U.S. patent application number 14/781763 was filed with the patent office on 2016-03-03 for radio communication system, radio base station apparatus, and user equipment.
This patent application is currently assigned to NTT DOCOMO, INC.. The applicant listed for this patent is NTT DOCOMO, INC.. Invention is credited to Yuichi Kakishima, Yoshihisa Kishiyama, Satoshi Nagata.
Application Number | 20160065388 14/781763 |
Document ID | / |
Family ID | 51658109 |
Filed Date | 2016-03-03 |
United States Patent
Application |
20160065388 |
Kind Code |
A1 |
Kakishima; Yuichi ; et
al. |
March 3, 2016 |
RADIO COMMUNICATION SYSTEM, RADIO BASE STATION APPARATUS, AND USER
EQUIPMENT
Abstract
A radio communication system includes a radio base station
having multiple antenna elements arranged two-dimensionally, and a
user equipment configured to perform radio communication with the
radio base station. The radio base station is configured to
transmit a reference signal used for channel estimation in two or
more different configurations using a part or all of the antenna
elements, and wherein the user equipment is configured to feed back
at least one of horizontal channel information, vertical channel
information, and inter-polarization channel information to the
radio base station.
Inventors: |
Kakishima; Yuichi; (Tokyo,
JP) ; Nagata; Satoshi; (Tokyo, JP) ;
Kishiyama; Yoshihisa; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NTT DOCOMO, INC. |
Tokyo |
|
JP |
|
|
Assignee: |
NTT DOCOMO, INC.
Tokyo
JP
|
Family ID: |
51658109 |
Appl. No.: |
14/781763 |
Filed: |
March 3, 2014 |
PCT Filed: |
March 3, 2014 |
PCT NO: |
PCT/JP2014/055294 |
371 Date: |
October 1, 2015 |
Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04B 7/10 20130101; H04L
5/0053 20130101; H04B 7/0478 20130101; H04B 7/0626 20130101; H04B
17/391 20150115; H04B 7/065 20130101; H04B 7/0469 20130101; H04B
7/0413 20130101; H04W 72/04 20130101; H04B 7/063 20130101; H04B
7/0641 20130101; H04L 25/0202 20130101 |
International
Class: |
H04L 25/02 20060101
H04L025/02; H04L 5/00 20060101 H04L005/00; H04B 7/04 20060101
H04B007/04; H04W 72/04 20060101 H04W072/04; H04B 7/06 20060101
H04B007/06 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 5, 2013 |
JP |
2013-079277 |
Claims
1. A radio communication system comprising: a radio base station
having multiple antenna elements arranged two-dimensionally; and a
user equipment configured to perform radio communication with the
radio base station, wherein the radio base station is configured to
transmit a reference signal used for channel estimation in two or
more different configurations using a part or all of the antenna
elements, and wherein the user equipment is configured to feedback
at least one of horizontal channel information, vertical channel
information, and inter-polarization channel information to the
radio base station.
2. The radio communication system according to claim 1, wherein the
radio base station transmits the reference signal in configurations
for channel estimation in a horizontal direction, in a vertical
direction, and between polarizations using a part of the antenna
elements including a reference antenna element, and wherein the
user equipment is configured to feedback the horizontal channel
information, the vertical channel information, and the
inter-polarization channel information relative to the reference
antenna element.
3. The radio communication system according to claim 1, wherein the
radio base station is configured to change over a group of the
antenna elements used to transmit the reference signal in terms of
time or frequency, based upon the different configurations.
4. The radio communication system according to claim 3, wherein the
radio base station is configured to prioritize a less frequent
transmit configuration over other configurations when transmission
timings of the two or more different configurations come into
collision.
5. The radio communication system according to claim 1, wherein the
user equipment is configured to feedback the horizontal channel
information and the vertical channel information using different
methods according to the referenced signal transmitted in the
different configurations.
6. The radio communication system according to claim 1, wherein the
user equipment is configured to create the horizontal channel
information and the inter-polarization channel information based
upon the reference signal transmitted in the different
configurations, then create a matrix in which the horizontal
channel information and the inter-polarization channel information
are incorporated in the vertical channel information, and feed back
the matrix to the radio base station.
7. The radio communication system according to claim 1, wherein the
user equipment is configured to select existing channel information
designed for a one-dimensional antenna array based upon the
reference signal transmitted in the different configurations,
create a matrix in which the existing channel information is
incorporated in the vertical channel information, and feed back the
matrix to the radio base station.
8. The radio communication system according to claim 1, wherein the
user equipment is configured to select existing channel information
designed for a one-dimensional antenna array based upon the
reference signal transmitted in the different configurations, and
feed back the selected existing channel information to the radio
base station.
9. A radio base station apparatus comprising: a plurality of
antenna elements arranged two-dimensionally; a signal generator
configured to generate a reference signal used for channel
estimation; a controller configured to select a part or all of the
antenna elements to cause the reference signal to be transmitted in
two or more different configurations including at least one of a
horizontal relationship, a vertical relationship, and an
inter-polarization relationship; and a transmitter configured to
transmit the reference signal in the different configurations based
upon an output from the controller.
10. The radio base station apparatus according to claim 9, wherein
the controller is configured to set up configurations for channel
estimation in a horizontal direction, in a vertical direction, and
between polarizations using a part of the antenna elements
including a reference antenna element, and the transmitter is
configured to transmit the reference signal in the configurations
set up by the controller.
11. The radio base station apparatus according to claim 9, wherein
the controller is configured to change over a group of the antenna
elements used to transmit the reference signal in terms of time or
frequency, based upon the different configurations.
12. The radio base station apparatus according to claim 11, wherein
the controller is configured to prioritize a less frequent transmit
configuration over other configurations when transmission timings
of the two or more different configurations come into
collision.
13. A user equipment comprising: a receiver configured to receive a
reference signal transmitted from a radio base station in two or
more different configurations; a feedback information generator
configured to generate feedback information including at least one
of horizontal channel information, vertical channel information,
and inter-polarization channel information, based upon the received
reference signal; and a transmitter configured to transmit the
feedback information to the radio base station.
14. The user equipment according to claim 13, wherein the feedback
information generator is configured to generate the horizontal
channel information and the vertical channel information as
different types of feedback information according to the different
configurations.
15. The user equipment according to claim 13, wherein the feedback
information generator is configured to create the horizontal
channel information and the inter-polarization channel information
based upon the reference signal transmitted in the different
configurations, and create a matrix in which the horizontal channel
information and the inter-polarization channel information are
incorporated in the vertical channel information, and wherein the
transmitter is configured to transmit the matrix or information
representing the matrix to the radio base station.
16. The user equipment according to claim 13, wherein the feedback
information generator is configured to select existing channel
information designed for a one-dimensional antenna array based upon
the reference signal transmitted in the different configurations,
and create a matrix in which the existing channel information is
incorporated in the vertical channel information, and wherein the
transmitter is configured to feed back the matrix or information
representing the matrix to the radio base station.
17. The user equipment according to claim 13, wherein the feedback
information generator is configured to select existing channel
information designed for a one-dimensional antenna array based upon
the reference signal transmitted in the different configurations,
and wherein the transmitter is configured to feed back the selected
existing channel information to the radio base station.
Description
TECHNICAL FIELD
[0001] The present invention relates to a radio communication
technology, and more particularly, to a three-dimensional multiple
input, multiple output (3D-MIMO) radio communication system, a
radio base station apparatus, and user equipment.
BACKGROUND ART
[0002] Long term evolution standardization in Third Generation
Partnership Project (which may be referred to as "3GPP LTE")
Release 8 to Release 11 have adopted horizontal beamforming using
linear array antenna elements arranged in the lateral
dimension.
[0003] In 3GPP LTE Release 12, a 3D-MIMO beamforming scheme that
uses a number of antenna elements arranged two-dimensionally in the
vertical and the lateral directions is being discussed. This new
technology allows a base station to perform vertical beamforming in
addition to horizontal beamforming. See, for example, non-Patent
documents 1 and 2 listed below. By providing directionality in both
the vertical (or elevation) and the horizontal (or azimuth)
directions, improvement of system characteristics is expected.
[0004] Up to the 3GPP LTE Release 11, closed loop MIMO precoding
has been achieved by returning channel state information (CSI) in
the horizontal direction and between polarized antenna elements
back to a MIMO base station. In order to reduce the CSI overhead, a
codebook describing a set of precoding matrices (linear filters) is
shared between the base station and user equipment. The user
equipment selects a desired precoding matrix from the codebook and
reports the selected matrix index together with the channel quality
indicator (CQI) to the base station. The base station applies the
precoding matrix to transmission data based upon the feedback
information supplied from the user equipment.
[0005] For downlink MIMO transmission, codebooks designed for 8
transmit antennas have come to agreement. See, for example,
non-patent Document 3 listed below.
LIST OF PRIOR ART DOCUMENTS
[0006] Document 1: "Study on Downlink Enhancement for Elevation
Beamforming for LTE," 3GPP TSG RAN#58, RP-121994 [0007] Document 2:
"New SID proposal: Study on Full Dimension for LTE," 3GPP TSG
RAN#58, RP-122015 [0008] Document 3: "Way Forward on 8Tx Codebook
for Rel. 10 DL MIMO," 3GPP TSG RAN WG1 Meeting #62, R1-105011
SUMMARY OF THE INVENTION
Technical Problem to be Solved
[0009] In 3D-MIMO, the transmission side needs to acquire channel
information in the vertical direction (which may be referred to as
"horizontal channel information), in addition to channel
information in the horizontal direction (which may be referred to
as "vertical channel information") and channel information between
polarized antenna elements (which may be referred to as
"inter-polarization channel information"). One of the important
issues is how to reduce the overhead in transmitting reference
signals (RSs) and feedback (FD) information.
[0010] For full-dimension MIMO (FD-MIMO) using antennas more than
8, it is forecast that the number of antenna elements will be
several tens to a hundred or more. It is strongly desired to
realize an efficient way of transmitting and receiving necessary
information for beamforming.
[0011] One of the objectives of the invention is to reduce at least
one of transmission overhead of reference signals and transmission
overhead of feedback information.
Means for Solving the Problem
[0012] To achieve the above-described objective, the following
techniques may be conceived.
(a) Reducing the RS overhead at a sender side by controlling
configuration of RS transmission systems, such as selection of
antenna elements used for RS transmission or RS transmission
timing. (b) Creating feedback information which includes vertical
channel information in an efficient manner.
[0013] In this context, "configuration" of RS transmission systems
includes configuration control information specifying a
multiplexing position of a reference signal in terms of time and/or
frequency, a reference signal transmission period, a combination of
antenna elements and/or a transmission sequence used for RS
transmission, etc.
[0014] In one aspect of the invention, a radio communication system
includes
[0015] a radio base station having multiple antenna elements
arranged two-dimensionally; and
[0016] a user equipment configured to perform radio communication
with the radio base station,
[0017] wherein the radio base station is configured to transmit a
reference signal used for channel estimation in two or more
different configurations using a part or all of the antenna
elements, and
[0018] wherein the user equipment is configured to feedback at
least one of horizontal channel information, vertical channel
information, and inter-polarization channel information to the
radio base station.
Advantageous Effect of the Invention
[0019] At least one of reference signal transmission overhead and
feedback information transmission overhead can be reduced in a
3D-MIMO radio communication system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic diagram of a radio communication
system according to the embodiment;
[0021] FIG. 2 is a diagram explaining the relationship between
two-dimensionally arranged antenna elements;
[0022] FIG. 3 illustrates examples of selection of antenna elements
for RS transmission;
[0023] FIG. 4 illustrates an example of transmission of reference
signals;
[0024] FIG. 5 illustrates another example of transmission of
reference signals;
[0025] FIG. 6 illustrates still another example of transmission of
reference signals;
[0026] FIG. 7 illustrates an information feedback technique from
user equipment to a base station;
[0027] FIG. 8 is an exemplified block diagram of a radio base
station; and
[0028] FIG. 9 is an exemplified block diagram of user
equipment.
EMBODIMENTS TO CARRY OUT THE INVENTION
[0029] FIG. 1 is a schematic diagram of a radio communication
system 1 according to the embodiment. The radio communication
system 1 includes a radio base station 10 and user equipment 20-1
and 20-2. In this example, multi-user MIMO (MU-MIMO) is employed to
spatially multiplex multiple signals transmitted from the radio
base station 10 to the user equipment 20-1 and 20-2.
[0030] The radio base station 10 has an antenna array 11 in which
antennas are arranged two-dimensionally in rows and columns. The
radio base station 10 uses a part or all of the antennas in the
antenna array 11 when transmitting a reference signal (RS) as
indicated by the arrow (1). The reference signal is received at the
user equipment 20-1 and 20-2, respectively, and used for channel
estimation. Detailed techniques of transmitting the reference
signal from the antenna array 11 will be explained later.
[0031] Each of the user equipment 20-1 and 20-2 feeds back channel
state information (CSI) estimated from the received reference
signal to the radio base station 10 as indicated by the arrow
(2).
[0032] The radio base station 10 creates transmission precoding
weights for the user equipment 20-1 and 20-2 using the feedback
information so as to suppress mutual interference between the user
equipment 20-1 and 20-2. The radio base station 10 performs
beamforming for transmitting data signals addressed to the user
equipment 20-1 and 20-2 and the reference signal for channel
estimation, and transmits the data signals as indicated by the
arrow (3).
[0033] The precoding vectors, i.e., the beamforming weight vectors
may be calculated at the radio base station 10 based upon the CSI
fed back from the user equipment 20-1 and 20-2. The calculated
precoding vectors may be reported to the user equipment 20-1 and
20-2. As an alternative, the user equipment 20-1 and 20-2 may
calculate the precoding vectors, respectively, based upon the
estimated channel information or channel matrixes. In this case,
the estimated precoding vectors may be fed back to the radio base
station 10. As another alternative, the radio base station 10 and
the user equipment 20-1 and 20-2 may share a common codebook
describing a set of precoding matrices. In this case, each of the
user equipment 20-1 and 20-2 may select a desired precoding vector
from the codebook based upon the estimated channel matrix.
[0034] As illustrated in the bottom part (B) of FIG. 1, the radio
base station 10 with the two-dimensionally arranged antenna array
11 is able to perform vertical or elevation beamforming, in
addition to horizontal or azimuth beamforming.
[0035] FIG. 2 illustrates several antenna configurations at the
radio base station 10. In example (A), eight antennas are arranged
in two rows and four columns. Each of the antenna #0 to antenna #7
may serve as an individual antenna element, or alternatively, two
adjacent antennas (e.g., antenna #0 and antenna #1) allocated to
mutually orthogonal polarizations may form one antenna element.
[0036] In example (B), four antenna elements are arranged in a
matrix, each antenna element including two antennas corresponding
to orthogonal polarizations. In example (C), four antennas are
arranged in a matrix. In example (D), two antenna elements are
arranged in the vertical direction, each of the antenna elements
including two antennas corresponding to orthogonal
polarizations.
[0037] When performing vertical or elevation beamforming using the
two-dimensionally arranged antenna array 11, vertical-direction
channel information is necessary, in addition to channel
information in the horizontal direction and channel information
between polarizations. The higher the number of antennas used for
RS transmission, the more serious is the problem of overhead of the
reference signal transmission from the radio base station 10.
[0038] One method for reducing transmission overhead is that the
radio base station 10 reduces the number of antenna ports used for
transmitting reference signals, compared with the actual number of
antenna ports, when transmitting the reference signals. It is
unnecessary to know all the relationships between antennas of the
two-dimensional array when carrying out beamforming; rather, only a
limited number of the antennas is needed. Important factors in the
inter-antenna relationship are channel information in the
horizontal direction, channel information in the vertical
direction, and channel information between polarizations. Channel
information includes, for example, absolute or relative values of
the amount of phase rotation, variation in amplitude, or other
parameters.
[0039] For example, with the antenna arrangement of example (B) of
FIG. 2, high-value antenna combinations are {0, 1}, {2, 3}, {4, 5},
and {6, 7} in the horizontal relationship; {0, 4}, {1, 5}, {2, 6},
and {3, 7} in the vertical relationship; and {0, 2}, {1, 3}, {4,
6}, and {5, 7} in the inter-polarization relationship. Other
antenna combinations are not so important. Accordingly, in the
embodiment, reference signals are transmitted using a part of the
antenna groups (combinations) of the two-dimensional antenna
array.
[0040] Although FIG. 2 illustrates examples of antenna
configurations with eight or less antennas, the same applies to the
case using more than 8 antennas. In FD-MIMO, a number of antennas
(greater than 8) are used to form a sharp beam for data
transmission, thereby improving the frequency gain. In contrast,
for transmission of reference signals, only high-value antenna
combinations may be used to reduce overhead.
[0041] FIG. 3 illustrates examples of selection of antennas for
transmission of reference signals. In examples (A) through (C) of
FIG. 3, a part of antennas of the 4.times.4 antenna array is used
for transmission of reference signals. In example (A), the antennas
#0, #1, #2, and #3 positioned at the center of the antenna array
are used. Assuming that the antenna #0 is the reference antenna,
then a phase difference or an amplitude difference in channels
between horizontally adjacent antennas can be estimated between
antenna #0 and antenna #1. A phase difference or an amplitude
difference in channels between vertically adjacent antennas can be
estimated between antenna #0 and antenna #2. A phase difference or
an amplitude difference in channels between the reference antenna
#0 and other antennas positioned outside the antennas #0 through #3
can be estimated by extrapolation.
[0042] In example (B) of FIG. 3, antennas #0, #1, #2 and #3
positioned at the corners of the antenna array are used for
transmission of reference signals. Between the reference antenna #0
and antenna #1 positioned at the horizontal corner with respect to
the reference antenna #0, a phase difference or an amplitude
difference in channels exists between horizontally arranged
antennas. Between the reference antenna #0 and antenna #2
positioned at the vertical corner with respect to the reference
antenna #0, a phase difference or an amplitude difference of
channels exists between vertically arranged antennas. A phase
difference or an amplitude difference in channels between the
reference antenna #0 and other antennas positioned inside the
antennas #0 through #3 can be estimated by interpolation.
[0043] In example (C) of FIG. 3, a row (or a horizontal line) and a
column (or a vertical line) of the antenna array are selected. The
antenna #0 positioned at the intersection between the selected row
and column is the reference antenna. A phase difference or an
amplitude difference in channels between horizontally arranged
antennas can be estimated from the mutual relationship between the
antennas #1, #0, #2 and #3 of the selected row. A phase difference
or an amplitude difference in channels between vertically arranged
antennas can be estimated from the mutual relationship between the
antennas #4, #0, #5 and #6 of the selected column.
[0044] In example (D) of FIG. 3, eight antennas are arranged, which
antennas form 4 polarization antenna elements. With antenna #0 as
the reference antenna, a phase difference (or an amplitude
difference) in channels between horizontally arranged antennas can
be estimated between the reference antenna #0 and antenna #1. A
phase difference or an amplitude difference between polarizations
can be estimated between the reference antenna #0 and antenna #2. A
phase difference (or an amplitude difference) in channels between
vertically arranged antennas can be estimated between the reference
antenna #0 and antenna #3.
[0045] The radio base station 10 transmits a reference signal using
only a part of the antennas, thereby reducing downlink resource
consumption. The user equipment 20-1 and 20-2 report channel
information relative to the reference antenna #0 to the radio base
station 10, thereby reducing the feedback overhead. The reference
antenna #0 may be determined in advance by selecting a suitable
antenna. As an alternative, the reference antenna #0 may be
configured semi-statically with a higher layer.
[0046] There is no limitation to the types of reference signals to
be transmitted. A channel status information reference signal
(CSI-RS), a cell-specific reference signal (CRS), a demodulation
reference signal (DRS), or a newly provided reference signal may be
transmitted as the reference signal.
[0047] FIG. 4 through FIG. 7 illustrate examples of transmission of
reference signals. In the example of FIG. 4, horizontal channel
estimation, vertical channel estimation, and inter-polarization
channel estimation are performed using two or more RS sequences.
CSI-RS configuration information supplied from an higher layer may
be used to perform horizontal, vertical, and inter-polarization
channel estimations.
[0048] For horizontal channel estimation, a reference signal is
transmitted in horizontal configuration C1. For inter-polarization
channel estimation, a reference signal is transmitted in
polarization configuration C2. For vertical channel estimation, a
reference signal is transmitted in vertical configuration C3.
[0049] In view of the conditions in a cell, channel fluctuation in
the vertical direction is generally slow and gentle, compared with
that in the horizontal direction. Therefore, the resource
allocation rate for RS transmission with the horizontal
configuration C1 is set the greatest. The resource allocation rate
for RS transmission with the polarization configuration C2 is set
the second greatest. The resource allocation rate for RS
transmission with the vertical configuration C3 is set smaller than
other configurations (C1 and C2 in this example).
[0050] The configuration group {C1, C2, C3} used for the RS
transmission may be instructed to be used by the higher layer.
Although three configurations are used in FIG. 4, the invention is
not limited to these examples and only two configurations may be
used. For example, a combination of antennas #0, #1, and #2 of
pattern (D) of FIG. 3 may be used as the first configuration C1,
which configuration may be named "horizontal-and-polarization
configuration". The vertical configuration C3 of FIG. 4 may be used
as the second configuration.
[0051] The user equipment 20-1 and 20-2 that received the
referenced signal in two or more configurations may employ
different feedback schemes corresponding to different
configurations. For example, a horizontal channel estimate value
and an inter-polarization channel estimate value (or precoding
vectors selected based upon these channel estimate values) may be
reported as feedback information to the radio base station 10 by
means of LTE 4 Tx codebooks. A vertical channel estimate value or
the precoding vector selected based upon the channel estimate value
may be fed back to the radio base station 10 by another codebook or
a precoded reference signal index (such as precoded CSI-RS index).
The radio base station 10 may designate and report the feedback
scheme to be used to the user equipment 20-1 and 20-2.
[0052] When transmitting a reference signal in two or more
configurations, the radio base station may dynamically change over
antenna groups used for multiplexing the reference signal.
[0053] FIG. 5 and FIG. 6 illustrate examples of changing over
antennas to which CSI-RS is mapped along the time axis. In the
previous example of FIG. 4, RS transmission overhead is reduced by
transmitting reference signals in two or more RS configurations
using a part of antenna elements. The overhead may also be reduced
by dynamically changing over antenna elements to which the
reference signal is multiplexed. In FIG. 5, a CIS-RS is transmitted
every 10 milliseconds in horizontal configuration C1, and
transmitted every 20 milliseconds in polarization configuration C2
and in vertical configuration C3.
[0054] The configuration to be used and the transmission period may
be reported from the radio base station 10 to the user equipment
20-1 and 20-2. The user equipment 20-1 and 20-2 may carry out
independent feedback operations for the respective
configurations.
[0055] FIG. 6 illustrates another example of changeover of RS
configurations. In FIG. 6, a reference signal is transmitted every
5 milliseconds in horizontal-and-polarization configuration C5, and
this configuration C5 is overridden by another configuration C6
every 20 milliseconds. The reference signal transmitted in
configuration C6 is used vertical channel estimation and
inter-polarization channel estimation. The overriding period (20
milliseconds in this example) may be reported from the radio base
station 10 to the user equipment 20-1 and 20-2.
[0056] When two or more configurations come into collision, several
countermeasures may be taken.
(1) More frequent RS (transmitted in configuration C5 in this
example) is overridden or prioritized over by less frequency RS
(transmitted in configuration C6 in this example). (2)
Configuration indexes are determined in advance, and the
transmission priority is determined according to the indexes. (3)
Priority order is reported.
[0057] Although in FIG. 6 the reference signal is transmitted every
20 milliseconds in configuration C6 for vertical and
inter-polarization channel estimations, the invention is not
limited to this example. Another configuration for horizontal,
vertical, and inter-polarization channel estimations may be used in
place of C6. In this case, the horizontal channel estimate value
and the inter-polarization channel estimate value may be fed back
by an LTE 4 Tx codebook, and the horizontal, vertical, and
inter-polarization channel estimate values may be fed back by an
LTE 8 Tx codebook.
[0058] Although changeover of RS transmission configurations in
time is illustrated in FIG. 5 and FIG. 6, the invention is not
limited to these examples. Reference signals may be multiplexed in
different configurations along the frequency axis. Multiplexing
granularity may be selected arbitrarily, for example, on the
sub-band basis, sub-carrier basis, or other frequency basis.
[0059] Next, a novel feedback method that enables reducing overhead
is explained.
[0060] FIG. 7 is a diagram explaining the feedback method according
to the embodiment. In the example of top part (A) of FIG. 7, 24
antenna ports, each antenna port corresponding to one antenna, are
arranged two-dimensionally. The phase rotation of channels between
horizontally adjacent antenna #0 and antenna #1 is .theta.1. The
phase rotation of channels between antenna #3 and antenna #4
serving as polarization elements is .theta.2. The phase rotation of
channels between vertically adjacent antenna #0 and antenna #2 is
.theta.3.
[0061] As illustrated in bottom part (B) of FIG. 7, channel
information in the vertical direction is incorporated in the
precoding vector W which is to be fed back to the radio base
station 10.
[0062] First, matrix X including phase rotation (or a phase
difference) .theta.1 between horizontally adjacent antennas is
created by formula (1).
X=[e.sup.j0.theta..sup.1 e.sup.j1.theta..sup.1 . . .
e.sup.jm.theta..sup.1]T (1)
where m denotes the number of antennas in the horizontal direction,
and T denote transposed matrix.
[0063] Then, matrix Y including phase rotation (or a phase
difference) .theta.2 between polarizations is created by formula
(2).
Y = [ X j.theta. 2 X ] ( 2 ) ##EQU00001##
The previously created matrix X representing the channel
information in the horizontal direction is incorporated in this
matrix Y.
[0064] Then, matrix W containing a phase rotation (or a phase
difference) .theta.3 between vertically adjacent antennas is
created by formula (3).
W=[e.sup.j0.theta..sup.3Y e.sup.j1.theta..sup.3Y . . .
e.sup.jn.theta..sup.3Y]T (3)
where n denotes the number of antennas in the vertical direction,
and T denotes transposed matrix. This matrix W incorporates the
matrix Y in which matrix X has been incorporated.
[0065] By simply transmitting the matrix W to the radio base
station 10, channel information between horizontally adjacent
antennas, channel information between polarization, and channel
information between vertically adjacent antennas can be fed back to
the radio base station 10. Consequently, overhead of feedback
information can be efficiently reduced.
[0066] The creation of the feedback information exemplified in FIG.
7 is for rank-1 transmission. By using a matrix orthogonal to the
matrix W, the feedback information creation scheme can be extended
to rank-2 or higher-rank transmission. In place of the relative
value .theta. of phase rotation, other values may be used as the
channel information. For example, an absolute value of phase
rotation, a relative or absolute value of amplitude fluctuation, or
a long-period or short-period fluctuation characteristic may be
used as the channel information.
[0067] In place of the scheme illustrated in FIG. 7, the existing
2D-MIMO codebooks may be extended to 3D-MIMO. In this case, the
extended vector W is expressed by formula (4).
W=[e.sup.j0.theta..sup.3X e.sup.j1.theta..sup.3X . . .
e.sup.jn.theta..sup.3X]T (4)
where n denotes the number of antennas in the vertical direction,
.theta.3 denotes a phase rotation (or phase difference) between
vertically adjacent antennas, and X denotes a codebook achieving
2D-MIMO. As the codebook for achieving 2D-MIMO, existing 2 Tx
codebooks, 4 Tx codebooks, 8 Tx codebooks, codebooks discussed in
LTE Release 12 DL-MIMO, or codebooks used in uplink transmission
may be used.
[0068] Although Formula (4) is for rank-1 transmission, it can be
extended to rank-2 or higher-rank transmission by using a matrix
orthogonal to the matrix W.
[0069] As still another scheme, existing 2D-MIMO codebooks may be
used as they are. Since 2D-MIMO codebooks are not designed for
including three factors, namely, horizontal channel information,
vertical channel information, and inter-polarization channel
information, the accuracy may be slightly degraded. However, using
2D-MIMO codebooks is still an effective method because the channel
fluctuation in the vertical direction is gentle and because there
is little impact on the exiting specification.
[0070] It is assumed that time selectivity and frequency
selectivity vary among the horizontal direction, the vertical
direction, and inter-polarization. Accordingly, temporal feedback
frequency and/or transmission bandwidth of feedback information may
be appropriately selected.
[0071] For example, in view of the facts that elevation beamforming
undergoes less channel fluctuation and that the beam pattern is
unlikely to be changed frequency because of less necessity for
avoiding inter-cell interference, feedback may be carried out less
frequently and at a broadband to reduce overhead.
[0072] Feedback may be carried out dynamically at a lower layer or
semi-statically at a higher layer, or alternatively, dynamic
feedback at a lower layer and semi-static feedback at a higher
layer may be combined. The number of bits for feedback information
in the horizontal direction, the vertical direction, and
inter-polarization may be the same or different from one
another.
[0073] The values of phase rotation (or phase difference) in the
horizontal direction, the vertical direction, and
inter-polarization may not be always acceptable. Especially, the
phase rotation .theta.3 in the vertical direction physically
corresponds to a tilt angle, as illustrated in the bottom part (B)
of FIG. 1. It may be effective to limit the selectable range of the
.theta.3 value. Several candidates for --83 may be set at each
radio base station 10. The candidate values may be reported by a
higher layer, and an appropriate value may be selected dynamically
at a lower layer for changing over the value.
[0074] FIG. 8 is a schematic diagram of the radio base station 10
used in the radio communication system 1 of FIG. 1. The radio base
station 10 has multiple antennas 11-1 to 11-N. Radio frequency (RF)
transmission circuits 16-1 to 16-N and RF receiving circuits 17-1
to 17-N are provided according to the number of antennas 21-1 to
21-N.
[0075] A reference signal generator 13 generates a reference signal
used for channel estimation. A precoding weight generator 19
generates a precoding weight vector based upon received feedback
information supplied via the RF receiving circuits 17-1 through
17-N. A precoding weight multiplier 14 performs precoding on the
reference signal and data signal using the generated precoding
weight vector.
[0076] The data signal input to the precoding weight multiplier 14
has been subjected to serial-to-parallel conversion, channel
coding, and modulation. Since these operations are not directly
related to the invention, processing blocks are omitted in the
figure.
[0077] A multiplexer (MUX) 15 multiplexes the precoded reference
signal and data signal. An RS configuration controller 18 controls
selection and changeover of configurations of reference signal (RS)
transmission. The RS configuration controller 18 controls
allocation of resources to different RS configurations, as
illustrated in FIG. 4. Alternatively, the RS configuration
controller 18 controls transmission timings or overriding timings
of RF configurations, as illustrated in FIG. 5 or FIG. 6. Under
such control, the reference signal is multiplexed to a sequence
corresponding the RS configuration to be used. The multiplexed
signal is transmitted via the RF transmission circuits 16-1 through
16-N and duplexers 12-1 through 12-N.
[0078] A feedback signal from user equipment 20 is received at
antennas 11-1 through 11-n, and supplied via the duplexers 12-1
through 12-N and the RF receiving circuits 17-1 through 17-N to a
feedback control information demodulator 31. Demodulation results
are supplied from the feedback control information demodulator 31
to the precoding weight generator 19, and a precoding weight vector
is generated according to the feedback information. Channel
estimation (performed by a channel estimator 32) based upon the
reference signal, demodulation of data signal (performed by a data
channel signal demodulator 33), or decoding operations are not
directly related to the invention, and explanation is omitted.
[0079] FIG. 9 is a schematic block diagram of user equipment 20.
The user equipment 20 receives the reference signal transmitted
from the radio base station 10 at RF receiving circuits 24-1 to
24-M through antennas 21-1 to 21-M and duplexers 22-1 to 22-M. A
channel estimator 25 estimates a channel based upon the received
reference signal. A precoding weight selector 27 selects a
precoding weight vector according to the channel estimation value.
A channel quality (CQI) measurement unit 26 measures a channel
quality based upon the received reference signal.
[0080] The channel quality measurement result and the selection
result of the precoding weight vector are supplied to a feedback
control signal generator 28. The feedback control signal generator
28 generates a feedback signal to be reported to the radio base
station 10. The feedback signal may include a precoding matrix W
expressed by formula (3), containing channel information of the
horizontal direction, the vertical direction, and
inter-polarization. The feedback signal may also include a
precoding matrix W expressed by formula (4), extending an existing
2D-MIMO codebook in the vertical direction. Alternatively, the
feedback signal may include an existing 2D-MIMO codebook.
[0081] A user reference signal and a user data signal are precoded
at a precoding weight multiplier 31, and supplied to a multiplexer
(MUX) 32. The multiplexer 32 multiplexes the precoded user
reference signal and user data signal to form the generated
feedback signal. The multiplexed signal is transmitted from
antennas 21-1 to 21-M, via the RF transmission circuits 23-1 to
23-M and duplexers 22-1 to 22-M.
[0082] With the above-described method and structure, transmission
overhead of channel-estimation reference signals can be reduced in
a 3D-MIMO radio communication system employing elevation
beamforming. Transmission overhead is also reduced in feedback of
channel characteristics.
[0083] Although in the embodiment explanation is made of downlink
3D-MIMO transmission, the invention is also applicable to uplink
transmission. If a mobile terminal (user equipment) has a
two-dimensional antenna array, the above-described reference signal
configurations or the feedback schemes can be applied. Especially
in a time division duplexing (TDD) system, three-dimensional
channel estimation can be performed based upon the receiving
results of the downlink reference signal, and uplink beamforming
making use of uplink-downlink duality.
[0084] Regarding phase rotation, discrete Fourier transform (DFT)
codebooks on the assumption of correlation antennas, or Householder
codebooks on the assumption of uncorrelated antennas may be
used.
[0085] This patent application is based upon and claims the benefit
of the priority of Japanese Patent Application No. 2013-079277
filed Apr. 5, 2013, which is incorporated herein by references in
its entirety.
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