1 through the antenna AN shown in FIG. 1.

Further, the radio communication unit 230 performs beam forming by a massive MIMO for controlling the phases of signals (transmission RF signals and/or reception RF signals) in a large number of antenna elements on the antenna ANn in accordance with instructions from the controller 200.

The radio communication unit 230 measures the reception signal intensity (for example, RSSI) from the mobile station UE and notifies the controller 200 of the measurement result in association with identification information of the mobile station UE.

The controller 200 is a control center that collectively controls each portion of the remote unit RUn. The controller 200 includes a work memory (not shown) and a processor (not shown) that executes processing based on a control program, control data, and the like read from a storage 240, which will be described later, into the work memory, thereby realizing various control functions.

As specific control functions, the controller 200 includes a communication control function for connecting the mobile station UE wirelessly connected to the remote unit RUn to the 5G core network 5GC via the master unit MU and the base station apparatus gNB2. In addition, the controller 200 includes at least a beam forming control function 200a, a search control function 200b, and a handover control function 200c, and also includes a processing function for integrating and executing these functions.

The beam forming control function 200a controls the massive MIMO by the radio communication unit 230, and performs beam forming according to a predetermined algorithm in accordance with the number of streams allocated to the mobile station UE, and the like.

FIG. 4 shows an example of beam forming. In this example, as shown in FIG. 4 (a), for example, the antenna ANn is divided into four groups Gr1 to Gr4 so that a large number of antenna elements correspond to four streams, and the directivity can be controlled in an arbitrary direction for each group.

In a case where one mobile station UE is located in the cover area of the antenna ANn and two streams are allocated to the mobile station UE, for example, as shown in FIG. 4 (b), beam forming is performed to direct the stream of the group Gr1 and the stream of the group Gr2 in the direction in which the mobile station UE is located.

In a case where one mobile station UE is located in the cover area of the antenna ANn and four streams are allocated to the mobile station UE, for example, as shown in FIG. 4 (c), beam forming is performed to direct the streams of the respective groups Gr1 to Gr4 in the direction in which the mobile station UE is located.

The search control function 200b controls the radio communication unit 230 to search for and estimate (detect) a direction and a range in which the mobile station UE is located. Specifically, the search control function 200b controls the massive MIMO by the radio communication unit 230 to repeatedly perform sweeping in the direction in which the beam is directed every 20 ms in an arbitrary range, for example, as shown in FIG. 5 (a), while monitoring the reception signal intensity sequentially detected by the radio communication unit 230, thereby detecting a direction and a range in which the mobile station UE is located. FIG. 5 (b) shows a timing of each of the beams shown in FIG. 5 (a), and the beams having the same shading in both figures represent the same beams.

The handover control function 200c is a control function for realizing a handover of the mobile station UE among the remote units RU1 to RU3 in cooperation with the master unit MU. To be more specific, the handover control function 200c notifies the master unit MU of the reception signal intensities sequentially detected by the radio communication unit 230, and in accordance with an instruction from the master unit MU (the handover control function), terminates the communication with the mobile station UE that is communicating at that time (for example, when moving in a direction out of the cover area), or newly starts communication with the mobile station UE (for example, when moving in a direction into the cover area).

The storage 240 stores a control program used by the controller 200 and a UE position information table 240a. The control program and the control data are installed in advance at the time of manufacturing, installed or updated through an external interface (not shown) at the time of process setting, or installed or updated by communicating with a server on the 5G core network 5GC, such as the core apparatus C.

The UE position information table 240a is a data table in which position information on the mobile station UE located in the cover area of the remote unit RUn is recorded, and items of information on the mobile station UE such as identification information, position information, and a date and time of updating are stored in association with each other. As an example of the position information, for example, a combination of identification information of the groups Gr1 to Gr4 of the antenna elements, azimuth information in which the beams are directed, information of the estimated distances, and the like is considered, but is not limited to this example.

Next, an operation will be described. In the following description, in particular, search control and stream allocation of the mobile station UE in the remote unit RUn of the distributed antenna system DAS will be described. FIG. 6 is a flowchart for explaining a control flow of the controller 200 of the remote unit RUn.

When the operation of the distributed antenna system DAS is started, in the remote unit RUn, the controller 200 executes several control flows in parallel, one of which is the control flow shown in FIG. 6. The control flow shown in FIG. 6 is repeatedly executed until the operation of the remote unit RUn is stopped or a stop command is received from the master unit MU or the like.

First, in step 601, the controller 200 starts or restarts a timer T, sets a value indicating OFF as a parameter of a time-out flag F stored in the work memory, and proceeds to step 602. That is, the controller 200 starts or restarts a time count by the timer T from the time point when the process proceeds to step 601.

In step 602, the controller 200 determines whether or not a new stream has been allocated to the mobile station UE from the master unit MU. Here, the new stream means a stream for newly performing communication in response to a request from the master unit MU. If a new stream has been allocated, the process proceeds to step 606, and if a new stream has not been allocated, the process proceeds to step 603.

In Step 603, the controller 200 determines whether or not a new communication start request has been generated from the mobile station UE. Here, the new communication start request means a request for newly performing communication in response to a request from the mobile station UE. If a new communication start request has been generated, the process proceeds to step 606, and if a new communication start request has not been generated, the process proceeds to step 604.

In step 604, the controller 200 checks the timer T started in step 601, and determines whether or not a preset time t has elapsed from step 601. If the time t has elapsed, the process proceeds to step 605, and on the other hand, if the time t has not elapsed, the process proceeds to step 602 again.

In step 605, the controller 200 sets a value indicating ON as the parameter of the time-out flag F described above, and proceeds to step 607. With this setting (ON setting), the time-out flag F indicates that the processing of the subsequent steps 607 to 609 is executed in response to the time-out in step 604.

In step 606, the controller 200 refers to the UE position information table 240a stored in the storage 240, compares the date and time of updating included in the table with the current time, and determines whether or not the preset time has elapsed.

If there is no information in the UE position information table 240a or if the preset time has elapsed, the process proceeds to step 607. On the other hand, if the preset time has not elapsed, the process proceeds to step 611.

That is, if there is no position information of the mobile station UE in the UE position information table 240a or if the position information is old, it is determined that a search for the mobile station UE is necessary, and the process proceeds to step 607 to search for the mobile station UE. On the other hand, if the position information of the mobile station UE that can be referred to is present in the UE position information table 240a, the process proceeds to step 611, and stream allocation is performed.

In step 607, the controller 200 uses the search control function 200b to perform a high-speed search for detecting a direction in which the mobile station UE is located, and proceeds to step 608.

Specifically, the controller 200 controls the massive MIMO by the radio communication unit 230 to set a variable range of a direction in which the beam is directed (for example, in the horizontal direction) to about 120 degrees as shown in FIG. 7 (a), repeatedly performs sweeping every 20 ms as shown in FIG. 7 (b), while monitoring the reception signal intensity sequentially detected by the radio communication unit 230, thereby detecting identification information of the mobile station UE and a direction and a range in which the mobile station UE is located. FIG. 7 (b) shows a reception signal intensity corresponding to each of the beams shown in FIG. 7 (a), and the beams having the same shading in both figures represent the same beams.

The high-speed search is independently performed for each of the four groups Gr1 to Gr4 of the antenna elements on the antenna ANn. The azimuth to be searched by each of the groups Gr1 to Gr4 may be arbitrarily set by an operator at the time of process setting, may be set by the search control function 200b on the basis of statistical data or learning data of the position information of the mobile station UE cumulatively stored in the storage 240, or may be updated by reviewing the setting.

In step 608, the controller 200 uses the search control function 200b to perform a low-speed search for detecting a direction in which the mobile station UE is located, and proceeds to step 609.

Specifically, the controller 200 controls the massive MIMO by the radio communication unit 230 to set a variable range of a beam pointing direction (for example, in the horizontal direction) to about 90 degrees, which is smaller than 120 degrees in the high-speed search, as shown in FIG. 8 (a), and repeatedly performs sweeping every 20 ms as shown in FIG. 8 (b). During this time, the controller 200 monitors the reception signal intensity sequentially detected by the radio communication unit 230 at the same frequency as the high-speed search, thereby detecting identification information of the mobile station UE and a direction and a range in which the mobile station UE is located. Similarly to FIG. 7, FIG. 8 (b) shows a reception signal intensity corresponding to each of the beams shown in FIG. 8 (a), and the beams having the same shading in both figures represent the same beams.

The low-speed search is independently performed for each of the four groups Gr1 to Gr4 of the antenna elements on the antenna ANn. The azimuth searched by each of the groups Gr1 to Gr4 has a center in a central portion of the direction detected by the high-speed search in step 607. The range of the search direction may be limited by an operator at the time of process setting, or may be limited by the search control function 200b on the basis of statistical data or learning data of the position information of the mobile station UE cumulatively stored in the storage 240, or the limited range may be reviewed.

At a time when the process proceeds to step 608, the parameter of the time-out flag F stored in the work memory may be referred to. If a value indicating ON is set at that time, the low-speed search in step 608 may be skipped and the process may proceed to step 609. That is, if a time-out occurs, the search with a higher accuracy may be omitted.

In step 609, the controller 200 stores the identification information of the mobile station UE detected in step 608, the information (position information) of the direction and a range in which the mobile station UE is located, and the time at the time of detection (date and time of update) in the UE position information table 240a in association with each other, and the process proceeds to step 610. Here, if information of the same mobile station is present in the UE position information table 240a, the information may be overwritten and stored, or information for a certain period (a certain number of times) may be cumulatively stored as a log. This log can be used by the controller 200 as the learning data described above.

In step 610, the controller 200 refers to the parameter of the time-out flag F stored in the work memory, and determines whether or not a value indicating ON is set. If a value indicating ON is set, the process proceeds to step 601, and if a value indicating OFF is set, the process proceeds to step 611.

That is, if a value indicating ON is set, only the updating of the information of the UE position information table 240a is performed, and on the other hand, if a value indicating OFF is set, the process proceeds to a subsequent stream allocation process.

In step 611, the controller 200 allocates a stream to the mobile station UE, and proceeds to step 612. Specifically, if it is determined that a new stream is allocated by the master unit MU in step 602, the controller