JP5925573B2 - Radio communication base station and radio communication base station control method - Google Patents

Description

  The present invention relates to a radio communication base station and a radio communication base station control method, and more particularly to a radio communication base station that performs coordinated transmission with other radio communication base stations and a radio communication base station control method. .

  As a world standard wireless communication system, LTE (Long Term Evolution) system is standardized by 3GPP (3rd Generation Partnership Project). In LTE, a transmission bandwidth in radio communication between a radio communication base station and a radio communication terminal (mobile terminal) is assigned with a resource block (RB) as a minimum unit. The bandwidth of the resource block is 180 kHz. The radio communication base station schedules radio resources to be allocated to each mobile terminal every subframe (1 ms). Hereinafter, in this specification, a radio communication base station is abbreviated as “base station” as appropriate, and a radio communication terminal (mobile terminal) is abbreviated as “terminal” as appropriate.

  In the LTE system, in order to efficiently use a radio communication band, a base station schedules radio resources to be allocated to each terminal based on a transmission request data size and radio quality between the base station and the terminal. A large number of resource blocks are allocated to a terminal having a large transmission data size, and a small number of resource blocks are allocated to a terminal having a small transmission data size. In addition, whether or not the radio quality with the base station is good, that is, whether or not SINR (Signal to Interference and Noise Ratio) is large is also considered when scheduling radio resources to be allocated to each terminal. The

  The radio quality is measured using a reference signal for both the uplink, which is data transmission from the terminal to the base station, and the downlink, which is data transmission from the base station to the terminal. The downlink radio quality is determined by the terminal measuring SINR with respect to a reference signal transmitted from the base station to the terminal. The terminal feeds back the radio quality to the base station by transmitting the measured SINR to the base station in correspondence with the CQI (Channel Quality Indicator). The terminal broadcasts CQI to the base station at a constant cycle. The uplink radio quality is determined by measuring the SINR of the reference signal transmitted from the terminal to the base station (for example, see Non-Patent Document 1). Thus, the base station schedules radio resources that are not allocated to each terminal based on the radio quality at the latest notification time. Thereby, a radio | wireless communication band can be utilized efficiently.

  In such a wireless communication system, the communication area can be expanded by adopting a cellular configuration in which a plurality of areas covered by a base station are arranged in a cell shape. The terminal usually selects and connects one base station with good communication quality (propagation path condition).

  In such a cellular configuration, by using different frequencies between adjacent cells (sectors), terminals existing in the cell edge (cell edge) region can communicate without receiving interference from adjacent cells. . However, in this case, there is a problem that the frequency use efficiency is lowered. Therefore, frequency utilization efficiency can be greatly improved by repeatedly using the same frequency in each cell (sector). However, in this case, it is necessary to take measures against interference from adjacent cells for terminals existing in the cell edge region.

  Therefore, a method for reducing interference with terminals existing in the cell edge region by performing cooperative transmission in which radio waves are transmitted to terminals in cooperation between adjacent cells of a plurality of base stations has been studied. For example, as such a scheme, a CoMP (Cooperative Multipoint) transmission scheme or the like has been studied (for example, see Non-Patent Document 2).

  In such a scheme, the base station can estimate communication quality between the terminal and the base station based on downlink quality information transmitted from the terminal, and determine whether to perform coordinated transmission. . Currently, several techniques related to such coordinated transmission have been proposed.

  For example, a technique for controlling cooperative transmission based on propagation path status information between a terminal and a plurality of base stations and schedule information in the plurality of base stations has been proposed (see, for example, Patent Document 1). .

  In addition, a technique for controlling cooperative transmission by performing threshold determination based on reception quality during cooperative transmission by a plurality of base stations has been proposed (see, for example, Patent Document 2).

JP 2010-258612 A JP 2011-142539 A

3GPP TS 36.213 V9.2.0 (2010-06), “Physical layer procedures” 3GPP TR 36.814 V1.1.1 (2009-06), “Further Advancements for E-UTRA Physical Layer Aspects”

  However, in the invention described in Patent Document 1, when a base station that performs cooperative transmission is determined based only on quality information between the terminal and each base station, cooperative transmission from a plurality of base stations is performed. In particular, the combined reception quality at the terminal is not always optimal in practice. In particular, when the terminal is moving, the positional relationship between the plurality of base stations and the terminal changes from moment to moment, so when performing coordinated transmission from a plurality of base stations, the combined reception quality at the terminal is optimal at a certain point in time. Even then, it is not always optimal. For this reason, in the invention described in Patent Document 1, it is not always possible to select a coordinated transmission base station so that an optimum throughput improvement effect is always obtained.

  Moreover, in the said patent document 2, although the opportunity which controls coordinated transmission is described, the method of optimizing coordinated transmission is not described.

  Therefore, an object of the present invention made in view of such circumstances is to provide a base station and a base station control method capable of optimizing communication throughput when performing cooperative transmission from a plurality of base stations in a wireless communication system. There is to do.

The invention according to the first aspect to achieve the above object is
A base station that performs coordinated transmission to transmit signals from a plurality of base stations to a terminal by wireless communication,
A receiver that receives communication quality information between the terminal and a base station that can communicate with the terminal from the terminal;
A control unit that controls to perform coordinated transmission with the coordinated base station,
Based on the communication quality information received by the receiving unit , the control unit performs control so as to select candidate base stations that are candidates for performing coordinated transmission among base stations that can communicate with the terminal, and The reception unit is controlled to receive communication quality information between the combination including at least one candidate base station and the terminal from the terminal for each combination, and the reception unit received from the combination A candidate base station that constitutes a combination with the highest communication quality based on communication quality information is the cooperative base station, and a candidate base station that does not constitute the highest combination is selected as a standby base station,
When the communication quality based on the communication quality information received by the receiving unit is reduced and falls below a predetermined threshold during the cooperative transmission by the cooperative base station, the standby base station to the cooperative base station Are re-sorted .

The invention according to the second aspect is the base station according to the first aspect,
The control unit reselects the cooperative base station from the standby base stations based on the communication quality information received by the receiving unit .

The invention according to a third aspect is the base station according to the first aspect,
During the coordinated transmission by the coordinated base station, when the communication quality based on the communication quality information received by the receiving unit is reduced and falls below a threshold smaller than the threshold ,
The control unit determines communication quality between a base station capable of communicating with the terminal and the terminal in order to select candidate base stations that are candidates for performing coordinated transmission among base stations communicable with the terminal. The terminal is notified to measure .

The invention according to a fourth aspect is the base station according to the first aspect,
The control unit performs control so as to select a base station capable of communicating with the terminal having a communication quality based on the communication quality information equal to or higher than a predetermined threshold as the candidate base station.

The invention according to a fifth aspect is the base station according to the first aspect,
The control unit performs control so that a base station capable of communicating with the terminal has a communication quality based on the communication quality information of a predetermined rank or higher is selected as the candidate base station.

The invention according to a sixth aspect is the base station according to the first aspect,
The control unit performs control so as to select a base station that can communicate with the terminal as a candidate base station that can be assigned radio resources used for the coordinated transmission.

The invention according to the second aspect is the base station according to the sixth aspect,
The control unit performs control so as to secure radio resources of the candidate base stations that have not been selected as the cooperative base station.

The invention according to an eighth aspect is the base station according to the first aspect,
While performing the cooperative transmission by the cooperative base station, when the fluctuation per unit time of communication quality based on the communication quality information received by the receiving unit exceeds a predetermined threshold,
The control unit receives communication quality information between the combination including at least one candidate base station and the terminal from the terminal for each combination at a frequency according to a fluctuation per unit time of the communication quality. The receiving unit is controlled.

The invention according to the ninth aspect for achieving the above object is as follows:
A control method of a base station that performs cooperative transmission in which signals are transmitted from a plurality of base stations to a terminal by wireless communication,
A receiving step of receiving communication quality information between the terminal and a base station capable of communicating with the terminal from the terminal;
Controlling to perform coordinated transmission with the coordinated base station;
Based on the communication quality information received in the receiving step, a combination of at least one candidate base station by selecting candidate base stations that are candidates for performing coordinated transmission among base stations that can communicate with the terminal Communication quality information between the terminal and the terminal is received from the terminal for each combination, a candidate base station that constitutes a combination having the highest communication quality based on the communication quality information among the combinations is a cooperative base station , Screening candidate base stations that do not constitute the highest combination as standby base stations;
During the coordinated transmission by the coordinated base station, when the communication quality based on the received communication quality information falls below a predetermined threshold, the coordinated base station is selected from the standby base station. Step to fix,
Is included.

  According to the base station and the base station control method of the present invention, it is possible to optimize communication throughput when performing coordinated transmission from a plurality of base stations in a wireless communication system.

It is a figure which shows schematic structure of the radio | wireless communications system which concerns on embodiment of this invention. It is a figure which shows schematic structure of the base station which concerns on embodiment of this invention. It is a figure which shows schematic structure of the terminal which concerns on embodiment of this invention. It is a figure which illustrates notionally the cooperative transmission which concerns on embodiment of this invention. It is a sequence diagram demonstrated centering on the start of the cooperative transmission which concerns on embodiment of this invention. It is a flowchart explaining the candidate base station selection process by the base station which concerns on embodiment of this invention. It is a flowchart explaining the radio | wireless resource allocation availability determination process by the base station which concerns on embodiment of this invention. It is a flowchart explaining the communication quality measurement process by the terminal which concerns on embodiment of this invention. It is a flowchart explaining the cooperation base station selection process by the base station which concerns on embodiment of this invention. It is a flowchart explaining the cooperative transmission preparation process by the base station which concerns on embodiment of this invention. It is a figure which illustrates notionally the cooperative transmission which concerns on embodiment of this invention. It is a sequence diagram explaining focusing on after the start of the cooperative transmission which concerns on embodiment of this invention.

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

  FIG. 1 is a diagram illustrating an example of a schematic configuration of a wireless communication system including a wireless communication base station according to an embodiment of the present invention. The radio communication system according to the present embodiment can be implemented by, for example, an LTE system.

  As shown in FIG. 1, the radio communication system according to the present embodiment includes a base station 100, a terminal 200, an MME (Mobility Management Equipment) 310, an S-GW (Serving-Gateway) 320, and a P-GW (PDN-Gateway). 330 and IMS (IP Multimedia Subsystem) 400. The example of the wireless communication system illustrated in FIG. 1 includes three base stations, which are represented as a base station 100A, a base station 100B, and a base station 100C in FIG. 1, and each forms a cell. In the wireless communication system according to the present embodiment, the number of base stations can be any number of two or more.

  The base station 100 assigns radio resources for the terminal 200 to perform radio communication. The terminal 200 performs wireless communication via the base station 100. The MME 310 performs mobility management such as location registration of the terminal 200, terminal call processing at the time of incoming call, and handover between the base stations 100. The S-GW 320 processes user data such as voice and packets. The P-GW 330 has an interface with the IMS 400. IMS 400 is a public communication network that supports multimedia services using SIP (Session Initiation Protocol).

  In FIG. 1, base station 100A, base station 100B, and base station 100C are connected by an interface called X2. That is, the base stations of the communication system according to the present invention are connected by backhaul, and each base station can communicate necessary data and control information as appropriate. Further, the description will be made assuming that the synchronization of the clock and timing phase between the base stations can be maintained by a synchronization process using a backhaul or a GPS device.

  As shown in FIG. 1, the MME 310 and the base station 100A, the base station 100B, and the base station 100C are connected by an interface called S1-MME. The S-GW 320 and the base station 100A, the base station 100B, and the base station 100C are connected by an interface called S1-U. The MME 310 and the S-GW 320 are connected by an interface called S11. The S-GW 320 and the P-GW 330 are connected by an interface called S5. Further, the P-GW 330 and the IMS 400 are connected by an interface called SGi.

  Further, it is assumed that a P-GW other than the P-GW 330 shown in FIG. Further, it is assumed that a P-GW other than the P-GW 330 is connected to a wireless communication system configured by the same node group as each base station 100, terminal 200, MME 310, and S-GW 320 illustrated in FIG. doing.

  FIG. 2 is a diagram illustrating an example of a schematic configuration of the base station 100 illustrated in FIG. As shown in FIG. 2, the base station 100 includes an RF antenna 101, an RF unit 102, a modem unit 103, a base station interface unit 104, an OMT (Operation Maintenance Tool) interface unit 105, a BH (Backhaul) interface unit 106, and a control unit. 107 and a storage unit 108.

  The RF antenna 101 transmits an RF signal to the terminal 200 and receives an RF signal from the terminal 200. The RF unit 102 converts an RF signal transmitted and received by the RF antenna 101 into an RF band and a band that can be digitally processed. In the present embodiment, the RF unit 102 constitutes a receiving unit according to the present invention, and this receiving unit receives from the terminal 200 communication quality information between the base station 100 that can communicate with the terminal 200 and the terminal 200. be able to. Here, the base station 100 that can communicate with the terminal 200 includes the base station 100 (master base station) that is the own station and the base stations 100 that exist around the terminal 200. The acquisition of communication quality information as described above will be described later.

  The modem 103 modulates the signal output to the RF unit 102 and demodulates the signal input from the RF unit 102. The base station interface unit 104 functions as an interface with other base stations. The OMT interface unit 105 functions as an interface when the operator manually controls the base station 100. The BH interface unit 106 functions as a core network interface. The control unit 107 controls and manages the entire base station 100 including each functional unit constituting the base station 100. In particular, in the present embodiment, the control unit 107 performs control related to allocation of radio resources used for radio communication with the terminal 200. Specifically, the control unit 107 includes a scheduler, and schedules radio resources to be allocated to each terminal. Control unique to the present embodiment by the control unit 107 will be further described later. The storage unit 108 is a memory that can store various data.

  FIG. 3 is a diagram illustrating an example of a schematic configuration of the terminal 200 illustrated in FIG. As shown in FIG. 3, the terminal 200 includes an RF antenna 201, an RF unit 202, a modem unit 203, a control unit 204, a storage unit 205, an input unit 206, a decoder unit 207, a microphone 208, a speaker 209, a display unit 210, and a GPS. An antenna 211 and a GPS unit 212 are provided.

  The RF antenna 201 transmits an RF signal to the base station 100 and receives an RF signal from the base station 100. The RF unit 202 converts an RF signal transmitted and received by the RF antenna 201 into an RF band and a band that can be digitally processed. The modem unit 203 modulates the signal output to the RF unit 202 and demodulates the signal input from the RF unit 202. The control unit 204 controls and manages the entirety of the terminal 200 including each functional unit constituting the terminal 200. In particular, in the present embodiment, the control unit 204 performs control such that the terminal 200 communicates with the base station 100 according to the radio resource assignment notified from the base station 100. The storage unit 205 is a memory that can store various data.

  The input unit 206 detects various input operations by the operator. The decoder unit 207 decodes the audio signal and the image signal. The microphone 208 detects sound and converts it into an electrical signal. The speaker 209 converts an electrical signal representing sound into sound and outputs the sound. The display unit 210 displays various images. The GPS antenna 211 receives a signal from an artificial satellite. The GPS unit 212 detects a position by GPS (Global Positioning System).

  The terminal 200 according to the present embodiment performs communication using a radio resource (RB: Resource Block) allocated by the schedule processing of the base station 100.

  Next, processing until cooperative transmission by the base station according to the embodiment of the present invention is started will be described.

  FIG. 4 is a diagram conceptually illustrating cooperative transmission according to the embodiment of the present invention. As shown in FIG. 4, it is assumed that terminal 200 is performing communication within the cell of base station 100A. The base station 100A will be described as a master base station that originally communicated with the terminal 200. FIG. 4 shows a state in which the terminal 200 communicates within the cell of the master base station 100A, and base stations 100B, 100C, 100D, and 100E that can communicate with the terminal 200 are arranged in the vicinity. .

  In this embodiment, as described above, for the purpose of reducing interference with a terminal existing in a cell edge region, coordinated transmission in which radio waves are transmitted to terminals in cooperation with each other between adjacent cells of a plurality of base stations. I do. That is, the base station 100 according to the present embodiment performs coordinated transmission in which signals are transmitted from the plurality of base stations 100 to the terminal 200 through wireless communication. In FIG. 4, the base station 100B performs cooperative transmission together with the base station 100A. Base stations 100C, 100D, and 100E do not perform cooperative transmission. Hereinafter, processing for optimizing the communication throughput when cooperative transmission is performed from a plurality of base stations as illustrated in FIG. 4 by the wireless communication system according to the present embodiment will be described.

  FIG. 5 is a sequence diagram that mainly describes up to the start of cooperative transmission according to the embodiment of the present invention. 5 shows information exchange in a wireless communication system including base stations 100B, 100C, and 100D in addition to master base station 100A and terminal 200 shown in FIG. Note that the processing performed by each element in the sequence shown in FIG. 5 is appropriately shown in the flowcharts of FIGS.

  As shown in the figure, it is assumed that the master base station 100A and the terminal 200 are in communication when the sequence shown in FIG. 5 starts. Further, it is assumed that none of the base stations 100 performs coordinated transmission when the sequence illustrated in FIG. 5 starts.

  First, if it is determined that it is possible to perform coordinated transmission to terminal 200 or that it is necessary to perform coordinated transmission, control unit 107 of base station 100A prepares terminal 200 for preparation for coordinated transmission. The RF unit 102 is controlled to make a notification. Here, when it is possible to perform coordinated transmission or when it is necessary to perform coordinated transmission, for example, the location information of the terminal (which can be acquired by GPS, for example), received power, throughput, and communication quality It is assumed that a determination is made based on a decrease or the like. However, in the present invention, when it is possible to perform coordinated transmission or it is necessary to perform coordinated transmission, the present invention is not limited to these conditions, and determination can be made based on various conditions.

  When notified from base station 100A to prepare for coordinated transmission, control section 204 of terminal 200 measures the reception quality from each of base stations 100A, 100B, 100C, and 100D, and uses the measurement result as communication quality information. Control to transmit to base station 100A. Here, the measurement of the reception quality from each base station refers to the terminal 200 and each base station 100 using a reference signal (for example, RS: Reference Signal) from the base station 100 that can receive a signal at the terminal 200. And can be done for each. What was measured in this way is hereinafter referred to as communication quality information as appropriate. The reception quality described above indicates downlink communication quality, and is measured by reference signals appropriately arranged in each frequency band. Since each base station 100 transmits reference signals unique to the base station and these reference signals are orthogonal, the terminal 200 can individually detect these reference signals. The communication quality information (CQI information and the like) measured in this way is transmitted to the base station 100 and used in the subsequent radio resource allocation process.

  When the communication quality information is transmitted from the terminal 200, the master base station 100A controls the RF receiving unit 102 to receive the communication quality information transmitted from the terminal 200. When the communication quality information is received, the control unit 107 of the base station 100A selects a candidate base station that is a candidate for the coordinated transmission among the base stations that can communicate with the terminal 200 based on the communication quality information. (Step S11).

  FIG. 6 is a diagram for explaining the details of the candidate base station selection process performed by the master base station 100A, which is started from step S11. In step S11, the control unit 107 of the base station 100A controls the candidate base stations to select, as candidate base stations, those having communication quality based on the communication quality information received from the terminal 200 that are equal to or higher than a predetermined threshold. Is preferred. In this way, in the coordinated transmission according to the present embodiment, it is possible to eliminate the base station 100 that has little effect of improving the communication quality or is assumed to cause the deterioration of the communication quality. In addition, by excluding the base station 100 whose communication quality is less than the predetermined threshold, the radio resources of the excluded base station can be used for communication to other terminals 200, so that the radio resources can be effectively used. We can expect to plan. In the present embodiment, it is preferable to set an appropriate threshold value so that such an effect can be obtained.

  In this example, the base stations 100A, 100B, 100C, and 100D shown in FIGS. 4 and 5 will be described on the assumption that the communication quality based on the communication quality information is equal to or higher than a predetermined threshold (first threshold). . That is, in this example, it is assumed that these base stations 100A, 100B, 100C, and 100D are selected as candidate base stations at this time.

  Next, the control unit 107 of the base station 100A performs control so as to confirm whether or not the radio resource (RB) to be used for cooperative transmission can be used for each of the base stations 100 selected in step S11. (Step S12). Specifically, the master base station 100A requests each of the candidate base stations 100 to check whether or not the target RB can be used, and each of the candidate base stations 100 The master base station 100A is answered as to whether it can be used.

  FIG. 7 shows a process performed by each candidate base station 100 in response to confirmation from the master base station 100A whether or not an RB to be used for coordinated transmission can be used, as shown in step S12 in FIG. It is a flowchart explaining the detail of.

  When the process shown in FIG. 7 starts, the control unit 107 of the candidate base station 100 determines whether or not there is a vacancy in the target RB (step S21). It is determined whether or not there is (step S22). When there are vacancies in a plurality of RBs, the control unit 107 selects at least one RB that can be cooperatively transmitted by the built-in scheduler (step S23), and does not use the RB for communication with other terminals. The RB is secured (step S24). For this reason, when there are a plurality of target RBs and these can be used in the candidate base station, the scheduler built in the control unit 107 determines whether or not to use the plurality of RBs, and determines the RBs used in cooperative transmission. Schedule processing is performed so that at least one is selected. In this case, the control unit 107 transmits information indicating that cooperative transmission is possible and information on usable RBs to the master base station 100A (step S25). If there is only one RB available in step S22, control unit 107 secures the RB (step S24), and informs that master transmission is possible and information on usable RBs. Transmit to the station 100A (step S25).

  On the other hand, if there is no vacancy in the target RB in step S21, the control unit 107 can change the target RB in use because there is no candidate base station that is cooperatively transmitting to other terminals at this time. It is determined whether or not (step S29). If the target RB in use can be changed, the control unit 107 determines whether or not a plurality of RBs can be changed (step S30). If the plurality of RBs can be changed, the control unit 107 selects the RB in step S23. If there is only one changeable RB, the RB is secured in step S24. If the target RB being used cannot be changed in step S29, the control unit 107 transmits to the master base station 100A that the candidate base station cannot perform coordinated transmission (step S31).

  In addition, in the candidate base station, when there is no vacancy in the target RB in step S21 and the coordinated transmission has already been performed to another terminal in step S26, the control unit 107 of the candidate base station 100 transmits to the other terminal. It is determined whether RB is used for cooperative transmission (step S27). When the target RB being used is used for cooperative transmission to another terminal, the control unit 107 determines whether or not a plurality of target RBs are used (step S28). If a plurality of target RBs are used, an RB is selected in step S23, and if there is only one target RB used, the control unit 107 determines whether the RB can be changed in step S29. Determine.

  In the example illustrated in FIG. 5, the candidate base stations 100B, 100C, and 100D perform the process for determining whether or not the target RB can be used, and a response indicating whether or not the target RB is used is transmitted from each candidate base station to the master base station 100A. Is shown. In the example shown in FIG. 5, the candidate base stations 100B and 100C reply that the target RB is usable, and the candidate base station 100D reply that the target RB is unusable. It is shown. As described above, in the present embodiment, the control unit 107 of the master base station 100A selects, as candidate base stations, the base stations 100 that can communicate with the terminal 200 that can be assigned radio resources used for coordinated transmission. It is preferable to control as described above.

  When the availability of the target RB is transmitted from each candidate base station 100 in step S25 or step S31 shown in FIG. 7, the master base station 100A receives the answer of the availability (step S13 shown in FIG. 6).

  When receiving the propriety of cooperative transmission in step S13, the control unit 107 of the master base station 100A ranks the communication quality of candidate base stations that can be used by the target RB, and selects the base stations having the communication quality of the top x or higher. Control is performed (step S14). Here, for the top x, an appropriate number such as the top five or more is set so that the number of candidate base stations that ultimately perform coordinated transmission does not become too large. As described above, in the present embodiment, the control unit 107 of the master base station 100A determines that the base station 200 that can communicate with the terminal 200 has a communication quality based on the communication quality information that is higher than or equal to a predetermined rank. It is preferable to control to sort as

  When the base station having the communication quality of the top x or higher is selected in step S14, the control unit 107 of the master base station 100A notifies the selection result to each candidate base station (steps S15 and S16).

  In the example shown in FIG. 5, candidate base stations 100B and 100C are shown to have been notified that they are the target base stations for coordinated transmission because it is determined that the communication quality is higher than the top x. On the other hand, since candidate base station 100D has been determined that the communication quality is lower than the top x, the candidate base station 100D has been notified that it is not the target base station for coordinated transmission.

  When the selection result is notified to the respective candidate base stations in steps S15 and S16, the control unit 107 of the master base station 100A notifies the terminal 200 of the candidate base stations that are to be subjected to coordinated transmission (step S17). ).

  As illustrated in FIG. 5, when information on a candidate base station to be subjected to coordinated transmission is notified from the master base station 100A in step S17, the terminal 200 receives information on the candidate base station (step S41). Here, the information on candidate base stations may include, for example, information on base station IDs and usable target RBs.

  FIG. 8 is a flowchart for explaining a communication quality measurement process performed by the terminal 200 when the master base station 100A is notified of information on candidate base stations to be subjected to coordinated transmission.

  As illustrated in FIG. 8, when information on a candidate base station is received from the master base station 100A in step S41, the control unit 204 of the terminal 200 determines whether the candidate base station to be subjected to coordinated transmission is already performing coordinated transmission. It is determined whether or not (step S42). Here, since it is assumed that no candidate base station performs cooperative transmission, the control unit 204 of the terminal 200 controls to measure the combined reception quality for each target RB (step S48). Here, the combined reception quality means a communication quality between the terminal 200 and a combination including at least one candidate base station 100 (a combined state when two or more candidate base stations 100 are included). Except for the case where there is only one candidate base station 100, there are a plurality of combinations including at least one candidate base station 100. Therefore, in this embodiment, in step S48, the communication quality in such a state is measured for all combinations.

  For example, when the master base station 100A and the candidate base stations are two stations (base stations 100B and 100C), the measurement of communication quality for all combinations in step S48 is performed for the following seven combinations. That is, there are seven combinations of base station 100A only, base station 100B only, base station 100C only, base stations 100A and 100B, base stations 100B and 100C, base stations 100C and 100A, and base stations 100A and 100B and 100C. Note that such combined reception quality can be measured, for example, by performing in-phase combining of the reference signals from each base station in the combination as described above.

  The combined reception quality measurement process for each combination may be performed simultaneously. In this case, assuming that the measurement processing time is completed in one subframe, the measurement for all combinations is completed in one subframe. Further, the measurement processing of the combined reception quality for each combination may be performed by changing one set for each subframe. In this case, assuming that the measurement processing time is completed in 1 subframe, the measurement for all combinations is completed in 7 subframes.

  When the combined reception quality is measured for all combinations in step S48, the control unit 204 of the terminal 200 calculates an average combined reception quality (step S49). Here, the average combined reception quality is calculated as a statistical average value, for example, an average of a predetermined period of time, which is obtained by measuring the combined reception quality for all the combinations.

  When the average combined reception quality is calculated in step S49, the control unit 204 of the terminal 200 performs control so as to notify the master base station 100A of the calculated communication quality information and information on the base stations constituting the combination ( Step S50). Here, the calculated communication quality information can be, for example, information on the target RB and information on combined reception quality for all combinations calculated for the RB. As shown in FIG. 5, when communication quality information or the like is notified in step S50, master base station 100A performs a process of selecting a base station that performs coordinated transmission as a coordinated base station.

  FIG. 9 is a flowchart for describing cooperative transmission preparation processing performed by the master base station 100A when communication quality information and the like are notified from the terminal 200.

  When the process illustrated in FIG. 9 starts, the master base station 100A performs different processes depending on whether or not the own base station has already performed coordinated transmission with other base stations 100 (step S61). Here, a case where master base station 100A does not perform cooperative transmission with other base stations 100 will be described. In this case, the control unit 107 of the master base station 100A receives the combined reception quality information and the like from the terminal 200 (step S66). Thus, in the present embodiment, the control unit 107 of the master base station 100A receives communication quality information between the combination including at least one candidate base station 100 and the terminal 200 from the terminal for each combination. The receiving unit (RF unit 102) is controlled.

  When the combined reception quality information or the like is received in step S66, the control unit 107 of the master base station 100A selects the base stations constituting the combination having the highest reception quality as cooperative base stations based on the combined reception quality information. (Step S67). When the cooperative base station is selected in step S67, the control unit 107 of the master base station 100A performs control so as to notify the selection result to each candidate base station (from step S68 to step S69 or S70).

  In the example illustrated in FIG. 5, the candidate base station 100B is assumed to be a candidate base station that constitutes a combination having the highest communication quality at this time, and a state in which it is notified that it is a cooperative base station is illustrated. On the other hand, the candidate base station 100C is assumed not to be a candidate base station that constitutes the combination having the highest communication quality at this time, and is shown to be notified that it is not a cooperative base station but a standby base station.

  FIG. 10 is a flowchart for explaining the cooperative transmission preparation process performed by each candidate base station when notified from the master base station 100A whether or not it is a cooperative base station.

  First, for example, a cooperative transmission preparation process performed in a candidate base station that has been notified that it is not a cooperative base station but a standby base station, such as the base station 100C illustrated in FIG. When the process illustrated in FIG. 10 starts, the control unit 107 of the base station 100 performs control so as to receive a notification from the master base station 100A as to whether or not it is a cooperative base station (step S81). Here, since it is notified that it is not a coordinated base station (step S82), the control unit 107 of the base station 100 performs control so as to secure an available RB. Thus, by securing usable RBs, the base station 100 can be immediately selected as a cooperative base station even when the state of communication quality changes. As a result, the base station 100 thereafter operates as a standby base station.

  Next, for example, a cooperative transmission preparation process performed in a candidate base station that is notified of the fact that it is a cooperative base station as in the base station 100B illustrated in FIG. 5 will be described. When the process illustrated in FIG. 10 starts, the control unit 107 of the base station 100 performs control so as to receive a notification from the master base station 100A as to whether or not it is a cooperative base station (step S81). Here, since it is notified that it is a cooperative base station (step S82), the control unit 107 of the base station 100 determines whether or not the target RB needs to be changed (step S84).

  When it is necessary to change the target RB in step S84, the control unit 107 of the base station 100 changes the RB allocation (step S85), and then performs control so as to secure a usable RB (step S85). S86). For example, when the target RB is used by another terminal, the allocation can be changed to another free RB, and the target RB can be secured. Also, when the target RB is used for cooperative transmission to other terminals, the allocation can be changed to another free RB, and the target RB can be secured. Furthermore, when the target RB is used for cooperative transmission to other terminals and a plurality of RBs are used, a part of the RBs can be released to secure the target RB.

  When it is not necessary to change the target RB in step S84, the control unit 107 of the base station 100 performs control so as to secure an available RB (step S86). For example, if the target RB is unused, it is not necessary to change the target RB in step S84. In this case, the target RB is secured until the cooperative transmission is started.

  When an RB that can be used is secured in step S86, the control unit 107 of the base station 100 performs control so as to notify the master base station 100A that the preparation for transmission is completed (step S87).

  In FIG. 5, the base station 100 </ b> B shows a state in which the master base station 100 </ b> A is notified that the preparation for cooperative transmission is completed by the cooperative transmission preparation process. On the other hand, FIG. 5 shows a state in which the base station 100C becomes a standby base station by the cooperative transmission preparation process. As shown in FIG. 5, upon receiving notification from the coordinated base station that preparation for coordinated transmission has been completed, master base station 100 </ b> A notifies terminal 200 that coordinated transmission starts.

  As shown in FIG. 5, when notified from the master base station 100A that cooperative transmission starts, the terminal 200 replies to the master base station 100A that the notification has been confirmed. By this reply, the master base station 100A can recognize that other base stations 100 are ready to perform coordinated transmission. Therefore, the master base station 100A performs coordinated transmission with other base stations 100 by transmitting transmission data to the coordinated base station via a backhaul or the like.

  For example, base station 100B, which is a coordinated base station, receives coordinated transmission data and control information from master base station 100A (step S88), and starts transmitting the received coordinated transmission data to terminal 200 based on the transmission timing. (Step S89). Here, the control information includes information necessary for communication with terminal 200, transmission start timing information, and the like. In this way, base station 100B can perform coordinated transmission to terminal 200 together with master base station 100A. In this embodiment, since the cooperative base station is selected based on the actual combined reception quality, when the base station 100B performs cooperative transmission to the terminal 200 together with the master base station 100A in the above-described example, communication is performed. Throughput is optimized. As described above, coordinated transmission as shown in FIG. 4 can be performed.

  As described above, in this embodiment, the control unit 107 of the master base station 100 </ b> A is a candidate base station that configures a combination having the highest communication quality based on the communication quality information received by the receiving unit (RF unit 102) among the combinations. Is selected as a coordinated base station and controlled to perform coordinated transmission. Further, in the present embodiment, it is preferable that the control unit 107 of the master base station 100A performs control so as to secure radio resources of candidate base stations that have not been selected as cooperative base stations.

  Next, processing after the cooperative transmission by the base station according to the embodiment of the present invention is started will be described.

  Even if the coordinated transmission is started in the state shown in FIG. 4, the situation of the combined reception quality may change due to, for example, the terminal 200 moving. In particular, when the combined reception quality is low, the communication throughput is low even if coordinated transmission is performed from a plurality of base stations. Therefore, in the present embodiment, for example, as illustrated in FIG. 11, when the situation of the combined reception quality changes, the base station that performs cooperative transmission is changed as appropriate.

  FIG. 11 shows that the base station 100B that has been cooperatively transmitting to the terminal 200 together with the master base station 100A at the stage shown in FIG. 4 has become a standby base station. Further, FIG. 11 shows a state where base station 100C, which is a standby base station at the stage shown in FIG. 4, has become a coordinated base station that performs coordinated transmission with terminal 200 together with master base station 100A.

  FIG. 12 is a sequence diagram illustrating mainly after the start of cooperative transmission according to the embodiment of the present invention. FIG. 12 shows information exchange in a wireless communication system including base stations 100B, 100C, and 100D in addition to master base station 100A and terminal 200 shown in FIG.

  In order to perform control after the coordinated transmission according to the present embodiment is started, the terminal 200 determines the combined reception quality of the candidate base station for each target RB while the signal by the coordinated transmission is received. Measure for all combinations. In addition, in the present embodiment, terminal 200 measures the communication quality of a signal that is being cooperatively transmitted, in addition to the combined quality for all combinations of candidate base stations described above. Such communication quality can be calculated from a block error rate after demodulation of PDSCH (Physical Downlink Shared Channel), etc., but combined reception quality may be used.

  The flowchart shown in FIG. 8 also describes the measurement of the combined reception quality as described above by the terminal 200.

  As shown in FIG. 8, in the present embodiment, even after the coordinated transmission is started, the control unit 204 of the terminal 200 receives the candidate base station information (step S41), and the average communication quality is obtained through step S42. Control is performed to calculate (step S43). If average communication quality is calculated in step S43, the control part 204 of the terminal 200 will calculate the fluctuation | variation per unit time of communication quality (step S44).

  When the fluctuation calculated in step S44 is equal to or greater than a predetermined (second) threshold, the control unit 204 of the terminal 200 changes the combination of base stations for the combined quality measurement so as to follow the fluctuation in communication quality, Control is performed to measure the combined reception quality (step S45 to step S46). On the other hand, when the fluctuation calculated in step S44 is less than the predetermined (second) threshold, the control unit 204 of the terminal 200 does not measure all the combinations of the combined reception quality at the same time, and combines the combinations at a constant period. Control is performed so as to be changed (from step S45 to step S47). Thereby, since it is not necessary to perform measurement for all combinations of the combined reception quality more frequently than necessary, the power consumption of the terminal 200 can be reduced. The predetermined (second) threshold value described above is set to an appropriate value so that the terminal 200 does not perform measurement more frequently than necessary and follows a certain amount of communication quality fluctuation. When measuring the combined reception quality, for example, the measurement may be performed by changing the combination of base stations for each subframe.

  As described above, in this embodiment, during the coordinated transmission by the coordinated base station, the fluctuation per unit time of the communication quality based on the communication quality information received by the receiving unit (RF unit 102) is a predetermined threshold value. In the case of exceeding, it is preferable to control as follows. That is, the control unit 107 of the master base station 100A sets the communication quality information for each combination of the communication quality information between the combination including at least one candidate base station and the terminal at a frequency corresponding to the fluctuation per unit time of the communication quality. It is preferable to control the receiving unit to receive from 200.

  In the present embodiment, after the cooperative transmission is started, the master base station 100A performs control according to the communication quality when the communication quality based on the received communication quality information is deteriorated. Specifically, the master base station 100A appropriately changes the cooperative base station according to the communication quality during cooperative transmission by the cooperative base station selection process described with reference to FIG.

  The flowchart shown in FIG. 9 also describes the control according to the communication quality as described above by the master base station 100A.

  As shown in FIG. 9, in the present embodiment, even after the coordinated transmission is started, the control unit 107 of the master base station 100A receives the communication quality information and the combined reception quality information from the terminal 200 as described above. (Step S61 to step S62).

  If such information is received in step S62, the control unit 107 of the master base station 100A determines whether or not the received communication quality is maintained at a predetermined (third) threshold value or more (step S63). When the communication quality is maintained at a predetermined (third) threshold value or higher in step S63, the cooperative base station maintains the current state without changing it. As described above, the predetermined (third) threshold value is set to a value that causes no problem even if the current state is maintained without changing the cooperative base station.

  As described above, in the present embodiment, the communication quality based on the communication quality information received by the receiving unit (RF unit 102) is reduced and falls below a predetermined threshold during the coordinated transmission by the coordinated base station. In this case, it is preferable to control as follows. That is, it is preferable that the control unit 107 of the master base station 100A performs control so as to reselect cooperative base stations.

  On the other hand, when the received communication quality is not maintained above the predetermined (third) threshold, the control unit 107 of the master base station 100A maintains the received communication quality above the predetermined (fourth) threshold. It is determined whether or not (step S64). The predetermined fourth threshold value is smaller than the above-described predetermined third threshold value.

  When the communication quality is maintained at or above a predetermined (fourth) threshold value in step S64, that is, when the communication quality is between the third threshold value and the fourth threshold value, the control unit 107 of the master base station 100A determines Then, control is performed so as to perform the process of step S67 described above. That is, in this case, the control unit 107 of the master base station 100A selects the base stations to be selected so as to select the base stations constituting the combination having the highest reception quality as the cooperative base station based on the combined reception quality information. Change as appropriate.

  On the other hand, when the communication quality is less than the predetermined (fourth) threshold value in step S64, the control unit 107 of the master base station 100A notifies the terminal 200 that the process of reselecting the candidate base stations is started. Control is performed as follows (step S65). That is, in this case, the master base station 100A notifies the terminal 200 to measure the reception quality from each base station in order to redo the selection of candidate base stations described in FIG. As described above, when the communication quality is lowered below the predetermined (fourth) threshold, it is determined that the communication quality is not improved even if the cooperative base station is changed from among the already selected candidate base stations. Set an appropriate value.

  As described above, in the present embodiment, the communication quality based on the communication quality information received by the receiving unit (RF unit 102) is reduced and falls below a predetermined threshold during the coordinated transmission by the coordinated base station. In this case, it is preferable to control as follows. That is, it is preferable that the control unit 107 of the master base station 100A performs control so as to reselect candidate base stations based on the communication quality information received by the reception unit.

  In the example shown in FIG. 12, since the communication quality has been maintained at or above the third threshold for a while after the coordinated transmission is started, the base station 100B performs coordinated transmission as a coordinated base station, while the base station 100C is on standby. The base station does not perform coordinated transmission. Thereafter, in the example illustrated in FIG. 12, since the communication quality has fallen below the third threshold, the cooperative base station is changed. That is, as a result of selecting the base stations constituting the combination having the highest reception quality as the cooperative base station, the base station 100B becomes a standby base station and does not perform cooperative transmission, while the base station 100C becomes a cooperative base station and cooperates. Transmission has started. Further, in the example shown in FIG. 12, since the communication quality has fallen below the fourth threshold value, the reception quality from each base station is measured for the terminal 200 to redo the selection of candidate base stations. To notify.

  In this way, in this embodiment, even after the cooperative transmission is started, the cooperative transmission as shown in FIG. 11 can be performed by appropriately changing the cooperative base station or the candidate base station.

  Thus, in this embodiment, since a cooperation base station is selected from communication quality after composition, communication throughput at the time of performing cooperation transmission can be optimized. Also, in this embodiment, the combined reception quality is measured for all combinations from among the candidate base stations, and the combination of base stations with the highest communication quality is selected from among them, so the communication throughput of cooperative transmission is further improved. You can expect Furthermore, in the present embodiment, base stations that are expected to have little effect on throughput improvement are appropriately excluded, so that interference with other cells can be reduced and radio resources can be effectively used.

  Furthermore, in this embodiment, since the candidate base station has secured RB, there is little time delay for processing when changing the cooperative base station, and even if the communication quality of the communication path changes, the change The followability to can be improved. In addition, since the measurement of the communication quality is appropriately performed following the change in the communication quality, the followability to the change in the quality of the communication path can be improved. For this reason, the information used as the judgment material at the time of selecting a cooperation base station becomes more accurate, and it can be expected that the communication throughput of cooperation transmission is further improved.

  Although the present invention has been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications and corrections based on the present disclosure. Therefore, it should be noted that these variations and modifications are included in the scope of the present invention. For example, functions included in each member, each means, each step, etc. can be rearranged so as not to be logically contradictory, and a plurality of means, steps, etc. can be combined or divided into one. Is possible.

  For example, in the above-described embodiments, description has been made assuming LTE as a mobile communication system. However, the present invention is not limited to this, and the present invention can be similarly applied to any wireless communication system including base stations that perform coordinated transmission by a plurality of base stations.

  Further, the above-described embodiment has been described as a mode in which the base station 100 communicating with the terminal 200 before starting cooperative transmission becomes a master base station and instructs other neighboring base stations. However, the present invention provides necessary information to the center station that supervises the base station via the base station 100 (master base station) with which the terminal 200 is communicating. A mode in which the station performs control can also be assumed.

100 wireless communication base station 101 RF antenna 102 RF unit 103 modem unit 104 base station interface unit 105 OMT interface unit 106 BH interface unit 107 control unit 108 storage unit 200 wireless communication terminal 201 RF antenna 202 RF unit 203 modem unit 204 control unit 205 Storage unit 206 Input unit 207 Decoder unit 208 Microphone 209 Speaker 210 Display unit 211 GPS antenna 212 GPS unit 310 MME
320 S-GW
330 P-GW
400 IMS

Claims (9)

A base station that performs coordinated transmission to transmit signals from a plurality of base stations to a terminal by wireless communication,
A receiver that receives communication quality information between the terminal and a base station that can communicate with the terminal from the terminal;
A control unit that controls to perform coordinated transmission with the coordinated base station,
Based on the communication quality information received by the receiving unit , the control unit performs control so as to select candidate base stations that are candidates for performing coordinated transmission among base stations that can communicate with the terminal, and The reception unit is controlled to receive communication quality information between the combination including at least one candidate base station and the terminal from the terminal for each combination, and the reception unit received from the combination A candidate base station that constitutes a combination with the highest communication quality based on communication quality information is the cooperative base station, and a candidate base station that does not constitute the highest combination is selected as a standby base station,
When the communication quality based on the communication quality information received by the receiving unit is reduced and falls below a predetermined threshold during the cooperative transmission by the cooperative base station, the standby base station to the cooperative base station Re-sort the base station. The base station according to claim 1, wherein the control unit reselects the cooperative base station from the standby base station based on the communication quality information received by the receiving unit . During the coordinated transmission by the coordinated base station, when the communication quality based on the communication quality information received by the receiving unit is reduced and falls below a threshold smaller than the threshold ,
The control unit determines communication quality between a base station capable of communicating with the terminal and the terminal in order to select candidate base stations that are candidates for performing coordinated transmission among base stations communicable with the terminal. The base station according to claim 1, which notifies the terminal to measure .   2. The base according to claim 1, wherein the control unit performs control so that a base station capable of communicating with the terminal has a communication quality based on the communication quality information equal to or higher than a predetermined threshold is selected as the candidate base station. Bureau.   2. The base according to claim 1, wherein the control unit performs control so that a base station capable of communicating with the terminal has a communication quality based on the communication quality information of a predetermined rank or higher is selected as the candidate base station. Bureau.   The base station according to claim 1, wherein the control unit performs control so as to select, as the candidate base station, a base station that can communicate with the terminal, to which radio resources used for the coordinated transmission can be allocated.   The base station according to claim 6, wherein the control unit performs control so as to secure radio resources of the candidate base stations that have not been selected as the cooperative base station. While performing the cooperative transmission by the cooperative base station, when the fluctuation per unit time of communication quality based on the communication quality information received by the receiving unit exceeds a predetermined threshold,
The control unit receives communication quality information between the combination including at least one candidate base station and the terminal from the terminal for each combination at a frequency according to a fluctuation per unit time of the communication quality. The base station according to claim 1, wherein the receiving unit is controlled. A control method of a base station that performs cooperative transmission in which signals are transmitted from a plurality of base stations to a terminal by wireless communication,
A receiving step of receiving communication quality information between the terminal and a base station capable of communicating with the terminal from the terminal;
Controlling to perform coordinated transmission with the coordinated base station;
Based on the communication quality information received in the receiving step, a combination of at least one candidate base station by selecting candidate base stations that are candidates for performing coordinated transmission among base stations that can communicate with the terminal Communication quality information between the terminal and the terminal is received from the terminal for each combination, a candidate base station that constitutes a combination having the highest communication quality based on the communication quality information among the combinations is a cooperative base station , Screening candidate base stations that do not constitute the highest combination as standby base stations;
During the coordinated transmission by the coordinated base station, when the communication quality based on the received communication quality information falls below a predetermined threshold, the coordinated base station is selected from the standby base station. Step to fix,
A base station control method including:

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