U.S. patent application number 15/967662 was filed with the patent office on 2018-08-30 for channel state feedback enhancement in downlink multiuser superposition transmission.
The applicant listed for this patent is MEDIATEK INC.. Invention is credited to Chien-Hwa Hwang, Pei-Kai Liao, Yi-Ju Liao, Lung-Sheng Tsai, Weidong Yang.
Application Number | 20180248594 15/967662 |
Document ID | / |
Family ID | 57248749 |
Filed Date | 2018-08-30 |
United States Patent
Application |
20180248594 |
Kind Code |
A1 |
Hwang; Chien-Hwa ; et
al. |
August 30, 2018 |
Channel State Feedback Enhancement in Downlink Multiuser
Superposition Transmission
Abstract
A method of performing downlink multiuser superposition
transmission (MUST) with enhanced channel state information (CSI)
feedback is proposed. When a user equipment (UE) reports CQI/SINR
feedback for RI=RANK-2, the UE also reports a single beam CQI/SINR
feedback for RI=RANK1. As a result, the scheduling base station can
calculate the actual SINRs based on different MUST scenarios and
thereby determining appropriate modulation and coding scheme (MCS)
for the UE. Furthermore, if the granularity of the CQI table cannot
reflect the high values of the single beam SINR, then a predefined
scaling factor (0<.beta.<1) known to both the base station
and the UE may be applied.
Inventors: |
Hwang; Chien-Hwa; (Hsinchu
County, TW) ; Tsai; Lung-Sheng; (Tainan City, TW)
; Liao; Yi-Ju; (Hsinchu City, TW) ; Liao;
Pei-Kai; (Nantou County, TW) ; Yang; Weidong;
(San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MEDIATEK INC. |
Hsinchu |
|
TW |
|
|
Family ID: |
57248749 |
Appl. No.: |
15/967662 |
Filed: |
May 1, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15150001 |
May 9, 2016 |
9985700 |
|
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15967662 |
|
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62160099 |
May 12, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 5/0026 20130101;
H04B 7/0452 20130101; H04L 27/3488 20130101; H04L 5/0037 20130101;
H04J 11/004 20130101; H04L 27/183 20130101; H04B 7/0632 20130101;
H04B 17/336 20150115; H04B 7/063 20130101; H04L 5/006 20130101;
H04W 72/1231 20130101; H04L 5/0069 20130101; H04L 5/0023 20130101;
H04L 5/0048 20130101; H04L 5/0028 20130101; H04L 27/0008
20130101 |
International
Class: |
H04B 7/0452 20060101
H04B007/0452 |
Claims
1. A method, comprising: transmitting reference signals to a
plurality of user equipments (UEs) by a base station in a wireless
communication network; receiving channel state information (CSI)
feedback from a first UE, wherein the CSI feedback comprises a
first channel quality indicator (CQI) associated with a first beam
group and a second beam group and a second CQI associated with a
single beam group; scheduling a downlink transmission to the first
UE and a second co-channel UE over an allocated time-frequency
radio resource using a multiuser transmission scheme; and
determining a modulation and coding scheme (MCS) for the first UE
based on the received CSI feedback and the multiuser transmission
scheme.
2. The method of claim 1, wherein the first CQI comprises a first
feedback signal to interference plus noise ratio (SINR) at the
first beam group and a second feedback SINR at the second beam
group, and wherein the second CQI comprises a third feedback SINR
at the single beam group.
3. The method of claim 2, wherein the base station calculates an
actual SINR of the first UE corresponds to the multiuser
transmission scheme using the first, second, and third feedback
SINRs.
4. The method of claim 2, wherein the third feedback SINR is a
result of multiplying a UE-measured single beam SINR with a
predefined scaling factor.
5. The method of claim 1, wherein the multiuser transmission scheme
is applied in the first beam group but not in the second beam
group.
6. The method of claim 1, wherein a first precoder is applied to
signals intended for the first UE, and wherein a second precoder is
applied to signals intended for the second co-channel UE.
7. The method of claim 1, wherein the first CQI is based on a first
precoding matrix, and wherein a second precoding matrix is applied
to signals intended for the first UE.
8. A method comprising: measuring reference signals from a base
station by a user equipment (UE) in a wireless communication
network; transmitting channel state information (CSI) feedback to
the base station, wherein the CSI feedback comprises a first
channel quality indicator (CQI) associated with a first beam group
and a second beam group and a second CQI associated with a single
beam group; receiving a downlink transmission scheduled to the UE
and a second co-channel UE over an allocated time-frequency radio
resource using a multiuser transmission scheme; and applying a
modulation and coding scheme (MCS) received from the base station,
wherein the MCS is determined based on the CSI feedback and the
multiuser transmission scheme.
9. The method of claim 8, wherein the first CQI comprises a first
feedback signal to interference plus noise ratio (SINR) at the
first beam group and the second feedback SINR at the second beam
group, and wherein the second CQI comprises a third feedback SINR
at the single beam group.
10. The method of claim 9, wherein an actual SINR of the UE
corresponds to the multiuser transmission scheme is calculated
using the first, the second, and the third feedback SINRs.
11. The method of claim 9, wherein the third feedback SINR is a
result of multiplying a UE-measured single beam SINR with a
predefined scaling factor.
12. The method of claim 8, wherein the multiuser transmission
scheme is applied in the first beam group but not in the second
beam group.
13. The method of claim 8, wherein a first precoder is applied to
signals intended for the UE, and wherein a second precoder is
applied to signals intended for the second co-channel UE.
14. The method of claim 8, wherein the first CQI is based on a
first precoding matrix, and wherein a second precoding matrix is
applied to signals intended for the UE.
15. A user equipment (UE) comprising: a radio signal detector that
detects and measures reference signals from a base station in a
wireless communication network; a transmitter that transmits
channel state information (CSI) feedback from the UE, wherein the
CSI feedback comprises a first channel quality indicator (CQI)
associated with a first beam group and a second beam group and a
second CQI associated with a single beam group; a receiver that
receives a downlink transmission scheduled to the UE and a second
co-channel UE over an allocated time-frequency radio resource using
a multiuser transmission scheme; and a detector that applies a
modulation and coding scheme (MCS) received from the base station,
wherein the MCS is determined based on the CSI feedback and the
multiuser transmission scheme.
16. The UE of claim 15, wherein the first CQI comprises a first
feedback signal to interference plus noise ratio (SINR) at the
first beam group and the second feedback SINR at the second beam
group, and wherein the second CQI comprises a third feedback SINR
at the single beam group.
17. The UE of claim 16, wherein an actual SINR of the UE
corresponds to the multiuser transmission scheme is calculated
using the first, the second, and the third feedback SINRs.
18. The UE of claim 16, wherein the third feedback SINR is a result
of multiplying a UE-measured single beam SINR with a predefined
scaling factor.
19. The UE of claim 15, wherein the multiuser transmission scheme
is applied in the first beam group but not in the second beam
group.
20. The UE of claim 15, wherein a first precoder is applied to
signals intended for the UE, and wherein a second precoder is
applied to signals intended for the second co-channel UE.
21. The UE of claim 15, wherein the first CQI is based on a first
precoding matrix, and wherein a second precoding matrix is applied
to signals intended for the UE.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation and claims priority under
35 U.S.C. .sctn. 120 from nonprovisional U.S. patent application
Ser. No. 15/150,001, entitled "Channel State Feedback Enhancement
in Downlink Multiuser Superposition Transmission," filed on May 9,
2016, the subject matter of which is incorporated herein by
reference. Application Ser. No. 15/150,001, in turn, claims
priority under 35 U.S.C. .sctn. 119 from U.S. Provisional
Application No. 62/160,099, entitled "Channel State Feedback
Enhancement in Downlink Multiuser Superposition Transmission,"
filed on May 12, 2015, the subject matter of which is incorporated
herein by reference.
TECHNICAL FIELD
[0002] The disclosed embodiments relate generally to mobile
communication networks, and, more particularly, to methods for
channel state feedback in downlink multiuser superposition
transmission.
BACKGROUND
[0003] Long Term Evolution (LTE) is an improved universal mobile
telecommunication system (UMTS) that provides higher data rate,
lower latency and improved system capacity. In LTE systems, an
evolved universal terrestrial radio access network includes a
plurality of base stations, referred as evolved Node-Bs (eNBs),
communicating with a plurality of mobile stations, referred as user
equipment (UE). A UE may communicate with a base station or an eNB
via the downlink and uplink. The downlink (DL) refers to the
communication from the base station to the UE. The uplink (UL)
refers to the communication from the UE to the base station. LTE is
commonly marketed as 4G LTE, and the LTE standard is developed by
3GPP.
[0004] In a wireless cellular communications system, multiuser
multiple-input multiple-output (MU-MIMO) is a promising technique
to significantly increase the cell capacity. In MU-MIMO, the
signals intended to different users are simultaneously transmitted
with orthogonal (or quasi-orthogonal) precoders. On top of that,
the concept of a joint optimization of MU operation from both
transmitter and receiver's perspective has the potential to further
improve MU system capacity even if the transmission and precoding
is non-orthogonal. For example, the simultaneous transmission of a
large number of non-orthogonal beams/layers with the possibility of
more than one layer of data transmission in a beam. Such
non-orthogonal transmission could allow multiple users to share the
same resource elements without spatial separation, and allow
improving the multiuser system capacity for networks with a small
number of transmit antennas (i.e. 2 or 4, or even 1), where MU-MIMO
based on spatial multiplexing is typically limited by wide
beamwidth.
[0005] An example of such joint Tx/Rx optimization associated with
adaptive Tx power allocation and codeword level interference
cancellation (CW-IC) receiver is recently a remarkable technical
trend, including non-orthogonal multiple access (NOMA) and other
schemes based on downlink multiuser superposition transmission
(MUST). In MUST, the signals intended for two users are superposed
and occupy the same time-frequency radio resource. To benefit from
MUST, the two co-scheduled users generally need to have a large
difference in the received signal quality, e.g., in terms of the
received signal-to-interference-plus-noise ratio (SINR). In a
typical scenario, one of the users is geometrically close to the
base station, and the other user is geometrically far away from the
base station. The former user and the latter user are also referred
to as the near-user and far-user respectively.
[0006] In order to apply MUST precoding, the transmitting station
is required to know the Channel State Information (CSI) of the
radio channels connecting it to each of the receiving stations for
transmission. In 3GPP LTE systems, it is common for the receiving
stations (e.g., UEs) to measure CSI and report CSI to the
transmitting station (e.g., eNB) via an uplink feedback channel.
The content of CSI feedback contains RI (rank indicator), CQI
(channel quality indicator), and PMI (precoding matrix indicator)
for each downlink channel.
[0007] In the current LTE communication system, the UE determines
the CQIs based on the output SINRs of an MMSE receiver. However,
the feedback SINRs may not be the same as the actual SINRs of the
UE. In a first scenario, when UE reports RI=1, but there are two
spatial layers in the actual transmission. In a second scenario,
when UE reports RI=2 with certain PMI, but the eNB uses a different
PMI for MU-MIMO transmission. As a result, the CSI feedback
received by the eNB does not reflect the actual channel state
information of the UE, causing the eNB unable to perform MUST
precoding effectively.
[0008] A solution is sought.
SUMMARY
[0009] A method of performing downlink multiuser superposition
transmission (MUST) with enhanced channel state information (CSI)
feedback is proposed. When a user equipment (UE) reports CQI/SINR
feedback for RI=RANK-2, the UE also reports a single beam CQI/SINR
feedback for RI=RANK1. As a result, the scheduling base station can
calculate the actual SINRs based on different MUST scenarios and
thereby determining appropriate modulation and coding scheme (MCS)
for the UE. Furthermore, if the granularity of the CQI table cannot
reflect the high values of the single beam SINR, then a predefined
scaling factor (0<.beta.<1) known to both the base station
and the UE may be applied.
[0010] In one embodiment, a base station transmits reference
signals to a plurality of user equipments (UEs) in a wireless
communication network. The base station receives channel state
information (CSI) feedback from a first UE. The CSI feedback
comprises a RANK-2 channel quality indicator (CQI) associated with
a first beam and a second beam and a RANK-1 CQI associated with a
single beam. The base station schedules a downlink transmission to
the first UE and a second co-channel UE over an allocated
time-frequency radio resource using a multiuser superposition
transmission (MUST) scheme. The base station determines a
modulation and coding scheme (MCS) for the first UE based on the
received CSI feedback and the MUST scheme. In one example, the
RANK-2 CQI comprises a first feedback signal to interference plus
noise ratio (SINR) at the first beam and a second feedback SINR at
the second beam, and the RANK-1 CQI comprises a third feedback SINR
at the single beam measured by the UE.
[0011] In another embodiment, a UE measures reference signals from
a base station in a wireless communication network. The UE
transmits channel state information (CSI) feedback to the base
station. The CSI feedback comprises a RANK-2 channel quality
indicator (CQI) associated with a first beam and a second beam and
a RANK-1 CQI associated with a single beam. The UE receives a
downlink transmission scheduled to the UE and a second co-channel
UE over an allocated time-frequency radio resource using a
multiuser superposition transmission (MUST) scheme. The UE applies
a modulation and coding scheme (MCS) received from the base
station, wherein the MCS is determined based on the CSI feedback
and the MUST scheme. In one example, the RANK-2 CQI comprises a
first feedback signal to interference plus noise ratio (SINR) at
the first beam and a second feedback SINR at the second beam, and
the RANK-1 CQI comprises a third feedback SINR at the single beam
measured by the UE.
[0012] Other embodiments and advantages are described in the
detailed description below. This summary does not purport to define
the invention. The invention is defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 illustrates a mobile communication network with
channel state information (CSI) feedback enhancement for multiuser
superposition transmission (MUST) in accordance with one novel
aspect.
[0014] FIG. 2 is a simplified block diagram of a base station and a
user equipment that carry out certain embodiments of the present
invention.
[0015] FIG. 3 illustrates a first embodiment for CSI feedback
enhancement in MUST scheme in accordance with one novel aspect.
[0016] FIG. 4 illustrates a second embodiment for CSI feedback
enhancement in MUST scheme in accordance with one novel aspect.
[0017] FIG. 5 illustrates a third embodiment for CSI feedback
enhancement in MUST scheme in accordance with one novel aspect.
[0018] FIG. 6 illustrates a downlink MUST procedure between a BS
and two UEs with enhanced CSI feedback in accordance with one novel
aspect.
[0019] FIG. 7 is a flow chart of a method of performing MUST with
enhanced CSI feedback from eNB perspective in accordance with one
novel aspect.
[0020] FIG. 8 is a flow chart of a method of performing MUST with
enhanced CSI feedback from UE perspective in accordance with one
novel aspect.
DETAILED DESCRIPTION
[0021] Reference will now be made in detail to some embodiments of
the invention, examples of which are illustrated in the
accompanying drawings.
[0022] FIG. 1 illustrates a mobile communication network 100 with
channel state information (CSI) feedback enhancement for multiuser
superposition transmission (MUST) in accordance with one novel
aspect. Mobile communication network 100 is an OFDM network
comprising a serving base station eNB 101, a first user equipment
102 (UE#1), and a second user equipment 103 (UE#2). In 3GPP LTE
system based on OFDMA downlink, the radio resource is partitioned
into subframes in time domain, each subframe is comprised of two
slots. Each OFDMA symbol further consists of a number of OFDMA
subcarriers in frequency domain depending on the system bandwidth.
The basic unit of the resource grid is called Resource Element
(RE), which spans an OFDMA subcarrier over one OFDMA symbol. REs
are grouped into resource blocks (RBs), where each RB consists of
12 consecutive subcarriers in one slot.
[0023] Several physical downlink channels and reference signals are
defined to use a set of resource elements carrying information
originating from higher layers. For downlink channels, the Physical
Downlink Shared Channel (PDSCH) is the main data-bearing downlink
channel in LTE, while the Physical Downlink Control Channel (PDCCH)
is used to carry downlink control information (DCI) in LTE. The
control information may include scheduling decision, information
related to reference signal information, rules forming the
corresponding transport block (TB) to be carried by PDSCH, and
power control command. For reference signals, Cell-specific
reference signals (CRS) are utilized by UEs for the demodulation of
control/data channels in non-precoded or codebook-based precoded
transmission modes, radio link monitoring and measurements of
channel state information (CSI) feedback. UE-specific reference
signals (DM-RS) are utilized by UEs for the demodulation of
control/data channels in non-codebook-based precoded transmission
modes.
[0024] In the example of FIG. 1, downlink multiuser superposition
transmission (MUST) scheme is used. In MUST, the signals intended
for two users are superposed and occupy the same time-frequency
radio resource. To benefit from MUST, the two co-scheduled users
generally need to have a large difference in the received signal
quality, e.g., in terms of the received
signal-to-interference-plus-noise ratio (SINR). In a typical
scenario, one of the users (e.g., UE#1) is geometrically close to
the base station, and the other user (e.g., UE#2) is geometrically
far away from the base station. The former user and the latter user
are also referred to as the near-user and far-user
respectively.
[0025] In order to apply MUST precoding, the transmitting station
is required to know the Channel State Information (CSI) of the
radio channels connecting it to each of the receiving stations for
transmission. In 3GPP LTE systems, it is common for the receiving
stations (e.g., UEs) to measure CSI and report CSI to the
transmitting station (e.g., eNB) via an uplink feedback channel.
The content of CSI feedback contains RI (rank indicator), CQI
(channel quality indicator), and PMI (precoding matrix indicator)
for each downlink channel.
[0026] Assume eNB 101 is equipped with N.sub.t transmit antennas,
and UE 102 has N.sub.r receive antennas. When UE#1 reports the Rank
Index (RI) equal to 2 and the precoding matrix index (PMI)
corresponding to the matrix [u.sub.1, u.sub.2], the determination
of the CQI is based on the received signal y which is obtained
after intercell-interference-plus-noise whitening. As shown in FIG.
1, UE#1 receives intra-cell interfering radio signal 112
transmitted from the same serving eNB 101 due to non-orthogonal
multiple access (NOMA) operation intended for multiple UEs (e.g.,
UE#2) in the same serving cell. UE#1 may be equipped with an
interference cancellation (IC) receiver that is capable of
cancelling the contribution of the interfering signal 112 from the
desired signal 111. For NOMA operation, the signals to the two UEs
are superposed and precoded with the same precoder having PMI
corresponding to the matrix [u.sub.1, u.sub.2], and transmitted
over two spatial beams. The received signal y at UE#1 is obtained
after intercell-interference-plus-noise whitening and is given as
the following equation:
y=H(u.sub.1 {square root over (P/2)}x.sub.1+u.sub.2 {square root
over (P/2)}x.sub.2)+w=h.sub.1 {square root over
(P/2)}x.sub.1+h.sub.2 {square root over (P/2)}x.sub.2+w (1)
Where
[0027] H is the wireless channel matrix, h.sub.1=Hu.sub.1,
h.sub.2=Hu.sub.2 [0028] P is the total transmit power [0029]
x.sub.1 and x.sub.2 are the modulated symbols on the two spatial
beams, and [0030] w is the contribution of the whitened
intercell-interference-plus-noise with the covariance matrix equal
to the identity matrix I.
[0031] In the current LTE communication system, UE#1 determines CQI
based on the output SINRs of an MMSE receiver, given as:
SINR fb ( 1 ) = ( P / 2 ) h 1 H ( I + ( P / 2 ) h 2 h 2 H ) - 1 h 1
= P 2 ( h 1 2 - ( P / 2 ) h 1 H h 2 2 1 + ( P / 2 ) h 2 2 ) ( 2 )
SINR fb ( 2 ) = P 2 ( h 2 2 - ( P / 2 ) h 2 H h 1 2 1 + ( P / 2 ) h
1 2 ) ( 3 ) ##EQU00001##
Where
[0032] SINR.sub.fb.sup.(1) and SINR.sub.fb.sup.(2) are the feedback
SINRs by UE#1 at the two beams for RI=RANK-2
[0033] However, the feedback SINRs may not be the same as the
actual SINRs of UE#1 due to different MUST scheduling scenarios. In
accordance with one novel aspect, when a UE reports RI=Rank-2 and
PMI=[u.sub.1, u.sub.2], besides two Rank-2 CQIs, one Rank-1 CQI is
also reported, as depicted by CQI feedback 120. In other words,
besides the two SINRs in equations (2) and (3), the UE additionally
report CQIs based on SINRs that corresponds to a RANK-1 single-beam
transmission:
SINR.sub.fb.sup.(3)=P.parallel.h.sub.1.parallel..sup.2 (4)
[0034] If the UE reports all SINRs given in equations (2), (3), and
(4) for RI=RANK-2, then the scheduling eNB can calculate the actual
SINRs based on different MUST scenarios and thereby determining
appropriate modulation and coding scheme (MCS) for the UE.
Furthermore, if the granularity of the CQI table cannot reflect the
high values of the SINRs in (4), then a predefined scaling factor
(0<.beta.<1) known to both the eNB and the UE may be
multiplied in front of power P. For example, a 4-bit CQI table can
reflect SNR range from 0-15 dB only, but cannot distinguish actual
SNR that is higher than 16 dB. If the RANK-1 single-beam SNR is 20
dB, then by multiplying a scaling factor .beta.=0.5, then the 4-bit
CQI table is able to reflect the high value of such SNR.
[0035] FIG. 2 is a simplified block diagram of a base station 201
and a user equipment 211 that carry out certain embodiments of the
present invention in a mobile communication network 200. For base
station 201, antenna 221 transmits and receives radio signals. RF
transceiver module 208, coupled with the antenna, receives RF
signals from the antenna, converts them to baseband signals and
sends them to processor 203. RF transceiver 208 also converts
received baseband signals from the processor, converts them to RF
signals, and sends out to antenna 221. Processor 203 processes the
received baseband signals and invokes different functional modules
to perform features in base station 201. Memory 202 stores program
instructions and data 209 to control the operations of the base
station. Similar configuration exists in UE 211 where antenna 231
transmits and receives RF signals. RF transceiver module 218,
coupled with the antenna, receives RF signals from the antenna,
converts them to baseband signals and sends them to processor 213.
The RF transceiver 218 also converts received baseband signals from
the processor, converts them to RF signals, and sends out to
antenna 231. Processor 213 processes the received baseband signals
and invokes different functional modules to perform features in UE
211. Memory 212 stores program instructions and data 219 to control
the operations of the UE.
[0036] Base station 201 and UE 211 also include several functional
modules and circuits to carry out some embodiments of the present
invention. The different functional modules are circuits that can
be configured and implemented by software, firmware, hardware, or
any combination thereof. The function modules, when executed by the
processors 203 and 213 (e.g., via executing program codes 209 and
219), for example, allow base station 201 to schedule (via
scheduler 204), encode (via codec 205), mapping (via mapping
circuit 206), and transmit control information and data (via
control circuit 207) to UE 211, and allow UE 211 to receive,
de-mapping (via de-mapper 216), and decode (via codec 215) the
control information and data (via control circuit 217) accordingly
with interference cancellation capability. In one example, base
station 201 provides assistant information that include parameters
related to interfering signals to UE 211. Upon receiving the
related parameters, UE 211 is then able to perform interference
cancellation via interference canceller 214 to cancel the
contribution of the interfering signals accordingly. In another
example, UE 211 performs reference signal detection and
measurements and provides enhanced CSI feedback information via a
detector and feedback module FB 220 to BS 201. Upon receiving the
CSI feedback information, BS 201 can calculate the actual SINRs
based on different MUST scenarios and thereby determining the MCS
for the UE accordingly.
[0037] FIG. 3 illustrates a first embodiment for CSI feedback
enhancement in MUST scheme in a wireless communication network 300
in accordance with one novel aspect. Wireless communication network
300 comprises a base station 301, a near-user 302, and a far-user
303. In the example of FIG. 3, MUST is implemented in the first
Beam#1 but not in the second Beam#2. The received signal of the
near-user after the intercell-interference-plus-noise whitening is
given as:
y=H(u.sub.1( {square root over (.alpha.P/2)}x.sub.1,N+ {square root
over ((1-.alpha.)P/2)}x.sub.1,F)+u.sub.2 {square root over
(P/2)}x.sub.2)+w=h.sub.1( {square root over (.alpha.P/2)}x.sub.1,N+
{square root over ((1-.alpha.)P/2)}x.sub.1,F)+h.sub.2 {square root
over (P/2)}x.sub.2+w (5)
Where
[0038] H is the wireless channel matrix, h.sub.1=Hu.sub.1,
h.sub.2=Hu.sub.2 [0039] P is the total transmit power [0040]
.alpha. is the power splitting factor for MUST [0041] x.sub.1,N is
the modulated symbols intended for the near-user at the first beam
[0042] x.sub.1,F is the modulated symbols intended for the far-user
at the first beam [0043] x.sub.2 is the modulated symbol carried at
the second beam
[0044] Assume the near-user reports RI equal to two and the PMI
corresponding to [u.sub.1, u.sub.2]. Assume an MMSE receiver is
used by the near-user to separate signals in two beams. In this
case, the near-user feedback CQIs at the two beams correspond to
the SINRs at the first and the second beams are:
SINR fb ( 1 ) = ( P / 2 ) h 1 H ( I + ( P / 2 ) h 2 h 2 H ) - 1 h 1
= P 2 ( h 1 2 - ( P / 2 ) h 1 H h 2 2 1 + ( P / 2 ) h 2 2 ) ( 6 )
SINR fb ( 2 ) = P 2 ( h 2 2 - ( P / 2 ) h 2 H h 1 2 1 + ( P / 2 ) h
1 2 ) ( 7 ) ##EQU00002##
[0045] Assume perfect intra-beam IC at the near-user receiver. It
can be shown the actual output SINRs at the near-user receiver
are:
SINR actual ( 1 ) = .alpha. P 2 ( h 1 2 - ( P / 2 ) h 1 H h 2 2 1 +
( P / 2 ) h 2 2 ) ( 8 ) SINR actual ( 2 ) = P 2 ( h 2 2 - ( .alpha.
P / 2 ) h 2 H h 1 2 1 + ( .alpha. P / 2 ) h 1 2 ) ( 9 )
##EQU00003##
[0046] Note that since the MMSE receiver is used for beam
separation, the actual output SINR at the second beam is given as
in equation (9) instead of being equal to
(P/2).parallel.h.sub.2.parallel..sup.2. It can be seen that with
feedback CQI SINRs in equations (6) and (7), the scheduler is in
general unable to determine the true output SINR given in equation
(9). The reason is that there are three unknown variables
.parallel.h.sub.1.parallel..sup.2,.parallel.h.sub.2.parallel..sup.2,
and |h.sub.2.sup.Hh.sub.1|.sup.2 in equation (9), while we have
only two given SINRs from equations (6) and (7). If the UE further
reports the SINR given in the following equation (10) for a RANK-1
single beam transmission (may be applied with a scaling factor
.beta.), then the scheduler can calculate the SINR given in
equation (9):
SINR.sub.fb.sup.(3)=P.parallel.h.sub.1.parallel..sup.2 and/or
SINR.sub.fb.sup.(3)=P.parallel.h.sub.2.parallel..sup.2 (10)
[0047] FIG. 4 illustrates a second embodiment for CSI feedback
enhancement in MUST scheme in a wireless communication network 400
in accordance with one novel aspect. Wireless communication network
400 comprises a base station 401, a near-user 402, and a far-user
403. In the example of FIG. 4, different precoders are applied to
signals intended for the near- and far-users. The received signal
of the near-user is given as:
y=H(u.sub.1 {square root over (.alpha.P/2)}x.sub.N+u.sub.2 {square
root over ((1-.alpha.)P/2)}x.sub.F)+w=h.sub.1 {square root over
(.alpha.P/2)}x.sub.N+h.sub.2 {square root over
((1-.alpha.)P/2)}x.sub.F+w (11)
Where
[0048] H is the wireless channel matrix, h.sub.1=Hu.sub.1,
h.sub.2=Hu.sub.2 [0049] P is the total transmit power [0050]
.alpha. is the power splitting factor for MUST [0051] x.sub.N is
the symbol intended for the near-user [0052] x.sub.F is the symbol
intended for the far-user
[0053] We assume the near-user reports the RI equal to two and the
PMI corresponding to [u.sub.1, u.sub.2]. If an MMSE receiver is
used, the CQIs at the two beams corresponding to SINRs are:
SINR fb ( 1 ) = ( P / 2 ) h 1 H ( I + ( P / 2 ) h 2 h 2 H ) - 1 h 1
= P 2 ( h 1 2 - ( P / 2 ) h 1 H h 2 2 1 + ( P / 2 ) h 2 2 ) ( 12 )
SINR fb ( 2 ) = P 2 ( h 2 2 - ( P / 2 ) h 2 H h 1 2 1 + ( P / 2 ) h
1 2 ) ( 13 ) ##EQU00004##
[0054] In the signal reception, suppose the near-user can perfectly
cancel the signal intended for the far-user. Therefore, the
near-user actual received SINR is:
SINR.sub.actual=.alpha.P.parallel.h.sub.1.parallel..sup.2 (14)
[0055] The far-user actual received SINR is:
SINR actual = ( 1 - .alpha. ) P ( h 2 2 - ( .alpha. P ) h 2 H h 1 2
1 + ( .alpha. P ) h 1 2 ) ( 15 ) ##EQU00005##
[0056] If the UE further reports the SINR given in the following
equation (16) for a RANK-1 single beam transmission (may be applied
with a scaling factor .beta.), then the scheduler can calculate the
SINR given in equation (14) and (15):
SINR.sub.fb.sup.(3)=P.parallel.h.sub.1.parallel..sup.2 and/or
SINR.sub.fb.sup.(3)=P.parallel.h.sub.2.parallel..sup.2 (16)
[0057] FIG. 5 illustrates a third embodiment for CSI feedback
enhancement in MUST scheme in a wireless communication network 500
in accordance with one novel aspect. Wireless communication network
500 comprises a base station 501, a near-user 502, and a far-user
503. In the example of FIG. 5, MUST is implemented in the second
Beam#2 but not in the first Beam#1. In addition, the actual
transmission uses precoders [u.sub.2, u.sub.3], while the PMI
reported by the near-user corresponds to the matrix [u.sub.1,
u.sub.2]. As a result, the received signal of the near-user after
the intercell-interference-plus-noise whitening is given as:
y=H(u.sub.2 {square root over (P/2)}x.sub.1+u.sub.3( {square root
over (.alpha.P/2)}x.sub.2,N+ {square root over
((1-.alpha.)P/2)}x.sub.2,F))+w=h.sub.2 {square root over
(P/2)}x.sub.1+h.sub.3 {square root over (.alpha.P/2)}x.sub.2,N+
{square root over ((1-.alpha.)P/2)}x.sub.2,F)+w (17)
Where
[0058] H is the wireless channel matrix, h.sub.2=Hu.sub.2,
h.sub.3=Hu.sub.3 [0059] P is the total transmit power [0060]
.alpha. is the power splitting factor for MUST [0061] x.sub.1 is
the symbol carried at the first beam [0062] x.sub.2,N is the
modulated symbols intended for the near-user at the second beam
[0063] x.sub.2,F is the modulated symbols intended for the far-user
at the second beam
[0064] We assume the near-user reports the RI equal to two and the
PMI corresponding to [u.sub.1, u.sub.2]. If an MMSE receiver is
used, the SINRs corresponding to the CQIs reported by the near-user
are:
SINR fb ( 1 ) = ( P / 2 ) h 1 H ( I + ( P / 2 ) h 2 h 2 H ) - 1 h 1
= P 2 ( h 1 2 - ( P / 2 ) h 1 H h 2 2 1 + ( P / 2 ) h 2 2 ) ( 18 )
SINR fb ( 2 ) = P 2 ( h 2 2 - ( P / 2 ) h 2 H h 1 2 1 + ( P / 2 ) h
1 2 ) ( 19 ) ##EQU00006##
[0065] However, the actual SINRs perceived by the near-user and the
far-user in the received signal are:
SINR actual ( 1 ) = .alpha. P 2 ( h 1 2 - ( P / 2 ) h 2 H h 3 2 1 +
( P / 2 ) h 3 2 ) ( 20 ) SINR actual ( 2 ) = .mu. F P 2 ( h 2 2 - (
P / 2 ) h 2 H h 3 2 1 + ( P / 2 ) h 3 2 ) .mu. N P 2 ( h 2 2 - ( P
/ 2 ) h 2 H h 3 2 1 + ( P / 2 ) h 3 2 ) + 1 ( 21 ) ##EQU00007##
[0066] If the UE reports SINR in equation (22) below (may be
applied with a scaling factor .beta.) along with those in equations
(18) and (19), then the BS can compute the quantities
.parallel.h.sub.1.parallel..sup.2,.parallel.h.sub.2.parallel..sup.2,
and |h.sub.2.sup.Hh.sub.1|.sup.2. This helps to the determination
of the actual SINR in equation (20) if the scheduler can perform
estimation based on the available quantities:
SINR.sub.fb.sup.(3)=P.parallel.h.sub.1.parallel..sup.2 and/or
SINR.sub.fb.sup.(3)=P.parallel.h.sub.2.parallel..sup.2 (22)
[0067] FIG. 6 illustrates a downlink MUST procedure between a BS
and two UEs with enhanced CSI feedback in accordance with one novel
aspect. In step 611, a base station BS 601 periodically transmits
cell-specific reference signals (CRS) to UE 602 and UE 603. In step
612, UE 602 performs radio signal measurements based on the
periodically received CRS reference signals. In step 613, UE 603
performs radio signal measurements based on the periodically
received CRS reference signals. In step 614, UE 602 reports channel
state information (CSI) feedback to BS 601. In step 615, UE 603
reports CSI feedback to BS 601. The content of CSI feedback
contains RI (rank indicator), CQI (channel quality indicator), and
PMI (precoding matrix indicator) for each downlink channel. The
determination of CQI is based on the received
signal-to-interference-plus-noise ratio (SINR). In accordance with
one novel aspect, when UE 602 or UE 603 reports RI=Rank-2 with a
precoding matrix (PMI), besides two Rank-2 CQIs, one Rank-1 CQI is
also reported.
[0068] In step 621, based on the CSI feedback, BS 601 determines
the modulation and coding scheme (MCS) for the next to-be-scheduled
downlink transmission involving different MUST scenarios. In step
622, BS 601 allocates a time-frequency resource to multiple UEs
including UE 602 and UE 603 for MU-MIMO or NOMA operation. In step
623, BS 601 determines which parameters about interfering signals
need to be signaled to the UEs. In step 631, BS 601 signals UE 602
information about interfering signals dedicated to UE 603. In step
632, BS 601 signals UE 603 information about interfering signals
dedicated to UE 602. In step 641, UE 602 performs IC based on the
received information. In step 642, UE 603 performs IC based on the
received information.
[0069] FIG. 7 is a flow chart of a method of performing MUST with
enhanced CSI feedback from eNB perspective in accordance with one
novel aspect. In step 701, a base station transmits reference
signals to a plurality of user equipments (UEs) in a wireless
communication network. In step 702, the base station receives
channel state information (CSI) feedback from a first UE. The CSI
feedback comprises a RANK-2 channel quality indicator (CQI)
associated with a first beam and a second beam and a RANK-1 CQI
associated with a single beam. In step 703, the base station
schedules a downlink transmission to the first UE and a second
co-channel UE over an allocated time-frequency radio resource using
a multiuser superposition transmission (MUST) scheme. In step 704,
the base station determines a modulation and coding scheme (MCS)
for the first UE based on the received CSI feedback and the MUST
scheme. In one example, the RANK-2 CQI comprises a first feedback
signal to interference plus noise ratio (SINR) at the first beam
and a second feedback SINR at the second beam, and the RANK-1 CQI
comprises a third feedback SINR at the single beam measured by the
UE.
[0070] FIG. 8 is a flow chart of a method of performing MUST with
enhanced CSI feedback from UE perspective in accordance with one
novel aspect. In step 801, a UE measures reference signals from a
base station in a wireless communication network. In step 802, the
UE transmits channel state information (CSI) feedback to the base
station. The CSI feedback comprises a RANK-2 channel quality
indicator (CQI) associated with a first beam and a second beam and
a RANK-1 CQI associated with a single beam. In step 803, the UE
receives a downlink transmission scheduled to the UE and a second
co-channel UE over an allocated time-frequency radio resource using
a multiuser superposition transmission (MUST) scheme. In step 804,
the UE applies a modulation and coding scheme (MCS) received from
the base station, wherein the MCS is determined based on the CSI
feedback and the MUST scheme. In one example, the RANK-2 CQI
comprises a first feedback signal to interference plus noise ratio
(SINR) at the first beam and a second feedback SINR at the second
beam, and the RANK-1 CQI comprises a third feedback SINR at the
single beam measured by the UE.
[0071] Although the present invention has been described in
connection with certain specific embodiments for instructional
purposes, the present invention is not limited thereto.
Accordingly, various modifications, adaptations, and combinations
of various features of the described embodiments can be practiced
without departing from the scope of the invention as set forth in
the claims.
* * * * *