KR20150111254A - Hand-over apparatus and method in a mobile communication system - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for adaptively applying a handover parameter according to a state of a mobile station in a mobile communication system. A handover apparatus of a mobile station in a mobile communication system including a small cell and a macro cell according to the present invention includes: a receiver for receiving a cell-specific handover parameter from a base station; And determining a state of a UE according to the measured SINR using the received cell-specific handover parameter, measuring a signal-to-noise ratio (SINR) of the small cell according to the measured SINR, And performing a handover using one of the set of handover parameters set based on the cell-specific handover parameter.

Description

TECHNICAL FIELD [0001] The present invention relates to a handover apparatus and method in a mobile communication system,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for adaptively applying a handover parameter according to a state of a mobile station in a mobile communication system.

In the mobile communication system, a handover is a change from a serving base station, in which a mobile station is currently provided, to a base station capable of providing a higher quality service. For example, if the service quality is deteriorated while receiving a service from a serving base station, the terminal performs handover to a target base station capable of providing a higher quality service and continuously receives service from the target base station.

Meanwhile, a heterogeneous network includes a macro base station communication area (i.e., macro cell) capable of transmitting / receiving service data with a macro base station, a small base station communication area capable of transmitting and receiving service data with a small base station Cell), and FIG. 1 shows an example of a heterogeneous network including a macro cell and a small cell.

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

The mobile terminals 101 and 103 in the communication area 100 shown in FIG. 1 perform a handover to maintain the service quality for the service to be provided.

When the serving cell of the terminals 101 and 103 is a macro cell capable of sending and receiving service data to and from the macro base station 105, the terminals 101 and 103 may transmit macro cells, which can transmit and receive service data, The macro cell or the small base station 117 capable of transmitting / receiving service data with the base station 107 can be handed over to a small cell capable of transmitting / receiving service data.

If 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 small base station 117, which are capable of transmitting and receiving service data with the small base station 117, Handover to a macro cell capable of transmitting and receiving service data and a small cell or macro base station 107 capable of transmitting and receiving data.

In such a heterogeneous network composed of a macro cell and a small cell, a terminal can perform handover from a macro cell to a macro cell, from a macro cell to a small cell, from a small cell to a macro cell, or from a small cell to a small cell. In order to efficiently perform the handover in such a heterogeneous network, it is necessary to adapt the handover parameters according to the state of the terminal and perform a handover.

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

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

In accordance with another aspect of the present invention, there is provided a handover method of a mobile station in a mobile communication system including a small cell and a macro cell, the method comprising: receiving a cell-specific handover parameter from a base station; And determining a state of a UE according to the measured SINR using the received cell-specific handover parameter, measuring a signal-to-noise ratio (SINR) of the small cell according to the measured SINR, And performing a handover using one of the set of handover parameters set based on the cell-specific handover parameter.

1 is a view showing an example of a heterogeneous network including a macro cell and a small cell,
2 is a diagram illustrating a result of a handover in a case where a handover method based on a relative signal quality is performed in a terminal,
FIG. 3 is a diagram illustrating a handover result when performing a handover method based on an absolute signal quality in a terminal,
4 is a diagram illustrating a trade-off relationship between a handover failure probability and a small cell ToS in an A2 / A4 event;
5 is a diagram illustrating a general cell range expansion method,
6 is a diagram illustrating a case where a hand-in and a hand-out to which an embodiment of the present invention is applied are shown,
7 is a diagram illustrating a situation in which hysteresis is applied to A2 / A4 events at the time of handover,
8 is a diagram illustrating a situation in which threshold values of A2 / A4 events are adjusted,
9 is a diagram illustrating a handover method according to a location of a terminal in an embodiment of the present invention;
10 is a diagram illustrating operations of a terminal and a base station according to an embodiment of the present invention;
11 is a diagram illustrating a change in SINR according to time when hand-out occurs in a terminal,
12 is a diagram illustrating a handover apparatus 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 processor 1233 according to an embodiment of the present invention.
FIG. 14 illustrates a method of performing a state change according to SINR of a small cell in a first processor 1233 according to an embodiment of the present invention.
15 is a diagram illustrating an example of performing a state change according to movement of a terminal in a first processor 1233 according to an embodiment of the present invention.
16 is a diagram illustrating 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,
17 is a diagram illustrating switching of a handover parameter set according to a change in SINR in an embodiment of the present invention;
18 illustrates a handover method according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, only parts necessary for understanding the operation according to the present invention will be described, and the description of other parts will be omitted so as not to obscure the gist of the present invention.

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

Hereinafter, a general handover method will be described, and then a handover method according to an embodiment of the present invention will be described in detail.

Hereinafter, a macro-to-small handover in which a UE connected to a macro cell moves to a small cell occurs in a heterogeneous network composed of a macro cell and a small cell, I will explain.

1. Handover method based on relative signal quality

The handover method based on the relative signal quality is a method in which the MS transmits a small cell (i.e., a target cell) close to a signal-to-interference ratio (SINR) of a macro cell (i.e., a serving macro cell) (target) small cell), and when the SINR of the target small cell is greater than or equal to a predetermined offset from the SINR of the serving macro cell, the handover is performed from the serving macro cell to the target small cell. In 3GPP LTE, this is defined as A3 event (RSRP / RSRQ of target cell> RSRP / RSRQ of serving cell + offset). Here, the SINR means an average SINR.

When the handover method based on the relative signal quality is applied, the UE has an advantage of performing handover to the cell with the best signal quality.

However, when the handover method based on the relative signal quality is applied, the following disadvantages arise. In general, macro cells provide a service in a wide area, so that the transmission power of the base station is large. However, since the small cell provides a service in a narrow area, the transmission power of the base station is small. Therefore, when the SINR of a macro cell and a small cell are compared, 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 lead to congestion of macro cells and under-utilization of small cells. The rate at which this phenomenon occurs is shown in Fig.

2 shows a handover result in the case where a handover method based on a relative signal quality is performed in a terminal.

According to the result of the handover shown in FIG. 2, the time during which the terminal stays in the macro cell (i.e., the time-to-stay ratio of the macro cell ToS) is very long while the time during which the terminal stays in the small cell .

2. Handover method based on absolute signal quality

In order to compensate for the disadvantage of the "relative signal quality based handover method" described in the above 1, the terminal can use the absolute signal quality based handover method. The absolute signal quality based handover method handover to the target small cell when the terminal considers only the SINR of the target small cell without considering the SINR of the serving macrocell and the SINR of the target small cell is higher than a specific threshold. 3GPP LTE defines it as A4 event (RSRP / RSRQ of target cell> threshold).

Also, when the UE performs the handover from the serving small cell to the target macro cell, considering the SINR of the serving small cell without considering the SINR of the target macro cell, if the SINR of the serving small cell is lower than the specific threshold value, Can be handed over. In 3GPP LTE, it is defined as an A2 event (RSRP / RSRQ of serving cell <threshold).

If the handover method based on the absolute signal quality is used, the UE can enter the small cell or escape from the small cell regardless of the signal quality of the macro cell. Therefore, by controlling the threshold value of the A2 / A4 event, it is possible to control the time for the terminal to stay in the small cell. The rate at which this phenomenon occurs is shown in Fig.

FIG. 3 shows a handover result in the case of performing a handover method based on an absolute signal quality in a terminal.

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

When the threshold value 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 escapes from the small cell when the signal quality of the small cell is very good. This allows small cell hand-in (i.e., macro-to-small handover) and hand-out (i.e., small-to-macro handover) ) Occurs. Here, the terms "hand-in" and "hand-out" are handovers based on a small cell. Therefore, although the time for the terminal to stay in the small cell is short, handover to the small cell occurs when the signal quality of the signal cell is very good, so there is a high possibility that the handover related information is successfully received from the base station located in the small cell. Therefore, if the threshold value of 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 bad and deviates from 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, when the terminal has a long time to stay in the small cell but the signal quality of the small cell is not good, the small cell handover occurs, and the possibility of successfully receiving the handover related information from the small cell base station is low. Therefore, if the threshold value of A2 / A4 event is low, the probability of handover failure is high.

These results can be shown briefly in FIG. FIG. 4 shows a trade-off relationship between the handover failure probability and the small cell ToS in the A2 / A4 event.

Referring to FIG. 4, when the threshold value of the A2 / A4 event is high (High SINR threshold), the handover failure probability is low, but the small cell ToS is also low, so that the small cell is used low. On the other hand, when the threshold value of A2 / A4 event is low (Low SINR threshold), the small cell ToS is high, but the probability of handover failure is also high. Therefore, there is a need to improve the trade-off relationship between the handover failure probability and the small cell ToS efficiently. That is, there is a need to increase the small cell ToS while maintaining a constant handover failure probability, or to reduce the handover failure probability while maintaining a constant small cell ToS.

3. Handover method using cell range expansion

The handover method using the 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. FIG. 5 shows a general cell range expansion method. As shown in FIG. 5, in the handover method using the cell range extension, in order to make the cell range of the small cell larger (500), the threshold value of the A3 event offset or the A2 / A4 event is adjusted, The handover to the small cell is performed. Accordingly, it is possible to solve the low usage of the small cell generated in the relative signal quality based handover method and the absolute signal quality based handover method. However, when the cell range extending method is used at the time of handover, the small cell ToS of the UE increases but the handover to the small cell occurs when the signal quality of the small cell is poor. Therefore, Is not likely to be successfully received. Therefore, the handover method using the cell range extension has a high probability of handover failure.

4. Other handover method

According to the 'ADAPTATION OF HANDOVER PARAMETERS (WO 2010/080849 A9)' filed by Qualcomm, when a handover failure or a ping-pong handover occurs, the cause (too early handover or too late hand Over) is reported to the base station to adaptively adjust the handover related parameters, i.e., the offset of the A3 event or the threshold of the A2 / A4 event. While this framework itself can be very useful, the patent does not include a specific algorithm for when and how to control parameters related to handover. Therefore, it is necessary to supplement the related technology.

Hereinafter, in the embodiment of the present invention, among the above-described handover methods, 2. Handover method based on absolute signal quality, that is, hand-in when a SINR of a small cell is higher than a specific threshold (i.e., macro- ) And performing a hand-out (i.e., a small-to-macro handover) when the SINR of the small cell is lower than a certain threshold, a trade-off relationship between the handover failure probability and the small cell ToS And suggests ways to improve it efficiently. That is, in the embodiment of the present invention, a method of lowering the probability of handover failure while maintaining a fixed handover failure probability while maintaining a small cell ToS or maintaining a constant small cell ToS is proposed.

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 handover method based on the absolute signal quality, when the terminal performs hand-in from the macrocell to the small cell when the SINR of the small cell is higher than the specific threshold and when the SINR of the small cell is lower than the specific threshold, Out to the macro cell.

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

Referring to FIG. 6, a 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 connected to the current macro cell, it receives the information necessary for performing the handover from the base station 610 of the macro cell. Therefore, except for the case where 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, there is a high possibility that the terminal 601 successfully receives information required for handover from the base station 610 of the macro cell . This means that the probability of the handover failure occurring in the hand-in 605 of the terminal 601 is not high.

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

In this way, when the hand-in and hand-out characteristics are compared, the handover failure probability is not high at the hand-in occurrence but the handover failure probability is high at the hand-out occurrence. The handover failure probability must be mitigated because it causes service interruption to the user. Therefore, in order to reduce the handover failure, it is necessary to improve the hand-out. On the other hand, when the hand-in occurs, it is necessary to increase the small cell usage by increasing the small cell ToS .

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

&Quot; (1) &quot;

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

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

FIG. 7 shows a situation in which hysteresis is applied to the A2 / A4 event at the time of handover.

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

FIG. 8 shows a situation where the threshold value of the A2 / A4 event is adjusted.

Referring to FIG. 8, hand-in 801 may be performed early to increase the small cell ToS during hand-in 801, hand-out when the signal quality between the subscriber station and the small cell base station 810 is good 803), thereby reducing the handover failure occurring in the hand-out (803). However, this threshold setting always causes a ping-pong handover 850. That is, as soon as the terminal performs the hand-in 801, the hand-out 803 condition is satisfied and the hand-out 803 is performed. When the hand-out 801 is performed, the hand- ) Condition is satisfied and hand-in 801 is performed. Therefore, in the handover, since such a ping-pong phenomenon is repeated, it is possible to increase the small cell ToS and reduce the handover failure, and such a threshold setting can not be applied to an actual system.

Therefore, the embodiment of the present invention proposes a method of increasing the small cell ToS at the time of hand-in and preventing a certain level of ping-pong handover while lowering the probability of handover failure at the time of hand-out. In the embodiment of the present invention, the experimental result as shown in FIG. 9 is utilized. FIG. 9 illustrates a handover method according to a location of a terminal in an embodiment of the present invention.

Referring to FIG. 9, among the terminals connected to the current macro cell, a terminal not yet adjacent to a small cell (i.e., a terminal having a signal quality of a small cell still poor) 901 does not consider hand- Consider hand-in only to increase.

A terminal 903 receiving a signal of a very high quality from a base station 907 located in a small cell among the terminals currently connected to a small cell does not consider the hand-in, and performs hand- out.

In addition, the base station 907 connected to the current macro cell receives a signal of a relatively high quality from the base station 907 located in the small cell, or receives a signal of a low quality from the base station 907 located in the small cell, Terminal 905 must consider both small cell hand-in and hand-out. Since ping-pong handover may occur in such a region, a method for preventing the ping-pong handover should be applied.

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

Referring to FIG. 10, the UE 1001 receives a cell-specific handover parameter from the Node B 1003 (1005). Here, the cell-specific handover parameters include a state threshold 1, a state threshold 2, a HO threshold 1 (or a common threshold + cell-specific offset 1), a HO threshold 2 (or a common threshold + cell-specific offset 2), a value of a first hysteresis (Hysteresis 1), and a value of a second hysteresis (Hysteresis 2) A detailed description of each parameter will be given in the description of the handover apparatus according to the embodiment of the present invention shown in FIG. Here, the cell-specific handover parameter may have a different value for each cell. This is because the transmission power of the base station and the corresponding cell coverage are different for each cell, and the interference environments in which the cells are configured are different from each other. That is, some cells are subject to strong interference from adjacent neighboring cells, while others are subject to insignificant interference. For example, FIG. 11 shows a change in SINR over time when a hand-out occurs in a terminal. Referring to FIG. 11, when a hand-out occurs in a terminal, when the interference is strong, the SINR rapidly deteriorates (1101) according to time (or movement of the terminal), whereas when the interference is weak, And slowly deteriorated (1103).

Therefore, considering that it takes a certain time to perform the handover and the handover must be terminated successfully before the minimum SINR, the time (or SINR) at which the handover should start at the UE 1101 depends on each cell They are different. Accordingly, the base station 1003 broadcasts a cell-specific handover parameter (1005).

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

Although not shown in FIG. 10, the MS 1001 may perform a handover and then transmit a handover status report to the BS 1003. Here, the handover status report may include information such as a handover timing, a reception SINR of the handover related message, and whether the handover is successful / unsuccessful. The base station 1003 may then update cell-specific parameters based on the received handover status report.

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

12, the handover apparatus includes a receiving unit 1210 and a control unit 1230. The control unit 1230 includes a measuring unit 1231, a first processing unit 1233, and a second processing unit 1235 do.

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

The measuring unit 1231 measures the SINR of the small cell for a predetermined time. Then, the first processor 1233 calculates the handover parameters using the state threshold value 1 and the state threshold value 2 of the cell-specific handover parameters as shown in Equation (2) And determines the state of the UE according to the small cell SINR at a UE (SINR) of the small cell.

&Quot; (2) &quot;

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

Where state threshold 1 is less than state threshold 2. Then, the terminal that satisfies the state 1 entry condition becomes the state 1, and the terminal that satisfies the state 2 entry condition becomes the state 2. In Equation (2), a terminal satisfying the condition of State threshold 1 <Small cell SINR at a UE <State threshold 2 maintains its current state as it is.

FIG. 13 illustrates an example of determining the state of a terminal in the first processor 1233 according to an embodiment of the present invention. 13, when the cell coverage is -6 dB, the state threshold 1 of Equation (2) is -8 dB and the state threshold 2 is 3dB, the first processing unit 1233 sets the SINR of the small cell to -8 dB The current state of the terminal is determined to be the first state (state 1), and if the SINR is higher than the SINR of the small cell, the current state of the terminal is determined to be the second state (state 2). A terminal with a SINR of between -8 dB and 3 dB maintains its state unchanged.

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.

FIG. 14 shows a method of performing a state change according to the SINR of a small cell in a first processor 1233 according to an embodiment of the present invention. FIG. 15 shows a state transition of a first processor 1233 according to an embodiment of the present invention, And the state transition is performed according to the movement of the terminal.

Referring to FIG. 14, if the SINR of the small cell transmitted from the measuring unit 1231 is different from that of the small cell previously transmitted, the first processing unit 1233 transmits the small cell using the Equation (2) Lt; / RTI &gt;

For example, when the terminal 1500 moves in the direction of reference numeral 1550, the terminal 1500 starts state 1 in region 1501 and maintains state 1 until it reaches region 1505. Next, when the terminal 1500 reaches the area 1505, the terminal 1500 switches to the state 2, and when the terminal 1500 reaches the area 1501 again, the terminal 1500 switches to the state 1. That is, according to state 1 and state 2 defined in Equation (2), a region near the small cell can be divided into the following three regions.

Region 1: A region always corresponding to state 1 (Small cell SINR <state threshold 1, area 1501 in FIG. 15)

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

Region 3: Region having different states according to what the previous state is (region 1503 in FIG. 15)

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

If the previous state is state 2, state 2 is maintained until the transition to state 1 is made

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

That is, the second processing unit 1235 applies different handover parameters according to the state of the terminal determined in the first processing unit 1233.

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

&Quot; (3) &quot;

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)

&Quot; (4) &quot;

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

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

In this case, the cell-specific handover parameters (HO threshold 1, HO threshold 2, hysteresis 2) used in Equation (3) and Equation (4) .

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 where the cell-specific handover parameter is set to satisfy Equation (5).

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 to a relatively low value of -7 dB. This means that the terminal performs hand-in early in a state where the signal quality of the small cell is relatively poor, thereby increasing the small cell ToS. Generally, a radio link failure (RLF) occurs at -8 dB. Therefore, if the HO threshold 1 is set to more than -8 dB, the time and frequency resources that are not used in the small cell can be used even if the signal quality of the small cell is not relatively good.

Also, the HO threshold 2 value in the HO parameter set 2 is set to a relatively high value of -2 dB. This means that the mobile station hand-outs 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 in which the small cell is located. This has the effect of lowering the probability of handover failure occurring during hand-out.

The second processing unit 1235 performs handover using HO parameter set 1 or HO parameter set 2 of Equation (3) and Equation (4) according to the state of the UE. That is, the second processor 1235 performs handover using the HO parameter set 1 when the current state of the UE is state 1, and performs handover using the HO parameter set 2 when the current state of the UE is state 2 .

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 area in FIG.

First area: area corresponding to state 1 (Small cell SINR <state threshold 1, area 1601 in FIG. 16)

- HO parameter set 1 applied (eg hand-in @ -6 dB, hand-out @ -8 dB)

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

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

Third area: area having different states depending on what previous state (area 1603 in FIG. 16)

- If the previous state is 1: Applies HO parameter set 1 (for example, hand-in @ -6 dB, hand-out @ -8 dB)

- If the previous state is 2: HO parameter set 2 applied (for example, hand-in @ -1 dB, hand-out @ -3 dB)

That is, when the terminal 1600 moves in the direction of the reference numeral 1650 in FIG. 16, the terminal 1500 performs a handover using the HO parameter set 1 in the area 1601 (i.e., the area corresponding to the state 1) , Handover is performed using the HO parameter set 2 in the 1605 area (i.e., the area corresponding to the state 2).

17 shows the switching of the handover parameter set according to the variation of the SINR in the embodiment of the present invention. That is, when the change of the SINR with respect to time when the mobile station moves is as shown in FIG. 17, the time (-6 dB) 1701 at which the hand-in occurs (-3 dB) 1705 and the time at which the hand- It is stated.

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

The receiving unit 1210 of the UE receives the cell-specific handover parameter from the base station (1801). The measuring unit 1231 measures the SINR of the small cell (1803). The first processor 1233 determines the state of the UE using the measured SINR of the small cell and the state threshold 1 and the state threshold 2 of the cell-specific handover parameters according to Equation (2) (1805). If the determined state of the UE is state 1, the second processor 1235 performs handover using HO parameter set 1 according to Equation (3) (1807). If the determined state of the UE is state 2, the second processor 1235 performs handover using the HO parameter set 2 according to Equation (4) (1809). The second processor 1235 keeps track 1811 the state change of the UE and performs handover if necessary.

When hysteresis is applied to the handover method based on the general absolute signal quality, the SINR at the hand-in time can not be lower than the SINR at the hand-out time. If the SINR at the hand-in time is lower than the SINR at the hand-out point, the hand-out condition is satisfied immediately after the hand-in and the hand-in condition is satisfied immediately after the hand-out, so that the ping- .

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

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

While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but is capable of various modifications within the scope of the invention. Therefore, the scope of the present invention should not be limited by the illustrated embodiments, but should be determined by the scope of the appended claims and equivalents thereof.

Claims (15)

A handover apparatus of a mobile station in a mobile communication system including a small cell and a macro cell,
A receiving unit for receiving a cell-specific handover parameter from a base station; And
Determining a state of the UE according to the measured SINR using the received cell-specific handover parameter, measuring a signal-to-noise ratio (SINR) of the small cell, And a control unit for performing a handover using one of the sets of handover parameters set based on the specific handover parameter.
The apparatus of claim 1,
A measuring unit for measuring an SINR of the small cell;
A first processor for determining a state of the UE according to the measured SINR; And
And a second processor for performing a handover by applying one of the handover parameter sets according to a state of the terminal.
2. The method of claim 1, wherein the cell-
A state first threshold, a state second threshold, a handover first threshold, a handover second threshold, a value of a first hysteresis, and a value of a second hysteresis.
The apparatus of claim 3,
If the measured SINR is less than the first threshold, the terminal enters a first state,
If the measured SINR is greater than the second threshold, the terminal enters a second state,
If the measured SINR is greater than the first threshold and less than the second threshold, the terminal maintains its current state,
And the state first threshold is lower than the state second threshold.
5. The apparatus of claim 4,
Setting a first handover parameter set including the handover first threshold value and the first hysteresis value,
Setting a second handover parameter set including a value of the handover second threshold and a value of the second hysteresis,
Wherein the handover first threshold value is smaller than the handover second threshold value and the sum of the handover first threshold value and the first hysteresis value is smaller than the sum of the handover second threshold value and the second hysteresis value The difference being smaller than the difference.
6. The apparatus of claim 5,
Performing handover using the first handover parameter when the state of the terminal is in the first state,
And performs a handover using the second handover parameter when the state of the terminal is the second state.
7. The apparatus as claimed in claim 5 or 6,
Performing handover from the macrocell to the small cell using the sum of the handover first threshold value and the first hysteresis value when the state of the UE is in the first state, And performing a handover from the small cell to the macro cell using a difference between the first hysteresis value and the first hysteresis value,
Wherein the value of the first hysteresis is greater than zero.
7. The apparatus as claimed in claim 5 or 6,
Performing handover from the macro cell to the small cell using the sum of the handover second threshold and the second hysteresis when the state of the MS is in the second state, And performing a handover from the small cell to the macro cell using a difference between the first hysteresis value and the second hysteresis value,
Wherein the value of the second hysteresis is greater than zero.
A handover method of a mobile station in a mobile communication system including a small cell and a macro cell,
Receiving a cell-specific handover parameter from a base station; And
Determining a state of the UE according to the measured SINR using the received cell-specific handover parameter, measuring a signal-to-noise ratio (SINR) of the small cell, And performing a handover using one of the set of handover parameters set based on the specific handover parameter.
10. The method of claim 9, wherein the cell-
A state first threshold, a state second threshold, a handover first threshold, a handover second threshold, a value of a first hysteresis, and a value of a second hysteresis.
11. The method according to claim 10,
Determining that the terminal enters a first state if the measured SINR is less than the first threshold;
And determining that the UE has entered a second state if the measured SINR is greater than the second threshold value,
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 the state first threshold is lower than the state second threshold Wherein the handover method comprises:
12. The method according to claim 11,
Setting a first handover parameter set including the handover first threshold value and the first hysteresis value;
And setting a second handover parameter set including a value of the second handover threshold and a value of the second hysteresis,
Wherein the handover first threshold value is smaller than the handover second threshold value and the sum of the handover first threshold value and the first hysteresis value is smaller than the sum of the handover second threshold value and the second hysteresis value And the difference is smaller than the difference.
13. The method according to claim 12,
Performing handover using the first handover parameter when the state of the terminal is in the first state,
Wherein the handover is performed using the second handover parameter when the state of the terminal is the second state.
14. The method according to claim 12 or 13,
Performing handover from the macrocell to the small cell using the sum of the handover first threshold value and the first hysteresis value when the state of the UE is in the first state, And performing a handover from the small cell to the macro cell using a difference between the first hysteresis value and the first hysteresis value,
Wherein the value of the first hysteresis is greater than zero.
14. The method according to claim 12 or 13,
Performing handover from the macro cell to the small cell using the sum of the handover second threshold and the second hysteresis when the state of the MS is in the second state, And performing a handover from the small cell to the macro cell using a difference between the first hysteresis value and the second hysteresis value,
Wherein the value of the second hysteresis is greater than zero.

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