KR20050115077A - Mehtod to assign subchannel in ofdm system - Google Patents

Abstract

The present invention relates to a subchannel allocation method in an orthogonal frequency division multiplexing system. According to the present invention, when the number of subchannels is determined according to the frequency reuse rate and the number of users that can be accommodated in a cell, a service area within a cell of a user terminal to which the subchannels are allocated is received when a subchannel allocation request is received from a plurality of base stations. After grasping, the subchannels are differentially allocated according to the service area of the cell of the user terminal. In this case, the differential allocation of the subchannels allocates the subchannels with a low frequency reuse rate as the service area in the cell of the user terminal approaches the cell boundary, and the high frequency reuse rate as the service area in the cell of the user terminal approaches the base station. A differential subchannel allocation method for allocating the subchannels is used.

The present invention divides a service area of a cell into a plurality of virtual service areas, increases cell yield by using a high frequency reuse rate to a user terminal located inside the cell, and a user terminal located outside a cell having poor channel conditions. By using the low frequency reuse rate to ensure the performance, there is an effect of increasing the frequency efficiency while ensuring the quality of service at the cell boundary in an OFDM system in a multi-cell environment.

Description

Subchannel Allocation Method in Orthogonal Frequency Division Multiplexing System

The present invention relates to an Orthogonal Frequency Multiplexing (OFDM) communication method, and more particularly, to improve frequency efficiency while guaranteeing quality of service at a cell boundary in an OFDM communication system using a multi-cell environment. The present invention relates to a subchannel allocation method for differentiating subchannel allocation according to a user's cell location.

The OFDM modulation scheme is a communication scheme in which transmission data is distributed and transmitted to a plurality of subcarriers, and inter-channel interference is maintained by maintaining orthogonality between subcarriers of each channel. In an OFDM communication system using the OFDM modulation scheme, a subcarrier allocation plan is required to reduce the influence of intercell interference and improve throughput, and thus, the ability to use the same frequency between cells is required. Currently, an OFDM communication system that combines a frequency hopping scheme with an OFDM modulation scheme to reduce the influence of interference between cells and implements a frequency reuse rate 1 is considered.

However, an OFDM communication system that implements frequency reuse rate 1 has a problem in that performance deteriorates due to collision between frequency hopping patterns when traffic load increases. That is, in the OFDM communication system having a frequency reuse rate of 1, when the same subcarriers are used between cells, there is a problem of deteriorating performance by generating co-channel interference. This problem is exacerbated at cell boundaries with poor channel conditions.

As a result, when the limit of performance provided by the system is determined due to the interference between cells, it is inevitable to use a high frequency reuse rate to satisfy the performance of users at the cell boundary. However, if frequency reuse is taken into consideration of the boundary users of the cell, it is inefficient for the user of the user inside the cell, which has a relatively good signal-to-noise ratio compared to the boundary users of the cell, and there is a problem that a disadvantage in terms of overall yield is expected. .

Disclosure of Invention The present invention has been made to solve a conventional problem, and an object of the present invention is to provide a subchannel allocation method in an orthogonal frequency division multiplexing system that differentially distributes subchannels according to user's cell location.

The present invention for achieving the above object is a first step of receiving a sub-channel allocation request from a plurality of base stations in a state of determining the number of sub-channels corresponding to the frequency reuse rate and the number of acceptable users per cell, and the sub-channel Identifying a service area in a cell of the user terminal to which the allocation is to be performed; and a third step of differentially allocating subchannels according to the service area in the cell of the user terminal, wherein the subchannels are differentially allocated. The subchannel is allocated with a low frequency reuse rate as the service area in the cell of the user terminal is closer to the cell boundary, and the subchannel is allocated with a high frequency reuse rate as the service area in the cell of the user terminal is closer to the base station. A subchannel allocation method in an orthogonal frequency division multiplexing system is provided.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, detailed descriptions of related well-known functions or configurations will be omitted when it is determined that they may unnecessarily obscure the subject matter of the present invention. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators. Therefore, the definition should be made based on the contents throughout the specification.

1 is a frequency configuration diagram illustrating a frequency used in the OFDM communication system of the present invention with reference to a frequency axis. As shown in FIG. 1, a frequency allocated to an OFDM communication system, that is, a base station controller for a session, includes an effective subcarrier band A1, which is a subcarrier band used for transmitting data, and a subcarrier band for not transmitting data. It consists of (A2). Here, if the frequency reuse rate of the OFDM communication system is R, the OFDM communication system uses RxU subchannels (Sm, m = 1, 2) when the number of users available for each cell in the cluster is U. , 3, ..., RU). The one subchannel consists of a plurality of subcarriers. The subcarriers of the subchannels maintain orthogonality with the subcarriers of other subchannels. In FIG. 1, subcarriers of each subchannel do not mean physically adjacent but logically divided.

Hereinafter, a network environment to which the present invention is applied will be described with reference to FIGS. 2, 3, 4, and 5.

In general, a mobile communication system including an OFDM communication system divides a service area into cell units, and each cell includes a base station that is in charge of wireless communication with a user terminal entering a cell.

In such a cell environment, the present invention divides cells into n cell units and allocates subchannels to each n cell units. In this case, n cell units for allocating subchannels are one cluster, and n is determined by the network designer as an arbitrary value, but preferably at least two. As a result, the present invention uses an OFDM scheme of a multi-cell environment in which subchannel allocation is performed in cluster units. Here, using n cells in one cluster unit means that a specific device of a network allocates subchannels in units of n cells, that is, in units of n base stations. In general, since a base station controller is used as a network configuration device that is in charge of base station control and participates in channel allocation, it is preferable that a specific device of a network managing a base station in a cluster unit is a base station controller.

2 illustrates a multi-cell environment in accordance with an embodiment of the present invention to facilitate understanding of the present invention. In the multi-cell environment shown in FIG. 2, the cluster is composed of seven cell units.

On the other hand, the present invention considers the location of the user terminal in the cell in the sub-channel allocation as described above. The present invention divides an area within a cell into a plurality of areas in order to determine a location in a cell of a user terminal. At this time, the plurality of areas are virtually divided by the base station controller, the base station controller performs the area division within the cell according to a specific condition.

There are three specific conditions that are used by the base station controller for cell division.

1. The first is to use the distance between the user terminal and the base station. The base station controller has reference distance information between the user terminal and the base station to distinguish the area. In this case, the reference distance information is proportional to the number of regions in the cell. For example, if the area within the cell is 2, it has one reference distance information. If the area within the cell is 3, it has two reference distance information. If the area within the cell is 4, it has three reference distance information.

The base station controller determines the location of the user's cell by determining whether the distance between the user terminal and the base station is greater than or less than the reference distance when the reference distance is one, and the distance between the user terminal and the base station when the reference distance is 2 or more. The location within the cell of the user is determined by determining whether it is within a reference distance range.

Here, the distance measurement between the user terminal and the base station includes a method using a global position system (GPS) and a method of measuring propagation delay.

The method of using GPS is limited to a user terminal having a built-in GPS module. That is, the base station receives the current coordinate value of the user terminal from the user terminal having a built-in GPS module, and provides the received current coordinate value of the user terminal to the base station controller. The base station controller then measures the distance between the base station and the user terminal through the known position coordinates of the base station and the current coordinate values of the user terminal received from the base station.

The method of measuring propagation delay is called TDOA (Time Difference Of Arrival) method. In the TDOA method, a propagation time difference is measured which is proportional to the difference in distance from two base stations to a user terminal. Since the intersection of the hyperbolic lines where the distance difference is constant at the two base stations, that is, the focus of the two base stations, is the position of the user terminal, A method of determining the distance between a user terminal and a base station by measuring the distance from one base station.

3 illustrates an example of cell area division using a distance between a user terminal and a base station, and shows a case where a cell area is divided into two areas. In FIG. 3, M1 and M2 are user terminals, Ls is a reference distance, L1 is a distance of the user terminal M1, and L2 is a distance of the user terminal M2. 3, the user terminal M1 is located in the cell boundary region B (hereinafter, referred to as an 'outer cell') because L1 is larger than Ls, and the user terminal M2 is located inside the cell because L2 is smaller than Ls. That is, it is located in an area A (hereinafter referred to as an "internal cell") adjacent to a base station (BS).

2. The second is to use the strength of the signal received by the user from the base station. The base station controller has reference signal strength information for each cell of the cluster in charge thereof. As in the first case, the reference signal strength information increases in proportion to the number of regions in the cell, and the base station controller determines the location of the user's cell by comparing the received signal strength of the user terminal received from the base station with the reference signal strength. do. The strength of the signal received by the user from the base station (ie, the received signal strength of the user terminal) is generally the strength of the pilot beacon signal transmitted by the base station of the cell at each periodic time. As the pilot beacon signal is transmitted every cycle time, the user terminal measures the signal strength of the received pilot beacon signal every cycle time and informs the base station of this. Then, the base station compares the received signal strength of the user with the reference signal strength to determine the location of the user in the cell.

Here, it will be apparent to those skilled in the art that a user may use a signal to noise ratio that is proportional to the distance instead of the strength of the signal received from the base station.

4 shows an example of cell area classification using the strength of a signal received from a base station by a user, and shows a case where the cell area is divided into two areas. In FIG. 4, M1 and M2 are user terminals, Ps is reference signal strength, P1 is received signal strength of user terminal M1, and P2 is received signal strength of user terminal M2. 4, the user terminal M1 is located in the outer cell B because P1 is larger than Ps, and the user terminal M2 is located in the inner cell A because P2 is smaller than Ps.

3. The third is a combination of the first condition (distance) and the second condition (signal strength).

In the case of dividing the service area based on the signal strength, the improvement in yield can be increased based on the low signal strength, but as the number of internal cell users increases, performance deterioration at the cell boundary becomes serious. Therefore, there is a need for a method of varying the signal strength criteria according to the distance of the user, and the third is for this.

An operation according to a third condition for dividing an internal region of a cell through the combination of the distance and the signal strength will be described with reference to FIG. 5. 5 is a flowchart illustrating a process of determining a location in a cell of a user using distance and signal strength according to an exemplary embodiment of the present invention. In this case, the process of FIG. 5 is for a case where an area within a cell is divided into two areas (an outer cell and an inner cell).

Each base station of the cluster provides the base station controller with distance information between the user terminal and the base station and received signal strength information of the user terminal with respect to the user terminal located in the cell. The base station controller calculates the distance between the user terminal and the base station through the received information and determines the received signal strength of each user terminal (S510).

Hereinafter, the distance between the user terminal and the base station is referred to as Dm, and the received signal strength of the user terminal is referred to as Pm. The base station controller compares the Dm of the user terminal with the stored reference distance (S520).

In the comparison, if the Dm of the user terminal is smaller than the reference distance, it is determined that the user terminal is temporarily located in the inner cell (S530). If the Dm of the user terminal is larger than the reference distance, it is determined that the user terminal is temporarily located in the foreign cell. (S560).

When the base station controller primarily determines the location of the user terminal in the cell based on the distance between the user terminal and the base station as described above, the base station controller compares the reference signal strength according to the location of the determined user terminal in the cell, ie, whether it is an inner cell or an outer cell. Vary in size.

That is, the base station controller compares the reference signal strength for the internal cell comparison with the Pm of the user terminal when the user terminal is temporarily in the inner cell (S540). If the user terminal is temporarily in the outer cell, the base station controller receives the reference for the outer cell comparison. The signal strength is compared with Pm of the user terminal (S570).

Here, the reference signal strength for comparing internal cells is lower than the reference signal strength for comparing external cells.

As a result of the comparison (S540), the base station controller determines that the Pm of the user terminal is in the inner cell if the Pm of the user terminal is greater than the reference cell signal strength for internal cell comparison (S550), and the Pm of the user terminal is the reference cell for internal cell comparison If it is smaller than the strength, it is finally determined that the foreign cell (S580).

As a result of the comparison (S570), the base station controller determines that the Pm of the user terminal is in the inner cell if the Pm of the user terminal is greater than the reference cell signal strength for comparing the outer cell (S550), and the Pm of the user terminal receives the reference cell for the outer cell comparison If it is less than the signal strength, it is finally determined that the outer cell (S580).

Cell area division, that is, service area division, according to the third condition with reference to FIG. 5 has a disadvantage in that the complexity of the system is increased, but the increase in yield is greater than that in the case of separating the service areas based only on distance or signal strength. There is an advantage to losing.

Hereinafter, a subchannel allocation method in an orthogonal frequency division multiplexing system according to an embodiment of the present invention will be described with reference to FIGS. 6 to 8. 6 is a flowchart illustrating a subchannel allocation method in an orthogonal frequency division multiplexing system according to an embodiment of the present invention.

First, in the multi-cell environment to which the present invention is applied, it is assumed that one cluster is composed of seven cells and that the frequency reuse rate is 7. In addition, it is assumed that there are two concurrent users per cell, and it is assumed that a service area of each cell is divided into two areas.

If the frequency reuse rate is 7 and the number of concurrent users is 2, the base station controller divides the effective subcarrier band A2 shown in FIG. 1 into 14 subchannels S1, S2, S3, ..., S14 (S610). ).

In this state, if two simultaneous users are generated in each of seven cells forming a cluster as shown in FIG. 7, the base station controller receives a subchannel allocation request for each user terminal from each base station (S620). At this time, the subchannel allocation request includes condition information of one of the three specific conditions described above.

The base station controller determines the location of the user's cell through the specific condition of each user terminal (S630). The location in the cell of the user is a service area. 7, the base station controller determines that the user area U1, user U2, user U5, user U7, user U10, user U11, user U12, and user U14 are service cells. The user U3, the user U4, the user U6, the user U8, the user U9, and the user U13 determine that the service area is an internal cell.

The base station controller preferentially allocates one subchannel to the user terminal located in the outer cell among the divided subchannels (S640). That is, after determining the base station having the user terminal in the outer cell, the base station controller instructs each base station to allocate one different subchannel for the corresponding user terminal. 8, U1, U2, U5, U7, U10, U11, U12, and U14, which are user terminals located in the outer cell, have a total of eight sub-channels S1, S2, One of S5, S7, S10, S11, S12, and S14).

As described above, when one different subchannel allocation is made to each of the user terminals located in the outer cell (S650), the base station controller determines the remaining six subchannels except for the subchannels allocated to the user terminals located in the outer cell. As shown in FIG. 8, the subchannels are commonly allocated to U6, U8, U9, and U13, which are user terminals located in an inner cell. However, since the user U3 and U4 are located in the inner cell of the same cell, the six subchannels are divided into two parts and allocated in order to maintain orthogonality between subcarriers (S660).

According to the above-described embodiment of the present invention, the present invention basically allocates one subchannel to a user terminal located in an outer cell, and then assigns the remaining unassigned subchannels to a user terminal located in an inner cell. When there are a plurality of user terminals in an inner cell of the same cell, the allocation is evenly allocated to have different subchannels.

Meanwhile, in the embodiment of the present invention with reference to FIGS. 6 to 8, the service area of the cell is divided into two areas. However, as described above, the present invention may divide the service area of the cell into two or more areas. This is to further subdivid the service area to differentially provide subchannels to user terminals located in two or more service areas. In this case, the present invention provides one subchannel for the user terminal located in the outermost service area, provides two subchannels for the user terminal located in the next outer service area, and then services Different subchannels can be allocated to a user terminal located in an area by providing three subchannels.

The technical spirit of the present invention has been described above with reference to the accompanying drawings, but this is by way of example only and not intended to limit the present invention. In addition, it is obvious that any person skilled in the art can make various modifications and imitations without departing from the scope of the technical idea of the present invention.

The present invention divides a service area of a cell into a plurality of virtual service areas, increases cell yield by using a high frequency reuse rate to a user terminal located inside the cell, and a user terminal located outside a cell having poor channel conditions. By using the low frequency reuse rate to ensure the performance, there is an effect of increasing the frequency efficiency while ensuring the quality of service at the cell boundary in an OFDM system in a multi-cell environment.

1 is a frequency configuration diagram showing the frequency used in the OFDM communication system with respect to the frequency axis.

2 is a state diagram of a multi-cell environment to which the present invention is applied.

3 is a state diagram of a cell that divides a service area of a cell using a distance between a user terminal and a base station according to the present invention.

4 is a state diagram of a cell that divides a service area of a cell using a signal strength received from a base station according to the present invention.

5 is a flowchart illustrating a process of determining a location in a cell of a user using distance and signal strength according to an exemplary embodiment of the present invention.

6 is a flowchart illustrating a subchannel allocation method in an orthogonal frequency division multiplexing system according to an embodiment of the present invention.

7 is a view showing a service area and a simultaneous user distribution of each cell of a cluster according to an embodiment of the present invention.

FIG. 8 illustrates subchannel allocation for each user terminal according to the distribution state of FIG. 7. FIG.

Claims (12)

With the number of subchannels corresponding to the frequency reuse rate and the number of acceptable users per cell determined, A first step of identifying a location in a cell of each user terminal accommodated in N cells forming a cluster; A second step of determining a service area in a cell to which the user belongs, based on the location in the cell of the user terminal identified above; And Based on the service area information to which each user terminal belongs, the subchannel is allocated at the lowest frequency reuse rate among the frequency reuse rates applied to the user terminal of the first service area closest to the cell boundary. And a third step of allocating the subchannels with the highest frequency reuse rate among the frequency reuse rates applied to the user terminal of the second service area closest to the base station. Channel assignment method. The method of claim 1, In the third step, the subchannel is allocated to the user terminal of the first service area first, and then the subchannel is allocated to the user terminal of another service area using the remaining subchannels. A subchannel allocation method in an orthogonal frequency division multiplexing system. The method according to claim 1 or 2, In the third step, when two or more user terminals exist in the same service area of the same cell when the subchannels are allocated to the user terminal of the service area to which a frequency usage rate higher than that used in the first service area is applied, the third channel is allocated. A subchannel allocation method in an orthogonal frequency division multiplexing system, characterized in that subchannels are equally distributed by the number of user terminals and allocated to each user terminal. The method of claim 3, The sub-channel allocation method in the orthogonal frequency division multiplexing system, wherein the frequency reuse rate applied to the first service area is 1. The method of claim 4, wherein The second step is a sub-channel allocation method in the orthogonal frequency division multiplexing system characterized in that the service area of the terminal is identified by comparing the distance between the base station and the user terminal with a preset stored reference distance. The method of claim 4, wherein The distance between the base station and the user terminal is calculated by using the current coordinate information of the user terminal provided by the user terminal with a built-in GPS module and the base station coordinate information of the fixed coordinate value in the quadrature frequency division multiple system Channel assignment method. The method of claim 4, wherein Subchannel allocation in an orthogonal frequency division multiplexing system characterized in that the distance between the base station and the user terminal is calculated through a time difference of arrival (TDOA) method for measuring the distance using the arrival delay time of the radio waves reaching the two base stations. Way. The method of claim 4, wherein In the second step, the sub-channel allocation in the orthogonal frequency division multiplexing system is to identify the service area of the terminal by comparing the strength of a signal periodically received from the base station by the user terminal with a preset reference received signal strength. Way. The method of claim 4, wherein In the second step, the sub-channel allocation method of the orthogonal frequency division multiplexing system is characterized in that the service area of the terminal is determined by comparing the signal-to-noise ratio of the user terminal with a reference signal-to-noise ratio. The method of claim 4, wherein In the second step, in the orthogonal frequency division multiplexing system, the service area in the cell of the user terminal is determined by mixing the strength of a signal periodically received from the base station by the user terminal and the distance between the user terminal and the base station. Subchannel allocation method. The method of claim 10, In the second step, when the service area in the cell is divided into an inner cell close to a cell boundary and an outer cell close to a base station, A first substep of determining a distance between the user and the base station and a strength of a signal periodically received by the user terminal from the base station; A second sub-step comparing the distance between the user and the base station with a preset stored reference distance, and temporarily determining that the internal cell is within the reference distance, and temporarily determining that the external cell is greater than the reference distance; In the case of the tentative inner cell, the user terminal periodically compares the strength of a signal periodically received from the base station with an internal reference received signal strength, and if it is larger than the reference received signal strength for the internal cell, finally determines that it is located in the internal cell and uses the internal cell. A third sub-step, which finally determines that it is located in an outer cell if it is smaller than the reference received signal; In the case of the provisional foreign cell, the user terminal periodically compares the strength of a signal periodically received from the base station with an external reference signal strength, and finally determines that the user terminal is located in an internal cell if it is greater than the reference signal strength for the external cell. If it is smaller than the reference received signal includes a fourth sub-step that finally determines that it is located in the outer cell, And the reference received signal strength for the inner cell is lower than the reference received signal strength for the outer cell. On a computer connected to the IP network, A first step of identifying a location in a cell of each user terminal accommodated in N cells forming a cluster; A second step of determining a service area in a cell to which the user belongs, based on the location in the cell of the user terminal identified above; And Based on the service area information to which each user terminal belongs, the subchannel is allocated at the lowest frequency reuse rate among the frequency reuse rates applied to the user terminal of the first service area closest to the cell boundary. A computer program for recording a program for executing the third step of allocating the subchannels at the highest frequency reuse rate among the frequency reuse rates applied to the user terminal of the second service area closest to the base station. Recordable media.