Communication apparatus and communication method

In the case of setting a transmission weight to a transmitted signal, when there exists a second known signal which is received by a plurality of antennas after a first known signal used for calculation of the transmission weight and is transmitted with a transmission frequency band different from the first known signal, a transmission weight processing unit of a weight processing unit corrects the transmission weight by use of a delay amount of reception timing concerning the second known signal. Then, the transmission weight processing unit sets the corrected transmission weight to a transmitted signal.

TECHNICAL FIELD

The present invention relates to communications technology for making communications by use of a plurality of antennas.

BACKGROUND ART

There have hitherto been proposed a variety of techniques concerning radio communications. For example, in Patent Document 1, a technique concerning LTE (Long Term Evolution) is disclosed. The LTE is also called “E-UTRA.”.

PRIOR ART DOCUMENT

Patent Document

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In a communication system such as LTE, an adaptive array antenna scheme for adaptively controlling directionality of an array antenna made up of a plurality of antennas may be adopted. When a communication apparatus communicates with a communication partner apparatus by use of the adaptive array antenna scheme, a transmission weight for controlling transmission directivity of the array antenna is calculated. Then, the communication apparatus sets the calculated transmission weight to a transmitted signal transmitted from the array antenna. In order to improve transmission performance of such a communication apparatus, it is desired to improve the accuracy in transmission weight.

Accordingly, the present invention was made in light of the foregoing respect, and has an object to provide a technique capable of improving the accuracy in transmission weight for controlling transmission directivity of a plurality of antennas.

Means for Solving the Problems

The communication apparatus according to one aspect is a communication apparatus for communicating with a communication partner apparatus, which includes: a plurality of antennas; a weight processing unit for calculating a transmission weight for controlling transmission directivity of the plurality of antennas based on a known signal from the communication partner apparatus which is received by the plurality of antennas, to set the transmission weight to a transmitted signal transmitted by the plurality of antennas; and a delay amount acquiring unit for obtaining a delay amount of reception timing for the known signal, wherein in the case of setting the transmission weight to the transmitted signal, when there exists a second known signal: which is the known signal received by the plurality of antennas after the reception of a first known signal as the known signal used for calculation of the transmission weight; and whose transmission frequency band is different from a transmission frequency band of the first known signal, the weight processing unit corrects the transmission weight, using the delay amount concerning the second known signal and sets the corrected transmission weight to the transmitted signal.

Further, a communication method according to one aspect is a communication method for communicating with a communication partner apparatus by use of a plurality of antennas, the method including the steps of: (a) calculating a transmission weight for controlling transmission directivity of the plurality of antennas based on a known signal from the communication partner apparatus which is received by the plurality of antennas, to set the transmission weight to a transmitted signal transmitted by the plurality of antennas; and (b) obtaining a delay amount of reception timing for the known signal, wherein in the case of setting the transmission weight to the transmitted signal in the step (a), when there exists a second known signal: which is the known signal received by the plurality of antennas after the reception of a first known signal as the known signal used for calculation of the transmission weight; and of which transmission frequency band is different from a transmission frequency band of the first known signal, the transmission weight is corrected using the delay amount obtained in the step (b) concerning the second known signal and the corrected transmission weight is set to the transmitted signal.

Effects of the Invention

According to the present invention, the accuracy in transmission weight is improved.

EMBODIMENT FOR CARRYING OUT THE INVENTION

FIG. 1is a diagram showing a configuration of a communication system100provided with a communication apparatus according to the present embodiment. The communication apparatus according to the present embodiment is, for example, a base station that communicates with a communication terminal. Hereinafter, the communication apparatus according to the present embodiment is referred to as a “base station1”.

The communication system100is, for example, LTE adopted with a TDD (Time Division Duplexing) scheme as a duplex scheme, and includes a plurality of base stations1. The respective base stations1communicate with a plurality of communication terminals2. In LTE, an OFDMA (Orthogonal Frequency Division Multiple Access) scheme is used for downlink communication, and an SC-FDMA (Single Carrier-Frequency Division Multiple Access) scheme is used for uplink communication. Therefore, the OFDMA scheme is used for transmission from the base station1to the communication terminal2, and the SC-FDMA scheme is used for transmission from the communication terminal2to the base station1. In the OFDMA scheme, an OFDM (Orthogonal Frequency Division Multiplexing) signal, formed by synthesizing a plurality of mutually orthogonal subcarriers, is used.

As shown inFIG. 1, a service area1aof each base station1is partially superimposed on service areas1aof peripheral base stations1. The plurality of base stations1are connected to a network, not shown, and are communicable with one another through the network. A server, not shown, is connected to the network, and each base station1is communicable with the server through the network.

FIG. 2is a diagram showing a configuration of each base station1. Each base station1can simultaneously communicate with a plurality of communication terminals2by individually allocating a radio resource, specified by a two dimension made up of a time axis and a frequency axis, to each of the plurality of communication terminals2. Each base station1has an array antenna as a transmission/reception antenna, and directionality of the array antenna can be controlled, using an adaptive array antenna scheme. In the present embodiment, as for the directionality of the array antenna, each base station1performs both beam-forming and null-steering, for example.

As shown inFIG. 2, the base station1is provided with a radio communication unit10, a plurality of D/A converting units14, a plurality of A/D converting units15, and a control unit16.

The control unit16is, for example, configured of a CPU, a DSP, a memory and the like, and has overall control on operations of the base stations1. The control unit16generates a baseband transmitted signal including bit data from the network connected with the base station1, and outputs the generated signal to each of the plurality of the D/A converting units14. Further, the control unit16regenerates bit data which is included in a received signal outputted from each of the plurality of A/D converting units15and which is generated by the communication terminal2. Out of the bit data regenerated by the control unit16, bit data intended for the network is transmitted from the base station1to the network.

The radio communication unit10is provided with an array antenna13made up of a plurality of antennas13a, a plurality of transmission units11, and a plurality of reception units12. Each antenna13areceives a transmitted signal from the communication terminal2. The received signals in the plurality of antennas13aare respectively inputted into the plurality of reception units12. Further, the transmitted signals outputted by the plurality of transmission units11are respectively inputted into the plurality of antennas13a. Thereby, the signal is radio-transmitted from each antenna13a.

Each D/A converting unit14converts a digital-form baseband transmitted signal, inputted from the control unit16, to an analog-form baseband transmission signal, and outputs the converted signal. The transmitted signals outputted from the plurality of D/A converting units14are respectively inputted into the plurality of transmission units11. Each transmission unit11converts the inputted baseband transmitted signal to a carrier-band transmitted signal, and outputs the converted signal.

Each reception unit12converts the inputted carrier-band received signal to a baseband received signal, and outputs the converted signal. Analog-form received signals outputted from the plurality of reception units12are respectively inputted into the plurality of A/D converting units15. Each A/D converting unit15converts the inputted analog-form received signal to a digital-form received signal, and outputs the converted signal.

FIG. 3is a block diagram showing a principal functional configuration of the control unit16. As shown inFIG. 3, in the control unit16, the CPU and the DSP execute a variety of programs in the memory, thereby to form a plurality of functional blocks such as a weight processing unit20, a transmitted signal generating unit23, a radio resource allocating unit24, a reception data acquiring unit25, a plurality of IDFT units26, a plurality of DFT units27and a delay amount acquiring unit28.

The received signals outputted from the plurality of A/D converting units15are respectively inputted into the plurality of DFT units27. Each DFT unit27performs discrete Fourier transform (DFT: Discrete Fourier Transform) on the inputted received signal. This gives each DFT unit27a plurality of complex signals (complex symbols) which respectively correspond to a plurality of subcarriers constituting the inputted received signal. Hereinafter, the complex signal obtained in the DFT unit27is referred to as a “received complex signal”. Further, a plurality of complex signals obtained in the DFT unit27are referred to as a “received complex signal train”. The received complex signal train obtained in each DFT unit27is inputted into the weight processing unit20.

The transmitted signal generating unit23generates bit data to be transmitted to the communication terminal2, which includes bit data from the network, and performs an encoding process and a scrambling process on the bit data. The transmitted signal generating unit23then converts the bit data after processed to a plurality of complex signals on an IQ plane, which correspond to a plurality of subcarriers constituting an OFDM signal, and inputs the plurality of complex signals to the weight processing unit20. Hereinafter, the complex signal generated by the transmitted signal generating unit23is referred to as a “transmission complex signal”. Further, the plurality of complex signals generated by the transmitted signal generating unit23are referred to as a “transmission complex signal train”.

Based on a known signal from the communication terminal2, the weight processing unit20calculates a transmission weight for controlling transmission directivity of the array antenna13, and a reception weight for controlling reception directionality of the array antenna13. The weight processing unit20is provided with a transmission weight processing unit21for calculating a transmission weight, and a reception weight processing unit22for calculating a reception weight.

The reception weight processing unit22calculates a reception weight that is set to the received signal in each antenna13a, namely the received complex signal train outputted from each DFT unit27, by use of MMSE (Minimum Mean Square Error), for example. The reception weight can be calculated based on a known complex signal included in the transmitted signal from the communication terminal2.

As for each of the plurality of received complex signal trains inputted respectively from the plurality of DFT units2, the reception weight processing unit22sets a corresponding reception weight to (performs complex multiplication of) each of the plurality of received complex signals constituting the received complex signal train. Then, the reception weight processing unit22adds up the plurality of received complex signals after setting of the reception weight concerning the same subcarrier, the signals being included in the plurality of received complex signal train. Accordingly, a beam concerning the reception directionality of the array antenna13, namely a beam concerning the reception directionality of the plurality of antennas13aas a whole, is directed to one subcarrier (desired wave) from the specific communication terminal2so that a desired complex signal concerning the one subcarrier can be acquired. That is, in a new complex signal obtained by adding up the plurality of received complex signals after setting of a reception weight concerning the same subcarrier, the signals being included in the plurality of received complex signal trains, an interference component has been removed, and the new complex signal is acquired as a wished complex signal. The reception weight processing unit22acquires the wished complex signal concerning each of the plurality of subcarriers constituting the received signal, and outputs the acquired signal. As thus described, by setting the reception weight to the received signal from the communication terminal2, the beam concerning the reception directionality of the array antenna13is directed to the communication terminal2, and hence, user data transmitted from the communication terminal2can be appropriately received.

The transmission weight processing unit21prepares the inputted transmission complex signal trains just in number corresponding to the number of antennas13a. These plurality of transmission complex signal trains are transmitted respectively from the plurality of antennas13a. The transmission weight processing unit21calculates a transmission weight that is set to each transmission complex signal train, namely a transmission weight that is set to a transmitted signal transmitted from each antenna13a. The transmission weight can be calculated based on the reception weight calculated in the reception weight processing unit22. Specifically, the transmission weight processing unit21corrects the reception weight calculated in the reception weight processing unit22based on calibration information, and regards the corrected reception weight as the transmission weight. The calibration information is information generated based on a difference in characteristics between a transmission-system circuit and a reception-system circuit in the base station1. Although the reception weight obtained in the reception weight processing unit22can be used as it is as the transmission weight, since there is a difference in characteristics between the transmission-system circuit and the reception-system circuit (e.g. difference in characteristics of an amplification unit between the transmission-system circuit and the reception-system circuit), it is possible, by correcting the reception weight so as to absorb the difference by use of the calibration information, to obtain an optimum transmission weight.

As for each of the inputted plurality of transmission complex signals trains, the transmission weight processing unit21sets a corresponding transmission weight to (performs complex multiplication of) each of the plurality of transmission complex signals constituting the transmission complex signal train. Then, the transmission weight processing unit21respectively inputs the plurality of transmission complex signal trains after being set with the transmission weight to the plurality of IDFT units26. As thus described, by setting a transmission weight to a transmitted signal to be transmitted to the communication terminal2, a beam concerning transmission directivity of the array antenna13is directed to the communication terminal2, and user data can be appropriately transmitted to the communication terminal2.

The reception data acquiring unit25performs an equalization process on the wished complex signal concerning each of the plurality of subcarriers constituting the received signal, which was outputted from the reception weight processing unit22, and thereafter performs inverse discrete Fourier transform (IDFT: Inverse DFT) on the processed signal. The reception data acquiring unit25then performs a demodulation process on the signal obtained by execution of the inverse discrete Fourier transform, to covert the processed signal to bit data. Subsequently, the reception data acquiring unit25performs a descrambling process and a decoding process on the obtained bit data. Thereby in the reception data acquiring unit25, the bit data for the base station1, which was generated in the communication terminal2, is regenerated. Out of this bit data, bit data intended for the network is transmitted from the base station1to the network.

Each IDFT unit26performs inverse discrete Fourier transform on the inputted transmission complex signal train. Thereby in the IDFT unit26, a baseband OFDM signal is obtained which was formed by synthesizing a plurality of subcarriers modulated by a plurality of transmission complex signals (complex symbols) constituting the transmission complex signal train. The baseband transmitted signals generated in the plurality of IDFT units26are respectively inputted into the plurality of D/A converting units14.

The radio resource allocating unit24allocates, to each communication terminal2as a communication subject, a radio resource (this may hereinafter be referred to as “downlink radio resource”) which is used at the time of the base station1transmitting a signal to the communication terminal2. Thereby, as for each communication terminal2, a frequency band (subcarrier) and a communication time zone which are used for downlink communication (OFDMA scheme) are decided. The control unit16generates a plurality of baseband transmitted signals based on a result of allocation of the downlink radio resource in the radio resource allocating unit24, and also inputs the plurality of transmitted signals respectively to the plurality of D/A converting units14on the timing based on the result of the allocation. Thereby, the radio communication unit10transmits a signal to each communication terminal2, using the downlink radio resource allocated to the communication terminal2.

The radio resource allocating unit24allocates, to each communication terminal2, a radio resource (this may hereinafter be referred to as “uplink radio resource”) which is used at the time of the communication terminal2transmitting a signal to the base station1. Thereby, as for each communication terminal2, a frequency band and a communication time zone which are used for uplink communication (SC-FDMA scheme) are decided. When the uplink radio resource is allocated to the communication terminal2in the radio resource allocating unit24, the control unit16generates a notification signal for notifying the communication terminal2of the uplink radio resource. Then, the control unit16generates a plurality of baseband transmitted signals including the generated notification signals, and respectively inputs these to the plurality of D/A converting units14. Accordingly, to each communication terminal2, the uplink radio resource allocated to the communication terminal2is notified in the base station1. Each communication terminal2transmits a signal to the base station1, using the uplink radio resource notified from the base station1.

The delay amount acquiring unit28obtains a delay amount of reception timing for a known signal which is used at the time of obtaining a transmission weight. An operation of the delay amount acquiring unit28will later be described in detail.FIG. 4is a block diagram showing a configuration of the reception weight processing unit22. As shown inFIG. 4, the reception weight processing unit22is provided with a plurality of complex multiplication units220, an addition unit221, and a reception weight calculating unit222.

received complex signals RS concerning the same subcarrier, which were acquired in the plurality of DFT units27, are respectively inputted into the plurality of complex multiplication units220. Further, a plurality of reception weights RW outputted from the reception weight calculating unit222are respectively inputted into the plurality of complex multiplication units220. Each complex multiplication units220performs complex multiplication of the inputted received complex signal RS by the inputted reception weight RW, and outputs the received complex signal RS multiplied by the reception weight RW. The addition unit221adds up the received complex signal RS which were multiplied by the reception weight RW and outputted from each of the plurality of complex multiplication units220, and outputs a new received complex signal thus obtained as a demodulated complex signal DS.

The reception weight calculating unit222generates an error signal indicating an error concerning the known demodulated complex signal DS obtained in the addition unit221with respect to a reference complex signal. This reference complex signal is a signal in an ideal state concerning the known demodulated complex signal DS obtained in the addition unit221. Based on a sequential estimation algorithm, e.g. an RLS algorithm, the reception weight calculating unit222updates a plurality of reception weights RW a predetermined number of times by use of a plurality of known demodulated complex signals DS so as to make an error signal to be generated small. When updating the plurality of reception weights RW the predetermined number of times, the reception weight calculating unit222completes updating of the plurality of reception weights RW. The plurality of reception weights RW after completion of updating are inputted into the transmission weight processing unit21. When the plurality of reception weights RW after completion of updating are respectively inputted into the plurality of complex multiplication units220, a demodulated complex signal DS with its interference component removed, namely a wished complex signal, is outputted from the addition unit221. Hence the wished complex signal concerning each subcarrier is outputted from the reception weight processing unit22.

Next, a TDD frame300used between the base station1and the communication terminal2will be described. The TDD frame300is specified by a two dimension made up of a time axis and a frequency axis. A frequency bandwidth (system bandwidth) of the TDD frame300is, for example, 20 MHz and a time length of the TDD frame300is 10 ms. The radio resource allocating unit24of the base station1decides an uplink radio resource and a downlink radio resource to be allocated from the TDD frame300to each communication terminal2.

FIG. 5is a diagram showing a configuration of the TDD frame300. As shown inFIG. 5, the TDD frame300is made up of two half frames301. Each half frame301is configured of five subframes302. That is, the TDD frame300is configured of ten subframes302. The subframes302has a time length of 1 ms. Hereinafter, the ten subframes302constituting the TDD frame300may be referred to as zeroth to ninth subframes302in order from the beginning.

Each subframe302is configured of two slots303in the time direction. Each slot303is configured of seven symbol periods304. Accordingly, each subframe302contains fourteen symbol periods304in the time direction. This symbol period304is one symbol period of an OFDM symbol in the OFDMA-system downlink, and is one symbol period of a DFTS (Discrete Fourier Transform Spread)—OFDM symbol in the SC-FDMA system uplink.

The TDD frame300configured as above contains the uplink communication-specific subframe302and the downlink communication-specific subframe302. Hereinafter, the uplink communication-specific subframe302is referred to as an “uplink subframe302”, and the downlink communication-specific subframe302is referred to as a “downlink subframe302”.

In LTE, in the TDD frame300, a region (radio resource) including a frequency bandwidth of 180 kHz in the frequency direction and a seven symbol periods304(one slot303) in the time direction is called a “resource block (RB). The resource block contains 12 subcarriers. Allocation of the uplink radio resource and the downlink radio resource to the communication terminal2in the radio resource allocating unit24is performed by units of one resource block in the frequency direction. It is to be noted that, since the SC-FDMA system is used for the uplink, when a plurality of resource blocks are allocated to one communication terminal2in one slot303of the uplink subframe302, a plurality of resource blocks continued in the frequency direction are allocated to the communication terminal2. Hereinafter, a frequency band of one resource block is referred to as an “allocation unit band”.

Further, in LTE, as for the configuration of the TDD frame300, seven kinds of configurations in different combinations of the uplink subframe302and the downlink subframe302are defined.FIG. 6is a diagram showing the seven kinds of configurations.

As shown inFIG. 6, in LTE, configurations of the TDD frames300from No. 0 to 6 are defined. In the communication system100, one configuration out of these seven kinds of configurations is used. InFIG. 6, the subframe302indicated by “D” represents the downlink subframe302, and the subframe302indicated by “U” represents the uplink subframe302. Further, the subframe302indicated by “S” represents the subframe302in which a switch from downlink to uplink is made in the communication system100. This subframe302is referred to as a “special subframe302”.

For example, in the TDD frame300having the configuration of No. 0, the zeroth and fifth subframes302are the downlink subframes302, the second to fourth subframes302and seventh to ninth subframes302are the uplink subframes302, and the first and sixth subframes302are the special subframes302. Further, in the TDD frame300having the configuration of No. 4, the zeroth subframe302and the fourth to ninth subframes302are the downlink subframes302, the second and third subframes302are the uplink subframes302, and the first subframe302is the special subframe302.

FIG. 7is a diagram showing in detail the configuration of the TDD frame300having the configuration of No. 1. As shown inFIG. 7, the special subframe302contains in the time direction a downlink pilot time slot (DwPTS)351, a guard period (GP)350, and an uplink pilot time slot (UpPTS)352. The guard period350is a no-signal period necessary for switching from downlink to uplink, and is not used for communication. In the following description, it is assumed that the TDD frame300having the configuration of No. 1 is used in the communication system100.

In LTE, as for the combination in time length among the down pilot time slot351, the guard period350and the uplink pilot time slot352, a plurality of kinds of combinations have been defined. In the example ofFIG. 7, a time length of the downlink pilot time slot351has been set to three symbol periods304, and a time length of the downlink pilot time slot352has been set to two symbol periods304.

In the communication system100according to the present embodiment, downlink can be performed not only in the downlink subframe302, but also in the downlink pilot time slot351of the special subframe302. Further, in the present communication system100, uplink can be performed not only in the uplink subframe302, but also in the uplink pilot time slot352of the special subframe302.

In the present embodiment, the base station1transmits data to the communication terminal2in each symbol period304of the downlink pilot time slot351. Further, the communication terminal2transmits a known signal called “sounding reference signal (SRS)” in the symbol period304included in the uplink pilot time slot352. The SRS is configured of a plurality of complex signals (complex symbols) that modulate a plurality of subcarriers. A symbol patterns indicated by the plurality of complex signals that constitute the SRS is known in the base station1. Hereinafter, the complex signal constituting the SRS is called an “SRS complex signal”.

In LTE, the SRS is often used at the time of estimating uplink quality, but in the present embodiment, the SRS transmitted in the uplink pilot time slot352is used for calculating a transmission weight. That is, the base station1controls the transmission directivity in the array antenna13based on the SRS transmitted by the communication terminal2in the uplink pilot time slot352.

It should be noted that as for a reception weight that is set to a received signal which includes user data from the communication terminal2and is received by the array antenna13, it is calculated based not on the SRS but on a known signal called “demodulation reference signal (DRS)” which is transmitted from the communication terminal2in the uplink subframe302.

Further, the SRS is also transmittable in the last symbol period304of the uplink subframe302. Hereinafter, the SRS means the SRS transmitted using the uplink pilot time slot352, unless otherwise specified.

Further, in the present embodiment, since the SRS is transmitted to every uplink pilot time slot352of the special subframe302, a period from the beginning of uplink pilot time slot352of the special subframe302to the beginning of uplink pilot time slot352of the subsequent special subframe302is called an “SRS transmission period360”.

Moreover, each symbol period304included in the uplink pilot time slot352is referred to as an “SRS transmission symbol period370”. In each special subframe302(in every SRS transmission period360), each communication terminal2transmits the SRS, using at least one of the two SRS transmission symbol periods370included in the uplink pilot time slot352.

<Transmission Frequency Band of SRS>

In the present communication system100, the special subframe302, in which a frequency band400usable for transmission of the SRS (hereinafter referred to as “SRS transmittable band400”) is arranged to the high frequency side of the system band, and the special subframe302, in which the SRS transmittable band400is arranged as pulled to the low frequency side of the system band, alternately appear. That is, the SRS transmittable band400is alternately arranged on the high frequency side or the low frequency side of the system band in every SRS transmission period360. InFIG. 7, the SRS transmittable band400is indicated by diagonal lines.

Further, in the communication system100according to the present embodiment, a frequency band that is used by one communication terminal2for transmission of the SRS (hereinafter referred to as “SRS transmission band”) changes in every special subframe302(in every SRS transmission period360) within the SRS transmittable band400, and by one communication terminal2transmitting the SRS a plurality of times, the SRS is transmitted over the entire band of the SRS transmittable band400. This operation is called “frequency hopping”.

FIG. 8is a view showing one example of manners in which an SRS transmission band450used by one communication terminal2performs frequency hopping. In the example ofFIG. 8, the SRS transmittable band400is divided into first to fourth partial frequency bands, and the SRS transmission band450is sequentially set to any one of the first to fourth partial frequency bands. Specifically, the SRS transmission band450with a bandwidth being a quarter of the bandwidth of the SRS transmittable band400sequentially changes to the first partial frequency band, the third partial frequency band, the second partial frequency band and the fourth partial frequency band in every SRS transmission period360.

A width of each of the first to fourth partial frequency bands is, for example, set to as large as a frequency bandwidth of 24 resource blocks, namely to 24 times as large as a width of the allocation unit band. Hereinafter, when distinguishing the first to fourth partial frequency bands is not necessarily required, each of those is referred to as a “partial frequency band”. Further, a period in which the SRS transmission band450changes over the whole area of the SRS transmittable band400is called a “hopping period”. In the example ofFIG. 8, the hopping period is configured of four SRS transmission periods360. Therefore, when the four SRS transmission periods360have elapsed, the SRS is transmitted over the entire area of the SRS transmittable band400.

In the base station1, the radio resource allocating unit24allocates a variety of information, necessary for transmitting the SRS, to each communication terminal2as a communication subject. Specifically, the radio resource allocating unit24allocates to each communication terminal2the bandwidth of the SRS transmission band (hereinafter referred to as “SRS transmission bandwidth”), the SRS transmission symbol period370and a frequency hopping method for the SRS transmission band (how to change the SRS transmission band). The SRS transmission bandwidth coincides with the width of the above partial frequency band. Therefore, when the SRS transmission bandwidth changes, the hopping period also changes.

The transmitted signal generating unit23generates a transmitted signal including a control signal for notifying the communication terminal2of the SRS transmission bandwidth and the like allocated to the communication terminal2, in the radio resource allocating unit24. This transmitted signal is transmitted from the radio communication unit10toward the communication terminal2. This allows each communication terminal2to recognize the SRS transmission bandwidth, the SRS transmission symbol period370and the frequency hopping method for the SRS transmission band, which have been allocated to its own. Each communication terminal2transmits the SRS in every SRS transmission period360based on the SRS transmission bandwidth allocated to its own, and the like.

It is to be noted that the above control signal is called an “RRCConnectionReconfiguration message” in LTE. Further, in LTE, a variety of parameters have been set for notifying the communication terminal2of the SRS transmission bandwidth and the like. For example, the SRS transmission bandwidth is decided by a parameter CSRScalled “srs-BandwidthConfig” and a parameter BSRScalled “srs-Bandwidth”, and by notifying the communication terminal2of values of the parameters CSRSand BSRS, it is possible to notify the communication terminal2of the SRS transmission bandwidth.

<Basic Operation of Base Station at the Time of Setting Transmission Weight to Transmitted Signal>

Subsequently, there will be described a basic operation at the time of the base station1setting a transmission weight to a transmitted signal to be transmitted to the communication terminal2. Hereinafter, the communication terminal2as a subject of description may be referred to as a “subject communication terminal2”.

In the base station1according to the present embodiment, as for a transmitted signal to be transmitted to the subject communication terminal2in one SRS transmission period360, a transmission weight is in principle calculated based on the SRS which have a transmission frequency band including a frequency band of the transmitted signal and which is transmitted by the subject communication terminal2in the SRS transmission period360(more precisely, the transmission weight is calculated based on a reception weight calculated based on the SRS), and the transmission weight is set to the transmitted signal.

However, when the SRS which have a transmission frequency band including the frequency band of the transmitted signal is not transmitted in one SRS transmission period360in which the transmitted signal is transmitted, a transmission weight is set to the transmitted signal, the weight having been calculated based on the SRS which have a transmission frequency including the frequency band of the transmitted signal and which is transmitted in another SRS transmission period360before that one SRS transmission period360and being as close to that one SRS transmission period360as possible. Hereinafter, a specific example of a setting method for a transmission weight to a transmitted signal will be described with reference toFIG. 9.

FIG. 9is a diagram showing an example of allocating the downlink radio resource to the subject communication terminal2. InFIG. 9, the downlink radio resource allocated to the subject communication terminal2in the radio resource allocating unit24is indicated by diagonal lines from top left to bottom right. Further inFIG. 9, six SRS transmission periods360, which appear in the Nth TDD frame300to the (N+2)th TDD frame300, are referred to as SRS transmission periods360ato360fin order from the beginning.

InFIG. 9, for example as for a transmitted signal in the first partial frequency band which is transmitted to the subject communication terminal2in the SRS transmission period360a, in accordance with the principle, a transmission weight is calculated based on an SRS of which SRS transmission band450is the first partial frequency band and which is transmitted from the subject communication terminal2in the SRS transmission period360a, and the transmission weight is set to the transmitted signal.

Further, as for a transmitted signal in the third partial frequency band which is transmitted to the subject communication terminal2in the SRS transmission period360b, a transmission weight is calculated based on an SRS of which SRS transmission band450is the third partial frequency band and which is transmitted from the subject communication terminal2in the SRS transmission period360b, and the transmission weight is set to the transmitted signal.

Moreover, as for a transmitted signal in the fourth partial frequency band which is transmitted to the subject communication terminal2in the SRS transmission period360d(a transmitted signal in the fourth partial frequency band which is transmitted in the ninth subframe of the (N+1)th TDD frame300and a transmitted signal in the fourth partial frequency band which is transmitted in the zeroth subframe of the (N+2)th TDD frame300), a transmission weight is calculated based on an SRS of which SRS transmission band450is the fourth partial frequency band and which is transmitted from the subject communication terminal2in the SRS transmission period360d, and the transmission weight is set to the transmitted signal.

As opposed to this, for example as for a transmitted signal in the first partial frequency band which is transmitted to the subject communication terminal2in the SRS transmission period360c, since an SRS, of which SRS transmission band450is first partial frequency band, is not transmitted from the subject communication terminal2in the SRS transmission period360c, a transmission weight is set to the transmitted signal, the weight having been calculated based on the SRS of which SRS transmission band450is first partial frequency band and which is transmitted in the SRS transmission period360abefore the SRS transmission period360c. At this time, the transmission weight is corrected and then set to the transmitted signal. This correction method will later be described in detail.

Further, as for a transmitted signal in the first partial frequency band which is transmitted in the SRS transmission period360d, since an SRS of which the SRS transmission band450is first partial frequency band is not transmitted in the SRS transmission period360d, a transmission weight is corrected and set to the transmitted signal, the weight having been calculated based on the SRS of which the SRS transmission band450is first partial frequency band and which is transmitted in the SRS transmission period360abefore the SRS transmission period360d.

Moreover, as for a transmitted signal in the second partial frequency band which is transmitted in the SRS transmission period360d, since an SRS of which the SRS transmission band450is second partial frequency band is not transmitted in the SRS transmission period360d, a transmission weight is corrected and set to the transmitted signal, the weight having been calculated based on the SRS of which the SRS transmission band450is second partial frequency band and which is transmitted in the SRS transmission period360cbefore the SRS transmission period360d.

Moreover, as for a transmitted signal in the first partial frequency band which is transmitted in the SRS transmission period360f(a transmitted signal in the first partial frequency band which is transmitted in the ninth subframe of the (N+2)th TDD frame300and a transmitted signal in the first partial frequency band which is transmitted in the zeroth subframe of the subsequent TDD frame300), since an SRS of which the SRS transmission band450is first partial frequency band is not transmitted in the SRS transmission period360f, a transmission weight is corrected and set to the transmitted signal, the weight having been calculated based on the SRS of which the SRS transmission band is first partial frequency band and which is transmitted in the SRS transmission period360ebefore the SRS transmission period360f.

In the weight processing unit20, at the time of calculating a transmission weight that is set to a transmitted signal to be transmitted to the communication terminal2, first in the reception weight processing unit22, a reception weight is calculated based on a plurality of SRS complex signals transmitted using the same frequency band as the frequency band of the transmitted signal, out of a plurality of SRS complex signals constituting the SRS for use in calculation of the transmission weight. Thereafter, in the transmission weight processing unit21, a transmission weight is calculated based on the reception weight obtained in the reception weight calculating unit222.

Further, in the weight processing unit20, the transmission weight is calculated for example in each allocation unit band. For example, assuming that the frequency band of the transmitted signal to be transmitted to the subject communication terminal2is configured of four allocation unit bands, a transmission weight is obtained concerning each of the four allocation unit bands. The transmission weight that is set to the transmitted signal to be transmitted to the subject communication terminal2by use of one allocation unit band is obtained based on 12 SRS complex signals transmitted using that allocation unit band, out of a plurality of SRS complex signals constituting the SRS received from the subject communication terminal2. With one resource block containing 12 subcarriers, 12 complex signals can be transmitted using one allocation unit band. There will hereinafter be described in detail a calculation method for a transmission weight that is set to a transmitted signal to be transmitted to the subject communication terminal2by use of one allocation unit band. Hereinafter, an allocation unit band as a subject of description is referred to as a “subject allocation unit band”.

In the reception weight processing unit22, as for the demodulated complex signal DS that is outputted from the addition unit221and corresponds to one SRS complex signal out of the 12 SRS complex signals transmitted using the subject allocation unit band, the reception weight calculating unit222obtains an error signal that indicates an error between the demodulated complex signal DS and a reference complex signal corresponding thereto. Then, using the obtained error signal, the reception weight calculating unit222updates a plurality of reception weights RW once. The reception weight calculating unit222performs this process on each of the 12 SRS complex signals which are transmitted using the subject allocation unit band. Thereby, the plurality of reception weights RW are updated twelve times, and updates of the plurality of reception weights RW is completed. The transmission weight processing unit21calculates a plurality of transmission weights respectively corresponding to the plurality of antennas13abased on the plurality of reception weights RW whose updates were completed in the reception weight processing unit22. Thereby, as for each antenna13a, the transmission weight is calculated which is set to the transmitted signal transmitted using the subject allocation unit band.

It is to be noted that as inFIG. 9, for example when a frequency band of a transmitted signal transmitted using at least part of the first partial frequency band in the SRS transmission period360ccoincides with a frequency band of a transmitted signal transmitted using at least part of the first partial frequency band in the SRS transmission period360a, it is not necessary to obtain a transmission weight in the SRS transmission period360c. In this case, a transmission weight calculated concerning the transmitted signal in the SRS transmission period360acan be set to the transmitted signal in the SRS transmission period360c.

As opposed to this, differently fromFIG. 9, when a frequency band of a transmitted signal transmitted using at least part of the first partial frequency band in the SRS transmission period360cdiffers from a frequency band of a transmitted signal transmitted using at least part of the first partial frequency band in the SRS transmission period360a, a transmission weight calculated concerning the transmitted signal in the SRS transmission period360acannot be set to the transmitted signal in the SRS transmission period360c. Hence in this case, a transmission weight that is set to a transmitted signal in the SRS transmission period360cis re-calculated based on an SRS transmitted in the SRS transmission period360a.

<Acquirement Method for Delay Amount of Reception Timing for SRS>

As understood from the foregoing description, a transmission weight concerning a transmitted signal transmitted using a subject allocation unit band is calculated based on a reception weight calculated using 12 SRS complex signals transmitted using the subject allocation unit band and 12 reference complex signals respectively corresponding to the 12 SRS complex signals. Hereinafter, a transmission weight that is set to a transmitted signal to be transmitted using one allocation unit band is referred to as a “unit transmission weight”, and a reception weight that is used at the time of obtaining the unit transmission weight is referred to as a “unit reception weight”. Further, the unit transmission weight and the unit reception weight are collectively referred to as a “unit weight”. Moreover, 12 SRS complex signals that are used at the time of obtaining a unit weight are collectively referred to as an “SRS signal train”. Then, 12 reference complex signals that are used at the time of obtaining a unit weight are collectively referred to as a “reference signal train”.

The delay amount acquiring unit28according to the present embodiment obtains a delay amount of reception timing for an SRS signal train in the base station1. Specifically, the delay amount acquiring unit28first calculates a correlation value between an SRS signal train received in one antenna13aand a reference signal train corresponding thereto, while gradually delaying a phase of the reference signal train. The delay amount acquiring unit28then regards, as a delay amount of reception timing for the SRS signal train in the base station1, a delay amount of the phase of the reference signal train where the calculated correlation value is the maximum, namely a delay amount of the phase of the reference signal train that correlates with the SRS signal train received in one antenna13a. From the fact that the reference signal train whose phase is delayed only by a correlates with the SRS signal train, reception timing for the SRS signal train in the base station1can be considered as delayed only by the time corresponding to α, whereby this a is regarded as a delay amount of reception timing for the SRS signal train in the base station1in the present embodiment.

The delay amount acquiring unit28acquires a delay amount of reception timing for the SRS signal train every time the reception weight calculating unit222calculates a unit reception weight based on an SRS signal train received in the array antenna13. When X (X is an integer equal to or greater than1) unit reception weights are calculated based on X SRS signal trains out of a plurality of SRS transmitted signal trains that constitute an SRS to be received in one SRS transmission period360, the delay amount acquiring unit28regards an average value of delay amounts of reception timing for the X SRS signal trains as a delay amount β of the reception timing for the SRS. However, when X=1, the delay amount of reception timing for the SRS signal trains, contained in the SRS, as it is becomes the delay amount β of reception timing for the SRS. This delay amount β is used for an undermentioned correction process for a transmission weight.

<Correction Process for Transmission Weight>

As described usingFIG. 9, in the base station1, when an SRS, whose transmission frequency includes a frequency band of a transmitted signal transmitted in one SRS transmission period360, is transmitted in the SRS transmission period360, it is possible to calculate a transmission weight based on the SRS transmitted in timing close to transmission timing for the transmitted signal. Hence in this case, the accuracy in transmission weight can be sufficiently ensured.

As opposed to this, when an SRS, whose transmission frequency includes a frequency band of a transmitted signal transmitted in one SRS transmission period360, is not transmitted in the SRS transmission period360, a transmission weight is set to the transmitted signal, the weight having been calculated based on the SRS transmitted in another SRS transmission period360before that one SRS transmission period360. In this case, since the transmission weight that is set to the transmitted signal is calculated based on the SRS transmitted in timing distant from the transmission timing for the transmitted signal, it may not be possible to sufficiently ensure the accuracy in the transmission weight.

Accordingly in the present embodiment, when a transmission weight, that is set to a transmitted signal, is calculated based on an SRS that is transmitted in timing distant from transmission timing for the transmitted signal, the transmission weight is corrected, thereby to improve the accuracy in transmission weight. Hereinafter, this correction process will be described in detail.

In the transmission weight processing unit21according to the present embodiment, in the case of setting a transmission weight to a transmitted signal, when another SRS exists which is received in the array antenna13after the SRS used for calculation of the above transmission weight and which is also transmitted in a different transmission frequency band from that for the above SRS, the above transmission weight is corrected using the delay amount β of reception timing for another SRS. At this time, it is desirable to use another SRS that is received in timing as close to the transmission timing for the above transmitted signal as possible. The transmission weight processing unit21then sets the transmission weight after corrected to a transmitted signal. In the transmission weight processing unit21, the correction process is performed on every unit transmission weight. In the transmission weight processing unit21, in the case of setting a unit transmission weight to a transmitted signal in one allocation unit band, when another SRS exists which is received in the array antenna13after the SRS (SRS signal train) used for calculation of the above unit transmission weight and which is also transmitted in a different transmission frequency band from that for the above SRS, the above unit transmission weight is corrected using the delay amount β of reception timing for another SRS.

FIG. 10is a diagram for explaining a correction process for a transmission weight concerning the subject communication terminal2in the transmission weight processing unit21. InFIG. 10, a downlink radio resource as inFIG. 9is allocated to the subject communication terminal2. A transmission weight that is set to a transmitted signal501in the first partial frequency band which is transmitted in the SRS transmission period360cofFIG. 10is considered as a subject of description, and a correction process for this transmission weight will be described in detail. Hereinafter, a transmission weight as a subject for the description is referred to as a “subject transmission weight”, and a transmitted signal to which the subject transmission weight is set is referred to as a “subject transmitted signal”. Further, the SRS used for calculation of the subject transmission weight is referred to as a “subject SRS”.

A subject transmission weight502that is set to a subject transmitted signal501is calculated based on a subject SRS503that is received in the array antenna13in the SRS transmission period360abefore the SRS transmission period360c. That is, the subject transmission weight502is calculated based on the subject SRS503transmitted in timing (SRS transmission period360a) distant from the transmission timing (SRS transmission period360c) for the subject transmitted signal501.

Meanwhile, as an SRS which is received in the array antenna13after the subject SRS503and of which SRS transmission band450is different from that of the subject SRS501and which is received in the array antenna13in timing as close to the transmission timing for the subject transmitted signal501as possible, there exists an SRS504of which SRS transmission band450is second partial frequency band and which is transmitted in the SRS transmission period360c. The transmission weight processing unit21corrects the subject transmission weight502based on a delay amount β of reception timing for this SRS504. Hereinafter, an SRS where a delay amount β that is used at the time of correcting the subject transmission weight is regarded as a delay amount of the reception timing is referred to as an “immediate SRS”.

Herein, upon a change in relative distance between the subject communication terminal2and the base station1, the delay amount β of the reception timing for the SRS from the subject communication terminal2in the base station1changes. Hence the delay amount β indicates the relative distance between the subject communication terminal2and the base station1. Since the foregoing immediate SRS504is transmitted in timing close to the transmission timing for the subject transmitted signal501, it can be said that the delay amount β of reception timing for the immediate SRS504indicates the relative distance between the subject communication terminal2and the base station1in timing close to the transmission timing of the subject transmitted signal501. Accordingly, by correcting the subject transmission weight502based on the delay amount β of reception timing for the immediate SRS504, namely by controlling the transmission directivity of the array antenna13at the time of transmitting the subject transmitted signal501based on the delay amount β, the subject transmitted signal501can be reliably transmitted to the subject communication terminal2.

<Example of Correction of Transmission Weight>

Next, there will be described specifically how the subject transmission weight is corrected using the delay amount β of reception timing for the immediate SRS.

At the time of correcting the subject transmission weight, the transmission weight processing unit21first compares the delay amount β of reception timing for the immediate SRS (hereinafter referred to as “first delay amount β”) with the delay amount β of reception timing for the subject SRS (hereinafter referred to as “second delay amount β”) used for calculation of the subject transmission weight. When the first delay amount β is larger than the second delay amount β, the transmission weight processing unit21determines that the subject communication terminal2is more distant from the base station1than when receiving the subject SRS, and the subject transmission weight is corrected (adjusted) so as to increase the transmission distance of the subject transmitted signal.FIG. 11is a view showing a beam600for the transmission directivity of the array antenna13which corresponds to the subject transmission weight before and after correction. InFIG. 11, the beam600for the transmission directivity of the array antenna13which corresponds to the subject transmission weight before correction is indicated by a wavy line, while the beam600for the transmission directivity of the array antenna13which corresponds to the subject transmission weight after correction is indicated by a solid line. Further, inFIG. 11, the subject communication terminal2at the time of the base station1receiving the subject SRS is indicated by a wavy line, while the subject communication terminal2at the time of the subject transmitted signal being transmitted from the base station1is indicated by a solid line. The same applies to undermentionedFIGS. 12 to 14.

When the first delay amount β is larger than the second delay amount β, as shown inFIG. 11, since the subject communication terminal2is more distant from the base station1than when receiving the subject SRS, the subject transmission weight is corrected such that the beam600extends while the orientation of the beam600remains unchanged, thereby increasing the possibility for the subject transmitted signal to be appropriately received in the subject communication terminal2. That is, the accuracy in subject transmission weight improves.

It should be noted that, since the subject communication terminal2does not necessarily move as shown inFIG. 11, namely since it does not necessary move in a direction connecting the position before the move and the base station1, it is desirable to correct the subject transmission weight so as to increase the width of the beam600while increasing the transmission distance of the subject transmitted signal, as shown inFIG. 12. This allows the subject communication terminal2to appropriately receive the subject transmitted signal regardless of the moving direction of the subject communication terminal2.

On the other hand, when the first delay amount β is smaller than the second delay amount β, the transmission weight processing unit21determines that the subject communication terminal2is closer to the base station1than when receiving the subject SRS, and the subject transmission weight is corrected (adjusted) so as to decrease the transmission distance of the subject transmitted signal.FIG. 13is a view showing the beam600for the transmission directivity of the array antenna13which corresponds to the subject transmission weight before and after correction in the case of the first delay amount β being smaller than the second delay amount β.

When the first delay amount β is smaller than the second delay amount β, as shown inFIG. 13, since the subject communication terminal2is closer to the base station1than when receiving the subject SRS, the subject transmission weight is corrected such that the beam600becomes shorter while the orientation of the beam600remains unchanged, thereby allowing transmission of the subject transmitted signal to the subject communication terminal2while suppressing interference with the periphery. That is, the accuracy in subject transmission weight improves.

It should be noted that, since the subject communication terminal2does not necessarily move in a direction connecting the position before the move and the base station1as shown inFIG. 13, it is desirable to correct the subject transmission weight so as to increase the width of the beam600while decreasing the transmission distance of the subject transmitted signal, as shown inFIG. 14. This allows the subject communication terminal2to appropriately receive the subject transmitted signal regardless of the moving direction of the subject communication terminal2.

When the first delay amount β and the second delay amount β are the same, the transmission weight processing unit21sets the subject transmission weight to the subject transmitted signal without correcting the subject transmission weight. Alternatively, the transmission weight processing unit21corrects the subject transmission weight such that the width of the beam600increases while the transmission distance of the subject transmitted signal remains the same. In the case of the latter, even when the subject communication terminal2moves such that the relative distance from the base station1remains unchanged, it is possible to make the subject communication terminal2appropriately receive the subject transmitted signal.

In addition, the correction amount with respect to the subject transmission weight may be changed in accordance with the amount of the difference between the first delay amount β and the second delay amount β.

For example, when the first delay amount β is larger than the second delay amount β, the correction amount of the subject transmission weight is changed in accordance with an absolute value of the difference therebetween such that a transmission distance of the transmitted signal at the time of the absolute value being not smaller than a predetermined amount is larger than a transmission distance of a transmitted signal at the time of the absolute value being smaller than the predetermined amount. This can further improve the accuracy in transmission weight.

Further, when the first delay amount β is smaller than the second delay amount β, the correction amount of the subject transmission weight is changed in accordance with an absolute value of the difference therebetween such that a transmission distance of the transmitted signal at the time of the absolute value being not smaller than a predetermined amount is smaller than a transmission distance of a transmitted signal at the time of the absolute value being smaller than the predetermined amount. This can further improve the accuracy in transmission weight.

As thus described, according to the present embodiment, when there exists another SRS which was received after the SRS used for calculation of a transmission weight that is set to a transmitted signal and whose transmission frequency band is different from the above SRS, the transmission weight is corrected based on the delay amount β of reception timing for another SRS. This can improve the accuracy in transmission weight. Hence the transmission performance of the base station1improves.

It is to be noted that, although the SRS has been exemplified as the known signal used at the time of calculating a transmission weight in the foregoing embodiment, even when a transmission weight is calculated based on another known signal, the transmission weight can be corrected in a similar manner. As described above, in LTE, the known signal called a demodulation reference signal (DRS), which is primarily used at the time of calculating a reception weight that is set to a received signal including user data, has been defined, but even in the case of calculating a transmission weight based on this known signal, the transmission weight can be corrected in a similar manner.

Further, although the case of applying the present invention to the base station of LTE in the foregoing embodiment, the present invention can be applied to a base station of another communication system. Moreover, the present invention can be applied to a communication device other than the base station.

EXPLANATION OF REFERENCE NUMERALS