Abstract:
A communication device for handling a user equipment (UE) Access Stratum (AS) context comprises a storage device for storing instructions and a processing circuit coupled to the storage device. The processing circuit is configured to execute the instructions stored in the storage device. The instructions comprise receiving a first radio resource control (RRC) message via a RRC connection from a first radio access network (RAN) of a first radio access technology (RAT), wherein the RRC message comprises a resume identity and indicates a RRC suspension; suspending the RRC connection in response to the first RRC message; keeping a UE AS context, when the RRC connection is suspended; selecting a cell of a second RAN of a second RAT different from the first RAT; and determining that the UE AS context and the resume identity are invalid in response to the selection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/343,803 filed on May 31, 2016, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    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 user equipment Access Stratum context. 
       2. Description of the Prior Art 
       [0003]    A long-term evolution (LTE) system provides high data rate, low latency, packet optimization, and improved system capacity and improved coverage. The LTE system is evolved continuously to increase peak data rate and throughput by using advanced techniques, such as carrier aggregation (CA), dual connectivity, licensed-assisted access, etc. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes at least one evolved Node-B (eNB) for communicating with at least one user equipment (UE), and for communicating with a core network. The core network may include a mobility management and a Quality of Service (QoS) control of the at least one UE. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention therefore provides a communication device and method for handling a user equipment Access Stratum context to solve the abovementioned problem. 
         [0005]    A communication device for handling a user equipment (UE) Access Stratum (AS) context comprises a storage device for storing instructions and a processing circuit coupled to the storage device. The processing circuit is configured to execute the instructions stored in the storage device. The instructions comprise being configured a radio resource control (RRC) connection and a data radio bearer (DRB) by a first radio access network (RAN) of a first radio access technology (RAT); receiving a first RRC message via the RRC connection from the first RAN, wherein the first RRC message comprises a resume identity and indicates a RRC suspension; suspending the RRC connection in response to the first RRC message; keeping a UE AS context, when the RRC connection is suspended; selecting a cell of a second RAN of a second RAT different from the first RAT; determining that the UE AS context and the resume identity are invalid in response to the selection; and transmitting a RRC connection request message to the first RAN, when the UE selects back a cell of the first RAN from the second RAN in response to the determination. 
         [0006]    A method for handling a user equipment (UE) Access Stratum (AS) context of a communication device comprises being configured a radio resource control (RRC) connection and a data radio bearer (DRB) by a first radio access network (RAN) of a first radio access technology (RAT); receiving a first RRC message via the RRC connection from the first RAN, wherein the first RRC message comprises a resume identity and indicates a RRC suspension; suspending the RRC connection in response to the first RRC message; keeping a UE AS context, when the RRC connection is suspended; selecting a cell of a second RAN of a second RAT different from the first RAT; determining that the UE AS context and the resume identity are invalid in response to the selection; and transmitting a RRC connection request message to the first RAN, when the UE selects back a cell of the first RAN from the second RAN in response to the determination. 
         [0007]    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 
         [0008]      FIG. 1  is a schematic diagram of a wireless communication system according to an example of the present invention. 
           [0009]      FIG. 2  is a schematic diagram of a communication device according to an example of the present invention. 
           [0010]      FIG. 3  is a flowchart of a process according to an example of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      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 composed of a communication device  100 , radio access networks (RANs)  102  and  104 . In  FIG. 1 , the communication device  100 , the RANs  102  and  104  and are utilized for illustrating the structure of the wireless communication system  10 . Practically, each of the RANs  102  and  104  may include an evolved NB (eNB) or a fifth generation (5G) (or called new radio (NR)) base station (BS). The 5G (or NR) BS supports orthogonal frequency-division multiplexing (OFDM) and/or non-OFDM, a transmission time interval (TTI) shorter than 1 ms (e.g., 100 or 200 microseconds) and a wider system bandwidth (e.g., 200 MHz). In  FIG. 1 , coverage areas of the RANs  102  and  104  may be overlapped or non-overlapped. In general, a BS may also be used to refer any of the eNB and the 5G BS. 
         [0012]    The communication device  100  may be a user equipment (UE), a mobile phone, a laptop, a tablet computer, an electronic book, a portable computer system, a vehicle, or an airplane. For uplink (UL), the communication device  100  is the transmitter and the RAN  102  and/or the RAN  104  is the receiver, and for downlink (DL), the RAN  102  and/or the RAN  104  is the transmitter and the communication device  100  is the receiver. 
         [0013]      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 the communication device  100 , the RAN  102  and/or the RAN  104  shown in  FIG. 1 , but is not limited herein. The communication device  20  includes a processing circuit  200  such as a microprocessor or Application Specific Integrated Circuit, a storage device  210  and a communication interfacing device  220 . The storage device  210  may be any data storage device that stores a program code  214 , accessed and executed by the processing circuit  200 . Examples of the storage device  210  include but are not limited to read-only memory, flash memory, random-access memory, hard disk, optical data storage device, non-volatile storage device, non-transitory computer-readable medium (e.g., tangible media), etc. The communication interfacing device  220  includes a transceiver used to transmit and receive signals (e.g., data, signals, messages and/or packets) according to processing results of the processing circuit  200 . 
         [0014]    In the following embodiments, a UE is used to represent the communication device  100  in  FIG. 1 , to simplify the illustration of the embodiments. 
         [0015]      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 UE (e.g., the communication device  100 ). The process  30  includes the following steps: 
         [0016]    Step  300 : Start. 
         [0017]    Step  302 : Be configured a radio resource control (RRC) connection and a data radio bearer (DRB) by a first RAN of a first radio access technology (RAT). 
         [0018]    Step  304 : Receive a first RRC message via the RRC connection from the first RAN, wherein the first RRC message comprises a resume identity and indicates a RRC suspension. 
         [0019]    Step  306 : Suspend the RRC connection in response to the first RRC message. 
         [0020]    Step  308 : Keep a UE AS context, when the RRC connection is suspended. 
         [0021]    Step  310 : Select a cell of a second RAN of a second RAT different from the first RAT. 
         [0022]    Step  312 : Determine that the UE AS context and the resume identity are invalid in response to the selection. 
         [0023]    Step  314 : Transmit a RRC connection request message to the first RAN, when the UE selects back a cell of the first RAN from the second RAN in response to the determination. 
         [0024]    Step  316 : End. 
         [0025]    According to the process  30 , the UE is configured a RRC connection (e.g., including a signaling radio bearer (SRB)) and a DRB by a first RAN of a first RAT (e.g., the RAN  102 ). The UE receives a first RRC message (e.g., RRCConnectionRelease) via the RRC connection from the first RAN, wherein the first RRC message comprises a resume identity and indicates a RRC suspension. Accordingly, the UE suspends the RRC connection in response to the first RRC message. The UE keeps (or maintains) a UE AS context, when the RRC connection is suspended. Later, the UE may select (or connect) a cell of a second RAN of a second RAT (e.g., the RAN  104 ) different from the first RAT. The UE determines that the UE AS context and the resume identity are invalid in response to the selection. For example, the UE discards, releases, removes, deletes or clears the UE AS context, or marks the UE AS context invalid. The UE transmits a RRC connection request message (e.g., RRCConnectionRequest) to the first RAN, when the UE selects back a cell of the first RAN from the second RAN in response to the determination. In other words, whether the UE AS context should be kept (i.e., considered as valid) is determined by the UE according to whether the cell of the second RAN is selected. Thus, a rule for handling validity of the UE AS context is provided, and the UE is able to communicate with the first RAN regularly. 
         [0026]    When the RRC connection is suspended, the UE may perform a cell selection and/or reselection according to the first RAT. When the RRC connection is suspended (i.e., the UE AS context and the resume identity are valid), the UE may transmit a resume request message to resume the RRC connection. 
         [0027]    In one example, the UE enters an idle mode in response to the first RRC message. The UE performs the cell selection and/or reselection in the idle mode according to the first RAT. 
         [0028]    The RRC connection request message may not include the resume identity. The RRC connection request message may include a NAS identity (e.g., S-Temporary Mobile Subscriber Identity (S-TMSI)). In one example, the UE receives a RRC connection setup message in response to the RRC connection request message, and enters a connected mode of the first RAT to communicate with the first RAN. 
         [0029]    In one example, when the UE AS context and the resume identity are valid, the UE transmits a resume request message (e.g., RRCConnectionResumeRequest) triggered by an initiation of a service request procedure or an extended service request procedure to the first RAN. The initiation of the service request procedure or the extended service request procedure may be due data pending for a transmission, a reception of a paging or a signaling pending for a transmission. The RRCConnectionResumeRequest message may include the resume identity or a truncated version of the resume identity. When the UE receives a connection resume message (e.g., RRCConnectionResume message) in response to the resume request message, the UE resumes the RRC connection according to the kept UE AS context which includes resuming the SRB and the DRB. The resume identity is a unique identification for the first RAN to identify the UE. When the UE AS context and the resume identity are invalid, the UE transmits the RRC connection request message triggered by an initiation of a service request procedure or an extended service request procedure to the first RAN, instead of the resume request message. 
         [0030]    Realization of the process  30  is not limited to the above description. The following examples may be applied to the process  30 . 
         [0031]    In one example, the UE selects the cell of the second RAN according to the second RAT, when the cell of the second RAN meets a cell selection (or reselection) criteria. In one example, after selecting the cell of the second RAN, the UE initiates a connection establishment to connect to the second RAN by transmitting a connection request message (e.g., a RRC connection request message of the second RAT) to the second RAN. Further, the UE receives a connection setup message (e.g., a RRC connection setup message of the second RAT) in response to the connection request message from the second RAN, to enter a connected mode of the second RAT. In one example, the UE considers that the UE AS context is not valid, in response to that the UE selects (or connects) to the second RAN. 
         [0032]    In one example, the UE determines that the UE AS context is not valid, when the UE connects to the second RAN. The UE determines the UE AS context is valid, when the UE selects the second RAN but does not connect to the second RAN. That is, the UE does not transmit any request message to the second RAN, or the UE transmits a request message to the second RAN but receives a reject message in response to the request message. Thus, the UE can still resume the RRC connection when selecting back to first RAN. In one example, the UE determines that the UE AS context is not valid, when the UE selects the second RAN and transmits a request message to the second RAN but receives a reject message in response to the request message. 
         [0033]    In one example, the UE is configured the RRC connection and the DRB by the first RAN of the first RAT by receiving a second RRC message (e.g., RRCConnectionReconfiguration) on the RRC connection from the first RAN. The second RRC message includes a DRB configuration configuring the DRB. The UE configures or setups the DRB according to the DRB configuration. The DRB configuration may include at least one of a DRB identity, a Packet Data Convergence Protocol (PDCP) configuration and a Radio Link Control (RLC) configuration. Similarly, the UE AS context may include the DRB configuration. The UE AS context may include RRC configuration (s) in addition to the DRB configuration. The RRC configuration(s) may include at least one of a measurement configuration, a physical layer configuration and Medium Access Control (MAC) configuration. The UE AS context may include a security context. 
         [0034]    In one example, the UE attaches to a core network (CN) via the first RAN according to the first RAT. Then, the UE establishes an evolved packet system (EPS) bearer context with the CN, wherein the DRB is associated with the EPS bearer context. Further, the UE may establish the EPS bearer context by transmitting a PDN Connectivity Request or a Bearer Resource Allocation Request message to the CN via the first RAN. The CN may transmit an ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message in response to the PDN Connectivity Request message via the first RAN. In one example, the CN transmits an ACTIVATE DEDICATED EPS BEARER CONTEXT REQUEST message via the first RAN to the UE to establish the EPS bearer context in response to the Bearer Resource Allocation Request message. 
         [0035]    The first RAT is different from the second RAT. The first RAT may be one of Narrow Band Internet of Things (NB-IoT), LTE and 5G. The second RAT may be one of Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), NB-IoT, LTE and 5G. 
         [0036]    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 . Any of the processes above may be compiled into the program code  214 . 
         [0037]    To sum up, the present invention provides a device and a method for handling a UE AS context. After a RRC connection of a first RAN is suspended, whether the UE AS context should be kept is determined according whether a cell of a second RAN is selected. Thus, a rule for handling validity of the UE AS context is provided, and the UE is able to operate regularly. 
         [0038]    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.