KR101925107B1 - Method for inteference coordination in wireless communication systems - Google Patents

Abstract

The present invention relates to a method for adjusting an interference in a wireless communication system comprising: a process of finding a first cell in which a service failure occurs most frequently; a process of requesting, by the first cell, service to adjacent second cells; a process of controlling transmission power of terminals of the cell in which no service failure has occurred in the second cells; a process of re-measuring RSRP at the terminal and performing a handover; a process of controlling transmission power of the first cell by comparing the number of terminals allocated to the first cell with the number of terminals capable of service; a process of requesting second cells adjacent to the first cell to adjust transmission power; and a process of re-measuring the RSRP on the terminal side and performing a handover.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for coordinating in wireless communication systems,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference adjusting method in a wireless communication system, and more particularly, to an interference adjusting method using a cell-zooming technique.

One of the functions supported by Long Term Evolution (LTE) communication standard is a self organizing network (SON), which is a technology for quickly performing functions of network configuration, optimization and restoration. Each function is subdivided so that the base station and the user terminal can perform efficient and stable operation itself, which minimizes the interference of the network element during operation and helps to maintain operation. Here, the self configuration is a function for establishing automatic connection and environment setting for the initial operation when a new base station is placed on the network. In addition, the self-optimization is a function for measuring the network state of a base station that is already in operation and maintaining optimal network load and quality. In addition, the self-restoration is a function for detecting a part where a problem occurs during operation so as to prevent a service interruption or to recover quickly.

Meanwhile, inter-cell interference is one of the factors affecting the communication performance of users in a cellular network. FIG. 1 is a schematic diagram for explaining inter-cell interference. FIG. 1 shows a situation in which a terminal of a particular cell receives not only a desired signal but also an interference signal from the base stations of adjacent cells in downlink communication. Inter-cell interference is stronger in the cell boundary region near the neighboring cells, which leads to cell boundary transmission degradation. To solve such inter-cell interference, a cell zooming technique can be used.

Cell zoning is a technique for increasing or decreasing a service range of a cell in accordance with a demand according to a network state or a communication state as shown in Fig. It is possible to efficiently distribute the communication load through the cell zooming and to use only the required transmission power and to save the power used by the entire system by not using the base station when there is a base station which does not need to be used . In other words, cell zooming has been developed as a technology to be used for a green cellular network for reducing power consumption of a system, and accordingly, the purpose and performance analysis of many studies have been focused on energy efficiency.

The cell zooming may be performed by adjusting the transmission power of the base station or adjusting the height and transmission angle of the base station antenna. For this, a coordination between a specific base station and a neighboring base station is required with the help of a cell-zooming server. In other words, implementation of cell zooming is limited by the fact that it needs to be supported by the network topology elements such as cell zooming server.

Korean Patent No. 10-1532436 Korean Patent No. 10-1173282

The present invention provides an interference coordination method in a wireless communication system that uses mutual cooperation among base stations so that user terminals providing services can communicate with each other optimally while focusing on the self optimization function of the SON function.

The present invention provides a method of adjusting interference in a wireless communication system that can mitigate inter-cell interference by using cell-zooming technology and thereby prevent transmission rate degradation of user terminals.

The present invention provides a method of coordinating interference in a wireless communication system using a distributed form of algorithm between base stations to facilitate cell zooming in SON.

An apparatus and method for interference coordination in a wireless communication system, in accordance with an aspect of the present invention, includes: (a) a process of finding a first cell in which a service failure occurs most frequently; (b) requesting a service from the first cell to neighboring second cells; (c) adjusting transmission power of terminals of a cell in which no service failure has occurred in the second cells; (d) re-measuring the RSRP at the terminal and performing handover; (e) adjusting transmission power of the first cell by comparing the number of terminals allocated to the first cell with the number of terminals capable of serving; (f) requesting second cells adjacent to the first cell to adjust transmission power; And (g) re-measuring the RSRP at the mobile station and performing a handover.

The above steps (a) to (g) are repeated until there is no cell in which a service failure has occurred.

The service failure cell includes a case where the specific terminal does not satisfy the target transmission rate and a case where the transmission rate of the terminal becomes 0 due to exceeding the number of terminals capable of serving at one time.

The step (c) adjusts the transmission power so that the terminals have a minimum transmission rate.

If the number of terminals allocated to the first cell is equal to or less than the number of available terminals, the step (e) includes the steps of: increasing the transmission power of the first cell by a predetermined value; And controlling transmission power of the first cell such that the terminal of the first cell has a minimum transmission rate when the number of available terminals exceeds the number of available terminals.

When the number of terminals allocated to the first cell exceeds the number of available terminals and the transmission rate of a predetermined terminal in the first cell exceeds a minimum transmission rate, the step of lowering the transmission power so that the predetermined terminal has a minimum transmission rate, And increasing the transmission power so that a predetermined terminal has a minimum transmission rate if the number of terminals allocated to the cell exceeds the number of serviceable terminals and the transmission rate of a predetermined terminal is less than a minimum transmission rate in the first cell.

The step (f) includes the steps of increasing the transmission power of the second cell by only a predetermined value if the number of terminals allocated to the second cell is equal to or less than the number of available terminals, And adjusting the transmission power of the second cell such that the terminal of the second cell has a minimum transmission rate when the number of available terminals exceeds the number of available terminals.

When the number of terminals allocated to the second cell exceeds the number of available terminals and the transmission rate of a predetermined terminal in the second cell exceeds a minimum transmission rate, the step of lowering the transmission power so that the predetermined terminal has a minimum transmission rate; And increasing the transmission power so that a predetermined terminal has a minimum transmission rate if the number of terminals allocated to the cell exceeds the number of serviceable terminals and the transmission rate of a predetermined terminal is less than a minimum transmission rate in the second cell.

The present invention finds a first cell in which a service failure has occurred most frequently and requests a service in a neighboring second cell in which a first cell in which a service failure has occurred most frequently causes transmission power of terminals of a cell in which no service failure occurs in second cells .

Except for the process of finding the cell with the most service failure, the communication process between the base stations is performed in a distributed fashion without central control. In addition, since the information exchanged between the base stations requires only the fact that a service failure has occurred in the corresponding cell, there is no great burden on the use of the backhaul.

Therefore, according to the present invention, even if the number of terminals in a cell increases, the probability of service failure due to the influence of interference is lowered.

1 is a schematic diagram for explaining inter-cell interference;
2 is a schematic view for explaining cell zooming;
3 is an RSRP receive schematic.
4 is a flow diagram of a method of coordinating interference in a wireless communication system in accordance with an embodiment of the present invention.
5 is an exemplary distribution diagram of a base station and a terminal.
FIGS. 6 to 8 are graphs illustrating a service failure probability according to the number of cells before and after cell zooming is applied using the method according to the present invention. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of other various forms of implementation, and that these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know completely.

The communication model of the present invention addresses downlink communication situations in a cellular network. In this case, neighbor base stations can exchange information necessary for operation through the backhaul link between the base stations. It is assumed that only a minimum amount of information necessary for cell-to-cell optimization can be delivered since the information conveyed through the backhaul may be limited. Therefore, the actually transmitted messages and channel coefficients are excluded from this information.

The user terminals measure RSRP (Reference Signals Received Power), which is the power intensity received from each base station. As shown in FIG. 3, the terminal receives a service of a base station that exhibits the strongest RSRP among the measured RSRP values, and recognizes RSRP from other base stations as interference. The RSRP values can be used to calculate the Signal to Interference plus Noise Ratio (SINR).

The method according to the present invention is centered on cells where service failures occur the most. Service failures are defined in the following two cases. 1) a particular terminal is located in an area that is difficult to service or geographically difficult to service due to interference, 2) the base station is unable to allocate more than one serviceable terminal at a time, The transmission rate of the terminal is 0. In this case, the service failure probability P _outage is calculated as a measure to evaluate the system.

In order to reduce the probability of service failure, the transmission power of the cell adjacent to the cell centered on the cell with the highest service failure is adjusted. To illustrate the method according to the present invention, some notations are defined as follows.

: 셀 i의 서비스를 받는 특정 단말기 k의 전송률

: The transmission rate of a specific terminal k receiving the service of the cell i

R _min : Minimum rate required by each terminal

P (i): transmission power of cell i

N (i): a set of cells adjacent to cell i

K: the number of terminals that the base station can service at one time

serve (i): number of terminals assigned to cell i

: : serve(i)>K인 셀 i에서 각 단말기의 각 전송률

을 내림차순으로 정렬했을 때 K번째 단말기의 전송률

:: serve (i)> k In each cell i in the cell i

In the descending order, the transmission rate of the Kth terminal

δ is any experimentally or empirically derived fixed value used to increase the transmitted power strength P (i) of i when a particular cell i is serve (i) ≤ K

FIG. 4 is a flowchart for explaining an interference adjusting method in a wireless communication system according to an embodiment of the present invention. Referring to FIG. 4, the present invention is described as follows.

))이 각 단말기가 필요로 하는 최소한의 전송률(R_min)보다 크거나 같은지 판단한다(min(

)≥R_min). 모든 셀 i가 min(

)≥R_min을 만족하여 서비스 실패가 일어나지 않은 셀이면 종료하고, 만족하지 않으면 서비스 실패가 가장 많이 일어난 셀 j를 찾는다(S120). 서비스 실패가 가장 많이 일어난 셀 j는 인접 셀들 l∈N(j)에 요청하여 이들 중 서비스 실패가 일어나지 않은 셀이 있다면 그 셀의 모든 단말기들이 R_min을 얻을 수 있을 정도로만 송출 전력을 낮추게 한다(S130, S140). 즉, 서비스 실패가 가장 많이 일어난 셀 j는 인접 셀들 l∈N(j)에 요청하여 min(

)≥R_min를 만족하는 셀이 있다면 그 셀의 모든 단말기들이 min(

)=R_min로 송출 전력을 낮추게 한다. 위 동작이 수행됐다면 단말기측에서의 RSRP의 재측정 및 핸드오버를 수행한다(S150). 한편, 서비스 실패가 가장 많이 일어난 셀 j는 인접 셀들 l∈N(j)에 요청하여 min(

)≥R_min를 만족하는 셀이 없다면 과정 S140 및 S150을 건너뛰어 S160으로 진행한다.First, it is determined whether all cells i are cells in which no service failure occurs (S110). That is, the minimum transmission rate (min (

) Is greater than or equal to the minimum transmission rate (R _min ) required by each terminal (min

) ≫ R _min ). If all cells i are in min (

If the cell satisfies R > _min and the service failure has not occurred, the process terminates. If not, the cell j having the most service failure is found (S120). The cell j having the highest service failure requests the neighboring cells l? N (j), and if there is a cell in which no service failure has occurred among all the cells, the transmission power is lowered to such an extent that all terminals of the cell can obtain R _min , S140). That is, the cell j which has the highest service failure requests the neighbor cells l∈N (j)

) If you have a cell that meets all the terminals of the cell ≥R _min to min (

) = R _min to lower the transmit power. If the above operation is performed, RSRP re-measurement and handover at the terminal side are performed (S150). On the other hand, the cell j that has the most service failures requests the neighbor cells l∈N (j)

) ≫ If there is no cell that satisfies R _min , the process proceeds to S160 by skipping steps S140 and S150.

>R_min이면

이 R_min을 얻을 정도로만 송출 전력 P(j)을 낮추게 하고, 셀 j에서 K번째 단말기의 전송률이 최소한의 전송률 이하이면, 즉

≤R_min이면

이 R_min을 얻을 정도로만 송출 전력 P(j)을 늘리게 한다(S170). 이제 j에 인접한 셀들 l∈N(j)에 요청하여 송출 전력 P(l)을 조절한다. 앞선 과정과 비슷한 과정을 반복하는데, 셀 l에 할당된 단말기의 개수가 K를 넘지 않는 경우(serve(l)≤K)(S180) l의 송출 전력에 δ만큼 더한다(P(l)=P(l)+δ)(S200). 그러나, K를 넘는 경우(serve(l)>K),

>R_min이면

이 R_min을 얻을 정도로만 송출 전력 P(l)을 낮추게 하고,

≤R_min이면

이 R_min을 얻을 정도로만 송출 전력 P(l)을 늘리게 한다(S190). 위 과정을 마치면 RSRP의 재측정 및 핸드오버를 수행하고(S220), 처음으로 돌아가 서비스 실패가 가장 많이 일어난 셀 j를 다시 찾는다. 이러한 과정을 모든 셀 i가 서비스 실패가 일어나지 않을 때까지 반복 실시한다.We now proceed to adjust the transmit power of cell j. First, the number of terminals allocated to cell j is compared with the number K of serviceable terminals (S160). (J) = P (j) +?) (S210) when the number of terminals allocated to cell j does not exceed K (serve (j)? K) j. However, if the transmission rate of the Kth terminal in the cell j exceeds the minimum transmission rate (i.e., serve (j) > K)

> R _min

R is lower the transmission power P (j) only enough to get _min and, when the rate of the K-th terminal in the cell j below the minimum rate, i.e.,

≪ = R _min

The transmission power P (j) is increased only to the extent that _Rmin is obtained (S170). We now request the cells l? N (j) adjacent to j to adjust the transmit power P (l). If the number of terminals allocated to the cell l does not exceed K (serve (l) K) (S180), the transmission power of 1 is added by delta (P (l) = P l) + [delta]) (S200). However, if K (serve (l) > K)

> R _min

The output power P (l) is lowered only to obtain _Rmin ,

≪ = R _min

The output power P (1) is increased only to the extent that _Rmin is obtained (S190). After completing the above procedure, RSRP performs remeasurement and handover (S220), and returns to the beginning to search cell j where the service failure has occurred the most. This process is repeated until all cells i fail to fail.

A series of processes of the interference adjusting method according to an embodiment of the present invention as described above will be summarized as follows.

1. Find the cell j where the service failure occurred most frequently. If there is no service failure, terminate the adjustment method.

)=R_min이 되도록 P(l)를 줄인다.2. If there is a cell l∈N (j) with no service failures in N (j), then min _k

) = R _min . ≪ / RTI >

3. Perform handover by measuring RSRP again.

4-1. If serve (j) ≤ K, add δ to P (j).

>R_min이면

=R_min이 되도록 P(j)를 줄인다. 그러나,

≤R_min이면

=R_min이 되도록 P(j)를 늘린다. 그리고, l∈N(j) 의 P(l)를 조정한다.4-2. If serve (j)> K

> R _min

Decrease P (j) to be R _min . But,

≪ = R _min

Increase P (j) to be R _min . Then, P (l) of l∈N (j) is adjusted.

4-2-1. If serve (l) ≤ K, add δ to P (l).

>R_min이면

=R_min이 되도록 P(l)를 줄인다. 그러나,

≤R_min이면

=R_min이 되도록 P(l)를 늘린다.4-2-2. If serve (l)> K

> R _min

Decrease P (l) to be R _min . But,

If? R_min

Let P (l) be equal to R _min .

5. Perform handover by measuring RSRP again.

6. Return to step 1.

The present invention is characterized in that the communication process between each base station is performed in a distributed manner without central control, except for the process of finding the cell with the most service failure. In addition, information exchanged between the base stations requires only a fact that a service failure has occurred in the corresponding cell, so there is no significant burden on the use of the backhaul.

The simulation was carried out by the method according to the present invention. In the simulation, a model in which terminals are distributed in Poisson Point Process (PPP) is used in cells arranged in a hexagonal grid shape. In the case of a model having 16 cells, an example of the distribution of the base station and the terminal is shown in FIG. The indicators used in the simulation are shown in [Table 1].

The average number of terminals per cell, λ 50, 60, 70, 80, 90, 100 Cell radius 200m Number of cells 16, 25, 64 Initial base station transmit power 50dBm Interference intensity -50dBm Total frequency width 10 ⁷ Hz Number of service terminals per base station, K 100 Frequency width used per terminal 10 ⁵ Hz The minimum transmission rate, R _min 20kbps Output power increase, δ 1 dB

In this simulation test, 1 dB was determined and the performance was observed. Smaller δ and more computation time can be used when terminals are densely distributed or when the delivery power intensity needs to be finer adjusted.

Also, the above method does not converge to the end point within a limited time, although the end point exists. This is because the terminal is located where the _Rmin can not be achieved on the system, or the number of terminals per cell is increased, so that there is not a bandwidth that can serve a specific terminal even after controlling the transmission power and handover. It is because it occurs. In such a case, the number of iterations of the above method should be limited to forced termination. In this simulation, the number of iterations is limited to a maximum of 25 repetitions.

Experimental results according to the number of 16, 25, and 64 cells are shown in FIGS. 6 to 8, respectively. Figures 6 to 8 show the service failure probability P _outage before and after applying cell zooming using the method according to the present invention. As the number of terminals in a cell increases, the probability of service failure increases due to the influence of interference, and the present invention has the effect of lowering the probability.

The present invention is not limited to the above-described embodiments, but may be embodied in various forms. In other words, the above-described embodiments are provided so that the disclosure of the present invention is complete, and those skilled in the art will fully understand the scope of the invention, and the scope of the present invention should be understood by the appended claims .

Claims (8)

(a) a process of finding a first cell in which a service failure occurs most frequently;
(b) requesting a service from the first cell to neighboring second cells;
(c) adjusting transmission power of terminals of a cell in which no service failure has occurred in the second cells;
(d) re-measuring the RSRP at the terminal and performing handover;
(e) adjusting transmission power of the first cell by comparing the number of terminals allocated to the first cell with the number of terminals capable of serving;
(f) requesting second cells adjacent to the first cell to adjust transmission power;
(g) re-measuring RSRP at the mobile station and performing a handover.
The method of claim 1, wherein the steps (a) to (g) are repeated until there is no cell in which a service failure has occurred.
The method of claim 2, wherein the service failure cell includes a case where the specific terminal does not satisfy the target transmission rate, and a case where the transmission rate of the terminal becomes zero due to the number of the terminals capable of serving at one time exceeding 0, Adjustment method.
3. The method of claim 2, wherein step (c) adjusts the transmission power so that the terminals have a minimum transmission rate.
The method of claim 2, wherein the step (e)
Increasing the transmission power of the first cell by a predetermined value when the number of terminals allocated to the first cell is equal to or less than the number of available terminals,
And adjusting a transmission power of the first cell such that a terminal of the first cell has a minimum transmission rate if the number of terminals allocated to the first cell exceeds the number of terminals capable of serving. .
The method of claim 5, wherein if the number of terminals allocated to the first cell exceeds the number of available terminals and the transmission rate of a predetermined terminal in the first cell exceeds a minimum transmission rate, Process,
When the number of terminals allocated to the first cell exceeds the number of serviceable terminals and the transmission rate of a predetermined terminal in the first cell is equal to or less than a minimum transmission rate, A method of adjusting interference in a system.
The method of claim 2, wherein the step (f)
If the number of terminals allocated to the second cell is equal to or less than the number of available terminals, increasing the transmitted power by only a predetermined value;
And adjusting a transmission power of the second cell such that a terminal of the second cell has a minimum transmission rate when the number of terminals allocated to the second cell exceeds the number of terminals capable of serving. .
The method of claim 7, wherein if the number of terminals allocated to the second cell exceeds the number of available terminals and the transmission rate of a predetermined terminal in the second cell exceeds a minimum transmission rate, Process,
When the number of terminals allocated to the second cell exceeds the number of available terminals and the transmission rate of a predetermined terminal is less than a minimum transmission rate in the second cell, the step of increasing the transmission power such that a predetermined terminal has a minimum transmission rate Method of adjusting interference in a communication system.

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