U.S. patent application number 15/065079 was filed with the patent office on 2016-09-08 for precoder for spatial multiplexing, multiple antenna transmitter.
The applicant listed for this patent is TELEFONAKTIEBOLAGET L M ERICSSON (PUBL). Invention is credited to GEORGE JONGREN.
Application Number | 20160261323 15/065079 |
Document ID | / |
Family ID | 48743918 |
Filed Date | 2016-09-08 |
United States Patent
Application |
20160261323 |
Kind Code |
A1 |
JONGREN; GEORGE |
September 8, 2016 |
PRECODER FOR SPATIAL MULTIPLEXING, MULTIPLE ANTENNA TRANSMITTER
Abstract
A method of spatially precoding data includes determining a
transmission rank of a communication channel and selecting a set of
one or more precoding filters derived from a single generator
matrix based on said transmission rank. The method also includes
precoding data for transmission to a remote device using said
precoding filters in a predetermined order according to a
predetermined precoding sequence. The precoding may include using
different ones of said precoding filters during different precoding
intervals in a predetermined precoding period of the predetermined
precoding sequence. Additionally, precoding data for transmission
may involve traversing an Orthogonal Frequency Division
Multiplexing (OFDM) resource block in an alternating pattern during
said predetermined precoding period.
Inventors: |
JONGREN; GEORGE;
(SUNDBYBERG, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TELEFONAKTIEBOLAGET L M ERICSSON (PUBL) |
STOCKHOLM |
|
SE |
|
|
Family ID: |
48743918 |
Appl. No.: |
15/065079 |
Filed: |
March 9, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13716603 |
Dec 17, 2012 |
9287951 |
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15065079 |
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12746448 |
Jun 4, 2010 |
8335274 |
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PCT/SE2008/051155 |
Oct 9, 2008 |
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13716603 |
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60991849 |
Dec 3, 2007 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 7/0486 20130101;
H04L 27/20 20130101; H04L 5/0026 20130101 |
International
Class: |
H04B 7/04 20060101
H04B007/04; H04L 5/00 20060101 H04L005/00; H04L 27/20 20060101
H04L027/20 |
Claims
1. A method of spatially precoding data in a channel independent
way, said method comprising: selecting a transmission rank based on
a transmission rank of a communication channel (16); said method
characterized by: selecting a set of one or more precoding filters
(124) derived from a single generator matrix based on said
transmission rank; and precoding data for transmission to a remote
device (200) using said precoding filters in a predetermined order
according to a predetermined precoding sequence, thus using
different ones of said precoding filters during different precoding
intervals in a precoding period of the predetermined precoding
sequence.
2. The method of claim 1 wherein selecting a set of one or more
precoding filters (124) derived from a single generator matrix
based on said transmission rank comprises selecting one or more
columns of the generator matrix for each precoding filter, and
wherein the number of columns selected for each precoding filter
equals the transmission rank.
3. The method of claim 2 wherein selecting a set of one or more
precoding filters (124) derived from a single generator matrix
based on said transmission rank further comprises selecting a set
of precoding filters such that the minimum subspace distance
between any two precoding filters in the set of precoding filters
is maximized according to a predetermined distance criterion.
4. The method of claim 3 wherein the distance criterion comprises
at least one of chordal, projection two-norm, or Fubini-Study
distance.
5. The method according to any of claims 2-4, wherein selecting a
set of one or more precoding filters (154) derived from a single
generator matrix based on said transmission rank comprises
selecting the set of precoding filters such that each column of the
generator matrix is used the same number of times.
6. The method according to any of claims 1-5, wherein said
generator matrix comprises a QPSK alphabet generator matrix.
7. A transmitter (100) for transmitting spatially precoding data in
a channel independent way comprising: a transmit controller (110)
configured to determine a transmission rank of said communication
channel and to select a set of one or more precoding filters (124)
derived from a single generator matrix based on said transmission
rank; and a transmit signal processor (120) including a precoder
(124) configured to precode said data for transmission using said
precoding filters in a predetermined order according to a
predetermined precoding sequence, thus using different ones of said
precoding filters in different precoding intervals of a
predetermined precoding period of the predetermined precoding
sequence.
8. The transmitter (100) of claim 7 wherein the transmit signal
processor (120) is further configured to select one or more columns
of the generator matrix for each precoding filter, and wherein the
number of columns in each precoding filter equals the transmission
rank.
9. The transmitter (100) of claim 8 wherein the transmit signal
processor (120) is further configured to select the set of filters
that maximizes a minimum subspace distance between any two
precoding filters in the set according to a predetermined distance
criterion.
10. The transmitter (100) of claim 11 wherein the distance
criterion comprises at least one of chordal, projection two-norm,
or Fubini-Study distance.
11. The transmitter (100) according to any of claims 9-10, wherein
the transmit signal processor (120) is further configured to select
the set of precoding filters such that the selected set of
precoding filters uses each column of the generator matrix the same
number of times.
12. A method of receiving spatially precoded data, precoded in a
channel independent way, received from a remote device (200), said
method comprising: determining a transmission rank applied at a
transmitter; said method characterized by: selecting a set of one
or more combining filters (224) based on said transmission rank,
wherein said combining filters corresponding to a set of precoding
filters derived from a single generator matrix; and combining said
spatially precoded data using said combining filters in a
predetermined order according to a predetermined combining
sequence, thus using different ones of said combining filters
during different combining intervals in a combining period of the
predetermined combining sequence.
13. The method of claim 12 wherein selecting a set of one or more
combining filters (224) derived from a single generator matrix
based on said transmission rank comprises selecting one or more
columns of the generator matrix for each combining filter, and
wherein the number of columns selected for each combining filter
equals the transmission rank.
14. The method according to claim 13, wherein selecting a set of
one or more combining filters (224) derived from a single generator
matrix based on said transmission rank comprises selecting the set
of combining filters such that each column of the generator matrix
is used the same number of times.
15. The method according to any of claims 12-14 applied to an
Orthogonal Frequency Division Multiplexing (OFDM) system wherein
combining said spatially precoded data comprises traversing an OFDM
resource block in an alternating pattern during said predetermined
combining period.
16. A receiver for receiving spatially precoded data, precoded in a
channel independent way, said method comprising: a receive
controller (210) configured to determine a transmission rank
applied at a transmitter (100) and to select a set of one or more
combining filters (224) derived from a single generator matrix
based on said transmission rank; and a receive signal processor
(220) including a combiner (124) configured to combine said
spatially precoded data using said combining filters in a
predetermined order according to the predetermined combining
sequence, thus using different ones of said combining filters in
different combining intervals in a predetermined combining period
of the predetermined combining sequence.
17. The receiver of claim 16 wherein said receive signal processor
(220) is further configured to select one or more columns of the
generator matrix for each combining filter, and wherein the number
of columns selected for each combining filter equals the
transmission rank.
18. The receiver according to any of claim 17, wherein said receive
signal processor (220) is further configured to select the set of
combining filters (224) such that each column of the generator
matrix is used the same number of times.
19. The receiver according to any of claims 16-18 applied to an
Orthogonal Frequency Division Multiplexing (OFDM) system, wherein
the receive signal processor (210) is configured to combine said
precoded data while traversing an OFDM resource block in an
alternating pattern during said predetermined combining period.
Description
CLAIMING BENEFIT OF PRIOR FILED U.S. APPLICATION
[0001] This application is a continuation of Ser. No. 13/716,603
filed Dec. 17, 2012, which is a continuation of U.S. patent
application Ser. No. 12/746,448, filed Jun. 4, 2010, which is a
National Phase Entry of International Application
PCT/SE2008/051155, with an international filing date of Oct. 9,
2008, which and which claims the benefit of U.S. Provisional
Application No. 60/991,849, filed Dec. 3, 2007, and the contents of
all of the preceding are hereby incorporated by reference
herein.
TECHNICAL FIELD
[0002] The invention relates generally to methods and apparatus for
transmitting signals using multiple transmit antennas and, more
particularly, to methods and apparatus for spatially precoding
signals transmitted from a multiple antenna transmitter.
BACKGROUND
[0003] In recent years, there has been much interest in multiple
input, multiple output (MIMO) systems for enhancing data rates in
mobile communication systems. MIMO systems employ multiple antennas
at the transmitter and receiver to transmit and receive
information. The receiver can exploit the spatial dimensions of the
signal at the receiver to achieve higher spectral efficiency and
higher data rates without increasing bandwidth.
[0004] One transmission scheme for MIMO systems that is receiving
significant attention is spatial multiplexing. In a spatial
multiplexing transmitter, the information symbols are precoded
before transmission to multiplex the information signal in the
spatial domain. The precoding may be channel dependent or channel
independent. With channel dependent precoding, also referred to as
closed loop precoding, the precoder matrix is chosen to match the
characteristics of the MIMO channel. With channel independent
precoding, also referred to as open-loop precoding, channel
characteristics are not considered in selecting the precoder
matrix.
[0005] With closed loop precoding, the user equipment performs
channel measurements on the forward link channel, and feeds back
channel information or precoder configurations to the base station.
One problem with closed loop precoding is that it takes time to
perform channel measurements and feed back information to the base
station. During that time, the channel conditions may have changed
so that the feedback information is outdated before it is used.
Consequently, closed loop precoding is typically used in low
mobility situations where the channel variations are slow.
[0006] In situations where the channel conditions vary more rapidly
and lack significant long-term properties, channel independent
precoding or open-loop precoding may be used. With open loop
precoding, the precoding matrix is selected independent of the
channel realizations. Channel independent precoding is generally
considered more suitable for high mobility situations.
[0007] One way to implement open loop precoding is to use a spatial
multiplexing precoder matrix to precode the information sequence
prior to transmission. In order to accommodate a wide range of
channel realizations, it is advantageous to apply multiple
precoders that are varied in a deterministic manner known to both
the transmitter and the receiver. For example, in an orthogonal
frequency division multiplexing (OFDM) system, the precoder may be
kept fixed for a set of one or more subcarriers and then changed
for the next set of subcarriers. This technique, referred to as
precoder cycling, serves to distribute the energy spatially in a
more isotropic manner, which in turn is useful for diversity and
reducing the tendency to bias the performance of the transmitter
for a particular set of channel realizations. When applying
precoder cycling, it is advantageous to have substantial precoding
variation over the smallest possible allocation unit, e.g., a
resource block (RB), since a codeword may potentially only span a
very small set of resource elements.
[0008] A number of drawbacks have been encountered in the past when
precoder cycling has been used. Interference rejection algorithms
implemented at the receiver need to characterize the spatial
properties of the channel to suppress interference. It is
beneficial that the interfering transmissions have roughly similar
properties over a large number of resource elements so that
averaging may be used to suppress noise and other impairments. In
systems where the cycling of the precoder is configurable, the
receiver can not be sure how fast the interference changes over a
resource block without a priori knowledge of the precoder sequence.
Also, the precoders are frequently chosen from a codebook designed
for channel dependent precoding. As a consequence, the precoders do
not distribute the energy uniformly over the vector space of the
MIMO channel. Finally, precoder cycling increases the computational
complexity of demodulation and CQI computation. The computational
complexity is bounded only by the codebook size, so the receiver
needs to be designed to handle the worst case scenario.
SUMMARY
[0009] The present invention relates to a method and apparatus for
spatially precoding data for transmission to a remote device over a
MIMO channel. In one exemplary embodiment, the transmitter selects
a transmission rank and uses a predetermined precoder sequence for
the selected transmission rank comprising one or more precoder
filters. During transmission, a precoder precodes data for
transmission to a remote device using different precoding filters
during different precoding intervals in a precoding period
according to the selected precoder sequence.
[0010] The invention offers an efficient way to support open-loop
MIMO transmission particularly targeting rank two or higher rank
transmissions. Computational complexity for demodulation and CQI
computation in the UE is reduced and the feasibility of
interference rejection is improved compared to existing solutions.
The increased uniformity of the transmission in the spatial domain
improves the robustness of the open-loop MIMO mode. The use of a
single generator matrix may result in considerable complexity
savings as many of the computations for CQI and demodulation may be
reused across several different ranks and when identifying the
characteristics of the inter-cell interference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates an exemplary MIMO channel.
[0012] FIG. 2 illustrates an exemplary transmit signal processor
for an OFDM system.
[0013] FIG. 3 illustrates the mapping of codewords to layers as
performed by the transmit signal processor.
[0014] FIG. 4 illustrates an exemplary method for precoding data
for transmission to a remote device.
[0015] FIG. 5 illustrates an exemplary receive signal processor for
an OFDM system.
[0016] FIG. 6 illustrates an exemplary method for receiving
precoded data.
DETAILED DESCRIPTION
[0017] FIG. 1 illustrates a multiple input/multiple output (MIMO)
wireless communication system 10 including a first station 12 and a
second station 14. The first station 12 includes a transmitter 100
for transmitting signals to the second station 14 over a
communication channel 16, while the second station 14 includes a
receiver 200 for receiving signals transmitted by the first station
12. Those skilled in the art will appreciate that the first station
12 and second station 14 may each include both a transmitter 100
and receiver 200 for bi-directional communications. In one
exemplary embodiment, the first station 12 comprises a base station
in a wireless communication network, and the second station 14
comprises a user terminal. The present invention is particularly
useful in Orthogonal Frequency Division Multiplexing (OFDM)
systems.
[0018] An information signal in the form of a binary data stream is
input to the transmitter 100 at the first station 12. The
transmitter 100 includes a controller 110 to control the overall
operation of the transmitter 100 and a transmit signal processor
120. The transmit signal processor 120 performs error coding, maps
the input bits to complex modulation symbols, and generates
transmit signals for each transmit antenna 130. After upward
frequency conversion, filtering, and amplification, transmitter 100
transmits the transmit signals from respective transmit antennas
130 through the communication channel 16 to the second station
14.
[0019] The receiver 200 at the second station 14 demodulates and
decodes the signals received at each antenna 230. Receiver 200
includes a controller 210 to control operation of the receiver 200
and a receive signal processor 220. The receive signal processor
220 demodulates and decodes the signal transmitted from the first
station 12. The output signal from the receiver 200 comprises an
estimate of the original information signal. In the absence of
errors, the estimate will be the same as the original information
signal input at the transmitter 12.
[0020] Because multiple data streams are transmitted in parallel
from different antennas 130, there is a linear increase in
throughput with every pair of antennas 130, 230 added to the system
without an increase in the bandwidth requirement. MIMO systems have
been the subject of extensive research activity worldwide for use
in wireless communication networks because of their potential to
achieve high spectral efficiencies, and therefore high data
rates.
[0021] In embodiments of the present invention, the transmit signal
processor 120 is configured to spatially multiplex the information
signal before transmission to realize further increases in spectral
efficiency by taking advantage of the spatial dimension of the
communication channel 16. FIG. 2 illustrates an exemplary transmit
signal processor 120 according to one embodiment of the invention
for an Orthogonal Frequency Division Multiplexing (OFDM) system.
The transmit signal processor 120 comprises a layer mapping unit
122, a precoder 124, and a plurality of Inverse Fast Fourier
Transform (IFFT) processors 126. The IFFT processors 126 may
perform a Discrete Fourier Transform or the inverse operation. A
sequence of information symbols is input to the layer mapping unit
122. The symbol sequence is divided into codewords that are mapped
by the transmitter 100 to corresponding OFDM symbols. The layer
mapping unit 122 maps the codewords into one or more layers
depending on the transmission rank. It should be noted that the
number of layers does not necessarily equal the number of antennas
130. Different codewords are typically mapped to different layers;
however, a single codeword may be mapped to one or more layers. The
number of layers L corresponds to the selected transmission
rank.
[0022] FIG. 3 illustrates the mapping of codewords to layers
according to one exemplary embodiment for transmission ranks from 1
to 4. For a transmission rank of 1, a single codeword is mapped to
a single layer. For a transmission rank of 2, two codewords are
mapped to two different layers. For a transmission rank of 3, two
codewords are mapped to three layers, and for a transmission rank
of 4, two codewords are mapped to four layers. It may be noted that
the transmission rank or number of layers need not be the same as
the number of antennas. In the subsequent discussion, it is assumed
that the transmitter 100 includes four transmit antennas 130.
[0023] Each layer output from the layer mapping unit 122 feeds into
the precoder 124. Precoder 124 spatially multiplexes the symbols in
each layer by multiplying a vector s of input symbols to the
precoder 124 with a precoding filter. The precoding filter is an
N.times.L matrix that multiplies each input symbol of the symbol
vector s by a corresponding column vector of the precoding matrix.
In order to achieve diversity, the precoder 124 cycles through
multiple precoding filters and outputs N coded symbol streams. Each
symbol stream is output to a corresponding IFFT processor 126. In
an orthogonal OFDM system, the precoding filter may be kept fixed
for a set of one or more subcarriers and then changed for the next
set of subcarriers according to the selected precoder sequence. The
precoding filters may be pre-stored in memory or generated on the
fly by the transmit signal processor 120 as hereinafter described.
The IFFT processors 126 transform the spatially coded symbols
output by the precoder 124 to the frequency domain to generate OFDM
symbols. The OFDM symbols output from each IFFT processor 124 are
then output to a respective antenna 130 via antenna ports 128 for
transmission to the receiver 200. By spatially coding the
information symbols, it is possible to transmit multiple symbols on
each resource element (RE) of the OFDM resource grid.
[0024] According to the present invention, precoder 124 cycles
through a fixed and predetermined set of precoding filters
determined based on the selected transmission rank. A precoder
sequence known a priori to the base station and user terminal
specifies the set of precoding filters to use for precoding and the
order in which the precoding filters in the set are applied. A
different precoder sequence is defined for each possible
transmission rank.
[0025] The precoding filters corresponding to each precoder
sequence are selected to satisfy the following criteria: [0026] the
precoder sequence is the same for each resource block or smallest
resource allocation unit; [0027] the precoding sequence should use
the different precoding filters an equal number of times, or as
close to equal as possible; [0028] the number of different
precoding filters in the precoding sequence should be small but
still distribute the subspaces sufficiently uniform over the
(complex) Grassmanian manifold; and [0029] the number of different
precoding filters corresponding to one period of the precoder
sequence should be applied to resource elements which are close to
one another in the resource grid. A precoding sequence meeting
these criteria is referred to as short uniformly varying precoding
sequence (SUVPS).
[0030] In one exemplary embodiment, the precoding filters may be
selected from a predetermined codebook. An exemplary codebook is
the House Holder codebook specified in the Long Term Evolution
(LTE) standard currently being developed. The House Holder codebook
comprises sixteen precoding filters. For each transmission rank,
four of the possible sixteen precoding filters in the House Holder
codebook may be selected to form a precoder sequence with a
periodicity of four. That is, each precoding filter is used once in
one period of the precoder sequence. The selection should be made
to optimize some predetermined criterion that strives for a uniform
distribution of subspaces over the Grassmanian manifold according
to some Grassmanian subspace packing principle. For example, the
precoding filters may be chosen to maximize the minimum distance
between subspaces, where the distance may correspond to measures
such as chordal, projection two-norm, or the Fubini-Study
distance.
[0031] In an orthogonal OFDM system, the precoding filter may be
kept fixed for a set of one or more subcarriers and then changed
for the next set of subcarriers according to the selected precoder
sequence. To ensure that a period of the precoder sequence is
localized in the OFDM resource grid, a precoder sequence with a
periodicity of four may be applied by traversing the resource
elements (REs) in a resource block (RB) of the OFDM resource grid
in a zig-zag like pattern. For example, the REs may be traversed in
a frequency first order from top to bottom in each odd-numbered
OFDM symbol period, and from bottom to top in each even-numbered
OFDM symbol period.
[0032] In one exemplary embodiment, the precoding filters in a
precoding sequence are selected from column subsets of a single
generator matrix. The elements of the generator matrix may, for
example, be taken from an 8-PSK or QPSK alphabet. An exemplary
generator matrix G for a 4 antenna transmitter is given by:
G = [ 1 1 1 1 1 j - 1 - j 1 - 1 1 - 1 1 - j - 1 j ] . Eq . ( 1 )
##EQU00001##
The precoding filters are derived from the generator matrix G by
selecting column subsets in G as precoding filters. In order to
meet the requirement for uniformity in the spatial properties, the
column subsets are selected such that: [0033] all the columns in
the generator matrix G are used an equal number of times in one
period of the precoder sequence for a given transmission rank;
[0034] each precoding filter is used the same number of times in
one period of the precoder sequence; and [0035] the maximum
possible period length is equal to the number of different column
subsets for a given transmission rank.
[0036] Table 1 below gives exemplary precoder sequences derived
from the generator matrix G for transmission ranks from 1 to 4,
where G.sub.[n.sub.1 .sub.. . . n.sub.K] denotes a filter matrix
with the columns n.sub.i . . . n.sub.K taken from G.
TABLE-US-00001 TABLE 1 Precoder Sequences for Transmission Ranks 1
to 4 Rank Precoder Sequence 1 G.sub.[1] G.sub.[2] G.sub.[3]
G.sub.[4] 2 G.sub.[12] G.sub.[34] G.sub.[13] G.sub.[24] G.sub.[14]
G.sub.[23] 3 G.sub.[123] G.sub.[124] G.sub.[134] G.sub.[234] 4
G
As seen in Table 1, the period of the precoder sequence for each
possible transmission rank equals the number of all possible column
combinations, and each precoder sequence uses each possible
precoding filter exactly once. For transmission rank 2, the
precoding filters are paired and ordered such that the full vector
space is covered by each pair. That is the first two filters form a
first pair, the next two filters form a second pair, and so forth.
The pairing is advantageous where the channel varies significantly
within one resource block because it is beneficial for uniformity
to cover the full vector space with as little channel variations as
possible.
[0037] FIG. 4 illustrates an exemplary method 150 of transmitting
signals from a multiple antenna transmitter 200. The transmit
controller 110 determines the rank of the channel and selects the
desired transmission rank (block 152). The channel rank may be
determined in a conventional manner. The transmission rank is
chosen to use as many transmission layers as the channel can
support. Once the transmission rank is determined, the transmit
controller 110 indicates the selected transmission rank to the
transmit signal processor 120. The transmit signal processor 120
selects the precoder sequence corresponding to the transmission
rank (block 154). As previously noted, the precoder sequence for
each possible transmission rank is known a priori to the
transmitter 100. The precoder sequence determines the set of
precoding filters to be used and the order in which the precoding
filters are applied. The precoding filters may be pre-stored in
memory. Alternatively, the generator matrix may be stored in memory
and the precoding filters may be constructed on-the-fly from the
generator matrix after the transmission rank is determined. With
the selected set of precoding filters, the transmit signal
processor 120 precodes the information symbols (block 156) and
transmits the precoded symbols (block 158). During the precoding,
the transmitter 100 changes or cycles the precoding filters while
traversing the OFDM resource grid. For example, the precoding
filter may be kept fixed for a set of one or more subcarriers and
then changed for the next set of subcarrier according to the
selected precoder sequence.
[0038] FIG. 5 illustrates an exemplary receive signal processor 220
according to one embodiment of the invention for decoding signals
transmitted by the transmitter 100. The receive signal processor
220 comprises a reverse layer mapping unit 222, a precoder 224, and
a plurality of Fast Fourier Transform (FFT) processors 226. The FFT
processors 226 may perform a Discrete Fourier Transform or the
inverse operation. The signal received at each antenna port 228 is
processed by a corresponding FFT processor 226. The output from
each FFT processor 226 is input to combiner 224. The combiner 224
combines the outputs from each FFT processor 226 and outputs a
received symbol stream corresponding to each transmitted layer. The
combiner 224 uses a set of combining filters that are selected
based on the transmission rank and which match the precoding
filters used by the transmitter 100. The combiner 224 cycles
through the set of combining filters, using a different one of the
combining filters during different combining intervals. The
combining intervals at the receiver correspond to precoding
intervals at the transmitter 100. The symbol streams output from
the combiner 224 are then combined into a single received symbol
stream by the reverse layer mapping unit 222. This symbol stream
may be subject to further processing, such as rate-dematching, soft
buffer combining, and turbo decoding.
[0039] FIG. 6 illustrates an exemplary method 250 of receiving
signals from a multiple antenna transmitter 200. The receive
controller 210 determines the transmission rank used by the
transmitter 100 (block 252). The transmitter 100 may inform the
receiver 100 of the transmission rank in a signaling message.
Alternatively, the receiver may determine the transmission rank
itself based on the channel rank. Once the transmission rank is
determined, the receive controller 210 indicates the selected
transmission rank to the receive signal processor 220. The receive
signal processor 220 selects a combining sequence corresponding to
the transmission rank, which is known a priori to the receiver 200
(block 254). The combining sequence determines the set of combining
filters to be used and the order in which the combining filters are
applied. The combining filters may be pre-stored in memory.
Alternatively, the generator matrix may be stored in memory and the
combining filters may be constructed on-the-fly from the generator
matrix after the transmission rank is determined. With the selected
set of combining filters, the receive signal processor 220 combines
the output of each FFT processor 226 to generate a symbol stream
corresponding to each layer (block 256). The symbol streams
corresponding to each layer are then output to the reverse layer
mapping unit 222 (block 258).
[0040] The invention offers an efficient way to support open-loop
MIMO transmission particularly targeting rank two or higher rank
transmissions. Computational complexity for demodulation and CQI
computation in the UE is reduced and the feasibility of
interference rejection is improved compared to existing solutions.
The increased uniformity of the transmission in the spatial domain
improves the robustness of the open-loop MIMO mode. The use of a
single generator matrix may result in considerable complexity
savings as many of the computations for CQI and demodulation may be
reused across several different ranks and when identifying the
characteristics of the inter-cell interference.
[0041] The present invention may, of course, be carried out in
other ways than those specifically set forth herein without
departing from essential characteristics of the invention. The
present embodiments are to be considered in all respects as
illustrative and not restrictive, and all changes coming within the
meaning and equivalency range of the appended claims are intended
to be embraced therein.
* * * * *