Patent Publication Number: US-11665761-B2

Title: Methods and related devices for secondary node addition

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 16/807,643, which is a continuation application of U.S. patent application Ser. No. 16/047,011 filed Jul. 27, 2018, which claims the benefit of and priority to provisional U.S. Patent Application Ser. No. 62/537,702 filed on Jul. 27, 2017. The contents of all above-named applications are fully incorporated herein by reference for all purposes. 
    
    
     FIELD 
     The present disclosure generally relates to wireless communication technology, and more particularly, to methods and related devices for secondary node addition. 
     BACKGROUND 
     In the next-generation (e.g., fifth generation (5G) New Radio (NR)) wireless network, multi-connectivity (MC) including dual-connectivity (DC) is envisioned to support more capacity, data, and services. A user equipment (UE) configured with multi-connectivity may have one master node as an anchor and one or more secondary nodes. For example, a UE in multi-connectivity may be configured with one master cell group (MCG) and one or more secondary cell groups (SCGs) for data delivery. Each cell group may be formed by one or more cells. All cell groups are not necessarily the same type. For example, one can be a Long Term Evolution (LTE) or an evolved LTE (eLTE) cell group, while another one can be an NR cell group. Regarding to the core network, taking E-UTRA (Evolved Universal Terrestrial Radio Access) for example, the core network that E-UTRA connects to can be the Evolved Packet Core (EPC) or NextGen Core (NGC) or 5G Core Network (5GC). eLTE is also known as LTE connected to 5GC. In NR-NR DC case and Multi-RAT (MR)-DC (e.g., NR-NR DC, EN (E-UTRAN New Radio)-DC, or New Radio E-UTRAN (NE)-DC) case, each network node may have its own Radio Resource Control (RRC) entity, but the UE&#39;s RRC entity may follow that of the master node. 
     While a UE in multi-connectivity may maintain simultaneous connections with the master node and the secondary node(s), in some cases, the UE may not camp to a cell even though the cell is suitable for being the UE&#39;s secondary node. 
     Thus, there is a need in the art for an improved secondary node addition mechanism for multi-connectivity. 
     SUMMARY 
     The present disclosure is directed to methods and related devices for secondary node addition. In an aspect of the present disclosure, a UE is provided. The UE includes a receiver, a processor coupled to the receiver, and a memory coupled to the processor. The memory stores at least one computer-executable program that, when executed by the processor, causes the processor to control the receiver to receive an RRC connection reconfiguration message from a master node when a Signaling Radio Bearer 3 (SRB3) connection with a secondary node has not been established, the RRC connection reconfiguration message comprising SCG information; and establish the SRB3 connection with the secondary node according to the RRC connection reconfiguration message. 
     In an aspect of the present disclosure, a method performed by a UE for multi-connectivity is provided. The method includes receiving, from a master node, an RRC connection reconfiguration message when an SRB3 connection with a secondary node has not been established, the RRC connection reconfiguration message comprising SCG information; and establishing the SRB3 connection with the secondary node according to the RRC connection reconfiguration message. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the exemplary disclosure are best understood from the following detailed description when read with the accompanying figures. Various features are not drawn to scale, dimensions of various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1 A  and  FIG.  1 B  are schematic diagrams illustrating two multi-connectivity scenarios. 
         FIG.  2    is a flowchart illustrating a method for secondary node blind addition, in accordance with an example implementation of the present disclosure. 
         FIG.  3    is a flowchart illustrating a method for secondary node blind addition, in accordance with an example implementation of the present disclosure. 
         FIG.  4    is a message flow diagram illustrating a secondary node blind addition procedure, in accordance with an example implementation of the present disclosure. 
         FIG.  5    is a message flow diagram illustrating operations performed between a UE and a master node, in accordance with an example implementation of the present disclosure. 
         FIG.  6    is a message flow diagram illustrating operations performed between a UE and a master node, in accordance with an example implementation of the present disclosure. 
         FIG.  7    is a message flow diagram illustrating a procedure for blind addition request and response, in accordance with an example implementation of the present disclosure. 
         FIG.  8    is a message flow diagram illustrating a procedure for blind addition request and response, in accordance with an example implementation of the present disclosure. 
         FIG.  9    is a message flow diagram illustrating operations performed among a UE, a master node and a secondary node, in accordance with an example implementation of the present disclosure. 
         FIG.  10    is a message flow diagram illustrating a normal secondary node addition procedure, in accordance with an example implementation of the present disclosure. 
         FIG.  11    is a block diagram illustrating a radio communication equipment, in accordance with an exemplary implementation of the present application. 
     
    
    
     DETAILED DESCRIPTION 
     The following description contains specific information pertaining to example implementations in the present disclosure. The drawings in the present disclosure and their accompanying detailed description are directed to merely example implementations. However, the present disclosure is not limited to merely these example implementations. Other variations and implementations of the present disclosure will occur to those skilled in the art. Unless noted otherwise, like or corresponding elements among the figures may be indicated by like or corresponding reference numerals. Moreover, the drawings and illustrations in the present disclosure are generally not to scale, and are not intended to correspond to actual relative dimensions. 
     For the purpose of consistency and ease of understanding, like features are identified (although, in some examples, not shown) by numerals in the example figures. However, the features in different implementations may be differed in other respects, and thus shall not be narrowly confined to what is shown in the figures. 
     References to “one implementation,” “an implementation,” “example implementation,” “various implementations,” “some implementations,” “implementations of the present application,” etc., may indicate that the implementation(s) of the present application so described may include a particular feature, structure, or characteristic, but not every possible implementation of the present application necessarily includes the particular feature, structure, or characteristic. Further, repeated use of the phrase “in one implementation,” or “in an example implementation,” “an implementation,” do not necessarily refer to the same implementation, although they may. Moreover, any use of phrases like “implementations” in connection with “the present application” are never meant to characterize that all implementations of the present application must include the particular feature, structure, or characteristic, and should instead be understood to mean “at least some implementations of the present application” includes the stated particular feature, structure, or characteristic. The term “coupled” is defined as connected, whether directly or indirectly through intervening components, and is not necessarily limited to physical connections. The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series and the equivalent. 
     Additionally, for the purposes of explanation and non-limitation, specific details, such as functional entities, techniques, protocols, standard, and the like are set forth for providing an understanding of the described technology. In other examples, detailed description of well-known methods, technologies, system, architectures, and the like are omitted so as not to obscure the description with unnecessary details. 
     Persons skilled in the art will immediately recognize that any network function(s) or algorithm(s) described in the present disclosure may be implemented by hardware, software or a combination of software and hardware. Described functions may correspond to modules may be software, hardware, firmware, or any combination thereof. The software implementation may comprise computer executable instructions stored on computer readable medium such as memory or other type of storage devices. For example, one or more microprocessors or general purpose computers with communication processing capability may be programmed with corresponding executable instructions and carry out the described network function(s) or algorithm(s). The microprocessors or general purpose computers may be formed of applications specific integrated circuitry (ASIC), programmable logic arrays, and/or using one or more digital signal processor (DSPs). Although some of the example implementations described in this specification are oriented to software installed and executing on computer hardware, nevertheless, alternative example implementations implemented as firmware or as hardware or combination of hardware and software are well within the scope of the present disclosure. 
     The computer readable medium includes but is not limited to random access memory (RAM), read only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), flash memory, compact disc read-only memory (CD-ROM), magnetic cassettes, magnetic tape, magnetic disk storage, or any other equivalent medium capable of storing computer-readable instructions. 
     A radio communication network architecture (e.g., a long term evolution (LTE) system, an LTE-Advanced (LTE-A) system, or an LTE-Advanced Pro system) typically includes at least one base station, at least one UE, and one or more optional network elements that provide connection towards a network. The UE communicates with the network (e.g., a core network (CN), an evolved packet core (EPC) network, an Evolved Universal Terrestrial Radio Access network (E-UTRAN), a Next-Generation Core (NGC), a 5G Core Network (5GC), or an internet), through a radio access network (RAN) established by the base station. 
     It should be noted that, in the present application, a UE may include, but is not limited to, a mobile station, a mobile terminal or device, a user communication radio terminal. For example, a UE may be a portable radio equipment, which includes, but is not limited to, a mobile phone, a tablet, a wearable device, a sensor, or a personal digital assistant (PDA) with wireless communication capability. The UE is configured to receive and transmit signals over an air interface to one or more cells in a radio access network. 
     A base station may include, but is not limited to, a node B (NB) as in the Universal Mobile Telecommunications System (UMTS), an evolved node B (eNB) as in the LTE-A, a radio network controller (RNC) as in the UMTS, a base station controller (BSC) as in the Global System for Mobile Communication (GSM)/GSM EDGE (Enhanced Data Rate for GSM Evolution) Radio Access Network (GERAN), an ng-eNB as in an E-UTRA base station in connection with the 5GC, a next generation node B (gNB) as in the 5G Access Network (5G-AN), and any other apparatus capable of controlling radio communication and managing radio resources within a cell. The base station may connect to serve the one or more UEs through a radio interface to the network. 
     A base station may be configured to provide communication services according to at least one of the following radio access technologies (RATs): Worldwide Interoperability for Microwave Access (WiMAX), GSM (often referred to as 2G), GERAN, General Packet Radio Service (GPRS), UMTS (often referred to as 3G) based on basic wideband-code division multiple access (W-CDMA), high-speed packet access (HSPA), LTE, LTE-A, eLTE (evolved LTE), New Radio (NR, often referred to as 5G), and/or LTE-A Pro. However, the scope of the present application should not be limited to the above mentioned protocols. 
     The base station is operable to provide radio coverage to a specific geographical area using a plurality of cells forming the radio access network. The base station supports the operations of the cells. Each cell is operable to provide services to at least one UE within its radio coverage. More specifically, each cell (often referred to as a serving cell) provides services to serve one or more UEs within its radio coverage, (e.g., each cell schedules the downlink and optionally uplink resources to at least one UE within its radio coverage for downlink and optionally uplink packet transmissions). The base station can communicate with one or more UEs in the radio communication system through the plurality of cells. A cell may allocate sidelink (SL) resources for supporting proximity service (ProSe). Each cell may have overlapped coverage areas with other cells. 
     As discussed above, the frame structure for NR is to support flexible configurations for accommodating various next generation (e.g., 5G) communication requirements, such as enhanced mobile broadband (eMBB), massive machine type communication (mMTC), ultra-reliable communication and low latency communication (URLLC), while fulfilling high reliability, high data rate and low latency requirements. The orthogonal frequency-division multiplexing (OFDM) technology as agreed in 3GPP may serve as a baseline for NR waveform. The scalable OFDM numerology, such as the adaptive sub-carrier spacing, the channel bandwidth, and the Cyclic Prefix (CP), may also be used in NR. Additionally, two coding schemes are considered for NR: (1) low-density parity-check (LDPC) code and (2) Polar Code. The coding scheme adaption may be configured based on the channel conditions and/or the service applications. 
     Moreover, it is also considered that in a transmission time interval TX of a single NR frame, a downlink (DL) transmission data, a guard period, and an uplink (UL) transmission data should at least be included, where the respective portions of the DL transmission data, the guard period, the UL transmission data should also be configurable, for example, based on the network dynamics of NR. In addition, sidelink resource may also be provided in an NR frame to support ProSe services. 
       FIG.  1 A  and  FIG.  1 B  are schematic diagrams illustrating two multi-connectivity scenarios. To simplify the description,  FIG.  1 A  and  FIG.  1 B  only depict a UE  102 , a cell N 1  and a cell N 2 . However, as can be readily appreciated by those of ordinary skill in the art, the multi-connectivity scenarios can be extended to include several UEs and several cells. For example, the multi-connectivity can be built by considering that the cell N 1  coordinates with multiple cells that function like the cell N 2 . 
     In  FIG.  1 A , the cell N 1  provides a coverage area partially overlapped with a coverage area of the cell N 2 . In  FIG.  1 B , a coverage area of the cell N 1  encompasses the coverage area of the cell N 2 . The cell N 1  and the cell N 2  may belong to the same or different Radio Access Technologies (RATs). For example, the cell N 1  and the cell N 2  may be NR cells using NR RAT. In another example, the cell N 1  and the cell N 2  may apply other types of RATs such as (e)LTE. 
     In the present implementation, the UE  102  may access to the cell N 1  as the master node. The cell N 1  may add the cell N 2  as a secondary node to support the UE&#39;s service requirement. For illustrative purposes, the master node and the secondary node are exemplified as the cell N 1  and the cell N 2 , respectively. However, it should be understood that the present disclosure is not limited thereto. For example, the master node and the secondary node may each include several cells. 
     One of the reasons the UE  102  does not camp to the cell N 2  directly may be because the UE  102  is barred by the cell N 2 , although the signal quality between the UE  102  and the cell N 2  may be good and the cell N 2  can support the service requirement of the UE  102 . Thus, the UE  102  may establish an RRC connection to another cell (e.g., cell N 1 ) as the master node, rather than the cell N 2 . Another reason may be that the coverage of the cell N 1  is more extensive than that of the cell N 2 , as shown in  FIG.  1 B . 
       FIG.  2    is a flowchart illustrating a method for secondary node blind addition (called “blind addition” for short), in accordance with an example implementation of the present disclosure. Compared to normal secondary node addition (called “normal addition” for short) procedure, blind addition is another type of secondary node addition procedure, in which the master node may be allowed to add one or more secondary nodes for a UE, without configuring the UE to perform specific measurements as a basis for selecting the secondary node(s). 
     One of ordinary skill in the art may understand that the illustrated order of actions is illustrative only and the order of the actions may change in response to the present disclosure. Additional actions can be added or fewer actions may be utilized, without departing from this disclosure. Moreover, for the convenience of description, the elements presented in the implementations sharing the same labeling are the same (or similar) elements, and the description of which are as aforementioned. 
     In action  202 , a UE (e.g., the UE  102  in  FIG.  1 A / 1 B) may receive an RRC connection reconfiguration message from a master node (e.g., the cell N 1  in  FIG.  1 A / 1 B). 
     In one implementation, the RRC connection reconfiguration message may include a secondary node&#39;s (e.g., the cell N 2  in  FIG.  1 A / 1 B) configuration, and/or the required information to access the secondary node. For example, the RRC connection reconfiguration message may include at least one of: a cell Identity (ID), a beam configuration of the cell N 2 , an association between Random Access Channel (RACH) resources and Channel State Information-Reference Signal (CSI-RS) configuration of the cell N 2 , an association between RACH resources and New Radio-Synchronization Signal (NR-SS) configuration of the cell N 2 , a Signaling Radio Bearer (SRB) configuration for the cell N 2 , a Data Radio Bearer (DRB) configuration for the cell N 2 , a Scheduling Request (SR) configuration of the cell N 2 , and an indication of access category in the cell N 2 . In one implementation, the SRB configuration may include the SRB3 configuration. The SRB3 is regarded as the signal radio bearer between the UE and the secondary node. 
     In action  204 , the UE  102  may add the cell N 2  as a secondary node in response to the RRC connection reconfiguration message. 
     In action  206 , the UE  102  may transmit an RRC connection reconfiguration complete message to the cell N 2 . 
     In one implementation, the RRC connection reconfiguration complete message is transmitted by the UE  102  to the cell N 2  via SRB3. 
       FIG.  3    is a flowchart illustrating a method for secondary node blind addition, in accordance with an example implementation of the present disclosure. In the present implementation, the method may be performed by a base station which provides at least one cell (e.g., the cell N 1  in  FIG.  1 A / 1 B), and acts as (or as a part of) the master node of a UE (e.g., the UE  102  in  FIG.  1 A / 1 B). 
     In action  302 , the base station may transmit a secondary node addition request to a cell (e.g., cell N 2 ). 
     In action  304 , the base station may receive a secondary node addition response from the cell N 2 . 
     In one implementation, the secondary node addition response may contain the required information for the UE  102  to access the cell N 2 . For example, the secondary node addition response may include at least one of: a UE ID, a beam configuration of the cell N 2 , an association between RACH resources and CSI-RS configuration of the cell N 2 , an association between RACH resources and NR-SS configuration of the cell N 2 , an SRB configuration (e.g., an SRB3 configuration) for the cell N 2 , a DRB configuration for the cell N 2 , an SR configuration of the cell N 2 , and an indication of access category in the cell N 2 . 
     In action  306 , the base station may add the cell N 2  as a secondary node in response to the secondary node addition response. 
     In action  308 , the base station may transmit an RRC connection reconfiguration message to the UE  102  in response to the secondary node addition response. For example, the base station may encapsulate the secondary node addition response into the RRC connection reconfiguration message. 
       FIG.  4    is a message flow diagram illustrating a secondary node blind addition procedure, in accordance with an example implementation of the present disclosure. 
     In the present implementation, the UE  102  may conduct a cell selection procedure to find suitable cell(s) for multi-connectivity. For example, before the UE  102  builds an RRC connection to the cell N 1 , the UE  102  may measure the signal quality to both the cell N 1  and the cell N 2 . The signal quality between the UE  102  to the cell N 1  and that between the UE  102  to the cell N 2  may be both acceptable and suitable for the UE  102 . In such a case, the cell N 1  and the cell N 2  may both satisfy the UE&#39;s cell (re)selection criteria (e.g., cell selection criterion S, cell ranking criterion R). For some criteria (e.g., the signal quality between the UE  102  and the cell N 2  can be better than that between the UE  102  and the cell N 1 ), the UE  102  may want to access the cell N 2  instead of the cell N 1 . However, it is possible that the UE  102  is barred by the cell N 2  because of an access barring mechanism or the UE  102  has an RRC connection failure (e.g., connection establishment failure) with the cell N 2 . In such a case, the UE  102  may turn to the cell N 1  and successfully establish an RRC connection to the cell N 1 . The cell N 1  may serve as the master node to the UE  102 . Thus, the procedure in  FIG.  4    can be helpful for the UE  102  to add the cell N 2  as a secondary node via the cell N 1 . 
     In one implementation, the procedure in  FIG.  4    may be applied for the situations that the UE  102  knows the measurement results of the cell N 2  and considers it as a suitable cell for data transmission and service support. However, for some reasons (e.g., the UE  102  is barred by the cell N 2 ), the UE  102  may access another cell (e.g., the cell N 1 ), and add the cell N 2  as a secondary node via the cell N 1 . 
     In one implementation, the UE  102  may not know the suggested cells for the secondary node addition (e.g., blind addition, normal addition), or the UE  102  may implicitly report the suggested cells to the master node (e.g., the cell N 1 ). In such case, the cell N 1  may then add the secondary node (e.g., the cell N 2 ) on its own. 
     In one implementation, the UE  102  may measure the signal quality to a couple of cells, which may be provided by the network, and/or are previously camped to, and/or nearby cells, so as to find the suitable cells for multi-connectivity. 
     In one implementation, one cell may broadcast inter-frequency cell IDs, intra-frequency cell IDs, neighboring cell IDs, zone IDs, and/or area IDs. Then, the UE  102  may base on the broadcast information (e.g., provided by the cell N 1 ) to measure the signal quality of other cells. For example, the UE  102  may measure the beam quality to an NR cell via NR-Synchronization Signal (NR-SS), and/or Channel State Information-Reference Signal (CSI-RS) of each NR cell, so that the UE  102  may derive the cell quality on its own based on the measurement results of NR-SS and/or CSI-RS. 
     In one implementation, the cells within a certain zone or area may broadcast the same zone ID or area ID. For example, the cell N 1  may broadcast its own zone ID and/or area ID, which may reveal the zone/area to which the cell N 1  belongs. The UE  102  may perform the measurement to a group of cells with the indicated zone IDs and/or area IDs, which are in the same zone/area as the cell N 1 . 
     In one implementation, the cell N 1  may broadcast neighboring zone IDs and/or area IDs. Such zone IDs and/or area IDs may indicate the UE  102  to perform the measurement to a group of cells with the indicated zone IDs and/or area IDs, which are in the neighboring zones/areas. 
     In one implementation, the UE  102  may be (pre)configured with a threshold for the cell selection procedure. The UE  102  may select one or more suitable cells for multi-connectivity, or rank the cells based on the threshold. For example, a cell considered suitable for the UE  102  to camp on may be at least with a signal quality better than the threshold. Among the suitable cells, the UE  102  may select one to camp, and perform an RRC connection establishment procedure. 
     As shown in  FIG.  4   , in action  402 , the UE  102  may perform an RRC connection establishment/resume procedure with the cell N 1 . 
     In action  402 , the UE  102  may have finished preamble transmission to the cell N 1  and random access response reception from the cell N 1 , and plan to move from an RRC idle state to an RRC connected state, or from an RRC inactive (or light connected) state to an RRC connected state. 
     For example, if the UE  102  is to move from the RRC idle state to the RRC connected state, the UE  102  may transmit an RRC connection request message via Signaling Radio Bearer 0 (SRB0) to the cell N 1 , and the cell N 1  may reply with an RRC connection setup message via SRB0 to the UE  102 , for the RRC idle to the RRC connected transition. In another example, when the UE  102  transitions from the RRC inactive (or light connected) state to the RRC connected state, the UE  102  may transmit an RRC connection resume request message via SRB0 to the cell N 1 , and the cell N 1  may reply with an RRC connection resume message via SRB1. 
     In the present implementation, if the UE  102  is to add a secondary node (e.g., the cell N 2 ) via the master node (e.g., the cell N 1 ) without measurement configurations from the master node (e.g., a UE-assisted blind addition), the UE  102  may transmit a blind addition request to the cell N 1  via an RRC message (e.g., an RRC connection request message or an RRC connection resume request message) in the RRC connection establishment/resume procedure. The cell N 1  may then reply the UE  102  with a blind addition response via an RRC connection setup message (or an RRC connection resume message) in the RRC connection establishment/resume procedure. 
     In one implementation, the blind addition response may indicate that the blind addition request is accepted, or the relative resources for the RRC establishment is granted while the blind addition request is rejected, or both the RRC establishment and the blind addition request are rejected. 
     In one implementation, the blind addition request may be realized by an indicator (e.g., at least one bit) added in the RRC connection request message (or in the RRC connection resume request message). For example, “1” means that the blind addition is required, “0” means no such requirement. 
     In one implementation, one field in the EstablishmentCause in the RRC connection request message (or the RRC connection resume request message) may be used to indicate that one of the causes of the RRC connection establishment (or RRC connection resume) is to execute the secondary addition (e.g., blind addition, normal addition). 
     In the present implementation, the EstablishmentCause may be reused for Narrow Band (NB) operations (e.g., NB Internet of Things (NB-IoT) in LTE network). However, the present disclosure is not limited thereto. In some implementations, when the cell N 1  receives the blind addition request, the cell N 1  may know that the UE  102  would like to report a cell selection result (e.g., a list of suitable cell IDs/zone IDs/area IDs) for the candidate secondary nodes in a subsequent RRC message (e.g., in the RRC connection setup complete message or in the RRC connection resume complete message). 
     In one implementation, the UE  102  may collect signal quality information for one or more cells, generate the cell selection result based on the signal quality information, and report the cell selection result to the master node (e.g., cell N 1 ) via an RRC message (e.g., the RRC connection setup complete message or the RRC connection resume complete message). The cell selection result may indicate at least one candidate secondary node. For example, the cell selection result may include at least one of: cell ID, zone ID, and area ID to which the candidate secondary node corresponds. 
     As shown in action  404 , the UE  102  may transmit an RRC connection setup complete message to the cell N 1  to confirm that the RRC connection is established, for the RRC idle state to the RRC connected state transition. 
     The RRC connection setup complete message may include a cell selection result which contains at least one cell ID of the candidate secondary node (e.g., ID of cell N 2 ). The cell ID may be, but not limited to, a physical cell ID (PCI), a global unique ID or a unique ID within the Mobility Management Entity (MME)/Access and Mobility Management Function (AMF). In response to the RRC connection setup complete message, the cell N 1  may add the indicated candidate secondary cell N 2  as the UE&#39;s secondary node. Although action  404  in  FIG.  4    shows a transmission of the “RRC connection setup complete” message, the present disclosure is not limited thereto. In some implementations, the UE  102  may reply with an RRC connection resume complete message to the cell N 1  in action  404 , to confirm that the RRC connection is resumed, for the RRC inactive (or light connected) state to the RRC connected state transition. The message content included in the RRC connection setup complete message is also applicable to the RRC connection resume complete message. 
     In one implementation, the UE  102  may generate the cell selection result based on the signal quality information. For example, if the signal quality between the UE  102  and a cell is above a (pre)configured threshold value, the ID of the cell will be included in the cell selection result by the UE  102 . 
     The threshold value and/or other required information for the UE  102  to report the list of cell IDs/zone IDs/area IDs may be configured by the cell N 1  during the RRC connection establishment/resume procedure via the system information message (e.g., minimum System Information (SI), on demand SI, or other SIs). For example, the cell N 1  may broadcast the threshold value. If the neighboring cell&#39;s signal strength (e.g. the Received Signal Strength Indication (RSSI), the Reference Signal Received Power (RSRP), or the Reference Signal Received Quality (RSRQ)) is above the threshold value, the UE  102  may provide such cell&#39;s ID in the reporting message (e.g., in the RRC connection setup complete message or in the RRC connection resume complete message). 
     In one implementation, the cell selection result may include more than one cell ID/zone ID/area ID. The cells indicated by the UE  102  in the RRC connection setup complete message (or in the RRC connection resume complete message) may be the UE&#39;s suitable cells that satisfy the UE&#39;s cell (re)selection criteria for choosing the secondary nodes. Upon receiving the list of cell IDs/zone IDs/area IDs indicated by the UE  102 , the cell N 1  may or may not filter them. 
     In one implementation, the cell N 1  may inform the UE  102  of at least one of: a maximum number of reported cell IDs in the cell selection result, a maximum number of reported zone IDs in the cell selection result, and a maximum number of reported area IDs in the cell selection result. For example, an indication, such as “maxReportedCellIDs”, may be configurable by the cell N 1 . The cell N 1  may configure the indication maxReportedCellIDs according to the length of each cell ID and the size of the RRC connection setup complete message. The cell N 1  may provide such indication via a system information broadcast message (e.g., minimum SI, on demand SI, or other SIs), or via the RRC connection setup message (or the RRC connection resume message). If the UE  102  receives the indication maxReportedCellIDs, the number of cell IDs included in the RRC connection setup complete message (or the RRC connection resume complete message) may not exceed the number indicated by maxReportedCellIDs. In another example, the UE  102  may report at least one zone ID (or area ID) in the RRC connection setup complete message (or the RRC connection resume complete message). The cell N 1  may perform the procedure of blind addition request and response with at least one cell with the reported zone IDs (or area IDs). In yet another example, the UE  102  may report at least one zone ID (or area ID) and at least one cell ID in the RRC connection setup complete message (or in the RRC connection resume complete message). In such a case, the cell N 1  may guarantee the secondary node added is tagged with the reported zone ID and belonged to one of the reported cell IDs. With the information of cell ID list and/or zone ID list, the cell N 1  may perform the filtering mechanism to add the suitable cells for the UE  102 . 
     In one implementation, the cell N 1  may broadcast a period value (e.g., 100 millisecond (ms)). The UE  102  may provide the ID of a cell with good signal quality over such period in the reporting message (e.g., in the RRC connection setup complete message or in the RRC connection resume complete message). For example, the suitable cells reported may be any cells with good signal quality since N−100 ms, where N is the timing when the UE  102  sends the RRC connection setup complete message. 
     In one implementation, the cell N 1  may specify the reporting order of the suitable cells (e.g., descending or ascending by RSSI value). For instance, the indication of “ascending” or “descending” may be configured in the RRC connection setup message, minimum SI, on demand SI, or other SIs. In such a case, the cell N 1  may perform the filtering mechanism for the procedure of blind addition request and response based on the order. 
     In one implementation, the UE  102  may inform the cell N 1  of the maximum number of secondary nodes that the UE  102  can support. For example, an indication, such as “maxAddedCells”, may be sent from the UE  102  to the cell N 1 . Such indication may be included in the UE&#39;s capability (e.g., in a UE capability message). Alternatively, the UE may provide the indication maxAddedCells together with at least one cell ID in the RRC connection setup complete message to inform the cell N 1  of the UE&#39;s maximum number of supported secondary nodes. For example, the cell N 1  may add the first “maxAddedCells” cells if the descending order is configured. 
     In action  406 , the cell N 1  may perform blind addition request and response with the cell (e.g., the cell N 2 ) indicated by the UE  102 . If the cell ID list/zone ID list/area ID list is filtered by the cell N 1 , in this action, the cell N 1  may only perform the procedure of blind addition request and response to the cells corresponding to the filtered cell IDs/zone IDs/area IDs. 
     During the procedure of blind addition request and response, the cell N 1  may perform the coordination through an X2/Xn interface to the cells indicated by the at least one cell ID in the RRC connection setup complete message (or in the RRC connection resume complete message). The cells to which the cell N 1  coordinates may or may not be filtered by the cell N 1 . Further, the cells to which the cell N 1  coordinates may have an X2/Xn interface with the cell N 1 . In such case, the cell N 1  may not send the blind addition request to all cells indicated by the UE  102 . 
     In action  408 , the cell N 1  may transmit an RRC connection reconfiguration message to the UE  102 . The RRC connection reconfiguration message may include the cell N 2 &#39;s configuration, and/or the required information to access the cell N 2 . Based on the configuration in the RRC connection reconfiguration message, the UE  102  may further add the cell N 2  as a secondary node. 
     In one implementation, if the cell N 1  receives the cell N 2 &#39;s configuration information, the cell N 1  may or may not filter the cell N 2 &#39;s information. If the filtering mechanism is applied, the cell N 1  may forward the configuration from cells not filtered or forward the configuration not filtered. For instance, the cell N 1  may receive more than one RRC resource response from different cells. The cell N 1  may only forward the information in RRC resource response from one or more (less than or equal to “maxAddedCells”) cells to the UE  102 . Based on the configuration in the RRC connection reconfiguration message, the UE  102  may further add the cell N 2  as the secondary node. Moreover, since the UE  102  may receive more than one RRC connection reconfiguration message, the UE  102  may add multiple secondary nodes. In such a case, one RRC connection reconfiguration message may correspond to one cell. In some implementations, the RRC connection reconfiguration message may correspond to more than one cells that the cell N 1  receives the RRC resource response from. 
     In one implementation, one RRC connection reconfiguration message may include all configuration information for different cells. Thus, in such an RRC connection reconfiguration message, there may be fields to explicitly indicate the cell ID and its corresponding configuration. Alternatively, in such an RRC connection reconfiguration message, there may be fields to implicitly indicate the configuration. For example, the order of the configuration may map to the order of cell IDs in the list reported by the UE  102 . In the implicit RRC connection reconfiguration message case, if the cell N 1  does not provide the configuration information for one cell, the field which supposes to carry such configuration may be empty/null. 
     Table 1 shows an exemplary cell ID list reported by the UE  102 . Table 2 shows fields of configurations contained in the RRC connection reconfiguration message. In this example, if the UE  102  reports the cell ID list in the RRC connection setup complete message, via the implicit RRC connection reconfiguration design, the order of configuration may correspond to the order of cell ID list. As shown in Table 1 and Table 2, the cell ID #1 may correspond to the configuration #1, and the cell ID #2 may correspond to the configuration #2. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Cell ID #1 
                 Cell ID #2 
                 Cell ID #3 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 Configuration #1 
                 Configuration #2 
                 Empty/Null 
               
               
                   
               
            
           
         
       
     
     If the cell N 1  is not going to add the cell with the cell ID #3 as the secondary node, the field of configuration corresponds to the cell ID #3 in the RRC connection reconfiguration message may be empty/null or a default value. 
     In one implementation, the cell N 1  may encapsulate the response from the cell N 2  (e.g., the RRC resource response) into the RRC connection reconfiguration message. In such case, the RRC resource response can be considered as an inter-node RRC message. 
     In action  410 , the UE  102  may transmit an RRC connection reconfiguration complete message to the cell N 1  in response to the RRC connection reconfiguration message. 
     In action  412 , the UE  102  may transmit an RRC connection setup complete message to the secondary node (e.g., the cell N 2 ), and build the RRC connection with the cell N 2 . 
     In action  414 , the UE  102  may perform multi-connectivity data transmission with the cell N 1  and the cell N 2 . 
     In one implementation, the UE  102  may receive multiple RRC connection reconfiguration messages from one or more cells. The UE  102  may or may not filter the RRC connection reconfiguration messages. If the UE  102  performs the filtering mechanism, the UE  102  may configure itself based on the RRC connection reconfiguration messages sent by the cells not filtered by the UE  102 . 
     In one implementation, the UE  102  may filter the RRC connection reconfiguration messages, and/or may filter even the configuration in the same RRC connection reconfiguration message. For example, the cell N 1  may provide the configuration information for cell ID #1 and cell ID #2. However, the UE  102  may only build the RRC connection to the cell with the cell ID #1. In such a case, the UE  102  may reply the cell N 1  with the RRC connection reconfiguration complete message including the cell ID #2, to indicate that the cell with the cell ID #2 is not added. The cell N 1  may further instruct the cell with the cell ID #2 to reset the configuration for the UE  102 . In one implementation, the UE  102  may reply the cell N 1  with the RRC connection reconfiguration complete message including the cell ID #1, to indicate that the cell with the cell ID #1 is successfully added. The cell N 1  may further instruct the cell with the cell ID #2 to reset the configuration for the UE  102 . 
     In one implementation, if none of the configurations of the secondary nodes is successfully configured, or if the “maxAddedCells” cell number is not successfully achieved, the secondary node addition (e.g., blind addition, normal addition) is considered unsuccessful. The UE  102  may further trigger the RRC connection re-establishment procedure to the cell N 1 . 
     Based on the received RRC connection reconfiguration message, the UE  102  may configure itself to connect to the cell N 2 . In one implementation, the UE  102  is provided with SRB configuration information in the cell N 2 , so that the UE  102  can build the RRC connection to the cell N 2 . The RRC connection may, for instance, be SRB3. 
     In one implementation, the UE  102  may send RRC messages (e.g., RRC connection request message, RRC connection setup complete message, and RRC connection reconfiguration complete message) to the cell N 2  to add the cell N 2  as the secondary node following the SRB configuration. The SRB configuration may include the configuration of a Packet Data Convergence Protocol (PDCP) entity, a Radio Link Control (RLC) entity, and a Medium Access Control (MAC) entity for SRB delivery. In these RRC messages, the UE  102  may provide its UE ID or any indicators provided by the cell N 2  and forwarded by the cell N 1  as the key to recognize the UE  102 . 
     In one implementation, the UE  102  may be provided with a DRB configuration. The UE  102  may configure its data radio bearers and/or send data to the cell N 2  based on the DRB configuration. The DRB configuration may include the configuration of a Service Data Adaptation Protocol (SDAP) entity, a PDCP entity, a RLC entity, and a MAC entity for DRB delivery. 
     In one implementation, the UE  102  may be provided with an SR configuration to the cell N 2 . The UE  102  may send scheduling request based on the SR configuration to the cell N 2  for uplink resource grants and/or uplink request. 
     In one implementation, the UE  102  may be configured with a beam configuration for the cell N 2 . The UE  102  may configure the beams to transmit and/or receive the beam-level signal to the cell N 2 . 
     In one implementation, the UE  102  is configured with an association between RACH resources and NR-SS configuration, and/or the association between RACH resources and CSI-RS configuration. The UE  102  may perform random access procedure to the cell N 2 . 
     In one implementation, the UE  102  may be provided with an indicator of special access category. The UE  102  may access the cell N 2  based on the indicator in the access control mechanism. For example, the barring parameters of the special access category is looser. For example, such special access category does not go through the access control mechanism, e.g., such special access category is not indicated in the access control parameters in the system information (e.g., SIB1). The configuration may, for instance, be a radio resource configuration including a physical configuration which includes a scheduling request configuration. 
       FIG.  5    is a message flow diagram illustrating operations performed between a UE (e.g., a UE  102 ) and a master node (e.g., a cell N 1 ), in accordance with an example implementation of the present disclosure. 
     In action  502 , the UE  102  may send an RRC connection request message to the cell N 1 . The RRC connection request message may include a blind addition request for the blind addition. 
     In action  504 , the cell N 1  may reply with an RRC connection setup message (or an RRC connection resume message, for the RRC inactive (or light connected) state to the RRC connected transition) to the UE  102 . 
     In one implementation, if the cell N 1  supports the blind addition, the cell N 1  may further ask the UE  102  for what it needs in the RRC connection setup message (or in the RRC connection resume message). If the cell N 1  supports the blind addition, the cell N 1  may allow the UE  102  to set up (or resume) the RRC connection and further report its suitable cell set. For example, the cell N 1  may provide a radio resource dedicated configuration (e.g., having a radio bearer configuration, a PDCP configuration, an RLC configuration, a MAC configuration, and/or a PHY configuration) in the RRC connection setup message (or in the RRC connection resume message), to configure the UE  102  to establish an RRC connection to the cell N 1 . 
     In one implementation, if the cell N 1  is to add the secondary nodes based on the list of cell IDs provided by the UE  102 , the cell N 1  may configure the indication, maxReportedCellIDs, in the RRC connection setup message (or in the RRC connection resume message) to inform the UE  102  of the maximum number of the cell IDs reported in the cell selection result. Thus, the cell N 1  may add the secondary nodes based on the list of cell IDs. 
     In one implementation, if the cell N 1  is to add the secondary nodes based on the list of zone IDs provided by the UE  102 , the cell N 1  may indicate the maxReportedZoneIDs in the RRC connection setup message (or in the RRC connection resume message) to inform the UE  102  of the maximum number of the zone IDs reported in the cell selection result. The cell N 1  may then add the secondary nodes with the provided zone IDs. 
     In one implementation, if the cell N 1  would add the secondary nodes based on the list of area IDs provided by the UE  102 , the cell N 1  may indicate the maxReportedAreaIDs in the RRC connection setup message (or in the RRC connection resume message) to inform the UE  102  of the maximum number of the area IDs reported in the cell selection result. The cell N 1  may then add the secondary nodes with the provided area IDs. 
     In one implementation, if the UE  102  is in the RRC inactive (or light connected) state and stores the previous added secondary node information in the UE context, the cell N 1  may indicate the UE  102  to add the previous secondary node in the UE context via an indicator in the RRC connection setup message (or in the RRC connection reconfiguration message). The cell N 1  may already have the UE context and confirm that the secondary node is available to be added. If the UE  102  has previously added more than one secondary node, and the information of the more than one secondary node is stored in the UE context, the cell N 1  may select the more than one secondary node (but less than maxAddedCells), and instruct the UE  102  to add them. In such a case, the UE  102  may not reply the cell N 1  with the suggested (to be added) cell IDs. 
     In action  506 , the UE  102  may transmit an RRC connection setup complete message (or an RRC connection resume complete message) including the cell selection result to the cell N 1 . The cell selection result may include, for instance, a cell ID list/zone ID list/area ID list indicating one or more suitable cells for being the secondary node(s). 
       FIG.  6    is a message flow diagram illustrating operations performed between a UE (e.g., a UE  102 ) and a master node (e.g., a cell N 1 ), in accordance with an example implementation of the present disclosure. 
     In the present implementation, the cell N 1  may reject the secondary node addition (e.g., blind addition, normal addition) with an RRC connection reject message. As shown in  FIG.  6   , in action  602 , the UE  102  may send an RRC connection request message including a blind addition request to the cell N 1 . If the cell N 1  rejects both the RRC establishment and the blind addition, then in action  604 , the cell N 1  may reply the UE  102  with an RRC connection reject message. 
     In another implementation, the cell N 1  may reject the secondary node addition but accept the RRC connection establishment, thereby replying the UE  102  with an RRC connection setup message. In such case, the cell N 1  may grant relative resources for the RRC establishment yet no secondary node addition. For example, the cell N 1  may send the RRC connection setup message to acknowledge the admission of the RRC connection establishment, provide the radio resource configuration to build the RRC connection establishment, but only grant a sufficient radio resource size for the UE  102  to enable the UE  102  to transmit the RRC connection setup complete message without any indication for blind addition, e.g., without appending the suitable cell ID list/zone ID list/area ID list. The radio resource configuration and suitable radio resource size for the RRC connection setup complete message may be included in a radio resource dedicated configuration. 
     In one implementation, if there is no indication for accepting the secondary node addition (e.g., blind addition, normal addition) or no information required for the secondary node addition in the RRC connection setup message, the UE  102  may know that the secondary node addition is rejected by the cell N 1 . 
     In one implementation, a normal secondary node addition (called “normal addition” for short) may, by default, take place when the secondary node addition (e.g., blind addition, normal addition) is rejected. Detailed description of the normal addition procedure will be illustrated in reference with  FIG.  10   . 
     In one implementation, the cell N 1  may provide an indication of secondary node addition (e.g., blind addition, normal addition) in the RRC connection setup message to explicitly let the UE  102  know whether it is admitted to report the cell selection result (e.g., suitable cell ID list/zone ID list/area ID list) in the following reporting RRC message by checking the indication. For example, an indication of blind addition may be one bit, e.g., “1” means accepting the blind addition, “0” means rejecting the blind addition, or “0” means that the blind addition is rejected but the normal addition takes over. 
       FIG.  7    is a message flow diagram illustrating a procedure for blind addition request and response, in accordance with an example implementation of the present disclosure. 
     In action  702 , a master node (e.g., a cell N 1 ) may send an inter-node blind addition request to a cell (e.g., a cell N 2 ). The cell (e.g., cell N 2 ) may be indicated by the UE  102  and not filtered out by the cell N 1 , if the cell N 1  performs the filtering mechanism. 
     In one implementation, the inter-node blind addition request may include at least one UE ID (e.g., ID of the UE  102 ), so as to inform the cell N 2  of which UE needs the RRC connection and/or data transmission to the cell N 2 . 
     In one implementation, the cell N 1  may send the inter-node blind addition request to multiple cells simultaneously. 
     In action  704 , the cell N 2  may reply an inter-node blind addition response with a positive feedback (e.g., an ACK (acknowledgement) indication) to the cell N 1 , if the blind addition is accepted by the cell N 2  (e.g., the cell N 2  determines that it can support the UE&#39;s requirement(s)). 
     In one implementation, the cell N 2  may, in response to the inter-node blind addition request, check with the core network about the UE&#39;s subscription and/or service requirement via the UE ID. 
     After receiving the positive feedback from the cell N 2 , it is possible that the cell N 1  may further ask for the configuration to access and/or build the RRC connection and/or perform data transmission to the cell N 2 . Thus, in action  706 , the cell N 1  may further send an RRC resource request to the cell N 2 . 
     In one implementation, the cell N 1  may or may not filter the cells sending the positive feedback. For example, if the cell N 2  is filtered out by the cell N 1 , the cell N 2  may not receive the RRC resource request, even if the cell N 2  replies with an ACK in the inter-node blind addition response. On the contrast, if the cell N 2  is not filtered or the cell N 1  does not perform the filtering mechanism, the cell N 2  may receive the RRC resource request from the cell N 1 . The RRC resource request may include, for instance, a UE ID for the cell N 2  to identify. 
     In action  708 , the cell N 2  may send an RRC resource response to the cell N 1 . 
     In one implementation, several pieces of information may be included in the RRC resource response, for the UE  102  to access and/or build the RRC connection and/or perform data transmission with the cell N 2 . The information may be at least one of: a UE ID, an SRB configuration for the UE  102 , a DRB configuration for the UE  102 , an SR configuration, a beam configuration, an association between RACH resources and NR-SS configuration, an association between RACH resources and CSI-RS configuration, and an indication of special access category. The purpose of the information may be to assist the UE  102  for adding the cell N 2  as a secondary node. The SRB configuration may comprise the configuration for the UE to set up the SRB3 between the UE and the cell N 2 . 
     In one implementation, the cell N 2  may be configured with a timer. The timer may be activated when the cell N 2  accepts the request from the cell N 1  (e.g., upon the cell N 2  sends the inter-node blind addition response with a positive feedback (e.g., ACK)), or when the cell N 2  prepares/reserves the resources for the UE  102  (e.g., upon the cell N 2  sends the RRC resource response with configurations). If the timer expires and the cell N 2  does not receive the UE&#39;s further information or response, the cell N 2  may clear the configuration and/or release the resources for the UE  102 . Alternatively, the cell N 2  may bar the UE  102 . In one implementation, the timer is configured by the cell N 1  to the cell N 2 , e.g., via inter-node blind addition request. In one implementation, the timer is configured by the cell N 2  itself. In one implementation, the cell N 2  sends the timer to the cell N 1  via inter-node blind addition response. 
     Although the procedure of blind addition request and response illustrated in  FIG.  7    includes four actions  702 ,  704 ,  706  and  708 , the present disclosure is not limited thereto. In one implementation, the procedure may be generalized into two actions: one is that the master node (e.g., cell N 1 ) transmits the secondary node addition request to the secondary node (e.g., cell N 2 ), and the other is that the master node receives the secondary node addition response from the secondary node. The secondary node addition request may be configured to request the preparation of resources for secondary node addition for a specific UE (e.g., UE  102 ). For example, the secondary node addition request may be an inter-node blind addition request or an RRC connection resource request. In one implementation, the secondary node addition request may include at least one of the following: timer and UE ID. 
     The secondary node addition response may be configured to confirm the master node about the secondary node addition, and provide the required configuration information for the UE  102  to access the secondary node. For example, the secondary node addition response may be an inter-node blind addition response or an RRC connection resource response. As discussed in  FIG.  4   , the secondary node addition response may include, for instance, at least one of: a timer, a UE ID, a beam configuration of the cell N 2 , an association between RACH resources and CSI-RS configuration of the cell N 2 , an association between RACH resources and NR-SS configuration of the cell N 2 , an SRB configuration for the cell N 2 , a DRB configuration for the cell N 2 , an SR configuration of the cell N 2 , and an indication of access category in the cell N 2 . 
       FIG.  8    is a message flow diagram illustrating a procedure for blind addition request and response, in accordance with another example implementation of the present disclosure. 
     In the present implementation, if the cell N 2  rejects the secondary node addition, the cell N 2  may reply an inter-node blind addition response with a negative feedback (e.g., an NACK indication) to the cell N 1 . 
     As shown in  FIG.  8   , in action  802 , a cell N 1  may transmit the inter-node blind addition request to a cell N 2 . Action  802  may substantially correspond to action  702  in  FIG.  7   , so the detailed description of this action is omitted. 
     In action  804 , the cell N 2  may reply an inter-node blind addition response with a negative feedback (e.g., an NACK indication) to the cell N 1 , if the cell N 2  rejects the requested secondary node addition. For example, the blind addition may be rejected because the cell N 2  cannot support the UE  102 &#39;s requirement(s). In such a case, the cell N 2  may reply to the cell N 1  with a negative feedback (e.g., an NACK indication) in the inter-node blind addition response. 
       FIG.  9    is a message flow diagram illustrating a secondary node blind addition procedure, in accordance with an example implementation of the present disclosure. 
     In action  902 , a UE  102  may perform an RRC connection establishment/resume procedure with a cell N 1 , for RRC idle to RRC connected transition, or for RRC inactive (or light connected) to RRC connected transition. 
     In action  904 , the UE  102  may send an RRC connection setup complete message including a cell selection result to the cell N 1 . 
     In action  906 , the cell N 1  may perform a procedure of blind addition request and response with the cell N 2  in response to the received RRC connection setup complete message. 
     In action  908 , the cell N 1  may forward a first RRC connection reconfiguration message to the UE  102 . 
     In one implementation, the first RRC connection reconfiguration message may include an indication to add the cell N 2  (e.g., cell ID of the cell N 2 ), and/or the required information to access the cell N 2  (e.g., a beam configuration of the cell N 2 , and/or an association between RACH resources and CSI-RS configuration of the cell N 2 , and/or an association between RACH resources and NR-SS configuration of the cell N 2 , and/or an indication of special access category in the cell N 2 ). 
     In response to the first RRC connection reconfiguration message, the UE  102  may then perform an RRC connection establishment procedure with the cell N 2 , as shown in action  910 . 
     In action  912 , the cell N 2  may configure the UE  102  with SCG/SN-specific configuration (e.g., an SRB configuration, and/or a DRB configuration, and/or an SR configuration in the cell N 2 ) via a second RRC connection reconfiguration message. The SRB configuration of the cell N 2  may comprise SRB3 configuration. 
     If the UE  102  adds the cell N 2  successfully, the UE  102  may send an RRC connection reconfiguration complete message to the cell N 2  and the cell N 1 , respectively, as shown in actions  914  and  916 . 
     In action  918 , the UE  102  may perform data transmission with the cell N 1  and the cell N 2  for multi-connectivity. 
     In some cases, the secondary node addition (e.g., blind addition, normal addition) is not guaranteed to be successful. For example, if the UE  102  fails to build an RRC connection to the cell N 2 , the UE  102  may declare a secondary node addition failure to the cell N 1 . In another example, if the UE  102  fails to add “maxAddedCells” secondary nodes, the UE  102  may declare a secondary node addition failure to the cell N 1 . 
     In one implementation, if the UE  102  cannot build the RRC connection to and/or transmit data to any (to be added) secondary nodes, the UE  102  may declare a secondary node addition failure. For example, the UE  102  may declare the secondary node addition failure in the RRC message (e.g., RRC connection reconfiguration complete message, RRC connection reestablishment request message). For example, one bit or a string in the RRC connection reconfiguration complete message may be the indicator of the secondary node addition failure. 
     In one implementation, the UE  102  may further include the cell IDs of cells which cause the secondary node addition failure, and/or provide the cause of failure (e.g. insufficient channel quality, timer expiry (e.g., running out of time before sending the RRC connection reconfiguration complete message), and no response from the cell N 2 ) in the RRC message (e.g., RRC connection reconfiguration complete message, RRC connection reestablishment request message). In one implementation, the result of no response from the cell N 2  may come from a timer in the cell N 2  times out. The timer may be activated by the cell N 2  when the cell N 2  grants or reserves the resources/configuration to the UE  102 , e.g., either during the RRC connection establishment/resume procedure (e.g., upon the transmission of the RRC resource response), or when the UE  102  accesses the cell N 2 . In one implementation, the UE starts the timer when the UE receives the RRC connection reconfiguration message from the cell N 1  or cell N 2 , if this timer is included in the RRC connection reconfiguration message. 
     If the timer times out in the cell N 2 , which means the cell N 2  does not receive the response from the UE  102  before the timer stops or expires, the cell N 2  may further reject or bar the UE  102 . In one implementation, if the timer in the cell N 2  times out, the cell N 2  may not reply to the UE  102  even when the UE  102  sends messages to the cell N 2 , resulting the secondary node addition failure. In one implementation, if the timer in the cell N 2  times out, the cell N 2  may reply to the UE  102  with an RRC reject message, resulting the secondary node addition failure. 
     In one implementation, the cell N 1  may coordinate with the unsuccessful cells and request them to release the configuration and resources for the UE  102  by using the combination of at least cell IDs and/or UE IDs. 
     In one implementation, if the secondary node addition failure is declared when all suggested secondary nodes with configurations (e.g., provided in the RRC connection reconfiguration message) fail, the UE  102  may not provide the cell IDs in the RRC message (e.g., RRC connection reconfiguration complete message, RRC connection reestablishment request message) to the cell N 1 . 
     Once a secondary node addition fails, the UE  102  may give up adding the secondary nodes, may continue on with the blind addition, or may switch to the normal addition. The normal addition is based on the UE&#39;s measurement report. If the UE  102  continues on the blind addition, the UE  102  may provide the cell selection result (e.g., a list of cell IDs or zone IDs) in the RRC connection reconfiguration complete message based on the latest measurement results. Upon receiving the RRC connection reconfiguration complete message, the cell N 1  may perform the procedure of blind addition request and response to those cells indicated by the UE  102 , and/or to those cells tagged with the zone IDs (or area IDs) indicated by the UE  102 . 
     In one implementation, the UE  102  may provide at least one field in the reporting message to indicate its subsequent behavior(s). For example, if the field is “1”, the UE  102  may continue performing the blind addition. If the field is “0”, the UE  102  may perform the normal addition. If the field is “null” (empty), the UE  102  may not add the secondary node at this time. 
       FIG.  10    is a message flow diagram illustrating a normal addition procedure, in accordance with an example implementation of the present disclosure. 
     In the present implementation, a UE  102  may indicate to perform a normal addition after a secondary node addition (e.g., blind addition, normal addition) failure. 
     As shown in  FIG.  10   , in action  1002 , the UE  102  may send an RRC connection reconfiguration complete message to a cell N 1 . The RRC connection reconfiguration complete message may include an indication of normal addition, e.g., 1 bit, to inform the cell N 1 . Alternatively, by default, the indication of secondary node addition failure may represent an indication of normal addition request. 
     Once the cell N 1  receives the UE  102 &#39;s normal addition requirement in an RRC connection reconfiguration complete message, in action  1004 , the cell N 1  may configure measurements for the UE  102  in the RRC connection reconfiguration message. For instance, the RRC connection reconfiguration message may include a measurement configuration. In one implementation, the UE sends the RRC message (e.g., RRC connection reconfiguration complete message, RRC connection reestablishment request message) including the addition failure, to the cell N 1 . The cell N 1  would reply to the UE with an RRC message (e.g., RRC connection reconfiguration message) including measurement configuration. 
     In action  1006 , the UE  102  may perform the measurements based on the measurement configuration, and send a measurement report including the measurement results to the cell N 1 . 
     In action  1008 , the cell N 1  may perform the normal addition based on the measurement report, and add a cell N 2  as a secondary node for the UE  102  accordingly. 
     In one implementation, when the secondary node addition is successful, the UE  102  may acknowledge the status via the RRC connection reconfiguration complete message. The master node (e.g., cell N 1 ) may further configure relative measurements toward the secondary node (e.g., cell N 2 ) that be successfully added during the secondary node addition procedure. 
     In one implementation, the cell N 1  may perform the blind addition without receiving the UE&#39;s suggested cell selection result (e.g., list of cell IDs/zone IDs/area IDs). This type of blind addition can be referred to as “master-node-initiated blind addition.” For example, the cell N 1  may directly send the RRC connection reconfiguration message to the UE  102 . The RRC connection reconfiguration messages may include at least one of: the cell N 2 &#39;s configuration (e.g., the SN/SCG-specific configuration), the indication to add the cell N 2 , and the required information to access the cell N 2 . The SN/SCG-specific configuration may be provided by the cell N 2 . The RRC connection reconfiguration message may encapsulate the SN/SCG-specific configuration sent from the cell N 2  to the cell N 1 , e.g., via inter-node SN addition response. 
     In one implementation, the UE  102  may already have the secondary node&#39;s information (e.g., the UE keeps the MC-related configuration in the UE context) when the UE  102  goes to the RRC inactive (or light connected) state. For example, the MC-related information can be the radio bearer configuration and/or the SCG configuration of the secondary nodes or secondary cells. In such a case, if the cell N 1  has the UE context, the cell N 1  may notify the UE  102  to add the secondary node (e.g., cell N 2 ). 
     In the master-node-initiated blind addition, the UE  102  may base on the configuration in the RRC connection reconfiguration message to add the secondary nodes. If the addition is successful, the UE  102  may build the RRC connection to the cell N 2  via an RRC message (e.g., RRC connection setup complete message). 
     In one implementation, the master-node-initiated blind addition may include the actions since the block of “blind addition request and response” in  FIG.  4    or  FIG.  9   . For example, if the addition is successful, the UE  102  may send the RRC connection reconfiguration complete message to indicate the success of blind addition. If the blind addition fails, the UE  102  may indicate the blind addition failure in the RRC connection reconfiguration complete message to inform the cell N 1 . 
     According to the implementation, the failure of master-node-initiated blind addition can be handled in a manner similar to the UE-assisted blind addition. For example, a blind addition failure can be by default followed by a normal addition. In such case, the UE  102  may indicate the blind addition failure to the cell N 1 , implicitly representing for the request of normal addition. In another example, the UE  102  may indicate the blind addition failure in the RRC connection reconfiguration complete message (e.g., by using one bit). The UE  102  may further indicate the request of the blind addition or the request of the normal addition (e.g., “1” bit means the request of the blind addition, and “0” bit means the request of the normal addition). In yet another example, the UE  102  may indicate the blind addition failure in the RRC connection reconfiguration complete message (e.g., by using one bit), and the cell N 1  may initiate the normal addition by configuring relative measurements after the RRC connection establishment. 
       FIG.  11    is a block diagram illustrating a radio communication equipment, in accordance with an exemplary implementation of the present application. The radio communication equipment may be a UE, a base station, or a network node as shown and described in the disclosure. 
     As shown in  FIG.  11   , the radio communication equipment  1100  may include a transceiver  1106 , a processor  1108 , a memory  1102 , one or more presentation components  1104 , and at least one antenna  1110 . The radio communication equipment  1100  may also include an RF spectrum band module, a base station communications module, a network communications module, and a system communications management module, input/output (I/O) ports, I/O components, and power supply (not explicitly shown in  FIG.  11   ). Each of these components may be in communication with each other, directly or indirectly, over one or more buses  1124 . 
     The transceiver  1106  having a transmitter  1116  and a receiver  1118  may be configured to transmit and/or receive time and/or frequency resource partitioning information. In some implementations, the transceiver  1106  may be configured to transmit in different types of subframes and slots including, but not limited to, usable, non-usable and flexibly usable subframes and slot formats. The transceiver  1106  may be configured to receive data and control channels. 
     The radio communication equipment  1100  may include a variety of computer-readable media. Computer storage media may include RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices. Computer storage media may not comprise a propagated data signal. Communication media may typically embody computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. 
     The memory  1102  may include computer-storage media in the form of volatile and/or non-volatile memory. The memory  1102  may be removable, non-removable, or a combination thereof. Example memory may include solid-state memory, hard drives, optical-disc drives, etc. As illustrated in  FIG.  11   , the memory  1102  may store data  1112  and computer-readable and computer-executable instructions  1114  (e.g., software codes) that are configured to, when executed, cause the processor  1108  to perform various functions described herein. Alternatively, the instructions  1114  may not be directly executable by the processor  1108  but be configured to cause the radio communication equipment  1100  (e.g., when compiled and executed) to perform various functions described herein. 
     The processor  1108  may include a central processing unit (CPU), a microcontroller, an ASIC, an intelligent hardware device, or any combination thereof configured to perform the functions described herein. The processor  1108  may process data  1120  and instructions  1122  received from the memory  1102 , and information through the transceiver  1106 , the base band communications module, and/or the network communications module. The processor  1108  may also process information to be sent to the transceiver  1106  for transmission through the antenna  1110 . 
     One or more presentation components  1104  may present data indications to a person or other devices. Examples of one or more presentation components  1104  may include a display device, speaker, printing component, vibrating component, etc. 
     From the above description it is manifest that various techniques can be used for implementing the concepts described in the present application without departing from the scope of those concepts. Moreover, while the concepts have been described with specific reference to certain implementations, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the scope of those concepts. As such, the described implementations are to be considered in all respects as illustrative and not restrictive. It should also be understood that the present application is not limited to the particular implementations described above, but many rearrangements, modifications, and substitutions are possible without departing from the scope of the present disclosure.