JP2014033387A - Radio communication system and handover optimization method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radio communication system and a handover optimization method that can accomplish optimization of handover by detecting a handover failure caused by handover restraint.SOLUTION: The radio communication system comprises: a handover restraint unit (104) for restraining inter-cell handover of a mobile terminal (10) per mobile terminal; a handover failure detection unit (105) for detecting a handover failure caused by handover restraint; and a handover restraint optimization unit (203) for adjusting the degree of handover restraint on the basis of an occurrence rate of handover failures.

Description

  The present invention relates to a radio communication system having a plurality of cells, and more particularly to a radio communication system having a handover control function and a handover optimization method.

  In a wireless communication system having a plurality of base stations that manage a plurality of cells, when a mobile terminal moves from a connected cell to another cell, a connection destination called a handover (hereinafter abbreviated as HO as appropriate) is used. A cell switching process is performed, whereby the communication of the mobile terminal can be continued between the cells. In order to implement handover of a mobile terminal, measurement is first performed when a base station that manages a connected cell (hereinafter referred to as a source cell) generates a predetermined event for the mobile terminal. Instruct them to send a report. As the predetermined event, there is, for example, deterioration of the radio quality of the source cell. The measurement report by the mobile terminal includes the measurement results of the radio quality of the source cell and its neighboring cells. When the base station receives the measurement report from the mobile terminal, the base station determines a switching destination cell (hereinafter referred to as a target cell) based on the measurement report, and includes a handover procedure including signaling between the mobile terminal and the target cell. To start.

  An example of an event that triggers measurement report transmission is defined in Non-Patent Document 1 by LTE (Long Term Evolution) / E-UTRAN (Evolved UTRAN). The essential part of the reporting event defined as Event A3 (Neighbor becomes offset better than serving) in Non-Patent Document 1 is expressed by the following equation (1).

P _s + O _s <P _t + O _t (1)
Here, P _s is the measurement result of the radio quality of the source cell, P _t is the measurement result of the radio quality of the neighboring cell, O _s is the offset value for the radio quality of the source cell, and O _t is the offset value for the radio quality of the neighboring cell. It is.

If LTE, employed is the received power RSRP of _{P s} and _{P t} are the downlink reference signal (Reference Signal Received Power) or reference signal received quality RSRQ (Reference Signal Received Quality). RSRQ is the ratio of RSRP to the total received power RSSI (Received Signal Strength Indicator). Further, O _s in Equation (1) affects the radio quality of the downlink reference signal of the source cell, and O _t affects the radio quality of the downlink reference signal of the adjacent cell.

O _t is generally a handover parameter called cell individual offset (CIO), and a different offset value can be set for each neighboring cell registered in the neighboring list. The CIO is included in the neighbor list notified by the base station to the mobile terminal connected to the cell managed by the base station.

When the operating condition of Equation (1) is set in the base station, the operating condition of Equation (1) is notified to the mobile terminal connected to the cell managed by the base station. If the measurement results P _s and P _t of the radio quality of the source cell and the radio quality of the neighboring cell satisfy the condition of the formula (1), the mobile terminal triggers to the base station that manages the source cell. Send measurement report.

  When receiving the measurement report from the mobile terminal, the base station determines a target cell based on the measurement report and starts a handover process to the target cell. At this time, if the start of the handover is too late, the wireless quality of the source cell falls below the required quality before the handover to the target cell is completed, and an abnormal disconnection of communication occurs. On the other hand, if the start of the handover is too early, the radio quality of the target cell falls below the required quality immediately after the handover to the target cell is completed, and an abnormal disconnection of communication occurs. In addition, when the handover starts too early, the handover condition for the original cell is satisfied again immediately after completing the handover to the target cell, and repetitive handover (hereinafter referred to as ping-pong HO as appropriate) occurs. There is.

If the timing of handover to the target cell is too late and fails (hereinafter referred to as “Too Late HO”), the handover timing is increased by increasing the CIO (ie, the offset value O _t in equation (1)). The success rate of handover can be improved early. On the other hand, if the timing of handover to the target cell is too early to fail (hereinafter referred to as “Too Early HO”), the handover success rate can be improved by reducing the CIO and thereby delaying the handover timing. Therefore, it is possible to reduce handover failures by dynamically adjusting handover parameters such as the offset value O _t (CIO) described above.

  For example, Patent Document 1 discloses a HO failure (Too Late HO, Too Early HO) by dynamically adjusting HO parameters such as TTT (Time-To-Trigger) and CIO on a cell basis based on a measurement report of a terminal. ) Has been disclosed.

  According to Non-Patent Document 2, three types of handover failure accompanied by radio link failure (RLF: Radio Link Failure), namely “Too Late Handover”, “Too Early Handover”, and “Handover to Wrong Cell” It is defined as follows (22.4.2). In this specification, these three types of handover failures are treated as handover failures (HO failures).

<Too Late HO>
Too Late Handover is when a mobile terminal (UE) that has experienced RLF at the source cell during the handover process tries to re-establish connection (including re-establishment of radio link) to the target cell (target). , Handover from the source cell to the target cell. Or it is RLF which arises in a source cell when the mobile terminal which experienced RLF in the source cell before the start of a handover tries to reestablish connection to a cell different from the source cell.

<Too Early HO>
Too Early Handover is a handover from a source cell to a target cell when a mobile terminal that has experienced RLF in the target cell during execution of the handover process or immediately after completion of the handover attempts to reestablish connection to the source cell.

<HO to Wrong Cell>
Handover to Wrong Cell is a mobile terminal that has experienced RLF in the source cell or target cell during the handover process or immediately after the handover is completed, and tries to reestablish connection to a cell (neighbor) different from both the source cell and the target cell. Is a handover from the source cell to the target cell.

International Patent Publication No. 2010 / 002926A1

3GPP TS36.331 v.9.3.0 (5.5.4.4, pages 75-76) 3GPP TS36.300 v.9.7.0 (22.4.2, pages 158-159)

  However, in a cell with various moving speeds of a mobile terminal that performs handover, if the HO parameter for each cell is adjusted in order to eliminate Too Late HO of a terminal that moves at high speed, ping is performed at the terminal that moves at low to medium speed. -pong HO increases. Hereinafter, an increase in ping-pong HO will be described with reference to FIG.

  FIG. 1 is a schematic system configuration diagram for explaining an example of handover failure. In the radio communication system shown in FIG. 1, base stations (eNBs) 30A and 30B manage a cell 35A and a cell 35B, respectively, and a cell 35B exists in the cell 35A. Mobile terminal 10a and high-speed mobile terminal 10b) pass while continuing communication.

  In this way, when the mobile terminal 10b moving at a high speed and the mobile terminal 10a moving at a low speed are mixed in the cell 35B, if trying to avoid the Too Late HO failure in the high-speed mobile terminal 10b, the CIO is increased and the measurement report It is necessary to advance the timing. However, when the CIO is increased so that the handover of the high-speed mobile terminal 10b is successful in this way, the low-to-medium-speed mobile terminal 10a satisfies the measurement report transmission condition again immediately after completion of HO because the HO timing is too early. Therefore, ping-pong HO frequently occurs between cells. Conversely, if the CIO is decreased and HO is suppressed so as to avoid ping-pong HO in the low-speed to medium-speed mobile terminal 10a, the Too Late HO failure increases in the high-speed mobile terminal 10b.

  As described above, in the above-described handover optimization control, HO failure is likely to occur by suppressing even HO necessary for suppressing ping-pong HO, and it is not possible to determine whether the cause of HO failure is HO suppression. Therefore, appropriate HO suppression cannot be performed.

  Therefore, an object of the present invention is to provide a radio communication system and a handover optimization method that can achieve handover optimization by detecting a handover failure caused by HO suppression.

A radio communication system according to the present invention includes a base station that manages a cell to which a mobile terminal is connected, and has a function of controlling inter-cell handover of a mobile terminal, and performs inter-cell handover of the mobile terminal. A handover suppression unit that suppresses handover every time, a handover failure detection unit that detects a handover failure caused by the handover suppression, and a handover suppression optimization unit that adjusts the degree of handover suppression based on the occurrence rate of the handover failure It is characterized by providing.
A handover optimization method according to the present invention is a handover optimization method in a radio communication system that includes a base station that manages a cell to which a mobile terminal is connected, and that has a function of controlling handover between cells of the mobile terminal. Inter-cell handover is suppressed for each mobile terminal, handover failure caused by the handover suppression is detected, and the degree of handover suppression is adjusted based on the occurrence rate of the handover failure.

  According to the present invention, handover failure due to HO suppression can be detected and handover optimization can be achieved.

FIG. 1 is a schematic system configuration diagram for explaining an example of handover failure. FIG. 2 is a block diagram showing a schematic configuration of a base station and a network management system in a wireless communication system according to an embodiment of the present invention. FIG. 3 is a flowchart showing the operation of the base station according to the first embodiment of the present invention. FIG. 4 is a schematic diagram showing an example of the HO history analysis in FIG. 5A is a sequence diagram showing Too Late HO, FIG. 5B is a sequence diagram showing Too Early HO, and FIG. 5C is a sequence diagram showing HO to Wrong Cell. FIG. 6 is a flowchart showing the operation of the HO failure detection unit in the first embodiment. FIG. 7 is a flowchart showing the HO optimization operation according to the first embodiment. FIG. 8 is a flowchart showing the operation of the base station according to the second exemplary embodiment of the present invention. FIG. 9 is a flowchart showing detection of Incoming HO failure by the HO failure detection unit in the second embodiment. FIG. 10 is a flowchart showing the operation of the base station according to the third embodiment of the present invention. FIG. 11 is a flowchart showing the detection of the incoming HO failure type by the HO failure detection unit in the third embodiment. FIG. 12 is a flowchart showing the detection of the Outgoing HO failure type by the HO failure detection unit in the third embodiment. FIG. 13 is a block diagram showing a schematic configuration of a wireless communication system for explaining a first example of handover suppression in an embodiment of the present invention. FIG. 14 is a flowchart showing the operation of the base station in FIG. FIG. 15 is a flowchart showing an example of handover failure detection in FIG. FIG. 16 is a block diagram showing a schematic configuration of a wireless communication system for explaining a second example of handover suppression in one embodiment of the present invention. FIG. 17 is a flowchart showing a first example of the operation of the base station in FIG. FIG. 18 is a flowchart showing a second example of the operation of the base station in FIG. FIG. 19 is a block diagram showing a schematic configuration of a wireless communication system for explaining a third example of handover suppression in one embodiment of the present invention. FIG. 20 is a flowchart showing a first example of the operation of the base station in FIG.

  According to the embodiments of the present invention described below, by detecting a handover failure caused by handover (HO) suppression, the selection criterion of a mobile terminal that should suppress handover, that is, the degree of handover suppression depends on the handover quality. Adjust. This makes it possible to reduce handover failures while effectively suppressing unnecessary handovers.

  According to one embodiment of the present invention, a mobile terminal that suppresses handover is selected based on the most recent handover frequency between cells experienced by the mobile terminal, and the cell is suppressed by suppressing handover to the selected mobile terminal. In addition to being able to effectively suppress unnecessary handovers during the period, unnecessary handovers can be further effectively suppressed by adjusting the degree of handover suppression according to handover quality such as a handover failure rate.

  FIG. 2 is a block diagram showing a schematic configuration of a base station and a network management system in a wireless communication system according to an embodiment of the present invention. In FIG. 2, base stations (eNBs) 30A and 30B manage a cell 35A and a cell 35B, respectively, and there is a cell 35B in the cell 35A. Furthermore, it is assumed that another cell 35C is arranged adjacent to the cells 35A and 35B, and the base station 30C manages the cell 35C. In addition, a network management system 20 is provided to perform optimization control of inter-cell handover. The present invention is not limited to the system configuration shown in FIG. 3 and can be applied to inter-cell handover in an arbitrary cell arrangement.

  In the following description, as shown in FIG. 2, the mobile terminal 10 enters the cell 35B from the cell 35A, fails to perform the handover to the cell 35A through the cell 35B, enters the other cell 35C as it is, and enters the base station 30C. The case of reconnecting will be described. When the mobile terminal 10 enters the cell 35B from the cell 35A (Incoming HO; incoming handover), the cell 35A becomes the source cell and the cell 35B becomes the target cell. Further, when the mobile terminal 10 leaves the cell 35B to the cell 35A / 35C (Outgoing HO), the cell 35B is the source cell and the cell 35A / 35C is the target cell.

  Although the base stations 30A, 30B, and 30C have the same configuration, FIG. 2 shows only the portion related to the description of the present embodiment. The base station 30A includes a handover control unit 101A that executes handover control, the base station 30B includes a handover control unit 101B, and the base station 30C includes a handover control unit 101C. Furthermore, the base station 30B that manages the cell 35B includes a handover history analysis unit 102, a mobile terminal selection unit 103, a handover suppression unit 104, and a handover failure detection unit 105 as functions related to the operation of this embodiment.

  The network management system 20 includes a handover optimization unit 201 and a quality statistics storage unit 202, and the handover optimization unit 201 is provided with a handover suppression optimization unit 203. As will be described later, the quality statistics storage unit 202 acquires handover statistical information from each base station, and the handover optimization unit 201 sets a handover suppression threshold for the base station.

  When the mobile terminal 10 reconnects with the base station 30C in the cell 35C, the base station 30C notifies handover failure information described later to the base station 30B. When the handover failure detection unit 105 of the base station 30B acquires the handover failure information from the cell 35C, the handover failure detection unit 105 determines whether the handover failure is caused by the handover suppression, and determines the handover statistical information including the number of handover failures as the network management system 20. Report to The handover suppression optimization unit 203 of the network management system 20 adjusts for each cell by sequentially increasing / decreasing the handover suppression threshold based on the handover statistical information reported from the base station.

  As described above, there are three types of handover failure (HO failure): Too Late HO, Too Early HO, and HO to Wrong Cell. As the handover failure information, for example, RLF Indication or Handover Report defined in 3GPP TS36.423 v.9.5.0 can be used (9.1.2.18 and 9.1.2.19). The RLF Indication includes identification information of a mobile terminal that has caused an abnormal disconnection, identification information of a cell that was connected immediately before the abnormal disconnection, and identification information of a cell that has been reconnected after the abnormal disconnection. The Handover Report includes the identification information of the source cell and the target cell and the type of handover failure. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

1. First Embodiment FIG. 3 is a flowchart showing an operation of a base station according to a first embodiment of the present invention. In FIG. 3, first, the handover suppression unit 104 of the base station 30B acquires a handover suppression threshold from the network management system 20 (operation S101). As will be described later, the handover suppression threshold is a threshold N _TH of the number of ping-pong HOs.

  Subsequently, when the handover control unit 101B receives the handover request (operation S102; YES), the handover history analysis unit 102 analyzes the handover history and ping-pong between the cell 35B and the cell 35A as described later. The frequency of HO is totaled, and the mobile terminal selection unit 103 selects a mobile terminal that should suppress handover between the cell 35B and the cell 35A based on the analysis result (frequency of ping-pong HO) (operation S103). ).

  As will be described later, the handover suppressing unit 104 suppresses handover for the selected mobile terminal (operation S104). Furthermore, the handover suppressing unit 104 counts the number of times of handover suppression (Operation S105), and records the identification information of the mobile terminal that has suppressed the handover (Operation S106).

  Subsequently, when the handover failure detection unit 105 acquires the handover failure information from the base station that manages another cell, the handover failure detection unit 105 analyzes the handover failure as described later (Operation S107), and the Outgoing HO failure due to the handover suppression. The number is counted (operation S108), and the handover statistical information is reported to the network management system 20 (operation S109). The handover statistical information includes the number of handover suppressions counted in operation S105 and the number of Outgoing HO failures counted in operation S108. Hereinafter, handover history analysis, handover suppression, and handover failure detection will be described in detail.

1.1) Handover History Analysis As a hand-over history of a mobile terminal, for example, UE History Information defined in 3GPP TS36.423 v9.5.0 (Non-Patent Document 3) (from the base station 30A on the source side at the time of a handover request) Can be used). UE History Information records up to 16 cells where the mobile terminal stayed during communication, and the history is discarded when the mobile terminal transitions to the idle state. Therefore, the latest handover history is recorded in UE History Information. As another form of the handover history, a list in which cell IDs and stay periods are recorded for handovers executed within a predetermined period in the past may be used.

  FIG. 4 is a schematic diagram showing an example of the HO history analysis in FIG. As shown in FIG. 4, in the handover history, the staying cell ID and the staying time are recorded in time series. Therefore, the handover history analysis unit 102 can count the number N of ping-pong HOs between the cell 35B and the cell 35A by tracing the handover history to the end in the order from the newest record. Specifically, the hand-over history is traced to the end in order of newest records, and the count N is incremented by 1 every time all the following ping-pong HO count conditions 1 and 2 are satisfied.

Condition 1: The sum of the stay times of the cells that have stayed between the mobile terminal staying in the own cell (cell 35B) and the next stay in the own cell is a predetermined minimum stay time (MTS: Minimum Time of Stay) , Eg MTS = 5 sec).
Condition 2: The cell staying next to the own cell is the handover request source cell (cell 35A).

  In the handover history shown in FIG. 4, since the conditions 1 and 2 are satisfied twice, the number of ping-pong HOs N = 2.

If the ping-pong HO count N exceeds the handover suppression threshold N _TH received from the network management system 20, the mobile terminal selection unit 103 should suppress handover between the cell 35B and the cell 35A for the mobile terminal to be handed over Select as mobile terminal.

  In addition to the above-described form, the handover history analysis unit 102 may be configured to count the frequency of ping-pong HO generated between the cell 35B and the cell 35A per unit time in units of cell pairs. In this case, the mobile terminal selection unit 103 performs all handover attempts between the cell 35B and the cell 35A when the frequency of the ping-pong HO in units of cell pairs between the cell 35B and the cell 35A exceeds a predetermined value. You may select a mobile terminal as a mobile terminal which should suppress the hand-over between the cell 35B and the cell 35A.

1.2) Handover failure detection The base station of the source cell can detect the handover failure type as shown below by referring to the handover failure information (ie, RLF Indication and Handover Report).

<Too Late HO>
As shown in FIG. 5 (A), the base station of the source cell receives the RLF-Indication message from the target cell (when handover is started) or a cell different from the source cell (when handover is not started). And the Too Late HO can be detected on the condition that the UE Context Release message has not been transmitted to another cell within a predetermined period ( _{Tstore_UE_cntxt} ) prior to the reception of RLF-Indication.

<Too Early HO>
As shown in FIG. 5B, when the base station of the target cell receives the RLF-Indication message from the base station of the source cell, the UE Context Release message is _sent within a predetermined period (T _{store_UE_cntxt} ) prior to the reception of the RLF-Indication. If already transmitted to the base station of the source cell, a Handover Report message indicating Too Early HO is transmitted to the base station of the source cell. Therefore, the base station of the source cell can detect Too Early HO on condition that the Handover Report message is received and Handover Report Type IE indicates Too Early HO.

<HO to Wrong Cell>
As shown in FIG. 5C, when the base station of the target cell receives the RLF-Indication message from the neighboring cell, the UE Context Release message is transmitted to the source cell within a predetermined period ( _{Tstore_UE_cntxt} ) prior to the reception of the RLF-Indication. If it has already been transmitted to the base station, a Handover Report message indicating HO to Wrong Cell is transmitted to the base station of the source cell. Therefore, the base station of the source cell can detect the HO to Wrong Cell on condition that the Handover Report message is received and that the Handover Report Type IE indicates HO to Wrong Cell.

1.3) Handover suppression operation (first example)
When the handover suppressing unit 104 receives a handover request from the base station 30A that manages the other cell 35A, the handover terminal 104 selects the mobile terminal selected by the mobile terminal selection unit 103 (for example, the number N of ping-pong HOs with the cell 35A is predetermined). The mobile terminal exceeding the value N _TH is notified of the CIO value for suppressing the ping-pong HO with the cell 35A, and the other mobile terminals are notified of the CIO value managed in cell units.

The CIO value that suppresses ping-pong HO is set to a value that satisfies the condition of the following equation (2) (a necessary condition for ensuring handover hysteresis).
(a3offset _s + a3offset _t ) − (CIO _st + CIO _ts )> 0 (2)
a3offset _s : a3-offset of cell 35A
a3offset _t : a3-offset of cell 35B
CIO _st : CIO from cell 35A to cell 35B
CIO _ts : CIO from cell 35B to cell 35A

That is, the CIO value is determined by the following equation (3).
CIO _ts : = Min (CIO _ts , a3offset _s + a3offset _t −CIO _st −δ) (3)
Here, δ is a predetermined margin value for the hysteresis of handover.

  Note that the set value of the handover parameter (a3-offset, CIO) of the cell 35A can be acquired from, for example, HandoverPreparationInformation a AS-Config a sourceMeasConfig IE defined in Non-Patent Document 1. The IE is notified from the base station that manages the source cell at the time of request for Incoming HO.

1.4) Handover suppression operation (second example)
If the handover margin (a3offset-CIO) of the cell 35B is too large, a handover failure may occur. Therefore, in addition to the first example described above, the handover suppressing unit 104 can also notify the mobile terminal after adjusting the CIO value of the cell 35B so as not to excessively increase the handover margin of the cell 35B.

In order not to excessively increase the handover margin of the cell 35B, the CIO value of the cell 35B is set to a value satisfying the condition of the following expression (4).
a3offset _t -CIO _ts <MaxHoMargin (4)
a3offset _t : a3-offset of cell 35B
CIO _ts : CIO from cell 35B to cell 35A
MaxHoMargin: Upper limit of handover margin

That is, the CIO value is determined by the following equation (5).
CIO _ts : = Max (CIO _{ts, PingPong} , a3offset _t -MaxHoMargin) (5)
However, CIO _{ts and PingPong} are the CIO values determined in the first example.

  According to the second example, the CIO value of the cell 35B can be adjusted so as not to become excessively small. Note that the reason why the Max value is adopted in Expression (5) is because priority is given to prevention of handover failure over suppression of ping-pong HO.

1.5) Outgoing handover failure detection due to handover suppression FIG. 6 is a flowchart showing the operation of the HO failure detection unit in the first embodiment. In FIG. 6, the handover failure detection unit 105 determines whether or not an RLF Indication message has been received (operation S201). If the RLF Indication message is received (operation S201; YES), identification information of the mobile terminal that has caused the abnormal disconnection (hereinafter referred to as terminal UEa) is acquired (operation S202). Subsequently, the handover failure detection unit 105 determines whether or not the terminal UEa is a terminal that should suppress handover (that is, the HO suppression target selected by the mobile terminal selection unit 103) (Operation S203), and the terminal UEa Is the HO suppression target (operation S203; YES), the identification information of the cell X to which the terminal UEa has just connected is acquired (operation S204). Subsequently, the handover failure detection unit 105 determines whether or not the handover completion message of the terminal UEa has been transmitted to the cell X within a predetermined period (Operation S205), and if not transmitted (Operation S205; NO), Outgoing HO failure is detected (operation S206), and the process is terminated. Note that if the terminal UEa is not a HO suppression target (operation S203; NO), or if a handover completion message of the terminal UEa is transmitted to the cell X within a predetermined period (operation S205; YES), the process ends.

  If no RLF Indication message has been received (operation S201; NO), the handover failure detection unit 105 of the base station 30B determines whether an HO Report message has been received (operation S207). When the HO Report message indicating the handover failure is received (operation S207; YES), identification information of the handover destination cell (cell Z) of the failed handover is acquired (operation S208), and the mobile terminal subject to HO suppression within a predetermined period. Is handed over to the cell Z (operation S209). If handed over to cell Z (operation 209; YES), an Outgoing HO failure is detected (operation S206), and the process is terminated. If the HO Report message is not received (operation S207; NO), or if the HO suppression target mobile terminal has not handed over to the cell Z within a predetermined period (operation S209; NO), the process is terminated.

As described above, detection of failure of handover (Outgoing HO) when the mobile terminal goes out to another cell is divided into two systems. First, when the RLF Indication message is received (operation S201; YES), for example, referring to FIG. 5, handover of the terminal UEa is completed within a predetermined period in the past (for example, T _{store_UE_cntxt} ) from the reception of the RLF Indication message. If not, an abnormal disconnection has been notified for the same mobile terminal, so it is determined that Outgoing handover has failed. On the other hand, when the RLF Indication message is not received (operation S201; NO), HO suppression is performed for the HO destination cell (cell Z) indicated by the HO Report message within a predetermined period in the past from the reception of the HO Report message. If the target mobile terminal has been handed over, it is determined that Outgoing handover has failed.

1.6) Handover Optimization Next, the operation of the handover optimization unit 201 of the network management system 20 will be described with reference to FIG.

FIG. 7 is a flowchart showing the HO optimization operation according to the first embodiment. In FIG. 7, the network management system 20 acquires handover statistical information from the base station and stores it in the quality statistical storage unit 202 (operation S301). The handover statistical information includes the number of handover suppressions and the number of Outgoing HO failures that are aggregated at each base station. The handover suppression optimization unit 203 calculates a handover failure rate Rf caused by handover suppression based on the acquired handover statistical information (operation S302). The handover failure rate Rf can be defined as follows:
Rf = (Number of Outgoing HO failures due to suppression) / Number of handover suppressions.

Subsequently, the handover suppression optimization unit 203 reads the handover statistical information stored in the quality statistics storage unit 202, and whether the handover failure rate has increased compared to before changing the handover suppression threshold (N _TH ). It is determined whether or not (operation S303). If the handover failure rate has increased from before the handover suppression threshold is changed (operation S303; YES), the handover suppression threshold is changed in a direction to mitigate handover suppression (operation S304). Handover suppression is alleviated by increasing the handover suppression threshold _NTH and enhanced by decreasing the threshold _NTH .

  If the handover failure rate has not increased from before the handover suppression threshold is changed (operation S303; NO), the handover suppression optimization unit 203 reduces the ping-pong handover rate compared to before changing the handover suppression threshold. It is determined whether or not (Operation S305). If the ping-pong handover rate has decreased (operation S305; YES), the handover suppression threshold is changed in a direction to strengthen handover suppression (operation S306). If the ping-pong handover rate has not decreased (operation S305; NO), the handover suppression threshold is not changed. Thus, the handover suppression threshold value determined in operation S304 or S306 is notified to the base station (operation S307).

  As described above, the handover optimization unit 201 sequentially changes the handover suppression threshold and notifies the base station based on the handover failure rate before and after the change of the handover suppression threshold and the amount of change in the ping-pong handover rate. To do.

  Note that if the handover failure rate has not increased before the handover suppression threshold is changed (operation S303; NO), the handover optimization unit 201 may immediately change the handover suppression threshold in a direction to strengthen the handover suppression. It does not matter (operation S306). In this case, the handover optimizing unit 201 sequentially changes the handover suppression threshold based on only the change amount of the handover failure rate before and after the change of the handover suppression threshold and notifies the base station.

1.7) Effect According to the first embodiment of the present invention, the threshold value for handover suppression can be adjusted for each cell based on the results of handover suppression, and handover failures due to HO suppression can be reduced.

2. Second Embodiment According to the second embodiment of the present invention, the handover failure detection unit 105 detects not only the Outgoing HO failure but also the Incoming HO failure for the HO failure caused by the handover suppression, and the handover suppression optimization unit 203 adjusts the degree of handover suppression considering Incoming HO failure.

2.1) Incoming handover failure detection due to handover suppression FIG. 8 is a flowchart showing the operation of the base station according to the second embodiment of the present invention. As shown in FIG. 8, the operation of the base station 30B is the same as that of the first embodiment shown in FIG. 3, except that both the number of incoming HO failures and the number of outgoing HO failures due to handover suppression are totaled ( The operations S401 and S402) are different. The detection of Outgoing HO failure is the same as the flow shown in FIG. 6, and thus the description thereof will be omitted. Hereinafter, detection of Incoming HO failure will be described in detail.

  FIG. 9 is a flowchart showing detection of Incoming HO failure by the HO failure detection unit in the second embodiment. In FIG. 9, the handover failure detection unit 105 determines whether an RLF Indication message has been received (operation S501). If an RLF Indication message is received (operation S501; YES), identification information of a mobile terminal that has caused an abnormal disconnection (hereinafter referred to as terminal UEa) is acquired (operation S502). Subsequently, the handover failure detection unit 105 determines whether or not the terminal UEa is a mobile terminal that should suppress handover (that is, a HO suppression target selected by the mobile terminal selection unit 103) (Operation S503). If UEa is a HO suppression target (operation S503; YES), the identification information of the cell X to which the terminal UEa is connected immediately before is acquired (operation S504). Subsequently, the handover failure detection unit 105 determines whether or not the handover completion message of the terminal UEa has been transmitted to the cell X within a predetermined period (operation S505), and if it has transmitted (operation S505; YES), Incoming HO failure is detected (operation S506), and the process is terminated. If the terminal UEa is not a HO suppression target (operation S503; NO), or if the terminal UEa's handover completion message has not been transmitted to the cell X within a predetermined period of time (operation S505; NO), the process ends. .

  If no RLF Indication message has been received (operation S501; NO), the handover failure detection unit 105 of the base station 30B determines whether there is a mobile terminal that has attempted reconnection to the cell 35B (operation S507), and reconnection is performed. If there is a mobile terminal (hereinafter referred to as a terminal UEb) that has tried (Operation S507; YES), identification information of the terminal UEb is acquired (Operation S508). Subsequently, the handover failure detection unit 105 determines whether or not the terminal UEb is a mobile terminal that should suppress handover (that is, a HO suppression target selected by the mobile terminal selection unit 103) (operation S509). If the UEb is a HO suppression target (operation S509; YES), the identification information of the cell Y to which the terminal UEb is connected immediately before is acquired (operation S510). Subsequently, the handover failure detection unit 105 determines whether or not the HO Report message of the terminal UEb is received from the cell Y (Operation S511). If the HO Report message is not received from the cell Y (Operation S511; NO) Further, it is determined whether or not the handover request of the terminal UEb has been received from the cell Y within a predetermined period in the past from the reconnection attempt (operation S512). If the handover request of the terminal UEb is received from the cell Y within the predetermined period (operation S512; YES), the handover failure detection unit 105 detects an Incoming HO failure (operation S506) and ends the process. When there is no terminal UEb that has attempted reconnection (operation S507; NO), when the terminal UEb is not a HO suppression target (operation S509; NO), when a HO Report message of the terminal UEb is received from the cell Y (operation) S511; YES), or if the handover request of the terminal UEb has not been received from the cell Y within a predetermined period (operation S512; NO), the process is terminated as it is.

As described above, detection of failure of handover (Incoming HO) in which the terminal enters the cell 35B is divided into two systems. First, when the RLF Indication message is received (operation S501; YES), for example, referring to FIG. 5, although the incoming HO of the terminal UEa has been completed before the reception of the RLF Indication message, Since the time when the handover is completed is within a predetermined period ( _{Tstore_UE_cntxt} ) before the reception of the RLF Indication message (that is, notification of abnormal disconnection), it is determined that Incoming HO has failed. On the other hand, when there is a terminal UEb that has attempted to reconnect to the cell 35B without receiving the RLF Indication (operation S501; NO), for example, referring to FIG. Since the handover request regarding the same terminal UEb is received within a predetermined period in the past from the reconnection, it is determined that the incoming handover has failed.

2.2) Handover Optimization As described above, the handover failure detection unit 105 detects an Outgoing HO failure and an Incoming HO failure due to handover suppression, and determines the number of handover suppressions, the number of Incoming HO failures, and the number of Outgoing HO failures. The network management system 20 is notified of the handover statistical information including it.

The handover suppression optimization unit 203 of the network management system 20 adjusts the handover suppression threshold using the following definition as the handover failure rate.
Handover failure rate Rf =
(Outgoing HO failure count due to suppression + Incoming HO failure count due to suppression) / Handover suppression count

  As described in FIG. 7, the handover optimizing unit 201 calculates the handover failure rate before and after the change of the handover suppression threshold using the calculation formula of the handover failure rate Rf, and determines the handover failure rate and the ping-pong handover rate. Based on the amount of change, the threshold for handover suppression is sequentially changed and notified to the base station.

2.3) Effect According to the second embodiment of the present invention, it is possible to evaluate the handover suppression performance in consideration of not only the Outgoing HO failure but also the Incoming HO failure, and adjust the handover suppression threshold for each cell. It is possible to reduce handover failures due to HO suppression.

3. Third Embodiment According to the third embodiment of the present invention, the handover failure detection unit 105 detects not only the number of incoming HO failures and the number of outgoing HO failures, but also the type of handover failure. The handover suppression threshold is adjusted in consideration of the failure type.

  FIG. 10 is a flowchart showing the operation of the base station according to the third embodiment of the present invention. As shown in FIG. 10, the operation of the base station 30B is the same as that of the second embodiment shown in FIG. 8, except that both the number of Incoming HO failures and the number of Outgoing HO failures due to handover suppression are determined as HO failures. The points (operations S401a and S402a) to be totaled for each type are different. The detection of Outgoing HO failure and Incoming HO failure is the same as the flow shown in FIG. 6 and FIG.

3.1) Detection of Incoming HO Failure Type Caused by Handover Suppression FIG. 11 is a flowchart showing detection of an Incoming HO failure type by the HO failure detection unit in the third embodiment. In FIG. 11, operations S601 to S605 are the same as the operations S501 to S505 described in the second embodiment (FIG. 9) described above, and a description thereof will be omitted. When the handover completion message of the terminal UEa is transmitted to the cell X within the predetermined period in operation S605 (operation S605; YES), the handover failure detection unit 105 sets the source cell of the RLF Indication message and the transmission destination of the handover completion message. It is determined whether or not the cell X is the same (operation S606). If the source cell of the RLF Indication message and the destination cell X of the handover completion message are the same (operation S606; YES), the incoming Too Early HO is detected (operation) as shown in FIG. If it is different (operation S606; NO), an incoming HO to Wrong Cell is detected as shown in FIG. 5C (operation S608).

  If no RLF Indication message is received (operation S601; NO), operations S609 to S614 are executed in the same manner as operations S507 to S512 described in the second embodiment (FIG. 9) described above, and the terminal UEb within a predetermined period. If the handover request is received from the cell Y (operation S614; YES), the handover failure detection unit 105 detects the incoming Too Late HO (operation S615) as shown in FIG. To do.

3.2) Detection of Outgoing HO Failure Type Caused by Handover Suppression FIG. 12 is a flowchart showing detection of an Outgoing HO failure type by the HO failure detection unit in the third embodiment. In FIG. 12, operations S701 to S705 and S707 to S709 are the same as the operations S201 to S205 and S207 to S209 in FIG. If the handover failure detection unit 105 does not transmit the handover completion message of the terminal UEa to the cell X within a predetermined period (operation S705; NO), the handover failure detection unit 105 detects Outgoing Too Late HO (operation S706) and ends the process. . Note that if the terminal UEa is not a HO suppression target (operation S703; NO), or if a handover completion message of the terminal UEa is transmitted to the cell X within a predetermined period (operation S705; YES), the process ends.

  If no RLF Indication message has been received (operation S701; NO), the handover failure detection unit 105 has performed operations S707 to S709 as described above, and the mobile terminal subject to HO suppression has handed over to the cell Z within a predetermined period. If there is (Operation 709; YES), Outgoing Too Early HO or Outgoing HO to Wrong Cell is detected (Operation S711, S712) depending on whether the type of handover failure is Too Early (Operation S710). If the HO Report message is not received (operation S707; NO), or if the mobile terminal subject to HO suppression has not handed over to the cell Z within a predetermined period (operation S709; NO), the processing is terminated as it is.

3.3) Handover Optimization As described above, the handover failure detection unit 105 detects the type of Incoming HO failure and the type of Outgoing HO failure caused by handover suppression, such type information and the number of times of handover suppression, Incoming Handover statistics information including the number of HO failures and the number of Outgoing HO failures is notified to the network management system 20.

The handover suppression optimization unit 203 of the network management system 20 adjusts the handover suppression threshold using the following definition as the handover failure rate.
Handover failure rate Rf =
(Number of Outgoing Too Late HOs due to suppression + Number of Incoming Too Early HOs due to suppression + Number of Incoming HO to Wrong Cells due to suppression) / Number of handover suppressions

  As described in FIG. 7, the handover optimizing unit 201 calculates the handover failure rate before and after the change of the handover suppression threshold using the calculation formula of the handover failure rate Rf, and determines the handover failure rate and the ping-pong handover rate. Based on the amount of change, the threshold for handover suppression is sequentially changed and notified to the base station.

3.4) Effect According to the third embodiment of the present invention, the handover suppression threshold can be adjusted for each cell in consideration of the type of handover failure, and the handover suppression threshold can be adjusted for each cell. Can be reduced.

4). Examples of handover suppression Several examples of handover suppression methods will be described below.

4.1) HO Suppression by Handover Rejection Response When a mobile terminal handover request is received from a base station managing another cell, the mobile terminal handover (Incoming HO) is suppressed based on the handover history of the mobile terminal. If it is a suppression target, a response rejecting the handover of the mobile terminal is returned to the handover requesting base station. Thereby, unnecessary handovers such as repeated handovers can be suppressed. In this example, the handover optimizing unit 201 uses the threshold N _TH of the ping-pong HO number as the handover suppression threshold set in the base station, and adjusts the threshold N _TH according to the handover failure rate, thereby performing the handover optimization. Can be achieved.

  FIG. 13 is a block diagram showing a schematic configuration of a wireless communication system for explaining a first example of handover suppression in an embodiment of the present invention. In FIG. 13, a cell 35A is a source cell and a cell 35B is a target cell. In response to a handover request from the base station 30A that manages the source cell 35A, the base station 30B that manages the target cell 35B responds with a handover permission / rejection. return it. Although the base stations 30A and 30B have the same configuration, only the portions related to the description of this example are shown in FIG. The base station 30A includes a handover control unit 101A that performs handover control, and the base station 30B includes a handover control unit 101B. Furthermore, the base station 30B that manages the cell 35B includes a handover history analysis unit 102, a mobile terminal selection unit 103, and a handover suppression unit 104 as functions related to the operation of the present embodiment.

  FIG. 14 is a flowchart showing the operation of the base station in FIG. In FIG. 14, when receiving a handover request from the base station 30A managing the cell 35A (Operation S801; YES), the handover history analysis unit 102 acquires the handover history of the mobile terminal 10 (Operation S802). The handover history can be received from the base station 30A. Subsequently, as described with reference to FIG. 4, the handover history analysis unit 102 analyzes the handover history and counts the frequency of ping-pong HO between the cell 35B and the cell 35A (operation S803).

  Based on the analysis result (frequency of ping-pong HO), the mobile terminal selection unit 103 selects a mobile terminal that should suppress handover between the cell 35B and the cell 35A (operation S804). The handover suppressing unit 104 determines whether the handover target mobile terminal is selected as the handover suppressing target (operation S805). If it is not a handover suppression target (operation S805; NO), the handover suppression unit 104 returns a handover permission response to the base station 101A of the cell 35A (operation S806). If it is a handover suppression target (operation S805; YES), the handover is performed. A rejection response is returned (operation S807).

  Also, in this example, mobile terminals that hand over from other cells that meet the predetermined criteria reject handover, so detection of Outgoing HO failure due to handover suppression is unnecessary, and Incoming Too Since Early HO and HO to Wrong Cell do not occur, only Incoming Too Late HO needs to be detected.

  Accordingly, the handover failure detection unit 105 detects Incoming Too Late HO caused by handover suppression. The fact that handover suppression is the cause of handover failure can be determined in consideration of the identity of terminal identification information and the elapsed time since handover suppression, as described below. Details will be described below.

  FIG. 15 is a flowchart showing an example of handover failure detection in FIG. In FIG. 15, the handover failure detection unit 105 of the base station 30B determines whether or not there is a mobile terminal that has attempted reconnection to the cell 35B (operation S901), and the mobile terminal that has attempted reconnection (hereinafter referred to as terminal UEa). .) (Operation S901; YES), the identification information of the terminal UEa is acquired (operation S902). Subsequently, the handover failure detection unit 105 determines whether or not the terminal UEa is a mobile terminal that should suppress handover (that is, a HO suppression target selected by the mobile terminal selection unit 103) (operation S903). If UEa is the HO suppression target (operation S903; YES), the identification information of the cell X to which the terminal UEa is connected immediately before is acquired (operation S904). Subsequently, the handover failure detection unit 105 determines whether or not the handover request of the terminal UEa has been received from the cell X within a predetermined period (operation S905), and if it has been received (operation S905; YES), Incoming Too Late HO is detected (operation S906), and the process is terminated. When there is no terminal UEa (operation S901; NO), when the terminal UEa is not a HO suppression target (operation S903; NO), or when a handover request for the terminal UEa is not received from the cell X within a predetermined period (operation) In S905; NO), the process is terminated as it is.

  As described above, when the mobile terminal subject to HO suppression attempts to reconnect to the cell 35B after the handover request to the cell 35B is rejected, it is determined that an abnormal disconnection has occurred in the cell X before the handover is started. Too Late HO is detected.

4.2) HO Suppression by Preventing Handover to Other Cell FIG. 16 is a block diagram showing a schematic configuration of a radio communication system for explaining a second example of handover suppression in an embodiment of the present invention. As shown in FIG. 16, after a handover from another cell (that is, Incoming HO) is not suppressed and the mobile terminal selected in the same manner as in the first embodiment described above completes Incoming HO. When there is a measurement report from the mobile terminal, the target cell is determined by excluding other cells connected immediately before from the target cell candidate for handover (ie, Outgoing HO) of the mobile terminal. That is, unnecessary handovers such as repeated handovers are suppressed by avoiding handovers to other cells that have frequently caused unnecessary handovers in the past.

4.2.1) Operation (first example)
FIG. 17 is a flowchart showing a first example of the operation of the base station in FIG. In FIG. 17, the description of operations S801 to S804 is the same as described with reference to FIG.

  When there is a measurement report from the mobile terminal connected to the cell 35B (operation S1001; YES), the handover suppressing unit 104 determines whether the mobile terminal is the mobile terminal selected in operation S804 (operation S1002). If it is not the selected mobile terminal (operation S1002; NO), the handover control unit 101B starts normal handover processing, determines the target cell of the mobile terminal (operation S1003), and transmits a handover request to the target cell. (Operation S1004), the process ends.

  If the mobile terminal is the mobile terminal selected in operation S804 (operation S1002; YES), the handover suppressing unit 104 excludes the cell 35A from the handover candidate cell of the handover control unit 101B (operation S1005), and the handover control unit 101B If handover candidate cells remain (operation S1006; YES), the target cell of the mobile terminal is determined (operation S1003), and a handover request is transmitted to the target cell (operation S1004). If there is no handover candidate cell (operation S1006; NO), the process ends.

In this first example, the handover optimizing unit 201 uses the threshold N _TH of the number of ping-pong HOs as the handover suppression threshold set in the base station, and adjusts the threshold N _TH according to the handover failure rate, so that the handover optimum Can be achieved.

4.2.2) Operation (second example)
The difference from the first example of FIG. 17 described above is that a comparison judgment operation between the radio quality Q _self of the own cell and the HO suppression threshold Q _min is entered. That is, whether to perform the operation of excluding the previous connected cell from the handover candidate cell depends on the radio quality of the cell 35B.

FIG. 18 is a flowchart showing a second example of the operation of the base station in FIG. In FIG. 18, operations S801 to S804 and operation S1001 are the same as those in the first example shown in FIG. When there is a measurement report from the mobile terminal connected to the cell 35B (operation S1001; YES), the handover suppressing unit 104 determines whether or not the radio quality Q _self of the cell 35B is smaller than a predetermined value _Qmin (operation). S1007). RSRP or RSRQ can be used as the radio quality Q _self of the cell 35B.

If Q _self <Q _min (operation S1007; YES), the handover control unit 101B starts normal handover processing, determines the target cell of the mobile terminal (operation S1003), and transmits a handover request to the target cell. (Operation S1004), the process ends. If the radio quality Q _self of the cell 35B is equal to or greater than the predetermined value Q _min (operation S1007; NO), the handover suppressing unit 104 determines whether the mobile terminal is the mobile terminal selected in operation S104 (operation S1002). . If it is not the selected mobile terminal (operation S1002; NO), operations S1003 and S1004 are executed as described above.

  If the mobile terminal is the mobile terminal selected in operation S804 (operation S1002; YES), the handover suppressing unit 104 excludes the cell 35A from the handover candidate cell of the handover control unit 101B (operation S1005), and the handover control unit 101B If handover candidate cells remain (operation S1006; YES), the target cell of the mobile terminal is determined (operation S1003), and a handover request is transmitted to the target cell (operation S1004). If there is no handover candidate cell (operation S1006; NO), the process ends.

In this second example, the handover optimizing unit 201 uses a radio quality threshold Q _min in addition to the threshold N _TH of the number of ping-pong HOs as the handover suppression threshold set in the base station, and the threshold N according to the handover failure rate. Adjust _TH and Q _min . When adjusting the threshold value Q _min , handover suppression is alleviated by increasing the threshold value Q _min and enhanced by reducing the threshold value Q _min . As a result, the second example has the advantage that handover failure due to HO suppression is less likely to occur, because whether or not to suppress handover is determined based on the radio quality of the mobile terminal.

4.3) Handover suppression by adjusting HO parameters No suppression is performed for handover from another cell (Incoming HO), and only the selected mobile terminal as described above is ping-pong HO with other cells. A handover parameter that suppresses unnecessary handover such as is notified. As a result, it is possible to suppress handovers to other cells (Outgoing HO) that frequently caused unnecessary handovers in the past, and to suppress unnecessary measurement reports from mobile terminals.

  FIG. 19 is a block diagram showing a schematic configuration of a wireless communication system for explaining a third example of handover suppression in one embodiment of the present invention. As shown in FIG. 19, it differs from the first example in FIG. 13 in that the incoming HO from other cells is not suppressed and the handover parameter is acquired from the base station that manages the other cells. Yes. Hereinafter, the HO suppression operation will be described focusing on this difference.

  FIG. 20 is a flowchart showing a first example of the operation of the base station in FIG. In FIG. 20, operations S801 to S804 are the same as the operations shown in FIG. 20, when notifying the handover parameter to the mobile terminal connected to the cell 35B, the handover suppressing unit 104 determines whether the mobile terminal is the mobile terminal selected in operation S804 (operation S1101). If it is not the selected mobile terminal (operation S1101; NO), the handover control unit 101B notifies the mobile terminal of the handover parameters common to the cell 35B (operation S1102).

  If the mobile terminal is the mobile terminal selected in operation S804 (operation S1101; YES), the handover suppressing unit 104 uses the equation (3) or the equation (5) to handover parameters that suppress ping-pong HO. Is calculated (operation S1103) and notified to the mobile terminal (operation S1104). In this way, only the mobile terminal selected by the mobile terminal selection unit 103 is notified of the handover parameter that suppresses the ping-pong HO with the cell 35A, so that an unnecessary measurement report from the mobile terminal can be obtained. The battery consumption of the mobile terminal and the processing load on the base station and the mobile terminal can be suppressed.

Also in this example, the handover optimizing unit 201 uses the threshold N _TH of the number of ping-pong HOs as the handover suppression threshold set in the base station, and adjusts the threshold N _TH according to the handover failure rate, thereby performing the handover optimization. Can be achieved.

  The present invention can be used for inter-cell handover control in a mobile communication system.

10, 10a, 10b Mobile terminal 20 Network management system 30, 30A, 30B Base station (eNB)
101, 101A, 101B, 101C Handover control unit 102 Handover history analysis unit 103 Mobile terminal selection unit 104 Handover suppression unit 105 Handover failure detection unit 201 Handover optimization unit 202 Quality statistics storage unit 203 Handover suppression optimization unit

Claims (27)

A wireless communication system comprising a base station for managing a cell to which a mobile terminal is connected and having a function of controlling inter-cell handover of a mobile terminal,
Handover suppressing means for suppressing inter-cell handover of a mobile terminal for each mobile terminal;
A handover failure detecting means for detecting a handover failure caused by the handover suppression;
Handover suppression optimization means for adjusting the degree of handover suppression based on the occurrence rate of the handover failure;
A wireless communication system comprising:   The handover failure detection means detects a handover failure caused by the suppression of the handover by determining the identity of the mobile terminal that has suppressed the handover and the mobile terminal that has abnormally disconnected the radio line during the handover. The wireless communication system according to claim 1.   The handover failure detection means detects a handover failure caused by the handover suppression when a mobile terminal that has suppressed the handover has an abnormal disconnection of a radio line within a predetermined time from the handover suppression. The wireless communication system according to claim 2.   The handover failure detection means uses the information acquired by the base station that manages the first cell, and the handover failure of the mobile terminal from the first cell to the second cell and the entry from the second cell to the first cell. The radio communication system according to any one of claims 1 to 3, wherein at least one of the handover failures is detected.   5. The wireless communication system according to claim 4, wherein the handover suppression optimization means calculates the occurrence rate of the handover failure using at least one of outgoing handover failure and incoming handover failure.   The handover suppression optimizing unit detects a change in the occurrence rate of the handover failure before and after changing the degree of handover suppression, and adjusts the degree of handover suppression based on the change. The radio | wireless communications system of any one of Claims 1-5.   The wireless communication system according to claim 6, wherein the handover suppression optimization means relaxes the degree of handover suppression when the occurrence rate of the handover failure increases after the change in the degree of handover suppression.   The wireless communication system according to claim 6, wherein the handover suppression optimization means enhances the degree of handover suppression when the occurrence rate of the handover failure does not increase after the change in the degree of handover suppression. .   The handover suppression optimizing unit, when selecting a mobile terminal to be suppressed for handover based on the frequency of handover, changes the degree of handover suppression by adjusting a frequency threshold as a selection criterion. The radio | wireless communications system of any one of Claims 1-8.   10. The wireless communication system according to claim 9, wherein the handover suppression optimization unit further changes the degree of handover suppression by adjusting a quality threshold of radio quality of the own cell.   The wireless communication system according to claim 1, wherein the handover failure detection unit determines a type of handover failure from a signal sequence of handover failure.   12. The wireless communication system according to claim 11, wherein the handover suppression optimization means calculates the handover failure occurrence rate using the type of handover failure frequency affected by the handover suppression. A handover optimization method in a radio communication system comprising a base station that manages a cell to which a mobile terminal is connected and having a function of controlling handover between cells of the mobile terminal,
Suppressing inter-cell handover of mobile terminals for each mobile terminal,
Detecting a handover failure due to the handover suppression;
Adjusting the degree of handover suppression based on the handover failure rate,
A handover optimization method characterized by the above.   The handover failure due to the suppression of the handover is detected by determining the identity of the mobile terminal that has suppressed the handover and the mobile terminal in which the abnormal disconnection of the radio line has occurred during the handover. Item 14. The handover optimization method according to Item 13.   15. The handover failure caused by the suppression of the handover is detected when a mobile terminal that has suppressed the handover is disconnected abnormally within a predetermined time from the handover suppression. The handover optimization method according to claim 1.   The handover failure due to the suppression of the handover is performed by using the information acquired by the base station that manages the first cell, the outgoing handover failure from the first cell to the second cell of the mobile terminal and the second cell from the second cell. The handover optimization method according to any one of claims 13 to 15, wherein at least one of the handover failures to the incoming handover to one cell is a failure.   The handover optimization method according to claim 16, wherein the occurrence rate of the handover failure is calculated using at least one of an outgoing handover failure and an incoming handover failure.   The degree of handover suppression is adjusted based on a change in the occurrence rate of the handover failure detected before and after changing the degree of handover suppression. The handover optimization method according to any one of the above.   The handover optimization method according to claim 18, wherein the degree of handover suppression is alleviated when the occurrence rate of the handover failure increases after the change of the degree of handover suppression.   The handover optimization method according to claim 18, wherein the degree of handover suppression is strengthened when the occurrence rate of the handover failure decreases after the change of the degree of handover suppression.   21. The degree of handover suppression is changed by using a frequency threshold as a selection criterion when selecting a mobile terminal to be subjected to handover suppression based on the frequency of handover. The handover optimization method according to claim 1.   The handover optimization method according to claim 21, wherein the degree of handover suppression is further changed by adjusting a quality threshold of radio quality of the own cell.   The handover optimization method according to any one of claims 13 to 22, further comprising determining a handover failure type from a handover failure signal sequence.   The handover optimization method according to claim 23, wherein the handover failure occurrence rate is calculated using the type of handover failure frequency affected by the handover suppression. A base station having a function of managing cells in a radio communication system and controlling inter-cell handover by a mobile terminal,
Handover suppressing means for suppressing inter-cell handover of a mobile terminal for each mobile terminal according to information indicating a degree of handover suppression;
A handover failure detecting means for detecting a handover failure caused by the handover suppression;
A base station characterized by comprising:   Further comprising control means for notifying the network management system of statistical information regarding the handover failure, receiving information indicating the degree of handover suppression adjusted based on the statistical information, and controlling the handover suppression means. The base station according to claim 25. A network management system that manages a base station that manages a cell to which a mobile terminal is connected,
Statistical information storage means for receiving and storing statistical information related to handover failure caused by the base station suppressing inter-cell handover of the mobile terminal for each mobile terminal;
Handover suppression optimization means for adjusting the degree of handover suppression of the base station based on the handover failure occurrence rate;
A network management system comprising:

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