KR20100003395A - Method for transmitting cell information in wireless communication system - Google Patents

Abstract

PURPOSE: A cell information transfer method in a wireless communications system is provided to perform the transmission power control of a terminal according to a sub band, thereby reducing efficiently intercell interference. CONSTITUTION: A terminal receives a cell list about an adjacent cell from a serving BS(Base Station)(S110). The terminal receives cell interference information from an adjacent BS through a cell information control channel(S120). Based on the cell interference information from the adjacent BS, the terminal controls uplink transmission power(S130). The terminal reports adjacent cell information to the serving BS through the uplink control channel(S140). The serving BS performs resource allocation with the adjacent cell information from the terminal(S150).

Description

Cell information transmission method in a wireless communication system {Method for transmitting cell information in wireless communication system}

The present invention relates to wireless communication, and more particularly, to a method for transmitting intercell control information.

The next generation multimedia wireless communication system, which is being actively researched recently, needs to process various information such as video, wireless data, etc. at a higher data rate beyond the initial voice-oriented service.

Orthogonal Frequency Division Multiplexing (OFDM), which can have a high data rate, has recently attracted attention. OFDM is a multi-carrier modulation technique for transmitting data by dividing a frequency band into a plurality of orthogonal subcarriers. Orthogonal frequency division multiple access (OFDMA) is a technique for providing multiplexing of multiple users by combining frequency division multiple access (FDMA) or time division multiple access (TDMA) or code division multiple access (CDMA) with OFDM.

The wireless communication system includes a base station (BS) and at least one user equipment (UE). The wireless communication system has a cell structure for efficient system configuration. A cell is a subdivision of a large area into smaller areas in order to use frequency efficiently. In general, a base station is installed in the center of a cell to relay a terminal, and a cell refers to a service area provided by one base station.

In a multi-cell environment, adjacent cells of an OFDM / OFDMA system may use the same subcarrier, and thus inter-cell interference may occur. Inter-cell interference is severely generated by terminals near the boundary of the cell. In order to reduce inter-cell interference, different subcarriers may be used between adjacent cells, but this reduces radio resources that can be used by one base station. As another method for reducing inter-cell interference, uplink transmit power of the UE may be controlled. In general, when the uplink signal state of the terminal is not good due to interference or topography, the base station instructs the terminal to increase the transmission power to control the uplink transmission power. When the transmission power control is performed based on the SINR (Signal to Interference plus Noise Ratio) of the uplink signal without using the interference situation or the information on the degree of interference provided from the neighbor cell, the uplink transmission power of the UE The maximum transmit power can be increased to increase interference to adjacent cells.

In order to maintain the maximum channel quality state to the terminal without increasing the inter-cell interference, it is preferable that the base stations share information about the inter-cell interference with each other and use it for uplink transmission power control of the terminal.

One method of sharing interference information between cells is to use a back-bone network. Inter-cell interference information such as an overload indicator or a high interference indicator may be exchanged between neighboring base stations using the backbone network. That is, each cell can share whether or not there is an overload due to interference from neighboring cells through the backbone network. When inter-cell interference information is shared using a backbone network, the data amount is not limited, but data delay may occur due to routing of data between base stations, and whether terminals are actually involved in inter-cell interference. I can not know.

Another method of sharing inter-cell interference information is a method in which a UE transmits inter-cell interference information through an uplink control channel. The terminal transmits the interference information of the serving cell (serving cell) that is receiving the service to the neighbor cell. When the terminal transmits the interference information of the serving cell to the neighbor cell, the data delay is less than when using the backbone network. However, there is a problem that the terminal consumes a lot of power for transmitting interference information, an uplink control channel must be allocated between a base station of a neighboring cell and the terminal, and a large amount of data cannot be transmitted through the uplink control channel.

There is a need for a method that can effectively reduce inter-cell interference.

An object of the present invention is to provide a method for transmitting control information for reducing inter-cell interference.

In a method of transmitting cell information in a wireless communication system according to an aspect of the present invention, the method comprises: collecting inter-cell interference information for each of a plurality of subbands divided in an entire frequency band, wherein the inter-band interference among the plurality of subbands is equal to or greater than a threshold; And selecting and transmitting a cell specific sequence through the selected subband.

In a method for transmitting cell information in a wireless communication system according to another aspect of the present invention, a cell-specific sequence indicating inter-cell interference information of the selected subband is transmitted through a subband selected from a plurality of subbands divided in all frequency bands. Searching for a downlink control channel and transmitting inter-cell interference information of the selected subband to a serving base station when the cell specific sequence is received through the downlink control channel.

By allowing the UE to know the inter-cell interference information for each subband through the downlink control channel of the adjacent cell, transmission power control of the UE can be performed for each subband, thereby effectively reducing inter-cell interference.

1 is an exemplary view showing a wireless communication system. Wireless communication systems are widely deployed to provide various communication services such as voice and packet data.

Referring to FIG. 1, a wireless communication system includes a user equipment (UE) 10 and a base station 20 (BS). The terminal 10 may be fixed or mobile and may be called by other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), and a wireless device. The base station 20 generally refers to a fixed station that communicates with the terminal 10, and in other terms, such as a Node-B, a Base Transceiver System, or an Access Point. Can be called. One or more cells may exist in one base station 20.

Hereinafter, downlink (DL) means transmission from the base station 20 to the terminal 10, and uplink (UL) means transmission from the terminal 10 to the base station 20. In downlink, the transmitter may be part of the base station 20 and the receiver may be part of the terminal 10. In uplink, the transmitter may be part of the terminal 10 and the receiver may be part of the base station 20.

There is no limitation on the multiple access scheme applied to the wireless communication system. Various multiple access techniques such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Single-Carrier FDMA (SC-FDMA), and Orthogonal Frequency Division Multiple Access (OFDMA) are available. Can be.

2 shows a radio frame according to an embodiment of the present invention.

2, a radio frame (or super frame) includes at least one frame, where the radio frame is shown to include four frames. When having a radio frame may have a size of 20ms.

A cell information region is allocated to any one of a plurality of frames included in the radio frame. The cell information area is an area to which cell information that should be shared between neighboring cells or notified to a terminal of a neighboring cell is allocated. The cell information area includes a plurality of subcarriers in a frequency domain. The cell information area may be allocated over the entire frequency band of the cell. The cell information area includes at least one OFDM symbol in a time domain.

The cell information area may be used for a downlink control channel for transmitting intercell interference information received from an adjacent cell. Hereinafter, the downlink control channel using the cell information area is called a cell information control channel. The cell information control channel may be a broadcast channel that can be received by all terminals, a multicast channel that can be received only by a specific terminal group, or a unicast channel that can be received only by a specific terminal. channel). Inter-cell interference information may be expressed as interference information for each subband. Inter-cell interference information may be expressed by dividing the inter-cell interference into several levels. For example, an inter-cell interference level may be indicated for a selected subband by selecting a subband having a large intercell interference among a plurality of subbands.

Meanwhile, the radio frame includes a radio frame based control region. The radio frame based control area is an area where system information and information on frames included in a radio frame are transmitted. The radio frame based control region may be allocated to the portion of the radio frame that is preceded in time and may be referred to as a radio frame header. Each frame may include a frame based control region. The frame-based control area is an area for transmitting information on resource units included in a frame. The frame-based control region may be allocated to the most advanced part of each frame in time, which may be referred to as a frame header.

The cell information area may be assigned to a fixed position in the radio frame or to an arbitrary position. The cell information area may be included in a radio frame based control area or a specific frame based control area. Alternatively, the cell information area may be included in a data area for transmitting downlink data, and the location of the cell information area may be indicated in a frame based control area or a radio frame based control area. When inter-cell interference information is transmitted through the cell information area, the inter-cell interference information may be transmitted in units of radio frames or multiples of radio frames.

3 illustrates a cell specific sequence allocated to a cell information area according to an embodiment of the present invention.

Referring to FIG. 3, the cell information area includes a plurality of subbands in a frequency domain, and the subband includes a plurality of subcarriers. The cell information area includes at least one OFDM symbol in a time domain. The base station may transmit information on the selected subband through the cell information control channel.

For example, the base station may measure intercell interference for each subband, and select a subband in which intercell interference is greater than a threshold. The base station carries a cell specific sequence only on the selected subband and transmits the cell specific sequence. The cell specific sequence may indicate a cell ID or cell specific information for identifying each cell. The base station may indicate the presence or absence of inter-cell interference for each subband using one cell specific sequence distinguished from neighboring base stations. Alternatively, the base station may indicate the level of intercell interference or the type of interference using a plurality of cell specific sequences.

Neighboring base stations may transmit a cell specific sequence by selecting the same subband as a large subband which is intercell interference. The cell-specific sequence should use an orthogonal sequence so that a plurality of cell-specific sequences can be transmitted through the same subband.

An example of an orthogonal sequence is a constant amplitude zero auto-correlation (CAZAC) sequence. In Zadoff-Chu CAZAC, one of the CAZAC sequences, N is the length of the positive integer CAZAC sequence, and index M is relative to N prime (M is a natural number less than N and is prime with N ), The k-th element of the M-th CAZAC sequence may be represented by Equation 1 below.

The CAZAC sequence P (k) has the following three characteristics.

Equation 2 means that the CAZAC sequence is always 1 in size, and Equation 3 means that the auto correlation of the CAZAC sequence is represented by a Dirac-delta function. The autocorrelation here is based on circular correlation. Equation 4 means that cross correlation is always constant.

The length of an orthogonal sequence used as a cell specific sequence may be determined according to the number of subcarriers occupied by one subband in the cell information region or the number of cell information to be distinguished. For example, when a cell information area is allocated to one OFDM symbol in the time domain and subbands are composed of 18 subcarriers in the frequency domain, an orthogonal sequence having a maximum length of 18 may be used as a cell-specific sequence. An orthogonal sequence of length 18 can distinguish up to 18 cell information. If the number of cell information to be distinguished is six, an orthogonal sequence having a length of six may be used. An orthogonal sequence of length 6 may be spread by being mapped to 18 subcarriers with a spreading factor of length 3.

Table 1 shows an example of indicating cell interference information when three cell specific sequences are given for each cell in a multi-cell environment.

Cell Index of cell specific sequence Cell interference level Cell 1 0 normal One high 2 very high Cell 2 3 normal 4 high 5 very high ... ... ...

When three cell-specific sequences are given for each cell, adjacent cells use different cell-specific sequences. The index of the cell specific sequence given for each cell indicates different cell specific sequences. Three cell specific sequences given to one cell may indicate a cell interference level. The cell-specific sequence of any one of indexes 0 to 2 corresponding to the cell interference level is mapped and transmitted to the subband selected by the first cell. The cell-specific sequence of any one of the indexes 3 to 5 corresponding to the cell interference level is mapped and transmitted to the subband selected by the second cell. Even if the first cell and the second cell select the same subband and transmit the cell specific sequence, the terminal may find the cell and the cell interference level using the correlation of the cell specific sequence.

When a plurality of cell interference levels are to be expressed by one cell specific sequence, the cell interference level may be represented by modulating the phase of the cell specific sequence. For example, in a cell using one cell specific sequence, the phase of the cell specific sequence is modulated by a binary phase shift keying (BPSK) scheme to indicate two cell interference levels. When the phase of the cell specific sequence is modulated by the QPSK scheme, four cell interference levels may be represented. Alternatively, when a spreading factor is applied to a cell specific sequence, a spreading factor may be used to indicate a cell interference level according to a cell interference level.

In the above, there is no limitation on the type of orthogonal sequence that can be used as the cell specific sequence. As orthogonal sequences, well-known Hadamard codes, DFT sequences, Walsh codes, and the like may be used. The cell-specific sequence for each subband may be mapped not only to the frequency domain but also to the time domain.

Now, control of uplink transmission power of a terminal and sharing of cell information between base stations using the cell information control channel described above will be described. The terminal receives cell interference information from a neighbor base station and uses the cell interference information to control uplink transmission power and share cell information between the base stations. The cell interference information is not limited to information for preventing interference between cells, and may be any control information that needs to be shared between cells.

Control information that needs to be shared between cells includes the following information in addition to inter-cell interference information. Inter-cell shared control information may be transmitted through at least one subband in a cell information control channel.

(1) Information about a terminal located at a cell boundary: Since inter-cell interference may occur by a terminal located at a cell boundary, it is necessary to inform a neighbor base station of terminal IDs of terminals located at a cell boundary.

(2) Scheduling information on subbands allocated to UEs located at the cell boundary: When the same frequency band is allocated to the UEs located at the cell boundary, inter-cell interference may occur, and thus, different frequency bands are adjacent to each other. It is desirable to reduce the inter-cell interference by assigning. Therefore, it is necessary to share information on subbands allocated to terminals located at cell boundaries between neighboring base stations.

(3) Power control information for the terminal located at the cell boundary: Since the terminal located at the cell boundary has a large path loss or interference loss, it is necessary to transmit a signal with high transmission power. However, the higher the transmission power, the greater the interference with neighboring base stations. In order to minimize interference while maintaining communication quality between adjacent cells, it is necessary to share power control information between base stations. The base station may determine an appropriate transmission power for the terminal located in its cell based on the information on the transmission power of the terminal located in the neighbor cell. The transmission power of the terminal may be set by the serving base station, or the terminal may directly set its own transmission power.

(4) Information on permutation used by UE: Permutation means mapping from logical radio resources to physical subcarriers. Permutations can be used for local / distributed resource allocation, frequency diversity, and the like. When all neighbor cells use the same permutation scheme, the base station can easily remove the signal received from the terminal of the neighbor cell from the valid data signal. However, when the permutation schemes used between neighboring cells are different from each other and the information on the permutation schemes used by each base station is not known to each other, it is difficult to remove inter cell interference. There is a need to share information about permutations used between base stations.

(5) Information on noise and interference: If each base station increases its transmission power of a cell in consideration of its own noise and interference level (NI level), this increases interference to neighboring cells. Let's go. It is necessary to determine the transmit power of the terminal in the cell within a range that does not interfere with the neighbor cell by sharing information on the NI level between neighbor base stations.

In addition, the shared control information between cells may include various information that should be known to each other between base stations.

4 is a flowchart illustrating a cell information transmission method according to an embodiment of the present invention.

Referring to FIG. 4, the terminal receives a cell list for a neighbor cell from a serving BS (S110). The cell list may be transmitted in order to reduce the burden on the terminal to search for all the neighboring cells in a limited time. The cell list may include a neighbor cell ID to be searched by the terminal. The cell list may further include information on subbands to which cell interference information is to be detected.

The cell list may be transmitted through a broadcast channel or a multicast channel. Alternatively, the cell list may be transmitted through a unicast channel for a specific terminal located at the cell edge. That is, the terminal may receive the cell interference information from the neighboring cell using the cell list for the neighboring cell obtained from the serving base station, and the neighboring cell information or the signal characteristics from the neighboring cell in advance without using the cell list. Etc., the neighboring cell to be searched can be directly selected. The signal characteristic refers to the strength of the received signal, the signal to noise ratio, and the like. The transmission period of the cell list may be determined to coincide with the transmission period of cell interference information from an adjacent cell. If the serving base station reports the cell interference information acquired from the neighbor cell by the terminal, the transmission period of the cell list may be determined based on the report result of the cell interference information of the terminal.

The terminal receives cell interference information from an adjacent base station through a cell information control channel (S120). The neighbor base station directly measures inter-cell interference or receives inter-cell interference information received from a terminal connected to the neighbor base station. The neighbor base station transmits its intercell interference information through the cell information control channel. The neighboring base station may measure or receive intercell interference for each subband in the entire frequency band, and transmit a cell specific sequence through subbands where intercell interference is greater than a threshold. In the cell specific sequence, a base station is assigned a specific sequence orthogonal to each other. One cell specific sequence may be allocated to each base station. Alternatively, a plurality of sequences may be allocated to one base station according to the type or level of reception performance or interference information. The transmission power of cell interference information through the cell information control channel may be selected according to the purpose of the cell interference information. For example, the cell interference information may be transmitted at a transmission power that can be transmitted by terminals of a neighboring cell, or may be transmitted at a higher transmission power so that cells located outside the neighboring cell group can receive.

Meanwhile, although all terminals may receive cell interference information transmitted from neighboring base stations, a specific terminal may receive cell interference information in the following manner to prevent unnecessary power loss of the terminal. (1) The serving base station may search for a cell information control channel and designate a terminal to receive cell interference information. At this time, the base station specifies the terminal to receive the cell interference information based on the uplink interference information by sharing the uplink interference information. (2) When the terminal detects interference above the threshold value from the downlink data, the terminal may search for the cell information control channel and receive the cell interference information. (3) Only users who are designated in advance to be allocated to cell boundaries may receive cell interference information.

The terminal controls the uplink transmission power based on the cell interference information received from the neighbor base station (S130). The terminal may receive cell interference information from at least one neighbor base station. The serving base station notifies the terminal of information about a cell specific sequence used by neighboring base stations in advance so that the terminal can detect the cell specific sequence of the neighboring base station. The UE detects the subband-specific cell specific sequence included in the cell interference information to determine whether the neighboring base station receives the subband interference. If the neighboring base station uses a plurality of cell-specific sequences, the terminal can also know the interference level for each subband. When the neighboring base station is being interfered with the subband used by the terminal, the terminal may transmit data by lowering its transmission power by one step. When none of a plurality of neighboring base stations is interfering with the subband used by the terminal, the terminal may transmit data by maintaining or increasing its transmission power.

The terminal reports the neighbor cell information to the serving base station through the uplink control channel (S140). The report of neighbor cell information may be sent periodically or aperiodically. The report of neighbor cell information may be transmitted at the same period as the transmission period of cell interference information. Alternatively, the serving base station may inform the terminals of the reporting period of neighbor cell information through a broadcasting channel. When a report of neighbor cell information is transmitted aperiodically, the terminal may perform radio resource allocation request to the serving base station and report neighbor cell information through an uplink radio resource allocated by the serving base station. Alternatively, the serving base station may first request a report of neighbor cell information to the terminal and report the neighbor cell information through an uplink radio resource to which the terminal is allocated. The serving base station designates terminals to report neighbor cell information and transmits a report request for neighbor cell information, and only a terminal receiving the report request for neighbor cell information searches for a cell information control channel.

The serving base station may perform resource allocation using the neighbor cell information received from the terminal (S150). The serving base station may know a subband having a large inter-cell interference in neighboring cell information received from terminals connected to the serving base station. The serving base station may allocate subbands with high inter-cell interference to terminals located near the cell center and not to terminals located at possible cell boundaries. When the serving base station allocates a subband having a large inter-cell interference, the serving base station may control the transmit power of the terminal by indicating a transmission power with little interference to the neighboring cells while maintaining communication quality.

5 illustrates a cell specific sequence allocated to a cell information area according to another embodiment of the present invention.

Referring to FIG. 5, a cell specific sequence allocated to a subband of a cell information region may indicate other control information along with cell interference information. The cell specific sequence indicating cell interference information may be mapped to some subcarriers of a plurality of subcarriers included in a subband, and the cell specific sequence indicating other control information may be mapped to the remaining subcarriers. The other control information may be information for frequency reuse. For example, the subband information, power control information, permutation information, assigned by the cell boundary user for the case where the ID information of the terminal located at the cell boundary, or the partial frequency reuse, are not applied. Noise and interference information may be mapped to some subcarriers and transmitted. Such information may be transmitted together with cell interference information, but only this information may be transmitted separately. For example, when a cell boundary user wants to share allocated subband information between cells, only a unique sequence for each cell may be transmitted in a subband designated by each cell to be assigned to a cell boundary user.

Assume that cell interference information and subband information are transmitted from a neighbor base station through a cell information control channel. When the terminal requests radio resources to the serving base station, it may transmit subband information used in the neighbor cell together. The serving base station may allocate a subband to the user equipment that has less intercell interference and is not used in neighboring cells. Frequency reuse efficiency can be improved by sharing unused subband information between neighboring cells.

All of the above functions may be performed by a processor such as a microprocessor, a controller, a microcontroller, an application specific integrated circuit (ASIC), or the like according to software or program code coded to perform the function. The design, development and implementation of the code will be apparent to those skilled in the art based on the description of the present invention.

Although the present invention has been described above with reference to the embodiments, it will be apparent to those skilled in the art that the present invention may be modified and changed in various ways without departing from the spirit and scope of the present invention. I can understand. Therefore, the present invention is not limited to the above-described embodiment, and the present invention will include all embodiments within the scope of the following claims.

1 is an exemplary view showing a wireless communication system.

2 shows a radio frame according to an embodiment of the present invention.

3 illustrates a cell specific sequence allocated to a cell information area according to an embodiment of the present invention.

4 is a flowchart illustrating a cell information transmission method according to an embodiment of the present invention.

5 illustrates a cell specific sequence allocated to a cell information area according to another embodiment of the present invention.

Claims (11)

In the cell information transmission method in a wireless communication system, Collecting inter-cell interference information for each of a plurality of subbands divided in all frequency bands; Selecting a subband having intercell interference greater than or equal to a threshold among the plurality of subbands; And And transmitting a cell specific sequence through the selected subband. The method of claim 1, wherein the subband includes a plurality of OFDM symbols in a time domain, and the cell specific sequence is mapped to at least one OFDM symbol of the plurality of OFDM symbols. The method of claim 1, wherein another cell specific sequence is further transmitted through the selected subband. The method of claim 1, wherein the cell specific sequence indicates whether inter-cell interference is present. The method of claim 1, wherein the cell specific sequence indicates subband information for frequency reuse. The method of claim 1, wherein the cell specific sequence indicates the level of intercell interference. In the cell information transmission method in a wireless communication system, Searching for a downlink control channel through which a cell specific sequence indicating intercell interference information of the selected subband is transmitted through a subband selected from a plurality of subbands divided in all frequency bands; And And when the cell specific sequence is received through the downlink control channel, transmitting inter-cell interference information of the selected subband to a serving base station. 8. The method of claim 7, further comprising the step of controlling a transmission power for the selected subband when the cell specific sequence is received through the downlink control channel. 8. The method of claim 7, wherein the inter-cell interference information of the selected subband is interference information for each subband. 8. The method of claim 7, further comprising receiving from the serving base station a list of neighbor base stations transmitting the cell specific sequence and subbands to search. 8. The method of claim 7, wherein the inter-cell interference information is periodically transmitted through the downlink control channel.

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