Abstract:
A radio communication method for performing radio communication between a base station and a terminal, including the steps of: comparing field strength of the base station which transmitted a reconnection instruction and filed strength of an adjacent base station which locates adjacent to the base station, when the terminal in a power saving standby mode receives the reconnection instruction from the base station, and controlling whether the terminal reconnects or not to the base station which transmitted the reconnection instruction, according to the result of the comparison.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-110897, filed on Apr. 22, 2008, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The present invention relates to a radio communication method and a terminal device which performs radio communication. 
       BACKGROUND 
       [0003]    WiMAX, which is one radio communication system, defines the hand over procedure for switching the connection of a mobile station (MS) from a serving Base Station (SBS) to an adjacent base station (BS) having higher field strength (e.g. see IEEE 802.16e-2005). 
         [0004]    Once hand over procedure occurs, the mobile station transmits and receives control messages to and from a newly connected destination base station, even if user data is not transmitted and received to and from the base station. An air band is used redundantly by the transmission and reception of the control message. 
         [0005]    To avoid this, WiMAX defines a connection state called “idle mode” (see IEEE 802.16e-2005). In the idle mode, the mobile station enters the power saving standby state, and need not transmit and receive the control message for hand over, even if the base station is switched. Thereby an air band resource can be saved. 
         [0006]      FIG. 9  shows a sequence example from entering idle mode until the mobile station starts the reconnection operation with a new base station. 
         [0007]    The mobile station MS exchanges a control message (DREG-REQ, DREG-CMD) with a base station BS# 1 , and enters idle mode (S 101 , S 102 ). DREG-CMD specifies the length of a listening interval (Paging Listening Interval) and an unavailable interval (Paging Unavailable Interval). 
         [0008]    In the idle mode, these two intervals are alternated repeatedly. The mobile station MS does not communicate with the base station BS# 1  during the unavailable interval, and receives a control message (Page Advertisement (MOB_PAG-ADV) message) in the listening interval (S 102 ). This control message specifies an action code (e.g. reconnection instruction to network) from the base station BS to the mobile station MS. 
         [0009]    When radio waves of the base station BS# 1  becomes weak, the mobile station MS switches to the base station BS# 2  (S 103 ). At this time, the mobile station MS, which is in the idle mode, does not exchange the control message for hand over with the new base station BS# 2 . The mobile station MS simply receives the MOB_PAG-ADV message from the base station BS# 2  in the listening interval (S 102 , S 104 ). 
         [0010]    Then the mobile station MS receives the MOB_PAG-ADV message, including an action code “Enter Network” (code to prompt reconnection of mobile station MS since the base station BS# 2  must send user data to the mobile station MS) from the base station BS# 2  (S 105 ). 
         [0011]    When this message is received, the mobile station MS executes the reconnection operation (Re-entry) to the base station BS# 2  (S 106 ). Thereby the mobile station MS clears the idle mode so that user data from the base station BS# 2  can be received. 
         [0012]    The above mentioned example concerns the downlink direction. In the case of an uplink direction, the mobile station MS itself executes the reconnection operation when the user data must be transmitted (S 106 ). The mobile station MS executes the reconnection operation by itself without receiving the MOB_PAG-ADV message, including the action code “Enter Network”. 
         [0013]    However when the mobile station MS positions near the cell boundary of the base station BS# 2 , the mobile station MS must execute the reconnection operation again for hand over with an adjacent base station immediately after exiting the idle mode and reconnecting with the base station BS# 2 . 
         [0014]    For example, as  FIG. 9  shows, after the reconnection operation with the base station BS# 2  (S 106 ), the mobile station MS detects a decline of field strength and executes the hand over procedure with the base station BS# 2  (S 108  to S 111 ), and executes the reconnection operation with the base station BS# 3  (S 112 ). 
         [0015]    As a result, the mobile station MS ends up with executing the reconnection operation with the two base stations BS# 2  and BS# 3  (S 106 , S 112 ), therefore the control message is transmitted and received redundantly, and the air band resource is used redundantly. 
         [0016]    In case user data occurs in the uplink direction as well, the mobile station MS executes the reconnection operation with the two base stations BS# 2  and BS# 3 . Therefore the air (radio) band resource is used redundantly in the same way. 
       SUMMARY 
       [0017]    With the foregoing in view, it is an object of the present invention to provide a radio communication method, terminal device and radio communication system in which waste of the air band is reduced. 
         [0018]    To achieve the above object, an embodiment of the present invention provides a radio communication method for performing radio communication between a base station and a terminal, including the steps of: comparing field strength of the base station which transmitted a reconnection instruction and field strength of adjacent base stations which locate adjacent to the base station, when the terminal in power saving standby mode receives the reconnection instruction from the base station, and controlling whether terminal reconnects or not to the base station which transmitted the reconnection instruction, according to the result of the comparison. 
         [0019]    To achieve the above object, another embodiment of the present invention provides a radio communication method for performing radio communication between a base station and a terminal, including the steps of: comparing field strength of the base station in which the terminal registers its position in the last time, and field strength of adjacent base stations which locate adjacent to the base station, when the terminal in a power saving standby state quits the state to transmit user data, and controlling whether the terminal reconnects or not to the base station in which the terminal registers its position in the last time, according to the result of the comparison. 
         [0020]    To achieve the above objects, another embodiment of the present invention provides a terminal for performing radio communication with a base station, including: a judgment unit which compares field strength of the base station which transmitted a reconnection instruction and field strength of adjacent base stations which locate adjacent to the base station, when terminal in a power saving standby state receives the reconnection instruction from the base station; and a transmission and reception processing unit which decides whether terminal reconnects or not to the base station which transmitted the reconnection instruction, according to the result of the comparison. 
         [0021]    To achieve the above object, another embodiment of the present invention provides a terminal for performing radio communication with a base station, including: a judgment unit which compares field strength of the base station in which the terminal registers its position the last time and field strength of adjacent base stations which locate adjacent to the base station, when the terminal in a power saving standby state quits the state to transmit user data; and a transmission and reception unit which switches whether the terminal reconnects or not to the base station in which the terminal registers its position in the last time, according to the result of the comparison. 
         [0022]    According to the present invention, a radio communication method, terminal device and radio communication system in which use of the air (radio) band resource is reduced, can be provided. 
         [0023]    Additional objects and advantages of the invention (embodiment) will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
         [0024]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  shows a diagram depicting a configuration example of a radio communication system; 
           [0026]      FIG. 2  shows a diagram depicting a configuration example of a base station; 
           [0027]      FIG. 3  shows a diagram depicting a configuration example of a mobile station; 
           [0028]      FIG. 4  shows a sequence diagram depicting an example of an idle mode transition phase; 
           [0029]      FIG. 5  shows a sequence diagram depicting an example of a network reconnection phase; 
           [0030]      FIG. 6  shows an example of an adjacent base station information table; 
           [0031]      FIG. 7  shows a sequence diagram depicting an example of a network reconnection phase; 
           [0032]      FIG. 8  shows a sequence diagram depicting an example of a network reconnection phase; and 
           [0033]      FIG. 9  shows a sequence diagram depicting an operation example of a conventional idle mode transition to reconnection. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    Embodiments of the present invention will now be described with reference to the drawings. 
         [0035]      FIG. 1  is a diagram depicting a network configuration example of a radio communication system  1 . The radio communication system  1  has base station devices (hereafter “base stations”) (BS# 1  to BS# 3 )  10 - 1  to  10 - 3 , a terminal device (hereafter “mobile station”) (MS)  20  and a host device  50 . 
         [0036]      FIG. 1  shows a state where the mobile station  20  moves from the base station  10 - 1  to the base station  10 - 3  via the base station  10 - 2 . A circle around each base station  10 - 1  to  10 - 3  indicates a radio range (cell range) from each base station  10 - 1  to  10 - 3 . 
         [0037]    The host device  50  is a gateway (ASN-GW), for example. The host device  50  has a paging controller  51 , and the paging controller  51  holds an MS context (connection parameters) of the mobile station  20 . 
         [0038]      FIG. 2  is a diagram depicting a configuration example of the base station  10 - 1  to  10 - 3 , and  FIG. 3  is a diagram depicting a configuration example of the mobile terminal  20 . Each base station  10 - 1  to  10 - 3  has an identical configuration, so it is described as base station  10  unless otherwise specified. 
         [0039]    The base station  10  has an on-the-air transmission and reception processing unit  11 , a network entry processing unit  12 , a hand over processing unit  13 , a back bone transmission and reception unit  14  and an idle mode processing unit  15 . 
         [0040]    The on-the-air transmission and reception unit  11  converts various messages from each processing unit  12  into radio waves, transmits them to the mobile station  20 , extracts various messages from the radio waves received from the mobile station  20 , and outputs them to each processing unit  12 . 
         [0041]    The network entry processing unit  12  processes the registration procedure protocol for the mobile station  20  to connect with the base station  10 . Specifically, the network entry processing unit  12  generates a control message and transmits to the mobile station  20  via the on-the-air transmission and reception processing unit  11 , and receives the control message from the mobile station  20  via the on-the-air transmission and reception processing unit  11 . When a new mobile station  20  is registered based on the registration procedure protocol, the network entry processing unit  12  notifies the registration of the mobile terminal  20  to the hand over processing unit  13 . At this time, the network entry processing unit  12  outputs the MS context (connection parameter) acquired from the mobile station  20  by the registration procedure to the hand over processing unit  13 . 
         [0042]    The hand over processing unit  13  executes the hand over procedure when the mobile station  20  executes hand over. The hand over processing unit  13  also holds the MS context and transmits the MS context to the base station at the hand over destination via the back bone transmission and reception unit  14 . When the transition of mobile station  20  to idle mode is notified by the idle mode processing unit  15 , the hand over processing unit  13  requests temporary buffering of the MS context to the host device  50 . When the registration of the mobile station  20  is notified from the network entry processing unit  12 , if the base station  10  is a base station at the hand over destination (Target BS), the hand over processing unit  13  requests the MS context of the mobile terminal  20  from the hand over source base station or host device  50  via the back bone transmission and reception unit  14 , and holds the MS context. 
         [0043]    The back bone transmission and reception unit  14  is connected with another base station or the host device  50  via the back bone network, transmits, and receives the MS context and various messages. 
         [0044]    The idle mode processing unit  15  performs protocol processing in the idle mode. For example, when DREG-REQ (idle mode transition request message) is received from the mobile station  20  via the on-the-air transmission and reception processing unit  11 , the idle mode processing unit  15  generates various messages (e.g. DREG-CMD) in the idle mode, and transmits them to the mobile station  20  via the on-the-air transmission and reception processing unit  11 . Also when the hand over processing unit  13  receives user data addressed to the mobile station  20 , for example, the idle mode processing unit  15  transmits a network reconnection instruction (MOB_PAG-ADV, including action code “Enter Network”), to the mobile station  20 . 
         [0045]      FIG. 3  shows a diagram depicting a configuration example of the mobile station  20 . The mobile station  20  has an on-the-air message transmission and reception processing unit (hereafter “message transmission and reception unit”)  21 , an adjacent information reception unit  22 , neighbor BS table (hereafter “adjacent table”)  23 , a field strength calculation unit  24 , time variation calculation unit  25 , network connection processing unit  26 , idle protocol processing unit  27 , BS switching processing unit  28 , and judgment unit  29 . 
         [0046]    The message transmission and reception unit  21  transmits and receives various messages synchronizing with a frame from the connected base station  10 , and measures the field intensity of a synchronized frame. 
         [0047]    The adjacent information reception unit  22  receives an adjacent base station information message from the communicating base station (serving BS)  10  via the message transmission and reception unit  21 , and stores the information included in this message in the adjacent table  23 . 
         [0048]    The adjacent table  23  stores information on the adjacent base stations (e.g. BSID). The adjacent table  23  also stores the time variations of field strength (CINR (Carrier to Interference plus Noise Ratio) in the case of the present embodiment) for each adjacent base station. 
         [0049]    The field strength calculation unit  24  calculates a value of field strength, by processing the field strength, based on the field strength measured by the message transmission and reception unit  21 . 
         [0050]    The time variation calculation unit  25  calculates the average of the field strength and time variation (differential value) which indicates how much the field strength changed over time, based on the field strength from the field strength calculation unit  24 , and stores this in the adjacent table  23 . Details on calculation will be described later. 
         [0051]    The network connection processing unit  26  processes the protocol message for the mobile station  20  to connect to the network. The network connection processing unit  26  transmits and receives the protocol message to and from the base station  10  via the message transmission and reception unit  21 . 
         [0052]    The idle protocol processing unit  27  processes a protocol message required for the idle mode. The idle protocol processing unit  27  generates an idle mode transition request message (e.g. DREG-REQ), and transmits it to the base station  10  via the message transmission and reception unit  21 , and processes a message on the idle mode (e.g. DREG-CMD, MOB_PAG-ADV) received from the base station  10  via the message transmission and reception unit  21 . 
         [0053]    The BS switching processing unit  28  performs processing to switch the connection of the base station  10 , generates a message for switching, and transmits and receives the message to and from the base station  10  via the message transmission and reception unit  21 . 
         [0054]    The judgment unit  29  judges a timing to exit the idle mode based on the time variation of the field strength from the adjacent table  23  and information from the idle protocol processing unit  27 , and instructs the network connection processing unit  26  and BS switching processing unit  28  to switch the network connection and the base station  10 . Details will be described later. 
         [0055]    Now the operation will be described. The general operation consists of two phases: an idle mode transition phase ( FIG. 4 ), and a network reconnection phase ( FIG. 5  or  FIG. 7 ). The network reconnection phase has a case of the mobile station  20  being distant from the cell boundary ( FIG. 5 ), and a case of the mobile station  20  being close to the cell boundary ( FIG. 7 ). 
         [0056]    The present operation example is an example when the mobile station  20 , which is connected with the base station  10 - 1 , transits to the idle mode, and moves from the base station  10 - 2  to the base station  10 - 3 , as shown in  FIG. 1 . 
         [0057]    When the mobile station  20  transits to the idle mode, the base station  10 - 1  saves the MS context to the host device  50 , and deletes the network registration state. Hence the mobile station  20  enters a state of not being connected with the network. However, even in the idle mode, the mobile station  20  must allow the base station  10 - 1  to know the position, so that the message (MOB_PAG-ADV) can be received. For this, according to the present embodiment, the base station  10 - 1  is called the “Preferred BS” (hereafter “PBS”) in the idle mode, so as to be distinguished from the serving BS. In the example in  FIG. 1 , after shifting to the idle mode, the base station  10 - 1  becomes PBS first, then after the mobile station  20  moves into the cell range of the base station  10 - 2 , the base station  10 - 2  becomes the PBS. The PBS  10 - 1  and PBS  10 - 2  accept the position registration of the mobile station  20  when the mobile station  20  moves in the respective cell range. 
         [0058]    Now the idle mode transition phase ( FIG. 4 ) will be described. When the transition to the idle mode is determined, the mobile station  20  requests the transition to the idle mode to the connected base station  10 - 1  (S 1 ). For example, the idle protocol processing unit  27  generates DREG-REQ based on the decision to transit to the idle mode, and transmits it to the base station  10 - 1  via the message transmission and reception unit  21 . 
         [0059]    When DREG-REQ is received, the base station  10 - 1  saves the MS context of the mobile station  20  in the host device  50 , and transmits the response message DREG-CMD to the mobile station  20  (S 101 ). For example, when DREG-REQ is received from the mobile station  20  via the on-the-air transmission and reception processing unit  11 , the idle mode processing unit  15  notifies this reception to the hand over processing unit  13 . The hand over processing unit  13  transmits the internally stored MS context of the mobile station  20  to the host device  50  via the back bone transmission and reception unit  14 . When DREG-REQ is received, the idle mode processing unit  15  generates DREG-CMD, and transmits it to the mobile terminal  20  via the on-the-air transmission and reception processing unit  11 . 
         [0060]    Then the mobile terminal  20  receives DREG-CMD, and transits to the idle mode (power saving standby state). The mobile terminal  20  reads the listening interval (Paging Listening Interval) included in this message, calculates the unavailable interval (Paging Unavailable Interval), and repeats these intervals alternately (S 2 , S 3 ). For example, when this message is received, the idle protocol processing unit  27  notifies each interval to the message transmission and reception unit  21 . The message transmission and reception unit  21  receives nothing in the unavailable interval, and receives MOB_PAG-ADV from the base station  10 - 1  in the listening interval (S 4 ). In the processing in S 4 , it is assumed that this message includes an action code “No Action Required” (mobile station  20  need do nothing). 
         [0061]    The base station  10 - 1 , on the other hand, generates and transmits MOB_PAG-ADV in the listening interval (S 102 ). For example, the idle mode processing unit  15  generates this message, and transmits it to the mobile station  20  via the on-the-air transmission and reception processing unit  11 . Each interval is stored in the memory of the idle mode processing unit  15 , for example, in advance. 
         [0062]    The mobile station  20  moves to the cell boundary of the base station  10 - 1 , and switches the preferred BS to the base station  10 - 2  (S 5 ). Since the base station  10 - 2  is in a same paging group as the base station  10 - 1 , the unavailable interval and listening interval are repeated at a same timing as the base station  10 - 1 . 
         [0063]    Then the mobile station  20  moves to the cell range of the base station  10 - 2 , the base station  10 - 2  transmits MOB_PAG-ADV in the listening interval (S 201 ), and the mobile station  20  receives this message in the listening interval (S 6 ). 
         [0064]    The base station  10 - 2  detects user data addressed to the mobile station  20  (S 201 ). For example, when the user data is received via the back bone transmission and reception unit  14 , the hand over processing unit  13  notifies this reception to the idle mode processing unit  15 , whereby the idle mode processing unit  15  detects the user data. 
         [0065]    Then the base station  10 - 2  generates MOB_PAG-ADV in the listening interval, and transmits it to the mobile station  20  (S 203 ). At this time, MOB_PAG-ADV includes the action code “Enter Network” (reconnection instruction to the network). For example, the idle mode processing unit  15  generates MOB_PAG-ADV including this action code, and transmits it to the mobile station  20  via the on-the-air transmission and reception processing unit  11 . 
         [0066]    Then the mobile station  20  receives this MOB_PAG-ADV (S 7 ). For example, the idle protocol processing unit  27  receives this message from the base station  10 - 2  via the message transmission and reception unit  21 . 
         [0067]    Then the processing shifts to the network reconnection phase ( FIG. 5 ), and the mobile station  20  measures the field strength of the adjacent base stations  10 - 1  and  10 - 3  in the unavailable interval (S 8 ). The mobile station  20  also measures the field strength of the base station (PBS)  10 - 2  which has transmitted the reconnection instruction (MOB_PAG-ADV, including the action code “Enter Network”). 
         [0068]    For example, the idle protocol processing unit  27  notifies the judgment unit  29  that “MOB_PAG-ADV, including “Enter Network”, was received, and the judgment unit  29  inquires the field strength calculation unit  24  about the measurement of the field strength between the adjacent base stations  10 - 1  and  10 - 3  and the base station (PBS)  10 - 2 . At this time, the judgment unit  29  also inquires the time variation calculation unit  25  about the time variation of the field strength of the adjacent base stations  10 - 1  and  10 - 3  and the base station (PBS)  10 - 2 . 
         [0069]    Responding to the inquiry about field strength measurement, the field strength calculation unit  24  instructs the message transmission and reception unit  21  to measure CINR (field strength) of the adjacent base stations  10 - 1  and  10 - 3  and the base station (PBS)  10 - 2 , and calculates each measured CINR. 
         [0070]    Responding to the inquiry about time variation, the time variation calculation unit  25  calculates time variation based on each CINR calculated by the field strength calculation unit  24 . The calculated time variation is stored in the adjacent table  23 . 
         [0071]      FIG. 6  shows an example of the adjacent table  23 . The adjacent table  23  includes each field of BSID, CINR Average, and ΔCINR. ΔCINR indicates the above mentioned time variation of the field strength. 
         [0072]    BSID is an identification code of the adjacent base station, and in the example in  FIG. 6 , the IDs of the base stations  10 - 1  and  10 - 3  (BS# 1 , BS# 3 ) are stored. BSID is included in adjacent base station information (Neighbor Advertisement message) which is periodically transmitted from the base stations  10 - 1  and  10 - 3 , for example, and the adjacent information reception unit  22  receives this information and stores it in the adjacent table  23 . 
         [0073]    CINR Average is a field for storing the average value of the previously measured CINR of the adjacent base station. The time variation calculation unit  25  calculates the average value of CINR according to the following expression. 
         [0000]        μ   CINR   [k ]=(1−α avg )  μ   CINR   [k− 1]+α avg CINR[ k]   [Expression 1] 
         [0074]    Here μCINR [k] is an average value of CINR of adjacent base station measured this time (time k), μCINR [k−1] is an average value of CINR of adjacent base station based the last time (time k−1), CINR [k] is a value of CINR of adjacent base station measured this time, and αavg is a constant (average parameter). The time variation calculation unit  25  determines the average value of CINR by adding the average value of CINR measured the previous time and CINR measured this time. The time variation calculation unit  25  stores the average value of CINR (μCINR [k]) in the adjacent base stations  10 - 1  and  10 - 3 , calculated by Expression 1, in the CINR Average field of the adjacent table  23  to implement updating. 
         [0075]    μCINR [k−1] is stored in the adjacent table  23 , and CINR [k] is calculated by the field strength calculation unit  24 , and αavg is included in a message, such as a DCD message, from the base station  10 - 2  (PBS). αavg may be stored in the time variation calculation unit  25  in advance as a parameter. The time variation calculation unit  25  calculates Expression (1) by reading these values from the adjacent table  23 . 
         [0076]    ΔCINR in the adjacent table  23  is a field to store a time variation (differential value) of CINR. The time variation calculation unit  25  calculates the time variation according to the following expression. 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0077]    Here ΔCINR [k] is a time variation of CINR, ΔCINR [k−1] is a time variation of CINR measured the last time (time k−1), μCINR [k] is an average value of CINR calculated with Expression (1) this time (time k), μCINR [k−1] is an average value of CINR calculated the last time, and Δt is an interval of measurement points. Here ΔCINR [k−1] and μCINR [k−1] are stored in the adjacent table  23 , and Δt is a value which the time variation calculation unit  25  holds in advance. The time variation calculation unit  25  calculates Expression (2) by reading these values from the adjacent table  23 , and stores the calculated time variation (ΔCINR [k]) in the ΔCINR field of the adjacent table  23  to update the value. 
         [0078]    In  FIG. 5 , the mobile terminal  20  compares the expected field strengths of the adjacent base stations  10 - 1  and  10 - 3 , and the field strength of the base station (PBS)  10 - 2  which has transmitted the reconnection instruction (MOB_PAG-ADV including “Enter Network”), and judges whether the expected field strengths of the base stations  10 - 1  and  10 - 3  are higher than the field strength of the base station (PBS)  10 - 2  which has transmitted the reconnection instruction (S 9 ). The judgment unit  29  calculates the expected field strengths of the adjacent base station  10 - 1  and  10 - 3  using the following expression. 
         [0000]      CINR NEI     —     PRE =ΔCINR×T NEXT +CINR Neighbor   [Expression 3] 
         [0079]    Here CINR NEI-PRE  is an expected field strength of an adjacent base station, ΔCINR is a time variation of CINR stored in the adjacent table  23 , and CINR Neighbor  is an average value of CINR of the adjacent base station at the current time (time k) (μCINR [k] of Expression 1) stored in the adjacent table  23 . T NEXT  is a time until the start of the next paging interval (unavailable interval or listening interval). 
         [0080]    The judgment unit  29  determines the change of CINR which is expected until the next paging interval start time by multiplying the time variation ΔCINR of CINR by T NEXT , adds the average value of CINR of the adjacent base station (CINR Neighbor ) at the current time, and determines the field strengths (CINR NEI-PRE ) of the adjacent base stations  10 - 1  and  10 - 3  which are expected at the next paging interval start time. 
         [0081]    The judgment unit  29  compares this expected field strength and the field strength of the base station (PBS)  10 - 2  calculated by the field strength calculation unit  24 , and judges YES in S 9  if the expected field strength is greater than the field strength of the base station (PBS), and NO if not. 
         [0082]    If S 9  is NO, that is in case that the field strength of the base station (PBS)  10 - 2  is greater, the mobile station  20  starts reconnection to the base station (PBS)  10 - 2  according to normal procedure (S 10 ). For example, if it is judged as NO in S 9 , the judgment unit  29  instructs the network connection processing unit  26  to reconnect to the base station (PBS)  10 - 2 . The network connection processing unit  26  transmits and receives the protocol message for reconnection to and from the base station (PBS)  10 - 2  so that connection is reestablished (S 11 ). 
         [0083]    The base station  10 - 2  requests the host device  50  to acquire the MS context of the mobile station  20 , and receives this context (S 204 ). The base station  10 - 2  transmits and receives a message based on this context, and transmits the user data to the mobile station  20  (S 205 ). 
         [0084]    If S 9  is YES, on the other hand, that is in case field strength of the adjacent base station  10 - 3  is greater than the field strength of the base station (PBS)  10 - 2 , the mobile station  20  executes the reconnection operation synchronizing with the adjacent base station  10 - 3 , without performing the reconnection operation to the base station (PBS)  10 - 2  (S 11  to S 12  in  FIG. 7 ). 
         [0085]    The base station  10 - 3  requests the MS context of the mobile station  20  to the host device  50 , and receives it (S 301 ). Based on the context, the base station  10 - 3  transmits and receives the control message for reconnection to and from the mobile terminal  20 , acquires user data from the base station  10 - 2 , and sends it to the mobile station  20  (S 302 ). 
         [0086]    In this way, when the field strength of the adjacent base station  10 - 3  is expected to be higher than the field strength of the base station (PBS)  10 - 2  from which the reconnection instruction is received (YES in S 9 ), the mobile station  20  executes the reconnection operation to the adjacent base station  10 - 3  without performing the reconnection operation to the base station  10 - 2  (S 12  to S 13 ). Since the mobile station  20  need not transmit and receive the message for reconnection to and from the base station  10 - 2 , the air band resource can be saved accordingly. 
         [0087]    The above mentioned example is an example of data occurrence for downlink direction. The same operation can also be performed for uplink direction.  FIG. 8  is a sequence diagram depicting an example of the reconnection phase in the uplink direction. The idle mode transition phase ( FIG. 4 ) is the same as the above mentioned example. 
         [0088]    When the mobile terminal  20  tries to send user data after transiting to idle mode (S 20 ), the mobile terminal  20  measures the field strengths of the adjacent base stations  10 - 1  and  10 - 3 , and the base station (PBS)  10 - 2  (S 8 ). Since the mobile station  20  can exit the idle mode at any time, the field strength of the adjacent base stations  10 - 1  and  10 - 3  and the base station (PBS)  10 - 2  can be measured when the transmission of user data is desired (S 20 ), regardless the paging interval (unavailable interval or listening interval). The rest of the operation is the same as the above mentioned downlink direction. 
         [0089]    In the above mentioned example, the judgment unit  29  uses the field strength of the base station (PBS)  10 - 2 , which has transmitted the reconnection instruction, as the judgment reference (S 8 , S 9 ). The judgment unit  29  may use a threshold held internally, for example, as the judgment reference. In this case, the judgment unit  29  judges as YES if the field strength of the adjacent base station is higher than the threshold, and as NO if not in S 9 . 
         [0090]    The above example was described regarding CINR as the field strength (Expression 1 to Expression 3). Instead, CIR (Carrier to Interference Ratio), CNR (Carrier to Noise Ratio), SINR (Signal to Interference plus Noise Ratio), SIR (Signal to Interference Ratio), SNR (Signal to Noise Ratio), and round trip delay or RSSI (Receive Signal Strength Indicator) may be regarded as the field strength. In this case, an average value or differential value of SINR and so on is stored in the adjacent table  23 , and the second term of the right side of Expression 1 becomes the measurement value of SINR or the like. 
         [0091]    In the above example, each base station  10 - 1  to  10 - 3  was described as base stations belonging to a same paging group. However each base station  10 - 1  to  10 - 3  may belong to different paging groups. The only difference is that in this case each base station  10 - 1  to  10 - 3  has a different paging interval (unavailable interval and listening interval), and transmits MOB_PAG-ADV in different paging interval, and the rest is the same as the case of the base stations belonging to the same paging group. 
         [0092]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention(s) has(have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.