U.S. patent application number 15/126162 was filed with the patent office on 2017-04-13 for beam selecting method, base station, 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, Chongning Na, Satoshi Nagata.
Application Number | 20170104517 15/126162 |
Document ID | / |
Family ID | 54144062 |
Filed Date | 2017-04-13 |
United States Patent
Application |
20170104517 |
Kind Code |
A1 |
Kakishima; Yuichi ; et
al. |
April 13, 2017 |
BEAM SELECTING METHOD, BASE STATION, AND USER EQUIPMENT
Abstract
A beam selection method in a mobile communication system
including a base station with multiple antennas and user equipment
conducting radio communication with the base station includes the
steps of, at the base station, detecting a direction in which the
user equipment is located, transmitting precoded reference signals
toward the detected direction by spatial multiplexing using same
frequency and time resources, and determining a beam for the user
equipment based upon feedback information from the user
equipment.
Inventors: |
Kakishima; Yuichi; (Tokyo,
JP) ; Nagata; Satoshi; (Tokyo, JP) ;
Kishiyama; Yoshihisa; (Tokyo, JP) ; Na;
Chongning; (Beijing, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NTT DOCOMO, INC. |
Tokyo |
|
JP |
|
|
Assignee: |
NTT DOCOMO, INC.
Tokyo
JP
|
Family ID: |
54144062 |
Appl. No.: |
15/126162 |
Filed: |
November 27, 2014 |
PCT Filed: |
November 27, 2014 |
PCT NO: |
PCT/JP2014/081418 |
371 Date: |
September 14, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 7/0697 20130101;
H04B 7/0617 20130101; H04B 7/0619 20130101; H04B 7/0695 20130101;
H04W 16/28 20130101; H04B 7/0456 20130101; H04B 7/10 20130101; H04B
7/0626 20130101; H04B 7/0684 20130101 |
International
Class: |
H04B 7/04 20060101
H04B007/04; H04B 7/06 20060101 H04B007/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 20, 2014 |
JP |
2014-059181 |
Claims
1. A beam selection method in a mobile communication system that
includes a base station with multiple antennas and user equipment
conducting radio communication with the base station, comprising
the steps of: at the base station, detecting a direction in which
the user equipment is located; transmitting precoded reference
signals toward the detected direction by spatial multiplexing using
same frequency and time resources; and determining a beam for the
user equipment based upon feedback information from the user
equipment.
2. The beam selection method as claimed in claim 1, wherein the
base station transmits information about signal sequences,
multiplexing positions on time and frequency axes, and a part or
all of orthogonal codes of the reference signals by signaling.
3. The beam selection method as claimed in claim 1, wherein when
receiving a first beam selection result indicating selection of a
beam that carries a first reference signal among the reference
signals from a first user equipment and receiving a second beam
selection result indicating selection of a beam that carries a
second reference signal from a second user equipment, the base
station determines the first user equipment and the second user
equipment as a user pair to which multi-user special multiplexing
is applied.
4. A base station that conducts radio communication with user
equipment using multiple antennas; comprising: a detector
configured to detect a direction in which the user equipment is
positioned; a precoding controller configured to determine
precoding vectors to be applied to a reference signal such that
multiple beams are formed in the detected direction; a reference
signal generator configured to generate precoded reference signals
with directivity by multiplexing the precoding vectors with the
reference signal; a transmitter configured to transmit the precoded
reference signals using the multiple antennas; a receiver
configured to receive feedback information from the user equipment;
and a feedback information processor configured to process the
feedback information to acquire a beam selection result contained
in the feedback information, wherein the precoding controller
determines a beam for the user equipment based upon the selection
result.
5. The base station as claimed in claim 4, further comprising: a
reporting block configured to supply information about signal
sequences, multiplexing positions on time and frequency axes, and a
part or all of orthogonal codes of the precoded reference signals
by signaling.
6. The base station as claimed in claim 4, wherein: the reference
signal generator generates mutually orthogonal reference signals;
and when the receiver receives a first beam selection result
indicating selection of a beam that carries a first reference
signal among the mutually orthogonal reference signals from a first
user equipment and a second beam selection result indicating
selection of a beam that carries a second reference signal from a
second user equipment, the precoding controller determines the
first user equipment and the second user equipment as a user pair
to which multi-user special multiplexing is applied.
7. The base station as claimed in claim 4, wherein: the transmitter
transmits a plurality of first precoded reference signals
simultaneously using a plurality of first beams toward the detected
direction, then, after receiving the feedback information as to the
first precoded reference signals, the transmitter transmits second
precoded reference signals using a plurality of second beams with a
beam width narrower than the first beams toward a direction
selected from the first beams and indicated by the feedback
information, and the receiver receives additional feedback
information as to the second precoded reference signals, and p1
wherein the precoding controller determines a beam for the user
equipment based upon the additional feedback information.
8. The base station as claimed in claim 7, wherein the receiver
receives the feedback information as to the first precoded
reference signals at a first period, and receives the additional
feedback information as to the second precoded reference signals at
a second period shorter than the first period.
9. The base station as claimed in claim 4, wherein the precoding
controller determines a rank index based upon a number of selected
beams contained in the feedback information.
10. A user equipment communicating with a base station using
multiple antennas, comprising: a receiver configured to receive
reference signals transmitted simultaneously from the base station
using multiple directional beams and signaling information about
the reference signals, the signaling information including signal
sequences, multiplexing positions on time and frequency axes, and a
part or all of orthogonal codes of the reference signals; a
measurement unit configured to measure a quality of each of the
reference signals; a controller configured to select one or more
beams from among the multiple directional beams based upon a
measurement result acquired the measurement unit; a feedback
information generator configured to generate feedback information
based upon a selection result selected by the controller; and a
transmitter configured to transmit the feedback information to the
base station.
Description
TECHNICAL FIELD
[0001] The present invention relates to the field of mobile
telecommunications, and more particularly, to a technique of beam
selection for three-dimensional multiple input multiple output
(3D-MIMO) mobile telecommunication systems.
BACKGROUND ART
[0002] Technical Specification Releases 8 to 11 of the Third
Generation Partnership Project (3GPP) for standardization of mobile
technologies adapt horizontal beam forming using multiple antenna
ports provided in a lateral direction at a base station.
[0003] In 3GPP Release 12, 3D-MIMO for achieving vertical beam
forming, in addition to the horizontal beam forming, has been
discussed. See, for example, Non-patent Documents 1 and 2 listed
below. By forming a beam in a vertical (or elevation) direction and
a horizontal (or azimuth) direction, system characteristics are
expected to be improved.
[0004] In 3GPP standardization, 3D-MIMO scheme using eight or less
transmission antenna ports is called "elevation beamforming" and
3D-MIMO with antenna ports greater than eight is called
full-dimensional (FD) MIMO. Apart from the standardization, FD-MIMO
is named a massive-MIMO and the antenna layout is not limited to
two-dimensional or three-dimensional layout.
[0005] FD-MIMO is a technique capable of greatly improving the
efficiency of frequency use by using a large number of antenna
ports or antenna elements at a base station to form a sharp
(directional) beam.
[0006] By providing a number of antenna ports to a base station,
beam forming in horizontal and vertical directions is achieved,
just like elevation beamforming.
LIST OF RELATED DOCUMENTS
[0007] Non-Patent Document 1: 3GPP TSG RAN#58, RP-121994, "Study on
Downlink Enhancement for Elevation Beamforming for LTE" [0008]
Non-Patent Document 2: 3GPP TSG RAN#58, RP-122015, "New SID
Proposal: Study on Full Dimension for LTE"
SUMMARY OF THE INVENTION
Technical Problem to be Solved
[0009] With FD-MIMO or massive-MIMO, the beam gain can be increased
by precoding, but the beam width becomes narrow. FIG. 1A through
FIG. 1C illustrate beam patterns formed by one-dimensional array
antennas with discrete Fourier transform (DFT) precoding applied.
FIG. 1A is a beam pattern (with 8 DFT beams) formed by four
antennas, FIG. 1B is a beam pattern (with 16 DFT beams) formed by
eight antennas, and FIG. 1C is a beam pattern (with 32 DFT beams)
formed by sixteen antennas.
[0010] To cover every direction, beam directions whose number is
proportional to (e.g., twice) the number of antenna ports or
elements are used. As the beam gain increases with the increased
number of antennas, the width of each beam becomes narrower. When
using a massive antenna array with sixteen or more antennas, it is
desired for a base station to form many beams or beam candidates
and select the optimum beams. To carry out beam selection involving
other cells or other sectors, the number of beams is likely to
increase to several times or dozens of times.
[0011] For the optimum beam selection, one of conceivable
approaches is to allow user equipment to select the best beam from
among as many beam candidates as possible. However, the greater the
number of beam candidates, the more the precoded reference signals
such as channel state information reference signals (CSI-RSs) are
to be transmitted and the reference signal overhead increases.
[0012] It is desired to provide a technique for efficient beam
transmission and beam selection, while preventing overhead from
increasing due to excessive traffic of reference signals and
feedback information.
Means for Solving the Problems
[0013] To solve the above-described technical problem, a novel beam
selecting method is provided for a mobile communication system that
includes a base station with multiple antennas and a user equipment
conducting radio communication with the base station. The beam
selecting method includes the steps of
[0014] at the base station, detecting a direction in which the user
equipment is located;
[0015] transmitting precoded reference signals toward the detected
direction by spatial multiplexinusing same frequency and time
resources; and
[0016] determining a beam for the user equipment based upon
feedback information from the user equipment.
Advantageous Effect of the Invention
[0017] In a mobile communication system using a 3D-MIMO scheme,
efficient beam transmission and beam selection can be achieved,
while preventing reference signal overhead from increasing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1A illustrates a directional beam pattern in accordance
with the number of antennas;
[0019] FIG. 1B illustrates a directional beam pattern in accordance
with the number of antennas;
[0020] FIG. 1C illustrates a directional beam pattern in accordance
with the number of antennas;
[0021] FIG. 2 illustrates a basic concept of a mobile communication
system of the embodiment;
[0022] FIG. 3 illustrates a step-by-step approach for beam
selection carried out in the embodiment;
[0023] FIG. 4 illustrates a first example of beam selection;
[0024] FIG. 5A illustrates a second example of beam selection;
[0025] FIG. 5B illustrates a second example of beam selection;
[0026] FIG. 6A illustrates a third example of beam selection;
[0027] FIG. 6B illustrates an example of a feedback table used in
beam selection of FIG. 6A;
[0028] FIG. 6C illustrates another example of a feedback table used
in beam selection;
[0029] FIG. 7A illustrates a fourth example of beam selection;
[0030] FIG. 7B illustrates an example of a feedback table used in
beam selection;
[0031] FIG. 8A illustrates a fifth example of beam selection;
[0032] FIG. 8B illustrates a feedback table used in beam
selection;
[0033] FIG. 9 is a diagram to explain beam tracking;
[0034] FIG. 10 illustrates a modification of beam selection;
[0035] FIG. 11 is a schematic diagram of a base station used in an
embodiment; and
[0036] FIG. 12 is a schematic diagram of user equipment used in an
embodiment.
EMBODIMENTS TO CARRY OUT THE INVENTION
[0037] FIG. 2 illustrates a basic concept of a mobile communication
system 1 according to the embodiment. The base station 10
simultaneously transmits two or more precoded (namely, directional)
reference signals from an antenna array 11 with multiple antenna
elements. The referenced signal is, for example, a precoded channel
state information reference signal (precoded CSI-RS).
[0038] User equipments UE 20-1 and 20-1 select appropriate beams
from among the precoded reference signals and feed the selection
results back to the base station 10. Appropriate beams can be
determined at each of the user equipments 20-1 and 20-2 based upon
signal to interference-plus-noise ratio (SINR), reference signal
received power (RSRP), reference signal received quality (RSRQ),
etc.
[0039] In realizing the system illustrated in FIG. 2, overhead may
increase due to the increased number of precoded reference signals.
To avoid the overhead increasing, a step-by-step beam selection is
proposed in the embodiment. With the step-by-step beam selection,
areas in or around which the respective user equipments 20-1 and
20-2 are located are specified. Then, precoded reference signals
are transmitted over candidate beams toward the specified areas to
allow each of the user equipments 20-1 and 20-2 (which may be
referred to collectively as "user equipment 20") to select an
appropriate beam from the finite number of candidate beams.
[0040] Depending on the contents of the feedback information from
the user equipment (abbreviated as "UE") 20, the base station 10
may immediately select the beam based upon the feedback
information, or determine a beam for the user equipment 20 after
further narrowing down the beam direction. Alternatively, the
earlier step(s) such as retransmission of reference signals toward
the detected area and/or detection of the UE location may be
repeated.
[0041] When the optimum beam for the user equipment 20 is
determined, beam tracking may be performed to let the beam
direction, namely, data transmission direction follow the user
equipment 20.
[0042] FIG. 3 is a diagram illustrating a step-by-step approach to
reduce the quantity or traffic of the reference signals and
feedback information. First, the base station 10 detects an
approximate location of the user equipment 20, namely, an
approximate direction of beam (S1). This step is a "rough
detection" step.
[0043] When the base station 10 illustrated in FIG. 2 is a small
base station or a remote base station, rough detection may be
performed under the assist of a macro base station or based upon
mutual relationship with neighboring base stations. The base
station 10 may perform rough detection by itself based upon
synchronization signals, positioning reference signals (PRSs),
global positioning system (GPS) information, etc. The rough
detection step (S1) may be repeated at prescribed time intervals or
based upon information from the user equipment 20.
[0044] When the approximate location of the user equipment 20 is
determined, the base station 10 narrows down the beam candidates
(S2). In the beam candidate narrow down step (S2), precoded
reference signals (such as precoded CSI-RSs) are transmitted
simultaneously over multiple streams toward the roughly detected
direction (S21). Then, the base station 10 selects the optimum beam
based upon feedback information from the user equipment 20 (S23).
Transmission of reference signals (S21) and/or beam selection (S23)
may be repeated at prescribed intervals or based upon the feedback
information from the user equipment 20. Depending on the contents
of the feedback information, rough detection (S1) and/or beam
candidates narrow down (S2) may be recommenced after the step S21
or S23.
[0045] When a beam is selected for the user equipment 20, beam
tracking may be performed (S3) to let the selected beam follow the
user equipment 20. If the beam deviates from or cannot follow the
user equipment 20 during the beam tracking, rough detection (S1)
and/or beam candidates narrow down (S2) may be recommenced.
[0046] Although in this example steps S1 to S3 are performed on a
step-by-step basis, one or two of the steps S1-S3 may be selected
to carry out less complicated beam selection. A part or all of the
steps S1-S3 may be combined with other beam selection
techniques.
[0047] Actual examples of the beam candidates narrow down step (S2)
are described in more detail below.
<Beam Selection Scheme 1>
[0048] FIG. 4 illustrates the first example of beam candidates
narrow down. In this example, the base station 10 transmits
precoded reference signals for channel measurement (e.g., precoded
CSI-RSs) by multiple streams using the same frequency and/or time
resources, applying spatial multiplexing, thereby preventing
overhead from increasing.
[0049] The base station 10 spatially multiplexes and transmits
beams (i.e., signal streams) A1 and A2 at timing A, and spatially
multiplexes and transmits beams (i.e., signal streams) B1 and B2 at
timing B. A set of beams is spatially multiplexed and transmitted
in the same manner at subsequent timings.
[0050] When user equipment (UE) 1 selects beam A1 and UE 2 selects
beam A2, then the base station 10 may apply multi-user MIMO
(MU-MIMO) by pairing UE 1 and UE 2. By transmitting multiple
precoded reference signals at the same time using different beams
(or signal streams), a user pair can be determined simultaneously
with beam selection, taking inter-user interference into
account.
[0051] In FIG. 4, precoded CSI-RSs denoted by different
alphabetical symbols are multiplexed along a time axis. Any type of
precoded reference signals may be multiplexed in a
frequency-division or code-division manner. The number of signal
streams simultaneously transmitted is not limited to two, but three
or more signal streams may be spatially multiplexed.
[0052] Because in this method two or more precoded CSI-RSs are
transmitted at once, TS overhead can be reduced.
[0053] By transmitting precoded CSI-RSs on multi-streams, received
signal qualities can be measured taking inter-user interference
into account.
[0054] By orthogonalizing candidate beams of precoded CSI-RSs
(e.g., A1 and A2) transmitted by multi-streams from the base
station 10, MU-MIMO with reduced inter-UE interference can be
applied.
[0055] When multi-stream transmission is employed in the scheme of
FIG. 4, the base station 10 provides signal sequences, multiplexed
positions (on time and/or frequency axes), orthogonal codes, etc.
to UEs as signaling information.
<Beam Selection Scheme 2>
[0056] FIG. 5A and FIG. 5B illustrate examples of beam candidates
narrow down. With the beam selection scheme 2, beam candidates
narrow down is performed on a step-by-step basis.
[0057] FIG. 5A illustrates a first half step 2-A of beam candidates
narrow down. In this step, the base station 10 transmits two or
more precoded CSI-RSs from the antenna array 11 with relatively
wide beams (Beam 1, Beam 2, Beam 3 and Beam 4 in the figure). The
reference signals being transmitted may be of other types, such as
cell-specific reference signals (CRSs). In the earlier stage of
beams candidates narrow down, wide beams are used to reduce RS
overhead, and simultaneously, the location of the user equipment 20
can be further narrowed.
[0058] The user equipment 20 selects the optimum beam (Beam 2 in
this example) from the multiple wide beams and feeds the selection
result back to the base station 10.
[0059] FIG. 5B illustrates a second half step 2-B of beam
candidates narrow down. Based upon the feedback information
acquired in the first half step 2-A, the base station 10 determines
two or more precoded CSI-RSs to be transmitted toward the direction
of Beam 2. The set of precoded CSI-RSs determined in this step are
named second precoded CSI-RSs for the sake of convenience. The
second precoded CSI-RSs may be transmitted by sharp and narrow
beams, compared with the wide beam used in the first half step
2-A.
[0060] The second precoded CSI-RSs are, for example, UE-specific
reference signals. By transmitting the second-half reference
signals over a UE-specific channel, especially on a physical
downlink shared channel (PDSCH), impact on legacy user equipment
(UE) can be reduced.
[0061] The user equipment 20 then selects the optimum beam (Beam 2C
in this example) from the second precoded CSI-RSs, and feeds the
selection result back to the base station 10. With this method,
beam candidates narrow down and selection of the optimum beam can
be performed more finely and more precisely. The number of the
optimum beam(s) fed back to the base station 10 is not limited to a
single beam, but plural beam indexes may be fed back to the base
station based upon the measurement levels and/or the channel
qualities of the received reference signals.
[0062] With the exemplified scheme illustrated in FIG. 5A and FIG.
5B, beam selection of the second half step 2-B is carried out using
the feedback information acquired in the first half step 2-A.
However, feedback operations may be performed separately from each
other between the first half step 2A and the second half step 2B.
For example, one or more beams may be selected in the vertical or
elevation direction in the first half step 2-A, and then one or
more beams may be selected in the horizontal or azimuth direction
in the second half step 2-B.
<Beam Selection Scheme 3>
[0063] FIG. 6A to FIG. 6C illustrates the third example of beam
candidates narrow down. In the above-described beam selection
scheme 2, the second half step 2-B may not be performed in some
cases. For example, when the moving speed of the user equipment 20
is fast, it is difficult to accurately narrow down the beams
candidates in the second half step 2-B because the location of the
user equipment 20 is likely to change from that detected in the
first half step 2-A. In this case, recommencing the first half step
2A or the previous step S1 is more efficient to narrow down the
beam candidates.
[0064] For this reason, the beam selection scheme 3 provides a
feedback method that accepts switch back to the previous step
(e.g., the first half step 2-A).
[0065] FIG. 6A schematically illustrates beam patterns transmitted
from the antenna array 11 of the base station 10, and FIG. 6B
illustrates an example of a feedback table 21 shared between the
base station 10 and the user equipment 20. The feedback table 21 of
FIG. 6B represents a situation in which the latest information
acquired in the first half step 2-A is "Beam 2".
[0066] With 3-bit feedback information, upon acquiring feedback
information indicating "Beam 2" selected in the first half step,
beam identification bits "100", "101", "110", and "111" are set up
in the table for sharp and narrow beams to 2A to 2D, while
maintaining beam identification information items "000", "001",
"010", and "011" for wide beams (Beam 1 to Beam 4) used in the
first half step.
[0067] When the feedback table 21 is in the FIG. 6B's state, the
user equipment 20 can monitor 8 beams. Two types of beams, namely,
wide beams used in the first half step 2-A and sharp narrow beams
used in the second half step 2-B are included in a single feedback
table 21. The base station 10 can switch between the beam types
based mainly upon the activities of the user equipment 20 (under
UE's initiative).
[0068] For example, when the user equipment 20 selects Beam 2C in
the latter step after selection of Beam 2, the user equipment 20
supplies bit information "110" to the base station 10. The base
station 10 sets up a precoding vector to form Beam 2C for the user
equipment 20.
[0069] On the other hand, if the feedback information transmitted
from the user equipment 20 is "000", "001", "010" or "011", then
the base station 10 applies precoding without using narrow beams.
This arrangement is advantageous for the user equipment 20 moving
at a high speed to narrow down beam patterns.
[0070] In comparing between wide beams and narrow beams, an offset
may be added. For example, the user equipment 20 may add an offset
value (e.g., 3 dB) to the received power level when measuring wide
beams "000", "001", "010", and "011". By adding an offset value,
beam qualities can be compared in a fair condition.
[0071] FIG. 6C illustrates an example of feedback table 22 shared
between the base station and the user equipment 20. Feedback table
22 exhibits the state in which Beam 2 has been selected in the
former step 2-A. Feedback table 22 has a value "000" in addition to
Beams 2A to 2D ("100", "101", "110", and "111") that subdivides the
direction of Beam 2. The value "000" may be used to report beam
deviation. When receiving feedback information "000" from the user
equipment 20 in the latter half step, the base station 10 may
recommence the first half step 2-A or return to rough detection in
S1.
[0072] With this scheme, step-by-step operations for beam
candidates narrow down can be performed based mainly upon the
activities of the user equipment 20 (under UE's initiative).
<Beam Selection Scheme 4>
[0073] FIG. 7A and FIG. 7B illustrate the fourth example of beam
candidates narrow down. With the beam selection scheme 3 described
above, reference signals are transmitted at different beam widths
to allow the step-by-step basis beam switching. The beam selection
scheme 4 achieves the same advantageous effect as in the method of
FIG. 6A to FIG. 6C without using wide beams.
[0074] In FIG. 7A and FIG. 7B, the base station 10 and the user
equipment 20 have a feedback table 23 which information is shared
and commonly used between the base station 10 and the user
equipment 20. The feedback table 23 represents the latest state
immediately after the selection of Beam 2 in the first half step
2-A.
[0075] The feedback table 23 has values "000" representing Beams 1A
to 1D, "010" representing Beams 3A to 3D, and "011" representing
Beams 4A to 4D, in addition to Beams 2A to 2D defined by
subdividing the direction of Beam 2. Beams 2A to 2D are represented
by values "100", "101", "110", and "111", respectively. An area for
"001" may be a reserved area.
[0076] When the feedback information supplied from the user
equipment 20 indicates a value "000", "010", or "011", the base
station 10 goes back to the first half step 2-A. With this scheme,
operations in the step-by-step basis beam candidates narrow down
can be switched under the initiative of the user equipment 20,
using 3-bit information.
<Beam Selection Scheme 5>
[0077] FIG. 8A and FIG. 8B illustrate the fifth example of beam
candidates narrow down. Although in the above-described beam
selection schemes 3 and 4, operations are switched between the
first half step 2-A and the second half step 2-B based mainly upon
the activities of the user equipment 20, the base station 10 may
control the step-by-step basis operations.
[0078] FIG. 8B illustrates a feedback table 24 shared between the
base station 10 and the user equipment 20. The feedback table 24
has two areas 24A and 24B: the area 24A represents wide beams, Beam
1 to Beam 4 using two bits, and the area 24B represent subdivided
beams, Beam A to Beam D using two bits. As illustrate in FIG. 8A,
the base station 10 allows the user equipment 20 to feed back a
wide beam index and a narrow beam index separately.
[0079] For example, by using different feedback periods between the
wide beams and the narrow beams, beam selection result can be fed
back independently from each other. By performing selection of a
narrow beam more frequently, efficient feedback operations can be
achieved.
[0080] The indexes of narrow beams may be in accordance with the
feedback information of a wide beam. For example, when Beam 2 is
selected in the first half step 2-A, the beam candidates narrow
down in the second half step 2-B may be carried out among
subdivided beams 2A to 2D.
[0081] The beam selection scheme 5 may be applied to step-by-step
basis beam candidates narrow down using only narrow beams as in the
beam selection scheme 4. In this case, for example, Beams 1A to 1D
are included in a beam group and represented by 2-bit
information.
[0082] Furthermore, the step-by-step basis operations may be
switched without feedback from the user equipment or configuration
at the base station 10. For example, when the channel quality
indicator (QI) is out of the range (which means that the radio
communication quality is unsatisfied), the operation may go back to
rough detection of step S1, or the first half step 2A of beam
candidates narrow down.
[0083] When the user equipment 20 transmits a random access channel
(RACH) (e.g., when disconnected from the cell), the process may
return to the rough detection (S1) or the first half step 2A of
beam candidates narrow down.
<Beam Tracking>
[0084] FIG. 9 is a diagram to explain beam tracking step (S3). FIG.
9 illustrates beam directions viewed from the antenna array 11 of
the base station 10. The lateral direction of the figure is a
horizontal or azimuth direction, and the top-to-bottom direction of
the figure is a vertical or elevation direction.
[0085] The base station 10 transmits tracking reference signals
using beams #1 to #6, in addition to a currently used beam #0
selected by the beam candidates narrow down step (S2). Beams #1 to
#6 are candidate beams to be used when the currently used beam #0
cannot follow the user equipment 20. The current beam #0 for data
transmission and the candidate beams #1 to #6 form a beam stream 51
for beam tracking.
[0086] Upon receiving the beam stream 51, the user equipment 20
measures the received strength or other parameters of each beam and
reports one or more beam indexes with satisfactory qualities to the
base station 10. The optimum beam index or the highest X beam
indexes may be reported. Alternatively, all the measurement results
of beams #0 to #6 may be reported. In this case, the measurement
results may be fed back ordered from the highest quality to the
lowest or from the lowest quality to the highest.
[0087] Based upon the feedback information about beam tracking, the
base station 10 selects and sets the optimum beam as the current
beam #0 for the user equipment 20, thereby letting the data
transmission direction follow the user equipment 20.
[0088] When beam tracking is deviated due to, for example, the fast
moving speed of the user equipment 20, the process may return to
rough detection (S1) or beam candidates narrow down (S2) as has
been described above.
<Modification>
[0089] FIG. 10 illustrates a modification of the embodiment. In a
mobile communication system, the number of streams is generally
switched using a rank indicator (RI) representing the number of
transmission streams.
[0090] When performing beam selection using a scheme of the
embodiment, the rank index can be grasped from the number of beams
selected and accordingly, feedback of the rank indicator becomes
unnecessary.
[0091] When the user equipment 20 transmits one or more beam
indexes using a conventional RI region, other channel state
information (CSI) such as CQI, precoding matrix indicator (PMI),
etc., may be transmitted.
[0092] Rank adaptation may be performed for each beam index. For
example, an RI may be transmitted for each beam index. When the
user equipment 20 receives a direct beam B1 from the base station
10 and an indirect beam B2 reflected from a building, an RI may be
transmitted on the beam-by-beam basis. It is generally known that
channel correlation is low between orthogonally polarized waves.
Depending on the antenna configuration, the number of streams may
be set separately. With an orthogonally polarized antenna
configuration, the number of streams may be fixed to two, and with
a single polarization antenna configuration, the number of streams
may be fixed to one. In this case, transmitting a rank index
semi-statically for each beam index may be effective, rather
dynamic RI transmission.
[0093] Particularly, it may be conceived to multiplex maximum two
streams using polarizations at a single beam index. When in FIG. 10
beam B1 includes two orthogonal polarized streams, the user
equipment 20 may transmit RI value "2" for beam B2 and RI value "1"
for beam B1.
[0094] The number of streams may be switched adaptively between one
and two. In this case, switching control can be performed by 1-bit
information. For example, bit "0" may indicate a signal stream (the
number of streams is one), and bit "1" may indicate two streams
layered.
<Apparatus Structures>
[0095] FIG. 11 is a schematic diagram of a base station 10 used in
the embodiment. The base station 10 has multiple antennas 110-1 to
110-N, a transmitter 106, a receiver 107, and a duplexer 108 for
switching between transmission and reception. The base station 10
roughly detects a location of the user equipment 20 at a UE
position detector 101.
[0096] A precoding controller 102 of the base station 10 determines
weighing factors (phase rotation and/or amplitude) of a precoding
vector for each of multiple reference signals such that the
reference signals are transmitted toward adjacent directions at or
around the detected location of the user equipment 20. A reference
signal generator 104 of the base station 10 multiplies the
precoding vector determined by the precoding controller 102 with
each of the reference signals to generate multiple reference
signals with directivities. The multiplication of precoding vectors
and associated reference signals may be performed before mapping to
subcarriers.
[0097] The multiple reference signals transmitted toward the user
equipment 20 may be multiplexed at a multiplexer 109 along a time
axis or a frequency axis, or undergo code-division multiplexing.
The directional reference signals are transmitted toward the user
equipment 20 from the antennas 110-1 to 110-N via the transmitter
106 and the duplexer 108. When polarized antennas are used, a beam
specified by a single beam index may be transmitted over two
orthogonal polarized waves (streams).
[0098] A feedback information processor 103 acquires feedback
information from the user equipment 20 via the antennas 110-1 to
110-N, the duplexer 108 and the receiver 107 and processes the
feedback information. The feedback information processor 103
identifies the beam index contained in the feedback information,
referring to a feedback table 105. The feedback table 105 may be
any one of the tables illustrated in FIG. 6 to FIG. 8.
[0099] The identified beam index is supplied to the precoding
controller 102, together with the associated information (such as
CQI, PMI, RI, etc.). The precoding controller 102 selects the
optimum beam from the reported information and controls beam
forming such that the data signal to the user equipment 20 is to be
weighted by an appropriate precoding vector corresponding to the
selected beam.
[0100] The precoding controller 102 may instruct the reference
signal generator 104 to generate directional reference signals
again, and/or instruct the UE position detector 101 to detect the
location of the user equipment 20 again, when a specific value is
contained in the feedback information.
[0101] The precoding controller 102 may determine precoding vectors
to be multiplied with reference signals for multiple user
equipments (UE1 and UE2, for example). In this case, the antennas
110-1 to 110-N transmit a reference signal for the first user
equipment and a reference signal for the second user equipment by
spatial multiplexing. It is preferable for the reference signals
addressed to the first user equipment and the second user equipment
to be orthogonal to each other. Upon receiving feedback information
indicating satisfactory beam indexes from the first user equipment
and the second user equipment, respectively, then the precoding
controller 102 may determine the first and the second user
equipments as a user pair with less interference.
[0102] The precoding controller 102 may determine two or more
precoding vectors further narrowing the direction of the user
equipment 20. In this case, the precoding controller 102 may
instruct the reference signal generator 104 to generate reference
signals with narrower and sharper directivities than the previously
transmitted reference signals. The generated reference signals are
transmitted from the antennas 110-1 to 110-N via the transmitter
106 and the duplexer 108.
[0103] FIG. 12 is a schematic diagram of a user equipment 20 used
in the embodiment. The user equipment 20 has multiple antennas
210-1 to 210-M, a transmitter 206, a receiver 207, and a duplexer
208 for switching transmission and reception. Upon receiving
reference signals from the base station, the user equipment 20
measures the quality of each of the reference signals at a channel
quality measurement block 201.
[0104] Based upon the measurement results, a reference signal
processing controller 202 of the user equipment 20 selects one or
more appropriate beam indexes among multiple reference signals,
referring to a feedback table 205. A feedback information generator
203 of the user equipment 20 generates feedback information that
contains the selected beam indexes. The reference signal processing
controller 202 may select a specific value, e.g., "000", from the
feedback table 205 when there is no reference signal received with
a quality beyond a predetermined level.
[0105] The reference signal processing controller 202 may
determines a rank index based upon the channel quality measurement
result and include the rank index in the feedback information. In
place of reporting the rank index, two or more satisfactory
reference signals may be selected and the corresponding beam
indexes may be fed back to the base station 10.
[0106] The generated feedback information is transmitted from the
antennas 210-1 to 210-M via the transmitter 206 and the duplexer
208.
[0107] By providing the base station 10 and the user equipment 20
with the above-described structures, efficient beam selection can
be achieved, while preventing reference signal overhead from
increasing.
[0108] This patent application is based upon and claims the benefit
of the priority of the Japanese Patent Application No. 2014-059181
filed Mar. 20, 2014, which is incorporated herein by reference in
its entirety. What is claims is:
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