Patent Publication Number: US-2023156534-A1

Title: Device and Method for Handling a Recovery of a Radio Resource Control Connection of a Non-terrestrial Network

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/278,516, filed on November 12th, 2021. The content of the application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a device and a method used in a wireless communication system, and more particularly, to a device and a method of handling a recovery of a radio resource control connection of a non-terrestrial network. 
     Description of the Prior Art 
     A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3GPP as a successor of the universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. 
     An LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an evolved Node-B (eNB), increases peak data rate and throughput, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmissions/reception, uplink (UL) multiple-input multiple-output (UL-MIMO), licensed-assisted access (LAA) (e.g., using LTE), etc. 
     A next generation radio access network (NG-RAN) is developed for further enhancing the LTE-A system. The NG-RAN includes one or more next generation Node-Bs (gNBs), and has properties of wider operation bands, different numerologies for different frequency ranges, massive MIMO, advanced channel codings, etc. 
     New advances in a NG (e.g., fifth generation (5G) new radio (NR)) standard create opportunities to integrate a non-terrestrial network (NTN) into an interoperable and standardized wireless experience across the globe. The NTN is able to satisfy requests of anywhere and anytime connection by offering wide-area coverage and ensuring service availability, continuity and scalability. However, the distance between gNBs or cells of gNBs in the NTN is far. It is difficult for the communication device to switch from a source gNB/cell to a target gNB/cell (e.g., from a cell of the gNB to another cell of the gNB, or from a cell of the gNB to another cell of another gNB), when a radio resource control (RRC) connection failure occurs. Thus, how to improve the performance of recovering the RRC connection of the NTN is an important problem to be solved. 
     SUMMARY OF THE INVENTION 
     The present invention therefore provides a communication device and method for handling a handover to solve the abovementioned problem. 
     A communication device for handling a recovery of a radio resource control (RRC) connection of a non-terrestrial network (NTN) includes at least one storage device; and at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores instructions, and the at least one processing circuit is configured to execute the instructions of: establishing an RRC connection with a first cell of the NTN; receiving a collaborator cell configuration message from the first cell, wherein the collaborator cell configuration message comprises at least one collaborator cell configuration associated with at least one cell of the NTN; initiating a RRC connection recovery procedure, after receiving the collaborator cell configuration message; selecting a second cell for the RRC connection recovery procedure, when initiating the RRC connection recovery procedure; and performing the RRC connection recovery procedure with the second cell. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a wireless communication system according to an example of the present invention. 
         FIG.  2    is a schematic diagram of a communication device according to an example of the present invention. 
         FIG.  3    is a flowchart of a process according to an example of the present invention. 
         FIG.  4    is a sequence diagram of a process according to an example of the present invention. 
         FIG.  5    is a sequence diagram of a process according to an example of the present invention. 
         FIG.  6    is a sequence diagram of a process according to an example of the present invention. 
         FIG.  7    is a sequence diagram of a process according to an example of the present invention. 
         FIG.  8    is a flowchart of a process according to an example of the present invention. 
         FIG.  9    is a flowchart of a process according to an example of the present invention. 
         FIG.  10    is a sequence diagram of a process according to an example of the present invention. 
         FIG.  11    is a flowchart of a process according to an example of the present invention. 
         FIG.  12    is a flowchart of a process according to an example of the present invention. 
         FIG.  13    is a flowchart of a process according to an example of the present invention. 
         FIG.  14    is a sequence diagram of a process according to an example of the present invention. 
         FIG.  15    is a flowchart of a process according to an example of the present invention. 
         FIG.  16    is a flowchart of a process according to an example of the present invention. 
         FIG.  17    is a flowchart of a process according to an example of the present invention. 
         FIG.  18    is a flowchart of a process according to an example of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a schematic diagram of a wireless communication system  10  according to an example of the present invention. The wireless communication system  10  is briefly composed of a network  12  and a plurality of communication devices  14 . The wireless communication system  10  may support a time-division duplexing (TDD) mode, a frequency-division duplexing (FDD) mode, a TDD-FDD joint operation mode, a non-terrestrial network (NTN) mode or a licensed-assisted access (LAA) mode. That is, the network  12  and a communication device  14  may communicate with each other via FDD carrier(s), TDD carrier(s), licensed carrier(s) (licensed serving cell (s)) and/or unlicensed carrier (s) (unlicensed serving cell (s)). In addition, the wireless communication system  10  may support a carrier aggregation (CA). That is, the network  12  and a communication device  14  may communicate with each other via multiple serving cells (e.g., multiple serving carriers) including a primary cell (e.g., primary component carrier) and one or more secondary cells (e.g., secondary component carriers). 
     In  FIG.  1   , the network  12  and the communication devices  14  are simply utilized for illustrating the structure of the wireless communication system  10 . Practically, the network  12  may be a universal terrestrial radio access network (UTRAN) including at least one Node-B (NB) in a universal mobile telecommunications system (UMTS). In one example, the network  12  may be an evolved UTRAN (E-UTRAN) including at least one evolved NB (eNB) and/or at least one relay node in a long term evolution (LTE) system, an LTE-Advanced (LTE-A) system, an evolution of the LTE-A system, etc. In one example, the network  12  may be a next generation radio access network (NG-RAN) including at least one next generation Node-B (gNB) and/or at least one fifth generation (5G) base station (BS). In one example, the gNB or the 5G BS of network  12  may include a NTN Gateway and a NTN payload. In one example, the network  12  may be any BS conforming to a specific communication standard to communicate with a communication device  14 . 
     A new radio (NR) is a standard defined for a 5G system (or 5G network) to provide a unified air interface with better performance. gNBs are deployed to realize the 5G system, which supports advanced features such as enhanced Mobile Broadband (eMBB), Ultra Reliable Low Latency Communications (URLLC), massive Machine Type Communications (mMTC), etc. The eMBB provides broadband services with a greater bandwidth and a low/moderate latency. The URLLC provides applications (e.g., end-to-end communication) with properties of a higher reliability and a low latency. The examples of the applications include an industrial internet, smart grids, infrastructure protection, remote surgery and an intelligent transportation system (ITS). The mMTC is able to support internet-of-things (IoT) of the 5G system which include billions of connected devices and/or sensors. 
     Furthermore, the network  12  may also include at least one of the UTRAN/E-UTRAN/NG-RAN and a core network, wherein the core network may include network entities such as Mobility Management Entity (MME), Serving Gateway (S-GW), Packet Data Network (PDN) Gateway (P-GW), Self-Organizing Networks (SON) server and/or Radio Network Controller (RNC), Access and Mobility Management Function (AMF), Session Management Function (SMF), User Plane Function (UPF), Authentication Server Function (AUSF), etc. In one example, after the network  12  receives information transmitted by a communication device  14 , the information may be processed only by the UTRAN/E-UTRAN/NG-RAN and decisions corresponding to the information are made at the UTRAN/E-UTRAN/NG-RAN. In one example, the UTRAN/E-UTRAN/NG-RAN may forward the information to the core network, and the decisions corresponding to the information are made at the core network after the core network processes the information. In one example, the information may be processed by both the UTRAN/E-UTRAN/NG-RAN and the core network, and the decisions are made after coordination and/or cooperation are performed by the UTRAN/E-UTRAN/NG-RAN and the core network. 
     A communication device  14  may be a user equipment (UE), a Very Small Aperture Terminal (VSAT), a low cost device (e.g., machine type communication (MTC) device), a device-to-device (D2D) communication device, a narrow-band internet of things (IoT) (NB-IoT), a mobile phone, a laptop, a tablet computer, an electronic book, a portable computer system, or combination thereof. In addition, the network  12  and the communication device  14  can be seen as a transmitter or a receiver according to direction (i.e., transmission direction), e.g., for an uplink (UL), the communication device  14  is the transmitter and the network  12  is the receiver, and for a downlink (DL), the network  12  is the transmitter and the communication device  14  is the receiver. 
       FIG.  2    is a schematic diagram of a communication device  20  according to an example of the present invention. The communication device  20  may be a communication device  14  or the network  12  shown in  FIG.  1   , but is not limited herein. The communication device  20  may include at least one processing circuit  200  such as a microprocessor or Application Specific Integrated Circuit (ASIC), at least one storage device  210  and at least one communication interfacing device  220 . The at least one storage device  210  may be any data storage device that may store program codes  214 , accessed and executed by the at least one processing circuit  200 . Examples of the at least one storage device  210  include, but are not limited to, a subscriber identity module (SIM), read-only memory (ROM), flash memory, random-access memory (RAM), Compact Disc Read-Only Memory (CD-ROM), digital versatile disc-ROM (DVD-ROM), Blu-ray Disc-ROM (BD-ROM), magnetic tape, hard disk, optical data storage device, non-volatile storage device, non-transitory computer-readable medium (e.g., tangible media), etc. The at least one communication interfacing device  220  is preferably at least one transceiver and is used to transmit and receive signals (e.g., data, messages and/or packets) according to processing results of the at least one processing circuit  200 . 
       FIG.  3    is a flowchart of a process  30  according to an example of the present invention. The process  30  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2   ), to handle a recovery of a radio resource control (RRC) connection of a non-terrestrial network (NTN). The process  30  may be compiled into the program codes  214  and includes the following steps:
     Step  300 : Start.   Step  302 : Establish an RRC connection with a first cell of a NTN.   Step  304 : Receive a collaborator cell configuration message from the first cell, wherein the collaborator cell configuration message comprises at least one collaborator cell configuration associated with at least one cell of the NTN.   Step  306 : Initiate a RRC connection recovery procedure, after receiving the collaborator cell configuration message.   Step  308 : Select a second cell for the RRC connection recovery procedure, when initiating the RRC connection recovery procedure.   Step  310 : Perform the RRC connection recovery procedure with the second cell.   Step  314 : End.   

     According to the process  30 , the communication device  20  establishes an RRC connection with a first cell of the NTN. The communication device  20  triggers a state transition to a connected mode and enters a connected mode (i.e., the access stratum of the mobile device  20  is in a RRC_CONNECTED state). The communication device  20  receives a collaborator cell configuration message from the first cell. The collaborator cell configuration message comprises the at least one collaborator cell configuration associated with at least one cell of the NTN (e.g., at least one collaborator cell). A collaborator cell is a NTN cell associated with one of the at least one collaborator cell configuration. The communication device  20  initiates a RRC connection recovery procedure, after receiving the collaborator cell configuration message. The communication device  20  selects a second cell for the RRC connection recovery procedure, when (e.g., upon) initiating the RRC connection recovery procedure. The communication device  20  performs the RRC connection recovery procedure with the second cell. 
     Realization of the process  30  is not limited to the above description. The following examples may be applied to realize the process  30 . 
     In one example, the first cell and the second cell are configured (e.g., served) by the same network entity (e.g., a gNB). In one example, the first cell and the second cell are configured by different network entities (e.g., different gNBs). 
     In one example, the at least one cell of the NTN receives a collaborator cell request message from the first cell, and transmits a collaborator cell response message to the first cell in response to the collaborator cell request message. Then, the first cell transmits a collaborator cell configuration message comprising information comprised in the collaborator cell response message to the communication device  20 . 
     In one example, the collaborator cell request message comprises mandatory information and optional information. The mandatory information comprises at least one of an identity of the first cell, an identity of the communication device  20  (e.g., a cell radio network temporary identifier (C-RNTI) of the communication device  20 ), a security verification information for the communication device  20  (e.g., a Message Authentication Code -Integrity (MAC-I) or a short MAC-I), an access stratum (AS) security configuration configured by and applied to the first cell. The optional information comprises at least one SRB (e.g., SRB1 and/or SRB2) configuration and/or at least one DRB configuration (e.g., a default DRB configuration). 
     In one example, the collaborator cell response message comprises at least one of cell access information for the at least one cell of the NTN (e.g., random access resource configuration information for the communication device  20  to perform random access with the collaborator cell, the system information associated with the collaborator cell), or a move-in identity of the communication device  20  (e.g., an identity to be utilized for the collaborator cell to identify the communication device  20 ). 
     In one example, the at least one cell of the NTN receives a handover request message from the first cell, and transmits a handover acknowledge (ACK) message to the first cell in response to the handover request message. The first cell generates at least one handover configuration according to the reception of the handover ACK message, and transmits the at least one handover configuration to the communication device  20 . In one example, the communication device  20  is configured with a handover configuration of the at least one handover configuration. 
     There are various alternatives to perform the RRC connection recovery procedure. In one example, the communication device  20  performs a RRC connection re-establishment procedure as the RRC connection recovery procedure. In detail, the communication device  20  performs the RRC connection re-establishment procedure with the second cell according to a collaborator cell configuration associated with the second cell, when the communication device  20  is configured with the collaborator cell configuration of the at least one collaborator cell configuration. In one example, the communication device  20  performs a RRC connection reconfiguration procedure (e.g., a handover or a conditional handover) as the RRC connection recovery procedure. In detail, the communication device  20  performs the RRC connection reconfiguration procedure with the second cell according to a handover configuration associated with the second cell, when the communication device is configured with the handover configuration of the at least one handover configuration. In one example, the communication device  20  performs a RRC connection re-establishment procedure with the second cell according to system information associated with the second cell, when the second cell is not associated with one of the at least one collaborator cell configuration associated with the at least one cell of the NTN (e.g., a cell other than the at least one cell of the NTN) and the communication device is not configured with a valid handover configuration associated with the second cell. In one example, the communication device  20  performs a random access procedure with the second cell. In one example, the communication device  20  transmits a RRC connection recovery request message to the second cell in the random access procedure. 
     In one example, the collaborator cell configuration associated with the second cell is a collaborator cell configuration of the at least one collaborator cell configuration comprised in the collaborator cell configuration message. In one example, the collaborator cell configuration is a handover configuration, and the handover configuration comprises a “ReconfigurationWithSync” associated with a NTN cell (e.g., the at least one cell of the NTN). In one example, the at least one handover configuration comprises at least one execution condition associated with the at least one cell of the NTN, respectively. That is, the at least one execution condition is associated with the at least one handover configuration, respectively. In one example, the RRC connection recovery request message comprises at least one of an identity of the communication device  20  to be applied in the second cell, an identity of the first cell, or an identity to be utilized by the second cell to perform authentication of the communication device  20 . In one example, the random access procedure is a 2-step random access procedure or a 4-step random access procedure. 
     In one example, the communication device  20  enters to an idle mode (i.e., the access stratum part of the communication device  20  is in RRC_IDLE state) from a connected mode, when failing to select the second cell. 
     In one example, the collaborator cell configuration message comprises a validity time associated with the at least one collaborator cell configuration associated with the at least one cell of the NTN. The validity time may be indicated by a starting time, a period of time comprising a starting time and a time duration, a period of time comprising a starting time and a stopping time, a period of time comprising a starting time and a timer, or a timer. The collaborator cell configuration associated with the at least one cell of the NTN is considered to be valid in the validity time. 
     In one example, the collaborator cell configuration of the at least one collaborator cell configuration comprises a validity period that the collaborator cell configuration is considered as valid in the validity period or a validity area that the collaborator cell configuration is considered as valid in the validity area. The validity period may be indicated by a period of time comprising a starting time and a time duration, a period of time comprising a starting time and a stopping time, a period of time comprising a starting time and a timer, or a timer. The collaborator cell configuration associated with the at least one cell of the NTN is considered to be valid in the validity period. The validity area may be indicated by the first cell and be included in the collaborator cell configuration message by at least one identity associated with at least one geographical area, at least one identity associated with at least one NTN tracking area, at least one identity associated with at least one NTN cell (e.g., the at least one cell of the NTN) or at least one identity associated with at least one identity of at least one NTN cell (e.g., a list or a set of the at least one cell of the NTN). A mapping of the at least one identity with the at least one geographical area is configured by the NTN and is provided (e.g., transmitted) to a communication device  20  via a NTN cell. The collaborator cell configuration associated with the at least one cell of the NTN is considered to be valid in the validity area. 
     In one example, the collaborator cell configuration of the at least one collaborator cell configuration comprises at least one of a cell identity associated with a cell (e.g., the second cell) of the at least one cell of the NTN, at least one frequency associated with the cell of the at least one cell of the NTN, at least one frequency associated with at least one reference signal of the cell of the at least one cell of the NTN, or the information to access the cell of the at least one cell of the NTN. In one example, the information to access the cell comprises at least one configuration of random access resource associated with the cell for the communication device  20  to perform the random access procedure with the cell. In one example, the at least one collaborator cell configuration comprises at least one of at least one synchronization signal block (SSB)-based radio resource management (RRM) measurement timing configuration (SMTC) associated with a frequency associated with the cell, an identity of the communication device  20  to be applied in the cell, or an identity to be utilized by the cell to perform authentication for the communication device  20 . 
     In one example, the communication device  20  selects the second cell by performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a cell selection configuration configured by system information of the first cell. In one example, the communication device  20  selects the second cell by performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to the at least one collaborator cell configuration associated with at least one cell of the NTN comprised in the collaborator cell configuration message. In one example, the communication device  20  selects the second cell by performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a handover configuration of at least one handover configuration. 
     Furthermore, there are other alternatives to select the second cell for the RRC connection recovery procedure. In one example, the communication device  20  performs at least one evaluation (or performs the at least one evaluation and at least one measurement) according to the at least one collaborator cell configuration associated with the at least one cell of the NTN, when the communication device is configured with at least one collaborator cell configuration associated with the at least one cell of the NTN and the communication device is configured with a handover configuration of at least one handover configuration. In one example, the communication device  20  performs at least one evaluation (or performs the at least one evaluation and at least one measurement) according to a handover configuration of at least one handover configuration, when the communication device is configured with the at least one collaborator cell configuration associated with the at least one cell of the NTN, the communication device is configured with the handover configuration of the at least one handover configuration and the handover configuration is considered as valid. 
     In one example, the communication device  20  initiates the RRC connection recovery procedure, after failing to perform a handover according to the handover configuration. In one example, the communication device  20  initiates the RRC connection recovery procedure, before any one of the at least one execution condition is satisfied. 
       FIG.  4    is a sequence diagram of a process  40  according to an example of the present invention. The process  40  is performed by a communication device  4000  (e.g., the communication device  20  in  FIG.  2   ), a cell  4002  (e.g., the first cell in the process  30 ) and a cell  4004  (e.g., the second cell in the process  30 ). The cells  4002  and  4004  are configured by a gNB  4006 , and the cell  4002  is a serving cell of the communication device  4000 . First, the communication device  4000  in a connected mode transmits a RRC re-establishment request message to the cell  4004  (Step  400 ) . The cell  4004  transmits a UE context request message to the cell  4002  (Step  402 ) if UE context of the communication device  4000  is not available in the cell  4004 , and receives a UE context response message from the cell  4002  (Step  404 ). The cell  4004  transmits a RRC re-establishment message to the communication device  4000  (Step  406 ) if UE context of the communication device  4000  is available in the cell  4004 , to resume an AS security and radio bearers for the communication device  4000 . The communication device  4000  transmits a RRC re-establishment complete message to the cell  4004  (Step  408 ). Accordingly, the communication device  4000  transmits UL data to the cell  4004  (Step  410 ) if UL resource is indicated by the cell  4004  for UL transmissions, and may receive a RRC reconfiguration message from the cell  4004  (Step  412 ). The cells  4002  and  4004  perform a path switch procedure (Step  414 ), may with involving core network entities (not shown). 
     In  FIG.  4   , the cells  4002  and  4004  are configured by the same gNB. Steps  402  and  404  are the intra-gNB behaviors resulting in a short latency. In one example, the cell  4004  retrieves the UE context of the communication device  4000  stored in the gNB  4006 , after authenticating the communication device  4000 . 
       FIG.  5    is a sequence diagram of a process  50  according to an example of the present invention. The process  50  is performed by a communication device  5000  (e.g., the communication device  20  in  FIG.  2   ), a cell  5002  (e.g., the first cell in the process  30 ) and a cell  5004  (e.g., the second cell in the process  30 ). The cell  5002  is configured by a gNB  5006 , and is a serving cell of the communication device  5000 . The cell  5004  is configured by a gNB  5008 . First, the cell  5002  transmits UE information (e.g., with a full UE context) of the communication device  5000  to the cell  5004  (Step  500 ). 
     The communication device  5000  in a connected mode transmits a RRC re-establishment request message to the cell  5004  (Step  506 ). The cell  5004  transmits a RRC re-establishment message to the communication device  5000  (Step  508 ). The RRC re-establishment message is used for resuming an AS security and establishing at least one signaling radio bearer (SRB), and may further used for establishing at least one data radio bearer (DRB). The communication device  5000  transmits a RRC re-establishment complete message to the cell  5004  (Step  510 ). The cells  5002  and  5004  may perform a path switch procedure (Step  512 ), may with involving core network entities (not shown). Before performing a path switch procedure between cell  5002  and cell  5004 , if cell  5002  determines to modify (e.g., update) the UE information (Step  502 ), the cell  5002  may transmit the modified (e.g., updated) UE information to the cell  5004  (Step  504 ). The communication device  5000  transmits UL data to the cell  5004  after transmitting the RRC re-establishment complete message (Step  514 ) if UL resource is indicated by the cell  4004  for UL transmissions, and may receive a RRC reconfiguration message from the cell  5004  (Step  506 ). 
     In  FIG.  5   , the cells  5002  and  5004  are configured by different gNBs. The cell  5002  transmits the UE information to the cell  5004  in advance. In one example, the cell  5004  activates the AS security and establishes a RRC connection for the communication device  5000   according to the UE information, when the cell  5004  is selected by the communication device  5000  and the cell  5004  receives the RRC re-establishment request message from the communication device  5000 . 
     In  FIG.  5   , the cells  5002  and  5004  perform the path switch procedure, may with involving core network entities (not shown), to switch connections (e.g., packet data unit (PDU) sessions and/or packet data network (PDN) connections) for the communication device  5000 . In one example, during the path switch procedure, the cell  5004  informs a core network entity (not shown) to communicate (e.g., transmit or receive) packets (e.g., user plane packets, packet data units) for the communication device  5000  with (e.g., to or from) the cell  5004  and stop communicating the packets for the communication device  5000  with the cell  5002 . 
       FIG.  6    is a sequence diagram of a process  60  according to an example of the present invention. The process  60  is performed by a communication device  6000  (e.g., the communication device  20  in  FIG.  2   ), a cell  6002  (e.g., the first cell in the process  30 ) and a cell  6004  (e.g., the second cell in the process  30 ). The cell  6002  is configured by a gNB  6006 , and is a serving cell of the communication device  6000 . The cell  6004  is configured by a gNB  6008 . The Steps  600 - 610  can be referred to the Steps  500 - 510  in  FIG.  5   , and are not narrated herein for brevity. The cell  6004  may transmit a notification to the cell  6002  (Step  612 ), after successfully receiving the RRC re-establishment request message from the communication device  6000  (Step  606 ), after successfully transmitting the RRC re-establishment message to the communication device  6000  (Step  608 ), or after successfully receiving RRC re-establishment complete message from the communication device  6000  (Step  610 ). Before receiving the notification from the cell  6004  (Step  602 ), the cell  6002  may transmit modification UE information to the cell  6004  (Step  604 ). After receiving the notification transmitted from the cell  6004  (Step  612 ), the cell  6002  stops modifying (e.g., updating) the UE information and releases the UE information, in response to the notification (Step  614 ). The communication device  6000  transmits UL data to the cell  6004  after transmitting the RRC re-establishment complete message (Step  616 ) if UL resource is indicated by the cell  6004  for UL transmissions, and may receive a RRC reconfiguration message from the cell  6004  (Step  618 ). The cells  6002  and  6004  perform a path switch procedure (Step  620 ), may with involving core network entities (not shown). 
     In  FIG.  6   , the cells  6002  and  6004  are configured by different gNBs. The cell  6002  transmits the UE information to the cell  6004  in advance. In one example, the cell  6004  activates the AS security and establishes a RRC connection for the communication device  6000  according to the UE information, when the cell  6004  is selected by the communication device  6000  and the cell  6004  receives the RRC re-establishment request message from the communication device  6000 . In one example, the cell  6004  transmits the notification to inform the cell  6002  to stop modifying/transmitting the UE information, after receiving the RRC re-establishment request message and successfully authenticating the communication device  6000 . 
     In  FIG.  6   , the cells  6002  and  6004  perform the path switch procedure, to switch connections (e.g., PDU sessions and/or PDN connections) for the communication device  6000 . In one example, during the path switch procedure, the cell  6004  informs a core network (not shown) to communicate (e.g., transmit or receive) packets (e.g., user plane packets) for the communication device  6000  with (e.g., to or from) the cell  6004  and stop communicating the packets for the communication device  6000  with the cell  6002 . 
       FIG.  7    is a sequence diagram of a process  70  according to an example of the present invention. The process  70  is performed by a communication device  7000  (e.g., the communication device  20  in  FIG.  2   ), a cell  7002  (e.g., the first cell in the process  30 ) and a cell  7004  (e.g., the at least one cell of the NTN in the process  30 ), and may be further performed by a cell  7006  (e.g., the at least one cell of the NTN in the process  30 ). The cell  7002  is a NTN serving cell of the communication device  7000 . First, the communication device  7000  establishes an RRC connection with the cell  7002  (successfully) (Step  700 ). The cell  7002  transmits a collaborator cell request message to the cell  7004  (Step  702 ) and may transmit the collaborator cell request message to the cell  7006  (Step  704 ), to negotiate with the cell  7004  (or with the cells  7004  and  7006 ). Each of the cells  7004  and  7006  transmits one collaborator cell response message to the cell  7002  in response to the collaborator cell request message (Steps  706  and  708 , respectively). The cell  7002  transits a collaborator cell configuration message to the communication device  7000  (Step  710 ), after receiving the collaborator cell response message (s) from the cell  7004  (or from the cells  7004  and  7006 ) . The communication device  7000  initiates a RRC connection recovery procedure with the cell  7002  (Step  712 ). 
     In one example, the cell  7002  comprises a SRB0 and a SRB1. In one example, the cell  7002  further comprises at least one of at least one other SRB or at least one DRB. In one example, the cell  7002  determines collaborator cell(s) (e.g., the cells  7004  and  7006 ) for the communication device  7000  according to, for example, a network deployment, a network policy, or UE information or measurement report(s) received from the communication device  7000 , to perform the RRC connection recovery procedure. 
     In one example, the cell  7002  generates the collaborator cell configuration message comprising collaborator cell configurations of the cells  7004  and  7006  according to the collaborator cell response messages. In one example, the cell  7002  modifies the collaborator cell configurations of the cells  7004  and  7006  (e.g., adds, changes, removes a part of or all content of the collaborator cell configurations) by transmitting another collaborator cell configuration message to the communication device  7000 . 
       FIG.  8    is a flowchart of a process  80  according to an example of the present invention. The process  80  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  7000  in  FIG.  7   ) . The process  80  includes the following steps:
     Step  800 : Start.   Step  802 : Initiate a RRC connection recovery procedure.   Step  804 : Does the communication device select a collaborator cell (e.g., one of the cells  7004  and  7006  in  FIG.  7   ) as a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 )? If yes, perform Step  806 . If no, perform Step  808 .   Step  806 : Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the collaborator cell according to the collaborator cell configuration associated with the collaborator cell, and perform Step  816 .   Step  808 : Select a suitable cell.   Step  810 : Does the communication device select the suitable cell successfully? If yes, perform Step  812 . If no, perform Step  814 .   Step  812 : Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell, and perform Step  816 .   Step  814 : Enter to an idle mode from a connected mode.   Step  816 : End.   

     In the Step  802 , if the communication device is configured with at least one collaborator cell configuration associated with at least one cell of the NTN and the at least one collaborator cell configuration is considered as valid, the communication device performs suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 ) evaluation according to the at least one collaborator cell configuration to select a suitable cell (e.g., only performs suitable cell evaluation among the cells that the identity of the cell, the frequency of the cell, or the frequency of the reference signal (s) of the cell is comprised in one of the at least one collaborator cell configuration). 
     The communication device may perform cell search and measurements according to the at least one collaborator cell configuration, before performing the suitable cell evaluation. 
     If a suitable cell is selected in the Step  802  and the suitable cell is a collaborator cell, the Step  806  is performed. In the Step  806 , the communication device initiates a RRC re-establishment procedure as a RRC connection recovery procedure with the suitable cell according to the collaborator cell configuration associated with the suitable cell. The RRC re-establishment procedure may comprise a random access procedure between the communication device and the suitable cell. 
     In the Step  804 , the communication device determines whether the suitable cell is one of the collaborator cell according to the collaborator cell configuration(s) 
     If a suitable cell is not selected in the Step  802 , the Step  808  is performed. In the Step  808 , the communication device selects a suitable cell. The selection of the suitable cell may comprise performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a cell selection configuration configured by system information of the serving cell of the communication device (i.e., the first cell in the process  30 ), or according to a stored cell selection configuration configured by the serving cell or by the previous serving cell of the communication device. 
     In the Step  810 , if a suitable cell is found according to the process in the Step  808 , the communication device performs the Step  812 . Otherwise, the communication device performs the Step  814 . In the step  812 , the communication device initiates a RRC re-establishment procedure as a RRC connection recovery procedure with the suitable cell. The RRC re-establishment procedure may comprise a random access procedure between the communication device and the suitable cell. The information required to access the suitable cell may be obtain from the system information associated with the suitable cell obtained by the communication device (e.g., the communication device may acquire the system information of the suitable cell from the system information broadcast by the suitable cell), or may be configured from previous serving cell and is stored by the communication device. In the Step  814 , the communication device enters an idle mode from a connected mode. An idle mode procedure may be initiated by the communication device accordingly. 
     In the process  80 , if a suitable cell is selected successfully and the suitable cell is a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the collaborator cell according to the collaborator cell configuration associated with the collaborator cell. If a suitable cell is selected successfully and the suitable cell is not a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to, for example, the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell. 
       FIG.  9    is a flowchart of a process  90  according to an example of the present invention. The process  90  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  7000  in  FIG.  7   ). The process  90  includes the following steps:
     Step  900 : Start.   Step  902 : Initiate a RRC connection recovery procedure.   Step  904 : Select a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 ) .   Step  906 : Does the communication device select the suitable cell successfully? If yes, perform Step  910 . If no, perform Step  908 .   Step  908 : Enter to an idle mode from a connected mode, and perform Step  916 .   Step  910 : Is the suitable cell a collaborator cell (e.g., one of the cells  7004  and  7006  in  FIG.  7   )? If yes, perform Step  912 . If no, perform Step  914 .   Step  912 : Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell, and perform Step  916 .   Step  914 : Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell.   Step  916 : End.   

     In the Step  904 , the communication device selects a suitable cell. The selection of the suitable cell may comprise performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a cell selection configuration configured by system information of the serving cell of the communication device (i.e., the first cell in the process  30 ), or according to a stored cell selection configuration configured by the serving cell or by the previous serving cell of the communication device. 
     In the Step  912 , the communication device initiates a RRC re-establishment procedure as a RRC connection recovery procedure with the suitable cell according to the collaborator cell configuration associated with the suitable cell. The RRC re-establishment procedure may comprise a random access procedure between the communication device and the suitable cell. 
     In the Step  914 , the RRC re-establishment procedure may comprise a random access procedure between the communication device and the suitable cell. The information required to access the suitable cell may be obtain from the system information associated with the suitable cell obtained by the communication device (e.g., the communication device may acquire the system information of the suitable cell from the system information broadcast by the suitable cell), or may be configured from previous serving cell and is stored by the communication device. 
     In the process  90 , if a suitable cell is selected successfully and the suitable cell is a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the collaborator cell according to the collaborator cell configuration associated with the collaborator cell. If a suitable cell is selected successfully and the suitable cell is not a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to, for example, the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell. 
       FIG.  10    is a sequence diagram of a process  100  according to an example of the present invention. The process  100  is performed by a communication device  10000  (e.g., the communication device  20  in  FIG.  2   ), a cell  10002  (e.g., the first cell in the process  30 ) and a cell  10004  (e.g., the at least one cell of the NTN in the process  30 ), and may be further performed by a cell  10006  (e.g., the at least one cell of the NTN in the process  30 ). The cell  10002  is a NTN serving cell of the communication device  10000 . First, the communication device  7000  establishes an RRC connection with the cell  10002  (successfully) (Step  1000 ). The cell  10002  transmits a collaborator cell request message to the cell  10004  (Step  1002 ), to negotiate with the cell  10004 . The cell  10004  transmits a collaborator cell response message to the cell  10002  (Step  1004 ). The cell  10002  transits a collaborator cell configuration message to the communication device  10000  (Step  1006 ). The communication device  10000  stores at least one collaborator cell configuration comprised in the collaborator cell configuration message. The cell  10002  determines a handover of the communication device  10000  (Step  1008 ), and transmits a handover request message to the cell  10006  (Step  1010 ), to negotiate with the cell  10006 . The cell  10002  receives a handover request ACK message from the cell  10006  (Step  1012 ), and transmits a RRC reconfiguration message associated with the cell  10006  to the communication device  10000  (Step  1014 ). The communication device  10000  performs a handover procedure according to the RRC reconfiguration message associated with the cell  10006 . If the communication device  10000  determines as failed to complete the handover procedure to handover from cell  10002  to cell  10006  (Step  1018 ) (e.g., the communication device  10000  failed to detect the cell  10006 , failed to perform random access procedure with the cell  10006 , or failed to transit a RRC reconfiguration complete message to the cell  10006  (Step  1016 )), the communication device  10000  initiates a RRC connection recovery procedure (Step  1020 ). 
     In one example, the cell  10002  comprises a SRB0 and a SRB1. In one example, the cell  10002  further comprises at least one of at least one other SRB or at least one DRB. In one example, the cell  10002  determines collaborator cell(s) (e.g., the cell  10004 ) for the communication device  10000  according to, for example, a network deployment, a network policy, or UE information or measurement report(s) received from the communication device  10000 , to perform the RRC connection recovery procedure. In one example, the cell  10002  generates the collaborator cell configuration message comprising a collaborator cell configuration of the cell  10004  according to the collaborator cell response message. 
     In one example, the cell  10002  modifies the collaborator cell configuration of the cell  10004  (e.g., adds, changes, removes a part of or all content of the collaborator cell configuration) by transmitting another collaborator cell configuration message to the communication device  10000 . In one example, the cell  10002  determines the handover of the communication device  10000  and determines handover cell(s) (a cell of the NTN associates with one of the at least one handover configuration comprised in the RRC reconfiguration message associated with the cell  10006  of the Step  1014 , e.g., the cell  10006 ) for the communication device  10000  according to measurement report(s) received from the communication device  10000 , the orbital information associated with the first cell and/or the cell  10006 , or the service time and/or area information associated with the first cell and/or the cell  10006 . In one example, the RRC reconfiguration message comprises a handover configuration with a “ReconfigurationWithSync”. 
     In  FIG.  10   , the cell  10002  transmits the collaborator cell request message to the cell  10004  and may transmit the collaborator cell request message to other collaborator cell(s), and each of the cell  10004  and the other collaborator cell(s) may transmit one collaborator cell response message to the cell  10002  in response to the collaborator cell request message. The cell  10002  transmits the handover request message to the cell  10006  and other handover cell(s), and each of the cell  10006  and the other handover cell(s) transmits one handover request ACK message to the cell  10002  in response to the handover request message (not shown). Accordingly, the RRC reconfiguration message may comprise handover configurations of the cell  10006  and the other handover cell(s). In one example, the other collaborator cell(s) and the other handover cell(s) are the same cell(s). In one example, the other collaborator cell(s) and the other handover cell(s) are different cells. 
       FIG.  11    is a flowchart of a process  110  according to an example of the present invention. The process  110  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  10000  in  FIG.  10   ). The process  110  includes the following steps:
     Step  1100 :Start.   Step  1102 :Initiate a RRC connection recovery procedure.   Step  1104 :Does the communication device select a collaborator cell (e.g., one of the cell  10004  and the other collaborator cell(s) in  FIG.  10   ) as a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 )? If yes, perform Step  1106 . If no, perform Step  1112 .   Step  1106 :Is the suitable cell selected by the communication device a handover cell (e.g., one of the cell  10006  and other handover cell(s) in  FIG.  10   )? If yes, perform Step  1108 . If no, perform Step  1110 .   Step  1108 :Perform a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell. After successfully performing Step  1108 , the communication device performs Step  1120 .   Step  1110 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell. After successfully performing Step  1110 , the communication device performs Step  1120 .   Step  1112 :Select a suitable cell.   Step  1114 :Does the communication device select the suitable cell successfully? If yes, perform Step  1116 . If no, perform Step  1118 .   Step  1116 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell. After successfully performing Step  1116 , the communication device performs Step  1120 .   Step  1118 :Enter to an idle mode from a connected mode.   Step  1120 :End.   

     In the Step  1102 , if the communication device is configured with at least one collaborator cell configuration associated with at least one cell of the NTN and the at least one collaborator cell configuration is considered as valid, and is configured with at least one handover configuration associated with at least one cell of the NTN and the at least one handover configuration is considered as valid. The communication device performs suitable cell evaluation according to the at least one collaborator cell configuration to select a suitable cell (e.g., only perform the suitable cell evaluation among the cells that the identity of the cell, the frequency of the cell, or the frequency of the reference signal(s) of the cell is comprised in one of the at least one collaborator cell configuration). 
     In the Steps  1104 , the communication device determines whether the suitable cell is one of the collaborator cell(s) (e.g., the cell  10004  and the other collaborator cell(s) in  FIG.  10   ) according to whether the suitable cell is associated with one of the at least one collaborator cell configuration. 
     In the Step  1106 , the communication device determines whether the suitable cell is a handover cell. 
     In the Step  1112 , the communication device selects a suitable cell. The selection of the suitable cell may comprise performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a cell selection configuration configured by system information of the serving cell of the communication device (i.e., the first cell in the process  30 ), or according to a stored cell selection configuration configured by the serving cell or by the previous serving cell. 
     In the Step  1116 , The RRC re-establishment procedure may comprise a random access procedure between the communication device and the suitable cell. The information required to access the suitable cell may be obtain from the system information associated with the suitable cell obtained by the communication device (e.g., the communication device may acquire the system information of the suitable cell from the system information broadcast by the suitable cell), or may be configured from previous serving cell and is stored by the communication device. 
     In the process  110 :
     (a) if a suitable cell is selected successfully and if the suitable cell is a handover cell, the communication device performs a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell;   (b) if a suitable cell is selected successfully and if the suitable cell is not a handover cell but a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to the collaborator cell configuration associated with the collaborator cell;   (c) if a suitable cell is selected successfully and the suitable cell is neither a handover cell nor a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to, for example, the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell.   

       FIG.  12    is a flowchart of a process  120  according to an example of the present invention. The process  120  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  10000  in  FIG.  10   ). The process  120  includes the following steps:
     Step  1200 :Start.   Step  1202 :Initiate a RRC connection recovery procedure.   Step  1204 :Select a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 ).   Step  1206 :Does the communication device select a suitable cell successfully? If yes, perform Step  1210 . If no, perform Step  1208 .   Step  1208 :Enter to an idle mode from a connected mode, and perform Step  1216 .   Step  1210 :Is the suitable cell a collaborator cell? If yes, perform Step  1212 . If no, perform Step  1214 .   Step  1212 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell, and perform Step  1216 .   Step  1214 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell.   Step  1216 :End.   

     In the Step  1204 , the communication device selects a suitable cell. The selection of the suitable cell may comprise performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a cell selection configuration configured by system information of the serving cell of the communication device (i.e., the first cell in the process  30 ), or according to a stored cell selection configuration configured by the serving cell or by the previous serving cell of the communication device. 
     In the Step  1206 , the communication device determines whether a suitable cell is selected successfully. In the Step  1210 , the communication device determines whether the suitable cell is a collaborator cell. 
     In the process  120 :
     (a) if a suitable cell is selected successfully and if the suitable cell is a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the collaborator cell according to the collaborator cell configuration associated with the collaborator cell;   (b) if a suitable cell is selected successfully and the suitable cell is not a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to e.g. the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell.   

       FIG.  13    is a flowchart of a process  130  according to an example of the present invention. The process  130  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  10000  in  FIG.  10   ). The process  130  includes the following steps:
     Step  1300 :Start.   Step  1302 :Initiate a RRC connection recovery procedure.   Step  1304 :Select a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 ).   Step  1306 :Does the communication device select the suitable cell successfully? If yes, perform Step  1310 . If no, perform Step  1308 .   Step  1308 :Enter to an idle mode from a connected mode, and perform Step  1320 .   Step  1310 :Is the suitable cell a handover cell? If yes, perform Step  1312 . If no, perform Step  1314 .   Step  1312 :Perform a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell, and perform Step  1320 .   Step  1314 :Is the suitable cell a collaborator cell? If yes, perform Step  1316 . If no, perform Step  1318 .   Step  1316 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell, and perform Step  1320 .   Step  1318 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell.   Step  1320 :End.   

     In the Step  1304 , the communication device selects a suitable cell. The selection of the suitable cell may comprise performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a cell selection configuration configured by system information of the serving cell of the communication device (i.e., the first cell in the process  30 ), or according to a stored cell selection configuration configured by the serving cell or by the previous serving cell of the communication device. 
     In the Step  1306 , the communication device determines whether a suitable cell is selected successfully. In the Step  1310 , the communication device determines whether the suitable cell is a handover cell. In the Step  1314 , the communication device determines whether the suitable cell is a collaborator cell. 
     In the process  130 :
     (a) if a suitable cell is selected successfully and if the suitable cell is a handover cell, the communication device performs a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell;   (b) if a suitable cell is selected successfully and if the suitable cell is not a handover cell but a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the collaborator cell according to the collaborator cell configuration associated with the collaborator cell;   (c) if a suitable cell is selected successfully and the suitable cell is neither a handover cell nor a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to e.g. the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell.   

       FIG.  14    is a sequence diagram of a process  140  according to an example of the present invention. The process  140  is performed by a communication device  14000  (e.g., the communication device  20  in  FIG.  2   ), a cell  14002  (e.g., the first cell in the process  30 ), a cell  14004  (e.g., the at least one cell of the NTN in the process  30 ) and a cell  14006  (e.g., the at least one cell of the NTN in the process  30 ). The cell  14002  is a NTN serving cell of the communication device  14000 . The Steps  1400 - 1406  can be referred to the Steps  1000 - 1006  in  FIG.  10   , and are not narrated herein for brevity. The cell  14002  determines a handover (e.g., a conditional handover) of the communication device  14000  (Step  1408 ), and transmits a handover request message to the cell  14006  (Step  1410 ), to negotiate with the cell  14006 . Then, the cell  14002  receives a handover request ACK message from the cell  10006  (Step  1412 ), and transmits a RRC reconfiguration message associated with the cell  14006  to the communication device  14000  (Step  1414 ). The communication device  14000  performs a handover procedure according to the RRC reconfiguration message associated with the cell  14006 . If the communication device  14000  determines as failed to complete the handover procedure to handover from cell  14002  to cell  14006  (e.g., the communication device  14000  failed to detect the cell  14006 , fails to perform a random access procedure with the cell  14006 , or fails to transits a RRC reconfiguration complete message to the cell  14006  (Step  1416 )), the communication device  14000  initiates a RRC connection recovery procedure (Step  1418 ). 
     In one example, the cell  14002  comprises a SRB0 and a SRB1. In one example, the cell  14002  further comprises at least one of at least one other SRB or at least one DRB. In one example, the cell  14002  determines collaborator cell (s) (e.g., the cell  14004 ) for the communication device  14000  according to, for example, a network deployment, a network policy, or UE information or measurement report(s) received from the communication device  14000 , to perform the RRC connection recovery procedure. In one example, the cell  14002  generates the collaborator cell configuration message comprising a collaborator cell configuration of the cell  14004  according to the collaborator cell response message. 
     In one example, the cell  14002  modifies the collaborator cell configuration of the cell  14004  (e.g., adds, changes, removes a part of or all content of the collaborator cell configuration) by transmitting another collaborator cell configuration message to the communication device  14000 . In one example, the cell  14002  determines the handover (e.g., the conditional handover) of the communication device  14000  and determines handover cell (s) (e.g., the cell  14006 ) for the communication device  14000  according to measurement report (s) received from the communication device  14000 , the orbital information associated with the first cell and/or the cell  14006 , or the service time and/or area information associated with the first cell and/or the cell  14006 . In one example, the RRC reconfiguration message comprises a handover configuration with a “ReconfigurationWithSync”. In one example, the RRC reconfiguration message may further comprise a handover configuration with an execution condition of the cell  14006 . 
     In one example, the communication device  14000  initiates a RRC connection recovery procedure, before the execution condition of the cell  14006  is satisfied. In one example, the communication device  14000  initiates a RRC connection recovery procedure, after failing to perform the handover from the cell  14002  to the cell  14006 . 
     In  FIG.  14   , the cell  14002  transmits the collaborator cell request message to the cell  14004  (or to the cell  14006  and other collaborator cell(s)), and each of the cell  14004  and the other collaborator cell(s) transmits one collaborator cell response message to the cell  14002  in response to the collaborator cell request message (not shown). The cell  14002  transmits the handover request message to the cell  14006  (or to the cell  14006  and other handover cell(s)), and each of the cell  14006  and the other handover cell(s) transmits one handover request ACK message to the cell  14002  in response to the handover request message (not shown). The RRC reconfiguration message may comprise handover configurations with execution conditions of the cell  14006  and other handover cell(s). In one example, the other collaborator cell (s) and the other handover cell(s) are the same cell(s). In one example, the other collaborator cell(s) and the other handover cell(s) are different cells. 
       FIG.  15    is a flowchart of a process  150  according to an example of the present invention. The process  150  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  14000  in  FIG.  14   ). The process  150  includes the following steps:
     Step  1500 :Start.   Step  1502 :Initiate a RRC connection recovery procedure.   Step  1504 :Does the communication device select a handover cell as a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 )? If yes, perform Step  1506 . If no, perform Step  1508 .   Step  1506 :Perform a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration with an execution condition of the suitable cell. After successfully performing Step  1506 , the communication device performs Step  1520 .   Step  1508 :Does the communication device select a collaborator cell as the suitable cell? If yes, perform Step  1510 . If no, perform Step  1512 .   Step  1510 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell. After successfully performing Step  1510 , the communication device performs Step  1520 .   Step  1512 :Select a suitable cell.   Step  1514 :Does the communication device select the suitable cell successfully? If yes, perform Step  1516 . If no, perform Step  1518 .   Step  1516 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell. After successfully performing Step  1506 , the communication device performs Step  1520 .   Step  1518 :Enter to an idle mode from a connected mode.   Step  1520 :End.   

     In the Step  1502 , if the communication device  1500  is configured with at least one collaborator cell configuration associated with at least one cell of the NTN and the at least one collaborator cell configuration is considered as valid, and is configured with at least one handover configuration associated with at least one cell of the NTN and the at least one handover configuration is considered as valid. The communication device performs suitable cell evaluation and may perform at least one measurement according to the at least one collaborator cell configuration to select a suitable cell (e.g., only perform suitable cell evaluation among the cells that the identity of the cell, the frequency of the cell, or the frequency of the reference signal(s) of the cell is comprised in one of the at least one collaborator cell configuration). 
     In the Step  1504 , the communication device determines whether the suitable cell is a handover cell. In the Step  1508 , the communication device determines whether the suitable cell is a collaborator cell. 
     In the process  150 :
     (a) if a suitable cell is selected successfully and if the suitable cell is a handover cell, the communication device performs a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell;   (b) if a suitable cell is selected successfully and if the suitable cell is not a handover cell but a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to the collaborator cell configuration associated with the suitable cell;   (c) if a suitable cell is selected successfully and the suitable cell is neither a handover cell nor a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to e.g. the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell.   

       FIG.  16    is a flowchart of a process  160  according to an example of the present invention. The process  160  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  14000  in  FIG.  14   ). The process  160  includes the following steps:
     Step  1600 :Start.   Step  1602 :Initiate a RRC connection recovery procedure.   Step  1604 :Does the communication device select a collaborator cell as a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 )? If yes, perform Step  1606 . If no, perform Step  1612 .   Step  1606 :Does the communication device select a handover cell as a suitable cell? If yes, perform Step  1608 . If no, perform Step  1610 .   Step  1608 : Perform a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration with an execution condition of the suitable cell. After successfully performing Step  1608 , the communication device performs Step  1620 .   Step  1610 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell. After successfully performing Step  1610 , the communication device performs Step  1620 .   Step  1612 :Select a suitable cell.   Step  1614 :Does the communication device select the suitable cell successfully? If yes, perform Step  1616 . If no, perform Step  1618 .   Step  1616 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell. After successfully performing Step  1616 , the communication device performs Step  1620 .   Step  1618 :Enter to an idle mode from a connected mode.   Step  1620 :End.   

     In the Step  1602 , if the communication device  1600  is configured with at least one collaborator cell configuration associated with at least one cell of the NTN and the at least one collaborator cell configuration is considered as valid, and is configured with at least one handover configuration associated with at least one cell of the NTN and the at least one handover configuration is considered as valid. The communication device performs suitable cell evaluation and may perform at least one measurement according to the at least one collaborator cell configuration to select a suitable cell (e.g., only perform suitable cell evaluation among the cells that the identity of the cell, the frequency of the cell, or the frequency of the reference signal(s) of the cell is comprised in one of the at least one collaborator cell configuration). 
     In the Step  1604 , the communication device determines whether the suitable cell is a collaborator cell. In the Step  1606 , the communication device determines whether the suitable cell is a handover cell. 
     In the process  160 :
     (a) if a suitable cell is selected successfully and if the suitable cell is a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to the collaborator cell configuration associated with the suitable cell;   (b) if a suitable cell is selected successfully and if the suitable cell is not a collaborator cell but a handover cell, the communication device performs a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell;   (c) if a suitable cell is selected successfully and the suitable cell is neither a collaborator cell nor a handover cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to e.g. the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell.   

       FIG.  17    is a flowchart of a process  170  according to an example of the present invention. The process  170  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  14000  in  FIG.  14   ). The process  170  includes the following steps:
     Step  1700 :Start.   Step  1702 :Initiate a RRC connection recovery procedure.   Step  1704 :Select a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 ).   Step  1706 :Does the communication device select the suitable cell successfully? If yes, perform Step  1710 . If no, perform Step  1708 .   Step  1708 :Enter to an idle mode from a connected mode.   Step  1710 :Is the suitable cell a handover cell (e.g., one of the cells  14006  and the other handover cell(s) in  FIG.  14   ) ? If yes, perform Step  1712 . If no, perform Step  1714 .   Step  1712 :Perform a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration with an execution condition of the suitable cell. After successfully performing Step  1712 , the communication device performs Step  1720 .   Step  1714 :Is the suitable cell a collaborator cell (e.g., one of the cells  14004  and the other collaborator cell(s) in  FIG.  14   )? If yes, perform Step  1716 . If no, perform Step  1718 .   Step  1716 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell. After successfully performing Step  1716 , the communication device performs Step  1720 .   Step  1718 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell.   Step  1720 :End.   

     In the Step  1704 , the communication device selects a suitable cell. The selection of the suitable cell may comprise performing at least one evaluation (or performing the at least one evaluation and at least one measurement) according to a cell selection configuration configured by system information of the serving cell of the communication device (i.e., the first cell in the process  30 ), or according to a stored cell selection configuration configured by the serving cell or by the previous serving cell of the communication device. 
     In the process  170 :
     (a) if a suitable cell is selected successfully and if the suitable cell is a handover cell, the communication device performs a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell;   (b) if a suitable cell is selected successfully and if the suitable cell is not a handover cell but a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to the collaborator cell configuration associated with the suitable cell;   (c) if a suitable cell is selected successfully and the suitable cell is neither a collaborator cell nor a handover cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to e.g. the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell.   

       FIG.  18    is a flowchart of a process  180  according to an example of the present invention. The process  180  may be utilized in a communication device (e.g., the communication device  20  in  FIG.  2    or the communication device  14000  in  FIG.  14   ). The process  180  includes the following steps:
     Step  1800 :Start.   Step  1802 :Initiate a RRC connection recovery procedure.   Step  1804 :Select a suitable cell (a cell on which a communication device may camp, e.g., the second cell in the process  30 ).   Step  1806 :Does the communication device select the suitable cell successfully? If yes, perform Step  1810 . If no, perform Step  1808 .   Step  1808 :Enter to an idle mode from a connected mode.   Step  1810 :Is the suitable cell a collaborator cell? If yes, perform Step  1812 . If no, perform Step  1814 .   Step  1812 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to a collaborator cell configuration associated with the suitable cell. After successfully performing Step  1812 , the communication device performs Step  1820 .   Step  1814 :Is the suitable cell a handover cell? If yes, perform Step  1816 . If no, perform Step  1818 .   Step  1816 :Perform a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration with an execution condition of the suitable cell. After successfully performing Step  1816 , the communication device performs Step  1720 .   Step  1818 :Perform a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell.   Step  1820 :End.   

     In the Step  1804 , the communication device selects a suitable cell. The selection of the suitable cell may comprise performing at least one evaluation and may perform at least one measurement according to a cell selection configuration configured by system information of the serving cell of the communication device (i.e., the first cell in the process  30 ), or according to a stored cell selection configuration configured by the serving cell or by the previous serving cell of the communication device. 
     In the process  180 :
     (a) if a suitable cell is selected successfully and if the suitable cell is not a collaborator cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to the collaborator cell configuration associated with the suitable cell;   (b) if a suitable cell is selected successfully and if the suitable cell is not a collaborator cell but a handover cell, the communication device performs a RRC connection reconfiguration procedure as the RRC connection recovery procedure with the suitable cell according to a handover configuration associated with the suitable cell;   (c) if a suitable cell is selected successfully and the suitable cell is neither a collaborator cell nor a handover cell, the communication device performs a RRC connection re-establishment procedure as the RRC connection recovery procedure with the suitable cell according to e.g. the system information associated with the suitable cell, or the stored information that may be configured by the serving cell or by a previous serving cell.   

     The operation of “determine” described above may be replaced by the operation of “compute”, “calculate”, “obtain”, “generate”, “output, “use”, “choose/select”, “decide” or “is configured to”. The operation of “detect” described above may be replaced by the operation of “monitor”, “receive”, “sense” or “obtain”. The phrase of “according to” described above may be replaced by “in response to”. The phrase of “associated with” described above may be replaced by “of” or “corresponding to”. The term of “via” described above may be replaced by “on”, “in” or “at”. The term of “when” described above may be replaced by “upon” and “after”. 
     Those skilled in the art should readily make combinations, modifications and/or alterations on the abovementioned description and examples. The abovementioned description, steps and/or processes including suggested steps can be realized by means that could be hardware, software, firmware (known as a combination of a hardware device and computer instructions and data that reside as read-only software on the hardware device), an electronic system, or combination thereof. An example of the means may be the communication device  20 . 
     Examples of the hardware may include analog circuit(s), digital circuit(s) and/or mixed circuit(s). For example, the hardware may include ASIC(s), field programmable gate array(s) (FPGA(s)), programmable logic device(s), coupled hardware components or combination thereof. In another example, the hardware may include general-purpose processor(s), microprocessor(s), controller(s), digital signal processor(s) (DSP(s)) or combination thereof. 
     Examples of the software may include set(s) of codes, set(s) of instructions and/or set(s) of functions retained (e.g., stored) in a storage unit, e.g., a computer-readable medium. The computer-readable medium may include SIM, ROM, flash memory, RAM, CD-ROM/DVD-ROM/BD-ROM, magnetic tape, hard disk, optical data storage device, non-volatile storage unit, or combination thereof. The computer-readable medium (e.g., storage unit) may be coupled to at least one processor internally (e.g., integrated) or externally (e.g., separated). The at least one processor which may include one or more modules may (e.g., be configured to) execute the software in the computer-readable medium. The set(s) of codes, the set(s) of instructions and/or the set(s) of functions may cause the at least one processor, the module(s), the hardware and/or the electronic system to perform the related steps. 
     Examples of the electronic system may include a system on chip (SoC), system in package (SiP), a computer on module (CoM), a computer program product, an apparatus, a mobile phone, a laptop, a tablet computer, an electronic book or a portable computer system, and the communication device  20 . 
     To sum up, embodiments of the present invention provide a communication device and method for handling a recovery of a RRC connection of a NTN. The initiation of the RRC connection recovery procedure is regarded as a trigger event to recovery the RRC connection of the NTN. In addition, the communication device performs the RRC connection recovery procedure according to a collaborator cell configuration associated with a target cell (e.g., the second cell in the process  30 ), a handover cell configuration associated with the target cell or system information associated with the target cell. In different conditions/environments, the RRC connection recovery procedure is performed in different ways. Thus, the performance of recovering the RRC connection of the NTN can be improved. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.