WO2015049779A1

WO2015049779A1 - Base station, radio communication system, and radio communication method

Info

Publication number: WO2015049779A1
Application number: PCT/JP2013/077029
Authority: WO
Inventors: 栄里子武田; 玉木　剛; 矢野　隆; 玉木　諭; 倫太郎片山; 山本　知史; 仁志石田
Original assignee: 株式会社日立製作所
Priority date: 2013-10-04
Filing date: 2013-10-04
Publication date: 2015-04-09

Abstract

The objective of the invention is to provide a radio communication system, a radio communication method and a base station apparatus for enabling a base station to form a desired communication area. Included are a base station and one or more terminals. The base station transmits a cell-specific reference signal to the terminals, and the terminals each select, on the basis of the received cell-specific reference signal, a precoding matrix indicator and feed back the selected precoding matrix indicator to the base station. The base station then collects information of the indicators fed back from the one or more terminals and uses the collected information to determine a factor by which a transmission signal of the base station is to be multiplied.

Description

Base station, radio communication system, and radio communication method

The present invention relates to a radio communication technique in a radio communication system including a base station and a terminal.

Various technologies have been proposed for the purpose and method of changing the communication area of a base station.

Patent Document 1 discloses that the zone control device changes the zone shape of the base station based on the position information of the mobile station and the base station and the zone determination result.

JP 2002-125258 A

Patent Document 1 shows that the zone control device selects a base station that is a zone change target based on a distance between a location where a new zone is set and a surrounding base station. As an example, the mobile station selects the base station that is closest to the position that the base station has determined not to be in the service zone, and changes the radio wave transmission direction and transmission power of the base station to change the zone. It is stated to change the shape of.

However, Patent Document 1 does not consider the actual propagation environment. Specifically, there is a possibility that a large shielding object or the like exists between the base station and a position determined not to be a zone, and generally, radio waves reflected by various objects reach the terminal. Considering these, even if the radio wave transmission direction and transmission power of the base station that are close to each other are changed, there is a possibility that sufficient radio waves may not reach the desired position.

In view of such circumstances, an object of the present invention is to provide a radio communication system, a radio communication method, and a base station apparatus for a base station to form a desired communication area.

Of the inventions disclosed in this application, the outline of typical ones will be briefly described as follows.

A radio signal transmitting / receiving unit that transmits a reference signal to a terminal and receives a precoding matrix indicator determined based on the reference signal from the terminal, and a precoding matrix indicator received from one or more terminals, And a processing unit that determines a coefficient to be multiplied with a transmission signal to be transmitted to the base station.

According to the present invention, the communication area of the base station can be changed in consideration of the propagation environment between the base station and the terminal. Thereby, the radio | wireless communications system which forms the communication area which has the desired communication characteristic of a base station can be provided.

Issues, configurations, and effects other than those described above will be clarified by the following description of the embodiments.

It is a figure which shows the concept of the information fed back from a base station, its communication area, a terminal, and a terminal. It is a figure which shows an example of a part of functional block of a base station. It is a figure which shows an example of the operation | movement sequence of a base station and a terminal. It is a figure which shows an example of the operation | movement flowchart of a base station. It is a figure which shows an example of a structure of a base station. It is a figure which shows an example of the hardware constitutions of the cell specific coefficient process part of a base station. It is a figure which shows an example of a structure of a terminal. It is a figure which shows an example of the list of a precoding matrix and its indicator. It is a figure which shows another example of the list of a precoding matrix and its indicator. It is a figure which shows an example of the hardware constitutions of the precoding matrix indicator determination part of a terminal. It is a figure which shows an example of a conversion list. It is a figure which shows an example of the weighting coefficient X (n). It is a figure which shows an example of the weighting coefficient X (n). It is a figure which shows the concept of a base station and its communication area, a terminal, and an adjacent base station and its communication area. It is a figure which shows an example of a structure of a base station. It is a figure which shows an example of a weighting coefficient. It is a figure which shows the concept of a base station, its communication area, a terminal, the communication area of the said base station, the other base station with which a part of communication area overlaps, and its communication area. It is a figure which shows an example of a weighting coefficient. It is a figure which shows an example of the operation | movement flowchart of a base station.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

In the following embodiment, when it is necessary for the sake of convenience, the description will be divided into a plurality of sections or embodiments, but they are not irrelevant unless otherwise specified. The other part or all of the modifications, details, supplementary explanations, and the like are related. Each embodiment may be implemented individually or in combination.

Also, in the following embodiments, when referring to the number of elements (including the number, numerical value, quantity, range, etc.), particularly when clearly indicated and when clearly limited to a specific number in principle, etc. Except, it is not limited to the specific number, and may be a specific number or more.

Further, in the following embodiments, the constituent elements (including element steps) are not necessarily essential unless explicitly stated or considered to be clearly essential in principle. Needless to say.

Similarly, in the following embodiments, when referring to the shape, positional relationship, etc., of components, etc., the shape is substantially the same unless otherwise specified or otherwise apparent in principle. And the like are included. The same applies to the numerical values and ranges.

Hereinafter, various examples will be described in detail.

FIG. 1 is a diagram illustrating a concept of a base station, its communication area, a terminal, and information fed back from the terminal in the first embodiment. The wireless communication system according to the first embodiment includes a base station (11) and a terminal (12) that communicates with the base station (11). The base station 11 has a communication area (13) shown in FIG.
In the wireless communication system of the present embodiment, the base station (11) transmits a cell-specific reference signal, which is one of broadcast signals, to the terminal. The terminal determines a precoding matrix indicator (PMI) associated with the precoding matrix for the signal transmitted from the base station based on the received cell-specific reference signal, A precoding matrix indicator is fed back to the base station. The precoding matrix indicator is an indicator that represents a precoding matrix that is expected to be optimal in order to increase the throughput from the base station to the own terminal and is multiplied by a signal destined for the own terminal.

A method for selecting an optimal precoding matrix is described below. The terminal estimates a spatial channel between the terminal and the base station from the received cell-specific reference signal. From the channel matrix obtained by the estimation of the spatial channel, the number of layers (rank) that can be transmitted by communication with the base station is determined. Next, when the terminal selects one of the precoding matrices set for each rank and uses the previously estimated channel matrix and the selected precoding matrix, the expected value of the throughput of the signal reaching the terminal Calculate This operation is performed for all the precoding matrices, a precoding matrix indicator corresponding to the precoding matrix having the largest expected value of throughput reaching the terminal is determined, and the precoding matrix indicator is fed back to the base station.

FIG. 1 shows that the precoding matrix indicator determined by each terminal (12) is “indicator: n” (14) and is fed back (15) to the base station.

In this embodiment, the base station (11) determines the cell specific coefficient using statistical information obtained by collecting the indicator (14) fed back from the one or more terminals (12).

The base station multiplies the determined cell specific coefficient by the transmission signal and transmits the transmission signal to the terminal. That is, the communication area of the base station is formed depending on the cell specific coefficient. Thus, since the indicator fed back from the terminal is used, a communication area can be formed in consideration of the actual propagation environment, and the communication characteristics of the base station can be improved.

Hereinafter, a method for determining a cell specific coefficient will be described by taking as an example a case where a base station transmits using two antennas. Equation 1 shows an equation used when the base station determines the cell specific coefficient by changing the weighting of two kinds of coefficients (matrix) based on the statistical information of the precoding matrix indicator fed back from the terminal.

In Equation 1, W is a cell specific coefficient to be multiplied by a transmission signal from the base station after the next change timing, Wc is a current cell specific coefficient, and W (PMI = n) is “indicator: n (n is a number)”. , X (n) is a weighting coefficient for W (PMI = n), and is a value determined corresponding to the ratio of “indicator: n”. In Equation 1, W (PMI = n ). W (PMI = n) and X (n) are obtained by processing the indicator information fed back from the terminal on the base station side. K is a normalization coefficient determined by the number of transmission antennas.

FIG. 2 shows a part of the functional blocks of the base station in this embodiment. Hereinafter, the multiplication of the cell specific coefficient to the transmission signal from the base station will be described with reference to FIG. In the base station, a unique signal (161) to each terminal is multiplied by a precoding matrix determined based on a precoding matrix indicator fed back by each terminal in a precoding matrix multiplication unit (164), and the output thereof is output. Is input to the layer mapping unit (165) and mapped to a predetermined position. The cell-specific reference signal (162) and other signals (163) are also input to the layer mapping unit (165) and mapped to a predetermined position. The signal output from the layer mapping unit (165) is multiplied by the cell specific coefficient in the cell specific coefficient multiplication unit (166), and the output is input to the IFFT unit (167). The IFFT unit converts the input signal on the frequency axis into a signal on the time axis.

Since the terminal estimates the spatial channel between the base station and the terminal based on the cell-specific reference signal, the cell-specific reference signal multiplied by the cell-specific coefficient is transmitted between the terminal and the base station. Is equivalent to changing the spatial channel of. In this embodiment, the cell specific coefficient is determined using an indicator fed back from the terminal. Therefore, the terminal feeds back the precoding matrix indicator to the base station based on the spatial channel changed in consideration of the actual propagation environment. Then, since precoding matrix multiplication section 164 multiplies signal 161 unique to each terminal by a precoding matrix corresponding to the indicator, throughput can be improved.

In this embodiment, since all the signals transmitted from the base station are multiplied by the cell specific coefficient, the output of the IFFT unit may be multiplied by the cell specific coefficient.

Hereinafter, the operation sequence of the base station and the terminal will be described with reference to FIG. FIG. 3 is a diagram illustrating an example of an operation sequence of the base station and the terminal in the first embodiment of the present embodiment. In the operation sequence of FIG. 3, only the sequence related to the present embodiment is extracted and described in the wireless communication system of the present embodiment.

The base station (11) sets the cell specific coefficient to an initial value (22) at time t0 and transmits a cell specific reference signal (23) to the terminal (12). The terminal determines a precoding matrix based on the received cell-specific reference signal (24), and feeds back the precoding matrix indicator to the base station (25). The base station collects the information of the indicator fed back from each terminal as cell specific coefficient related information (26).

When a precoding matrix indicator is attached to each rank, for example, a precoding matrix indicator “1” is assigned to a precoding matrix with rank 1, and a precoding matrix is also assigned to a precoding matrix with rank 2. When the indicator “1” is assigned, the terminal may feed back a rank indicator indicating rank information and a precoding matrix indicator for the rank. Also, the frequency and timing at which the rank indicator and the precoding matrix indicator are fed back to the base station may be different, and the cycle for feeding back the rank indicator may be an integer multiple of the cycle for feeding back the precoding matrix indicator. In this case, the base station may use the currently held rank indicator value until the rank indicator information is updated.

Also, when the precoding matrix indicator held by the terminal and the base station is different from the precoding matrix for the different ranks, the base station can return the rank only by feeding back the precoding matrix indicator. Since the precoding matrix can be determined, only the precoding matrix indicator may be fed back.

In addition, the system may be configured so that information necessary for the base station to determine the cell specific coefficient is fed back from the terminal to the base station. Examples of other information will be described in examples described later.

The base station changes the cell-specific coefficient using the statistical information collected so far after a certain time (t1 hour) (27). Thereafter, a cell-specific reference signal multiplied by the changed cell-specific coefficient is transmitted to the terminal (23), and the terminal determines a precoding matrix based on the cell-specific reference signal after the coefficient change (24), and the result Is fed back to the base station (29). The base station continues to collect cell specific coefficient related information while changing the cell specific coefficient every t1 time.

In the present embodiment, the base station is configured to return the cell specific coefficient to the initial value (22) after a predetermined time (t2) has elapsed while continuing these operations. The subsequent operation is a repetition of the above-described operation.

As in this embodiment, the cell specific coefficient is periodically reset to the initial value, so that the mobile station communicates with the base station because the area has moved from the initial state and moved to the vicinity of the base station. A terminal that has not been able to receive a signal from the base station may be able to communicate with the base station.

In the present embodiment, the cell specific coefficient is periodically returned to the initial value, but it goes without saying that the timing for returning the cell specific coefficient to the initial value is not limited to this. For example, when the base station of this embodiment is linked with another base station and detects that the load of the other base station exceeds the threshold, the terminal is handed over from the other base station to the own base station. In order to facilitate, the cell specific coefficient may be returned to the initial value.

Next, the operation of the wireless communication system according to the first embodiment will be described using the operation flowchart of the base station shown in FIG. First, the base station sets a cell specific coefficient to an initial value (31). In this embodiment, a unit matrix is used as the initial value, but other values may be used as the initial value. Next, as described with reference to FIG. 3, cell-specific coefficient related information fed back from the terminal is collected (32). Next, for example, when the cell specific coefficient is determined using the expression described in Expression 1, the weighting coefficient X (n) illustrated in Expression 1 is determined by statistical processing of the collected results, and the determined X ( The cell specific coefficient is calculated using n) and W (PMI = n) (33). Next, the cell specific coefficient is changed to the newly calculated cell specific coefficient at the change timing of the cell specific coefficient (34). The above operation is repeated for a certain time. When the certain time has elapsed (35), the cell specific coefficient is set to the initial value (31), and the same operation is repeated.

A configuration of the base station in the first embodiment will be described with reference to FIG.

The base station (11) includes a wired interface (43), a radio signal transmission / reception unit (41), a bus (44), and a base station operation control unit (42). Although not shown in the drawing, a plurality of antennas are included in the wireless transmission / reception unit (41).
The wired interface (43) is an interface with a backhaul or the like. The wireless signal transmission / reception unit (41) performs wireless communication with the terminal via the antenna. A signal received via the wired interface (43) or a signal created in the base station operation control unit, for example, in the base station is converted into a radio signal by the radio signal transmission / reception unit (41). The radio signal received via the antenna is converted into a digital signal by the radio signal transmitting / receiving unit (41). The wireless signal transmitting / receiving unit (41) includes at least a wireless front end unit. The base station operation control unit (42) includes a signal processing unit (421) and another base station operation control unit (429). The signal processing unit (421) includes a control signal processing unit (422) and a data signal processing unit (428). The control signal processing unit (422) includes a cell specific coefficient processing unit (423), a cell specific reference signal creation unit (426), and other signal processing units (427). The cell specific coefficient processing unit (423) includes a cell specific coefficient related information collection unit (424) and a weighting coefficient determination unit (425).

Of these, functions other than the cell-specific coefficient processing unit (423) have the same functions as those of a general base station, and thus description thereof is omitted here. The operation of the cell specific coefficient processing unit (423) specific to the present embodiment is as described with reference to FIGS.

FIG. 6 is a diagram illustrating an example of a hardware configuration of the cell specific coefficient processing unit 423 in the first embodiment. The cell intrinsic coefficient processing unit can be realized by a normal computer configuration, and in addition to the bus 55, a central processing unit (CPU) 51 as a processing unit, a memory 53 as a storage unit, and further, as necessary. Accordingly, an input / output unit 54 may be provided. The CPU 51 determines the weighting coefficient X (n) shown in Formula 1 from the collected cell specific coefficient related information, calculates the cell specific coefficient using the determined X (n) and W (PMI = n), A cell specific coefficient processing program (52) is executed to perform processing to multiply the signal by the changed coefficient in accordance with the change timing of the cell specific coefficient. This program is stored in the memory 53 or the like, and is executed by the CPU 51 as necessary, thereby realizing the functions described above. Also, a program for performing other operations necessary for the base station in the present embodiment can be realized by a hardware configuration similar to the configuration shown in FIG.

Next, a configuration of the terminal according to the first embodiment will be described with reference to FIG.

The terminal 12 includes at least a radio signal transmission / reception unit 61 including an antenna, a radio communication control unit 62 including a precoding matrix indicator determination unit 621 and other radio communication control units 624, and an application control unit 63. The wireless signal transmission / reception unit 61, the wireless communication control unit 62, and the application control unit 63 are connected to each other by a bus 64 and can exchange signals with each other.

The radio signal transmission / reception unit transmits / receives radio signals to / from the base station and includes at least a radio front end unit. The wireless communication control unit 62 and the application control unit 63 can be configured by a normal CPU and a memory, respectively. Various functions can be realized by the CPU executing various programs stored in the memory.

Below, the precoding matrix indicator determination part (621) which is the characteristic of a present Example is demonstrated. Since other functions are the same as those of a general terminal, description thereof is omitted. In the terminal according to the first embodiment, the precoding matrix indicator determination unit (621) is included in the wireless communication control unit 62. The precoding matrix indicator determination unit (621) includes a precoding matrix indicator selection unit (622) and a precoding matrix related information feedback signal generation unit (623) that generates a signal for feeding back the precoding matrix related information. It is configured to include at least.
The precoding matrix indicator selection unit (622) uses the received cell-specific reference signal to determine a precoding matrix suitable for the base station to transmit a signal to its own terminal. As a result, one precoding matrix indicator is selected. Select coding matrix indicator.

FIG. 8 is an example of a list (71) showing the relationship between the precoding matrix (73) and the indicator (72) when the rank is 2 in the first embodiment. FIG. 9 shows an example of the list (81) of the precoding matrix (83) and its indicator (82) when the rank is 1 in the first embodiment.

These lists may be stored in a memory in the precoding matrix indicator selection unit, or may be stored in a separate memory so that information can be read there.
Also, the values and types of precoding matrices are not limited to those described here. If the number of antennas used in a wireless communication system increases, the types of ranks and types of precoding matrices for the ranks increase. It is clear to do. Needless to say, the system may be configured using a precoding matrix determined by 3GPP.

The precoding matrix related information feedback signal creation unit selects an indicator for one precoding matrix selected by the precoding matrix indicator selection unit, and creates a signal for feeding back the information to the base station. The information that the terminal feeds back to the base station is not limited to the precoding matrix indicator. As described above, if the terminal maintains a list of precoding matrices for each rank, the base station And the information on the indicator for the precoding matrix at that rank may be fed back to the base station.

FIG. 10 shows a hardware configuration of the precoding matrix indicator determination unit of the terminal in the first embodiment. This configuration is the same as the hardware configuration of the cell specific coefficient processing unit of the base station shown in FIG. The precoding matrix indicator determination unit can be realized by a normal computer configuration, and in addition to the bus 95, a central processing unit (CPU) 91 as a processing unit, a memory 93 as a storage unit, and further necessary It is also possible to provide an input / output unit 94 according to the above. The CPU 91 executes a precoding matrix related information determination program (92) for determining a precoding matrix indicator from the reception result of the cell-specific reference signal. This program is stored in the memory 93 or the like, and is executed by the CPU 91 as necessary, thereby realizing the above-described functions.

Also, a program for performing other operations necessary for the terminal in the present embodiment can be realized with the same hardware configuration as the configuration shown in FIG.

It is desirable that both the base station and the terminal maintain the same list of precoding matrices and their indicators. The base station may maintain a list for converting one rank of the precoding matrix indicator fed back from the terminal into another rank of the precoding matrix indicator.

For example, in the example described in FIG. 1, the base station is a system that can transmit a signal to a terminal in a maximum of two layers, in which one terminal that feeds back 1 as a precoding matrix indicator, It is shown that there are 4 terminals that feed back 2 and 2 terminals that feed back 3 as an indicator. In the example described in FIG. 1, it is assumed that only two terminals that feed back the indicator 3 have a rank of 1 and all other terminals have a rank of 2.

Thus, depending on the propagation environment between the base station and each terminal, the base station may communicate with both rank 1 and rank 2 terminals, and rank 1 terminals are ranked 1 precoding matrices. An indicator may be fed back, and a rank 2 terminal may feed back a rank 2 precoding matrix indicator. In this case, the base station needs to convert the rank-1 precoding matrix indicator into a rank-2 precoding matrix indicator to obtain statistical information. For this purpose, the base station may maintain a conversion list as shown in FIG. 11, for example. The conversion list (191) is a list for converting the indicator (192) of the rank-1 precoding matrix fed back from the terminal into the rank-2 indicator used in the cell-specific coefficient processing unit of the base station. Using the list shown in FIG. 11, for example, when the rank-1 precoding matrix indicator (192) fed back from the terminal is 1 or 2, the base station feeds back the rank-2 precoding matrix indicator 1 from the terminal. The cell specific coefficient processing unit performs processing.

FIG. 12 is a diagram showing the relationship between the ratio of terminals that feed back the indicator n and the weighting coefficient X (n). In Example 1, the statistical information of the indicator fed back from the terminal is taken, and the weighting coefficient X (n) is determined by the ratio of the terminal that fed back the same indicator with respect to the total number of terminals. In FIG. 12, X (n) is a weighting coefficient when the indicator is n, N (PMI = n) is the number of terminals that fed back n as an indicator, and N (total) is the total number of terminals.

In the system shown in FIG. 1, the base station collects information for a certain period of time from the terminals and obtains statistical information. It is assumed that the ratio of terminals that perform feedback is 4/7 and the ratio of terminals that feed back the indicator 1 of rank 1 is 2/7. The base station uses the statistical information and the conversion list shown in FIG. 11, and X (1) is the value on the vertical axis 100 when the value on the horizontal axis 101 is 1/7 in FIG. In (2), the cell specific coefficient is calculated by Equation 1 using the value on the vertical axis when the value on the horizontal axis is 6/7 in FIG. Also, when the indicator fed back from the terminal is n, W (PMI = n), which is a coefficient to be multiplied on the base station side, corresponds to indicator 1 of rank 2 when the indicator is 1. The unitary matrix created by referring to the precoding matrix to be used, and when the indicator is 2, the unitary matrix created by referring to the precoding matrix corresponding to the indicator 2 of rank 2 is used.

FIG. 12A shows that the weighting coefficient increases as the proportion of terminals that feed back n as an indicator increases. That is, as the proportion of terminals that show the same indicator in the communication area increases, a signal is transmitted so that a signal from the base station can easily reach a terminal that has fed back the indicator.

As a result, the SN ratio (signal-to-noise ratio) is improved, that is, the communication environment is improved, and it is possible to transmit a signal in a state in which the number of modulation multi-values is larger than in the past. The effect of improving the throughput during transmission can be obtained.

Also, the change of communication environment, that is, the speed of change of communication area can be changed by the value of X (n). That is, when the ratio of terminals that feed back the same indicator is the same, if X (n) is set to a large value, the communication environment changes in a short time, and if X (n) is set to a small value, the communication environment changes. It happens little by little. Therefore, the value of X (n) may be set so as to match the speed of change in the distribution of terminals within the communication area. That is, if the terminal distribution changes slowly, the value of X (n) is decreased, and if it changes rapidly, the value of X (n) may be increased.

Thus, by using a base station that provides a communication area that can follow changes in the distribution of terminals, radio resources possessed by the base station can be used effectively, and as a result, system throughput can be improved.

Also, as an example of the weighting coefficient, a section where the coefficient is 0 may be provided when the ratio of terminals is equal to or less than a certain value as shown in FIG. By adopting such a configuration, only the ratio of terminals that feed back the same indicator is included in the calculation of the weighting factor only, and those below the predetermined value are not included in the calculation. Can be simple. The weighting coefficient in the first embodiment may have a shape such that the value increases as the proportion of terminals that feed back the same indicator increases, or increases stepwise while taking the same value every fixed interval. Needless to say, the number is not limited to twelve.

Next, a second embodiment of the present invention will be described. In the second embodiment, as shown in FIG. 13, a terminal 113 at the boundary between a base station 111 constituting the communication area 112 and another base station 114 constituting the communication area 115 performs handover between a plurality of communication areas. A method of determining the weighting coefficient X (n) of the cell specific coefficient to prevent repetition will be described.

As shown in FIG. 14, the base station according to the second embodiment is configured to receive a terminal currently communicating with the base station and a terminal that has communicated with the base station within a certain period of time. A handover history management unit (430) that manages a history related to handover such as the number of times the terminal has been handed over to the base station.

In Example 1, when determining the weighting coefficient, the weighting coefficient X (n) is determined based on the ratio of the number of terminals that feed back the same indicator among all terminals.

However, in the second embodiment, the weighting coefficient X (n) is determined based on the ratio of the number of terminals that feed back the same indicator among the total number of terminals for which the number of handovers per unit time exceeds the threshold. .

FIG. 15 is a diagram illustrating the relationship between the ratio of terminals that feed back the same indicator and the weighting coefficient X (n) in the second embodiment. In FIG. 15, N ′ (total) indicates the total number of terminals whose number of handovers per unit time exceeds the threshold, and N ′ (PMI = n) indicates the number of terminals that have fed back n as an indicator.

For example, the terminal described in FIG. 13 includes two terminals that feed back the indicator 1 of rank 2, three terminals that feed back the indicator 2 of rank 2, and one terminal that feeds back the indicator 3 of rank 1. It has become. Among these, terminals whose number of handovers within a certain time exceeds a predetermined threshold are only two terminals that have fed back the rank 1 indicator 1 and one terminal that has fed back the rank 1 indicator 3. In this case, the parameter number of the terminal for calculating the weight of the cell specific coefficient is 3. As in the first embodiment, when the same ratio is shown when statistical information is taken, X (1) is the value on the vertical axis when the horizontal axis 121 corresponds to 2/3 in FIG. In 2), the cell specific coefficient is calculated by Equation 1 using the value of the vertical axis when the horizontal axis corresponds to 1/3 in FIG.

By multiplying the transmission signal from the base station by the cell specific coefficient calculated by the above method, a terminal located near the boundary between the communication area of the other base station and the communication area of the own base station and having repeated the handover On the other hand, the signal from the own base station can be made easier to reach than before. As a result, the number of terminal handovers can be reduced, and a stable communication environment can be provided. In addition, it is possible to reduce the processing of the base station required for handover.

Next, a third embodiment of the present invention will be described. In the third embodiment, as shown in FIG. 16, two base stations, a base station 131 and a base station 134, are installed, and at least a part of each communication area (132, 135) overlaps.

In the third embodiment, for example, it is assumed that the base station 131 determines that the filling rate of the resource block used for data signal transmission and the number of connected terminals are lower than a predetermined threshold and that there is a margin in radio resources. is doing. As described above, when it is possible to be an offload target of a terminal from another base station, a terminal communicating with the base station 134 is required to effectively use radio resources of the base station 131. A method for determining a cell specific coefficient for facilitating a handover to communication with 131 will be described.

In the present embodiment, the base station 131 is the same as that in the first embodiment until statistical information of the indicator fed back from the terminal is obtained. In the example shown in FIG. 16, there are three terminals that feed back the indicator 2 of rank 2, three terminals that feed back the indicator 3 of rank 1, and one terminal that feeds back the indicator 1 of rank 2 It has become. In this case, when the transmission direction from the base station is not changed, or when the method shown in the first embodiment is applied, the base station 131 is located where the rank 1 indicator 1 becomes an appropriate precoding matrix indicator. May be difficult to reach.

In such a case, in the third embodiment, contrary to the first embodiment, as shown in FIG. 17, the weighting coefficient X (( n) is set to be large. In FIG. 17, X (n) is a weighting coefficient when the indicator is n, N (PMI = n) is the number of terminals that fed back n as an indicator, and N (total) is the total number of terminals.

Therefore, in the case of the example shown in FIG. 16, in Example 3, the weighting coefficient X (1) for the indicator 1 of rank 2 is larger than the weighting coefficient X (2) for the indicator 2 of rank 2.

By transmitting such a weighted signal from the base station, it is possible to make the signal easier to reach even where the signal is difficult to reach than when no weighting is performed. As a result, it is possible to increase the probability that a terminal (for example, the terminal 136) connected to the base station 134 will be handed over to the base station 131. When the usage rate of the radio resource of the base station 134 is high and the usage rate of the radio resource of the base station 131 is low, the base station 134 transmits the cell-specific reference signal as described above to the base station 131. If the terminal can be handed over to the base station, the usage rate of the radio resources of both base stations can be averaged.

As described above, according to the present embodiment, there is a surplus in the usage rate of the radio resources of the own base station, and it is possible to provide an environment in which the terminal can be easily handed over when the terminal is handed over from another base station. . As a result, it is possible to average the radio resource usage rate of each base station and average the load applied to each base station.

Further, in the present embodiment, since there may be no terminal at a position where the indicator 1 of rank 2 is desired, if there is no terminal to be handed over from another base station even after a certain period of time, When a terminal that has been connected to the base station so far is handed over to another base station, the cell specific coefficient may be returned to before the change. Further, as in the first embodiment, an indicator having a large proportion of terminals that feed back the same indicator may be changed to a cell specific coefficient with a large weighting coefficient.

In the above embodiment, as shown in FIG. 4, the cell specific coefficient is set back to the initial value after a certain time has elapsed, but as shown in FIG. 18, when there are no terminals connected to the own base station (155 ), The cell specific coefficient may be returned to the initial value. Note that steps 31 to 34 in FIG. 4 and steps 151 to 154 in FIG. 18 are the same.

If you reset to the initial value at regular intervals, you can reset the communication area periodically. However, if the communication area immediately before the reset is significantly different from the communication area when the weighting coefficient is reset to the initial value, there is a possibility that a terminal whose communication environment changes rapidly due to the resetting of the weighting coefficient may occur. is there. On the other hand, if the reset is performed at a timing when there is no terminal communicating with the base station, it is possible to prevent a sudden change in the communication environment with respect to the terminal.

The embodiment of the present invention has been described above, but the configuration of the present invention is not limited to this, and it is obvious that various modifications may be made without departing from the gist of the present invention. The method for determining the cell specific coefficient is not limited to Formula 1, but may be determined using Formula 2, for example, or may be determined using Formulas other than Formula 1 and Formula 2. The meanings of W, Wc, W (PMI = n), X (n), and K in Equation 2 are the same as those in Equation 1. However, in Equation 2, as shown in Equation 2, X (n) is not an index of W (PMI = n).

Also, instead of calculating using mathematical formulas, for example, the base station holds in advance a table indicating the relationship between the statistical information state of the precoding matrix indicator and the corresponding cell specific coefficient, and refers to that table. The cell specific coefficient may be determined from the state of the statistical information.

In the embodiment, the case of the layer 2 wireless communication system using two antennas for transmission and reception has been described. However, the present invention may be applied to a wireless communication system having a larger number of antennas, that is, a larger number of layers. In that case, the

formulas

1 and 2 may be modified according to the number of layers.

In addition, the list of precoding matrices and their indicators held by the base station and the terminal are not limited to the types and values described in the present specification, and the types and values may be wirelessly communicated without departing from the gist of the present invention. It goes without saying that various settings may be made according to the use and configuration of the system.

11, 111, 114, 131, 134:

base station

12, 113, 133, 136: terminal 13, 112, 115, 132, 135: communication area 14: indicator 41, 61: wireless signal transmission / reception unit 43: wired IF
44, 64: Bus 423: Cell specific coefficient processing unit
621: Precoding matrix indicator determination unit

Claims

A radio signal transmitting / receiving unit that transmits a reference signal to a terminal and receives a precoding matrix indicator determined based on the reference signal from the terminal;
A base station comprising: a processing unit that determines a coefficient by which a transmission signal to be transmitted to the terminal is multiplied by using the precoding matrix indicator received from one or more of the terminals.
The base station according to claim 1, wherein
The base station, wherein the processing unit determines the coefficient based on a ratio of terminals that transmit the same precoding matrix indicator to all terminals connected to the base station.
The base station according to claim 1, wherein
The processing unit determines the coefficient based on a ratio of terminals transmitting the same precoding matrix indicator with respect to a total number of terminals whose number of handovers per unit time exceeds a predetermined threshold. base station.
The base station according to claim 1, wherein
An initial value of the coefficient is a unit matrix.
The base station according to claim 1, wherein
The base station is characterized in that the processing unit periodically changes the coefficient and returns the coefficient to an initial value after a predetermined time has elapsed or when there is no terminal connected to the base station.
The base station according to claim 1, wherein
A base station comprising a coefficient multiplier for multiplying the reference signal to the terminal by the coefficient.
The base station according to claim 6, wherein
A precoding matrix multiplication unit;
The radio signal transmitting / receiving unit receives a precoding matrix indicator determined based on the reference signal multiplied by the coefficient from the terminal, and the precoding matrix multiplying unit receives a precoding matrix indicator corresponding to the precoding matrix indicator. A base station that multiplies a coding matrix by a unique signal to the terminal.
A terminal,
A base station that communicates with the terminal, comprising:
The base station has a first radio signal transmission / reception unit that transmits a reference signal to the terminal,
The terminal includes a precoding matrix indicator determination unit that determines a precoding matrix indicator based on the reference signal, and a second radio signal transmission / reception unit that transmits the precoding matrix indicator to the base station,
The base station includes a processing unit that determines a coefficient for multiplying a transmission signal to be transmitted to the terminal using the precoding matrix indicator received from one or more of the terminals.
A wireless communication system according to claim 8,
The wireless communication system, wherein the processing unit determines the coefficient based on a ratio of terminals transmitting the same precoding matrix indicator to all terminals connected to the base station.
A wireless communication system according to claim 8,
The processing unit determines the coefficient based on a ratio of terminals transmitting the same precoding matrix indicator with respect to a total number of terminals whose number of handovers per unit time exceeds a predetermined threshold. Wireless communication system.
A wireless communication system according to claim 8,
The wireless communication system, wherein an initial value of the coefficient is a unit matrix.
A wireless communication system according to claim 8,
The wireless communication system, wherein the processing unit periodically changes the coefficient and returns the coefficient to an initial value after a fixed time has elapsed or when there is no terminal connected to the base station.
A wireless communication system according to claim 8,
The base station includes a coefficient multiplier that multiplies the reference signal to the terminal by the coefficient.
A wireless communication system according to claim 13,
The base station further includes a precoding matrix multiplication unit,
The first radio signal transmitting / receiving unit receives a precoding matrix indicator determined based on the reference signal multiplied by the coefficient from the terminal,
The wireless communication system, wherein the precoding matrix multiplication unit multiplies a signal specific to the terminal by a precoding matrix corresponding to the precoding matrix indicator.
A wireless communication method in a wireless communication system having a terminal and a base station that communicates with the terminal,
The base station transmits a reference signal to the terminal,
The terminal determines a precoding matrix indicator based on the reference signal, and transmits the precoding matrix indicator to the base station;
The base station determines a coefficient by which a transmission signal to be transmitted to the terminal is multiplied using the precoding matrix indicator received from one or more of the terminals.