U.S. patent application number 14/147084 was filed with the patent office on 2014-04-17 for method and arrangement in a mobile communications network.
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 Yang Hu, Yin Liu, Xinghua Song, Hai Wang, Jianfeng Wang.
Application Number | 20140105320 14/147084 |
Document ID | / |
Family ID | 41481260 |
Filed Date | 2014-04-17 |
United States Patent
Application |
20140105320 |
Kind Code |
A1 |
Wang; Hai ; et al. |
April 17, 2014 |
METHOD AND ARRANGEMENT IN A MOBILE COMMUNICATIONS NETWORK
Abstract
The present invention relates to an arrangement and method to
design orthogonal UE-specific reference signals so that multiple
UE-specific reference signals are feasible to support
non-codebook-based multi-stream beamforming/precoding. This is
achieved by re-using an existing UE-specific reference signal to
obtain additional UE-specific reference signals being orthogonal to
the existing UE reference signal.
Inventors: |
Wang; Hai; (Beijing, CN)
; Hu; Yang; (Beijing, CN) ; Liu; Yin;
(Beijing, CN) ; Song; Xinghua; (Beijing, CN)
; Wang; Jianfeng; (Beijing, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget L M Ericsson (PUBL) |
Stock |
|
SE |
|
|
Assignee: |
Telefonaktiebolaget L M Ericsson
(PUBL)
Stockholm
SE
|
Family ID: |
41481260 |
Appl. No.: |
14/147084 |
Filed: |
January 3, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13063966 |
Mar 18, 2011 |
8638722 |
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PCT/IB2009/006861 |
Sep 16, 2009 |
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14147084 |
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61098130 |
Sep 18, 2008 |
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Current U.S.
Class: |
375/267 |
Current CPC
Class: |
H04L 1/0606 20130101;
H04L 5/0026 20130101; H04L 25/0226 20130101; H04L 27/2613 20130101;
H04B 7/0421 20130101; H04L 1/0668 20130101; H04B 7/0658 20130101;
H04B 7/0413 20130101; H04L 1/0618 20130101; H04L 25/0204 20130101;
H04B 7/0617 20130101; H04L 25/0248 20130101; H04L 5/0051
20130101 |
Class at
Publication: |
375/267 |
International
Class: |
H04B 7/04 20060101
H04B007/04; H04L 1/06 20060101 H04L001/06 |
Claims
1. A method in a network node of a mobile communication network,
wherein the network node is associated with an antenna comprising a
plurality of antenna ports using non-codebook-based multi-stream
beamforming, the method comprises the steps of: applying a first UE
specific reference signal on a first antenna port, and reusing the
first UE specific reference signal to be applied orthogonally on an
antenna port.
2. The method according to claim 1, wherein the reused first UE
specific reference signal are multiplexed on a second antenna port
in different time-frequency resource elements than the first UE
specific reference signal applied on the first antenna port.
3. The method according to any of claims 1-2, wherein the reused
first UE specific reference signal are applied on the first antenna
port sharing the same time-frequency resource elements as the first
set of UE specific reference signals.
4. The method according to claim 3, wherein the step of re-using
the first UE specific reference signal further comprises: code
division multiplexing the reused first UE specific reference signal
in a set of resource elements.
5. The method according to claim 4, wherein the reused first UE
specific reference signal are cyclic shifts of the first set of UE
specific reference signals.
6. The method according to claim 4, wherein the reused first UE
specific reference signal are orthogonal covers of the first set of
UE specific reference signals.
7. The method according to any of claims 1-4, wherein the reused
first UE specific reference signal are space-frequency block codes
based on the first set of UE specific reference signals.
8. A network node of a mobile communication network, wherein the
network node is associated with an antenna comprising a plurality
of antenna ports using non-codebook-based multi-stream beamforming,
the network node comprises a processor configured to apply a first
UE specific reference signal on a first antenna port, and to reuse
the first UE specific reference signal to be applied orthogonally
on an antenna port.
9. The network node according to claim 8, wherein the processor is
configured to multiplex the reused first UE specific reference
signal on a second antenna port in different time-frequency
resource elements than the first UE specific reference signal
applied on the first antenna port.
10. The network node according to any of claims 8-9, wherein the
processor is configured to apply the reused first UE specific
reference signal on the first antenna port sharing the same
time-frequency resource elements as the first set of UE specific
reference signals.
11. The network node according to claim 10, wherein the processor
is further configured to code division multiplex the reused first
UE specific reference signal in a set of resource elements.
12. The antenna according to any of claims 8-11, wherein the reused
first UE specific reference signal are cyclic shifts of the first
UE specific reference signal.
13. The antenna according to any of claims 8-11, wherein the reused
first UE specific reference signal are orthogonal covers of the
first UE specific reference signal.
14. The antenna according to any of claims 8-11, wherein the reused
first UE specific reference signal are space-frequency block codes
based on the first UE specific reference signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/063,966, filed Mar. 18, 2011, pending, which claims the
benefit of PCT/IB2009/006861 filed on Sep. 16, 2009, pending, which
claims the benefit of U.S. Provisional Application No. 61/098,130,
filed Sep. 18, 2008, the disclosure of which is incorporated herein
by reference.
TECHNICAL FIELD
[0002] The present invention relates to method and arrangement in a
mobile communications network having multiple antennas. In
particular, the present invention relates to design of multiple
UE-specific reference signals for non-codebook based multi-stream
beamforming/precoding.
BACKGROUND
[0003] Multiple antenna techniques play an important role in modern
wireless communication systems to provide improved system
performance, including increased capacity and coverage, as well as
improved service provisioning. One challenge with the
implementation of multi-antenna techniques is the acquisition of
channel state information (CSI) at the transmitter or the receiver.
In general, the channel can be estimated through a predefined
training sequence, which is often referred to as reference signal.
Taking the downlink transmission as an example, the base station
should transmit reference signals to mobile terminals so that the
channel matrix can be estimated at the receiver side. With this
estimated channel matrix, coherence demodulation can be carried
out. Consequently the potential beamforming gain, spatial diversity
gain and spatial multiplexing gain can be obtained. In addition,
the reference signals can be used for channel quality measurements
to support link adaptation. In the case of orthogonal frequency
division multiplexing (OFDM) transmission, a straightforward design
of reference signal is to insert known reference signals into the
OFDM time-frequency grid.
[0004] In the 3GPP specifications 3GPP TS 36.211 V8.3.0 (2008-05),
"Evolved Universal Terrestrial Radio Access (E-UTRA); Physical
channels and modulation", 3GPP TS 36.212 V8.3.0 (2008-05) "Evolved
Universal Terrestrial Radio Access (E-UTRA); Multiplexing and
channel coding" and 3GPP TS 36.213 V8.3.0 (2008-05) "Evolved
Universal Terrestrial Radio Access (E-UTRA); Physical layer
procedures", two kinds of downlink reference signals are defined,
i.e. the cell-specific reference signal and the UE-specific
reference signal. (The above mentioned 3GPP specification refers to
LTE release 8.)
[0005] Up to four cell-specific reference signals are defined
corresponding to four antenna ports 0 to 3, as shown in FIG. 1,
targeting to support codebook based multiple streams spatial
multiplexing transmission
[0006] A codebook is a predefined finite set consisting of a number
of precoding matrices with different ranks. In codebook based
precoding, the user equipment (UE) will first estimate the channel
matrix based on the multiple cell-specific reference signals and
then the UE applies an exhaustive search over all precoding
matrices and reports the preferred precoding matrix indicator (PMI)
to the base station, referred to as eNodeB in LTE, under certain
criterions, e.g., maximizing system throughput. Note that the PMI
can be overridden by the eNodeB. It should be noted that the
channel matrix refers to the precoded channel information by
multiplying precoding matrix, i.e. Y=H*W, where Y is the precoded
channel matrix, H is channel matrix and W is precoding matrix.
[0007] However, only one UE-specific reference signal on antenna
port 5 is defined, which is transmitted only on the resource blocks
upon which the corresponding physical downlink shared channel
(PDSCH) is mapped, as shown in FIG. 2, targeting to support
non-codebook based single stream beamforming transmission
[0008] In non-codebook based precoding, the precoding weight matrix
applied both on UE-specific reference signals and the data signals
is not retrieved from the codebook set but is directly calculated
by the eNodeB in terms of some criterions, e.g.,
eigen-decomposition based or direction of arrival based criterion.
In TDD (time Division Duplex) system, due to channel reciprocity,
non-codebook based beamforming/precoding can reduce further uplink
feedbacks and improve beamforming gain.
[0009] In the downlink transmission of the LTE system, both
codebook based precoding and non-codebook based
beamforming/precoding are supported for up to 4 transmit
antennas.
[0010] The transmission mode switch between codebook based multiple
streams spatial transmission is semi-statically configured via
higher layer signalling.
[0011] However, future communication systems, e.g., LTE-Advance,
are likely to employ more transmit antennas in order to reach more
aggressive performance targets. Especially, for systems with e.g.,
8 transmit antennas, extension of current codebook based precoding
is needed from precoder and reference signal perspective.
[0012] On the other hand, non-codebook based multiple stream
beamforming is considered to be one of the most important candidate
technologies for future wireless communication systems. This
enhanced multi-antenna technique can simultaneously make use of the
beamforming gain to improve receive signal-to-noise-ratio (SNR) as
well as multi-layer transmissions to improve the peak data rate.
Compared to codebook based spatial multiplexing transmission,
non-codebook based multi-stream beamforming has potentially the
advantages of signalling overhead reduction and performance
improvement.
[0013] However, in order to support non-codebook based multi-stream
beamforming, it is necessary to define multiple UE-specific
reference signals with least effort, especially based on existing
antenna port 5 and the UE-specific reference signal as defined in
LTE release 8.
SUMMARY
[0014] The present invention relates to an arrangement and method
to design orthogonal UE-specific reference signals so that multiple
UE-specific reference signals are feasible to support
non-codebook-based multi-stream beamforming/precoding. This is
achieved by re-using an existing UE-specific reference signal to
obtain additional UE-specific reference signals being orthogonal to
the existing UE reference signal.
[0015] Hence, according to a first aspect of the present invention
a method in a network node of a mobile communication network is
provided. The network node is associated with an antenna comprising
a plurality of antenna ports using non-codebook-based multi-stream
beamforming. In the method, a first UE specific reference signal is
applied on a first antenna port, and the first UE specific
reference signal is reused to be applied orthogonally on an antenna
port. This antenna port may be the same as the first antenna port
wherein the reused UE-specific reference signals are code division
multiplexed in the same time-frequency resource elements as the
first set of UE-specific reference signals or the antenna port may
be a second antenna port wherein the reused UE-specific reference
signals are multiplexed in different time-frequency resource
elements than the first set of UE specific reference signals.
[0016] According to a second aspect of the present invention a
network node of a mobile communication network is provided. The
network node is associated with an antenna comprising a plurality
of antenna ports using non-codebook-based multi-stream beamforming.
Further, the network node comprises a processor configured to apply
a first UE specific reference signal on a first antenna port, and
to reuse the first UE specific reference signal to be applied
orthogonally on an antenna port.
[0017] In order to keep the orthogonality among the multiple
UE-specific reference signals in a cell, different embodiments are
proposed with some cost, in terms of e.g., overhead, channel
estimation performance etc. These embodiments can be divided into
three categories as:
[0018] From performance perspective, multiple UE-specific reference
signals are in this embodiment multiplexed in different
time-frequency resource elements. This is used in scheme 1 where
multiple antenna ports reuse the existing UE-specific reference
signal.
[0019] From overhead perspective, multiple UE-specific reference
signals are code-division multiplexed in a set of resource elements
in the allocated resource blocks to share the same antenna port 5
in other embodiments. The requirement of multiple stream
transmission moves the operating point to higher SNR, which is
helpful to satisfy the separation of multiple UE-specific reference
signals and further channel estimation accuracy/performance. This
is used in schemes 2-4. In scheme 2, cyclic shifts of the existing
UE-specific reference signal are used for the additional
UE-specific reference signals. In scheme 3, orthogonal covers based
on the existing UE-specific reference signal, are used for the
additional UE-specific reference signals and in scheme 4,
space-frequency block codes (SFBC) are applied on the existing
UE-specific reference signal to obtain the additional UE-specific
reference signals. In addition, a combination of scheme 1 and at
least one of schemes 2-4 can also be used if a compromise between
overhead and performance is desired.
[0020] At the receiver, the effective channel state information
used to demodulate and decode different streams can be estimated
via multiple UE-specific reference signals, e.g., pilot
signals.
[0021] An advantage with embodiments of the present invention is
that support of non-codebook based multiple stream downlink
transmission is provided. The effective channel state information
used to demodulate and decode different streams can be estimated
via multiple UE-specific reference signals.
[0022] A further advantage with embodiments of the present
invention is the low complexity to generate multiple UE-specific
reference signals, since more antenna ports or simple operation is
based on the existing UE-specific reference signal.
[0023] A further advantage with embodiments of the present
invention is the flexible switch for rank adaptation, i.e., the
adjustment of the number of antenna ports or UE-specific reference
signals.
[0024] A further advantage with embodiments of the present
invention is that efficient overhead reduction is provided.
Multiple UE-specific reference signals in code-division
multiplexing share the same resource elements in antenna port 5,
i.e., Schemes 2-4. In addition, Scheme 5 considers both overhead
and performance.
[0025] A yet further advantage with embodiments of the present
invention is that it can easily be introduced into the later LTE
releases. The introduction of multiple UE-specific reference
signals is easily based on the existing antenna port 5 or the
existing UE-specific reference signal
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 illustrates multiple downlink cell-specific reference
signals for LTE with normal CP (cyclic prefix) according to prior
art.
[0027] FIG. 2 illustrates one downlink UE-specific reference signal
for LTE with normal CP according to prior art.
[0028] FIG. 3 illustrates reuse of the same UE-specific reference
signals for two antenna ports according to one embodiment of the
present invention.
[0029] FIG. 4 illustrates generation of multiple UE-specific
reference signals by cyclic shifts according to embodiments of the
present invention.
[0030] FIG. 5 illustrates generation of multiple UE-specific
reference signals by length-4 orthogonal covers according to one
embodiment of the present invention.
[0031] FIG. 6 illustrates generation of 2 UE-specific reference
signals by a 2.times.2 Alamouti code according to an embodiment of
the present invention.
[0032] FIG. 7 is a flowchart of the method according to embodiments
of the present invention.
[0033] FIG. 8 illustrates schematically the network node with the
associated antenna ports according to embodiments of the present
invention.
[0034] FIG. 9 illustrates schematically a LTE network wherein the
embodiments of the present invention may be implemented.
DETAILED DESCRIPTION
[0035] The present invention will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. The invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. In the drawings, like
reference signs refer to like elements.
[0036] Moreover, those skilled in the art will appreciate that the
means and functions explained herein below may be implemented using
software functioning in conjunction with a programmed
microprocessor or general purpose computer, and/or using an
application specific integrated circuit (ASIC). It will also be
appreciated that while the current invention is primarily described
in the form of methods and devices, the invention may also be
embodied in a computer program product as well as a system
comprising a computer processor and a memory coupled to the
processor, wherein the memory is encoded with one or more programs
that may perform the functions disclosed herein.
[0037] FIG. 9 illustrates a Long Term Evolution (LTE) network 900
wherein the embodiments of the present invention may be
implemented. Radio base stations 801 referred to as eNode Bs are
connected to a core network 903 and are also interconnected. Each
eNode B 801 has an antenna 802 comprising a plurality of antenna
ports. The eNode Bs communicate wirelessly with user equipments
904.
[0038] Although the embodiments of the present invention are
described in the context of a mobile communication network based on
the LTE (Long Term Evolution) standard, the present invention is
lot limited to LTE but can also be used in networks using multiple
UE-specific reference signals for enabling non-codebook based
multi-stream beamforming.
[0039] In order to obtain multiple UE-specific reference signals
for multiple streams, a first UE specific reference signal is
applied 701 on a first antenna port, and the first UE specific
reference signal is reused 702 to be applied orthogonally on an
antenna port according to the flowchart of FIG. 7.
[0040] According to a first embodiment, referred to as scheme 1,
the reused first UE specific reference signal are multiplexed 704
on a second antenna port in different time-frequency resource
elements than the first UE specific reference signal applied on the
first antenna port.
[0041] In LTE, release 8, the antenna port 5 has been specified for
the UE-specific reference signals as described above. In order to
reuse the reference signal for multiple stream transmission
according to this embodiment, more antenna ports can be defined, in
which the resource elements used for UE-specific reference signals
are occupied by only one antenna port, the same as the mapping of
multiple Cell-specific reference signals. For instance, 4 antenna
ports of 5 to 8 can support up to 4 streams downlink data
transmission. FIG. 3 shows the basic structure of two antenna ports
only for illustration. More antenna ports have similar design.
Hence UE-specific reference signals are transmitted from port 5 and
from port 6, where the resource elements 301 of port 5 used for the
UE-specific reference signals are orthogonal to the resource
elements 302 of port 6 used for the UE-specific reference
signals.
[0042] Scheme 1 implies that higher channel estimation accuracy can
be achieved due to interference among the antenna ports is
avoided.
[0043] In contrast to scheme 1, the UE-specific reference signals
are orthogonally code-division multiplexed 703 in a set of resource
elements in the allocated resource blocks to share the same antenna
port (antenna port 5) according to further embodiments referred to
as scheme 2-4, i.e., the reused first UE specific reference signal
is applied on the first antenna port sharing the same
time-frequency resource elements as the first UE specific reference
signal by code division multiplexing 703 the reused first UE
specific reference signal in a set of resource elements.
[0044] Since the existing UE-specific reference signals on antenna
port 5 in LTE are a pseudo-random sequence, multiple UE-specific
reference signals can be generated by introducing cyclic shifts to
guarantee good auto-correlation (orthogonality) among the
UE-specific reference signals according to a second embodiment
referred to as scheme 2. FIG. 4 shows 4 cyclic shifted sequences
based on the existing UE-specific reference signals as an example.
Thus, a first UE-specific reference signal is sent on antenna port
5. In addition a second, a third and a fourth UE-specific reference
signal are also applied on antenna port 5 on the same resource
elements as the first UE-specific reference signal. However, the
second UE-specific reference signal is a cyclic version of the
first UE-specific reference signal, the third UE-specific reference
signal is a cyclic version of the second UE-specific reference
signal and the fourth UE-specific reference signal is a cyclic
version of the third UE-specific reference signal. It should be
noted that more and less than 4 UE-specific reference signals can
be generated in the similar way, i.e., antenna port 5 can by using
scheme 2 transmit multiple UE-specific reference signals wherein
each reference signal is a cyclic shifted version of the existing
UE-specific reference signal.
[0045] Accordingly, scheme 2 results in lower overhead than scheme
1 since the same antenna port (e.g., antenna port 5) can be used
for multiple UE-specific reference signals and the same sequence
for the transmission of multiple streams can be kept, i.e., in
scheme 2, a code division multiplex scheme is used, where each
layer can share the same reference signals but with different
orthogonal cover.
[0046] Further, flexible switch for rank adaptation is possible due
to selection of multiple UE-specific reference signals instead of
multiple antenna ports. Another advantage with scheme 2 is the
flexible generation of multiple UE-specific reference signals,
e.g., different shift intervals.
[0047] A further scheme, referred to as scheme 3 follows the same
principle as scheme 2, i.e., multiple UE-specific reference signals
code-division multiplexed (CDM) in the time-frequency domain share
the same antenna port. In order to keep orthogonality between the
multiple UE-specific reference signals, orthogonal code sequences
with length-M can be applied on the existing UE-specific reference
signal. The number of the UE-specific reference signals is decided
by the length of the orthogonal codes. FIG. 5 shows the generation
of 4 UE-specific reference signals being orthogonal covers with
length 4 of the existing UE reference signal. Thus, a first
UE-specific reference signal is sent on antenna port 5 (with an
orthogonal cover of [1 1 1 1]. In addition a second, a third and a
fourth UE-specific reference signal are also applied on antenna
port 5 on the same resource elements as the first UE-specific
reference signal. However, the second UE-specific reference signal
is an orthogonal cover of the first UE-specific reference signal,
wherein the orthogonal code [1 1 -1 -1] is applied. The third
UE-specific reference signal is an orthogonal cover of the first
and second UE-specific reference signal, wherein the orthogonal
code [1 -1 -1 1] is applied. The fourth UE-specific reference
signal is an orthogonal cover of the first, second and third
UE-specific reference signal, wherein the orthogonal code [1 -1 1
-1] is applied. The placement of the orthogonal covers is marked by
the dashed boxes 501 and is just an example, giving the estimated
area of each UE-specific reference signal.
[0048] Accordingly, scheme 3 provides lower overhead than scheme 1
due to the sharing of the same antenna port and the same sequence
for the transmission of multiple streams can be kept. Flexible
switch for rank adaptation due to selection of multiple UE-specific
reference signals instead of multiple antenna ports and flexible
generation of multiple UE-specific reference signals, e.g.,
length-2 Walsh codes, length-3 DFT codes and length-4 Walsh codes
and so on are achieved.
[0049] A further scheme, referred to as scheme 4 follows the same
principle as schemes 2 and 3, i.e., multiple UE-specific reference
signals code-division multiplexed (CDM) in the time-frequency
domain share the same antenna port. In order to keep orthogonality
between the multiple UE-specific reference signals, space frequency
block codes (SFBC) in the space-frequency domain can be applied to
generate multiple UE-specific reference signals. An example of SFBC
structures is a 2.times.2 Alamouti matrix. FIG. 6 shows the
generation of 2 UE-specific reference signals based on a 2.times.2
Alamouti matrix 602, where two adjacent resource elements are
grouped 601 as an example. In the first UE-specific reference
signal, two adjacent resource elements are grouped 601 and a SFBC
603 is applied on the group 601 resulting in the second UE-specific
reference signal 603.
[0050] Two UE-specific reference signals can be generated by e.g.,
applying the SFBC according to the matrix below. X.sub.1 and
X.sub.2 are reference signals of 1.sup.st stream and X.sub.1.sup.x
and X.sub.2.sup.x are the reference signals of 2.sup.nd stream.
[ X 1 X 2 X 2 * - X 1 * ] 1 st UE - specific reference signals 2 nd
UE - specific reference signals ##EQU00001##
[0051] Three UE-specific reference signals can be generated by
e.g., applying the SFBC according to the matrix below. X.sub.1,
X.sub.2 and X.sub.3 are reference signals of 1.sup.st stream and
X.sub.1.sup.x, X.sub.2.sup.x and X.sub.3.sup.x are the complex
conjugation of X1, X2 and X3, accordingly.
[ X 1 X 2 * X 3 * 0 - X 2 X 1 * 0 - X 3 * - X 3 0 X 1 * X 2 * ] 1
st UE - specific reference signals 2 nd UE - specific reference
signals 3 r d UE - specific reference signals ##EQU00002##
[0052] Four UE-specific reference signals can be generated by e.g.,
applying the SFBC according to the matrix below. X.sub.1, X.sub.2,
X.sub.3 and X.sub.4 are reference signals of 1.sup.st stream and
X.sub.1.sup.x, X.sub.2.sup.x, X.sub.3.sup.x and X.sub.4.sup.x are
the complex conjugation of X1, X2 and X3, accordingly.
[ X - X 2 X 3 * 2 X 3 * 2 X 2 X 1 X 3 * 2 - X 3 * 2 X 3 2 X 3 2 - X
1 - X 1 * + X 2 - X 2 * 2 X 2 - X 2 * + X 1 - X 1 * 2 X 3 2 - X 3 2
- X 2 - X 2 * + X 1 - X 1 * 2 - X 1 - X 1 * - X 2 + X 2 * 2 ] 1 st
UE - specific reference signals 2 nd UE - specific reference
signals 3 r d UE - specific reference signals 4 th UE - specific
reference signals ##EQU00003##
[0053] Accordingly, scheme 4 provides lower overhead than scheme 1
due to the sharing of the same antenna port and the same sequence
for the transmission of multiple streams can be kept. Flexible
switch for rank adaptation due to selection of multiple UE-specific
reference signals instead of multiple antenna ports.
[0054] According to a yet further embodiment, scheme 1 is combined
with at least one of schemes 2-4. This is referred to as scheme 5.
This embodiment provides a compromise between overhead and channel
estimation accuracy. For instance, 2 antenna ports with 2
code-division multiplexed UE-specific reference signals can support
the transmission of up to 4 streams, where 2 UE-specific reference
signals are generated using any of the schemes 2-4. As an example,
when one UE-specific reference signal is required (transmission of
one stream), antenna port 1 may be used, when two UE-specific
reference signals are required, antenna ports 5 and 6 may be used.
In addition if three UE-specific reference signals are required,
one reference signal may be sent from antenna port 5 and two
reference signals may be sent from antenna port 6 using any of the
schemes 2-4.
[0055] Moreover, the present invention relates to a network node
801 such as a base station, also referred to as an eNodeB in LTE as
illustrated in FIG. 8. The network node is associated with an
antenna 802 comprising a plurality of antenna ports using
non-codebook-based multi-stream beamforming. The network node
comprises a processor 803 configured to apply a first UE specific
reference signal 804 on a first antenna port 802a, and to reuse the
first UE specific reference signal to be applied orthogonally on an
antenna port 802a;b.
[0056] According to the first embodiment, the processor 803 is
configured to multiplex the reused first UE specific reference
signal 805' on a second antenna port in different time-frequency
resource elements than the first UE specific reference signal
applied on the first antenna port.
[0057] According to further embodiments, as explained above, the
processor 803 is configured to apply the reused first UE specific
reference signal on the first antenna port sharing the same
time-frequency resource elements as the first set of UE specific
reference signals. The orthogonality between the plurality of
UE-specific reference signals is achieved in these embodiments by
the processor 803 which is configured to code division multiplex
the reused first UE specific reference signal 805 in a set of
resource elements.
[0058] It should be noted that the network node 801 also comprises
other standard devices. However, these devices are not considered
to be essential for the present invention and are therefore not
shown in figures or explained further.
[0059] The present invention is not limited to the above-described
preferred embodiments. Various alternatives, modifications and
equivalents may be used. Therefore, the above embodiments should
not be taken as limiting the scope of the invention, which is
defined by the appending claims.
* * * * *