Communication control method, communication control device, and wireless communication system

Abstract

According to the present invention, in a beam search period for searching for a beam that is used for wireless communication with a terminal device, each of a plurality of distributed antennas is caused to perform an all-beam search that transmits beams in all directions in which transmission is possible. If one beam identifier has been acquired indicating the best beam among the beams transmitted by the all-beam search, the all-beam search is stopped, and the beam identified by the acquired beam identifier and information indicating the distributed antenna that has transmitted the beam indicated by the beam identifier is used as a detection reference beam. A beam identifier of a distributed antenna that did not perform the all-beam search during the beam search period and that has been selected together with the detection reference beam is detected from a beam combination history storing unit as a candidate beam identifier for the relevant distributed antenna. Then, it is determined, on the basis of the detection result, whether or not to cause the distributed antenna that did not perform the all-beam search during the beam search period to perform the all-beam search.

Description

Technical Field

[0001] The present invention relates to a communication control method, a communication control apparatus, and a wireless communication system.

Background Art

[0002] (Beamforming in High Frequency Bands) In high frequency bands such as millimeter wave bands and terahertz bands, free space propagation loss is large compared to low frequency bands such as microwave bands. Therefore, it is necessary to use beamforming technology to form a beam that concentrates power in a specific direction in order to compensate for this loss (see, for example, Non-Patent Document 1).

[0003] In P-P (Point-to-Point) type communication where the combination of wireless stations that always communicate is fixed, and the positional relationship of the wireless stations and the propagation environment around the wireless stations do not change, beamforming can be performed fixedly by determining the beam formation direction in advance at the time of installation of the wireless stations. On the other hand, in a P-MP (Point-to-Multi Point) type that accommodates a plurality of wireless stations, or when at least one of the wireless stations moves, fixed beamforming cannot be performed. In this case, it is necessary to perform adaptive beamforming that adaptively controls the beam formation direction in accordance with the position of the wireless station that requires communication among the plurality of wireless stations, the movement of the wireless station, and changes in the propagation environment around the wireless station.

[0004] Adaptive beamforming is generally performed by adjusting the phase relationship of radio waves radiated between a plurality of antenna elements without using a mechanical drive unit to control the beam formation direction. However, in order to appropriately adjust the phase relationship, it is necessary to derive an appropriate phase relationship after grasping the phase relationship between each antenna element of both the transmitting and receiving wireless stations. That is, it is necessary to grasp the state of the propagation path between each antenna element of both the transmitting and receiving wireless stations in all combinations of the antenna elements.

[0005] The state of the propagation path can be grasped by transmitting and receiving a known signal between the wireless station on the transmission side and the wireless station on the reception side. However, during this transmission and reception, other communications cannot be performed, and since it is necessary to accurately transmit the state of the propagation path from the wireless station on the reception side to the wireless station on the transmission side, the communication overhead increases.

[0006] To suppress the increase in overhead, in adaptive beamforming, a signal including a beam identifier (hereinafter referred to as beam ID (Identifier)) associated with a plurality of candidate beams set discretely in advance is transmitted with each candidate beam, and the beam ID of the beam determined to be most suitable for communication is selected from among them. This technology is defined as a specification of wireless communication systems such as 3GPP 5G (5th Generation) and IEEE802.11ad that have been increasingly put into practical use in recent years, and has also been implemented (see, for example, Non-Patent Documents 1, 2, and 3).

[0007] (Beam selection procedure) When selecting the transmission-side beam of a wireless station, the wireless station on the transmission side transmits a signal that allows the wireless station on the reception side to uniquely identify each beam used for transmission. As such a signal, for example, there is a beam search signal in which the beam ID of the beam used for transmission is embedded as digital information. The wireless station on the transmission side transmits each beam carrying a beam search signal embedded with a different beam ID while switching the direction in time. The wireless station on the reception side receives a plurality of beams, reads out the beam IDs included in the beam search signals of the received plurality of beams, measures the reception quality of each beam, and determines which transmission-side beam has the best reception quality. By the wireless station on the reception side transmitting a feedback signal that allows the wireless station on the transmission side to uniquely identify the beam ID of the transmission-side beam with the best reception quality to the wireless station on the transmission side, the wireless station on the transmission side can select the transmission-side beam.

[0008] Regarding the receiving-side beam selection of a wireless station, in a system such as TDD (Time Division Duplex) that uses the same frequency for transmission and reception, it is also possible to select the same beam as the transmitting side. In contrast, in a system such as FDD (Frequency Division Duplex) that uses different frequencies for transmission and reception, similar to the transmitting-side beam selection, it is necessary to perform beam selection for the receiving-side beam as well. When selecting the receiving-side beam of a wireless station, the receiving-side wireless station transmits a signal requesting a reception beam search procedure to the transmitting-side wireless station. The receiving-side wireless station switches the direction temporally according to the signal transmitted by the transmitting-side wireless station and receives it, and measures the reception quality of the received signal. Thereby, the receiving-side wireless station can select the receiving-side beam by determining which reception beam has the best reception quality.

[0009] (Distributed antenna system) FIG. 17 is a diagram showing the configuration of a wireless communication system 500 which is an example of a conventional general wireless communication system. In FIG. 17, as an example, a configuration in which five cells 100-1 to 100-5 exist in the wireless communication system 500 is shown. The wireless communication system 500 has a configuration in which one antenna is installed for each cell. That is, in the wireless communication system 500, antenna devices 200-1 to 200-5 are installed in each of the cells 100-1 to 100-5. Digital signal processing devices 210-1 to 210-5 for transmitting and receiving signals are connected to each of the antenna devices 200-1 to 200-5. Looking at cell 100-1, the antenna device 200-1 and the digital signal processing device 210-1 form a so-called base station device. When one terminal station is located in cell 100-1, for example, the terminal station will be connected by radio waves from one antenna device 200-1.

[0010] Here, as described above, in high-frequency bands such as the millimeter-wave band and the terahertz band, since beamforming technology is used, the influence of reflected waves and diffracted waves is reduced. Therefore, in high-frequency bands, when the beam is blocked, the possibility of communication interruption increases, and line-of-sight communication becomes fundamental. By the way, there is a technology called MIMO (Multiple Input Multiple Output), which is a powerful spatial multiplexing technology. In MIMO, by using multiple antennas for transmission and reception, the transmission speed can be improved by several times the maximum number of antennas through spatial multiplexing at the same time and on the same frequency resource. However, since line-of-sight communication is fundamental in high-frequency bands, when applying MIMO technology, the spatial correlation between multiple antennas for transmission and reception increases, making spatial multiplexing difficult.

[0011] Therefore, in high-frequency bands, distributed antenna systems that have the effect of improving shielding resistance and reducing spatial correlation are being studied (see, for example, Non-Patent Documents 4 and 5). FIG. 18 is a diagram showing the configuration of a wireless communication system 500a, which is an example of a high-frequency band distributed antenna system. Similar to the wireless communication system 500 in FIG. 17, there are five cells 100-1 to 100-5 in the wireless communication system 500a. However, different from the wireless communication system 500, the wireless communication system 500a has a configuration in which a plurality of antennas are distributed and installed for one cell. Hereinafter, each of the plurality of antennas distributed and installed is referred to as a distributed antenna. Looking at cell 100-1, distributed antenna devices 200a-1-1 to 200a-1-4, each having one distributed antenna, are distributed and installed, and a single digital signal processing device 210a-1 is connected to the distributed antenna devices 200a-1-1 to 200a-1-4. In the wireless communication system 500a, the distributed antenna devices 200a-1-1 to 200a-1-4 and the digital signal processing device 210a-1 form a so-called base station device, and the same configuration is used for cells 100-2 to 100-5. In the wireless communication system 500a, when a single terminal station is located in, for example, cell 100-1, the terminal station will be connected by radio waves from the plurality of distributed antenna devices 200a-1-1 to 200a-1-4.

[0012] (Beam Selection Procedure in Distributed Antenna) By applying MIMO, i.e., single-user MIMO, between a radio station equipped with a plurality of antennas installed dispersedly, i.e., a distributed antenna, and a terminal station equipped with a plurality of antennas, spatial correlation is reduced and spatial multiplexing becomes possible. However, it is essential to select a beam in advance in the link between each of the plurality of antennas of the radio station and each of the plurality of antennas of the terminal station. In the following description, MIMO between a radio station equipped with a distributed antenna and a terminal station equipped with a plurality of antennas is referred to as distributed MIMO.

[0013] Here, a general transmission beam selection method for performing distributed MIMO in a high-frequency band will be described. A beam search signal in which a beam ID associated with each of a plurality of candidate beams set discretely in advance for each of a plurality of transmission antennas of a radio station and an antenna ID associated with each of the plurality of transmission antennas are embedded as digital information is generated in a plurality for each combination of the beam ID and the antenna ID. Each of the generated plurality of beam search signals is transmitted by a transmission beam in which a transmission antenna corresponding to the antenna ID included therein is switched temporally and is superimposed on a transmission beam corresponding to the beam ID included therein.

[0014] The terminal station on the communication partner side receives a plurality of beams with each of a plurality of receiving antennas, reads out the beam ID included in the beam search signal of each of the received plurality of beams and the transmission antenna ID, and measures the reception quality of the received beam. The terminal station selects, for each transmission antenna ID, the beam ID with the best reception quality, and feeds back to the wireless station on the transmission side the data combining the transmission antenna ID, the beam ID selected for the transmission antenna ID, and the reception quality corresponding to the beam ID. The wireless station that has received this feedback selects a plurality of transmission beams for the number of spatial multiplexes by MIMO based on the reception quality. In addition to this, the terminal station on the communication partner side sequentially selects a plurality of received beams by reception-side beam selection. Thereby, transmission and reception by MIMO are possible between a plurality of transmission and reception beams in the high frequency band.

[0015] It is assumed that distributed MIMO is applied to the wireless communication system 500a shown in FIG. 18. For example, when one terminal station is located in cell 100-1, each of the plurality of distributed antenna devices 200a-1-1 to 200a-1-4 located in cell 100-1 performs a beam search for transmitting a beam search signal to the terminal station, and it is necessary to select the beam with the best reception quality from among the plurality of beams by the beam search. Therefore, in the wireless communication system 500a, the overhead increases for the number of beam searches of the distributed antenna devices 200a-1-1 to 200a-1-4, and the efficiency of data transmission decreases. That is, the problem is that the overhead increases by increasing the distributed antenna devices 200a-1-1 to 200a-1-4.

[0016] Regarding the problem of this increase in overhead, for example, a method of storing the combination of beams selected for each of the plurality of distributed antennas and reducing the number of beam searches based on the history of the beam combination is disclosed in Non-Patent Document 6.

[0017] FIG. 19 and FIG. 20 are diagrams showing the outline of the technology disclosed in Non-Patent Document 6. In the wireless communication system 500b shown in FIG. 19(a), there is a cell 100 corresponding to any one of the cells 100-1 to 100-5 in the wireless communication system 500a shown in FIG. 18. In the cell 100, three distributed antenna devices 200a-1 to 200a-3 are distributed and installed, and one digital signal processing device 210 is connected to the distributed antenna devices 200a-1 to 200a-3. The communication control device 220 stores the history of the combination of beams selected for the plurality of distributed antenna devices 200a-1 to 200a-3. The communication control device 220 is a device that executes a method of reducing the number of beam searches based on the history of the combination of beams, and is connected to the digital signal processing device 210. In the cell 100, there is one terminal device 300 corresponding to the above-described terminal station, and it moves within the cell 100.

[0018] In the high frequency band, due to large free space propagation loss and diffraction loss and the use of beamforming, a small number of paths centered on the line-of-sight wave become dominant. Therefore, the beam combination selected by each of the distributed antenna devices 200a-1 to 200a-3 for distributed MIMO is a limited combination at each position of the terminal device 300. Therefore, one of the distributed antenna devices 200a-1 to 200a-3 transmits a beam carrying beam search signals in all directions that can be transmitted. One beam selected based on the beam search signal and the beams of the other distributed antenna devices 200a-1 to 200a-3 that were previously selected in combination are used as candidate beams, and the number of beam searches can be reduced by performing a partial beam search only for the candidate beams. In order to perform the process of reducing the number of beam searches, the communication control device 220 performs two types of mode processes: a storage mode for storing the combination of beams selected for the distributed antenna devices 200a-1 to 200a-3, and a reference mode for reducing the number of beam searches based on the history of the combination of beams. FIG. 19 is a diagram showing the outline of the process in the storage mode, and FIG. 20 is a diagram showing the outline of the process in the reference mode.

[0019] In the memory mode, the communication control device 220 instructs the digital signal processing device 210 to transmit a beam with beam search signals in all directions that each of the distributed antenna devices 200a-1 to 200a-3 can transmit. When the terminal device 300 receives each of the beams with the beam search signals transmitted by each of the distributed antenna devices 200a-1 to 200a-3, it reads out the beam ID included in the beam search signal and measures the reception quality of the beam. The terminal device 300 determines for each of the distributed antenna devices 200a-1 to 200a-3 based on the value indicating the reception quality measured for which beam has the best reception quality, and transmits a feedback signal that allows the communication control device 220 to uniquely identify the beam ID with the best reception quality.

[0020] When the communication control device 220 acquires the feedback signal through the distributed antenna devices 200a-1 to 200a-3 and the digital signal processing device 210, it generates a record in the beam combination history table shown in Fig. 19(b) based on the acquired feedback signal. In the table shown in Fig. 19(b), "1" under the "Antenna" item indicates the distributed antenna device 200a-1, "2" indicates the distributed antenna device 200a-2, and "3" indicates the distributed antenna device 200a-3. The numbers with "#" in the table indicate the beam ID. For example, the record in the first row indicates that in each of the distributed antenna devices 200a-1 to 200a-3, the beams with beam IDs "#3", "#4", and "#5" were selected as the best beams. While the terminal device 300 moves within the cell 100 and the communication control device 220 repeatedly performs the processing in the memory mode, records of different combinations will be added in the row direction to the beam combination history table.

[0021] In the reference mode, for example, as shown in Fig. 20(a), the communication control device 220 instructs the digital signal processing device 210 to transmit the beams of the beam search signal in all directions that can be transmitted to the distributed antenna device 200a-1. As a result, it is assumed that the beam with the beam ID "#3" is selected as the best beam in the distributed antenna device 200a-1. In this case, the communication control device 220 sets the beam with the beam ID "#3" of the distributed antenna device 200a-1 as the detection reference beam, and as shown in Fig. 20(b), detects the beam IDs "#4" and "#5", which indicate the beams of the distributed antenna device 200a-2 that have been selected in combination with the detection reference beam from the table of the beam combination history, as the beam IDs indicating the candidate beams. The communication control device 220 detects the beam IDs "#4", "#5", and "#6", which indicate the beams of the distributed antenna device 200a-3 that have been selected in combination with the detection reference beam, as the beam IDs indicating the candidate beams. The communication control device 220 instructs the digital signal processing device 210 to transmit the beams of the beam search signal in the directions corresponding to the beam IDs "#4" and "#5" to the distributed antenna device 200a-2. The communication control device 220 instructs the digital signal processing device 210 to transmit the beams of the beam search signal in the directions corresponding to the beam IDs "#4", "#5", and "#6" to the distributed antenna device 200a-3. In this way, without performing beam search in all directions that can be transmitted in the distributed antenna devices 200a-2 and 200a-3, the beam search is narrowed down to the candidate beams and partially performed, so that the number of beam searches can be reduced.

Prior Art Documents

Non-Patent Documents

[0022]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0023] The total number of combinations of the beams transmitted by the plurality of distributed antenna devices 200a-1 to 200a-3 is determined by the range where the terminal device 300 exists, the positions of the distributed antenna devices 200a-1 to 200a-3, the angular interval of the discrete beams transmitted by the distributed antenna devices 200a-1 to 200a-3, and the like. Therefore, during the process performed by the communication control device 220 in the storage mode, for example, in an area near an area where the terminal device 300 did not exist in the past, such as the area indicated by reference numeral 400 in FIG. 21, it may not be possible to acquire candidate beams that have been selected in combination with the detection reference beam. In this case, since a sufficient number of records are not accumulated in the table of the beam combination history, appropriate beam search and beam selection may not be possible, and in such a case, the transmission capacity may decrease. In order not to decrease the transmission capacity, for example, as shown in FIG. 22, during the process performed in the storage mode, the terminal device 300 is gradually moved at intervals that do not decrease the transmission capacity throughout the entire cell 100 that is the service providing area, and a sufficient number of records are accumulated in the table of the beam combination history. As a method for accumulating a sufficient number of records, for example, there are a method performed by the operator and a method performed using the user's terminal device 300.

[0024] When performed by the operator, there is a problem that it is very costly to gradually move the terminal device 300 at intervals that do not decrease the transmission capacity throughout the entire cell 100 that is the service providing area. On the other hand, when using the user's terminal device 300, the cost can be suppressed, but there is a low possibility that the user will move in an ideal manner on the operator side of the wireless communication system 500b. Therefore, when using the user's terminal device 300, there is a problem that it takes an enormous amount of time to obtain a sufficient number of records. In particular, as the carrier frequency increases, the beam width decreases, so in order not to decrease the transmission capacity, it is necessary to shorten the length of the interval when moving the terminal device 300, and the time required to obtain a sufficient number of records will further increase.

[0025] In view of the above circumstances, an object of the present invention is to provide a technology that enables accumulation of a sufficient number of records for reducing the number of beam searches without reducing the transmission capacity when performing beam search processing, without generating a record showing a history of beam combination by moving terminal devices little by little at intervals that do not reduce the transmission capacity throughout the service area in advance.

Means for Solving the Problems

[0026] One aspect of the present invention is to perform a full beam search in which, during a beam search period for searching for beams used for wireless communication with a terminal device, each of a plurality of distributed antennas transmits beams in all directions in which transmission is possible. When one beam identifier indicating the best beam among the beams obtained by the full beam search is acquired, the full beam search is stopped, and a beam specified by the acquired beam identifier and information indicating the distributed antenna that transmitted the beam indicated by the beam identifier is set as a detection reference beam. A candidate beam detection step of detecting, from a beam combination history storage unit, a beam identifier of a beam that is a beam identifier of a distributed antenna that has not performed the full beam search during the beam search period and that has been selected together with the detection reference beam as a candidate beam identifier for the distributed antenna; a beam search execution determination step of determining whether to perform a full beam search for the distributed antennas that have not performed the full beam search during the beam search period based on the detection result of the candidate beam detection step; and a beam combination recording step of generating a record indicating a combination of beam identifiers indicating the beams that have been the best beams in each of the distributed antennas during the beam search period, and recording the generated record in the beam combination history storage unit.

[0027] One aspect of the present invention is to perform a full beam search in which, during a beam search period for searching for a beam used for wireless communication with a terminal device, beams are transmitted in all directions that can be transmitted for each of a plurality of distributed antennas. When one beam identifier indicating the best beam among the beams obtained by the full beam search is acquired, the full beam search is stopped, and a beam specified by the acquired beam identifier and information indicating the distributed antenna that transmitted the beam indicated by the beam identifier is set as a detection reference beam. A candidate beam detection unit that detects, from a beam combination history storage unit, a beam identifier of a beam that is a beam identifier of the distributed antenna that has not performed the full beam search during the beam search period and that has been selected together with the detection reference beam as a candidate beam identifier for the distributed antenna; a beam search execution determination unit that determines whether to perform a full beam search for the distributed antenna that has not performed the full beam search during the beam search period based on the detection result of the candidate beam detection unit; and a beam combination recording unit that generates a record indicating a combination of beam identifiers indicating the beams that have been the best beams in each of the distributed antennas during the beam search period and records the generated record in the beam combination history storage unit.

[0028] One aspect of the present invention is a wireless communication system including a terminal device, a plurality of distributed antenna devices each including one distributed antenna, and a communication control device. During a beam search period for searching for a beam used for wireless communication with the terminal device, the communication control device causes each of the plurality of distributed antennas to perform a full beam search by transmitting beams in all directions that can be transmitted. When one beam identifier indicating the best beam among the beams obtained by the full beam search is acquired, the communication control device stops the full beam search, and designates as a detection reference beam a beam specified by the acquired beam identifier and information indicating the distributed antenna that transmitted the beam indicated by the beam identifier. A candidate beam detector that detects, from a beam combination history storage unit, a beam identifier of a beam that is a beam identifier of a distributed antenna that has not performed the full beam search during the beam search period and that has been selected together with the detection reference beam as a candidate beam identifier for the distributed antenna, A beam search execution determination unit that determines whether to perform a full beam search on the distributed antennas that have not performed the full beam search during the beam search period based on the detection result of the candidate beam detector, and a beam combination recorder that generates a record indicating a combination of beam identifiers indicating the beams that have been determined to be the best beams in each of the distributed antennas during the beam search period, and records the generated record in the beam combination history storage unit.

Advantages of the Invention

[0029] According to the present invention, when performing beam search processing, it is possible to accumulate a sufficient number of records for reducing the number of beam searches without reducing the transmission capacity, without generating records indicating the history of beam combinations by moving the terminal device little by little at intervals that do not reduce the transmission capacity over the entire service area in advance.

Brief Description of the Drawings

[0030]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

Figure 22

Best Mode for Carrying Out the Invention

[0031] (First Embodiment) Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an example of the configuration of a wireless communication system 1 in the first embodiment. The wireless communication system 1 includes at least one cell 100. Inside the area of the cell 100, distributed antenna devices 30-1, 30-2, 30-3, 30-4 and a terminal device 40 are provided. The wireless communication system 1 includes a communication control device 10 and a digital signal processing device 20. The communication control device 10 is connected to the digital signal processing device 20. The digital signal processing device 20 is connected to each of the distributed antenna devices 30-1 to 30-4. The communication control device 10, the digital signal processing device 20, and the distributed antenna devices 30-1 to 30-4 will constitute a so-called base station device.

[0032] Note that the wireless communication system 1 shown in FIG. 1 is a configuration as an example. In the wireless communication system 1, a plurality of distributed antenna devices 30-1 to 30-N may be installed in the cell 100, and a digital signal processing device 20 may be connected to each of the distributed antenna devices 30-1 to 30-N. Here, N is an integer of 2 or more. The wireless communication system 1 may have a configuration as shown in FIG. 18, that is, a configuration including a plurality of cells 100. In this case, it includes a digital signal processing device 20 in a number corresponding to the number of cells 100 and a communication control device 10. Each of the communication control devices 10 is connected to the corresponding digital signal processing device 20, and each of the digital signal processing devices 20 is connected to the distributed antenna devices 30-1 to 30-N in the corresponding cell 100.

[0033] Each of the distributed antenna devices 30-1 to 30-4 is capable of beamforming to switch directions and form a beam of radio waves, and is connected to the terminal device 40 by radio waves. In FIG. 1, as an example, an example in which the distributed antenna devices 30-1 to 30-4 form beams in nine directions is shown. However, the number of directions in which the distributed antenna devices 30-1 to 30-4 can form beams may be two or more. The maximum number of directions in which the distributed antenna devices 30-1 to 30-4 can form beams is determined in advance according to the specifications of the distributed antenna devices 30-1 to 30-4, and the operator may be able to arbitrarily determine the number of beams. Each of the distributed antenna devices 30-1 to 30-4 includes one distributed antenna 31-1 to 31-4 and one main body device 32-1 to 32-4. Each of the distributed antennas 31-1 to 31-4 is pre-assigned a distributed antenna ID that can uniquely identify each of them.

[0034] Each of the main body devices 32-1 to 32-4 performs transmission and reception of radio frequency analog signals through the distributed antennas 31-1 to 31-4 connected thereto. That is, each of the main body devices 32-1 to 32-4 modulates a carrier wave based on the digital signal of the transmission data output by the digital signal processing device 20 to generate a radio frequency analog signal. Each of the main body devices 32-1 to 32-4 transmits the generated analog signal by radio waves from the distributed antennas 31-1 to 31-4 connected thereto. Each of the main body devices 32-1 to 32-4 demodulates the analog signal output by receiving radio waves by the distributed antennas 31-1 to 31-4 connected thereto and converts it into a digital signal. Each of the main body devices 32-1 to 32-4 outputs the converted digital signal to the digital signal processing device 20.

[0035] From the perspective of beamforming, each of the main body devices 32-1 to 32-4 captures the digital signal of the beam search signal as the transmission data output by the digital signal processing device 20. Each of the main body devices 32-1 to 32-4 modulates a carrier wave based on the beam search signal so as to form a beam in the direction corresponding to the beam ID included in the captured beam search signal. Each of the main body devices 32-1 to 32-4 transmits the radio frequency analog signal carrying the beam search signal generated by modulation through the distributed antennas 31-1 to 31-4 connected thereto.

[0036] Here, the beam ID is an identifier to which a character string "Beam ID#" is assigned to consecutive integer values starting from, for example, 1, and is an identifier predetermined for each of the distributed antenna devices 30-1 to 30-4. For example, when the distributed antenna device 30-1 can form beams in 40 different directions, beam IDs from "Beam ID#1" to "Beam ID#40" are fixedly assigned in advance to each of the 40-direction beams, and data indicating the correspondence between the beam ID and the direction is stored in advance in the storage area inside the main body device 32-1. In other words, when "Beam ID#1" is specified by the beam search signal for the main body device 32-1 of the distributed antenna device 30-1, the direction of the beam formed by the distributed antenna 31-1 of the distributed antenna device 30-1 will be uniquely determined. The same applies to the other distributed antenna devices 30-2 to 30-4. In this case, there may be the same beam ID in each of the distributed antenna devices 30-1 to 30-4.

[0037] The digital signal processor 20 outputs the digital signal of the transmission data to the main body devices 32-1 to 32-4. When the digital signal processor 20 receives a beam search instruction signal from the communication control device 10, it uses the distributed antenna ID included in the beam search instruction signal as the transmission source antenna ID, and generates a beam search signal including the transmission source antenna ID and the beam ID included in the beam search instruction signal. The digital signal processor 20 outputs the generated beam search signal to the main body devices 32-1 to 32-4 corresponding to the distributed antenna ID included in the beam search instruction signal. As a result, the terminal device 40 that receives the beam carrying the beam search signal can identify the distributed antennas 31-1 to 31-4 that are the transmission sources of the beam search signal by referring to the transmission source antenna ID included in the beam search signal, and can further identify the beam carrying the beam search signal by referring to the beam ID included in the beam search signal.

[0038] The digital signal processing device 20 detects received data contained in the digital signal from the digital signals output by the main body devices 32-1 to 32-4. When the detected received data is a feedback signal transmitted by the terminal device 40 by being loaded on a radio wave, the digital signal processing device 20 outputs the feedback signal to the communication control device 10.

[0039] (Configuration of the communication control device according to the first embodiment) As shown in FIG. 2, the communication control device 10 includes a beam search execution instruction unit 11, a feedback signal reception unit 12, a beam combination history generation unit 13, a beam combination history storage unit 14, a candidate beam detection unit 15, a beam search execution determination unit 16, and a beam combination recording unit 17.

[0040] The beam search execution instruction unit 11 causes each of the distributed antenna devices 30-1 to 30-4 to perform a full beam search in which beams are transmitted by switching time for each of all directions in which each can transmit via the digital signal processing device 20, or causes a partial beam search in which beams are transmitted by switching time for one or a plurality of specific directions. The beam search execution instruction unit 11 stores in its internal storage area a beam number table 110 shown in FIG. 3 in advance. The beam number table 110 has items of "distributed antenna ID" and "maximum beam ID". In the item of "distributed antenna ID", "distributed antenna ID #1", "distributed antenna ID #2", "distributed antenna ID #3", and "distributed antenna ID #4", which are the distributed antenna IDs assigned to each of the distributed antennas 31-1 to 31-4, are written in advance. Here, "distributed antenna ID #1" corresponds to the distributed antenna ID assigned to the distributed antenna 31-1, "distributed antenna ID #2" corresponds to the distributed antenna ID assigned to the distributed antenna 31-2, "distributed antenna ID #3" corresponds to the distributed antenna ID assigned to the distributed antenna 31-3, and "distributed antenna ID #4" corresponds to the distributed antenna ID assigned to the distributed antenna 31-4.

[0041] In the item of "maximum beam ID", the maximum value of the beam ID for each of the distributed antennas 31-1 to 31-4, that is, the number of transmission directions of the beams that each of the distributed antennas 31-1 to 31-4 can transmit, is written in advance. In FIG. 3, the values of the "maximum beam ID" of the distributed antennas 31-1 to 31-4 are all set to "40", but different values may also be used.

[0042] When the beam search execution instruction unit 11 receives all beam search request signals including the distributed antenna ID, it refers to the beam number table 110 and reads out the maximum beam ID corresponding to the distributed antenna ID included in all beam search request signals. The beam search execution instruction unit 11 generates beam search instruction signals with a number corresponding to the read maximum beam ID, and each beam ID included in each of them is different from each other, and each beam ID from 1 to the maximum beam ID is included one by one. The beam search execution instruction unit 11 writes the distributed antenna ID included in all beam search request signals for each of the generated beam search instruction signals.

[0043] For example, when the beam search execution instruction unit 11 receives all beam search request signals with the distributed antenna ID being "Distributed Antenna ID #1", it will generate 40 beam search instruction signals. More specifically, since the beam search execution instruction unit 11 generates "Beam ID #1" to "Beam ID #40", each of the 40 generated beam search instruction signals will include ("Distributed Antenna ID #1", "Beam ID #1"), ("Distributed Antenna ID #1", "Beam ID #2"), …, ("Distributed Antenna ID #1", "Beam ID #40"). Note that the "Beam ID #1" to "Beam ID #40" generated by the beam search execution instruction unit 11 are fixedly associated in advance with each of the 40 beams formed by the distributed antenna 31-1 in the main body device 32-1 of the distributed antenna device 30-1 as described above. Therefore, the process of generating beam IDs performed by the beam search execution instruction unit 11 is not to generate and associate new beam IDs with the directions of the beams transmitted by the distributed antenna device 30-1 again, but simply to perform the process of generating beam IDs for all directions.

[0044] When the beam search execution instruction unit 11 receives a partial beam search request signal including a distributed antenna ID and one or more beam IDs, it generates the same number of beam search instruction signals as the number of beam IDs included in the partial beam search request signal. The beam search execution instruction unit 11 writes each of the beam IDs included in the partial beam search request signal into each of the generated beam search instruction signals so that the beam IDs included in each of the generated beam search instruction signals are all different beam IDs. The beam search execution instruction unit 11 writes the distributed antenna ID included in the partial beam search request signal into each of the generated beam search instruction signals. The beam search execution instruction unit 11 outputs the beam search instruction signals generated by receiving all beam search request signals or partial beam search request signals to the digital signal processing device 20 one by one in the order of generation at a predetermined fixed time interval.

[0045] When the beam combination generation process for generating beam combinations starts, the beam combination history generation unit 13 causes all of the distributed antenna devices 30-1 to 30-4 to perform a full beam search for each trial period, thereby generating a record indicating a combination of beam IDs of the beams that are the best beams in each of the distributed antenna devices 30-1 to 30-4.

[0046] The beam combination history storage unit 14 stores, for example, the beam combination history table 140 shown in FIG. 4. As shown in FIG. 4, the beam combination history table 140 has an item of "distributed antenna ID" on the horizontal axis and an item of "record ID" on the vertical axis. In the item of "distributed antenna ID", "distributed antenna ID #1", "distributed antenna ID #2", "distributed antenna ID #3", and "distributed antenna ID #4", which are the distributed antenna IDs assigned to each of the distributed antenna devices 31-1 to 31-4, are written in advance.

[0047] In the item of "record", record IDs that are assigned to the records generated for each trial period and are different identifiers are written. For example, "record ID #1" is the record ID assigned to the record generated in the first trial period, and "record ID #2" is the record ID assigned to the record generated in the second trial period. Information indicating the beam ID is written in each of the elements specified by the distributed antenna ID on the horizontal axis and the record ID on the vertical axis. That is, "beam ID #23" in the element of "record 1" of "distributed antenna ID #1" indicates that the beam ID of the beam that is the best beam in the distributed antenna device 30-1 in the first trial period is "beam ID #23".

[0048] When the feedback signal reception unit 12 receives the output destination switching instruction signal, it sets any one of the beam combination history generation unit 13, candidate beam detection unit 15, and beam search execution determination unit 16 specified by the output destination switching instruction signal as the output destination of the feedback signal. When the feedback signal reception unit 12 captures the feedback signal output by the digital signal processing device 20, it outputs the captured feedback signal to the set output destination.

[0049] When the beam search process for searching for a beam is started, the candidate beam detection unit 15 causes each of the distributed antenna devices 30-1 to 30-4 to perform a full beam search one by one in order. That is, the candidate beam detection unit 15 sequentially designates one by one the distributed antenna IDs of the distributed antennas 31-1 to 31-4, and outputs a full beam search request signal including the designated one distributed antenna ID to the beam search execution instruction unit 11. After starting the beam search process, when the candidate beam detection unit 15 captures the first feedback signal, it stops the full beam search, and uses the beam specified by the transmission source antenna ID and the beam ID included in the feedback signal as the detection reference beam. The candidate beam detection unit 15 detects, from the beam combination history table 140, the beam IDs of the distributed antenna devices 30-1 to 30-4 that have not performed a full beam search during the beam search period, which are the beam IDs of the beams that have been selected together with the detection reference beam, and the distributed antenna IDs corresponding to the beam IDs. Here, the beam search period is, for example, the time allocated to one round of processing of the beam search process. The beam search process is a periodically performed process, and this period is called the beam search period. In each of the beam search periods, there are a beam search period in which one round of the beam search process is performed, and a data transmission period that is performed between the distributed antennas 31-1 to 31-4 of the distributed antenna devices 30-1 to 30-4 and the terminal antennas 41-1 to 41-M of the terminal device 40 after the beam search period. Note that the above-mentioned "beam search period" and the "beam search period" described below indicate the beam search period for one round included in the period of one beam search cycle, unless otherwise specified as having a different meaning. The candidate beam detection unit 15 sets the detected beam ID as the beam ID (hereinafter also referred to as the candidate beam ID) indicating the candidate beam in the distributed antenna devices 30-1 to 30-4 corresponding to the detected distributed antenna ID, and uses the combination of the detected distributed antenna ID and the candidate beam ID as the detection result.

[0050] Based on the detection results of the candidate beam detection unit 15, the beam search execution determination unit 16 determines whether to cause the distributed antenna devices 30-1 to 30-4 that have not performed full beam search during the beam search period to perform full beam search. More specifically, the beam search execution determination unit 16 determines, as the distributed antenna devices 30-1 to 30-4 that will perform full beam search, the distributed antenna devices 30-1 to 30-4 that have not performed full beam search during the beam search period and whose detection results do not contain the candidate beam ID in the detection results of the candidate beam detection unit 15.

[0051] For the distributed antenna devices 30-1 to 30-4 whose detection results of the candidate beam detection unit 15 contain the candidate beam ID, the beam search execution determination unit 16 causes them to perform partial beam search using the candidate beam ID included in the detection results. That is, the beam search execution determination unit 16 outputs a partial beam search request signal including the distributed antenna ID and the candidate beam ID included in the detection results to the beam search execution instruction unit 11. Based on the received power value included in the feedback signal transmitted by the terminal device 40 that has received the beam by partial beam search and a predetermined threshold value, the beam search execution determination unit 16 determines whether to cause the distributed antenna devices 30-1 to 30-4 corresponding to the transmission source antenna ID included in the feedback signal to perform full beam search. When the beam search execution determination unit 16 determines to perform full beam search, it designates the transmission source antenna ID included in the feedback signal to be determined. The beam search execution determination unit 16 outputs a full beam search request signal including the designated transmission source antenna ID to the beam search execution instruction unit 11.

[0052] During the beam search period, based on the combination of the beam ID of the beam that is finally determined to be the best beam in each of the distributed antenna devices 30-1 to 30-4 and the transmission source antenna ID corresponding to the beam ID, the beam combination recording unit 17 generates one record. The beam combination recording unit 17 assigns a new record ID to the generated one record and writes it into the beam combination history table 140.

[0053] (Configuration of the Terminal Device in the First Embodiment) FIG. 5 is a block diagram showing the configuration of the terminal device 40. The terminal device 40 includes M terminal antennas 41-1 to 41-M, an analog signal transceiver 42, a digital signal processing unit 43, a beam search signal receiver 44, an optimal beam selection unit 45, and a feedback signal generation unit 46. Here, M is an integer of 2 or more. Wireless communication using distributed MIMO is performed between the terminal antennas 41-1 to 41-M and the distributed antennas 31-1 to 31-4 included in each of the distributed antenna devices 30-1 to 30-4.

[0054] The analog signal transceiver 42 modulates a carrier wave based on the digital signal of the transmission data output by the digital signal processing unit 43 to generate an analog signal of a radio frequency. The analog signal transceiver 42 transmits the generated analog signal as radio waves through the terminal antennas 41-1 to 41-M. The analog signal transceiver 42 demodulates the analog signal output when the terminal antennas 41-1 to 41-M receive radio waves and converts it into a digital signal. The analog signal transceiver 42 outputs the converted digital signal to the digital signal processing unit 43. The analog signal transceiver 42 measures the received power of the beam received by the terminal antennas 41-1 to 41-M. The analog signal transceiver 42 outputs the received power value obtained by the measurement to the digital signal processing unit 43 in association with the digital signal corresponding to the beam to be measured.

[0055] The digital signal processing unit 43 outputs the digital signal of the feedback signal output by the feedback signal generation unit 46 to the analog signal transceiver 42. The digital signal processing unit 43 takes in the digital signal output by the analog signal transceiver 42 and the received power value associated with the digital signal. The digital signal processing unit 43 associates the received power value taken in with the beam search signal included as reception data in the taken-in digital signal and outputs it to the beam search signal receiver 44.

[0056] The beam search signal receiving unit 44 captures the beam search signal output by the digital signal processing unit 43 and the received power value associated with the beam search signal. The beam search signal receiving unit 44 combines the transmission source antenna ID and beam ID included in the captured beam search signal with the captured received power value, writes the combination as a set of data into an internal storage area, and stores it. When the beam search signal receiving unit 44 captures all the beam search signals for any one of the distributed antenna devices 30-1 to 30-4, among the data stored in the internal storage area, it detects and reads out all the data including the transmission source antenna ID corresponding to the distributed antenna devices 30-1 to 30-4 for which all the beam search signals have been captured. The beam search signal receiving unit 44 outputs all the read data as a set of data to the best beam selection unit 45.

[0057] The best beam selection unit 45 captures a set of data output by the beam search signal receiving unit 44. The best beam selection unit 45 selects the data corresponding to the maximum received power value among the captured set of data. In other words, the best beam selection unit 45 selects the beam indicated by the beam ID corresponding to the selected maximum received power value as the best beam in the distributed antenna devices 30-1 to 30-4 corresponding to the transmission source antenna ID. The best beam selection unit 45 outputs the transmission source antenna ID, beam ID, and received power value included in the selected data to the feedback signal generation unit 46. The feedback signal generation unit 46 generates a feedback signal including the transmission source antenna ID, beam ID, and received power value output by the best beam selection unit 45. The feedback signal generation unit 46 outputs the generated feedback signal to the digital signal processing unit 43.

[0058] (Processing by the terminal device according to the first embodiment) The processing by the terminal device 40 will be described with reference to FIGS. 6 and 7. FIG. 6 is a flowchart showing the flow of processing performed when the terminal device 40 receives the beams transmitted by the distributed antenna devices 30-1 to 30-4.

[0059] The analog signal transceiver unit 42 waits for the reception of the beams transmitted by the distributed antennas 31-1 to 31-4 of the distributed antenna devices 30-1 to 30-4 (step Sta1), and repeatedly determines whether the beams have been received through the terminal antennas 41-1 to 41-M (step Sta2). When the analog signal transceiver unit 42 determines that the beam has not been received (step Sta2, No), it continues the process of step Sta1, that is, waits for the reception of the beam.

[0060] On the other hand, when the analog signal transceiver unit 42 determines that the beam has been received (step Sta2, Yes), it measures the reception power of the beam received through the terminal antennas 41-1 to 41-M. The analog signal transceiver unit 42 converts the received beam into a digital signal. The analog signal transceiver unit 42 associates the reception power value obtained by measurement with the digital signal obtained by the conversion and outputs it to the digital signal processing unit 43. The digital signal processing unit 43 takes in the digital signal output by the analog signal transceiver unit 42 and the reception power value associated with the digital signal. The digital signal processing unit 43 detects the received data from the taken-in digital signal to obtain the beam search signal included as the received data in the digital signal. The digital signal processing unit 43 associates the obtained beam search signal with the taken-in reception power value and outputs it to the beam search signal reception unit 44.

[0061] The beam search signal reception unit 44 takes in the beam search signal output by the digital signal processing unit 43 and the reception power value associated with the beam search signal. The beam search signal reception unit 44 reads out the transmission source antenna ID and the beam ID included in the taken-in beam search signal (step Sta3).

[0062] The beam search signal receiving unit 44 is capable of generating a plurality of timers internally, and determines whether the read transmission source antenna ID is associated with any of the timers (step Sta4). When the beam search signal receiving unit 44 determines that the read transmission source antenna ID is not associated with any of the timers (step Sta4, No), it generates and activates one timer in association with the read transmission source antenna ID. When activating the timer, the beam search signal receiving unit 44 sets, in the timer, the time required for all the distributed antenna devices 30-1 to 30-4 that transmit the largest number of beams to transmit all the beams. It is assumed that the time is a predetermined time (step Sta5).

[0063] On the other hand, when the beam search signal receiving unit 44 determines that the read transmission source antenna ID is associated with any of the timers (step Sta4, Yes), or after the process of step Sta5, it combines the read transmission source antenna ID and beam ID with the captured received power value to form a set of data, and writes and stores the set of data in an internal storage area (step Sta6). Thereafter, the processes after step Sta1 are repeated.

[0064] Putting it another way for the determination process in step Sta4 above, in this determination process, the beam search signal receiving unit 44 determines whether the captured beam search signal is the first beam search signal in all beam searches or partial beam searches performed by each of the distributed antenna devices 30-1 to 30-4, based on whether the timer is activated. If no timer is generated in association with the transmission source antenna ID included in the beam search signal captured by the beam search signal receiving unit 44, the beam search signal is the first beam search signal, and if a timer is generated, the beam search signal is the beam search signal after the second time.

[0065] FIG. 7 is a flowchart showing the flow of processing performed when the timer started by the beam search signal receiving unit 44 expires. The beam search signal receiving unit 44 waits for the expiration of the timer started in the process of step Sta5 in FIG. 6. It is assumed that the timer outputs a timer expiration notification signal when the measured time reaches the set time (step Stb1).

[0066] The beam search signal receiving unit 44 repeatedly determines whether it has received a timer expiration notification from any of the timers (step Stb2). If it determines that it has not received a timer expiration notification (step Stb2, No), it continues the process of step Stb1, that is, waits for the expiration of the timer. On the other hand, when the beam search signal receiving unit 44 determines that it has received a timer expiration notification from any of the timers (step Stb2, Yes), it acquires the transmission source antenna ID associated with the expired timer and deletes the timer. The beam search signal receiving unit 44 detects and reads out all the data including the acquired transmission source antenna ID from the data stored in the internal storage area, that is, the data combining the transmission source antenna ID, the beam ID, and the received power value. After reading, the beam search signal receiving unit 44 deletes the data read from the internal storage area. The beam search signal receiving unit 44 outputs all the read data as a set of data to the best beam selection unit 45 (step Stb3).

[0067] The best beam selection unit 45 captures a set of data output by the beam search signal reception unit 44. The best beam selection unit 45 selects the data including the maximum received power value from the captured set of data (step Stb4). The best beam selection unit 45 outputs the transmission source antenna ID, the beam ID, and the received power value included in the selected data to the feedback signal generation unit 46. The feedback signal generation unit 46 captures the transmission source antenna ID, the beam ID, and the received power value output by the best beam selection unit 45, and generates a feedback signal including the captured transmission source antenna ID, the beam ID, and the received power value. The feedback signal generation unit 46 outputs the generated feedback signal to the digital signal processing unit 43 (step Stb5).

[0068] The digital signal processing unit 43 captures the feedback signal output by the feedback signal generation unit 46. The digital signal processing unit 43 outputs the captured feedback signal to the analog signal transceiver unit 42. The analog signal transceiver unit 42 generates a radio frequency analog signal from the feedback signal output by the digital signal processing unit 43. The analog signal transceiver unit 42 transmits the generated analog signal by radio waves through the terminal antennas 41-1 to 41-M (step Stb6).

[0069] (Processing by the wireless communication system of the first embodiment) FIG. 8 is a flowchart showing the overall processing performed by the wireless communication system 1. In the communication control device 10, for example, in response to an operation by an operator who operates the wireless communication system 1, first, in order to generate the beam combination history table 140, the beam combination history generation unit 13 performs a beam combination generation process (step S1). When the beam combination generation process by the beam combination history generation unit 13 ends, in response to an operation by the operator, the candidate beam detection unit 15 starts a beam search process (step S2). When the beam search process ends, for example, between the communication device connected to the digital signal processing device 20 and the terminal device 40, data transmission processing using the beams of the distributed antenna devices 30-1 to 30-4 that are selected by the beam search process is started (step S3). During the data transmission process, the candidate beam detection unit 15 repeatedly determines whether or not a beam search cycle timer described later has expired at regular intervals in order to determine whether or not the beam search cycle has elapsed (step S4). When the candidate beam detection unit 15 determines that the beam search cycle timer has not expired (step S4, No), the data transmission process continues. On the other hand, when the candidate beam detection unit 15 determines that the beam search cycle timer has expired (step S4, Yes), it outputs a data transmission end instruction signal for ending the data transmission process to the digital signal processing device 20. When receiving the data transmission end instruction signal, the digital signal processing device 20 stops the output of data to be transmitted to the distributed antenna devices 30-1 to 30-4. As a result, the data transmission process that has been performed between the communication device connected to the digital signal processing device 20 and the terminal device 40 via the distributed antenna devices 30-1 to 30-4 ends.

[0070] The operator determines whether to stop the beam search process by the communication control device 10 (step S5). If the operator determines not to stop the beam search process by the communication control device 10 (step S5, No), the process proceeds to step S2, and the candidate beam detection unit 15 starts the process for the next beam search cycle. On the other hand, if the operator determines to stop the beam search process by the communication control device 10 (step S5, Yes), the communication control device 10 is operated to stop the beam search process. Hereinafter, the details of the beam combination generation process in step S1 and the beam search process in step S2 will be described.

[0071] (Beam Combination Generation Process of the First Embodiment) FIG. 9 is a flowchart showing the flow of the beam combination generation process performed in the process of step S1 of FIG. 8. In response to the operation of the operator of the wireless communication system 1, the beam combination history generation unit 13 of the communication control device 10 starts the process for the first trial cycle of the beam combination generation process. The beam combination history generation unit 13 reads out all the distributed antenna IDs written in the distributed antenna ID item of the beam combination history table 140 stored in the beam combination history storage unit 14. The beam combination history generation unit 13 provides a counter i in its internal storage area and initializes it to i = 1. Here, i is an integer value from 1 to N. However, the wireless communication system in FIG. 1 includes four distributed antenna devices 30-1 to 30-4. The beam combination history generation unit 13 reads out four distributed antenna IDs, namely, "distributed antenna ID #1", "distributed antenna ID #2", "distributed antenna ID #3", and "distributed antenna ID #4" from the beam combination history table 140. Therefore, hereinafter, the description will be made with N = 4. The i-th distributed antenna device is referred to as the distributed antenna device 30-i, the i-th distributed antenna is referred to as the distributed antenna 31-i, and the i-th main body device is referred to as the main body device 32-i for the following description.

[0072] The beam combination history generation unit 13 outputs an output destination switching instruction signal whose output destination is the beam combination history generation unit 13 to the feedback signal reception unit 12. When the feedback signal reception unit 12 receives the output destination switching instruction signal whose output destination is the beam combination history generation unit 13 from the beam combination history generation unit 13, it sets the output destination of the feedback signal to the beam combination history generation unit 13.

[0073] In order to cause the distributed antenna device 30-i including the i-th distributed antenna 31-i to perform a full beam search, the beam combination history generation unit 13 outputs a full beam search request signal including the distributed antenna ID of the i-th distributed antenna 31-i to the beam search execution instruction unit 11. After the beam combination history generation unit 13 outputs the full beam search request signal to the beam search execution instruction unit 11, it starts a feedback signal timer provided therein. When starting the feedback signal timer, the beam combination history generation unit 13 sets the time required until the feedback signal from the full beam search performed by the distributed antenna devices 30-1 to 30-4 with the largest number of transmitted beams is acquired after the beam combination history generation unit 13 outputs the full beam search request signal to the beam search execution instruction unit 11 to the feedback signal timer. Note that the time is a predetermined time and is set in the beam combination history generation unit 13 in advance.

[0074] The beam search execution instruction unit 11 captures the full beam search request signal output by the beam combination history generation unit 13 and reads out the distributed antenna ID of the i-th distributed antenna 31-i included in the captured full beam request instruction signal. The beam search execution instruction unit 11 reads out the maximum beam ID corresponding to the distributed antenna ID of the i-th distributed antenna 31-i read from the beam number table 110 in the internal storage area. Here, as an example, it is assumed that the beam search execution instruction unit 11 reads out "40" as the maximum beam ID.

[0075] The beam search execution instruction unit 11 generates beam search instruction signals that match the maximum value of the read beam IDs, i.e., "40", and each contains a different beam ID from 1 to the maximum beam ID value one by one. The beam search execution instruction unit 11 writes the distributed antenna ID of the i-th distributed antenna 31-i read out for each of the generated beam search instruction signals. The beam search execution instruction unit 11 outputs the 40 generated beam search instruction signals to the digital signal processing device 20 one by one in the order of generation at a predetermined fixed time interval.

[0076] The digital signal processing device 20 sequentially captures the 40 beam search instruction signals output by the beam search execution instruction unit 11. The digital signal processing device 20 generates beam search signals from the captured beam search instruction signals. The digital signal processing device 20 outputs the generated beam search signals to the i-th main body device 32-i corresponding to the distributed antenna ID included in the beam search instruction signal in the order of generation. The i-th main body device 32-i captures the beam search signal output by the digital signal processing device 20. The i-th main body device 32-i modulates the carrier wave based on the beam search signal so as to form a beam in the direction corresponding to the beam ID included in the captured beam search signal. The i-th main body device 32-i generates an analog signal of a radio frequency with the beam search signal generated by modulation. By the i-th main body device 32-i outputting the generated analog signal of the radio frequency to the i-th distributed antenna 31-i, the i-th distributed antenna 31-i transmits a beam with the beam search signal in the direction of the beam ID included in the beam search signal (step Sa1).

[0077] The terminal device 40 receives all the beams transmitted by the distributed antenna 31-i. For each of the received beams, the terminal device 40 performs the processing described with reference to FIGS. 6 and 7. The distributed antenna 31-i receives the radio wave carrying the feedback signal transmitted by the terminal device 40. The distributed antenna 31-i outputs the received radio wave to the main body device 32-i as an analog signal. The main body device 32-i converts the analog signal including the feedback signal into a digital signal and outputs it to the digital signal processing device 20. The digital signal processing device 20 detects and acquires the feedback signal included in the digital signal output by the main body device 32-i. The digital signal processing device 20 outputs the acquired feedback signal to the feedback signal receiving unit 12 of the communication control device 10. The feedback signal receiving unit 12 takes in the feedback signal output by the digital signal processing device 20 and outputs it to the beam combination history generation unit 13 whose output destination is set.

[0078] The beam combination history generation unit 13 determines whether a feedback signal including the distributed antenna ID of the i-th distributed antenna 31-i as the transmission source antenna ID has been taken in before the time measured by the feedback signal timer reaches the time set in the feedback signal timer and the feedback signal timer expires (step Sa2).

[0079] If the beam combination history generation unit 13 determines that a feedback signal including the distributed antenna ID of the i-th distributed antenna 31-i as the transmission source antenna ID has been taken in before the feedback signal timer expires (step Sa2, Yes), the beam ID included in the taken-in feedback signal is set as the beam ID indicating the best beam in the i-th distributed antenna device 30-i. The beam combination history generation unit 13 writes and records the data obtained by combining the transmission source antenna ID included in the taken-in feedback signal and the beam ID in the internal storage area (step Sa3).

[0080] On the other hand, assume that the beam combination history generation unit 13 determines that it has not captured the feedback signal in which the distributed antenna ID of the i-th distributed antenna 31-i is included as the transmission source antenna ID before the feedback signal timer expires (step Sa2, No). Here, the case where the beam combination history generation unit 13 cannot capture the feedback signal includes, for example, when there is no line of sight between the i-th distributed antenna 31-i and the terminal device 40 and the beam transmitted by the i-th distributed antenna 31-i does not reach the terminal device 40; when the beam transmitted by the i-th distributed antenna 31-i reaches the terminal device 40 but the received level of the beam reaching the terminal device 40 is lower than the reception sensitivity of the terminal device 40, so the analog signal transceiver 42 of the terminal device 40 cannot demodulate it and discards it; when the reception level of the radio wave carrying the feedback signal transmitted by the terminal device 40 is lower than the reception sensitivity of the distributed antenna device 30-i, so the main body device 32-i cannot demodulate it and discards it; when the transmission of the radio wave carrying the feedback signal is delayed due to a delay in the internal processing of the terminal device 40 or the like, and after the feedback signal timer expires, the feedback signal reception unit 12 outputs the feedback signal to the beam combination history generation unit 13.

[0081] After the processing of step Sa3 or after making a "No" determination in the processing of step Sa2, if the value of i at that time is not N (here, N = 4), the value obtained by adding 1 to i is set as the new value of i, and the processing of steps Sa1 to Sa3 is performed again (loops La1s to La1e). When the value of i at that time is N (here, N = 4), the beam combination history generation unit 13 ends the processing of loops La1s to La1e. The beam combination history generation unit 13 reads out all the data written in the internal storage area, and after the reading, deletes all the data from the internal storage area. The beam combination history generation unit 13 generates a record indicating the best beam combination in the distributed antenna devices 30-1 to 30-4 based on all the read data (step Sa4).

[0082] The beam combination history generation unit 13 detects the number of records in the beam combination history table 140 that match the combination of beam IDs in the generated record from the combination of beam IDs of the records already recorded in the beam combination history table 140, and determines whether the detected number is less than a predetermined number of records (step Sa5). When the beam combination history generation unit 13 determines that the detected number is less than the predetermined number of records (step Sa5, No), the beam combination history generation unit 13 generates a new row in the beam combination history table 140. The beam combination history generation unit 13 generates a new record ID. The beam combination history generation unit 13 writes the generated new record ID into the "Record ID" item of the new row. The beam combination history generation unit 13 writes the corresponding beam ID into each element of "Distributed Antenna ID #1", "Distributed Antenna ID #2", "Distributed Antenna ID #3", and "Distributed Antenna ID #4" in the new row based on the combination of the transmission source antenna ID and the beam ID (step Sa6). Thereafter, as the processing for the next trial period, the beam combination history generation unit 13 performs the processing of loops La1s to La1e again.

[0083] On the other hand, assume that the beam combination history generation unit 13 determines that the detected number is not less than a predetermined number of records (step Sa5, Yes). In this case, during the beam combination generation process, within the range where the terminal device 40 has moved within the cell 100, it can be considered that the pattern of the best beam combination in the distributed antenna devices 30-1 to 30-4 has been sufficiently obtained. Therefore, the beam combination history generation unit 13 ends the process. Incidentally, before ending the process, if there are multiple records that are the same as the combination of beam IDs stored in the beam combination history table 140, the beam combination history generation unit 13 may delete other records leaving only one of the records. Also, in the beam combination generation process, even if the pattern of the best beam combination in the distributed antenna devices 30-1 to 30-4 has not been sufficiently obtained, instead of the determination process in step Sa5, the beam combination history generation unit 13 may perform the process in step Sa6 until the number of records stored in the beam combination history table 140 reaches a predetermined number of records, and when the predetermined number of records is reached, perform a determination process to end the process in FIG. 9. By doing so, it is possible to start the beam search process in step S2 of FIG. 8 when the number of records stored in the beam combination history table 140 reaches a certain amount of records.

[0084] As a result, for example, when the beam combination history generation unit 13 repeats the processing of loops La1s to La1e M times, that is, when the trial periods for M times are completed, as shown in FIG. 4, M records are generated in the beam combination history table 140. Note that in the beam combination history table 140 shown in FIG. 4, beam IDs are not written in "Record ID #2", "Record ID #4", and "Record ID #6" of "Distributed Antenna ID #3". This indicates that in the trial periods of "Record ID #2", "Record ID #4", and "Record ID #6", any one of the events where the beam combination history generation unit 13 cannot capture the feedback signal occurred, and thus the beam combination history generation unit 13 could not obtain the feedback signal from all beam searches performed by the distributed antenna device 30-3 corresponding to "Distributed Antenna ID #3".

[0085] (Beam Search Processing of the First Embodiment) FIG. 10 is a flowchart showing the flow of beam search processing performed in the processing of step S2 in FIG. 8. It is assumed that the beam combination history table 140 shown in FIG. 4 is generated in the beam combination history storage unit 14 as a premise for starting the beam search processing shown in FIG. 8.

[0086] Receiving the operation of the operator of the wireless communication system 1, the candidate beam detection unit 15 of the communication control device 10 starts the beam search processing. The candidate beam detection unit 15 activates a beam search period timer provided therein. When activating the beam search period timer, the candidate beam detection unit 15 sets a time indicating the length of a preset one beam search period.

[0087] The candidate beam detection unit 15 reads out all the distributed antenna IDs written in the distributed antenna ID item of the beam combination history table 140 stored in the beam combination history storage unit 14. The candidate beam detection unit 15 sets a counter i in its internal storage area and initializes it to i = 1. Here, i is an integer value from 1 to N. However, the wireless communication system in FIG. 1 includes four distributed antenna devices 30-1 to 30-4. Since the candidate beam detection unit 15 reads out four distributed antenna IDs, namely "distributed antenna ID #1", "distributed antenna ID #2", "distributed antenna ID #3", and "distributed antenna ID #4" from the beam combination history table 140, in the following, the description will be made with N = 4. The i-th distributed antenna device is referred to as the distributed antenna device 30-i, the i-th distributed antenna is referred to as the distributed antenna 31-i, and the i-th main body device is referred to as the main body device 32-i for the following description.

[0088] The candidate beam detection unit 15 outputs an output destination switching instruction signal to the feedback signal receiving unit 12 with the output destination being the candidate beam detection unit 15. When the feedback signal receiving unit 12 receives the output destination switching instruction signal with the output destination being the candidate beam detection unit 15 from the candidate beam detection unit 15, it sets the output destination of the feedback signal to the candidate beam detection unit 15.

[0089] When i = 1, the candidate beam detection unit 15 performs the following processing. That is, the candidate beam detection unit 15 outputs a full beam search request signal including "distributed antenna ID #1", which is the distributed antenna ID assigned to the first distributed antenna 31-1, to the beam search execution instruction unit 11 in order to cause the distributed antenna device 30-1 including the first distributed antenna 31-1 to perform a full beam search. After the candidate beam detection unit 15 outputs the full beam search request signal to the beam search execution instruction unit 11, it activates a feedback signal timer provided inside. When activating the feedback signal timer, the candidate beam detection unit 15 sets the same time as that set by the beam combination history generation unit 13 for the feedback signal timer in the process of step Sa1 in FIG. 9. Note that the time is a predetermined time and is set in advance in the candidate beam detection unit 15. When the beam search execution instruction unit 11 captures the full beam search request signal output by the candidate beam detection unit 15, then, in the process of step Sa1 in FIG. 9, after the beam search execution instruction unit 11 captures the full beam search request signal, the processing is performed by the beam search execution instruction unit 11, the digital signal processing device 20, the main body device 32-1, the distributed antenna 31-1, and the terminal device 40 with i = 1 (step Sb1 when i = 1).

[0090] The candidate beam detection unit 15 determines whether or not it has captured a feedback signal including "distributed antenna ID #1", which is the distributed antenna ID of the first distributed antenna 31-1, as the transmission source antenna ID before the feedback signal timer expires (step Sb2 when i = 1).

[0091] Here, assume that any event occurs when the beam combination history generation unit 13 described above cannot capture the feedback signal. In this case, the candidate beam detection unit 15 determines that it has not captured the feedback signal in which the "diversity antenna ID #1", which is the diversity antenna ID of the first diversity antenna 31-1, is included as the transmission source antenna ID before the feedback signal timer expires (step Sb2, No). If the value of i at that time is not N (here, N = 4), the candidate beam detection unit 15 sets the value obtained by adding 1 to i as the new value of i, and the processes of steps Sb1 and Sb2 are performed again (loops Lb1s to Lb1e). Here, since the value of i at that time is "1", the candidate beam detection unit 15 sets "2" as the new value of i.

[0092] When i = 2, the candidate beam detection unit 15 performs the following processing as the second time of processing in loops Lb1s to Lb1e. That is, the candidate beam detection unit 15 outputs a full beam search request signal including the "diversity antenna ID #2", which is the diversity antenna ID given to the second diversity antenna 31-2, to the beam search execution instruction unit 11 in order to cause the diversity antenna device 30-2 including the second diversity antenna 31-2 to perform a full beam search. After the candidate beam detection unit 15 outputs the full beam search request signal to the beam search execution instruction unit 11, it starts the feedback signal timer provided inside. When starting the feedback signal timer, the candidate beam detection unit 15 sets the same time as that set by the beam combination history generation unit 13 for the feedback signal timer in the process of step Sa1 in FIG. 9. When the beam search execution instruction unit 11 captures the full beam search request signal output by the candidate beam detection unit 15, then, in the process of step Sa1 in FIG. 9, after the beam search execution instruction unit 11 captures the full beam search request signal, the subsequent processing is performed by the beam search execution instruction unit 11, the digital signal processing device 20, the main body device 32-2, the diversity antenna 31-2, and the terminal device 40 with i = 2 (step Sb1 when i = 2).

[0093] The candidate beam detection unit 15 determines whether it has captured a feedback signal that includes the "diversity antenna ID #2," which is the diversity antenna ID of the second diversity antenna 31-2, as the transmission source antenna ID before the feedback signal timer expires (step Sb2 when i = 2). Here, it is assumed that none of the events occur when the beam combination history generation unit 13 cannot capture the feedback signal, and the feedback signal reception unit 12 outputs the feedback signal to the candidate beam detection unit 15. In this case, the candidate beam detection unit 15 determines that it has captured a feedback signal that includes the "diversity antenna ID #2," which is the diversity antenna ID of the second diversity antenna 31-2, as the transmission source antenna ID before the feedback signal timer expires (step Sb2 when i = 2, Yes).

[0094] The candidate beam detection unit 15 exits the processing of loops Lb1s to Lb1e, and sets the beam ID included in the captured feedback signal as the beam ID indicating the best beam in the second diversity antenna device 30-2. The candidate beam detection unit 15 designates the beam specified by the beam ID and the "diversity antenna ID #2," which is the transmission source antenna ID included in the feedback signal, as the detection reference beam. Here, it is assumed that the beam ID included in the feedback signal captured by the candidate beam detection unit 15 is "beam ID #33."

[0095] The candidate beam detection unit 15 combines the transmission source antenna ID, the beam ID, and the received power value included in the feedback signal into a set of data, and outputs the set of data to the beam combination recording unit 17. The beam combination recording unit 17 captures the set of data output by the candidate beam detection unit 15. The candidate beam detection unit 15 outputs an output destination switching instruction signal to the feedback signal reception unit 12 to change the output destination to the beam search execution determination unit 16. When the feedback signal reception unit 12 receives the output destination switching instruction signal from the candidate beam detection unit 15 to change the output destination to the beam search execution determination unit 16, it sets the output destination of the feedback signal to the beam search execution determination unit 16 (step Sb3).

[0096] Stated in another way regarding the processing of the above steps Sb2 and Sb3, the candidate beam detection unit 15 performs a process of causing all the distributed antenna devices 30-1 to 30-4 to perform full beam search in order. During this process, when one feedback signal is captured, the processing of loops Lb1s to Lb1e ends, and the beam specified by the transmission source antenna ID and the beam ID included in the captured feedback signal is set as the detection reference beam.

[0097] At this point, during the beam search period, the distributed antenna devices for which full beam search has not been performed are the two devices 30-3 and 30-4. If the wireless communication system 1 includes N distributed antenna devices 30-1 to 30-N, the number of distributed antenna devices 30-1 to 30-N for which full beam search has not been performed is represented by N - i using the value of i at the time of exiting loops Lb1s to Lb1e and the value of N. The candidate beam detection unit 15 provides a counter k in its internal storage area and initializes it to k = i + 1. k is an integer value from (i + 1) to N. Here, k takes the values of "3" and "4".

[0098] When k = 3, the candidate beam detection unit 15 performs the following processing. That is, the candidate beam detection unit 15 detects from the beam combination history table 140, as candidate beams, the beams of the third distributed antenna device 30-3 that have been selected together with the detection reference beam. Here, the detection reference beam is the beam specified by "beam ID #33" of "distributed antenna ID #2". In the beam combination history table 140, "beam ID #33" of "distributed antenna ID #2" corresponding to the detection reference beam is included in "record ID #2", "record ID #4", and "record ID #6", but the items of "distributed antenna ID #3" in "record ID #2", "record ID #4", and "record ID #6" are blank. Therefore, the candidate beam detection unit 15 detects as a detection result that there is no candidate beam ID indicating a candidate beam that is a beam of the third distributed antenna device 30-3 and has been selected together with the detection reference beam. The candidate beam detection unit 15 generates data indicating a detection result including only "distributed antenna ID #3" (step Sb4 when k = 3).

[0099] The candidate beam detection unit 15 outputs data indicating a detection result including only "distributed antenna ID #3" to the beam search execution determination unit 16 (step Sb5 when k = 3). When the beam search execution determination unit 16 receives the data indicating the detection result from the candidate beam detection unit 15, it starts the subroutine of the all beam search execution determination process shown in FIG. 11 (step Sb6 when k = 3).

[0100] The beam search execution determination unit 16 fetches the data indicating the detection result output by the candidate beam detection unit 15 (step Sc1 when k = 3). The beam search execution determination unit 16 determines whether the candidate beam ID is included in the fetched data indicating the detection result (step Sc2 when k = 3). Here, since the candidate beam ID is not included in the data indicating the detection result, the beam search execution determination unit 16 determines that the candidate beam ID is not included in the fetched data indicating the detection result (step Sc2, No when k = 3).

[0101] The processing of steps Sc3 to Sc6 will be described below. The beam search execution determination unit 16 reads out the "distributed antenna ID #k" included in the data indicating the detection result, and based on the read "distributed antenna ID #k", in order to cause the distributed antenna device 30-k including the k-th distributed antenna 31-k to perform a full beam search, it outputs a full beam search request signal including the "distributed antenna ID #k" to the beam search execution instruction unit 11. After the beam search execution determination unit 16 outputs the full beam search request signal to the beam search execution instruction unit 11, it activates the feedback signal timer provided inside. When activating the feedback signal timer, the beam search execution determination unit 16 sets the same time as that set by the beam combination history generation unit 13 for the feedback signal timer in the process of step Sa1 in FIG. 9. Note that the time is a predetermined time and is set in advance in the beam search execution determination unit 16. When the beam search execution instruction unit 11 captures the full beam search request signal output by the beam search execution determination unit 16, then, in the process of step Sa1 in FIG. 9, after the beam search execution instruction unit 11 captures the full beam search request signal, the subsequent process is performed by the beam search execution instruction unit 11, the digital signal processing device 20, the main body device 32-k, the distributed antenna 31-k, and the terminal device 40 with i = k (step Sc3).

[0102] The beam search execution determination unit 16 determines whether it has captured a feedback signal including the "distributed antenna ID #k", which is the distributed antenna ID of the k-th distributed antenna 31-k, as the transmission source antenna ID before the feedback signal timer expires (step Sc4).

[0103] For example, assume that none of the events occur when the beam combination history generation unit 13 described above cannot capture the feedback signal, and the feedback signal reception unit 12 outputs the feedback signal to the beam search execution determination unit 16. In this case, the beam search execution determination unit 16 determines that it has captured the feedback signal in which the "diversity antenna ID #k", which is the diversity antenna ID of the k-th diversity antenna 31-k, is included as the transmission source antenna ID before the feedback signal timer expires (step Sc4, Yes). The beam search execution determination unit 16 sets the beam ID included in the feedback signal as the beam ID indicating the best beam in the k-th diversity antenna device 30-k. The beam search execution determination unit 16 combines the transmission source antenna ID, the beam ID, and the received power value included in the feedback signal into a set of data, and outputs the set of data to the beam combination recording unit 17. The beam combination recording unit 17 captures the set of data output by the beam search execution determination unit 16. (Step Sc5).

[0104] On the other hand, assume that any one of the events occurs when the beam combination history generation unit 13 described above cannot capture the feedback signal. In this case, the beam search execution determination unit 16 determines that it has not captured the feedback signal in which the "diversity antenna ID #k", which is the diversity antenna ID of the k-th diversity antenna 31-k, is included as the transmission source antenna ID before the feedback signal timer expires (step Sc4, No).

[0105] After the process of step Sc5, or after making a "No" determination in the process of step Sc4, the beam search execution determination unit 16 outputs an end notification signal indicating that the process for the k-th diversity antenna device 30-k has ended, and the end notification signal includes the value of the counter k, to the candidate beam detection unit 15 (step Sc6), and ends the processing of the subroutine.

[0106] Returning to FIG. 10, when the candidate beam detection unit 15 captures the end notification signal output by the beam search execution determination unit 16, if the value of k at that time is not N (here, N = 4), the value obtained by adding 1 to k is set as the new value of k, and the processes of steps Sb4 to Sb6 are performed again (loops Lb2s to Lb2e). Here, since the value of k at that time is "3", the candidate beam detection unit 15 sets "4" as the new value of k.

[0107] When k = 4, the candidate beam detection unit 15 performs the following process. That is, the candidate beam detection unit 15 detects from the beam combination history table 140 the beam ID of the beam that is the beam of the fourth distributed antenna device 30-4 and has been selected together with the detection reference beam. Here, the detection reference beam is the beam specified by "beam ID #33" of "distributed antenna ID #2". In the beam combination history table 140, "beam ID #33" of "distributed antenna ID #2" corresponding to the detection reference beam is included in "record ID #2", "record ID #4", and "record ID #6". In the "distributed antenna ID #4" items of "record ID #2", "record ID #4", and "record ID #6", "beam ID #15", "beam ID #16", and "beam ID #15" are written.

[0108] Therefore, the candidate beam detection unit 15 detects, from the beam combination history table 140, the beam IDs "Beam ID #15" and "Beam ID #16" as the beams of the fourth distributed antenna device 30-4 that have been selected together with the detection reference beam. The candidate beam detection unit 15 sets "Beam ID #15" and "Beam ID #16" as candidate beam IDs indicating the candidate beams of the distributed antenna device 30-4 corresponding to "Distributed Antenna ID #4". The candidate beam detection unit 15 generates data indicating a detection result including "Distributed Antenna ID #4", "Beam ID #15" which is the candidate beam ID, and "Beam ID #16" (step Sb4 when k = 4). The candidate beam detection unit 15 outputs the generated data indicating the detection result to the beam search execution determination unit 16 (step Sb5 when k = 4). When receiving the data indicating the detection result from the candidate beam detection unit 15, the beam search execution determination unit 16 starts the subroutine of the all beam search execution determination process shown in FIG. 11 (step Sb6 when k = 4).

[0109] The beam search execution determination unit 16 fetches the data indicating the detection result output by the candidate beam detection unit 15 (step Sc1 when k = 4). The beam search execution determination unit 16 determines whether the candidate beam ID is included in the fetched data indicating the detection result (step Sc2 when k = 4). Here, since the data indicating the detection result includes "Beam ID #15" and "Beam ID #16", the beam search execution determination unit 16 determines that the candidate beam ID is included in the fetched data indicating the detection result (step Sc2 when k = 4, Yes).

[0110] The beam search execution determination unit 16 outputs a partial beam search request signal including "distributed antenna ID #4", "beam ID #15", and "beam ID #16" to the beam search execution instruction unit 11 in order to cause the distributed antenna device 30-4 including the fourth distributed antenna 31-4 to perform a partial beam search for the candidate beam ID. After the beam search execution determination unit 16 outputs the partial beam search request signal to the beam search execution instruction unit 11, it activates the feedback signal timer provided inside. When activating the feedback signal timer, the beam search execution determination unit 16 sets the same time as that set by the beam combination history generation unit 13 for the feedback signal timer in the process of step Sa1 in FIG. 9.

[0111] The beam search execution instruction unit 11 captures the partial beam search request signal output by the beam search execution determination unit 16, and reads out "distributed antenna ID #4", "beam ID #15", and "beam ID #16" included in the captured partial beam request instruction signal. The beam search execution instruction unit 11 generates a beam search instruction signal including "distributed antenna ID #4" and "beam ID #15", and a beam search instruction signal including "distributed antenna ID #4" and "beam ID #16". The beam search execution instruction unit 11 outputs the two generated beam search instruction signals to the digital signal processing device 20 one by one in the order of generation at a predetermined fixed time interval. The digital signal processing device 20 sequentially captures the two beam search instruction signals output by the beam search execution instruction unit 11. Thereafter, in the process of step Sa1 in FIG. 9, after the digital signal processing device 20 captures the beam search instruction signal, the processing is performed by the digital signal processing device 20, the main body device 32-4, the distributed antenna 31-4, and the terminal device 40 with i = 4 (step Sc7 when k = 4).

[0112] The beam search execution determination unit 16 determines whether or not it has captured a feedback signal including "distributed antenna ID #4", which is the distributed antenna ID of the fourth distributed antenna 31-4, as the transmission source antenna ID before the feedback signal timer expires (step Sc8).

[0113] For example, assume that any event occurs when the beam combination history generation unit 13 described above cannot capture the feedback signal. In this case, the beam search execution determination unit 16 determines that it has not captured the feedback signal in which the "diversity antenna ID #4", which is the diversity antenna ID of the fourth diversity antenna 31-4, is included as the transmission source antenna ID before the feedback signal timer expires (step Sc8, No when k = 4), and proceeds to step Sc3.

[0114] On the other hand, assume that any event does not occur when the beam combination history generation unit 13 described above cannot capture the feedback signal, and the feedback signal reception unit 12 outputs a feedback signal to the beam search execution determination unit 16. In this case, the beam search execution determination unit 16 determines that it has captured the feedback signal in which the "diversity antenna ID #4", which is the diversity antenna ID of the fourth diversity antenna 31-4, is included as the transmission source antenna ID before the feedback signal timer expires (step Sc8, Yes when k = 4).

[0115] In this case, the beam search execution determination unit 16 determines whether the received power value included in the feedback signal exceeds a predetermined threshold (step Sc9 when k = 4). When the beam search execution determination unit 16 determines that the received power value included in the feedback signal does not exceed the predetermined threshold (step Sc9, No when k = 4), it proceeds to step Sc3. On the other hand, when the beam search execution determination unit 16 determines that the received power value included in the feedback signal exceeds the predetermined threshold (step Sc9, Yes when k = 4), it proceeds to step Sc5.

[0116] Putting it another way for the processing of the above steps Sc7, Sc8, and Sc9, the beam search execution determination unit 16 causes the distributed antenna device 30-4 corresponding to the "distributed antenna ID #4" to perform a partial beam search for transmitting the beam corresponding to the "beam ID #15" and the beam corresponding to the "beam ID #16". When the feedback signal from the partial beam search cannot be obtained, the beam search execution determination unit 16 advances the process to step Sc3 and causes the distributed antenna device 30-4 corresponding to the "distributed antenna ID #4" to perform a full beam search to re-search for the best beam for the distributed antenna device 30-4.

[0117] On the other hand, when the feedback signal from the partial beam search is obtained, if the received power value included in the feedback signal, that is, the received power value of the beam selected as the best beam in the terminal device 40, does not exceed the threshold value, the beam search execution determination unit 16 determines that the beam indicated by the feedback signal is an inappropriate beam that cannot be used for normal operation. Therefore, the beam search execution determination unit 16 advances the process to step Sc3 and causes the distributed antenna device 30-4 corresponding to the "distributed antenna ID #4" to perform a full beam search to re-search for the best beam for the distributed antenna device 30-4.

[0118] On the other hand, when the received power value included in the feedback signal exceeds the threshold value, the beam search execution determination unit 16 determines that the beam is the best beam in the distributed antenna device 30-4. In this case, since the beam search execution determination unit 16 can set the beam as an appropriate beam that can be used for normal operation without performing a full beam search for the distributed antenna device 30-4, the number of beam searches can be reduced. The beam search execution determination unit 16 advances the process to step Sc5, so that the data related to the beam is used as the data to be added to the beam combination history table 140.

[0119] Returning to FIG. 10, when the candidate beam detection unit 15 captures the end notification signal output by the beam search execution determination unit 16, if the value of k at that time is N (here, N = 4), the processing of loops Lb2s to Lb2e is terminated, and the processing proceeds to step Sb7.

[0120] The beam combination recording unit 17 generates one record in the beam combination history table 140 based on the remaining data obtained by removing the received power value from the set of data captured in the processing of step Sb3 and step Sc5, that is, the data combining the transmission source antenna ID, the beam ID, and the received power value. That is, the beam combination recording unit 17 generates a new record ID "Record ID#M + 1" by generating a new row in the beam combination history table 140. The beam combination recording unit 17 writes the newly generated record ID "Record ID#M + 1" in the "Record ID" item of the newly generated row. The beam combination recording unit 17 writes the corresponding beam ID in each of the elements of "Diversity Antenna ID#1", "Diversity Antenna ID#2", "Diversity Antenna ID#3", and "Diversity Antenna ID#4" in the row of "Record ID#M + 1" based on the combination of the transmission source antenna ID and the beam ID (step Sb7).

[0121] In the wireless communication system 1 of the above-described first embodiment, during the beam search period in which the candidate beam detection unit 15 searches for a beam to be used for wireless communication with the terminal device 40, for each of the plurality of distributed antennas 31-1 to 31-4, all beam searches are performed by transmitting beams in all directions in which transmission is possible. When one beam identifier indicating the best beam among the beams obtained by the all beam search is acquired, the all beam search is stopped, and the beam specified by the acquired beam identifier and the information indicating the distributed antennas 31-1 to 31-4 that transmitted the beam indicated by the beam identifier is set as the detection reference beam. The beam identifier of a beam that is a beam identifier of the distributed antennas 31-1 to 31-4 that have not performed the all beam search during the beam search period and that has been selected together with the detection reference beam is detected from the beam combination history storage unit 14 as the candidate beam identifier for the distributed antennas 31-1 to 31-4. The beam search execution determination unit 16 determines whether to perform the all beam search for the distributed antennas 31-1 to 31-4 that have not performed the all beam search during the beam search period, based on the detection result of the candidate beam detection unit 15. The beam combination recording unit 17 generates a record indicating a combination of beam identifiers indicating the beams that have been the best beams at each of the distributed antennas during the beam search period. The beam combination recording unit 17 records the generated record in the beam combination history storage unit. As a result, in the beam combination generation process performed by the beam combination history generation unit 13, even if sufficient records indicating the history of the beam combination cannot be generated by moving the terminal device 40 little by little at intervals that do not reduce the transmission capacity over the entire cell 100 within the service area, when performing the beam search process, it is possible to accumulate a sufficient number of records to reduce the beam search count without reducing the transmission capacity.

[0122] While referring to FIG. 12, the content of the beam search process of the above-described first embodiment will be organized. For the distributed antenna device 30-1, in the process of step Sb2 when i = 1, the candidate beam detection unit 15 cannot obtain a feedback signal by all beam searches performed on the distributed antenna device 30-1 until the feedback signal timer expires, so it means that the beam cannot be searched. For the distributed antenna device 30-2, in the process of step Sb2 when i = 2, the candidate beam detection unit 15 can obtain a feedback signal by all beam searches performed on the distributed antenna device 30-2, and the beam indicated by the beam ID of "Beam ID #33" included in the obtained feedback signal can be searched as the best beam in the distributed antenna device 30-2.

[0123] For the distributed antenna device 30-3, the candidate beam detection unit 15 cannot detect the beam ID of the beam that has been selected together with the "Beam ID #33", which is the best beam in the distributed antenna device 30-2, from the beam combination history table 140. Therefore, as shown in FIG. 12(a), the beam search execution determination unit 16 proceeds to the process of step Sc3 in FIG. 11 to cause the distributed antenna device 30-3 to perform all beam searches. If the beam search execution determination unit 16 can obtain a feedback signal by all beam searches by the distributed antenna device 30-3, the best beam in the distributed antenna device 30-3 can be searched, and if the feedback signal cannot be obtained, the best beam in the distributed antenna device 30-3 cannot be searched.

[0124] Regarding the distributed antenna device 30-4, the candidate beam detection unit 15 can detect "beam ID #15" and "beam ID #16" from the beam combination history table 140 as the beam IDs of the beams that have been selected together with "beam ID #33", which is the best beam in the distributed antenna device 30-2. Therefore, as shown in Fig. 12(b), the beam search execution determination unit 16 causes the distributed antenna device 30-4 to perform a partial beam search using the beams indicated by "beam ID #15" and "beam ID #16" as candidate beams in step Sc7 of Fig. 11. The beam search execution determination unit 16 has acquired a feedback signal through the partial beam search by the distributed antenna device 30-4. Further, if the received power value included in the acquired feedback signal exceeds a predetermined threshold value, the beam ID included in the feedback signal can be searched as the best beam in the distributed antenna device 30-4. On the contrary, if the beam search execution determination unit 16 has not obtained a feedback signal through the partial beam search by the distributed antenna device 30-4, or if the received power value included in the feedback signal does not exceed the predetermined threshold value when the feedback signal has been obtained, the beam search execution determination unit 16 proceeds to the process of step Sc3 in Fig. 11 and causes the distributed antenna device 30-4 to perform a full beam search. If the beam search execution determination unit 16 can acquire a feedback signal through the full beam search by the distributed antenna device 30-4, the best beam in the distributed antenna device 30-4 can be searched. If the feedback signal cannot be acquired, the best beam in the distributed antenna device 30-4 cannot be searched.

[0125] Therefore, all the beam search processes performed in the processes of steps Sb1 and Sb2 in FIG. 10 and steps Sc3 and Sc4 in FIG. 11 can be regarded as processes for covering the shortage of records when a sufficient number of records are not recorded in the beam combination history table 140 by the beam combination generation process by the beam combination history generation unit 13. On the other hand, the partial beam search process performed in the processes of steps Sc7 to Sc9 in FIG. 11 is a process for searching for beams while reducing the number of beam searches. However, when a sufficient number of records are not recorded in the beam combination history table 140, when the candidate beams are narrowed down, the best beam may not be included in the narrowed candidate beams, resulting in a decrease in the transmission capacity. In such a case, the process proceeds to step Sc3 to perform a full beam search to newly select the best beam. As a result, even if there are not a sufficient number of records in the beam combination history table 140, it is possible to accumulate a sufficient number of records for performing the beam search process without reducing the transmission capacity and for reducing the number of beam searches.

[0126] (Second Embodiment) FIG. 13 is a block diagram showing the configuration of the communication control device 10a in the second embodiment. The communication control device 10a is a device used in place of the communication control device 10 in the first embodiment. Hereinafter, for convenience of explanation, the wireless communication system 1 including the communication control device 10a in place of the communication control device 10 is referred to as a wireless communication system 1a. In the communication control device 10a, the same components as those of the communication control device 10 in the first embodiment are denoted by the same reference numerals, and the different components will be described below.

[0127] The communication control device 10a includes a beam search execution instruction unit 11, a feedback signal reception unit 12, a beam combination history generation unit 13a, a beam combination history storage unit 14a, a candidate beam detection unit 15a, a beam search execution determination unit 16a, and a beam combination recording unit 17a.

[0128] The beam combination history storage unit 14a stores a beam combination history table 140a shown in FIG. 14. The beam combination history table 140a has the same data format as the beam combination history table 140 of the first embodiment shown in FIG. 4, except that the data written to the elements is different. The beam combination history table 140 of the first embodiment has a data configuration in which only the beam ID is written as an element. In contrast, the beam combination history table 140a of the second embodiment has the beam ID and the received power value measured by the terminal device 40 when the terminal device 40 receives the beam indicated by the beam ID written as elements. Note that, as the received power value, for example, a numerical value represented in the unit of "dBm" is written.

[0129] The beam combination history generation unit 13a has the same configuration as the beam combination history generation unit 13 of the first embodiment, except for the configuration described below. In the process of step Sa3 in FIG. 9, the beam combination history generation unit 13 of the first embodiment writes and records the data obtained by combining the transmission source antenna ID included in the captured feedback signal and the beam ID in an internal storage area. In contrast, in the process of step Sa3 in FIG. 9, the beam combination history generation unit 13a of the second embodiment adds the received power value included in the captured feedback signal, and writes and records the data obtained by combining the transmission source antenna ID, the beam ID, and the received power value in an internal storage area.

[0130] In the record generated by the beam combination history generation unit 13 of the first embodiment in the process of step Sa4 in FIG. 9, the received power value is not included. In contrast, the beam combination history generation unit 13a of the second embodiment generates a record in which the received power value is added to the transmission source antenna ID and the beam ID in the process of step Sa4 in FIG. 9. Therefore, the beam combination history generation unit 13a writes the record including the received power value into the beam combination history table 140a in the process of step Sa6. At this time, the beam combination history generation unit 13a writes each of the data obtained by combining the beam ID and the received power value included in the record as an element at the location specified by the newly generated row for the record in the beam combination history table 140a and the column of the transmission source antenna ID corresponding to the data obtained by combining the beam ID and the received power value. In the first embodiment, it is stated that when there are a plurality of records in the beam combination history table 140 in which the combinations of beam IDs are the same before the process ends, the beam combination history generation unit 13 may leave any one of the records and delete the other records. However, in the second embodiment, since there may be cases where the received power values are different even if the combinations of beam IDs are the same, it is necessary to leave all the records written in the beam combination history table 140a.

[0131] The candidate beam detection unit 15a has the same configuration as the candidate beam detection unit 15 of the first embodiment, except for the configuration described below. When the candidate beam detection unit 15a detects the beam ID of the distributed antenna devices 30-1 to 30-4 that have not performed a full beam search during the beam search period and that has been selected together with the detection reference beam, and the distributed antenna ID corresponding to the beam ID from the beam combination history table 140a, the candidate beam detection unit 15a detects the received power value written as an element together with the detected beam ID in the beam combination history table 140a. Based on the combination of the detected beam ID and the detected received power value, the candidate beam detection unit 15a calculates the average value of the received power values for each beam ID, and sets the maximum value of the calculated average values as the average received power value for the detected distributed antenna ID. The candidate beam detection unit 15a includes the calculated average received power value in the data indicating the detection result.

[0132] The beam search execution determination unit 16a has the same configuration as the beam search execution determination unit 16 of the first embodiment, except for the configuration described below. When the data indicating the detection result output by the candidate beam detection unit 15 includes the candidate beam ID, the beam search execution determination unit 16a performs the following processing. That is, the beam search execution determination unit 16a reads out the average received power value included in the data indicating the detection result, and sets the value obtained by adding a margin to the read average received power value as the threshold for the distributed antenna ID included in the data indicating the detection result. Here, the reason for setting the value obtained by adding a margin to the average received power value as the threshold is to allow a received power value slightly smaller than the average received power value. Specifically, the beam search execution determination unit 16a sets the value of the error occurring in the pre-calculated received power value as a predetermined value, and sets the subtraction value obtained by subtracting the predetermined value from the average received power value as the threshold.

[0133] The beam search execution determination unit 16a selects, from among the calculated thresholds, a threshold for the transmission source antenna ID included in the feedback signal obtained when causing any of the distributed antenna devices 30-1 to 30-4 to perform the partial beam search, and based on the selected threshold and the received power value included in the feedback signal, determines whether to cause the distributed antenna devices 30-1 to 30-4 corresponding to the transmission source antenna ID included in the feedback signal to perform the full beam search.

[0134] The beam combination recording unit 17a has the same configuration as the beam combination recording unit 17 of the first embodiment, except for the configuration described below. The beam combination recording unit 17 of the first embodiment generated one record based on the remaining data obtained by removing the received power value from the data combining the transmission source antenna ID, the beam ID, and the received power value in the process of step Sb7 in FIG. 10. In contrast, the beam combination recording unit 17a of the second embodiment generates one record from the data combining the transmission source antenna ID, the beam ID, and the received power value without removing the received power value in the process of step Sd7 in FIG. 15 corresponding to the process of step Sb7 in FIG. 10. The beam combination recording unit 17a writes each of the data combining the beam ID and the received power value included in the record as an element at the location specified by the newly generated row for the record in the beam combination history table 140a and the column of the transmission source antenna ID corresponding to the data combining the beam ID and the received power value.

[0135] (Processing by the wireless communication system of the second embodiment) Similar to the first embodiment, in the wireless communication system 1a of the second embodiment, the beam combination generation process in step S1, the beam search process in step S2, and the determination process in step S3 shown in FIG. 8 are also performed. However, in the points described below, the beam combination generation process and the beam search process performed in the second embodiment are different from the processes performed in the first embodiment.

[0136] (Beam combination generation process of the second embodiment) In the beam combination generation process shown in FIG. 9, except that the processes in steps Sa3, Sa4, and Sa6 are replaced with the above-described processes, the same processes as those in the first embodiment are performed by the beam search execution instruction unit 11, the feedback signal reception unit 12, the digital signal processing device 20, and the distributed antenna devices 30-1 to 30-4, and the process that was performed by the beam combination history generation unit 13 is performed by the beam combination history generation unit 13a.

[0137] (Beam Search Processing of the Second Embodiment) FIG. 15 is a flowchart showing the beam search process of the second embodiment, and FIG. 16 is a flowchart showing the process of the subroutine for determining the execution of all beam searches performed in step Sd6 of FIG. 15. In FIG. 15, the processes in steps Sd1 and Sd2, the processes in the loop Ld1s to Ld1e that repeats the processes in steps Sd1 and Sd2, and the process in step Sd3 are the same as the processes in steps Sb1 and Sb2, the processes in the loop Lb1s to Lb1e that repeats the processes in steps Sb1 and Sb2, and the process in step Sb3 in FIG. 10, and are performed by the beam search execution instruction unit 11, the feedback signal reception unit 12, the digital signal processing device 20, and the distributed antenna devices 30-1 to 30-4. The process that was performed by the candidate beam detection unit 15 is performed by the candidate beam detection unit 15a. The process in step Sd7 is performed by the beam combination recording unit 17a with the above-described process.

[0138] In the process of the loop Ld2s to Ld2e, when k = 3, similar to the first embodiment, the candidate beam detection unit 15a cannot detect the candidate beam ID corresponding to "distributed antenna ID #3" from the beam combination history table 140a. Therefore, in this case, the candidate beam detection unit 15a cannot detect the received power value either. Thus, the candidate beam detection unit 15a does not calculate the average received power value, and as in the process of step Sb4 of the first embodiment, there is no candidate beam ID indicating the beam of the third distributed antenna device 30-3 that has been selected together with the detection reference beam, which is regarded as the detection result. The candidate beam detection unit 15a generates data indicating a detection result including only "distributed antenna ID #3" (step Sd4 when k = 3).

[0139] The candidate beam detection unit 15a outputs data indicating a detection result including only "distributed antenna ID #3" to the beam search execution determination unit 16a (step Sb5 when k = 3). When the beam search execution determination unit 16a receives the data indicating the detection result from the candidate beam detection unit 15a, it starts the subroutine of the all beam search execution determination process shown in FIG. 16 (step Sd6 when k = 3).

[0140] Regarding the subroutine of the all beam search execution determination process performed in step Sd6 of the beam search process of the second embodiment, it will be described with reference to FIG. 16. The processes from step Se1 to Se6 are the same as the processes of step Sc1 to Sc6 shown in FIG. 11, which are performed by the beam search execution instruction unit 11, the feedback signal reception unit 12, the digital signal processing device 20, and the distributed antenna devices 30-1 to 30-4. The process that was performed by the beam search execution determination unit 16 is now performed by the beam search execution determination unit 16a. Therefore, when k = 3, in the determination process of step Se2, the beam search execution determination unit 16a determines "No", and the process proceeds to step Se3. After that, the same process as when k = 3 in the first embodiment is performed.

[0141] In the process of the loop Ld2s to Ld2e shown in FIG. 15, assume that the process has advanced to k = 4. Similar to the first embodiment, the candidate beam detection unit 15a detects from the beam combination history table 140a the beam ID of the beam of the fourth distributed antenna device 30-4 that has been selected together with the detection reference beam. Here, the detection reference beam is the beam specified by "beam ID #33" of "distributed antenna ID #2". In the beam combination history table 140a, "beam ID #33" of "distributed antenna ID #2" corresponding to the detection reference beam is included in "record ID #2", "record ID #4", and "record ID #6". In the items of "distributed antenna ID #4" of "record ID #2", "record ID #4", and "record ID #6", "beam ID #15", "beam ID #16", and "beam ID #15" are written.

[0142] Therefore, the candidate beam detection unit 15a detects "beam ID #15" and "beam ID #16" from the beam combination history table 140 as the beam IDs of the beams of the fourth distributed antenna device 30-4 that have been selected together with the detection reference beam. The candidate beam detection unit 15a uses "beam ID #15" and "beam ID #16" as candidate beam IDs indicating candidate beams of the distributed antenna device 30-4 corresponding to "distributed antenna ID #4".

[0143] The candidate beam detection unit 15a further detects, for "Beam ID #15", "(Received power value 2 - 4)" of "Record ID #2" and "(Received power value 6 - 4)" of "Record ID 6", and for "Beam ID #16", detects "(Received power value 4 - 4)" of "Record ID #4". The candidate beam detection unit 15a calculates the average value of "(Received power value 2 - 4)" and "(Received power value 6 - 4)" corresponding to "Peak ID #15". For "Beam ID #16", since there is only one "(Received power value 4 - 4)", "(Received power value 4 - 4)" is used as the average value. The candidate beam detection unit 15a sets the maximum average value among the average value corresponding to "Peak ID #15" and the average value of "Beam ID #16" as the average received power value for "Diversity antenna ID #4". The candidate beam detection unit 15 generates data indicating a detection result including "Diversity antenna ID #4", "Beam ID #15" which is the candidate beam ID, "Beam ID #16", and the calculated average received power value (step Sd4 when k = 4). The candidate beam detection unit 15a outputs the data indicating the generated detection result to the beam search execution determination unit 16a (step Sd5 when k = 4).

[0144] In the process of step Se1 of the subroutine of the all beam search execution determination process of FIG. 16 performed in the subsequent step Sd6 process, the beam search execution determination unit 16a fetches the detection result data output by the candidate beam detection unit 15a, and in the process of step Se2, determines that the candidate beam ID is included in the data indicating the fetched detection result (step Se2, Yes when k = 4).

[0145] The processes of steps Se7 and Se8 are respectively the same processes as steps Sc7 and Sc8 in FIG. 11, which are performed by the beam search execution instruction unit 11, the feedback signal reception unit 12, the digital signal processing device 20, and the distributed antenna device 30-4. The process that was performed by the beam search execution determination unit 16 is now performed by the beam search execution determination unit 16a. When the beam search execution determination unit 16a determines "Yes" in the determination process of step Se8, it reads out the average received power value included in the data indicating the detection result, gives a margin to the read average received power value, and calculates a threshold value for "Distributed Antenna ID #4" (step Se9). The beam search execution determination unit 16a determines whether the received power value included in the feedback signal exceeds the threshold value calculated for the transmission source antenna ID (here, "Distributed Antenna ID #4") included in the feedback signal (step Se10 when k = 4).

[0146] When the beam search execution determination unit 16a determines that the received power value included in the feedback signal exceeds the threshold value calculated for the transmission source antenna ID included in the feedback signal (step Se10, Yes), the process then proceeds to the process of step Se5. On the other hand, when the beam search execution determination unit 16a determines that the received power value included in the feedback signal does not exceed the threshold value calculated for the transmission source antenna ID included in the feedback signal (step Se10, No), the process then proceeds to the process of step Se3.

[0147] Thus, in the second embodiment, when the beam search execution determination unit 16a determines to cause any one of the distributed antenna devices 30-1 to 30-4 to perform a partial beam search, if the received power value included in the feedback signal obtained by the partial beam search is about the past average received power value of the distributed antenna devices 30-1 to 30-4 targeted by the partial beam search, the beam indicated by the beam ID included in the feedback signal can be searched as the best beam of the distributed antenna devices 30-1 to 30-4 corresponding to the transmission source antenna ID included in the feedback signal. The beam search execution determination unit 16a can add data related to the beam to the beam combination history table 140a. On the other hand, if the received power value included in the feedback signal obtained by the partial beam search is not about the past average received power value of the distributed antenna devices 30-1 to 30-4 targeted by the partial beam search, the beam search execution determination unit 16a causes the distributed antenna devices 30-1 to 30-4 corresponding to the transmission source antenna ID included in the feedback signal to perform a full beam search. If the beam search execution determination unit 16 can obtain a feedback signal by the full beam search, the best beam in the distributed antenna devices 30-1 to 30-4 targeted by the full beam search can be searched, and if the feedback signal cannot be obtained, the best beam in the distributed antenna devices 30-1 to 30-4 targeted by the full beam search cannot be searched. Therefore, in the second embodiment, in addition to the effects achieved by the communication control device 10 of the first embodiment, when causing a partial beam search to be performed, the best beam can be searched more accurately than in the first embodiment.

[0148] (Supplementary form) In the above-described first and second embodiments, the processes of searching for transmission-side beams in the distributed antenna devices 30-1 to 30-4, the digital signal processing device 20, and the communication control devices 10 and 10a are shown. For example, when the wireless communication systems 1 and 1a are systems such as FDD that use different frequencies for transmission and reception, the terminal device 40 on the receiving side needs to perform a process of searching for a receiving-side beam. The process of searching for a receiving-side beam is performed, for example, as follows. The terminal device 40 on the receiving side transmits a signal requesting a reception beam search procedure to each of the distributed antennas 31-1 to 31-4 on the transmission side. When the digital signal processing device 20 captures a signal requesting a reception beam search procedure via the distributed antenna devices 30-1 to 30-4, it causes each of the distributed antenna devices 30-1 to 30-4 to periodically transmit a signal through the distributed antennas 31-1 to 31-4 provided therein. The terminal device 40 switches the direction of the receiving-side beam formed by the plurality of terminal antennas 41-1 to 41-M and receives the signal periodically transmitted by the distributed antennas 31-1 to 31-4. The terminal device 40 measures the reception power of the received signal. Thereby, the terminal device 40 can select a receiving-side beam by determining which direction of the receiving-side beam formed by the terminal antennas 41-1 to 41-M has the best reception power value when receiving. Note that when each of the distributed antenna devices 30-1 to 30-4 is periodically transmitting a signal through the distributed antennas 31-1 to 31-4 provided therein, the terminal device 40 may receive the signal and select a receiving-side beam without transmitting a signal requesting a reception beam search procedure. In this process of selecting a receiving-side beam, the mechanisms of the above-described first and second embodiments may also be applied. Conversely, even in a configuration where the terminal device 40 searches for a transmission-side beam and the distributed antenna devices 30-1 to 30-4, the digital signal processing device 20, and the communication control devices 10 and 10a search for a receiving-side beam, the mechanisms of the above-described first and second embodiments may be applied.

[0149] In the above-described first and second embodiments, the terminal device 40 measures the received power and selects the best beam based on the received power value obtained by the measurement. Here, the received power value is an example, and the terminal device 40 may measure other reception quality indicators such as the carrier-to-noise ratio and the signal-to-noise ratio, and select the best beam based on the value indicating the measured reception quality.

[0150] In the above-described first and second embodiments, the terminal device 40 selects the best beam in each of the distributed antenna devices 30-1 to 30-4 based on a plurality of beam search signals transmitted by each of the distributed antennas 31-1 to 31-4. On the other hand, the communication control devices 10 and 10a may select the best beam in each of the distributed antenna devices 30-1 to 30-4. For example, if the terminal device 40 does not include the best beam selection unit 45, the beam search signal reception unit 44 outputs all the read data as a set of data to the feedback signal generation unit 46 in the process of step Stb3 in FIG. 7. The feedback signal generation unit 46 generates one feedback signal including the transmission source antenna ID common to all of the set of data, the plurality of beam IDs included in the set of data, and the received power value corresponding to the beam ID. The beam combination history generation units 13 and 13a, the candidate beam detection units 15 and 15a, and the beam search execution determination units 16 and 16a select the best beam for the distributed antenna devices 30-1 to 30-4 corresponding to the transmission source antenna ID included in the feedback signal based on the combination of the plurality of beam IDs included in the one feedback signal taken in and the received power values corresponding to each of the plurality of beam IDs.

[0151] In the above-described first and second embodiments, it is assumed that the operator designates the timing of starting the beam combination generation process in step S1 and the timing of starting the beam search process in step S2 by operating the communication control devices 10 and 10a. On the other hand, when the operator operates the communication control device 10 and the beam combination generation process in step S1 is started, after the process in step S1, the beam combination history generation unit 13 may activate the candidate beam detection unit 15 so that the beam search process in step S2 is automatically started without the operator's operation.

[0152] In the above-described first and second embodiments, the beam search process in step S2 may be started without performing the beam combination generation process in step S1. In this case, in the state before the beam search process is started, there are no records in the beam combination history tables 140 and 140a of the beam combination history storage units 14 and 14a. Therefore, the candidate beam detection units 15 and 15a cannot detect candidate beams. Thus, in the first embodiment, the entire beam search processes in steps Sb1 and Sb2 in FIG. 10 and steps Sc3 and Sc4 in FIG. 11, and in the second embodiment, the entire beam search processes in steps Sd1 and Sd2 in FIG. 15 and steps Se3 and Se4 in FIG. 16 are mainly performed. By repeatedly performing the entire beam search processes in these beam search processes, records are accumulated in the beam combination history tables 140 and 140a of the beam combination history storage units 14 and 14a, and gradually, the partial beam search processes are performed. Therefore, in this case, since the communication control devices 10 and 10a do not need to include the beam combination history generation units 13 and 13a, when performing the beam search process, it is possible to accumulate a sufficient number of records to reduce the transmission capacity and the number of beam searches without generating records indicating the history of beam combinations in advance.

[0153] In the above-described first and second embodiments, as shown in the flowchart of FIG. 7, the beam search signal receiving unit 44 of the terminal device 40 performs the process of step Stb3 after the timer expires. In contrast, if the beam search signal receiving unit 44 can determine that it has received all the beams carrying the beam search signals transmitted by the distributed antenna devices 30-1 to 30-4 corresponding to a certain transmission antenna ID, as described below, it may perform the process of step Stb2 without waiting for the timer to expire. For example, assume that the number of beams that the distributed antenna devices 30-1 to 30-4 can transmit is the same for all and this number is known. In this case, each time the beam search signal receiving unit 44 captures the beam search signal output by the digital signal processing unit 43, it counts the number of captured beam search signals for each transmission source antenna ID included in the beam search signal. Each time the beam search signal receiving unit 44 counts, it determines whether the counted number for each of the transmission source antenna IDs matches the number of beams that the known distributed antenna devices 30-1 to 30-4 can transmit. Assume that the beam search signal receiving unit 44 determines that the counted number for each of any of the transmission source antenna IDs matches the number of beams that the known distributed antenna devices 30-1 to 30-4 can transmit. In this case, since the beam search signal receiving unit 44 can consider that it has captured all the beam search signals transmitted by the distributed antenna devices 30-1 to 30-4 corresponding to the transmission source antenna ID, it can perform the process of step Stb3 without waiting for the expiration of the timer associated with the transmission source antenna ID.

[0154] In the above-described first and second embodiments, when the beam search execution instruction unit 11 generates a plurality of beam search instruction signals corresponding to a certain one distributed antenna ID, the beam search instruction signal that is first output to the digital signal processing device 20 among the plurality of generated beam search instruction signals is generated by adding the number of generated beam search instruction signals, in other words, the number of beams transmitted by the distributed antennas 31-1 to 31-4 corresponding to the distributed antenna ID, and the transmission timing which is the interval for transmitting the beams. Here, the transmission timing may be a time appropriately determined in the wireless communication system 1, or may be a time determined in the specifications or the like. In this case, when the digital signal processing device 20 receives a beam search instruction signal including the number of beams, the transmission timing, the distributed antenna ID, and the beam ID, the digital signal processing device 20 generates a beam search signal including the number of beams, the transmission timing, the transmission source antenna ID, and the beam ID from the received beam search instruction signal. Since the digital signal processing device 20 outputs the generated beam search signal to the main body devices 32-1 to 32-4 corresponding to the transmission source antenna ID in the order of generation, the beam search signal including the number of beams and the transmission timing will reach the terminal device 40 first. The beam search signal receiving unit 44 of the terminal device 40 can calculate the time to be set in the timer based on the number of beams and the transmission timing included in the received beam search signal in the process of step Sta5, and set the calculated time in the timer. By doing so, the terminal device 40 can grasp the number of beams transmitted by a certain one distributed antenna device 30-1 to 30-4 during the beam search period when receiving the first beam search signal corresponding to the certain one distributed antenna device 30-1 to 30-4, and can set a more appropriate time in the timer, thereby shortening the time required for the beam search process.

[0155] In the above-described second embodiment, the candidate beam detection unit 15a detects the received power value written in the same element location of the beam combination history table 140a together with the detected candidate beam ID in the process of step Sd4 in FIG. 15, and calculates the average received power value. On the other hand, the average received power value may be calculated by the following procedure. For example, the candidate beam detection unit 15a only detects the candidate beam ID in the process of step Sd4 in FIG. 15, and generates data indicating a detection result including the detected candidate beam ID, the distributed antenna ID of the processing target, and data indicating the detection reference beam, and outputs the data to the beam search execution determination unit 16a. The beam search execution determination unit 16a may detect the received power value written in the same element location together with the candidate beam ID based on the distributed antenna ID and the data indicating the detection reference beam included in the data indicating the detection result, for example, after the process of step Se7 in FIG. 16, and calculate the average received power value. By doing so, although the process of detecting the same candidate beam ID from the beam combination history table 140a is performed in the two processes of step Sd4 and step Se7, the calculation process of the average received power value can be performed only when the determination in step Se2 is "Yes". In the process of step Se8, the average received power value can be calculated using the time until the feedback signal is captured, and the threshold value can be calculated from the calculated average received power value.

[0156] In the above-described first and second embodiments, the terminal device 40 includes a plurality of terminal antennas 41-1 to 41-M, but it may be configured to include only one terminal antenna. In this case, when the distributed antenna devices 30-1 to 30-4 are on the transmission side and the terminal device 40 is on the reception side, MISO (Multiple Input Single Output) will be performed. When the distributed antenna devices 30-1 to 30-4 are on the reception side and the terminal device 40 is on the transmission side, SIMO (Single Input Multiple Output) will be performed. Further, when the terminal device 40 includes only one terminal antenna, site diversity may be performed in which one of the distributed antenna devices 30-1 to 30-4 with the best received power value is adaptively selected from a plurality of candidate distributed antenna devices 30-1 to 30-4 for wireless communication.

[0157] In the configurations of the above-described first and second embodiments, in the processes of step Sc9 shown in FIG. 11 and step Se10 shown in FIG. 16, a determination process of whether the received power value exceeds a threshold is performed. However, the present invention is not limited to the above embodiment, and the determination process of "whether it exceeds or not" is merely an example, and depending on how the threshold is determined, it may be replaced with a determination process of whether the received power value is equal to or greater than the threshold.

[0158] In the above-described first and second embodiments, the communication control devices 10, 10a may be configured to include a digital signal processing device 20 therein. The beam search execution instruction unit 11 and the feedback signal reception unit 12 of the communication control devices 10, 10a may be configured to be provided in the digital signal processing device 20.

[0159] In the above-described first and second embodiments, when performing full beam search, beam search signals are transmitted for all beam directions. In contrast, for example, when performing full beam search, a two-stage beam search may be applied in which a rough beam search is performed with a beam having a wide beam width in the first stage, and a precise beam search is performed with a beam having a narrow beam width within the range selected in the first stage in the second stage. Alternatively, when the position information of the terminal device 40 is known, the beam search may be performed by narrowing down to the vicinity of the direction in which the terminal device 40 exists. Alternatively, the beam search may be performed only in the vicinity of the beam that was previously connected to the terminal device 40.

[0160] In the above-described first and second embodiments, when the terminal device 40 is stationary, it is assumed that the beam combination history generation units 13, 13a and the beam combination recording units 17, 17a may generate records with the same combination of beam IDs in a short period of time. In this case, in order to prevent records with the same combination of beam IDs from being continuously recorded in the beam combination history tables 140, 140a, for example, the beam combination history generation units 13, 13a and the beam combination recording units 17, 17a store the record written immediately before in the internal storage area with respect to the beam combination history tables 140, 140a. If a record with the same combination of beam IDs as the record written immediately before stored in the internal storage area is generated within a predetermined fixed time, the generated record may be discarded without being written into the beam combination history tables 140, 140a.

[0161] The communication control devices 10 and 10a in the above-described first and second embodiments may be implemented by a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on this recording medium may be read into a computer system and executed to realize it. Here, the "computer system" shall include hardware such as an OS and peripheral devices. Further, the "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, etc., and a storage device such as a hard disk built into a computer system. Furthermore, the "computer-readable recording medium" also includes, like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, something that dynamically holds a program for a short time, and also includes something that holds a program for a certain time, like a volatile memory inside a computer system that serves as a server or a client in that case. Also, the above program may be for realizing a part of the aforementioned functions, and may further be something that can be realized in combination with a program already recorded in a computer system, and may also be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0162] As described above, the embodiments of this invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs etc. within the scope not departing from the gist of this invention are also included.

Industrial Applicability

[0163] It can be applied to a wireless communication system equipped with distributed antennas.

Explanation of Signs

[0164] 1…Wireless communication system, 10…Communication control device, 11…Beam search execution instruction unit, 12…Feedback signal reception unit, 13…Beam combination history generation unit, 14…Beam combination history storage unit, 15…Candidate beam detection unit, 16…Beam search execution determination unit, 17…Beam combination recording unit, 20…Digital signal processing device, 30-1 to 30-4…Distributed antenna device, 31-1 to 31-4…Distributed antenna, 32-1 to 32-4…Main body device, 40…Terminal device

Claims

1. In a beam search period for searching for a beam used for wireless communication with a terminal device, for each of a plurality of distributed antennas, perform a full beam search by transmitting beams in all directions that can be transmitted, and when one beam identifier indicating the best beam among the beams obtained by the full beam search is acquired, stop the full beam search, and use the acquired beam identifier and the information indicating the distributed antenna that transmitted the beam indicated by the beam identifier as a detection reference beam, and a candidate beam detection step of detecting, from a beam combination history storage unit, as a candidate beam identifier for the distributed antenna, a beam identifier of a beam that is a beam identifier of the distributed antenna that has not performed the full beam search during the beam search period and that has been selected together with the detection reference beam; a beam search execution determination step of determining whether to perform a full beam search on the distributed antennas that have not performed the full beam search during the beam search period based on the detection result of the candidate beam detection step; a beam combination recording step of generating a record indicating a combination of beam identifiers indicating the beams that have been the best beams in each of the distributed antennas during the beam search period, and recording the generated record in the beam combination history storage unit; A communication control method including the above steps.

2. In the beam search execution determination step, determine, as the distributed antenna for which the full beam search is to be performed, the distributed antenna that has not performed the full beam search during the beam search period and for which the candidate beam identifier has not been detected by the candidate beam detection step; in the beam combination recording step, the beam that has been the best beam in each of the distributed antennas during the beam search period is the beam that the terminal device has determined to be the best among the beams transmitted by the distributed antennas for which the full beam search has been performed during the beam search period; The communication control method according to Claim 1.

3. In the beam search execution determination step, Causing a partial beam search to be performed on the distributed antenna on which the candidate beam identifier has been detected by the candidate beam detection step, to transmit the beam indicated by the candidate beam identifier, and based on a value indicating the reception quality of the beam that the terminal device has determined to be the best among the beams obtained by the partial beam search, and a threshold value, determining whether to cause the distributed antenna to perform a full beam search; In the beam combination recording step, the beam determined to be the best among the beams of each of the distributed antennas during the beam search period is, when it is determined by the beam search execution determination step not to cause the distributed antenna to perform a full beam search, the beam that the terminal device has determined to be the best among the beams obtained by the immediately preceding partial beam search performed by the distributed antenna, or, when it is determined by the beam search execution determination step to cause the distributed antenna to perform a full beam search, the beam that the terminal device has determined to be the best among the beams obtained by the full beam search to be performed by the distributed antenna; The communication control method according to claim 1 or claim 2.

4. The threshold value is a predetermined value. The communication control method according to claim 3.

5. In the beam combination history storage unit, a value indicating the reception quality when the terminal device receives the beam indicated by the beam identifier is stored in association with each of the beam identifiers for each distributed antenna; The beam search execution determination step is as follows: Based on the threshold value calculated from the value indicating the reception quality associated with the candidate beam identifier in the beam combination history storage unit and the value indicating the reception quality of the beam indicated by the beam identifier, determining whether to cause the distributed antenna to perform a full beam search; The communication control method according to claim 3.

6. A beam combination history generation step of recording, for each trial period, in the beam combination history storage unit, a record indicating a combination of beam identifiers indicating the beams that the terminal device has determined to be the best for each of the distributed antennas among the beams obtained by the full beam search to be performed on all of the distributed antennas for each trial period, before the beam search period; The communication control method according to any one of claims 1 to 5, further comprising:

7. In a beam search period for searching for a beam used for wireless communication with a terminal device, for each of a plurality of distributed antennas, perform a full beam search by transmitting beams in all directions in which transmission is possible, and when one beam identifier indicating the best beam among the beams obtained by the full beam search is acquired, stop the full beam search, and use the beam identifier and the information indicating the distributed antenna that transmitted the beam indicated by the beam identifier to identify a beam as a detection reference beam. A candidate beam detection unit that detects, from a beam combination history storage unit, as a candidate beam identifier for the distributed antenna, a beam identifier of a beam that is a beam identifier of the distributed antenna that has not performed the full beam search during the beam search period and that has been selected together with the detection reference beam. A beam search execution determination unit that determines whether to perform a full beam search on the distributed antennas that have not performed the full beam search during the beam search period, based on the detection result of the candidate beam detection unit. A beam combination recording unit that generates a record indicating a combination of beam identifiers indicating the beams that have been determined to be the best beams in each of the distributed antennas during the beam search period, and records the generated record in the beam combination history storage unit. A communication control device comprising the above.

8. A wireless communication system comprising a terminal device, a plurality of distributed antenna devices each having one distributed antenna, and a communication control device, wherein the communication control device In a beam search period for searching for a beam used for wireless communication with the terminal device, for each of a plurality of distributed antennas, perform a full beam search by transmitting beams in all directions in which transmission is possible, and when one beam identifier indicating the best beam among the beams obtained by the full beam search is acquired, stop the full beam search, and use the beam identifier and the information indicating the distributed antenna that transmitted the beam indicated by the beam identifier to identify a beam as a detection reference beam. A candidate beam detection unit that detects, from a beam combination history storage unit, as a candidate beam identifier for the distributed antenna, a beam identifier of a beam that is a beam identifier of the distributed antenna that has not performed the full beam search during the beam search period and that has been selected together with the detection reference beam. A beam search execution determination unit that determines whether to perform a full beam search on the distributed antennas that have not performed the full beam search during the beam search period, based on the detection result of the candidate beam detection unit; A beam combination recording unit that generates a record indicating a combination of beam identifiers indicating the beams that have been determined to be the best beams in each of the distributed antennas during the beam search period, and records the generated record in the beam combination history storage unit; A wireless communication system comprising the above.

Images