JP4659385B2 - Handover control method and mobile station - Google Patents

Description

  The present invention relates to a handover control method for continuing communication by switching from a sector in communication to another sector, and in particular, suppressing handover to a sector of another band or another system covering the same area. The present invention relates to a handover control method and a mobile station.

  In a mobile communication system, a service area is generally divided into a plurality of cells, one base station is provided for each cell, and one cell is further divided into a plurality of sectors. In such a system, a mobile station communicates with a base station corresponding to the sector in which the mobile station exists via a wireless line.

  In addition, the control for switching from the currently communicating sector to another adjacent sector by movement and continuing the communication is called “handover”. When performing handover, the mobile station measures the perch channel quality (reception level, etc.) of the adjacent sector designated by the base station. Based on the measurement result, the handover start timing and the handover destination are determined. Specifically, whether or not the difference between the measurement result of the handover destination sector and the measurement result of the handover source sector exceeds a predetermined value is set as a handover start condition. When satisfied, that is, when the difference exceeds a predetermined value, a handover request is transmitted to the base station, and communication is switched to the adjacent sector in accordance with the handover instruction sent from the base station.

  In addition, the reception level of each sector of the handover candidate is, for example, a slow fluctuation over a section of about several tens of meters due to a change due to a change in the distance between the base station and the mobile station (long-term median fluctuation) and a change in the influence of the feature. Is affected by a large fluctuation (short-term median fluctuation) and a sudden fluctuation (instantaneous value fluctuation) in a section of about several tens of meters due to the influence of multipath. That is, the measurement result of the reception level during movement fluctuates drastically due to fading due to the influence of features or multipaths. When the handover start condition is determined using this measurement result, an appropriate handover start point and handover destination are determined. May not be determined. Therefore, in the conventional mobile communication system, a method is used in which the measurement results of the reception level are averaged, and the handover start condition is determined using the averaged measurement results (see Non-Patent Document 1).

  Also, in the prior art, for example, when the upper limit of transmission power approaches in a mobile station and the possibility of quality degradation becomes high, a handover control method for optimizing the handover start point by relaxing the handover start condition Has been proposed (see Patent Document 1).

  In addition, in a mobile communication system, a multiband scheme using a plurality of frequencies is adopted in order to avoid exhaustion of radio resources accompanying an increase in the number of users. For example, in the PDC (Personal Digital Cellar) method, which is the second generation method, services are performed in two bands, an 800 MHz band and a 1.5 GHz band, and handover control to another band is necessary. In addition, in the W-CDMA (Wideband Code Division Multiple Access) system, which is one of the third generation international standards, the 2 GHz band is divided into a plurality of parts, so handover control to a different frequency band is required. . In the W-CDMA system, handover control to another mobile communication system using a different frequency band is also standardized.

  In such a multi-band system, there are cases where the adjacent sector of the handover destination designated by the base station includes a sector of another band or another system that covers the same area as the sector with which the mobile station is communicating.

ARIB STANDARD "Digital System Car Telephone System RCR STD-27J 1st Volume" The Radio Industry Association May 30, 2002 P147-148 JP-A-9-322222, pages 4 to 5, FIG. 4

  Basically, considering only the fluctuation (long-term median fluctuation) associated with the change in the distance between the base station and the mobile station, there is no need to perform handover to another band or other system sector covering the same area as the communicating sector. is there. That is, the measurement result of the reception level in the other band or the sector of another system covering the same area as the communicating sector is not essentially different from the measured result of the communicating sector, so it should not satisfy the handover start condition. is there.

  However, in the conventional mobile communication system, due to the influence of fading or the like, the measurement result of the reception level of the sector being communicated and the measurement result of the reception level of the surrounding sector are fluctuating, for example, the same as the sector being communicated A sector of another band or another system that covers the area may satisfy the handover start condition. Therefore, there is a problem that the mobile station performs unnecessary handover to another band or sector of another system that covers the same area as the communicating sector, and the sound is interrupted by this unnecessary handover.

  In addition, when a mobile station performs a handover to another band or sector of another system that covers the same area as the currently communicating sector, the reception quality will not change after the handover. In order to continue, it is necessary to perform the handover again, which causes a problem that the load on the network increases.

  In addition, there is a problem that the handover start point is delayed and the call is likely to be cut off due to repeated handover to another band or sector of another system that covers the same area as the communicating sector as described above. . In particular, in the above-mentioned Patent Document 1, since the handover start condition is relaxed, the mobile station frequently generates handovers to other bands or other system sectors that cover the same area as the communicating sector. There has been a problem that the load on the network increases and the possibility of call disconnection increases.

  The present invention has been made in view of the above, and provides a handover control method and a mobile station that can reduce handover to a sector of another band or another system that covers the same area as a communicating sector. The purpose is to obtain.

  In order to solve the above-described problems and achieve the object, a handover control method according to the present invention is a handover control method by a mobile station that performs handover between sectors, for example, a measurement result of communication quality of a handover source sector. And a correlation calculation step for calculating a correlation between the communication quality measurement result of the handover candidate sector designated by the base station, and if the correlation is greater than a predetermined threshold as a result of the correlation calculation, the handover candidate And a handover control step for suppressing handover to the sector.

  According to the present invention, the correlation value of the communication quality between the handover source sector (own station) and the handover candidate sector (peripheral station) is obtained, so that the other station covering the same area as the own station or the peripheral station of another system In addition, a negative offset is applied to the communication quality of peripheral stations having a high correlation.

  According to the present invention, there is an effect that unnecessary handover to a peripheral station of another band or another system covering the same area as the own station can be suppressed. In addition, by making it possible to suppress unnecessary handover, there is an effect that the sound interruption of the mobile station can be reduced. In addition, the network signaling load can be reduced. In addition, since there is no delay in handover to the optimum cell, there is an effect that the call disconnection of the mobile station can be reduced.

  Embodiments of a handover control method and a mobile station according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment 1 FIG.
FIG. 1 is a diagram showing a configuration example of a mobile station 1 (common to both TDMA and CDMA systems) that implements a handover control method according to the present invention, and includes a control unit 2, a radio circuit unit 3, and movement speed detection. Part 4. Further, the control unit 2 includes a handover condition determining unit 5 that determines a handover condition, a memory 6 that stores a threshold value and an offset value used for determining the handover condition, and a correlation value of a measurement result of communication quality of a handover candidate sector. And a correlation calculation unit 7 for calculating. The contents of the memory 6 can be rewritten by an external memory rewriting device 8.

  Here, the operation of the mobile station 1 configured as described above will be described in detail with reference to the drawings. For example, in a TDMA (Time Division Multiple Access) system such as PDC, the communication quality (reception level, etc.) of the perch channel of the sector (local station) in communication in the Rx slot is measured, and the handover candidate sector (in the idle slot) Measure the perch channel of the peripheral station. FIG. 2 is a diagram showing a slot format in the TDMA system. Since the number of peripheral stations that can be measured in one idle slot is limited, in order to measure all peripheral stations, it is necessary to measure multiple idle slots. In FIG. 2, since one peripheral station is measured in one slot as an example, for example, when there are N peripheral stations that are candidates for handover, it is necessary to measure the Idle slot N times.

  FIG. 3 is a flowchart showing processing of mobile station 1 according to the present embodiment. Here, the processing of the TDMA mobile station will be described, but the same processing is also executed in the CDMA mobile station. Here, a case where the reception level is measured will be described as an example of the communication quality of the perch channel.

  First, the handover condition determination unit 5 of the control unit 2 receives the reception result of the control information from the radio circuit unit 3 (step S1), and obtains a list of peripheral stations as handover candidates designated by the base station from the control information. Extract (step S2).

  Next, the handover condition determination unit 5 instructs the radio circuit unit 3 to measure the local station in communication and the peripheral stations that are candidates for handover (step S3). Then, the radio circuit unit 3 notifies the control unit 2 of the measurement result of the reception level of the own station and the measurement result of the reception level of the designated peripheral station # 1, for example (step S4).

  Next, the correlation calculation unit 7 of the control unit 2 calculates a correlation value between the notified reception level of the local station and the reception level of the peripheral station # 1 (step S5). The reception level of peripheral station # 1 is notified to the handover condition determination unit 5 (step S6).

  Next, the handover condition determining unit 5 averages the received reception levels of the local station and the peripheral station # 1 (step S7). Further, in the handover condition determination unit 5, based on the moving speed information (corresponding to stationary, low speed, and high speed shown in FIG. 4) periodically reported from the moving speed detection unit 8 (step S8), the received correlation value and The predetermined threshold value stored in the memory 6 is compared, and, for example, when the correlation value is equal to or lower than the threshold value, the offset value stored in the memory 6 is applied (step S9), and the above is obtained. Lower the average received level (correction). FIG. 4 is a diagram illustrating an example of threshold values and offset values stored in the memory 6. The threshold value and the offset value stored in the memory 6 can be changed by the memory rewriting device 7, for example. Each value in FIG. 4 can be set according to the purpose of handover to another band, handover to another system, and the moving speed (stationary, low speed, high speed) of the mobile station. As an example, the mobile station 1 of PDC (800 MHz) compares the received correlation value with the threshold value Th13 stored in the memory 6 during high-speed movement, and the correlation value is equal to or less than the threshold value Th13. In this case, it is determined that the designated peripheral station is a sector of another band that covers the same area as the own station, and the offset value: Offset13 stored in the memory 6 is applied (step S9), and obtained in step S7. The average value of the received level is lowered (see FIG. 4).

  On the other hand, when the correlation value is larger than the predetermined threshold value, the average value of the reception levels obtained in step 7 is held. Note that a known technique such as fading pitch detection is applied to the method of detecting the movement speed by the movement speed detector 8.

  After carrying out the processing of step S4 to step S9 for all the specified stations (# 2 to #N), the handover condition determination unit 5 next receives the average value of the reception level of the own station and the reception of the peripheral stations. Compare the average level (including both uncorrected and corrected) and whether or not the handover conditions are satisfied (the average of the reception level of the local station and the reception level of the neighboring stations) Whether the difference from the average value exceeds a specific value) is determined (step S10).

  Next, a correlation value calculation process (corresponding to step S5) in the case of suppressing handover to a sector (peripheral station) in another band covering the same area as the own station in the TDMA system will be described. FIG. 5 is a diagram showing a 3-slot multiplexed slot format used in the TDMA system and correlation value calculation processing. For example, the reception level Meas (1) of the own station measured in the Rx slot, and the reception level Meas (2) of the peripheral station (any one of # 1 to #N) measured in the Idle slot continuous with the Rx slot In step S21, first, the correlation calculation unit 7 corrects the difference between the base station transmission power of the local station and the base station transmission power of the peripheral station with respect to Meas (2). 'Is calculated (step S22). Next, correction regarding the difference between the frequency of the local station and the frequency of the peripheral station is performed, and Meas (2) '' is calculated (step S23). And the value which calculated the square of the difference of Meas (1) and Meas (2) '' is output as a correlation value (step S24). In the present embodiment, the smaller the correlation value, the greater the correlation.

  Next, a process (corresponding to step S9) for determining whether or not a sector (peripheral station) of another band covering the same area as the own station using the correlation value in the TDMA system will be described. For example, the reception level varies due to changes in the distance between the base station and mobile station (long-term median fluctuation), and slow fluctuations (short-term median fluctuation) over several tens of meters due to changes in the influence of features. ), Affected by sudden fluctuation (instantaneous value fluctuation) in a section of about several tens of meters due to the influence of multipath. Further, when comparing the own station and a sector (peripheral station) of another band covering the same area as the own station, since the distance between the mobile station and the base station is the same, the long-term median fluctuations coincide. Moreover, since the influence of the features between the mobile station and the base station is the same, the short-term median fluctuations also coincide. However, since the measurement timing is slightly different, the influence of multipath is different, and the instantaneous value fluctuations do not match.

  Even if the instantaneous value fluctuations do not match, the spatial correlation increases if the moving speed is slow, so the instantaneous value fluctuations do not differ so much. The fluctuation will be very different. Therefore, in the present embodiment, the threshold value and the offset value are set according to the moving speed (see FIG. 4). Thereby, the estimation accuracy of sectors in other bands covering the same area as the own station is maintained.

  Also, the square of the difference between the measurement result of the reception level of the own station and the measurement result of the reception level of the sector (peripheral station) of another band covering the same area as the own station is calculated (corresponding to step S24). ) And the difference between the instantaneous value fluctuations remain, but the square of the difference between the reception level measurement result of the local station and the reception level measurement results of the peripheral stations covering other areas is calculated (step S24). And a difference other than the difference of the instantaneous value fluctuation also exists, so that the calculated correlation value becomes larger. That is, it can be seen that the correlation is smaller. Therefore, the handover condition determination unit 5 can determine whether the sector is a sector (peripheral station) in another band that covers the same area as the own station by comparing the correlation value with a predetermined threshold value. Yes (corresponding to the comparison process in step S9).

  However, since the behavior of the median value of the long interval varies depending on the base station transmission power, it is necessary to perform a correction to absorb the difference in the base station transmission power before calculating the square of the difference between the measurement results (steps). Corresponding to S22). For example, when the base station transmission power is 2 W and 4 W, correction of about 3 dB is required.

Moreover, since the behavior of the median value of the long interval and the median value of the short interval varies depending on the frequency, it is necessary to perform correction for absorbing the difference in frequency before calculating the square of the difference. For example, if the median of the long interval follows free space propagation loss (10 * log (4πd / λ) ² : d represents distance and λ represents wavelength), the frequency is about 0 when the frequency is 800 MHz and 1.5 GHz. A correction of .55 dB is required.

  Next, a correlation value calculation process (corresponding to step S5) when suppressing handover to a sector (peripheral station) in another band that covers the same area as the own station in the CDMA system will be described. FIG. 6 is a diagram showing a slot format and correlation value calculation process at the time of handover between different frequencies used in the CDMA system. For example, the reception level Meas (1) of the own station measured in the Rx slot and the reception level Meas (2) of the peripheral station (any one of # 1 to #N) measured by “Transmission Gap” continuous with the Rx slot. )) (Step S31), first, Meas (2) is corrected for the difference between the base station transmission power of the local station and the base station transmission power of the peripheral station, and Meas ( 2) 'is calculated (step S22). Next, correction regarding the difference between the frequency of the local station and the frequency of the peripheral station is performed, and Meas (2) '' is calculated (step S23). And the value which calculated the square of the difference of Meas (1) and Meas (2) '' is output as a correlation value (step S24). Here, as in the TDMA system, the smaller the correlation value, the greater the correlation.

  In W-CDMA, correction for absorbing the difference in base station transmission power is performed based on the base station transmission power of other sectors broadcast from the base station (corresponding to step S32). In addition, the process of determining whether or not the sector (peripheral station) is in another band that covers the same area as the own station (corresponding to step S9) is the same as in the case of the FDMA system.

  Subsequently, a correlation value calculation process (corresponding to step S5) when suppressing handover to a sector (peripheral station) of another system that covers the same area as the own station in the CDMA system will be described. FIG. 7 is a diagram illustrating a slot format and correlation value calculation process during inter-system handover used in the CDMA system. For example, the reception level Meas (1) of the own station measured in the Rx slot and the reception level Meas (2) of the peripheral station (any one of # 1 to #N) measured by “Transmission Gap” continuous with the Rx slot. )) (Step S41), first, Meas (2) is corrected for the difference between the base station transmission power of the local station and the base station transmission power of the peripheral station, and Meas ( 2) 'is calculated (step S22). Next, correction regarding the difference between the frequency of the local station and the frequency of the peripheral station is performed, and Meas (2) '' is calculated (step S23). And the value which calculated the square of the difference of Meas (1) and Meas (2) '' is output as a correlation value (step S24). Here, as in the TDMA system, the smaller the correlation value, the greater the correlation.

  Note that the process for determining whether or not the sector (peripheral station) is in another system that covers the same area as the local station (corresponding to step S9) is the same as in the case of the TDMA system.

  As described above, in the present embodiment, by determining the correlation value between the reception levels of the local station and the peripheral station, it is determined whether it is a sector of another band or another system that covers the same area as the local station. Furthermore, a negative offset is applied to the measurement results (reception level) of peripheral stations that are below a predetermined threshold based on the purpose (handover to another band, handover to another system) and the moving speed. did. Thereby, as shown in FIG. 8, an unnecessary handover to a sector of another band or another system covering the same area as the own station can be suppressed. FIG. 8 is a diagram illustrating a state in which unnecessary handover is suppressed. For example, as a result of performing a negative correction on the reception level of the peripheral station, the difference between the reception level of the local station and the peripheral station is The state of being below the threshold for performing is described.

  Further, in this embodiment, by enabling unnecessary handover to be suppressed, it is possible to reduce the sound interruption of the mobile station. In addition, the signaling load on the network can be reduced. Moreover, since there is no delay in handover to the optimum cell, it is possible to reduce the call disconnection of the mobile station.

  In this embodiment, the threshold value and the offset value are set for each purpose such as handover to another band and handover to another system. As a result, even if the value to be corrected is originally unknown in the current standard, by setting a value obtained by field evaluation or the like, another band covering the same area as the own station or Sectors of other systems can be estimated with high accuracy. In addition to the above purpose, the threshold value and the offset value are set according to the moving speed. As a result, even if the instantaneous value fluctuation becomes uncorrelated because the moving speed is fast, it is possible to estimate the other band or the sector of the other system covering the same area as the own station with high accuracy.

  In FIGS. 5 to 7 of the present embodiment, a correlation value is calculated from the measurement result of one reception level, and another band or other area covering the same area as the own station is calculated using this correlation value. Although the system sector was estimated, the present invention is not limited to this. For example, the above-mentioned estimation is performed by using a value obtained by averaging correlation values calculated based on N reception level measurement results (FIG. 9, step S25). Processing may be performed. FIG. 9 is a diagram illustrating, as an example, a three-slot multiplexed slot format used in the TDMA scheme and correlation value calculation processing. In this case, the number N of reception level measurements can be set by the memory rewriting device 7 and stored in the memory 6. Thereby, the estimation error of the sector of the other band or other system covering the same area as the own station can be reduced. Note that the process of step S25 is not limited to FIG. 5 but can be applied to FIGS.

  In FIG. 3 of the present embodiment, the necessity of applying the offset is determined by one correlation value determination (step S9). However, the present invention is not limited to this. For example, the offset is determined by N correlation value determinations. The offset value to be applied may be changed depending on the number of times it is determined that the application of is necessary. FIG. 10 is a diagram illustrating a modification of offset application. For example, if the offset increment is 2 dB and it is determined that the offset needs to be applied M = 1 time in N correlation value determinations (step S51), an offset of 1 × 2 dB = 2 dB is applied (step S52). , S53), when it is determined that the application of the offset is required only twice (step S51), an offset of 2 × 2 dB = 4 dB is applied (steps S52, S53), and it is determined that the application of the offset is necessary. The offset is changed in accordance with the number of times of being performed. Thereby, the estimation error of the sector of the other band or other system covering the same area as the own station can be further reduced. When FIG. 10 is applied, the offset in FIG. 4 is an offset increment and is set by the memory rewriting device 7. The number of times of reception level measurement: N is also set by the memory rewriting device 7.

  As described above, the handover control method according to the present invention is useful for a mobile station that continues communication by performing a handover, and in particular, a sector of another band or another system that covers the same area as the sector being communicated. Suitable for mobile stations that suppress handover to

It is a figure which shows the structural example of the mobile station which implement | achieves the handover control method concerning this invention. It is a figure which shows the slot format in a TDMA system. It is a flowchart which shows the process of a mobile station. It is a figure which shows an example of the threshold value and offset value which were stored in memory. It is a figure which shows the calculation process of the slot format of 3 slot multiplexing used with the TDMA system, and a correlation value. It is a figure which shows the slot format at the time of the handover between different frequencies used by the CDMA system, and the calculation process of a correlation value. It is a figure which shows the slot format at the time of the system handover used by the CDMA system, and the calculation process of a correlation value. It is a figure which shows a mode that an unnecessary handover is suppressed. It is a figure which shows the calculation process of the slot format of 3 slot multiplexing used with the TDMA system, and a correlation value. It is a figure which shows the modification of offset application.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mobile station 2 Control part 3 Radio equipment circuit part 4 Movement speed detection part 5 Handover condition determination part 6 Memory 7 Correlation calculation part

Claims (7)

In a handover control method by a mobile station that performs handover between sectors,
A correlation calculation step for calculating a correlation between the measurement result of the communication quality of the handover source sector and the measurement result of the communication quality of the handover candidate sector designated by the base station;
As a result of the correlation calculation, if the correlation is greater than a predetermined threshold, it is determined that the handover source sector and the handover candidate sector are sectors in the same area, and a handover suppressing step for suppressing handover to the handover candidate sector; ,
A handover control method comprising: The correlation calculation step includes:
A correction related to a difference in base station transmission power between the handover source sector and the handover candidate sector, and a correction related to a difference in frequency between the handover source sector and the handover candidate sector with respect to the measurement result of the communication quality of the handover candidate sector. A correction step for performing
A correlation value calculating step of outputting, as a correlation value, the square of the difference between the communication quality of the handover source sector and the corrected communication quality of the handover candidate sector;
Including
The handover control method according to claim 1, wherein the correlation is larger as the correlation value is smaller. The handover suppressing step includes:
An averaging step of averaging communication quality of the handover source sector and the handover candidate sector;
If the correlation is greater than a predetermined threshold as a result of the correlation calculation, a predetermined offset value is applied to the average communication quality of the handover candidate sector, and the communication quality of the handover candidate sector is A communication quality adjustment step for holding an average value of the communication quality of the handover candidate sectors when the correlation is not more than a predetermined threshold value,
The handover control method according to claim 1 or 2, characterized by comprising:   The handover control method according to claim 3, wherein the threshold value and the offset value are rewritable.   The handover control method according to claim 3 or 4, wherein the threshold value and the offset value are set for each handover to be suppressed.   The handover control method according to claim 3, 4 or 5, wherein the threshold value and the offset value are set according to a moving speed of a mobile station. In a mobile station that performs handover between sectors,
Correlation calculating means for calculating a correlation between the measurement result of the communication quality of the handover source sector and the measurement result of the communication quality of the handover candidate sector designated by the base station;
Based on the correlation calculation result, when the correlation is larger than a predetermined threshold, it is determined that the handover source sector and the handover candidate sector are sectors in the same area, and the handover condition for suppressing the handover to the handover candidate sector A determination means;
A mobile station comprising:

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