JP2012050131A - Communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a communication system capable of avoiding interference from a neighboring sector.SOLUTION: The communication system includes: sector units which constitute base stations 10 through 16; and mobile stations 2, 3 which are respectively positioned in sectors 4, 5 corresponding to the sector units. The mobile stations 2, 3 transmit a transmission path information signal to the sector units corresponding to the sectors 4, 5 and sector units corresponding to sectors 4, 5 that neighbor those sectors 4, 5. A sector unit transmits data to the mobile station 2 using a transmitting beam which is oriented to the mobile station 2 but is not to the neighboring mobile station 3 based on a transmission path information signal received from the mobile station 2 which is positioned in the sector 4 corresponding to that sector unit and on a transmission path information signal received from the mobile station 3 which is positioned in the neighboring sector 5 that neighbors the sector 4 corresponding to that sector unit.

Description

  The present invention relates to a communication system including a plurality of base stations and mobile stations respectively present in areas corresponding to the base stations. For example, in a cellular mobile land mobile communication system applied to a Mobile WiMAX (Worldwide Interoperability for Microwave Access) system and the like, and particularly to a communication system that avoids mutual interference in a wireless communication system using the same frequency It is.

  In a radio communication system that develops a surface like a cellular system, a base station (BS) needs to ensure mutual independence between adjacent base stations. Conventionally, methods such as using different frequencies, using different spreading codes, or using different time domains have been used. In recent years, IEEE 802.16e (Mobile WiMAX) that employs an OFDM (Orthogonal Frequency Division Multiplexing) method for performing communication by dividing a usable frequency band into narrow-band orthogonal subcarrier frequencies has been standardized. In Mobile WiMAX, a method of using the same frequency band in adjacent sectors is used on the assumption that no allocation is made to all subcarriers (see Non-Patent Document 1).

  In the case of FDMA (Frequency-division multiple access) using different frequencies, it is clearly disadvantageous in terms of frequency utilization efficiency. Further, even in CDMA (Code-division multiple access) using the same frequency by code spreading, signals from adjacent sectors become interference and cause a reduction in communication capacity. Even in Mobile WiMAX, when the communication load increases, the collision probability increases and the influence of interference cannot be ignored.

  In Mobile WiMAX, an adaptive array system (AAS) using multiple antennas is standardized (see Non-Patent Document 2). It is possible to avoid interference with a mobile station (Mobile Subscriber Station: MSS) in its own sector within the range of freedom of the array. However, no consideration is given to avoiding interference with mobile stations existing in adjacent sectors.

JP 2002-319894 A US Pat. No. 6,067,290

IEEE 802.16-2004 (8.4.4.4 Allocation of subchannels for FCH, and logical subchannel numbering) IEEE 802.16-2004 / Cor 1-2005 (8.4.6.3.3 AMC support for SDMA)

  As described above, in cellular systems and the like, there is no technology that satisfies all of the convenience of using the same frequency, the improvement of frequency utilization efficiency, and the avoidance of interference from other sectors, and the system is based on these trade-offs. Has been established.

  An object of the present invention is to provide a communication system that can avoid interference from adjacent areas.

The present invention is a communication system including a plurality of base stations and mobile stations respectively present in an area corresponding to the base station,
The mobile station transmits a transmission path estimation signal to a base station corresponding to the area and a base station corresponding to an adjacent area adjacent to the area,
The base station receives a transmission path estimation signal received from the mobile station existing in an area corresponding to the base station, and a transmission path received from an adjacent mobile station existing in an adjacent area adjacent to the area corresponding to the base station. Based on the estimated signal, the data is transmitted to the mobile station using a transmission beam directed to the mobile station and not directed to the adjacent mobile station,
The adjacent area includes a plurality of adjacent areas, and at least one of the plurality of adjacent areas includes an out-cell area of a base station corresponding to the area.

  According to the present invention, interference from adjacent areas can be avoided, so that it is possible to perform good communication with a base station corresponding to an area to which a mobile station belongs, which greatly contributes to effective use of frequencies. be able to.

It is a figure which shows the structure of the communication system which concerns on Embodiment 1 of this invention. It is a figure which shows the format of a transmission frame. It is a figure which shows the correspondence of a sector and a subcarrier. It is a block diagram which shows the structure of the transmission part of a sector unit. It is a control flowchart which shows the procedure of interference avoidance control. It is a figure which shows the example of a frame structure in consideration of the combination of a mobile station. It is a figure which shows the communication system which performs interference control per cell. It is a control flowchart which shows the procedure of the interference avoidance control which concerns on Embodiment 2 of this invention. It is a control flowchart which shows the procedure of the interference avoidance control which concerns on Embodiment 3 of this invention.

Embodiment 1 FIG.
FIG. 1 is a diagram showing a configuration of a communication system according to Embodiment 1 of the present invention. The communication system includes mobile stations 2 and 3, base stations 10 to 16, and a base station control device 17. The mobile stations 2 and 3 are terminals carried by the user, and perform wireless communication with the base stations 10 to 16 while moving. The base stations 10 to 16 are fixedly installed and have a cell as an area for wireless communication with the mobile stations 2 and 3 around them. The base stations 10 to 16 are composed of three sector units that perform radio communication with the mobile stations 2 and 3 existing in the sectors 4 and 5 that are areas obtained by dividing the cell into azimuths of 120 degrees. The base station control device 17 is a host device that controls the base stations 10 to 16.

  Each cell of the base stations 10 to 16 is drawn as a hexagonal region, and the base stations 10 to 16 are respectively arranged at the centers of the cells. The hexagonal cell is composed of three sectors which are diamond-shaped regions. Pay attention to sector 4, which is a diamond-shaped region surrounded by a thick solid line. The sector 4 includes the mobile station 2 (black star). A sector 5 adjacent to the sector 4 is illustrated as a diamond-shaped region surrounded by a thick dotted line. Each adjacent sector 5 includes a mobile station 3 (black circle). In the case of a 3-sector configuration, there are a total of 10 adjacent sectors 5. When the sector unit of the base station 10 tries to transmit data to the mobile station 2 existing in the sector 4 corresponding to the sector unit, the sector unit is directed to the mobile station 2 and A transmission beam not directed to the existing mobile station 3 is formed. The transmission beam pattern at this time is drawn at the position of the base station 10. Forming a beam pattern not directed to the mobile station 3 is called null steering.

  In this way, a beam pattern directed to the mobile station 2 in the sector 4 and not directed to the mobile station 3 in the adjacent sector 5 is formed, and data is transmitted to the mobile station 3 in the adjacent sector 5. Interference can be prevented.

  The area referred to in the present invention may be a cell or a sector. When a sector is an area, a sector unit corresponding to each sector is regarded as a base station corresponding to that area.

  In order to form a beam pattern directed to the mobile station 2 included in the sector 4 and not directed to the mobile station 3 included in the adjacent sector 5, information indicating the transmission path status of the mobile station 2 included in the sector 4 In addition, information indicating the transmission path status of the mobile station 3 included in the adjacent sector 5 is necessary. Here, the transmission path status of the mobile station 2 refers to the status of the transmission path between the mobile station 2 included in the sector 4 and the sector unit corresponding to the sector 4. Similarly, the transmission path status of the mobile station 3 refers to the status of the transmission path between the mobile station 3 included in the adjacent sector 5 and the sector unit corresponding to the sector 5. This information is based on the reception state when the transmission path estimation signal 8 is transmitted in the uplink (UL) from the mobile stations 2 and 3 to the base stations 10 to 16 and received by the base stations 10 to 16. It is obtained by estimating the state of the transmission line.

  FIG. 2 is a diagram illustrating a format of a transmission frame. The transmission frame is composed of an uplink frame 7 and a downlink frame 8. The transmission path estimation signal 8 is also called an uplink sounding pilot symbol (UL Sounding pilot symbol) and is added to the uplink frame 7.

  FIG. 3 is a diagram illustrating a correspondence relationship between sectors and subcarriers. The transmission path estimation signal is transmitted using subcarriers assigned to each sector (sct1 to 21). The subcarriers have different frequencies. As a result, transmission path estimation signals can be transmitted and received for each sector, and transmission paths of mobile stations can be estimated.

  In addition, in the subcarriers assigned to each sector, the transmission path estimation signals for a plurality of mobile stations are multiplexed using spreading codes. As a result, transmission path estimation can be performed for each mobile station. Note that Walsh codes and GOLD codes that are orthogonal to each other are used as spreading codes. Since the number of orthogonal codes is limited in the Walsh code, the GOLD code is substituted for a mobile station exceeding that number. For example, when sub-carriers for 7 base stations x 3 sectors = 21 sectors are secured in consideration of sector repetition, transmission path estimation is performed for 1,344 mobile stations with FFT (Fast Fourier Transform) size 2096 and spreading factor 64 Can be done.

  FIG. 4 is a block diagram showing the configuration of the transmission unit of the sector unit. The transmission unit of the sector unit includes a weight generation unit 22, a transmission signal generation unit 23, and an antenna 24. Each sector unit is time-synchronized with each other, and transmission / reception timing is the same in a cellular system composed of a plurality of base stations. A transmission channel estimation signal (Mobile Sounding symbol: MSS) transmitted from each mobile station is received by the sector unit including the other sectors and other sectors. The received transmission path estimation signal is converted into the frequency domain, and then the transmission path is estimated through a despreading process for each subcarrier assigned to each sector.

  Next, the number of antenna elements of the transmission antenna will be described. The transmission antenna 24 is a multi-antenna composed of a plurality of antenna elements. Assuming that the number of mobile stations communicating in the own sector at a certain symbol timing is k (k is a natural number) and the number of mobile stations in communication in other sectors is m (m is a natural number), the number of antenna elements n = (k + m) is required as n (n is a natural number).

  In the downlink communication of IEEE 802.16e, mobile stations that perform communication in units of subcarriers and symbols are allocated. In an adjacent sector that is serviced at the same symbol timing as a mobile station that is serving in its own sector, transmission timing information of the scheduled mobile station is acquired in advance. Based on these transmission timing information, a combination of mobile stations is recognized for each symbol, and a downlink transmission weight is generated by the combination.

  Next, a downlink transmission weight generation method will be described. Here, as an example, a method of calculating according to the MMSE (Minimum Mean Square Error) standard using transmission path information obtained by transmission path estimation is used. Based on the scheduling information notified from the base station controller, a combination of k mobile stations communicating in its own sector and m mobile stations communicating in another sector is assumed. At this time, the transmission weight to be obtained is given by the following equation (1).

However, “*” represents a conjugate matrix and “T” represents a transposed matrix. Also, σ ² is an average noise power, P is a transmission power, I is a unit matrix of n rows and n columns, and a channel matrix H is expressed by the following equation (2).

In actual transmission, the first k columns of the transmit weight matrix is multiplied to the transmission symbol vector toward the k mobile stations communicating in the local sector, transmission signal X ^T of each antenna is obtained.

However, the transmission vector S is represented by S = [s ₁ ... S _k ]. The transmission weight can be calculated by other methods such as the ZF method.

  FIG. 5 is a control flowchart showing the procedure of interference avoidance control. First, when a mobile station registers its location, the base station controller notifies each base station of adjacent base station information and a sounding carrier number corresponding to the mobile station. Alternatively, for the mobile station that is starting communication, the base station controller notifies each base station of the adjacent base station information and the sounding carrier number corresponding to the mobile station.

  Each base station that has received them notifies each mobile station in its own sector of the sounding carrier number and spreading code number notification. Each mobile station that has received these transmits a transmission path estimation signal using the sounding carrier of the designated number.

  Next, each base station transmits downlink transmission scheduling information to the base station controller. The base station controller that has acquired information from each base station transmits downlink transmission scheduling information of another adjacent base station to each base station. Each base station performs downlink transmission beam formation not directed to a mobile station existing in another sector, using downlink transmission scheduling information of another adjacent base station and transmission path information from the mobile station.

  Thereafter, each mobile station periodically transmits a transmission path estimation signal, and each base station transmits downlink transmission scheduling information to the base station controller. The base station controller notifies each base station of information when there is a change in downlink transmission scheduling information.

  With this procedure, downlink transmission beam pattern formation can be systematically performed.

  FIG. 6 is a diagram illustrating a frame configuration example in consideration of a combination of mobile stations. Mobile stations included in the cell of the base station 10 are indicated as A to H, and mobile stations included in the cell of the base station 11 are indicated as I to P. The scheduling information from the base station control device includes information on which mobile station each base station communicates with at each timing when the radio frame is time-divided. At timing 1, mobile stations A, B, C, and D communicate with base station 10, and mobile stations I, J, K, and L communicate with base station 11. This combination is determined by each base station, and a transmission weight is calculated using transmission path information estimated based on sounding carriers transmitted from the mobile stations A to D and I to L. For example, when the base station 10 communicates with the mobile station A, a transmission beam pattern directed to the mobile station A and not directed to the other mobile stations B to D and I to K is formed.

  Note that “downlink transmission by transmission beamforming” in FIG. 5 described above collectively represents a series of transmission beamforming performed according to the combination with the mobile station shown in FIG.

  Further, in order to determine the timing of the combination of mobile stations that perform communication and mobile stations that need not cause interference, it is necessary to synchronize the time between the base stations. At the timing when the combination of mobile stations changes, the base station forms a transmission beam according to the combination and changes the directivity.

  As described above, by acquiring transmission path information from other sectors and performing transmission beamforming based on a combination reflecting the communication status in each sector at each symbol timing, a communication system that is not subject to interference with each other is constructed. be able to.

  FIG. 7 is a diagram illustrating a communication system that performs interference control in units of cells. Each cell corresponding to the base stations 10 to 16 is drawn as a hexagonal region, and the base stations 10 to 16 are respectively arranged at the centers of the cells. Attention is paid to the cell 4, which is a hexagonal region surrounded by a thick solid line. The cell 4 includes a mobile station 2 (black star). A cell 5 adjacent to the cell 4 is illustrated as a hexagonal region surrounded by a thick dotted line. Each adjacent cell 5 includes a mobile station 3 (black circle).

  In such a configuration, the total number of adjacent cells 5 for the cell 4 is six. Seven types of subcarriers are sufficient to transmit the transmission path estimation signal. When the FFT size is 2096, the spreading factor is 256, and the transmission path estimation signal can be transmitted to 256 mobile stations per sector.

  As described above, the present invention can be applied without depending on the arrangement of areas corresponding to base stations such as sectors or cells.

Embodiment 2. FIG.
FIG. 8 is a control flowchart showing a procedure of interference avoidance control according to Embodiment 2 of the present invention. When the mobile station registers its location, the base station controller transmits neighboring base station information and a sounding carrier number notification corresponding to the mobile station to each base station. Alternatively, the base station control device transmits adjacent base station information and a sounding carrier number notification corresponding to the mobile station to each base station for the mobile station that is starting communication. Each received base station notifies each mobile station in its own sector of the sounding carrier number and spreading code number notification. Each received mobile station transmits a channel estimation signal using a sounding carrier.

  Each base station selects a mobile station on the basis of the reception level of a transmission path estimation signal received from a mobile station in another sector for each timing at which a radio frame in which a combination transmitted by a mobile station in its own sector changes is time-divided. Specifically, m mobile stations are selected from the higher reception level. However, if the number of transmitting antennas of the base station is n, and the number of mobile stations in its own sector transmitting at the timing is k, m is given by the following equation.

m = n-k-1 ... (4)
The transmission path information of the mobile station of the selected other sector is added to the transmission path information of the mobile station of the own sector transmitted by the base station, and the transmission weight is determined by, for example, the algorithm shown in the equations (1) to (3) .

  As described above, since a transmission beam pattern that is not directed to m mobile stations in other sectors having a large influence of interference is formed, it is possible to construct a system that can communicate with mobile stations in its own sector without causing interference.

Embodiment 3 FIG.
FIG. 9 is a control flowchart showing a procedure of interference avoidance control according to Embodiment 3 of the present invention. When the mobile station registers its location, the base station controller transmits neighboring base station information and a sounding carrier number notification corresponding to the mobile station to each base station. Alternatively, the base station control device transmits adjacent base station information and a sounding carrier number notification corresponding to the mobile station to each base station for the mobile station that is starting communication. Each received base station notifies each mobile station in its own sector of the sounding carrier number and spreading code number notification. Each received mobile station transmits a channel estimation signal and scheduling information using a sounding carrier or the like.

  The transmission path estimation signal and scheduling information can transmit information from a large number of mobile stations by, for example, combining known pilot information and scheduling information and transmitting them using a code spreading method.

  Each base station uses a combination of a mobile station in another sector and a mobile station in its own sector at each timing when a radio frame in which the combination transmitted by the mobile station changes is time-divided based on the received scheduling information from the mobile station in another sector. Considering the above, for example, the transmission weight is determined by the algorithm shown in equations (1) to (3).

  As described above, since the scheduling information is instructed from the mobile station to the base station, the procedure can be simplified and the scheduling information can be transmitted promptly. Therefore, the transmission beam pattern can be formed quickly without delay.

Claims (7)

A communication system including a plurality of base stations and mobile stations respectively present in an area corresponding to the base station,
The mobile station transmits a transmission path estimation signal to a base station corresponding to the area and a base station corresponding to an adjacent area adjacent to the area,
The base station receives a transmission path estimation signal received from the mobile station existing in an area corresponding to the base station, and a transmission path received from an adjacent mobile station existing in an adjacent area adjacent to the area corresponding to the base station. Based on the estimated signal, the data is transmitted to the mobile station using a transmission beam directed to the mobile station and not directed to the adjacent mobile station,
The adjacent region includes a plurality of adjacent regions, and at least one of the plurality of adjacent regions includes an out-cell region of a base station corresponding to the region.   The communication system according to claim 1, wherein the mobile station transmits a transmission path estimation signal using a carrier having a frequency assigned to each region.   The base station acquires the transmission timing at which the mobile station and the adjacent mobile station transmit data, and receives the data transmitted to the mobile station simultaneously with other data at the adjacent mobile station. 2. The communication system according to claim 1, wherein the data is transmitted to the mobile station using a transmission beam directed to the mobile station and not directed to an adjacent mobile station.   The communication system according to claim 1, wherein the mobile station transmits a transmission path estimation signal using spreading codes assigned to the mobile stations and orthogonal to each other.   The communication system according to claim 1, wherein the mobile station transmits a transmission path estimation signal using a gold spreading code assigned to each mobile station.   3. The communication system according to claim 2, wherein the number of carriers allocated for transmitting the transmission path estimation signal is equal to or greater than the number of adjacent areas for one area.   The communication system according to claim 3, wherein the base station acquires transmission timing at which the mobile station and the adjacent mobile station transmit data from the mobile station and the adjacent mobile station.

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