KR20080030238A - Apparatus and method for executing bi-casting in wireless network - Google Patents

Abstract

A device for performing bi-casting in a mobile communication system and a method thereof are provided to enable a reference base station **to differently start** data bi-casting start time according to mobile speed of each terminal, thereby reducing backhaul resource consumption as well as generation of backhaul overloads. A mobile speed estimator(303) estimates mobile speed of terminals. A bi-casting threshold value setup unit(307) sets bi-casting threshold values based on the estimated mobile speed. A controller(301) compares the bi-casting threshold values with signal strength of a neighbor base station, and demands bi-casting execution to an upper system if the signal strength of the neighbor base station is larger than the bi-casting threshold values.

Description

Apparatus and method for performing bicasting in mobile communication system {APPARATUS AND METHOD FOR EXECUTING BI-CASTING IN WIRELESS NETWORK}

1 illustrates a bicasting execution time in the prior art,

2 is a schematic diagram of bicasting execution time according to the present invention;

3 is an apparatus block diagram for setting a bicasting threshold value according to a moving speed according to an embodiment of the present invention;

4 is a flowchart of setting a bicasting threshold value according to a moving speed according to an embodiment of the present invention;

5 is a bicasting example considering a moving speed of a mobile communication terminal according to an embodiment of the present invention;

6 is an IEEE 802.20 Mobile Broadband Wireless Access (MBWA) system experiment environment according to an embodiment of the present invention;

7 is a table designating a group according to a moving speed according to an embodiment of the present invention;

8 is an example of a movement scenario used in an experiment according to an embodiment of the present invention;

9 is a bicasting threshold table set for each moving speed group according to an embodiment of the present invention;

10 is a measure used to analyze the performance of the present invention,

11 is a result table for a low moving speed group of a moving scenario in a downtown cell environment according to an embodiment of the present invention;

12 is a result table for an intermediate movement speed group of a movement path 1 movement scenario in a downtown cell environment according to an embodiment of the present invention;

FIG. 13A is a table showing a comparison result of a low movement speed group of a movement path 1,2 movement scenario in a downtown cell environment according to an embodiment of the present invention; FIG.

FIG. 13B is a table showing a result of comparing a high speed group of movement paths 1 and 2 in a downtown cell environment according to an embodiment of the present invention; FIG.

FIG. 13C is a result table for a very high movement speed group of a movement path 1 movement scenario in a downtown cell environment in accordance with an embodiment of the present invention. FIG.

The present invention relates to an apparatus and a method for performing bicasting in a mobile communication system, and more particularly, to an apparatus and a method for performing bicasting by setting a bicasting threshold value differentially according to a moving speed of a terminal.

Services that require real-time support (eg, voice over IP, video streaming) can degrade performance due to packet loss due to latency during handoff. As such, there is a need to minimize delay in handoff.

Recently, 3GPP LTE (Long-Term Evolution) system performs a bicasting technique to minimize the delay. The by-casting technique enables the mobile communication terminal to grasp information on a base station that is likely to move in advance, and deliver data packets to two or more base stations in advance of actual handoff. Thereafter, when the mobile communication terminal actually moves to the target base station and the handoff procedure is completed, the packet for the mobile communication terminal is received through a new path so that the packet can be received without interruption.

By performing bicasting as described above, the target base station can transmit data to the mobile communication terminal without delay by retaining data to be transmitted immediately after the mobile communication terminal is connected with the mobile station in advance.

Figure 1 is a schematic of the bicasting execution time performed in the prior art.

Referring to FIG. 1, an SINR value of a mobile communication terminal moving from a serving base station to a neighbor base station is shown. As the mobile communication terminal moves away from the serving base station, the signal-to-interference-and noise ratio (SINR) value of the serving base station decreases with time. On the other hand, since it is getting closer to the neighboring base station, the SINR value of the neighboring base station increases with time. In performing the bicasting due to the handoff, the bicasting time is from the time t1 corresponding to the set update threshold for the handoff to the handoff determination time t2. That is, in the related art, a bicasting of a corresponding base station is performed immediately after performing an active set update for handoff.

As in the prior art, when the bicasting is started immediately after the active set update, unnecessary data is transmitted to the base stations belonging to the active set in advance to increase the backhaul resource consumption. Recently, handover techniques in 3GPP LTE system and IEEE 802.20 MBWA system are actively utilizing communication through backhaul to optimize handover performance. As a result, as the number of handover users increases, the backhaul resource consumption for the handover may be significantly increased compared to the existing system.

Accordingly, there is a need for an apparatus and method for reducing backhaul resource consumption and reducing backhaul overload when performing bicasting.

Accordingly, an object of the present invention is to provide an apparatus and method for reducing backhaul resource consumption and reducing backhaul overload by varying the time of performing bicasting for each mobile speed group of a mobile communication terminal.

Another object of the present invention is to provide an apparatus and method for minimizing a delay that occurs during intercell handoff of a mobile communication terminal.

It is another object of the present invention to provide an apparatus and method for satisfying QoS of a service requiring real-time feature support.

In accordance with an aspect of the present invention, there is provided a device for performing bicasting in a mobile communication system, comprising: a moving speed estimating unit estimating a moving speed of a terminal; and based on the estimated moving speed. By-casting threshold setting unit for setting a threshold value, and comparing the signal strength of the neighboring base station with the bi-casting threshold value, if the signal strength of the neighboring base station is greater than the bi-casting threshold value to request to perform the upper-casting to the upper system It characterized in that it comprises a control unit.

In accordance with an aspect of the present invention, a method for performing bicasting in a mobile communication system includes estimating a moving speed of a mobile communication terminal and a bicasting threshold based on the estimated moving speed. Setting a value, and comparing the bicasting threshold with a signal strength of a neighboring base station and requesting a higher-level system to perform a bicasting when the signal strength of the neighboring base station is greater than the bicasting threshold. It features.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In describing the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

Hereinafter, the present invention will be described an apparatus and method for reducing the backhaul resource consumption and backhaul overload by varying the time of performing the bicasting for each mobile speed group of the mobile communication terminal.

Figure 2 illustrates the bicasting execution time according to the present invention.

Referring to FIG. 2, an SINR value of a mobile communication terminal moving from a serving base station to a neighbor base station is shown. Since the mobile communication is farther away from the serving base station, the SINR value of the serving base station decreases with time. On the other hand, since it is getting closer to the neighboring base station, the SINR value of the neighboring base station increases with time. In performing bicasting due to handoff, the present invention operates by setting a bicasting threshold in consideration of the moving speed of the mobile communication terminal. For example, since a mobile communication terminal having a fast moving speed can perform a handoff to a neighboring base station quickly, a bicasting threshold is set small so that bicasting occurs quickly. On the contrary, since a mobile communication terminal with a slow moving speed may be handed off to a neighboring base station late, the bicasting occurs late by setting a large bicasting threshold.

In FIG. 2, t1 is an active set update time point for handoff, t2 is bicasting time according to a moving speed, and t3 is a time point at which the mobile communication terminal is handed off to a neighbor base station. Therefore, the time from t2 to t3 time is the bicasting time. Thus, the bicasting time can be reduced by t1 to t2 compared with the prior art.

3 is a block diagram of an apparatus for setting a bicasting threshold value according to a moving speed according to an exemplary embodiment of the present invention.

Referring to FIG. 3, the base station 300 includes a radio access termination function for transmitting and receiving voice and video and data traffic with a mobile communication terminal according to a wireless access standard, multi-frequency allocation, and multi-sector. -Sector) Accepts transmit / receive antennas through the structure of receiving (for example, 2FA / 3sector) and softer handoff, and transmits / receives information and voice and video call processing functions to the mobile communication terminal wirelessly. To perform. In particular, according to the present invention, the base station 300 controls the function block of the control unit 301, the moving speed estimating unit 303, the group designation unit 305, the bicasting threshold value determining unit 307, and the RF processing unit 309. By setting the by-casting threshold value in consideration of the moving speed of the mobile communication terminal and if the signal strength of the neighboring base station is greater than the by-casting threshold value, requests to perform the by-casting to the upper system.

The moving speed estimator 303 estimates a moving speed of a mobile communication terminal. The moving speed estimating unit 303 estimates a speed using a frequency error of a received signal generated in proportion to the moving speed of a mobile station with respect to a base station. Velocity is estimated using the envelope of the received signal, i.e., the number of times the power passes a predetermined reference level (number of crossings), and the covariance between received samples with a certain time difference. The moving speed of the mobile communication terminal is estimated using one of the estimating techniques.

The group designation unit 305 groups the mobile communication terminal based on the estimated moving speed. The group designates the first group as 3 km / h to 15 km / h, the second group as 16 km / h to 90 km / h, and the third group as 91 km / h to 250 km / h.

The by-casting threshold setting unit 307 sets a corresponding by-casting threshold according to the designated group. The bicasting threshold decreases as the movement speed increases and increases as the movement speed decreases.

The controller 301 compares the corresponding bicasting threshold with the signal strength of the neighboring base station and requests bicasting when the signal strength of the neighboring base station is greater than the bicasting threshold.

4 is a flowchart for setting a bicasting threshold value according to a moving speed according to an exemplary embodiment of the present invention.

Referring to FIG. 4, first, the base station estimates a moving speed of a mobile communication terminal within a service range in order to set a bicasting threshold in step 400.

As a technique for estimating the moving speed of the mobile communication terminal, a method of estimating the speed using a characteristic in which the frequency error of the received signal due to Doppler shift is proportional to the moving speed of the mobile station (Patent Application No .: P2004-0040773), a given time interval Level crossing rate estimating technique (patent application number: P2003-0032819), which estimates the velocity by using an envelope of a received signal, that is, the number of times the power passes a predetermined reference level (number of crossings), And a covariance estimating technique (patent application number: P2003-0032819) for estimating a velocity by using a covariance value between received samples having a number.

In step 402, the base station groups the terminals based on the estimated moving speed. For example of group designation, see FIG. 7.

Thereafter, the base station proceeds to step 404 to determine the bicasting threshold value for each of the designated groups. An example of determining the bicasting threshold value for each group is described with reference to FIG. 9.

In step 406, the base station compares the signal strength of the neighboring base station with the determined bicasting threshold value. When the signal strength of the neighboring base station is greater than the bicasting threshold value, the base station proceeds to step 408. Make a bicasting request. If the signal strength of the neighboring base station is less than the bicasting threshold in step 406, the base station proceeds to step 400.

Thereafter, the base station proceeds to step 410 where bicasting is performed by the higher-level system to the corresponding neighboring base station.

The base station then terminates the algorithm of the present invention.

5 illustrates an example of bicasting considering a moving speed of a mobile communication terminal according to an exemplary embodiment of the present invention.

Referring to FIG. 5, m mobile communication terminals receiving services from base station A 500 are moving to base station B 510 at speeds of v ₁ , v ₂ , ... v _m , respectively. The base station A 500 estimates the moving speed of each mobile communication terminal, groups the groups into groups 1, 2, and m under a predetermined condition (see FIG. 7 below), and sets a bicasting threshold for each group, and sets If the casting threshold is greater than the signal strength of the neighboring base station, the bicasting is not requested to the reference base station 520. If the setting of the casting threshold is less than the signal strength of the neighboring base station, the bicasting is requested to the reference base station. . t1, t2, and t3 are bicasting start points of Group 1, Group 2, and Group m, respectively, and have a relationship of t1 <t2 <t3 if the speed of Group 1> speed of Group 2> speed of Group 3 respectively. On the contrary, if there is a relationship between the speed of the group 1 and the speed of the group 2 and the speed of the group 3, there is a relationship of t 1> t 2> t 3.

6 is a table showing an IEEE 802.20 MBWA (Mobile Broadband Wireless Access) system experiment environment according to an embodiment of the present invention.

Referring to FIG. 6, the network topology used in the experimental environment was arranged with 19 cells having a hexagonal cell shape, and the distance between cells was set to 1 km in an urban scenario and 2.5 km in a semi-urban scenario. The propagation model is a model used in urban and sub-urban environments, the carrier frequency band is 1.9GHz band, the bandwidth is 10MHz. The standard deviation of log-normal shadowing for the slow fading effect is set to 10 dB and the distance at which the slow fading correlates for each moving speed is 15 km / h. Up to 5m was set, for more than 20m was set. The maximum transmit power of the base station was simulated by setting 53dBm.

7 is a table showing group designations according to movement speeds according to an embodiment of the present invention.

Referring to FIG. 7, the speed groups set in the present invention and representative speeds for each group used in the experiment are shown. The requirements of the 3GPP Long-Term Evolution (LTE) system require optimal system performance for low travel speeds up to 15 km / h, high performance from 15 km / h to 120 km / h, and 350 km at 120 km / h. It is recommended that mobility be guaranteed for speeds up to / h or up to 500 km / h. Based on this, in the experimental environment, as shown in FIG. 7, the speed groups are classified and 3km / h and 10km / h are used in the moving speed group up to 15km / h as the representative speed belonging to each group, and 120km at 15km / h. 15 km / h, 30 km / h, 60 km / h, 90 km / h for movement speed groups up to / h; 120 km / h, 150 km / h, 200 km / h, 250 km / h for movement speed groups above 120 km / h Use the speed of

8 is a diagram illustrating a movement scenario used in an experiment according to an embodiment of the present invention.

Referring to FIG. 8A, the movement path 1 movement scenario according to the embodiment of the present invention is illustrated. The MS shows a scenario in which the MS moves from the base station 1 to the base station 2 without a corner effect. In the present invention, based on the movement scenario (Movement Path 1), the movement speed group up to 15km / h and the movement speed group up to 120km / h performs an experiment to move the urban cell deployment environment, and from 120km / h to 250km / h A group of speeds up to perform experiments to move the quasi-city cell deployment environment.

Referring to FIG. 8B, a movement path 2 movement scenario according to an embodiment of the present invention is illustrated. A corner effect occurs when the MS moves from base station 1 to base station 2. In this movement scenario (Movement Path 2), as the mobile communication terminal (MS) moves toward the base station 2, edge loss occurs in the signal strength of the serving base station in the cell boundary region, and in the signal strength of the target base station. Edge gain occurs. In the present invention, as shown in Table 1, based on this movement scenario, it is set to have a boundary loss and edge gain of 6 dB up to 15 km / h, and to have a boundary loss and edge gain of 3 dB up to 120 km / h. . In the present invention, it is assumed that a corner phenomenon does not occur at a moving speed of 120 km / h or more.

3km / h to 15km / h 6 dB 15 km / h to 120 km / h 3 dB

Table 1 shows boundary loss (gain) for each movement speed group in the Movement Path 2 movement scenario.

9 illustrates a bicasting threshold set for each moving speed group according to an embodiment of the present invention.

Referring to FIG. 9A, a bicasting threshold set for each moving speed group in a downtown cell environment is shown. As shown in FIG. 9A, a threshold value of -5 dB is set for a low moving speed group of 3 km / h to 15 km / h, and a threshold value of -6 dB for an intermediate moving speed group of 15 km / h to 90 km / h. Set a threshold value of -7 dB for a high moving speed group of 90 km / h or more. That is, when the signal strength of the neighboring base station measured by the terminal becomes larger than the bicasting threshold set in the group to which the mobile station belongs, the serving base station requests the base station to perform bicasting to the base station, and the reference base station requests this request. In accordance with the base station performs the by-casting for the mobile communication terminal.

Referring to FIG. 9B, a bicasting threshold set in a very high moving speed group in a suburban cell environment is shown. As shown in FIG. 9B, the bicasting value is set to -5 dB from 120 km / h to 200 km / h, -6 dB to 200 km / h to 250 km / h, and -7 dB to a speed of 250 km / h or more. Set it.

Figure 10 shows the metrics used to analyze the performance of the present invention.

Referring to FIG. 10, the first measure is a prior art bicasting time. In the prior art, bicasting is performed immediately after one base station is included in an active set. The second measure is the bicasting time performed when a sudden handover occurs due to the corner effect in the cell edge region by the Movement Path 2 movement scenario. The third measure is the bicasting time according to the present invention.

FIG. 11 illustrates the results for a low movement speed group of a mobility scenario in a downtown cell environment according to an embodiment of the present invention.

Referring to FIG. 11A, it is a result graph for a low movement speed group in a Movement Path 1 movement scenario in a downtown cell environment. As shown in FIG. 11A, the signal strength change and the active set update of the serving base station and the target base station for 3 km / h and 10 km / h, which are representative speeds of 3 km / h to 15 km / h, which are low moving speed groups, are updated. Results for the time point and the time of bicasting execution. Table 2 below shows the results in FIG. 11A. As a result, the bicasting time performed in the prior art is longer than the bicasting time of the present invention. Long bicasting time means more resources are used in the backhaul.

speed Original Bi-casting time Bi-casting time Proposed 3km / h 118.7 s 60.7 s 10 km / h 34.6 s 20.3 s

FIG. 11B illustrates the results for the low movement speed group of the movement path 2 movement scenario in a downtown cell environment according to an embodiment of the present invention.

Referring to FIG. 11B, [FIG. 6] is a result graph for a low movement speed group in a Movement Path 2 movement scenario in an urban cell environment. As shown in FIG. 11B, the signal strength change and the active set update of the serving station and the target base station are performed for 3 km / h and 10 km / h, which are representative speeds of 3 km / h to 15 km / h, which are low moving speed groups. Result graphs for the time point and the time of bicasting execution. Table 3 below shows the results in FIG. 11B. Likewise, the bicasting time performed in the prior art is longer than the bicasting time of the present invention.

speed Original Bi-casting time by corner effect Bi-casting time Proposed 3km / h 49s 15 s 10 km / h 19s 3.1 s

FIG. 12A illustrates a result for an intermediate movement speed group of a movement path 1 movement scenario in a downtown cell environment according to an exemplary embodiment of the present invention.

Referring to FIG. 12A, the signal strength change of the serving base station and the target base station and the active set update time point for 15 km / h, 30 km / h, and 60 km / h, which are representative speeds of 15 km / h to 90 km / h, which are intermediate moving speed groups And a result graph with respect to when the bicasting is performed. Table 10 below shows the results in FIG. 12A.

speed Original Bi-casting time Bi-casting time Proposed 15 km / h 22.9 s 18.7 s 30 km / h 13.3 s 10.9s 60km / h 6.5s 4.9 s

12B illustrates the results for the middle movement speed group of the movement path 2 movement scenario in a downtown cell environment according to an embodiment of the present invention.

Referring to FIG. 7B, the signal strength change of the serving base station and the target base station and the active set update time point for 15 km / h, 30 km / h, and 60 km / h, which are representative speeds of 15 km / h to 90 km / h, which are intermediate moving speed groups. And a result graph for the time of performing the bicasting. Table 5 below shows the results in FIG. 12B.

speed Original Bi-casting time Bi-casting time Proposed 15 km / h 10.3 s 4.9 s 30 km / h 5.3 s 2.9 s 60km / h 2.6 s 1.4 s

12C illustrates the results for a very high movement speed group of the movement path 1 movement scenario in a downtown cell environment according to an embodiment of the present invention.

Referring to FIG. 12C, the signal strength change of the serving base station and the target base station and the active set update time point for 120 km / h, 150 km / h, and 200 km / h, which are representative speeds of 120 km / h to 250 km / h, which are high moving speed groups. And a result graph for the time of performing the bicasting. Table 6 below shows the results in FIG. 12C.

speed Original Bi-casting time Bi-casting time Proposed 120 km / h 6.3 s 4.5 s 150 km / h 4.5 s 2.3 s 200 km / h 3.5 s 2.8s

FIG. 13A is a comparison result of a low movement speed group of movement paths 1,2 movement scenarios in a downtown cell environment according to an exemplary embodiment of the present invention. FIG.

Referring to FIG. 13A, as shown in the left figure of FIG. 13A, the 3km / h and the 10km / h of the representative speeds of 3km / h to 15km / h, which are low movement speed groups, in the Movement Path 1 movement scenario The bicasting time according to the present invention is compared with the bicasting time according to the present invention. As shown in the right figure of FIG. 13A, the bicasting time by the corner effect in the cell edge region is compared with the bicasting time according to the present invention for each representative velocity in the Movement Path 2 movement scenario. Table 7 below shows the results of the ratios of the bicasting time according to the prior art and the time reduced due to the bicasting according to the present invention at the representative speeds of 3 km / h and 10 km / h.

speed Reduced backhaul time ratio (%) Movement Path 1 Reduced backhaul time ratio (%) Movement Path 2 3km / h 49.4% 70% 10 km / h 41.5% 83.7%

FIG. 13B is a result of comparing a high speed group of movement paths 1 and 2 in a downtown cell environment according to an embodiment of the present invention. FIG.

Referring to FIG. 13B, as shown in the left figure of FIG. 13B, in the Movement Path 1 movement scenario, 15km / h, 30km / h, and 60km / h which are representative speeds of 15km / h to 90km / h, which are intermediate movement speed groups, For h, the bicasting time according to the present invention is compared with the bicasting time according to the present invention. As shown in the right figure of FIG. 13B, the bicasting time by the corner effect in the cell edge region is compared with the bicasting time according to the present invention for each representative velocity in the Movement Path 2 movement scenario. Table 8 below shows the ratio of time reduced by bicasting according to the present invention to bicasting time according to the prior art at the representative speeds of 15 km / h, 30 km / h, and 60 km / h. It is shown.

speed Reduced backhaul time ratio (%) Movement Path 1 Reduced backhaul time ratio (%) Movement Path 2 15 km / h 18.4% 52.3% 30 km / h 18.1% 45.5% 60km / h 24.5% 46.1%

FIG. 13C is a result of a very high movement speed group of the movement path 1 movement scenario in a downtown cell environment according to an embodiment of the present invention. FIG.

Referring to FIG. 13C, the bicasting time and the pattern according to the prior art for the representative speeds of 120 km / h to 250 km / h, which are very high moving speed groups, are 120 km / h, 150 km / h, 200 km / h, and 250 km / h. The bicasting time according to the invention is compared. Table 15 shows the ratio of time reduced by bicasting according to the present invention to bicasting time according to the prior art at the representative speeds of 120 km / h, 150 km / h, 200 km / h, and 250 km / h. It shows the result for.

speed Reduced backhaul time ratio (%) Movement Path 1 120 km / h 28.6% 150 km / h 46.5% 200 km / h 19.7%

Referring to the experimental results of FIGS. 11, 12, and 13, the bicasting time performed in the prior art is longer than the bicasting time of the present invention. This is because bicasting occurs after the active set update time for handoff.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

As described above, the start time of data by-casting at the reference base station can be started differently according to the moving speed of each user equipment, thereby reducing backhaul resource consumption and reducing the occurrence of backhaul overload. There is an advantage to manage.

Claims (8)

An apparatus for performing bicasting in a mobile communication system, A moving speed estimating unit estimating a moving speed of the terminal; A by-casting threshold setting unit for setting a by-casting threshold based on the estimated moving speed; And a controller for comparing the signal strength of the neighboring base station with the signal strength of the neighboring base station and requesting a higher system to perform the bicasting when the signal strength of the neighboring base station is greater than the bicasting threshold. The method of claim 1, A technique for estimating speed using a frequency error of a received signal generated in proportion to the moving speed of a mobile station relative to a base station, the envelope of the received signal during a given time interval, that is, the number of times the power passes a predetermined reference level ( Estimating the moving speed of the mobile communication terminal using one of a technique of estimating speed using a number of level crossings) and a technique of estimating speed using a covariance value between received samples having a predetermined time difference. Bicasting device. The method of claim 1, And the bicasting threshold value decreases as the movement speed increases and increases as the movement speed decreases. The method of claim 1, And wherein the time of bicasting is from the time at which the set bicasting threshold is reached to the time at which the handoff is determined. In the method for performing bicasting in a mobile communication system, Estimating a moving speed of the mobile communication terminal; Setting a bicasting threshold based on the estimated moving speed; And comparing the signal strength of the neighboring base station with the signal strength of the neighboring base station and requesting a higher-level system to perform a bicasting when the signal strength of the neighboring base station is greater than the bicasting threshold. The method of claim 5, wherein A technique for estimating speed using a frequency error of a received signal generated in proportion to the moving speed of a mobile station relative to a base station, the envelope of the received signal during a given time interval, that is, the number of times the power passes a predetermined reference level ( Estimating the moving speed of the mobile communication terminal using one of a technique of estimating speed using a number of level crossings) and a technique of estimating speed using a covariance value between received samples having a predetermined time difference. By-casting method. The method of claim 5, wherein And the bicasting threshold value decreases as the movement speed increases and increases as the movement speed decreases. The method of claim 5, wherein And wherein the time of bicasting is from the time at which the set bicasting threshold is reached to the time at which the handoff is determined.

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