U.S. patent application number 14/184164 was filed with the patent office on 2014-06-19 for methods and systems for combined cyclic delay diversity and precoding of radio signals.
This patent application is currently assigned to Telefonaktiebolaget L M Ericsson (publ). The applicant listed for this patent is Telefonaktiebolaget L M Ericsson (publ). Invention is credited to Bo Goransson, George Jongren.
Application Number | 20140169498 14/184164 |
Document ID | / |
Family ID | 39685805 |
Filed Date | 2014-06-19 |
United States Patent
Application |
20140169498 |
Kind Code |
A1 |
Jongren; George ; et
al. |
June 19, 2014 |
Methods and Systems for Combined Cyclic Delay Diversity and
Precoding of Radio Signals
Abstract
In a transmitter or transceiver, signals can be precoded by
multiplying symbol vectors with various matrices. For example,
symbol vectors can be multiplied with a first column subset of
unitary matrix which spreads symbols in the symbol vectors across
virtual transmit antennas, a second diagonal matrix which changes a
phase of the virtual transmit antennas, and a third precoding
matrix which distributes the transmission across the transmit
antennas.
Inventors: |
Jongren; George; (Stockhom,
SE) ; Goransson; Bo; (Sollentuna, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget L M Ericsson (publ) |
Stockholm |
|
SE |
|
|
Assignee: |
Telefonaktiebolaget L M Ericsson
(publ)
Stockholm
SE
|
Family ID: |
39685805 |
Appl. No.: |
14/184164 |
Filed: |
February 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13304870 |
Nov 28, 2011 |
8693566 |
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14184164 |
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12029548 |
Feb 12, 2008 |
8068555 |
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13304870 |
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PCT/SE2008/050161 |
Feb 12, 2008 |
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12029548 |
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Current U.S.
Class: |
375/296 |
Current CPC
Class: |
H04B 7/068 20130101;
H04L 27/2626 20130101; H04B 7/0671 20130101; H04L 5/0023 20130101;
H04L 25/03898 20130101; H04B 7/0617 20130101; H04B 7/0456 20130101;
H04L 27/2647 20130101; H04B 7/0465 20130101; H04B 7/0697 20130101;
H04B 7/0426 20130101 |
Class at
Publication: |
375/296 |
International
Class: |
H04B 7/06 20060101
H04B007/06; H04B 7/04 20060101 H04B007/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 13, 2007 |
SE |
0700367-6 |
Claims
1. A method for transmitting information signals having a plurality
of symbol vectors associated therewith on a radio channel
comprising: precoding said symbol vectors by multiplying said
symbol vectors with: a first column subset of a unitary matrix
which spreads symbols in said symbol vectors across all virtual
transmit antennas, a second diagonal matrix which changes a phase
of said virtual transmit antennas, and a third precoding matrix
which distributes transmit energy across physical transmit
antennas, further processing said precoded symbol vectors to
generate said information signals, and transmitting said
information signals.
2. The method according to claim 1, wherein said physical transmit
antennas are antenna ports.
3. The method according to claim 1, wherein said symbol vectors are
first multiplied by said first column subset of unitary matrix,
next multiplied by said second diagonal matrix and then multiplied
by said third precoding matrix.
4. The method according to claim 1, wherein when transmitting using
r layers, said third precoding matrix has l columns, said second
diagonal matrix has l rows and l columns, said first column subset
of unitary matrix has l rows and r columns, and said symbol vectors
have r elements.
5. The method according to claim 1, wherein when transmitting using
r layers, said third precoding matrix has r columns, said second
diagonal matrix has r rows and r columns, said first column subset
of unitary matrix is a unitary matrix having r rows and r columns,
and said symbol vectors have r elements.
6. The method according to claim 1, wherein said step of further
processing further comprises: mapping precoded symbols to resource
blocks to be transmitted via at least one of said transmit
antennas; and distributing said resource blocks over the resource
element grid of an orthogonal frequency division multiplexing
(OFDM) type of transmission.
7. The method according to claim 1, wherein phase shifts induced by
said second diagonal matrix are varied with respect to a parameter
that is a function of a position of the resource element used for
transmitting a particular symbol vector.
8. The method according to claim 7, wherein said parameter is a
subcarrier index.
9. The method according to claim 7, wherein said parameter is a
data resource element index.
10. The method according to claim 1, wherein said first column
subset of unitary matrix and said second diagonal matrix together
exhibit the same structure as cyclic delay diversity (CDD) for
spatial multiplexing when represented in the frequency domain.
11. The method according to claim 1, wherein said third precoding
matrix is performing channel dependent precoding.
12. A transmitter for transmitting information signals having a
plurality of symbol vectors associated therewith on a radio channel
comprising: a plurality of physical transmit antennas; a processor
for precoding said symbol vectors by multiplying said symbol
vectors with: a first column subset of a unitary matrix which
spreads symbols in said symbol vectors across all virtual transmit
antennas, a second diagonal matrix which changes a phase of said
virtual transmit antennas, and a third precoding matrix which
distributes transmit energy across said physical transmit antennas,
and for further processing said precoded symbol vectors to generate
said information signals; and a transmit chain of elements for
transmitting said information signals.
13. The transmitter according to claim 12, wherein said physical
transmit antennas are antenna ports.
14. The transmitter according to claim 12, wherein said symbol
vectors are first multiplied by said first column subset of unitary
matrix, next multiplied by said second diagonal matrix and then
multiplied by said third precoding matrix.
15. The transmitter according to claim 12, wherein when
transmitting using r layers, said third precoding matrix has l
columns, said second diagonal matrix has l rows and l columns, said
first column subset of unitary matrix has l rows and r columns, and
said symbol vectors have r elements.
16. The transmitter according to claim 12, wherein when
transmitting using r layers, said third precoding matrix has r
columns, said second diagonal matrix has r rows and r columns, said
first column subset of unitary matrix is a unitary matrix having r
rows and r columns, and said symbol vectors have r elements.
17. The transmitter according to claim 12, wherein said step of
further processing further comprises: mapping precoded symbols to
resource blocks to be transmitted via at least one of said transmit
antennas; and distributing said resource blocks over the resource
element grid of an orthogonal frequency division multiplexing
(OFDM) type of transmission.
18. The transmitter according to claim 12, wherein phase shifts
induced by said second diagonal matrix are varied with respect to a
parameter that is a function of a position of the resource element
used for transmitting a particular symbol vector.
19. The transmitter according to claim 18, wherein said parameter
is a subcarrier index.
20. The transmitter according to claim 18, wherein said parameter
is a data resource element index.
21-36. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention generally relates to radio
communication systems, devices, software and methods and, more
particularly, to mechanisms and techniques for combining precoding
and cyclic delay diversity associated therewith.
BACKGROUND
[0002] At its inception radio telephony was designed, and used for,
voice communications. As the consumer electronics industry
continued to mature, and the capabilities of processors increased,
more devices became available use that allowed the wireless
transfer of data between devices and more applications became
available that operated based on such transferred data. Of
particular note are the Internet and local area networks (LANs).
These two innovations allowed multiple users and multiple devices
to communicate and exchange data between different devices and
device types. With the advent of these devices and capabilities,
users (both business and residential) found the need to transmit
data, as well as voice, from mobile locations.
[0003] The infrastructure and networks which support this voice and
data transfer have likewise evolved. Limited data applications,
such as text messaging, were introduced into the so-called "2G"
systems, such as the Global System for Mobile (GSM) communications.
Packet data over radio communication systems became more usable in
GSM with the addition of the General Packet Radio Services (GPRS).
3G systems and, then, even higher bandwidth radio communications
introduced by Universal Terrestrial Radio Access (UTRA) standards
made applications like surfing the web more easily accessible to
millions of users (and with more tolerable delay).
[0004] Even as new network designs are rolled out by network
manufacturers, future systems which provide greater data
throughputs to end user devices are under discussion and
development. For example, the so-called 3GPP Long Term Evolution
(LTE) standardization project is intended to provide a technical
basis for radiocommunications in the decades to come. Among other
things of note with regard to LTE systems is that they will provide
for downlink communications (i.e., the transmission direction from
the network to the mobile terminal) using orthogonal frequency
division multiplexing (OFDM) as a transmission format and will
provide for uplink communications (i.e., the transmission direction
from the mobile terminal to the network) using single carrier
frequency division multiple access (FDMA).
[0005] Another interesting feature of LTE is its support for
multiple antennas at both the transmit side and the receive side.
This provides the opportunity to leverage several different
techniques to improve the quality and/or data rate of received
radio signals. Such techniques include, for example, diversity
against fading (e.g., spatial diversity), shaping the overall
antenna beam to maximize gain in the direction of the target
(beamforming), and the generation of what can be seen as multiple,
parallel "channels" to improve bandwidth utilization (spatial
multiplexing or multi-input multi-output (MIMO).
[0006] Precoding is a popular technique used in conjunction with
multi-antenna transmission. The basic principle involved in
precoding is to mix and distribute the modulation symbols over the
antennas while potentially also taking the current channel
conditions into account. Precoding can be implemented by, for
example, multiplying the information carrying symbol vector
containing modulation symbols by a matrix which is selected to
match the channel. Sequences of symbol vectors thus form a set of
parallel symbol streams and each such symbol stream is referred to
as a "layer". Thus, depending on the choice of precoder in a
particular implementation, a layer may directly correspond to a
certain antenna or a layer may, via the precoder mapping, be
distributed onto several antennas.
[0007] Cyclic delay diversity (CDD) is a form of open-loop
precoding in which the precoding matrix is intentionally varied
over the frequency within the transmission (or system) bandwidth.
Typically, this is realized by introducing different cyclic time
delay for the different antennas, or alternatively realized by
varying the phase of the transmitted signals from the different
antennas. This kind of phase shift means that the effective
channel, comprising the true channel and the CDD precoding, varies
faster over frequency than the original channel. By distributing
the transmission over frequency, this kind of artificially induced
frequency-selectivity is useful in achieving frequency
diversity.
[0008] One of the more significant characteristics of the radio
channel conditions to consider in the context of high rate,
multi-antenna transmission is the so-called channel rank. Generally
speaking, the channel rank can vary from one up to the minimum of
number of transmit and receive antennas. For example, given a
4.times.2 system as an example, i.e., a system with four transmit
antennas and two receive antennas, the maximum channel rank is two.
The channel rank associated with a particular connection varies in
time and frequency as the fast fading alters the channel
coefficients. Moreover, the channel rank determines how many
layers, also referred to as the transmission rank, can be
successfully transmitted simultaneously. For example, if the
channel rank is one at the instant of the transmission of two
layers, there is a strong likelihood that the two signals
corresponding to the two layers will interfere so much that both of
the layers are erroneously detected at the receiver. In conjunction
with precoding, adapting the transmission to the channel rank
involves striving for using as many layers as the channel rank.
[0009] FIG. 1 illustrates a transmission structure 108 for
combining CDD and, possibly channel dependent, precoding. Therein,
each layer 110 created by the transmitter presents a stream of
information carrying modulation symbols to the CDD based precoder
112 as a sequence of symbol vectors 114. The CDD precoder 112
applies the two matrices 116 and 118 illustrated therein to each
incoming symbol vector to perform the precoding process. More
specifically, the CDD precoder 112 first applies the matrix
U.sub.N.sub.T.sub..times.r 118 to the symbol vector 114, followed
by the diagonal CDD matrix 116. U.sub.N.sub.T.sub..times.r matrix
118 is a column subset of a (possibly scaled) unitary matrix, r
denotes the transmission rank and N.sub.T is the number of transmit
antennas in the transmitting device. The notation A.sub.k.times.l
means a matrix A having k rows and/columns. The diagonal CDD matrix
116 has non-zero values along the diagonal including an antenna
phase shift value .theta. indexed by a parameter k which may be a
function of frequency. If OFDM is used for the transmission, k may
e.g. represent the subcarrier index or the closely related data
resource element index (which excludes resource elements containing
reference symbols). It should also be noted that k may be a more
arbitrary function of the position of the resource elements on the
resource grid in OFDM. The resulting, precoded modulation symbol
vector is then output for, e.g., resource mapping and OFDM
modulation 120, prior to being transmitted via antennas 122 (also
referred to as antenna ports).
[0010] The transmission structure 108 illustrated in FIG. 1 can be
utilized in several ways. For example, one option is to use a
fixed, channel independent, unitary matrix
U.sub.N.sub.T.sub..times.r 118 with a certain number of columns r
corresponding to the transmission rank. The unitary matrix 118
serves to distribute each symbol on all antennas 122, while the
diagonal CDD matrix 116 varies (shifts) the phase of each antenna
122. This increases the frequency selectivity of the effective
channel each layer 110 experiences which, as mentioned above, can
be useful for achieving frequency diversity (as well as multi-user
diversity when frequency domain scheduling is used).
[0011] There are, however, certain problems associated with using
the transmission structure 108 illustrated in FIG. 1 to perform
precoding. The spatial correlation properties vary as a function of
k and these variations need to be fast in order to ensure
sufficient frequency diversity over even rather narrowband
transmissions. This makes it difficult for a receiver to estimate
the properties of interference stemming from such kind of
transmissions. The transmission structure 108 also does not provide
sufficient freedom to design the precoding onto the antenna ports.
Furthermore, considering for example a r=1 rank one transmission,
the transmission structure 108 will inherently use one column of
the U.sub.N.sub.T.sub..times.r matrix 118 to apply to the incoming
symbol vector 114. This column would for example (in a two transmit
antenna scenario) be equal to [1, 1]. Thus, this column together
with the diagonal CDD matrix 116, forms a frequency selective
beamformer which may be varied in a periodic fashion over the
scheduled bandwidth. The period will depend on the selected speed
of the phase variations. However, such beamforming may be
problematic because, if the MIMO channel is correlated at the
transmit side, severe cancellation of signals may occur at some
frequencies. If the coding rate is not low enough over the
scheduled bandwidth, this will in turn result in communication
errors. Similar cancellation can occur even for transmission ranks
greater than one. So, generally, it will be difficult to use high
coding rates in conjunction with the transmission structure 108
(and its technique for precoding) if the scheduled bandwidth is
over a significant portion of the previously mentioned beamformer
period. Such a scenario, however, typically occurs when large delay
CDD is used, i.e., corresponding to fast phase shift variations in
the frequency domain.
[0012] Accordingly, it would be desirable to provide precoding
systems, methods, devices and software which avoid the
afore-described problems and drawbacks.
SUMMARY
[0013] According to one exemplary embodiment, a method for
transmitting information signals having a plurality of symbol
vectors associated therewith on a radio channel includes precoding
the symbol vectors by multiplying the symbol vectors with: a first
column subset of a unitary matrix which spreads symbols in the
symbol vectors across all virtual transmit antennas, a second
diagonal matrix which changes a phase of the virtual transmit
antennas, and a third precoding matrix which distributes transmit
energy across physical transmit antennas, further processing the
precoded symbol vectors to generate the information signals, and
transmitting the information signals.
[0014] According to another exemplary embodiment, a transmitter for
transmitting information signals having a plurality of symbol
vectors associated therewith on a radio channel includes: a
plurality of physical transmit antennas, a processor for precoding
the symbol vectors by multiplying the symbol vectors with: a first
column subset of a unitary matrix which spreads symbols in the
symbol vectors across all virtual transmit antennas, a second
diagonal matrix which changes a phase of the virtual transmit
antennas, and a third precoding matrix which distributes transmit
energy across the physical transmit antennas, and for further
processing the precoded symbol vectors to generate the information
signals; and a transmit chain of elements for transmitting the
information signals.
[0015] According to another exemplary embodiment, a method for
equalizing received information signals having a plurality of
symbol vectors associated therewith includes forming a channel
estimate associated with the received information signals by
multiplying an initial channel estimate with a plurality of
matrices, the plurality of matrices including: a first column
subset of a unitary matrix, a second diagonal matrix, and a third
precoding matrix, and equalizing the information signals using the
formed channel estimate.
[0016] According to another exemplary embodiment, a processor forms
a channel estimate associated with received information signals by
multiplying an initial channel estimate with a plurality of
matrices, the plurality of matrices including: a first column
subset of a unitary matrix, a second diagonal matrix, and a third
precoding matrix, and wherein the processor uses the formed channel
estimate to equalize the received information signals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate one or more
embodiments and, together with the description, explain these
embodiments. In the drawings:
[0018] FIG. 1 illustrates a transmission structure including a
conventional precoder;
[0019] FIG. 2 illustrates an exemplary LTE access network in which
exemplary embodiments can be implemented;
[0020] FIG. 3 depicts exemplary LTE physical layer information
signal processing with which exemplary embodiments can be
associated;
[0021] FIG. 4 shows an example of an antenna mapping function in
more detail;
[0022] FIG. 5 illustrates a transmission structure including a
precoder according to an exemplary embodiment;
[0023] FIG. 6 is a block diagram of an exemplary transmitting
device in which precoding according to these exemplary embodiments
can be implemented;
[0024] FIG. 7 is a flowchart illustrating a method for transmitting
information signals according to an exemplary embodiment;
[0025] FIG. 8 is a block diagram of an exemplary receiving device
in which signals which have been precoded according to these
exemplary embodiments can be processed; and
[0026] FIG. 9 is a flowchart illustrating a method for processing
received information signals according to an exemplary
embodiment.
DETAILED DESCRIPTION
[0027] The following description of the exemplary embodiments of
the present invention refers to the accompanying drawings. The same
reference numbers in different drawings identify the same or
similar elements. The following detailed description does not limit
the invention. Instead, the scope of the invention is defined by
the appended claims.
[0028] As mentioned above, the transmission structure 108
illustrated in FIG. 1 and, more particularly, the CDD precoder 112,
suffer from certain drawbacks when considering its applicability in
the context of matrices 118 which are channel independent. In
addition to the problem described in the Background section,
another problem with the transmission structure 108 can occur if
channel dependent precoding is to be used in conjunction with CDD.
Since the diagonal CDD matrix 116 is applied to the symbol vector
114 before the, in this example, channel dependent precoding matrix
118, the precoding matrix 118 will then need to deal with a more
frequency-selective effective channel, i.e., comprising the true
channel and the applied CDD diagonal matrix 118. In order to ensure
an effective precoding scheme under these circumstances, the
precoder 112 must then switch the elements representing matrix 118
at a finer frequency granularity than if only the original channel
was present. This, in turn, may lead to substantially higher
signaling overhead because the precoder elements which are used to
precode transmitted symbols are typically identified to the
receiver in the form additional (overhead) signaling.
[0029] According to exemplary embodiments these problems are
addressed by providing a different transmission structure in which,
for example, an additional (channel dependent or channel
independent) precoder element is applied to the symbol vector
output from the CDD operation comprising application of diagonal
CDD matrix and column subset of a channel independent unitary
matrix as described above. This can be seen by, for example, noting
the additional matrix 515 in FIG. 5, where for future reference, it
should be noted that the symbol vector after application of the
diagonal CDD matrix is referred to as virtual antennas. The
resulting vector x(k) transmitted on a resource indexed by k can
thus be written as
x(k)=W.sub.N.sub.T.sub..times.l(k)D(k)U.sub.l.times.rs(k) (1)
where D(k) is the second diagonal CDD matrix 516 and it is
emphasized that the third precoding matrix 515,
W.sub.N.sub.T.sub..times.l(k), may potentially be different for
different values of k. The parameter/would here typically be set to
equal the transmission rank r. These exemplary embodiments can be
used to, for example, add a channel dependent precoding stage
directly at the input of the true channel (i.e., outputting onto
the antenna ports), which in turn allows CDD to be combined with
channel dependent precoding without requiring finer precoding
granularity, thus saving signaling overhead. Even if the third
precoding matrix is not channel dependent, the structure indicated
by the exemplary embodiments provides additional freedom in
selecting suitable precoders for the third precoding stage so as to
avoid some of the previously mentioned problems associated with the
use of the structure 108.
[0030] To provide some context for the more detailed discussion of
combined CDD and precoding according to these exemplary
embodiments, consider first the exemplary radiocommunication system
illustrated in FIGS. 2-4. Beginning with the radio access network
nodes and interfaces in FIG. 2, it will be seen that this
particular example is provided in the context of LTE systems.
Nonetheless, the present invention is not limited in its
applicability to transmitters or transmissions associated with LTE
systems and can instead be used in any system wherein multiple
transmit antennas and precoding are employed, including, but not
limited to Wideband Code Division Multiple Access (WCDMA), GSM,
UTRA, E-UTRA, High Speed Packet Access (HSPA), UMB, WiMaX and
other, systems, devices and methods. Since, however, the example in
FIG. 2 is provided in terms of LTE, the network node which
transmits and receives over the air interface is termed an eNodeB,
several of which eNodeBs 200 are illustrated therein.
[0031] In the context of the air interface, each eNodeB 200 is
responsible for transmitting signals toward, and receiving signals
from, one or more cells 202. Each eNodeB includes multiple
antennas, e.g., 2, 4, or more transmit antennas, and handles
functions including, but not limited to coding, decoding,
modulation, demodulation, interleaving, de-interleaving, etc., with
respect to the physical layer of such signals. Note that the phrase
"transmit antennas" as used herein is specifically meant to
include, and be generic to, physical antennas, virtual antennas and
antenna ports. The eNodeBs 200 are also responsible for many higher
functions associated with handling communications in the system
including, for example, scheduling users, handover decisions, and
the like. The interested reader who desires more information
regarding transmit or receive functions associated with LTE or
other systems in which these exemplary embodiments may be deployed
is directed toward the book entitled "3G Evolution--HSPA and LTE
for Mobile Broadband", to Erik Dahlman et al., published by
Elsevier Ltd., 2007, the disclosure of which is incorporated by
reference.
[0032] Nonetheless, to briefly discuss the baseband processing
associated with the transmission of signals in the downlink (i.e.,
possibly transferred through the core network 203 to an eNodeB 200
and then into the cells 202 toward target mobile terminal or
stations, e.g., MS 204 in FIG. 2), consider FIG. 3. Therein, two
transport blocks of data 300 are being processed for transmission
by an eNodeB 200 using spatial multiplexing. Cyclic redundancy
check (CRC) bits are inserted at steps 302 to be used by the
receiver to detect errors. Channel coding is applied to the
transport blocks at steps 304 to provide protection to the payload
data against the impairments presented by the radio channel. The
hybrid automatic retransmission request (HARQ) steps 306 operate to
extract or repeat code bits from the blocks of code bits provided
by the channel encoder to generate a precise set of bits to be
transmitted within a transmit time interval (TTI), e.g., based upon
various criteria such as the number of assigned resource blocks,
the selected modulation scheme and the spatial multiplexing
order.
[0033] At step 308, the code words output from the HARQ block are
scrambled (multiplied) by a bit-level scrambling sequence or mask,
which aids the receive in suppressing interference to the radio
signal. The selected data modulation, e.g., Quadrature Phase-Shift
Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), or 64 QAM,
is then applied at step 310 to transform blocks of scrambled bits
into corresponding blocks of modulation symbols. These modulation
symbols are then mapped to different antennas and/or different
antenna ports at step 312. In LTE nomenclature, an antenna port
corresponds to the transmission of a particular downlink reference
signal which may, or may not, correspond to an actual, physical
antenna. The symbols to be transmitted on each antenna (1-n in FIG.
3, e.g., 2, 4, 8, 16) are then mapped to respective resource blocks
314 and sent off for OFDM processing (not shown) prior to
transmission by the eNodeB 200.
[0034] Of particular interest in the downlink processing for these
exemplary embodiments is the antenna mapping step/block 312. The
antenna mapping process can be further subdivided into layer
mapping of the codewords output from the modulation block 310 and
precoding of the resulting symbol vectors to generate the antenna
(or antenna port) mapped symbols, as shown in FIG. 4. Therein an
example is provided with two sets of codewords being mapped by
layer mapping function 400 into three layers. Two symbol vectors v1
and v2 associated with the three layers are illustrated in FIG. 4.
These symbol vectors are then precoded by applying one or more
precoding matrices by precoding function 402, i.e., by matrix
multiplication of the precoding matrix or matrices with the
incoming symbol vectors. According to one exemplary embodiment, the
precoding function 402 can apply three different matrices as will
be described below with respect to FIG. 5. It will be appreciated
that the selections of mapping to three layers and four antennas in
FIG. 4 is purely exemplary. Selection of the number of layers will,
as described earlier, vary based upon the channel rank (among
possibly other criteria) and the number of antennas may vary from
system to system or even among transmit devices within systems.
[0035] FIG. 5 illustrates a precoder according to exemplary
embodiments which can be used to perform precoding, e.g., as
described with respect to blocks 312 and 402 above. Therein, each
layer 510 created by the transmitter presents a stream of
modulation symbols to the CDD based precoder 512 as a sequence of
symbol vectors 514. The CDD precoder 512 applies the three matrices
515, 516 and 518 illustrated therein to each incoming symbol vector
to perform the precoding process. More specifically, the CDD
precoder 512 according to this exemplary embodiment first applies
the matrix U.sub.l.times.r 518, which is a column subset of a
possibly scaled unitary/x/matrix, to the symbol vector 514,
followed by diagonal CDD matrix 516, followed then by a precoding
matrix N.sub.N.sub.T.sub..times.l 515 resulting in the transmit
vector previously given in equation (1).
[0036] The columns of the matrix 518 are taken from a possibly
scaled unitary matrix. A unitary matrix exhibits the property that
its inverse is equal to the complex conjugate transpose of the
unitary matrix of interest. Thus, the columns of the matrix 518 are
orthogonal and of equal norm. The first applied, matrix 518
operates to spread the symbols across the antenna ports. The second
applied CDD matrix 516 will have the qualities of a diagonal
matrix, i.e., elements on one diagonal are non-zero and the
remaining matrix elements are zero. This CDD matrix 516 operates to
vary (shift) the phase of each antenna or antenna port 522. The
third applied, precoding matrix 515 operates to distribute the
transmission energy across the antennas or antenna ports. It may be
determined in either a channel independent manner or based upon, at
least in part, current radio channel conditions resulting in a
channel dependent precoder operation. As discussed above,
application of these matrices to the incoming symbol vectors can be
performed by a processing unit within the transmitter by way of
matrix multiplication.
[0037] The parameter/is introduced in this exemplary embodiment as
a size parameter of the three matrices used to perform precoding,
i.e., the number of columns in the last applied precoding matrix
515, the number of rows and columns in the second applied, diagonal
CDD matrix 516 and the number of rows in the first applied, unitary
matrix 518. Thus, unlike the transmission structure illustrated in
FIG. 1, the size of the matrices involved in performing precoding
according to these exemplary embodiments may vary dynamically for a
given transmitter according to the transmission rank of the channel
(or the number of layers), e.g., the number of rows in the unitary
matrix 518 may be different than the number of transmit antennas.
As previously mentioned, the parameter/is typically set equal to
the transmission rank r. By way of contrast, the matrices 116 and
118 discussed above with respect to FIG. 1 were fixedly sized to
the number of transmit antennas associated with the particular
transmitter performing the precoding.
[0038] Looking more closely at the three matrices used to perform
precoding according to this exemplary embodiment shown in FIG. 5,
the matrix U.sub.l.times.r 518 is, like matrix 118, a column subset
of a (possibly scaled) unitary matrix where/denotes the number of
rows in the matrix and r denotes the transmission rank and number
of columns. The diagonal CDD matrix 516 includes
exp(j.theta..sub.nk) elements along the diagonal wherein
.theta..sub.n represents a phase value associated with a particular
antenna or antenna port and k is an index associated with a
particular resource element (e.g. indices of all subcarriers or
indices of only those subcarriers which carry data rather than
those which carry reference symbols). The matrix
W.sub.N.sub.T.sub..times.l 515 is a precoding matrix which can have
various values, e.g., to perform channel dependent beamforming or
precoding in a channel independent manner, some examples of which
are described below, and which has a size of N.sub.T (the number of
transmit antennas/antenna ports in the transmitting device) by l.
The resulting, precoded modulation symbol vector is then output
for, e.g., resource mapping and OFDM modulation 520, prior to being
transmitted via antennas 522.
[0039] According to theses exemplary embodiments, the precoding
matrix 515 W.sub.N.sub.T.sub..times.l is now applied directly on
the MIMO channel matrix. This means that, in for example the case
of a channel-dependent precoding, W.sub.N.sub.T.sub..times.l can
"see" the true channel which is unaffected by any potential CDD
operation. The diagonal CDD matrix and U.sub.l.times.r can then be
used to perform CDD operation on the new, improved, effective
channel comprising the true channel and W.sub.N.sub.T.sub..times.l.
The number of rows/can moreover be adapted so that CDD operation is
only performed among the virtual antennas taken as input to
W.sub.N.sub.T.sub..times.l.
[0040] For example, for transmission rank one (and therefore, the
number of layers is one given that these examples are concerned
with spatial multiplexing), l could be set to one, the diagonal
matrix would be 1, and W.sub.N.sub.T.sub..times.l would be one
column vector performing possibly channel dependent beamforming. If
different W.sub.N.sub.T.sub..times.l are used for different indices
k frequency dependent precoding is possible. Similarly, for
transmission rank two, l could be two, U.sub.l.times.r could have
two columns, and W.sub.N.sub.T.sub..times.l could be channel
dependent and have two columns as well. The diagonal matrix
together with U.sub.l.times.r then performs CDD like operation on
the virtual antennas, meaning that the two layers see a mixture of
the virtual antenna channels which in turn are formed from the true
channel and W.sub.N.sub.T.sub..times.l. Thus, the three matrices
515, 516 and 518 could, for example, be selected from the following
table:
TABLE-US-00001 Maximum Number of Layers W D U 1 [ a b ]
##EQU00001## [1] [1] 2 [ c d e f ] ##EQU00002## [ g 0 0 h ]
##EQU00003## [ k l m n ] ##EQU00004##
Therein, variables a, b, c, d, e, f, g, h, k, l, m, and n
represent, potentially complex, values which are selected to
provide the functions or matrix-types described above, resulting in
a so-called precoder codebook. Examples of these values can be
found in, for example, the standards specification 3GPP TS 36.211
V1.3.1, (2007-08), at section 6.3.3.2. By using precoding as
described above and illustrated in FIG. 5, beamforming on the
virtual antennas (antenna ports) spreads energy in designated
sub-spaces, which sub-spaces focus the transmission energy toward
the intended recipient (e.g., mobile station) of the transmission.
Channel independent precoding is also possible in for example the
sense of varying the selection of precoders in a more random manner
so as to avoid focusing the energy in any particular direction.
[0041] According to another exemplary embodiment, a transmission
structure could provide for precoding wherein the precoding matrix
515 i.e., W.sub.N.sub.T.sub..times.l, is instead set to be a fixed
channel and frequency independent matrix with orthogonal and equal
norm columns, the diagonal CDD matrix set to be of size
N.sub.T.times.N.sub.T (i.e., a square matrix equal to the number of
transmit antennas) and the matrix 518 U.sub.l.times.r can then be a
single column of all ones. This exemplary embodiment provides
another form of CDD which does not suffer from the previously
mentioned cancellation problem when correlated fading is present on
the transmit side.
[0042] As mentioned above, the transmit processing techniques
described herein may be used for various communication systems such
as Code Division Multiple Access (CDMA) systems, Time Division
Multiple Access (TDMA) systems, Frequency Division Multiple Access
(FDMA) systems, Orthogonal FDMA (OFDMA) systems, Single-Carrier
FDMA (SC-FDMA) systems, etc. The transmitter may, for example, be
disposed within a radio base station, NodeB, eNodeB, or the like,
to transmit information signals on a downlink radio channel.
Alternatively, the transmitter may, for example, be disposed in a
mobile unit, terminal device, user equipment, or the like to
transmit information signals on an uplink radio channel. Regardless
of the particular type of communication system in which these
exemplary embodiments are presented, the transmit device will
typically include the components illustrated generally in FIG.
6.
[0043] Therein, the transmitter includes a plurality of physical
transmit antennas 602 in this example numbering four, although more
or fewer than four transmit antennas can be used. The physical
transmit antennas 602 are connected to a processor 606 via transmit
(TX) chain elements 604 which can include one or more of filters,
power amplifiers and the like, as will be appreciated by those
skilled in the art. Processor(s) 606, in conjunction with memory
device(s) 608 (and potentially other devices not shown) can operate
to perform the transmit processes discussed above with respect to
FIGS. 3-5, e.g., by way of software stored therein, additional
hardware or some combination of software and hardware. Thus, the
precoding functionality described above can, for example, be
performed in software by executing computer-readable instructions
from memory device 608 to perform the matrix multiplications
described above with respect to FIG. 5. Thus, it will be apparent
that exemplary embodiments also relate to software, e.g., program
code or instructions which are stored on a computer-readable medium
and which, when read by a computer, processor or the like, perform
certain steps associated with transmitting information signals
which are precoded in the manner described above. An example of
such steps is illustrated in the flowchart of FIG. 7.
[0044] Therein, at step 700, symbol vectors are precoded by
multiplying them with a first unitary matrix which spreads symbols
in the symbol vectors across the virtual transmit antennas, a
second diagonal matrix which changes a phase of the virtual
transmit antennas, and a third precoding matrix which distributes
the transmission across the transmit antennas. After precoding the
symbol vectors, they can undergo further processing at step 702 to
generate information signals. For example, such additional signal
processing can include mapping precoded symbols to resource blocks
to be transmitted via at least one of the transmit antennas and
orthogonal frequency division multiplexing (OFDM) the resource
blocks, although other processing, e.g., for non-OFDM systems,
could alternately be performed downstream of the precoding
operation. Then, at step 704, the resultant information signals are
transmitted.
[0045] Exemplary embodiments also provide for receive side
processing of signals which have been transmitted using the
foregoing exemplary precoding embodiments. In systems using common
pilots (common reference symbols (RS)), the receiver needs to be
aware of the transmission structure in order to be able to properly
decode the transmission. LTE is one example of such a system where
this transmission mode is using common reference symbols and is
thus not transparent to the UE. Thus, all of the involved matrices
described above (i.e., W, D and U) need to be known on the receive
side (e.g., at the UE) to be used for equalizing the channel. For
example, the UE may first form the effective channel H_eff=HWDU,
where H is a channel estimate obtained from the common RS, equalize
the effective channel, e.g. by using a linear filter inv(H_eff
*H_eff)H_eff *), producing the equalized vector sequence z, which
is input to a demodulator, producing soft values of coded bits
which are finally input to, e.g., a turbo decoder to produce an
estimate of the transmitted information bits.
[0046] It will be appreciated that there are numerous
implementations for receiving and decoding wirelessly received
information symbols and that the foregoing is simply one exemplary
implementation. The receive side processing according to these
exemplary embodiments will essentially provide a mirrored
processing to that performed on the transmit side. The receiver
will use its knowledge of the precoding performed by the
transmitter to perform its channel estimation/equalization
function. Such knowledge on the part of the receiver may be
predefined a priori or it may be passed on to the receiver
explicitly as part of the transmitted information.
[0047] Thus, an exemplary receiver 800 for receiving and processing
information signals which have been precoded as described above is
illustrated in FIG. 8. Therein, one (or more) receive antennas 802
receive the information signals which have been precoded during
transmit side processing. After passing through one or more receive
(RX) chain processing elements 804 (e.g., filters, amplifiers or
the like), processor(s) 806 will process the received information
signals to extract the information contained therein, e.g., in
conjunction with processing software stored on memory device(s)
808, by using its knowledge of the precoding performed on those
information signals to calculate a channel estimate used in
subsequent receive side processing. For example, as shown in the
flowchart of FIG. 9, a method for equalizing received information
signals includes the steps of forming a channel estimate associated
with the received information signals by multiplying an initial
channel estimate with a plurality of matrices, the plurality of
matrices including a first column subset of a unitary matrix, a
second diagonal matrix, and a third precoding matrix at step 900,
and equalizing the information signals using the formed channel
estimate at step 902.
[0048] The foregoing description of exemplary embodiments provides
illustration and description, but it is not intended to be
exhaustive or to limit the invention to the precise form disclosed.
Modifications and variations are possible in light of the above
teachings or may be acquired from practice of the invention. For
example, the exemplary embodiments also include
W.sub.N.sub.T.sub..times.l and U.sub.l.times.r matrices of more
general form and, potentially, also a more general form of CDD
matrix, e.g., not limited to a diagonal matrix. The following
claims and their equivalents define the scope of the invention.
* * * * *