KR102431464B1 - Handover method for minimizing unnecessary handovers and apparatus for the same - Google Patents

Abstract

A handover control method for frequent handover mitigation (FHM) includes the steps of: (a) receiving handover history information and/or a handover event measurement report from one or more terminals; (b) determining a terminal causing unnecessary handover among the one or more terminals based on handover history information and/or a handover event measurement report; and moving the terminal causing the unnecessary handover to a macro cell (c).

Description

Handover method for minimizing unnecessary handovers and apparatus for the same}

The present invention relates to a handover method and apparatus for a mobile communication system, and more particularly, to a handover method and apparatus for reducing unnecessary handover in a small cell environment.

The small cell base station started from the concept of a femto cell and expanded to the concept of a small cell base station to improve call quality and reduce operating costs by expanding indoor coverage. Small cell base station research has been discussed by standardization organizations since the second half of 2007. In particular, in 3GPP, standardization began in TSG RAN in March 2007, and 3GPP WCDMA (UTRA) small cells were defined as Home NB (HNB) and LTE Home eNB (HeNB). The 3GPP LTE (EUTRA) small cell concept was established in Release 8, and in Release 9 (December 2009), a function for a femtocell (or HeNB) was added, and more HeNB functions were defined in Release 10. Meanwhile, at the 3GPP Summit in 2013, the importance of small cell base stations is being strengthened enough to be defined as an important technology element of 5G networks.

As a UE (User Equipment) moves in an area where small cells are located, handover moving from a currently accessed cell to another cell may occur. If you move away from the serving cell you are currently accessing, the connection with the serving cell is cut off and you need to connect to another cell existing in the vicinity called a target cell. In this process, if the connection with the serving cell is disconnected before accessing the target cell, the user quality is deteriorated. Therefore, it is necessary to maintain user quality while the terminal is moving by using the handover technology. In addition, in a heterogeneous network (HetNet) environment in which small cells having narrow coverage with low transmission power are densely arranged, the problem of inter-cell mobility is more important.

Dense deployment of small cells in a mobile wireless network can increase the throughput of the network. However, it causes several problems such as performing multiple handovers and interference. A high-speed terminal serving in a small cell deployment densely in a small range may undergo frequent handover between small cells. In addition, the slow-moving UE may suffer frequent handover due to shadowing or channel fading when pilots of different small cell base stations are located in an area having almost the same strength.

An object of the present invention for solving the above problems is to alleviate unnecessary handover that may occur in terminals moving at high speed in an environment in which small cells are densely arranged or terminals moving the boundary of adjacent small cells ( It is to provide an optimized handover control method capable of mitigation.

Another object of the present invention for solving the above problems is to alleviate unnecessary handover that may occur in terminals moving at high speed in an environment in which small cells are densely arranged or terminals moving the boundary of adjacent small cells. An object of the present invention is to provide an apparatus for controlling a handover that performs an optimized method for controlling a handover.

In order to achieve the above object, the present invention provides a handover control method for alleviating unnecessary handover, comprising the steps of: (a) receiving handover history information and/or a handover event measurement report from a terminal; (b) determining a terminal causing a ping-pong handover based on the received handover history information, (c) determining a high-speed moving terminal based on the received handover history information, and performing at least one of the step (d) of determining a terminal whose movement speed is abruptly changed based on the received handover event measurement report; And it may be configured to include the step (e) of moving the UE causing unnecessary handover, determined in at least one of the steps (b) to (d), to the macro cell.

Here, the terminal inducing the ping-pong handover serves when, based on the handover history information, the dwelling time for a predetermined number of serving cells in which the terminal last stayed is lower than a predetermined threshold value. It can be determined by checking the repeating pattern of the cell.

Here, when the residence time predicted based on the handover history information of the terminal is lower than a predetermined threshold value, the terminal may be determined as a terminal moving at a high speed.

Here, the terminal whose movement speed is rapidly changed can be determined by continuously monitoring the measurement report according to the handover event A1 and calculating the remaining service time based on the RSRP reduction rate of the corresponding terminal.

According to the configuration of the present invention, unnecessary (self-organizing network) that may be caused by terminals moving at high speed in an environment in which small cells are densely arranged by a SON (self-organizing network) algorithm or terminals moving the boundary of adjacent small cells too frequent) handover can be mitigated.

1A and 1B are conceptual diagrams for explaining a situation in which handover frequently occurs in an environment in which a small cell exists.
2 is a flowchart illustrating a method for reducing unnecessary handover according to an embodiment of the present invention.
3 is a block diagram illustrating the configuration of a handover control apparatus according to an embodiment of the present invention.

Since the present invention can have various changes and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention. In describing each figure, like reference numerals have been used for like elements.

Terms such as first, second, A, and B may be used to describe various elements, but the elements should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, a first component may be referred to as a second component, and similarly, a second component may also be referred to as a first component. and/or includes a combination of a plurality of related listed items or any of a plurality of related listed items.

When a component is referred to as being “connected” or “connected” to another component, it may be directly connected or connected to the other component, but it is understood that other components may exist in between. it should be On the other hand, when it is said that a certain element is "directly connected" or "directly connected" to another element, it should be understood that the other element does not exist in the middle.

The terms used in this application are only used to describe specific embodiments, and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as “comprise” or “have” are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one or more other features It is to be understood that this does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in a commonly used dictionary should be interpreted as having a meaning consistent with the meaning in the context of the related art, and should not be interpreted in an ideal or excessively formal meaning unless explicitly defined in the present application. does not

Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

1A and 1B are conceptual diagrams for explaining a situation in which handover frequently occurs in an environment in which a small cell exists.

Figure 1a illustrates a case in which handover occurs while a car moving at high speed passes through a densely arranged small cell service area, and the car 110 is in the coverage area 130 of a macro base station (macro eNB) 120. Even in existence, depending on the movement path from the cell (macro cell) of the macro base station to the small cell 141, from the small cell 142 to the small cell 143, from the small cell 142 to the small cell 143, the small cell From 143 to the small cell 145, from the small cell 145 to the small cell 146, and finally from the small cell 146 to the macro cell 130 to illustrate a situation in which unnecessary handover continuously occurs and have.

In this case, although the actual staying time in each small cell is only an extremely short time, an overhead for performing a repeated handover procedure is unnecessary.

On the other hand, Figure 1b illustrates a case in which a terminal (eg, a terminal carried by a pedestrian; 150) moving at a low speed moves along the boundary of two small cells (161, 162), the terminal ( 150) is exemplified as a situation in which handover continuously occurs between the small cells while crossing the boundaries of the small cells 161 and 162 . In particular, handover in such a situation can be called a ping-pong handover in that the handover to the small cell 161 and the small cell 162 is repeated in a send-and-receive manner.

Frequent handover between such small cells is not desirable for the following reasons.

First, packet loss or packet delay may result in voice quality degradation or user quality degradation.

Second, it can bring a large signaling load in the small cell gateway or core network.

Third, the risk of poor user quality and call disconnection may increase.

Therefore, in order to effectively remove unnecessary handover in the network, a solution through coordination between cells is required. For this purpose, frequent handover mitigation (FHM) methods have been introduced. FHM can use overall mobility parameter optimization to improve handover performance in the following ways.

(1) FHM classifies terminals into high-speed mobile terminals or ping-pong handover-inducing terminals based on the handover history.

(2) Then, the terminal with high mobility is handed over to the macro cell, and the ping-pong handover-inducing terminal adjusts the handover parameter.

(3) If the ping-pong handover situation is not improved by adjusting the handover parameter, the terminal causing the ping-pong handover is handed over to the macro cell.

Many studies have proposed various approaches to overcome the FHM problem. For example, in "Enabling Hyper-Dense Small Cell Deployments with UltraSON", Qualcomm, 2014.2, a method of using handover history information to determine a terminal experiencing frequent handover is proposed. According to the method proposed in this paper, unnecessary handovers are classified according to the cause, and actions are taken according to the classification. However, this solution does not detect FHM problems quickly. The system detects frequent handovers after too many handovers.

“Mitigation of redundant handovers to femtocells by estimation of throughput gain”, Mobile Information Systems 9, pp. 315-330, 2013 proposed a handover decision algorithm based on the estimation of the throughput gain reached by handover to a small cell. In the proposed scheme, handover is initiated only when the user's throughput gain estimate exceeds a predefined threshold. Therefore, this approach is represented as a throughput gain estimate for handover decisions. The increase in throughput is calculated from the signal level calculation estimate of all relevant cells measured by the terminal and the estimated time spent in the femtocell.

Alternatively, "Adaptive Hysteresis Margin for Handover in Femtocell Networks", ICMWC pp. In 256-261, 2010, an adaptive history width (MH: Hysteresis Margin) scheme was proposed to remove unnecessary handover. In fact, it is proposed to modify the existing history width method, which is a parameter commonly used to remove unnecessary handover. In particular, this paper proposes a method of using the CINR (Carrier to Interference and Noise Ratio) level to adapt the actual handover history value. This approach also improves the user's information throughput.

“A Novel Handover Mechanism between Femtocell and Macrocell for LTE based Networks”, ICCSN, 2010, uses a combination of additional parameters such as the user’s speed and quality of service (QoS) requirements to improve handover decisions to enable gateway-based femtocells. A modified handover signaling procedure of the network architecture is presented. The number of unnecessary handovers is reduced, but the user's throughput is negatively affected. Finally, “Self-Optimized Coverage Coordination in Femtocell Networks”, IEEE transactions on Wireless communications pp. The method proposed in 2977-2982, 2010 attempts to eliminate unnecessary handover by adapting the transmit power of the small cell base station. The proposed method eliminates most of the redundant handovers, but is more suitable for a Closed Subscriber Group (CSG) small cell.

As mentioned above, frequent handovers have a negative impact on network performance. Therefore, an FHM algorithm is needed to improve performance and reduce unnecessary handover. The present invention proposes an FHM algorithm for reducing unnecessary handover.

The algorithm proposed in the present invention can be applied to a base station belonging to a distributed SON system in which each base station (eNB) has a SON function, and a hybrid SON system in which multiple base stations are managed to perform the SON function. Hereinafter, a base station having a distributed SON function performing the algorithm proposed in the present invention or a control device having a SON function of a separate entity will be collectively described as a 'handover control device'.

The FHM algorithm proposed in the present invention continuously monitors terminals, identifies a terminal or a high-speed mobile terminal causing a ping-pong handover, and then forcibly handover a terminal causing frequent handover with a problem to a macro cell. have.

Alternatively, in the case of an FHM for a terminal that induces a ping-pong handover, a handover history parameter may be adjusted. However, adjustment of the handover history (Hysteresis) parameter affects the entire network. Therefore, if there is no improvement after performing optimization for a certain number of times, in the present invention, the terminal causing the problem can be handed over directly to the macro cell.

2 is a flowchart illustrating a method for reducing unnecessary handover according to an embodiment of the present invention.

Referring to FIG. 2 , in the method for reducing unnecessary handover according to an embodiment of the present invention, receiving handover history information and/or a handover event measurement report from a terminal (S210), the received Determining a terminal causing a ping-pong handover based on handover history information (S220), determining a terminal that moves at high speed based on the received handover history information (S230), and measuring the handover event Step (S240) of determining the terminal whose movement speed is rapidly changed based on the report, and the step of moving the terminal causing unnecessary handover determined in the steps (S220) to (S240) to the macro cell (S250) may be included.

Here, in the method for reducing unnecessary handover according to an embodiment of the present invention, all steps S220 to S240 may be performed, or at least one of steps S220 to S240 may be performed. That is, steps (S220) to (S240) may be all or alternatively performed. On the other hand, when only the terminal having a sudden speed change is determined through step S240, handover history information may not be received at step S210. In addition, when only a ping-pong handover-inducing terminal or a high-speed mobile terminal is determined through steps S220 and S230, the handover event measurement report may not be received in step S210.

Hereinafter, specific implementation methods of each of steps S220 to S240 and examples of applicable algorithms will be described in detail.

How to identify a ping-pong terminal

In order to identify the terminal causing the ping-pong handover, the method according to an embodiment of the present invention is to continuously monitor the terminals. It can be said that the method pays attention to the fact that the terminal causing the ping-pong handover repeatedly visits the same serving cells.

Therefore, in order to identify the ping-pong terminal, the ping-pong terminal can be identified by identifying the serving cell to which the corresponding terminal is repeatedly handed over. To this end, when handover occurs, the handover control apparatus according to an embodiment of the present invention may receive HO history information of the corresponding terminal. In this case, the handover history information includes 'last visited cell information' of the corresponding terminal, information (eg, identifier) on the serving cell in which the corresponding terminal stayed, and the dwelling time for each serving cell. It may include. . Thereafter, the handover control device refers to the collected and accumulated 'last visited cell information', and the total number of serving cells in which the terminal last stayed (eg, three; a predetermined number can be changed and applied depending on the situation). The dwelling time can be calculated. If the total residence time is less than a predefined threshold, the handover control device checks the repetition pattern of the corresponding serving cell. If it is determined that the terminal is repeatedly performing handover to the same serving cells, the handover control apparatus may determine the corresponding terminal as a terminal inducing ping-pong handover.

When the terminal causing the ping-pong handover is determined, the handover control apparatus may forcibly move the terminal from the small cell to the macro cell. An example of an algorithm for identifying a terminal causing a ping-pong handover may be configured as follows.

[Algorithm 1]

1. Start

2. Get UE HO history information

3. Calculate total_dwelling_time for last 3 serving cell

4. If total_dwelling_time < Threshold, then

5. Check serving cell pattern

6. If serving cell repeats, then

7. Ping-pong UE, connect to macro

8. End If

9. End If

10. End

How to identify a high-speed mobile terminal/abrupt speed change terminal

On the other hand, in the case of a terminal moving at a high speed, it does not have the same characteristics as the terminal causing the ping-pong handover described above. Also, there are two types of high-speed mobile terminals. One is constantly moving fast and the other is a sudden increase in speed. The handover history information of the terminal collected using the aforementioned handover history information is used to identify a fast moving user by estimating the residence time.

When handover occurs, the handover control device estimates the expected residence time (S1) in the current serving cell by applying exponential smoothing, as shown in Equation 1 below, in order to identify a fast moving terminal. can

Here, the parameter α is the smoothing coefficient, and 0 < α <1. Each smoothed value is a weighted average of previous observations whose weights decrease exponentially with the value of the parameter α. If α = 1, previous observations are completely ignored (short memory). If α = 0, the current observation is completely ignored (long memory). The value that results in the smallest mean square error (MSE) is the best value for α. The parameter x _-n is the residence time in the nth previous serving cell, and x ₀ means the residence time in the immediately previous cell.

If the predicted residence time is less than the predefined threshold, the handover control apparatus according to an embodiment of the present invention considers that the corresponding terminal moves relatively quickly. An example of an algorithm for detecting a high-speed mobile terminal is as follows.

[Algorithm 2]

1. Start

2. Get UE HO history information

3. List dwelling time according to HO history

4. Calculate dwelling_time_forecast (S ₁ ) as in (Equation 1)

5. If dwelling_time_forecast < Threshold, then

6. UE is moving fast, connect to macro cell

7. End If

8. End

On the other hand, in order to detect a sudden change in the speed of the terminal, it is necessary to continuously monitor the terminal from the time it enters a new cell. Therefore, it utilizes the measurement report according to handover event A1, which is useful for tracking the rapid decrease of the reference signal. The conditions of the event A1 measurement report are as follows.

parameter Explanation Ms. Measurement results of neighboring cells without considering the offset Hys Historical parameters of event A1 Thresh Threshold parameter for event A1

Ms is expressed in dBm in the case of Reference Signal Received Power (RSRP) or in dB in the case of Reference Signal Received Quality (RSRQ). Hys and Thresh are expressed in dB.

The handover control apparatus continuously monitors the terminal in order to confirm the sudden increase in the speed of the terminal. For this purpose, a periodic report of event A1 is used. When the terminal leaves the currently accessed cell, the handover control apparatus for FHM calculates the remaining service time to be in the current serving cell. The remaining service time may be calculated through Equation 4, which will be described later.

The handover control device continuously monitors RSRP (Reference Signal Received Power) information of all terminals under event A1. While event A3 occurs, the handover control device stores the M _p value of the measurement report.

When R _x1 is the measured value at time t1, R _x2 is the measured value at time t2, and Δt (= t2-t1) is the reporting interval, the RSRP reduction rate (RRR) may be calculated as in Equation 3 below.

Using the RRR and _MP stored when the event A3 occurs, the remaining service time of a specific user can be estimated as in Equation 4 below.

If the remaining time is lower than a predetermined threshold, the terminal is suspected of sudden high-speed movement. However, even when the terminal is already suspected of a sudden high-speed movement by the previous serving cell, the terminal may be regarded as a sudden high-speed movement user. An example of an algorithm for detecting a high-speed moving (rapid acceleration) terminal is as follows.

[Algorithm 3]

1. Start

2. Get RSRP measurement under A1 event

3. If UE is leaving, then

4. Calculate remaining_time as in (Equation 4)

5. If remaining_time < Threshold, then

6. If UE suspected as suddenly fast moving by the previous serving cell, then

7. UE is moving suddenly fast, connect to macro

8. Else

9. UE is suspected as fast moving by the present serving cell

Set a flag as “suspected”.

10. End If

11. End If

12. End If

13. End

When it is determined by the above-described steps (S220) to (S240) as a terminal causing a ping-pong handover, a terminal moving at a high speed, or a terminal having a sudden speed change, in step S250, the handover control device is The UE may be forcibly handed over to the macro cell. In this case, a command for forced handover of the corresponding terminal to the macro cell may be transmitted to the corresponding terminal and the target macro cell base station.

3 is a block diagram illustrating the configuration of a handover control apparatus according to an embodiment of the present invention.

Referring to FIG. 3 , the handover control apparatus 300 according to an embodiment of the present invention includes a processor 310 , a memory 320 , a network interface 330 connected to a network to perform communication, and a storage device 340 . ) and a bus 350 . The processor 310 , the memory 320 , the network interface 330 , and the storage device 340 of the handover control device 300 may be connected to the bus 350 to communicate with each other. Alternatively, the processor 310 , the memory 320 , the network interface 330 , and the storage device 340 may be directly connected through a dedicated 1-to-1 interface.

Meanwhile, as mentioned above, the handover control device 300 may be a base station having a distributed SON function or a separate device having a SON function managing a plurality of base stations. In this case, when the handover control device 300 is a base station having a distributed SON function, the network interface 330 may be a transceiver for wireless transmission/reception with a terminal, and a separate device that manages a plurality of base stations In one case, the device 300 may be configured as a wired network interface connecting the base stations.

The processor 310 may execute a program command stored in at least one of the memory 320 and the storage device 340 . The processor 310 may mean a central processing unit (CPU), a graphics processing unit (GPU), or a dedicated processor on which methods according to embodiments of the present invention are performed. Each of the memory 320 and the storage device 340 may be configured as at least one of a volatile storage medium and a non-volatile storage medium. For example, the memory 320 may be configured as at least one of a read only memory (ROM) and a random access memory (RAM).

When described in parallel with FIG. 2 described above, the program command executed by the processor 310 may be configured to execute steps S210 to S250. At this time, the handover history information or handover event measurement report collected from the terminal in step S210 may be collected through the network interface 330, and the command for forcibly handover the terminal to the macro cell in step S250 is It may be transmitted to the corresponding terminal and the target macrocell base station through the network interface 330 .

Although the above has been described with reference to the preferred embodiment of the present invention, those skilled in the art can variously modify and change the present invention within the scope without departing from the spirit and scope of the present invention as set forth in the claims below. You will understand that it can be done.

110: high-speed mobile (vehicle) 120: macro cell base station
130: macro cell 141 to 146: small cell
150: low-speed moving object (pedestrian) 161 ~ 162: small cell

Claims (16)

As a handover control method performed in a handover control device for frequent handover mitigation (FHM),
Receiving handover history information and/or a handover event measurement report from one or more terminals (a);
(b) determining a terminal causing unnecessary handover among the one or more terminals based on handover history information and/or a handover event measurement report; and
Including the step (c) of moving the terminal causing the unnecessary handover to a macro cell,
In step (b), the terminal whose movement speed is rapidly changed determined based on the received handover event measurement report is determined as the terminal causing the unnecessary handover, and monitoring of the handover event measurement report The terminal whose remaining service time based on the received signal received power (RSRP) reduction rate calculated by is lower than a predetermined threshold is determined as the terminal whose movement speed is rapidly changed,
Handover control method. The method according to claim 1,
The handover control device is included in a base station having a self-organizing network (SON) function or exists as a control device that manages a plurality of base stations to perform a SON function,
Handover control method. The method according to claim 1,
In the step (b), it is determined that the terminal causing the ping-pong handover determined based on the received handover history information is the terminal causing the unnecessary handover,
Handover control method. 4. The method of claim 3,
The sum of dwelling times for a predetermined number of serving cell(s) that have finally stayed, calculated based on the handover history information, is lower than a predetermined threshold, and a repetitive handover pattern for the same serving cell is induced. is determined as the terminal causing the ping-pong handover,
Handover control method. The method according to claim 1,
In the step (b), it is determined that the high-speed mobile terminal determined based on the received handover history information is the terminal causing the unnecessary handover,
Handover control method. 6. The method of claim 5,
A terminal whose residence time predicted based on the handover history information is lower than a predetermined threshold is determined as the terminal moving at the high speed,
Handover control method. delete delete As a handover control device for frequent handover mitigation (FHM),
processor;
a memory storing instructions executed by the processor; and
a transceiver controlled by the processor;
When executed by the processor, the instructions cause the processor to:
(a) receiving handover history information and/or a handover event measurement report from one or more terminals using the transceiver;
(b) determining a terminal causing unnecessary handover among the one or more terminals based on handover history information and/or a handover event measurement report; and
It is configured to perform the step (c) of controlling the terminal causing the unnecessary handover to move to a macro cell using the transceiver,
In step (b), the terminal whose movement speed is rapidly changed determined based on the received handover event measurement report is determined as the terminal causing the unnecessary handover, and monitoring of the handover event measurement report A terminal whose remaining service time based on a received signal received power (RSRP) reduction rate calculated by is lower than a predetermined threshold is determined as a terminal whose movement speed is rapidly changed,
handover control unit. 10. The method of claim 9,
The handover control device is included in a base station having a self-organizing network (SON) function or exists as a control device that manages a plurality of base stations to perform a SON function,
handover control unit. 10. The method of claim 9,
In the step (b), it is determined that the terminal causing the ping-pong handover determined based on the received handover history information is the terminal causing the unnecessary handover,
handover control unit. 12. The method of claim 11,
The sum of dwelling times for a predetermined number of serving cell(s) that have finally stayed, calculated based on the handover history information, is lower than a predetermined threshold, and a repetitive handover pattern for the same serving cell is induced. is determined as the terminal causing the ping-pong handover,
handover control unit. 10. The method of claim 9,
In the step (b), it is determined that the high-speed mobile terminal determined based on the received handover history information is the terminal causing the unnecessary handover,
handover control unit. 14. The method of claim 13,
A terminal whose residence time predicted based on the handover history information is lower than a predetermined threshold is determined as the terminal moving at the high speed,
handover control unit. delete delete

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