JP2011004127A - Base station device, mobile communication system, and handover control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology which successfully executes handover according to moving speed of a terminal.SOLUTION: A speed acquisition means acquires the moving speed of the terminal connected with a self-device. A determination means compares a level difference between received signal strength of a signal from a base station device at a moving origin, which is the self-device in the terminal and received signal strength of a signal from a base station device at a moving destination in the terminal with a first threshold to be determined based on the moving speed of the terminal, and when the level difference is equal to or more than the first threshold, determines that the terminal is to be handed over from the base station device at the moving origin to the base station device at the moving destination.

Description

  The present invention relates to a technique for switching a base station connected to a terminal by handover in a wireless communication system capable of moving the terminal connected to the base station.

  In a wireless communication system in which a terminal is connected to a base station via a wireless line, in order to enable stable communication, it is required to maintain the received signal strength of the signal from the base station at a certain level or higher. However, the movement of the terminal may change the environment of the wireless line, and the received signal strength at the terminal may deteriorate. When the received signal strength is improved by switching the connection destination from the connected base station to another base station, the terminal performs switching (handover) of the connection destination base station.

  Handover significantly improves the received signal strength and prevents the handover from being repeated near the boundary between base stations, so that the difference between the reception level from the destination base station and the reception level of the source base station is It is executed on condition that it is above a certain threshold.

  In Patent Document 1, a mobile terminal acquires location information of its own device using GPS (Global Positioning System), calculates the distance between itself and each base station, and based on the distance, A technique for performing handover is disclosed.

  Further, Patent Document 2 detects a moving speed and a moving direction from terminal position information, and is connected based on an offset for each base station calculated based on the information and a reception level of a signal from each base station. A technique for selecting a previous base station is disclosed.

  Further, in Patent Document 3, a threshold for a reception level from a connected base station is used as a threshold for determining whether or not to perform handover from a connected base station to another base station. The threshold value is changed based on the moving speed of the terminal.

  Further, in Patent Document 4, in a handover in which a terminal is connected simultaneously with a plurality of base stations, a threshold is set to include a base station having a reception level close to the reception level from the connected base station in the base station connected to the terminal. Used. The threshold value is changed based on the moving speed of the terminal.

Japanese Laid-Open Patent Publication No. 2002-199428 Japanese Laid-Open Patent Publication No. 2005-012429 JP 2000-197093 A JP 2002-10313 A

  As described above, to execute handover on condition that the difference between the reception level from the destination base station and the reception level of the source base station is equal to or greater than the threshold, it is possible to prevent the handover from being repeated between base stations. There is a meaning to do. This is because if the handover is repeated near the boundary, resources such as the radio frequency and the processing capability of the apparatus are wasted.

  However, the likelihood of handover when the terminal moves varies depending on the moving speed of the terminal. This is because the received signal strength at the terminal is affected by the attenuation of the signal depending on the distance from the base station to the terminal and the fluctuation of the radio environment such as fading. When the moving speed of the terminal is fast, the change in the received signal strength from the base station is highly dependent on the change in the distance between the terminal and the base station. On the other hand, when the moving speed of the terminal is slow, a change in received signal strength from the base station is highly dependent on radio environment fluctuations such as fading.

  7A to 7C are diagrams for explaining the behavior of the system in the handover when the moving speed of the terminal is low.

  When the moving speed of the terminal is low, as shown in FIG. 7A, the change in the signal strength of the received signal from the movement source base station and the movement destination base station is slow.

  If the threshold for the difference between the reception level from the destination base station and the reception level of the source base station is small, switching from the source base station to the destination base station can be performed quickly. However, as shown in FIG. 7B, if there is a fading effect, handover may be unnecessarily repeated between the source base station and the destination base station with a small threshold (α in the figure).

  On the other hand, if the threshold value (β in the figure) is large as shown in FIG. 7C, unnecessary handover as described above can be reduced.

  8A to 8C are diagrams for explaining the behavior of the system in the handover when the moving speed of the terminal is high.

  When the moving speed of the terminal is high, as shown in FIG. 8A, the signal strength of the received signal from the moving base station and the moving base station changes sharply.

  As shown in FIG. 8B, if the threshold is large, unnecessary handover is reduced as described above. However, if the moving speed of the terminal is high, the occurrence of unnecessary handover is unlikely to occur. On the other hand, if the threshold is large, activation of the handover is delayed. Even if the signal can be received with good signal strength by switching to the destination base station, the connection with the source base station may be continued. Therefore, there is a high possibility that the signal strength is deteriorated and the connection is disconnected.

  On the other hand, when the moving speed of the terminal is high, the handover can be executed quickly by reducing the threshold value as shown in FIG. 8C, and the occurrence of unnecessary handover is small.

  As described above, Patent Documents 2-4 disclose a handover technique considering the moving speed of a terminal. However, since the threshold value changed according to the moving speed in Patent Documents 2 and 3 is relative to the reception level of the movement-source base station, the repetition of handover is effectively reduced depending on the fluctuation of the reception level from the movement-destination base station. In some cases, it was not possible. In addition, the threshold value that is changed according to the moving speed in Patent Document 4 is for selecting a base station with a reception level close to the reception level from the connected base station. It did not contribute.

  An object of the present invention is to provide a technique that enables a favorable handover to be executed according to the moving speed of a terminal.

In order to achieve the above object, a base station apparatus of the present invention is a base station apparatus included in a mobile communication system that covers a communication area by a plurality of base station apparatuses,
Speed acquisition means for acquiring the moving speed of the terminal device connected to the own device;
The level difference between the received signal strength at the terminal device of the signal from the source base station device which is the own device and the received signal strength at the terminal device of the signal from the destination base station device, and the terminal A first threshold value determined based on the moving speed of the device is compared. If the level difference is equal to or greater than the first threshold value, the terminal device is moved from the source base station device to the destination base station device. Determining means for determining that a handover should be performed;
Have

The mobile communication system of the present invention is a mobile communication system that covers a communication area by a plurality of base station devices,
A terminal device capable of performing handover between base station devices;
The movement speed of the terminal device connected to the own device is acquired, and the received signal strength of the signal from the source base station device that is the own device and the signal from the destination base station device are obtained. A level difference between the received signal strength at the terminal device and a first threshold value determined based on a moving speed of the terminal device are compared, and if the level difference is equal to or greater than the first threshold value, the terminal device A base station apparatus that determines that a handover from the source base station apparatus to the destination base station apparatus is to be performed,
Have

A handover control method of the present invention is a handover control method in a mobile communication system that covers a communication area by a plurality of base station devices,
Get the moving speed of the terminal device,
The level difference between the received signal strength at the terminal device of the signal from the source base station device and the received signal strength at the terminal device of the signal from the destination base station device, and the moving speed of the terminal device The terminal device is to be handed over from the source base station device to the destination base station device if the level difference is equal to or greater than the first threshold value. Judgment.

  According to the present invention, since the base station apparatus uses the first threshold value that differs depending on the moving speed of the terminal apparatus, it is possible to execute a good handover even if the moving speed of the terminal apparatus varies.

It is a block diagram which shows the structural example of the WiMAX system by this embodiment. It is a sequence chart of network connection in this embodiment. 2 is a block diagram showing a configuration of a base station 110. FIG. It is a flowchart which shows an example of the communication control in the handover process in this embodiment. It is a figure for demonstrating the method of calculating the moving speed of the terminal. It is a figure for demonstrating the method of calculating the moving speed of the terminal. It is a flowchart which shows the other example of the communication control in the handover process in this embodiment. It is a figure for demonstrating the behavior of the system in a handover when the moving speed of a terminal is low. It is a figure for demonstrating the behavior of the system in a handover when the moving speed of a terminal is low. It is a figure for demonstrating the behavior of the system in a handover when the moving speed of a terminal is low. It is a figure for demonstrating the behavior of the system in a handover when the moving speed of a terminal is high. It is a figure for demonstrating the behavior of the system in a handover when the moving speed of a terminal is high. It is a figure for demonstrating the behavior of the system in a handover when the moving speed of a terminal is high.

  Embodiments of the present invention will be described in detail with reference to the drawings.

  Here, a WiMAX (Worldwide Interoperability for Microwave Access) system to which the present invention is applied is exemplified.

  FIG. 1 is a block diagram illustrating a configuration example of a WiMAX system according to the present embodiment. Referring to FIG. 1, the WiMAX system includes a terminal 100, a base station 110, an ASN-GW 120, and a CSN 130.

  A terminal (MS: Mobile Station) 100 is a WiMAX client such as a personal computer or a CPE (Customer Premises Equipment) device installed in a home. The terminal 100 can be handed over between the base stations 110.

  Each base station 110 forms a cell and is connected to the terminal 100 via a wireless line, thereby enabling communication by the terminal 100. The connection to the base station 110 by the terminal 100 includes a case of newly connecting to the base station 110 and a case of connecting to the base station 110 by a handover from another base station 110.

  FIG. 2 is a sequence chart of network connection in the present embodiment.

  The terminal 100 and the base station 110 perform a ranging process to start connection of the terminal 100 to the base station 110 (step S101). A CDMA code is used for the ranging process.

  Subsequently, the terminal 100 and the base station 110 execute a process for adjusting the transmission power of the terminal 100 (step S102). Through this process, the transmission power of terminal 100 is adjusted to correspond to the distance between terminal 100 and base station 110.

  Furthermore, the terminal 100 and the base station 110 execute processing for adjusting the transmission timing of the terminal 100, and synchronize the terminal 100 and the base station 110 (step S103).

  After the above processing is completed, the terminal 100 can be connected to the network through function registration and authentication procedures of the terminal 100.

  Also, the base station 110 controls the handover between the base stations 110 by the terminal 100 by cooperating with the terminal 100.

  In the handover process, the base station 110 acquires the received signal strength between the terminal 100 and the base station 110, and calculates the moving speed of the terminal 100. Further, the base station 110 determines whether the terminal 100 should be handed over from the source base station to the destination base station, and the received signal strength and movement of the signal from the source base station 110 in the terminal 100 The level difference between the received signal strength of the signal from the previous base station 110 and the first threshold value determined based on the moving speed of the terminal 100 are compared.

  The source base station 110 is the base station 110 to which the terminal 100 is currently connected. The destination base station 110 is a target base station 110 to which the terminal 100 is to be handed over.

  If the level difference is greater than or equal to the first threshold, the base station 110 determines that the terminal 100 should be handed over from the source base station 110 to the destination base station 110.

  Further details of the handover process and the configuration of the base station 110 related thereto will be described later.

  An ASN (Access Service Network) -GW (Gateway) 120 is a gateway for relaying communication of the terminal 100 connected to the base station 110 to the CSN 130.

  Base station 110 and ASN-GW 120 are included in the ASN. The ASN is an access network in a WiMAX system including a single or a plurality of base stations 110 and a single or a plurality of ASN-GWs 120, and is similar to a radio access network (RAN: Radio Access Network) in a cellular phone network. It provides functions such as Quality of Service management and radio resource management.

  A CSN (Connectivity Service Network) 130 is responsible for the IP core network function in the WiMAX service.

  The CSN 130 integrates core network functions including AAA (Authentication, Authorization, and Accounting) functions, Mobile IP HA (Home Agent) functions, and proxy functions and connection functions with other networks.

  The CSN 130 includes an AAA unit 131, a DNS DHCP unit 132, and an HA unit 133. The AAA unit 131 is responsible for the AAA function, the DNS DHCP unit 132 is responsible for the proxy function and connection function with other networks, and the HA unit. 133 is responsible for the HA function.

  FIG. 3 is a block diagram showing the configuration of base station 110. Referring to FIG. 3, a signal intensity acquisition unit 111, a speed acquisition unit 112, and a determination unit 113 are included.

  The signal strength acquisition unit 111 acquires the received signal strength between the terminal 100 and the base station 110. For example, the signal strength of a signal received from the base station 110 to which the terminal 100 is connected is measured and notified to the base station 110.

  The speed acquisition unit 112 acquires the moving speed of the terminal 100. For example, the speed acquisition unit 112 may calculate the moving speed of the terminal 100 from the time change of the signal intensity acquired by the signal intensity acquisition unit 111.

  As a specific example, the speed acquisition unit 112 calculates the distance between the terminal 100 and the base station 110 from the received signal strength of the terminal 100 acquired by the signal strength acquisition unit 111 using a theoretical formula of free space propagation loss, The moving speed of the terminal 100 is calculated from the time change of the distance.

  The determination unit 113 is determined based on the level difference between the received signal strength of the signal from the source base station 110 in the terminal 100 and the received signal strength of the signal from the destination base station 110 and the moving speed of the terminal 100. The first threshold value is compared. The higher the moving speed of the terminal 100, the smaller the first threshold value is set.

  Note that the determination unit 113 receives, for example, notification from the terminal 100, the received signal strength at the terminal 100 of the signal from the source base station 110 that is the own device, and the destination base station 110 that is the other device. The received signal strength at the terminal 100 of the signal can be acquired. Usually, the destination base station 110 is a base station adjacent to the source base station 110.

  As a specific example of the method for determining the first threshold, the determination unit 113 selects a plurality of threshold candidates that are candidates for the first threshold and the first threshold from the threshold candidates based on the moving speed. For this purpose, at least one fourth threshold value may be given in advance. Then, the determination unit 113 may compare the moving speed with the fourth threshold value and select one of the threshold candidates as the first threshold value based on the comparison result.

  If the determination unit 113 compares the level difference with the first threshold and the level difference is greater than or equal to the first threshold, the determination unit 113 should hand over the terminal 100 from the source base station 110 to the destination base station 110. judge. If the level difference is smaller than the first threshold, the determination unit 113 determines that the terminal 100 should not be handed over from the source base station 110 to the destination base station 110.

  FIG. 4 is a flowchart illustrating an example of communication control in the handover process according to the present embodiment.

  Referring to FIG. 4, first, the terminal 100 finishes the ranging process, terminal function registration, and authentication process, and then connects to a certain base station 110 (step S201). Here, the base station 110 to which the terminal 100 is connected at this time is the handover source base station.

  Next, the base station 110 compares the received signal strength between the terminal 100 and the base station 110 with a threshold (second threshold), and determines whether to start the handover process from the comparison result (step S202). ).

  Here, as an example, the condition for starting the handover process is that the received signal strength is lower than a certain value. The second threshold value used here is a threshold value for determining whether or not the condition is satisfied.

  If the received signal strength is equal to or greater than the second threshold, the base station 110 continues to monitor the received signal strength without starting the handover process.

  Also, if the received signal strength is lower than the second threshold, the base station 110 determines to start the handover process, and calculates the moving speed of the terminal 100 (step S203). At that time, the base station 110 calculates the moving speed of the terminal 100 from the received signal strength from the base station 110 at the terminal 100 using the theoretical formula of the free space propagation loss.

  5A and 5B are diagrams for explaining a method of calculating the moving speed of the terminal 100. FIG. As shown in FIG. 5A, if the terminal 100 moves, moves away from the source base station 110, and approaches the destination base station 110, the received signal strength of the signal from the source base station 110 at the terminal 100 decreases. Then, the received signal strength at the terminal 100 of the signal from the destination base station 110 increases. If the theoretical formula of the free space propagation loss is used, the distance between the terminal 100 and the base station 110 can be obtained from the received signal strength. If the change in the distance between the terminal 100 and the base station 110 is divided by the time, the moving speed of the terminal 100 can be obtained.

  At that time, the base station 110 acquires the received signal strength for each frame transmitted by the base station 110 from the terminal 100, and uses the received signal strength for each frame and the frame period of the frame transmitted by the base station 110. The moving speed of the terminal 100 may be calculated. For example, the frame period of the WiMAX system is 5 msec.

  According to this, since the frame period which is a fixed value is used for the calculation of the moving speed, the calculation is easy. Further, the base station 110 can calculate the moving speed without the information of the time when the received signal strength is measured.

  Returning to FIG. 4, the base station 110 next determines whether or not the moving speed of the terminal 100 is equal to or higher than a certain threshold (fourth threshold) (step S204). The fourth threshold value used here is a threshold value for determining whether the moving speed of the terminal 100 is faster or slower than a certain value.

  If the moving speed of the terminal 100 is equal to or higher than the fourth threshold, the base station 110 selects a relatively small value as the first threshold (step S205). Conversely, if the moving speed of the terminal 100 is slower than the fourth threshold value, the base station 110 selects a relatively large value as the first threshold value (step S206). The value selected in step S206 is larger than the value selected in step S205.

  Although an example in which the fourth threshold value is one and the first threshold value is selected from two candidates is shown here, the present invention is not limited to this. If the number of first threshold candidates is increased by using a plurality of fourth threshold values, the first threshold value can be set to a more appropriate value for the moving speed of the terminal 100.

  Next, base station 110 compares the level difference between the received signal strength of the signal from source base station 110 in terminal 100 and the received signal strength of the signal from destination base station 110 with the first threshold value. (Step S207).

  If the level difference is equal to or greater than the first threshold, the base station 110 determines that the terminal 100 should be handed over from the source base station 110, which is its own device, to the destination base station 110, and executes the handover. (Step S208). If the level difference is smaller than the first threshold, the base station 110 returns to step S202 without executing the handover, and continues monitoring the received signal strength at the terminal 100.

  As described above, according to the present embodiment, since the base station 110 uses a different threshold (first threshold) according to the moving speed of the terminal 100, it is favorable even if the moving speed of the terminal 100 varies. A handover can be performed. Specifically, in the present embodiment, both quickness of handover and reduction of unnecessary handover can be achieved.

  In the present embodiment, as shown in FIG. 4, the condition for starting the handover process is that the received signal strength of the terminal 100 is lower than a certain value, but the present invention is not limited to this. is not. As another example, a condition for starting the handover process may be that the carrier-to-noise (interference) ratio of the radio channel between the base station 110 and the terminal 100 is lower than a certain value. As yet another example, the handover process is started when the received signal strength of the terminal 100 is lower than a certain value and the carrier-to-noise (interference) ratio of the radio channel between the base station 110 and the terminal 100 is lower than the certain value. It is good also as conditions for doing.

  FIG. 6 is a flowchart illustrating another example of communication control in the handover process according to the present embodiment. Here, the fact that the received signal strength of terminal 100 is lower than a certain value and the carrier-to-noise (interference) ratio of the radio channel between base station 110 and terminal 100 is lower than a certain value is for starting the handover process. An example of the condition is illustrated.

  Referring to FIG. 6, step S301 is added between step S202 and step S203 in the flowchart shown in FIG. For other steps S201 to S208, FIG. 6 and FIG. 4 are the same.

  In step S301, the base station 110 determines whether the carrier-to-noise (interference) ratio between the base station 110 and the terminal 100 is smaller than a certain threshold value (third threshold value). If the carrier-to-noise (interference) ratio is smaller than the third threshold, the base station 110 proceeds to step S203, and if the carrier-to-noise (interference) ratio is greater than or equal to the third threshold, the base station 110 proceeds to step S202. Return.

  According to this, it is possible to prevent the carrier-to-noise (interference) ratio from being deteriorated by performing a handover in spite of the favorable carrier-to-noise (interference) ratio in the source base station 110. it can.

  In the present embodiment, an example in which the moving speed of the terminal 100 is calculated from the received signal strength from the base station 110 is shown, but the present invention is not limited to this, and the moving speed of the terminal 100 is not limited to this. May be obtained. For example, the terminal 100 may be provided with a GPS (Global Positioning System) receiver, and the position of the own device may be measured by the GPS receiver. In that case, the moving speed of the terminal 100 can be calculated from the time change of the position of the own device measured by the terminal 100.

  As mentioned above, although embodiment of this invention was described, this invention is not limited only to this embodiment, You may change a some structure within the scope of the technical idea of this invention.

100 terminal 110 base station 111 signal strength acquisition unit 112 speed acquisition unit 113 determination unit 120 GW
130 CSN
131 AAA part 132 DHCP part 133 HA part

Claims (11)

A base station device included in a mobile communication system that covers a communication area by a plurality of base station devices,
Speed acquisition means for acquiring the moving speed of the terminal device connected to the own device;
The level difference between the received signal strength at the terminal device of the signal from the source base station device which is the own device and the received signal strength at the terminal device of the signal from the destination base station device, and the terminal A first threshold value determined based on the moving speed of the device is compared. If the level difference is equal to or greater than the first threshold value, the terminal device is moved from the source base station device to the destination base station device. Determining means for determining that a handover should be performed;
A base station apparatus. The determination means sets the first threshold value to a smaller value as the moving speed of the terminal device increases.
The base station apparatus according to claim 1. The determination means uses a condition that a received signal strength between the terminal device and a source base station device is lower than a second threshold as a condition for performing a handover.
The base station apparatus according to claim 1 or 2. The determination means uses a carrier-to-noise ratio between the terminal apparatus and the base station apparatus as a movement source that is lower than a third threshold as a condition for performing handover.
The base station apparatus of any one of Claims 1-3. A plurality of threshold candidates that are candidates for the first threshold and at least one fourth threshold for selecting the first threshold from the threshold candidates based on the moving speed are predetermined. And
The determination means compares the moving speed with the fourth threshold, and selects one of the threshold candidates as the first threshold according to the comparison result.
The base station apparatus of any one of Claims 1-4. Further comprising signal strength acquisition means for acquiring the received signal strength at the terminal device of the signal from the own device;
The speed acquisition means calculates a distance between the terminal apparatus and the base station apparatus from a received signal intensity of the terminal apparatus acquired by the signal intensity acquisition means, using a theoretical expression of free space propagation loss, Calculate the moving speed of the terminal device from the time change of the distance,
The base station apparatus of any one of Claims 1-5. The signal strength acquisition means acquires the received signal strength for each frame transmitted by the base station device,
The speed acquisition means calculates the moving speed of the terminal device using the received signal strength for each frame acquired by the signal strength acquisition means and the frame period of the frame transmitted by the base station device.
The base station apparatus according to claim 6. A mobile communication system that covers a communication area by a plurality of base station devices,
A terminal device capable of performing handover between base station devices;
The movement speed of the terminal device connected to the own device is acquired, and the received signal strength of the signal from the source base station device that is the own device and the signal from the destination base station device are obtained. A level difference between the received signal strength at the terminal device and a first threshold value determined based on a moving speed of the terminal device are compared, and if the level difference is equal to or greater than the first threshold value, the terminal device A base station apparatus that determines that a handover from the source base station apparatus to the destination base station apparatus is to be performed,
A mobile communication system. The base station device sets the first threshold value to a smaller value as the moving speed of the terminal device increases.
The mobile communication system according to claim 8. A handover control method in a mobile communication system that covers a communication area by a plurality of base station devices,
Get the moving speed of the terminal device,
The level difference between the received signal strength at the terminal device of the signal from the source base station device and the received signal strength at the terminal device of the signal from the destination base station device, and the moving speed of the terminal device The terminal device is to be handed over from the source base station device to the destination base station device if the level difference is equal to or greater than the first threshold value. judge,
Handover control method. The faster the moving speed of the terminal device, the smaller the first threshold value,
The handover control method according to claim 10.

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