JP2008182313A - Handover method of mobile communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a handover method of a mobile communication system that can suitably make a handover. <P>SOLUTION: In a step S4, a radio terminal refers to a radio base station list to acquire signal intensities RDx of down-link signals from other radio base stations (x) put in operation using the same frequency with the radio base station 1 that the radio terminal belongs to. Then differences (RDx-RD1) between the signal intensities RDx and a signal intensity RD1 of a down-link signal from the radio base station 1 are compared with a relative threshold R6. When it is found through the comparison that there is a radio base station (x) having a relation of (RDx-RD1)>R6, it is decided that a handover to the radio base station (x) is made, and an advance to a step S7 is made. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a handover method for a mobile communication system, and particularly to a technique for appropriately performing a handover of a radio terminal.

  In the conventional mobile communication system, when the downlink signal strength from the radio base station to which the radio terminal belongs is below a predetermined first threshold (fixed value) preset in the radio terminal, When the strength of the downlink signal from another radio base station is measured and a radio base station having a predetermined second threshold value (fixed value) or more is found, a handover is made to that radio base station (for example, a patent) Reference 1).

  In addition, when the uplink signal strength from the radio terminal falls below a predetermined third threshold (fixed value) preset in the radio base station at the radio base station to which the radio terminal belongs, When there is another radio base station that measures the uplink signal strength from the terminal and exceeds the predetermined fourth threshold value (fixed value), the radio terminal is handed over to the base station by the base station initiative (For example, patent document 2).

JP 2000-50337 A JP 2000-506320 A

  Among mobile communication system services, for example, a small cell system with a radius of 30 m or less such as an indoor service, and since the usable frequency is limited, a cell design is simplified by operating a plurality of adjacent cells at the same frequency. However, the following system has the following problems.

  That is, if the threshold for the downlink signal strength when the wireless terminal is handed over is too low, the wireless terminal moves and moves away from the wireless base station to which it belongs, and the wireless base station in the adjacent cell is closer. No handover. Therefore, there is a problem that the voice quality deteriorates during a call because the opportunity is not given even though communication in a better communication environment is possible.

  In addition, if the threshold for the downlink signal strength when the wireless terminal is handed over is too high, the threshold is reached when the wireless terminal is moving and the adjacent cell is still far away, so communication between adjacent cells is impossible. There is a problem that the voice is interrupted when the call is determined to be outside the service area and the call is in progress.

  In order to solve these problems, it is necessary to adjust the optimum threshold value according to the distance between the radio base stations between adjacent cells. Even in the same service, the distance between radio base stations between adjacent cells often differs depending on the location. Therefore, the conventional mobile communication system handover method using a fixed value as a threshold has a problem in that handover cannot always be performed at an appropriate location.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a handover method for a mobile communication system capable of appropriately performing a handover.

  A handover method of a mobile communication system according to the present invention is a handover method of a mobile communication system including a radio terminal and a plurality of radio base stations that communicate with the radio terminal via a radio channel, the radio terminal A first downlink signal strength for measuring a first downlink signal strength, which is a strength of a downlink signal from a first radio base station to which a wireless terminal belongs and is included in a plurality of wireless base stations In the measurement process, the wireless terminal is included in a plurality of wireless base stations, is operated at the same frequency as the first wireless base station, and is down from the second wireless base station to which the wireless terminal does not belong. A second downlink signal strength measuring step for measuring a second downlink signal strength, which is a strength of the link signal, and a first downlink signal strength and a second downlink signal strength. The difference between the comparison with the predetermined threshold value, provided in accordance with the comparison result, and a switching step of the wireless terminal is switched to belong from the first radio base station to a second radio base station.

  The handover method of the mobile communication system according to the present invention compares the difference between the first downlink signal strength and the second downlink signal strength with a predetermined threshold value, and performs switching according to the comparison result. Therefore, compared to the conventional handover method of a mobile communication system using a fixed threshold value, the handover can be performed at an optimal place by using the relative threshold value, so that the voice quality can be improved. Further, it is possible to make adjustment of the threshold value unnecessary. That is, it becomes possible to perform a handover appropriately.

<Embodiment 1>
FIG. 1 is a schematic diagram schematically showing a mobile communication system according to Embodiment 1 of the present invention. The mobile communication system in FIG. 1 includes wireless terminals 11 and wireless base stations 1 to 3 that communicate with the wireless terminals 11 via wireless lines.

  In FIG. 1, the switch 10 is composed of, for example, an Ethernet (registered trademark) switch, and a message or signal addressed to each of the radio base stations 1 to 3 or a message or signal transmitted from each of the radio base stations 1 to 3 is appropriately transmitted. It is a device to switch to.

  In FIG. 1, it is assumed that the wireless terminal 11 is initially in a voice call belonging to the wireless base station 1. The wireless base stations 1 to 3 are assumed to have service providing areas, that is, cells adjacent to each other and operated at the same frequency f1. As shown in FIG. 1, the wireless terminal 11 includes a current signal strength measurement unit 31, a peripheral signal strength measurement unit 32, a radio base station list 33, a signal strength absolute comparison unit 34, a signal strength relative comparison unit 35, and a handover determination unit. 36 and a handover execution unit 37. The radio base station list 33 is for storing the signal strength of the downlink signal RDx from each radio base station to the radio terminal 11, and is arranged in the storage means.

  FIG. 2 is a schematic diagram schematically showing the cell arrangement of the radio base stations 1 to 3 and the like in FIG.

  In FIG. 2, as described above, in the radio base stations 1 to 3, the cells are adjacent to each other and are operated at the same frequency f1. In the radio base stations 4 to 6, the cells are adjacent to each other and are operated at the same frequency f2. In the radio base stations 7 to 9, cells are adjacent to each other and are operated at the same frequency f3. In FIG. 2, radio base station groups operated at the same frequency are each surrounded by a dotted line. In the same radio base station group, it is assumed that adjacent cells overlap each other. Further, FIG. 2 shows a case where the number of radio base stations constituting one radio base station group is three, but the number is not limited to three and may be two or more. In the present invention, the same frequency is not limited to a frequency value that completely matches, and even if the value is slightly different, it is used for the same channel in a predetermined frequency band. To be included.

  3 to 4 are flowcharts showing a handover method of the mobile communication system of FIG. Note that steps S1 to S7 in FIG. 3 relate to handover to a radio base station operated at the same frequency as the radio base station to which it belongs, and steps S8 to S13 in FIG. Is related to a handover to a radio base station operated at a frequency different from that of the radio base station to which is assigned.

  First, in step S1, the wireless terminal 11 in a call is based on a control signal such as a beacon transmitted by the wireless base station (wireless base station 1) to which the wireless terminal 11 belongs or a communication signal such as call data. The signal strength RD1 of the downlink signal 21 from 1 is measured (periodically) by the current signal strength measuring unit 31. Similarly, a control signal such as a beacon transmitted by a wireless base station (wireless base station 2 or 3) that is operated at the same frequency as the wireless base station to which it belongs and does not belong to itself or a call Based on communication signals such as data, the signal strengths RD2 and RD3 of the downlink signals 22 and 23 from the radio base stations 2 and 3 are measured (periodically) by the peripheral signal strength measuring unit 32, respectively. The signal strengths RD1 to RD3 may be measured as a moving average. Since both of the radio base stations 2 and 3 are operated at the same frequency f1 as that of the radio base station 1, the radio terminal 11 communicates with the radio base station 1 (to which the radio terminal 11 belongs). Without interruption, the strength of the downlink signals 22 and 23 from the radio base stations 2 and 3 is continuously measured as time series data, and the measured value is obtained by moving average that averages the time series data during a certain period. Obtainable. In other words, in the case of radio, the strength of a received signal constantly fluctuates, so that a stable handover can be performed at a more appropriate location by taking a moving average of the signal strength.

  Next, in step S <b> 2, the radio terminal 11 saves / updates the signal strengths RD <b> 1 to RD <b> 3 measured in step S <b> 1 in the radio base station list 33.

  Next, in step S3, the wireless terminal 11 compares the signal strength RD1 measured in step S1 with the handover guard threshold value R5 by the signal strength absolute comparison unit 34. The handover guard threshold value R5 is a signal strength threshold (absolute threshold value) of a downlink signal that allows the wireless terminal 11 in a call to absolutely ensure call quality in both uplink and downlink. Is set to As a result of the comparison, if RD1 <R5, it is determined that it is preferable to perform a handover. Therefore, the process proceeds to step S4. If RD1 ≧ R5, it is determined that a handover is not necessary. Proceed to step S1.

  Next, in step S4, the wireless terminal 11 refers to the wireless base station list 33, and thereby has the same frequency (here, frequency f1) as the wireless base station to which it belongs (here, the wireless base station 1). Signal strengths RDx (here, signal strengths RD2, RD3) of downlink signals (here, downlink signals 22, 23) from other radio base stations (here, radio base stations 2 and 3) operated in FIG. . Then, the signal strength relative comparison unit 35 compares the difference (RDx−RD1) between the signal strength RDx and the signal strength RD1 with a relative threshold value R6. This relative threshold value R6 is set in advance in the wireless terminal 11 as a threshold value (handover start relative threshold value) that serves as a trigger for starting handover during a call. As a result of the comparison, if there is a radio base station x where (RDx−RD1)> R6, the handover decision unit 36 determines that a handover to the radio base station x is performed, and the process proceeds to step S7 ( If there is no radio base station x where RDx−RD1)> R6, the handover determination unit 36 cannot yet determine that the handover is to be performed, and the process proceeds to step S5.

  Next, in step S7, the wireless terminal 11 uses the handover execution unit 37 to perform handover to the wireless base station x (for example, (RD2-RD1)> R6 in the wireless base station 2 in step S4). 2). Then, the process proceeds to step S1.

  That is, in this case, the radio base stations 1 and 2 function as first to second radio base stations according to the present invention, respectively. Further, the signal strengths RD1 to RD2 function as first to second downlink signal strengths according to the present invention, respectively. Further, the relative threshold value R6 functions as a predetermined threshold value according to the present invention, is compared with the difference between the first downlink signal strength and the second downlink signal strength, and is switched according to the comparison result. .

  In principle, the relative threshold R6 should always be handed over to the radio base station with the stronger downlink signal even when it is set to a minimum value (for example, 1 dB) that can be set. However, in practice, the signal strength of the downlink signal is likely to fluctuate depending on the multipath environment. Therefore, when the relative threshold R6 is too small, a phenomenon in which handover is frequently repeated (fluctuates) between the same radio base stations occurs. Therefore, the call quality is deteriorated. Therefore, in order to avoid such a phenomenon, it is preferable that the relative threshold value R6 can be set with a slight margin.

  Next, in step S5, the radio terminal 11 uses the signal strength of the downlink signal from the radio base station to which it belongs (here, the signal strength RD1 of the downlink signal 21 from the radio base station 1) as a signal. The strength absolute comparison unit 34 compares with the forced handover threshold R7. This forced handover threshold value R7 (<R5) is set in advance in the wireless terminal 11 as a minimum signal strength threshold value (absolute threshold value) of the downlink signal that can ensure call quality. As a result of the comparison, if RD1 <R7, it is determined that the handover should be performed, so the process proceeds to step S6, and if RD1 ≧ R7, it is determined that the handover is not required, step S1 Proceed to

  Next, in step S6, the wireless terminal 11 refers to the wireless base station list 33, and thereby has the same frequency (here, frequency f1) as the wireless base station to which it belongs (here, the wireless base station 1). Signal strengths RDx (here, signal strengths RD2, RD3) of downlink signals (here, downlink signals 22, 23) from other radio base stations (here, radio base stations 2 and 3) operated in FIG. . Then, the signal strength absolute comparison unit 34 compares the signal strength RDx with the forced handover threshold R7. As a result of the comparison, if there is a radio base station x where RDx> R7, the handover decision unit 36 determines that a handover to this radio base station x is performed, and proceeds to step S7, where RDx> R7. If the radio base station x does not exist, the handover determination unit 36 cannot yet determine that the handover is to be performed, and thus the process proceeds to step S8.

  That is, if it cannot be determined in step S6 that handover is to be performed, the radio terminal 11 determines that handover at the same frequency is difficult, and attempts handover at a different frequency after step S8.

  As described above, in the same radio base station group shown in FIG. 2, adjacent service providing areas, that is, cells, overlap each other. Therefore, when the signal strength RD1 decreases until RD1 <R7 (step S5), there is a high possibility that the radio base station x satisfying (RDx−RD1)> R6 exists (step S4). . Therefore, it is usually considered that the process often proceeds from step S4 to step S7, and the process proceeds from step S5 to step S6.

  Next, in step S8, the wireless terminal 11 transmits a control signal such as a beacon or a call transmitted by a wireless base station (wireless base stations 4 to 9) operated at a frequency different from that of the wireless base station to which the wireless terminal 11 belongs. Based on communication signals such as data, the signal strength of the downlink signals from the radio base stations 4 to 9 is measured (periodically) by the peripheral signal strength measuring unit 32, respectively.

  Next, in step S9, the radio terminal 11 saves / updates the signal strength measured in step S8 in the radio base station list 33.

  Next, in step S10, the wireless terminal 11 refers to the wireless base station list 33, and thereby has a frequency (here, frequencies f2, f3) different from the wireless base station to which the wireless terminal 11 belongs (here, the wireless base station 1). ) To obtain the signal strength RDx of each downlink signal from another radio base station (in this case, the radio base stations 4 to 9) operated. Then, the signal strength absolute comparison unit 34 compares the signal strength RDx with an absolute threshold value R11 (> R7) set in the wireless terminal 11 in advance. If there is a radio base station x with RDx> R11 as a result of the comparison, the handover decision unit 36 determines that a handover to this radio base station x is to be performed, and proceeds to step S12, where RDx> R11. If the radio base station x does not exist, the handover determination unit 36 cannot yet determine that the handover is to be performed, and thus the process proceeds to step S11.

  Next, in step S12, the radio terminal 11 performs handover to the radio base station x at the handover execution unit 37. Then, the process proceeds to step S1.

  Next, in step S11, the wireless terminal 11 refers to the wireless base station list 33, and thereby has a frequency (here, frequencies f2, f3) different from the wireless base station to which the wireless terminal 11 belongs (here, the wireless base station 1). ) To obtain the signal strength RDx of each downlink signal from another radio base station (in this case, the radio base stations 4 to 9) operated. Then, the signal strength absolute comparison unit 34 compares the signal strength RDx with the forced handover threshold R7. As a result of the comparison, if there is a radio base station x where RDx> R7, the handover decision unit 36 determines that a handover to this radio base station x is performed, and proceeds to step S12, where RDx> R7. If the radio base station x does not exist, the handover determination unit 36 cannot determine that the handover is to be performed, and thus the process proceeds to step S13.

  Next, in step S13, the wireless terminal 11 performs out-of-service processing. That is, the signal strength of the downlink signal from the radio base station to which it belongs (here, the radio base station 1) and the surrounding radio base stations (here, the radio base stations 2 to 9) are insufficient. Therefore, it will be in a waiting state for return to the area.

  As described above, in the handover method of the mobile communication system according to the present embodiment, the difference obtained by subtracting the first downlink signal strength (signal strength RD1) from the second downlink signal strength (for example, signal strength RD2) ( For example, when (RD2-RD1)) is larger than the relative threshold value R6, the wireless terminal 11 belongs to the second wireless base station (for example, the wireless base station 2) from the first wireless base station (wireless base station 1). Switch as follows. Therefore, compared to the conventional handover method of a mobile communication system using a fixed threshold value, the handover can be performed at an optimal place by using the relative threshold value, so that the voice quality can be improved. Further, it is possible to make adjustment of the threshold value unnecessary. That is, it becomes possible to perform a handover appropriately.

  In recent years, there are an increasing number of cases of introducing indoor wireless telephone systems that are mainly used for extension telephones using wireless LAN technology. However, the frequency that can be used is limited especially when the IEEE802.11b standard is used. There is a problem that. As a solution, a so-called virtual cell system having a configuration in which a plurality of base stations having the same frequency are arranged together is attracting attention. However, the present invention may be used as a handover method for a mobile communication system in a virtual cell system. is there.

<Embodiment 2>
In the first embodiment, the case has been described in which whether or not handover can be performed is determined using a downlink signal from the radio base station to the radio terminal. However, whether or not the handover can be performed is not limited to the downlink signal from the radio base station to the radio terminal, or may be determined using an uplink signal from the radio terminal to the radio base station.

  5 to 6 are schematic diagrams schematically showing a mobile communication system according to Embodiment 2 of the present invention.

  In FIG. 5, the radio base station controller 51 has a radio base station list 81 for storing the signal strength of the uplink signal RUx from the radio terminal 11 to each radio base station in the storage means, and performs handover. Determine whether or not. 5-6, the same code | symbol is attached | subjected about the member similar to FIG. 1, and detailed description here is abbreviate | omitted.

  In FIG. 5, it is assumed that the wireless terminal 11 is initially in a voice call belonging to the wireless base station 1. The wireless base stations 1 to 3 are assumed to have service providing areas, that is, cells adjacent to each other and operated at the same frequency f1. As shown in FIG. 5, the radio base station 1 includes a current signal strength measurement unit 71, a peripheral signal strength measurement unit 72, and a radio base station controller report unit 73. The radio base station 2 includes a current signal strength measurement unit 74, a peripheral signal strength measurement unit 75, and a radio base station controller report unit 76. The radio base station 3 includes a current signal strength measurement unit 77, a peripheral signal strength measurement unit 78, and a radio base station controller report unit 79. As shown in FIG. 6, the radio base station controller 51 includes a radio base station list 81, a signal strength absolute comparison unit 82, a signal strength relative comparison unit 83, a handover determination unit 84, and a handover execution unit 85.

  Moreover, it is assumed that the cell arrangement of the radio base stations 1 to 3 in FIGS.

  FIG. 7 is a flowchart showing a handover method of the mobile communication system of FIG.

  First, in step S21, the wireless base station (wireless base station 1) to which the wireless terminal 11 in communication belongs belongs to the uplink from the wireless terminal 11 based on communication signals such as call data transmitted by the wireless terminal 11. The signal intensity RU1 of the signal 61 is measured (periodically) by the current signal intensity measuring unit 71. Similarly, the radio base stations (radio base stations 2 and 3) that are operated at the same frequency as the radio base station to which the radio terminal 11 belongs, and to which the radio terminal 11 does not belong, Based on communication signals such as call data to be transmitted, the signal strengths RU2 and RU3 of the uplink signals 62 and 63 from the wireless terminal 11 are measured (periodically) by the peripheral signal strength measuring unit 72, respectively. The signal strengths RU1 to RU3 may be measured as a moving average. Since both of the radio base stations 2 and 3 are operated at the same frequency f1 as that of the radio base station 1, the radio base stations 2 and 3 are connected to the radio terminal 11 and the radio base station to which the radio terminal 11 belongs. A moving average that continuously measures the strength of the uplink signals 62 and 63 from the wireless terminal 11 as time series data without interrupting communication with the station 1 and averages the time series data during a certain period. A measurement value can be obtained. In other words, in the case of radio, the strength of a received signal constantly fluctuates, so that a stable handover can be performed at a more appropriate location by taking a moving average of the signal strength.

  Next, in step S22, the signal strengths RU1 to RU3 measured in step S21 are transmitted from the radio base station controller reporting units 73, 76, and 79 to the radio base station controller 51, respectively.

  Next, in step S23, the radio base station controller 51 saves / updates the signal strengths RU1 to RU3 received in step S22 in the radio base station list 81.

  Next, in step S24, the radio base station controller 51 compares the signal strength RU1 measured in step S21 with the handover guard threshold value R8 in the signal strength absolute comparison unit 82. This handover guard threshold value R8 is a radio base station in advance as a signal strength threshold value (absolute threshold value) of an uplink signal that allows the radio terminal 11 in a call to absolutely ensure call quality in both uplink and downlink. It is set in the controller 51. As a result of the comparison, if RU1 <R8, it is determined that it is preferable to perform a handover. Therefore, the process proceeds to step S25, and if RU1 ≧ R8, it is determined that a handover is not necessary. Proceed to step S21.

  Next, in step S25, the radio base station controller 51 refers to the radio base station list 81, so that the same frequency (here, the radio base station 1) to which the radio terminal 11 belongs (here, the radio base station 1). Then, the signal strength RUx (here, signal strength RU2, here) of the uplink signals (here, uplink signals 62, 63) from the other radio base stations (here, radio base stations 2, 3) operated at the frequency f1). RU3) is obtained. Then, the signal strength relative comparison unit 83 compares the difference (RUx−RU1) between the signal strength RUx and the signal strength RU1 with the relative threshold value R9. The relative threshold value R9 is set in advance in the radio base station controller 51 as a threshold value (handover start relative threshold value) that serves as a trigger for starting handover during a call. As a result of the comparison, if there is a radio base station x where (RUx−RU1)> R9, the handover decision unit 84 determines that a handover to the radio base station x is performed, and the process proceeds to step S28. If there is no radio base station x where RUx−RU1)> R9, the handover determination unit 84 cannot yet determine that the handover is to be performed, and the process proceeds to step S26.

  Next, in step S28, the radio base station controller 51 performs a handover from the handover instruction unit 85 to the radio base station x to the radio base station to which the radio terminal 11 belongs (here, the radio base station 1). (For example, if (RU2-RU1)> R9 in the radio base station 2 in step S25, a handover instruction message 91 instructing the handover to the radio base station 2) is sent.

  Next, in step S29, the radio base station 1 sends a handover instruction message 92 for instructing handover to the radio base station 2 to the radio terminal 11, based on the handover instruction 91 received in step S28. As a result, the wireless terminal 11 performs handover to the wireless base station 2. Then, the process proceeds to step S21.

  That is, in this case, the radio base stations 1 and 2 function as first to second radio base stations according to the present invention, respectively. Further, the signal strengths RU1 to RU2 function as first to second uplink signal strengths according to the present invention, respectively. Further, the relative threshold value R9 functions as a predetermined threshold value according to the present invention, is compared with the difference between the first uplink signal strength and the second uplink signal strength, and is switched according to the comparison result. .

  In principle, the relative threshold value R9 should always be handed over to a radio base station with a stronger uplink signal even when the relative threshold value R9 is set to a settable minimum value (for example, 1 dB). However, in reality, the signal strength of the uplink signal is likely to fluctuate depending on the multipath environment. Therefore, when the relative threshold R9 is too small, a phenomenon in which handover is frequently repeated (fluctuates) between the same radio base stations occurs. Therefore, the call quality is deteriorated. Therefore, in order to avoid such a phenomenon, it is preferable that the relative threshold value R9 can be set with a slight margin as in the case of the relative threshold value R6.

  The means for instructing handover from the radio base station 1 to the radio terminal 11 is a wireless LAN management frame defined in IEEE 802.11, instead of transmitting a handover instruction message 92 instructing handover to the radio base station 2. The disassociation message 93 may be transmitted. As a result, the wireless terminal 11 that has received the Disassociation message 93 performs a new association operation and expects handover to be performed by connecting to the wireless base station having the highest downlink signal strength at this time (this It is highly possible that the condition is the radio base station 2).

  Further, the means for instructing handover from the radio base station 1 to the radio terminal 11 replaces the transmission of the handover instruction message 92 instructing handover to the radio base station 2, and the call data addressed to the radio terminal 11 from the radio base station 1. It is also possible to reduce the radio signal output of only the communication signal such as (transmission of the output down instruction message 94). As a result, the signal strength RD1 of the downlink signal 21 from the radio base station 1 is reduced in the radio terminal 11 and is expected to be handed over by being lower than a threshold such as the handover guard threshold R5 (corresponding to this condition). It is highly likely that the wireless base station 2 will do this).

  Next, in step S26, the radio base station controller 51 determines the signal strength of the uplink signal from the radio base station to which the radio terminal 11 belongs (here, the signal strength RU1 of the uplink signal 61 from the radio base station 1). ) Is compared with the forced handover threshold R10 by the signal strength absolute comparison unit 82. The forced handover threshold value R10 (<R8) is set in advance in the radio base station controller 51 as a threshold value (absolute threshold value) of the minimum uplink signal that can ensure call quality. As a result of the comparison, if RU1 <R10, it is determined that the handover should be performed, so the process proceeds to step S27. If RU1 ≧ R10, it is determined that the handover need not be performed. Proceed to

  Next, in step S27, the radio base station controller 51 refers to the radio base station list 81, so that the radio base station to which the radio terminal 11 belongs (here, the radio base station 1) has the same frequency (here Then, the signal strength RUx (here, signal strength RU2, here) of the uplink signals (here, uplink signals 62, 63) from the other radio base stations (here, radio base stations 2, 3) operated at the frequency f1). RU3) is obtained. Then, the signal strength absolute comparison unit 82 compares the signal strength RUx with the forced handover threshold R10. As a result of the comparison, if there is a radio base station x where RUx> R10, the handover decision unit 84 determines that a handover to this radio base station x is to be performed, and proceeds to step S28, where RUx> R10. If the radio base station x does not exist, the handover determination unit 84 cannot determine that the handover is to be performed, and thus the process proceeds to step S30.

  Next, in step S30, the wireless terminal 11 performs a handover independently. That is, the radio base station controller 51 is an uplink from any of the radio base station to which the radio terminal 11 belongs (in this case, the radio base station 1) and its surrounding radio base stations (in this case, the radio base stations 2 to 3). Since the signal strength of the signal is also insufficient, it is determined that it is difficult to perform the handover by the radio base station controller 51 itself by transmitting the handover instruction message 91, and the radio terminal 11 voluntarily performs the handover. .

  As described above, in the same radio base station group shown in FIG. 2, adjacent service providing areas, that is, cells, overlap each other. Therefore, when the signal strength RU1 decreases until RU1 <R10 (step S26), there is a high possibility that a radio base station x satisfying (RUx−RU1)> R9 exists (step S25). . Therefore, it is usually considered that the process often proceeds from step S25 to step S28, and the process proceeds from step S26 to step S27.

  As described above, in the handover method of the mobile communication system according to the present embodiment, the difference obtained by subtracting the first uplink signal strength (signal strength RU1) from the second uplink signal strength (for example, signal strength RU2) ( For example, when (RU2-RU1)) is larger than the relative threshold value R9, the wireless terminal 11 belongs to the second wireless base station (for example, the wireless base station 2) from the first wireless base station (wireless base station 1). Switch as follows. Therefore, compared to the conventional handover method of a mobile communication system using a fixed threshold value, the handover can be performed at an optimal place by using the relative threshold value, so that the voice quality can be improved. Further, it is possible to make adjustment of the threshold value unnecessary. That is, as in the first embodiment, handover can be performed appropriately.

1 is a schematic diagram schematically showing a mobile communication system according to Embodiment 1. FIG. 6 is a schematic diagram schematically showing a cell arrangement of radio base stations according to Embodiment 1 and Embodiment 2. FIG. 3 is a flowchart showing a handover method of the mobile communication system according to the first embodiment. 3 is a flowchart showing a handover method of the mobile communication system according to the first embodiment. 5 is a schematic diagram schematically showing a mobile communication system according to Embodiment 2. FIG. 5 is a schematic diagram schematically showing a mobile communication system according to Embodiment 2. FIG. 6 is a flowchart showing a handover method of the mobile communication system according to the second embodiment.

Explanation of symbols

  1-9 radio base station, 10 switch, 11 radio terminal, 21-23 downlink signal, 31, 71, 74, 77 current signal strength measuring unit, 32, 72, 75, 78 peripheral signal strength measuring unit, 33, 81 Radio base station list, 34, 82 signal strength absolute comparison unit, 35, 83 signal strength relative comparison unit, 36, 84 handover determination unit, 37, 85 handover execution unit, 51 radio base station controller, 61-63 uplink signal, 73, 76, 79 Radio base station controller report section, 91, 92 Handover instruction message, 93 Disassociation message, 94 Output down instruction message.

Claims (6)

A handover method of a mobile communication system comprising a wireless terminal and a plurality of wireless base stations that communicate with the wireless terminal via a wireless line,
In the wireless terminal, a first downlink signal strength that is a strength of a downlink signal from a first wireless base station included in the plurality of wireless base stations and to which the wireless terminal belongs is measured first. The downlink signal strength measurement process of
The wireless terminal is included in the plurality of wireless base stations, is operated at the same frequency as the first wireless base station, and is down from the second wireless base station to which the wireless terminal does not belong. A second downlink signal strength measurement step for measuring a second downlink signal strength that is the strength of the link signal;
The difference between the first downlink signal strength and the second downlink signal strength is compared with a predetermined threshold value, and the wireless terminal determines whether the second wireless base station transmits the second downlink signal strength from the first wireless base station according to the comparison result. A switching method for a mobile communication system comprising a switching step of switching to belong to a wireless base station. A handover method of a mobile communication system comprising a wireless terminal and a plurality of wireless base stations that communicate with the wireless terminal via a wireless line,
A first radio base station, which is included in the plurality of radio base stations and to which the radio terminal belongs, measures a first uplink signal strength that is the strength of an uplink signal from the radio terminal. The uplink signal strength measurement process of
In a second radio base station included in the plurality of radio base stations and operated at the same frequency as the first radio base station and to which the radio terminal does not belong, A second uplink signal strength measurement step for measuring a second uplink signal strength that is the strength of the link signal;
The difference between the first uplink signal strength and the second uplink signal strength is compared with a predetermined threshold, and according to the comparison result, the wireless terminal transmits the second uplink signal strength from the first wireless base station to the second A switching method for a mobile communication system comprising a switching step of switching to belong to a wireless base station. A method for handover of a mobile communication system according to claim 1,
In the first to second downlink signal strength measuring steps, the strength of the downlink signal is continuously measured as time series data, and a measurement value is obtained by a moving average that averages the time series data during a predetermined time. A handover method of a mobile communication system characterized by the above. A handover method for a mobile communication system according to claim 2,
In the first to second uplink signal strength measuring steps, the strength of the uplink signal is continuously measured as time-series data, and a measurement value is obtained by a moving average that averages the time-series data during a predetermined time. A handover method of a mobile communication system characterized by the above. A handover method for a mobile communication system according to claim 2 or 4,
The handover method of a mobile communication system, wherein the switching step includes a step of transmitting a disassociation message by a management frame for wireless LAN defined by IEEE 802.11 to the wireless terminal. A handover method for a mobile communication system according to claim 2 or 4,
The handover method of a mobile communication system, wherein the switching step includes a step in which the first radio base station reduces a radio signal output to the radio terminal.

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