WO2015147510A1 - Handover device and method in mobile communication system - Google Patents

Abstract

The present invention relates to a method and device for performing handover by adaptively applying a handover parameter according to the state of a terminal in a mobile communication system. According to the present invention, a handover device of a terminal in a mobile communication system including a small cell and a macro cell comprises: a reception unit for receiving a cell-specific handover parameter from a base station; and a control unit for measuring a signal to interference and noise ratio (SINR) of the small cell, determining the state of the terminal according to the measured SINR using the received cell-specific handover parameter, and performing handover, according to the state of the terminal, using one among handover parameter sets configured on the basis of the cell-specific handover parameter.

Description

Handover Device and Method in Mobile Communication System

The present invention relates to a method and apparatus for handover by adaptively applying a handover parameter according to a state of a terminal in a mobile communication system.

In a mobile communication system, the handover is to change a serving base station from which a terminal is currently provided to a base station capable of providing a better quality of service. For example, if a service quality is deteriorated while receiving a service from a serving base station, the terminal hands over to a target base station capable of providing better quality of service, and continues to receive service from the target base station.

Heterogeneous networks include a macro base station communication area capable of transmitting and receiving service data with a macro base station (i.e., a macro cell), and a small base station communication area capable of transmitting and receiving service data with a small base station (ie, small). FIG. 1 shows an example of a heterogeneous network including a macro cell and a small cell.

Referring to FIG. 1, a heterogeneous network includes a plurality of macro cells, and each macro cell includes at least one small cell. Here, it is assumed that one macro cell includes one macro base station 105, 107, 109, 111 and at least one small base station 113, 117, 119, 121, 123, 125, 157 for transmitting and receiving service data with at least one terminal 101 and 103. The small base station may be, for example, a micro base station, a pico base station, a femto base station, or the like.

In the communication area 100 shown in FIG. 1, the mobile terminals 101 and 103 perform a handover in order to maintain a quality of service for a service they receive.

When the serving cell of the terminal (101,103) is a macro cell capable of transmitting and receiving service data with the macro base station 105, the terminal (101, 103) is a macro base station from the macro cell capable of transmitting and receiving service data with the macro base station 105 according to the moving position It is possible to handover to a macro cell capable of transmitting and receiving service data with the 107 or a small cell capable of transmitting and receiving service data with the small base station 117.

In addition, when the serving cell of the terminals 101 and 103 is a small cell capable of transmitting and receiving service data with the small base station 117, the small base station 119 and the service from the small base station 117 capable of transmitting and receiving service data with the small base station 117 according to the movement position. Handover to the small cell or macro base station 107 capable of transmitting and receiving data and the macro cell capable of transmitting and receiving service data may be handed over.

As described above, in a heterogeneous network composed of a macro cell and a small cell, a terminal may handover from a macro cell to a macro cell, a macro cell to a small cell, a small cell to a macro cell, or a small cell to a small cell. In order to efficiently perform the handover in such a heterogeneous network, it is necessary to apply a handover parameter by applying a handover parameter according to the state of the UE.

The present invention provides an apparatus and method for handover by adaptively applying a handover parameter according to a state of a terminal in a mobile communication system.

In a mobile communication system including a small cell and a macro cell according to the present invention, a handover apparatus of a terminal includes: a receiver configured to receive a cell-specific handover parameter from a base station; And measuring a signal-to-noise ratio (SINR) of a small cell, determining a state of the terminal according to the measured SINR using the received cell-specific handover parameter, and determining the state of the terminal according to the state of the terminal. And a control unit for performing a handover using one of the handover parameter sets set based on the cell-specific handover parameter.

In addition, in a mobile communication system including a small cell and a macro cell according to the present invention, a handover method of a terminal includes: receiving a cell-specific handover parameter from a base station; And measuring a signal-to-noise ratio (SINR) of a small cell, determining a state of the terminal according to the measured SINR using the received cell-specific handover parameter, and determining the state of the terminal according to the state of the terminal. And performing a handover using one of the handover parameter sets set based on the cell-specific handover parameter.

1 is a diagram illustrating an example of a heterogeneous network including a macro cell and a small cell;

2 is a diagram illustrating a handover result when a terminal performs a handover method based on relative signal quality;

3 is a diagram illustrating a handover result when a terminal performs an absolute signal quality based handover method;

4 illustrates a trade-off relationship between a probability of handover failure and a small cell ToS in an A2 / A4 event;

5 is a diagram illustrating a general cell range extension method;

6 is a view illustrating a case where hand-in and hand-out to which an embodiment of the present invention is applied are generated;

7 is a diagram illustrating a situation in which hysteresis is applied to an A2 / A4 event during handover;

8 is a diagram illustrating a situation in which a threshold value of an A2 / A4 event is adjusted;

9 is a view showing a handover method according to the position of a terminal in an embodiment of the present invention;

10 is a view showing the operation of a terminal and a base station according to an embodiment of the present invention;

11 is a view showing a change in SINR with time when a hand-out occurs in a terminal;

12 is a view showing a handover device according to an embodiment of the present invention,

13 is a diagram illustrating an example of determining a state of a terminal in a first processing unit 1233 according to an embodiment of the present invention;

FIG. 14 is a diagram illustrating a method of performing state switching according to an SINR of a small cell in the first processor 1233 according to an embodiment of the present invention; FIG.

FIG. 15 is a diagram illustrating an example of performing state switching according to a movement of a terminal in a first processing unit 1233 according to an embodiment of the present invention; FIG.

16 is a diagram illustrating a method of applying a handover parameter set according to a state of a terminal in a second processing unit 1235 according to an embodiment of the present invention;

17 is a view illustrating switching of a handover parameter set according to a change in SINR according to an embodiment of the present invention;

18 is a diagram illustrating a handover method according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that in the following description, only parts necessary for understanding the operation according to the present invention will be described, and descriptions of other parts will be omitted so as not to disturb the gist of the present invention.

The main subject of the present invention is that a UE located in a small cell in a heterogeneous network performs handover by adaptively applying a handover parameter according to a current state.

To this end, after explaining a general handover method, a handover method according to an embodiment of the present invention will be described in detail.

Hereinafter, a handover method in which a handover (hereinafter, referred to as a macro-to-small handover) in which a terminal connected to a macro cell moves to a small cell occurs in a heterogeneous network including a macro cell and a small cell. Let's explain.

1. Handover Method Based on Relative Signal Quality

In the relative signal quality based handover method, a small cell (that is, a target) that is getting closer to the Signal to Interference Noise Ratio (SINR) of a macro cell (ie, a serving macro cell) to which the UE is currently connected. When the SINR of the target small cell is compared to the SINR of the serving macro cell by more than a predetermined offset, the handover from the serving macro cell to the target small cell is compared. In 3GPP LTE, this is defined as an A3 event (RSRP / RSRQ of target cell> RSRP / RSRQ of serving cell + offset). Here, SINR means average SINR.

When the handover method based on the relative signal quality is applied, the UE may perform handover to a cell having the best signal quality.

However, the following disadvantages occur when the relative signal quality based handover method is applied. In general, a macro cell provides a large area for service, and thus a base station has a large transmit power. However, a small cell provides a service in a small area, and thus a base station has a small transmit power. Therefore, when comparing the SINR of the macro cell and the small cell, it is very likely that the SINR of the macro cell is better than the SINR of the small cell. Therefore, macro-to-small handover is unlikely to occur, which may cause congestion of macro cells and under-utilization of small cells. The rate at which this phenomenon occurs is shown in FIG. 2.

2 shows a handover result when a terminal performs a handover method based on relative signal quality.

According to the handover result of FIG. 2, the time that the UE stays in the macro cell (ie, macro cell time of stay ratio) is very long, while the time that the UE stays in the small cell (ie, small cell ToS) is very short. It can be seen.

2. Handover Method Based on Absolute Signal Quality

In order to compensate for the shortcomings of the "relative signal quality based handover method" described in 1 above, the UE may use an absolute signal quality based handover method. In the absolute signal quality-based handover method, the UE does not consider the SINR of the serving macro cell but considers only the SINR of the target small cell and handover to the target small cell when the SINR of the target small cell is higher than a specific threshold. In 3GPP LTE, this is defined as an A4 event (RSRP / RSRQ of target cell> threshold).

In addition, even when the UE performs a handover from the serving small cell to the target macro cell, if the SINR of the serving small cell is lower than a specific threshold value by considering only the SINR of the serving small cell without considering the SINR of the target macro cell, the UE moves to the target macro cell. Handover may be performed. In 3GPP LTE, this is defined as an A2 event (RSRP / RSRQ of serving cell <threshold).

When using the absolute signal quality based handover method, the UE may enter or exit the small cell regardless of the signal quality of the macro cell. Therefore, by adjusting the threshold of the A2 / A4 event it is possible to control the time the terminal stays in the small cell. The rate at which this phenomenon occurs is shown in FIG. 3.

3 shows a handover result when the terminal performs an absolute signal quality based handover method.

According to the handover result of FIG. 3, when the threshold value of the A2 / A4 event is high, the handover failure rate of the UE is low, and the small cell ToS is also short. On the other hand, if the threshold value of the A2 / A4 event is low, the probability of handover failure of the UE is high, and the small cell ToS is also long. This can be explained as follows.

When the threshold of the A2 / A4 event is high, the UE enters the small cell when the signal quality of the small cell is very good and exits from the small cell when the signal quality of the small cell is very good. This can be accomplished by small cell hand-in (ie, macro-to-small handover) and hand-out (ie, small-to-macro handover) at a point close to the small cell base station. ) Is generated. Here, the terms "hand-in" and "hand-out" refer to handover based on a small cell. Accordingly, although the UE stays in the small cell for a short time, when the signal quality of the signal cell is very good, since the handover occurs to the small cell, it is highly likely to successfully receive the handover related information from the base station located in the small cell. Therefore, if the threshold of the A2 / A4 event is high, the probability of handover failure is low.

On the other hand, when the threshold value of the A2 / A4 event is low, the UE enters the small cell when the signal quality of the small cell is poor and leaves the small cell when the signal quality of the small cell is bad. This means that hand-in and hand-out occur at a point far from the base station located in the small cell. Therefore, since the small cell handover occurs when the UE stays in the small cell for a long time but the signal quality of the small cell is not good, the possibility of successfully receiving the handover related information from the small cell BS is low. Therefore, when the threshold value of the A2 / A4 event is low, the probability of handover failure is high.

This result can be briefly represented as shown in FIG. 4 illustrates a trade-off relationship between a handover failure probability and a small cell ToS in an A2 / A4 event.

Referring to FIG. 4, when the A2 / A4 event has a high SINR threshold, the probability of handover failure is low, but the small cell ToS is also low, resulting in low utilization of the small cell. On the other hand, if the threshold of the A2 / A4 event is low (Low SINR threshold), the small cell ToS is high but the probability of handover failure is high. Therefore, there is a need for a method for efficiently improving the trade-off relationship between the probability of handover failure and the small cell ToS. That is, there is a need for a method of increasing the small cell ToS while maintaining a constant handover failure probability or decreasing the handover failure probability while maintaining a constant small cell ToS.

3. Handover Method using Cell Range Expansion

The handover method using cell range extension is a concept included in the relative signal quality based handover method and the absolute signal quality based handover method described above. 5 shows a general cell range extension method. As shown in FIG. 5, in the handover method using cell range extension, in order to make the cell range of the small cell appear large to the UE (500), the offset of the A3 event or the threshold value of the A2 / A4 event is adjusted to the macro cell. This is to induce the located UE to perform handover to the small cell. Through this, it is possible to solve the low utilization of small cells generated in the relative signal quality based handover method and the absolute signal quality based handover method. However, when handover is used, the handover related information from the base station located in the small cell is increased because the handover for the small cell occurs when the small cell ToS of the UE increases but the signal quality of the small cell is not good. Is unlikely to receive successfully. Therefore, the handover method using cell range extension has a high probability of handover failure.

4. Other Handover Methods

According to the patent 'ADAPTATION OF HANDOVER PARAMETERS (WO 2010/080849 A9)' filed by Qualcomm, the cause of a handover failure or ping-pong handover (too early or too late) A framework has been proposed for adaptively adjusting parameters related to handover, i.e., an offset of an A3 event or a threshold of an A2 / A4 event. This framework itself can be very useful, but the patent does not include specific algorithms for when and how to control parameters related to handover. Therefore, there is a need to complement the related technology.

Hereinafter, according to the embodiment of the present invention, the handover method based on the absolute signal quality based on the above-described handover method, that is, when the SINR of the small cell is higher than a specific threshold value (ie, macro-to-small handover). And hand-out (i.e., small-to-macro handover) when the SINR of the small cell is lower than a certain threshold, the trade-off relationship between the probability of handover failure and the small cell ToS Suggest ways to improve efficiently. That is, an embodiment of the present invention proposes a method of increasing the small cell ToS while maintaining a constant handover failure probability or decreasing the handover failure probability while maintaining a constant small cell ToS during handover.

First, a hand-in situation and a hand-out situation to which an embodiment of the present invention is applied will be described in detail. As described above, in the absolute signal quality-based handover method, when the UE performs hand-in from the macro cell to the small cell when the SINR of the small cell is higher than the specific threshold, the small cell when the SINR of the small cell is lower than the specific threshold Perform a hand-out from the macro cell to the cell.

6 illustrates a case where hand-in and hand-out to which an embodiment of the present invention is applied are generated.

Referring to FIG. 6, the hand-in 605 occurs where the base station 630 of the small cell in the macro cell is located. At this time, since the terminal 601 performing the hand-in 605 is currently connected to the macro cell, the terminal 601 receives information necessary for performing handover from the base station 610 of the macro cell. Therefore, except when the small cell is located at the boundary of the macro cell, since the terminal 601 is provided with a good signal quality in the macro cell, it is highly likely to successfully receive information necessary for handover from the base station 610 of the macro cell. . This means that the terminal 601 does not have a high probability of handover failure when hand-in 605.

And hand-out 607 always occurs at the boundary of the small cell. In this case, since the terminal 603 performing the hand-out 607 is currently connected to the small cell, the terminal 603 receives the information necessary to perform the handover from the base station 630 of the small cell. In general, since the signal quality between the base station 630 and the terminal 603 of the small cell is poor at the boundary of the small cell, it is difficult for the terminal 603 to successfully receive information necessary for handover from the base station 630 of the small cell. . This means that the terminal 603 has a high probability of handover failure when hand-out 607.

As such, when comparing different features of hand-in and hand-out, it can be seen that the probability of handover failure is not high when the hand-in occurs, whereas the probability of handover failure is high when the hand-out occurs. The probability of handover failure must be mitigated as it causes service interruption for the user. Therefore, when a hand-out occurs, an improvement plan for reducing handover failure is needed. On the contrary, when a hand-in occurs, a method for increasing small cell use by increasing small cell ToS rather than a handover failure is needed. .

Next, the A2 / A4 event and the ping-pong handover to which the embodiment of the present invention is applied will be described.

In the general handover method including the absolute signal quality based handover method described above, hysteresis is applied to prevent ping-pong handover. When hysteresis is applied to A2 / A4 event, handover occurs under the condition of Equation 1.

<Equation 1>

(A4 event based hand-in) RSRP / RSRQ of small cell> threshold + hysteresis

(A2 event based hand-out) RSRP / RSRQ of small cell <threshold hysteresis

7 illustrates a situation in which hysteresis is applied to an A2 / A4 event during handover.

Referring to FIG. 7, since the threshold applied to the hand-in 701 increases by the hysteresis value 730, the hand-in 701 is delayed and the small cell ToS is lowered. On the other hand, since the threshold applied to the hand-out 703 is lowered by the hysteresis value 730, the hand-out 703 is also delayed. This increases the small cell ToS but increases the probability of handover failure because the hand-out 703 occurs when the signal quality between the base station 750 and the terminal of the small cell becomes worse. However, since a margin equal to the value 730 of 2 × hysteresis exists between the hand-in 701 and the hand-out 703, the occurrence of the ping-pong handover can be prevented to a certain level.

8 illustrates a situation in which the threshold of the A2 / A4 event is adjusted.

Referring to FIG. 8, the hand-in 801 is performed early to increase the small cell ToS during the hand-in 801, and when the signal quality between the terminal and the base station 810 of the small cell is good, the hand-out ( By performing the operation 803, it is possible to reduce the handover failure that occurs during the hand-out 803. However, this threshold setting always results in ping-pong handover 850. That is, the terminal performs the hand-out 803 by satisfying the hand-out 803 condition as soon as the terminal performs the hand-in 801 and immediately hand-in 801 after performing the hand-out 803. ) Satisfies the condition and performs hand-in 801. Therefore, even in the case of handover, even though this ping-pong phenomenon is repeated, it is possible to increase the small cell ToS and reduce the handover failure, but this threshold setting is not applicable to the actual system.

Accordingly, an embodiment of the present invention proposes a method of increasing a small cell ToS when a hand-in occurs and preventing a ping-pong handover to a certain level while lowering the probability of handover failure when a hand-out occurs. And the embodiment of the present invention utilizes the experimental results as shown in FIG. 9 shows a handover method according to the position of the terminal in an embodiment of the present invention.

Referring to FIG. 9, among the terminals currently connected to the macro cell, a terminal not yet adjacent to the small cell (ie, a terminal in which the signal quality of the small cell is still poor) 901 does not consider hand-out and the small cell ToS Consider only hand-in to increase

Among the terminals currently connected to the small cell, the terminal 903 that receives a very high quality signal from the base station 907 located in the small cell does not consider hand-in and does not consider hand-in, and reduces hand-over failure probability. Only consider out.

It is also receiving a relatively high quality signal from a base station 907 that is currently connected to a macro cell but located in a small cell, or a low quality signal being received from a base station 907 that is currently connected to a small cell but located in a small cell. The terminal 905 should consider both small cell hand-in and hand-out. Ping-Pong handover may occur in this area, and a method to prevent this should be applied.

10 illustrates operations of a terminal and a base station according to an embodiment of the present invention.

Referring to FIG. 10, the terminal 1001 receives a cell-specific handover parameter from a base station 1003 (1005). Here, the cell-specific handover parameter may include a state threshold 1, a state threshold 2, and a handover threshold 1 (or common threshold + cell-specific offset). 1)), the handover second threshold (HO threshold 2 (or common threshold + cell-specific offset 2)), the value of the first hysteresis (Hysteresis 1) and the value of the second hysteresis (Hysteresis 2), A detailed description of each parameter will be given later in the description of the handover apparatus according to the embodiment of the present invention. Here, the cell-specific handover parameter may have a different value for each cell. This is because the transmission power and the cell coverage of the base station are different for each cell, and the interference environment in which each cell is configured is different. That is, some cells receive strong interference from neighboring neighboring cells, but some cells receive minimal interference. For example, FIG. 11 illustrates a change in SINR according to time when a hand-out occurs in a terminal. Referring to FIG. 11, when hand-out occurs in a terminal, SINR is rapidly deteriorated 1101 according to time (or movement of the terminal) when interference is strong, whereas signal quality is improved according to time when interference is weak. It is slowly degraded 1103.

Therefore, considering that it takes a certain time to perform the handover and must successfully terminate the handover before the minimum SINR, the time point (or SINR) that the handover should be started in the UE 1101 according to each cell. Are different. Accordingly, the base station 1003 broadcasts the cell-specific handover parameter (1005).

The terminal 1001 performs a handover by applying a handover parameter set according to a current state using the cell-specific handover parameter received from the base station 1003 (1007). Here, the method for the terminal 1001 to apply the handover parameter set according to the current state will be described in the description of the handover apparatus according to the embodiment of the present invention of FIG. 12.

Although not shown in FIG. 10, the terminal 1001 may transmit a handover status report to the base station 1003 after performing the handover. Here, the handover status report may include information such as handover timing, SINR of handover related messages, and handover success / failure. The base station 1003 may update the cell-specific parameter based on the received handover status report.

12 illustrates a handover apparatus according to an embodiment of the present invention.

Referring to FIG. 12, the handover apparatus includes a receiver 1210 and a controller 1230, and the controller 1230 includes a measurer 1231, a first processor 1233, and a second processor 1235. do.

The receiver 1210 receives a cell-specific handover parameter from a base station. Here, the cell-specific handover parameter may include a state threshold 1, a state threshold 2, and a handover threshold 1 (or common threshold + cell-specific). offset 1)), a handover second threshold (HO threshold 2 (or common threshold + cell-specific offset 2)), a value of first hysteresis (Hysteresis 1) and a value of second hysteresis (Hysteresis 2) .

The measuring unit 1231 measures the SINR of the small cell for a predetermined time. The first processor 1233 measures the measured state using a state threshold 1 and a state threshold 2 among cell-specific handover parameters as shown in Equation 2 below. The state of the terminal is determined according to the small cell SINR at a UE.

<Equation 2>

State 1 entry condition: Small cell SINR at a UE <state threshold 1

State 2 entry condition: Small cell SINR at a UE> state threshold 2

Here, state threshold 1 is smaller than state threshold 2. The terminal that satisfies the state 1 entry condition becomes state 1, and the terminal that satisfies the state 2 entry condition becomes state 2. In addition, in Equation 2, the terminal satisfying the State threshold 1 <Small cell SINR at a UE <State threshold 2 condition maintains its current state.

13 illustrates an example of determining a state of a terminal by the first processor 1233 according to an embodiment of the present invention. As shown in FIG. 13, when the cell coverage is -6 dB, the state threshold 1 of Equation 2 is -8 dB, and the state threshold 2 is 3 dB, the first processor 1233 has an SINR of -8 dB. If it is lower, it is determined that the current state of the terminal is the first state (state 1). If it is higher than 3 dB of the SINR of the small cell, it is determined that the current state of the terminal is the second state (state 2). The UE having an SINR between -8 dB and 3 dB is maintained without changing its state.

When the SINR of the small cell is changed, the first processor 1233 may perform a state transition according to the changed SINR of the small cell, as shown in FIGS. 14 and 15.

14 illustrates a method of performing state switching according to the SINR of the small cell in the first processor 1233 according to an embodiment of the present invention, and FIG. 15 is a diagram of a first processor 1233 according to an embodiment of the present invention. An example of performing a state switch in accordance with the movement of the terminal is shown.

Referring to FIG. 14, when the SINR of the small cell transferred from the measuring unit 1231 is different from the SINR of the previously transmitted small cell, the first processor 1233 may use the terminal using Equation 2 above. Re-determine the current state of.

For example, when the terminal 1500 moves in the direction of reference number 1550, the terminal 1500 maintains state 1 until it reaches the region 1505 after starting with state 1 in the region 1501. Next, when the terminal 1500 reaches the region 1505, the terminal 1500 switches to state 2, and when the terminal 1500 reaches the region 1501, the terminal 1500 switches to state 1. That is, according to state 1 and state 2 defined in Equation 2, an area near the small cell may be divided into three areas as follows.

◎ first region (Region 1): always the region 1 (Small cell SINR <state threshold 1, region 1501 in Figure 15)

◎ Second region: Region 2 always corresponding to state 2 (Small cell SINR> state threshold 2, region 1505 in FIG. 15)

◎ Third region (Region 3): region having different states depending on what the previous state is (region 1503 in FIG. 15)

If the previous state is state 1, state 1 remains until transition to state 2

If the previous state is state 2, state 2 remains until transition to state 1

The third region is a region that appears because state threshold 1 and state threshold 2 are different from each other and state threshold 1 < According to an embodiment of the present invention, if the terminal is in the first region, hand-in is performed to increase the ToS of the small cell, and if the terminal is in the second region, the hand-out is performed to reduce the probability of handover failure. Being in zone 3 provides a way to prevent ping-pong handovers while maintaining the hand-in / hand-out conditions of the previous state.

That is, the second processor 1235 applies different handover parameters according to the state of the terminal determined by the first processor 1233.

To this end, two sets of handover parameters based on the A2 / A4 event are defined as shown in Equations 3 and 4 below.

<Equation 3>

HO parameter set 1: HO threshold 1 and hysteresis 1

Hand-in: HO threshold 1 + hysteresis 1

-Hand-out: HO threshold 1-hysteresis 1 (hysteresis 1> 0)

<Equation 4>

HO parameter set 2: HO threshold 2 and hysteresis 2

Hand-in: HO threshold 2 + hysteresis 2

-Hand-out: HO threshold 2-hysteresis 2 (hysteresis 2> 0)

Equation 3 is a value set for increasing small cell ToS and is mainly used when a terminal located in a macro cell performs hand-in. That is, in Equation 3, the HO parameter set 1 is applied when the state of the terminal is state 1.

Equation 4 is set to reduce the probability of handover failure and is used when a terminal located in the center of the small cell performs hand-out. That is, in Equation 4, the HO parameter set 2 is applied when the state of the terminal is state 2.

In this case, the cell-specific handover parameters (HO threshold 1, hysteresis 1, HO threshold 2, and hysteresis 2) used in Equations 3 and 4 satisfy the relationship in Equation 5 below. Should be set to

<Equation 5>

Condition 1: HO threshold 1 <HO threshold 2

Condition 2: HO threshold 1 + hysteresis 1 (hand-in) <HO threshold 2-hysteresis 2 (hand-out)

Table 1 below shows an example in which the cell-specific handover parameter is set such that Equation 5 is satisfied.

Table 1

HO parameter set 1	HO parameter set 2
HO threshold 1 = -7 dB Hysteresis 1 = 1 dB	HO threshold	2 = -2 dB Hysteresis 2 = 1 dB
Hand-in @ -6 dB Hand-out @ -8 dB	Hand-in @ -1 dB Hand-out @ -3 dB

In Table 1, the HO threshold 1 value in the HO parameter set 1 is set relatively low as -7 dB. This means that the UE performs hand-in early in a state where the signal quality of the small cell is relatively poor, thereby increasing the small cell ToS. In general, radio link failure (RLF) occurs at -8 dB. Therefore, if the HO threshold 1 value is set to -8 dB or more, even though the signal quality of the small cell is relatively poor, the time and frequency resources that are not used in the small cell can be used a lot, thereby improving the performance of the UE.

In HO parameter set 2, the HO threshold 2 value was set relatively high as -2 dB. This means that the terminal performs hand-out early in a state where the signal quality of the small cell is relatively good and successfully receives information necessary for handover from the base station where the small cell is located. This lowers the probability of handover failure that occurs during hand-out.

The second processor 1235 performs handover using HO parameter set 1 or HO parameter set 2 of Equations 3 and 4 according to the state of the terminal. That is, the second processor 1235 performs handover using HO parameter set 1 when the current state of the terminal is state 1, and performs handover by using HO parameter set 2 when the current state of the terminal is state 2. To perform.

FIG. 16 illustrates a method of applying a handover parameter set according to a state of a terminal in a second processor 1235 according to an embodiment of the present invention. That is, the second processor 1235 performs handover using the following handover parameter set according to the state corresponding to each region in FIG. 16.

◎ first region: always corresponding to state 1 (Small cell SINR < state state 1, region 1601 in FIG. 16)

HO parameter set 1 (for example, Hand-in @ -6 dB, hand-out @ -8 dB)

◎ Second area: always corresponding to state 2 (Small cell SINR> state threshold 2, area 1605 in FIG. 16)

HO parameter set 2 (for example, Hand-in @ -1 dB, hand-out @ -3 dB)

◎ Third region: region having different states depending on what the previous state is (region 1603 in FIG. 16)

If the previous state is 1: Apply HO parameter set 1 (eg, Hand-in @ -6 dB, hand-out @ -8 dB)

If the previous state is 2: Apply HO parameter set 2 (e.g. Hand-in @ -1 dB, hand-out @ -3 dB)

That is, when the terminal 1600 moves in the direction of reference number 1650 in FIG. 16, the terminal 1500 performs a handover using the HO parameter set 1 in an area 1601 (that is, an area corresponding to state 1). In the 1605 region (that is, the region corresponding to state 2), the handover is performed using the HO parameter set 2.

17 illustrates switching of a handover parameter set according to a change in SINR according to an embodiment of the present invention. That is, when the change in SINR with time as the UE moves as shown in FIG. 17, the time when hand-in occurs (-6 dB) 1701 and the time when hand-out occurs (-3 dB) 1705 It is stated.

18 shows a handover method according to an embodiment of the present invention.

The receiving unit 1210 of the terminal receives a cell-specific handover parameter from the base station (1801). The measurer 1231 measures the SINR of the small cell (1803). The first processor 1233 determines the state of the terminal using state threshold 1 and state threshold 2 of the measured SINR and the cell-specific handover parameter according to Equation 2 (1805). If the determined state of the terminal is state 1, the second processor 1235 performs a handover using HO parameter set 1 according to Equation 3 (1807). In contrast, when the determined state of the terminal is state 2, the second processor 1235 performs a handover using HO parameter set 2 according to Equation 4 (1809). The second processor 1235 continuously tracks the state change of the terminal 1811 and performs handover when necessary.

When hysteresis is applied to the general absolute signal quality based handover method, the SINR at the hand-in point cannot be lower than the SINR at the hand-out point. If the SINR at the hand-in point is lower than the SINR at the hand-out point, the ping-pong handover always occurs because the hand-out condition is satisfied immediately after the hand-in and the hand-in condition is satisfied immediately after the hand-out. .

However, in the handover method according to an embodiment of the present invention, the SINR at the hand-in time point may be set lower than the SINR at the hand-out time point. That is, in the handover method according to an embodiment of the present invention, since the state of the terminal is determined by state threshold 1 and state threshold 2 having a predetermined difference, the HO parameter set may be adaptively set according to the state of the terminal.

Therefore, if the SINR at the hand-in point is set lower than the SINR at the hand-out point as in the embodiment of the present invention, the small cell ToS can be increased at the hand-in point and the probability of handover failure at the hand-out point is increased. Can be reduced. Therefore, the handover method according to an embodiment of the present invention can effectively improve the trade-off relationship between small cell ToS and handover failure probability.

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 defined not only by the appended claims, but also by the equivalents of the claims.

Claims (15)

1. In a handover device of a terminal in a mobile communication system including a small cell and a macro cell,

   A receiver for receiving a cell-specific handover parameter from a base station; And

   The signal-to-noise ratio (SINR) of the small cell is measured, the state of the terminal is determined according to the measured SINR using the received cell-specific handover parameter, and the cell is determined according to the state of the terminal. A control unit for performing a handover by using one of the handover parameter sets set based on a specific handover parameter.
2. The method of claim 1, wherein the control unit,

   A measuring unit measuring an SINR of the small cell;

   A first processor which determines a state of a terminal according to the measured SINR; And

   And a second processor configured to perform handover by applying one of the handover parameter sets according to the state of the terminal.
3. The method of claim 1, wherein the cell-specific handover parameter,

   A handover device comprising a state first threshold, a state second threshold, a handover first threshold, a handover second threshold, a value of first hysteresis and a value of second hysteresis.
4. The method of claim 3, wherein the control unit,

   If the measured SINR is less than the state first threshold, the terminal enters a first state,

   If the measured SINR is greater than the state second threshold, the terminal enters a second state,

   If the measured SINR is greater than the state first threshold and less than the state second threshold, the terminal maintains the current state.

   And wherein the state first threshold is lower than the state second threshold.
5. The method of claim 4, wherein the control unit,

   Set a first handover parameter set that includes the handover first threshold and the value of the first hysteresis,

   Set a second handover parameter set that includes the handover second threshold and the value of the second hysteresis,

   The handover first threshold is less than the handover second threshold, and the sum of the handover first threshold and the value of the first hysteresis is a value of the handover second threshold and the value of the second hysteresis. Handover device characterized by less than the difference.
6. The method of claim 5, wherein the control unit,

   If the state of the terminal is the first state, handover is performed using the first handover parameter;

   And performing a handover using the second handover parameter when the state of the terminal is the second state.
7. The method of claim 5 or 6, wherein the control unit,

   When the state of the terminal is the first state, the handover is performed from the macro cell to the small cell using the sum of the handover first threshold value and the first hysteresis value, or the handover first threshold value. Performing a handover from the small cell to the macro cell by using a difference between the value of the first hysteresis and

   And the first hysteresis has a value greater than zero.
8. The method of claim 5 or 6, wherein the control unit,

   When the state of the terminal is the second state, the handover is performed from the macro cell to the small cell by using a sum of the handover second threshold value and the second hysteresis value, or the handover second threshold value. Performing a handover from the small cell to the macro cell by using a difference between the value of the second hysteresis and

   And the second hysteresis has a value greater than zero.
9. In a handover method of a terminal in a mobile communication system including a small cell and a macro cell,

   Receiving a cell-specific handover parameter from a base station; And

   The signal-to-noise ratio (SINR) of the small cell is measured, the state of the terminal is determined according to the measured SINR using the received cell-specific handover parameter, and the cell is determined according to the state of the terminal. A handover method comprising performing a handover using one of the handover parameter sets set based on a specific handover parameter.
10. 10. The method of claim 9, wherein the cell-specific handover parameter is

    A handover method comprising a state first threshold, a state second threshold, a handover first threshold, a handover second threshold, a value of first hysteresis and a value of second hysteresis.
11. The method of claim 10, wherein the control process,

    Determining that the terminal enters a first state when the measured SINR is smaller than the state first threshold value;

    If the measured SINR is greater than the state second threshold, determining that the terminal has entered the second state;

    When the measured SINR is greater than the state first threshold and less than the state second threshold, the terminal maintains the current state, and the state first threshold is lower than the state second threshold. A handover method characterized by the above.
12. The method of claim 11, wherein the control process,

    Setting a first handover parameter set that includes the handover first threshold and the value of the first hysteresis;

    Setting a second handover parameter set that includes the handover second threshold and the value of the second hysteresis,

    The handover first threshold is less than the handover second threshold, and the sum of the handover first threshold and the value of the first hysteresis is a value of the handover second threshold and the value of the second hysteresis. Handover method characterized by less than the difference.
13. The method of claim 12, wherein the control process,

    If the state of the terminal is the first state, handover is performed using the first handover parameter;

    If the state of the terminal is the second state, the handover method using the second handover parameter.
14. The method of claim 12 or 13, wherein the control process,

    When the state of the terminal is the first state, the handover is performed from the macro cell to the small cell using the sum of the handover first threshold value and the first hysteresis value, or the handover first threshold value. And performing a handover from the small cell to the macro cell by using a difference between the value of the first hysteresis and

    The value of the first hysteresis has a value greater than zero.
15. The method of claim 12 or 13, wherein the control process,

    When the state of the terminal is the second state, the handover is performed from the macro cell to the small cell by using a sum of the handover second threshold value and the second hysteresis value, or the handover second threshold value. And performing a handover from the small cell to the macro cell by using a difference between the value of the second hysteresis and

    And the second hysteresis has a value greater than zero.

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