KR20120020539A - Method for frequency reuse with power control in orthogonal frequency division multiple access systems - Google Patents

Abstract

PURPOSE: A power controlled frequency reuse method in OFDMA(Orthogonal Frequency Division Multiple Access) system is provided to allocate a segment of subcarrier set to each cell following a predetermined order, thereby reducing co-channel interference between cells. CONSTITUTION: Subcarriers in a system bandwidth is divided into M sub-channel set(S210). The sub-channel set is divided into M segments(S220). The segments are classified into a plurality of classes(S230). A transmission power is differentially allocated following the classes(S240). A segment allocation sequence is determined by the class which is determined by a cell type and a channel state of a user terminal A subcarrier is allocated to the user terminal according to a determined segment allocation sequence(S250).

Description

TECHNICAL FOR FREQUENCY REUSE WITH POWER CONTROL IN ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS SYSTEMS

The present invention relates to a frequency reuse method, and more particularly, power control sequential frequency reuse (SqFR) for improving performance of a cell edge user in an orthogonal frequency division multiple access (OFDMA) system. ) Method.

In a cellular environment based on Orthogonal Frequency Division Multiple Access (OFDMA), the performance of the system is highly dependent on the location of the user. Frequency reuse factor that reuses the entire frequency band in all cells. In the case where FRF) is 1, the user at the cell boundary experiences severe co-channel interference (CCI) because the transmission power received from the base station is significantly low. As a result, the transmission speed of the user is greatly reduced, and the user is not provided with a satisfactory quality service. In order to solve the interference problem between the cells, techniques such as a frequency reuse strategy using a FRF of 3 and a partial frequency reuse (FFR) technique have been proposed.

) 는 셀 중앙 주파수 대역의 크기를 결정하게 된다. FFR은 인접한 셀 간에 셀 경계 주파수 대역을 겹치지 않게 할당함으로써 셀 경계 사용자의 성능을 향상시키게 된다. 그런데, 셀 중앙 주파수 대역이 모든 셀에서 재사용되어 시스템의 용량을 증가시키기는 하지만, 각 셀은 타입에 따라 특정한 셀 경계 주파수 대역만을 사용하도록 허용되기 때문에, 사용 가능한 주파수 대역에 제한이 생긴다. 따라서, 각각의 셀은 셀 경계 주파수 대역의 1/3만 사용할 수밖에 없는 문제점이 발생한다. 1 is a diagram illustrating a frequency division and allocation method in a conventional FFR scheme. FIG. 1 illustrates an example of frequency division when three cell types are used. Hereinafter, a method of allocating a frequency to a user will be described. Prior art FFR divides the entire frequency band into two parts. One portion is the cell boundary frequency band assigned to the cell edge user, and the other portion is the cell center frequency band assigned to the cell center user. The cell center frequency band is commonly reused in all cells in the system, and the cell boundary frequency band is partially reused depending on the type of cell. Ratio of cell center frequency band to total frequency band (

) Determines the size of the cell center frequency band. The FFR improves the performance of cell boundary users by allocating non-overlapping cell boundary frequency bands between adjacent cells. By the way, although the cell center frequency band is reused in all cells to increase the capacity of the system, each cell is allowed to use only a specific cell boundary frequency band depending on the type, which places limitations on the usable frequency band. Accordingly, a problem arises in that each cell can only use one third of the cell boundary frequency band.

The technical problem to be solved by the disclosed technology is to provide a power control sequential frequency reuse (SqFR) method for improving the performance of a cell boundary user of an orthogonal frequency division multiple access system. The present invention reduces the co-channel interference between cells by allocating segments, which are sets of subcarriers, to each cell in a predetermined order while efficiently utilizing the entire frequency resource with a frequency reuse coefficient (FRF) of 1. In addition, the present invention provides a frequency reuse method capable of effectively improving the performance of a cell edge user by allocating high transmit power to a subcarrier allocated to the cell edge user.

In order to achieve the above technical problem, a first aspect of the disclosed technology is a method of reusing a frequency by a base station in an Orthogonal Frequency Division Multiple Access (OFDMA) system in which cells in a system are classified into M types. Dividing subcarriers in a system band into the M subchannel sets and dividing each of the subchannel sets into the M segments; Classifying the segments into a plurality of classes and differentially allocating transmit power according to the classes; And determining a segment allocation sequence based on a type of a cell in which a user terminal is located and a class determined according to a channel state of the user terminal, and allocating a subcarrier to the user terminal according to the determined segment allocation sequence. It provides a frequency reuse method.

Embodiments of the disclosed technology can have the effect of including the following advantages. However, the embodiments of the disclosed technology are not meant to include all of them, and thus the scope of the disclosed technology should not be understood as being limited thereto.

According to an embodiment of the disclosed technology, the frequency reuse method reduces co-channel interference (CCI) and allocates high transmit power to subcarriers allocated to cell edge users, thereby improving performance of cell edge users. It is effective to let.

1 is a diagram illustrating a frequency division and allocation method in a conventional FFR scheme.
2 is a flowchart illustrating a power control frequency reuse method according to an embodiment of the disclosed technology.
3 is a diagram illustrating an example of frequency division and subcarrier allocation sequences when three cell types are used (M = 3) according to an embodiment of the present invention.
4 is a diagram illustrating an example of frequency division and subcarrier allocation sequences when four cell types (M = 4) according to an embodiment of the present invention.
FIG. 5 is a graph showing the overall throughput of cell i of each frequency reuse method in the uniform traffic load distribution when M = 3.
6 is a graph showing the overall throughput of cell i of each frequency reuse method in the uniform traffic load distribution when M = 4.
7 is a graph showing the overall throughput of the cell edge users of the cells of each frequency reuse method in the uniform traffic load distribution.

The description of the disclosed technique is merely an example for structural or functional explanation and the scope of the disclosed technology should not be construed as being limited by the embodiments described in the text. That is, the embodiments may be variously modified and may have various forms, and thus the scope of the disclosed technology should be understood to include equivalents capable of realizing the technical idea.

On the other hand, the meaning of the terms described in the present application should be understood as follows.

The terms " first ", " second ", and the like are used to distinguish one element from another and should not be limited by these terms. For example, the first component may be named a second component, and similarly, the second component may also be named a first component.

When a component is referred to as being "connected" to another component, it should be understood that there may be other components in between, although it may be directly connected to the other component. On the other hand, when an element is referred to as being "directly connected" to another element, it should be understood that there are no other elements in between. On the other hand, other expressions describing the relationship between the components, such as "between" and "immediately between" or "neighboring to" and "directly neighboring to", should be interpreted as well.

Singular expressions should be understood to include plural expressions unless the context clearly indicates otherwise, and terms such as "include" or "have" refer to features, numbers, steps, operations, components, parts, or parts thereof described. It is to be understood that the combination is intended to be present, but not to exclude in advance the possibility of the presence or addition of one or more other features or numbers, steps, operations, components, parts or combinations thereof.

Each step may occur differently from the stated order unless the context clearly dictates the specific order. That is, each step may occur in the same order as specified, may be performed substantially simultaneously, or may be performed in the reverse order.

All terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed technology belongs, unless otherwise defined. Terms defined in commonly used dictionaries should be interpreted to be consistent with meaning in the context of the relevant art and can not be construed as having ideal or overly formal meaning unless expressly defined in the present application.

According to an embodiment, in an orthogonal frequency division multiple access (OFDMA) system in which cells in a system are classified into M types, a frequency reuse factor (FRF) of each cell is 1; Is assigned to. Each cell has a FRF of 1, so that all subcarriers are reused and allocated in all cells.

2 is a flowchart illustrating a power control frequency reuse method according to an embodiment of the disclosed technology.

In step S210, the subcarriers in the system band are divided into M subchannel sets. In this case, each subchannel set includes a plurality of subchannels. In this case, if the total number of subcarriers is N, the subcarriers of the cell are indexed from 0 to N-1. Subsequently, M subchannels are divided according to index numbers corresponding to subcarriers. Subchannel division uses Equation 1.

는 k번째 부 반송파이고,

는 i 번째 부 채널 집합,

는

이다.

는

를 넘지 않는 최대 정수이다. In Equation 1,

Is the kth secondary bounce pie,

Is the i th subchannel set,

Is

to be.

Is

Maximum integer not exceeding.

In step S220, the subchannel set is divided into M segments. At this time, each segment is divided using Equation 2.

는 i번째 부 채널 집합 중에서 j번째 세그먼트이고,

는

이다. here,

Is the jth segment of the i th subchannel set,

Is

to be.

In step S230, the segments are classified into a plurality of classes. The plurality of classes include a boundary user class and a central user class, and the boundary user class performs a modular M operation on each segment to assign an index. The boundary user class and the central user class are expressed as in Equation 3.

는, 경계 사용자 클래스이고,

는 중앙 사용자 클래스이다. F는 부 채널 집합 전체를 뜻한다. 한편, 사용자 단말이 복수의 클래스 가운데 어느 클래스에 해당되는지에 대한 결정은, 상기 사용자 단말의 신호 대 간섭 및 잡음비가

(

은 문턱 SINR(Signal-to-Interference plus Noise Ratio; SINR))과 같거나 크면 상기 중앙 사용자 클래스로 결정하고, 상기 사용자 단말의 신호 대 간섭 및 잡음비가

보다 작으면 상기 경계 사용자 클래스로 결정한다. here

Is the boundary user class,

Is the central user class. F means the entire subchannel set. On the other hand, the determination of which class among the plurality of classes the user terminal, the signal-to-interference and noise ratio of the user terminal is

(

Is equal to or greater than the threshold Signal-to-Interference plus Noise Ratio (SINR), and is determined as the central user class, and the signal-to-interference and noise ratio of the user terminal is

If smaller, the boundary user class is determined.

클래스의 부 반송파의 송신 전력은

이다. 타입 i셀에서 셀 경계 클래스에 해당하는 사용자 단말에게 할당되는

클래스의 부 반송파의 전력은

이다. 여기서

는 전력 증폭 계수(PAF, Power Amplification Factor)로서, 셀 경계 클래스에 해당하는 사용자 단말에 할당되는 부 반송파에 더 높은 송신 전력을 할당 함으로서 SqFR은 셀 경계 사용자의 서비스 품질을 향상시킨다.In step S240, the transmission power is differentially allocated according to the classified class. Assigned to the user terminal corresponding to the cell center class in the type i cell

The transmit power of the subcarrier of the class

to be. Assigned to a user terminal corresponding to a cell boundary class in a type icell

The power of the subcarriers of the class

to be. here

SqFR is a power amplification factor (PAF). By assigning a higher transmission power to a subcarrier allocated to a user terminal corresponding to a cell boundary class, SqFR improves the quality of service of a cell boundary user.

In step S250, the segment allocation sequence is determined based on the class determined according to the type of the cell in which the user terminal is located and the channel state of the user terminal, and the subcarrier is allocated to the user terminal according to the determined segment allocation sequence.

는 세그먼트 할당 시퀀스 0≤i<M의 일때, 각각의 셀 타입별로 정해진 클래스에 따라 부 반송파를 수학식 4에 의한 순서에 따라 각각 사용자 단말에 할당한다.

When the segment allocation sequence 0 ≦ i <M, the subcarriers are allocated to the user terminals according to the equation (4) according to the class determined for each cell type.

는 i번째 셀 타입의 경계 사용자 클래스에 속하는 사용자 단말의 시퀀스를

로, i번째 셀 타입의 중앙 사용자 클래스에 속하는 사용자 단말의 시퀀스를

로 결정하는 순서이다. More specifically, segment allocation sequence

Is a sequence of user terminals belonging to the boundary user class of the i-th cell type.

A sequence of user terminals belonging to the central user class of the i-th cell type

The order is decided.

={S_{0 ,0}, S_1,0, S_{2 ,0}}과

={S_{0 ,1}, S_{0 ,2}, S_{1 ,1}, S_{1 ,2}, S_{2 ,1}, S_{2 ,2}}로 표현된다. 각각의 셀 타입에 대하여, 부 반송파는 수학식 4에 의하여 다음과 같이 순차적으로 할당된다. 3 is a diagram illustrating an example of frequency division and subcarrier allocation sequences when three cell types are used (M = 3) according to an embodiment of the present invention. Each subchannel set is represented by F ₀ , F ₁ , and F ₂ , and each segment is S _{0 , 0} , S _{0 , 1} , S _{0 , 2} , S _{1 , 0} , S _{1 , 1} , S _{1 , 2} , S _{2 , 0} , S _{2 , 1} , and S ₂ , ₂ . The arrow of the subchannel allocation in the figure indicates the subcarrier allocation sequence of each subcarrier allocation sequence type. If the cell type is 0, each class

= {S _{0 , 0} , S _1,0 , S _{2 , 0} }

= {S _{0 , 1} , S _{0 , 2} , S _{1 , 1} , S _{1 , 2} , S _{2 , 1} , S _{2 , 2} }. For each cell type, subcarriers are sequentially assigned as follows by equation (4).

When a cell boundary user requests a new resource for a type 0 cell, firstly, a subcarrier in S _0,0 is sequentially assigned to the user terminal. If all subcarriers in S _{0 , 0} are assigned to the cell edge user, the subcarriers in S _{1 , 0} are allocated to the cell edge user for new incoming resource requests. And S _{0, 0} and S _1, after all the sub-carriers of ₀ is allocated to the cell edge users are assigned sub-carriers of S _{2, 0} to cell edge users. Similarly, when a cell central user terminal requests a new resource, firstly, a subcarrier in S _{0 , 1} is sequentially assigned to the cell central user terminal _, and a subcarrier in S _{0 , 1} is assigned to the cell central user terminal. When all are allocated, the subcarriers in S _{0 , 2} , S _{1 , 1} , S _1,2 , S _{2 , 1} and S _{2 , 2} , are sequentially assigned to the cell central user. Subcarriers are also assigned to cells of type 1 and type 2 in the same process as the above method.

4 is a diagram illustrating an example of frequency division and subcarrier allocation sequences when four cell types (M = 4) according to an embodiment of the present invention. In this case, the subcarriers are allocated to the user terminal in the same manner as the case of the three cell types (M = 3).

가 0.5인 FFR과 FRF가 1인 보편적인 주파수 재사용 방법(Universal Frequency Reuse, 이하UFR)을 고려하도록 한다. 이때,

는 FFR에서 셀 중앙 주파수 대역 대 전체 주파수 대역의 비를 나타내는 파라미터로서 0과 1 사이의 값을 갖는다. To compare the performance of SqFR according to one embodiment of the disclosed technology

Consider a FFR of 0.5 and a Universal Frequency Reuse (UFR) of FRF 1. At this time,

Is a parameter representing the ratio of the cell center frequency band to the total frequency band in the FFR and has a value between 0 and 1.

Hereinafter, the results of simulating the performance of the system according to the SqFR, FFR and UFR applied according to the present invention.

The number of cells is set to 19, the distance between base stations is 1 km, and the transmission power of the base stations is set to 20W. In addition, the user is generated evenly in each cell, and Table 1 shows other simulation parameters.

Parameter Value Carrier frequency 2.3 GHz Diameter of the cell 577 m Sampling frequency 10 MHz FFT size 1024 Number of data subcarriers 768 Symbol rate 9.76 ksymbols / sec Noise density -174 dBm / Hz

)Shadowing standard deviation (

) 8 dB

Considering the carrier frequency and cell radius, the COST-WI urban micromodel with shadowing effect is applied to the channel model.

는 평균이 0이고 분산이

인 실외 쉐도우잉을 나타낸다. 또한, 모듈레이션 스킴과 에러 정정 코드는 CINR에 의해 결정된다. 표 2는 SqFR과 FFR 및 UFR에 대한 모듈레이션 및 코딩 스킴(MCS) 테이블을 나타낸다.Where d is the distance between the base station and the user, and

Is 0 and the variance is

Outdoor shadowing. In addition, the modulation scheme and the error correction code are determined by the CINR. Table 2 shows the Modulation and Coding Schemes (MCS) table for SqFR and FFR and UFR.

Modulations Code rate CINR QPSK 1/12 -4.34 QPSK 1/8 -2.80 QPSK 1/6 -1.65 QPSK 1/4 0.13 QPSK 1/3 1.51 QPSK 1/2 4.12 QPSK 2/3 6.35 16QAM 1/2 9.50 16QAM 2/3 12.21 64QAM 1/2 13.32 64QAM 2/3 16.79 64QAM 5/6 20.68

FIG. 5 is a graph showing the overall throughput of cell i of each frequency reuse method in the uniform traffic load distribution when M = 3. 6 is a graph showing the overall throughput of cell i of each frequency reuse method in the uniform traffic load distribution when M = 4. As shown in Figures 5 and 6, in a uniform traffic load distribution, SqFR has more improved performance at medium and low traffic loads. As the traffic load increases, the SqFR plot becomes smaller at each critical point as the secondary carriers reused in each cell are duplicated. That is, as shown in FIG. 5, when M = 3, the threshold of SqFR appears when the traffic loads are 0.33 and 0.66. At 0.33 traffic load, when the PAF was changed from 3 to 1, SqFR improved the throughput by 24% to 34% compared to UFR, and increased the throughput by 14% to 24% compared to FFR. In addition, as shown in FIG. 6, when M = 4, the threshold of SqFR appears when the traffic load is 0.25, 0.5, and 0.75. At a traffic load of 0.25, when the PAF was changed from 3 to 1, SqFR improved throughput from 32% to 40% compared to UFR, and increased throughput from 23% to 30% compared to FFR. It can be seen that throughput performance is better when M = 4 than when M = 3, because the number of subcarriers allocated to the cell center user when M = 4 is larger than when M = 3. On the other hand, the FFR saturates the throughput when the traffic load is 0.66, which is a result of the limitation of the available frequency band.

7 is a graph showing the overall throughput of cell edge users of cells of each frequency reuse method in uniform traffic load distribution. As the PAF increases, the overall throughput of SqFR decreases insignificantly, but as shown in FIG. 7, the throughput of the cell boundary user is significantly improved. As PAF increases, SqFR significantly improves throughput compared to FFR and UFR. On the other hand, the SqFR according to the present invention has a higher throughput when M = 4 because the transmission power of the subcarrier allocated to the cell edge user is higher than when M = 3.

The disclosed method and apparatus have been described with reference to the embodiments illustrated in the drawings for ease of understanding, but these are merely exemplary, and various modifications and equivalent other embodiments are possible to those skilled in the art. Will understand. Therefore, the true technical protection scope of the disclosed technology should be defined by the appended claims.

Claims (8)

A method for reusing a frequency by a base station in an orthogonal frequency division multiple access (OFDMA) system in which cells in a system are classified into M types,
Dividing subcarriers in a system band into the M sub-channel sets and dividing each of the sub-channel sets into the M segments;
Classifying the segments into a plurality of classes and differentially allocating transmit power according to the classes; And
Determining a segment allocation sequence based on a type of a cell in which a user terminal is located and a class determined according to a channel state of the user terminal, and allocating a subcarrier to the user terminal according to the determined segment allocation sequence; Frequency reuse method. The method of claim 1, wherein the segment allocation sequence is
The segments included in the segment allocation sequence are different according to the class,
Frequency reuse method according to the type of the cell is a different sequence of segments allocated by the segment allocation sequence. The method of claim 1, wherein the dividing comprises:

(here,

Is the kth subcarrier,

Is the i th subchannel set

Is

,

Is

,

Is

Dividing the subcarriers in the system band into M subchannel sets by a maximum integer not exceeding); And
Each of the subchannel sets

(

Is partitioned into M segments by the j-th segment of the i-th subchannel set. The method of claim 1, wherein the plurality of classes,
A boundary user class and a central user class, wherein the boundary user class comprises:

(

Is the boundary user class,

Means index allocation via modular M operation),
The central user class is

(

Is the central user class,

Is a full set of subchannels). The method of claim 4, wherein
The transmit power of the subcarrier of the central user class is

The power of the subcarrier of the boundary user class is

(

Is a power amplification factor). The method of claim 1, wherein the plurality of classes include a boundary user class and a central user class,
Allocating the subcarrier,
The signal-to-interference and noise ratio of the user terminal

(

Is equal to or greater than the threshold Signal-to-Interference plus Noise Ratio (SINR), and is determined as the central user class, and the signal-to-interference and noise ratio of the user terminal is

Frequency reuse method that determines the boundary user class if smaller. The method of claim 1, wherein assigning the subcarrier comprises:
Segment allocation sequence

(I is 0≤i <M),
Sequence of user terminals belonging to the boundary user class of the i-th cell type

in,
Sequence of user terminals belonging to the central user class of the i-th cell type

Decide how to reuse the frequency. The method according to claim 7, wherein when the M value is 3,
Each segment

For each cell type,

(

,

And

(B) assigns the subcarriers of the segment to each user terminal according to the order of segment allocation sequence type when i values are 0, 1 and 2, respectively.

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