U.S. patent application number 13/045944 was filed with the patent office on 2012-09-13 for method for determining beamforming parameters in a wireless communication system and to a wireless communication system.
This patent application is currently assigned to Fraunhofer-Gesellschaft zur Foerderung der angewandten Forschung e. V.. Invention is credited to Wilhelm KEUSGEN, Andreas KORTKE, Michael PETER.
Application Number | 20120230380 13/045944 |
Document ID | / |
Family ID | 45774231 |
Filed Date | 2012-09-13 |
United States Patent
Application |
20120230380 |
Kind Code |
A1 |
KEUSGEN; Wilhelm ; et
al. |
September 13, 2012 |
METHOD FOR DETERMINING BEAMFORMING PARAMETERS IN A WIRELESS
COMMUNICATION SYSTEM AND TO A WIRELESS COMMUNICATION SYSTEM
Abstract
A method for determining a beamforming vector or a beamforming
channel matrix in a communication system including a transmitting
station and a receiving station, and a communication system are
described. The transmitting and receiving stations include
respective antenna groups and respective codebooks include a
plurality of predefined beamforming vectors for the antenna
group.
Inventors: |
KEUSGEN; Wilhelm; (Berlin,
DE) ; PETER; Michael; (Berlin, DE) ; KORTKE;
Andreas; (Berlin, DE) |
Assignee: |
Fraunhofer-Gesellschaft zur
Foerderung der angewandten Forschung e. V.
Munich
DE
|
Family ID: |
45774231 |
Appl. No.: |
13/045944 |
Filed: |
March 11, 2011 |
Current U.S.
Class: |
375/227 ;
375/224 |
Current CPC
Class: |
H04B 7/0691 20130101;
H04B 7/088 20130101; H04B 7/0465 20130101; H04B 7/0695 20130101;
H04B 7/0874 20130101; H04B 7/0482 20130101; H04B 7/0617 20130101;
H04B 7/0413 20130101; H04B 7/0669 20130101 |
Class at
Publication: |
375/227 ;
375/224 |
International
Class: |
H04B 17/00 20060101
H04B017/00 |
Claims
1. A method for determining a beamforming vector of an antenna
group of a transmitting station in a wireless communication system
and a beamforming vector of an antenna group of a receiving station
in the wireless communication system, wherein each of the
transmitting station and the receiving station comprises a codebook
including a plurality of predefined beamforming vectors, the method
comprising: performing a test transmission from the transmitting
station to the receiving station using a test signal and a
beamforming vector pair, the beamforming vector pair including a
beamforming vector selected from the codebook of the transmitting
station and a beamforming vector selected from the codebook of the
receiving station, determining a transmission characteristic of the
test transmission at the receiving station, repeating the test
transmission and the determination of the transmission
characteristic using different beamforming vector pairs, wherein
the beamforming vectors in the beamforming vector pair are selected
such that each beamforming vector from the codebook of the
transmitting station encounters all beamforming vectors from the
codebook of the receiving station, and determining the beamforming
vectors of the transmitting and receiving stations from the
beamforming vector pair for which the transmission characteristic
has a predefined value.
2. The method of claim 1, wherein determining the transmission
characteristic is done at the receiving station, and wherein the
transmitting station is informed about the beamforming vector
determined from the beamforming vector pair.
3. The method of claim 2, wherein informing the transmitting
station comprises sending the determined beamforming vector or an
information identifying the determined beamforming vector to the
transmitting station.
4. The method of claim 1, wherein the wireless communication system
comprises a plurality of stations allowing for a bidirectional
transmission there between, wherein the method is performed for
both directions for obtaining for the station a transmit
beamforming vector when the station operates as a transmitting
station, and for obtaining a receive beamforming vector when the
station operates as a receiving station.
5. The method of claim 1, wherein the wireless communication system
comprises a plurality of stations allowing for a bidirectional
transmission there between, wherein a station uses the same
antennas for transmitting and receiving, and wherein a beamforming
vector determined for the station is used both for transmitting and
receiving.
6. The method of claim 1, wherein the beamforming vectors of the
transmitting and receiving stations are provided in respective
codebook matrices of the transmitting and receiving stations, and
wherein training matrices for the transmitting and receiving
stations are provided, wherein the training matrix for the
transmitting station comprises K.sub.R-times the beamforming
vectors of the transmitting station, K.sub.R being the number of
beamforming vectors in the codebook of the receiving station,
wherein the training matrix for the receiving station comprises
K.sub.T-times the beamforming vectors of the receiving station,
K.sub.T being the number of beamforming vectors in the codebook of
the transmitting station, and wherein the order of the beamforming
vectors in the transmitting matrices is selected such that each
beamforming vector from the codebook of a transmitting station
encounters all beamforming vectors from the codebook of the
receiving station.
7. The method of claim 6, wherein the training matrices are
determined as follows: T.sub.T=1.sub.1,K.sub.RC.sub.T, or
T.sub.R=C.sub.R1.sub.1,K.sub.T, wherein: T.sub.T=training matrix
for beamforming at the transmitting station, T.sub.R=training
matrix for beamforming at the receiving station, C.sub.T=codebook
matrix of the transmitting station, C.sub.R=codebook matrix of the
receiving station, 1.sub.1,K.sub.T=a row vector having K.sub.T
elements that are each 1, 1.sub.1,K.sub.R=a row vector having
K.sub.R elements that are each 1.
8. The method of claim 1, wherein the transmission characteristic
comprises a receive power, a signal-to-noise ratio (SNR), a
signal-to-interference ratio (SIR), and a signal to
interference-plus-noise ratio (SINR), and wherein the predefined
value comprises a maximum of the receive power, of the
signal-to-noise ratio (SNR), of the signal-to-interference ratio
(SIR), and of the signal to interference-plus-noise ratio
(SINR).
9. A wireless communication system comprising: a transmitting
station comprising an antenna group and a codebook comprising a
plurality of predefined beamforming vectors for the antenna group
of the transmitting station, and a receiving station comprising an
antenna group and a codebook comprising a plurality of predefined
beamforming vectors for the antenna group of the receiving station,
wherein, for determining a beamforming vector of the antenna groups
of the transmitting and receiving stations, the wireless
communication system is configured to: perform a test transmission
from the transmitting station to the receiving station using a test
signal and a beamforming vector pair, the beamforming vector pair
including a beamforming vector selected from the codebook of the
transmitting station and a beamforming vector selected from the
codebook of the receiving station, determine a transmission
characteristic of the test transmission at the receiving station,
repeat the test transmission and the determination of the
transmission characteristic using different beamforming vector
pairs, wherein the beamforming vectors in the beamforming vector
pair are selected such that each beamforming vector from the
codebook of the transmitting station encounters all beamforming
vectors from the codebook of the receiving station, and determine
the beamforming vectors of the transmitting and receiving stations
from the beamforming vector pair for which the transmission
characteristic has a predefined value.
10. A method for determining a beamforming channel matrix
describing a radio channel between a transmitting station and a
receiving station of a wireless communication system, the
transmitting and receiving stations comprising respective antenna
groups and respective codebooks comprising a plurality of
predefined beamforming vectors for the antenna group, the method
comprising: performing a plurality of test transmissions from the
transmitting station to the receiving station using a test signal,
wherein for each of the plurality of test transmissions the
beamforming vectors at the transmitting station and at the
receiving station are varied on the basis of a transmit estimate
matrix and a receive estimate matrix, wherein each element of an
estimate matrix defines the beamforming weight for a specific
antenna from the antenna group used during a specific test
transmission, and determining from all test transmissions the
beamforming channel matrix.
11. The method of claim 10, wherein an estimate matrix comprises
the beamforming vectors for the transmitting station or the
receiving station in chronological order starting with column 1,
and wherein the estimate matrix for the transmitting and receiving
stations is determined on the basis of a base estimate matrix for
the transmitting station and the receiving station,
respectively.
12. The method of claim 11, wherein the base estimate matrix
comprises the beamforming weights and is a square matrix.
13. The method of claim 12, wherein the wireless communication
system uses equal-gain beamforming, and wherein the base estimate
matrix is a unitary matrix.
14. The method of claim 13, wherein the unitary base estimate
matrix comprises a Hadamard matrix, a matrix having four
equidistant phase states, a matrix having {square root over (N)}
equidistant phase states, or a matrix having N equidistant phase
states.
15. The method of claim 10, wherein the transmit and receive
estimate matrices are defined as follows: E.sub.T=1.sub.1,NB.sub.T,
and E.sub.R=B.sub.R1.sub.1,M, wherein: E.sub.T=transmit estimate
matrix, B.sub.T=base transmit estimate matrix having the dimension
M.times.M for a transmitting station having M transmit antennas,
E.sub.R=receive estimate matrix, B.sub.R=base receive estimate
matrix having the dimension N.times.N for a receiving station
having N receive antennas, 1.sub.1,N=a row vector having N elements
that are equal 1, and 1.sub.1,M=a row vector having M elements that
are equal 1, wherein [E.sub.T].sub.m,k describes a beamforming
weight for the m-th transmit antenna during the k-th test
transmission of NM test transmissions, and wherein
[E.sub.R].sub.n,k describes a beamforming weight for the n-th
receive antenna during the k-th transmission of the NM test
transmissions.
16. The method of claim 10, wherein the beamforming channel matrix
is estimated as follows: h=S.sup.-1d, wherein: h=vec(H)=vectorized
beamforming channel matrix, S=S=(B.sub.RB.sub.T).sup.T B.sub.R=base
receive estimate matrix, B.sub.T=base transmit estimate matrix, and
d=transfer coefficient vector for each test signal.
17. The method of claim 16, wherein the base estimate matrices are
unitary matrices, and wherein the beamforming channel matrix is
estimated as follows: h=S.sup.Hd.
18. The method of claim 10, wherein the wireless communication
system comprises a plurality of stations allowing for a
bidirectional transmission there between, wherein the method is
performed for both directions for obtaining for the station a
transmit beamforming channel matrix when the station operates as a
transmitting station, and for obtaining a receive beamforming
channel matrix when the station operates as the receiving
station.
19. The method of claim 10, wherein the wireless communication
system comprises a plurality of stations allowing for a
bidirectional transmission there between, wherein a station uses
the same antennas for transmitting and receiving, and wherein the
beamforming channel matrix determined for the station is used both
for transmitting and receiving.
20. A wireless communication network comprising: a transmitting
station comprising an antenna group and a codebook comprising a
plurality of predefined beamforming vectors for the antenna group
of the transmitting station, and a receiving station comprising an
antenna group and a codebook comprising a plurality of predefined
beamforming vectors for the antenna group of the receiving station,
wherein, for determining a beamforming channel matrix describing a
radio channel between a transmitting station and a receiving
station of the wireless communication system, the wireless
communication system is configured to: perform a plurality of test
transmissions from the transmitting station to the receiving
station using a test signal, wherein for each of the plurality of
test transmissions the beamforming vectors at the transmitting
station and at the receiving station are varied on the basis of a
transmit estimate matrix and a receive estimate matrix, wherein
each element of an estimate matrix defines the beamforming weight
for a specific antenna from the antenna group used during a
specific test transmission, and determine from all test
transmissions the beamforming channel matrix.
21. A method for determining a beamforming vector of an antenna
group of a transmitting station in a wireless communication system
and a beamforming vector of an antenna group of a receiving station
in the wireless communication system, wherein each of the
transmitting station and the receiving station comprises a codebook
including a plurality of predefined beamforming vectors, the method
comprising: determining from the codebook of the transmitting or
receiving station the beamforming vector yielding a first
predefined result when applying the beamforming weights defined in
the beamforming vector to a known beamforming channel matrix
describing the radio channel between the transmitting station and
the receiving station, and determining from the codebook of the
receiving or transmitting station the beamforming vector yielding a
second predefined result when applying the beamforming weights
defined in the beamforming vector to a combination of the known
beamforming channel matrix and the determined transmit or receive
beamforming vector.
22. The method of claim 21, wherein for determining the beamforming
vectors an optimization method or a search across all beamforming
vectors of the respective codebook is made.
23. The method of claim 21, wherein determining the beamforming
vector for the transmitting station comprises selecting the
beamforming vector w.sub.CH from the codebook C.sub.T of the
transmitting station in accordance with the following equation: w
CH = arg max w .di-elect cons. C T H w 1 ##EQU00021## wherein:
H=known beamforming channel matrix, w=beamforming vector from the
codebook C.sub.T, and .parallel. .parallel..sub.1=L.sub.1 Norm,
Taxi Cab Norm or Manhattan Norm.
24. The method of claim 23, wherein determining the beamforming
vector for the receiving station comprises selecting the
beamforming vector z.sub.CH from the codebook C.sub.R of the
receiving station in accordance with the following equation: z CH =
arg max z .di-elect cons. C R z T H w CH = arg max z .di-elect
cons. C R z T h w , CH ##EQU00022## wherein: z=beamforming vector
from the codebook Z.sub.R.
25. The method of claim 23, wherein determining the beamforming
vector for the receiving station comprises determining z.sub.H as
follows and selecting from the codebook the beamforming vector
z.sub.CH having the maximum correlation with z.sub.H: z H = 1 N exp
( - j.angle. ( H w CH ) ) ##EQU00023## z CH = arg max z .di-elect
cons. C R z H H z ##EQU00023.2##
26. The method of claim 21, wherein determining the beamforming
vector for the receiving station comprises selecting the
beamforming vector z.sub.CH from the codebook C.sub.R of the
receiving station in accordance with the following equation: z CH =
arg max z .di-elect cons. C R H T z 1 ##EQU00024##
27. The method of claim 26, wherein determining the beamforming
vector for the transmitting station comprises selecting the
beamforming vector w.sub.CH from the codebook C.sub.T of the
transmitting station in accordance with the following equation: w
CH = arg max z .di-elect cons. C T w T H T z CH ##EQU00025##
28. The method of claim 26, wherein determining the beamforming
vector for the transmitting station comprises determining W.sub.H
as follows and selecting from the codebook the beamforming vector
w.sub.CH having the maximum correlation with w.sub.H: w CH = 1 N
exp ( - j.angle. ( H T z CH ) ) ##EQU00026## w CH = arg max w
.di-elect cons. C T w H H w ##EQU00026.2##
29. The method of claim 21, wherein the wireless communication
system comprises a multicarrier system having K subcarriers, and
wherein from the plurality of beamforming matrices H.sup.(k) for
the respective subcarriers the beamforming channel matrix H.sup.(1)
which has the largest sum of the absolute values of the matrix
values is selected for determining the beamforming vectors.
30. The method of claim 21, wherein the beamforming channel matrix
H.sup.(l) is determined from a subset of the plurality of
beamforming matrices H.sup.(k) for the respective subcarriers, and
wherein l is determined as follows: l = arg max k .di-elect cons. K
n = 1 N m = 1 M h n , m ( k ) ##EQU00027## K { 1 , 2 , K } .
##EQU00027.2##
31. The method of claim 21, wherein the wireless communication
system comprises a plurality of stations allowing for a
bidirectional transmission there between, wherein the method is
performed for both directions for obtaining for the station a
transmit beamforming vector when the station operates as a
transmitting station, and for obtaining a receive beamforming
vector when the station operates as a receiving station.
32. The method of claim 21, wherein the wireless communication
system comprises a plurality of stations allowing for a
bidirectional transmission there between, wherein a station uses
the same antennas for transmitting and receiving, and wherein a
beamforming vector determined for the station is used both for
transmitting and receiving.
33. A wireless communication system comprising: a transmitting
station comprising an antenna group and a codebook comprising a
plurality of predefined beamforming vectors for the antenna group
of the transmitting station, and a receiving station comprising an
antenna group and a codebook comprising a plurality of predefined
beamforming vectors for the antenna group of the receiving station,
wherein, for determining a beamforming vector of the antenna groups
of the transmitting and receiving stations, the wireless
communication system is configured to: determine from the codebook
of the transmitting or receiving station the beamforming vector
yielding a first predefined result when applying the beamforming
weights defined in the beamforming vector to a known beamforming
channel matrix describing the radio channel between the
transmitting station and the receiving station, and determine from
the codebook of the receiving or transmitting station the
beamforming vector yielding a second predefined result when
applying the beamforming weights defined in the beamforming vector
to a combination of the known beamforming channel matrix and the
determined transmit or receive beamforming vector.
34. A method for determining a beamforming vector for a
transmitting station in a wireless communication system and a
beamforming vector for a receiving station in the wireless
communication system, wherein at least one of the transmitting
station and the receiving station comprises a hybrid MIMO
beamforming configuration including a plurality of MIMO branches,
each MIMO branch comprising a plurality of antennas, the method
comprising: splitting the hybrid MIMO beamforming system into a
plurality of subsystems, and determining the beamforming parameters
for each subsystem separately.
35. The method of claim 34, wherein both the transmitting station
and the receiving station comprise a hybrid MIMO beamforming
configuration.
36. The method of claim 35, wherein splitting the hybrid MIMO
beamforming system comprises assigning each MIMO transmit branch to
a MIMO receive branch and each MIMO receive branch to a MIMO
transmit branch.
37. The method of claim 36, wherein the branches are assigned such
that the number of MIMO receive branches assigned to the same MIMO
transmit branch or the number of MIMO transmit branches assigned to
the same MIMO receive branch is minimized.
38. The method of claim 36, wherein assigning comprises: assigning
the branches such that, in case a plurality of MIMO receive
branches is allocated to the same MIMO transmit branch, the MIMO
receive branches whose MIMO antennas are spatially as far as
possible apart from one another are assigned to the same MIMO
transmit branch, or assigning the branches such that, in case a
plurality of MIMO transmit branches is allocated to the same MIMO
receive branch, the MIMO transmit branches whose MIMO antennas are
spatially as far as possible apart from one another are assigned to
the same MIMO receive branch.
39. The method of claim 38, wherein dependent on the MIMO signal
processing not the MIMO branches having the most distant antennas
but those MIMO branches having their antennas as close as possible
are used.
40. The method of claim 36, wherein the hybrid MIMO beamforming
system is split into asymmetric subsystems comprising only one MIMO
branch on the transmitting side or on the receiving side.
41. The method of claim 40, wherein splitting the hybrid MIMO
beamforming system comprises: dividing the hybrid MIMO beamforming
system into P M p .times. q = 1 Q N q ##EQU00028## subsystems,
wherein: P=number of MIMO transmit branches, M.sub.p=the number of
transmit beamforming branches of the p-th MIMO transmit branch,
Q=number of MIMO receive branches, N.sub.q=number of the receive
beaming branches of the q-th MIMO receive branch, dividing the MIMO
beamforming system into Q p = 1 P N p .times. M q ##EQU00029##
subsystems, wherein: Q=number of MIMO receive branches, P=number of
MIMO transmit branches, N.sub.p=number of receive beamforming
branches of the p-th MIMO receive branch, and M.sub.q=number of
transmit beamforming branches of the q-th MIMO transmit branch, and
wherein the transmit and receive beamforming vectors are determined
for the P subsystems and the Q subsystems separately.
42. The method of claim 41, wherein the hybrid MIMO beamforming
system is a hybrid SIMO beamforming system with P=1 and Q>1, or
wherein the hybrid MIMO beamforming system is a hybrid MISO
beamforming system with P>1 and Q=1.
43. The method of claim 42, comprising the following steps for the
SIMO beamforming system: determining for the M .times. q = 1 Q N q
##EQU00030## beamforming system a suitable transmitting beamforming
vector; and splitting the SIMO beamforming system into Q
M.times.N.sub.q subsystems, wherein for every subsystem a suitable
receiving beamforming vector is determined taking into account the
transmitting beamforming vector.
44. The method of claim 42, comprising the following steps for the
MISO beamforming system: determining for the p = 1 P M p .times. N
##EQU00031## beamforming system a suitable receiving beamforming
vector without considering transmitting beamforming vectors; and
dividing the MISO beamforming system into P M.sub.p.times.N
subsystems, wherein for every beamforming subsystem a suitable
transmitting beamforming vector is determined taking into account
the receiving beamforming vector.
45. The method of claim 35, wherein determining the beamforming
parameters for each subsystem comprises one or more of the
following: (1) determining the beamforming vector of the
transmitting station and of the receiving station in the wireless
communication system, wherein each of the transmitting station and
the receiving station comprises a codebook including a plurality of
predefined beamforming vectors, by performing a test transmission
from the transmitting station to the receiving station using a test
signal and a beamforming vector pair, the beamforming vector pair
including a beamforming vector selected from the codebook of the
transmitting station and a beamforming vector selected from the
codebook of the receiving station, determining a transmission
characteristic of the test transmission at the receiving station,
repeating the test transmission and the determination of the
transmission characteristic using different beamforming vector
pairs, wherein the beamforming vectors in the beamforming vector
pair are selected such that each beamforming vector from the
codebook of the transmitting station encounters all beamforming
vectors from the codebook of the receiving station, and determining
the beamforming vectors of the transmitting and receiving stations
from the beamforming vector pair for which the transmission
characteristic has a predefined value, or (2) determining the
beamforming channel matrix describing a radio channel between the
transmitting station and the receiving station, the transmitting
and receiving stations comprising respective antenna groups and
respective codebooks comprising a plurality of predefined
beamforming vectors for the antenna group, by performing a
plurality of test transmissions from the transmitting station to
the receiving station using a test signal, wherein for each of the
plurality of test transmissions the beamforming vectors at the
transmitting station and at the receiving station are varied on the
basis of a transmit estimate matrix and a receive estimate matrix,
wherein each element of an estimate matrix defines the beamforming
weight for a specific antenna from the antenna group used during a
specific test transmission, and determining from all test
transmissions the beamforming channel matrix, or (3) determining a
beamforming vector the transmitting station and the receiving
station, wherein each of the transmitting station and the receiving
station comprises a codebook including a plurality of predefined
beamforming vectors, by determining from the codebook of the
transmitting or receiving station the beamforming vector yielding a
first predefined result when applying the beamforming weights
defined in the beamforming vector to a known beamforming channel
matrix describing the radio channel between the transmitting
station and the receiving station, and determining from the
codebook of the receiving or transmitting station the beamforming
vector yielding a second predefined result when applying the
beamforming weights defined in the beamforming vector to a
combination of the known beamforming channel matrix and the
determined transmit or receive beamforming vector.
46. A wireless communication system comprising: a transmitting
station, and a receiving station, wherein at least one of the
transmitting station and the receiving station comprises a hybrid
MIMO beamforming configuration including a plurality of MIMO
branches, each MIMO branch comprising a plurality of antennas, and
wherein the system is configured to split the hybrid MIMO
beamforming system into a plurality of subsystems, and determine
the beamforming vectors for each subsystem separately.
47. A non-transitory computer readable medium including a computer
program including instructions for performing a method for
determining a beamforming vector of an antenna group of a
transmitting station in a wireless communication system and a
beamforming vector of an antenna group of a receiving station in
the wireless communication system, wherein each of the transmitting
station and the receiving station comprises a codebook including a
plurality of predefined beamforming vectors, when executing the
instructions by a computer, the method comprising: performing a
test transmission from the transmitting station to the receiving
station using a test signal and a beamforming vector pair, the
beamforming vector pair including a beamforming vector selected
from the codebook of the transmitting station and a beamforming
vector selected from the codebook of the receiving station,
determining a transmission characteristic of the test transmission
at the receiving station, repeating the test transmission and the
determination of the transmission characteristic using different
beamforming vector pairs, wherein the beamforming vectors in the
beamforming vector pair are selected such that each beamforming
vector from the codebook of the transmitting station encounters all
beamforming vectors from the codebook of the receiving station, and
determining the beamforming vectors of the transmitting and
receiving stations from the beamforming vector pair for which the
transmission characteristic has a predefined value.
48. A non-transitory computer readable medium including a computer
program including instructions for performing a method for
determining a beamforming channel matrix describing a radio channel
between a transmitting station and a receiving station of a
wireless communication system, the transmitting and receiving
stations comprising respective antenna groups and respective
codebooks comprising a plurality of predefined beamforming vectors
for the antenna group, when executing the instructions by a
computer, the method comprising: performing a plurality of test
transmissions from the transmitting station to the receiving
station using a test signal, wherein for each of the plurality of
test transmissions the beamforming vectors at the transmitting
station and at the receiving station are varied on the basis of a
transmit estimate matrix and a receive estimate matrix, wherein
each element of an estimate matrix defines the beamforming weight
for a specific antenna from the antenna group used during a
specific test transmission, and determining from all test
transmissions the beamforming channel matrix.
49. A non-transitory computer readable medium including a computer
program including instructions for performing a method for
determining a beamforming vector of an antenna group of a
transmitting station in a wireless communication system and a
beamforming vector of an antenna group of a receiving station in
the wireless communication system, wherein each of the transmitting
station and the receiving station comprises a codebook including a
plurality of predefined beamforming vectors, when executing the
instructions by a computer, the method comprising: determining from
the codebook of the transmitting or receiving station the
beamforming vector yielding a first predefined result when applying
the beamforming weights defined in the beamforming vector to a
known beamforming channel matrix describing the radio channel
between the transmitting station and the receiving station, and
determining from the codebook of the receiving or transmitting
station the beamforming vector yielding a second predefined result
when applying the beamforming weights defined in the beamforming
vector to a combination of the known beamforming channel matrix and
the determined transmit or receive beamforming vector.
50. A non-transitory computer readable medium including a computer
program including instructions for performing a method for
determining a beamforming vector for a transmitting station in a
wireless communication system and a beamforming vector for a
receiving station in the wireless communication system, wherein at
least one of the transmitting station and the receiving station
comprises a hybrid MIMO beamforming configuration including a
plurality of MIMO branches, each MIMO branch comprising a plurality
of antennas, when executing the instructions by a computer, the
method comprising: splitting the hybrid MIMO beamforming system
into a plurality of subsystems, and determining the beamforming
parameters for each subsystem separately.
Description
BACKGROUND
[0001] Embodiments of the invention relate to a method for
determining beamforming parameters in a wireless communication
system, and to a wireless communication system. More specifically,
embodiments of the invention may be used for improving the
transmission in wireless communication systems and may be
particularly interesting for mobile radio systems and wireless
millimeter wave transmission systems.
[0002] For improving the performance of wireless communication
networks or radio systems, multi antenna techniques using group
antennas (antenna arrays) at the transmitting side and at the
receiving side may be used. One approach is called beamforming, and
in accordance with this approach a signal is split at the
transmitter and multiplied by a complex weighting factor (having a
magnitude and a phase) for every transmitter antenna individually.
At the receiver, the signals of the individual receiving antennas
are also weighted with complex factors and added. Weighting the
signals of a group antenna is implemented by a beamformer. If the
weights all have constant amplitude and differ only in phase, this
is referred to as equal-gain beamforming or as a phased array.
Contrary to the beamforming signal processing, in MIMO signal
processing (MIMO=Multiple-Input Multiple-Output), not only complex
weightings but also costly digital signal processing operations
need to be performed in every branch. The MIMO operations may each
have a different effect on certain portions of the antenna signals
(samples in time or frequency), whereas in beamforming all signal
portions are weighted identically. Equal-gain beamformers may be
implemented in analog circuitry with relatively little effort and
are hence particularly interesting when a large number of antennas
is used. In contrast, systems using MIMO signal processing require
a higher effort in the analog and digital circuitry and are hence
generally limited to moderate numbers of antennas, e.g. to only 2
or 4 antennas.
[0003] FIG. 1 shows a schematic equivalent baseband representation
of a unidirectional wireless communication system comprising M
antennas at the transmitter and N antennas at the receiver. The
system 100 comprises a transmitter 102 having an input 104 at which
an input data signal d.sub.s to be transmitted in the wireless
communication system or radio system 100 is received. The
transmitter comprises a plurality of antennas 105.sub.1, 105.sub.2,
. . . 105.sub.m, i.e. the transmitter 102 comprises M antennas. The
input data signal received at the input 104 is processed by a
transmitter signal processing unit 106 which outputs a signal x to
be transmitted. The signal x received at the beamformer input 107
is distributed via a transmit beamformer 108 to the respective
antennas 105.sub.1 to 105.sub.m. The beamformer 108 comprises a
dividing or splitting circuit 109 and a plurality of weighting
elements 110.sub.1, 110.sub.2, . . . 110.sub.m applying to the
input signal x received at the beamformer input 107 respective
weighting factors w.sub.1, w.sub.2, . . . , W.sub.M. The weighted
input signals are transmitted from the antennas 105.sub.1 to
105.sub.m via a radio channel 112 to a receiver 114. The receiver
114 comprises a plurality of receive antennas 116.sub.1, 116.sub.2,
. . . , 116.sub.N. The signals received from the respective
antennas 116.sub.1 to 116.sub.N are fed into a receive beamformer
118. The receive beamformer 118 comprises a plurality of weighting
elements 120.sub.k, 120.sub.2, . . . 120.sub.N that are provided
for applying to the respective signals received from the antennas
116.sub.1 to 116.sub.N the respective weighting factors z.sub.1,
z.sub.2, . . . z.sub.N and an adding circuit 122. The adding
circuit adds the weighted receive signals to form the output signal
y of the beamformer 118 that is provided at an output 124. The
signal y is fed into the receiver signal processing unit 126
providing the received data signal d, at the output 128 of the
receiver 114. In case beamforming is done at the transmitter and at
the receiver, a beamforming system comprises a transmit beamformer,
transmit antennas, receive antennas and a receive beamformer. For
example, the transmit beamformer 108, the transmit antennas
105.sub.1 to 105.sub.m, the receive antennas 116.sub.1 . . .
116.sub.N and the receive beamformer 118 shown in FIG. 1 form a
beamforming system. When beamforming is only applied at the
transmitter, the beamforming system comprises the transmit
beamformer, the transmit antennas, and the receive antennas.
Alternatively, when using beamforming only at the receiver, the
beamforming system comprises the transmit antennas, the receive
antennas, and the receive beamformer.
[0004] At the transmitter 102 M beamforming branches are formed,
each of the beamforming branches comprises one of the weighting
elements of the beamformer 108 and one of the antennas of the
transmitter. For example, a first beamforming branch is formed by
the weighting element 110.sub.1 of the beamformer 108 and the
antenna 105.sub.1. Likewise, at the receiver 114 N beamforming
branches are formed, the respective branches comprises one of the
weighting elements of the beamformer 118 and one of the antenna
elements of the receiver. For example, a first beamforming branch
at the receiver 114 is formed by the antenna element 116.sub.1 and
the weighting element 120.sub.1 of the receive beamformer 118.
[0005] By beamforming at the transmitter 102, the power radiated in
certain space directions is increased, while it is reduced in other
space directions. Beamforming at the receiver 114 has the effect
that signals from certain space directions are received in an
amplified manner and from other space directions in an attenuated
manner. Because the transmission attenuation increases with rising
transmission frequencies, beamforming is considered as promising
and inexpensive means for increasing the performance of systems
having high transmission frequencies, e.g. future 60 GHz
systems.
[0006] The weighting factors w.sub.1, w.sub.2, . . . , w.sub.m or
z.sub.r, Z.sub.2, . . . , z.sub.m for the individual antennas
105.sub.1 to 105.sub.m or 116.sub.1 to 116.sub.N at the transmitter
102 or at the receiver 114 may each be combined into one
beamforming vector. FIG. 1 shows an example of an unidirectional
wireless communication system allowing for a transmission using M
beamforming branches at the transmitter 102 and N beamforming
branches at the receiver 114. The adjustment of the signals
provided by the transmitter 102 using the transmit beamformer 108
is described by the transmit beamforming vector W:
w = ( w 1 w 2 w M ) ##EQU00001##
[0007] The adjustment of the signals received at the receiver 114
using the receive beamformer 118 is described by the receive
beamforming vector z:
z = ( z 1 z 2 z N ) ##EQU00002##
[0008] In the case of using the equal-gain beamforming, the
elements of the beamforming vectors have a constant modulus. If the
magnitude of the beamforming vectors is defined to 1, the
beamforming vectors are given as follows:
w = 1 M ( exp ( j 1 ) exp ( j 2 ) exp ( j M ) ) ##EQU00003## and
##EQU00003.2## z = 1 N ( exp ( j.PHI. 1 ) exp ( j.PHI. 2 ) exp (
j.PHI. N ) ) ##EQU00003.3##
wherein .theta..sub.2=phase values .theta..sub.m.epsilon.[0, 2.pi.]
for the transmitter 102, and .phi..sub.n=phase values
.phi..sub.n.epsilon.[0, 2.pi.] for the receiver 114.
[0009] Many known systems may use discrete (quantized) phase values
only, so that the number of possible beamforming vectors is
limited.
[0010] The wireless transmission between the antenna groups 106 and
116 at the transmitting side 102 and at the receiving side 114 is
performed via the radio channel 112 including all possible
connection paths between all transmitting antennas 106.sub.1 to
106.sub.m and all receiving antennas 116.sub.1 to 116.sub.N. The
radio channel 112 is defined using a matrix, the so called channel
matrix H.
[0011] The presented beamforming techniques are considered for a
unidirectional trans-mission between a transmitter 102 and a
receiver 114. Conventionally, wireless communications systems are
provided for a bidirectional transmission between stations. Each
station needs to be provided with a transmitter and a receiver.
Both in the transmitter and in the receiver beamforming techniques
may be used. FIG. 1(a) depicts a bidirectional, wireless
beamforming transmission system 900 having two stations 902 and
904. Each station is provided with a transmitter 906, 910 and a
receiver 908, 912 having a structure as described in FIG. 1. Up to
four beamforming vectors may be involved in case of such a
bidirectional transmission between the two stations, station 902
and station 904: for a transmission from the station 902 to the
station 904 the beamformer 914 at the station 902 may use for a
transmitting beamforming at station 902, and the beamformer 916 at
the station 904 may use for a receiving beamforming at station 904;
and for a trans-mission from the station 904 to the station 902 the
beamformer 918 at the station 904 may use for a transmitting
beamforming at station 904, and the beamformer 920 at the station
902 may use for a receiving beamforming at station 902. Since a
bidirectional transmission can always be split into two
unidirectional transmissions in opposite directions, with respect
to the beamforming techniques it is sufficient to consider a
unidirectional transmission and a unidirectional transmission
system, respectively, including one transmitter and one
receiver.
[0012] A problem for the operation of a multi-antenna system using
beamforming is the adaptive (dynamic) adjustment of the beamforming
vectors for maximizing the transmission quality in dependence on
the propagation conditions. The methods for determining beamforming
vectors may be divided into two categories: Methods with explicit
beamforming channel knowledge, and methods without beamforming
channel knowledge. In the former case, beamforming channel
knowledge means that the radio channel 112 between any transmitting
beamformer antenna element 106.sub.1 to 106.sub.M and any receiving
beamformer antenna element 116.sub.1 to 116.sub.N, i.e. the
beamforming channel matrix, is known. In the latter case,
estimating the channel matrix presents a significant additional
challenge. In bidirectional transmission, in general, two channel
matrices are to be considered: one for the forward direction and
one for the backward direction, and they have to be acquired in
practice by a beamforming channel estimation.
[0013] The following problems occur when using a beamforming system
with beamforming signal processing according to FIG. 1: [0014] 1.
Determining optimal beamforming vectors at transmitter and receiver
without explicit channel knowledge. [0015] 2. Estimating a
multi-antenna channel in systems with beamforming signal
processing. [0016] 3. Determining suitable beamforming vectors at
the transmitter and at the receiver using channel knowledge for
systems with pure beamforming signal processing.
[0017] The following problem occurs when using a hybrid MIMO
beamforming system with MIMO signal processing and beamforming
signal processing according to FIG. 5: [0018] 4. Determining
suitable beamforming vectors at the transmitter and at the receiver
in hybrid MIMO beamforming systems.
[0019] For determining suitable beamforming vectors, known methods
without explicit channel knowledge provide for a training phase,
during which test signals or training symbols are transmitted and
evaluated within a training frame at different suitably selected
beamforming vectors (see e.g. ECMA-387 Standard: High Rate 60 GHz
PHY, MAC and HDMI PAL, 2008, Ecma International). The temporal
sequence of beamforming adjustments may be described by a matrix (a
training matrix), which consists of the respective beamforming
vectors. In a bidirectional radio system using two-way beamforming
in the transmitting and receiving branches, transmission of
training frames is performed in both directions. Optimizing the
beamforming vectors is obtained by repeating the alternating
transmission several times and iteratively adapting the beamforming
vectors.
[0020] At present methods for determining the beamforming channel
matrix are only known for systems where a group antenna is used
only on one side (at the transmitter or at the receiver). In such a
case, the beamforming channel matrix transitions into a beamforming
channel vector, which is calculated using side information. The
side information relate to the direction of incidence of the
receive signal or the desired transmitting direction of the
transmit signal and the geometry of the group antenna. This
requires the presence of definite a-priori directional information
and only little multipath propagation may exist in the radio
channel (a typical field of such an application is the
communication to a geostationary satellite, a communication from a
vehicle, or a target tracking radar). Estimating the directional
information for the receiver merely from the receive signals
without a-priori information is possible, requires, however, MIMO
signal processing see e.g. Chung, Pei-Jung and Bohme, J. F.,
"Recursive EM and SAGE-inspired algorithms with application to DOA
estimation" Signal Processing, IEEE Transactions on,
53(8):2664--2677, 2005; Schmidt, R., "Multiple emitter location and
signal parameter estimation", Antennas and Propagation, IEEE
Transactions on, 34(3):276--280, 1986; or Stoica, P. and Sharman,
K. C., "Maximum likelihood methods for direction-of-arrival
estimation", Acoustics, Speech and Signal Processing, IEEE
Transactions on, 38(7):1132--1143, 1990).
[0021] Methods for determining a beamforming vector on the
transmitter side or on the receiver side using channel knowledge
from the directional information have been known for a long time
for phased-array applications. However, these direction-based
methods may only be applied with little or non-existing multipath
propagation. Methods for determining the optimal beamforming
vectors on the transmitter side and on the receiver side using
channel knowledge--also with multipath propagation--have so far
only been known for systems having MIMO signal processing (see e.g.
Heath, R. W., Jr. and Paulraj, A., "Multiple antenna arrays for
transmitter diversity and space-time coding", Communications, 1999.
ICC '99. 1999 IEEE International Conference on, pages 36--40 vol.
1, 1999). For MIMO systems, different approaches for determining
the channel matrix are known. Transferring such techniques to
systems having only beamforming signal processing has not been
possible so far, since, on the one hand, channel knowledge without
side information (directional information) was not available for
these systems and, on the other hand, it was unclear how a common
beamforming vector is to be determined for all possibly different
signal portions (in time and frequency).
[0022] For hybrid methods the principle of combining beamforming
and MIMO signal processing is described e.g. by Dammann, A. and
Raulefs, R. and Kaiser, S., "Beamforming in combination with
space-time diversity for broadband OFDM systems", Communications,
2002. ICC 2002. IEEE International Conference on, pages 165--171,
2002. Smart antennas are controlled via an adaptive antenna
processor. The aim of beamforming is the transmission of the signal
via several ideally statistically independent propagation paths. On
the transmitting side, the data stream is split into several
sub-streams based on the diversity principle, and combined again on
the receiving side. Among others, space-time coding (STC) as a form
of MIMO signal processing is suggested as method. Further, when
using beamforming at the transmitter and receiver, a mutual
allocation of the transmitting and the receiving antenna groups may
be performed, wherein every group generates one data channel.
However, a method for the allocation is not presented by Dammann,
A. and Raulefs, R. and Kaiser, S., "Beamforming in combination with
space-time diversity for broadband OFDM systems", Communications,
2002. ICC 2002. IEEE International Conference on, pages 165--171,
2002. Further, it is assumed that the antenna processor provides
the directions into which the beams are to be formed. Methods for
determining the beamforming vectors are not discussed. In
Morelos-Zaragoza, R. H. and Ghavami, M., "Combined beamforming and
space-time block coding with a sparse array antenna", Wireless
Personal Multimedia Communications, 2002. The 5th International
Symposium on, pages 432--434 vol. 2, 2002, beamforming is also
considered in the context of STC. The research focus lies on the
influence of a correlation between different antenna beams on the
performance of the system. Methods for determining suitable
beamforming vectors are not considered.
[0023] Heath, R. W., Jr. and Paulraj, A., "Multiple antenna arrays
for transmitter diversity and space-time coding", Communications,
1999. ICC '99. 1999 IEEE International Conference on, pages 36--40
vol. 1., 1999 examine what gains may be obtained with different
transmitting side diversity technologies in combination with
beamforming, and what effect beamforming vectors deviating from the
optimum have. The considerations are limited to a system having
several antenna groups at the transmitter and one antenna at the
receiver (MISO) and only apply under the assumption that only one
propagation path exists between one antenna group and the receiver.
Further, the research relates to a single user, wherein it is noted
that in a multi-user system beamforming is not only to be used for
maximizing the received power for the desired user, but at the same
time for reducing interference for other users. This principle is
also described by Wu, Sau-Hsuan and Chiu, Lin-Kai and Lin, Ko-Yen
and Chung, Shyh-Jong, "Planar arrays hybrid beamforming for SDMA in
millimeter wave applications" Personal, Indoor and Mobile Radio
Communications, 2008. PIMRC 2008. IEEE 19th International Symposium
on, pages 1--6, 2008; Wu, Sau-Hsuan and Lin, Ko-Yen and Chiu,
Lin-Kai, "Hybrid beamforming using convex optimization for SDMA in
millimeter wave radio", Personal, Indoor and Mobile Radio
Communications, 2009 IEEE 20th International Symposium on, pages
823--827, 2009; and Smolders, A. B. and Kant, G. W., "THousand
Element Array (THEA)" Antennas and Propagation Society
International Symposium, 2000. IEEE, pages 162--165 vol. 1, 2000,
where hybrid beamforming is considered. It is to be noted that the
term "hybrid" refers to the combination of beamforming in the
baseband and in the RF-range. The approach does include a
transceiver architecture having several parallel transmitting and
receiving branches in the digital baseband, however, no MIMO signal
processing but beamforming signal processing is performed on the
branches. Hence, the same are no hybrid methods in the sense of the
above definition.
[0024] Thus, there is a need for methods for determining suitable
beamforming parameters in a wireless communications system or
network including beamforming systems.
SUMMARY OF THE INVENTION
[0025] Embodiments of a first aspect of the invention provide a
method for determining a beamforming vector of an antenna group of
a transmitting station in a wireless communication system and a
beamforming vector of an antenna group of a receiving station in
the wireless communication system, wherein each of the transmitting
station and the receiving station comprises a codebook including a
plurality of predefined beamforming vectors, the method comprising:
[0026] performing a test transmission from the transmitting station
to the receiving station using a test signal and a beamforming
vector pair, the beamforming vector pair including a beamforming
vector selected from the codebook of the transmitting station and a
beamforming vector selected from the codebook of the receiving
station, [0027] determining a transmission characteristic of the
test transmission at the receiving station, [0028] repeating the
test transmission and the determination of the transmission
characteristic using different beamforming vector pairs, wherein
the beamforming vectors in the beamforming vector pair are selected
such that each beamforming vector from the codebook of the
transmitting station encounters all beamforming vectors from the
codebook of the receiving station, and [0029] determining the
beamforming vectors of the transmitting and receiving stations from
the beamforming vector pair for which the transmission
characteristic has a predefined value.
[0030] Embodiments of the first aspect of the invention provide a
wireless communication system comprising: [0031] a transmitting
station comprising an antenna group and a codebook comprising a
plurality of predefined beamforming vectors for the antenna group
of the transmitting station, and [0032] a receiving station
comprising an antenna group and a codebook comprising a plurality
of predefined beamforming vectors for the antenna group of the
receiving station, [0033] wherein, for determining a beamforming
vector of the antenna groups of the transmitting and receiving
stations, the wireless communication system is configured to:
[0034] perform a test transmission from the transmitting station to
the receiving station using a test signal and a beamforming vector
pair, the beamforming vector pair including a beamforming vector
selected from the codebook of the transmitting station and a
beamforming vector selected from the codebook of the receiving
station, [0035] determine a transmission characteristic of the test
transmission at the receiving station, [0036] repeat the test
transmission and the determination of the transmission
characteristic using different beamforming vector pairs, wherein
the beamforming vectors in the beamforming vector pair are selected
such that each beamforming vector from the codebook of the
transmitting station encounters all beamforming vectors from the
codebook of the receiving station, and [0037] determine the
beamforming vectors of the transmitting and receiving stations from
the beamforming vector pair for which the transmission
characteristic has a predefined value.
[0038] In accordance with an embodiment of the first aspect of the
invention, the trans-mission characteristics may be determined at
the receiving station, and the transmitting station is informed
about the beamforming vector determined from the beamforming vector
pair, for example by sending the determined beamforming vector or
an information identifying the determined beamforming vector from
the receiving station to the transmitting station.
[0039] In accordance with a further embodiment of the first aspect
of the invention the wireless communication system may comprise a
plurality of stations allowing for a bidirectional transmission
there between, wherein the method is performed for both directions
for obtaining for the station a transmit beamforming vector when
the station operates as a transmitter station, and for obtaining a
receive beamforming vector, when the station operates as a
receiving station. In accordance with the embodiments a station may
operate using a frequency division duplex (FDD) technique so that
the station may simultaneously transmit and receive signals. Also
in such embodiments the method in accordance with the first aspect
is performed for both the transmitting part and the receiving part.
In accordance with another embodiment of the first aspect, the
wireless communication system comprises a plurality of stations
allowing for a bidirectional transmission there between, wherein a
station uses the same antennas for transmitting and receiving, and
wherein a beamforming vector determined for the station is used
both for transmitting and receiving.
[0040] In accordance with an embodiment of the first aspect of the
invention, the beamforming vectors of the transmitting and
receiving stations are provided in respective codebook matrices of
the transmitting and receiving stations, and the training matrices
for the transmitting and receiving stations are provided, wherein
training matrix for the transmitting station comprises
K.sub.R-times the beamforming vectors of the transmitting station,
K.sub.R being the number of beamforming vectors in the codebook of
the receiving station, and wherein the training matrix for the
receiving station comprises K.sub.T-times the beamforming vectors
of the receiving station, K.sub.T being the number of beamforming
vectors in the codebook of the transmitting station. In accordance
with this embodiment, the order of the beamforming vectors in the
training matrices is selected such that each beamforming vector
from the codebook of the transmitting station encounters all
beamforming vectors from the codebook of the receiving station.
[0041] In accordance with an embodiment of the first aspect of the
invention, the trans-mission characteristics comprises a receive
power, a signal-to-noise ratio (SNR), a signal-tointerference ratio
(SIR), and a signal to interference-plus-noise ratio (SINR), and
the predefined value comprises a maximum of the receive power, of
the signal-to-noise ratio (SNR), of the signal-to-interference
ratio (SIR), and of the signal to interference-plus-noise ratio
(SINR).
[0042] Embodiments of the second aspect of the invention provide a
method for determining a beamforming channel matrix describing a
radio channel between a transmitting station and a receiving
station of a wireless communication system, the transmitting and
receiving stations comprising respective antenna groups and
respective codebooks comprising a plurality of predefined
beamforming vectors for the antenna group, the method comprising:
[0043] performing a plurality of test transmissions from the
transmitting station to the receiving station using a test signal,
wherein for each of the plurality of test transmissions the
beamforming vectors at the transmitting station and at the
receiving station are varied on the basis of a transmit estimate
matrix and a receive estimate matrix, wherein each element of an
estimate matrix defines the beamforming weight for a specific
antenna from the antenna group used during a specific test
transmission, and [0044] determining from all test transmissions
the beamforming channel matrix.
[0045] Embodiments of the second aspect of the invention provide a
wireless communication system comprising: [0046] a transmitting
station comprising an antenna group and a codebook comprising a
plurality of predefined beamforming vectors for the antenna group
of the transmitting station, and [0047] a receiving station
comprising an antenna group and a codebook comprising a plurality
of predefined beamforming vectors for the antenna group of the
receiving station, [0048] wherein, for determining a beamforming
channel matrix describing a radio channel between a transmitting
station and a receiving station of a wireless communication system,
the wireless communication system is configured to: [0049] perform
a plurality of test transmissions from the transmitting station to
the receiving station using a test signal, wherein for each of the
plurality of test transmissions the beamforming vectors at the
transmitting station and at the receiving station are varied on the
basis of a transmit estimate matrix and a receive estimate matrix,
wherein each element of an estimate matrix defines the beamforming
weight for a specific antenna from the antenna group used during a
specific test transmission, and [0050] determine from all test
transmissions the beamforming channel matrix.
[0051] In accordance with an embodiment of the second aspect of the
invention, an estimate matrix may comprise the beamforming vectors
for the transmitting station and the receiving station,
respectively, in chronological order and starting at column 1,
wherein the estimate matrix for the transmitting and receiving
stations is determined on the basis of a base estimate matrix for
the transmitting and receiving stations. In general there is a
matrix for the transmitter and a matrix for the receiver which are
different. However, in accordance with embodiments, in specific
cases the matrices for the transmitter and the receiver are
identical. The base estimate matrix may comprise the beamforming
weights and may be a square matrix. In accordance with embodiments,
the base estimate matrix comprises all beamforming vectors used for
an estimation at the transmitter/receiver only once. Further, the
wireless communication system may use equal-gain beamforming, and
the base estimate matrix may be a unitary matrix. The unitary base
estimate matrix may be a Hadamard matrix, a matrix having four
equidistant phase states, a matrix having {square root over (N)}
equidistant phase states, or a matrix having N equidistant phase
states.
[0052] In accordance with another embodiment of the second aspect
of the invention, the wireless communication system may comprise a
plurality of stations allowing for a bidirectional transmission
there between, wherein the method is performed for both directions
for obtaining for the station a transmit beamforming channel matrix
when the station operates as a transmitting station, and for
obtaining a receive beamforming channel matrix when the station
operates as a receiving station. In accordance with yet another
embodiment of the second aspect of the invention, the wireless
communication system may comprise a plurality of stations allowing
for a bidirectional transmission there between, wherein a station
uses the same antennas for transmitting and receiving, and wherein
a beamforming channel matrix determined for the station is used
both for transmitting and receiving.
[0053] Embodiments of the third aspect of the invention provide a
method for determining a beamforming vector of an antenna group of
a transmitting station in a wireless communication system and a
beamforming vector of an antenna group of a receiving station in
the wireless communication system, wherein each of the transmitting
station and the receiving station comprises a codebook including a
plurality of predefined beamforming vectors, the method comprising:
[0054] determining from the codebook of the transmitting or
receiving station the beamforming vector yielding a first
predefined result when applying the beamforming weights defined in
the beamforming vector to a known beamforming channel matrix
describing the radio channel between the transmitting station and
the receiving station, and [0055] determining from the codebook of
the receiving or transmitting station the beamforming vector
yielding a second predefined result when applying the beamforming
weights defined in the beamforming vector to a combination of the
known beamforming channel matrix and the determined transmit or
receive beamforming vector.
[0056] Embodiments of the third aspect of the invention provide a
wireless communication system comprising: [0057] a transmitting
station comprising an antenna group and a codebook comprising a
plurality of predefined beamforming vectors for the antenna group
of the transmitting station, and [0058] a receiving station
comprising an antenna group and a codebook comprising a plurality
of predefined beamforming vectors for the antenna group of the
receiving station, [0059] wherein, for determining a beamforming
vector of the antenna groups of the transmitting and receiving
stations, the wireless communication system is configured to:
[0060] determine from the codebook of the transmitting or receiving
station the beamforming vector yielding a first predefined result
when applying the beamforming weights defined in the beamforming
vector to a known beamforming channel matrix describing the radio
channel between the transmitting station and the receiving station,
and
[0061] determine from the codebook of the receiving or transmitting
station the beamforming vector yielding a second predefined result
when applying the beamforming weights defined in the beamforming
vector to a combination of the known beamforming channel matrix and
the determined transmit or receive beamforming vector.
[0062] In accordance with an embodiment of the third aspect of the
invention for determining the beamforming vectors an optimization
method or a search across all beamforming vectors of the respective
codebook is made.
[0063] In accordance with an embodiment of the third aspect of the
invention, the wireless communication system may comprise a
plurality of stations allowing for a bidirectional transmission
there between, wherein the method is performed for both directions
for obtaining for the station a transmit beamforming vector when
the station operates as a transmitting station, and for obtaining a
receive beamforming vector when the station operates as a receiving
station.
[0064] In another embodiment of the third aspect of the invention,
the wireless communication system may comprise a plurality of
stations allowing for a bidirectional transmission there between,
wherein a station uses the same antennas for transmitting and
receiving, and wherein a beamforming vector determined for the
station is used both for transmitting and receiving.
[0065] Embodiments of the fourth aspect of the invention provide a
method for determining beamforming vectors for a transmitting
station in a wireless communication system and beamforming vectors
for a receiving station in the wireless communication system,
wherein at least one of the transmitting station and the receiving
station comprises a hybrid MIMO beamforming configuration including
a plurality of MIMO branches, each MIMO branch comprising a
plurality of antennas, the method comprising: [0066] splitting the
hybrid MIMO beamforming system into a plurality of subsystems, and
[0067] determining the beamforming parameters for each subsystem
separately.
[0068] Embodiments of the fourth aspect of the invention provide a
wireless communication system comprising: [0069] a transmitting
station, and [0070] a receiving station, [0071] wherein at least
one of the transmitting station and the receiving station comprises
a hybrid MIMO beamforming configuration including a plurality of
MIMO branches, each MIMO branch comprising a plurality of antennas,
and [0072] wherein the system is configured to [0073] split the
hybrid MIMO beamforming system into a plurality of subsystems, and
[0074] determine the beamforming vectors for each subsystem
separately.
[0075] In accordance with an embodiment of the fourth aspect of the
invention, both the transmitting station and the receiving station
may comprise a hybrid MIMO beamforming configuration. In accordance
with this embodiment, splitting the hybrid MIMO beamforming system
into a plurality of beamforming subsystems may comprise the
assignment of each MIMO transmit branch to a MIMO receive branch
and each MIMO receive branch to a MIMO transmit branch, wherein
this may comprise assigning the branches such that a number of MIMO
receive branches assigned to the same MIMO transmit branch, or a
number of MIMO transmit branches assigned to the same MIMO receive
branch is minimal. In accordance with another embodiment of the
fourth aspect of the invention, the branches may be assigned such
that in case a plurality of MIMO receive branches is allocated to
the same MIMO transmit branch, MIMO receive branches whose MIMO
antennas are spatially as far as possible apart from one another
are assigned to the same MIMO transmit branch, or assigning the
branches is such that in case a plurality of MIMO transmit branches
is allocated to the same MIMO receive branch, the MIMO transmit
branches whose MIMO antennas are spatially as far as possible apart
from one another are assigned to the same MIMO receive branch.
Dependent on the MIMO signal processing, in accordance with
embodiments, it may be advantageous to use not the MIMO branches
having the most distant antennas but those MIMO branches having
their antennas as close as possible.
[0076] In another embodiment of the fourth aspect of the invention
the splitting of the hybrid MIMO beamforming system may comprise
splitting the system into asymmetric subsystems comprising only one
MIMO branch on the transmitting side or on the receiving side.
[0077] In accordance with an embodiment of the fourth aspect of the
invention, the subsystems may be considered as beamforming systems
and the beamforming parameters for each subsystem may be determined
in accordance with one or more of the methods of the first, second
and third aspects of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0078] FIG. 1 is a schematic representation of an unidirectional
wireless communication system using beamforming comprising M
antennas at the transmitter and N antennas at the receiver.
[0079] FIG. 1(a) is a is a schematic representation of a
bidirectional wireless communication system using beamforming.
[0080] FIG. 2 is a flow diagram of an embodiment of the invention
in accordance with the first aspect for determining beamforming
vectors for the transmitting and receiving station.
[0081] FIG. 3 is a flow diagram showing the respective steps of a
method in accordance with embodiments of the second aspect of the
invention for determining a beamforming channel matrix of a channel
between the transmitting and receiving stations.
[0082] FIG. 4 is a flow diagram of an embodiment of the invention
in accordance with the third aspect for determining beamforming
vectors for the transmitting and receiving station.
[0083] FIG. 5 is an example of a unidirectional hybrid MIMO
beamforming system having two MIMO branches at the transmitter and
at the receiver each having associated therewith two beamforming
branches.
[0084] FIG. 6 is a flow diagram representing the steps of a method
in accordance with the embodiments of the fourth aspect of the
invention.
[0085] FIG. 7 shows examples for the static allocation of
subsystems of the 2.times.2 MIMO system of FIG. 5.
[0086] FIG. 8 shows examples for the asymmetric splitting of a
2.times.2 MIMO system as it is for example described in FIG. 5.
DESCRIPTION OF THE EMBODIMENTS
[0087] In the following the different aspects of the invention will
be described. It is noted that the respective aspects, while being
described separately may be used in combination, e.g. in a wireless
communication system the beamforming parameters may be determined
applying one or more of the subsequently described aspects
(approaches).
[0088] In the subsequent description, the following notation is
used: Small letters in italics (e.g. a) describe complex- or
real-valued quantities, capital letters in italics (e.g. A)
describe complex- or real-valued constants, bold small letters
(e.g. a) describe complex or real valued vectors, and bold capital
letters (e.g. A) describe complex- or real-valued matrices. The
dimensions of a matrix having N rows and M columns is N.times.M.
The k-th element of vector a is indicated by [a].sub.k, and
[A].sub.n,m is the element of the n-th row and m-th column of
matrix A. A row vector having K elements that are each 1 is
described as 1.sub.1,K, a matrix having N rows and M columns that
are each 1 is describe as 1.sub.N,W. The transpose and hermitian of
a matrix A are symbolized by A.sup.T and A.sup.H. The Kronecker
product between matrices or vectors is represented by O. The
abbreviation A.sup.-1 represents the inverse matrix of A. A
diagonal matrix having the values of the vector a on the diagonal
is generated by diag(a). The operation vec(H)=[[H].sub.1,m . . .
[H].sub.N,m].sup.T with m=[1 . . . M] generates a column vector of
length NM from the joined rows of matrix H.
1.sup.st Aspect: Training with the Help of Complete Training
Matrices
[0089] In the following, embodiments in accordance with the first
aspect of the invention will be described. The first aspect of the
invention, in accordance with embodiments, concerns the training of
a beamforming system, as it is for example depicted in FIG. 1,
using complete training matrices. A beamforming vector w for the
antenna group 106 of the transmitter 102 shown in FIG. 1 is
determined. Also, a beamforming z of the antenna group 116 of the
receiver 114 of the wireless communication system 100 of FIG. 1 is
determined. The transmitter 102 and the receiver 114 use respective
codebooks, each including a plurality of predefined beamforming
vectors. The transmit codebook of the transmitter 102 may be stored
in a memory provided by the beamformer 108 of the transmitter 102.
Alternatively, the codebook may be provided at another location
inside or external from the transmitter 102. Likewise, a receive
codebook for the receiver 114 may be stored in a memory of the
beamformer 118 or may be provided somewhere else in the receiver
114 or may be provided from an external source. The wireless
communication system 100 as shown in FIG. 1 is configured to
perform a method for determining a beamforming vector for the
respective antenna groups of the transmitting and receiving
stations, as it is depicted and described in the following with
regard to FIG. 2. The respective method steps may be implemented in
the control circuitry of the overall system or may be part of the
control circuitry of the respective beamformers 108 and 118.
[0090] FIG. 2 shows a flow diagram of an embodiment of the
invention in accordance with the first aspect for determining a
beamforming vector for the transmitting and receiving stations, in
a first step S100 a test transmission from the transmitting station
to the receiving station using a test signal or a test symbol and a
beamforming vector pair is performed. The beamforming vector pair
includes a beamforming vector selected from the codebook of the
transmitting station 102 and a beamforming vector selected from the
codebook of the receiving station 114. Following the test
transmission in step S100, in step S102 a transmission
characteristic, for example a receive power, a signal-to-noise
ratio (SNR), a signal-to-interference ratio (SIR) or a signal to
interference-plus-noise ratio (SINR), of the test transmission is
determined at the receiving station 114. At step S104, following
the determination of the receive power, the SNR, the SIR or the
SINR for a test transmission, it is determined as to whether all
possible test trans-missions were performed or not. In case not all
possible test transmissions were performed, a new beamforming
vector pair is selected at step S106 and the method returns to step
S100 for performing a test transmission using the new beamforming
vector pair and the test signal. Thus, by means of steps S104 and
S106 the test transmission and the determination of the
transmission characteristic are repeated using different
beamforming vector pairs. In accordance with an embodiment of the
first aspect of the invention, the beamforming vectors in the
beamforming vector pairs are selected such that each beamforming
vector from the codebook of the transmitting station encounters all
beamforming vectors from the codebook of the receiving station.
Once all possible test transmissions were performed, e.g. all
possible combinations of beamforming vectors from the transmitting
station and from the receiving station were used for performing the
test transmission the method proceeds to step S108, in accordance
with which the beamforming vectors for the transmitting and
receiving stations are determined from that beamforming vector pair
for which the transmission characteristic, for example the receive
power, the SNR, the SIR or the SINR at the receiver 114 had a
predefined value, for example which of the beamforming vector pairs
resulted in a maximum receive power, SNR, SIR or SINR at the
receiver 114. In accordance with other embodiments, it is not
necessary to evaluate the receive power, SNR, SIR or SINR after
each step. Rather, the received test symbols may be recorded at the
receiver and the evaluation of some or all of received test symbols
and the selection may be done after all or a predefined number of
test symbols has been transmitted.
[0091] The thus determined beamforming vectors are used for a
transmission from the transmitter 102 to the receiver 114.
[0092] As just described, the 1.sup.St aspect of the invention
relates to the use of suitable training matrices T, i.e. a specific
selection and temporal sequence of beamforming vectors for the
training. In accordance with embodiments, the method operates
without knowledge of the beamforming channel matrix. It is assumed
that the beamforming is performed based on codebooks. A codebook C
is the (finite) magnitude of all possible and allowable beamforming
vectors. Basically, an individual codebook may be defined for every
antenna group in the system, however, group antennas having the
same number of antenna elements may also use the same codebook. The
codebook may also be expressed as codebook matrix C, into which the
beamforming vectors of the codebooks are entered column by column.
The selection of the codebook may be arbitrary, as long as the rows
and columns of the codebook matrix are not linearly dependent. The
maximum diversity gain, visible in the maximum increase of the bit
error frequency curve for large signal/interference power intervals
is obtained for unitary codebook matrices (see e.g. Love, D. J. and
Heath, R. W., Jr., "Equal gain transmission in multiple-input
multiple-output wireless systems", Communications, IEEE
Transactions on, 51(7):1102--1110, 2003). In a unitary codebook
matrix, all beamforming vectors (columns) are pairwise orthogonal
and have the norm one (orthonormal). By adding further
non-orthonormal beamforming vectors, additionally, antenna gain may
be realized, which is expressed in an improvement of the
signal-to-noise ratio. Further optimization criteria for codebooks
are, for example, minimal phase numbers for equal-gain beamformers
(see e.g. ECMA-387 Standard: High Rate 60 GHz PHY, MAC and HDMI
PAL, 2008, Ecma International).
[0093] In the following, the codebook matrices for the transmitter
102 and the receiver 114 are referred to by C.sub.T and C.sub.R,
respectively. For a unidirectional transmission between the two
stations 102 and 114 two different training matrices are provided:
The matrix T.sub.T for beamforming at the transmitter 102, and the
matrix T.sub.R for beamforming at the receiver 114. The matrices
T.sub.T and T.sub.R form a matrix pair. Each of the matrices of the
matrix pair has the same number of columns. If the codebook of the
transmitter 102 includes K.sub.T vectors and the codebook of the
receiver 114 includes K.sub.R vectors, the training matrices
T.sub.T and T.sub.R will each have K.sub.T-K.sub.R columns. The
vectors of the codebook of the transmitter 102 are included
K.sub.R-times in the training matrix T.sub.T of the transmitter. In
the same way, for the receiver 114, the training matrix T.sub.R
includes the vectors of the codebook of the receiver 102
K.sub.T-times.
[0094] For the training, the beamforming vectors for the
transmitter 102 and for the receiver 114 are each taken column by
column, starting with column 1, successively from the respective
training matrices, and the test transmission using suitable
training signals or training symbols is performed. Hence, for the
training, K.sub.TK.sub.R beamforming configurations and test
transmissions are required. The method in accordance with this
aspect is based on selecting the order of beamforming vectors in
the training matrices such that every vector from the codebook of
the transmitter 102 encounters all vectors from the codebook of the
receiver 114--and vice versa. In accordance with an embodiment, a
simple design rule for obtaining the training matrices may be
stated using the Kronecker product. A matrix pair T.sub.T, T.sub.R
may be calculated as follows:
T.sub.T=1.sub.1,K.sub.RC.sub.T, (1)
T.sub.R=C.sub.R1.sub.1,K.sub.T. (2)
[0095] Equations (1) and (2) may be exchanged, which means
T.sub.T=C.sub.T1.sub.1,K.sub.R, T.sub.R=1.sub.1,K.sub.TC.sub.R.
Also, simultaneously exchanging columns in T.sub.T and T.sub.R is
possible.
[0096] In accordance with an embodiment, in a bidirectional
transmission, the method is performed for both directions according
to the duplex method used in the system. Every station requires
both transmitting and receiving beamforming vectors and a training
matrix T.sub.T for the transmitter or for the receiver T.sub.R. The
training is then performed separately for both directions of
transmission, wherein the respective matrix pairs of training
matrices are used. In accordance with an embodiment, a station may
use the same antennas for transmitting and receiving. In such an
embodiment the beamforming vectors for one direction of
transmission may be determined and used also in the other direction
of transmission.
[0097] After the complete run of all test transmissions, those
beamforming vectors on the transmitter side and on the receiver
side for which the highest received power, SNR, SIR or SINR has
been obtained during the training phase are obtained. These
beamforming vectors are optimal for the selected codebooks at
transmitter 102 and at the receiver 114, independent of the used
data transmission method. In accordance with embodiments, the
optimization may take place at the receiver 114 so that the
determined transmitting beamforming vector (codebook entry) has to
be transmitted to the transmitter 102. The method in accordance
with the first aspect determining optimal beamforming vectors for
the transmitter 102 and for the receiver 114 in a single training
phase--without any iterative feedbacks from the receiver 114 to the
transmitter 102.
[0098] Embodiments in accordance with the first aspect are
advantageous, since for the training of the transmitting and
receiving beamformers 108 and 118 (unidirectional), the
trans-mission of training symbols in one direction is sufficient
using the respective training matrix pair. Consequently, for a
complete training of the beamformers 108, 118, for both directions
of transmission (bidirectional), only a single transmission in each
direction (station 102 to station 114 as well as station 114 to
station 102) is required. The method allows not only determining
particularly suitable adjustments but allows for the determination
of optimal beamformer adjustments with respect to the codebooks and
the chosen optimization criterion (e.g. received power, SNR, SIR,
SINR). Optimizing the beamformer weights by several transmissions
in both directions and iterative adoption of the weights is
omitted. The training is simplified, accelerated and the
performance of the data transmission system is maximized. If
small-scale codebooks are used, the method is also interesting for
mobile applications with quickly changing radio channels.
2.sup.nd Aspect: Estimating the Beamforming Channel Matrix
[0099] Subsequently, embodiments of the second aspect of the
invention are described, in accordance with which a beamforming
channel matrix is estimated without side information and without
MIMO signal processing. Again, a wireless communication system 100
as depicted in FIG. 1 is assumed, and a beamforming channel matrix
is to be determined which describes the radio channel 112 between
the transmitter 102 and the receiver 114. The transmitter 102 and
the receiver 114 comprise the respective antenna groups 106.sub.1
to 106.sub.M and 116.sub.1 to 116.sub.N. Further, as already
described above with regard to the first aspect, respective
codebooks for the transmitter 102 and 114 are provided, each of the
codebooks comprising a plurality of predetermined beamforming
vectors for the antenna group of the transmitter 102 or for the
antenna group of the receiver 114.
[0100] FIG. 3 is a flow diagram showing the respective steps of a
method in accordance with embodiments of the second aspect of the
invention. In a first step S200 a test transmission from the
transmitter 102 to the receiver 114 using beamforming vectors for
the receiver and for the transmitter and using a test symbol is
performed. At step S202 the beamforming vectors at the receiver and
at the transmitter are varied in accordance with a scheme, wherein
in accordance with embodiments of the invention, the scheme allows
for a variation of the beamforming vectors at the transmitting
station and at the receiving station on the basis of a transmit
estimate matrix and a receive estimate matrix, wherein each element
of an estimate matrix defines the beamforming weight for a specific
antenna form the antenna group used during a specific test
transmission. Basically the approach in accordance with the second
aspect is similar to the first approach except that other matrices
are used. For each new beamforming setup a test symbol is
transmitted. At step S204 it is determined as to whether a
variation of the beamforming vectors in accordance with a scheme
was completed. In case it was not completed, the method returns to
step S200 and performs the next test transmission on the basis of
the varied beamforming vectors. Otherwise, in case the beamforming
vector variation was completed, the method proceeds to step S206
and the beamforming channel matrix is determined from the test
transmissions.
[0101] As just described, the 2.sup.nd aspect of the invention
relates to a method for estimating the beamforming channel matrix H
without side information and without MIMO signal processing. In a
system with M transmitting antennas 106 and N receiving antennas
116, NM test transmissions are required. The beamforming channel
matrix H has M columns and N rows, and the element
[H].sub.n,m=h.sub.n,m describes the transmission from the
transmitting antenna m to the receiving antenna n:
H = ( h 1 , 1 h 1 , M h N , 1 h N , M ) ( 3 ) ##EQU00004##
[0102] The beamforming channel matrix H is estimated by performing
several test transmissions, i.e. transmitting several estimation
symbols subsequently, while varying the beamforming vectors at the
transmitter 102 and at the receiver 114 according to a specific
scheme. In accordance with an embodiment, the scheme may be
described mathematically using two matrices, a transmitting
estimation matrix E.sub.T and an associated receiving estimation
matrix E.sub.R. In accordance with this embodiment, the
transmitting estimation matrix E.sub.T includes the beamforming
vectors for the transmitter 102 as column entries in chronological
order, beginning with column 1. E.sub.T is derived from a base
transmitting estimation matrix B.sub.T. For a beamforming system
having M transmitting antennas, the base transmitting estimation
matrix B.sub.T has the dimension M.times.M. The transmitting
estimation matrix E.sub.T may be defined using the Kronecker
product:
E.sub.T=1.sub.1,NB.sub.T. (4)
[0103] It follows that the transmitting estimation matrix E.sub.T
has the dimension M.times.NM, and that the element
[E.sub.T].sub.m,k (=value in the m-th row and at the k-th column of
the transmitting estimation matrix E.sub.T) describes the
beamforming weight for the m-th transmitting antenna 106 in the
k-th of NM transmissions.
[0104] The receiving estimation matrix E.sub.R includes the
beamforming vectors for the receiver 114. The receiving estimation
matrix E.sub.R is also derived from a base receiving estimation
matrix B.sub.R, wherein the base receiving estimation matrix
B.sub.R has the dimension N.times.N for a system having N receiving
antennas:
E.sub.R=B.sub.R1.sub.1,m. (5)
[0105] It follows that the receiving estimation matrix E.sub.R has
the dimension N.times.NM, and that the element [E.sub.T].sub.n,k
(=value in the n-th row and at the k-th column of the receiving
estimation matrix E.sub.R) describes the weight for the n-th
receiving antenna 116 in the k-th transmission. The base estimation
matrices B.sub.T and B.sub.R for the transmitter 102 and for the
receiver 114 may be selected to be the same or different.
[0106] In the k-th transmission, a training symbol
x.sub.k=[x].sub.k is transmitted, and the symbol y.sub.k=[y].sub.k
is received at the receiver 114.
[0107] In accordance with an embodiment, the beamforming channel
matrix is estimated as follows. All transmissions may be presented
in matrix vector notation as follows:
y=SXh (6)
[0108] wherein X=diag(x) includes the transmitting vector x. The
matrix S includes the base estimation matrices according to
S=(B.sub.RB.sub.T).sup.T (7)
and h=vec(H) is the vectorized channel matrix H. Using the
transmission coefficient d.sub.k=[d].sub.k for every training
symbol
d=X.sup.-1y (8)
equation (6) reads as follows
d=Sh. (9)
[0109] The estimation of the beamforming channel matrix or is
performed by:
h=S.sup.-1d. (10)
[0110] Since the matrix S is independent of the channel H and
previously known, S.sup.-1 may be calculated and stored in advance,
such that equation 10 may be implemented efficiently. In accordance
with embodiments, unitary matrices are used for the base estimation
matrices, and S is also unitary and the following applies for
equation 10:
h=S.sup.Hd. (11)
[0111] In accordance with embodiments, for the base estimation
matrices B.sub.T and B.sub.R, basically, any square matrices or
codebooks may be used, as long as the elements are valid settings
for the beamforming weights w.sub.m=[w].sub.m or z.sub.n=[z].sub.n
and the matrices have full rank, i.e. the rows or the columns are
not linearly dependent. In the case of equal-gain beamforming, the
beamforming weights only differ in phase and the implementation of
the beamforming signal processing in accordance with embodiments
allows only a discrete number of equidistant phase states. For
channel estimation matrices, only certain phase states are possible
and low phase numbers are generally advantageous.
[0112] In accordance with further embodiments, regarding the
magnitude, unitary matrices have the same eigenvalues and are hence
optimal with respect to a low estimation error in error-prone
transmissions. Above that, unitary matrices, may be inverted in an
particularly easy manner. Embodiments of the invention suggest the
following unitary matrices for equal-gain beamforming: [0113] 1.
Hadamard matrices having the two phase states {0, .pi.}
corresponding to beamformer weights {1, -1}. Hadamard matrices of
the dimensions N.times.N are known for N=2 or N=4k with k E N.
[0114] 2. Matrices having four equidistant phase states {0, .pi.,
.pi./2, -.pi./2} corresponding to beamformer weights {1, -1, j, -j}
and j= {square root over (-1)}. These matrices cannot only be
stated for N=4k with k.epsilon.N but also for further even N.
[0115] 3. Matrices having {square root over (N)} equidistant phase
states may be stated for all N forming a square number by the
cyclical shift (Toplitzmatrix) of a minimum-phase uniform sequence
having perfect periodical autocorrelation. [0116] 4. Matrices
having N equidistant phase states may be constructed for all N. For
this, DFT matrices may be used.
[0117] In accordance with other embodiments, the unitarity of the
base estimation matrices may be abandoned and suitable matrices may
be determined by selecting certain rows and columns from suitable
larger matrices. For example, from a Hadamard matrix that is larger
than the desired base estimation matrix, smaller matrices having
two phase states may be derived.
[0118] Basically, the estimation method may also be performed using
non-square base estimation matrices. In such a case, B.sub.T and
B.sub.R include more columns than rows and the determination of the
vector h is performed with an over determined equation system. The
over determined equation system may be solved according to the
criterion of least error squares, and the estimation accuracy may
be improved in case the transmission is interfered by noise.
However, correspondingly more test transmissions are required.
[0119] If beamforming is used for both directions of transmission,
the method is generally performed for both directions, i.e. one
beamforming channel matrix for each direction is determined. In
case the same antennas are used for transmitting and receiving at a
station, estimating the channel matrix for one direction of
transmission may be sufficient. The beamforming channel matrix
derived may be used both for transmitting and for receiving.
[0120] In the following some examples for unitary base estimation
matrices are given:
Matrices Having Two Phase States:
[0121] Lists of Hadamard matrices for N=4k with k.epsilon.N are
available in mathematical literature, example for N=8:
B = 1 8 ( 1 1 1 1 1 1 1 1 1 - 1 1 - 1 1 - 1 1 - 1 1 1 - 1 - 1 1 1 -
1 - 1 1 - 1 - 1 1 1 - 1 - 1 1 1 1 1 1 - 1 - 1 - 1 - 1 1 - 1 1 - 1 -
1 1 - 1 1 1 1 - 1 - 1 - 1 - 1 1 1 1 - 1 - 1 1 - 1 1 1 1 ) ( 12 )
##EQU00005##
Matrices Having Four Phase States:
[0122] Matrices having four phase states are described for all even
N with N<16 (see e.g.
http://chaos.if.uj.edu.pl/.about.karol/hadamard/), example for
N=8:
B = 1 8 ( 1 1 1 1 1 1 1 1 1 j - 1 - j 1 j - 1 - j 1 j 1 - 1 1 - 1 1
- 1 1 - j - 1 j 1 - j - 1 j 1 1 1 1 - 1 - 1 - 1 - 1 1 j - 1 - j - 1
- j 1 j 1 - 1 1 - 1 - 1 1 - 1 1 1 - j - 1 1 - 1 j 1 - j ) ( 13 )
##EQU00006##
Matrices Having {square root over (N)} Phase States:
[0123] Matrices having {square root over (N)} phase states may be
generated by cyclically shifting from Frank sequences (see e.g.
Frank, R. and Zadoff, S. and Heimiller, R. Phase shift pulse codes
with good periodic correlation properties. Information Theory
(Corresp.), IRE Transactions on, 8(6):381--382, 1962), example for
N=16:
B = 1 4 ( 1 j - 1 - j 1 - 1 1 - 1 1 - j - 1 j 1 1 1 1 1 1 j - 1 - j
1 - 1 1 - 1 1 - j - 1 j 1 1 1 1 1 1 j - 1 - j 1 - 1 1 - 1 1 - j - 1
j 1 1 1 1 1 1 j - 1 - j 1 - 1 1 - 1 1 - j - 1 j 1 1 1 1 1 1 j - 1 -
j 1 - 1 1 - 1 1 - j - 1 j j 1 1 1 1 1 j - 1 - j 1 - 1 1 - 1 1 - j -
1 - 1 j 1 1 1 1 1 j - 1 - j 1 - 1 1 - 1 1 - j - j - 1 j 1 1 1 1 1 j
- 1 - j 1 - 1 1 - 1 1 1 - j - 1 j 1 1 1 1 1 j - 1 - j 1 - 1 1 - 1 -
1 1 - j - 1 j 1 1 1 1 1 j - 1 - j 1 - 1 1 1 - 1 1 - j - 1 j 1 1 1 1
1 j - 1 - j 1 - 1 - 1 1 - 1 1 - j - 1 j 1 1 1 1 1 j - 1 - j 1 1 - 1
1 - 1 1 - j - 1 j 1 1 1 1 1 j - 1 - j - j 1 - 1 1 - 1 1 - j - 1 j 1
1 1 1 1 j - 1 - 1 - j 1 - 1 1 - 1 1 - j - 1 j 1 1 1 1 1 j j - 1 - j
1 - 1 1 - 1 1 - j - 1 j 1 1 1 1 1 ) ( 14 ) ##EQU00007##
Matrices Having N Phase States:
[0124] The descriptive matrices of the discrete Fourier
transformation (DFT matrices) may be used as base estimation
matrices having N phase states, by cyclically shifting or by
permuting rows and columns, further base estimation matrices having
the same characteristics may be generated. Example for N=8:
B = 1 8 ( 1 1 1 1 1 1 1 1 1 1 2 - j 1 2 - j - 1 2 - j 1 2 - 1 - 1 2
+ j 1 2 j 1 2 + j 1 2 1 - j - 1 j 1 - j - 1 j 1 - 1 2 - j 1 2 j 1 2
- j 1 2 - 1 1 2 + j 1 2 - j - 1 2 + j 1 2 1 - 1 1 - 1 1 - 1 1 - 1 1
- 1 2 + j 1 2 - j 1 2 + j 1 2 - 1 1 2 - j 1 2 j - 1 2 - j 1 2 1 j -
1 - j 1 j - 1 - j 1 1 2 + j 1 2 j - 1 2 + j 1 2 - 1 - 1 2 - j 1 2 -
j 1 2 - j 1 2 ) ( 15 ) ##EQU00008##
[0125] Embodiments in accordance with the second aspect of the
invention are advantageous, since for estimating the beamforming
channel matrix for one direction of transmission, one transmission
phase using the respective estimation matrix pair is sufficient.
For estimating the channel matrices for both transmission
directions, one transmission phase for each direction (station 102
to station 114 as well as station 114 to station 102) is required.
The number of estimation symbols to be transmitted may be minimized
and consequently also the period for which this transmission phase
is required.
3.sup.rd Aspect: Determining the Beamforming Vectors by Using the
Beamforming Channel Matrix
[0126] In the following, embodiments of the third aspect of the
invention are described in further detail. In accordance with
embodiments beamforming vectors are determined using the known
beamforming channel matrix. Again, a wireless communication system
as described with regard to FIG. 1 is assumed, and a beamforming
vector of the antenna group 105 of the transmitter 102 as well as a
beamforming vector of the antenna group 116 of the receiver 114 is
determined. Again, as described above, the transmitter 102 and the
receiver 114 comprise a codebook including a plurality of
predefined beamforming vectors.
[0127] FIG. 4 depicts a flow diagram of an embodiment of the
invention in accordance with the just mentioned third aspect. In a
first step S300 from the codebook of the transmitter 102 or from
the codebook of the receiver 114 the beamforming vector is
determined that yields a first predetermined result when applying
the beamforming weights defined in the beamforming vector to a
known beamforming channel matrix describing the radio channel 112
between the transmitter 102 and the receiver 114. In a subsequent
step S302 for the receiver or the transmitter the beamforming
vector is selected from the codebook, wherein that beamforming
vector is selected that yields a second result when applying the
weights of the selected beamforming vector to a combination of the
beamforming channel matrix and the beamforming vector determined in
the preceding step S300.
[0128] As just described, the 3.sup.rd aspect of the invention
relates to the determination of the optimal beamforming vectors
using beamforming channel matrix knowledge. The beamforming vectors
for transmitter and receiver are determined based on the
beamforming channel matrix H. Any codebooks C may be used. In
accordance with an embodiment, the system use equal-gain
beamforming at the transmitter 102 and at the receiver 114. In the
following, the codebook for the transmitting beamformer 108 is
referred to as C.sub.T, and the codebook for the receiving
beamformer 118 is referred to as C.sub.R.
[0129] In accordance with an embodiment, the beamforming vectors
are determined in two steps. In the first step, a beamforming
vector w.sub.CH is determined for the transmitter 102, which
optimizes the expression Hw according to the criterion of the L1
Norm .parallel. .parallel..sub.1 (also named Taxi Cab Norm or
Manhattan Norm):
w CH = arg max w .di-elect cons. C T H w 1 . ( 16 )
##EQU00009##
[0130] For determining w.sub.CH, an optimization method may be used
or a search across all vectors of the codebook C.sub.T may be
performed.
[0131] In a second step, the beamforming vector z for the receiver
114 is determined. The term z.sup.THw.sub.CH=z.sup.Th.sub.w,CH is
maximized according to the criterion of the largest absolute value.
This may again be performed by a search across all vectors of the
codebook C.sub.R:
z CH = arg max z .di-elect cons. C R z T H w CH = arg max z
.di-elect cons. C R z T h w , CH ( 18 ) ( 17 ) ##EQU00010##
[0132] Alternatively,
z H = 1 N exp ( - j .angle. ( Hw CH ) ) ( 19 ) ##EQU00011##
may be determined and the vector having the maximum correlation
with z.sub.H may be selected from the codebook:
z CH = arg max z .di-elect cons. C R z H H z ( 20 )
##EQU00012##
[0133] The order in which the beamforming vectors are determined
may be changed. In such a case, a reciprocal system is considered
and the transmitting and receiving beamforming vectors are
exchanged in the equations (exchanging w and z) and the transposed
beamforming channel matrix (H.fwdarw.H.sup.T) is used. In that
manner, at first, a suitable receiving beamforming vector may be
determined without considering the transmitting beamforming vector,
and subsequently, a suitable transmitting beamforming vector
considering the determined from the receiving beamforming
vector.
[0134] In a bidirectional transmission system using beamforming in
both directions of transmission, the method may be performed for
both directions. In case the stations use the same antennas and
beamformers for transmitting and receiving, determining the
beamforming vectors for one direction of transmission may be
sufficient. A beamforming vector may then be used both for
transmitting and for receiving.
[0135] In a multi-carrier system having K subcarriers (K spectral
components), basically, for every subcarrier k, a beamforming
channel matrix H.sup.(k) may be determined:
H ( k ) = ( h 1 , 1 ( k ) h 1 , M ( k ) h N , 1 ( k ) h N , M ( k )
) . ( 21 ) ##EQU00013##
[0136] In a system using MIMO signal processing, individual
beamforming vectors would be determined for every subcarrier and
adjusted separately. This is not possible in a system with
beamforming signal processing, since only one beamforming vector
may be adjusted for all spectral components. Therefore, in
accordance with embodiments an easy-to-calculate solution is
suggested. For determining the beamforming vectors the method is
based only on the beamforming channel matrix having the highest
modulus sum norm (sum of the absolute values of matrix
entries):
H := H ( l ) with ( 22 ) l = arg max 1 .ltoreq. k .ltoreq. K n = 1
N m = 1 M h n , m ( k ) ( 23 ) ##EQU00014##
[0137] For reducing the effort further, only a sub range of the K
channel matrices may be considered in (23):
l = arg max 1 .ltoreq. k .ltoreq. K n = 1 N m = 1 M h n , m ( k )
with ( 24 ) K { 1 , 2 , K } . ( 25 ) ##EQU00015##
[0138] In that way, for example, only every second subcarrier may
be considered. The further steps for determining the beamforming
vectors starting from H correspond to the above described
method.
[0139] Embodiments in accordance with the third aspect are
advantageous, since suitable beamforming vectors are determined
based on the estimated beamforming channel matrix. The gain
obtainable for a given system by beamforming depends, apart from
the hardware, on the performance of the algorithms used. With a
suitable algorithm, not only particularly suitable but optimal
beamformer settings with respect to the codebooks may be
determined. A time-consuming training phase and/or iterative
optimization of the beamforming vectors by repeated transmission of
training symbols are not required. Hence, the method may operate
quickly and transmission resources are saved. It is of particular
interest for large-scale codebooks, since the number of required
estimation symbols does not depend on the scale of the codebook but
merely on the number of the transmitting and receiving beamformer
branches.
[0140] Beamforming vectors may also be determined for a
multi-carrier system based on an individual beamforming channel
matrix. This reduces the computational overhead. At the same time,
the selection criterion ensures that the vectors are optimized for
a suitable carrier. In that way, higher gains are possible by
beamforming, compared to the use of a fixed carrier. After the
selection of the channel matrix, any method may be used for
determining the beamforming vectors that are based on the knowledge
of the channel matrix.
4.sup.th Aspect: Determining the Beamforming Vectors in a Hybrid
MIMO Beamforming System
[0141] In accordance with the embodiments of a fourth aspect of the
invention an approach is described for determining the beamforming
vectors in a hybrid MIMO beamforming system. FIG. 5 shows an
example of a hybrid MIMO beamforming system having two MIMO
branches at the transmitter and at the receiver each having
associated therewith two beamforming branches. To be more specific,
FIG. 5 shows a unidirectional radio system comprising a transmitter
502 having an input 504, and a receiver 506 having an output 508.
The transmitter 502 comprises a transmitter signal processing unit
510, beamformers 512.sub.1 and 512.sub.2 having inputs 514.sub.1
and 514.sub.2, and antennas 516.sub.1 to 516.sub.4. The antennas
516.sub.1 and 516.sub.2 are connected to the beamformer 512.sub.1
and form a first antenna group. Together with the beamformer
512.sub.1 the antennas 516.sub.1 and 516.sub.2 form a first MIMO
branch 511.sub.1 of the transmitter 502. The antennas 516.sub.3 and
516.sub.4 are connected to the beamformer 512.sub.2 and form a
second antenna group. Together with the beamformer 512.sub.2 the
antennas 516.sub.3 and 516.sub.4 form a second MIMO branch
511.sub.2 of the transmitter 502. The beamformer 512.sub.1
comprises a splitting circuit 520.sub.1 and two weighting elements
522.sub.1 and 522.sub.2. Also the beamformer 512.sub.2 comprises a
splitting circuit 520.sub.2 and two weighting elements 522.sub.3
and 522.sub.4.
[0142] The data signal d.sub.s fed to the input 504 of the
transmitter 502 is processed by the transmitter signal processing
unit 510. In the transmitter signal processing unit 510 also the
MIMO signal processing of the transmit signal takes place. The
output transmission signal x.sup.(1) of the first MIMO branch
511.sub.1 is fed via the input 514.sub.1 into the beamformer
512.sub.1, and is split using the splitting circuit 520.sub.1. The
split signal is subsequently weighted using the weighting elements
522.sub.1 and 522.sub.2, and is forwarded to the antennas 516.sub.1
and 516.sub.2 Likewise, the output transmission signal x.sup.(2) of
the second MIMO branch 511.sub.2 is fed via the input 514.sub.2 of
the beamformer 512.sub.2 into the beamformer 512.sub.2, and is
split using the splitting circuit 520.sub.2. The split signal is
subsequently weighted using the weighting elements 522.sub.3 and
522.sub.4, and is forwarded to the antennas 516.sub.3 and
516.sub.4.
[0143] The signal radiated by the antennas 516.sub.1 to 516.sub.4
is transmitted via a radio channel and is received by a receiver
506. The receiver 506 comprises a receiver signal processing unit
526, beamformers 528.sub.1 and 528.sub.2 having outputs 530.sub.1
and 530.sub.2 and antennas 532.sub.1 to 532.sub.4. The antennas
532.sub.1 and 532.sub.2 are connected to the beamformer 528.sub.1
and form an antenna group. Together with the beamformer 528.sub.1
the antennas 532.sub.1 and 532.sub.2 form a first MIMO branch
521.sub.1 of the receiver 506. The antennas 532.sub.3 and 532.sub.4
are connected to the beamformer 528.sub.2 and also form an antenna
group. Together with the beamformer 528.sub.2 the antennas
532.sub.3 and 532.sub.4 form a second MIMO branch 521.sub.2 of the
receiver 506. The beamformer 528.sub.1 comprises an adding circuit
536.sub.1 and two weighting elements 538.sub.1 and 538.sub.2. Also
the beamformer 528.sub.2 comprises an adding circuit 536.sub.2 and
two weighting elements 538.sub.3 and 538.sub.4.
[0144] The signals received via the antennas 532.sub.1 and
532.sub.2 are fed to the beamformer 528.sub.1, are weighted by the
weighting elements 538.sub.1 and 538.sub.2, and are added using the
adding circuit 536.sub.1. At the output 530.sub.1 of the beamformer
528.sub.1 the signal y.sub.1 of the first MIMO branch 521.sub.1 is
present, which is input into the receiver signal processing unit
526. Likewise, the signals received via the antennas 532.sub.3 and
532.sub.4 are fed to the beamformer 528.sub.2, are weighted by the
weighting elements 538.sub.3 and 538.sub.4, and are added using the
adding circuit 536.sub.2. At the output 530.sub.2 of the beamformer
528.sub.2 the signal y.sub.2 of the second MIMO branch 521.sub.2 is
present, which is input into the receiver signal processing unit
526. In the receiver signal processing unit 526 the signals y.sub.1
and y.sub.2 are processed. In the receiver signal processing unit
526 also the MIMO signal processing of the receive signals occurs.
At the output 508 of the receiver 506 the received data signal
d.sub.r is present.
[0145] As just described, the 4th aspect of the invention relates
to a multi-antenna radio system with MIMO signal processing. In
every MIMO branch beamforming signal processing can be applied
using a beamformer and an antenna group, which results in a hybrid
MIMO beamforming configuration. The system has P MIMO branches at
the transmitter 502 and Q MIMO branches at the receiver 506. Every
MIMO transmitting branch p, p=1 . . . . P, comprises M.sub.p
beamforming transmitting branches. Every MIMO receiving branch q,
q=1 . . . . Q consists of N.sub.q beamforming receiving branches.
FIG. 5 shows an example of a hybrid MIMO beamforming system 500
having the two MIMO branches 511.sub.1, 511.sub.2 at the
transmitter 502 and the two MIMO branches 521.sub.1, 521.sub.2 at
the receiver 506, wherein each of the beamformers 512.sub.1,
512.sub.2 and 528.sub.1, 528.sub.2 has two branches.
[0146] FIG. 6 depicts a flow diagram representing the steps of a
method in accordance with the embodiments of the fourth aspect of
the invention. The method is implemented, for example, by a system
as described with regard to FIG. 5 and comprises as a first step
S600 the splitting of the hybrid MIMO beamforming system into a
plurality of subsystems. In a subsequent step S602 for each
subsystem the beamforming vectors are determined separately.
[0147] In accordance with embodiments of the 4th aspect of the
invention, a method for determining suitable beamforming vectors
for hybrid MIMO beamforming systems having any number of MIMO
branches at the transmitter and at the receiver is described. The
basic idea is to suitably split the hybrid MIMO beamforming system
into several subsystems. Then, for every subsystem, any known
method for determining suitable beamforming parameters may be used.
If the subsystems are beamforming systems or are considered as
beamforming systems one or more of the above described methods in
accordance with the 1.sup.St to 3.sup.rd aspect of the invention
for determining suitable beamforming parameters may be used.
Considering a subsystem with more than one MIMO branch at one side
as beamforming system, means to assume for the optimization that
the beamformers of all MIMO branches at this side form one large
beamformer and that only beamforming signal processing can be
applied.
[0148] For splitting an overall system, two approaches may be
used.
[0149] In accordance with embodiments, the first approach comprises
a fixed or static allocation, where the hybrid MIMO beamforming
system is split into a plurality of beamforming subsystems. Every
MIMO transmitting branch is allocated to one MIMO receiving branch,
and every MIMO receiving branch is allocated to one MIMO
transmitting branch. For optimizing the performance of the system,
the following allocation rules are defined: [0150] 1. The
allocation is performed "as evenly as possible", i.e. the number of
MIMO receiving branches allocated to a MIMO transmitting branch or
the number of MIMO transmitting branches allocated to a MIMO
receiving branch is minimized. [0151] 2. The information on the
spatial arrangement of the MIMO transmitting and receiving antennas
are considered as follows: If several MIMO receiving branches are
allocated to a MIMO transmitting branch (several MIMO transmitting
branches to one MIMO receiving branch) those receiving branches are
respectively allocated to a MIMO transmitting branch whose MIMO
antennas are spatially as far as possible apart from one another
(those transmitting branches are allocated to a MIMO receiving
branch whose MIMO antennas are spatially as far as possible apart
from one another). Dependent on the MIMO signal processing, in
accordance with embodiments, it may be advantageous to use not the
MIMO branches having the most distant antennas but those MIMO
branches having their antennas as close as possible.
[0152] FIGS. 7(a) and (b) show examples for the static allocation
on the basis of the MIMO beamforming system of FIG. 5. As can be
seen, the hybrid 2.times.2 MIMO beamforming system is separated
into two beamforming subsystems, and in accordance with the
embodiment depicted in FIG. 7(a) a first beamforming subsystem 700
is formed of the MIMO branches 511.sub.1 of the transmitter 502 and
the MIMO branch 521.sub.1 of the receiver 514. The second
beamforming subsystem 702 comprises the second MIMO branch
511.sub.2 of the transmitter 502 and the second MIMO branch
521.sub.2 of the receiver 514. In the embodiment of FIG. 7(b) the
first beamforming subsystem 704 comprises the first MIMO branch
511.sub.1 of the transmitter 502 and the second MIMO branch
521.sub.2 of the receiver 514. The second beamforming subsystem 706
comprises the second MIMO branch 511.sub.2 of the transmitter 502
and the first MIMO branch 521.sub.1 of the receiver 514.
[0153] By the allocation, the MIMO beamforming overall system is
split into max(P, Q) beamforming subsystems. FIG. 7 illustrates the
two options for splitting a hybrid MIMO beamforming system having
two MIMO branches at the transmitter and at the receiver. For every
beamforming subsystem, suitable beamforming vectors for the
transmitter and the receiver are to be determined. For this
purpose, the methods in accordance with the 1st to 3rd aspect
described may be used.
[0154] In methods with training matrices (see the first aspect),
the different beamforming subsystems are considered sequentially in
any order. Thereby only the MIMO branches of the currently
considered subsystem are active. Transmit branches that do not
belong to the currently considered subsystem are turned off and
receive branches that do not belong to the currently considered
subsystem remain unconsidered. Only the training matrices of the
currently considered subsystem are used.
[0155] In methods for determining beamforming vectors having
channel knowledge (see the third aspect), only those beamforming
channel submatrices describing each of the channels between the
allocated MIMO branches have to be known. For the channel
estimation, the subsystems may be considered sequentially in any
order. Thereby, on the transmitter side, only the MIMO branch of
the currently considered subsystem is active. Transmit branches
that do not belong to the currently considered subsystem are turned
off and receive branches that do not belong to the currently
considered subsystem remain unconsidered.
[0156] In case there are more MIMO branches in the transmission
system on the transmitter side than on the receiver side, or vice
versa, for determining the beamforming vectors some MIMO branches
are considered several times. For example, in case of three MIMO
branches at the transmitter and two MIMO branches at the receiver,
one MIMO branch of the receiver is considered twice. Since for each
beamformer, finally, only one beamforming vector is used, two
different approaches are suggested:
[0157] 1. The beamforming vector determined while considering the
MIMO branch for the first time is maintained. In case the same MIMO
branch is considered again, this beamforming vector will only be
used for optimizing the beamforming vector on the opposite
side.
[0158] 2. At the beginning no attention is paid to the fact whether
MIMO branches are considered multiple times. Thus, for some MIMO
branches a plurality of beamforming vectors are determined. From
these vectors for each branch that beamforming vector is selected
and maintained which provides the best performance in accordance
with a predefined optimization criterion. Subsequently a new
optimization of all those MIMO branches on the opposite side is
done, which are associated with the currently considered MIMO
branch and which in accordance with the predefined optimization
criterion showed a worse performance. On the basis of the already
determined beamforming vector only the respective beamforming
vector on the opposite side is optimized.
[0159] Using the static allocation has little complexity, since the
MIMO branches are firmly allocated to one another and the
beamforming subsystems have, at the most, the dimension
max 1 .ltoreq. p .ltoreq. P M p .times. max 1 .ltoreq. q .ltoreq. Q
N q . ##EQU00016##
[0160] In accordance with embodiments, the second approach
comprises asymmetric splitting of the beamforming system. The basic
idea of this approach is splitting the hybrid MIMO beamforming
overall system into asymmetric subsystems, each having only one
MIMO branch on the transmitter side or on the receiver side. The
method may be divided into two steps:
[0161] Step 1: The hybrid MIMO beamforming overall system is split
into
P M p .times. q = 1 Q N q ##EQU00017##
subsystems. For every subsystem, a suitable transmitting
beamforming vector is determined. By considering the subsystem as
beamforming system, the methods described above can be used.
[0162] Step 2: The hybrid MIMO beamforming overall system is split
into
Q p = 1 P N p .times. M q ##EQU00018##
subsystems. For every subsystem, a suitable receiving beamforming
vector is determined. By considering the subsystem as beamforming
system, the methods described above may be used. (Method without
considering the transmission beam, determining the beamforming
vectors in reverse order).
[0163] After these steps, all transmitting and receiving
beamforming vectors are determined. If the methods described above
are used, step 1 and step 2 may be considered independently of one
another and are exchangeable. Further, the subsystems may be
considered in any order.
[0164] FIG. 8 shows an example of the asymmetric splitting of a
hybrid 2.times.2 MIMO beamforming system as it is for example
described in FIG. 5. In FIG. 8(a) the first subsystem 800 which is
an asymmetric system, comprises the first MIMO branch 511.sub.1 of
the transmitter 502 and the first and second MIMO branches
521.sub.1 and 521.sub.2 of the receiver 514. The second asymmetric
subsystem 802 comprises the second MIMO branch 511.sub.2 of the
transmitter 502 and the two MIMO branches 521.sub.1 and 521.sub.2
of the receiver 514. In the embodiment shown in FIG. 8(b) an
asymmetric subsystem 804 comprises the MIMO branches 511.sub.1 and
511.sub.2 of the transmitter 502 and the first MIMO branch
521.sub.1 of the receiver 514. The asymmetric, subsystem 806
comprises the two MIMO branches 511.sub.1 and 511.sub.2 of the
transmitter 502, and the second MIMO branch 521.sub.2 of the
receiver 514.
[0165] FIG. 8 illustrates the two steps for splitting a hybrid MIMO
beamforming system having two MIMO branches at the transmitter and
receiver. For the specific cases that a hybrid SIMO (single-input
multiple-output) or a MISO (multiple-input single-output)
beamforming system is treated and the subsystems are considered as
beamforming systems, the modifications described below will be
considered, which increase the gain obtainable by beamforming.
[0166] For the special case of a SIMO beamforming system (P=1,
Q>1) first, the
M .times. q = 1 Q N q ##EQU00019##
beamforming system is considered and a suitable transmitting
beamforming vector is determined. Then, the SIMO beamforming
overall system is split into Q M.times.N.sub.q subsystems. For
every subsystem, a suitable receiving beamforming vector is
determined. The difference to the basic method is that during
determining the receiving beamforming vectors, the previously
determined transmitting beamforming vector is considered.
[0167] For the special case of a MISO beamforming system (P>1,
Q=1), first, the
p = 1 P M p .times. N ##EQU00020##
beamforming system is considered and a suitable receiving
beamforming vector is determined without considering transmitting
beamforming vectors. Then, the MISO beamforming overall system is
divided into P M.sub.p.times.N subsystems. For every beamforming
subsystem, a suitable transmitting beamforming vector is
determined. The difference to the basic method is that during
determining the transmitting beamforming vectors, the previously
determined receiving beamforming vector is considered.
[0168] In accordance with embodiments splitting a hybrid MIMO
beamforming system into several subsystems is advantageous as this
allows the determination of suitable beamforming vectors in hybrid
MIMO beamforming systems by applying known methods for determining
the suitable beamforming vectors in beamforming systems.
[0169] Although some aspects have been described in the context of
an apparatus, it is clear that these aspects also represent a
description of the corresponding method, where a block or device
corresponds to a method step or a feature of a method step.
Analogously, aspects described in the context of a method step also
represent a description of a corresponding block or item or feature
of a corresponding apparatus.
[0170] Depending on certain implementation requirements,
embodiments of the invention can be implemented in hardware or in
software. The implementation can be performed using a digital
storage medium, for example a floppy disk, a DVD, a CD, a ROM, a
PROM, an EPROM, an EEPROM or a FLASH memory, having electronically
readable control signals stored thereon, which cooperate (or are
capable of cooperating) with a programmable computer system such
that the respective method is performed. Some embodiments according
to the invention comprise a data carrier having electronically
readable control signals, which are capable of cooperating with a
programmable computer system, such that one of the methods
described herein is performed.
[0171] Generally, embodiments of the present invention may be
implemented as a computer program product with a program code, the
program code being operative for performing one of the methods when
the computer program product runs on a computer. The program code
may for example be stored on a machine readable carrier.
[0172] Other embodiments comprise the computer program for
performing one of the methods described herein, stored on a machine
readable carrier. In other words, an embodiment of the inventive
method is, therefore, a computer program having a program code for
performing one of the methods described herein, when the computer
program runs on a computer. A further embodiment of the inventive
methods is, therefore, a data carrier (or a digital storage medium,
or a computer-readable medium) comprising, recorded thereon, the
computer program for performing one of the methods described
herein. A further embodiment of the inventive method is, therefore,
a data stream or a sequence of signals representing the computer
program for performing one of the methods described herein. The
data stream or the sequence of signals may for example be
configured to be transferred via a data communication connection,
for example via the Internet.
[0173] A further embodiment comprises a processing means, for
example a computer, or a programmable logic device, configured to
or adapted to perform one of the methods described herein. A
further embodiment comprises a computer having installed thereon
the computer program for performing one of the methods described
herein. In some embodiments, a programmable logic device (for
example a field programmable gate array) may be used to perform
some or all of the functionalities of the methods described herein.
In some embodiments, a field programmable gate array may cooperate
with a microprocessor in order to perform one of the methods
described herein. Generally, the methods are preferably performed
by any hardware apparatus.
[0174] The above described embodiments are merely illustrative for
the principles of the present invention. It is understood that
modifications and variations of the arrangements and the details
described herein will be apparent to others skilled in the art. It
is the intent, therefore, to be limited only by the scope of the
impending patent claims and not by the specific details presented
by way of description and explanation of the embodiments
herein.
* * * * *
References