Wireless communication apparatus

A wireless communication apparatus, for example a wireless base station that spatially multiplexes data for transmission, includes plural array antennas and a beam shape controller that controls the array antennas to form beams of different shapes and each transmit signals for communication quality measurement in an area to be covered by the apparatus, and determines an initial value of a beam shape to be used in data transmission to a counterpart device that has received the signal for communication quality measurement, and after the initial value is determined, repeatedly executes processing to control the array antennas to tentatively change a beam shape in use in data transmission to the counterpart device, and redetermine a beam shape to be used in data transmission to the counterpart device, based on communication quality measured when the post-tentative-change beam shape is used and communication quality measured when the pre-tentative-change beam shape is used.

FIELD

The present invention relates to a wireless communication apparatus that spatially multiplexes data for transmission.

BACKGROUND

In order to transmit large-volume data by the use of limited frequencies, multiple-input and multiple-output (MIMO) systems for performing spatial multiplex transmission using a plurality of transmitting and receiving antennas are being developed. The number of spatial multiplexing is expected to continue to increase for further improvements in frequency utilization efficiency.

In the next-generation of mobile communication systems, the use of high frequencies exceeding 6 GHz is being studied. However, the use of a high-frequency band disadvantageously increases propagation loss. On the other hand, for the use of a high-frequency band, antenna elements can be increased to form a transmission beam with high gain. Thus, it is being studied to compensate, by forming a transmission beam, for the disadvantage of propagation loss increase. A technology of combining MIMO transmission and an array antenna formed of a multi-element antenna is being developed to avoid a reduction in the cover area of a cell while achieving large-volume data transmission.

Patent Literature 1 describes an invention that combines MIMO transmission and a plurality of array antennas, and controls the antennas to form transmission beams that reduce correlation between MIMO streams and increase reception quality. The invention described in Patent Literature 1 sweeps transmission beams at small angular intervals when determining transmission beam shapes, thereby searching for a plurality of beams of low correlation and good reception quality.

Patent Literature 2 describes an invention that selects a combination of beam antennas so as to maximize the signal-to-interference-and-noise ratio (SINR) of each MIMO stream, and further minimize the correlation coefficient of a channel transfer matrix.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent 4536733

SUMMARY

Technical Problem

In the inventions described in Patent Literatures 1 and 2, a system including MIMO processing and a plurality of array antennas (beam antennas) on the wireless base station side selects, for each array antenna, a beam shape that reduces correlation between MIMO streams and increases reception quality. Here, channel information on all beam shapes are necessary to select beam shapes, and it is necessary to perform a transmission beam sweep to cover all beam shapes. In this case, radio resources for a beam sweep the number of which corresponds to the multiplication of the number of array antennas by the number of beam shapes, are consumed. To apply the above-described system that combines MIMO processing and a plurality of array antennas to a next-generation mobile communication system, a large number of array antennas are necessary for multiplexing a large number of MIMO streams. This results in enormous radio resource consumption for a beam sweep.

The present invention has been made in view of the above, and has an object of providing a wireless communication apparatus capable of reducing the amount of consumption of radio resources when determining beam shapes.

Solution to Problem

In order to solve the above-described problem and attain the object, an aspect of the present invention provides a wireless communication apparatus that spatially multiplexes data for transmission and includes a plurality of array antennas. The wireless communication apparatus controls the plurality of array antennas so that the array antennas form beams of different shapes and each transmit signals for communication quality measurement in an area to be covered by the apparatus, and determines an initial value of a beam shape to be used in data transmission to a counterpart device that has received the signal for communication quality measurement, based on communication quality of each beam measured by the counterpart device. The wireless communication apparatus repeatedly executes processing to control the array antennas to tentatively change a shape of a beam in use in data transmission to the counterpart device, and redetermine a beam shape to be used in data transmission to the counterpart device, based on communication quality measured when the post-tentative-change beam is used and communication quality measured when the pre-tentative-change beam is used.

Advantageous Effects of Invention

The wireless communication apparatus according to the present invention achieves an effect of being able to reduce the amount of consumption of radio resources when determining beam shapes.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a wireless communication apparatus according to embodiments of the present invention will be described in detail with reference to the drawings. The embodiments are not intended to limit the invention.

First Embodiment

FIG. 1is a diagram illustrating a configuration example of a communication system to which a wireless communication apparatus according to a first embodiment is applied. The communication system illustrated inFIG. 1includes a wireless base station1, a wireless terminal2, and a higher-level apparatus3. The wireless base station1is the wireless communication apparatus according to the first embodiment.

The wireless base station1is configured to be capable of forming a plurality of beams5using array antennas, and communicates with the wireless terminal2, a counterpart device, using one or more beams5.FIG. 1illustrates one wireless terminal2; however, this is an example. A plurality of wireless terminals2can simultaneously communicate with the wireless base station1. The wireless terminal2can be configured to have a plurality of antennas. The higher-level apparatus3is an apparatus on the core network side. A gateway, a mobility management entity (MME), or the like corresponds to the higher-level apparatus3. The wireless base station1is connected to the higher-level apparatus3via a communication line, and the higher-level apparatus3is connected to a network4. The network4is a network different from the wireless communication network that includes the wireless base station1, the wireless terminal2, and the higher-level apparatus3.

FIG. 2is a diagram illustrating a configuration example of the wireless base station1in the first embodiment.FIG. 2describes only the main components of the wireless base station1, and omits the description of components for processes that are not directly related to the implementation of the invention, e.g. components for communicating with the higher-level apparatus3.FIG. 2illustrates an example in a case where the present invention is applied to the wireless base station1, which is a wireless communication apparatus that performs orthogonal frequency-division multiplexing (OFDM) processing. However, the present invention is not limited to a wireless communication apparatus that performs multicarrier transmission. The present embodiment illustrates an example of a wireless base station; however, the present invention is not limited to a wireless base station. For example, the wireless terminal2may have the same function.

With reference toFIG. 2, the configuration and operation of the wireless base station1will be described.

The wireless base station1includes baseband processing units11and16, a plurality of digital-to-analog converters (DACs)12, a local oscillator13, a plurality of mixers14and18, a plurality of array antennas (hereinafter, referred to as antennas)15, and a plurality of analog-to-digital converters (ADCs)17. The wireless base station1provides a function of spatially multiplexing and simultaneously transmitting, to a plurality of users, signals addressed to the users (including multi-user MIMO and single-user MIMO).

The baseband processing unit11includes a MIMO processing unit112, a plurality of OFDM processing units113, and a beam shape controller114. The baseband processing unit11generates transmission signals to be transmitted to wireless terminals2, and controls the antennas15.

When the MIMO processing unit112of the baseband processing unit11is fed streams111, which constitute a signal stream group to be transmitted to the wireless terminals2by spatial multiplexing, the MIMO processing unit112distributes the fed streams111to the antennas15, and performs MIMO processing including precoding and others on the distributed streams. The streams111are data strings that are transmitted to different wireless terminals. Precoding is processing for weighting by multiplying streams distributed to the antennas15by transmission weights. The transmission weights are calculated by the MIMO processing unit112, based on the channel states between the wireless base station1and the wireless terminals2. A way of determining channel states and calculating transmission weights is known as described in Patent Literature 1 described above and others, and thus will not be described in detail. The OFDM processing units113perform modulation processing, inverse fast Fourier transform (IFFT) processing, cyclic prefix (CP) addition processing, and others on signals fed from the MIMO processing unit112, to generate transmission signals to be transmitted to the wireless terminals2. In the modulation processing, input signals are modulated in accordance with a modulation scheme such as quadrature phase-shift keying (QPSK) or quadrature amplitude modulation (QAM). The beam shape controller114controls the antennas15based on communication quality information and others fed back from the wireless terminals2to be the destinations of the streams111, to cause the antennas15to form transmission beams. Details of an operation of the beam shape controller114to control the antennas15will be described separately.

The DACs12convert transmission signals generated by the baseband processing unit11from digital signals to analog signals. The mixers14up-convert analog signals output from the DACs12to a carrier frequency, based on a locally generated signal output from the local oscillator13, and feed them to the antennas15.

The antennas15each include a plurality of phase shifters151, and control the phase shifters151based on an instruction of the beam shape controller114of the baseband processing unit11, that is, a control signal indicating the phase shift amounts of the phase shifters151, thereby forming a beam in a direction specified by the beam shape controller114. For example, each of the antennas15can direct a transmission beam in a direction in which a wireless terminal2easily receives signals, according to an instruction from the beam shape controller114. Note that, to specify the phase shift amounts of the phase shifters151to the antennas15, the beam shape controller114may specify phase shift amounts themselves, or may specify them in a different way. In the different way, for example, the antennas15store a plurality of predetermined phase shift amounts and identification numbers of the phase shift amounts in memory, the beam shape controller114notifies the antennas15of the identification numbers of phase shift amounts, and the antennas15read the phase shift amounts of the indicated identification numbers from the memory.

The antennas15receive signals transmitted from the wireless terminals2. The antennas15can form reception beams when receiving signals from the wireless terminals2.

Signals received by the antennas15are fed to the mixers18. The mixers18down-convert analog received signals of a carrier frequency fed from the antennas15into signals of a baseband frequency, based on a locally generated signal output from the local oscillator13. The ADCs17convert analog signal reception of the baseband frequency output from the mixers18into digital signals. The received signals converted into the digital signals by the ADCs17are fed to the baseband processing unit16.

The baseband processing unit16includes a feedback information extractor161, a MIMO processing unit162, and a plurality of OFDM processing units163. The baseband processing unit16processes signals received from the wireless terminals2via the antennas15, the mixers18, and the ADCs17, and restores data transmitted from the wireless terminals2. When the restored data contains information to be used in beam shape control, e.g. communication quality information and others, the baseband processing unit16outputs the information to the beam shape controller114of the baseband processing unit11.

The OFDM processing units163of the baseband processing unit16perform CP removal processing, FFT processing, demodulation processing, and others on signals fed from the ADCs17for demodulation. The MIMO processing unit162weights and combines demodulated received signals fed from the OFDM processing units163. In weighting and combining performed by the MIMO processing unit162, for example, channel estimation is performed based on a known sequence contained in received signals from the wireless terminals2, and from resultant channel estimation values, weights of the received signals fed from the OFDM processing units163are calculated, and the received signals are multiplied by the calculated weights for weighting, and then combined. The feedback information extractor161extracts communication quality information from demodulated data, which are demodulated signals that have been weighted and combined by the MIMO processing unit162, and outputs the communication quality information to the beam shape controller114of the baseband processing unit11.FIG. 3is a flowchart illustrating an operation example of the feedback information extractor161. When receiving demodulated data from the MIMO processing unit162(step S101), the feedback information extractor161extracts feedback information (step S102). Then, the feedback information extractor161outputs the extracted feedback information to the beam shape controller114(step S103).

Next, a beam shape control operation by the wireless base station1will be described with reference toFIG. 4.FIG. 4is a sequence diagram illustrating an example of the beam shape control operation by the wireless base station1in the first embodiment.FIG. 4describes the plurality of antennas15included in the wireless base station1as antennas151to15N. In the following description, the antennas15are sometimes described as antennas151to15N. The example ofFIG. 4illustrates a case where the communication system complies with the Long Term Evolution (LTE) standard of the Third Generation Partnership Project (3GPP). This is an example and is not intended to limit the communication system to LTE.

In the beam shape control operation by the wireless base station1, first, the wireless base station1transmits a signal containing a common reference signal (CRS) standardized by LTE from all the antennas15, i.e. the antennas151to15N to all the wireless terminals2(wireless terminals2A,2B, . . . ) (step S11). In this step S11, the antennas15of the wireless base station1form beams of different shapes. Specifically, in the wireless base station1, the beam shape controller114specifies different phase shift amounts to the phase shifters151of the antennas15. Different beam shapes mean different beam directions, that is, directions in which beams are formed are different to each other. Even though two beams have the same width, when the two beams are formed in different directions, both become beams of different shapes. The beam shape controller114specifies phase shift amounts to the antennas15such that beams are formed all over an area that should be covered by the wireless base station1, that is, beams are formed in all directions within the area that should be covered by the wireless base station1. Here, when it is difficult for the wireless base station1to form beams all over the area that should be covered even by forming beams in different directions using all the antennas15, that is, when the number of beams necessary to form beams all over the area that should be covered is larger than the number of the antennas15, the beam shape controller114controls one or more antennas15such that they form beams of a plurality of shapes over a plurality of radio frames. That is, the beam shape controller114controls one or more antennas15such that the shape of a beam is changed for a radio frame basis, to form beams all over the area that should be covered by the wireless base station1. On the contrary, when the number of beams necessary to form beams all over the area that should be covered by the wireless base station1is smaller than the number of the antennas15, the beam shape controller114may control some of the antennas15such that they do not form beams, or may control a plurality of antennas15such that they generate beams of the same shape.

In the present embodiment, in order to avoid complicated description, the widths of beams formed by the antennas15are fixed, and only the directions of the beams are changed to change the shapes of the beams.

A CRS is a signal used by the wireless terminals2to measure communication quality, that is, a signal for communication quality measurement. Communication quality here is, for example, a received power value, an SINR, a correlation value between a received CRS and a CRS held by the wireless terminals2. In step S11, the wireless base station1transmits from each of the antennas15the CRS and a beam ID indicating the shape of a beam formed by the antenna. The CRS and the beam ID to be transmitted from each of the antennas15are, for example, output from the beam shape controller114to the OFDM processing unit113that corresponds to each of the antennas15. The CRS and the beam ID fed to each of the OFDM processing units113are subjected to modulation processing and others, and then transmitted from the corresponding antenna15through the DAC12and the mixer14in later stages.

When the wireless terminals2receive the CRS transmitted from the antennas15in step S11, each wireless terminal2measures the communication quality of each beam, and notifies the wireless base station1of the beam ID of a beam of high communication quality, e.g. a beam of the received CRS with a high received power value, and the number of a radio frame in which the beam of the high received power value has been received (step S12). In step S12, the number of a radio frame is indicated together with a beam ID on the assumption that one or more antennas15of the wireless base station1form beams of a plurality of shapes over a plurality of radio frames in step S11described above. The wireless terminals2may notify the beam IDs of a plurality of beams of high communication quality, for example, of a beam having the highest communication quality to a beam having the Mth highest communication quality and a radio frame number. Although, for explanatory convenience,FIG. 4describes that only the wireless terminal2A notifies the wireless base station1of a beam ID, wireless terminals2other than the wireless terminal2A also notify the wireless base station1of a beam ID. The wireless terminals2may identify the beam ID of a transmitted beam by the use of a pseudorandom sequence value obtained when demodulating the CRS, for example. In LTE, when a CRS is transmitted, the CRS is converted into a pseudorandom sequence using information unique to a beam, e.g. a beam ID. Accordingly, a pseudorandom sequence is a pattern unique to each beam, and the CRS-receiving end can determine a received beam from a pseudorandom sequence. By providing each of the wireless terminals2with a table of correspondence between pseudorandom sequences and beam IDs in advance, each wireless terminal2can determine a beam ID from a pseudorandom sequence.

Here, the configuration of a wireless terminal2will be described.FIG. 5is a diagram illustrating a configuration example of a wireless terminal2in the first embodiment.FIG. 5only describes the main components of the wireless terminal2, and omits the description of components for processing that is not directly related to the implementation of the invention.

The wireless terminal2includes an antenna21, a local oscillator22, mixers23,27, an analog-to-digital converter (ADC)24, a baseband processing unit25, and a digital-to-analog converter (DAC)26.

At the wireless terminal2, the antenna21receives radio signals transmitted from the wireless base station1, and transmits signals fed from the mixer27to the wireless base station1. The mixer23down-converts an analog received signal of a carrier frequency fed from the antenna21into a signal of a baseband frequency, based on a locally generated signal output from the local oscillator22. The ADC24converts the analog signal reception of the baseband frequency output from the mixer23into a digital signal. The received signal converted into the digital signal by the ADC24is fed to the baseband processing unit25.

The baseband processing unit25includes OFDM processing units251,254, a CRS processing unit252, and a feedback information transmitter253. The baseband processing unit25processes signals received from the wireless base station1via the antenna21, the mixer23, and the ADC24, to restore data transmitted from the wireless base station1. When the restored data contains a CRS and a beam ID, the baseband processing unit25measures communication quality using the CRS, and notifies the wireless base station1of the result of the communication quality measurement, the beam ID of a beam in which the CRS used in the measurement has been received, and the number of a radio frame in which the CRS and the beam ID have been received, as needed.

The OFDM processing unit251of the baseband processing unit25performs CP removal processing, FFT processing, demodulation processing, and others on received signals fed from the ADC24. Using a CRS contained in a received signal, the OFDM processing unit251measures the communication quality of a beam in which the CRS has been transmitted (transmitted beam).

FIG. 6is a flowchart illustrating an operation example of the CRS processing unit252. The CRS processing unit252obtains a beam ID from a demodulated signal output from the OFDM processing unit251(step S201), then generates communication quality information based on communication quality measured by the OFDM processing unit251(step S202). Next, the CRS processing unit252outputs, to the feedback information transmitter253, the beam ID obtained in step S201, the communication quality information generated in step S202, and a radio frame number (step S203). The radio frame number output together with the beam ID in step S203is the number of a radio frame in which the beam ID has been received.

The feedback information transmitter253transmits, some out of the beam ID, the communication quality information, and the radio frame number that are fed from the CRS processing unit252, from the antenna21to the wireless base station1through the OFDM processing unit254, the DAC26, and the mixer27. That is, the beam ID and the radio frame number are transmitted, or the beam ID, the communication quality information, and the radio frame number are transmitted. When the feedback information transmitter253executes step S12illustrated inFIG. 4, for example, the feedback information transmitter253transmits, to the wireless base station1, the beam ID of a transmission beam having the highest communication quality among beam IDs fed from the CRS processing unit252, together with the number of a radio frame in which the beam ID has been received. Alternatively, in step S12, the feedback information transmitter253may transmit, to the wireless base station1, the beam IDs of M number of transmission beams in descending order starting from the transmission beam having the highest communication quality to a transmission beam having the Mth highest communication quality, together with communication quality information and a radio frame number.

FIG. 7is a flowchart illustrating an operation example of the feedback information transmitter253. When the feedback information transmitter253acquires a beam ID, communication quality information, and a radio frame number from the CRS processing unit252(step S301), the feedback information transmitter253generates feedback information containing some out of the acquired beam ID, communication quality information, and radio frame number. That is, the beam ID and the radio frame number are contained, or the beam ID, the communication quality information, and the radio frame number are contained (step S302). Next, the feedback information transmitter253outputs the generated feedback information to the OFDM processing unit254(step S303). The feedback information output from the feedback information transmitter253is transmitted from the antenna21to the wireless base station1through the OFDM processing unit254, the DAC26, and the mixer27. When the feedback information transmitter253executes step S12illustrated inFIG. 4, for example, the feedback information transmitter253transmits, to the wireless base station1, the beam ID of a transmission beam having the highest communication quality among beam IDs fed from the CRS processing unit252, together with the number of a radio frame in which the beam ID has been received. That is, in step S302, the feedback information transmitter253generates feedback information containing the beam ID of a transmission beam having the highest communication quality and the number of a radio frame in which the beam ID has been received.

Alternatively, in step S12, the feedback information transmitter253may transmit, to the wireless base station1, the beam IDs of M number of transmission beams in descending order starting from a transmission beam having the highest communication quality to a transmitted beam having the Mth highest communication quality, together with communication quality information and a radio frame number.

The OFDM processing unit254executes modulation processing, IFFT processing, CP addition processing, and others on a signal fed from the feedback information transmitter253to generate a transmission signal to be transmitted to the wireless base station1.

The DAC26converts the transmission signal generated by the OFDM processing unit254of the baseband processing unit25from a digital signal to an analog signal. The mixer27up-converts the analog signal output from the DAC26to a carrier frequency, based on a locally generated signal output from the local oscillator22, and feeds the converted signal to the antenna21.

The description returns to operations illustrated inFIG. 4. Only the operations of the wireless base station1and the wireless terminal2A will be described hereinafter. Assume that the wireless base station1performs the same processing for the wireless terminals2other than the wireless terminal2A. Assume that the wireless terminals2other than the wireless terminal2A perform the same processing as that of the wireless terminal2A.

When the wireless base station1receives a notification of a beam ID from the wireless terminal2A, the wireless base station1determines an initial value of beam shape of beams to be used for data transmission to the wireless terminal2A, that is, an initial beam shape (step S13). At the wireless base station1, the beam shape controller114determines a beam shape initial value. When only one set of a beam ID and a radio frame number is indicated in step S12described above, the beam shape controller114sets the beam shape of a beam corresponding to the indicated beam ID and radio frame number for the initial value. When a plurality of sets of a beam ID, a radio frame number, and communication quality information is indicated in step S12described above, the beam shape controller114selects a beam shape having the highest communication quality and sets the beam shape for an initial value, or selects a specified number of beam shapes in descending order in communication quality and sets the beam shapes for initial values.

In the present embodiment, the wireless base station1uses a plurality of antennas, the antennas form beams of different shapes to transmit a CRS all over an area that should be covered, and an initial value of a beam shape to be directed to a wireless terminal2is determined based on communication quality of the beams. However, a way of determining an initial value is not limited to this. The wireless base station1may search for a direction from which radio waves have arrived from a wireless terminal2, and determine a beam shape initial value based on a search result. Alternatively, the wireless base station1may cause a wireless terminal2to notify the location of the wireless terminal2, and determine a beam shape initial value based on the indicated location.

Next, using a beam shape indicated by the initial value determined in step S13, the wireless base station1transmits, to the wireless terminal2A, data (Data) and a demodulation reference signal (DMRS) prescribed by LTE (step S14). The example illustrated inFIG. 4shows an example in which two antennas15(antennas151,152) are used for the wireless terminal2A to transmit data and a DMRS. In step S14immediately after step S13in which an initial value is determined, all antennas used, that is, both of the antennas151and152form beams of the same shape (beam shape #2inFIG. 4). The DMRS is a signal necessary for the wireless terminals2to demodulate data. The DMRS is a signal for communication quality measurement that is necessary for measuring communication quality. When two beam shapes (assume that they are beam shapes #1and #2) are set as beam shape initial values in step S13described above, the wireless base station1may set the shape of a beam formed by the antenna151to beam shape #1, and set the shape of a beam formed by the antenna152to beam shape #2, for example, so that the antennas151and152form beams of different shapes.

When the wireless terminal2A receives the data and the DMRS, the wireless terminal2A demodulates the data using the DMRS and measures communication quality, to generate communication quality information indicating a measurement result. In the wireless terminal2A, the OFDM processing unit251performs demodulation of data and measurement of communication quality. Communication quality is measured for each beam. The OFDM processing unit251measures received power, an SINR, or the like as the communication quality. The communication quality information is, for example, a received power value, a modulation coding scheme (MCS) value, a rank indicator (RI) value, or the like. The MCS value and the RI value are prescribed by LTE, and are information varying in value depending on communication quality. Thus, LTE allows these to be used as communication quality information. The generation of communication quality information may be performed by the OFDM processing unit251or may be performed by the CRS processing unit252.

The wireless terminal2A feeds back to the wireless base station1communication quality information indicating the communication quality of the signals transmitted in step S14described above, that is, communication quality information on the signal received from the antenna151and communication quality information on the signal received from the antenna152(step S15). At the wireless terminal2A, the feedback information transmitter253transmits communication quality information on signals received from antennas to the wireless base station1. At this time, the feedback information transmitter253also transmits the beam IDs of the beams in which the data has been received or the antenna IDs. When the wireless base station1receives the communication quality information from the wireless terminal2A in step S15, the wireless base station1uses beams of the same shape as the beams used in data transmission in step S14. AlthoughFIG. 4illustrates the sequence example in which the wireless terminals2measure communication quality and feed it back to the wireless base station1, the wireless base station1may instruct the wireless terminals2to transmit a sounding reference signal (SRS) prescribed by LTE, and use the SRS transmitted from the wireless terminals2to measure communication quality. The SRS is a signal used for the wireless base station1to measure communication quality in the uplink, i.e. channels from the wireless terminals2to the wireless base station1.

Then, the wireless base station1tentatively changes the shape of one or both of the beam formed by the antenna151and the beam formed by the antenna152used in communication with the wireless terminal2A (step S16). That is, the beam shape controller114of the wireless base station1controls one or both of the antennas151and152used in communication with the wireless terminal2A to tentatively change the beam shape. The beam shape controller114controls, for example, the antenna151to tentatively change the shape of the beam formed by the antenna151. The beam shape controller114controls the antenna151so that the antenna151forms a beam of a shape similar to the shape of the beam used up to that time. The beam shape controller114, for example, controls the antenna151so that the antenna151forms a beam at an angle in a boresight direction close to that of the beam used up to that time, that is, the antenna151forms a beam in a direction close to a direction in which the beam used up to that time has been formed. Here, description will be continued on the assumption that the beam shape of the antenna151has been tentatively changed to beam shape #3.

Next, using the post-tentative-change beams, the wireless base station1transmits data and a DMRS from the antennas15(antennas151and152) (step S17). The operation in step S17is identical to the operation in step S14described above except that the shape of a beam used is different. When the wireless terminal2A receives the data and the DMRS, the wireless terminal2A measures communication quality and feeds back communication quality information to the wireless base station1as in step S15described above (step S18).

The wireless base station1may randomly select an antenna to be tentatively changed in beam shape and a post-tentative-change beam shape, or may select them in a predetermined order.

When the wireless base station1receives the communication quality information fed back in step S18, the wireless base station1compares the newly fed back communication quality information with the communication quality information fed back last time in step S15described above. As a result of the comparison, when the communication quality information received in step S18indicates better communication quality, it is determined that the beam of the shape tentatively changed in step S16be continuously used (step S19). In the example illustrated inFIG. 4, the beam shape controller114of the wireless base station1controls the antenna151so as to form a beam of beam shape #3and controls the antenna152so as to form a beam of beam shape #2. When the communication quality information received in step S15indicates better communication quality than the communication quality information received in step S18, the wireless base station1returns the beam shape tentatively changed in step S16to the shape before the tentative change. That is, the wireless base station1determines that the beam of the pre-tentative-change shape be used. In the example inFIG. 4, the wireless base station1returns the shape of the beam formed by the antenna151to beam shape #2. Steps S14to S19show processing of redetermining beam shapes.

Then, using the post-change beams, the wireless base station1transmits data and a DMRS from the antennas15(antennas151and152) (step S20). Hereinafter, the wireless base station1and the wireless terminal2A repeatedly execute an operation to execute the same processing as that in steps S15to S16described above to tentatively change a beam shape, and further to execute the same processing as that in steps S17to S19to change the shape of a beam, that is, processing to redetermine beam shapes, to control the antennas15such that the shapes of beams formed by the antennas15become optimum. This enables communication using beams of optimum shapes to maintain high communication quality even when the states of channels between the wireless base station1and the wireless terminals2change, for example.

The wireless base station1repeatedly executes, with each wireless terminal2(wireless terminals2A,2B, . . . ), processing to tentatively change the shape of a beam, and collect communication quality information when a post-tentative-change beam is used, to change the beam shape to the post-tentative-change beam shape or return the beam shape to the pre-tentative-change beam shape, specifically, processing corresponding to steps S14to S19described above.

Although the number of array antennas assigned to one wireless terminal2is two in the example illustrated inFIG. 4, the number is not necessarily limited to two, and may be one or three or more.

As for the beam shape search procedure (redetermination procedure) in steps S16to S19, for example, when a certain extent of performance improvement or more has been made, or a fixed number of searches have been made, the search processing in steps S16to S19may be stopped. “When a certain extent of performance improvement or more has been made” corresponds to a case where, when received power is used as communication quality, for example, the difference between received power after a beam is changed by the beam shape search procedure and received power before the beam shape search procedure is started reaches a threshold.

Next, the operation of the beam shape controller114of the wireless base station1will be described in detail.FIG. 8is a flowchart illustrating an example of an operation by the beam shape controller114to determine the shape of a beam to be formed by each antenna15.

First, the beam shape controller114determines a beam shape initial value for each of the wireless terminals2connected to the wireless base station1(step S21). In this step S21, the beam shape controller114performs processing, following the procedure in steps S11to S13inFIG. 4already described to determine a beam shape initial value to communicate with each wireless terminal2.

Next, the beam shape controller114repeatedly performs steps S22to S30. These steps are repeated a specified number of times. Specifically, these are repeated a number of times corresponding to a predetermined number of radio frames. For example, when steps S22to S30are set to be repeated over one hundred radio frames, they are repeated one hundred times.

In the repeated processing for the predetermined number of radio frames, the beam shape controller114first determines transmission target wireless terminals in a certain radio frame (step S22). That is, the beam shape controller114determines wireless terminals2to which data are transmitted. Transmission target wireless terminals2may be determined in any manner. For example, the beam shape controller114sets wireless terminals2to which data stored in a transmission buffer (not illustrated) in the MIMO processing unit112are addressed, for transmission targets. When data of different priorities are stored in the transmission buffer, the beam shape controller114may determine transmission target wireless terminals2, considering the priorities of the data. Alternatively, the beam shape controller114may determine transmission target wireless terminals, using beam shapes directed to the wireless terminals2determined in step S21described above as a basis for decision. For example, transmission target wireless terminals are selected so that the shapes of beams to be used in signal transmission to transmission target wireless terminals are different to each other as much as possible.

When the beam shape controller114determines transmission target wireless terminals2, the beam shape controller114repeatedly executes steps S23to S30for each of the transmission target wireless terminals2. For example, when the number of the transmission target wireless terminals2is ten, the steps are repeated ten times. In the repeated processing, the beam shape controller114first assigns an antenna15to be used in signal transmission to one selected from among the transmission target wireless terminals2(step S23). That is, the beam shape controller114determines an antenna to be used in signal transmission to a wireless terminal2selected from among the transmission targets (hereinafter, referred to as a selected terminal). The beam shape controller114assigns an antenna15, based on the ability of a selected terminal, communication quality to be satisfied, and the like. The ability of a selected terminal is, for example, the number of antennas that the selected terminal has, that is, the number of spatial multiplexing supported by the selected terminal. The communication quality to be satisfied is the Quality of Service (QoS), the contracted band, and the like of a selected terminal. In step S23, one or more antennas are assigned to the wireless terminal2. Next, the beam shape controller114determines whether the selected terminal is moving or not (step S24). For example, the beam shape controller114determines whether the selected terminal is moving or not, based on the amount of change per specified time in the reception quality of a signal transmitted from the selected terminal, the amount of change per specified time in communication quality indicated from the selected terminal, or the like. It is a conceivable case that although the selected terminal is not moving, the amount of change in reception quality, communication quality, or the like is increased by a change in the surrounding environment of radio wave propagation. In this case, it is determined that the selected terminal is “moving.” When the beam shape controller114determines that the selected terminal is moving (step S24: Yes), the beam shape controller114sets the shapes of beams formed by all antennas15assigned to the selected terminal in step S23to an initial beam shape (step S27). The initial beam shape is a beam shape corresponding to the initial value determined in step S21. In this case, initial beam shapes at the antennas15assigned to the selected terminal are the same. On the other hand, when the beam shape controller114determines that the selected terminal is not moving (step S24: No), the beam shape controller114determines whether the selected terminal corresponds to a newly connected terminal or not, that is, whether the selected terminal is a wireless terminal newly connected to the wireless base station1or not (step S25). Whether the selected terminal is a wireless terminal newly connected to the wireless base station1or not may be determined in any manner. As an example, when there have been no records of data transmission to the selected terminal for a certain past period, or when no notifications of communication quality information have been received from the selected terminal for a certain past period, the beam shape controller114determines that the selected terminal is a wireless terminal newly connected to the wireless base station1. When the selected terminal is a newly connected terminal (step S25: Yes), the beam shape controller114executes step S27. On the other hand, when the selected terminal is not a newly connected terminal (step S25: No), the beam shape controller114determines whether the antenna assignment to the selected terminal has been changed or not, that is, the antenna15assigned in step S23described above has been changed from an antenna15assigned in step S23last time (step S26). When the antenna assignment to the selected terminal has been changed (step S26: Yes), the beam shape controller114executes step S27.

When the antenna assignment to the selected terminal has not been changed (step S26: No), the beam shape controller114determines whether or not to execute a beam shape search (step S28). For example, when the beam shape controller114determines that communication quality indicated by communication quality information obtained from the selected terminal is less than a threshold, that is, desired communication quality has not been achieved, the beam shape controller114determines that a beam shape search, that is, processing to redetermine a beam shape be executed. Alternatively, when elapsed time since the last communication quality information has been obtained from the selected terminal exceeds a threshold, that is, no communication quality information has been obtained from the selected terminal for a long period of time, the beam shape controller114determines that a beam shape search be executed. The way of determining whether to execute a beam shape search or not is not limited to these. Any manner of determination may be used.

When the beam shape controller114determines that a beam shape search be executed (step S28: Yes), the beam shape controller114tentatively changes a beam shape (step S29). When the beam shape controller114tentatively changes a beam shape, the beam shape controller114stores a wireless terminal for which a beam shape has been tentatively changed. Then, data is transmitted to the wireless terminal, and the beam shape controller114waits for communication quality information to be fed back. When communication quality information is fed back from the wireless terminal for which the beam shape has been tentatively changed, the beam shape controller114determines whether to continuously use the post-tentative-change beam shape thereafter, or to return the beam shape to the original pre-tentative-change beam shape. That is, when the beam shape controller114determines that a beam shape search be executed in step S28, the beam shape controller114executes processing illustrated in steps S16to S19inFIG. 4to redetermine beam shapes. When the beam shape controller114determines that no beam shape search be executed (step S28: No), the beam shape controller114determines that beams of the same shape as that of the last beam be used (step S30).

After the beam shape controller114executes steps S23to S30for all the transmission target terminals, and further executes steps S22to S30for the predetermined number of radio frames, the beam shape controller114returns to step S21to repeat the processing in steps S21to S30.

FIG. 8illustrates the operation of the beam shape controller114, and thus does not describe processing of the wireless base station1to transmit data to the wireless terminals2. The wireless base station1transmits data every time steps S22to S30are executed for each radio frame.

The beam shape controller114executes steps S22to S30illustrated inFIG. 8for a plurality of radio frames and wireless terminals, thereby determining whether the initial beam shape determined in step S21needs to be changed or not. When the initial beam shape needs to be changed, the beam shape controller114executes step S29. As a result, a beam shape is changed to an optimal beam shape. Further, the beam shape controller114determines whether further change is necessary or not on a post-change beam shape. When a further change is necessary, the post-change beam shape is changed again to an optimal beam shape.

The repetition of steps S22to S30may be terminated before they are repeated the number of times corresponding to the predetermined number of radio frames. For example, when communication quality after tentative change of a beam shape becomes lower than communication quality before the tentative change, or both are on the same level, that is, no improvement in communication quality is expected by change of a beam shape, the repetition of steps S22to S30may be terminated. In this case, radio resources can be prevented from being consumed more than necessary by unnecessary repetition of beam shape adjustment.

As above, the wireless base station1in the present embodiment forms beams all over an area that should be covered to transmit a signal for communication quality measurement, and determines an initial value of the shape of beam used in data transmission to each wireless terminal, that is an initial beam shape, based on communication quality of each beam measured by each wireless terminal that has received the signal for communication quality measurement. At this time, initial values of the beam shapes of array antennas used in data transmission to one wireless terminal are set to be same. Thereafter, the wireless base station1tentatively changes a beam shape in use to a different beam shape for each wireless terminal, and determines which of the beam of the pre-tentative-change beam shape and the beam of the post-tentative-change beam shape be continuously used, based on communication quality in the post-tentative-change beam shape and communication quality in the pre-tentative-change beam shape. Specifically, when the communication quality in the post-tentative-change beam shape is better than the communication quality in the pre-tentative-change beam shape, the post-tentative-change beam shape is set for a beam shape to be newly used, and is continuously used. When a beam shape is tentatively changed, it is changed to a beam shape similar to the beam shape in use. Thus, once a beam shape is determined, a new beam shape can be determined based on communication quality when beams of some beam shapes are used instead of communication quality when each of all formable beam shapes is used, and the amount of consumption of radio resources when a beam shape is determined can be reduced. Further, reception wait time for a wireless terminal to detect all patterns of a common reference signal from all array antennas is shortened, so that connection time for a wireless communication terminal to be connected to a wireless base station can be shortened.

Here, hardware configurations of the baseband processing units11and16and the array antennas15of the wireless base station1, and the baseband processing unit25of the wireless terminal2will be described.

The MIMO processing unit112of the baseband processing unit11that the wireless base station1has is an electronic circuit that performs precoding on the streams111fed. The OFDM processing units113are each an electronic circuit that performs modulation processing, IFFT processing, CP addition processing, and others on signals fed from the MIMO processing unit112. The beam shape controller114is implemented by a processor301illustrated inFIG. 9executing a program stored in memory302. Specifically, the beam shape controller114is implemented such that the processor301reads a program for performing the operation of the beam shape controller114from the memory302and executes the program. The processor301is a CPU (also called a central processing unit, a central processor, a processing unit, an arithmetic unit, a microprocessor, a microcomputer, a processor, and a digital signal processor (DSP)), a system large-scale integration (LSI), or the like. The memory302is nonvolatile or volatile semiconductor memory such as random-access memory (RAM), read-only memory (ROM), flash memory, an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM), or a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, a digital versatile disk (DVD), or the like. The memory302is used as a storage area for communication quality information fed back from the wireless terminals2, information on beam shapes used in communication with the wireless terminals2, and others.

The array antennas15that the wireless base station1has are each formed of a plurality of phase shifters and antenna elements.

The MIMO processing unit162of the baseband processing unit16that the wireless base station1has is an electronic circuit that weights and combines received signals fed from each of the OFDM processing units163. The OFDM processing units163are each an electronic circuit that performs CP removal processing, FFT processing, demodulation processing, and others on signals fed from the ADCs17. The feedback information extractor161is implemented by the processor301illustrated inFIG. 9executing a program stored in the memory302. Specifically, the feedback information extractor161is implemented by the processor301reading a program for performing the operation of the feedback information extractor161from the memory302and executing the program.

The OFDM processing unit251of the baseband processing unit25that the wireless terminal2has is an electronic circuit that performs CP removal processing, FFT processing, demodulation processing, and others on received signals fed from the ADC24. The OFDM processing unit254is an electronic circuit that performs modulation processing, IFFT processing, CP addition processing, and others on signals fed from the feedback information transmitter253. The CRS processing unit252and the feedback information transmitter253are implemented by a processor301illustrated inFIG. 9executing a program stored in memory302. Specifically, the CRS processing unit252and the feedback information transmitter253are implemented by the processor301reading a program for performing the operations of the CRS processing unit252and the feedback information transmitter253from the memory302and executing the program.

Second Embodiment

A wireless communication apparatus according to a second embodiment will be described. The configuration of a communication system to which the wireless communication apparatus is applied and the configuration of the wireless communication apparatus are both identical to those in the first embodiment.

The wireless base station1as the wireless communication apparatus according to the first embodiment tentatively changes a beam shape in step S16illustrated inFIG. 4. In addition to change of a beam shape, antennas used in transmission of data and a DMRS may be tentatively changed. For example, a wireless base station1according to the present embodiment changes antennas in rotation illustrated inFIG. 10each time the wireless base station1executes processing corresponding to step S16illustrated inFIG. 4.FIG. 10is a diagram illustrating an example of change of antenna positions when the number of antennas15is eight, and two in eight are assigned to a wireless terminal2to perform communication. InFIG. 10, eight rectangles aligned in two rows and four columns indicate the positions of the eight antennas. Instead of simultaneously performing tentative change of antennas and tentative change of a beam shape, in step S16executed repeatedly, tentative change of antennas may be performed at a certain time, and tentative change of a beam shape may be performed at another time. In the example inFIG. 10, two antennas15are assigned to a wireless terminal2in four assignment change patterns. There is no need to be particularly limited to that, and three or more antennas15may be assigned to one wireless terminal2. As for assignment change patterns, there is no need to comply with those inFIG. 10. Change may be made in various change patterns. When communication quality after tentative change of antenna assignment and a beam shape is better than communication quality before the tentative change, the wireless base station1changes setting to continuously use post-tentative-change antennas and beam shapes. When pre-tentative-change communication quality is better than post-tentative-change communication quality, the wireless base station1uses the same antennas and beam shapes as those up to that time, that is, pre-tentative-change antennas and beam shapes.

Thus, the wireless base station in the present embodiment tentatively changes antennas15used also when a beam shape is tentatively changed. This, in addition to the effects in the first embodiment, further allows antennas15used in transmission to a wireless terminal2to be changed to antennas15that reduce correlation between a plurality of streams transmitted in MIMO, and enables further improvement in MIMO transmission performance.

Third Embodiment

A wireless communication apparatus according to a third embodiment will be described. The configuration of a communication system to which the wireless communication apparatus is applied, and the configuration of the wireless communication apparatus are both identical to those in the first embodiment.

When the wireless base station1as the wireless communication apparatus according to the first embodiment determines a beam shape initial value as illustrated inFIG. 4, the wireless base station1receives from a wireless terminal2a notification of a beam ID indicating a beam shape that provides the best communication quality. On the other hand, a wireless base station1according to the present embodiment collects, from each wireless terminal2, not only a beam shape that provides the best communication quality but also the beam IDs of all beam shapes or the beam IDs of a plurality of beam shapes of higher ranks in communication quality together with communication quality information when beams of the beam shapes are used. That is, when each of the wireless terminals2according to the third embodiment receives a CRS transmitted from each antenna15of the wireless base station1in step S11, in processing corresponding to step S12illustrated inFIG. 4, each of the wireless terminals2measures communication quality of each beam, and notifies the wireless base station1of the beam IDs of all beams and communication quality information, or the beam IDs of a plurality of beams of higher ranks in communication quality and communication quality information, together with the number of a radio frame in which the beams have been received.

FIG. 11is a flowchart illustrating an example of an operation by a beam shape controller114in the wireless base station1according to the third embodiment to determine the shape of a beam formed by each antenna15.FIG. 11is the flowchart illustrated inFIG. 8to which steps S31and S32are added. An operation in steps S21to S30illustrated inFIG. 11is identical to that in the first embodiment, and thus will not be described.

When the wireless base station1and the wireless terminals2constitute an orthogonal frequency-division multiple access (OFDMA) system, for example, there is a possibility that after the wireless base station1executes processing in steps S23to S30on each transmission target wireless terminal determined in step S22and assigns radio resources, radio resources not assigned to any wireless terminals2remain. Assuming that a case like this happens, the beam shape controller114of the wireless base station1according to the present embodiment executes steps S31and S32. Radio resources not assigned to any wireless terminals2are antennas15that have not been assigned to any transmission target wireless terminals in step S23that has been executed on each of the transmission target wireless terminals determined in step S22.

In step S31, the beam shape controller114checks whether there are available radio resources and there is a wireless terminal that needs radio resources. That is, the beam shape controller114checks whether or not there are radio resources left unassigned to transmission target wireless terminals determined in step S22, and data addressed to a wireless terminal2that has not been set as a transmission target wireless terminal in step S22is queued in a transmission buffer in a MIMO processing unit112.

When there are no available radio resources, or there are no wireless terminals that need radio resources although there are available radio resources (step S31: No), the beam shape controller114returns to step S21and repeats processing in steps S21to S32.

On the other hand, when there are available radio resources and there is a wireless terminal that needs radio resources (step S31: Yes), the beam shape controller114assigns radio resources to one wireless terminal that needs radio resources (step S32). When there is a plurality of wireless terminals that needs radio resources, the beam shape controller114determines a wireless terminal to be assigned with radio resources, based on communication quality information indicated from each wireless terminal that needs radio resources. “Communication quality information indicated from a wireless terminal” used at this time is communication quality information on each of a plurality of beam shapes indicated from each wireless terminal that needs radio resources when a beam shape initial value is determined in step S21. After executing step S32, the beam shape controller114returns to step S31, and hereinafter, repeatedly executes steps S31and S32until there are no available radio resources, or there are no wireless terminals that need radio resources although there are available radio resources.

When the beam shape controller114determines in step S31that there are no available radio resources, or when the beam shape controller114determines that there are no wireless terminals that need radio resources although there are available radio resources, the wireless base station1in the present embodiment transmits data to the wireless terminals assigned with radio resources.

Thus, the wireless base station1in the present embodiment checks whether there are available radio resources or not after the completion of radio resource assignment to once-determined transmission target wireless terminals. When there are available radio resources, the wireless base station1assigns remaining radio resources to a wireless terminal that needs radio resources. This allows radio resources to be assigned without being wasted, and allows system communication capacity to be increased.

Fourth Embodiment

A wireless communication apparatus according to a fourth embodiment will be described. The configuration of a communication system to which the wireless communication apparatus is applied, and the configuration of the wireless communication apparatus are identical to those in the first embodiment.

A wireless base station1according to the present embodiment determines the shapes of beams to be used in data transmission to wireless terminals2, following a sequence illustrated inFIG. 12.FIG. 12is a sequence diagram illustrating an example of a beam shape control operation by the wireless base station1in the fourth embodiment.

As illustrated inFIG. 12, like the wireless base station1according to the first embodiment, the wireless base station1according to the fourth embodiment transmits a CRS from all antennas15, i.e. antennas151to15Nto all wireless terminals2(wireless terminals2A,2B, . . . ) (step S41). In this step S41, the antennas15of the wireless base station1form beams of different shapes.

The wireless terminals2that have received the CRS transmitted from the antennas15of the wireless base station1in step S41each measure communication quality of each transmitted beam, and, like the wireless base station1according to the third embodiment, notifies the wireless base station1of the beam IDs of all beam shapes and communication quality information, or the beam IDs of a plurality of beams of higher ranks in communication quality and communication quality information, together with the number of a radio frame in which the beams have been received (step S42).

In step S42, each wireless terminal2notifies the wireless base station1of communication quality information of a configuration illustrated inFIG. 13, for example.FIG. 13illustrates an example of communication quality information when the wireless terminals2receive eight beams of beam shapes #1to #8in step S41inFIG. 12. The example illustrated inFIG. 13is an example of a case where received power is used as communication quality, and the wireless terminals2notify received power in each beam shape and information indicating the associated beam shapes as communication quality information.

Returning to the description ofFIG. 12, the wireless base station1that has received notifications of communication quality information from the wireless terminals assigns antennas15to wireless terminals2to which data is transmitted, and then determines an initial value of the beam shape of a beam formed by each of the antennas15assigned to the wireless terminals2, based on communication quality information on transmitted beams indicated from the wireless terminals2(step S43). Then, using beams of the determined shapes, the wireless base station1transmits data and a DMRS to the wireless terminals2(step S44). At this time, a beam shape controller114assigns each of the antennas15assigned to each wireless terminal2a beam shape in descending order of rank in communication quality to assign different beam shapes to the antennas. For example, in step S43, first, the beam shape controller114assigns a beam shape that provides the best communication quality to the antenna151, and assigns a beam shape that provides the second best communication quality to the antenna152. Hereinafter, in the same manner, a beam shape that provides the Nth best communication quality is assigned to the antenna15N. To simplify explanation, a beam shape of the Kth (K=1, 2, . . . , N) best communication quality is assigned to an antenna15K; however, this is an example. Assignment may be made in a different way. For example, a beam shape that provides the best communication quality can be assigned to an antenna15that is assigned to a wireless terminal2to which a large amount of data is addressed, among data addressed to wireless terminals stored in a transmission buffer in the wireless base station1.FIG. 12illustrates an example in which the wireless base station1assigns the antennas151and152to a wireless terminal2A, forms a beam of beam shape #3of the best communication quality by the antenna151, and forms a beam of beam shape #2of the second best communication quality by the antenna152, to transmit data and a DMRS.

Thus, receiving notifications of communication quality information on beam shapes from the wireless terminals2, the wireless base station1in the present embodiment determines an initial value of the shape of a beam formed by each antenna15, based on the communication quality information. Specifically, the wireless base station1determines a beam shape initial value of a beam formed by each antenna15so that the antennas15form beams of different shapes. This enables determination of beam shapes of antennas so as to reduce correlation between MIMO streams to increase system communication capacity.

Although the beam shape controller114performs radio resource assignment, i.e. the processing in steps S22to S23inFIG. 8and the processing in steps S22to S23and S31to S32inFIG. 11in the embodiments, radio resource assignment may be performed outside the beam shape controller114. For example, a radio resource assignment unit to assign radio resources to the wireless terminals2may be additionally provided independently of the beam shape controller114.

The configurations illustrated in the above embodiments illustrate an example of the subject matter of the present invention, and can be combined with another known art, and can be partly omitted or changed without departing from the scope of the present invention.

REFERENCE SIGNS LIST