The calibration coefficient calculation unit 112 calculates a ratio (UL/DL channel coefficient ratio) ratio of the DL channel coefficient to the UL channel coefficient for each of the plurality of calibration measurements and for each of the plurality of distributed antennas DA, based on the plurality of DL channel coefficients and the plurality of UL channel coefficients for each of the plurality of calibration measurements and for each of the plurality of distributed antennas DA. The calibration coefficient calculation unit 112 calculates a calibration coefficient for calibrating a radio DL signal, based on the DL channel coefficient and the UL channel coefficient. The DL channel coefficient and the UL channel coefficient also include a transmission/reception characteristic of the distributed antenna DA. However, it is assumed that a propagation channel included in the DL channel coefficient is equal to a propagation channel included in the UL channel coefficient. In other words, it is assumed that the propagation channels are equal since the DL and the UL are the same propagation path.

The calibration coefficient calculation unit 112 calculates a ratio between a UL/DL channel coefficient ratio between a calibration antenna in a reference calibration measurement among calibration measurements for a plurality of times and a reference distributed antenna, and a UL/DL channel coefficient ratio between a calibration antenna in a calibration measurement other than the reference calibration measurement and the reference distributed antenna. The ratio is referred to as a correction coefficient of a calibration measurement.

By using a correction coefficient between a plurality of calibration measurements, a proportion of the DL channel coefficient to the UL channel coefficient is corrected for the plurality of calibration measurements.

The reference distributed antenna described above may include a plurality of reference distributed antennas, and an average of correction coefficients calculated by the plurality of reference distributed antennas described above may be newly set as a correction coefficient.

The calibration coefficient calculation unit 112 performs calibration measurements for a plurality of times, and obtains a weighted coefficient, based on received power of a calibration DL signal or a calibration UL signal, when a plurality of DL channel coefficients and UL channel coefficients are acquired (when there are a plurality of effective measurements) for one distributed antenna DA.

The calibration coefficient calculation unit 112 calculates a UL/DL channel coefficient ratio for each of the calibration measurements from the DL channel coefficient and the UL channel coefficient described above.

The calibration coefficient calculation unit 112 performs, by using the correction coefficient described above, correction between the calibration measurements on the UL/DL channel coefficient ratio for each of the calibration measurements described above.

The calibration coefficient calculation unit 112 calculates, for each of the plurality of distributed antennas DA, a weighted average UL/DL channel coefficient ratio in which weighted averaging between the plurality of calibration measurements based on the weighted coefficient is performed on the UL/DL channel coefficient ratio for each of the corrected calibration measurements.

The weighted average UL/DL channel coefficient ratio is referred to as a calibration coefficient.

The calibration coefficient calculation unit 112 selects the DL channel coefficient associated with the calibration DL signal whose received power is equal to or more than predetermined power from the DL channel coefficients in the plurality of measurements. The calibration coefficient calculation unit 112 selects the DL channel coefficient associated with the calibration UL signal whose received power is equal to or more than predetermined power from the UL channel coefficients in the plurality of measurements. The calibration coefficient calculation unit 112 may calculate a calibration coefficient for each of the plurality of distributed antennas DA by using the selected DL channel coefficient and the selected UL channel coefficient.

The calibration control unit 111 may add the calibration DL signal or the calibration UL signal to the radio DL signal being transmitted from each of the plurality of distributed antennas, and perform a calibration measurement.

Overview of Operation of Radio Communication Apparatus

Hereinafter, a D-MIMO apparatus will be described as an example of one radio communication apparatus. Further, the D-MIMO apparatus 11 that is associated with the MIMO transmission and uses the TDD method will be described as an example.

FIG. 2 is a schematic diagram illustrating installation of the distributed antenna according to the first example embodiment.

FIG. 3 is a schematic diagram illustrating installation of the distributed antenna according to the first example embodiment.

FIGS. 2 and 3 are diagrams when an operation place, for example, indoor installation of the distributed antenna of the D-MIMO apparatus according to the first example embodiment is viewed from above.

FIG. 2 illustrates a communication state during calibration. FIG. 3 illustrates a communication state during operation. In the first example embodiment, it is assumed that the number of the distributed antennas DA is four, and the distributed antennas DA are installed in four corners in a room. As illustrated in FIG. 2, a radio wave transmitted from the distributed antenna DA1 is blocked by a pole and does not thus reach a distributed antenna DA4 and vice versa. Further, a dotted line illustrated in FIG. 2 indicates that the plurality of distributed antennas DA can communicate with each other.

It is assumed that all of the distributed antenna DA1 to a distributed antenna DA4 are calibrated. Calibration is performed by transmitting and receiving a calibration signal being a known calibration signal by the distributed antennas DA. In the first example embodiment, calibration can be performed by using an external measuring instrument and user equipment (UE), but it is assumed that calibration is performed by only the distributed antennas DA.

In the method for performing calibration by the distributed antennas DA according to the first example embodiment, transmission and reception of all of the distributed antennas DA are not switched at the same timing unlike a normal operation. In the first example embodiment, transmission or reception of one of any distributed antennas DA is operated in an opposite manner to the other distributed antenna DA. In other words, one distributed antenna DA is used like an external measuring instrument. The one distributed antenna DA is referred to as a calibration antenna. In this way, calibration can be performed only with the D-MIMO apparatus 11.

The distributed antennas DA of the D-MIMO apparatus 11 according to the first example embodiment are installed at various places depending on an operation place. Thus, as in the distributed antenna DA1 and the distributed antenna DA4 illustrated in FIG. 2, a signal may not reach each antenna. Thus, calibration (measurement) using a calibration signal of the D-MIMO apparatus 11 is performed for a plurality of times by changing the distributed antenna DA used as the calibration antenna. The D-MIMO apparatus 11 obtains a calibration coefficient for calibrating a transmission portion of the D-MIMO apparatus 11 by using measurement results for a plurality of times. Subsequently, the D-MIMO apparatus 11 performs calibration by adding the obtained calibration coefficient to a DL signal to be transmitted from each of the distributed antennas DA.

Details of Configuration of Radio Communication Apparatus

FIG. 4 is a block diagram illustrating a configuration of the D-MIMO apparatus according to the first example embodiment.

FIG. 5 is a block diagram illustrating a digital front end DFE according to the first example embodiment.

As illustrated in FIG. 4, the D-MIMO apparatus 11 according to the first example embodiment includes a distributed antenna unit DA1, a distributed antenna unit DA2, a distributed antenna unit DA3, a distributed antenna unit DA4, and a radio unit RU. Each of the distributed antenna units DA mainly includes an antenna and an RF up/down converter unit. The distributed antenna unit D1, the distributed antenna unit DA2, the distributed antenna unit DA3, and the distributed antenna unit DA4 are collectively referred to as the distributed antenna unit DA.

The digital front end DFE illustrated in FIG. 4 includes the calibration control unit 111, the calibration coefficient calculation unit 112, and the calibration application unit 113 illustrated in FIG. 1. The RF up/down converter unit of the distributed antenna unit DA illustrated in FIG. 4 is included in a transmitter-receiver 114 illustrated in FIG. 1.

The radio unit RU converts a digital signal being a transmission signal into an analog signal in the down link (DL) communication, and converts an analog signal being a reception signal into a digital signal in the up link (UL) communication. The radio unit RU and each of the four distributed antenna units DA (from the distributed antenna unit DA1 to the distributed antenna unit DA4) are connected by a cable. The distributed antenna unit DA can be freely installed within a range reached by the cable. In the first example embodiment, the number of the distributed antenna units DA is four, which is not limited thereto. The number of the distributed antenna units DA may be three or more.

The radio unit RU includes the digital front end DFE that performs digital signal processing, and a modulator-demodulator MODEM that converts a digital signal and an analog modulation signal. The radio unit RU includes four modulator-demodulators MODEM in the same number as the number of the distributed antenna units DA, and performs digital beam forming. The radio unit RU and the four distributed antenna units DA are connected by a coaxial cable, and transmit an analog modulation signal by using the coaxial cable.

The distributed antenna unit DA performs up-conversion on an analog modulation signal transmitted from the radio unit RU into an RF signal in the RF up/down converter unit RF, and then transmits the RF signal from the antenna. Further, the distributed antenna unit DA performs down-conversion on a signal received by the antenna into an analog modulation signal in the RF up/down converter unit RF, and then transmits the analog modulation signal to the radio unit RU.

Processing of a DL signal of the digital front end DFE will be indicated below.

As illustrated in FIG. 5, in the processing of a DL signal, the digital front end DFE divides a transmission signal for each layer in a layer mapper unit 1151, and outputs a layer signal X. In this example, it is assumed that a layer number is two, for example. A precoding unit 1152 generates four DL signals WX by assigning a weight to each layer signal X according to a precoding weight coefficient W and adding each other. The calibration application unit 113 multiplies the DL signal WX after the precoding by a coefficient C that calibrates an error of the transmitter-receiver, and outputs a DL signal CWX. An OFDM signal generation unit 1153 performs OFDM modulation on the DL signal CWX subjected to the precoding and the calibration, and generates a digital baseband signal. The digital baseband signal is transmitted to the modulator-demodulator MODEM, and is converted into an analog modulation signal.

Processing of a UL signal of the digital front end DFE will be indicated below.

An OFDM signal demodulation unit 1154 performs demodulation processing on a received baseband signal (OFDM signal) being input from the modulator-demodulator MODEM, and outputs a signal U. In this way, the received baseband signal is converted from a signal in a time domain into a signal in a frequency domain. In other words, the received baseband signal is converted into a signal for each subcarrier. The signal U subjected to the OFDM demodulation is a signal having a phase and an amplitude changing due to an influence of interference between pieces of multiplexing communication with a plurality of UE, a propagation environment, and the like.

A channel estimation unit 1155 obtains channel coefficients H_uand H_dof a propagation path (communication path) by using a known reference signal included in the OFDM signal or a distributed antenna unit DA calibration signal. The channel coefficient obtained by channel estimation from the UL signal or a distributed antenna unit DA calibration UL signal is assumed to be H_u. The channel coefficient obtained by channel estimation from a distributed antenna unit DA calibration DL signal is assumed to be H_d. The D-MIMO apparatus 11 removes signal interference and corrects a change in phase and amplitude by a spatial multiplexing technique by using the channel coefficient H_u, and acquires the UL signal for each layer.

A precoding weight calculation unit 1156 calculates the precoding weight coefficient W for the DL from the channel coefficient H_uobtained in the channel estimation unit 1155 by using the spatial multiplexing technique. The calibration coefficient calculation unit 112 calculates the calibration coefficient C by using the channel coefficients H_uand H_d. The calibration application unit 113 performs calibration of the DL signal by using the calibration coefficient C.

Details of Operation of Radio Communication Apparatus

FIG. 6 is a flowchart illustrating an operation of the D-MIMO apparatus according to the first example embodiment.

FIG. 6 illustrates an operation of the D-MIMO apparatus according to the first example embodiment during calibration.

FIG. 7 is a schematic diagram illustrating a frequency arrangement of a calibration DL signal according to the first example embodiment.

FIG. 7 illustrates the frequency arrangement of the calibration DL signal being transmitted from the distributed antenna DA1.

FIG. 8 is a schematic diagram illustrating a frequency arrangement of a calibration DL signal according to the first example embodiment.

FIG. 8 illustrates the frequency arrangement of the calibration DL signal being transmitted from the distributed antenna DA2.

FIG. 9 is a schematic diagram illustrating a frequency arrangement of a calibration DL signal according to the first example embodiment.

FIG. 9 illustrates the frequency arrangement of the calibration DL signal being transmitted from the distributed antenna DA3.

FIG. 10 is a schematic diagram illustrating a frequency arrangement of a calibration DL signal according to the first example embodiment.

FIG. 10 illustrates the frequency arrangement of the calibration DL signal being transmitted from the distributed antenna DA4.

FIG. 11 is a schematic diagram illustrating a frequency arrangement of a calibration UL signal according to the first example embodiment.

The D-MIMO apparatus 11 may start a calibration operation, i.e., an operation of adjusting a phase and an amplitude of a DL signal with, as an opportunity, any operation indicated below.

- At a time of activation (at a time of turning-on) of the D-MIMO apparatus 11.
- When a difference between a temperature of any of the plurality of distributed antennas DA and a temperature during previous calibration is equal to or more than a predetermined temperature, or when a difference between an average temperature of all of the distributed antennas DA and a temperature during previous calibration is equal to or more than the predetermined temperature.
- When a predetermined time or longer has elapsed since previous calibration.
- When communication quality of a DL signal is lower than predetermined quality during a predetermined period or longer.

With the operation described above as an opportunity, as illustrated in FIG. 6, 1 is set as a variable k (step S101). Note that the variable k is assumed to be an integer.

Since the variable k=1, a distributed antenna DAk is the distributed antenna DA1. The distributed antenna DA1 is set as a calibration antenna, and a calibration measurement is performed by using the distributed antenna DA1 (step S102). Note that a k-th distributed antenna DA is assumed to be the distributed antenna DAk.

A known calibration DL signal is transmitted from all of the distributed antennas DA other than the distributed antenna DAk, and is received by the distributed antenna DAk (step S103). In order to avoid interference between the distributed antennas DA, an OFDM signal disposed at a frequency (subcarrier) different for each of the plurality of distributed antennas DA is used as the calibration DL signal.

As illustrated in FIGS. 7 to 10, the calibration DL signal disposed in a subcarrier different for each of the plurality of distributed antennas DA is assigned. One subcarrier in four subcarriers is disposed for the calibration DL signal. The calibration DL signal is not transmitted from the distributed antenna DAk used as the calibration antenna. The calibration DL signal (data) being transmitted from the distributed antenna DA other than the distributed antenna DAk used as the calibration antenna and received and measured by the distributed antenna DAk is demodulated similarly to a time of reception of a normal OFDM signal, subjected to channel estimation, and then held in a storage unit (not illustrated). In this example, the example from the distributed antenna DA1 to the distributed antenna DA4 is illustrated, which is not limited thereto.

After step S103, a known calibration UL signal is transmitted from the distributed antenna DAk, and is received by the distributed antennas DA other than the distributed antenna DAk (step S104).

As illustrated in FIG. 11, the calibration UL signal is different from the calibration DL signal, and does not change a subcarrier disposed for each of the distributed antennas DA. Subcarrier mapping (frequency arrangement) of the calibration UL signal is different from that of the calibration DL signal, and the calibration UL signal is disposed in all subcarriers. The calibration UL signal (data) being received by each of the distributed antennas DA is demodulated similarly to a time of reception of a normal OFDM signal, subjected to channel estimation, and then held in the storage unit.

After step S104, 1 is added to the variable k (step S105).

When the variable k is equal to or less than 4 (step S106: No), the processing returns to step S102. Further, when the variable k is greater than 4 (step S106: Yes), the processing proceeds to step S107. In the first example embodiment, a measurement is performed in such a way that the calibration antenna is set from the distributed antenna DA1 in order, and all of the distributed antennas DA are used once as the calibration antenna.

After Yes in step S106, a calibration coefficient is calculated based on the calibration DL signal or the calibration UL signal (data) being measured by repeating step S103 and step S104, and is applied to the calibration application unit 113 (step S107).

After step S107, a measurement for calibration coefficient calculation ends, and the processing returns to a normal operation (step S108).

Calculation Method of Calibration Coefficient

Herein, a calculation method of a calibration coefficient performed in step S107 illustrated in FIG. 6 will be described.

As illustrated in FIGS. 7 to 10, the calibration DL signal output from the distributed antenna DA is disposed in a subcarrier different for each of the plurality of distributed antennas DA. For example, the calibration DL signal transmitted from the distributed antenna DA2 to the distributed antenna DA4 is received by the distributed antenna DA1 as the calibration antenna. The calibration DL signal is subjected to OFDM modulations and divided for each subcarrier, and is thus received without interference between the plurality of distributed antennas DA.

Herein, a subcarrier used for communication with an i-th distributed antenna DAi is assumed to be s_i. The calibration DL signal and the calibration UL signal may be collectively referred to as a calibration signal.

In the first example embodiment, the variable i is assumed to be an integer from 1 to 4. As illustrated in FIGS. 7 to 10, a subcarrier s₁is, for example, a first, fifth, ninth, . . . subcarrier. A subcarrier s₂is, for example, a second, sixth, tenth . . . subcarrier. Data about an s_i-th subcarrier being acquired by demodulating an OFDM signal received by the k-th calibration antenna are assumed to be represented as d_k,i(s_i). When k=i, it is assumed that d_k,i(s_i)=0.

In a measurement of the calibration UL signal, a signal of an s_i-th subcarrier of the calibration UL signal being acquired by transmitted from the k-th distributed antenna DAk and received by the i-th distributed antenna DAi is assumed to be u_k,i(s_i). When k=i, it is assumed that u_k,i(s_i)=0. As illustrated in FIG. 11, the calibration UL signal is disposed in all subcarriers, and calculation of a calibration coefficient is performed by using a subcarrier used for the calibration DL signal of each of the distributed antennas DA.

In the first example embodiment, a measurement of the calibration DL signal and a measurement of the calibration UL signal are each performed for four times by changing the calibration antenna. Since the calibration antenna is changed, a transmission/reception characteristic varies by each calibration antenna. A transmission complex gain of the k-th calibration antenna is assumed to be ε_k,t, and a reception complex gain is assumed to be ε_k,r. The gain herein is a value represented by a complex number, and includes information about an amplitude and a phase. Thus, the gain is referred to as the transmission complex gain and the reception complex gain, which may be referred to as a transmission gain and a reception gain by omitting “complex” in the following description.

Similarly, an error is also present in the individual distributed antenna DA. A transmission complex gain of the i-th calibration antenna is assumed to be e_i,t, and a reception complex gain is assumed to be e_i,r. An object of the first example embodiment is to correct an influence of the gain on a DL signal during operation. In the first example embodiment, since one of the distributed antennas DA is used as the calibration antenna, when k=i, the transmission gain ε_k,tand the transmission gain e_i,tare equal or the reception gain ε_k,rand the reception gain e_i,rare equal. However, the gains may be different in a second example embodiment and a third example embodiment described below, and are thus handled as different gains.

The calibration DL signal d_k,i(s_i) measured for a k-th time can be represented as follows.

d

k
,
i

(

S
i

)

=

ε

k
,
r

(

s
i

)

·

h

k
,
i

(

s
i

)

·

e

i
,
t

(

s
i

)

·

x
i

(

s
i

)

(
1
)

h_k,iis a channel coefficient of a propagation path from the distributed antenna DAi to the distributed antenna DAk, and x_iis a known calibration DL signal transmitted from the distributed antenna DAi. (s_i) in an equation (1) indicates to be changed depending on a frequency (subcarrier), but will be omitted from description below for simplification. Further, the calibration DL signal d_k,i(s_i) is measured by over the air (OTA) and thus includes noise, which will be omitted in this example.

Similarly, the signal u_k,i(s_i) received by the i-th distributed antenna DAi in the calibration UL signal measured for the k-th time can be represented as follows.

u

k
,
i

(

s
i

)

=

e

i
,
r

(

s
i

)

·

h

k
,
i

(

s
i

)

·

ε

k
,
t

(

s
i

)

·

y
k

(

s
i

)

(
2
)

y_kis a known calibration UL signal being transmitted from the calibration antenna CAk. (s_i) in an equation (2) indicates to be changed depending on a frequency (subcarrier), but will be omitted from description below for simplification. Further, the calibration UL signal u_k,i(s_i) is measured by OTA and thus includes noise, which will be omitted in this example.

The channel coefficients h_k,iincluded in the measured calibration DL signal d_k,iand the measured UL signal u_k,iare assumed to be equal on an assumption that the calibration signals propagate through the same propagation path. Channel estimation is performed from each of the equation (1) and the equation (2). A channel coefficient estimated from the calibration DL signal d_k,iis assumed to be h_d,k,i.

h

d
,
k
,
i

=

d

k
,
i

·

x
i
*

(
3
)

=

ε

k
,
r

·

h

k
,
i

·

e

i
,
t

(
4
)

The known calibration DL signal x_iis assumed to be x_i·x_i*=1. (*) represents a conjugate complex number.

Similarly, a channel coefficient h_u,k,iestimated from the calibration UL signal is calculated as in an equation (5) or an equation (6).

h

u
,
k
,
i

=

u

k
,
i

·

y
k
*

(
5
)

=

e

i
,
r

·

h

k
,
i

·

ε

k
,
t

(
6
)

The known calibration UL signal y_kis assumed to be y_k·y_k*=1.

A correction coefficient γ_kthat corrects the transmission gain ε_k,tand the reception gain ε_k,rof the k-th calibration antenna is obtained from the channel coefficient h_d,k,iand the channel coefficient h_u,k,iobtained in an equation (3) and the equation (5). The correction coefficient γ_kis obtained with reference to any measurement of k=1 to 4. Hereinafter, a calibration signal (data) being acquired by communicating with an i0-th distributed antenna DAi0 in a k0-th measurement is used as a reference.

γ
k

=

h

u
,
k
,

i
⁢
0

·

h

d
,

k
⁢
0

,

i
⁢
0

h

d
,
k
,

i
⁢
0

·

h

u
,

k
⁢
0

,

i
⁢
0

(
7
)

=

ε

k
,
t

·

ε

k
⁢
0

,
r

ε

k
,
r

·

ε

k
⁢
0

,
t

(
8
)

k, k0, and i0 are assumed to be different numbers.

In an equation (7) and an equation (8), the correction coefficient γ_kis obtained from only a measurement result of the i0-th distributed antenna DAi0, but the correction coefficient γ_kmay be calculated by calculating the calibration signal described above from communication with the plurality of distributed antennas DA by changing a number of i0, and taking an average. The correction coefficient γ_kis calculated for all k (in this example, 1 to 4). However, the number i0 of the distributed antenna DAi0 may be changed each time a value of k changes. A k0-th correction coefficient γ_k0being a reference is assumed to be 1.

A radio wave may not be reached depending on an arrangement of the calibration antenna used for the k-th time and the k0-th calibration antenna being the reference, and a calibration signal (data) may not be able to be acquired. A number of the distributed antenna DA to which the radio wave from the calibration antenna being the reference is not reached is assumed to be newly set as k′. Such a k′-th correction coefficient ε_k+ is obtained as in an equation (9) and an equation (10) by using a k1-th measurement in which a correction coefficient is already obtained.

γ

k
′

=

h

u
,

k
′

,

i
⁢
0

·

h

d
,

k
⁢
1

,

i
⁢
0

h

d
,

k
′

,

i
⁢
0

·

h

u
,

k
⁢
1

,

i
⁢
0

⁢

γ

k
⁢
1

(
9
)

=

ε

k
′

,
t

·

ε

k
⁢
0

,
r

ε

k
′

,
r

·

ε

k
⁢
0

,
t

(
10
)

As in the equation (7) and the equation (9), two propagation paths are needed in order to obtain the correction coefficient γ_k. When the propagation path is one or less, γ_kis assumed to be zero. Further, a propagation path whose received power is equal to or less than a predetermined threshold value may be determined to be absent. When the received power is equal to or less than the predetermined threshold value, the measurement results d_k,iand u_k,iof the k-th calibration antenna and the i-th distributed antenna DAi being associated are both zero.

The correction coefficient γ_kdepends on a frequency and varies by each subcarrier. The correction coefficient γ_kmay be obtained by taking an average for all subcarriers or each fixed number of subcarriers. Further, when an amplitude gain is small, an amplitude component of the correction coefficient γ_kmay be ignored, and only a phase component of the correction coefficient γ_kmay be used.

A calibration coefficient c_ithat calibrates the transmission gain e_i,tand the reception gain e_i,rof the distributed antenna DA in the transmission portion of the D-MIMO apparatus 11 is as in an equation (11).

c
i

=

∑

k
=
1

N
m

⁢

h

u
,
k
,
i

·

h

d
,
k
,
i

*

·

γ
k

-
1

∑

k
=
1

N

⁢

❘
"[LeftBracketingBar]"

h

d
,
k
,
i

❘
"[RightBracketingBar]"

2

(
11
)

=

ε

k
⁢
0

,
t

ε

k
⁢
0

,
r

⁢

∑

k
=
1

N
m

⁢

❘
"[LeftBracketingBar]"

ε

k
,
r

❘
"[RightBracketingBar]"

2

⁢

❘
"[LeftBracketingBar]"

h

k
,
r

❘
"[RightBracketingBar]"

2

⁢

e

i
,
r

⁢

e

i
,
t

-
1

∑

k
=
1

N

⁢

❘
"[LeftBracketingBar]"

ε

k
,
r

❘
"[RightBracketingBar]"

2

⁢

❘
"[LeftBracketingBar]"

h

k
,
r

❘
"[RightBracketingBar]"

2

(
12
)

Note that the number of times of calibration measurements performed by changing the calibration antenna is assumed to be N_m. In a case of the first example embodiment, N_m=4. The number of the distributed antennas DA is assumed to be N. When the correction coefficient is γ_k=0, it is assumed that (γ_k)⁻¹=0. The equation (11) can be expanded as in an equation (12). According to the equation (12), the calibration coefficient c_iis an average of (e_i,r/e_i,t) by assigning a weight of (|ε_k,r|²|h_k,r|²). Further, (|ε_k,r|²|h_k,r|²) is proportional to received power of a calibration signal.

When a calibration signal is actually measured, received power is greatly changed by an arrangement of the distributed antennas DA and the like. Normally, an influence of noise is reduced with greater received power, and an accurate calibration signal can be acquired. Thus, by taking an average by assigning a weight of (|ε_k,r|²|h_k,r|²), an influence of a measurement having a high degree of accuracy is increased and an influence of a measurement having a low degree of accuracy is reduced, and thus the calibration coefficient c_ibeing accurate as a whole can be acquired.

When the calibration coefficient c_iis actually applied to the D-MIMO apparatus 11, the calibration coefficient c_iis normalized according to a dynamic range of the D-MIMO apparatus 11. Further, (ε_k0,t/ε_k0,r) in the equation (12) is a ratio of transmission/reception gains of a reference calibration antenna of the correction coefficient γ_k. Since this is fixed in all of the distributed antennas DA, accuracy of a DL signal is not affected. Thus, (ε_k0,t/ε_k0,r) is omitted in response to the normalization, and the equation (12) can be approximately represented as in an equation (13).

c
i

=

e

i
,
r

⁢

e

i
,
t

-
1

(
13
)

Thus, by using a ratio of the transmission/reception gains of the distributed antenna DA, calibration of the transmission portion of the D-MIMO apparatus 11 is performed.

The calibration coefficient c_iis obtained for each subcarrier s_ibeing used in calibration. The calibration coefficient c_iof a subcarrier not being used in calibration such as a subcarrier being used in calibration of the other distributed antenna DA is obtained by interpolation and extrapolation from the calibration coefficient of the subcarrier s_i. Further, the calibration coefficient may be obtained by taking an average in a certain subcarrier range. In this way, calibration can be performed also in consideration of a frequency characteristic of an error between the distributed antennas DA.

In the first example embodiment, a calibration signal is disposed in all subcarriers, but calibration can also be performed by using only some of subcarriers. A subcarrier in which a calibration signal is not disposed is calibrated by interpolation and extrapolation from a calibration result of a neighboring subcarrier. Thus, calibration can be performed by using a subcarrier and a timing of a reference signal and the like even during operation.

Effect

An effect of the first example embodiment will be described. Herein, an effect of a normal operation state (see FIG. 3) of communicating with two UEs by using spatial multiplexing will be described. In the following description, the number of the UEs is assumed to be M=2. The number of the UEs is generally equal to or less than the number of the antennas of the D-MIMO apparatus 11, and, when the number of the distributed antennas DA is N, M may take any integer from 1 to N.

A UL signal being transmitted from each of the UEs and received (measured) by the distributed antenna DA is assumed to be U. The UL signal U is a matrix with N rows and 1 column (hereinafter described as N×1) and can be represented as in an equation (14).

U
=

E
r

⁢
HY

(
14
)

H is a channel coefficient of a transmission path and is represented by a matrix of N×M, and Y is a UL signal being transmitted from the UE and is represented by M×1. E_rrepresents a reception gain of the distributed antenna DA, is represented by using e_i,r, and is a diagonal matrix of N×N.

E
r

=

[

e

1
,
r

⋯

0

⋮

⋱

⋮

0

⋯

e

N
,
r

]

(
15
)

The channel coefficient H_uobtained by channel estimation from the received UL signal U is in an equation (16).

H
u

=

E
r

⁢
H

(
16
)

It can be seen that the reception gain E_ris included in the channel coefficient H_u.

The precoding weight coefficient W is obtained by using a zero-forcing (ZF) algorithm being one of spatial multiplexing techniques as in an equation (17).

W
T

=

H
u
+

=

(

E
r

⁢
H

)

+

(
17
)

A superscript T of the precoding weight coefficient W indicates a transposed matrix, and a superscript + indicates a pseudo-inverse matrix. There are a plurality of spatial multiplexing techniques such as an MMSE algorithm in addition to the ZF algorithm, but an effect of calibration can be acquired similarly to the ZF algorithm.

A DL signal acquired by multiplying the precoding weight coefficient W and the calibration coefficient c_iindicated in the equation (11) is transmitted from each of the distributed antennas DA. Then, the DL signal D received by each of the UEs is represented as follows.

D
=

H
T

·

E
t

⁢
CWX

(
18
)

X is a matrix of M×1 and is an original signal to be transmitted to each of the UEs. In the first example embodiment, a signal is transmitted by one layer per one UE. C is a diagonal matrix of N×N as indicated in an equation (19) including the calibration coefficient c_iobtained by the technique in the first example embodiment.

C
=

[

c
1

⋯

0

⋮

⋱

⋮

0

⋯

c
N

]

(
19
)

E_trepresents a transmission gain of the distributed antenna DA and is a diagonal matrix of N×N as in an equation (20).

E
t

=

[

e

1
,
t

⋯

0

⋮

⋱

⋮

0

⋯

e

N
,
t

]

(
20
)

The calibration coefficient matrix C is represented as in an equation (21) by using the equation (13) to an equation (15) and the equation (20).

C
=

E
t

-