JP2013150064A - Base station, and communication control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce interference of radio signals in a downlink control channel transmitted by a plurality of base stations at the same timing.SOLUTION: In a base station 12, a detection unit 26 detects the number of mobile stations 14 connected to the base station; and a transmission power control unit 30 controls, when there are other base stations that transmit radio signals in a downlink control channel at the same timing, transmission power at which the base station 12 transmits radio signals in the downlink control channel in accordance with the number of connection detected by the detection unit 26.

Description

  The present invention relates to a base station and a communication control method.

  Conventionally, there is a problem that signals transmitted from a plurality of base stations interfere with each other in a mobile station. And in order to prevent this problem, the method of controlling transmission power in a base station is known.

  For example, in Patent Document 1 (Japanese Patent Application Laid-Open No. 2010-239203), a straight line distance and a moving direction between a mobile station existing in a cell area and a base station are determined, and the mobile station moves away from the base station and near the cell area boundary By suppressing the increase in downlink transmission power of the mobile station, the increase in downlink interference and the fluctuation of interference in the cell are suppressed, and the communication quality in the cell is stabilized and the throughput is improved. .

JP 2010-239203 A

  The transmission power control described in Patent Document 1 described above is for controlling the transmission power of a speech channel (TCH) signal. Transmission power control of a speech channel (TCH) has been conventionally performed by beam forming that directs a beam in a specific direction, and is relatively easy. However, the control channel is transmitted omni, not beamforming. Further, in order to improve the success rate of link establishment when the mobile station connects to the base station, the base station increases the transmission power for transmitting the radio signal through the control channel. For this reason, when a plurality of adjacent base stations transmit radio signals on the downlink control channel at the same timing, interference occurs in the radio signals.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a base station and a communication control method capable of preventing interference of control channel signals from a plurality of radio base stations. To do.

In order to solve the above problems, a base station according to the present invention includes a transmission unit that transmits a radio signal,
The number of connections detected by the detecting means when there is a detecting means for detecting the number of connected mobile stations connected to the own station and another base station that transmits a radio signal on the downlink control channel at the same timing as the own station. And a control means for controlling the transmission power of the radio signal in the downlink control channel by the transmission means.

  In the aspect of the invention, the control unit may reduce the transmission power of the radio signal in the downlink control channel by the transmission unit in accordance with the increase in the number of connections detected by the detection unit.

  In this aspect, the control unit may increase the transmission power of the radio signal in the downlink control channel by the transmission unit according to the decrease in the number of connections detected by the detection unit.

  Further, the communication control method according to the present invention includes a step in which the base station detects the number of mobile stations connected to the own station, and another base station that transmits a radio signal using a downlink control channel at the same timing as the own station. If present, the base station includes a step of controlling transmission power of a radio signal in a downlink control channel according to the detected number of connections.

It is a figure which shows the structure of the mobile communication system which concerns on embodiment of this invention. It is a figure which shows the radio channel structure in the mobile communication system which concerns on this embodiment. It is a figure which shows the effective cell radius which fluctuates (enlargement / reduction) according to the number of mobile stations connected to the base station according to the present embodiment. It is a functional block diagram of the base station which concerns on this embodiment. It is a figure which shows an example of the table which defined the correspondence of the connection number of a mobile station, and the transmission power in a downlink control channel. It is a flowchart which shows the process of the transmission power control in the downlink control channel of the base station which concerns on this embodiment.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing a configuration of a mobile communication system 10 according to an embodiment of the present invention. As shown in FIG. 1, a mobile communication system 10 includes a plurality of base stations 12 (only one is shown here) and a plurality of mobile stations 14 (here, mobile stations 14 located in a cell 16 of the base station 12). Only -1 to 14-4 are shown).

  Each base station 12 uses an OFDMA (Orthogonal Frequency Division Multiple Access) method and a TDMA / TDD (Time Division Multiple Access / Time Division Duplex) method. Wireless communication with the mobile station 14 located in the cell 16. Examples of the mobile station 14 include a mobile phone, a communication card, and a portable information terminal with a built-in communication function.

  FIG. 2 is a diagram showing a radio channel configuration in the mobile communication system 10 (vertical axis: frequency, horizontal axis: time). As shown in FIG. 2, in the mobile communication system 10, a TDMA frame having a predetermined period (here, 5 ms) is divided into an uplink subframe (2.5 ms) and a downlink subframe (2.5 ms), and each subframe is further divided. Are divided into a plurality of time slots (here, Slot 1 to Slot 4). The transmission / reception timing of the radio signal is synchronized between the base station 12 and its neighboring base stations so that the boundary of the TDMA frame (radio signal transmission / reception cycle) falls within a predetermined time (for example, within ± 10 μs) ( Frame synchronization is done). A plurality of OFDMA subchannels (here, Sch1 to Sch18) are defined in a predetermined frequency band.

  The minimum unit of the radio channel that the base station 12 allocates to the mobile station 14 is called a PRU (Physical Resource Unit), and each PRU is either one of time slots (Slot 1 to Slot 4) and one of subchannels (Sch 1 to Sch 18). , Belonging to. In addition, it is determined that each PRU is identified by consecutive PRU numbers (1, 2, 3,...) Starting from 1, for example, in both the uplink subframe and the downlink subframe. It is supposed to be used in upper and lower pairs. That is, PRUs are assigned symmetrically.

  Among these, a PRU belonging to a specific subchannel (here, Sch1) is defined as a CCH (Common Channel) shared by one or more mobile stations 14. Also, each base station 12 is assigned a CCH transmission / reception timing (hereinafter simply referred to as “CCH timing”) that does not overlap each other and is repeated at a predetermined period. Specifically, any one of 80 time slot pairs (4 pairs of upper and lower time slots × 20 frames) included in 20 consecutive frames is assigned to each base station 12 as CCH timing. Each base station 12 transmits / receives various control signals to / from one or more mobile stations 14 located in the cell 16 of the own station at the CCH timing assigned to the own station.

  On the other hand, PRUs belonging to subchannels other than the specific subchannel (here, Sch2 to Sch18) are used as ICH (Individual Channel) individually assigned to each mobile station 14. An ICH is assigned to each mobile station 14 in principle as an individual control channel and is mainly used for transmission of control information. An CHCH (Anchor Channel) is assigned to each mobile station 14 as one or more communication channels. There are EXCH (Extra Channel) used for the transmission of the.

  In the mobile communication system 10, the mobile station 14 has received and received a downlink control channel radio signal transmitted from each of the base stations 12 (including the connected base station 12) located in the vicinity thereof. The base station 12 to be connected is selected based on the reception level of the radio signal (for example, RSSI (Received Signal Strength Indicator)). For example, the mobile station 14 selects the base station 12 with the highest reception level of the received radio signal as the connection destination. Then, the mobile station 14 transmits a connection request (link channel allocation request) to the base station 12 selected as the connection destination in the uplink control channel, and after performing a series of connection processes with the base station 12, the base station 12 Connect to station 12.

  The radio signal transmitted from the base station 12 to the mobile station 14 includes BCCH (Broadcasting Control Channel), PCH (Paging Channel), and ICH transmitted via the CCH. There are control signals, communication signals, etc. transmitted through the network. These wireless signals include known signals for timing synchronization. Then, the mobile station 14 detects the correlation value between the radio signal transmitted from the base station 12 and the known signal, and acquires the timing when the correlation value equal to or greater than the predetermined value is detected as the reception timing of the radio signal.

  On the other hand, the base station 12 receives the radio signal of the downlink control channel transmitted from each of the other base stations 12 located in the vicinity periodically (for example, every few hours). Then, the base station 12 detects the presence of another base station 12 that transmits the radio signal of the downlink control channel at the same timing as that at which the own station transmits the radio signal on the downlink control channel. When another base station 12 transmitting a radio signal using the downlink control channel at the same timing is detected, the base station 12 transmits the radio signal using the downlink control channel based on the number of connected mobile stations 14 connected to the own station. Controls the transmission power when transmitting. Specifically, the base station 12 performs control to reduce the transmission power of the radio signal transmitted through the downlink control channel in accordance with the increase in the number of connections of the mobile station 14 to the own station, whereby the mobile station 14 However, the range (substantial coverage range (effective cell) of the own station) in which radio signals transmitted on the downlink control channel can be received without error is expanded or reduced. Here, in general, when the number of mobile station connections increases, it becomes difficult for the base station to assign the EXCH used for transmission of communication data to the mobile station 14 that has newly requested a connection. In such a situation, the mobile station cannot connect even if a connection request is made to the base station. Therefore, by causing the base station 12 that is in a situation in which the mobile station 14 is difficult to connect in this way to perform control to reduce the transmission power of the radio signal transmitted through the downlink control channel preferentially, Thus, it is possible to reduce the interference of radio signals transmitted through downlink control channels at the same timing, while ensuring the ease of connection of the mobile station 14 to other base stations.

  FIG. 3 shows fluctuation (enlargement / reduction) depending on the number of connections of the mobile station 14 to the base station 12 in the base station 12 when another base station 12 that transmits a radio signal is detected using the downlink control channel at the same timing. It is a figure which shows the effective cell radius to do. At some point, the effective cell of the base station 12 is in the range indicated by the cell 16-A, and the mobile stations 14-1 to 14-4 located in the cell 16-A are connected to the base station 12. To do.

  Here, if it is determined that the number of mobile stations 14 connected to the base station 12 has increased, the base station 12 reduces the transmission power of the radio signal transmitted on the downlink control channel by an amount corresponding to the number of connections. Thereby, since the effective cell of the base station 12 is reduced, it is possible to reduce the influence of interference on the radio signal of the downlink control channel transmitted from the neighboring base station at the same timing. For example, if the effective cell of the base station 12 is reduced from the cell 16-A to the range indicated by the cell 16-B, the mobile stations 14-2 and 14-3 connected to the base station 12 until then are A handover to another base station 12 is performed or the communication is terminated. As a result, the number of mobile stations 14 connected to the base station 12 decreases, and the communication load on the base station 12 also decreases.

  Conversely, if it is determined that the communication load of the base station 12 has decreased in a state where the cell 16 of the base station 12 is in the range indicated by the cell 16-A, the base station 12 will only increase the amount corresponding to the communication load. The transmission power of the radio signal transmitted on the downlink control channel is increased. Thereby, the effective cell of the base station 12 is expanded. For example, if the effective cell of the base station 12 viewed from the mobile station 14 is expanded from the cell 16-A to the range indicated by the cell 16-C (initial setting), the movement that has been outside the effective cell of the base station 12 until then is performed. The stations 14-5 to 14-7 can newly connect to the base station 12. As a result, the number of mobile stations 14 connected to the base station 12 increases, and the communication load of the base station 12 also increases.

  As described above, in the mobile communication system 10, each base station 12 controls the transmission power for transmitting a radio signal on the downlink control channel in accordance with the number of mobile stations 14 connected to the own station. Enlarge or reduce the cell. For this reason, each base station 12 can reduce the influence of the interference with the radio signal of the downlink control channel transmitted from a neighboring base station at the same timing.

  Below, the structure with which the base station 12 is provided in order to implement | achieve the said process is demonstrated concretely.

  FIG. 4 is a functional block diagram of the base station 12. As shown in the figure, the base station 12 includes an antenna 20, a reception RF unit 22, a reception baseband unit 24, a detection unit 26, a storage unit 28, a transmission power control unit 30, a transmission baseband unit 32, and a transmission RF unit. 34 is comprised.

  The antenna 20 receives a radio signal transmitted from the mobile station 14 and outputs the received radio signal (reception signal) to the reception RF unit 22. Further, the antenna 20 transmits a radio signal supplied from the reception RF unit 22 to the mobile station 14.

  The reception RF unit 22 includes a low noise amplifier, a frequency converter, a band pass filter, and an A / D converter. The reception RF unit 22 amplifies the radio signal input from the antenna 20 with a low noise amplifier, down-converts it to an intermediate frequency signal, further converts it into a digital signal, and outputs it to the reception baseband unit 24 To do.

  The reception baseband unit 24 includes a serial-parallel converter, an FFT (Fast Fourier Transform) unit, a parallel-serial converter, and a demodulation unit (not shown). The reception baseband unit 24 converts the digital signal input from the reception RF unit 22 from serial-to-parallel conversion, CP (Cyclic Prefix) removal, primary demodulation (fast Fourier transform), parallel-serial conversion, secondary demodulation (symbol decoding). Mapping) and the like, and output the received data to an upper layer (not shown).

  The detection unit 26 periodically receives a radio signal of a downlink control channel transmitted from each of a plurality of other base stations 12 located in the vicinity, and a timing at which the own station transmits a radio signal on the downlink control channel When the presence of another base station 12 that transmits a radio signal using the downlink control channel at the same timing is detected, the transmission power control unit 30 is notified of this. In addition, the detection unit 26 detects the number of connections of the mobile station 14 to the base station 12 constantly or periodically, and notifies the transmission power control unit 30 of the detected number of connections. More specifically, the detection unit 26 detects the number of connections of the base station 12 based on the number of mobile stations 14 to which PRUs that can be allocated by the base station 12 are assigned.

  The storage unit 28 is composed of, for example, a semiconductor memory element, and stores a program, data, a table shown in FIG. The table shown in FIG. 5 is an example of a table that defines a correspondence relationship between the number of mobile stations 14 connected to the base station 12 and the transmission power of radio signals transmitted from the base station 12 through the downlink control channel.

  The transmission power control unit 30 determines the presence of another base station 12 that transmits a radio signal on the downlink control channel at the same timing detected by the detection unit 26, and the number of mobile stations 14 connected to the base station 12. Accordingly, the transmission power of the radio signal transmitted through the downlink control channel is controlled. That is, the transmission power control unit 30 indicates the transmission power corresponding to the number of connections of the mobile station 14 to the base station 12 detected by the detection unit 26, and the correspondence between the number of connections stored in the storage unit 28 and the transmission power. Read from the defined table (see FIG. 5). Then, the transmission power control unit 30 instructs the transmission baseband unit 32 to transmit the radio signal (BCCH, PCH, etc.) of the downlink control channel with the read transmission power.

  Therefore, according to the table shown in FIG. 5, for example, when the number of mobile stations 14 connected to the base station 12 increases from 7 to 13, the transmission power of the radio signal transmitted on the downlink control channel is changed from B [dB] to C [ dB] (B> C).

  Conversely, when the number of mobile stations 14 connected to the base station 12 decreases from 7 to 3, the transmission power increases from B to A (A> B) (returns to the initial transmission power A). The effective cell radius of 12 expands (returns to the default execution cell radius).

  The transmission baseband unit 32 includes a serial-parallel converter, an IFFT (Inverse Fast Fourier Transform) unit, a parallel-serial converter, and a modulation unit (not shown). The transmission baseband unit 32 performs primary modulation (symbol mapping), serial-parallel conversion, on transmission data addressed to the connected base station 12 or the connected base station 12 input from an upper layer (not shown). Secondary modulation (inverse fast Fourier transform), addition of CP, parallel serial conversion, and the like are performed, and the obtained digital signal is output to the transmission RF unit 34.

  The transmission RF unit 34 includes a power amplifier, a frequency converter, a band pass filter, and a D / A converter. The transmission RF unit 34 converts the digital signal input from the transmission baseband unit 32 into an analog signal, then up-converts it to a radio signal, amplifies it to a transmission output level with a power amplifier, and then supplies it to the antenna 20 .

  Next, an example of the operation of the base station 12 will be described based on FIG. FIG. 6 is a diagram illustrating an example of transmission power control processing of the base station 12.

  As shown in FIG. 6, the base station 12 monitors the number of connections of the mobile station 14 to the own station constantly or periodically (S100). When the base station 12 detects another base station 12 that transmits a radio signal on the downlink control channel at the same timing (S102: Y), the base station 12 transmits the radio signal on the downlink control channel based on the current number of connections. The power is determined (S104). Then, the base station 12 transmits a radio signal (BCCH, PCH, etc.) in the downlink control channel with the transmission power determined in S102 (S106). Further, when the number of connections of the mobile station 14 to the own station changes (S108: Y), the base station 12 determines the transmission power of the radio signal in the downlink control channel based on the number of connections as shown in FIG. (S104). That is, the base station 12 reduces the transmission power of the radio signal in the downlink control channel according to the increase in the number of connections of the mobile station 14 to the own station, and conversely, according to the decrease of the mobile station 14 relative to the own station, The transmission power of the radio signal in the downlink control channel is increased.

  Thereby, when the number of connections of the mobile station 14 to the base station 12 is large, the effective cell of the base station 12 is reduced. Conversely, when the number of connections of the mobile station 14 to the base station 12 is small, the base station 12 12 effective cells are expanded (the execution cell returns to the initial setting).

  According to the mobile communication system 10 described above, when each base station 12 detects another base station 12 that transmits a radio signal using the downlink control channel at the same timing, it depends on the number of mobile stations 14 connected to the own station. Thus, the effective cell of the own station is enlarged or reduced by controlling the transmission power of the radio signal in the downlink control channel. For this reason, each base station 12 can reduce the influence of the interference with the radio signal of the downlink control channel transmitted from a neighboring base station at the same timing.

  The present invention is not limited to the above embodiment.

  For example, the present invention is not limited to mobile communication systems that employ OFDMA and TDMA / TDD, but can be widely applied to all mobile communication systems in which the influence of interference is problematic.

  Further, although the present invention defines three types of transmission power when transmitting a radio signal on the downlink control channel according to the number of mobile stations connected to the base station, the present invention is not limited to this. Furthermore, the present invention controls the transmission power when transmitting a radio signal on the downlink control channel according to the number of mobile stations connected to the base station. You may control the transmission power at the time of transmitting a radio signal with a control channel according to load and the utilization factor of a subchannel.

10 mobile communication systems, 12 base stations, 14 mobile stations, 16 cells, 20 antennas, 22 reception RF units, 24 reception baseband units, 26 detection units, 28 storage units, 30 transmission power control units, 32 transmission baseband units, 34 Transmitting RF unit.

Claims (4)

A transmission means for transmitting a radio signal;
Detection means for detecting the number of mobile stations connected to the own station;
When there is another base station that transmits a radio signal on the downlink control channel at the same timing as the own station, the transmission power of the radio signal on the downlink control channel by the transmission unit depends on the number of connections detected by the detection unit Control means for controlling
A base station comprising: In the base station according to claim 1,
The control means reduces the transmission power of the radio signal in the downlink control channel by the transmission means according to the increase in the number of connections detected by the detection means,
A base station characterized by that. In the base station according to claim 2,
The control means increases the transmission power of the radio signal in the downlink control channel by the transmission means according to the decrease in the number of connections detected by the detection means.
A base station characterized by that. Detecting the number of mobile stations connected to the base station by the base station;
When there is another base station that transmits a radio signal using a downlink control channel having the same timing as the own station, the base station controls the transmission power of the radio signal in the downlink control channel according to the detected number of connections. When,
The communication control method characterized by including.

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