U.S. patent application number 15/566380 was filed with the patent office on 2018-05-03 for radio transmission apparatus.
This patent application is currently assigned to Mitsubishi Electric Corporation. The applicant listed for this patent is Mitsubishi Electric Corporation. Invention is credited to Hiroki IURA, Akinori TAIRA, Shigeru UCHIDA.
Application Number | 20180123667 15/566380 |
Document ID | / |
Family ID | 55747717 |
Filed Date | 2018-05-03 |
United States Patent
Application |
20180123667 |
Kind Code |
A1 |
TAIRA; Akinori ; et
al. |
May 3, 2018 |
RADIO TRANSMISSION APPARATUS
Abstract
A radio transmission apparatus includes a plurality of antennas
each having an amplifier, a transmission signal generation unit
(modulation units, S/P units, and FFT units) which generates a
signal to be transmitted to a terminal via the antennas, and a
weighting processing unit (a precoder unit, a maximum power
calculation unit, and multipliers) which executes a weighting
process on a signal to be transmitted to the terminal, which has
been generated by the transmission signal generation unit, based on
channel state information between the radio transmission apparatus
and the terminal and an output limit value of the amplifier.
Inventors: |
TAIRA; Akinori; (Tokyo,
JP) ; UCHIDA; Shigeru; (Tokyo, JP) ; IURA;
Hiroki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mitsubishi Electric Corporation |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Assignee: |
Mitsubishi Electric
Corporation
Chiyoda-ku, Tokyo
JP
|
Family ID: |
55747717 |
Appl. No.: |
15/566380 |
Filed: |
August 5, 2015 |
PCT Filed: |
August 5, 2015 |
PCT NO: |
PCT/JP2015/072269 |
371 Date: |
October 13, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 7/0626 20130101;
H04L 27/2646 20130101; H04B 7/0619 20130101; H04B 7/0465 20130101;
H04B 1/0475 20130101; H04B 2001/0433 20130101; H04B 7/0417
20130101; H04B 7/0634 20130101; H04L 27/2615 20130101 |
International
Class: |
H04B 7/06 20060101
H04B007/06; H04B 7/04 20060101 H04B007/04; H04B 7/0417 20060101
H04B007/0417; H04B 1/04 20060101 H04B001/04 |
Claims
1: A radio transmission apparatus comprising: a plurality of
antennas each having an amplifier; a transmission signal generation
unit to generate a signal to be transmitted to a terminal via the
antennas; and a weighting processing unit to execute a weighting
process on a signal to be transmitted to the terminal, which has
been generated by the transmission signal generation unit, based on
channel state information between the radio transmission apparatus
and the terminal and an output limit value of the amplifier,
wherein the weighting processing unit comprises: a precoder unit to
generate a transmission weight for weighting a signal to be
transmitted to the terminal based on the channel state information,
and multiply the signal to be transmitted to the terminal by the
generated transmission weight to perform weighting; and a power
correction unit to correct power of a signal weighted by the
precoder unit based on the transmission weight generated by the
precoder unit and a threshold determined in advance based on the
output limit value of the amplifier.
2. (canceled)
3: The radio transmission apparatus according to claim 1, wherein
the power correction unit comprises: a correction value calculation
unit to calculate an average power value of a signal outputted from
the precoder unit based on the transmission weight, and calculate a
power correction value for correcting power of the signal weighted
by the precoder unit based on the calculated average power value
and the threshold; and a multiplier to multiply the signal weighted
by the precoder unit by the power correction value.
4: The radio transmission apparatus according to claim 1,
comprising a correction value selection unit to select a correction
value for correcting an amplitude and a phase of a signal inputted
to the plurality of antennas for each of the antennas from among a
plurality of correction values determined in advance, based on the
transmission weight, wherein the antennas correct the amplitude and
the phase of the inputted signal by using the correction value
selected by the correction value selection unit.
5: The radio transmission apparatus according to claim 4, wherein
the correction value selection unit calculates an average power
value of a signal outputted from the precoder unit for each of the
antennas to which the signal is inputted based on the transmission
weight, and selects the correction value for each of the antennas
based on the calculated average power value.
6: A radio transmission apparatus comprising: a plurality of
antennas each having an amplifier; a transmission signal generation
unit to generate a signal to be transmitted to a terminal via the
antennas; and a weighting processing unit to execute a weighting
process on a signal to be transmitted to the terminal, which has
been generated by the transmission signal generation unit based on
channel state information between the radio transmission apparatus
and the terminal and an output limit value of the amplifier,
wherein the weighting processing unit comprises: a precoder unit to
generate a transmission weight for weighting a signal to be
transmitted to the terminal based on the channel state information,
and multiply the signal to be transmitted to the terminal by the
generated transmission weight; and a maximum power calculation unit
to calculate a power correction value to be used when the precoder
unit generates the transmission weight, the number of the antennas
is equal to or larger than the number of signal streams to be
spatially multiplexed, the precoder unit calculates a first
transmission weight for achieving eigenmode transmission, and
calculates a second transmission weight by which a signal to be
transmitted to the terminal is multiplied, based on the first
transmission weight and the power correction value calculated by
the maximum power calculation unit, and the maximum power
calculation unit calculates, based on the first transmission
weight, an average power value of a weighted signal obtained when a
signal to be transmitted to the terminal is weighted with the first
transmission weight, and calculates the power correction value
based on the calculated average power value and a threshold
determined in advance based on an output limit value of the
amplifier.
7: The radio transmission apparatus according to claim 6, wherein
the precoder unit calculates an average power value of a weighted
signal obtained when a signal to be transmitted to the terminal is
weighted with the first transmission weight, for each antenna to
which the signal to be transmitted to the terminal is inputted,
generates a power upper limit value by correcting a maximum value
of the calculated average power value using the power correction
value, and generates the second transmission weight by linearly
combining basis vectors of the first transmission weight so that an
average power value of a weighted signal obtained when the signal
to be transmitted to the terminal is weighted with the second
transmission weight is equal to or less than the power upper limit
value.
8: The radio transmission apparatus according to claim 6,
comprising a correction value selection unit to select a correction
value for correcting an amplitude and a phase of a signal inputted
to the plurality of antennas for each of the antennas from
correction values determined in advance, based on the second
transmission weight, wherein the antennas correct the amplitude and
the phase of the inputted signal by using the correction value
selected by the correction value selection unit.
9: The radio transmission apparatus according to claim 8, wherein
the correction value selection unit calculates an average power
value of a signal outputted from the precoder unit for each of the
antennas to which the signal is inputted, based on the second
transmission weight, and selects the correction value for each of
the antennas based on the calculated average power value.
Description
FIELD
[0001] The present invention relates to a radio transmission
apparatus which spatially multiplex-transmits data using a
plurality of antennas.
BACKGROUND
[0002] In order to transmit large volume data at limited
frequencies, there has been advanced development of a multiple
input multiple output (MIMO) system in which spatial multiplex
transmission is performed using a plurality of transmitting and
receiving antennas. In recent years, with the aim of further
improving frequency utilization efficiency, use of higher
frequencies and multi-element configuration of antenna elements
have been promoted, and it is expected that the number of spatially
multiplexed objects will continue to increase in the future.
[0003] In a radio transmission apparatus, a high power amplifier
(HPA) for emitting a signal from an antenna is necessary. A
multicarrier scheme such as orthogonal frequency division
multiplexing (OFDM) is used as a wireless transmission system
suitable for high volume transmission, but this scheme is known to
have a large peak to average power ratio (PAPR). In order to
accurately transmit a signal with a large PAPR, the HPA is required
to have precise linearity, and there is concern that the cost will
increase. When combining the multicarrier scheme and the MIMO
system, it is necessary to take account of a larger PAPR, so the
above-mentioned linearity problem becomes more serious.
[0004] As a technique for dealing with a large PAPR, for example,
in Non Patent Literature 1, a PAPR is minimized by applying a time
filter to an instantaneous value of a waveform with a large time
fluctuation thereby to mitigate an influence thereof on an
amplifier. In Non Patent Literature 2, a technique is disclosed in
which a multi user (MU)-MIMO system is assumed, and with the use of
a carrier and noise power ratio (CNR) fed back from a receiving
side, a transmitting side adaptively controls backoff based on a
predicted carrier and interference noise power ratio (CINR)
calculated from backoff of an HPA.
CITATION LIST
Non Patent Literature
[0005] Non Patent Literature 1: Kageyama, Muta, HarisGACANIN,
Furukawa, "Performance Evaluation of OFDM-SDMA Systems using
Adaptive Peak Cancellation under Restriction of Out-of-band
Radiation and In-band Distortion," THE INSTITUTE OF ELECTRONICS,
INFORMATION AND COMMUNICATION ENGINEERS, IEICE Technical Report,
RCS2015-1
[0006] Non Patent Literature 2: Takebuchi, Maruko, Osada, Maehara,
"A study on application of OFDM clipping & filtering employing
transmit power control to MU-MIMO systems," THE INSTITUTE OF
ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, IEICE
Technical Report, RCS2013-311
SUMMARY
Technical Problem
[0007] As the number of antenna elements increases and the number
of spatially multiplexed objects increases, the above-mentioned
problem of the PAPR becomes more serious. In addition, as the
number of antenna elements increases, it becomes difficult to
implement an analog to digital converter (ADC) and a digital to
analog converter (DAC) for each of the elements from the viewpoints
of cost and size. In order to deal with this difficulty, a
technique is adopted, in which an array antenna is constructed of
an analog circuit to achieve cost reduction. However, in this
technique, it is necessary to perform calibration between the
elements in order to emit a beam accurately from the antenna.
Furthermore, in a time division duplex (TDD) type system which
forms a beam using symmetry of a transmission path, it is necessary
to perform calibration between a reception circuit on a signal
receiving side and a transmission circuit including an amplifier on
a signal transmitting side, thus leading to difficulty of adjusting
the circuits. In particular, there is a variation in frequency
characteristics in antenna elements for an inexpensive
configuration, and so it is necessary to change a transmission
weight for spatial multiplex for each frequency. However, in the
technique of Non Patent Literature 2, it is difficult to predict a
CINR with frequency characteristics taken into account. In short,
it is difficult to apply the technique of Non Patent Literature 2
when there is a variation in frequency characteristics in antenna
elements.
[0008] In addition, as the number of multiplexed users increases,
rapid feedback from each user becomes difficult, and fading
fluctuation of a transmission path in the same section becomes
noticeable in a high frequency band, so that more frequent feedback
is required to acquire an accurate reception CNR, which is a factor
responsible for pressuring an uplink bandwidth.
[0009] The present invention has been made in view of the above
circumstances, and it is an object of the present invention to
provide a radio transmission apparatus capable of controlling input
backoff of a high power amplifier.
Solution to Problem
[0010] In order to solve the above-mentioned problems and to
achieve the object, a radio transmission apparatus according to the
present invention includes a plurality of antennas each having an
amplifier, and a transmission signal generation unit to generate a
signal to be transmitted to a terminal via the antenna. In
addition, the radio transmission apparatus has a weighting
processing unit to execute a weighting process on a signal to be
transmitted to the terminal, which has been generated by the
transmission signal generation unit, based on channel state
information between the radio transmission apparatus and the
terminal and an output limit value of the amplifier.
Advantageous Effects of Invention
[0011] The present invention achieves an effect of obtaining a
radio transmission apparatus capable of controlling input backoff
of a high power amplifier.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIG. 1 is a diagram illustrating a configuration example of
a radio transmission apparatus according to a first embodiment.
[0013] FIG. 2 is a flowchart illustrating an operation example of a
precoder unit of the first embodiment.
[0014] FIG. 3 is a flowchart illustrating an operation example of a
maximum power calculation unit of the first embodiment.
[0015] FIG. 4 is a flowchart illustrating an operation example of a
correction value selection unit of the first embodiment.
[0016] FIG. 5 is a diagram illustrating an example of a hardware
configuration for realizing the precoder unit, the maximum power
calculation unit, and the correction value selection unit of the
first embodiment.
[0017] FIG. 6 is a diagram illustrating another example of the
hardware configuration for realizing the precoder unit, the maximum
power calculation unit, and the correction value selection unit of
the first embodiment.
[0018] FIG. 7 is a diagram illustrating an example of a radio
communication system to which a radio transmission apparatus of a
second embodiment is applied.
[0019] FIG. 8 is a diagram illustrating a configuration example of
the radio transmission apparatus according to the second
embodiment.
[0020] FIG. 9 is a flowchart illustrating an example of an
operation of calculating a transmission weight, performed by the
precoder unit and the maximum power calculation unit of the second
embodiment being cooperated with each other.
DESCRIPTION OF EMBODIMENTS
[0021] Hereinafter, a radio transmission apparatus according to an
embodiment of the present invention will be described in detail
with reference to the drawings. The invention is not necessarily
limited to the embodiment.
First Embodiment
[0022] FIG. 1 is a diagram illustrating a configuration example of
a radio transmission apparatus according to a first embodiment of
the present invention. The radio transmission apparatus 100
illustrated in FIG. 1 constitutes, for example, a base station of a
mobile communication system, and provides a function of forming one
or more beams, spatially multiplexing signals directed to
respective users, and simultaneously transmitting the resultant
multiplexed signals to the users, that is, mobile terminals
(hereinafter referred to as terminals) (including multi-user MIMO
and single-user MIMO). Hereinafter, an example of the case where
the radio transmission apparatus constitutes the base station for
the mobile communication system will be described.
[0023] As illustrated in FIG. 1, the radio transmission apparatus
100 according to the present embodiment includes modulation units
1.sub.1 to 1.sub.Ns, serial-to-parallel conversion units (S/P
units) 2.sub.1 to 2.sub.Ns, fast fourier transform (FFT) units
3.sub.1 to 3.sub.Ns, a precoder unit 4, inverse fast fourier
transform (IFFT) units 5.sub.1 to 5.sub.Nt, parallel-to-serial
conversion units (P/S units) 6.sub.1 to 6.sub.Nt, multipliers
7.sub.1 to 7.sub.Nt, a maximum power calculation unit 8, a
correction value selection unit 9, and antennas 10.sub.1 to
10.sub.Nt. Ns denotes the number of signal streams to be spatially
multiplex-transmitted. The modulation units 1.sub.1 to 1.sub.Ns,
the serial-to-parallel conversion units 2.sub.1 to 2.sub.Ns, and
the FFT units 3.sub.1 to 3.sub.Ns operate as a transmission signal
generation unit. The precoder unit 4, the maximum power calculation
unit 8, and the multipliers 7.sub.1 to 7.sub.Nt operate as a
weighting processing unit. The maximum power calculation unit 8 and
the multipliers 7.sub.1 to 7.sub.Nt constitute a power correction
unit of the weighting processing unit.
[0024] In addition, the precoder unit 4 is constituted by a
plurality of precoders 41, and the antennas 10.sub.1 to 10.sub.Nt
that are array antennas are each constituted by a plurality of
array elements 11. Each array element 11 includes a phase shifter
11A and a high power amplifier (HPA) 11B, and can adjust a phase
and an amplitude of a signal to be transmitted. The HPAs 11A are
amplifiers included in the antennas 10.sub.1 to 10.sub.Nt.
[0025] Although FIG. 1 illustrates a configuration example of a
radio communication apparatus for a multicarrier signal, the
present invention is not limited to a case of transmitting a
multicarrier signal, and can also be applied to a case of
transmitting a single carrier signal.
[0026] Hereinafter, an operation of each unit of the radio
transmission apparatus 100 of the present embodiment will be
described.
[0027] When signal streams #1 to #Ns that are signals to be
transmitted to the users are inputted, the modulation units 1.sub.1
to 1.sub.Ns modulate the signal streams #1 to #Ns and output the
resultant signal streams to the serial-to-parallel conversion units
2.sub.1 to 2.sub.Ns. The modulation units 1.sub.1 to 1.sub.Ns
modulate the input signal streams #1 to #Ns in conformity to a
determined modulation scheme such as quadrature phase shift keying
(QPSK) or quadrature amplitude modulation (QAM).
[0028] The serial-to-parallel conversion units 2.sub.1 to 2.sub.Ns
perform serial-to-parallel conversion of the input signals from the
modulation units 1.sub.1 to 1.sub.Ns and output the resultant
signals to the FFT units 3.sub.1 to 3.sub.Ns.
[0029] The FFT units 3.sub.1 to 3.sub.Ns convert the signals
inputted from the serial-to-parallel conversion units 2.sub.1 to
2.sub.Ns from signals on a time axis to signals on a frequency
axis, and output the resultant signals to the precoder unit 4. The
signals on a frequency axis outputted from the FFT units 3.sub.1 to
3.sub.Ns are inputted to the corresponding precoders 41 of the
precoder unit 4 for each frequency component.
[0030] In the precoder unit 4, each precoder 41 performs a
weighting process between antennas for spatial multiplex for each
of frequency bins f1 to fm, that is, for each frequency component,
based on channel state information (CSI) between the radio
transmission apparatus and each terminal. In the present
embodiment, a TDD-type system is assumed, which does not require
CSI feedback from a terminal that is on a signal receiving side.
That is, based on a known signal transmitted from the terminal, the
CSI in an uplink direction is obtained on the side of the radio
transmission apparatus 100, and with the use thereof as the CSI in
a downlink direction, a weighting process between antennas is
performed. The CSI is calculated, for example, by a radio receiving
apparatus whose illustration is omitted in FIG. 1, that is, by a
radio reception apparatus constituting the base station together
with the radio transmission apparatus 100, and the calculated CSI
is inputted to the precoder unit 4. Since a method of calculating
CSI is widely known, description thereof is omitted.
[0031] FIG. 2 is a flowchart illustrating an operation example of
the precoder unit 4. The precoder unit 4 first acquires CSI (Step
S11), and then calculates a transmission weight of each antenna
based on the acquired CSI (Step S12). In Step S12, the precoder
unit 4 calculates the transmission weight of each antenna for each
signal stream. The precoder unit 4 calculates the transmission
weight by, for example, a publicly known block diagonalization (BD)
method. In this BD method, a transmission weight for each antenna
is calculated so that a beam is formed such that transmission of a
signal stream addressed to one terminal does not interfere with
other terminals. A method of calculating a transmission weight is
not limited to the BD method. The transmission weight may be
calculated in other publicly known methods. The precoder unit 4
next weights a transmission signal using the calculated
transmission weight (Step S13). Specifically, each precoder 41
multiplies the transmission signal, which is a signal inputted from
each of the FFT units 3.sub.1 to 3.sub.Ns, by the transmission
weight calculated in Step S12, thereby performing the
weighting.
[0032] The transmission signals obtained after the weighting in the
precoder unit 4 are outputted to the IFFT units 5.sub.1 to 5.sub.Nt
in accordance with a destination terminal. The IFFT units 5.sub.1
to 5.sub.Nt convert the transmission signals inputted from the
precoder unit 4 into signals on a time axis and output the
resultant signals to the parallel-to-serial conversion units
6.sub.1 to 6.sub.Nt.
[0033] The parallel-to-serial conversion units 6.sub.1 to 6.sub.Nt
perform parallel-to-serial conversion of the input signals from the
IFFT units 5.sub.1 to 5.sub.Nt. In general, a guard interval
addition process and an up-conversion process are subsequently
carried out, but since they are not indispensable for realizing the
present invention, constituent elements performing these processes
are omitted in FIG. 1.
[0034] Here, the transmission weight generated by the precoder unit
4 changes variously depending on the CSI between the radio
transmission apparatus 100 performing spatial multiplex and a
terminal receiving a spatially multiplexed signal. Also, depending
on conditions, power concentrates on a limited one of the antennas,
and an average power value of a signal transmitted from the limited
antenna may become high. As already described, the HPA for
amplifying a signal is mounted on the radio transmission apparatus
100, but output of the HPA has a limit, that is, a saturation power
value, and for a high power signal, linearity of the signal at the
time of amplification cannot be maintained. In addition, in
massive-MIMO with a large number of antenna elements, a large
amount of CSI feedback is required when a frequency division duplex
(FDD) type system is realized. For this reason, in the present
embodiment, transmission path estimation using propagation path
reversibility of a TDD system is performed so that CSI feedback is
not required.
[0035] On the other hand, in order to utilize the propagation path
reversibility of the TDD, calibration between transmitting and
receiving blocks is indispensable. To this end, in the radio
transmission apparatus 100 of the present embodiment, a correction
value corresponding to an average power value of an input signal
for the HPA is selected from a correction table, and a phase and an
amplitude of the input signal for the HPA are corrected.
Specifically, the correction value selection unit 9 performs a
process for selecting a correction value corresponding to the
average power value of the input signal from the correction table,
and the antennas 10.sub.1 to 10.sub.Nt use the correction values
selected by the correction value selection unit 9 to correct the
phase and the amplitude of the transmission signals. The correction
table is a table in which a plurality of average power values of an
input signal to the HPA and correction values of a plurality of
phases and amplitudes respectively corresponding to the average
power values are registered, and the correction table is created at
the time of designing the radio transmission apparatus 100 and is
stored in advance by the correction value selection unit 9.
[0036] Also in a correction function of the correction value
selection unit 9 and the antennas 10.sub.1 to 10.sub.Nt, it is
difficult to correct a signal having a certain level of power or
more. That is, since there is an allowable maximum power value for
achieving spatial multiplex, it is necessary to adjust power of
input signals for the antennas 10.sub.1 to 10.sub.Nt by means of
limiting the magnitude of the transmission weight generated by the
precoder unit 4 to a certain value or less, or by the like means.
Therefore, the radio transmission apparatus 100 according to the
present embodiment includes the maximum power calculation unit 8
and the multipliers 7.sub.1 to 7.sub.Nt, and is configured to
adjust the power of the input signals for the antennas 10.sub.1 to
10.sub.Nt. The maximum power calculation unit 8 that is a
correction value calculation unit determines a power correction
value 3 for adjusting power of the transmission signal, and the
multipliers 7.sub.1 to 7.sub.Nt multiply the transmission signal by
the power correction value .beta. to adjust the power.
[0037] FIG. 3 is a flowchart illustrating an operation example of
the maximum power calculation unit 8. First, the maximum power
calculation unit 8 acquires a transmission weight of each antenna,
calculated by the precoder unit 4, from the precoder unit 4 (Step
S21). Next, the maximum power calculation unit 8 calculates a power
correction value 3 based on the acquired transmission weight (Step
S22).
[0038] Now detailed description will be given for an operation in
which the maximum power calculation unit 8 calculates the power
correction value .beta. in Step S22. In Step S22, the maximum power
calculation unit 8 first calculates an average power value for each
antenna when signal streams to be transmitted respectively to all
users are combined. A transmission weight w.sub.k,f, where a stream
number is k and a frequency bin number is f in the precoder unit 4,
is represented by the following formula (1). In the following
formula (1), Nt denotes the number of transmission antennas.
[ Formula 1 ] w k , f = [ w k , f 1 w k , f 2 w k , f Nt ] ( 1 )
##EQU00001##
[0039] In the formula (1), elements of a vector are transmission
weights of the respective antennas. An average power value P.sub.m
(m=1, 2, . . . , Nt) for each antenna when signal streams to be
transmitted respectively to all users are combined can be obtained
according to the following formula (2). In the formula (2), Ns
denotes the number of signal streams to be transmitted, and Nf
denotes the number of frequency bins.
[ Formula 2 ] P m = k = 1 Ns f = 1 Nf w k , f m 2 ( 2 )
##EQU00002##
[0040] Next, the maximum power calculation unit 8 obtains the power
correction value 3 based on the average power value P.sub.m
obtained according to the above formula (2). Here, it is assumed
that in the maximum power calculation unit 8, a maximum power
threshold Th is set in advance, in consideration of nonlinear
characteristics of the HPAs provided in the antennas 10.sub.1 to
10.sub.Nt and a limit value at which calibration for an antenna
element 11 and a high-frequency circuit whose illustration is
omitted in FIG. 1 can be maintained. This maximum power threshold
Th is determined, for example, by simulation or the like at the
time of designing the radio transmission apparatus 100. The maximum
power calculation unit 8 calculates the power correction value
.beta. according to the following formula (3) using the average
power value P.sub.m of the signals inputted to the antennas
10.sub.m (m=1, 2, . . . , Nt) and the maximum power threshold Th.
In the following formula (3), Pmax denotes the maximum value among
the average power values P.sub.m of the signals inputted to the
respective antennas, which have been obtained according to the
above formula (2).
[ Formula 3 ] P max = max .A-inverted. m [ P m ] .beta. = Th / P
max ( 3 ) ##EQU00003##
[0041] Returning to the description of FIG. 3, when the maximum
power calculation unit 8 calculates the power correction value
.beta. in Step S22, the maximum power calculation unit 8 outputs
the calculated power correction value .beta. to the multipliers
7.sub.1 to 7.sub.Nt (Step S23).
[0042] The multipliers 7.sub.1 to 7.sub.Nt multiply the input
signals for the antennas 10.sub.1 to 10.sub.Nt by the power
correction value .beta. calculated by the maximum power calculation
unit 8 in Step S22 described above, and thereby the calibration
problem can be avoided without disturbing orthogonality among
users.
[0043] FIG. 4 is a flowchart illustrating an operation example of
the correction value selection unit 9. First, the correction value
selection unit 9 acquires a transmission weight of each antenna
calculated by the precoder unit 4 from the precoder unit 4 (Step
S31). Next, the correction value selection unit 9 calculates
average power values of signals respectively corresponding to
antennas based on the acquired transmission weights (Step S32).
That is, the correction value selection unit 9 calculates the
average power value P.sub.m (m=1, 2, . . . , Nt) for each antenna
when signal streams to be transmitted respectively to all users are
combined. The average power value P.sub.m for each antenna is the
same as the average power value P.sub.m calculated when the maximum
power calculation unit 8 obtains the power correction value .beta.,
and similarly to the maximum power calculation unit 8, the average
power value P.sub.m is calculated according to the above formula
(2). It should be noted that the correction value selection unit 9
may acquire the average power value P.sub.m for each antenna from
the maximum power calculation unit 8 instead of calculating the
average power value P.sub.m for each antenna based on the
transmission weight of each antenna. That is, instead of Steps S31
and S32, the correction value selection unit 9 may execute a step
of acquiring the average power value P.sub.m for each antenna from
the maximum power calculation unit 8.
[0044] After calculating the average power value P.sub.m for each
antenna, the correction value selection unit 9 then selects a
correction value to be used in a correction process for each of the
antennas 10.sub.1 to 10.sub.Nt from the correction table, based on
the average power value P.sub.m for each antenna (Step S33). That
is, the correction value selection unit 9 selects, for each of the
antennas 10.sub.1 to 10.sub.Nt, correction values respectively
corresponding to the average power values P.sub.1 to P.sub.Nt for
the respective antennas 10.sub.1 to 10.sub.Nt.
[0045] Upon selecting the correction value to be used in the
correction process for each of the antennas 10.sub.1 to 10.sub.Nt,
the correction value selection unit 9 then outputs the selected
correction values to the corresponding antennas 10.sub.1 to
10.sub.Nt, and instructs the antennas to correct amplitudes and
phases of signals inputted from the multipliers 7.sub.1 to 7.sub.Nt
(Step S34).
[0046] When signals are inputted from the multipliers 7.sub.1 to
7.sub.Nt, the antennas 10.sub.1 to 10.sub.Nt correct the amplitudes
and the phases thereof using the correction values received from
the correction value selection unit 9, and emit the resultant
signals in a space. In each array element 11 of the antennas
10.sub.1 to 10.sub.Nt, the phase shifter 11A corrects a phase of
the signal, and the HPA 11B corrects an amplitude of the signal,
that is, amplifies the signal.
[0047] In the present embodiment, the multipliers 7.sub.1 to
7.sub.Nt are provided between the parallel-to-serial conversion
units 6.sub.1 to 6.sub.Nt and the antennas 10.sub.1 to 10.sub.Nt,
the input signals for the antennas 10.sub.1 to 10.sub.Nt are
multiplied by the power correction value .beta. thereby to perform
the power adjustment. However, the position where the multipliers
7.sub.1 to 7.sub.Nt are provided may be anywhere between the
precoder unit 4 and the antennas 10.sub.1 to 10.sub.Nt.
[0048] As described above, the radio transmission apparatus 100
according to the present embodiment corrects the power of the input
signal for each antenna to be equal to or less than a prescribed
threshold based on the transmission weight of each antenna, and
corrects, for each antenna, the amplitude and the phase of the
signal to be transmitted from each antenna by using the correction
value according to the average power value of the input signal for
the antenna. As a result, it is possible to control the input
backoff of the high power amplifier. In addition, since the input
backoff of the high power amplifier can be controlled, it is
possible to solve calibration between the transmitting side and the
receiving side and calibration between the antennas, which result
in a problem for the TDD-type system. Therefore, it is possible to
provide a radio transmission apparatus capable of achieving highly
accurate user multiplex without requiring CSI feedback from the
receiving side of the spatially multiplexed signal.
[0049] Although the description has been given for the case where
the TDD-type system includes the maximum power calculation unit 8
and the correction value selection unit 9 in the present
embodiment, the maximum power calculation unit 8 and the correction
value selection unit 9 may be included in an FDD-type system, that
is, a system which calculates a transmission weight based on the
CSI in a direction from a base station to a terminal (downlink
direction), which has been fed back from the terminal. Operations
of the maximum power calculation unit 8 and the correction value
selection unit 9 in that case are equivalent to those in the
present embodiment.
[0050] A hardware configuration for realizing the radio
transmission apparatus 100 according to the present embodiment will
be described. Among the constituent elements of the radio
transmission apparatus 100, the modulation units 1.sub.1 to
1.sub.Ns can be realized by a modulator, a modem, or the like. The
serial-to-parallel conversion units 2.sub.1 to 2.sub.Ns, the FFT
units 3.sub.1 to 3.sub.Ns, the IFFT units 5.sub.1 to 5.sub.Nt, the
parallel-to-serial conversion units 6.sub.1 to 6.sub.Nt, and the
multipliers 7.sub.1 to 7.sub.Nt are realized by an electronic
circuit or circuits configured to include various kinds of logic
circuits. The antennas 10.sub.1 to 10.sub.Nt are realized by an
electronic circuit configured to include a phase shifter, an HPA,
and the like.
[0051] The precoder unit 4, the maximum power calculation unit 8,
and the correction value selection unit 9 are realized, for
example, by a processor 101 illustrated in FIG. 5 executing a
program stored in a memory 102. That is, the precoder unit 4, the
maximum power calculation unit 8, and the correction value
selection unit 9 are realized by the processor 101 reading a
program for performing operations of the precoder unit 4, the
maximum power calculation unit 8, and the correction value
selection unit 9 from the memory 102, and executing the program.
The processor 101 is a central processing unit (CPU, also referred
to as a processing unit, an arithmetic unit, a microprocessor, a
microcomputer, a processor, or a digital signal processor (DSP)),
system large scale integration (LSI), or the like. The memory 102
may be a non-volatile or volatile semiconductor memory such as a
random access memory (RAM), a read only memory (ROM), a flash
memory, an erasable programmable read only memory (EPROM), or an
electrically erasable programmable read-only memory (EEPROM), a
magnetic disk, a flexible disk, an optical disk, a compact disc, a
mini disk, a digital versatile disc (DVD), or the like. The memory
102 is used also as a storage area for the maximum power threshold
Th to be used by the maximum power calculation unit 8 to obtain the
power correction value .beta., and as a storage area for the
correction table to which reference is made by the correction value
selection unit 9 to select a correction value to be used in the
correction process for the antennas 10.sub.1 to 10.sub.Nt.
[0052] The precoder unit 4, the maximum power calculation unit 8,
and the correction value selection unit 9 may be realized by
dedicated hardware, or some of them may be realized by dedicated
hardware, and the rest thereof may be realized by software,
firmware, or a combination of software and firmware. A hardware
configuration in the case where these units are realized by
dedicated hardware is as illustrated in FIG. 6, for example. That
is, the precoder unit 4, the maximum power calculation unit 8 and
the correction value selection unit 9 are realized by a processing
circuit 110. The processing circuit 110 is an electronic circuit
which executes a process of weighting a signal to be transmitted to
the terminal, a process of calculating a power correction value for
adjusting power of a transmission signal, and a process of
selecting a correction value for correcting an input signal for an
antenna from the correction table. The processing circuit 110
corresponds to, for example, a single circuit, a composite circuit,
a programmed processor, a parallelly programmed processor, an
application specific integrated circuit (ASIC), a field
programmable gate array (FPGA), or a combination thereof.
Second Embodiment
[0053] The radio transmission apparatus according to the first
embodiment obtains the power correction value P for adjusting the
power of the input signal for each antenna to be equal to or less
than the maximum power threshold Th, based on the transmission
weight of each antenna, and adjusts the input signal for each
antenna using the power correction value .beta.. On the other hand,
in a radio transmission apparatus of the present embodiment, power
of an input signal for each antenna is limited to or below a
maximum power threshold Th by adjusting a transmission weight of
each antenna.
[0054] FIG. 7 is a diagram illustrating an example of a radio
communication system to which the radio transmission apparatus
according to the second embodiment is applied. FIG. 7 illustrates a
configuration example of MU-MIMO in which four users, that is,
terminals #1 to #4, and a base station including the radio
transmission apparatus according to the present embodiment perform
simultaneous communication. For the base station, only antennas are
illustrated. As illustrated in the figure, the base station has 16
antennas, each of the terminals #1 to #4 has four antennas, and 16
streams, in total, are spatially multiplexed and transmitted to the
terminals #1 to #4 from the base station. The base station
transmits a signal with allocating four streams to one terminal. In
that case, it is unnecessary for the base station to perform
nulling for the four streams addressed to a specific terminal, and
it is only necessary to achieve nulling for the 12 antennas
included in the other terminals.
[0055] In the case of the configuration illustrated in FIG. 7, for
example, when a transmission weight of a stream #m
(1.ltoreq.m.ltoreq.4) of a user #k (1.ltoreq.k.ltoreq.4) that is a
terminal #k is v (bold) .sub.k.sup.m, a signal weighted by the
transmission weight v (bold) .sub.1.sup.m and transmitted to a user
#1 is nulled for users #2, #3, and #4. That is, the transmission
weight addressed to the user #1 can be arbitrarily selected within
a subspace having four basis vectors v (bold) .sub.1.sup.1 to v
(bold) .sub.1.sup.4. In other words, a vector (transmission weight)
generated by a linear combination of these four vectors does not
give interference to users #2, #3, and #4. In general, the base
station calculates the transmission weight so as to achieve
eigenmode transmission to the four antennas of each user
(terminal), weights the signal for each user accordingly, and
transmits the resultant signal. Weighting of signals by
transmission weights is performed for each frequency bin, but the
notation of a frequency bin number is omitted here. When the number
of transmission antennas equipped in the base station is larger
than the number of spatially multiplexed objects, a spatial degree
of freedom, that is, the number of basis vectors, increases
accordingly. The number of spatially multiplexed objects herein
corresponds to the number of signal streams addressed to each user,
that is, the number of signal streams to be spatially
multiplexed.
[0056] In the present embodiment, what is assumed is the case where
the number of antennas included in the base station is equal to or
larger than the number of spatially multiplexed objects of the
signal streams to be transmitted to each terminal, as illustrated
in FIG. 7, and the radio transmission apparatus limits power of the
input signal for each antenna to the maximum power threshold Th or
less by adjusting the transmission weight of each antenna.
[0057] Various methods are conceivable as a method of calculating a
transmission weight for limiting the power of an input signal for
each antenna to the maximum power threshold Th or less, and an
example thereof will be described below.
[0058] FIG. 8 is a diagram illustrating a configuration example of
the radio transmission apparatus according to the second
embodiment. The radio transmission apparatus 100a of the present
embodiment is obtained by eliminating the multipliers 7.sub.1 to
7.sub.Nt from the radio transmission apparatus 100 of the first
embodiment and replacing the precoder unit 4 and the maximum power
calculation unit 8 with a precoder unit 4a and a maximum power
calculation unit 8a. The constituent elements other than the
precoder unit 4a and the maximum power calculation unit 8a are
equivalent to those of the radio transmission apparatus 100 of the
first embodiment, and thus the description thereof will be
omitted.
[0059] The maximum power calculation unit 8a of the radio
transmission apparatus 100a calculates the above-described power
correction value (by a procedure equivalent to that of the maximum
power calculation unit 8 described in the first embodiment.
Furthermore, when calculating the power correction value .beta.,
the maximum power calculation unit 8a outputs the value to the
precoder unit 4a.
[0060] As with the precoder unit 4 of the radio transmission
apparatus 100 according to the first embodiment, the precoder unit
4a calculates a transmission weight of each antenna for each signal
stream. At this time, the radio transmission apparatus 100a
cooperates with the maximum power calculation unit 8a to obtain a
transmission weight such that the power of the input signal for
each antenna becomes equal to or less than the maximum power
threshold Th. The maximum power threshold Th mentioned herein is
equal to the maximum power threshold Th described in the first
embodiment.
[0061] FIG. 9 is a flowchart illustrating an example of an
operation of calculating a transmission weight performed by the
precoder unit 4a and the maximum power calculation unit 8a in
cooperation with each other. In the calculation of the transmission
weight performed by the precoder unit 4a and the maximum power
calculation unit 8a in cooperation with each other, the precoder
unit 4a first calculates a transmission weight of each antenna
using the BD method or the like as with the precoder unit 4 of the
radio transmission apparatus 100 of the first embodiment (Step
S41). This transmission weight is called a provisional transmission
weight that is a first transmission weight. Also in a case of
calculating a transmission weight in a method other than the BD
method, a transmission weight is calculated such that the eigenmode
transmission is achieved. Next, the maximum power calculation unit
8a calculates the power correction value .beta. based on the
provisional transmission weight and the maximum power threshold Th,
and outputs the value to the precoder unit 4a (Step S42). The
average power value for each antenna calculated based on the
provisional transmission weight when the maximum power calculation
unit 8a calculates the power correction value .beta. is an average
power value of weighted signals (average power value for each
antenna) obtained when the signals inputted from FFT units 3.sub.1
to 3.sub.Ns to the precoder unit 4a are weighted with the
provisional transmission weight.
[0062] When the power correction value .beta. is inputted, the
precoder unit 4a uses this value to calculate a final transmission
weight that is a second transmission weight, that is, a
transmission weight to be used in a weighting process of a
transmission signal (Step S43).
[0063] In Step S43, the precoder unit 4a calculates the final
transmission weight of each antenna so as to satisfy a condition
represented by the following formula (4). In the formula (4), bold
w.sub.k,f with `.about.` added thereto is the final transmission
weight. In the formula (4), f denotes a frequency bin number, m
denotes an antenna number, and k denotes a user number. Q.sub.f,m
is an average power value before correction of a signal of the
frequency bin f to be inputted to an antenna m, and M.sub.max is an
antenna number of an antenna having the highest average power value
of the input signal thereof. Q.sub.f with `.about.` added thereto
is an average power value after correction of the signal of the
frequency bin f with the power correction value .beta.. The average
power value before the correction means an average power value of
the weighted signal obtained when weighting is performed with the
provisional transmission weight. u.sub.j is a weighting coefficient
for a j-th basis vector of the provisional transmission weight
calculated in Step S41, and Nr is the number of multiplexed streams
per user. When the number of transmission antennas is larger than
the number of spatially multiplexed objects, the number of basis
vectors is larger than Nr.
[ Formula 4 ] Q f , m = k = 1 Nr w k , f m 2 , Q ~ f = .beta. Q f ,
Mmax w ~ k , f = j = 1 Nr u j w k , f , provided that ( k = 1
.A-inverted. n Nr w ~ k , f n 2 .ltoreq. Q ~ f ) ( 4 )
##EQU00004##
[0064] When the final transmission weight of each antenna is
calculated in the precoder unit 4a, a precoder 41 multiplies a
signal of each frequency bin by the final transmission weight,
thereby weighting the transmission signal.
[0065] In the present embodiment, the maximum power calculation
unit 8a calculates the power correction value .beta., but the power
correction value .beta. may be calculated by the precoder unit
4a.
[0066] A correction value selection unit 9 which calculates an
average power value P.sub.m for each antenna based on the
transmission weight calculated by the precoder unit 4a calculates
the average power value P.sub.m using the above-mentioned final
transmission weight calculated by the precoder unit 4a.
[0067] As described above, the radio transmission apparatus 100a of
the present embodiment is configured to determine a transmission
weight of each antenna such that power of an input signal for each
antenna becomes equal to or less than a prescribed threshold.
Specifically, first, a transmission weight with which eigenmode
transmission is achieved is calculated and set as a provisional
transmission weight, a power correction value is calculated based
on an average power value of an input signal for each antenna which
can be calculated from the provisional transmission weight and a
threshold determined in advance, and a final transmission weight is
calculated in consideration of the power correction value. By doing
so, a maximum power value can be limited without disturbing
orthogonality among the users, and an influence on a transmission
signal, that is, an influence of an HPA that is a nonlinear
amplifier and a calibration error can be minimized.
[0068] A hardware configuration for realizing the radio
transmission apparatus 100a is equivalent to that of the radio
transmission apparatus 100 according to the first embodiment.
[0069] The configuration described in the embodiments above
indicates one example of the content of the present invention and
can be combined with other publicly known techniques, and a part
thereof can be omitted or modified without departing from the gist
of the present invention.
REFERENCE SIGNS LIST
[0070] 1.sub.1 to 1.sub.Ns modulation unit; 2.sub.1 to 2.sub.Ns
serial-to-parallel conversion unit (S/P unit); 3.sub.1 to 3.sub.Ns
FFT unit; 4, 4a precoder unit; 5.sub.1 to 5.sub.Nt IFFT unit;
6.sub.1 to 6.sub.Nt parallel-to-serial conversion unit (P/S unit);
7.sub.1 to 7.sub.Nt multiplier; 8, 8a maximum power calculation
unit; 9 correction value selection unit; 10.sub.1 to 10.sub.Nt
antenna; 11 array element; 11A phase shifter; 11B HPA (High Power
Amplifier); 41 precoder; 100, 100a radio transmission apparatus;
101 processor; 102 memory; 110 processing circuit.
* * * * *