Handover type judgement scheme for CDMA mobile communication system

Abstract: A handover type judgement scheme for a CDMA mobile communication system which is capable of judging an appropriate type of handover from multiple types of handover available. At the mobile station, one type of handover for which a handover start condition is weakest among available types of handover is judged. Then, whether the handover start condition for that one type of handover is satisfied or not is checked, and each base station for carrying out that one type of handover is notified when the handover start condition for said one type of handover is satisfied.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a CDMA handover type judgement method for a CDMA mobile communication system, further a CDMA mobile communication system for carrying out any of multiple types of handover with different representing criteria for starting respective types of handover or, in other words, handover start conditions, further a CDMA mobile station apparatus for use as a mobile station in a CDMA mobile communication system, and a CDMA base station apparatus for use as a base station in a CDMA mobile communication system. DESCRIPTION OF THE BACKGROUND ART In US-A-5,257,402 there is described a method of effecting handoff of a communication unit from. a first communication resource to a communication resource. After determining that a handoff from the first communication resource to the second communication resource should likely be made, the method makes determinations regarding relative levels of communication degradation that will likely occur when using different handoff methodologies to effect the handoff. In general, the method selects a particular handoff methodology that will provide a best level of handoff reliability while simultaneously posing at least no more than an acceptable level of communication degradation. In WO-A-95 16329 there is described a method for performing a handoff in a communication system wherein the power of a first signal is measured and then compared with a threshold. If the first signal power meets a handoff threshold, the system enters the handoff procedure. Based on the fact whether estimated signal levels meet certain handoff thresholds, the service to the subscriber is either continued or not. Further, WO-A-96 02117 there is described another handover method and cellular communications system wherein at the border of two or more base station systems the service the service areas of base stations under different base station controllers at least partly overlap. Further, as a terminal equipment moves from one base station system to another, the handover is carried out so that when the terminal equipment moves into a cell served by two or more base stations, a soft handover is executed from the old cell to the new cell. In the conventional mobile communication system such as PDC (Personal Digital Cellular) system that is currently providing the mobile communication service in Japan, there is only one available type of handover called hard handover. In contrast, in the CDMA mobile communication system, the soft handover is also available in addition to the hard handover, and this soft handover further includes multiple types of soft handovers such as the same code same frequency soft handover and the different frequency soft handover. Here, these multiple types of handovers are associated with mutually different handover start conditions. In order for each mobile station to carry out the handover properly, each base station is transmitting the perch channel (or the pilot channel). The mobile station receives the perch channel transmitted from each base station of each nearby cell, compares the receiving level of the perch channel received from each nearby cell with the receiving level of the perch channel of a currently located cell, and judges the start of the handover according to the obtained difference. Here, as already mentioned above, the available types of handover includes the hard handover, the same code same frequency soft handover, and the different frequency soft handover, and the handover start conditions for these three types of handover are as indicated in Fig. 3. As can be seen from Fig. 3, when an influence on the radio channel capacity is taken into account, the handover start condition is weakest for the same code same frequency soft handover, second weakest for the different frequency soft handover, and strongest for the hard handover. As already mentioned above, the conventional mobile communication system has been designed to use only one type of handover, so that there has been no means for judging an appropriate type of handover to be executed out of multiple types of handover. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a handover type judgement scheme for a CDMA mobile communication system which is capable of judging an appropriate type of handover from multiple types of handover available. According to the present invention this object is achieved through a CDMA handover type judgement method having the features of claim 1, a CDMA mobile communication system for carrying out any of multiple types of handovers having the features of claim 11, a CDMA mobile station apparatus having the features of claim 21, and a CDMA base station apparatus having the features of claim 33. Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a mobile station apparatus for realizing a handover type judgement scheme for a CDMA mobile communication system according to the present invention. Fig. 2 is a block diagram of a base station apparatus for realizing a handover type judgement scheme for a CDMA mobile communication system according to the present invention. Fig. 3 is a graph for explaining handover start conditions for three types of handover used in a handover type judgement scheme for a CDMA mobile communication system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 1 to Fig. 3, various embodiments of a handover type judgement scheme for a CDMA mobile communication system according to the present invention will be described in detail. Fig. 1 and Fig. 2 respectively show a mobile station apparatus and a base station apparatus for realizing the handover type judgement scheme for a CDMA mobile communication system according to the present invention. The mobile station apparatus of Fig. 1 has an antenna 1 for receiving radio signals from base stations and transmitting radio signals to base stations. The signals from base stations received by the antenna 1 are supplied to a demodulation unit 7 through a duplexer 3 and a reception unit 5, and applied with the despreading using a spread code and the usual demodulation at this demodulation unit 7. The demodulated signals are then supplied to a decoding unit 9 which decodes the demodulated signals into transmission signals and control signals. The decoded control signals are supplied to a control unit 11, while the decoded transmission signals are supplied to a man-machine interface (not shown). On the other hand, transmission signals such as speech signals from the man-machine interface are supplied to an encoding unit 13 along with the control signals from control unit 11, encoded at this encoding unit 13, applied with the primary modulation and the spreading using a spread code at a modulation unit 15, and then transmitted to base stations from the antenna 1 through a transmission unit 17 and the duplexer 3. In addition, the mobile station apparatus of Fig. 1 also has a memory 51 connected to the control unit 11, which stores different handover start conditions for multiple types of handover along with an order of weakness among these handover start conditions, where the multiple types of handover includes the same code same frequency soft handover, the different frequency soft handover, and the hard handover. The CDMA mobile communication system of this embodiment is capable of carrying out any of these multiple types of handover. In addition, the memory 51 is also capable of storing an unexecutable handover type which is to be notified from a base station as will be described in detail below. Moreover, the mobile station apparatus of Fig. 1 also has a measurement unit 53 provided between the reception unit 5 and the control unit 11, which measures a total receiving level or an interference receiving level of a frequency bandwidth by which this mobile station is currently communicating with base stations, and supplies the measured value to the control unit 11. The base station apparatus of Fig. 2 has an antenna 21 for receiving radio signals from mobile stations and transmitting radio signals to mobile stations. The signals from mobile stations received by the antenna 21 are supplied to a demodulation unit 27 through a duplexer 23 and a reception unit 25, and applied with the despreading using a spread code and the usual demodulation at this demodulation unit 27. The demodulated signals are then supplied to a decoding unit 29 which decodes the demodulated signals into transmission signals and control signals. The decoded control signals are supplied to a control unit 31, while the decoded transmission signals are transmitted to an upper level exchange station (not shown) through a wire transmission line transmission unit 39. On the other hand, transmission signals the exchange station are received by a wire transmission line reception unit 41, and decomposed into the transmission signals and the control signals. The decomposed control signals are supplied to the control unit 31, while the decomposed transmission signals are encoded at an encoding unit 33, applied with the primary modulation and the spreading using a spread code at a modulation unit 15, and then transmitted to mobile stations from the antenna 21 through a transmission unit 7 and the duplexer 23. Moreover, the base station apparatus of Fig. 2 also has a measurement unit 55 provided between the reception unit 25 and the control unit 31, which measures a radio channel state for each cell/sector, or a total receiving level or an interference receiving level of each frequency bandwidth implemented at this base station for each cell/sector, and supplies the measured value to the control unit 31. In the mobile communication system having the mobile station device of Fig. 1 and the base station of Fig. 2 as described above, it is possible to carry out any of the multiple types of handover including the same code same frequency soft handover, the different frequency soft handover, and the hard handover, under different handover start conditions as indicated in Fig. 3. Now, in one embodiment of a handover type judgement scheme according to the present invention, the mobile station judges and selects a handover type which has the weakest handover start condition among these multiple types of handover as the most effective one from a viewpoint of the radio channel capacity, and when the handover start condition for the selected handover type is satisfied, the mobile station notifies this fact to the base station so as to carry out the selected type of handover. As can be seen from a level difference for judging the handover start for each handover type indicated in Fig. 3, the handover start condition is weakest for the same code same frequency soft handover, second weakest for the different frequency soft handover, and strongest for the hard handover among three types of handover available in this mobile communication system, and as already mentioned above, the memory 51 provided in the mobile station device stores these different handover start conditions for multiple types of handover along with an order of weakness among these handover start conditions. At the mobile station, normally, whether the weakest handover start condition among the handover start conditions for multiple types of handover as stored in the memory 51, that is the handover start condition for the same code same frequency soft handover, is satisfied or not is regularly checked by the control unit 11, and when this handover start condition is satisfied, this fact is notified from the control unit 11 to the base station in a form of a control signal which is transmitted through the encoding unit 13, the modulation unit 15, the transmission unit 17, and the duplexer 3, so as to start the same code same frequency soft handover. In addition, there are cases in which a certain base station cannot use a particular frequency or cannot carry out the soft handover so that only the hard handover is possible, and when a particular type of handover cannot be carried out at a certain base station because of the reason not changing in time such as that related to the system configuration, this base station notifies this fact to the mobile station in advance. Then, the mobile station judges and selects a handover type which has the weakest handover start condition among the available types of handover excluding the notified type of handover that cannot be carried out at the base station, and when the handover start condition for the selected handover type is satisfied, the mobile station notifies this fact to the base station so as to carry out the selected type of handover. As one concrete example of this embodiment, the base station notifies the available frequency bandwidth of each surrounding cell/sector to the mobile station in advance, and the mobile station stores this available frequency bandwidth of each surrounding cell/sector notified from the base station into the memory 51. Then, the mobile station identifies each cell/sector at which the same code same frequency soft handover cannot be carried out according to the stored available frequency bandwidth, and makes the judgement of the handover start condition for the different frequency soft handover with respect to each identified cell/sector, without making the judgement of the handover start condition for the same code same frequency soft handover. Then, when the handover start condition for the different frequency soft handover is satisfied, the mobile station notifies this fact to the base station. As another concrete example of this embodiment, when a certain base station cannot carry out the soft handover so that only the hard handover is possible, this base station notifies this fact to the mobile station in advance, and the mobile station stores this notification into the memory 51. Then, according to this stored notification, the mobile station makes the judgement of the handover start condition for the hard handover with respect to this base station, without making the judgement of the handover start condition for the soft handover. Then, when the handover start condition for the hard handover is satisfied, the mobile station notifies this fact to this base station. Next, in another embodiment of a handover type judgement scheme according to the present invention, the mobile station measures the downlink radio channel state at the measurement unit 53, and judges whether it is possible to carry out each type of handover or not at the control unit 11 according to the measured value, in order to cope with a case in which a particular type of handover cannot be carried out because of the reason changing in time, as in a case where the interference receiving level of the frequency bandwidth currently used for the communication at the mobile station is greater than a prescribed threshold level and the radio channel capacity of this frequency bandwidth is already full so that the same code same frequency soft handover using this frequency bandwidth cannot be carried out. Then, the mobile station judges and selects a handover type which has the weakest handover start condition among the available types of handover excluding the type of handover that has been judged as not possible to carry out, and when the handover start condition for the selected handover type is satisfied, the mobile station notifies this fact to the base station so as to carry out the selected type of handover. As one concrete example of this embodiment, the mobile station measures the interference receiving level or the total receiving level of the frequency bandwidth which is currently used for the communication at the measurement unit 53, and compares the measured value with a prescribed threshold value at the control unit 11. Then, when the measured value is greater than the prescribed threshold value, the mobile station makes the judgement of the handover start condition for the different frequency soft handover, without making the judgement of the handover start condition for the same code same frequency soft handover using this frequency bandwidth, and when the handover start condition for the different frequency soft handover is satisfied, the mobile station notifies this fact to this base station. Next, in another embodiment of a handover type judgement scheme according to the present invention, in order to cope with a case in which a particular type of handover cannot be carried out because of the reason changing in time such as the capacity overflow as described above, each base station measures the uplink radio channel state for each cell/sector at the measurement unit 55, judges whether it is possible to carry out each type of handover or not according to the measured value, and notifies an information on this judgement result to the surrounding base stations. Then, each surrounding base station notifies this information to the currently communicating mobile station. Then, the mobile station judges and selects a handover type which has the weakest handover start condition among the available types of handover excluding the type of handover that has been judged as not possible to carry out with respect to a relevant cell/sector according to the notified information, and when the handover start condition for the selected handover type is satisfied, the mobile station notifies this fact to the base station so as to carry out the selected type of handover. As one concrete example of this embodiment, each base station measures the interference receiving level or the total receiving level of each frequency bandwidth that is implemented at that base station, for each cell/sector at the measurement unit 55, and compares the measured value with a prescribed threshold value at the control unit 31. Then, when the measured value is greater than the prescribed threshold value, each mobile station notifies the fact that the frequency bandwidth corresponding to this measured value cannot be used to the surrounding base stations. Then, each surrounding base station notifies this notified frequency bandwidth that cannot be used to the currently communicating mobile station. At the mobile station which received this notification and which is using this notified frequency bandwidth, the judgement of the handover start condition for the different frequency soft handover is made with respect to a relevant cell/sector, without making the judgement of the handover start condition for the same code same frequency soft handover using this frequency bandwidth, and when the handover start condition for the different frequency soft handover is satisfied, the mobile station notifies this fact to this base station. Note here that the notification from the base station in this embodiment may be made only with respect to the mobile station which is currently using the relevant frequency bandwidth for the communication. As described, according to the handover type judgement scheme of the present invention, the mobile station judges and selects a handover type which has the weakest handover start condition among the available multiple types of handover, and when the handover start condition for the selected handover type is satisfied, this fact is notified to the base station so as to carry out the selected type of handover. Consequently, the each type of handover among the available multiple types of handover can be carried out appropriately according to its handover start condition. In addition, in a case where a particular type of handover cannot be carried out because of either the reason not changing in time such as that related to the system configuration or the reason changing in time such as the capacity overflow, this fact is notified in advance or such a case is detected. Then, the mobile station judges and selects a handover type which has the weakest handover start condition among the available types of handover excluding this particular type of handover that cannot be carried out, and when the handover start condition for the selected handover type is satisfied, this fact is notified to the base station so as to carry out the selected type of handover. Consequently, it is possible to realize an efficient handover type judgement scheme by eliminating an unnecessary judgement of the handover start condition for that particular type of handover that cannot be carried out as well as an unnecessary increase of the traffic due to the notification resulting from such an unnecessary judgement. It is to be noted that, besides those already mentioned above, many modifications and variations of the above embodiments may be made without departing from the novel and advantageous features of the present invention. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.

Claims: CDMA handover type judgement method for a CDMA mobile communication system which is capable of carrying out any of multiple types of handover including at least a soft handover and a hard handover, the multiple types of handover having different handover start conditions representing criteria for starting respective types of handover, wherein the handover start condition is weaker for the soft handover than the hard handover, the method comprising the steps of: a) determining which of said multiple types of handover are possible; b) selecting at said mobile station the type of said possible types of handover for which a handover start condition is weakest; and c) checking whether the handover start condition for said one type of handover is satisfied or not at the mobile station (1-17, 51, 53), and notifying each base station (21-41, 55) for carrying out said one type of handover when the handover start condition for said one type of handover is satisfied. Method according to claim 1, characterized in that it further comprises the step of: notifying from one base station (21-41, 55) to the mobile station (1-17, 51, 53) in advance a particular type of handover among said multiple types of handover that cannot be carried out at said one base station (21-41, 55), wherein at a time of handover with respect to said one base station (21-41, 55), the selecting step selects said one type of handover from said available types of handover which include said multiple types of handover except for said particular type of handover. Method according to claim 1, characterized in that it further comprises the steps of: notifying from one base station (21-41, 55) to the mobile station (1-17, 51, 53) in advance an available frequency bandwidth for each surrounding cell and/or sector of said one base station (21-41, 55); and determining one cell and/or sector for which a same code same frequency soft handover cannot be carried out according to the available frequency bandwidth for said one cell and/or sector as notified by the notifying step; wherein said multiple types of handover includes a same code same frequency soft handover and a different frequency soft handover, and at a time of handover with respect to said one cell and/or sector determined by the determining step, the selecting step selects the different frequency soft handover as said one type of handover so that the checking step does not check the handover start condition for the same code same frequency soft handover with respect to said cell and/or sector. Method according to claim 1, characterized in that it further comprises the steps of: notifying from one base station (21-41, 55) to the mobile station (1-17, 51, 53) in advance an information as to whether the soft handover is possible or not with respect to each surrounding cell and/or sector of said one base station (21-41, 55); and determining one cell and/or sector for which the soft handover is not possible according to the information for said one cell and/or sector as notified by the notifying step; wherein said multiple types of handover includes the soft handover and the hard handover, and at a time of handover with respect to said one cell and/or sector determined by the determining step, the selecting step selects the hard handover as said one type of handover so that the checking step does not check the handover start condition for the soft handover with respect to said one cell and/or sector. Method according to claim 1, characterized in that it further comprises the steps of: measuring a radio channel state at the mobile station (1-17, 51,53); and determining each impossible type of handover that cannot be carried out by the mobile station (1-17, 51, 53) according to the radio channel state measured by the measuring step; wherein the selecting step selects said one type of handover from said available types of handover which include said multiple types of handover except for said each impossible type of handover. Method according to claim 1, characterized in that it further comprises the step of: measuring an interference receiving level or a total receiving level of a frequency bandwidth currently used for communication at the mobile station (1-17, 51, 53); wherein said multiple types of handover includes a same code same frequency soft handover and a different frequency soft handover, and when a measured value obtained by the measuring step is greater than a prescribed threshold value, the selecting step selects the different frequency soft handover as said one type of handover so that the checking step does not check the handover start condition for the same code same frequency soft handover. Method according to claim 1, characterized in that it further comprises the steps of: measuring a radio channel state for each cell and/or sector of each base station (21-41, 55); determining each impossible type of handover that cannot be carried out for each cell and/or sector by said each base station (21-41, 55) according to the radio channel state measured by the measuring step; and notifying an information indicating said each impossible type of handover from said each base station (21-41, 55) to the mobile station (1-17, 51, 53); wherein at a time of handover with-respect to said each base station (21-41, 55), the selecting step selects said one type of handover from said available types of handover which include said multiple types of handover except for said each impossible type of handover according to said information. Method according to claim 7, characterized in that at the notifying step, said each base station (21-41, 55) notifies said information to surrounding base stations (21-41, 55) and the mobile station (1-17, 51, 53) receives said information via one of said surrounding base stations (21-41, 55) with which the mobile station (1-17, 51, 53) is currently communicating. Method according to claim 1, characterized in that it further comprises the steps of: measuring an interference receiving level or a total receiving level of each frequency bandwidth that is implemented at each base station (21-41, 55) for each cell and/or sector of said each base station (21-41, 55); and notifying an information indicating that one frequency bandwidth cannot be used for one cell and/or sector from said each base station (21-41, 55) to the mobile station (1-17, 51, 53), when a measured value for said one frequency bandwidth and said one cell and/or sector is greater than a prescribed threshold value; wherein said multiple types of handover includes a same code same frequency soft handover and a different frequency soft handover, and at a time of handover with respect to said one cell and/or sector while the mobile station (1-17, 51, 53) is currently using said one frequency bandwidth as indicated by said information, the selecting step selects the different frequency soft handover as said one type of handover so that the checking step does not check the handover start condition for the same code same frequency soft handover with respect to said one cell and/or sector. Method according to claim 9, characterized in that at the notifying step, said each base station (21-41, 55) notifies said information to surrounding base stations (21-41, 55) and the mobile station (1-17, 51, 53) receives said information via one of said surrounding base stations (21-41, 55) with which the mobile station (1-17, 51, 53) is currently communicating. CDMA mobile communication system for carrying out any of multiple types of handover including at least a soft handover and a hard handover, the multiple types of handover having different handover start conditions representing criteria for starting respective types of handover, wherein the handover start condition is weaker for the soft handover than the hard handover, comprising: a determing unit for determining which of said multiple types of handover are impossible; a notification unit for notifying possible types of handover; a plurality of base stations; and at least one mobile station having: a selecting unit for selecting the type of possible types of handover for which a handover start condition is weakest; and a checking unit (11) for checking whether the handover start condition for said one type of handover is satisfied or not, and notifying each base station for carrying out said one type of handover when the handover start condition for said one type of handover is satisfied. System according to claim 11, characterized in that one base station has a notifying unit (31) for notifying to the mobile station in advance a particular type of handover among said multiple types of handover that cannot be carried out at said one base station; and wherein at a time of handover with respect to said one base station, the selecting unit (55, 31) selects said one type of handover from said available types of handover which include said multiple types of handover except for said particular type of handover. System according to claim 11, characterized in that one base station has a notifying unit (31) for notifying to the mobile station in advance an available frequency bandwidth for each surrounding cell and/or sector of said one base station; the mobile station also has a determining unit (11) for determining one cell and/or sector for which the same code same frequency soft handover cannot be carried out according to the available frequency bandwidth for said one cell and/or sector as notified by the notifying unit; and wherein said multiple types of handover include a same code same frequency soft handover and a different frequency soft handover, and at a time of handover with respect to said one cell and/or sector determined by the determining unit (11), the selecting unit (55, 31) selects the different frequency soft handover as said one type of handover so that the checking unit (31) does not check the handover start condition for the same code same frequency soft handover with respect to said one cell and/or sector. System according to claim 11, characterized in that one base station has a notifying unit (31) for notifying to the mobile station in advance an information as to whether the soft handover is possible or not with respect to each surrounding cell and/or sector of said one base station; the mobile station also has a determining unit (11) for determining one cell and/or sector for which the soft handover is not possible according to the information for said one cell and/or sector as notified by the notifying unit; and wherein said multiple types of handover include the soft handover and the hard handover, and at a time of handover with respect to said one cell and/or sector determined by the determining unit (11), the selecting unit (31) selects the hard handover as said one type of handover so that the checking unit does not check the handover start condition for the soft handover with respect to said one cell and/or sector. System according to claim 11, characterized in that the mobile station also has: a measuring unit (53) for measuring a radio channel state; and a determining unit (11) for determining each impossible type of handover that cannot be carried out by the mobile station according to the radio channel state measured by the measuring unit (53); and wherein the selecting unit (11) selects said one type of handover from said available types of handover which include said multiple types of handover except for said each impossible type of handover. System according to claim 11, characterized in that the mobile station further comprises: a measuring unit (53) for measuring an interference receiving level or a total receiving level of a frequency bandwidth currently used for communication; and wherein said multiple types of handover includes a same code same frequency soft handover and the different frequency soft handover, and when a measured value obtained by the measuring unit (53) is greater than a prescribed threshold value, the selecting unit (31) selects the different frequency soft handover as said one type of handover so that the checking unit (11) does not check the handover start condition for the same code same frequency soft handover. System according to claim 11, characterized in that each base station has: a measuring unit (55) for measuring a radio channel state for each cell and/or sector of said each base station; a determining unit (31) for determining each impossible type of handover that cannot be carried out for each cell and/or sector by said each base station according to the radio channel state measured by the measuring unit (55); and a notifying unit (31) for notifying an information indicating said each impossible type of handover to the mobile station; and wherein at a time of handover with respect to said each base station, the selecting unit (31) selects said one type of handover from said available types of handover which include said multiple types of handover except for said each impossible type of handover according to said information. System according to claim 17, characterized in that the notifying unit (31) notifies said information to surrounding base stations and the mobile station receives said information via one of said surrounding base stations with which the mobile station is currently communicating. System according to claim 11, characterized in that each base station has: a measuring unit (55) for measuring an interference receiving level or a total receiving level of each frequency bandwidth that is implemented at said each base station for each cell and/or sector of said each base station; and a notifying unit (31) for notifying an information indicating that one frequency bandwidth cannot be used for one cell and/or sector to the mobile station, when a measured value for said one frequency bandwidth and said one cell and/or sector is greater than a prescribed threshold value; and wherein said multiple types of handover include a same code same frequency soft handover and a different frequency soft handover, and at a time of handover with respect to said one cell and/or sector while the mobile station is currently using said one frequency bandwidth as indicated by said information, the selecting unit (31) selects the different frequency soft handover as said one type of handover so that the checking unit (31) does not check the handover start condition for the same code same frequency soft handover with respect to said one cell and/or sector. System according to claim 19, characterized in that the notifying unit (31) notifies said information to surrounding base stations and the mobile station receives said information via one of said surrounding base stations with which the mobile station is currently communicating. CDMA mobile station apparatus for use as a mobile station in a CDMA mobile communication system for carrying out any of multiple types of handover including at least a soft handover and a hard handover, the multiple types of handover having different handover start conditions representing criteria for starting respective types of handover, wherein the handover start condition is weaker for the soft handover than the hard handover, comprising: among notified possible types of handover a selecting unit for selecting the type of possible handover for which a handover start condition is weakest a checking unit (11) for checking whether the handover start condition for said one type of handover is satisfied or not, and notifying each base station for carrying out said one type of handover when the handover start condition for said one type of handover is satisfied. Apparatus according to claim 21, characterized in that at a time of handover with respect to one base station, the selecting unit (11) receives in advance a particular type of handover among said multiple types of handover that cannot be carried out at said one base station, and the selecting unit (11) selects said one type of handover from said available types of handover which include said multiple types of handover except for said particular type of handover. Apparatus according to claim 21, characterized in that it further comprises: a determining unit (11) for receiving from one base station in advance an available frequency bandwidth for each surrounding cell and/or sector of said one base station, and determining one cell and/or sector for which the same code same frequency soft handover cannot be carried out according to the available frequency bandwidth for said one cell and/or sector as notified from said one base station; wherein said multiple types of handover include a same code same frequency soft handover and a different frequency soft handover, and at a time of handover with respect to said one cell and/or sector determined by the determining unit (11), the selecting unit (11) selects the different frequency soft handover as said one type of handover so that the checking unit (11) does not check the handover start condition for the same code same frequency soft handover with respect to said one cell and/or sector. Apparatus according to claim 21, characterized in that it further comprises: a determining unit (11) for receiving from one base station in advance an information as to whether the soft handover is possible or not with respect to each surrounding cell and/or sector of said one base station, and determining one cell and/or sector for which the soft handover is not possible according to the information for said cell and/or sector as notified from said one base station; and wherein said multiple types of handover include the soft handover and the hard handover, and at a time of handover with respect to said one cell and/or sector determined by the determining unit (11), the selecting unit (11) selects the hard handover as said one type of handover so that the checking unit (11) does not check the handover start condition for starting respective types of handover for the soft handover with respect to said one cell and/or sector. Apparatus according to claim 21, characterized in that it further comprises: a measuring unit (53) for measuring a radio channel state; and a determining unit (11) for determining each impossible type of handover that cannot be carried out according to the radio channel state measured by the measuring unit (53); and wherein the selecting unit (11) selects said one type of handover from said available types of handover which include said multiple types of handover except for said each impossible type of handover. Apparatus according to claim 21, characterized in that it further comprises: a measuring unit (53) for measuring an interference receiving level or a total receiving level of a frequency bandwidth currently used for communication; and wherein said multiple types of handover include a same code same frequency soft handover and a different frequency soft handover, and when a measured value obtained by the measuring unit (53) is greater than a prescribed threshold value, the selecting unit (11) selects the different frequency soft handover as said one type of handover so that the checking unit (11) does not check the handover start condition for the same code same frequency soft handover. Apparatus according to claim 21, characterized in that the selecting unit (11) is adapted to receive information transmitted from each base station measuring a radio channel state for each cell and/or sector of said each base station and determining each impossible type of handover that cannot be carried out for each cell and/or sector by said each base station according to the radio channel state measured by said each base station, and notifying an information indicating said each impossible type of handover to the mobile station; and wherein at a time of handover with respect to said each base station, the selecting unit (11) selects said one type of handover from said available types of handover which include said multiple types of handover except for said each impossible type of handover according to said information. Apparatus according to claim 27, characterized in that selecting unit (11) receives said information via a surrounding base station with which the mobile station is currently communicating after said each base station notifies said information to surrounding base stations. Apparatus according to claim 21, characterized in that the selecting unit (11) is adapted to receive information transmitted from each base station measuring an interference receiving level or a total receiving level of each frequency bandwidth that is implemented at said each base station for each cell and/or sector of said each base station, and notifying the information indicating that one frequency bandwidth cannot be used for one cell and/or sector to the mobile station, when a measured value for said one frequency bandwidth and said one cell and/or sector is greater than a prescribed threshold value; and wherein said multiple types of handover include a same code same frequency soft handover and a different frequency soft handover, and at a time of handover with respect to said one cell and/or sector while the mobile station is currently using said one frequency bandwidth as indicated by said information, the selecting unit (11) receives said information and selects the different frequency soft handover as said one type of handover so that the checking unit does not check the handover start condition for the same code same frequency soft handover with respect to said one cell and/or sector. Apparatus according to claim 29, characterized in that the selecting unit (11) receives said information via a surrounding base station with which the mobile station is currently communicating after said each base station notifies said information to surrounding base stations. Apparatus according to claim 21, characterized in that it further comprises: a memory (51) for storing the representing criteria for starting respective types of handover of said multiple types of handover; wherein the selecting unit (11) selects said one type of handover according to the handover start condition stored in the memory (51). Apparatus according to claim 31, characterized in that the memory (51) also stores an order of weakness of the handover start conditions among said multiple types of handover. CDMA base station apparatus for use as a base station in a CDMA mobile communication system for carrying out any of multiple types of handover including at least a soft handover and a hard handover, the multiple types of handover having different handover start conditions representing criteria for starting respective types of handover, wherein the handover start condition is weaker for the soft handover than the hard handover, comprising: a measuring unit (55) for measuring a radio channel state for each cell and/or sector of the base station; a determining unit (31) for determining each impossible type of handover that cannot be carried out for each cell and/or sector by the base station according to the radio channel state measured by the measuring unit (55); and a notifying unit (31) for notifying an information indicating said each impossible type of handover to a mobile station for judgement - at a time of handover with respect to the base station - one type of handover for which a handover start condition is weakest among available types of handover which include said multiple types of handover except for said each impossible type of handover according to said information, said one type of handover being a type of handover that is most effective from a viewpoint of radio channel capacity, and checking whether the handover start condition for said one type of handover is satisfied or not; wherein the base station is adapted to carry out said one type of handover upon being notified from the mobile station that the handover start condition for said one type of handover is satisfied. Apparatus according to claim 33, characterized in that the notifying unit (31) notifies said information to surrounding base stations and the mobile station receives said information via one of said surrounding base stations with which the mobile station is currently communicating.

Inventors: NAKAMURA TAKEHIRO, SATO TAKAAKI, NAKAMURA, TAKEHIRO, SATO, TAKAAKI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1997
Main classification: H04Q7
IPC codes: H04W36