U.S. patent application number 14/298763 was filed with the patent office on 2014-11-27 for method and system of operating a multi-user system.
The applicant listed for this patent is Mediatek Inc.. Invention is credited to Ravishankar H. MAHADEVAPPA, Thomas E. PARE, JR., Yung-Szu TU, Kiran ULN, Cheng-Hsuan WU.
Application Number | 20140348082 14/298763 |
Document ID | / |
Family ID | 44901859 |
Filed Date | 2014-11-27 |
United States Patent
Application |
20140348082 |
Kind Code |
A1 |
PARE, JR.; Thomas E. ; et
al. |
November 27, 2014 |
METHOD AND SYSTEM OF OPERATING A MULTI-USER SYSTEM
Abstract
A method and system is disclosed for grouping the multiple
stations connected to an access point (AP). The system and method
comprise sending a sounding packet to a plurality of stations,
wherein the stations may be all or part of the stations that are
located within the range of the AP. The stations that receive the
sounding packets respond to the AP, and the AP determines the
channel state information (CSI) from the responses. According to
the CSI, the AP divides the multiple stations into several groups.
According to an embodiment of the present invention, a confirmation
step is performed to each group of stations, respectively. The AP
sends a second sounding packet to each group of stations, and
verifies the CSI between each station group by group. Therefore,
the method and system provides for monitoring the validation of
each group by periodically sending sounding packets to each
group.
Inventors: |
PARE, JR.; Thomas E.;
(Mountain View, CA) ; WU; Cheng-Hsuan; (Jhubei
City, TW) ; TU; Yung-Szu; (Jhubei City, TW) ;
ULN; Kiran; (Pleasanton, CA) ; MAHADEVAPPA;
Ravishankar H.; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mediatek Inc. |
Hsin-Chu |
|
TW |
|
|
Family ID: |
44901859 |
Appl. No.: |
14/298763 |
Filed: |
June 6, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13041177 |
Mar 4, 2011 |
8824386 |
|
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14298763 |
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61331476 |
May 5, 2010 |
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61345108 |
May 15, 2010 |
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Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04B 7/0456 20130101;
H04W 24/00 20130101; H04W 8/186 20130101; H04W 4/08 20130101; H04B
7/0626 20130101; H04W 4/21 20180201; H04B 7/0465 20130101; H04W
24/10 20130101; H04B 7/0452 20130101 |
Class at
Publication: |
370/329 |
International
Class: |
H04B 7/04 20060101
H04B007/04; H04W 8/18 20060101 H04W008/18; H04B 7/06 20060101
H04B007/06; H04W 4/08 20060101 H04W004/08 |
Claims
1. A method for a transmitter in a multi-user system, comprising:
sending a non-precoded sounding request to a plurality of
receivers, wherein the transmitter gathers channel state
information from each receiver and divides the plurality of
receivers into at least one group; sending a precoded sounding
request to the at least one group; and receiving signal to
interference and noise ratio (SINR) information measured at the
plurality of receivers.
2. The method of claim 1, further comprising, sending an initial
sounding to the plurality of receivers before the step of sending
the non-precoded sounding request; and gathering channel state
information from the plurality of receivers.
3. The method of claim 2, wherein the initial sounding is a null
data packet.
4. The method of claim 1, wherein the non-precoded sounding request
is a null data packet.
5. The method of claim 1, further comprising, sending a multi-user
packet to the at least one group of receivers; and measuring SINR
information by the at least one group of receivers after receiving
the multi-user packet.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Under 35 U.S.C. 120, this application is a Continuation
Application and claims priority to U.S. application Ser. No.
13/041,177, filed Mar. 4, 2011, entitled "METHOD AND SYSTEM OF
OPERATING A MULTI-USER SYSTEM," which claims the benefit of U.S.
Provisional Application No. 61/331,476 filed on May 5, 2010,
entitled "METHOD FOR GROUPING WITH SOUNDING PACKETS," and U.S.
Provisional Application No. 61/345,108, filed on May 15, 2010,
entitled "METHOD OF OPERATING MULTI-USER SYSTEM," all of which are
incorporated herein by reference.
[0002] The present invention is related to co-pending U.S. patent
application Ser. No. 13/041,068 (2010-P012US/4924P), entitled
"METHOD AND SYSTEM OF OPERATING A MULTI-USER SYSTEM," filed on Mar.
4, 2011 and assigned to the assignee of the present invention,
which is also incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to a method of operation in a
multi-user system, and more particularly, to a method of grouping
operation in a multi-user system.
BACKGROUND
[0004] A wireless local area network (WLAN) is widely used to
provide access to the internet with mobile devices. To improve the
throughput in a WLAN, the IEEE 802.11n standard adopts a multiple
input multiple output (MIMO) system that transmits a plurality of
streams with multiple antennas and, at the same time, receives the
streams with multiple antennas. However, the IEEE 802.11n standard
is still based on a point-to-point transmission scheme. When there
are more stations connected to the access point (AP), the AP has to
hold the transmission for some stations while transmitting to
another and wait for an empty time slot.
[0005] In order to transmit to multiple stations (STA) on the same
channel (frequency band) in a multi-user (MU) system, the downlink
(DL) channels may be orthogonalized to create virtual spatial
channels. Some conventional systems are described in "Zero-Forcing
Methods for Downlink Spatial Multiplexing in Multi-User MIMO
Channels", IEEE Transactions On Signal Processing, Vol. 52, No. 2,
February 2004, Q. Spencer, A. L. Swindlehurst, and M Haardt. In
this publication, and in later references, a set of pre-coding
matrices is computed at the AP for a group of downstream users. The
effective pre-coded channel is block diagonalized so that the
interference between stations is minimized.
[0006] For this system to work effectively there must be
cooperation between the AP and the remote stations. First, the
stations must feedback channel information to the AP for pre-coder
computation. In certain cases where the individual station channels
are highly correlated, pairs of stations may not be compatible to
be grouped for MU transmission due to a poor solution space for MU
pre-coding matrices. In addition, MU channels are more sensitive to
changing channel conditions, since variations in the channel can
lead to loss of pre-coded orthogonality and quickly lead to high
interference conditions. Many designs only consider grouping
without regard to compatibility of the stations, while other
systems do not provide real time metrics to assess the status of
the channel condition. These two factors can limit the performance
of a MU-MIMO downlink (DL). Hence, there's a need to provide a
method to form stations into compatible multi-user (MU) groups, and
the need for a mechanism to quickly identify interference
conditions caused by variations in the wireless MU channel.
[0007] Therefore, a multi-user (MU), or multi-station, transmission
system with MIMO system can be provided. The AP in a MU-MIMO system
can transmit data to multiple stations either with multiple
antennas or a single antenna at the same time, such that the AP can
serve more stations at the same time. However, there's still a need
for a method to improve the reliability and functionality of a MU
system.
SUMMARY OF INVENTION
[0008] It is therefore an object of the present invention to
provide a method for grouping the multiple stations connected to an
AP. The method comprises sending a sounding packet to a plurality
of stations, wherein the stations may be all or part of the
stations that are located within the range of the AP. The stations
that receive the sounding packets respond to the AP, and the AP
determines the channel state information (CSI) from the responses.
According to the CSI, the AP divides the multiple stations into
several groups. According to an embodiment of the present
invention, a confirmation step is performed to each group of
stations, respectively. The AP sends a second sounding packet to
each group of stations, and verifies the CSI between AP and each
station group by group.
[0009] In another embodiment of this invention, the AP will compute
the pre-coding matrices and determine the compatibility between
stations of a certain group by computing a certain gain metric and
comparing it to a preset threshold limit.
[0010] It is therefore another object of the present invention to
provide a method for monitoring the validation of each group by
periodically sending sounding packets to each group.
[0011] It is therefore another object of the present invention to
provide a method for monitoring the validation of each group by
periodically sending sounding packets to each group in order to
measure the interference between each of the stations and feed this
information back the MU AP.
[0012] In another embodiment of this invention, the AP will
continuously monitor the interference levels between each of the
stations transmit streams, resound the down stream channels and
re-compute the pre-coding matrices when the interference levels
reach a pre-set threshold.
[0013] In yet another embodiment of this invention the AP will
continuously monitor the interference levels between each of the
stations transmit streams and eliminate a particular station from
the group when the interference levels reach a pre-set
threshold.
BRIEF DESCRIPTION OF DRAWINGS
[0014] The accompanying drawings illustrate an embodiment of the
present invention and, together with the description, serve to
explain the principle of the invention. One skilled in the art will
recognize that the particular embodiments illustrated in the
drawings are merely exemplary, and are not intended to limit the
scope of the present invention.
[0015] FIG. 1 illustrates a flow chart of grouping stations
validation according to an embodiment of the present invention.
[0016] FIG. 2 illustrates a block diagram of DL MIMO AP to 3
stations according to an embodiment of the present invention.
[0017] FIG. 3 illustrates a flowchart for forming groups for MU
operation according to an embodiment of the present invention.
[0018] FIG. 4 illustrates a flowchart of multi-station AP sounding
the channels to 3 stations with (a) unbeamformed CSI feedback, and
(b) beamformed sounding for MU group validation according to an
embodiment of the present invention.
[0019] FIG. 5 illustrates a flowchart of group formation/validation
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] The present invention relates to a method of operation in a
multi-user system, and more particularly, to a method of grouping
operation in a multi-user system. The following description is
presented to enable one of ordinary skill in the art to make and
use the invention and is provided in the context of a patent
application and its requirements. Various modifications to the
embodiments and the generic principles and features described
herein will be readily apparent to those skilled in the art. Thus,
the present invention is not intended to be limited to the
embodiments shown, but is to be accorded the widest scope
consistent with the principles and features described herein.
[0021] FIG. 1 illustrates a flow chart 100 of grouping stations
according to an embodiment of the present invention. There are
three modes for grouping stations, the initial sounding mode 102,
then a grouping mode 104, and then a validation mode 106.
[0022] A method and system that provides for the grouping of
stations in accordance with the present invention can take the form
of an entirely hardware implementation, an entirely software
implementation, or an implementation containing both hardware and
software elements. In one implementation, this grouping system and
method is implemented in software, which includes, but is not
limited to, application software, firmware, resident software,
microcode, etc.
[0023] Furthermore, the detection procedure can take the form of a
computer program product accessible from a computer-usable or
computer-readable medium providing program code for use by or in
connection with a computer or any instruction execution system. For
the purposes of this description, a computer-usable or
computer-readable medium can be any apparatus that can contain,
store, communicate, propagate, or transport the program for use by
or in connection with the instruction execution system, apparatus,
or device.
[0024] The medium can be an electronic, magnetic, optical,
electromagnetic, infrared, or semiconductor system (or apparatus or
device) or a propagation medium. Examples of a computer-readable
medium include a semiconductor or solid state memory, magnetic
tape, a removable computer diskette, a random access memory (RAM),
a read-only memory (ROM), a rigid magnetic disk, and an optical
disk. Current examples of optical disks include DVD, compact
disk-read-only memory (CD-ROM), and compact disk-read/write
(CD-R/W). To describe the features of the present invention in more
detail, refer now to the following description in conjunction with
the accompanying Figures.
[0025] Initial Sounding 102. The initial sounding mode 102 is
executed to sound the channels without targeting any specific
group. The sounding could be in the form of a null data packet
(NDP) dedicated for sounding or it could be accomplished as part of
a MU downstream data packet. In this stage, the packets are not
beamformed or pre-coded, and the AP requests the channel state
information from the stations. In response to the initial sounding
from the access point (AP), the stations (STAs) send back channel
state information (CSI) to the AP. Then, the AP uses the CSI from
each of the STAs to determine the group definitions and informs
STAs of the group membership.
[0026] In order to improve the performance in a MU-MIMO system, the
AP first performs an initial sounding by sending a sounding packet
to a large number of stations. These stations may be all or part of
the stations within the range covered by the AP. The stations that
receive the sounding packet send a respond to the AP. The AP
obtains channel state information (CSI) from the responses. The
stations are then divided into groups in some logical order. One
possible way is to roughly group them according to some measure of
orthogonalization.
[0027] For example, the CSI returned to the AP for each station is
stacked in a column matrix Htot_csi, as,
Htot_csi=[H0; H1; H2 . . . Hn]
wherein n is the number of stations sending a response to the
AP.
[0028] Since the grouping is determined based on CSI, the stations
within the same group might be stations physically close to each
other. On the contrary, stations within the same group might be
spaced apart, such that the interference is minimized. Moreover,
each station may be grouped into one or more groups.
[0029] After the initial grouping, the AP may then sound to a
particular group by sending a sounding packet again. This sounding
step may be performed with a beamformed (or pre-coded) packet or a
non-beamformed packet. If the packet is not beamformed, this step
may be combined with the initial sounding.
[0030] Group sounding 104. The group sounding could be executed to
sound the channels of specific groups. They could be either
null-data-packet (NDP) sounding or they could be performed with
multicast (to a specific group) data. Packets are not beamformed or
precoded. During group sounding, packets are transmitted from AP,
and then the STAs in the targeted group send back CSI to the AP.
Then, the AP determines the number of space-time streams for each
STA and, accordingly, the steering/precoding matrix.
[0031] For example, if the packet is not beamformed, for a
particular group having 3 stations, the group CSI Hgroup at the AP
may be expressed as:
Hgroup = [ h 00 h 01 h 02 ; h 10 h 11 h 12 ; h 20 h 21 h 22 ] ;
##EQU00001##
[0032] In this case, each element could be a row vector, and the
"coupling terms" are the off-diagonal terms. If they are large
compared to the diagonal terms h00, h11, h22, then there will be
considerable interference between the stations, which is not good
for performance.
[0033] Therefore, the AP computes a pre-coding, or steering matrix
V=[V1 V2 V3] such that
Hgroup .times. V * .apprxeq. [ hv 00 0 0 ; 0 hv 11 0 ; 0 0 hv 22 ]
; ##EQU00002##
[0034] Because of imperfections at the transmitter, and any
variable channel conditions, the zero terms will not be identically
zero, but close to zero, or comparatively small with the diagonal
terms.
[0035] Validation sounding 106. These could be executed to sound
the effective channels of specific groups with beamforming or
precoding. They could be NDP sounding or performed with DL MU-MIMO
(to a specific group) data. Packets are beamformed or precoded.
After validation, sounding packets are transmitted from the AP. The
STAs in the targeted group either: (1) Send back the CSI
corresponding the pre-coded channel to the AP; or (2) compute the
signal-to-interference ratio (SINR) of the preamble or data, and
send it back to the AP. Then, the AP determines whether the
grouping is valid or the steering/precoding matrix has to be
updated.
[0036] If the grouping is not valid, then: (1) first go to the
initial sounding mode to determine the new group; or (2) directly
determine the new group definitions and go to the group sounding
mode.
[0037] If the steering/precoding has to be updated, then: (1) go to
the group sounding mode to determine the new steering/precoding
matrix.
[0038] According to another embodiment of the present invention, a
validation step is performed in which the AP sends a beamformed
sounding packet, which could be null data packet (NDP). The CSI
beamformed at this step should be "diagonal" as above. The AP
double-checks the magnitude of the off-diagonal terms in the
matrix. The relative size of these terms (using any vector norm, or
Frobenius norm, for example, if these are MIMO stations, since
these off-diagonal terms could be matrices) with respect to the
diagonal terms will determine how much interference there will be
between the stations, i.e. signal-to-interference ratio (SINR)
value. After this step, the AP is able to finally choose a proper
modulation and coding scheme (MCS), based on signal to noise (SNR)
(RSSI), and SINR, and prescribe the number of long training frames
(LTF) per station. If the metrics are satisfactory, data
transmission mode can start after this step.
[0039] According to another embodiment of this invention, a steady
state mode is also performed where the AP may periodically send
non-beamformed sounding packets to the group to periodically update
a steering matrix V. Intermittently, the AP can send beamformed
sounding packets to keep track of the SINR/SNR of the downlink
condition of the multi-station (MU-DL). This metric can be used to
re-assess the group, such as removing a particular station, which
has lost orthogonality, or re-grouping, such as effectively killing
the group, and starting the grouping process over, when the channel
changes too drastically.
[0040] Moreover, once SINR validation is used, SINR can also be
used to assign the number of spatial streams (Nss) to each
station.
[0041] At the validation/assignment step, the system may re-compute
the steering matrices. For example, a 2Rx STA may only be assigned
Nss=1, because the residual coupling on one Rx chain was high. In
this case, the second Rx chain will be ignored. The second chain
will then correspond to a direction not used for data; instead,
this direction will contain cross-coupling interference. In this
case, the Rx may choose to shut down (power down) the second
receiver chain in order to minimize effective noise from entering
the Rx front end.
[0042] According to an embodiment, the AP selects the Rx-chain,
rather than the antenna, for each station and re-computes the
steering matrices, using unused directions to improve isolation (or
reduce interference) between stations.
Implementation
[0043] According to an embodiment 200 of the present invention,
when there are 3 stations within a MU network 202, such as in the
case in FIG. 2, the simultaneous channels 204, 206 and 208 to the
three STAs 210, 212 and 214 can be expressed as:
H = [ H 1 H 2 H 3 ] ##EQU00003##
and the signal received at the remote stations 210, 212 and 214 can
be expressed as (neglecting noise terms):
[ Y ] = [ Y 1 Y 2 Y 3 ] = [ H 1 H 2 H 3 ] X ##EQU00004##
[0044] Therefore, in this embodiment, the transmitted signal X may
contain components for each station and each is pre-coded by the
matrix V:
X = V [ X 1 X 2 X 3 ] = [ V 1 V 2 V 3 ] [ X 1 X 2 X 3 ]
##EQU00005##
[0045] In general each station can be transmitted multi-stream
data:
X i = [ x 1 x n ] . ##EQU00006##
[0046] The precoding matrices
V=[V1 V2 V3]
are computed such that the effective channels 204, 206 and 208 to
the independent STAs 210, 212 and 214 are orthogonal, or
independent of one another. In other words, transmission to one STA
does not interfere with the other STA in the MU group.
[0047] According to an embodiment of the present invention, the
matrices V.sub.i in general need to be orthogonal to the channel
matrices corresponding to the other stations in the group. That
is;
H.sub.jV.sub.i=0, i.noteq.j.
[0048] In this case, the columns of the precoding matrices V.sub.i
are said to lie in the Null Space of H.sub.j. Candidate precoding
matrices V.sub.i can be computed using the QR decomposition.
[0049] The QR decomposition can be used to compute null spaces.
Consider a general complex matrix A, which is m.times.n, m<n.
Then, the QR decomposition can be used to decompose the matrix
as
A*P=QR=[Q.sub.1 Q.sub.2]R,
where P is a permutation matrix, and the relative dimensions are
specified as:
Q:n.times.n
R:n.times.m
Q.sub.1:n.times.m
Q.sub.2:n.times.(n-m)
[0050] Given this partition, the orthogonality condition
results:
AQ.sub.2=0.
[0051] In this case, the columns of Q.sub.2 span the nullspace (or,
right nullspace) of the matrix A. This is expressed as:
N(A)=Q.sub.2.
[0052] Hence, according to an embodiment of the present invention,
in order to find the precoding steering matrices for the 3 stations
system described above, the following conditions must hold:
V 1 .di-elect cons. N ( [ H 2 H 3 ] ) = def N 1 ##EQU00007## V 2
.di-elect cons. N ( [ H 1 H 3 ] ) = def N 2 ##EQU00007.2## V 3
.di-elect cons. N ( [ H 1 H 2 ] ) = def N 3 ##EQU00007.3##
[0053] More precisely, the columns of the precoding matrices
V.sub.i might have belonged to the nullspace N.sub.i, which is
called Condition 1, and could be expressed as:
CONDITION 1 : { N 1 = { v .di-elect cons. C n [ H 2 H 3 ] v = 0 } N
2 = { v .di-elect cons. C n [ H 1 H 3 ] v = 0 } N 3 = { v .di-elect
cons. C n [ H 1 H 2 ] v = 0 } ##EQU00008##
[0054] Condition 1 simply means that the precoding matrices for a
particular station must lie in a Nullspace of the other stations,
to guarantee low interference between the stations. While this
condition will guarantee low interference in the MU group, for good
performance, the transmission to the stations has to be strong. In
other words, there's a need for a second condition to guarantee
reasonable transmission:
CONDITION 2 : { H 1 N 1 .noteq. 0 H 2 N 2 .noteq. 0 H 3 N 3 .noteq.
0 ##EQU00009##
[0055] Here, condition 2 means that the channel to each station can
not lie in the Nullspace of the other stations. This can happen, in
general, if two stations are strongly correlated. The chance that
any of the conditions in Condition 2 will not happen "precisely"
(meaning that one of the equations in (condition 2) is exactly
"zero") is probably small. The design of the precoding matrices
should be done to minimize the chance of Condition 2 occurring, as
described below.
[0056] To minimize the chance that Condition 2 occurs, according to
an embodiment of the present invention, the precoding matrices are
selected to maximize the received strength of transmission to each
station. That is, after the corresponding nullspaces are found, the
following optimization problem has to be solved:
max.sub.V.sub.i.sub..di-elect
cons.N.sub.i.parallel.H.sub.iV.sub.i.parallel..
[0057] For example, in a particular scenario, where the stations
are either 1 or 2 spatial streams capable. For these cases,
consider the dimensions such that H is 5.times.8, and the
individual stations are defined by the following scenario:
Scenario : H ( 5 .times. 8 ) = { H 1 is 2 .times. 8 H 2 is 2
.times. 8 H 3 is 1 .times. 8 ##EQU00010##
[0058] For these stations, the corresponding Nullspaces dimensions
are
Nullspaces : { N 1 is 8 .times. 5 N 2 is 8 .times. 5 N 3 is 8
.times. 4 ##EQU00011##
[0059] For STA3 214, which is one stream, the dimension of N.sub.3
is 8.times.4, and can be written a set of vectors:
N.sub.3=[n.sub.1 . . . n.sub.4]
[0060] The precoding matrix V.sub.3 is 8.times.1, and can be
written as a linear combination of the columns of N.sub.3, as:
V 3 = N 3 a , where is a is a vector of coefficients as a = [ a 1 a
4 ] . ##EQU00012##
[0061] In order to choose a to maximize the signal strength at the
3.sup.rd receiver. The received signal is expressed as:
Y 3 = H 3 V 3 X 3 = H 3 N 3 aX 3 . ##EQU00013##
[0062] The problem then reduces to finding the vector a that gives
good transmission through the effective channel:
H.sub.3N.sub.3h.sub.3.
[0063] In this one spatial stream (1SS) case, h.sub.3 is a
1.times.4 complex row vector. The energy maximizer is then:
.sub.3h*.sub.3/.parallel.h*.sub.3.parallel.
[0064] That is, the maximizer is the complex conjugate of the
vector H.sub.3. In this case h.sub.3 is the projection of the
Nullspace of the STA1 210 and STA2 212 channels onto the channel of
STA3 214. The maximizer .sub.3 will maximize the energy gain
through this projection space.
[0065] The optimal precoding matrix is then:
{N.sub.3 .sub.3}(k)=V.sub.3(k).
[0066] The final step is to assess the MU-MIMO gain through the
channel, to determine if the MU constraint was too restrictive for
this particular station. One possible metric approach is to compare
the MU-Gain to the unrestricted BF gain:
MU-GAIN(k)=|H.sub.3V.sub.3|/.parallel.H.sub.3.parallel.(k).
[0067] This metric is summed over all N.sub.sc subcarriers. It
provides a measure of how much energy was lost by restricting the
precoder to lie in the Nullspace N.sub.3. Only include this 1SS
station if the MU-GAIN is above a threshold:
[0068] MU Group Requirement:
k = 0 k = N sc H 3 V 3 H 3 ( k ) > THRESH . ##EQU00014##
[0069] Please note that, in practice, {N.sub.3
.sub.3}(k)=V.sub.3(k) would be computed for each subcarrier k. Each
subcarrier then has an 8-element precoding coefficient. The squared
sum for each Tx component is computed across all subcarriers, and
the total is stored for each Tx. The maximum of the 8-energies is
determined, and this value is scaled so that the maximum energy at
the strongest transmitter does not overdrive the Tx PA. The final
normalizing step should be done after all the precoding matrices
are computed for each STA, to make sure the MU power constraint is
satisfied.
[0070] Note also that the solution is the 2-norm maximizer which is
also the maximum power gain through the channel sub-space. However,
other solutions may be chosen to maximize the 1-norm or
.infin.-norm, for different emphasis.
[0071] Note also that if the MU Group requirement is not satisfied,
the station can be removed from the group, the number of stations
reduced by 1, and the Nullspaces recalculated.
[0072] Note also that the MU-Gain threshold may be dependent on
RSSI. It could happen that the signal strength to the particular
station is so strong that the threshold is relaxed.
[0073] According to another embodiment of the present invention,
for STA1 210 and STA2 212, which are 2-stream devices, the
dimensions of N.sub.1,2 are 8.times.5, and can be written according
to a set of vectors:
N.sub.1,2=[n.sub.1 . . . n.sub.5].
[0074] In this case, the precoding matrices V.sub.1,2 are
8.times.2, and of the form: V.sub.1=N.sub.1A, where N.sub.1 is
8.times.5 and A is a 5.times.2 complex matrix, and the received
signal expressed as:
Y 1 = H 1 V 1 X 1 = H 1 N 1 AX 1 . ##EQU00015##
[0075] The problem then reduces to finding the directions that give
good transmission through the effective 2.times.5 channel:
H.sub.1N.sub.1h.sub.1.
[0076] In this case, the maximum power gain objective leads to an
SVD solution for the matrix A. Specifically, if h.sub.1=U.SIGMA.V*
then the optimal solution for the 2SS transmission is: =V. For a 2
spatial stream (2SS) channel, the SVD has a closed form
solution.
[0077] At this step, the algorithm can check the condition of the
channel to assess whether or not 2 spatial streams is feasible
under the MU channel assumptions. This can be done by comparing the
relative strength of the two singular values in .SIGMA., to make
sure the smaller value is not attenuated too much. If the MU-AP
concludes only 1SS is possible to STA1 210, or STA2 212, it can
assign 1SS.
[0078] Please note that two normalization steps are needed, one
assuming 1SS transmission, the other assuming 2SS is sent. Both
need to normalize the power at each of the 8 Tx so that none are
over-driven into a poor EVM state. The first is just the same as in
the 1SS station, since {N.sub.3 .sub.1}(k)=V.sub.1(k), for 1SS,
only .sub.1, the first column of =[ .sub.1 .sub.2] is used for
transmission.
[0079] For 2SS, the magnitude of each row of {N.sub.1
.sub.1}(k)=V.sub.1(k) is computed, and summed over the subcarriers.
Again, the output of the precoding matrices corresponding to each
of the transmitters is scaled to prevent the maximum power from
saturating any of the eight transmitters.
[0080] In summary, the Precoding Design described above can be
integrated into a process that can be executed by a MU capable AP.
In the process of computing the precoders, the membership in each
group is also assessed using a metric; i.e., the MU groups are
formed as part of the computation of the MU precoders.
[0081] Note also, that although the preceding grouping process
required CSI as measured by the channel matrices, the identical
process can be achieved using alternate representation for CSI. As
an example, consider the singular value decomposition for each
channel given by: H=U.SIGMA.V*. The same steps above can be
followed by substituting the H=.SIGMA.V*. In this case, the
requisite null spaces and peak input gain directions can be
calculated using this alternate CSI information.
[0082] Please refer to FIG. 3, which illustrates a process flow 300
according to an embodiment of the present invention. At Step 302,
the candidate MU capable STA associated with the AP are sounded,
and the channel state information (CSI) for each station is
gathered to form a pool of candidate stations. A subset of these
stations is formed into a candidate group of N.sub.C stations. The
criteria for initial group formation at Step 302 might include
similar types of data traffic, or some initial orthogonality test
to ascertain a group of stations' compatibility for MU
operation.
[0083] At Step 304, the Null spaces of all the candidate stations
in the group are computed.
[0084] At Step 306, the precoding matrix V.sub.i are computed for
each station.
[0085] At Step 308, the transmission quality of the precoder is
computed, and tested against a threshold. If the threshold
condition is met at Step 310, this particular station is considered
a group member, and the next station is considered, at Step 312.
The process Steps 306, Step 308, and Step 310 are then repeated for
the next indexed group candidate.
[0086] At step 310, if a particular candidate is not considered a
valid group member, that station is disqualified from the group, as
shown in Step 314. In this case, that candidate CSI is eliminated
from the set of channel matrices (Step 302), and the corresponding
Nullspaces for each candidate recomputed (Step 304). The iterative
process of Steps 306, Step 308 and Step 310 are repeated for each
station in the candidate group.
[0087] After the final membership in the Group is established, by
using the flow 300 shown in FIG. 3, the final precoding scheme can
be validated by issuing a sounding request to the group. This
sounding request packet, via step 316, should be sent prior to the
start of data, and can be used to assess the actual levels of
interference, via step 318, between the stations. After the final
group assessment and during the MU data transmission, a periodic
MU-Sounding frame can be issued to the active members of the group
in order to continuously monitor the interference levels, via step
310, between stations. The interference can be expected to change
with varying channel conditions, and occasionally the members of
the group will need to be adjusted. Periodic MU-sounding will
enable the AP to move some stations out of the group, by detecting
interference conditions that exceed preset limits at step 318, via
step 320, and validate new stations as members.
[0088] The formation and validation of a station group is shown as
a two-step sounding process in FIGS. 4a and 4b. In FIG. 4a, the
initial sounding is done to the three stations 410-414 so that the
AP 402 has the CSI information of these channels 406-409 for each
station. The MU CSI is the aggregated channel response:
H ^ = [ H ^ 1 H ^ 2 H ^ 3 ] ##EQU00016##
[0089] The precoding matrices V1, V2, and V3 are computed as
described above. Then the second sounding stage as shown in FIG. 4B
is shown to ensure that the interference between the stations
410-414 is acceptable. The interference levels are measured in the
off-diagonal terms of the aggregate sounding response:
H ^ precoded = [ H ^ 1 H ^ 2 H ^ 3 ] [ V 1 V 2 V 3 ]
##EQU00017##
MU-Sounding Response:
[0090] = [ H ^ 1 V 1 H ^ 1 V 2 H ^ 1 V 3 H ^ 2 V 1 H ^ 2 V 2 H ^ 2
V 3 H ^ 3 V 1 H ^ 3 V 2 H ^ 3 V 3 ] ##EQU00018##
[0091] In this aggregated response, each row corresponds to the
MU-Sounding response. The diagonal terms correspond to the
particular station's beamformed channel, such as H.sub.1V.sub.1 for
STA1 410, whereas the off-diagonal responses, such as
H.sub.1V.sub.2 and H.sub.1V.sub.3 represent the measured
interference levels. In the system design, the each STA 410-414 can
send MU-Sounding response so that the AP 402 can estimate the
interference, and validate the MU group, or the each STA 410-414
can estimate the interference from the other stations by comparing
relative strength of the diagonal terms to the off-diagonal terms
(this is referred to as the Signal-to-Interference Ratio, or SINR,
as also shown in FIG. 5). The off-diagonal terms should be
relatively small to indicate good isolation between the
stations.
[0092] An example from the above 3 station's scenario of
interference metrics that can be measured at the station 1 and sent
back to the AP as part of an interference feedback can be
represented as:
SINR 1 ( i ) = H ^ 1 V 1 H ^ 1 V i , i = 2 , 3. ##EQU00019##
[0093] In this case, the metrics SINR.sub.1(1) and SINR.sub.1(2)
are simply the ratio of measured energy from the other stations, 2
and 3, respectively, with respect to the signal energy for the
intended station, which is represented by:
.parallel.H.sub.1V.sub.1.parallel..
[0094] Through period MU-sounding, if the interference is detected
to be too large, then the interfering stations can be separated
into different groups (regrouping) by removing one of the
particular stations. This detection and regrouping is indicated by
steps 316, 318 and 320 in FIG. 3.
[0095] The MU group process 500 described can be implemented as a
three stage process shown in FIG. 5, whereby the initial group is
formed by computing the Vi precoding matrices. The stages comprise
a group initialized state 502, a validation state 504 and a MU Data
stage 500. The initial candidate group is then validated prior to
commencing MU-data 500 operation by issuing a MU sounding packet,
as discussed above. The interference levels between the stations
are then measured at the individual stations and fed back to the
AP, via step 510, or at the AP using the MU Sounding response
(beamformed). This is shown as the Validation Phase 504 in FIG. 5.
If the measured interference levels between all the stations are
acceptable, then MU-Data 506 mode can start. If not, the stations
are recognized via step 513. In this mode, the validate precoding
matrices are used to transmit data to the remote stations
simultaneously.
[0096] During the MU-Data stage 506, periodic MU-soundings, via
step 512, are issued to the stations in the group to make sure
channel conditions have not corrupted the MU data 506 operation,
via step 516. In addition, the process extends to allow an
operation MU-AP to form new groups, as channel conditions change
(discussed above, by removing an interfering station), or by
accommodating new MU STAs 514 that may join the network, via step
518. As a new station joins (as shown in FIG. 5), via step 514,
this station can be grouped with existing stations, via step 508,
into a candidate group after an initial unbeamformed sounding
request/response. The new group is then validated prior to the
start of MU-data 512 transmission, as discussed above.
[0097] Although the present invention has been described in
accordance with the embodiments shown, one of ordinary skill in the
art will readily recognize that there could be variations to the
embodiments and those variations would be within the spirit and
scope of the present invention. Accordingly, many modifications may
be made by one of ordinary skill in the art without departing from
the spirit and scope of the appended claims.
* * * * *