Abstract:
A handover method for controlling communication between a mobile station and base station(s) in a wireless communication system. The method includes measuring signal qualities transmitted from the base station(s), the measured signal qualities including a measured quality of a signal received from a certain base station; identifying the base station as an addition candidate based on the measured quality exceeding an addition threshold; thereafter detecting that the measured quality of the signal received from the addition candidate has further exceeded an active threshold which is greater than the addition threshold. The active threshold is determined in accordance with a quality of a signal transmitted from one or more active base stations. In response, the base stations are notified of information about the addition candidate; and a handover process is performed on the bases on this information.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional application of common assigned, co-pending U.S. patent application Ser. No. 11/555,174 filed Oct. 31, 2006, which patent application is incorporated herein by reference in its entirety. That application is a continuation application of commonly assigned, U.S. patent application Ser. No. 11/345,927 filed Feb. 2, 2006, and entitled “Mobile Communication System, Mobile Station and Diversity Handover Branch Control Method”, (now U.S. Pat. No. 7,289,809; issued on Oct. 30, 2007) which is incorporated herein by reference in its entirety. That application is a divisional application of commonly assigned, co-pending U.S. patent application Ser. No. 10/810,263, filed Mar. 26, 2004, and entitled “Mobile Communication System, Mobile Station and Diversity Handover Branch Control Method”, which is incorporated herein by reference in its entirety. That application is a divisional application of commonly assigned, U.S. patent application Ser. No. 10/384,314, filed Mar. 7, 2003, and entitled “Mobile Communication System, Mobile Station and Diversity Handover Branch Control Method”, (now U.S. Pat. No. 6,807,420; issued on Oct. 19, 2004) which is incorporated herein by reference in its entirety. That application is a divisional application of commonly assigned, U.S. patent application Ser. No. 09/230,956, filed Feb. 4, 1999 and entitled “Mobile Radio Communication System, Mobile Station, and Method for Controlling Diversity Hand-Over Branch”, (now U.S. Pat. No. 6,728,227; issued on Apr. 27, 2004) which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. The Field of the Invention  
         [0003]     The present invention relates to a branch candidate selecting method in a mobile station in the case where the mobile station detects a plurality of addition branch candidates and deletion branch candidates, and to a diversity handover branch control method on a network side in executing diversity handover.  
         [0004]     2. The Relevant Technology  
         [0005]     Recently, code division multiple access (CDMA) technology has been proposed as one of promising radio transmission methods for implementing multimedia communications in the next generation mobile communication network systems. A CDMA system carries out diversity handover (DHO) that establishes multiple communication links (branches) between a mobile station and a plurality of base stations while the mobile station is moving near a boundary of a radio area, and communicates by performing signal combining (selection combining) on the multiple communication links. In the diversity condition, it is possible to increase a radio capacity by controlling such that the mobile station and the base stations can communicate at minimum transmission power (transmission power control). Furthermore, it is possible in the diversity condition to eliminate instantaneous chopping during handover, which can occur in the conventional time division multiple access (TDMA).  
         [0006]     Establishing a new branch in the DHO is specifically referred to as “addition DHO” in the DHO, and deleting a communication branch not contributing to the communications in the diversity condition is specifically referred to as “deletion DHO” in the DHO. To carry out the addition DHO and deletion DHO, the mobile station normally detects a candidate of the addition DHO (called “addition DHO candidate” or “addition branch candidate”), and a candidate of the deletion DHO (called “deletion DHO candidate” or “deletion branch candidate”). Detecting an addition or deletion branch candidate by monitoring a radio condition (transmission loss, for example) of a perch in the current sector or peripheral sectors, the mobile station notifies the network side of the detected candidate. The network side performs the DHO (addition DHO or deletion DHO) based on the notified candidate. The term “network side” usually refers to a switching center or a control center of base stations, it is possible to provide this function to the base stations.  
         [0007]     The mobile station communicates with the base stations using individual radio links (radio branches) associated with the base stations. A switching center comprises cable links (cable branches) connecting it with the base stations that are communicating with the mobile station, and carries out, with a handover (HO) processor or a diversity handover trunk (DHT), the selection combining of the signals sent from the base stations. The fundamental operation of the diversity handover branch is disclosed in Japanese Patent Application Laid-open No. 9-508773 (1997), and network configurations and control methods of the diversity handover trunks are disclosed in Japanese Patent Application No. 8-348900 (1996).  
         [0008]     Because of hardware implementation or the like, there are provided an upper limit to the number of radio branches (a maximum radio branch number) that the mobile station can establish simultaneously, and an upper limit to the number of cable branches (a maximum cable branch number) that the DHT in the switching center can connect or process simultaneously. The upper limit to the number of communication branches (the maximum communication branch number) that can be established in the diversity condition equals the smaller one of the maximum numbers of the radio branches and cable branches. In ordinary systems, it is designed that the maximum radio branch number becomes equal to the maximum communication branch number to make effective use of radio resources by giving priority to them.  
         [0009]     However, conventional papers (for example, Shimizu, et al. “Handover equipment and control method in next generation mobile communication systems”, General assembly of the Institute of Electronics, Information and Communication Engineers of Japan, 1997) handle the subject only under the assumption that the control of branches to be added or deleted is limited to a single branch, and do not handle simultaneous control of a plurality of addition branches and deletion branches. It is not specified in ITU-T recommendation Q.FIF version 6, as well.  
         [0010]     As mentioned above, since the control of the branches to be added and deleted is carried out on one by one basis in the prior art, N times of control is required in principle to control N branches. However, the mobile station can detect a plurality of deletion branch candidates and addition branch candidates at the same time, because their detection depends on the ambient radio condition.  
         [0011]     In such a case, a number of times of similar control operations are repeated between the mobile station and the switching center, which is not only inefficient, but also takes an extra time for the control until the entire handover control is completed.  
         [0012]     Taking account of the addition DHO of a single branch, the prior art sets the maximum cable branch number at the maximum radio branch number plus one. Thus, the network side can prepare the branches in cable sections by the number greater than the maximum radio branch number by one. This enables a simple switching operation in the radio sections to complete the addition DHO by adding one cable branch from among the prepared cable branches without deleting the communication branch even in the case where one branch is added to the maximum number of branches.  
         [0013]     However, such a method that follows the prior art, in which the maximum cable branch number is set at the number greater than the maximum radio branch number by one, has a problem of impairing effective control because of the network side control repeated by the number of addition branch candidates. Furthermore, no prior art takes account of handling any addition branch candidates that have not been prepared in the cable section, even though they have higher DHO priority than the communication branches, because the control unit is for a single branch and the maximum cable branch number is limited.  
       BRIEF SUMMARY  
       [0014]     Therefore, the present invention is implemented to solve the foregoing problems. An object of the present invention is to provide a handover method for controlling a communication between a mobile station and at least one base station, in a wireless communication system including the mobile station and a plurality of base stations.  
         [0015]     In a first aspect, there is provided a handover method that includes an act of measuring qualities of signals transmitted from at least some of said plurality of base stations, the measured qualities of signals including a measured quality of a signal received from a certain base station; an act of identifying the base station as an addition candidate based on the measured quality of the signal received from the certain base station exceeding an addition threshold; after the act of identifying the base station as an addition candidate, an act of detecting that the measured quality of the signal received from the addition candidate has further exceeded an active threshold which is greater than the addition threshold, wherein the active threshold is determined in accordance with a quality of a signal transmitted from one or more active base stations; in response to the act of detecting, an act of notifying one or more of the plurality of base stations of information about the addition candidate; and an act of performing a handover process based on the information in the notification; wherein the notification includes the measured quality.  
         [0016]     In a second aspect, the handover method includes an act of measuring qualities of signals transmitted from at least some of said plurality of base stations, the measured qualities of signals including a measured quality of a signal received from a certain base station; an act of detecting that the measured quality of the signal received from the certain base station has dropped below a deletion threshold, wherein the deletion threshold is determined in accordance with a quality of a signal transmitted from one or more active base stations; in response to the act of detecting, an act of notifying one or more of the plurality of based stations of information about the detected base station; and an act of performing a handover process based on the information in the notification; wherein the notification includes the measured quality.  
         [0017]     These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0019]      FIG. 1  is a schematic diagram illustrating an embodiment of a diversity handover branch control method in accordance with the present invention;  
         [0020]      FIG. 2  is a diagram illustrating images of branch connection and release on a mobile station side and a network side in respective DHO operations;  
         [0021]      FIG. 3  is a diagram illustrating images of an addition DHO threshold value and a deletion DHO threshold value;  
         [0022]      FIG. 4A  is a diagram showing a format of addition information parameters;  
         [0023]      FIG. 4B  is a diagram showing a format of deletion information parameters;  
         [0024]      FIG. 5  is a diagram illustrating an image of the DHO performance in terms of a threshold value;  
         [0025]      FIG. 6  is a diagram illustrating relationships between the branch numbers on a network side;  
         [0026]      FIG. 7  is a flowchart illustrating the details of hand over trigger signal transmission algorithm in a mobile station;  
         [0027]      FIG. 8  is a diagram illustrating an example, in which no addition branch candidate is notified during maximum radio branch communications;  
         [0028]      FIG. 9A  is a diagram illustrating an oscillation inhibiting threshold value in the case where the addition branch candidate is added;  
         [0029]      FIG. 9B  is a diagram illustrating an oscillation inhibiting threshold value in the case where no addition branch candidate is added;  
         [0030]      FIG. 10  is a flowchart illustrating the details of DHO performance algorithm in a network;  
         [0031]      FIG. 11  is a diagram illustrating branches for activating a DHO sequence;  
         [0032]      FIG. 12  is a diagram illustrating the relationship between  FIGS. 12A, 12B  and  12 C linked in this order;  
         [0033]      FIG. 12A  is a diagram illustrating a DHO processing sequence when the radio condition as illustrated in  FIG. 11  takes place in a mobile station;  
         [0034]      FIG. 12B  is a diagram illustrating the DHO processing sequence when the radio condition as illustrated in  FIG. 11  takes place in the mobile station;  
         [0035]      FIG. 12C  is a diagram illustrating the DHO processing sequence when the radio condition as illustrated in  FIG. 11  takes place in the mobile station;  
         [0036]      FIG. 13  is a diagram illustrating an example of candidates that cannot be added because of restrictions on a maximum cable branch number;  
         [0037]      FIG. 14  is a diagram illustrating a deletion sequence;  
         [0038]      FIG. 15  is a diagram illustrating a deletion sequence; and  
         [0039]      FIG. 16  is a diagram illustrating a deletion sequence.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]     The invention will now be described with reference to the accompanying drawings.  
         [0041]      FIG. 1  is a schematic diagram illustrating an embodiment of a diversity handover branch control method in accordance with the present invention.  
         [0042]     In  FIG. 1 , a mobile station (MS)  20  is a device such as a mobile phone and a mobile information terminal, which has radio communication functions, and base stations (BS&#39;s)  10 ,  14  and  16  are radio stations that communicate with the mobile station  20 . The mobile station communicates with the base stations  10 ,  14  and  16  at first. Mobile services switching center (MSC)  30 ,  32  or  34 , which is connected with one of the base stations  10 ,  12 ,  14 ,  16  and  18  through a cable, has base station control functions that perform radio control of the base stations  10  and others, and a paging control function that provides the mobile station  20  and others with communication services through the base stations  10  and others. It will be needless to say that the mobile switching center  30 ,  32  or  34  can be connected with the base station  10 ,  12 ,  14 ,  16  or  18  through fixed radio channels instead of the cable. One mobile switching center can handle a plurality of base stations. Although the base station  10  is connected double through cable channels to both the mobile services switching centers  30  and  32  in  FIG. 1 , this is for the purpose of load distribution or switching between operating and standby systems, and each base station is connected to one of the mobile services switching centers during a single communication term. When the mobile station  20  moves to the area of the mobile station  22 , and acquires quality communication condition with the base stations  12  and  18 , the base stations  12  and  18  each become an addition branch candidate. In this case, if the radio condition between the mobile station  20  and the base stations  10  and  14  deteriorates, the base stations  10  and  14  each become a deletion branch candidate. Thus, because the addition branch candidate is present, it does not necessarily follow that the deletion branch candidate is present. The mobile services switching centers  30 ,  32  and  34  are interconnected through a trunk circuit network.  
         [0043]     Next, types of control of the DHO the present invention employs will be described.  
         [0044]     The DHO is roughly divided into the addition DHO, deletion DHO and addition deletion DHO which is a special case of the addition DHO. The addition DHO is one that adds a new branch candidate, whereas the deletion DHO is one that deletes a branch candidate under communications which does not contribute to any communications. Control procedures of these DHO will now be described.  
         [0045]     [Addition DHO] 
         [0046]      FIG. 2  illustrates images of the branch connection and release on the mobile station side and network side in the respective DHO operations.  
         [0047]     The radio branch connection and release image of the addition DHO in  FIG. 2  indicates that a new radio branch  52  is added at a time  60  in the presence of a communicating branch  50 . The cable branch connection and release image of the addition DHO indicates that a new cable branch  54  is added. In the branch control sequence in the addition DHO, the addition (cable) branch  54  comes first, and then the addition (radio) branch  52 .  
         [0048]     The mobile station monitors radio conditions (for example, monitors transmission losses on the perch) of peripheral sectors, and selects addition branch candidates from among the branches that meet the addition branch candidate decision conditions, that is, the mobile station&#39;s detection conditions of the addition branch candidate.  
         [0049]      FIG. 3  shows an image of an addition DHO threshold value and a deletion DHO threshold value. In  FIG. 3 , the vertical axis indicates the transmission loss level, and the horizontal axis represents the distance the mobile station travels.  
         [0050]     As shown in  FIG. 3 , when the mobile station moves from left to right on the horizontal axis and reaches the DHO area, it enters the diversity condition, and communicates through two branches at the same time. During this state, the handover (HO) is carried out. The mobile station is communicating through one branch on the left-hand side of the DHO area. Thus, one addition branch candidate takes place when the mobile station enters the DHO area during one branch communications, and one deletion branch candidate appears when it passes through the DHO area to the right-hand side during the two branch communications. The mobile station detects the addition and deletion branch candidates in accordance with the difference between the HO (handover) source BS transmission loss and the HO destination BS transmission loss (addition/deletion branch candidate decision conditions). If the difference becomes smaller than the addition DHO threshold value (meets the addition branch candidate decision condition), the mobile station detects the addition branch candidate, whereas if the difference becomes greater than the deletion DHO threshold value (meets the deletion branch candidate decision condition), it detects the deletion branch candidate. In both the DHO operations (the addition and deletion DHO operations), the least transmission loss among the branches is used as a reference HO source BS transmission loss during communications through a plurality of communication branches. The deletion DHO threshold value is made greater than the addition DHO threshold value, so that a difference (hysteresis) is provided to the two threshold values, thereby preventing the deletion DHO branch from being added soon again depending on changes in the radio conditions.  
         [0051]     The mobile station notifies the network of both the addition branch candidates and communicating branches in such a manner that they are arranged in ascending order of the transmission loss, or in a given order with information indicative of the order of precedence, and are added to addition information parameters in a handover trigger signal. The deletion branch candidates, if there is any, are also notified in a similar manner in the form of deletion information parameters.  
         [0052]     It is possible for the mobile services switching center to acquire information about the communicating branches, and this will make it unnecessary for the mobile station to transmit the information on the communicating branches to the mobile services switching center.  
         [0053]     The information indicative of the order of precedence may be the foregoing arranged orders of the transmission losses of the addition branch candidates, or their absolute values or relative values. For example, values indicative of the lowness of the transmission losses can be transmitted in connection with the information about the branches independently of the arranged orders.  
         [0054]      FIG. 4A  illustrates an example of the format of the addition information parameters, and  FIG. 4B  illustrates an example of the format of the deletion information parameters.  
         [0055]     In  FIG. 4A , the communicating visitor location sector number indicates the number of sectors in DHO communications when the mobile station sends the handover trigger signal; the addition DHO candidate sector number indicates the number of sectors that can be added newly as the DHO candidates; a base station number indicates the number of a base station in DHO communications with the mobile station, or that of a base station of an addition branch candidate; a sector number indicates the number of a sector within the service area of the base station; a perch channel received SIR indicates the received SIR of a broadcasting control channel; and a perch channel transmission power level indicates the transmission power level of the broadcasting control channel. In the parameter list, sets of the base station number, sector number, perch channel received SIR and perch channel transmission power level are repeated by the number of times of the sum total of the communicating visitor location sector number and the addition DHO candidate sector number.  
         [0056]     In  FIG. 4B , the deletion DHO candidate sector number indicates the number of sectors of the DHO candidates that can be newly deleted, and sets of a base station number and a sector number are repeated by the number of times of the deletion DHO candidate sector number as in  FIG. 4A .  
         [0057]      FIG. 5  illustrates a DHO performance image in terms of the threshold values.  
         [0058]     In  FIG. 5 , the highest priority communicating branch is a branch  110 , and the next highest one is a branch  112 . Threshold values  120  and  122  are based on the highest priority branch  110  in the communicating branches. The threshold value  120  designates the deletion DHO threshold value, and the threshold value  122  designates the addition DHO threshold value. The network side extracts branches in order of the addition DHO precedence (in order of the branches  114 ,  116  and  118 ) by the number of branches the network side can handle, considering the number of branches in communications at present, and carries out the addition DHO. It is clear that since the addition DHO branch candidate  114  has a lower transmission loss than the communicating branch  110 , it satisfies the addition branch decision conditions. Even the branch  116 , which has a greater transmission loss than the branch  110  with a minimum transmission loss among the communicating branches, can be considered to have a higher precedence than the branch  118 . In other words, the high priority means that the branch is given higher precedence in the addition processing.  
         [0059]     The branch candidate selection by the mobile station in accordance with the present invention means that the mobile station carries out its control such that it does not notify the network side of the candidates that are unlikely to be subjected to the addition DHO among the addition branch candidates detected by the mobile station.  
         [0060]     [Deletion DHO] 
         [0061]     The radio branch connection and release image of the deletion DHO in  FIG. 2  indicates that a radio branch  72  is deleted at a time  80  in the presence of a communicating branch  70 . The cable branch connection and release image of the deletion DHO indicates that a cable branch  74  is deleted. In the branch control sequence in the deletion DHO, the deletion (radio) branch  72  is deleted first, and then the deletion (cable) branch  74  is deleted.  
         [0062]     The mobile station monitors the radio conditions (for example, monitors the transmission losses on the perch) of the communicating sectors, and selects all the branches that meet the deletion branch candidate decision conditions as the deletion DHO candidates. In the deletion DHO, the DHO precedence is not considered. The deletion DHO candidates are deleted as soon as they are detected, because they do not contribute to the communications. The mobile station notifies the network side of the deletion DHO candidates by placing them into the deletion information parameters in the handover trigger signal. The deletion information parameters include the deletion DHO sector number, and the sets of the base station number and sector number which are repeated by the number of times of the deletion DHO sector number (see, the deletion information parameter format of  FIG. 4B ). As illustrated in  FIG. 5 , the network side carries out the deletion DHO, and deletes the deletion DHO candidates, the branch  113 , for example. Alternatively, since the deletion DHO candidates are sure to be deleted when detected, the mobile station can autonomously delete them and notify the network side of that.  
         [0063]     [Addition Deletion DHO] 
         [0064]     The addition deletion DHO is one of the addition DHO operations based on the restrictions on the maximum radio branch number, and is activated when the sum total of the communicating branches and addition branches exceeds the maximum radio branch number. To achieve the addition deletion DHO effectively, the maximum cable branch number is set greater than the maximum radio branch number. Higher priority branches are added up to the maximum cable branch number in order of decreasing precedence in the addition deletion DHO.  
         [0065]      FIG. 6  illustrates relationships between these branch numbers seen from the network side.  
         [0066]     As illustrated in  FIG. 6 , a (transitional) cable branch number  126 , which is the sum total of the communicating branch number  128  and the addition branch number  127 , exceeds the maximum radio branch number  125 . The cable branches corresponding to the excess, that is, the (transitional) cable branch number  126 −the maximum radio branch number  125 =M cable branches, are provided transitionally for carrying out effective addition DHO. The network side eventually deletes M branches in order of increasing precedence (in order of decreasing transmission loss, for example) within the (transitional) cable branches, inclusive of communicating branches, where M is equal to or less than N, where N equals the maximum cable branch number  124 −the maximum radio branch number  125 . In the radio section, addition of the radio branches and switching of them are carried out in response to the finally selected cable branches. It is also possible to take such a control method that omits the addition of those candidates which are most likely to be deleted because of their lower precedence than the communicating branches even though they are added as the cable branches.  
         [0067]     The radio branch connection and release image of the addition deletion DHO in  FIG. 2  indicates that a new branch  92  is added in the presence of branches  90  and  91  at a time  100 , the branch  91  is deleted by switching at a time  102 , and then a branch  93  is added by switching after the time  102 . The point of intersection of the time  102  and the branch  93  is represented by an open circle because the maximum radio branch number is set at three. On the other hand, the cable branch connection and release image indicates that cable branches  96  and  98  are added, and a cable branch  94  is deleted. The branch control sequence in the addition deletion DHO takes place in order of (1) the addition of the branches  96  and  98 ; (2) addition of the branch  92 ; (3) deletion of the branch  91  by switching; (4) addition of the branch  93  by switching; and (5) deletion of the branch  94 .  
         [0068]      FIG. 7  is a flowchart illustrating the details of handover trigger signal transmission algorithm in the mobile station.  
         [0069]     In  FIG. 7 , the mobile station monitors the radio conditions of its current sector and peripheral sectors to detect addition branch candidates and deletion branch candidates at step S 10 . The mobile station places the communicating branches and addition branch candidates into the addition information parameters in the handover trigger signal, and the deletion branch candidates into the deletion information parameters in the same signal. The mobile station sets the addition information parameters in the following conditions, and sends them through steps S 20 -S 90 .  
         [0070]     When one or more deletion branch candidates are present (step S 20 ), the mobile station places all the communicating branches and addition branch candidates into the addition information parameters, and sends them through steps S 40  and S 50  (Condition  1 ).  
         [0071]     When no deletion branch candidate is present and the number of the communicating radio branches is less than the maximum value (step S 30 ), the mobile station places all the detected addition branch candidates and the communicating branches in the addition information parameters, and sends them through steps S 40  and S 50  (Condition  2 ).  
         [0072]     If the number of the communicating radio branches equals the maximum value (step S 30 ), the mobile station decides as to the communicating branches and the addition branch candidates whether their difference (in the transmission loss, for example) is greater than the oscillation inhibiting threshold value by comparing them at step S 60 , and sends only the addition branch candidates with a difference greater than the oscillation inhibiting threshold value by placing them into the handover trigger signal through steps S 80  and S 90  (Condition  3 ), without sending them otherwise (step S 70 ).  
         [0073]     A first reason for imposing a rigid condition such as the condition  3  on the candidate transmission when the number of communicating radio branches is maximum is that it is known beforehand that those addition branch candidates which meet the normal addition branch decision conditions but have lower precedence than any other communicating branches are not subjected the addition processing even though they are sent to the network side. Thus, notifying the network side of them as the addition branch candidates is avoided.  
         [0074]      FIG. 8  illustrates an example that does not notify the network side of the addition branch candidates during communications using the maximum number of radio branches.  
         [0075]     In  FIG. 8 , branches  130 ,  132  and  134  are communicating branches, and the threshold value  142  indicates an addition DHO threshold value. Although the branches  136  and  138  are addition branch candidates because they meet the addition branch candidate decision conditions (because the differences in the transmission losses between them and the branch  130  are less than the addition DHO threshold value  142 ), they are highly unlikely to be added because their precedence is lower than the lowest communicating branch  134 . Furthermore, the oscillation inhibiting threshold value is set to prevent the repetition of the addition and deletion, that is, “oscillation”, by imposing more strict condition (second reason). Those branches that are deleted at the start of the addition deletion DHO are deleted not because they meet the deletion branch candidate conditions, but because they become lower than other branches. If a branch that meets the requirements for a communication branch and contributes communications is replaced by an addition branch candidate with a slightly higher priority, it is very likely that the replaced branch is added again instead of the added addition branch candidate because of fluctuations in the radio section. Such an operation, if repeated, will increase the control load of the network side. To prevent such oscillation, the oscillation inhibiting threshold value is set. The oscillation inhibiting threshold value is set under the assumption that the differences are considered between the addition branch candidates and the communicating branch with the lowest priority (because of a large transmission loss, for example) during the communications using the maximum number of radio branches, and the decision is made by comparing the differences with the threshold value so that the addition is made if the differences are greater than the oscillation inhibiting threshold value.  FIGS. 9A and 9B  are diagrams illustrating the oscillation inhibiting threshold value.  FIG. 9A  illustrates a case when an addition branch candidate is added, whereas  FIG. 9B  illustrates a case when it is not added. An addition branch candidate lower in precedence than the communicating branches, that is, lower than the lowest priority communicating branch among them is not added because it is unlikely to be added. If it has a slightly higher priority, it is not added because it will cause the oscillation.  
         [0076]     In  FIG. 9A , branches  150 ,  152  and  154  are communicating branches and a branch  156  is an addition branch candidate with precedence above the oscillation inhibiting threshold value  157 .  
         [0077]     In  FIG. 9B , branches  160 ,  162  and  164  are communicating branches and a branch  166  is an addition branch candidate with precedence below the oscillation inhibiting threshold value  167 .  
         [0078]     When one or more deletion branch candidates are present, or when the number of the communicating branches is less than maximum, the mobile station notifies the network side of all the candidates. In this way, the network side can obtain alternative addition branch candidate, even if it rejects a particular addition branch candidate owing to some reason.  
         [0079]     It will be obvious for those skilled in the art that such control can be implemented without any inconvenience that adds only addition branch candidates which are very likely to be added. This is analogous to the foregoing case in which it is possible to circumvent any cable branch addition processing of the candidates which are very likely to be deleted because of their lower precedence than the communicating branches even though they are added as the cable branch candidates.  
         [0080]      FIG. 10  is a flowchart illustrating the details of the DHO performance algorithm in the network.  
         [0081]     In  FIG. 10 , receiving the handover trigger signal at step S 100 , the network side carries out the deletion DHO first at step S 110 , and then the addition DHO at step S 130  after completing the deletion DHO. The deletion DHO sequence (step S 200 ) is for increasing the number of branches that are available simultaneously in the addition DHO by releasing beforehand those branches not contributing to the communications by performing the deletion DHO prior to the addition DHO. When the mobile station carries out the deletion DHO autonomously, it can perform the deletion DHO independently of the addition DHO which is carried out after receiving the handover trigger. It is preferable, however, to carry out the deletion DHO before triggering the addition DHO. The procedure of the addition DHO is divided depending on whether the sum total of the communicating branch number (#Br)−the deletion DHO branch number (#Del) (that is, the communicating branch number after completing the deletion DHO) and the addition DHO branch number (#Add) exceeds the maximum radio branch number (#Brmax) or not.  
         [0082]     First, when the sum total (#Br−#Del+#Add) of the communicating branch number and the addition branch candidate number does not exceed the maximum radio branch number (#Brmax), the network side carried out the addition DHO in a normal procedure (steps S 120  and S 130  of the addition DHO sequence S 210 ).  
         [0083]     In contrast, when the sum total (#Br−#Del+#Add) of the communicating branch number and the addition branch candidate number exceeds the maximum radio branch number (#Brmax), the network side carried out the addition deletion DHO (the addition deletion DHO sequence S 220 ). The cable branches are added up to the maximum cable branch number (step S 140 ). Subsequently, the addition and switching of the radio branches are carried out up to the maximum radio branch number (#Brmax) (step S 150 ). Finally, unneeded cable branches exceeding the maximum radio branch number (#Brmax) is released (step S 160 ).  
         [0084]      FIG. 11  is a branch diagram for activating the DHO sequence.  
         [0085]     In  FIG. 11 , a branch  1  ( 190 ), branch  2  ( 191 ) and branch  3  ( 196 ) are communicating branches, in which the branch  3  ( 196 ) is below a deletion DHO threshold value  194 . A branch  4  ( 192 ) and branch  5  ( 193 ) are addition branch candidates, in which the branch  5  ( 193 ) is within an addition DHO threshold value  195 , and the branch  4  ( 192 ) has higher precedence than the branch  2  ( 191 ).  
         [0086]      FIGS. 12A, 12B  and  12 C are linked in this order, and illustrate the DHO processing sequence of performing under the control of the network the addition deletion DHO after the deletion DHO in the case where the precedence of the branches is as shown in  FIG. 11 , where it is assumed that the maximum radio branch number is three and the maximum cable branch number is four.  
         [0087]      FIG. 12A  shows the deletion DHO sequence, in which an unneeded radio branch is deleted.  
         [0088]      FIG. 12A  shows three radio branches  1 - 3 , three cable branches  1 - 3 , and five base stations with base station numbers  1 - 5 . A mobile station  500  detects addition DHO candidates and deletion DHO candidates ( 501 ), and decides addition DHO candidates and deletion DHO candidates ( 505 ). The mobile station  500  sends to a network  503  a handover trigger request (branch deletion or branch addition) ( 510 ). The network  503  extracts the deletion DHO candidates ( 517 ), and sends to the mobile station  500  a handover execution request (deletion of the branch  3 ) ( 515 ). The mobile station  500  sends back to the network  503  a handover execution response ( 520 ). Then, the mobile station  500  deletes the radio branch  3  ( 525 ), and suspends the maximal ratio combining of the radio branch  3  ( 530 ), as represented by the deletion of the line of the radio branch  3  at  530  as indicated by a broken line.  
         [0089]     The network  503  sends to the base station with the base station number  3  a radio and cable bearer release request ( 535 ). The base station with the base station number  3  halts the reception of the reverse radio channel ( 540 ), halts the transmission of the forward radio channel ( 545 ), releases base station number resources ( 550 ), and sends back to the network  503  a radio and cable bearer release response ( 560 ). The network  503  deletes the DHT cable branch  3  ( 565 ).  
         [0090]      FIGS. 12B and 12C  illustrate an addition deletion DHO sequence which adds cable branches  4  and  5  in advance, adds a radio branch  4  corresponding to the added cable branches, carries out switching between the radio branches  1  and  5 , and finally deletes the cable branch  1  that becomes unnecessary.  
         [0091]     In  FIG. 12B , the network  503  detects addition DHO candidates ( 600 ), decides an addition DHO destination ( 605 ), and adds the cable branches  4  and  5  ( 610 ). The addition of the cable branches  4  and  5  is represented in  FIG. 12B  by lines of the cable branches  4  and  5  appearing from the position indicated by a broken line. The network  503  sends to the base stations with the base station numbers  4  and  5  a radio and cable bearer establishing request ( 615  and  620 ). The base stations with the base station numbers  4  and  5  each send to the mobile station  500  a forward radio channel transmission start ( 625  and  630 ), receive a reverse radio channel reception start ( 635  and  640 ), and sends to the network  503  a radio and cable bearer establishment response ( 645  and  650 ).  
         [0092]     In  FIG. 12C , the network  503  checks the cable branches  4  and  5  ( 700 ). The network  503  sends to the mobile station  500  a handover execution request (for adding the branch  4  and switching between the branches  1  and  5 ) ( 705 ), and the mobile station  500  sends back to the network  503  a handover execution response ( 710 ). The mobile station  500  adds the radio branch  4  ( 715 ), establishes synchronization of the new branch  4  ( 720 ), starts the maximal ratio combining of the radio branch  4  ( 725 ), switches the radio branches  1  and  5  ( 730 ), and establishes the synchronization of the new branch  5  ( 735 ). On the other hand, the network  503  adds the cable branches  4  and  5  ( 727 ). The base station with the base station number  4  detects the reverse radio channel synchronization ( 740 ), and sends back to the network  503  a radio and cable bearer establishment response ( 745 ). The base station with the base station number  5  detects the reverse radio channel synchronization ( 750 ), and sends back to the network  503  a radio and cable bearer establishment response ( 755 ). The network  503  adds the radio branch  4  and checks the switching between the radio branches  1  and  5  ( 757 ), and sends to the base station with the base station number  1  a radio and cable bearer release request ( 760 ). The base station with the base station number  1  halts the reception of the reverse radio channel ( 765 ), halts the transmission of the forward radio channel ( 770 ), releases the base station number resources ( 775 ), and sends back to the network  503  a radio and cable bearer release response ( 780 ). The network  503  deletes the DHT cable branch  1  ( 785 ).  
         [0093]     Next, control will be described of a candidate which is not added because of the restrictions on the cable branch number (maximum cable branch number), though the mobile station notifies the network side of the candidate. The maximum number of branches that can be handled in single network control is limited to the maximum cable branch number. Thus, when the sum total of the communicating branches and the addition branch candidates exceeds the maximum cable branches, it is probable that the single control cannot add some addition branch candidates although they have higher precedence than the communicating branches.  
         [0094]      FIG. 13  illustrates such addition candidates which cannot be added because of the maximum cable branch number.  
         [0095]     In  FIG. 13 , branches  170 ,  172  and  174  are communicating branches, and branches  176 ,  178  and  180  are addition branch candidates. Although the branches  178  and  180  have higher precedence than the communicating branch  170 , they are not added because of the maximum cable branch number of four. Only the addition candidate branch  176  with the highest precedence is added by the single control.  
         [0096]     In such a case, a control method can be introduced that notifies the network again of the unadded addition branch candidates  178  and  180  by putting them into the handover trigger signal in response to the measurement on the mobile station side. The network side can perform the addition DHO of the addition branch candidates  178  and  180  that are not added in the first control, at the point of time it receives the handover trigger signal including them. Alternatively, the network side can choose a method that stores the addition branch candidates  178  and  180 , and successively carries out the addition DHO for those candidates that are omitted in the first control after completing the series of the operations as shown in  FIG. 10 .  
         [0097]     In the present invention, selecting the branch candidates considering the maximum cable branch number and the maximum radio branch number, the mobile station notifies the network side of only the candidates that are most likely to undergo the DHO processing by excluding in advance the branch candidates that are unlikely to be handled in the DHO processing even though the mobile station notifies the network side of them. In this case, the following two methods can be employed.  
         [0098]     First, considering the case where the network side cannot add an addition branch candidate of a higher precedence because of some reason, the mobile station notifies the network side of alternative addition branch candidates that are unlikely to be added if the branch candidate of a higher precedence is added. Thus, all the addition branch candidates are notified of Nevertheless, the mobile station keeps performing the control that does not notify the network side of the addition branch candidates with lower precedence than the communicating branches during communications using the maximum number of the communicating branches. This is because it cannot be expected that these branches with the lower precedence than the communicating branches are added because the maximum number of communicating branches are already used.  
         [0099]     Second, the mobile station notifies the network side of only the addition branch candidates that are likely to be added in the normal DHO processing without notifying of the alternative addition branch candidates. Although this method has an advantage over the first method of being able to reduce an amount of signals, the mobile station must consider the relationships between the number of the communicating branches and the number of the deletion branch candidates. That is, since the deletion DHO is carried out previously, such control is required that selects only addition branch candidates that are most likely to be added after the deletion DHO which is executed beforehand.  
         [0100]     In the present invention, the mobile station notifies the network side not only of the addition branch candidates by putting them into the notification signal (handover trigger signal), but also of the communicating branches in combination with the information about the precedence by putting them into the handover trigger signal. For example, magnitudes of the transmission loss are retained in the notification signal in order of precedence as an indicator of the precedence. It is not necessary to notify the network side of the deletion branch candidates because they do not contribute to the communications. Thus, they are detected and deleted without being sent to the network side with the communicating branches that are transmitted in combination with the information about their precedence. Since the network side can compare the precedence between the communicating branches and the addition branch candidates, it can decide the addition branch candidates to be added, and the communicating branches to be retained or deleted as needed.  
         [0101]     The mobile station places into one handover trigger signal all the DHO candidates, that is, a plurality of addition branch candidates and deletion branch candidates, and notifies the network side of them. More specifically, the addition branch candidates are put into the addition information parameters in the handover trigger signal, and the deletion branch candidates are inserted into the deletion information parameters. It is needless to say that the communicating branches are put into the addition information parameters together with the addition branch candidates. This will make it possible to reduce the number of transmission of the handover trigger signal, enabling effective use of the radio resources. To simplify the processing, the network side separates the addition branch candidates from the deletion branch candidates, and sequentially carries out their DHO independently. Carrying out the deletion DHO in advance to delete unneeded branches makes it possible to add more addition branch candidates through a single transmission operation of the handover trigger signal, enabling more effective use of the radio and cable resources.  
         [0102]     To achieve the deletion DHO under the control of the mobile station, the mobile station puts the deletion branch candidates into the handover trigger signal, notifies the network side of them, and releases the radio branches. Detecting the break of the radio channels associated with the deletion branch candidates notified, the network side releases the cable branches corresponding thereto.  
         [0103]      FIG. 14  illustrates the deletion DHO sequence under the control of the mobile station. The addition DHO is carried out independently of this sequence.  
         [0104]     By setting the maximum cable branch number at the maximum radio branch number plus N (N≧1), it becomes possible to add a greater number of cable branches in advance than when the maximum cable branch number is set at the maximum radio branch number plus one. This makes it possible to achieve more efficient control of the addition operation of a plurality of multiple addition branch candidates.  
         [0105]     In the foregoing control, it is likely that the restrictions on the maximum cable branch number may result in an addition branch candidate to which no cable branch is added. In this case, the present invention can propose the following two methods.  
         [0106]     1. The network side does not carry out any special control. The mobile station decides the addition branch candidates again in the condition of using new communication branches, notifies the network of the handover signal again so that the addition DHO of the unexecuted candidates is carried out.  
         [0107]     2. The network side stores the addition branch candidates that are not handled by the first control, and autonomously carries out the second control.  
         [0108]      FIG. 15  shows an embodiment of a configuration of base stations, a switching center and so on in accordance with the present invention.  
         [0109]     In  FIG. 15 , a base station (BS 1 )  200  and a base station (BS 2 )  210  are connected to a MSC (Mobile services switching center)  220 , to which a diversity handover trunk (DHT)  230 , a voice encoder (VXC)  240 , data service controller (DSC)  250  and processor (PRC)  260  are connected.  
         [0110]      FIG. 16  shows another embodiment of a configuration of the base stations, switching centers and so on in accordance with the present invention.  
         [0111]     In  FIG. 16 , blocks having the same functions as those of  FIG. 15  are designated by the same reference numerals, and the description thereof is omitted here. The configuration of  FIG. 16  differs from that of  FIG. 15  in that it comprises a new additional base station control office (MSC 1 )  320  with a function of controlling the base stations, which is connected to a mobile services switching center (MSC 2 )  330  with a normal switching function. The base station control office (MSC 1 )  320  can be installed near the base station BS 1  or BS 2 . It is also possible to utilize a switching center of a fixed network as the switching center (MSC 2 )  330  without any change. The DHT  230  and PRC  260  are connected to the base station control office (MSC 1 )  320 , and the VXC  240 , DSC  250  and PRC  260  are connected to the switching center (MSC 2 ).  
         [0112]     As described above, the present invention provides a diversity handover branch control method in the DHO performance, in which a mobile station selects, when it detects a plurality of addition branch candidates and deletion branch candidates, branch candidates considering the maximum cable branch number and maximum radio branch number.  
         [0113]     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.