Patent Publication Number: US-2013235740-A1

Title: Method and mobile terminal for managing circuit switched fallback procedure and network registration

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit under 35 U.S.C. §119(a) of a Korean Patent Application No. 10-2012-0023355, filed on Mar. 7, 2012, which is incorporated herein by reference for all purposes. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a method and mobile terminal for managing a circuit switched fallback procedure and a network registration, and more particularly to, a method for controlling a mobile communication terminal to improve receiving/sending voice call by reducing missing of a voice call and a registration delay or failure to a packet switching network when switching from a circuit switching network to the packet switching network after a circuit switched fall back, and a mobile communication terminal using the same. 
     2. Discussion of the Background 
     Long Term Evolution (LTE) refers to a next-generation communication technology evolved from 3rd-generation (3G) mobile communication technology, such as Wideband Code Division Multiple Access (WCDMA), Time Division Synchronous CDMA (TD-SCDMA), and CDMA2000. LTE is also called 3.9th-generation (3.9G) mobile communication since it is regarded as an intermediate standard between the 3rd-generation (3G) mobile communication technology represented by Wideband Code Division Multiple Access (WCDMA) and Code Division Multiple Access 2000 (CDMA2000) and  4 th-generation (4G) mobile communication technology, and is one of leading candidates for the 4th-generation mobile communication technology in addition to WiBro evolution. Mobile communication methods prior to a new communication method, such as LTE and WiMAX, are sometimes collectively expressed as legacy mobile communication methods. 
     LTE is configured to support a voice call based on IP Multimedia Subsystem (IMS), but most service providers have not adopted IMS, and small number of service providers have an interest to adopt IMS. Accordingly, in the LTE standard supporting only the packet switching network (hereinafter, a PS network), the circuit switched fall back (hereinafter, CSFB) service is proposed to support the voice call. 
     In the CSFB service, if a voice call or phone call is received while a mobile communication terminal supporting LTE is in connection with an LTE network, the LTE connection is disconnected and fall-back is made to a circuit switching network (hereinafter, referred to as a CS network) of 3G WCDMA or 2G GSM to provide a voice call service. 
     Further, if the mobile communication terminal supporting LTE is in connection with a PS network (e.g., LTE), the mobile communication terminal is shifted to the CS network to make a voice call according to the CSFB process. After the termination of the voice call, the mobile communication terminal shifts from the CS network to the PS network and is registered to the PS network. 
     However, while the mobile communication terminal attempts to make a reconnection with the PS network, an electric field or wireless communication environment of the PS network may be bad. Thus, the registration to the PS network may be frequently delayed or unsuccessful. Since the mobile communication terminal attempts a connection to the PS network in a state where the connection to the CS network is released after terminating a voice call, the terminal is in a disconnected state from both the CS network and the PS network, and thus the mobile communication terminal cannot send or receive a call. If the registration attempt to the PS network is delayed or unsuccessful, there is a higher possibility for the mobile communication terminal to miss a voice call from another mobile communication terminal during the time period. 
     Generally, the CS network has a call sending/receiving success rate of about 99% or above, and the PS network has a call sending/receiving success rate of just about 90% to about 5%. This phenomenon frequently occurs in the downtown area or when the mobile communication terminal is moving. 
     Therefore, there is a demand for a technique capable of reducing the registration to the PS network from being delayed or failing, and ensuring a terminal to stably send or receive a call without missing a mobile terminating voice call, when the terminal shifts from the CS network to the PS network after CSFB. 
     SUMMARY 
     Exemplary embodiments of the present invention provide a method and mobile terminal for managing a circuit switched fallback procedure and a network registration. 
     According to the exemplary embodiments of the present invention, the possibility of switching to the PS network is determined based on information about the PS network, which is previously collected and stored, and the switching to the PS network is attempted based on the determination result. Therefore, the continual connection of the mobile communication terminal to a communication network is ensured, and a communication-unavailable state may be avoided. In addition, the registration delay or failure to the PS network may be adaptively managed after CSFB. 
     Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
     Exemplary embodiments of the present invention provide a method that uses a processor to manage a circuit switched fall back (CSFB) procedure including connecting to a circuit switched network for the CSFB procedure, processing, using the processor, a voice call using the circuit switched network, and determining whether to register to a packet switched network according to a connection possibility of the packet switched network. 
     Exemplary embodiments of the present invention provide a mobile terminal including a first mobile communication modem to connect to a packet switched network, a second mobile communication modem to switch from the packet switched network to a circuit switched network, and a controller to determine whether to re-register to the packet switched network according to a connection possibility of the packet switched network. 
     Exemplary embodiments of the present invention provide a method that uses a processor to manage a circuit switched fall back (CSFB) procedure including connecting to a circuit switched network for the CSFB procedure, processing, using the processor, a voice call using the circuit switched network, and determining whether to register to a packet switched network if a guard time expires. 
     It is to be understood that both forgoing general descriptions and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a block diagram showing a mobile communication terminal according to an exemplary embodiment of the present invention. 
         FIG. 2  is a flowchart illustrating a method for controlling a mobile communication terminal in a CSFB procedure for sending a voice call according to an exemplary embodiment of the present invention. 
         FIG. 3  is a flowchart illustrating a method for controlling a mobile communication terminal in a CSFB procedure for receiving a voice call according to an exemplary embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating a method for measuring and storing PS network information according to an exemplary embodiment of the present invention. 
         FIG. 5  shows a table including radio resource measurement data for a PS network according to an exemplary embodiment of the present invention. 
         FIG. 6  shows a table including radio resource measurement data for a PS network measured by an inter-RAT measurement according to an exemplary embodiment of the present invention. 
         FIG. 7  is a flowchart illustrating a method for managing a registration to a PS network after terminating a CSFB procedure according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. It will be understood that for the purposes of this disclosure, “at least one of X, Y, and Z” can be construed as X only, Y only, Z only, or any combination of two or more items X, Y, and Z (e.g., XYZ, XZ, XYY, YZ, ZZ). Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals are understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity. 
     Hereinafter, a method for controlling a mobile communication terminal for an improved voice call receiving performance and a mobile communication terminal using the same will be described in detail with reference to the drawings. 
       FIG. 1  is a block diagram showing a mobile communication terminal according to an exemplary embodiment of the present invention. Although some components of a mobile communication terminal  10  are not illustrated in  FIG. 1 , the configuration of the mobile communication terminal  10  may include one or more processors, memories, antennas, display panels (e.g., touch screen display), input/output interfaces, and the like to support dual mode connections to a packet switched (PS) network and a circuit switched (CS) network. 
     Referring to  FIG. 1 , a mobile communication terminal  10  includes a first mobile communication modem  110  for performing a data communication through a packet switching network (a PS network), a second mobile communication modem  120  for performing a voice communication through a circuit switching network (a CS network), and a controller  130  for controlling the first and second mobile communication modems  110  and  120  and communication operations. Further, the mobile communication terminal  10  may include a separate database  140  for storing data relating to the communication operations. The first and second mobile communication modems  110  and  120  are illustrated separately for description purposes only; however, aspects need not be limited thereto such that the first and second mobile communication modems  110  and  120  may be integrated. 
     The mobile communication terminal  10  may perform a data communication through the first mobile communication modem  110 . While connecting to a PS network and staying in an idle state, a voice call may be received or may be requested by the mobile communication terminal  10  and the mobile communication terminal  10  may connect through the second mobile communication modem  120  to the CS network and perform a voice communication. Various data for such network-shifting communication may be stored in the database  140 , and such communication operations may be controlled by the controller  130 . However, since the first mobile communication modem  110 , the second mobile communication modem  120 , the controller  130 , and the database  140  are included in the mobile communication terminal  10 , the operations of each component may be described as operations of the mobile communication terminal  10 . 
       FIG. 2  is a flowchart illustrating a method for controlling a mobile communication terminal in a CSFB procedure for sending a voice call according to an exemplary embodiment of the present invention, and  FIG. 3  is a flowchart illustrating a method for controlling a mobile communication terminal in a CSFB procedure for receiving a voice call according to an exemplary embodiment of the present invention. Referring to  FIG. 2  and  FIG. 3 , a method for switching from a CS network to a PS network after circuit switched fall back (CSFB) will be described.  FIG. 2 ,  FIG. 3 ,  FIG. 4 , and  FIG. 7  will be described as if performed by the mobile communication terminal  10  shown in  FIG. 1 , but is not limited as such. 
     The mobile communication terminal  10  connecting to the PS network  30  is in an idle state in operation S 000 . 
     The PS network  30  splits data into blocks, which are also called packets, and adds receiver address information or the like to a header of each packet and sends the packet. In the PS network  30 , the data is transmitted to a receiver according to the information added to the header of each packet. A private network may be used for a point-to-point communication between two users without a call switching, and allows a rapid transport of 64 kbps to 1,920 kbps. 
     The PS network  30  may include Long Term Evolution (LTE), Wireless Broadband Internet (WirBro), Worldwide Interoperability for Microwave Access (WiMAX)-Advanced (IEEE 802.16m), LTE-Advanced, WiBro Evolution, or the like. 
     The mobile communication terminal  10  may include various types of mobile communication terminals supporting a voice call and data communication, such as smart phones, personal digital assistant (PDA), and tablet computers. The mobile communication terminal  10  may use the PS network  30  when performing a data communication with other mobile communication terminals, and may use the CS network  50  when making a voice call. 
     If a connection between two points is requested, in the CS network  50 , an exchanger continuously connects the requested line for a communication until the communication ends. The CS network  50  may include various types of 3-generation (3G) mobile communication networks such as Wideband Code Division Multiple Access (WCDMA) and Code Division Multiple Access (CDMA) 2000. Further, the CS network  50  may include other circuit switched networks of the 3rd-generation networks or 2nd-generation networks (e.g. GSM network). 
     In operation S 000 , the mobile communication terminal  10  is in an idle state in connection with the PS network  30  without performing a communication. If the mobile communication terminal  10  is in an active state and performing a data communication, the location of the mobile communication terminal  10  is updated in the PS network  30  based on the unit of a cell. However, if the mobile communication terminal  10  is in an idle state without performing a data communication, the location of the mobile communication terminal  10  is updated in the PS network  30  based on the unit of tracking area (TA). In the PS network  30 , several neighboring eNBs (base stations) are grouped and defined as a single TA. If the mobile communication terminal  10  is in an idle state and data traffic toward the mobile communication terminal  10  is generated, the TA wakes up the mobile communication terminal  10  to receive the data, and this wake up operation becomes a unit of paging. 
     In the PS network  30 , the latest location information of the mobile communication terminal  10  is obtained in order to support the communication of the mobile communication terminal  10 . If the mobile communication terminal  10  falls into an idle state, tracking area update (hereinafter, referred to as TAU) may be performed. The mobile communication terminal  10  may transmit a TAU message requesting TAU to the PS network  30  whenever TA is changed, in order to perform the TAU. The mobile communication terminal  10  transmits the TAU message through a random access channel (hereinafter, referred to as RACH), which is an uplink dedicated channel, to the PS network  30 . 
     However, a problem may occur while the mobile communication terminal  10  is transmitting the TAU message through the RACH to the PS network  30 . For example, when a terminal transmits a TAU message through the RACH to the LTE network, the TAU may fail or the connection to the RACH may fail. 
     If the TAU fails or the connection to the RACH fails, back data (first information) about each TA of the PS network  30  may be collected and used as a criterion for determining the possibility of switching to the PS network after the CSFB (“connection possibility of the PS network after CSFB”). A database  140  (see  FIG. 1 ) about each TA may be built with the back data. The back data may be used to determine a registration procedure to the PS network after terminating a voice call in the CSFB procedure. 
       FIG. 4  is a flowchart illustrating a method for measuring and storing PS network information according to an exemplary embodiment of the present invention. Operations S 000 , S 010 , and S 030  illustrated in  FIG. 4  may be performed during the operations S 000  illustrated in  FIG. 2 ,  FIG. 3 , or  FIG. 7 . Referring to  FIG. 4 , while the mobile communication terminal  10  is in an idle state in operation S 000 , the mobile communication terminal  10  may attempt TAU and RACH connection. In operation S 010 , it may be determined whether the TAU attempt or the RACH connection attempt is failed. If the mobile communication terminal  10  fails in the TAU or fails in the connection to the RACH as determined in operation S 010 , the back data about each TA may be stored in the database  140  in operation S 030 . The back data indicates a connection failure rate in TAU or in RACH connection, and the like. 
     The back data stored in the database  140  may include at least one of a TAU failure frequency and a RACH connection failure frequency, and the database  140  may further store the information of the PS network  30  with which the mobile communication terminal fails in TAU or in RACH connection along with the back data indicating a connection failure rate. The information of the PS network  30  stored in the database  140  may include a cell ID, a tracking area code (TAC), and a RACH ID associated with the PS network  30 . 
     In order to enhance successful switching from the CS network  50  to the PS network  30  after the CSFB and to evaluate the success possibility of the switching, the back data (first information) of the PS network  30  collected in an idle state of the mobile communication terminal  10  in operation S 000  may be used as a criterion for determining the possibility of switching to the PS network. Based on the determination associated with the back data, the failure of the switching to the PS network  30  may be prevented or reduced by attempting the switching to the PS network  30  if the back data of the PS network  30  is good. For example, the mobile communication terminal may determine the back data of the PS network is good if the TAU failure frequency and/or the RACH connection failure frequency is less than or equal to a threshold failure rate. 
     If a CSFB voice call is received by the mobile communication terminal  10  in an idle state or the mobile communication terminal  10  requests a CSFB voice call to another terminal, the connection to the PS network  30  may be released in operations S 100  of  FIG. 2  or  FIG. 7 , or in operation S 200  of  FIG. 3 . 
     Referring back to  FIG. 2 , the mobile communication terminal  10  sends a voice call (a CSFB call), e.g., a user inputs a dial or operates a send key to send a voice call in operation S 100 . If the user of the mobile communication terminal  10  attempts a CSFB voice call in operation S  100 , the mobile communication terminal  10  has a Radio Resource Control (RRC) connection with the PS network  30  in operation S 110 , and releases the RRC connection with the PS network  30  in operation S 130 . 
     Referring back to  FIG. 3 , if the mobile communication terminal  10  receive a voice call (a CSFB call), the mobile communication terminal  10  receives a paging signal for the voice call from the PS network  30  in operation S 220 . Next, the mobile communication terminal  10  having a RRC connection with the PS network  30 , in operation S 240 , releases the RRC connection with the PS network  30  in operation S 260 . Subsequent CSFB operations S 300 , S 310 , S 330 , S 500 , S 550 , S 700 , S 900 , S 910 , and S 930  illustrated in  FIG. 3  correspond to those operations illustrated in  FIG. 2 . 
     Referring to  FIG. 2  and  FIG. 3 , the mobile communication terminal  10  may switch to CS network  50  in operation S 300 . The mobile communication terminal  10  may connect to the CS network  50  and transmit a response signal in operation S 310 , and the mobile communication terminal  10  may perform a voice communication via the CS network  50  in operation S 330 . After terminating the voice communication in operation S 500 , the mobile communication terminal  10  may receive an instruction to switch to the PS network  30  in operation S 550 . In order to reduce the re-registration failure to the PS network  30 , the mobile communication terminal  10  may determine whether to switch back to the PS network  30  based on accumulated data including the back data collected in idle state in operation S 700 . If the mobile communication terminal  10  determines to switch back to the PS network  30  in operation S 700 , the mobile communication terminal  10  may attempt to switch to the PS network  30  in operation S 910 . If the switching attempt is successful, registration to the PS network  30  is completed in operation S 930 . If the mobile communication terminal  10  determines not to switch back to the PS network  30  in operation S 700 , the mobile communication terminal  10  may evaluate communication environment of the PS network  30  and may set a timer to delay the switching back to the PS network  30 . 
     In operations S 100  of  FIG. 2  or  FIG. 7 , or in operation S 200  of  FIG. 3 , the mobile communication terminal  10  may collect and store wireless environment data (second information) about the PS network  30  to which the mobile communication terminal  10  connects. 
       FIG. 5  shows a table including radio resource measurement data for a PS network according to an exemplary embodiment of the present invention. Referring to  FIG. 5 , the mobile communication terminal  10  may build a data table in the database  140  with the wireless environment data of the PS network  30 . The wireless environment data of the PS network  30  is used as a criterion for determining the possibility of switching to the PS network after CSFB. 
     The wireless environment data of the PS network  30  may be collected through a network access at a point of starting sending or receiving a voice call request, or before or after starting sending/receiving the voice call request. The collected wireless environment data of the PS network  30  may include at least one of a reference signal received power (RSRP), a reference signal received quality (RSRQ), and a signal-to-noise ratio (SNR). RSRP represents an intensity of a signal obtained from the PS network  30 , RSRQ represents a quality of the obtained signal intensity, and SNR represents a signal-to-noise ratio (in the unit of dB). 
     If the mobile communication terminal  10  starts sending or receiving a CSFB call, the wireless environment data (second information) of the PS network  30  may be collected and used as a criterion for determining the possibility of switching to the PS network after CSFB. 
     Referring back to  FIG. 2  and  FIG. 3 , after sending or receiving a CSFB call and the connection to the PS network  30  being released, the mobile communication terminal  10  may switch to the CS network  50  and perform a voice call in operation S 300 . The mobile communication terminal  10  may connect to the CS network  50  in operation S 310 , and perform a voice call in operation S 330 . 
     Since the mobile communication terminal  10  performs a communication by using the PS network  30  and then switches to the CS network  50  when sending or receiving a CSFB voice call, the switching process is expressed as a fall back from a higher-level network to a lower-level network, e.g., from 4G to 3G. This method is called circuit switched fall back (CSFB), since a circuit switched network of the lower-level network is used for the voice communication. 
     In operation S 300 , wireless environment data (third information) about each cell of a PS network  30  neighboring to the CS network  50  to which the mobile communication terminal  10  has been connected is collected and stored while a voice call is performed. 
       FIG. 6  shows a table including radio resource measurement data for a PS network measured by an inter-RAT measurement according to an exemplary embodiment of the present invention. Referring to  FIG. 6 , the wireless environment data of a neighboring PS network  30  may build the database  140  for each cell. While performing the voice call, the mobile communication terminal  10  collects wireless environment data of the connected CS network  50 . However, the mobile communication terminal  10  collects additionally wireless environment data of the neighboring PS network  30  and uses it as a criterion for determining the possibility of switching to the PS network after CSFB. 
     For the wireless environment data of neighboring PS networks  30 , information of neighboring networks is collected by means of the inter radio access technology measurement (hereinafter, referred to as Inter RAT measurement) while the voice call is performed. The collected wireless environment data of the PS network  30  may include at least one of a reference signal received power (RSRP), a reference signal received quality (RSRQ), and a signal-to-noise ratio (SNR). 
     The types of information collected in operation S 300  may be identical to the information collected in operation S  100 . However, even though the wireless environment data of the PS network  30  to which the mobile communication terminal  10  connects is collected in operation S 100 , both the wireless environment data of the CS network  50  to which the mobile communication terminal  10  connects and the wireless environment data of the PS network  30  neighboring to the connected CS network  50  may be collected in operation S 300 . Therefore, even though the mobile communication terminal  10  is moving to another region while performing the voice call, the information of a PS network  30  in a region where the mobile communication terminal  10  passes last may be collected. 
     While the mobile communication terminal  10  performs a voice call, the information (third information) of neighboring networks may be collected by means of Inter RAT measurement and used as a criterion for determining the possibility of switching to the PS network after CSFB. 
     If the voice call ends in operation S 500 , the mobile communication terminal  10  may receive redirection information from the CS network  50  to redirect to the PS network  30  in operation S 550 . 
     In the related art, if the redirection information (redirection info) to the PS network  30  is received, the mobile communication terminal  10  releases the connection to the CS network  50 , switches to the PS network  30 , and attempts registration. However, if an electric field or wireless environment of the PS network  30  is bad, the registration to PS network  30  is frequently delayed or fails. Therefore, the mobile communication terminal  10  may not be capable of providing a communication during a period when the registration to the PS network  30  is being delayed or unsuccessful. 
     In order to address this problem, even though redirection info to the PS network  30  is received, the switching to the PS network  30  may not be instantly performed. The mobile communication terminal may determine whether to switch to the PS network based on the information (the first information, the second information, and/or the third information) collected respectively in operations S 000 , S 100  (or S 200 ), and S 300  in operation S 700 . 
     By determining whether or not to switch to the PS network, it is judged to switch to the PS network  30  when the PS network  30  has a good electric field or other wireless environments, thereby determining the possibility of switching to the PS network. In order to determine the possibility of switching to the PS network, at least one of the information (the first to third information) collected in S 000  to S 300  is used. 
     For example, the switching to the PS network  30  may be decided by determining whether the corresponding information satisfies the one or more criteria. As a result of the determination, in a case where the corresponding information satisfies the one or more criteria, it is attempted to switch to the PS network  30  (S 900 ). However, in a case where the corresponding information does not satisfy the one or more criteria, the redirection info for switching to the PS network  30  may be neglected for a guard time period, and the switching to the PS network  30  may be blocked for a while. 
     Hereinafter, S 700  will be described in more detail.  FIG. 7  is a flowchart illustrating a method for managing a registration to a PS network after terminating a CSFB procedure according to an exemplary embodiment of the present invention. 
     In operation S 711 , the mobile communication terminal  10  may determine the possibility of switching to the PS network at the location of the mobile communication terminal  10  when the voice call is terminated by using the back data (first information) of the PS network  30  collected during the operation S 000 . 
     The back data of the PS network  30  is a TAU failure frequency or a RACH connection failure frequency of each TA collected when the mobile communication terminal  10  is in an idle state (S 000 ), and the back data may be used as a criterion for determining the possibility of switching to the PS network. For example, if the TAU failure frequency exceeds a first threshold value, the criterion for switching to the PS network is not satisfied and the mobile communication terminal  10  may determine not to switch back to the PS network  30  for a while. 
     As a result of determining the possibility of switching to the PS network in operation S 711 , if it is determined that the criterion for switching to the PS network is not satisfied, the switching to the PS network  30  may be blocked and the connection to the CS network  50  may be maintained in operation S 730 . If it is determined that the criterion for switching to the PS network is satisfied after evaluating the possibility of switching to the PS network  30 , the switching to the PS network  30  may be performed in operation S 900 , or the mobile communication terminal  10  may perform the operation S 713 . 
     In operation S 713 , the mobile communication terminal  10  may further determine the possibility of switching to the PS network at the location of the mobile communication terminal  10  when the voice call is terminated by using the wireless environment data (second information) of the PS network  30  collected during operation S 100 . 
     The wireless environment data of the PS network  30  may be wireless environment data collected when the mobile communication terminal  10  starts sending or receiving a voice call in operation S 100 , and may be used as a criterion for determining the possibility of switching to the PS network. For example, if the RSRP or RSRQ is less than a second threshold value, the criterion for switching to the PS network is not satisfied. 
     If it is determined that the criterion for switching to the PS network is not satisfied after evaluating the possibility of switching to the PS network  30  in operation S 713 , the switching to the PS network  30  may be blocked, and the connection to the CS network  50  may be maintained in operation S 730 . If the criterion for switching to the PS network  30  is satisfied after evaluating the possibility of switching to the PS network  30  in operation S 713 , the switching to the PS network  30  may be performed in operation S 900 , or the mobile communication terminal may perform operation S 715 . 
     In operation S 715 , the mobile communication terminal  10  may determine the possibility of switching to the PS network at the location of the mobile communication terminal  10  when the voice call is terminated by using the wireless environment data (third information) of the PS network  30  neighboring to the CS network  50  collected in operation S 300 . 
     The wireless environment data of the neighboring PS network  30  is wireless environment data about each cell collected while the mobile communication terminal  10  performs a voice call in operation S 300 ), and may be used as a criterion for determining the possibility of switching to the PS network  30 . For example, if the RSRP or RSRQ is less than a third threshold value, the criterion for switching to the PS network is not satisfied. 
     If it is determined that the criterion for switching to the PS network is not satisfied after evaluating the possibility of switching to the PS network  30  in operation S 715 , the switching to the PS network  30  may be blocked, and the connection to the CS network  50  may be maintained in operation S 730 . If it is determined that the criterion for switching to the PS network  30  is satisfied after evaluating the possibility of switching to the PS network  30  in operation S 715 , the switching to the PS network  30  may be performed in operation S 900 . 
     As described above, if the mobile communication terminal  10  receives redirection info for switching back to the PS network  30 , the possibility of switching to the PS network may be determined by using the first information, the second information, and/or the third information in operations S 711 , S 713 , and/or S 715 . 
     Operations for determining the possibility of switching to the PS network (S 711 , S 713 , and S 715 ) may be performed in the order as shown in  FIG. 7 . However, Operations for determining the possibility of switching to the PS network (S 711 , S 713 , and S 715 ) may be performed in a different order or selectively. Further, the operations S 711 , S 713 , and S 715  may be performed individually or simultaneously based on the accumulated information including the first information, the second information, and the third information. Further, one of the operations S 711 , S 713 , and S 715  may be omitted. 
     If the criterion for switching to the PS network is not satisfied, the mobile communication terminal  10  may maintain the connection to the CS network  50  in operation S 730 , and the mobile communication terminal  10  may collect wireless environment data (fourth information) of a PS network  30  neighboring to the connected CS network  50  in operation S 750 . 
     In the operation S 750 , the mobile communication terminal  10  is in an idle state where the mobile communication terminal  10  connects to the CS network  50  but may not perform a communication, and the duration of the idle state may be measured by a timer. In addition, information of neighboring networks (fourth information) may be collected by means of Inter RAT measurement as performed in operation S 300 . 
     Similar to operation S 300 , the database  140  about each cell may be built with the wireless environment data of the neighboring PS network  30 . The wireless environment data of the neighboring PS network  30  may be used as a criterion for determining the possibility of switching to the PS network after CSFB. 
     The wireless environment data of the neighboring PS network  30  may include at least one of a reference signal received power (RSRP), a reference signal received quality (RSRQ), and a signal-to-noise ratio (SNR). 
     The mobile communication terminal  10  may collect information of neighboring networks by means of Inter RAT measurement while maintaining the connection to the CS network  50  after quitting the voice call, and may use the information as a criterion for determining the possibility of switching to the PS network after CSFB. 
     Further, if the mobile communication terminal  10  stays in an idle state in the CS network  50 , it may be determined whether the duration of the idle state passes a guard time before determining whether to attempt switching to the PS network in operation S 770 . 
     If the duration of the idle state passes the guard time, in operation S 790 , the possibility of switching to the PS network at the location of the mobile communication terminal  10  when the guard time expires is determined by using the wireless environment data of the neighboring PS network  30  collected during operation S 750 . If the duration of the idle state does not pass the guard time, the idle state may be maintained until the guard time expires. 
     The wireless environment data (fourth information) of the neighboring PS network  30  is wireless environment data about each cell collected when the mobile communication terminal  10  connects to the CS network  50  and stays in an idle state in operation S 750 , and may be used as a criterion for determining the possibility of switching to the PS network. For example, if the RSRP, RSRQ, and/or SNR are less than a fourth threshold value, the criterion for switching to the PS network is not satisfied. 
     If the criterion for switching to the PS network is not satisfied after evaluating the possibility of switching to the PS network  30  in operation S 790 , the mobile communication terminal  10  may perform operation S 750  again and another guard time period may be triggered. The second guard time may be set differently or equal to the first guard time. If the criterion for switching to the PS network is satisfied after evaluating the possibility of switching to the PS network  30  in operation S 790 , the switching to the PS network  30  may be performed in operation S 900 . 
     Thus, if the standard for switching to the PS network is satisfied, in operation S 700 , the switching to the PS network  30  may be performed in operation S 900 . 
     According to one operation or the combination of at least two operations for determining the possibility of switching to the PS network S 711 , S 713 , S 715 , and S 790 , the switching to the PS network  30  may be attempted when the criterion for switching to the PS network is satisfied (S 910 , see  FIG. 2 ), and the registration to the PS network  30  may be completed (S 930 , see  FIG. 2 ). The connection possibility of the PS network may be determined to be good or satisfied if the first information is less than (or equal) to a threshold value or if the second, third, and/or fourth information is greater than (or equal to) a threshold value. 
     According to the method for switching to the PS network after CSFB illustrated in the present disclosure, while the mobile communication terminal is in an idle state, back data (first information) about TA of the PS network  30  may be collected (S 000  of  FIG. 2 ,  FIG. 3  or  FIG. 7 ), and the possibility of switching to the PS network  30  may be determined by using the collected back data (S 711 ). In addition, if it is detected to send or receive a voice call, wireless environment data (second information) about the connected PS network may be collected (S 100  in  FIG. 2  or  FIG. 7 ), and the possibility of switching to the PS network may be determined by using the collected wireless environment data (S 713 ). 
     Further, while the voice call is being performed, wireless environment data (third information) about a cell of a PS network neighboring to the connected CS network may be collected (S 300 ), and the possibility of switching to the PS network may be determined by using the collected wireless environment data (S 715 ). Further, while the CS network  50  maintains connection after the voice call ends, wireless environment data (fourth information) about a cell of the neighboring PS network is collected (S 750 ), and the possibility of switching to the PS network is determined by using the collected wireless environment data (S 790 ). 
     As illustrated in the aspects of the present disclosure, even though redirection info for switching to the PS network  30  is received, the switching to the PS network  30  may not be instantly performed, but the possibility of switching to the PS network may be determined based on the first, second, third, and/or fourth information. As a result, since the connection to the CS network  50  is maintained until the wireless environment of the PS network  30  to which the mobile communication terminal  10  is to switch becomes good, the continual connection of the mobile communication terminal  10  to the CS network is ensured. Therefore, the rate of success of sending or receiving a voice call may be enhanced by maintaining the connection to the CS network regardless of the registration delay or failure to the PS network  30 . Further, the registration to the PS network may be delayed until the success probability for the switching back to the PS network is ensured while maintaining the connection to the CS network. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.