JP2833551B2 - Forced handover method for mobile communication systems - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a handover system for mobile communication, and more particularly to a priority call transmission used for contacting a newly-created police, fire department, etc. in a radio base station area where radio channels are congested. The present invention relates to a forced handover method in which a call during a call that has the lowest probability of causing a call loss when handed over is promptly switched to a radio channel of an adjacent radio base station in order to allocate a call channel to the request. is there.

[0001]

2. Description of the Related Art An outline of a typical mobile communication system will be described with reference to FIG.

[0002] A mobile communication network includes a mobile communication switching center (hereinafter abbreviated as MSC) connected to a fixed communication network, and a plurality of radio base stations (hereinafter, BSs) connected to the MSC.
) And a mobile station (hereinafter abbreviated as MS) that performs communication while moving. The MSC is connected to a fixed communication network, performs connection processing between MSs and communication between a terminal in the fixed communication network and the MS, and performs location registration of the MS which is a control specific to mobile communication, switching of a communication channel, and the like. Is performed. The number of BSs is appropriately set according to the size of the area for providing the mobile communication service, the terrain condition, and the like, configures a radio communication area by radio waves, and performs communication with the MS existing in the radio communication area.

In FIG. 1, the installed BSs are BS1 to BS1.
BS7 (reference numerals 21 to 27), and it is assumed that MS1 to MS4 (reference numerals 31 to 34) are present as MSs in the wireless communication area of BS1.

In such a mobile communication system, there is a limit to the number of radio wave channels that can be used for communication within a radio communication area provided by each BS. Therefore, when a large number of MSs concentrate in a specific wireless communication area and perform communication there, a congestion state in which a communication channel is insufficient occurs.
In this congestion state, if a certain MS makes a priority call for communication to the fire department, police, etc., one of the communications in use is forcibly transmitted by the radio wave emitted by the adjacent BS. A forced handover process of switching to a channel and allocating a vacant communication channel to a priority call is performed. In this case, it is assumed that adjacent BSs are not in a congestion state.

[0005] In this forced handover process, various techniques have been proposed as to which MS in communication is to be forcibly handed over.

As the simplest method, there is a method in which the terminal identification number of the communicating MS is the youngest.
However, in this method, the handover is performed ignoring the situation of the target MS altogether, and it is unclear whether communication is possible on the communication channel to which the MS has handed over. This leads to call loss, which is not preferable.

Therefore, a mobile station terminal assisted handover technique disclosed in Japanese Patent Application Laid-Open No. 4-344723 and a communication network driven handover technique disclosed in Japanese Patent Application Laid-Open No. 62-159529 are disclosed. Has been proposed. The respective technologies will be described below.

[0008] The mobile station terminal assisted handover means that the electric field strength of a radio wave emitted by a BS currently communicating and the electric field strength of a radio wave emitted by an adjacent BS are determined by each MS.
Measure and periodically report to the MSC via the BS. Then, the MSC, which has become congested and determines the necessity of forced handover, identifies the most suitable MS for handover to the adjacent BS from the latest field strength report information and issues a handover instruction to the MS. It is.

FIG. 7 is a sequence diagram for explaining the operation of the mobile station terminal assisted handover. With reference to FIG. 1, in FIG.
1 to MS4 exist and the communication channel CH usable by BS1
It is assumed that the mobile stations 1 to 3 are in the congestion state by using the mobile stations 1 to 3 respectively. In this state, it is assumed that the MS 4 requests transmission of a priority call through a control channel.

Each of the MSs during a call communicates with the receiving field strength of the radio wave emitted by the BS1 currently communicating with the adjacent BS2 to BS7.
Of the radio waves emitted by each of the BSs are periodically measured and reported to the MSC via the BS1 by a period measurement report message.

When the MS 4 issues a priority call transmission request, the MSC makes the following calculations and judgments based on the information of the received latest cycle measurement report message.

(1) For each MS, find the difference between the received field strength of BS1 and the received field strength of each adjacent BS,
Determine the maximum difference for each MS.

(2) The maximum difference between the MSs is compared to identify the MS having the largest maximum difference.

(3) The MS having the largest difference is set as a candidate for forced handover.

As described above, for example, if the difference between the MS 2 and the BS 2 is the largest, the MSC sends a handover execution message to the MS 2 to forcibly execute the handover. Perform handover and BS
MS2, which has switched to the communication channel using radio wave 2,
Send a handover complete message to the MSC.

[0016] The MSC allocates the vacant communication channel to the priority call of the MS 4 by the above operation, and sends a channel assignment message to the MS 4.

Next, the operation of the communication network initiated handover will be described with reference to a sequence diagram for explaining the operation of the communication network initiated handover of FIG. 8 and FIG.

In this communication network-driven handover, each MS selects the most suitable MS for the handover.

The presupposed conditions are the same as those in the mobile station terminal assisted handover. It is assumed that congestion occurs in the communication area of the BS 1 and the MS 4 requests transmission of a priority call.

The MS that has determined the necessity of forced handover
C sends a forced call channel switching instruction order a to BS1 to determine the MS to be handed over, and instructs the selection of the MS. In the forced communication channel switching instruction order a, a communication level value to be compared and compared by the BS for selecting the MS is set.

The BS1 measures the communication level of each MS existing in the communication area of the BS1 and compares it with the communication level value set in the compulsory communication channel switching instruction order a. An MS having a communication level lower than the communication level value set in the compulsory communication channel switching instruction order a is reported to the MSC as a handover target. The information transmitted at this time is the call channel switching request order b, and includes, for example, information indicating that the handover target is MS2.

The MSC that has received the handover target MS information sends the MS2 to the other BSs (BS2 to BS7).
It is sequentially determined whether or not communication with is possible. This is MS
2 notifies each BS of the frequency of the channel communicating with BS1, and makes a determination by measuring the electric field strength at each BS. FIG. 8 shows a case where BS2 to BS4 determine that communication is not possible and BS5 determines that communication is possible. At this point, the MSC instructs the MS 2 to execute a handover to the traffic channel of the BS 5.

The MS2 that has completed the handover is BS5.
And report the handover completion to the MSC via. MS
The MSC that has received the handover completion of No. 2 allocates a free communication channel in BS2 to MS4.

[0024]

There are conventional techniques as described above in the forced handover scheme of the mobile communication system, but these conventional techniques have the following problems.

The mobile station terminal assisted handover technique disclosed in Japanese Patent Application Laid-Open No. 4-344723 has a problem that it takes time to determine an MS to be handed over.

This is because the MSC has to perform calculations for determining the most suitable MS for handover based on the information of the period measurement report received from each MS. In this calculation, the difference of the electric field strength between the own BS and each of a plurality of adjacent BSs is calculated for each MS, the maximum difference is calculated from the differences, and then the MSs are compared with each other to obtain the maximum difference. The MS having the difference is set as a handover candidate. Therefore, as the number of MSs present in the wireless communication area of the BS increases, and as the number of adjacent BSs increases, the load on the calculation process increases, and it takes time to determine an MS as a handover candidate.

Further, the communication network-driven handover technique disclosed in Japanese Patent Laid-Open No. 62-159529 has a problem that the MS to be handed over is not always determined optimally.

This is because each BS measures and determines the electric field strength of the uplink radio wave emitted by the candidate MS. The radio wave emitted by the MS performs transmission output control called power control so that the MS emits a radio wave having an optimum intensity. Therefore, it cannot be said that the electric field strength of the radio wave of the MS received by each BS is not necessarily proportional to the distance to the MS, and even if a certain BS determines that handover is possible and switches the communication channel, the communication is performed. Quality degradation may occur.

[0029]

SUMMARY OF THE INVENTION A forced handover method for a mobile communication system according to the present invention solves the above-mentioned problem, and quickly switches an optimal MS to be handed over to switch a communication channel. The present invention provides a forced handover scheme that does not burden the MSC and has high reliability.

In the forced handover method of the mobile communication system according to the present invention, a mobile communication switching center for controlling a mobile communication switching process and a plurality of radio base stations constituting a wireless communication area under the control of the mobile communication switching center are provided. In a mobile communication system composed of a station and a plurality of mobile stations communicating with the radio base station in a radio communication area provided by each radio base station, in a mobile station terminal-driven handover,
The mobile switching center has the following means.

(1) In a congestion state in which all the communication channels in the radio communication area are used, when a request for a priority call that needs to be connected with priority over other communication is received, the congestion state is entered. Means for transmitting a first handover message instructing all mobile stations that are talking in the wireless communication area to perform a handover to an adjacent wireless base station at the discretion of each mobile station, (2) When a response indicating that handover is impossible is received from all mobile stations in response to the first handover message, a specific mobile station is selected, and the mobile station is forcibly instructed to execute handover. Means for transmitting a second handover message; (3) performing a handover according to the instructions of the first handover message and the second handover message; Means for assigning a traffic channel the mobile station had been used was the priority call.

Further, the first handover message includes threshold information for allowing the mobile station to determine whether or not handover is possible. The mobile station includes the following means.

(1) When the first handover message is received, the radio wave of the currently communicating radio base station and the radio wave of each adjacent radio base station are measured, and the measurement result is referred to as the first handover message. Means for judging whether handover is possible or not by comparing it with threshold information included in the message; (2) when it is judged that handover is impossible, mobile communication switching including measurement results optimal for handover among measurement results Means for sending a handover disabled report message to the station.

When the mobile switching center receives the handover failure report message from all the mobile stations, the mobile switching center compares the best measurement result for the handover contained in the handover failure report message between the mobile stations. In addition, there is provided a means for selecting a mobile station which has reported an optimum measurement result for handover as a target of forced handover.

In the mobile station terminal assist type handover, the mobile communication switching center has the following means.

(1) In a congestion state in which all communication channels in the wireless communication area are used, when a request for a priority call that needs to be connected with priority over other communication is received, the state is changed to the congestion state. Means for transmitting a report instruction message for instructing all mobile stations communicating in a wireless communication area to report the radio wave reception status of the currently communicating wireless base station and each adjacent wireless base station, (2) reporting When a response message to the instruction message is received from each of the mobile stations, the mobile station that has reported the one with the largest electric field strength difference included in these response messages is selected, so that the mobile station can move with the optimal adjacent radio base station for handover. Means for determining a station and sending a forced handover message for instructing the mobile station to execute handover; (3) forced handover Means for allocating, to the priority call, a communication channel used by the mobile station that has performed the handover in response to the instruction of the over message, the mobile station (1) upon receiving the report instruction message, radio of the electric field intensity of radio waves were measured and measurement
Optimal adjacent radio base station for handover by selecting the adjacent radio base station with the maximum electric field strength difference that is the value obtained by subtracting the electric field strength of the radio base station currently communicating from the electric field strength of the base station radio wave Means for judging the station; and (2) means for including the optimum adjacent radio base station information for the judged handover and the measured electric field strength difference in a response message and sending the response message to the mobile communication switching center.

[0037]

[0038]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 2 is a block diagram showing the configuration of a mobile communication system to which the forced handover method of the present invention is applied. This mobile communication system includes an MSC 1 connected to a fixed communication network 4 for performing mobile communication exchange processing, and a plurality of BSs 1 to 3 forming a wireless communication area under the control of the MSC 1.
BSn and B in the wireless communication area provided by each BS
S and a plurality of MSs 3 (only one is shown) that communicate with each other.

The MS 3 has the following functional units as components related to the forced handover method of the present invention.

(1) Adjacent base station radio wave measuring function unit 31 for measuring the received electric field strength of the radio wave from the BS currently communicating and the received electric field strength of the radio wave from each adjacent BS. Upon receiving the over message, a field strength difference calculation function unit 32 that calculates a difference between the received field strength of the radio wave from the BS currently communicating and the received field strength of the radio wave from each adjacent BS (3) the field strength The electric field strength maximum difference detection function part 34 for obtaining the maximum value of the difference between the electric field strengths obtained by the difference calculation function part 32 (4) The quasi-handover threshold value included in the quasi-handover message (described later) received from the MSC and The handover candidate determination function unit 33 which compares the maximum difference obtained by the electric field strength maximum difference detection function unit 34 to determine whether or not quasi-handover can be performed (5) If the handover candidate determination function unit 33 determines that quasi-handover is possible, the handover function unit 36 performs handover. (6) The handover candidate determination function unit 33 cannot perform quasi-handover. When it is determined, the handover rejection notification message transmitting function unit 37 for transmitting a handover rejection notification message to notify the MSC that the quasi-handover is not to be performed will be described later. In this case, the forced handover function unit 35 for performing the forced handover is performed. The MSC 1 includes the following function units as a configuration related to the forced handover method of the present invention.

(1) When a priority call request is received from a radio communication area of a BS in a congested state, a semi-handover message to be described later is transmitted to all MSs in the radio communication area. Handover message transmission function unit 11 (2) When a handover rejection notification message is received from all MSs in response to the transmission of the quasi-handover message, the electric field having the maximum difference from the information included in the handover rejection notification message Field strength maximum difference mobile station detection function unit 13 for identifying an MS having a strength (3) The MS having the field strength of the maximum difference identified by the above-described field strength maximum difference mobile station detection function unit 13 is a candidate for forced handover. (4) MS selected by the candidate mobile station selection function unit 14
A forced handover message transmitting function unit 12 for transmitting a forced handover message to a priority call channel allocating function unit 15 for allocating, to a priority call, a communication channel that has become free as a result of handover 15 Next, the operation of the forced handover according to the present invention will be described with reference to the operation of each functional unit described above and other drawings.

First, with reference to FIG. 4, a description will be given of a handover operation led by a mobile station terminal, in which the MS independently determines whether or not a handover is possible.

The entire premise is the same as in FIG. When MS4 makes a priority call origination request in the radio communication area of BS1 in which all communication channels are used by MS1 to MS3 and congested, based on the request transmitted to MSC via control channel. ,
The MSC sends a priority assignment message to MS4,
Set S4 to the channel assignment waiting state.

Next, the MSC uses the quasi-handover message transmitting function unit 11 to transmit a quasi-handover message to all the active MS1 to MS3 in the radio communication area of the BS1. This quasi-handover message includes a quasi-handover threshold, as shown in FIG. The quasi-handover threshold is such that, for example, -2 dBm, even if the electric field strength of the electric field received from the currently communicating BS is slightly lower than the electric field strength of the radio wave received by the adjacent BS,
If it is possible to receive from the terminal, a value to be handed over is set.

When each of the MS1 to MS3 receives the quasi-handover message, the adjacent base station radio measurement function unit 31
To measure the electric field strength of the radio wave received from the BS currently communicating and the electric field strength of the radio wave received from each adjacent BS, and further, based on the measurement result, the electric field strength difference calculation function unit 32 The difference between the electric field strength of the radio wave received from the BS currently communicating and the electric field strength of the radio wave received from each adjacent BS is calculated.

Next, the largest difference among the pieces of difference information is obtained by the maximum electric field strength difference detection function unit 34. The calculated maximum difference is compared with the quasi-handover threshold included in the quasi-handover message, and the handover candidate determination function unit 33 determines whether or not handover (quasi-handover) is possible.

At this time, if the obtained maximum difference is larger than the quasi-handover threshold, it is determined that handover is possible, and the handover function unit 36 sends a handover request to an adjacent BS having the maximum difference. Send a message and perform handover.

However, if the obtained maximum difference is smaller than the quasi-handover threshold, it is determined that handover is impossible, and the handover rejection notification message transmitting function unit 37 performs handover without performing handover. Return an over-rejection notification message to the MSC. The handover rejection notification message has a format as shown in FIG.
4 includes the maximum difference information obtained.

As described above, as a result of instructing the quasi-handover to perform the handover based on the judgment of each MS,
If S is not in a state capable of handover and receives a handover rejection notification message from all MSs,
The MSC then performs forced handover control.

The MSC communicates with the communicating BS and the neighbor B included in the handover rejection notification message received from each MS.
The maximum electric field intensity difference information with the S is compared by the electric field intensity maximum difference mobile station detection function unit 13 and the candidate mobile station selection function unit 1
In step 4, the MS having the maximum difference information is selected as a candidate for forced handover. (For example, MS2.) MSC selects M selected as a candidate for forced handover.
A forced handover message is sent from the forced handover message transmission function unit 12 to S2. In the MS 2 having received the forced handover message, the forced handover function unit 35 sends a handover request message to the adjacent BS having the maximum difference in the electric field strength to perform the handover.

The MSC sends a channel assignment message to the MS 4 in the channel assignment waiting state because the communication channel used by the handed-over MS 2 has become vacant.

In this way, for a priority call generated in a radio communication area in a congested state, a forced handover can be performed to allocate a communication channel. When the MS determines that handover is possible at the stage of receiving the quasi-handover message and sends a handover request message to an adjacent BS to execute handover, the MS 4 transmits a channel to the MS at that stage. The communication channel that has become free after the allocation message is transmitted is allocated to the MS 4 (see FIG. 5).

Next, with reference to FIG. 6, a description will be given of a handover operation assisted by a mobile station terminal, which executes a handover based on the handover propriety information notified from the MS.

The premise of the overall configuration is the same as that shown in FIG. 1. In the radio communication area of BS1 in which all communication channels are used by MS1 to MS3 and congestion occurs, M1 is used.
S4 is an operation when a priority call transmission request is made.

Based on the priority call transmission request transmitted to the MSC via the control channel, the MSC transmits a priority assignment message to the MS 4 and sets the MS 4 to a channel assignment waiting state.

Next, the MSC uses the quasi-handover message transmitting function unit 11 to transmit the quasi-handover message to all the active MS1 to MS3 in the radio communication area of the BS1. This quasi-handover message includes a quasi-handover threshold, as shown in FIG.

When each of the MS1 to MS3 receives the quasi-handover message, the adjacent base station radio measurement function unit 31
To measure the electric field strength of the radio wave received from the BS currently communicating and the electric field strength of the radio wave received from each adjacent BS, and further, based on the measurement result, the electric field strength difference calculation function unit 32 The difference between the electric field strength of the radio wave received from the BS currently communicating and the electric field strength of the radio wave received from each adjacent BS is calculated.

Next, the largest difference among the pieces of difference information is obtained by the maximum electric field strength difference detecting function unit 34. The calculated maximum difference is compared with the quasi-handover threshold included in the quasi-handover message, and the handover rejection notification message transmitting function unit 37 returns a handover rejection notification message to the MSC. This handover rejection notification message includes the maximum difference obtained by the maximum electric field strength difference detection function unit 34 and the information on the adjacent BS, as shown in the format of FIG. 3C.

In this way, the MSC compares the maximum electric field strength difference information between the communicating BS and the adjacent BS included in the handover rejection notification message received from each MS by the electric field strength maximum difference mobile station detection function unit 13. Then, the MS having the maximum difference information is selected by the candidate mobile station selection function unit 14 as a candidate for forced handover. (Eg M
S2 is assumed, and BS2 is set as the adjacent BS in the handover rejection notification message. ) The MSC determines which M has been selected as a candidate for forced handover.
A handover execution message to BS2 is sent to S2. The MS 2 having received the handover execution message performs a handover by transmitting a handover completion message to the adjacent BS 2.

The MSC sends a channel assignment message to the MS 4 in the channel assignment waiting state because the communication channel used by the handed-over MS 2 has become vacant.

[0062]

As described above, in the forced handover method of the mobile communication system according to the present invention, the MS receiving the handover request from the MSC communicates with the radio wave of the BS currently communicating with the radio wave of the adjacent BS. Measure the difference in electric field strength of
Since the calculation is reported to the MSC, the MSC can easily select the MS having the largest difference among them as a candidate for forced handover. Therefore, an MS that is a candidate for forced handover can be quickly selected without imposing a burden on the MSC. In addition, since each MS selects a candidate MS for forced handover based on the measured and calculated values, the most suitable MS for handover is always selected, and the communication quality after handover is highly reliable. It can be.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an outline of a configuration of a mobile communication system.

FIG. 2 is a block diagram showing a configuration of a forced handover method of the mobile communication system of the present invention.

FIG. 3 is a diagram illustrating a format of a message used in a forced handover method of the mobile communication system according to the present invention.

FIG. 4 is a sequence diagram illustrating an operation of a mobile station terminal-initiated handover of a forced handover method of the mobile communication system according to the present invention, which is a sequence in a case where all MSs reject quasi-handover.

FIG. 5 is a sequence diagram illustrating an operation of a mobile station terminal-initiated handover of a forced handover method of the mobile communication system according to the present invention.
This is a sequence when S exists.

FIG. 6 is a sequence diagram illustrating an operation of a mobile station terminal assisted handover of a forced handover method in the mobile communication system of the present invention.

FIG. 7 is a sequence diagram illustrating an operation of a mobile station terminal assisted handover according to the related art.

FIG. 8 is a sequence diagram illustrating an operation of a communication network initiated assisted handover according to the related art.

[Explanation of symbols]

 Reference Signs List 1 mobile communication switching center (MSC) 2 radio base station (BS) 3 mobile station (MS) 4 fixed communication network

Claims (3)

(57) [Claims] 1. A mobile communication switching center for performing mobile communication switching processing, a plurality of wireless base stations constituting a wireless communication area under the control of the mobile communication switching center, and a wireless communication provided by each wireless base station. In a mobile communication system composed of a plurality of mobile stations communicating with the radio base station in the area, the mobile communication switching center is in a congestion state where all the communication channels of the radio communication area are used, and When a request for a priority call that needs to be connected prior to communication is received, all mobile stations that are talking in the congested wireless communication area are determined to be adjacent to each other by the mobile station. Means for transmitting a first handover message instructing the base station to perform a handover, and in response to the first handover message, a hand from all mobile stations. Means for selecting a specific mobile station when receiving a response indicating that the mobile station cannot perform a handover, and for transmitting a second handover message for instructing the mobile station to execute a handover; Means for allocating, to the priority call, a communication channel used by the mobile station that has performed the handover in accordance with the instructions of the handover message and the second handover message. . 2. The mobile station according to claim 1, wherein the first handover message includes threshold information for determining whether or not the mobile station can perform a handover. The radio wave of the communicating radio base station and the radio wave of each adjacent radio base station are measured, and the measurement results are compared with threshold information included in the first handover message to determine whether handover is possible. Means for judging, and means for sending a handover impossible report message to the mobile switching center including a measurement result optimum for handover among the measurement results when it is judged that handover is impossible, When the switching center receives the handover not possible report message from all mobile stations, it includes the handover not possible report message. Mobile station that compares the best measurement result for the handover that has been performed between mobile stations and selects the mobile station that has reported the best measurement result for the handover as a target for forced handover. The forced handover method for a mobile communication system according to claim 1, wherein 3. A mobile communication switching center for performing mobile communication switching processing, a plurality of wireless base stations constituting a wireless communication area under the control of the mobile communication switching station, and a wireless communication service provided by each wireless base station. In a mobile communication system composed of a plurality of mobile stations communicating with the radio base station in the area, the mobile communication switching center is in a congestion state where all the communication channels of the radio communication area are used, and When a request for a priority call that needs to be connected prior to communication is received, all mobile stations that are talking in the congested wireless communication area are notified to the currently communicating wireless base station and adjacent mobile stations. Means for transmitting a report instruction message for instructing the report of the radio wave reception status of each radio base station, wherein the mobile station receives the report instruction message,
Measures the electric field strength of the radio wave of each adjacent wireless base station and measures
The optimal neighbor for handover is selected by selecting the adjacent wireless base station with the largest electric field strength difference, which is the value obtained by subtracting the electric field strength of the radio wave of the currently communicating radio base station from the electric field strength of the radio base station radio wave Means for determining a radio base station, and means for transmitting the determined adjacent radio base station information optimal for handover and the measured electric field strength difference to the mobile communication switching center in a response message, and The exchange further receives a response message to the report instruction message from each of the mobile stations, and selects the mobile station reporting the one with the largest electric field strength difference included in these response messages to thereby optimize the handover. Handover for determining a neighboring wireless base station and a mobile station and forcibly instructing the mobile station to perform a handover Means for transmitting a message, and means for allocating, to the priority call, a communication channel used by the mobile station that has executed the handover in accordance with the instruction of the forced handover message. Over method.