  208  and the NR PDCP sublayer  210  receive packets (e.g., from the Internet Protocol (IP) layer, layered directly or indirectly over the PDCP layer  208  or  210 ) that can be referred to as service data units (SDUs), and output packets (e.g., to the RLC layer  206 A or  206 B) that can be referred to as protocol data units (PDUs). Except where the difference between SDUs and PDUs is relevant, this disclosure for simplicity refers to both SDUs and PDUs as “packets.” 
     On a control plane, the EUTRA PDCP sublayer  208  and the NR PDCP sublayer  210  provide SRBs to exchange Radio Resource Control (RRC) messages, for example. On a user plane, the EUTRA PDCP sublayer  208  and the NR PDCP sublayer  210  provide DRBs to support data exchange. 
     When the UE  102  operates in EUTRA/NR DC (EN-DC), with the base station  104  operating as a MeNB and the base station  106 A or  106 B operating as a SgNB, the network can provide the UE  102  with an MN-terminated bearer that uses EUTRA PDCP  208  or MN-terminated bearer that uses NR PDCP  210 . The network in various scenarios also can provide the UE  102  with an SN-terminated bearer, which use only NR PDCP  210 . The MN-terminated bearer can be an MCG bearer or a split bearer. The SN-terminated bearer can be a SCG bearer or a split bearer. The MN-terminated bearer can be an SRB (e.g., SRB1 or SRB2) or a DRB. The SN-terminated bearer can an SRB (e.g., SRB) or a DRB. 
     Next, several example scenarios in which a base station initiates a conditional PSCell addition or change (CPAC) procedure, a conditional SN addition or change (CSAC) procedure, or a condition handover procedure are discussed.  FIG.  3    (i.e.,  3 A through  3 F) depict scenarios in which a base station initiates a CPAC procedure for a UE and  FIG.  4    (i.e.,  4 A through  4 C) depict scenarios in which a base station initiates a CSAC procedure for a UE.  FIG.  5    (i.e.,  5 A through  5 F) depict scenarios in which a base station initiates a CPAC or CSAC procedure for a UE.  FIG.  6    (i.e.,  6 A and  6 B),  FIG.  7    (i.e.,  7 A and  7 B),  FIG.  8    and  FIG.  9    depict handover scenarios in which a base station initiates a conditional handover procedure for a UE. 
     Referring first to  FIG.  3 A , in a scenario  300 A, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a secondary source CU (referred to here as S-CU  172 ), a secondary source DU (referred to here as S-DU  174 A) and a C-DU  174 B. Initially, the UE  102  (operating in DC) communicates  302 A data (e.g., uplink and/or downlink data PDUs) with the MN  104  (via cell  124 ), e.g., in accordance with an MN configuration, and communicates  302 A data (e.g., uplink and/or downlink data PDUs) with the S-CU  172  via the S-DU  174 A (via cell  126 A) in accordance with a source SN configuration. 
     The S-CU  172  at some point determines  304 A that it should prepare a conditional PSCell change to a C-PSCell (e.g., C-PSCell  125 A) operated by the C-DU  174 B for the UE  102 . The S-CU  172  can make this determination based on one or more measurement results received from the UE  102 , for example, or another suitable event. In response to this determination, the S-CU  172  sends  305 A an UE Context Setup Request message to the C-DU  174 B to obtain a C-DU configuration. In response to receiving UE Context Setup Request message, the C-DU  174 B includes a first C-DU configuration in a UE Context Setup Response message for the UE  102 . The first C-DU configuration included in this message can include one or more configuration parameters for communication on the C-PSCell  125 A. The DU  174 B then sends  307 A the UE Context Setup Response message to the S-CU  172 . After receiving the UE Context Setup Response message, the S-CU  172  generates  308 A a first C-SN configuration including the first C-DU configuration. Then, the S-CU  172  sends  310 A the first C-SN configuration to the MN  104 , which in turn transmits  312 A an RRC container message including the first C-SN configuration to the UE  102 . The events  305 A and  307 A are collectively referred to in  FIG.  3 A  as the UE Context Setup procedure  306 A. The events  304 A,  305 A,  307 A,  308 A,  310 A and  312 A are collectively referred to in  FIG.  3 A  as the CPAC configuration procedure  320 A. 
     In some implementations, the S-CU  172  can indicate to the C-DU  174 B a particular candidate cell (e.g., a C-PSCell  125 A or a C-PSCell  127 A) for which the C-DU  174 B generates a C-DU configuration. The S-CU  172  can include an identity of the particular candidate cell in a UE Context Setup Request message. In other implementations, the C-DU  174 B indicates a particular candidate cell (e.g., a C-PSCell  125 A or a C-PSCell  127 A) for which the C-DU  174 B generates a C-DU configuration in a UE Context Setup Response message in response to the UE Context Setup Request message. In such implementations, the S-CU  172  can determine an association between the C-DU configuration (or a C-SN configuration including the C-DU configuration) and identity of the particular candidate cell. The S-CU  172  can store the association for determining to use the C-DU configuration (or the C-SN configuration). For example, the S-CU  172  can associate an identity of the C-PSCell  125 A with the first C-SN configuration (or the first C-DU configuration) at event  308 A or upon receiving the UE Context Setup Response message  307 A in the CPAC configuration procedure  320 A. The S-CU  172  can store the association for the determination  340 A. 
     In some implementations, the S-CU  172  can generate an RRC reconfiguration message including the first C-SN configuration and send  310 A the RRC reconfiguration message to the MN  104 . In turn, the MN  104  transmits  312 A the RRC container message including the RRC reconfiguration message to the UE  102 . In one implementation, the S-CU  172  can send  310 A an SN message (e.g., SN Modification Required message, RRC Transfer message, etc.) including the first C-SN configuration or the RRC reconfiguration message to the MN  104 . In some implementations, the UE  102  can transmit an RRC container response message to the MN  104  in response to the RRC container message. In one implementation, the UE  102  transmits an RRC container response message including an RRC reconfiguration complete message. The MN  104  can send an SN message (e.g., SN Reconfiguration Complete message) including the RRC reconfiguration complete message to the S-CU  172 . The RRC reconfiguration complete message can respond to the RRC reconfiguration message. The RRC container response message can respond to the RRC container message. 
     The S-CU  172 , in some implementations, can perform  322 A the CPAC configuration procedure with the C-DU  174 B, the MN  104  and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A operated by the C-DU  174 B), similar to the CPAC configuration procedure  320 A. In other implementations, the S-CU  172  can perform  322 A the CPAC configuration procedure with the S-DU  174 A, the MN  104  and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A operated by the S-DU  174 A), similar to the CPAC configuration procedure  320 A. In these implementations, the S-CU  172  may perform a UE Context Modification procedure with the S-DU  174 A to obtain a second C-DU configuration instead of the UE Context Setup procedure. In the UE Context Modification procedure, the S-CU  172  can send a UE Context Modification Request message to the S-DU  174 A, similar to the UE Context Setup Request message and the S-DU  174 A responds with a UE Context Modification Response message including a second DU configuration. In these implementations, event  342 A and  348 A would occur between the UE  102  and the S-DU  174 A. The S-CU  172  can perform the CPAC configuration procedures  322 A in parallel with or after the CPAC configuration procedure  320 A. In some implementations, the S-CU  172  can associate an identity of the C-PSCell  127 A with the second C-SN configuration (or the second C-DU configuration) at event  308 A or upon receiving the UE Context Setup Response message  307 A (or the UE Context Modification Response message) in the CPAC configuration procedure  322 A. The S-CU  172  can store the association for the determination  340 A. 
     In some implementations, the S-CU  172  can include a first C-CU configuration in the first C-SN configuration and a second C-CU configuration in the second C-SN configuration. The first C-CU configuration and the second C-CU configuration can have the same content or different contents. In other implementations, the S-CU  172  does not include a C-CU configuration in the first C-SN configuration and the S-CU  172  does not include a C-CU configuration in the second C-SN configuration. The first C-DU configuration and the second C-DU configuration can have some portions that are different. 
     Later in time, the UE  102  determines (or detects)  334 A that a condition for connecting to a C-PSCell  127 A is met and initiates a random access procedure on the C-PSCell  127 A in response to the detection. For convenience, this discussion may refer to the condition or a configuration in the singular, but it will be understood that there may be multiple conditions, and that the RRC reconfiguration message generated by the S-CU  172  can include one or multiple configuration parameters to specify the condition or the multiple conditions. 
     In response to the determination  334 A, the UE  102  transmits  336 A an RRC reconfiguration complete message including an identity of the C-PSCell  127 A to the MN  104 , which in turn sends  338 A the RRC reconfiguration complete message to the S-CU  172 . In some implementations, the UE  102  can include frequency information (e.g., absolute radio-frequency channel number and/or frequency band number) of the C-PSCell  127 A in the RRC reconfiguration message  336 A. The S-CU  172  determines  340 A to use the second C-SN configuration (or the second C-DU configuration and/or the second CU configuration) as a new S-SN configuration (as a new S-DU configuration) based on the identity of the C-PSCell  127 A. In one implementation, the MN  104  can send  338 A an SN message (e.g., an SN Reconfiguration Complete message, an SN Modification Request message, or an RRC Transfer message) including the RRC reconfiguration complete message to the S-CU  172 . Alternatively, the MN  104  obtains the identity of the C-PSCell  127 A and optionally the frequency information (if included) from the RRC reconfiguration message, includes the identity of the C-PSCell  127 A and the frequency information (if included) in at least one IE, and sends an SN message (e.g., an SN Reconfiguration Complete message, an SN Modification Request message, or an RRC Transfer message) including the at least one IE to the S-CU  172 . In another implementation, the RRC reconfiguration message  336 A may be transparent to the MN  104  so that the S-CU  172  can send the MN  104  one or more SN messages (e.g., SN Modification Required message, SN configuration update message, SN information update message, etc.) including the identity of the C-PSCell  127 A and optionally the frequency information (if received or derived by the S-CU  172  based on the identity of the C-PSCell  127 A). 
     In some implementations, the UE  102  can generate an RRC container message (e.g., ULInformationTransferMRDC message) including the RRC reconfiguration complete message and transmit  336 A the RRC container message to the MN  104 . The MN  104  in turn extracts the RRC reconfiguration complete message from the RRC container message and sends  338 A the RRC reconfiguration complete message to the S-CU  172 . In one implementation, the UE  102  can include the identity of the C-PSCell  127 A and optionally include the frequency information in the RRC container message. In other implementations, the UE  102  can generate an RRC container response message (similar to the RRC container message described above) including the RRC reconfiguration complete message and transmit  336 A the RRC container response message to the MN  104 . The MN  104  in turn extracts the RRC reconfiguration complete message from the RRC container response message and sends  338 A the RRC reconfiguration complete message to the S-CU  172 . In one implementation, the UE  102  can include the identity of the C-PSCell  127 A and optionally include the frequency information in the RRC container response message. 
     In response to the determination  334 A, the UE  102  then performs  342 A a random access procedure with the C-DU  174 B via the C-PSCell  127 A, e.g., using one or more random access configurations in the second C-DU configuration. If the UE  102  successfully completes the random access procedure (e.g., succeeds the contention resolution in the random access procedure), the UE  102  communicates  348 A with the C-DU  174  via the C-PSCell  127 A using the second C-DU configuration and communicates with the S-CU  172  via the C-DU  174  using the second CU configuration. In some implementations, the UE  102  may disconnect from the PSCell  126 A to perform the random access procedure, i.e., to connect the C-PSCell  127 A. In other implementations, the UE  102  does not disconnect from the PSCell  126 A while performing the random access procedure. If the C-DU  174 B identifies the UE  102  in the random access procedure, the C-DU  174 B becomes an S-DU  174 B and communicates  348 A with the UE  102  via the C-PSCell  127 A. The S-DU  174 B can send a message (e.g., a DL Data Delivery Status message in  FIG.  3 C ) to indicate to the S-CU  172  that the UE  102  is connected, after or response to identifying the UE  102  in the random access procedure. Later on, if the S-CU  172  initiates an immediate DU change from the S-DU  174 B to a DU of the S-CU  172  (e.g., the DU  174 A or another DU not shown in  FIG.  3 A ), the S-CU  172  can send the second C-SN configuration (i.e., the new S-SN configuration) or the second C-DU configuration (e.g., the new S-DU configuration) to the DU. Later on, if the S-CU  172  initiates immediate SN change to the base station  106 B or if the MN  104  requests the latest SN configuration, the S-CU  172  can send the second C-SN configuration to the MN  104 . 
     In some implementations, the random access procedure can be a four-step random access procedure or a two-step random access procedure. The UE  102  can transmit a Message A including a UE identity (ID) to the C-DU  174 B in the two-step random access procedure. In other implementations, the random access procedure can be a contention-based random access procedure or a contention-free random access procedure. In the contention-based random access procedure, the UE  102  can transmit a Message  3  including a UE ID to the C-DU  174 B. The C-DU  174 B can assign the UE ID in the second DU configuration. The UE ID can be a cell-radio network temporary identifier (C-RNTI). In the contention-free random access procedure, the UE  102  can transmit a dedicated preamble to the C-DU  174 B. The C-DU  174 B can assign the dedicated preamble in the second DU configuration. The C-DU  174 B can assign the UE ID in the second DU configuration. If the C-DU  174 B receives the UE ID or the dedicated preamble, the C-DU  174 B identifies the UE  102 . After the UE  102  successfully completes  342 A the random access procedure (e.g., succeeds the contention resolution in the random access procedure), the C-PSCell  127 A begins to operate as the PSCell  127 A, and the UE  102  begins to operate  348 A in DC with the MN  104  via the PCell  124  and the SN  106 A via the PSCell  127 A. In particular, the UE  102  communicates  348 A with the SN  106 A via the C-PSCell  127 A (i.e., new PSCell  127 A) in accordance with the second C-SN configuration. 
     Because the S-CU  172  receives the identity of the C-PSCell  127 A, the S-CU  172  does not select the first C-SN configuration (or the first C-DU configuration) for the C-PSCell  125 A. The S-CU  172  uses the second C-SN configuration, and does not use the first C-SN configuration, to communicate with the UE  102  while the UE  102  is connected to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     In some implementations, the identity of the C-PSCell  127 A can be a cell global identity (CGI). In other implementations, the identity of the C-PSCell  127 A can be a cell identity in a system information block broadcast on the C-PSCell  127 A. In yet other implementations, the identity of the C-PSCell  127 A can be a physical cell identity (PCI) that the UE  102  obtains from synchronization signals received by the UE  102  on the C-PSCell  127 A. In various implementations, the S-CU  172  maintains a table for mapping between a CGI and a PCI or another suitable identifier of a particular cell in the wireless communication system  100  for the purpose of identifying a particular C-SN configuration. 
     In some implementations, S-CU  172  can include at least one first security configuration parameter in the second C-SN configuration. In other implementations, the S-CU  172  can send the at least one first security configuration parameter with the first C-SN configuration at event  310 A and the MN  104  include the at least one first security configuration parameter in the RRC container message at event  312 A. The S-CU  172  can generate the at least one first security key (security key(s)) from the at least one first security configuration parameters and a first security base key (e.g., K SN  or K SN* ). For example, the first security key(s) can include a first ciphering key for encryption and decryption and/or include a first integrity key for integrity protection and check. 
     In other implementations, S-CU  172  can include at least one second security configuration parameter in the second C-SN configuration. In other implementations, the S-CU  172  can send the at least one second security configuration with the second C-SN configuration during procedure  322 A at an event similar to event  310 A, and the MN  104  can include the security configuration in the RRC container message during procedure  322 A at an event similar to event  312 A. The S-CU  172  can generate at least one second security key (security key(s)) from the at least one second security configuration parameter and a second security base key (e.g., K SN  or K SN* ). For example, the second security key(s) can include a second ciphering key for encryption and decryption and/or include a second integrity key for integrity protection and check. In one implementation, the S-CU  172  determines to use the at least one second security configuration parameter and the second security base key to generate the second security key(s) according to the identity of the C-PSCell  127 A. In another implementation, the S-CU  172  determines to use the second security key(s) according to the identity of the C-PSCell  127 A. The UE  102  can generate the second security key(s) (which is the same as the second security key(s) generated by the SN  106 A) from the at least one second security configuration parameter and the security base key. In one implementation, the UE  102  can generate the second security key(s) from the at least one second security configuration parameter and the security base key after event  334 A or receiving the RRC container message during the CPAC procedure  322 A. Thus, the UE  102  in DC communicates  348 A with the S-CU  172  via the S-DU  174 B using the second C-SN configuration and the second security key(s). In one implementation, the first security base key and the second security base key can be the same or identical. In another implementation, the first security base key and the second security base key can be different. The S-CU  172  can determine which security base key or which security key(s) based on the identity of the C-PCell  126 A. 
     In yet other implementations, if the RRC container message at event  312 A (in CPAC procedure  320 A or CPAC procedure  322 A) does not include any security configuration parameter for the UE  102  to communicate with the S-CU  172  via the C-DU  174 B, the UE  102  in DC communicates  348 A with the S-CU  172  via the S-DU  174 B using the second C-SN configuration and security key(s) which was configured at a dual connectivity configuration (e.g., an SN Addition procedure) procedure at event  302 A. 
     In some implementations, the first C-DU configuration can include multiple configuration parameters such as physical layer configuration parameters, MAC configuration parameters, RLC configuration parameters, and/or the one or more random access configurations needed by the UE  102  to perform  342 A the random access procedure with the C-DU  174 B on the C-PSCell  125 A (if the UE  102  determines that a condition for connecting the C-PSCell  125 A is satisfied). The second C-DU configuration can include multiple configuration parameters such as physical layer configuration parameters, MAC configuration parameters, RLC configuration parameters, and/or the one or more random access configurations needed by the UE  102  to perform  342 A the random access procedure with the C-DU  174 B on the C-PSCell  127 A. In other implementations, the first C-DU configuration can be CellGroupConfig information element (IE) that configures the C-PSCell  125 A and zero, one, or more C-SCells of the C-DU  174 B. The second C-DU configuration can be CellGroupConfig IE that configures the C-PSCell  127 A and zero, one, or more C-SCells of the C-DU  174 B. In yet other implementations, the first C-DU configuration can include configurations in ConfigPartSCG-r12 IE and the second C-DU configuration can include configurations in ConfigPartSCG-r12 IE. 
     In some implementations, the first C-CU configuration may include a radio bearer configuration and/or measurement configuration. The second C-CU configuration may include a radio bearer configuration and/or measurement configuration. For example, the radio bearer configuration can be a RadioBearerConfig IE, DRB-ToAddModList IE or SRB-ToAddModList IE, DRB-ToAddMod IE or SRB-ToAddMod IE. The measurement configuration can be a MeasConfig IE. 
     In some implementations, the first C-SN configuration can be an RRCReconfiguration message or an RRCReconfiguration-IEs conforming to 3GPP TS 38.331. The second C-SN configuration can be an RRCReconfiguration message or an RRCReconfiguration-IEs conforming to 3GPP TS 38.331. In other implementations, the first C-SN configuration can be an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. The second C-SN configuration can be an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. 
     In some implementations, the MN configuration includes a RadioBearerConfig IE, an RRCReconfiguration message, an RRCReconfiguration-IEs, a CellGroupConfig IE and/or MeasConfig IE conforming to 3GPP TS 38.331. In other implementations, the MN configuration includes an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. In still other implementations, the MN configuration includes configurations in the RadioBearerConfig IE, the CellGroupConfig IE, RRCReconfiguration-IEs, and/or or RRCConnectionReconfiguration-IEs. 
     In some implementations, the S-SN configuration includes a RadioBearerConfig IE, an RRCReconfiguration message, an RRCReconfiguration-IEs, a CellGroupConfig IE and/or MeasConfig IE conforming to 3GPP TS 38.331. In other implementations, the S-SN configuration includes an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. In still other implementations, the S-SN configuration includes configurations in the RadioBearerConfig IE, the CellGroupConfig IE, RRCReconfiguration-IEs, and/or or RRCConnectionReconfiguration-IEs. In some implementations, the S-SN configuration can include a S-DU configuration. The S-DU configuration can include multiple configuration parameters such as physical layer configuration parameters, MAC configuration parameters, RLC configuration parameters. The S-DU configuration can be CellGroupConfig IE or include configurations in ConfigPartSCG-r12 IE. 
     In some implementations, if the MN  104  is a gNB, the RRC container message message is an RRCReconfiguration message, and the RRC container response message is an RRCReconfigurationComplete message, respectively. In other implementations, if the MN  104  is an eNB or an ng-eNB, the RRC container message is an RRCConnectionReconfiguration message, and the RRC container response message is an RRCConnectionReconfigurationComplete message, respectively. 
     In some implementations, if the SN  106 A is a gNB, the RRC reconfiguration and RRC reconfiguration complete messages are RRCReconfiguration and RRCReconfigurationComplete messages, respectively. In other implementations, if the SN  106 A is an eNB or ng-eNB, the RRC reconfiguration and RRC reconfiguration complete messages are RRCConnectionReconfiguration and RRCConnectionReconfigurationComplete messages, respectively. 
     Now referring to  FIG.  3 B , a scenario  300 B involves a CPAC without SN change, i.e., a conditional change of a PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a secondary CU (referred to here as S-CU  172 ), a secondary DU (referred to here as S-DU  174 A) and a C-DU  174 B. Events in this scenario similar to those discussed above are labeled with the same reference numbers and the examples and implementations for  FIG.  3 A  can apply to  FIG.  3 B . The differences between the scenarios of  FIG.  3 A  and  FIG.  3 B  are discussed below. 
     In the scenario  300 B, the UE  102  may not transmit the RRC reconfiguration complete message  336 B or may transmit the  336 B RRC reconfiguration complete message not including the identity of the C-PSCell  127 A to the MN  104 . After the C-DU  174 B identifies the UE  102  during the random access procedure at event  342 B, the C-DU  174  can transmit  344 B a DU to CU message including the identity of the C-PSCell  127 A to the S-CU  172  to indicate that the UE  102  is connected on the C-PSCell  127 A, so that the S-CU  172  can determine  346 B to use the second C-SN configuration (or the second C-DU configuration) as a new S-SN configuration (or as a new S-DU configuration) based on the identity of the C-PSCell  127 A. In some implementations, the DU to CU message can be a F1AP message or a DL Data Delivery Status message. For example, the F1AP message can be a UL RRC Message Transfer message including a dummy RRC message or excluding an RRC message. In another example, the F1AP message can be a new F1AP message excluding an RRC message. In another example, the F1AP message can be a UL RRC Message Transfer message including an RRC container IE. The C-DU  174 B can include a dummy or fake RRC message (i.e., an RRC message not received from the UE  102 ) in the RRC container IE. The C-DU  174 B can include an indicator indicating ignoring (or discarding) the RRC container IE in the UL RRC Message Transfer message. The S-CU  172  ignores (or discard, does not use) the RRC container IE (i.e., the dummy or fake RRC message) in response to the indicator. 
     Because the S-CU  172  receives the identity of the C-PSCell  127 A, the S-CU  172  does not select the first C-SN configuration (or the first C-DU configuration) for the C-PSCell  125 A. Thus, the S-CU  172  does not use the first C-SN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Now referring to  FIG.  3 C , a scenario  300 C involves a CPAC without SN change, i.e., a conditional change of a PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a secondary CU (referred to here as S-CU  172 ), a secondary DU (referred to here as S-DU  174 A) and a C-DU  174 B. Events in this scenario similar to those discussed above are labeled with the same reference numbers and the examples and implementations for  FIG.  3 A  can apply to  FIG.  3 C . The differences between the scenarios of  FIG.  3 C  and  FIGS.  3 A- 3 B  are discussed below. 
     In the scenario  300 C, the UE  102  may not transmit the RRC reconfiguration complete message  336 C or may transmit the  336 C RRC reconfiguration complete message not including the identity of the C-PSCell  127 A to the MN  104 . 
     After the C-DU  174 B identifies the UE  102  during the random access procedure at event  342 C, the C-DU  174  can send  345 C a DL Data Delivery Status message to the S-CU  172 . In some implementations, the C-DU  174  can send  345 C a General Packet Radio Service (GPRS) Tunneling Protocol (GTP) packet including the DL Data Delivery Status message to the S-CU  172 . The GTP packet can include one or more tunnel endpoint identifiers (TEID(s)). The TEID(s) can include at least one of (1) a TEID for the S-CU  172  endpoint of a F1 transport bearer for delivery of UL PDUs or (2) a TEID for the C-DU  174 B endpoint of the F1 transport bearer for delivery of DL PDUs. The S-CU  172  can assign the TEID(s) with the C-DU  174 B in a UE Context Setup procedure  306 C. The S-CU  172  can assign a TEID for the S-CU  172  endpoint of a F1 transport bearer for delivery of UL PDUs in the UE Context Setup Request message. The C-DU  174 B can assign a TEID for the C-DU  174 B endpoint of the F1 transport bearer for delivery of DL PDUs in the UE Context Setup Response message in the UE Context Setup procedure  306 C. The TEID(s) can be the same value or different values. The S-CU  172  can associate the TEID(s) with a C-DU configuration (obtained in the UE Context Setup procedure) and a C-CU configuration (if generated by the S-CU  172 ) so that the S-CU  172  can determine to use a particular C-SN configuration (or a particular C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) based on the TEID(s) received in the GTP packet. In the scenario  300 C, the S-CU  172  can determine  346 C to use the second C-SN configuration (or the second C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) based on the TEID(s) received in a GTP packet including the DL Data Delivery Status message  345 C. The S-CU  172  can establish or store an association between particular TEID(s) and a particular C-SN configuration (or a particular C-DU configuration included in the particular C-SN configuration) so that the S-CU  172  can determine the particular C-SN configuration (or the particular C-DU configuration) based on the particular TEID(s) received in a GTP packet including a DL Data Delivery Status message. For example, the S-CU  172  can establish particular TEID(s) and a particular C-SN configuration (or a particular C-DU configuration included in the C-SN configuration) after performing a UE Context Setup procedure (or a UE Context Modification procedure) with the C-DU  174 B (or S-DU  174 A) to obtain a C-DU configuration for a candidate cell. The S-CU  172  can also associate an identity of a particular candidate cell (e.g., C-PSCell  125 A or C-PSCell  127 A) with the particular C-SN configuration (or the particular C-DU configuration included in the particular C-SN configuration) as described for  FIG.  3 A . Thus, the S-CU  172  can establish or store an association between the identity of the particular candidate cell, the particular TEID(s), and the particular C-SN configuration (or the particular C-DU configuration included in the particular C-SN configuration). The S-CU  172  can determine the identity of the particular candidate cell based on the TEID(s) received in the GTP packet including the DL Data Delivery Status message. The S-CU  172  can send the identity of the particular candidate cell to the MN  104  as described above. 
     For example, in the UE Context Setup procedure in the CPAC configuration  320 C, the S-CU  172  can assign a first TEID for the S-CU  172  endpoint of a F1 transport bearer for delivery of UL PDUs in the UE Context Setup Request message. The C-DU  174 B can assign a second TEID for the C-DU  174 B endpoint of the F1 transport bearer for delivery of DL PDUs in the UE Context Setup Response message in the UE Context Setup procedure  306 C. The first and second TEIDs can be the same or different. The S-CU  172  associates the first C-SN configuration (or the C-DU configuration) with the first and/or second TEIDs. In the UE Context Setup procedure in the CPAC configuration  322 C, the S-CU  172  can assign a third TEID for the S-CU  172  endpoint of a F1 transport bearer for delivery of UL PDUs in the UE Context Setup Request message. The C-DU  174 B can assign a fourth TEID for the C-DU  174 B endpoint of the F1 transport bearer for delivery of DL PDUs in the UE Context Setup Response message in the UE Context Setup procedure  306 C. The third and fourth TEIDs can be the same or different and are different from the first and second TEIDs. The S-CU  172  associates the first C-SN configuration (or the C-DU configuration) with the third and/or fourth TEIDs. 
     The C-DU  174 B includes the third and/or the fourth TEIDs in the GTP packet including the DL Data Delivery Status message and transmits  345 C the GTP packet to the S-CU  172 , the S-CU  172  can determine to use the second C-SN configuration (or the second C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) from the third and/or the fourth TEIDs received in the GTP packet. 
     Based on the TEID(s) in the GTP packet including the DL Data Delivery Status message  345 C, the S-CU  172  does not select the first CU configuration for the C-PSCell  125 A. Thus, the S-CU  172  does not use the first CU configuration for the C-PSCell  125 A to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Now referring to  FIG.  3 D , a scenario  300 D involves a CPAC without SN change, i.e., a conditional change of a PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a secondary CU (referred to here as S-CU  172 ), a secondary DU (referred to here as S-DU  174 A) and a C-DU  174 B. Events in this scenario similar to those discussed above are labeled with the same reference numbers and the examples and implementations for  FIG.  3 A  can apply to  FIG.  3 D . The differences between the scenarios of  FIG.  3 D  and  FIG.  3 A  are discussed below. 
     The CPAC configuration procedure  321 D is generally similar to the CPAC configuration procedure  320 A of  FIG.  3 A . However, in the scenario  300 D the S-CU  172  sends  311 D the first C-SN configuration to the S-DU  174 A, which in turn transmits  313 D the first C-SN configuration to the UE  102 , rather than transmitting the first C-SN configuration to the UE  102  via the MN  104 , as the S-CU  172  does in the scenario  300 A of  FIG.  3 A . In some implementations, the S-CU  172  configures a first SRB for the UE  102  via the MN  104  and transmits the first C-SN configuration via the first SRB to the UE  102  via the S-DU  174 A. For example, the SN  106 A transmits an SRB configuration configuring the first SRB (e.g., SRB3) to the MN  104 , and the MN  104  transmits the SRB configuration to the UE via a second SRB (e.g., SRB1) between the MN  104  and the UE  102 . 
     In some implementations, the S-CU  172  can generate an RRC reconfiguration message including the first C-SN configuration and send  311 D,  313 D the RRC reconfiguration message on the first SRB via the S-DU  174 A. In one implementation, the S-CU  172  can send  311 D a F1AP message (e.g., DL RRC Message Transfer message, UE Context Modification Request message, etc.) including the RRC reconfiguration message to the S-DU  174 A. In some implementations, the UE  102  can transmit an RRC reconfiguration complete message on the first SRB to the S-DU  174 A in response to the RRC reconfiguration message. In turn, the S-DU  174 A sends the RRC reconfiguration complete message to the S-CU  172 . In one implementation, the S-DU  174 A can send a F1AP message (e.g., UL RRC Message Transfer message, UE Context Modification Response message, etc.) including the RRC reconfiguration complete message to the S-CU  172 . 
     The S-CU  172 , in some implementations, can perform  323 D the CPAC configuration procedure with the C-DU  174 B, the S-DU  174 A, and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A operated by the C-DU  174 B), similar to the CPAC configuration procedure  321 D. In other implementations, the S-CU  172  can perform  323 D the CPAC configuration procedure with the S-DU  174 A and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A operated by the S-DU  174 A), similar to the CPAC configuration procedure  321 D. In these implementations, the S-CU  172  may perform a UE Context Modification procedure with the S-DU  174 A to obtain a second C-DU configuration instead of the UE Context Setup procedure. In the UE Context Modification procedure, the S-CU  172  can send a UE Context Modification Request message to the S-DU  174 A, similar to the UE Context Setup Request message and the S-DU  174 A responds with a UE Context Modification Response message including a second DU configuration. In these implementations, event  342 D and  348 D would occur between the UE  102  and the S-DU  174 A. The S-CU  172  can perform the CPAC configuration procedures  323 D in parallel with or after the CPAC configuration procedure  321 D. 
     Later in time, the UE  102  determines (or detects)  334 D that a condition for connecting to a C-PSCell  127 A is met and initiates a random access procedure on the C-PSCell  127 A in response to the detection. For convenience, this discussion may refer to the condition or a configuration in the singular, but it will be understood that there may be multiple conditions, and that the RRC reconfiguration message generated by the S-CU  172  can include one or multiple configuration parameters to specify the condition or the multiple conditions. 
     In response to the determination  334 D, the UE  102  then performs  342 D a random access procedure with the C-DU  174 B via the C-PSCell  127 A, e.g., using one or more random access configurations in the second C-DU configuration. If the UE  102  successfully completes the random access procedure (e.g., succeeds the contention resolution in the random access procedure), the UE  102  communicates  348 D with the C-DU  174  via the C-PSCell  127 A using the second C-DU configuration and communicates with the S-CU  172  via the C-DU  174  using the second CU configuration. In some implementations, the UE  102  may disconnect from the PSCell  126 A to perform the random access procedure, i.e., to connect the C-PSCell  127 A. In other implementations, the UE  102  does not disconnect from the PSCell  126 A while performing the random access procedure. If the C-DU  174 B identifies the UE  102  in the random access procedure, the C-DU  174 B becomes an S-DU  174 B and communicates  348 D with the UE  102  via the C-PSCell  127 A. The S-DU  174 B can send a message (e.g., a DL Data Delivery Status message as in  FIG.  3 C ) to indicate to the S-CU  172  that the UE  102  is connected, after or response to identifying the UE  102  in the random access procedure. Later on, if the S-CU  172  initiates an immediate DU change from the S-DU  174 B to a DU of the S-CU  172  (e.g., the DU  174 A or another DU not shown in  FIG.  3 D ), the S-CU  172  can send the second C-SN configuration (i.e., the new S-SN configuration) or the second C-DU configuration (e.g., the new S-DU configuration) to the DU. Later on, if the S-CU  172  initiates an immediate SN change to the base station  106 B or the MN  104  requests the latest SN configuration, the S-CU  172  can send the second C-SN configuration to the MN  104 . 
     During or after the random access procedure, the UE  102  transmits  337 D an RRC reconfiguration complete message to the C-DU  174 B, which in turn sends  339 D a UL RRC Message Transfer message including the RRC reconfiguration complete message and the identity of the C-PSCell  127 A to the S-CU  172 . The S-CU  172  determines  340 C to use the second C-SN configuration (or the second C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) based on the identity of the C-PSCell  127 A. In one implementation, the UE  102  may not include the identity of the C-PSCell  127 A in the RRC reconfiguration complete message  337 D. 
     In some implementations, the random access procedure can be a four-step random access procedure or a two-step random access procedure. The UE  102  can include a UE ID and the RRC reconfiguration complete message  337 D in a Message A and transmit the Message A to the C-DU  174 B in the two-step random access procedure. In other implementations, the random access procedure can be a contention-based random access procedure or a contention-free random access procedure. In the contention-based random access procedure, the UE  102  can include a UE ID and the RRC reconfiguration complete message  337 D in a Message  3  and transmit the Message  3  to the C-DU  174 B. The C-DU  174 B can assign the UE ID in the second DU configuration. The UE ID can be a C-RNTI. In the contention-free random access procedure, the UE  102  can transmit a dedicated preamble to the C-DU  174 B. The C-DU  174 B can assign the dedicated preamble in the second DU configuration. The C-DU  174 B can assign the UE ID in the second DU configuration. If the C-DU  174 B receives the UE ID or the dedicated preamble, the C-DU  174 B identifies the UE  102 . After the UE  102  successfully completes  342 D the random access procedure (e.g., succeeds the contention resolution in the random access procedure), the C-PSCell  127 A begins to operate as the PSCell  127 A, and the UE  102  begins to operate  348 D in DC with the MN  104  via the PCell  124  and the SN  106 A via the PSCell  127 A. In particular, the UE  102  communicates  348 D with the SN  106 A via the C-PSCell  127 A (i.e., new PSCell  127 A) in accordance with the second C-SN configuration. 
     In some implementations, before the event  337 D, the UE  102  can transmit an RRC message (e.g., RRC reconfiguration complete message, UEAssistanceInformation message or ULInformationTransferMRDC message) including the identity of the C-PSCell  127 A to the MN  104 , which in turn sends the identity of the C-PSCell  127 A to the S-CU  172 . In one implementation, the MN  104  can send an SN message (e.g., an SN Reconfiguration Complete message, an SN Modification Request message, or an RRC Transfer message) including the identity of the C-PSCell  127 A or the RRC message to the S-CU  172 . In such implementations, the C-DU  174 B may not include the identity of the C-PSCell  127 A in the UL RRC Message Transfer message at event  339 D. 
     Because the S-CU  172  receives the identity of the C-PSCell  127 A, the S-CU  172  does not select the first C-SN configuration (or the first C-DU configuration) for the C-PSCell  125 A. Thus, the S-CU  172  does not use the first C-SN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Now referring to  FIG.  3 E , a scenario  300 E involves a CPAC without SN change, i.e., a conditional change of a PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a secondary CU (referred to here as S-CU  172 ), a secondary DU (referred to here as S-DU  174 A) and a C-DU  174 B. Events in this scenario similar to those discussed above are labeled with the same reference numbers and the examples and implementations for  FIGS.  3 A and  3 D  can apply to  FIG.  3 E . The differences between the scenarios of  FIG.  3 E  and  FIGS.  3 A and  3 D  are discussed below. 
     Unlike the event  339 D, the C-DU  174 B does not include the identity of the C-PSCell  127 A in the UL RRC Message Transfer message at event  339 E. The S-CU  172  can determine to use a particular C-SN configuration (or a particular C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) based on one or more UE ID(s) included in a UL RRC Message Transfer message. In the scenario, the S-CU  172  determines  346 E to use the second C-SN configuration (or the second C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) based on the UE ID(s) included in the UL RRC Message Transfer message  339 E. 
     In some implementations, the UE ID(s) can include UE F1AP ID(s) which can include at least one of a CU F1AP ID and a DU F1AP ID. The S-CU  172  can include a CU F1AP ID in a UE Context Setup Request message and the C-DU  174 B can include a DU F1AP ID in a UE Context Setup Response message. For example, the S-CU  172  can include a first CU F1AP ID in the UE Context Setup Request message in the CPAC configuration procedure  321 E and include a second CU F1AP ID in the UE Context Setup Request message in the CPAC configuration procedure  323 E. In another example, the C-DU  174 B can include a first DU F1AP ID in the UE Context Setup Response message in the CPAC configuration procedure  321 E and include a second DU F1AP ID in the UE Context Setup Response message in the CPAC configuration procedure  323 E. Thus, the S-CU  172  can associate the first CU UE F1AP ID and/or the first DU FLAP ID to the first C-SN configuration (or the first C-DU configuration) and associate the second CU UE F1AP ID and/or the second DU F1AP ID to the second C-SN configuration (or the second C-DU configuration), respectively. After the C-DU  174 B identifies the UE  102  in the random access procedure on the C-PSCell  127 A, the C-DU  174 B can include the second CU UE F1AP ID and/or the second DU UE F1AP ID in the UL RRC Message Transfer message  339 E. Thus, the S-CU  172  determines  346 E to use the second C-SN configuration (or the second C-DU configuration) based on the second CU UE F1AP ID and/or the second DU UE F1AP ID in the UL RRC Message Transfer message  339 E. 
     As described above, the S-CU  172  can establish or store an association between particular UE ID(s) and a particular C-SN configuration (or a particular C-DU configuration included in the particular C-SN configuration) so that the S-CU  172  can determine the particular C-SN configuration (or the particular C-DU configuration) based on the particular UE ID(s) received in a UL RRC Message Transfer message. For example, the S-CU  172  can establish particular UE ID(s) and a particular C-SN configuration (or a particular C-DU configuration included in the C-SN configuration) after performing a UE Context Setup procedure with the C-DU  174 B to obtain a C-DU configuration for a candidate cell. The S-CU  172  can also associate an identity of a particular candidate cell (e.g., C-PSCell  125 A or C-PSCell  127 A) with the particular C-SN configuration (or the particular C-DU configuration included in the particular C-SN configuration) as described for  FIG.  3 A . Thus, the S-CU  172  can establish or store an association between the identity of the particular candidate cell, the particular UE ID(s), and the particular C-SN configuration (or the particular C-DU configuration included in the particular C-SN configuration). The S-CU  172  can determine the identity of the particular candidate cell based on the UE ID(s) received in the GTP packet including the DL Data Delivery Status message. The S-CU  172  can send the identity of the particular candidate cell to the MN  104  as described above. 
     In other implementations, the UE ID can include a C-RNTI. The C-DU  174 B can include a first C-RNTI in the UE Context Setup Response message or the first C-DU configuration in the CPAC configuration procedure  321 E, and include a second C-RNTI in the UE Context Setup Response message or the second C-DU configuration in the CPAC configuration procedure  323 E. Thus, the S-CU  172  can obtain the first and second C-RNTIs from the UE Context Setup Response messages, and associate the first and second RNTIs to the first C-SN configuration (or the first C-DU configuration) and second C-SN configuration (or the second C-DU configuration), respectively. After the C-DU  174 B identifies the UE  102  in the random access procedure on the C-PSCell  127 A, the C-DU  174 B can include the second C-RNTI in the UL RRC Message Transfer message  339 E. Thus, the S-CU  172  determines  346 E to use the second C-SN configuration (or the second C-DU configuration) from the second C-RNTI in the UL RRC Message Transfer message  339 E. 
     Based on the UE ID(s) in the UL RRC Message Transfer message, the S-CU  172  does not select the first C-SN configuration (or first C-DU configuration) for the C-PSCell  125 A. The S-CU  172  does not use the first C-SN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Now referring to  FIG.  3 F , a scenario  300 F involves a CPAC without SN change, i.e., a conditional change of a PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a secondary CU (referred to here as S-CU  172 ), a secondary DU (referred to here as S-DU  174 A) and a C-DU  174 B. Events in this scenario similar to those discussed above are labeled with the same reference numbers and the examples and implementations for  FIGS.  3 C and  3 D  can apply to  FIG.  3 F . 
     In the scenario  300 F, after the C-DU  174 B identifies the UE  102  during the random access procedure at event  342 F, the C-DU  174 B can send  345 F a DL Data Delivery Status message to the S-CU  172  (similar to event  345 C). Similar to event  346 C, the S-CU  172  determines  346 F to use the second C-SN configuration based on TEID(s) for the DL Data Delivery Status message received at event  345 F. 
     Several example scenarios involving CSAC are discussed next with reference to  FIGS.  4 A-C . 
     Referring first to a scenario  400 A of  FIG.  4 A , which involves a CSAC, i.e., a conditional addition of a C-PSCell of a C-SN when the UE is in SC with the MN, or a conditional change of a PSCell of an SN to a C-PSCell of a C-SN when the UE is already in DC with the MN and SN (e.g., base station  106 B). In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a C-CU (referred to here as C-CU  172 ) and a C-DU  174 B. At the beginning of this scenario, the UE  102  communicates  402 A UL PDUs and/or DL PDUs in SC with the MN  104  (via a PCell  124 ) or in DC with the MN  104  (via a PCell  124 ) and SN  106 B (via a PCell  126 B). Events  406 A,  408 A,  434 A,  436 A,  438 A,  440 A,  442 A and  448 A are similar to events  306 A,  308 A,  334 A,  336 A,  338 A,  340 A,  342 A and  348 A. The description, examples and implementations for  FIG.  3 A  can apply to  FIG.  4 A . The differences between the scenarios of  FIG.  3 A  and  FIG.  4 A  are discussed below. 
     The MN  104  at some point determines  403 A that it should initiate a CSAC procedure to configure the base station  106 A as a C-SN for the UE  102 . The MN  104  can make this determination based on one or more measurement results received from the UE  102  or based on a Conditional SN Change Required message from SN  106 B, for example, or another suitable event. In response to this determination, the MN  104  sends  404 A an SN Addition Request message to the C-CU  172  to initiate a conditional SN Addition procedure. In response to receiving  404 A the SN Addition Request message, the C-CU  172  performs a UE Context Setup procedure with the C-DU  174 B to obtain a first C-DU configuration for configuring a C-PSCell (e.g. C-PSCell  125 A), similar to the UE Context Setup procedure  306 A. The C-CU  172  generates  408 A a first C-SN configuration including the first C-DU configuration and includes the first C-SN configuration in an SN Addition Request Acknowledge message for the UE  102 . The SN  106 A then sends  410 A the SN Addition Request Acknowledge message to the MN  104 , in response to the SN Addition Request message. The first C-SN configuration included in this message can include one or more configuration parameters for the C-PSCell  125 A. In turn, the MN  104  transmits  412 A an RRC container message including the first C-SN configuration to the UE  102 . The events  403 A,  404 A,  406 A,  408 A,  410 A and  412 A are collectively referred to in  FIG.  4 A  as the CSAC configuration procedure  420 A. 
     In some implementations, the UE  102  can transmit an RRC container response message to the MN  104  in response to the RRC container message. In one implementation, the MN  104  can generate an RRC reconfiguration message including the first C-SN configuration, include the RRC reconfiguration message in the RRC container message and send  412 A the RRC container message to the UE  102 . 
     The C-CU  172  can perform  422 A the CSAC configuration procedure with the C-DU  174 B and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A), similar to the CSAC configuration procedure  420 A. The C-CU  172  can perform the CSAC configuration procedure  422 A in parallel with or after the CSAC configuration procedure  420 A. 
     Later in time, the UE  102  determines (or detects)  434 A that a condition for connecting to a C-PSCell  127 A is met and initiates a random access procedure on the C-PSCell  127 A in response to the detection. For convenience, this discussion may refer to the condition or a configuration in the singular, but it will be understood that there may be multiple conditions, and that the RRC reconfiguration message generated by the MN  104  can include one or multiple configuration parameters to specify the condition or the multiple conditions. 
     In response to the determination  434 A, the UE  102  transmits  436 A an RRC reconfiguration complete message including an identity of the C-PSCell  127 A to the MN  104 , which in turn sends  438 A the RRC reconfiguration complete message to the C-CU  172 . In some implementations, the UE  102  can include frequency information (e.g., absolute radio-frequency channel number and/or frequency band number) of the C-PSCell  127 A in the RRC reconfiguration message  436 A. The C-CU  172  determines to use the second C-SN configuration (or the second C-DU configuration) as a new SN configuration (as a new S-DU configuration) based on the identity of the C-PSCell  127 A. In one implementation, the MN  104  can send  438 A an SN message (e.g., an SN Reconfiguration Complete message, a SN Modification Request message, or an RRC Transfer message) including the RRC reconfiguration complete message to the C-CU  172 . Alternatively, the MN  104  obtains the identity of the C-PSCell  127 A and optionally the frequency information (if included) from the RRC reconfiguration message, includes the identity of the C-PSCell  127 A and the frequency information (if included) in at least one IE and sends an SN message (e.g., an SN Reconfiguration Complete message, an SN Modification Request message, or an RRC Transfer message) including the at least one IE to the S-CU  172 . In another implementation, the RRC reconfiguration message  436 A may be transparent to the MN  104  so that the C-CU  172  can send the MN  104  one or more SN messages (e.g., SN Modification Required message, SN configuration update message, SN information update message, etc.) including the identity of the C-PSCell  127 A and optionally the frequency information (if received or derived by the C-CU  172  based on the identity of the C-PSCell  127 A). 
     In some implementations, the UE  102  can generate an RRC container message (e.g., ULInformationTransferMRDC message) including the RRC reconfiguration complete message and transmit  436 A the RRC container message to the MN  104 . The MN  104  in turn extracts the RRC reconfiguration complete message from the RRC container message and sends  438 A the RRC reconfiguration complete message to the C-CU  172 . In one implementation, the UE  102  can include the identity of the C-PSCell  127 A and optionally include the frequency information in the RRC container message. In other implementations, the UE  102  can generate an RRC container response message (similar to the RRC container message described above) including the RRC reconfiguration complete message and transmit  436 A the RRC container response message to the MN  104 . The MN  104  in turn extracts the RRC reconfiguration complete message from the RRC container response message and sends  438 A the RRC reconfiguration complete message to the C-CU  172 . In another implementation, the UE  102  can include the identity of the C-PSCell  127 A and optionally include the frequency information in the RRC container response message. 
     If the C-DU  174 B identifies the UE  102  in the random access procedure, the C-CU  172 , the C-DU  174 B and the C-PSCell  127 A become an S-CU  172 , an S-DU  174 B, and an PSCell  127 A, respectively. The S-DU  174 B communicates  448 A with the UE  102  via the PSCell  127 A using the S-DU configuration after identifying the UE  102 . The S-DU  174 B can send a message (e.g., a DL Data Delivery Status message in  FIG.  3 C ) to indicate to the S-CU  172  that the UE  102  is connected, after or response to identifying the UE  102  in the random access procedure. Later on, if the S-CU  172  initiates an immediate DU change from the S-DU  174 B to a DU of the S-CU  172  (e.g., the DU  174 A or another DU not shown in  FIG.  4 A ), the S-CU  172  can send the second C-SN configuration (i.e., the new SN configuration) or the second C-DU configuration (e.g., the new S-DU configuration) to the DU. Later on, if the S-CU  172  initiates immediate SN change to another base station (e.g., the base station  106 B or a base station not shown in  FIG.  1 A ) or is requested by the MN  104  to provide the latest SN configuration, the S-CU  172  can send the second C-SN configuration to the MN  104 . 
     Because the C-CU  172  receives the identity of the C-PSCell  127 A, the C-CU  172  does not select the first C-SN configuration (or the first C-DU configuration) for the C-PSCell  125 A. Thus, the C-CU  172  does not use the first C-SN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Now referring to  FIG.  4 B , a scenario  400 B involves a CSAC, i.e., a conditional addition of a C-PSCell of a C-SN when the UE is in SC with the MN, or a conditional change of a PSCell of an SN to a C-PSCell of a C-SN when the UE is already in DC with the MN and SN (e.g., base station  106 B). In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a C-CU (referred to here as C-CU  172 ) and a C-DU  174 B. Events in this scenario similar to those discussed above are labeled with the same reference numbers. Events  406 B,  408 B,  434 B,  436 B,  438 B,  440 B,  442 B,  444 B and  448 B are similar to events  306 B,  308 B,  334 B,  336 B,  338 B,  340 A,  342 B,  344 B and  348 B. The description, examples and implementations for  FIGS.  4 A and  3 B  can apply to  FIG.  4 B . The differences between the scenarios of  FIG.  4 B  and  FIG.  4 A  are discussed below. 
     In the scenario  400 B, the UE  102  may not transmit the RRC reconfiguration complete message  436 B or may transmit the  436 B RRC reconfiguration complete message not including the identity of the C-PSCell  127 A to the MN  104 . After the C-DU  174 B identifies the UE  102  during the random access procedure at event  442 B, the C-DU  174  can transmit  444 B a DU to CU message including the identity of the C-PSCell  127 A to the S-CU  172  to indicate that the UE  102  is connected on the C-PSCell  127 A, so that the S-CU  172  can determine to use the second C-SN configuration (or the second C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) based on the identity of the C-PSCell  127 A. In some implementations, the DU to CU message can be a F1AP message or a DL Data Delivery Status message. For example, the F1AP message can be a UL RRC Message Transfer message including a dummy RRC message or excluding an RRC message. In another example, the FLAP message can be a new F1AP message excluding an RRC message. In another example, the F1AP message can be a UL RRC Message Transfer message including an RRC container IE. The C-DU  174 B can include a dummy or fake RRC message (i.e., an RRC message not received from the UE  102 ) in the RRC container IE. The C-DU  174 B can include an indicator indicating ignoring (or discarding) the RRC container IE in the UL RRC Message Transfer message. The C-CU  172  ignores (or discard, does not use) the RRC container IE (i.e., the dummy or fake RRC message) in response to the indicator. 
     Because of the receives the C-PSCell  127 A, the C-CU  172  does not select the first C-SN configuration (or the first C-DU configuration) for the C-PSCell  125 A. Thus, the C-CU  172  does not use the first C-SN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Now referring to  FIG.  4 C , a scenario  400 C involves a CSAC, i.e., a conditional addition of a C-PSCell of a C-SN when the UE is in SC with the MN, or a conditional change of a PSCell of an SN to a C-PSCell of a C-SN when the UE is already in DC with the MN and SN (e.g., base station  106 B). In this scenario, the base station  104  operates as an MN and the base station  106 A operates as an SN that includes both a C-CU (referred to here as C-CU  172 ) and a C-DU  174 B. Events in this scenario similar to those discussed above are labeled with the same reference numbers. Events  406 C,  408 C,  434 C,  436 C,  438 C,  440 C,  442 C,  445 C and  448 C are similar to events  306 C,  308 C,  334 C,  336 C,  338 C,  340 C,  342 C,  345 C and  348 C. The description, examples and implementations for  FIGS.  4 A and  3 C  can apply to  FIG.  4 C . The differences between the scenarios of  FIG.  4 C  and  FIG.  4 A  are discussed below. 
     In the scenario  400 C, the UE  102  may not transmit the RRC reconfiguration complete message  436 C or may transmit the  436 C RRC reconfiguration complete message not including the identity of the C-PSCell  127 A to the MN  104 . After the C-DU  174 B identifies the UE  102  during the random access procedure at event  442 C, the C-DU  174 B can send  445 C a DL Data Delivery Status message to the C-CU  172 . Similar to events  346 C and  346 F, the C-CU  172  determines  446 C to use the second C-SN configuration based on TEID(s) for the DL Data Delivery Status message received at event  445 C. 
     Several example scenarios involving CPAC or CSAC are discussed next with reference to  FIGS.  5 A- 5 C . Several example scenarios involving CPAC are also discussed with reference to  FIGS.  5 D- 5 F . 
     Referring first to a scenario  500 A of  FIG.  5 A , which involves a CPAC, or a CSAC, i.e., a conditional addition of a C-PSCell of a C-SN when the UE is in SC with the MN, a conditional change of a PSCell of an SN to a C-PSCell of a C-SN when the UE is already in DC with the MN and SN (e.g., base station  106 B). In this scenario, the base station  106 A operates as an MN that includes a CU  172  (operated as a master CU (M-CU)  172 ) and a master DU (M-DU)  174 C, and the base station  106 A can operate as a C-SN that includes the CU  172  (operated as a S-CU  172 ) and a C-DU  174 B. Alternatively, the base station  106 A can operate as an SN that includes the CU  172  (operated as a S-CU  172 ), a C-DU  174 B and a S-DU  174 A. 
     At the beginning of this scenario, the UE  102  communicates  502 A UL PDUs and/or DL PDUs in SC with the MN  106 A (via a PCell  126 A) or in DC with the MN  106 A (via a PCell  126 A) and SN  106 A (via a PSCell not shown in  FIG.  1 A ). In cases where the UE  102  is in DC with the MN  106 A and the SN  106 A, events  504 A,  506 A,  508 A,  510 A,  512 A,  522 A,  534 A,  536 A,  538 A,  542 A and  548 A are similar to events  304 A,  306 A,  308 A,  310 A,  312 A,  322 A,  334 A,  336 A,  338 A,  340 A,  342 A and  348 A. In cases where the UE  102  is in SC with the MN  106 A or in DC with the MN  106 A and another base station (e.g., the base station  106 B), event  504 A is similar to event  304 A, and events  506 A,  508 A,  510 A,  512 A,  522 A,  534 A,  536 A,  538 A,  542 A and  548 A are similar to events  406 A,  408 A,  410 A,  412 A,  422 A,  434 A,  436 A,  438 A,  440 A,  442 A and  448 A. The description, examples and implementations for  FIGS.  3 A and  4 A  can apply to  FIG.  5 A . The differences between the scenarios of  FIG.  5 A  and  FIGS.  3 A and  4 A  are discussed below. 
     In the scenario  500 A, the M/S-CU  172  at some point determines  504 A that it should prepare a conditional PSCell change to a C-PSCell (e.g., C-PSCell  125 A) operated by the C-DU  174 B for the UE  102 . The M/S-CU  172  can make this determination based on one or more measurement results received from the UE  102  or based on a Conditional SN Change Required message from SN  106 B, for example, or another suitable event. In response to this determination, the M/S-CU  172  performs a UE Context Setup procedure with the C-DU  174 B to obtain a first C-DU configuration for configuring a C-PSCell (e.g. C-PSCell  125 A), similar to the UE Context Setup procedure  306 A. The first C-DU configuration can include one or more configuration parameters for communication on the C-PSCell  125 A. The M/S-CU  172  generates  508 A a first C-SN configuration including the first C-DU configuration. After generating the first C-SN configuration, the M/S-CU  172  sends  510 A the first C-SN configuration to the M-DU  174 C, which in turn transmits  512 A an RRC container message including the first C-SN configuration to the UE  102 . If the UE  102  is in DC with the MN  106 A and the SN  106 A, the events  504 A,  506 A,  508 A,  510 A and  512 A can be collectively referred to in  FIG.  5 A  as the CPAC configuration procedure  520 A, similar to the CPAC configuration procedure  320 A. If the UE  102  is in SC with the MN  106 A or in DC with the MN  106 A and the SN  106 B, the events  504 A,  506 A,  508 A,  510 A and  512 A can be collectively referred to in  FIG.  5 A  as the CSAC configuration procedure  520 A, similar to the CSAC configuration procedure  420 A. 
     If the UE  102  is in DC with the MN  106 A and the SN  106 A, the S-CU  172 , in some implementations, can generate an RRC reconfiguration message including the first C-SN configuration. Then, the M-CU  172  can include the RRC reconfiguration message in the RRC container message and send  510 A a F1 application protocol (F1AP) message (e.g., DL RRC Message Transfer message, UE Context Modification Request message, UE Context Setup Request message, etc.) including the RRC container message to the M-DU  174 C. In turn, the M-DU  174 C transmits  512 A the RRC container message to the UE  102 . The M-DU  174 C may send a F1AP response message (e.g., UE Context Modification Response message, UE Context Setup Response message, etc.) to the M-CU  172  in response to the F1AP message (e.g., UE Context Modification Request message, UE Context Setup Request message, etc.). 
     If the UE  102  is in SC with the MN  106 A or in DC with the MN  106 A and the SN  106 B, the C-CU  172 , in some implementations, can generate an RRC reconfiguration message including the first C-SN configuration. Then, the M-CU  172  can include the RRC reconfiguration message in the RRC container message and send  510 A a F1 application protocol (F1AP) message (e.g., DL RRC Message Transfer message, UE Context Modification Request message, UE Context Setup Request message, etc.) including the RRC container message to the M-DU  174 C. In turn, the M-DU  174 C transmits  512 A the RRC container message to the UE  102 . The M-DU  174 C may send a F1AP response message (e.g., UE Context Modification Response message, UE Context Setup Response message, etc.) to the M-CU  172  in response to the F1AP message (e.g., UE Context Modification Request message, UE Context Setup Request message, etc.). 
     If the UE  102  is in DC with the MN  106 A and the SN  106 A, the S-CU  172  can perform  522 A the CPAC configuration procedure with the C-DU  174 B and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A), similar to the CPAC configuration procedure  322 A. The S-CU  172  can perform the CPAC configuration procedure  522 A in parallel with or after the CPAC configuration procedure  520 A. In other implementations, the S-CU  172  can perform  522 A the CPAC configuration procedure with the S-DU  174 A and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A operated by the S-DU  174 A), similar to the CPAC configuration procedure  520 A. 
     If the UE  102  is in SC with the MN  106 A or in DC with the MN  106 A and the SN  106 B, the M-CU  172  can perform  522 A the CSAC configuration procedure with the C-DU  174 B and the UE  102  to configure the UE  102  a second C-SN configuration including a second C-DU configuration for another C-PSCell (e.g., C-PSCell  127 A), similar to the CSAC configuration procedure  422 A. The M-CU  172  can perform the CSAC configuration procedure  522 A in parallel with or after the CSAC configuration procedure  520 A. 
     Later in time, the UE  102  determines (or detects)  534 A that a condition for connecting to a C-PSCell  127 A is met and initiates a random access procedure on the C-PSCell  127 A in response to the detection. For convenience, this discussion may refer to the condition or a configuration in the singular, but it will be understood that there may be multiple conditions, and that the RRC reconfiguration message generated by the CU  172  can include one or multiple configuration parameters to specify the condition or the multiple conditions. 
     In response to the determination  534 A, the UE  102  transmits  536 A an RRC reconfiguration complete message including an identity of the C-PSCell  127 A to the M-DU  174 C, which in turn sends  538 A the RRC reconfiguration complete message to the M/S/C-CU  172 . In some implementations, the UE  102  can include frequency information (e.g., absolute radio-frequency channel number and/or frequency band number) of the C-PSCell  127 A in the RRC reconfiguration message  536 A. The S/C-CU  172  determines  540 A to use the second C-SN configuration (or the second C-DU configuration) as a new SN configuration (as a new S-DU configuration) based on the identity of the C-PSCell  127 A. 
     In some implementations, the UE  102  can generate an RRC container message (e.g., ULInformationTransferMRDC message) including the RRC reconfiguration complete message and transmit  536 A the RRC container message to the M-DU  174 C, which in turn sends  538 A the RRC container message to the M/S/C-CU  172 . In one implementation, the UE  102  can include the identity of the C-PSCell  127 A and optionally include the frequency information in the RRC container message. In other implementations, the UE  102  can generate an RRC container response message (similar to the RRC container message described above) including the RRC reconfiguration complete message and transmit  536 A the RRC container response message to the M-DU  174 C, which in turn sends  538 A the RRC container message to the M/S/C-CU  172 . In one implementation, the UE  102  can include the identity of the C-PSCell  127 A and optionally include the frequency information in the RRC container response message. 
     Because the S/C-CU  172  receives the identity of the C-PSCell  127 A, the S/C-CU  172  does not select the first C-SN configuration (or the first C-DU configuration) for the C-PSCell  125 A. Thus, the S/C-CU  172  does not use the first C-SN configuration for the C-PSCell  125 A to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Now referring to  FIG.  5 B , a scenario  500 B involves a CPAC, or a CSAC, i.e., a conditional addition of a C-PSCell of a C-SN when the UE is in SC with the MN, a conditional change of a PSCell of an SN to a C-PSCell of a C-SN when the UE is already in DC with the MN and SN (e.g., base station  106 B). In this scenario, the base station  106 A operates as an MN that includes a CU  172  (operated as a master CU (M-CU)  172 ) and a master DU (M-DU)  174 C, and the base station  106 A can operate as a C-SN that includes the CU  172  (operated as a S-CU  172 ) and a C-DU  174 B. Alternatively, the base station  106 A can operate as an SN that includes the CU  172  (operated as a S-CU  172 ), a C-DU  174 B and a S-DU  174 A. Events in this scenario similar to those discussed above are labeled with the same reference numbers. The description, examples and implementations for  FIG.  5 A  and  FIG.  3 B  can apply to  FIG.  5 B . The differences between the scenarios of  FIG.  5 B  and  FIG.  5 A  are discussed below. 
     In the scenario  500 B, the UE  102  may not transmit the RRC reconfiguration complete message  536 B or may transmit the  536 B RRC reconfiguration complete message not including the identity of the C-PSCell  127 A to the MN  106 A (i.e., to the M-CU  172  via the M-DU  174 C). 
     After the C-DU  174 B identifies the UE  102  during the random access procedure at event  542 B, the C-DU  174  can transmit  544 B a DU to CU message including the identity of the C-PSCell  127 A to the S/C-CU  172  to indicate that the UE  102  is connected on the C-PSCell  127 A, so that the S/C-CU  172  can determine  546 B to use the second C-SN configuration (or the second C-DU configuration) as a new S-SN configuration (as a new S-DU configuration) based on the identity of the C-PSCell  127 A. In some implementations, the DU to CU message can be a FLAP message or a DL Data Delivery Status message. For example, the F1AP message can be a UL RRC Message Transfer message including a dummy RRC message or excluding an RRC message. In another example, the F1AP message can be a new F1AP message excluding an RRC message. In another example, the F1AP message can be a UL RRC Message Transfer message including an RRC container IE. The C-DU  174 B can include a dummy RRC message or a fake RRC message (i.e., an RRC message not received from the UE  102 ) in the RRC container IE. The C-DU  174 B can include an indicator indicating ignoring (or discarding) the RRC container in the UL RRC Message Transfer message. The C-CU  172  ignores (or discard, does not use) the RRC container IE (i.e., the dummy or fake RRC message) in response to the indicator. 
     Because the S/C-CU  172  receives the identity of the C-PSCell  127 A, the S/C-CU  172  does not select the first C-SN configuration (or the first C-DU configuration) for the C-PSCell  125 A. Thus, the S/C-CU  172  does not use the first C-SN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PSCell  127 A, thereby avoiding communication failure due to configuration mismatch. 
     Now referring to  FIG.  5 C , a scenario  500 C involves a CPAC, or a CSAC, i.e., a conditional addition of a C-PSCell of a C-SN when the UE is in SC with the MN, a conditional change of a PSCell of an SN to a C-PSCell of a C-SN when the UE is already in DC with the MN and SN (e.g., base station  106 B). In this scenario, the base station  106 A operates as an MN that includes a CU  172  (operated as a master CU (M-CU)  172 ) and a master DU (M-DU)  174 C, and the base station  106 A can operate as a C-SN that includes the CU  172  (operated as a S-CU  172 ) and a C-DU  174 B. Alternatively, the base station  106 A can operate as an SN that includes the CU  172  (operated as a S-CU  172 ), a C-DU  174 B and a S-DU  174 A. Events in this scenario similar to those discussed above are labeled with the same reference numbers. The description, examples and implementations for  FIG.  5 A  and  FIG.  3 C  can apply to  FIG.  5 C . The differences between the scenarios of  FIG.  5 C  and  FIG.  5 A  are discussed below. 
     In the scenario  500 C, the UE  102  may not transmit the RRC reconfiguration complete message  536 C or may transmit the  536 C RRC reconfiguration complete message not including the identity of the C-PSCell  127 A to the MN  106 A (i.e., to the M-CU  172  via the M-DU  174 C). The S/C-CU  172  receives  545 C a DL Data Delivery Status message from the C-DU  174 B, and determines  546 C to use the second C-SN configuration based on TEID(s) for the DL Data Delivery Status message. 
     Now referring to  FIG.  5 D , a scenario  500 D involves a CPAC, i.e., a conditional addition of a C-PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  106 A operates as an MN that includes a CU  172  (operated as a master CU (M-CU)  172 ) and a master DU (M-DU)  174 C, and the base station  106 A can operate as an SN that includes the CU  172  (operated as a S-CU  172 ), a C-DU  174 B and a S-DU  174 A. Events in this scenario similar to those discussed above are labeled with the same reference numbers. Events  504 D,  506 D,  508 D,  511 D,  513 D,  523 D,  534 D,  542 D,  537 D,  539 D,  540 D,  548 D are similar to events  304 D,  306 D,  308 D,  311 D,  313 D,  323 D,  334 D,  342 D,  337 D,  339 D,  340 D,  348 D. The description, examples and implementations for  FIG.  5 A  and  FIG.  3 D  can apply to  FIG.  5 D . 
     Now referring to  FIG.  5 E , a scenario  500 E involves a CPAC, i.e., a conditional addition of a C-PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  106 A operates as an MN that includes a CU  172  (operated as a master CU (M-CU)  172 ) and a master DU (M-DU)  174 C, and the base station  106 A can operate as an SN that includes the CU  172  (operated as a S-CU  172 ), a C-DU  174 B and a S-DU  174 A. Events in this scenario similar to those discussed above are labeled with the same reference numbers. Events  521 E,  523 E,  534 E,  542 E,  537 E,  539 E,  546 E,  548 E are similar to events  321 E,  323 E,  334 E,  342 E,  337 E,  339 E,  346 E,  348 E. The description, examples and implementations for  FIG.  5 A  and  FIG.  3 E  can apply to  FIG.  5 E . 
     Now referring to  FIG.  5 F , a scenario  500 F involves a CPAC, i.e., a conditional addition of a C-PSCell of an SN when the UE is already in DC with the MN and SN. In this scenario, the base station  106 A operates as an MN that includes a CU  172  (operated as a master CU (M-CU)  172 ) and a master DU (M-DU)  174 C, and the base station  106 A can operate as an SN that includes the CU  172  (operated as a S-CU  172 ), a C-DU  174 B and a S-DU  174 A. Events in this scenario similar to those discussed above are labeled with the same reference numbers. Events  521 F,  523 F,  534 F,  542 F,  537 F,  539 F,  545 F,  548 F are similar to events  321 F,  323 F,  334 F,  342 F,  337 F,  339 F,  345 F,  348 F. The description, examples and implementations for  FIG.  5 A  and  FIG.  3 F  can apply to  FIG.  5 F . 
       FIGS.  6 A- 6 B,  7 A- 7 B,  8 , and  9    depict handover scenarios in which a base station initializes a conditional handover procedure for the UE  102 . 
     Referring to  FIG.  6 A , in a scenario  600 A, the base station  104  operates as an MN and the base station  106 A operates as a candidate base station that includes both a candidate CU (referred to here as C-CU  172 ), a candidate DU (referred to here as C-DU  174 B) and optionally another C-DU  174 A. Initially, the UE  102  communicates  602 A data (e.g., uplink and/or downlink data PDUs) with the MN  104  (via cell  124 ), e.g., according to an S-MN configuration. Several events in the scenario  600 A are similar to events in the scenarios  300 A. The differences between the scenarios of  FIG.  3 A  and  FIG.  6 A  are described below. 
     The MN  104  at some point determines  604 A that it should prepare a conditional handover to a C-PCell (e.g., C-PCell  125 A) operated by the C-DU  174 B for the UE  102 . The MN  104  can make this determination based on one or more measurement results received from the UE  102 , for example, or another suitable event. In response to this determination, the MN sends  606 A a Handover Request message to the C-CU  172 . The C-CU  172  performs  608 A UE Context Setup Procedures with the C-DU  174 B to obtain a C-DU configuration, similar to the procedure  306 A in  FIG.  3   . In some implementation, the Handover Request message includes a target Cell ID (e.g. CGI of the C-PCell  125 A) and the C-CU  172  determines the C-DU to perform the UE Context Setup procedures based on the Cell ID. The C-CU  172  generates  610 A a first C-MN configuration including the obtained C-DU configuration. The C-CU  172  sends  612 A a Handover Response message including an RRC Reconfiguration message, which includes the first C-MN configuration, to the MN  104 . The MN  104  transmits  614 A an RRC reconfiguration message including the C-MN configuration to the UE  102 . The events  604 A,  606 A,  306 A,  610 A,  612 A, and  614 A are collectively referred to in  FIG.  6 A  as the conditional handover (CHO) configuration procedure  620 A. 
     The MN  104  can perform  622 A the CHO configuration procedure with the C-DU  174 B and the UE  102  to configure the UE  102  a second C-MN configuration including a second C-DU configuration for another C-PCell (e.g., C-PCell  126 A), similar to the CHO configuration procedure  620 A. The C-CU  172  can perform the CHO configuration procedures  620 A,  622 A in parallel or sequentially. 
     In some implementations, the MN  104  can include a first C-CU configuration in the first C-MN configuration and a second C-CU configuration in the second C-MN configuration. The first C-CU configuration and the second C-CU configuration can have the same content or different contents. In other implementations, the MN  104  does not include a C-CU configuration in the first C-MN configuration and the MN  104  does not include a C-CU configuration in the second C-MN configuration. The first C-DU configuration and the second C-DU configuration can have some portions that are different. 
     Later in time, the UE  102  determines (or detects)  634 A that a condition for connecting to a C-PCell  126 A is met and initiates a random access procedure on the C-PCell  126 A in response to the detection. For convenience, this discussion may refer to the condition or a configuration in the singular, but it will be understood that there may be multiple conditions, and that the conditional configuration can include one or multiple configuration parameters to specify the condition or the multiple conditions. In response to the determination, the UE  102  transmits  638 A an RRC reconfiguration complete message to the C-DU  174 B, and the C-DU  174 B in turn sends  640 A a UL RRC Message Transfer including the RRC reconfiguration complete message and an identity of the C-PCell  126 A to the C-CU  172 . The C-CU  172  determines  650 A to use the second C-MN configuration (or the second C-DU configuration and/or second C-CU configuration) as a new S-MN configuration (as a new M-DU configuration and/or a new M-CU configuration) based on the identity of the C-PCell  126 A. 
     In some implementations, the identity of the C-PSCell  126 A can be a cell global identity (CGI). In other implementations, the identity of the C-PSCell  126 A can be a cell identity in a system information block broadcast on the C-PSCell  126 A. In yet other implementations, the identity of the C-PSCell  126 A can be a PCI that the UE  102  obtains from synchronization signals received by the UE  102  on the C-PSCell  126 A. In various implementations, the C-CU  172  maintains a table for mapping between a CGI and a PCI or another suitable identifier of a particular cell in the wireless communication system  100  for the purpose of identifying a particular C-MN configuration. 
     In response to the determination  634 A, the UE  102  then performs  636 A a random access procedure with the C-DU  174 B via the C-PCell  126 A, e.g., using one or more random access configurations in the second C-DU configuration. If the UE  102  successfully completes the random access procedure, the UE  102  communicates  642 A with the C-DU  174 B via the C-PCell  126 A using the second C-DU configuration and communicates with the C-CU  172  via the C-DU  174 B using the second C-CU configuration. The UE  102  can transmit  638 A the RRC reconfiguration complete message during or after the random access procedure. In some implementations, the UE  102  may disconnect from the PCell  124  to perform the random access procedure, i.e., to connect the C-PCell  126 A. In other implementations, the UE  102  does not disconnect from the PCell  124  while performing the random access procedure. If the C-DU  174 B identifies the UE  102  in the random access procedure, the C-DU  174 B becomes DU  174 B and communicates  642 A with the UE  102  via the C-PCell  126 A. The DU  174 B can send a message (e.g., a DL Data Delivery Status message in  FIG.  3 C ) to indicate to the CU  172  that the UE  102  is connected, after or response to identifying the UE  102  in the random access procedure. 
     In some implementations, the random access procedure can be a four-step random access procedure or a two-step random access procedure. The UE  102  can transmit the RRC reconfiguration complete message  638 A in a Message A of the two-step random access procedure. In other implementations, the random access procedure can be a contention-based random access procedure or a contention-free random access procedure. In the contention-based random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  638 A in a Message  3  of the contention-based random access procedure. In case of the contention-free random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  638 A after the contention-free random access procedure. 
     Because the C-CU  172  receives the C-PCell  126 A in the UL RRC Message Transfer message  640 A, the C-CU  172  does not select the first C-MN configuration (or the first C-DU configuration and/or the first C-CU configuration) for the C-PCell  125 A. Thus, the C-CU  172  does not use the first C-MN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PCell  126 A, thereby avoiding a communication failure due to configuration mismatch can be avoided. 
     In some implementations, C-CU  172  can include at least one first security configuration parameter in the second C-MN configuration. In other implementations, the C-CU  172  can send the at least one first security configuration parameter with the first C-MN configuration at event  612 A and the MN  104  forwards it at event  614 A. The C-CU  172  can generate the at least one first security key (security key(s)) from the at least one first security configuration parameters and a first security base key (e.g., K MN  or K NG-RAN* ). For example, the first security key(s) can include a first ciphering key for encryption and decryption and/or include a first integrity key for integrity protection and check. 
     In other implementations, C-CU  172  can include at least one second security configuration parameter in the second C-MN configuration. In other implementations, the C-CU  172  can send the at least one second security configuration with the second C-MN configuration during procedure  622 A at an event similar to  612 A and the MN  104  can forward it at an event similar to  614 A. The C-CU  172  can generate at least one second security key (security key(s)) from the at least one second security configuration parameter and a second security base key (e.g., K MN  or K NG-RAN* ). For example, the second security key(s) can include a second ciphering key for encryption and decryption and/or include a second integrity key for integrity protection and check. In one implementation, the C-CU  172  determines to use the at least one second security configuration parameter and the second security base key to generate the second security key(s) according to the identity of the C-PCell  126 A. In another implementation, the C-CU  172  determines to use the second security key(s) according to the identity of the C-PCell  126 A. The UE  102  can generate the second security key(s) (same as the second security key(s) generated by the candidate base station  106 A) from the at least one second security configuration parameter and the second security base key. In one implementation, the UE  102  can generate the second security key(s) from the at least one second security configuration parameter and the second security base key after event  634 A or receiving the RRC reconfiguration message during the CHO configuration procedure  622 A. Thus, the UE  102  communicates  642 A with the C-CU  172  via the C-DU  174 B using the second C-MN configuration and the second security key(s). In one implementation, the first security base key and the second security base key can be the same or identical. In another implementation, the first security base key and the second security base key can be different. The C-CU  172  can determine which security base key or which security key(s) based on the identity of the C-PCell  126 A. 
     In some implementations, the first C-DU configuration can include multiple configuration parameters such as physical layer configuration parameters, MAC configuration parameters, RLC configuration parameters, and/or the one or more random access configurations needed by the UE  102  to perform  636 A the random access procedure with the C-DU  174 A on the C-PCell  125 A. The second C-DU configuration can include multiple configuration parameters such as physical layer configuration parameters, MAC configuration parameters, RLC configuration parameters, and/or the one or more random access configurations needed by the UE  102  to perform  636 A the random access procedure with the C-DU  174 A on the C-PCell  125 A. In other implementations, the first C-DU configuration can be CellGroupConfig information element (IE) that configures the C-PCell  125 A and zero, one, or more C-SCells of the C-DU  174 A. The second C-DU configuration can be CellGroupConfig IE that configures the C-PCell  126 A and zero, one, or more C-SCells of the C-DU  174 B. 
     In some implementations, the first C-CU configuration may include a radio bearer configuration and/or measurement configuration. The second C-CU configuration may include a radio bearer configuration and/or measurement configuration. For example, the radio bearer configuration can be a RadioBearerConfig IE, DRB-ToAddModList IE or SRB-ToAddModList IE, DRB-ToAddMod IE or SRB-ToAddMod IE. The measurement configuration can be a MeasConfig IE. 
     In some implementations, the first C-MN configuration can be an RRCReconfiguration message or an RRCReconfiguration-IEs conforming to 3GPP TS 38.331. The second C-MN configuration can be an RRCReconfiguration message or an RRCReconfiguration-IEs conforming to 3GPP TS 38.331. In other implementations, the first C-SN configuration can be an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. The second C-SN configuration can be an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. 
     In some implementations, the S-MN configuration includes a RadioBearerConfig IE, an RRCReconfiguration message, an RRCReconfiguration-IEs, a CellGroupConfig IE and/or MeasConfig IE conforming to 3GPP TS 38.331. In other implementations, the S-MN configuration includes an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. In still other implementations, the S-MN configuration includes configurations in the RadioBearerConfig IE, the CellGroupConfig IE, RRCReconfiguration-IEs, and/or or RRCConnectionReconfiguration-IEs. In some implementations, the Handover Request message can be a Handover Request message defined in 3GPP TS 36.423 or TS 38.423 and the Handover Response message can be Handover Request Acknowledge message defined in 3GPP TS 36.423 or TS 38.423. In some implementation, the S-MN configuration includes an M-CU configuration and/or an M-DU configuration. The M-CU configuration in some implementation is as defined for the C-CU configuration in this disclosure. The M-DU configuration in some implementation is as defined for the C-DU configuration in this disclosure for a PCell and zero, one, or more SCells operated by the M-DU. 
     In some implementations, if the base station  106 A is a gNB, the RRC reconfiguration and RRC reconfiguration complete messages are RRCReconfiguration and RRCReconfigurationComplete messages, respectively. In other implementations, if the SN  106 A is an eNB or ng-eNB, the RRC reconfiguration and RRC reconfiguration complete messages are RRCConnectionReconfiguration and RRCConnectionReconfigurationComplete messages, respectively. 
     Referring next to  FIG.  6 B , in a scenario  600 B similar to  600 A, the base station  104  again operates as an MN and the base station  106 A operates as a candidate base station that includes both a C-CU  172 , a C-DU  174 B and optionally a C-DU  174 A. Several events in the scenario  600 B are similar to events in the scenarios  600 A. The differences between the scenarios of  FIG.  6 B  and  FIG.  6 A  are described below. 
     The MN  104  can perform  620 B a CHO configuration procedure with the C-DU  174 B and the UE  102  to configure the UE  102  a first C-MN configuration including a first C-DU configuration for a C-PCell (e.g., C-PCell  125 A), similar to the CHO configuration procedure  620 A. The MN  104  can perform  622 B another CHO configuration procedure with the C-DU  174 B and the UE  102  to configure the UE  102  a second C-MN configuration including a second C-DU configuration for another C-PCell (e.g., C-PCell  126 A), similar to the CHO configuration procedure  620 A. The MN  104  can perform the CHO configuration procedures  620 B,  622 B in parallel or sequentially. 
     Later in time, the UE  102  determines (or detects)  634 B that a condition for connecting to a C-PCell  126 A is met and initiates a random access procedure on the C-PCell  126 A in response to the detection. In response to the determination, the UE  102  transmits  638 B an RRC reconfiguration complete message to the C-DU  174 B, which in turn the C-DU  174 B sends  640 B a UL RRC Message Transfer including the RRC reconfiguration complete message to the C-CU  172 . The C-CU  172  determines  646 B to use the second C-MN configuration (or the second C-DU configuration and/or second C-CU configuration) as a new S-MN configuration (as a new M-DU configuration and/or a new M-CU configuration) based on the UE ID(s) in the UL RRC Message Transfer message. In some implementation, the UE ID(s) are the gNB-CU UE F1AP ID and the gNB-DU UE F1AP ID as defined in 3GPP TS 38.401 or TS 38.473. 
     Based on the UE ID(s) in the UL RRC Message Transfer message  640 B, the C-CU  172  does not select the first C-MN configuration (or the first C-DU configuration and/or the first C-CU configuration) for the C-PCell  125 A. Thus, the C-CU  172  does not use the first C-MN configuration for the C-PCell  125 A to communicate with the UE  102  while the UE  102  connects to the C-DU  174 B on the C-PCell  126 A, thereby avoiding a communication failure due to configuration mismatch. 
     Referring next to  FIG.  7 A , in a scenario  700 A, the base station  106 A operates as a (master) base station that includes a CU  172 , a source DU (referred to here as S-DU  174 A), a candidate DU (referred to here as C-DU  174 B) and optionally another C-DU  174 C. Initially, the UE  102  communicates  702 A data (e.g., uplink and/or downlink data PDUs) with the CU  172  via the S-DU  174 A and PCell  125 A, e.g., according to an S-MN configuration. Several events in the scenario  700 A are similar to events in the scenarios  600 A. The differences between the scenarios of  FIG.  7 A  and  FIG.  6 A  are described below. 
     The CU  172  at some point determines  704 A that it should prepare a conditional handover to a C-PCell (e.g., C-PCell  126 A) operated by the C-DU  174 C for the UE  102 . The CU  172  can make this determination based on one or more measurement results received from the UE  102 , for example, or another suitable event. In response to this determination, the CU  172  performs  708 A UE Context Setup Procedures with the C-DU  174 C to obtain a C-DU configuration, similar to the procedure  306 A in  FIG.  3   . The CU  172  generates  710 A a first C-MN configuration including the obtained C-DU configuration. The CU  172  sends  712 A a DL RRC Message Transfer message including an RRC Reconfiguration message, which includes the first C-MN configuration. The S-DU  174 A transmits  714 A the received RRC Reconfiguration message to the UE  102 . The events  704 A,  708 A,  710 A,  712 A, and  714 A are collectively referred to in  FIG.  7 A  as the conditional handover (CHO) configuration procedure  720 A for intra-base-station handover. 
     The CU  172  can perform  722 A the CHO configuration procedure with the C-DU  174 C and the UE  102  to configure the UE  102  a second C-MN configuration including a second C-DU configuration for another C-PCell (e.g., C-PCell  127 A), similar to the CHO configuration procedure  720 A. The CU  172  can perform the CHO configuration procedures  720 A,  722 A in parallel or sequentially. 
     In some implementations, the CU  172  can include a first C-CU configuration in the first C-MN configuration and a second C-CU configuration in the second C-MN configuration. The first C-CU configuration and the second C-CU configuration can have the same content or different contents. In other implementations, the CU  172  does not include a C-CU configuration in the first C-MN configuration and the CU  172  does not include a C-CU configuration in the second C-MN configuration. The first C-DU configuration and the second C-DU configuration can have some portions that are different. 
     Later in time, the UE  102  determines (or detects)  734 A that a condition for connecting to a C-PCell  127 A is met and initiates a random access procedure on the C-PCell  127 A in response to the detection. For convenience, this discussion may refer to the condition or a configuration in the singular, but it will be understood that there may be multiple conditions, and that the conditional configuration can include one or multiple configuration parameters to specify the condition or the multiple conditions. In response to the determination, the UE  102  transmits  738 A an RRC reconfiguration complete message to the C-DU  174 C, which in turn the C-DU  174 C sends  740 A a UL RRC Message Transfer including the RRC reconfiguration complete message and an identity of the C-PCell  127 A to the CU  172 . The CU  172  determines  750 A to use the second C-MN configuration (or the second C-DU configuration and/or the second C-CU configuration) as a new S-MN configuration (as a new M-DU configuration and/or a new M-CU configuration) based on the identity of the C-PCell  127 A. 
     In response to the determination  734 A, the UE  102  then performs  736 A a random access procedure with the C-DU  174 C via the C-PCell  127 A, e.g., using one or more random access configurations in the second C-DU configuration. If the UE  102  successfully completes the random access procedure, the UE  102  communicates  742 A with the C-DU  174 C via the C-PCell  127 A using the second C-DU configuration and communicates with the CU  172  via the C-DU  174 C using the second C-CU configuration. The UE  102  can transmit  738 A the RRC reconfiguration complete message during or after the random access procedure. In the contention-based random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  738 A in a Message  3  of the contention-based random access procedure. In case of the contention-free random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  738 A after the contention-free random access procedure. In some implementations, the UE  102  may disconnect from the PCell  125 A to perform the random access procedure, i.e., to connect the C-PCell  127 A. In other implementations, the UE  102  does not disconnect from the PCell  125 A while performing the random access procedure. If the C-DU  174 C identifies the UE  102  in the random access procedure, the C-DU  174 C becomes DU  174 C and communicates  742 A with the UE  102  via the C-PCell  127 A. The DU  174 C can send a message (e.g., a DL Data Delivery Status message in  FIG.  3 C ) to indicate to the CU  172  that the UE  102  is connected, after or response to identifying the UE  102  in the random access procedure. 
     In some implementations, the random access procedure can be a four-step random access procedure or a two-step random access procedure. The UE  102  can include a UE ID and the RRC reconfiguration complete message  738 A in a Message A and transmit the Message A to the C-DU  174 C in the two-step random access procedure. In other implementations, the random access procedure can be a contention-based random access procedure or a contention-free random access procedure. In the contention-based random access procedure, the UE  102  can include a UE ID and the RRC reconfiguration complete message  738 A in a Message  3  and transmit the Message  3  to the C-DU  174 C. The C-DU  174 C can assign the UE ID in the second DU configuration. The UE ID can be a C-RNTI. In the contention-free random access procedure, the UE  102  can transmit a dedicated preamble to the C-DU  174 C. The C-DU  174 C can assign the dedicated preamble in the second DU configuration. The C-DU  174 C can assign the UE ID in the second DU configuration. If the C-DU  174 C receives the UE ID or the dedicated preamble, the C-DU  174 C identifies the UE  102 . 
     Based on the identity of the C-PCell  126 A in the UL RRC Message Transfer message  740 A, the CU  172  does not select the first C-MN configuration for the C-PCell  126 A. Thus, the C-CU  172  does not use the first C-MN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 C on the C-PCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     In some implementations, CU  172  can include at least one first security configuration parameter in the second C-MN configuration. In other implementations, the CU  172  can send the at least one first security configuration parameter with the first C-MN configuration at event  712 A and the S-DU  174 A forwards it at event  714 A. The CU  172  can generate the at least one first security key (security key(s)) from the at least one first security configuration parameters and a security base key (e.g., K MN  or K NG-RAN* ). For example, the first security key(s) can include a first ciphering key for encryption and decryption and/or include a first integrity key for integrity protection and check. 
     In other implementations, CU  172  can include at least one second security configuration parameter in the second C-MN configuration. In other implementations, the CU  172  can send the at least one second security configuration with the second C-MN configuration during procedure  722 A at an event similar to event  712 A, and the S-DU  174 A can forward it at an event similar to event  714 A. The CU  172  can generate at least one security key (security key(s)) from the at least one second security configuration parameter and a security base key (e.g., K MN  or K NG-RAN* ). For example, the second security key(s) can include a second ciphering key for encryption and decryption and/or include a second integrity key for integrity protection and check. In one implementation, the CU  172  determines to use the at least one second security configuration parameter and the security base key to generate the security key(s) according to the identity of the C-PCell  127 A. In another implementation, the CU  172  determines to use the second security key(s) according to the identity of the C-PCell  127 A. The UE  102  can generate the second security key(s) (which is the same as the second security key(s) generated by the CU  172 ) from the at least one second security configuration parameter and the security base key. In one implementation, the UE  102  can generate the second security key(s) from the at least one second security configuration parameter and the security base key after event  734 A or receiving the RRC reconfiguration message at event during the procedure  722 A. Thus, the UE  102  communicates  742 A with the CU  172  via the C-DU  174 C using the second C-MN configuration and the second security key(s). 
     In some implementations, the first C-DU configuration can include multiple configuration parameters such as physical layer configuration parameters, MAC configuration parameters, RLC configuration parameters, and/or the one or more random access configurations needed by the UE  102  to perform  736 A the random access procedure with the C-DU  174 B on the C-PCell  126 A. The second C-DU configuration can include multiple configuration parameters such as physical layer configuration parameters, MAC configuration parameters, RLC configuration parameters, and/or the one or more random access configurations needed by the UE  102  to perform  736 A the random access procedure with the C-DU  174 C on the C-PCell  127 A. In other implementations, the first C-DU configuration can be CellGroupConfig information element (IE) that configures the C-PCell  126 A and zero, one, or more C-SCells of the C-DU  174 B. The second C-DU configuration can be CellGroupConfig IE that configures the C-PCell  127 A and zero, one, or more C-SCells of the C-DU  174 C. 
     In some implementations, the first C-CU configuration may include a radio bearer configuration and/or measurement configuration. The second C-CU configuration may include a radio bearer configuration and/or measurement configuration. For example, the radio bearer configuration can be a RadioBearerConfig IE, DRB-ToAddModList IE or SRB-ToAddModList IE, DRB-ToAddMod IE or SRB-ToAddMod IE. The measurement configuration can be a MeasConfig IE. 
     In some implementations, the first C-MN configuration can be an RRCReconfiguration message or an RRCReconfiguration-IEs conforming to 3GPP TS 38.331. The second C-MN configuration can be an RRCReconfiguration message or an RRCReconfiguration-IEs conforming to 3GPP TS 38.331. In other implementations, the first C-SN configuration can be an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. The second C-SN configuration can be an RRCConnectionReconfiguration message or RRCConnectionReconfiguration-IEs conforming to 3GPP TS 36.331. In some implementations, if the base station  106 A is a gNB, the RRC reconfiguration and RRC reconfiguration complete messages are RRCReconfiguration and RRCReconfigurationComplete messages, respectively. In other implementations, if the SN  106 A is an eNB or ng-eNB, the RRC reconfiguration and RRC reconfiguration complete messages are RRCConnectionReconfiguration and RRCConnectionReconfigurationComplete messages, respectively. 
     Referring next to  FIG.  7 B , in a scenario  700 B similar to  700 A, the base station  106 A operates as a (master) base station that includes both a CU  172 , a S-DU  174 A, a C-DU  174 B and optionally another C-DU  174 C. Several events in the scenario  700 B are similar to events in the scenarios  700 A. The differences between the scenarios of  FIG.  7 B  and  FIG.  7 A  are described below. 
     The CU  172  can perform  720 B a CHO configuration procedure with the C-DU  174 C and the UE  102  to configure the UE  102  a first C-MN configuration including a first C-DU configuration for a C-PCell (e.g., C-PCell  126 A), similar to the CHO configuration procedure  720 A. The CU  172  can perform  722 B another CHO configuration procedure with the C-DU  174 C and the UE  102  to configure the UE  102  a second C-MN configuration including a second C-DU configuration for another C-PCell (e.g., C-PCell  127 A), similar to the CHO configuration procedure  720 A. The CU  172  can perform the CHO configuration procedures  720 B,  722 B in parallel or sequentially. 
     Later in time, the UE  102  determines (or detects)  734 B that a condition for connecting to a C-PCell  127 A is met and initiates a random access procedure on the C-PCell  127 A in response to the detection. In response to the determination, the UE  102  transmits  738 B an RRC reconfiguration complete message to the C-DU  174 C, which in turn the C-DU  174 C sends  740 B a UL RRC Message Transfer including the RRC reconfiguration complete message to the CU  172 . The CU  172  determines  746 B to use the second C-MN configuration (or the second C-DU configuration and/or the second C-CU configuration) as a new S-MN configuration (as a new M-DU configuration and/or a new M-CU configuration) based on the UE ID(s) in the UL RRC Message Transfer message. In some implementation, the UE ID(s) are the gNB-CU UE F1AP ID and the gNB-DU UE F1AP ID as defined in 3GPP TS 38.401 or TS 38.473. 
     Based on the UE ID(s) in the UL RRC Message Transfer message  740 B, the CU  172  does not select the first C-MN configuration for the C-PCell  126 A. Thus, the CU  172  does not use the first C-MN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 C on the C-PCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     Referring to  FIG.  8   , the candidate base station  106 B in some cases can manage conditional configurations based on the Tunnel Endpoint ID(s) (TEID(s)) for DL Data Delivery Status frame in an inter-base-station conditional handover. 
     An example scenario  800  of  FIG.  8    involves an inter-base-station condition handover procedure. The base station  104  in the scenario  800  operates as an MN (or source base station), the base station  106 A operates as a candidate base station made up of a C-CU  172  and C-DU  174 A and C-DU  174 B. Several events in the scenario  800  are similar to events in the scenarios  600 A. The differences between the scenarios of  FIG.  8    and  FIG.  6 A  are described below. 
     Initially, the UE  102  communicates  802  with the MN  104  and uses an S-MN configuration to communicate data (e.g., UL Data PDUs and/or DL Data PDUs). The MN  104  can determine  804  that it should initiate a conditional handover to a C-PCell (e.g., C-PCell  125 A, in response to one or more measurement results received from the UE  102  or from measurements on signals received from the UE  102 ). The MN  104  can make the determination to perform the conditional handover based on one or more measurement results received from the UE, e.g., if the one or more measurement results are above a first threshold. The MN  104  can make this determination to perform the conditional handover based on one or more measurement results received from the UE, e.g., if the one or more measurement results are above the first threshold or a second threshold. The second threshold can be different from the first threshold. 
     In response to the determination of the event  804 , the MN  104  transmits  806  a Handover Request message which includes the target Cell ID (e.g. CGI of the C-PCell  125 A) and Security Information to the C-CU  172 . The C-CU  172  performs  808  UE Context Setup procedures with C-DU  174 B to obtain a C-DU configuration for the C-PCell, similar to the procedure  306 A in  FIG.  3   . In some implementations, the C-CU  172  determines the C-DU to perform the UE Context Setup procedures based on the Cell ID. The C-CU  172  generates  810  a first C-MN configuration including the first C-DU configuration. In response to event  806 , the C-CU  172  transmits  812  a Handover Response message including an RRC Reconfiguration message which further includes the first C-MN configuration. The MN  104  then transmits  814  the RRC Reconfiguration message to the UE  102 . The events  804 ,  806 ,  808 ,  810 ,  812 , and  814  are collectively referred to in  FIG.  8    as the CHO configuration procedure  820 . In some implementations, the Handover Request message is a Handover Request message as defined in 3GPP TS 38.423 or TS 36.423 with an indication of conditional operation, and the Handover Response message is a Handover Request Acknowledge message as defined in 3GPP TS 38.423 or TS 36.423. In some implementations, the UE Context Setup procedures are as defined in 3GPP TS 38.401 and TS 38.473. 
     At a later time, the MN  104  can determine to configure  822  the UE  102  with another CHO configuration procedure to configure a second C-MN configuration for another C-PCell (e.g., C-PCell  126 A) operated by the C-DU  174 B, similar to the procedure  820 . The MN  104  can perform the CHO configuration procedures  820 ,  822  in parallel or sequentially. 
     Later in time, the UE  102  determines (or detects)  834  that a condition for connecting to a C-PSCell  126 A is met and initiates a random access procedure on the C-PSCell  126 A in response to the detection. In response to the determination  834 , the UE  102  then performs  836  the random access procedure with the C-DU  174 B via the C-PCell  126 A, e.g., using one or more random access configurations included in the second C-MN configuration. As soon as the C-DU  174 B detects the successful RACH access by the UE  102  for the corresponding data radio bearer(s), the C-DU  174 B sends  837  a DL Data Delivery Status frame to the C-CU  172 . The C-CU  172  determines  850  to use the second C-MN configuration (or the second C-DU configuration and/or the second C-CU configuration) as a new S-MN configuration (as a new M-DU configuration and/or a new M-CU configuration) based on the TEID(s) for the DL Data Delivery Status frame. The UE  102  transmits  838  an RRC Reconfiguration Complete message to the C-DU  174 B. The C-DU  174 B transmits  840  the RRC Reconfiguration Complete message in a UL RRC Message Transfer message to the C-CU  172 . The UE  102  can transmit  838  the RRC reconfiguration complete message during or after the random access procedure. In the contention-based random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  838  in a Message  3  of the contention-based random access procedure. In case of the contention-free random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  838  after the contention-free random access procedure. The UE communicates  842  with the C-CU  172  via the C-DU  174 B via the C-PCell  126 A in accordance with the configurations in the second C-MN configuration. To access the C-PCell, the UE  102  in some implementations disconnects from the PCell hosted by MN  104 . 
     Based on the TEID(s) for the DL Data Delivery Status frame  837 , the C-CU  172  does not select the first C-MN configuration for the C-PCell  125 A. Thus, the C-CU  172  does not use the first C-MN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 C on the C-PCell  126 A, thereby avoiding a communication failure due to configuration mismatch. 
     In some implementations, C-CU  172  can include at least one first security configuration parameter in the second C-MN configuration. In other implementations, the C-CU  172  can send the at least one first security configuration parameter with the first C-MN configuration at event  812  and the MN  104  forwards it at event  814 . The C-CU  172  can generate the at least one first security key (security key(s)) from the at least one first security configuration parameters and a security base key (e.g., K MN  or K NG-RAN* ). For example, the first security key(s) can include a first ciphering key for encryption and decryption and/or include a first integrity key for integrity protection and check. 
     In other implementations, C-CU  172  can include at least one second security configuration parameter in the second C-MN configuration. In other implementations, the C-CU  172  can send the at least one second security configuration with the second C-MN configuration during procedure  822  at an event similar to event  812 , and the MN  104  can forward it at an event similar to event  814 . The C-CU  172  can generate at least one security key (security key(s)) from the at least one second security configuration parameter and a security base key (e.g., K NG-RAN* ). For example, the second security key(s) can include a second ciphering key for encryption and decryption and/or include a second integrity key for integrity protection and check. In one implementation, the C-CU  172  determines to use the at least one second security configuration parameter and the security base key to generate the security key(s) according to the identity of the C-PCell  126 A. In another implementation, the C-CU  172  determines to use the second security key(s) according to the identity of the C-PCell  126 A. The UE  102  can generate the second security key(s) (which is the same as the second security key(s) generated by the C-CU  172 ) from the at least one second security configuration parameter and the security base key. In one implementation, the UE  102  can generate the second security key(s) from the at least one second security configuration parameter and the security base key after event  834  or receiving the RRC reconfiguration message during the procedure  822 . Thus, the UE  102  communicates  842  with the C-CU  172  via the C-DU  174 B using the second C-MN configuration and the second security key(s). 
     In some implementation, the TEID is as defined in 3GPP TS 29.281 for General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U). In some implementations, the DL Data Delivery Status frame is as defined in 3GPP TS 38.425 and the transmission of the frame is as defined in 3GPP TS 38.401 and TS 38.470. The UL RRC Message Transfer message in some implementations is as defined in 3GPP TS 38.473. 
     Now referring to  FIG.  9   , the base station  106 A in some cases can manage security configurations based on the TEID(s) for DL Data Delivery Status frame in an intra-base-station conditional handover. 
     An example scenario  900  of  FIG.  9    involves an intra-base-station condition handover procedure. In a scenario  900  similar to  700 A, the base station  106 A operates as a (master) base station that includes both a CU  172 , a S-DU  174 A, a C-DU  174 B and a another C-DU  174 C. Several events in the scenario  900  are similar to events in the scenarios  700 A and  800 . The differences between the scenarios of  FIG.  9   ,  FIG.  7 A , and  FIG.  8    are described below. 
     Initially, the UE  102  communicates  902  with the CU  172  via the S-DU  174 A on PCell  125 A and uses an S-MN configuration to communicate data (e.g., UL Data PDUs and/or DL Data PDUs). The CU  172  can determine  904  that it should initiate an intra-base-station (or, inter-DU) conditional handover to a C-PCell (e.g., C-PCell  126 A, in response to one or more measurement results received from the UE  102  or from measurements on signals received from the UE  102 ). The CU  172  can make the determination to perform the conditional handover based on one or more measurement results received from the UE, e.g., if the one or more measurement results are above a first threshold. The CU  172  can make this determination to perform the conditional handover based on one or more measurement results received from the UE, e.g., if the one or more measurement results are above the first threshold or a second threshold. The second threshold can be different from the first threshold. 
     In response to the determination of the event  904 , the CU  172  performs  908  UE Context Setup procedures with C-DU  174 C to obtain a C-DU configuration, similar to the procedure  306 A in  FIG.  3   . The CU  172  generates  910  a first C-MN configuration including a first C-DU configuration. The CU  172  transmits  912  a DL RRC Message Transfer message including an RRC Reconfiguration message which further includes the first C-MN configuration to the S-DU  174 A. The S-DU  174 A then transmits  914  the RRC Reconfiguration message to the UE  102 . The events  904 ,  906 ,  910 ,  912 , and  914  are collectively referred to in  FIG.  9    as the CHO configuration procedure  920  for intra-base-station handover. 
     The CU  172  later can determine to configure  922  the UE  102  with another CHO configuration procedure to configure a second C-MN configuration for another C-PCell (e.g., C-PCell  127 A) operated by C-DU  174 C, similar to event  920 . The CU  172  can perform the CHO configuration procedures  920 ,  922  in parallel or sequentially. 
     The UE  102  later determines  934  that a condition for connecting to C-PCell  127 A is satisfied and initiates a random access procedure on C-PCell  127 A. The UE  102  then performs  936  the random access procedure with the C-DU  174 C via the C-PCell  127 A, e.g., using one or more random access configurations included in the second C-MN configuration. As soon as the C-DU  174 C detects the successful RACH access by the UE  102  for the corresponding data radio bearer(s), the C-DU  174 C sends  937  a DL Data Delivery Status frame to the CU  172 , similar to the event  345 C. The CU  172  determines  950  to use the second C-MN configuration (or the second C-DU configuration and/or the second C-CU configuration) as a new S-MN configuration (as a new M-DU configuration and/or a new M-CU configuration) based on the TEID(s) for the DL Data Delivery Status frame, similar to the event  346 C. The UE  102  transmits  938  an RRC Reconfiguration Complete message to the C-DU  174 C. The C-DU  174 C transmits  940  the RRC Reconfiguration Complete message in a UL RRC Message Transfer message to the CU  172 . The UE  102  can transmit  938  the RRC reconfiguration complete message during or after the random access procedure. In the contention-based random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  938  in a Message  3  of the contention-based random access procedure. The UE  102  can include a UE ID in the Message  3 . In case of the contention-free random access procedure, the UE  102  can transmit the RRC reconfiguration complete message  938  after the contention-free random access procedure. The UE communicates  942  with the CU  172  via the C-DU  174 C via the C-PCell  127 A in accordance with the configurations in the second C-MN configuration. To access the C-PCell, the UE  102  in some implementations disconnects from the PCell  125 A hosted by S-DU  174 A. In other implementations, the UE  102  continues communication with the PCell  125 A hosted by S-DU  174 A, while perform the random access procedure on the C-PCell  127 A. 
     Based on the TEID(s) for the DL Data Delivery Status frame  937 , the CU  172  does not select the first C-MN configuration for the C-PCell  126 A. Thus, the CU  172  does not use the first C-MN configuration to communicate with the UE  102  while the UE  102  connects to the C-DU  174 C on the C-PCell  127 A, thereby avoiding a communication failure due to configuration mismatch. 
     In some implementations, CU  172  can include at least one first security configuration parameter in the second C-MN configuration. In other implementations, the CU  172  can send the at least one first security configuration parameter with the first C-MN configuration at event  912  and the S-DU  174 A forwards it at event  914 . The CU  172  can generate the at least one first security key (security key(s)) from the at least one first security configuration parameters and a security base key (e.g., K MN  or K NG-RAN* ). For example, the first security key(s) can include a first ciphering key for encryption and decryption and/or include a first integrity key for integrity protection and check. 
     In other implementations, CU  172  can include at least one second security configuration parameter in the second C-MN configuration. In other implementations, the CU  172  can send the at least one second security configuration with the second C-MN configuration during the procedure  922  at an event similar to event  912 , and the S-DU  174 A can forward it at an event similar to event  914 . The CU  172  can generate at least one security key (security key(s)) from the at least one second security configuration parameter and a security base key (e.g., K NG-RAN* ). For example, the second security key(s) can include a second ciphering key for encryption and decryption and/or include a second integrity key for integrity protection and check. In one implementation, the CU  172  determines to use the at least one second security configuration parameter and the security base key to generate the security key(s) according to the identity of the C-PCell  127 A. In another implementation, the CU  172  determines to use the second security key(s) according to the identity of the C-PCell  127 A. The UE  102  can generate the second security key(s) (same as the second security key(s) generated by the CU  172 ) from the at least one second security configuration parameter and the security base key. In one implementation, the UE  102  can generate the second security key(s) from the at least one second security configuration parameter and the security base key after event  934  or receiving the RRC reconfiguration message during the procedure  922 . Thus, the UE  102  communicates  942  with the CU  172  via the C-DU  174 C using the second C-MN configuration and the second security key(s). 
     In some implementation, the TEID is as defined in 3GPP TS 29.281 for General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U). In some implementations, the DL Data Delivery Status frame is as defined in 3GPP TS 38.425 for NR user plane protocol and the transmission of the frame is as defined in 3GPP TS 38.401 and TS 38.470. The DL RRC Message Transfer and UL RRC Message Transfer message in some implementations are as defined in 3GPP TS 38.473. 
     Next, several example methods which a base station, a base station CU, or a UE can implement to support conditional configuration handling and management in conditional mobility scenarios are discussed with reference to  FIGS.  10 - 17   . 
     Referring first to  FIG.  10   , an example method  1000  is depicted for determining to use a particular conditional configuration based on a received Cell ID related to conditional mobility to a UE, such as the UE  102 , which can be implemented in a base station  106 A such as the secondary base station of  FIGS.  3 A,  3 B,  3 D , the target secondary base station of  FIGS.  4 A and  4 B , the (source) base station of  FIGS.  5 A,  5 B,  5 D, and  7 A , and the candidate base station of  FIG.  6 A  for example. In various implementations, the referred Cell ID is a cell global ID (CGI), and the base station or base station CU maintains a table for mapping between the CGI and a physical cell ID (PCI, e.g., as specified in 3GPP TS 36.423 or 38.423) or another suitable identifier of a particular cell in the wireless communication system  100  for the purpose of management of conditional configurations. 
     The method  1000  begins at block  1002 , where the base station CU transmits multiple conditional configurations for a UE. At block  1004 , the base station CU receives a message including a Cell ID from a DU (event  344 B of  FIG.  3 B ; event  339 D of  FIG.  3 D ; event  444 B of  FIG.  4 B ; event  544 B of  FIG.  5 B ; event  539 D of  FIG.  5 D ; event  640 A of  FIG.  6 A ; event  740 A of  FIG.  7 A ) or a UE (event  338 A of  FIG.  3 A ; event  438 A of  FIG.  4 A ; event  538 A of  FIG.  5 A ). At block  1006 , the base station CU determines to use a particular conditional configuration based on the received Cell ID (event  340 A of  FIG.  3 A ; event  346 B of  FIG.  3 B ; event  340 D of  FIG.  3 D ; event  440 A of  FIG.  4 A ; event  446 B of  FIG.  4 B ; event  540 A of  FIG.  5 A ; event  546 B of  FIG.  5 B ; event  540 D of  FIG.  5 D ; event  650 A of  FIG.  6 A ; event  750 A of  FIG.  7 A ). As mentioned above, the determination may be made based on a mapping table of the Cell ID and a PCI maintained by the base station CU. The base station CU at block  1008  communicates with the UE via the DU on the Cell using the particular conditional configuration (event  348 A of  FIG.  3 A ; event  348 B and  3 B; event  348 D of  FIG.  3 D ; event  448 A of  FIG.  4 A ; event  448 B of  FIG.  4 B ; event  548 A of  FIG.  5 A ; event  548 B of  FIG.  5 B ; event  548 D of  FIG.  5 D ; event  642 A of  FIG.  6 A ; event  742 A of  FIG.  7 A ). 
       FIG.  11    illustrates an example method  1100  for determining a particular conditional configuration based on UE ID(s) and/or DU IP address from a message from DU related to conditional mobility to a UE, such as the UE  102 , which can be implemented in a base station  106 A such as the secondary base station of  FIG.  3 E , the (source) base station of  FIGS.  5 E and  7 B , and the candidate base station of  FIG.  6 B  for example. In some implementation, the UE ID(s) are the gNB-CU UE F1AP ID and the gNB-DU UE F1AP ID as defined in 3GPP TS 38.401 or TS 38.473. 
     The method  1100  begins at block  1102 , where the base station CU transmits multiple conditional configurations for a UE. At block  1104 , the base station CU receives a message including UE ID(s) from a DU (event  339 E of  FIG.  3 E ; event  539 E of  FIG.  5 E ; event  640 B of  FIG.  6 B ; event  740 B of  FIG.  7 B ). At block  1106 , the base station CU determines to use a particular conditional configuration from at least one of the UE ID(s) and the DU&#39;s IP address (event  346 E of  FIG.  3 E ; event  546 E of  FIG.  5 E ; event  646 B of  FIG.  6 B ; event  746 B of  FIG.  7 B ). The base station CU at block  1108  communicates with the UE via the DU on the Cell using the particular conditional configuration (event  348 E of  FIG.  3 E ; event  548 E of  FIG.  5 E ; event  642 B of  FIG.  6 B ; event  742 B of  FIG.  7 B ). 
       FIG.  12    illustrates an example method  1200  for determining a particular conditional configuration based on TEID(s) and/or the DU&#39;s IP address from a message from DU related to conditional mobility to a UE, such as the UE  102 , which can be implemented in a base station  106 A such as the secondary base station of  FIGS.  3 C and  3 F , the target secondary base station of  FIG.  4 C , the (source) base station of  FIGS.  5 C,  5 F, and  9   , and the candidate base station of  FIG.  8    for example. In some implementation, the TEID is as defined in 3GPP TS 29.281 for General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U). 
     The method  1200  begins at block  1202 , where the base station CU transmits multiple conditional configurations for a UE. At block  1204 , the base station CU receives a User Plane frame/message from a DU (event  345 C of  FIG.  3 C ; event  345 F of  FIG.  3 F ; event  445 C of  FIG.  4 C ; event  545 C of  FIG.  5 C ; event  545 F of  FIG.  5 F ; event  837  of  FIG.  8   ; event  937  of  FIG.  9   ). At block  1206 , the base station determines to use a particular conditional configuration from at least one of TEID(s) for the User Plane frame/message, and the DU&#39;s IP address (event  346 C of  FIG.  3 C ; event  346 F of  FIG.  3 F ; event  446 C of  FIG.  4 C ; event  546 C of  FIG.  5 C ; event  546 F of  FIG.  5 F ; event  850  of  FIG.  8   ; event  950  of  FIG.  9   ). The base station CU at block  1208  communicates with the UE via the DU on the Cell using the particular conditional configuration (event  348 C of  FIG.  3 C ; event  348 F of  FIG.  3 F ; event  448 C of  FIG.  4 C ; event  548 C of  FIG.  5 C ; event  548 F of  FIG.  5 F ; event  842  of  FIG.  8   ; event  942  of  FIG.  9   ). 
       FIG.  13 A  illustrates an example method  1300 A for configuring a conditional configuration (e.g., a C-MN configuration, C-SN configuration or C-DU configuration) for a UE and avoiding the necessity of managing multiple conditional configurations, which can be implemented in a first network node such as the (C-)SN (or the CU of the (C-)SN) of  FIGS.  3 A-F ,  4 A-C and  5 A- 5 F, or the candidate base station or (C-)MN (or the CU of the (C-)MN) of Figs. of  FIGS.  6 A-B ,  7 A-B,  8  and  9 , for example. 
     The method  1300 A begins at block  1302 A, where the first network node configures a first candidate cell for a UE. At block  1304 A, the first network node refrains from configuring a second candidate cell for the UE. Because there is only one candidate cell configured to a UE, second network node operating the candidate cell directly or indirectly via a child node does not need to manage multiple conditional configurations for the UE. 
       FIG.  13 B  illustrates a similar example method  1300 B for configuring a conditional configuration for a UE and avoiding the necessity of managing multiple conditional configurations associated to the same network node, which can be implemented in a first network node such as the (C-)SN (or the CU of the (C-)SN) of  FIGS.  3 A-F ,  4 A-C and  5 A- 5 F or the candidate base station or (C-)MN (or the CU of the (C-)MN) of Figs. of  FIGS.  6 A-B ,  7 A-B,  8  and  9 , for example. 
     The method  1300 B begins at block  1302 B, where a first network node configures a first candidate cell of a second network node for a UE. At block  1304 B, the first network node refrains from configuring a second candidate cell of the second network node for the UE. The second network node can be a DU, which can be a M-DU, S-DU or C-DU, a (C-) MN or a (C-)SN of  FIGS.  3 A-F ,  4 A-C and  5 A- 5 F,  6 A-B,  7 A-B,  8  and  9 , for example. The first and second network nodes can be the same or different. Because there is only one candidate cell configured to a UE, second network node operating the candidate cell directly or indirectly via a child node does not need to manage multiple conditional configurations for the UE. 
     Unlike the method  1300 A, the first network node with the method  1300 B can configure a second candidate cell of a third network node for the UE. 
       FIG.  14    depicts an example method  1400  for configuring a conditional base station configuration for a UE and avoiding the necessity of managing multiple conditional configurations, which can be implemented in a base station of  FIGS.  3 A-F ,  4 A-C,  5 A-F,  6 A-B,  7 A-B,  8  and  9  for example. 
     The method  1400  begins at block  1402 , where the base station receives measurement result(s) of a cell from a UE. At block  1404 , the base station determines if the threshold for conditional configuration is met for the measurement result(s). The method ends when the threshold is not met. Otherwise, the flow proceeds to block  1406 , where the base station further determines if the cell belongs to a network node where a candidate cell has been configured to the UE. If the cell does belong to a network node where a candidate cell has been configured to the UE, the flow proceeds to block  1410  where the base station does not transmit a conditional configuration which configures the cell as a candidate cell to the UE. Otherwise, the flow proceeds to block  1408  where the base station transmit a conditional configuration which configures the cell as a candidate cell to the UE. 
     Now referring to  FIG.  15   , an example method  1500  is depicted for performing a conditional mobility and informing a network node of a candidate cell ID, which can be implemented in a UE such as the UE  102  discussed above. According to this method, the UE determines whether a condition for connecting a candidate cell is satisfied and then transmits an RRC response message to the candidate base station including a cell ID of the configured candidate cell based on this determination. The network node can be a M-CU, S-CU, C-CU, (C-)MN or a (C-)SN, for example. 
     The method  1500  begins at block  1502 , where the UE receives a conditional configuration configuring a candidate cell. The UE at block  1504  determines that a condition for connecting to a candidate cell is satisfied. At block  1506 , the UE transmits an RRC response message including a cell identity of the candidate cell in response to the determination at block  1504  (event  336 A of  FIG.  3 A ; event  436 A of  FIG.  4 A ; event  536 A of  FIG.  5 A  for example). The UE at block  1508  connects to the candidate cell in response to the determination of block  1504 . 
     Next,  FIG.  16    illustrates a similar example method  1600  for a conditional mobility and informing a network node of a candidate cell ID, which can be implemented in a UE such as the UE  102  discussed above. The network node can be a M-CU, S-CU, C-CU, (C-)MN or a (C-)SN, for example. 
     The method  1600  begins at block  1602 , where the UE determines to transmit an RRC response message. At block  1604 , if the determination at block  1602  is triggered by a condition for connecting a candidate cell being satisfied, the flow proceeds to block  1606  where the UE includes a cell identity of the candidate cell in the RRC response message and then the UE transmits the RRC response message at block  1608  (event  336 A of  FIG.  3 A ; event  436 A of  FIG.  4 A ; event  536 A of  FIG.  5 A  for example). If, at block  1604 , the determination at block  1602  is NOT triggered by a condition for connecting a candidate cell being satisfied, the flow proceeds directly to block  1608  where the UE transmits the RRC response message. 
       FIG.  17    illustrates another example method  1700  for a conditional mobility and informing a network node of a candidate cell ID, which can be implemented in a UE such as the UE  102  discussed above. The network node can be a M-CU, S-CU, C-CU, (C-)MN or a (C-)SN, for example. 
     The method  1700  begins at block  1702 , where the UE determines to transmit an RRC response message. At block  1704 , if the determination at block  1702  is triggered by a condition for connecting a candidate PSCell being satisfied, the flow proceeds to block  1706  where the UE determines whether the conditional configuration configuring the candidate PSCell is received from SRB1 or SRB3. If it is from SRB3, the flow proceeds to block  1712 , where the UE transmits the RRC response message on SRB3. Otherwise, if it is from SRB1, the flow proceeds to block  1708  where the UE includes a cell identity of the candidate cell in the RRC response message and then the UE transmits the RRC message on SRB1 (event  336 A of  FIG.  3 A ; event  436 A of  FIG.  4 A ; event  536 A of  FIG.  5 A  for example). If at block  1704 , the determination at block  1702  is NOT triggered by a condition for connecting a candidate PSCell being satisfied, the flow proceeds directly to block  1710  where the UE transmits the RRC response message on SRB1. 
       FIG.  18    illustrates another example method  1800  for processing a message for a conditional mobility to a UE, which can be implemented in a first network node such as the (C-)SN (or the CU of the (C-)SN) of  FIGS.  3 B,  4 B and  5 B . 
     The method  1800  begins at block  1802 , where the base station CU transmits a conditional configuration configuring a cell for a UE (event  320 B of  FIG.  3 B ). At block  1804 , the base station CU receives an interface message including an RRC container and a cell ID of the cell for the UE from a DU (event  344 B of  FIG.  3 B ; event  444 B of  FIG.  4 B ; event  544 B of  FIG.  5 B ). At block  1806 , the base station CU determines whether the interface message includes an indicator indicating ignoring the RRC container. If the interface message includes the indicator, the base station CU at block  1808  ignores the RRC container. If the interface message does not include the indicator, the base station CU at block  1810  processes the RRC container, i.e., decodes an RRC message in the RRC container and process content in the RRC message. The base station CU at block  1812  communicates with the UE via the DU on the cell using the conditional configuration (event  348 B and  3 B; event  448 B of  FIG.  4 B ; event  548 B of  FIG.  5 B ). In some implementations, the interface message can be a UL RRC Message Transfer message. 
       FIG.  19    illustrates another example method  1900  for processing a message for a conditional mobility to a UE, which can be implemented in a first network node such as (C-) SN (or the DU of the (C-)SN)) of  FIGS.  3 B,  4 B and  5 B . 
     The method  1900  begins at block  1902 , where the base station DU performs a random access procedure with a UE via a cell (event  342 B of  FIG.  3 B ; event  442 B of  FIG.  4 B ; event  542 B of  FIG.  5 B ). At block  1904 , the base station DU determines whether the cell is a candidate cell for the UE. If the cell is a candidate cell, the base station DU at block  1906  generates an interface message including a cell ID of the cell, an RRC container IE and an indicator indicating ignoring the RRC container IE. If the cell is not a candidate cell, the base station DU at block  1908  generates an interface message including an RRC container IE and excluding an indicator indicating ignoring the RRC container IE. The base station DU at block  1910  transmits the interface message to a base station CU (event  344 B and  3 B; event  444 B of  FIG.  4 B ; event  544 B of  FIG.  5 B ). In some implementations, the interface message can be a UL RRC Message Transfer message. 
     In some implementation, the base station DU can store a candidate cell identity for the UE when performing the UE Context Setup procedure or a UE Context Modification procedure with the CU for preparing a conditional PSCell change. If the cell ID of the cell is the same as the candidate cell ID, the base station DU can determine the cell is the candidate cell. Otherwise, the base station DU can determine the cell is not the candidate cell. In other implementations, the base station DU can store a C-DU configuration for the UE. The C-DU configuration includes a UE identifier. If the base station DU receives the UE identifier from the UE on the cell, the base station DU can determine the cell is the candidate cell for the UE. Otherwise, the base station can determine the cell is not the candidate cell for the UE. 
     The following description may be applied to the description above. 
     A user device in which the techniques of this disclosure can be implemented (e.g., the UE  102 ) can be any suitable device capable of wireless communications such as a smartphone, a tablet computer, a laptop computer, a mobile gaming console, a point-of-sale (POS) terminal, a health monitoring device, a drone, a camera, a media-streaming dongle or another personal media device, a wearable device such as a smartwatch, a wireless hotspot, a femtocell, or a broadband router. Further, the user device in some cases may be embedded in an electronic system such as the head unit of a vehicle or an advanced driver assistance system (ADAS). Still further, the user device can operate as an internet-of-things (IoT) device or a mobile-internet device (MID). Depending on the type, the user device can include one or more general-purpose processors, a computer-readable memory, a user interface, one or more network interfaces, one or more sensors, etc. 
     Certain embodiments are described in this disclosure as including logic or a number of components or modules. Modules may can be software modules (e.g., code, or machine-readable instructions stored on non-transitory machine-readable medium) or hardware modules. A hardware module is a tangible unit capable of performing certain operations and may be configured or arranged in a certain manner. A hardware module can comprise dedicated circuitry or logic that is permanently configured (e.g., as a special-purpose processor, such as a field programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), a digital signal processor (DSP), etc.) to perform certain operations. A hardware module may also comprise programmable logic or circuitry (e.g., as encompassed within a general-purpose processor or other programmable processor) that is temporarily configured by software to perform certain operations. The decision to implement a hardware module in dedicated and permanently configured circuitry, or in temporarily configured circuitry (e.g., configured by software) may be driven by cost and time considerations. 
     When implemented in software, the techniques can be provided as part of the operating system, a library used by multiple applications, a particular software application, etc. The software can be executed by one or more general-purpose processors or one or more special-purpose processors. 
     Upon reading this disclosure, those of skill in the art will appreciate still additional and alternative structural and functional designs for handling mobility between base stations through the principles disclosed herein. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes and variations, which will be apparent to those of ordinary skill in the art, may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope defined in the appended claims. 
     The following list of examples reflects additional embodiments explicitly contemplated by the present disclosure 
     Example 1. A method in a central unit (CU) of a distributed base station for configuring a connection with a UE, the method comprising: providing, by processing hardware to the UE, a conditional configuration for a cell of a distributed unit (DU) of the base station; receiving, by the processing hardware, an identifier of the cell of the DU; determining, by the processing hardware, that the UE connects to the cell based on the identifier of the cell; and communicating with the UE in accordance with the conditional configuration for the cell. 
     Example 2. The method according to example 1, wherein the identifier of the cell is a cell global identifier (CGI) and further comprising: storing, by the processing hardware, a mapping of the CGI and corresponding physical cell identifiers (PCI); and determining, by the processing hardware, that the UE connects to the cell based on the received CGI and the stored mapping. 
     Example 3. The method according to any of the preceding examples, wherein the identifier of the cell of the DU is received from the DU. 
     Example 4. The method according to any of the preceding examples, wherein the identifier of the cell of the DU is received from the UE. 
     Example 5. The method according to any of the preceding examples, wherein the cell is a first cell, the conditional configuration is a first conditional configuration, and further comprising: providing, by processing hardware to the UE, a second conditional configuration for a second cell of the DU; and determining, by the processing hardware, that the UE selected the first conditional configuration based on the identifier of the first cell. 
     Example 6. The method according to any of the preceding examples, wherein receiving an identifier of the cell of the DU includes: receiving, by the processing hardware from the DU, a status message or an F1 Application Protocol (AP) message including the identifier of the cell of the DU. 
     Example 7. The method according to any of the preceding examples, wherein the identifier is a tunnel endpoint identifier (TEID). 
     Example 8. The method according to any of the preceding examples, wherein the cell is a first cell, the conditional configuration is a first conditional configuration, providing the first conditional configuration for the first cell of the DU includes providing a first TEID and further comprising: providing, by processing hardware to the UE, a second conditional configuration for a second cell of the DU including a second TEID; wherein: receiving the status message includes receiving, by the processing hardware, the status message with the first TEID, and determining that the UE connects to the first cell includes determining, by the processing hardware, that the UE connects to the first cell based on receiving the first TEID. 
     Example 9. The method according to any of the preceding examples, further comprising: storing, by the processing hardware, one or more associations between particular TEIDs and particular conditional configurations; and determining, by the processing hardware, the particular conditional configuration that the UE selected based on the one or more stored associations and the received TEID. 
     Example 10. The method according to any of the preceding examples, wherein: providing the first conditional configuration for the first cell of the DU further includes obtaining, by the processing hardware, a first TEID; and providing the second conditional configuration for the second cell of the DU further includes obtaining, by the processing hardware, a second TEID. 
     Example 11. A method in a central unit (CU) of a distributed base station for configuring a connection with a UE, the method comprising: receiving, by processing hardware, measurement results from the UE; determining, by the processing hardware based on the measurement results, that a condition has been satisfied for initiating a conditional configuration for a cell of a distributed unit (DU) of the base station; in response to determining that the condition has been satisfied, generating, by the processing hardware, the conditional configuration for the cell of the DU; providing, by the processing hardware to the UE, the conditional configuration for the cell of DU; and refraining from configuring additional conditional configuration for additional cells of the DU. 
     Example 12. The method according to example 11, further comprising: receiving, by the processing hardware, additional measurement results from the UE; determining, by the processing hardware based on the measurement results, that a condition has been satisfied for initiating an additional conditional configuration for an additional cell; determining, by the processing hardware, whether the additional cell belongs to the DU; and in response to determining that the additional cell belongs to the DU, refraining from generating the additional conditional configuration. 
     Example 13. The method according to either one of example 11 or example 12, further comprising: in response to determining that the additional cell does not belong to the DU, generating, by the processing hardware, the additional conditional configuration. 
     Example 14. A method in a distributed unit (DU) of a distributed base station for configuring a connection with a UE, the method comprising: receiving, by processing hardware from a central unit (CU) of the base station, a request message to obtain a conditional configuration for connecting to a cell of the DU; providing, by the processing hardware to the CU, the conditional configuration for the cell; performing, by the processing hardware, a random access procedure with a UE to connect the UE to the cell; and providing, by the processing hardware to the CU, an identifier of the cell of the DU to indicate to the CU that the UE is connected to the cell corresponding to the conditional configuration. 
     Example 15. The method according to example 14, wherein the identifier of the cell is a cell global identifier (CGI). 
     Example 16. The method according to either example 14 or example 15, wherein providing an identifier of the cell includes: providing, by the processing hardware to the CU, a CU to DU interface message including the identifier of the cell. 
     Example 17. The method according to any one of examples 14-16, wherein the CU to DU interface message is an F1 Application Protocol (AP) message. 
     Example 18. The method according to any one of examples 14-17, wherein the F1 AP message does not include a radio resource control (RRC) message. 
     Example 19. The method according to any one of examples 14-18, wherein the cell is a first cell, the conditional configuration is a first conditional configuration, and further comprising: providing, by processing hardware to the CU, a second conditional configuration for a second cell of the DU; and in response to performing the random access procedure with the UE to connect the UE to the first cell, providing, by the processing hardware to the CU, an identifier of the first cell of the DU to indicate to the CU that the UE is connected to the first cell corresponding to the first conditional configuration. 
     Example 20. The method according to any of the preceding examples, wherein the distributed base station is a node in a radio access network (RAN) that operates in multi-radio dual connectivity (MR-DC), and wherein the first and second conditional configurations are provided for a candidate primary secondary cell (C-PSCell) of an SN in a conditional PSCell addition or change (CPAC) configuration procedure. 
     Example 21. The method according to any of the preceding examples, wherein the distributed base station is a node in a radio access network (RAN) that operates in multi-radio dual connectivity (MR-DC), and wherein the first and second conditional configurations are provided for a candidate secondary node (C-SN) in a conditional SN addition or change (CSAC) configuration procedure. 
     Example 22. The method according to any of the preceding examples, wherein the distributed base station includes a first DU operating as a master DU in MR-DC and a second DU operating as a secondary DU in MR-DC. 
     Example 23. The method according to any of the preceding examples, wherein the distributed base station is a node in a radio access network (RAN) that operates in single connectivity (SC), and wherein the first and second conditional configurations are provided for a conditional handover (CHO) procedure. 
     Example 24. The method according to any of the preceding examples, wherein the distributed base station includes a first DU operating as a source DU in SC and a second DU operating as a candidate DU. 
     Example 25. A base station comprising processing hardware and configured to implement a method according to any of the preceding claims. 
     Example 26. A method in a UE for configuring a connection with a distributed unit (DU) of a base station, the method comprising: receiving, by processing hardware from a central unit (CU) of the base station, a first conditional configuration for a first cell of a DU of the base station; receiving, by the processing hardware from the CU, a second conditional configuration for a second cell of the DU; selecting, by the processing hardware, the second conditional configuration in response to determining that a condition for the second conditional configuration is satisfied; and connecting, by the processing hardware, to the second cell of the DU in response to the selection. 
     Example 27. The method according to example 26, further comprising: transmitting, by the processing hardware, a radio connection reconfiguration complete message including an identifier of the second cell of the DU. 
     Example 28. The method according to either one of example 26 or example 27, further comprising: transmitting, by the processing hardware, a radio connection reconfiguration complete message including a UE identifier for identifying the selected conditional configuration, wherein the radio connection reconfiguration complete message does not include an identifier of the second cell of the DU. 
     Example 29. The method according to any one of examples 26-28, wherein: receiving the first conditional configuration for the first cell of the DU further includes receiving, by the processing hardware, a first UE identifier; receiving the second conditional configuration for the second cell of the DU further includes receiving, by the processing hardware, a second UE identifier; wherein the processing hardware transmits the radio connection reconfiguration complete message with the second UE identifier. 
     Example 30. The method according to any one of examples 26-29, wherein transmitting the radio connection reconfiguration complete message includes transmitting, by the processing hardware, the radio connection reconfiguration complete message including a radio network temporary identifier (RNTI). 
     Example 31. The method according to any one of examples 25-30, further comprising: transmitting, by the processing hardware, a radio connection reconfiguration message, wherein the radio connection reconfiguration complete message does not include an identifier of the second cell of the DU. 
     Example 32. The method according to any one of examples 26-31, wherein the UE operates in multi-radio dual connectivity (MR-DC), and wherein the first and second conditional configurations are provided for a candidate primary secondary cell (C-PSCell) of an SN in a conditional PSCell addition or change (CPAC) configuration procedure. 
     Example 33. The method according to any one of examples 26-32, wherein the UE operates in multi-radio dual connectivity (MR-DC), and wherein the first and second conditional configurations are provided for a candidate secondary node (C-SN) in a conditional SN addition or change (CSAC) configuration procedure. 
     Example 34. The method according to any one of examples 26-33, wherein the UE operates in single connectivity (SC), and wherein the first and second conditional configurations are provided for a conditional handover (CHO) procedure. 
     Example 35. A user equipment (UE) comprising processing hardware and configured to implement a method according to any one of examples 26-34.