U.S. patent application number 13/256659 was filed with the patent office on 2012-01-05 for method and apparatus for codebook-based precoding in mimo systems.
Invention is credited to Kari Juhani Hooli, Kari Pekka Pajukoski, Esa Tapani Tiirola.
Application Number | 20120002750 13/256659 |
Document ID | / |
Family ID | 41319628 |
Filed Date | 2012-01-05 |
United States Patent
Application |
20120002750 |
Kind Code |
A1 |
Hooli; Kari Juhani ; et
al. |
January 5, 2012 |
Method and Apparatus for Codebook-Based Precoding in MIMO
Systems
Abstract
A method including using a precoding code book for controlling
transmissions from four antennas of a device, said code book
including a plurality of entries, wherein said entries are such
that a single layer is mapped to each selected antenna, said code
book entries including different antenna pair combinations whereby
one or two antenna pairs are selected for transmission.
Inventors: |
Hooli; Kari Juhani; (Oulu,
FI) ; Pajukoski; Kari Pekka; (Oulu, FI) ;
Tiirola; Esa Tapani; (Kempele, FI) |
Family ID: |
41319628 |
Appl. No.: |
13/256659 |
Filed: |
March 17, 2009 |
PCT Filed: |
March 17, 2009 |
PCT NO: |
PCT/EP2009/053162 |
371 Date: |
September 15, 2011 |
Current U.S.
Class: |
375/295 |
Current CPC
Class: |
H04B 7/0639 20130101;
H04B 7/046 20130101; H04B 7/0469 20130101; H04B 7/0417 20130101;
H04B 7/0619 20130101; H04B 7/0486 20130101; H04B 7/0691
20130101 |
Class at
Publication: |
375/295 |
International
Class: |
H04B 7/04 20060101
H04B007/04 |
Claims
1. A method comprising: using a precoding code book for controlling
transmissions from four antennas of a device, said code book
comprising a plurality of entries, wherein said entries are such
that a single layer is mapped to each selected antenna, said code
book entries comprising different antenna pair combinations whereby
one or two antenna pairs are selected for transmission.
2. A method as claimed in claim 1, comprising providing a plurality
of precoding books, each of said precoding books being associated
with a respective rank.
3. A method as claimed in claim 2, comprising providing four
precoding books for ranks 1 to 4.
4. A method as claimed in claim 3, wherein a size of a codebook of
one rank is different from a size of a codebook of at least one
other rank.
5. A method as claimed in claim 4, wherein a size of a codebook of
one rank is bigger than a size of a codebook of a higher rank.
6. A method as claimed in any of claim 2, comprising determining a
rank and selecting one of a plurality of codebooks in dependence on
the determined rank.
7. A method as claimed in claim 6, comprising determining the rank
is one and selecting an associated codebook which allocates said
antenna to the same layer.
8. A method as claimed in claim 6, comprising determining the rank
is two and selecting an associated codebook which allocated two
antenna to each layer.
9. A method as claimed in claim 6, comprising determining that the
rank is three and selecting an associated codebook which allocates
two antenna to one layer and a single antenna to each of two other
layers.
10. A method as claimed in claim 1, wherein a plurality of entries
of at least one precoding book is arranged to divide said antennas
into at least one pair, with at least one pair having the same
phase rotation.
11. A method as claimed in claim 1, wherein a plurality of entries
of at least precoding book is arranged to divide said antenna into
at least one pair, with different antenna in at least one pair
having different phase rotation.
12. A method as claimed in claim 1, wherein at least a plurality of
different entries of at least one precoding book provide different
respective phase rotation combinations.
13. A method as claimed in claim 1, wherein a plurality of entries
of at least one precoding book is arranged to only use one pair of
said antennas.
14. A method as claimed in claim 1, comprising controlling power
with which an antenna transmits in accordance with a number of
antenna which are transmitting at the same time.
15. A method as claimed in claim 1, wherein at least one antenna
pair is arranged in accordance with spatial correlation properties
of said antennas.
16. A method as claimed in claim 15, wherein said different spatial
correlation properties comprise at least one of antenna
polarization and antenna position.
17. Method as claimed in claim 1, wherein at least one entry in
said codebook provides transmit diversity applied over two antennas
of a selected antenna pair.
18. A method as claimed in claim 1, comprising transmitting at
least one transport block from a plurality of antenna.
19. A computer program comprising program code means adapted to
perform the steps of claim 1.
20. Apparatus comprising: a processor configured to use a precoding
code book for controlling transmissions from four antennas of a
device, said code book comprising a plurality of entries, wherein
said entries are such that a single layer is mapped to each
selected antenna, said code book entries comprising different
antenna pair combinations whereby one or two antenna pairs are
selected for transmission.
21. Apparatus as claimed in claim 20, wherein said processor is
configured to control power with which an antenna transmits in
accordance with a number of antenna which are transmitting at the
same time.
22. Apparatus as claimed in claim 20, wherein said processor is
configured to control the transmission of at least one transport
block from a plurality of antenna and further configured to control
power with which an antenna transmits in accordance with a number
of antenna which are transmitting at the same time.
23. Apparatus comprising: a processor configured to select one of a
plurality of entries in a precoding code book for controlling
transmissions from four antennas of a device, wherein said entries
are such that a single layer is mapped to each selected antenna,
said code book entries comprising different antenna pair
combinations whereby one or two antenna pairs are configured to be
selected for transmission.
24. Apparatus as claimed in claim 23, wherein said processor is
configured to determine a rank and selecting one of a plurality of
codebooks in dependence on the determined rank.
25. Apparatus as claimed in claim 24, wherein said processor is
configured, when the rank is determined to be one to select an
associated codebook which allocates said antenna to the same
layer.
26. Apparatus as claimed in claim 24, wherein said processor is
configured, when the rank is determined to be two to select an
associated codebook which allocates two antenna to each layer.
27. Apparatus as claimed in claim 25, wherein said processor is
configured, when the rank is determined to be three to select an
associated codebook which allocates two antenna to one layer and a
single antenna to each of two other layers.
28. Apparatus as claimed in claim 20, comprising a memory storing a
plurality of precoding books, each of said precoding books being
associated with a respective rank.
29. Apparatus as claimed in claim 28, wherein four precoding books
for ranks 1 to 4 are provided.
30. Apparatus as claimed in claim 28, wherein a size of a codebook
of one rank is different from a size of a codebook of at least one
other rank.
31. Apparatus as claimed in claim 30, wherein a size of a codebook
of one rank is bigger than a size of a codebook of a higher
rank.
32. Apparatus as claimed in claim 20, wherein a plurality of
entries of at least one precoding book is arranged to divide said
antennas into at least one pair, with at least one pair having the
same phase rotation.
33. Apparatus as claimed in claim 20, wherein a plurality of
entries of at least precoding book is arranged to divide said
antenna into at least one pair, with different antenna in at least
one pair having different phase rotation.
34. Apparatus as claimed in claim 20, wherein at least a plurality
of different entries of at least one precoding book provide
different respective phase rotation combinations.
35. Apparatus as claimed in claim 20, wherein a plurality of
entries of at least one precoding book is arranged to only use one
pair of said antennas.
36. Apparatus as claimed in claim 20, wherein at least one antenna
pair entry is arranged in accordance with spatial correlation
properties of said antennas.
37. Apparatus as claimed in claim 20, wherein said different
spatial correlation properties comprise at least one of antenna
polarization and antenna position.
38. Apparatus as claimed in claim 20, wherein at least one entry in
said codebook provides transmit diversity applied over two antennas
of a selected antenna pair.
39. An integrated circuit or chip set comprising an apparatus as
claimed in claim 20.
40. A user equipment comprising an apparatus as claimed in claim
20.
41. (canceled)
42. A base station comprising an apparatus as claimed in claim 23.
Description
FIELD OF THE INVENTION
[0001] Embodiments of the present invention relate to a method and
apparatus and, in particular but not exclusively, to apparatus and
a method for use in a multiple input multiple output wireless
telecommunications network.
BACKGROUND
[0002] It has been proposed to improve the coverage and capacity of
communication by use of spatial diversity or spatial multiplexing.
By using spatial multiplexing, the data rate can be increased by
transmitting independent information streams from different
antennas but using the same channel as defined by frequency and
time resource and possibly spreading code.
[0003] These systems may be referred to as multiple input multiple
output (MIMO) systems. These systems require complex controllers to
control both the transmission and receiving elements of the mobile
station and the base station.
[0004] Multi-stream single user MIMO transmission has been proposed
and forms part of WCDMA (wideband code division multiple access),
3GPP LTE (Third generation partnership project--long term
evolution) and WiMax system standards. In single user multiple
input multiple output (SU-MIMO), a MIMO receiver with multiple
antennas and receiving circuitry receives the multiple streams,
separates the multiple streams and determines the transmission
symbols sent over each stream of the spatially multiplexed data
streams.
[0005] In the 3GPP forum, LTE-Advanced has been proposed to be an
evolution of LTE Rel'8 system to address the ITU-R (International
Telecommunications Union Radio communication Sector) requirements
for IMT (International Mobile Telecommunications)-Advanced. 3GPP
approved a new Study Item on LTE-Advanced in RAN#39 (March 2008).
It has been proposed that SU-MIMO with 2-4 transmission antennas at
the UE (user equipment) will be part of LTE-Advanced [TR 36.913
v8.0.0].
[0006] It has been proposed that SU-MIMO UL (uplink) transmissions
will involve transmission precoding techniques and that this
precoding utilizes fixed codebooks.
[0007] In a submission made to 3GPP in R1-090915, Ericsson has
proposed a 4Tx (transmission) precoding codebook that attempts to
preserve a favourable PAPR (peak to average power ratio) property
of the transmitted signal. However, the inventors have identified
that this proposal is limited to rank 2 transmissions.
[0008] In the R1-090590 submission to 3GPP, Texas Instruments noted
that large codebook sets at full transmission rank do not provide
significant gain.
[0009] Antenna imbalance has been considered in the codebook
design, proposed in R1-062355 to 3GPP by Nokia. However, the
inventors have identified that these designs have focused on 2 Tx
schemes.
[0010] The Householder codebook used in LTE Rel'8 DL (downlink)
increases PAPR but the inventors have identified that this scheme
does not take the potential transmit antenna imbalance (e.g. due to
movement of the user equipment in a user's hand) into account.
SUMMARY
[0011] According to one aspect of the present invention, there is
provided a method comprising using a precoding code book for
controlling transmissions from four antennas of a device, said code
book comprising a plurality of entries, wherein said entries are
such that a single layer is mapped to each selected antenna, said
code book entries comprising different antenna pair combinations
whereby one or two antenna pairs are selected for transmission.
[0012] According to another aspect of the present invention, there
is provided an apparatus comprising a processor configured to use a
precoding code book for controlling transmissions from four
antennas of a device, said code book comprising a plurality of
entries, wherein said entries are such that a single layer is
mapped to each selected antenna, said code book entries comprising
different antenna pair combinations whereby one or two antenna
pairs are selected for transmission.
[0013] According to a further aspect of the present invention,
there is provided an apparatus comprising a processor configured to
select one of a plurality of entries in a precoding code book for
controlling transmissions from four antennas of a device, wherein
said entries are such that a single layer is mapped to each
selected antenna, said code book entries comprising different
antenna pair combinations whereby one or two antenna pairs are
configured to be selected for transmission.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Various embodiments of the present invention will now
described by way of example only with reference to the accompanying
Figures, in which:--
[0015] FIG. 1 shows a schematic view of a system including an
schematic base station and user equipment configuration within
which embodiments of the invention may be implemented;
[0016] FIG. 2 shows a codebook embodying the present
invention--rank 1;
[0017] FIG. 3 shows a codebook embodying the present
invention--rank 2;
[0018] FIG. 4 shows a codebook embodying the present
invention--rank 3;
[0019] FIG. 5 shows a flowchart of steps taken at the mobile
station; and
[0020] FIG. 6 shows a flowchart of steps taken at the base
station
DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0021] Embodiments of the present invention are described herein by
way of particular examples and specifically with reference to
preferred embodiments. It will be understood by one skilled in the
art that the invention may not be limited to the details of the
specific embodiments given herein.
[0022] FIG. 1 shows a communication network 30 in which some
embodiments of the present invention may be implemented. In
particular, some embodiments of the present invention may relate to
the implementation of radio modulators/demodulators (modems) for a
range of devices that may include: user equipment 201, relays,
access points or base stations 101 which communicate over a
wireless environment 151.
[0023] Embodiments of the present invention may be applicable to
communication networks implemented according to a range of
standards and their evolution including: WCDMA (Wideband Code
Division Multiple Access), 3GPP LTE (Long Term Evolution), WiMax
(Worldwide interoperability for Microwave Access), UMB (Ultra
Mobile Broadband), CDMA (Code Division Multiple Access), 1xEV-DO
(Evolution-Data Optimized), WLAN (Wireless Local Area Network), and
UWB (Ultra-Wide Band) receivers.
[0024] With respect to FIG. 1, a schematic view of a system within
which embodiments of the invention may be implemented is shown. The
communication system 30 is shown with a base station 101 which may
be a node B (NB), an enhanced node B (eNB) or any access server
suitable for enabling user equipment 201 to access wirelessly a
communication system.
[0025] FIG. 1 shows a system whereby the base station (BS) 101 may
transmit to the user equipment (UE) 201 via the wireless
environment communications channel 151, which may be known as the
downlink (DL), and the user equipment (UE) 201 may transmit to the
base station (BS) 101 via the wireless environment communications
channel 151, which may be known as the uplink (UL).
[0026] The base station 101 can comprise a processor 105 which may
be configured to control the operation of the receiver/transmitter
circuitry 103. The processor may be configured to run software
stored in memory 106.
[0027] The memory 106 may be further configured to store data
and/or information to be transmitted and/or received. The memory
106 may further be used to store configuration parameters used by
the processor 105 in operating the base station 101.
[0028] The transmitter/receiver circuitry 103 may be configured to
operate as a configurable transmitter and/or receiver converting
between radio frequency signals of a specific protocol for
transmission over (or reception via) the wireless environment and
baseband digital signals. The transmitter/receiver circuitry 103
may be configured to use the memory 106 as a buffer for data and/or
information to be transmitted over or received from the wireless
environment 151.
[0029] The transmitter/receiver circuitry 103 may further be
configured to be connected to at least one antenna for receiving
and transmitting the radio frequency signals over the wireless
environment to the user equipment 201. In FIG. 1 the base station
is shown comprising 2 antennas, the first antenna 107.sub.1 and the
second antenna 107.sub.2 both configured to transmit and receive
signals. In other embodiments of the invention the base station may
have more antennas represented by the dotted antenna 107.sub.m in
FIG. 1. In one preferred embodiment, m may be 4. Four receiving
antennas is needed in order to support rank-4 transmission.
[0030] The base station 101 may be connected to other network
elements via a communications link 111. The communications link 111
may receive data to be transmitted to the user equipment 201 via
the downlink and transmits data received from the user equipment
201 via the uplink. This data may comprise data for all of the user
equipment within the cell or wireless communications range operated
by the base station 101. The communications link 111 is shown in
FIG. 1 as a wired link. However it would be understood that the
communications link may further be a wireless communications
link.
[0031] in FIG. 1, there is shown two user equipment 201 within the
range of the base station 101. However it would be understood that
there may be more or fewer user equipment 201 within range of the
base station 101. The user equipment may be a mobile station, or
any other apparatus or electronic device suitable for communication
with the base station. For example in further embodiments of the
invention the user equipment may be personal data organizers or
laptop computers suitable for wireless communication in the
environment as described hereafter. It should be appreciated that
embodiments of the invention may also be applied to a relay
station.
[0032] FIG. 1 in particular shows a first user equipment UE.sub.1
201.sub.1 and a second user equipment UE.sub.2 201.sub.2.
Furthermore FIG. 1 shows in more detail the first user equipment
UE.sub.1 201.sub.1. The first user equipment 201.sub.1 may comprise
a processor 205 configured to control the operation of a
receiver/transmitter circuitry 203. The processor may be configured
to run software stored in memory 207. The processor may further
control and operate any operation required to be carried out by the
user equipment such as operation of the user equipment display,
audio and/or video encoding and decoding in order to reduce
spectrum usage, etc.
[0033] The memory 207 may be further configured to store data
and/or information to be transmitted and/or received. The memory
207 may further be used to store configuration parameters used by
the processor 205 in operating the user equipment 201.sub.1. The
memory may be solid state memory, optical memory (such as, for
example, CD or DVD format data discs), magnetic memory (such as
floppy or hard drives), or any media suitable for storing the
programs for operating the processors, configuration data or
transmission/reception data.
[0034] The transmitter/receiver circuitry 203 may be configured to
operate as a configurable transmitter and/or receiver converting
between radio frequency signals of a specific protocol for
transmission over (or reception via) the wireless environment and
baseband digital signals. The transmitter/receiver circuitry 203
may be configured to use the memory 207 as a buffer for data to be
transmitted over or received from the wireless environment 151.
[0035] The transmitter/receiver circuitry 203 is configured to be
connected to at least one antenna for receiving and transmitting
the radio frequency signals over the wireless environment to the
base station 101. In FIG. 1 the user equipment is shown comprising
4 antennas, the first antenna 251.sub.11 to the fourth antenna
251.sub.14.
[0036] Although FIG. 1 and the examples described hereafter
describe the user equipment and the bases station as having a
processor arranged to carry out the operations described below, it
would be understood that in embodiments of the invention the
respective processors may comprise a single processor or a
plurality of processors. The processors may be implemented by one
or more integrated circuits.
[0037] Some embodiments of the present invention maybe used in the
LTE-Advanced system which may be part of 3GPP LTE Rel. 10. However,
it should be appreciated protocol for transmission over (or
reception via) the wireless environment and baseband digital
signals. The transmitter/receiver circuitry 203 may be configured
to use the memory 207 as a buffer for data to be transmitted over
or received from the wireless environment 151.
[0038] The transmitter/receiver circuitry 203 is configured to be
connected to at least one antenna for receiving and transmitting
the radio frequency signals over the wireless environment to the
base station 101. In FIG. 1 the user equipment is shown comprising
4 antennas, the first antenna 251.sub.11 to the fourth antenna
251.sub.14.
[0039] Although FIG. 1 and the examples described hereafter
describe the user equipment and the bases station as having a
processor arranged to carry out the operations described below, it
would be understood that in embodiments of the invention the
respective processors may comprise a single processor or a
plurality of processors. The processors may be implemented by one
or more integrated circuits.
[0040] Some embodiments of the present invention maybe used in the
LTE-Advanced system which may be part of 3GPP LTE Rel. 10. However,
it should be appreciated that this is by way of example only and
embodiments of the invention may be used in alternative
systems.
[0041] A PUSCH (physical uplink shared channel) precoding scheme
for single user MIMO (SU-MIMO), with a precoding codebook design
for 4 Tx (Transmission) antennas is discussed. In another
embodiment, these techniques could be applied also to PUCCH Format
2 (for example with single stream precoding). The same techniques
may be applied to sounding reference signals.
[0042] In some embodiments of the invention, the SU-MIMO precoding
codebooks are arranged to take into account the properties specific
for the uplink of LTE-Advanced system.
[0043] Some embodiments are arranged to take into account: [0044]
Imbalance between Tx antennas in the UE due to for example the grip
of the hand and changing of the antenna orientation, which provide
different responses to the vertical and horizontal polarization
components. [0045] Power efficiency decrement due to increased
PAPR
[0046] In a MIMO system, the performance of a radio system is
improved by using spatial precoding at a transmitter and spatial
postcoding at the receiver. Spatial precoding may comprise spatial
beamforming and spatial coding. The spatial precoding is done to
enhance the signal power at the destination and to diminish the
interfering power.
[0047] In single-layer beamforming, the same signal is emitted from
each of the transmit antennas with appropriate phase (and
optionally gain) weighting such that the signal power is maximized
at the receiver input. The benefits of beamforming are to increase
the signal gain from constructive combining and to reduce the
multipath fading effect. When the receiver has multiple antennas,
the transmit beamforming cannot simultaneously maximize the signal
level at all of the receive antenna and precoding is used.
Precoding requires knowledge of the channel state information (CSI)
at the transmitter.
[0048] Some embodiments of the invention use a precoding codebook
design for simultaneous transmission of up to 2 codewords. In this
embodiment, the codeword can be regarded as a transport block which
contains data that is encoded with e.g. turbo code. In the
following, the terminology transport block will be used. The
transport blocks are transmitted from 4 transmit antennas. In some
embodiments of the invention, the precoding codebook is designed
based on one or more of the following criteria: [0049] Precoding
contains precoding matrices that preserve the PAPR of SC-FDMA
(single carrier frequency division multiple access) transmission.
Thus, only one layer is mapped for each antenna. [0050] Precoding
takes potential transmit antenna imbalance into account by one or
more of: [0051] Containing antenna or polarization selection in the
codebook for rank 1 transmission [0052] Ensuring that each
transport block is transmitted from a plurality of antennas that
can be selected according to the current channel state, when
transmission rank is less than number of transmit antennas. [0053]
Containing Tx diversity elements with antenna selection or antenna
grouping in the codebook in order to minimize the size of
codebook
[0054] Based on the criteria, a codebook for a specific rank is
designed with following steps:
[0055] For a single-stream transmission: [0056] transmit antennas
are grouped into two groups, with 2 antennas per group. [0057]
After that, all possible antenna-to-antenna group mappings are
listed [0058] Different phase rotation combinations between the
antennas within the antenna group are listed as codebook entries.
[0059] Additionally, selection of single antenna group for
transmission is included into the codebook. Different
antenna-to-antenna group options are included, and either phase
rotation or a transmission Tx diversity method is applied between
the antennas within the antenna group.
[0060] For a multi-stream transmission [0061] Number of transmit
antennas per layer is selected so that each transport block is
transmitted from multiple antennas [0062] After that, all possible
layer-to-antenna mappings (i.e. antenna groups for each layer are
formed) are listed with restriction that only one layer is mapped
per antenna. [0063] When a layer is mapped to multiple antennas
(i.e. antenna groups), different phase rotation combinations
between the antennas within the antenna group are listed as
codebook entries. [0064] Alternatively, a transmission Tx diversity
method can be applied between the antennas within the antenna
group.
[0065] It should be noted that a codebook design containing both
PAPR preserving and antenna selection precoding options is not
contradictory. PARP preserving precoding is used when UE
transmission is power limited, whereas precoding with antenna
selection (and, thus, power boosting of remaining transmit
antennas) may be used when UE transmission is not power
limited.
[0066] The codebook may be designed to contain PAPR preserving
precoding matrices. Nevertheless, codebook can also contain
precoding matrices that do not preserve PAPR.
[0067] When looking on the specific codebook designs, it should be
appreciated that embodiments of the invention may be used with
transmission ranks 1, 2, and 3. Rank 4 is not considered in
following. A rank 4 codebook may be provided, using known
techniques. Rank can be regarded as the number of different
transmit streams.
[0068] In the case that pilot signals are typically precoded and
the codebook contains entries with a Tx diversity method requiring
an antenna specific pilot, two pilot sequences need to be allocated
to the UE. The second pilot sequence is used only when Tx diversity
is used.
Rank 1
[0069] The values in the tables represent the amplitude and phase
when a layer X is mapped to antenna Y.
[0070] Antennas are grouped into 2 groups with 2 antennas per
group. After that, precoding vectors with QPSK rotation
combinations between antennas within antenna groups are formed.
Additionally, precoding vectors for antenna group selection are
included. In the case of antenna group selection, there can be
either phase rotation, e.g., BPSK (binary shift keying, between the
transmitting antennas, or simply Tx diversity, e.g., Space-Time
Block-Coding STBC. The benefit of using Tx diversity is that it
allows for more compact (smaller) codebook design. Such codebook
design results in 22 or 16 precoding matrix indices if BPSK or STBC
is used. An example of resulting codebook options are shown in FIG.
2. There are three tables shown. The first table is where STBC is
used and the second table is where BPSK is used. In the third
table, the codebook is designed by taking the spatial correlation,
that is the polarization or position of antennas into account. In
the codebook design, precoding vectors with antenna selection
elements are designed so that antennas with high spatial
correlation, that is the same polarization direction or adjacent
antenna positions can be selected. Thus, only a subset of possible
antenna selection combinations is included in the codebook, in one
embodiment of the invention.
[0071] In the example shown 3-PSK rotation between the selected
antennas is applied (in PM's where antenna selection is presented).
It should be noted that also other phase rotation constellations
can be used, based, e.g., on QPSK or 8-PSK rotations.
[0072] In one alternative embodiment of the invention, entries
11-16 (i.e. antenna selection entries) are taken from table 3, and
entries 1-10 are selected from Householder precoding vectors used,
e.g. in the 3GPP LTE Release 8 DL 4Tx antenna codebook.
[0073] In column 1, the precoding matrix indicator is listed. These
indicators are from 1 to 22. The tables lists for each of the four
antennas the required rotation or Tx diversity.
[0074] The zeros indicate that the associated antenna has not been
used for transmission. Rather the transmission power (which may be
kept constant for UE) is concentrated on the antennas having
favorable channels.
[0075] In both of the tables, the first 10 entries show that each
of the antenna has a rotation of 0.5, -0.5, 0.5j and -0.5j.
[0076] The combinations represent antenna pairing and phase
rotation between antenna pairs. For example with table 1, PMIs 1
to-4 represent the following antenna grouping: antenna group number
1 comprises antennas 1 and 2 and antenna group number 2 comprises
antennas 3 and 4, with QPSK rotation between groups. Thus antennas
3 and 4 have same phase in these rows, since they belong to the
same antenna group. In PMIs 5 to 8, the antenna groups are antennas
1 and 3 and antennas 2 and 4, respectively.
[0077] As can be seen, the antennas can be regarded as being two
pairs, with each pair being allocated the same rotation. (This is
the case for the first 10 entries).
[0078] For the first of the tables, the 11.sup.th to 16.sup.th
entries have STBC on two out of the four antennas. This is so when
the UE is not at, for example, a cell edge, it may have power
headroom on power amplifiers. Then it may be better to concentrate
transmission power on good antennas.
[0079] In the second of the tables, there are 12 entries where
there is rotation provided for only two of the antennas. One
rotation value is always 0.5 in this example. In one embodiment, it
an aim to normalize the total transmission power to 1 (4.times.0.5
2=1). However the absolute value may be varied. In one embodiment,
considerations for the phase and the relation of amplitudes may be
more important. The other rotation value is -0.5 or 0.5.
[0080] As an example, the precoding vector that is expected to
maximise the SINR (signal to interference noise ratio) at the
output of equalizer in the base station receiver is selected. This
can be estimated based on existing channel estimates obtained from
a sounding reference signal.
[0081] As mentioned, the absolute values in the table may be
changed. The amplitude may be the same for all antennas in a given
precoding vector, and that amplitudes may be increased when only
two transmit antennas out of the four are used so that the same
power is distributed between two instead of four antenna.
Rank 2
[0082] Antennas are grouped into 2 groups with 2 antennas per
group, with each antenna group mapped to one layer or data stream.
After that, precoding vectors with phase rotation, e.g. BPSK
(Binary phase shift keying), combinations between the antennas
within each antenna groups are formed. Alternatively, there can be
Tx diversity, e.g., Space-Time Block-Coding between the antennas
mapped to the same layer/antenna group. Such codebook design
results in 12 or 3 precoding matrix indices, depending if on BPSK
or if STBC is used. The resulting codebook options are shown in
FIG. 3.
[0083] The benefit of using Tx diversity is that it allows for more
compact (smaller) codebook design.
[0084] The first table of FIG. 3 is the BPSK codebook design and
the second table is the STBC codebook design. As with FIG. 2, the
first column represents the precoder matrix indicator. The second
column indicates the antenna number. The third column represent
layer 1 and the fourth column, layer 2. As can be seen, two of the
four antennas are allocated to each of the two layers. Different
precoder matrix indicators have different ones of the first to
fourth antennas allocated to each of the first and second data
streams or layers.
[0085] The values assigned to each antenna in the first table are
0.5 and -0.5. With BPSK elements and two layers, there are 12
options. There is no need to have phase rotation between antennas
that are mapped to different layers, in some embodiments of the
invention.
[0086] The values in the table relate to the used phase rotation;
in here, the BPSK constellation is used (to keep the codebook size
reasonable). Alternatively, the QPSK constellation could be used
with the associated values in the table which may also include 0.5j
and -0.5j.
[0087] In the second table, there are three options given where the
antenna each have STBC are applied thereto but different ones of
the antennas are assigned to the respective layers.
Rank 3
[0088] When transport block-to-layer mapping from LTE Rel'8 DL is
assumed, transport block #1 is mapped to layer #1, and transport
block #2 is mapped to layers #2 & #3. To ensure that each
transport block is transmitted from multiple antennas, layer #1
mapped to 2 antennas whereas layer #2 and 3 are mapped to a single
antenna each. Thus each codeword or transport block is mapped to 2
transmit antennas. The precoding codebook contains 6 different
layer-to-antenna mappings and applies BPSK rotation or Tx diversity
(e.g. STBC) between the antennas used by layer #1. Such a codebook
design results in 12 or 6 precoding matrix indices, depending on if
BPSK or STBC is used. The resulting codebook options are shown in
FIG. 3.
[0089] In FIG. 4, the first table applies BPSK rotation and the
second table applies STBC. The first column is the PMI and the
second column lists the antenna number. The next columns are for
the first to third layers. In each table, layer 1 has two antennas
assigned to it, whilst layers 2 and 3 each have a single antenna
assigned to them. The last columns indicate which ones.
[0090] In the first of the tables, the values of 0.5 and -0.5 can
be assigned. In the second table, STBC or the value 0.5 can be
applied. In rank 3, layer 1 uses two antennas. Since the same data
is transmitted from two antennas, STBC is applied over the antennas
mapped to layer 1. However, layer 2 and 3 use single antenna each,
and thus, different data is sent from these antennas. Thus, STBC is
not applied.
[0091] Some embodiments of the invention may have one or more of
the following advantages: [0092] Single carrier properties are
maintained which is suitable for LTE-Advanced UL. [0093] Codebook
size can be kept small which means that there is a small signalling
overhead in DL--
[0094] The antenna pairing is useful in keeping the codebook size
small. The antenna pairing reduces the number of combinations for
phase rotations. Further by limiting antenna pairs to contain
antennas with significant spatial correlation, e.g., the same
polarization direction or adjacent antenna positions, the codebook
can be kept small. Finally, there is the use of transmit diversity
which again assists in keeping the codebook small.
[0095] Reference is made to FIG. 5 which shows a flow chart
embodying the present invention. This is carried out by the user
equipment. Additionally layer mapping (i.e. how transport blocks
(code words) are mapped to spatial layers is performed. This may be
performed in the UE, corresponding to layer de-mapping in the base
station
[0096] In S1, a codebook is received, or information identifying
the codebook. Information identifying one of the entries of the
codebook is also received.
[0097] In S2, the received information is stored in the memory
207.
[0098] In S3, the data stream(s) are precoded in accordance with
the information of selected entry. The precoded data streams are
then transmitted by the respective antennas. If necessary the
selected precoding may be used in any necessary retransmission.
[0099] In this regard, reference is made to FIG. 6 which shows
steps which may be carried out by the base station.
[0100] In T1, the base station is arranged to determine the channel
conditions. Preferably, the instantaneous channel conditions are
determined. The channel is the channel between the user equipment
and the base station.
[0101] A determination is made as to the rank of the
communication--T2. In other words the number of data streams which
are to be transmitted at the same time is determined. In the
embodiment discussed, the number of data streams can be up to m
where m is the number of antenna which the UE has.
[0102] In the next step T3, the base station selects a codebook
based on rank and a codebook entry based on the channel conditions
and/or the nature of the data streams. Preferably the codebook
entry is selected based on the instantaneous channel
conditions.
[0103] In T4, the selected codebook entry and codebook is sent to
the user equipment. Alternatively, information identifying the
codebook may be sent, with the codebook entry.
[0104] In alternative embodiments of the invention, at the receiver
side, for example, in the case that demodulation reference signals
are not precoded, the processor 105 of the BTS receiver needs to
calculate the effective channel by combining the selected precoding
matrix with channel estimates
[0105] Embodiments of the invention may be used with fewer antennas
than four or more than four antennas.
[0106] It is noted that whilst embodiments may have been described
in relation to user equipment or mobile devices such as mobile
terminals, embodiments of the present invention may be applicable
to any other suitable type of apparatus suitable for communication
via access systems. A mobile device may be configured to enable use
of different access technologies, for example, based on an
appropriate multi-radio implementation.
[0107] It is also noted that although certain embodiments may have
been described above by way of example with reference to the
exemplifying architectures of certain mobile networks and a
wireless local area network, embodiments may be applied to any
other suitable forms of communication systems than those
illustrated and described herein. It is also noted that the term
access system may be understood to refer to any access system
configured for enabling wireless communication for user accessing
applications.
[0108] The above described operations may require data processing
in the various entities. The data processing may be provided by
means of one or more data processors. Similarly various entities
described in the above embodiments may be implemented within a
single or a plurality of data processing entities and/or data
processors. Appropriately adapted computer program code product may
be used for implementing the embodiments, when loaded to a
computer. The program code product for providing the operation may
be stored on and provided by means of a carrier medium such as a
carrier disc, card or tape. A possibility may be to download the
program code product via a data network. Implementation may be
provided with appropriate software in a server.
[0109] For example the embodiments of the invention may be
implemented as a chipset, in other words a series of integrated
circuits communicating among each other. The chipset may comprise
microprocessors arranged to run code, application specific
integrated circuits (ASICs), or programmable digital signal
processors for performing the operations described above.
[0110] Embodiments of the inventions may be practiced in various
components such as integrated circuit modules. The design of
integrated circuits can be by and large a highly automated process.
Complex and powerful software tools may be available for converting
a logic level design into a semiconductor circuit design ready to
be etched and formed on a semiconductor substrate.
[0111] Programs, such as those provided by Synopsys, Inc. of
Mountain View, Calif. and Cadence Design, of San Jose, Calif. may
automatically route conductors and locate components on a
semiconductor chip using well established rules of design as well
as libraries of pre-stored design modules. Once the design for a
semiconductor circuit may have been completed, the resultant
design, in a standardized electronic format (e.g., Opus, GDSII, or
the like) may be transmitted to a semiconductor fabrication
facility or "fab" for fabrication.
[0112] It is noted herein that while the above describes
exemplifying embodiments of the invention, there are several
variations and modifications which may be made to the disclosed
solution without departing from the scope of the present invention
as defined in the appended claims.
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