Patent Publication Number: US-2012034920-A1

Title: Handover control method, mobile communication system, and mobile communication terminal

Description:
TECHNICAL FIELD 
     The present invention relates to a handover control method, a mobile communication system and a mobile communication terminal. 
     BACKGROUND ART 
     As a next generation mobile communication system, a system to build one mobile communication area by a plurality of cells using a plurality of base stations such as a LTE (Long Term Evolution) system is considered. In this system, when a communicating mobile communication terminal (also referred to as a communication terminal at below) moves across a cell (hereinafter, referred to as a handover), handover control is considered as it will be possible to move while continuing the communication state. 
     A communication terminal of related art receives notification information transmitted from a base station, and carries out a handover by grasping information on a cell of a shift destination. Here, as notification information, there is a plurality of parameters necessary for a communication terminal to move to another cell. These parameters, for example, include a company code, location information, a timer, a cell reselection parameter, common channel information, the surrounding cell information, and the like. Further, as parameters, a cell ED and radio resource information on a cell are also included. 
     As a handover control system of a LTE system, from a view of the radio resource use efficiency and processing load reduction in communication terminal, it is proposed that a movement destination base station should notify the difference information, as radio resource information notified to a communication terminal. Further, the difference information hereof is information with a difference in parameter value extracted by transmitting radio resource information, which a movement source base station notified a communication terminal, to the movement destination base station, and comparing with the radio resource information which the movement destination base station was considering to notify. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent document 1: Japanese Patent Application Laid-Open No. 2008-103865 
     Non-Patent Documents 
     non-patent document 1: LTE/3GPPTS36.300v8.7.0 2008-12, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network(E-UTRAN); Overall description; Stage 2 (Release 8) 
     non-patent document 2: LTE/3GPPTS36.331 v8.4.0 2008-12, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 8) 
     non-patent document 3: LTE/3GPPTS36.423 v8.4.0 2008-12, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); X2 application protocol (X2AP) (Release 8) 
     non-patent document 4: LTE/3GPPTS36.413 v8.4.0 2008-12, Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); S1 application protocol(S1AP) (Release 8) 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in a handover control scheme according to the proposed LTE system, a part of notification information is also included in the radio resource information which is a parameter. Therefore, when notification information is updated after a movement source base station transmitted a part of notification information to a movement destination base station, a difference occurs between difference information of which a movement destination base station notifies a communication terminal and notification information after update acquired from a movement source base station. As a result, there was a possibility of a handover to fail. 
     The object of the present invention is to provide a handover control method, a mobile communication system and a mobile communication terminal capable of preventing an occurrence of a failure of a handover. 
     Means for Solving the Problems 
     A handover control method according to the present invention is a handover control method of a mobile communication system and comprises giving a time stamp to a parameter necessary for a mobile communication terminal to move to another cell and transmitting the parameter to a movement destination base station from a movement source base station, wherein the above-mentioned movement destination base station transmits difference information on parameter difference with the movement source base station to the mobile communication terminal together with the time stamp as notification information, and the mobile communication terminal compares the time stamp of the difference information acquired from the movement destination base station and the time stamp of notification information acquired from the movement source base station, and selects whether or not to perform a handover when the time stamps do not match. 
     A mobile communication system according to the present invention includes a movement source base station, a movement destination base station and a mobile communication terminal that carries out a handover, wherein the movement source base station includes a transmission means for giving a time stamp to a parameter necessary for the mobile communication terminal to move to another cell and transmitting the parameter to .the movement destination base station from the movement source base station, the movement destination base station includes a transmission means for transmitting difference information on parameter difference with the movement source base station to the mobile communication terminal together with the time stamp, as notification information, and the mobile communication terminal has a selecting means for comparing the time stamp of the difference information acquired from the movement destination base station and the time stamp of notification information acquired from the movement source base station, and selecting whether or not to perform a handover when the time stamps do not match. 
     A mobile communication terminal according to the present invention has a selecting means for comparing a time stamp of difference information acquired from a movement destination base station and a time stamp of notification information acquired from a movement source base station, and selecting whether or not to perform a handover when the time stamps do not match. 
     The Effect of the Invention 
     In the present invention, in a communication terminal, by grasping whether the notification information was updated or not at the time of a handover, whether or not to carry out a handover can be selected, therefore a failure of a handover to occur can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [ FIG. 1 ] It is a system configuration example and a control flow chart according to the embodiments of the present invention. 
       [ FIG. 2 ] It is a control table example of the latest Value Tag of a parameter of which a change has occurred. 
       [ FIG. 3 ] It is a control table example of changed part history information in notification information for each Value Tag. 
       [ FIG. 4 ] It is a flowchart which determines whether or not to carry out a handover by Value Tag. 
       [ FIG. 5 ] It is a flowchart showing a process for eliminating a difference when the Value Tag is different. 
       [ FIG. 6 ] It is a system configuration example according to a second embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Mode for Carrying Out the Invention 
     (A First Embodiment) 
     Hereinafter, by referring to the drawings, an embodiment of the present invention will be described. 
     In  FIG. 1 , an exemplary configuration of a mobile communication system of one embodiment of the present invention is shown. A radio control system and a handover system of LTE/3GPP indicated by non-patent documents 1-4 are applied to the present system. The present system includes a movement source base station A 101 , a movement destination base station B 102  and a mobile communication terminal (hereinafter simply referred to as a communication terminal)  103  which moves between cells of the base stations  101  and  102 . X2 interface indicated in the non-patent document 3 is used for interface which connects between two base stations A 101 -B 102 , and transmission of information on a communication terminal at the time of a handover and transmission of user data are performed via the X2 interface. Further, in  FIG. 2  and  FIG. 3 , control table example of parameters which the communication terminal  103  has is shown.  FIG. 2  shows a control table example which has stored the value of parameters for each parameter and the latest Value Tag of a Value Tag which is a time stamp.  FIG. 3  shows a control table example which has stored history information for the changed part (difference) of a parameter of notification information for each Value Tag. 
     At below, it is described using an example of which the communication terminal  103  located in the cell in the movement source base station A 101  moves to a cell in the movement destination base station B 102  and a handover is carried out as having the movement destination base station B 102  as a movement destination base station. 
     The movement source base station A 101  which determined to start a handover using a transmission line  104  to the movement destination base station B 102  transmits to the movement destination base station B 102 , a part of notification information  201  included in “Handover Preparation Information (handover preparation information)” in X2-AP: HANDOVER REQUEST which is handover request information specified in the non-patent document 3. To the part of notification information  201 , parameter information necessary for the communication terminal  103  to carry out a handover is included. Further, a Value Tag  202  which is a time stamp is included in parameters of the part of notification information  201  (Value Tag is given for each parameter). The Value Tag  202  hereof is a Value Tag corresponding to notification information being notified at the time when the movement source base station A 102  determined to start a handover. 
     The movement destination base station B 102  compares information on the part of notification information  201  and notification information which should be notified to the communication terminal  103  in order to make the communication terminal  103  to continue communicating in a movement destination cell. The movement destination base station B 102 , as a result of comparing, notifies the communication terminal  103  of the different part as a difference information  203  to a part of notification information, included in RRC: RRCConnection Reconfiguration (connection re-setting information) in X2-AP:HANDOVER REQUEST ACKNOWLEDGE which is handover request approval information specified in the non-patent document 3. At the same time, the movement destination base station B 102  notifies the communication terminal  103  of the Value Tag  202  received from the movement source base station A 101 . Further, while in  FIG. 1 , the difference information  203  is described as to be notified to the communication terminal  103  via the movement source base station A 101 , it may be directly notified via a radio link. 
     The communication terminal  103  always acquires a notification information  205  notified from the movement source base station A 101  and a Value Tag  206  included in the notification information. The communication terminal  103 , whenever receiving the notification information  205  corresponding to a different Value Tag, controls the changed part of the notification information as history information.  FIG. 2  shows a list of parameters which a change has occurred in the past in the notification information  205  received from the movement source base station A 101 , and a control table storing the latest Value Tag among the Value Tags when a change in the parameter occurs. Further, to the control table of  FIG. 2 , notification information on the latest Value Tag  206  (as of Value Tag=N) is also held. 
       FIG. 3  is a control table storing a changed part of a parameter, for which a change has occurred in the past in the notification information  205  received from the movement source base station A 101 , for each Value Tag. Further, the changed part is indicating a changed part from a value corresponding to a Value Tag received previously from a Value Tag which a change has occurred. For example, when the value received in Value Tag=N- 4  is p, and the value held before receiving Value Tag=N- 4  is q, the changed part in N- 4  is p-q. 
     The communication terminal  103  compares the Value Tag  202  received via the movement destination base station B 102  and the latest Value Tag  206  received directly from the movement source base station A 101 , and when there is a difference in the received Value Tag  202  and the Value Tag  206 , executes the following process. 
     (1) Selects whether a handover is canceled or carried out. 
     (2) When the cancel is selected, the movement source base station A 101  is notified of the fact of cancellation by a processing cancellation notice  207 . The communication terminal  103 , by canceling a handover, can prevent an occurrence of a failure of a handover due to a parameter being different. In a relatively simple system by canceling handover processing at this stage, there is a merit to prevent a malfunction of a system. In this case, handover operation may be repeated from the beginning until the above-mentioned comparison result matches. Further, when wanting to continue a handover from this state, the communication terminal  103 , in order to match parameters, executes processing of (3) or later. 
     (3) A parameter which was not received from the movement destination base station B 102  and a parameter for which a change has occurred, in other words among the Value Tags stored in the control table ( FIG. 2 ) of the latest Value Tag, a parameter of the last received Value Tag  202  or later from the movement destination base station B 102 , are extracted. For example, when the Value Tag  202  last received from the movement destination base station B 102  is N- 2 , parameters A, B, and C with any changed parts are relevant. 
     (4) At the communication terminal  103 , to the parameters extracted at (3), for each parameter, the changed part (the changed part shown in  FIG. 3 ) occurred by the time the movement destination base station B 102  received the Value Tag  202  is added to the notification information  205  ( FIG. 2 ) held at the time (as of Value Tag=N: as of the latest) when the communication terminal  103  received the difference information  203  of notification information. By this calculation, the value of the notification information  205  transmitted by the movement source base station A 101  at the time when receiving the Value Tag  202 , is calculated. As an example, the actual value of parameter A as of Value Tag=N- 2 , is calculated as follows. That is, to the actual value (V) of the notification information  205  ( FIG. 2 ) held at the time of receiving the difference information  203  of notification information (Value Tag=N), changed part AN- 2  and changed part AN- 1  to Value Tag=N- 2  are added. To a part with no changed part, 0 is added. 
     (5) As difference information to a part of the notification information, to a parameter for which the difference information  203  have been received from the movement destination base station B 102 , a parameter calculated at (4) is set. 
     (6) A parameter which is not relevant to (3), that is, to a parameter for which a change has not occurred, a parameter which is already held at the terminal is set. 
     By performing the above-mentioned procedures, the communication terminal  103 , when the notification information is updated after the movement source base station transmitted a part of information on the notification information to the movement destination base station, can select whether to cancel or to carry out a handover in the terminal. The communication terminal  103  operates as follows when a handover is carried out. That is, the difference that occurs by the difference information to a part of the notification information notified to the terminal from the movement destination base station and the notification information acquired after updating from the movement source base station is eliminated and set. As a result, it is possible to prevent a failure of a handover. 
     In  FIG. 4  and  FIG. 5 , a setup steps of the above-mentioned parameter is shown as a flowchart.  FIG. 4  indicates an example in which the notification information notified from a movement source base station A and a Value Tag of parameter information in the notification information notified from a movement destination base station are compared, and a handover is canceled when they are different. Steps S 11 -S 14  indicate process until just before the above-mentioned (1) which compares a movement source base station and a part of Value Tag of the notification information received from a movement destination base station. S 15  indicates processing of the above-mentioned (1) which judges whether to cancel or to carry out a handover. S 16  indicates the above-mentioned processing of (2) which cancels a handover when a comparison result does not match.  FIG. 5  corresponds to S 17  of  FIG. 4 , and indicates operation of executing a handover of (3)-(6) by setting a parameter by adding a difference of a parameter when the Value Tags do not match. 
     As has been described above, according to the present embodiment, when the notification information is updated after a movement source base station transmitted parameter information necessary for a handover to a movement destination base station by a part of notification information, whether to cancel or to carry out a handover can be selected in a terminal. Further, when a handover is carried out, the difference that occurs by the difference information to a part of the notification information notified to a communication terminal from a movement destination base station and the notification information after updating acquired from a movement source base station is eliminated and set. As a result, it will be possible to prevent a failure of a handover. 
     While the above-mentioned description has been described by an example of giving a Value Tag for each parameter and transmitting, it is equivalent to give one Value Tag corresponding to a parameter group and to transmit. In this case, when a parameter is transmitted from a movement source base station to a movement destination base station, and to a communication terminal, notification information is transmitted by giving one Value Tag for each transmission opportunity of a parameter. In a communication terminal, in the control table shown in  FIG. 2 , by controlling a Value Tag for which a change has occurred for each parameter, by performing addition or subtraction processing on a parameter for which a change has occurred, the latest parameter value may be calculated. Processing can be simplified by performing comparison processing of a plurality of parameters by the group unit. 
     (A Second Embodiment) 
     Next, a second embodiment of the present invention will be described with reference to  FIG. 6 . 
     In  FIG. 6 , a composition of a mobile communications system of the second embodiment is shown. 
     In  FIG. 6 , a movement source base station  601  has a transmission means  604  for giving a time stamp to a parameter necessary for a communication terminal  603  to move to another cell and transmitting the parameter to a movement destination base station  602  from the the movement source base station  601 . The above-mentioned movement destination base station  602  has a transmission means  605  for transmitting difference information with the movement source base station  601  of the above-mentioned parameter to the above-mentioned communication terminal  603  together with the above-mentioned time stamp as notification information. The above-mentioned communication terminal  603  compares the time stamp of difference information acquired from the above-mentioned movement destination base station  602  and the time stamp of notification information acquired from the movement source base station  601 . The communication terminal  603  includes a selecting means  606 , which selects whether or not to perform a handover when the comparison indicates that time stamps do not match. 
     According to the second embodiment, when notification information is updated after the movement source base station  601  transmits a part of information of notification information to the movement destination base station  602 , whether to cancel or to carry out a handover can be selected at the mobile communication terminal  603 . As a result, it will be possible to prevent a failure of a handover by performing a handover in the state that parameters do not match. 
     (A Third Embodiment) 
     A third embodiment of the present invention is a handover control method of a mobile communications system. In the handover control method hereof a time stamp for each parameter necessary for a mobile communication terminal to move to another cell is given, or one time stamp corresponding to a parameter group is given, and the parameter or the parameter group is transmitted to a movement destination base station from a movement source base station. Afterwards, the movement destination base station transmits difference information with the movement source base station of a parameter to a mobile communication terminal together with a time stamp as notification information. The mobile communication terminal compares the time stamp of difference information acquired from the movement destination base station and the time stamp of notification information acquired from the movement source base station, and selects whether or not to perform a handover when the time stamps do not match. 
     (A Fourth Embodiment) 
     A fourth embodiment of the present invention is related to a mobile communications system. The mobile communications system has a base station and a mobile communication terminal, and the mobile communication terminal includes a means to move to another cell which a base station composes and executes a handover. A movement source base station gives a time stamp to each parameter necessary for a mobile communication terminal to move to another cell or gives one time stamp corresponding to a parameter group. The movement source base station has a means to transmit the given time stamp to a movement destination base station from the movement source base station. The movement destination base station has a means to transmit difference information with the movement source base station of a parameter to the mobile communication terminal together with a time stamp as notification information. The mobile communication terminal compares the time stamp of difference information acquired from the movement destination base station and the time stamp of notification information acquired from the movement source base station. The mobile communication terminal has a means to select whether or not to perform a handover when a compared time stamps do not match. 
     (A Fifth Embodiment) 
     A fifth embodiment of the present invention relates to a mobile communication terminal and includes a base station and a mobile communication terminal, wherein the mobile communication terminal has a means to execute a handover by moving to another cell which the base station composes. The movement source base station has a means to give a time stamp to each parameter necessary for the mobile communication terminal to move to another cell or to give one time stamp corresponding to a parameter group and to transmit the parameter or the parameter group to the movement destination base station from the movement source base station. The movement destination base station has a means to transmit difference information with the movement source base station of the above-mentioned parameter to the above-mentioned mobile communication terminal together with a time stamp as notification information. The mobile communication terminal is a mobile communication terminal applied to a mobile communications system equipped with the above-mentioned means. The mobile communication terminal further includes a means to compare a time stamp of difference information acquired from the movement destination base station and a time stamp of notification information acquired from the movement source base station, and to select whether or not to perform a handover when the time stamps do not match. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
     Further, the function of the communication terminal carrying out the above-mentioned handover can be realized by installing a program to an information processing device. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2009-108007, filed on Apr. 27, 2009, the disclosure of which is incorporated herein in its entirety by reference. 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to a handover control method, a mobile communications system, and a mobile communication terminal and has the applicability on the industry. 
     Description of the Codes 
       101  movement source base station A 
       102  movement destination base station B 
       103  communication terminal 
       104  transmission line 
       201  a part of notification information 
       205  notification information 
       202  and  206  Value Tag 
       203  difference information 
       207  handover processing cancellation notice