Method and Apparatus for Sub-sampling of a Codebook in LTE-A
System
TECHNICAL FIELD
The present invention relates to Long Term Evolution Advanced
(LTE-A), especially to a method and apparatus for sub-sampling of a
codebook in LTE-A system.
BACKGROUND OF THE INVENTION
Nowadays, LTE technology is one of leading technologies in the
field of radio technology. Various precoding technologies are widely
used in the LTE system. In accordance with the 8 transmitting antenna
codebook prescribed in Rl-10501 1 of LTE-A Rel-10 (8Tx codebook for
short), a precoder for subbands is a product of two matrices i and W2,
in which matrix Wi corresponds to wideband and/or long-term channel
properties, and the other matrix W2 corresponds to frequency-selective
and/or short-term channel properties. For rank 1 and rank 2, the
codebook sizes for i and W2 are both 4 bits. Thus, in Channel State
Information (CSI ) reporting mode 1-1 (broadband mode), 8 bits are
needed to represent Precoding Matrix Indicator (PMI), however, it is
prescribed that only 4 bits could be used for PMI feedback. Thus, the
currently prescribed 8Tx codebook cannot be directly used in this CSI
reporting mode 1-1, and thus sub-sampling of the currently prescribed
8Tx codebook is necessary and important.
In Rl-10501 1, "Way Forward on 8Tx Codebook for Rel.10 DL
MIMO", RAN1#62, Madrid, Spain, August 2010, it is prescribed only
the 8Tx codebook, while no solution is presented for sub-sampling of
the 8Tx codebook.
SUMMARY OF THE INVENTION
In view of the problems existing in the prior art, the embodiments
of the present invention provide a method and an apparatus for
sub-sampling of a codebook in the LTE-A system.
According to an embodiment of the invention, it is provided a
method for sub-sampling of a codebook in a LTE-A system where a
precoding matrix W is a product of two matrices i and W2, i.e. W =
WiW 2, codebooks for W, i and W2 are denoted as C, Ci and C2
respectively, and r indicates a rank. The method comprises:
sub-sampling the codebook C such that the sub-sampled codebook
C has a size of equal to or less than 4 bits.
According to another embodiment of the invention, it is provided an
apparatus for sub-sampling of a codebook in a LTE-A system where a
precoding matrix W is a product of two matrices i and W2, i.e. W =
WiW 2, codebooks for W, i and W2 are denoted as C, Ci and C2
respectively, and r indicates a rank. The apparatus comprises:
sub-sampling means for sub-sampling the codebook C such that the
sub-sampled codebook C has a size of equal to or less than 4 bits.
BRIEF DESCRIPTION OF THE INVENTION
The other features, objects and advantages of this invention are
made more evident, when the following detailed description to the
non-limiting embodiments is read in conjunction with the accompanying
drawings.
Figure 1 illustrates a method for sub-sampling of a codebook in a
LTE-A system according to an embodiment of the invention.
Figure 2 illustrates an apparatus for sub-sampling of a codebook in
a LTE-A system according to an embodiment of the invention.
DETAILED DESCRIPTION
A method and an apparatus for sub-sampling of a codebook in the
LTE-A system is described in conjunction with drawings and with
reference to the embodiments.
Prior to introducing embodiments of the invention, let's have a
review to the 8Tx codebook C presented in Rl-10501 1 of LTE-A Rel-10,
in which the precoding matrix W is defined as a product of i and W2,
i.e., Codebooks for W, i and W2 are denoted as C, Ci and
C2 respectively. In other words, the 8Tx codebook C is comprised of two
constituent codebooks (namely, codebooks Ci and C2) .
A first Codebookl Ci of rank 1 codebook and rank 2 codebook may
be expressed as:
B = [b0 b ... b3 1 ] , m= 0,1,2,3, n= 0,1,• -,31
X( )
[^2imod32 ^(2i+l)mod32 ^(2i+2)mod32 ^(2i+3)mod32 ] • " " " '
(*) 0
=
0 x
Codebook 1: C ={ , ,W 2 ,•••, }
A second Codebook2 C of rank 1 codebook may be expressed as:
,k =0,1,2,3;/ = 0,1,2,3
Codebook 2: C2 ={ W2
(0) ,W2
(1) ,W2
(2) ,•••,W (15) }
where = , = J = ~ = j e i an 4x1 selection vector for
which the k-t element has a value of 1 and other elements are all zero.
A second codebook Codebook2 C2 of rank 2 codebook may be
expressed as:
Codebook 2: C ={ W (0) ,W (1) ,W (2) ,•••,W (1 }
where = , = J e* i an 4x1 selection vector for which the k-t
element has a value of 1 and other elements are all zero, and
(Y°, Y°) =(1 ) =(e ,e ) (Y ,Y2)=(e ,e ) (Y3,Y2
3)=(e ,e )
(Y1
4,Y2
4)=(e ,e ) (Y1
5,Y2
5)=(e ,e ) (Y ,Y2
6) =(el e4) (Y ,Y2
7)=(e ,e )
A first codebook Codebookl Ci of rank 3 codebook and rank 4
codebook presented in Rl-105011 may be expressed as:
B=[b b, - b = 0,1,2,3, n = 0,1,• --15
X { [b' 4 imodl6 "(4i+l)modl6 '(4i+7)modl6 k = 0,1,2,3}
X 0
1 = 0 x
Codebook 1: ={ ,W ",W ,W }
A second codebook Codebook2 C of rank 3 codebook may be
expressed as:
i e ]),(e ,[e e ]),(e ,[e e ]),(e ,[e e ]),
e ]),(e ,[e e ]),(e ,[e e ]),(e ,[e e ]),
e ],e ),([e e ],e ),([e e ],e ),([e e ],e ),
fl e e ],e ),([e e ],e ),([e e ],e ) J
A second codebook Codebook2 C2 of rank 4 codebook may be
expressed as:
Y { e ],[e e ],[e e ],[e e ] }
In the context of the application, e indicates a 4x1 selection
vector for which the -th element has a value of 1 and other elements
are all zero, unless otherwise indicated.
As to rank 5-8 codebooks presented in Rl-105011, no sub-sampling
is required in CSI reporting mode 1-1. Therefore, detailed description
for it is omitted here. As to rank 5-8 codebooks, please refer to
Rl-105011 for more information.
Payload sizes for rank 1-8 codebooks are summarizes as follows.
If the rank is 1 or 2, Ci has a size of 4 bits, and C2 has a size of 4
bits;
If the rank is 3, Ci has a size of 2 bits, and C2 has a size of 4 bits;
If the rank is 4, Ci has a size of 2 bits, and C2 has a size of 3 bits;
If the rank is 5-7, Ci has a size of 2 bits, and C2 has a size of 0 bit;
If the rank is 8, Ci has a size of 0 bit, and C2 has a size of 0 bit.
With regard to CSI reporting mode 1-1, only 4 bits may be used for
feeding back i and W2. Thus, in CSI reporting mode 1-1, rank 1-4
codebooks should be sub-sampled to 4 bits at most.
Figure 1 illustrates a method for sub-sampling of a codebook in the
LTE-A system according to an embodiment of the invention. In the
system, a precoding matrix W is a product of two matrices i and W2,
1.e. W = WiW 2, codebooks for W, i and W2 are denoted as C, Ci and
C2 respectively, and r indicates a rank. As shown in Figure 1, in step
101, a codebook C is sub-sampled such that the sub-sampled codebook
C has a size of equal to or less than 4 bits. Codebook C is a 8Tx
codebook prescribed in Rl-10501 1, for example. When the rank is 1 or
2, the codebook C has a size of 8 bits, i.e., it contains 256 codewords.
After the sub-sampling, 16 codewords evenly distributed in the
codebook C are extracted from the 256 codewords such that the
sub-sampled codebook C may represent PMI with 4 bits.
According to an embodiment of the invention, when r = 3 or 4, Ci
still has the size of 2 bits, but C2 is sub-sampled as:
(Y
i ' Y 2 ) {(e i ,[e 1,e ]),(e 2 ,[e 2 ,e ]),(e 3 ,[e 3 , e7 ]),(e 4 ,[e 4 ,e ])}
Y { es ],[e ,e6],[e3,e7],[e4,e8]} .
where indicates a 4x1 selection vector for which the k-t element
has a value of 1 and other elements are all zero. It can be seen that the
above sub-sampled rank 3 codebook and rank 4 codebook both have a
nesting property.
According to another embodiment of the invention, when r = 1 or 2,
Ci still has the size of 4 bits, but C2 is defined as:
the sub-sampled C isC 2 = for r= 1,
the sub-sampled C is = for r=2,
where the sub-sampled rank 1 codebook, rank 2 codebook as well as
the above sub-sampled rank 3 codebook and rank 4 codebook all have a
nesting property.
According to a further embodiment of the invention, when r = 1 or
2, Ci still has the size of 4 bits. For each codeword in Ci, only one
codeword in C2 is corresponding thereto. According to a preferred
embodiment of the invention, when r = 1, for the codeword 4k+l) ,
(A: =0,1,2,3, =0,1,2,3) in Ci, the codeword in C is extracted to
correspond thereto, where 0
= , =i =-\ and «3 = . When r =
2, for the codeword ( =0,i,-,7 , z=o,i) in Ci, the codeword
Ί in C is extracted to correspond thereto, where 0
= ,
=j .
According to another embodiment of the invention, when r = 1 or 2,
Ci and C are sub-sampled respectively. In particular, Ci is
sub-sampled to 3 bits, and C is sub-sampled to 1 bit. According to a
preferred embodiment of the invention, the sub-sampled C is
the sub-sampled C2 is C = for
r=l; and the sub-sampled C is C2= for r=2.
According to another embodiment of the invention, in the
sub-sampling, codewords evenly distributed in the codebook C are
extracted, where some or all of the codewords are in a form of discrete
Fourier transform (DFT) vector to be suitable for evenly linear arrays,
and the other codewords are suitable for cross-polarized linear arrays.
Figure 2 illustrates an apparatus 200 for sub-sampling of a
codebook in the LTE-A system according to an embodiment of the
invention. In the system, a precoding matrix W is a product of two
matrices and W2, i.e. W = WiW 2, codebooks for W, i and W2 are
denoted as C, Ci and C respectively, and r indicates a rank. As shown
in Figure 2, the apparatus 200 includes a sub-sampling means 201
adapted to sub-sample a codebook C such that the sub-sampled
codebook C has a size of equal to or less than 4 bits.
According to an embodiment of the invention, when r = 3 or 4, Ci
still has the size of 2 bits, and C is sub-sampled as:
for r = 3 Y - Y (3)
Yi' Y 2 ) {(ei,[e ,e ]),(e 2,[e2,e ]),(e 3,[e3,e7]),(e 4,[e4,e
Ye {[e ,e ],[e 2,e ],[e 3,e7],[e 4,e ]} .
where indicates a 4x1 selection vector for which the k-t element
has a value of 1 and other elements are all zero. It can be seen that the
above sub-sampled rank 3 codebook and rank 4 codebooks both have a
nesting property.
According to an embodiment of the invention, when r = 1 or 2, Ci
still has the size of 4 bits, but C2 is defined as:
the sub-sampled C is C. for r= 1,
the sub-sampled C is C2 = for r=2,
where the sub-sampled rank 1 codebook, rank 2 codebook as well as
the above sub-sampled rank 3 codebook and rank 4 codebook have a
nesting property.
According to an embodiment of the invention, when r = 1 or 2, Ci
still has the size of 4 bits. For each codeword in Ci, only one codeword
in C is corresponding thereto. According to a preferred embodiment of
the invention, when r = 1, fo the codeword 4k+l) ( = 0,1,2,3 1= 0,1,2,3)
the codeword in C is extracted to correspond thereto,
where «0 =l, = j , 2 = - l and 3 = - j . When r = 2, for the codeword
W 2i+ fc in C is
extracted to correspond thereto, where = 1, = j .
According to an embodiment of the invention, when r = 1 or 2, Ci
and C2 are sub-sampled respectively. In particular, Ci is sub-sampled to
3 bits, and C is sub-sampled to 1 bit. According to a preferred
embodiment of the invention, the sub-sampled C is
for
2.
According to another embodiment of the invention, in the
sub-sampling, codewords evenly distributed in the codebook C are
extracted, where some or all of the codewords are in a form of discrete
Fourier transform (DFT) vector to be suitable for evenly linear arrays,
and the other codewords are suitable for cross-polarized linear arrays.
Next, the method for sub-sampling of the codebook in the LTE-A
system according to embodiments of the invention is further described
in connection with the detailed embodiments of the invention.
Sub-sampling scheme for CSI mode 1-1 according to
embodiments of the invention
In the following, the sub-sampling schemes of rank 1-4 for CSI
mode 1-1 are presented according to embodiments of the invention.
1. Sub-sampling of rank 3 or 4 codebook
In order to achieve the nesting property of rank 3-7 codebooks,
sub-sampling of rank 3 or 4 codebooks are proposed as follows. Note
that since the rank 8 codebook corresponds to only one codeword, the
nesting property thereof is unnecessary to be considered.
rank 3 or 4 : the first codebook Ci is the same as the
above-mentioned original 2 bits codebook, while the second codebook
C2 is sub-sampled as:
when rank =
Codebook 2: C2
(Yi' Y 2 ) {(ei,[e1,e ]),(e 2,[e2,e ]),(e 3,[e3,e7]),(e 4,[e4,e ])}
when rank
Y Y
Codebook 2:
Y - Y (6)
Y {[e i ,e ],[e 2,e ],[e3,e7],[e4,e ]}
The sub-sampled rank 3 and rank 4 codebooks obtained in this
sub-sampling manner may have a nesting property together with the
corresponding sub-sampled rank 1 and rank 2 codebooks having
nesting property as well as rank 5-7 codebooks. This nesting property
is very important for precoding, since base stations may acquire
codeword information for various ranks according to the feedback
PMI.
2. Sub-sampling of rank 1 or 2 codebooks
As to sub-sampling of rank 1 or 2 codebooks, there may by a
number of solutions to extract 16 codewords evenly distributed in the
256 codewords of codebook C. However, the sampling according to
embodiments of the invention should obey the following principles:
(1) Due to the 4-bit limit in CSI mode 1-1, with sub-sampling there
are at most 16 4TxDFT beams for X in Wl design.
(2) Accordingly, the sub-sampling scheme may select from Ci 16 or
8 or 4 4TxDFT beams for X.
(3) Furthermore, W1*W2 may generate at least 4 8TxDFT vectors,
which is suitable for evenly linear arrays.
Next, some examples for sub-sampling rank 1 or 2 codebooks are
provided. For rank r, codeword m , n r represents a codeword with
rank r constructed by the multiplication of codeword with index m in
Ci and codeword with index n in C .
Sub-sampling scheme 1 for rank 1 or 2 codebooks
A 16-point 4-TxDFT for X is selected during design of Wi, and
four 8-TxDFT vectors are generated from i and W2. These 8-TxDFT
vectors are particularly suitable for evenly linear arrays, and the other
12 codewords in the sub-sampled codebook are suitable for
cross-polarized arrays. With this sub-sampling and the sub-sampling
for rank 3 or 4 as shown in equations (5) and (6), the sub-sampled
codebooks with ranks 1 to 7 have the nesting property.
In this scheme, the first Codebookl Ci is the same as original 4 bits
codebook, while the second Codebook2 C2 is defined as:
Rank 1: Codebook 2: C2 =
Rank 2 : Codebook 2: C2 =
Thereby, the sub-sampled codewords may be indexed as follows.
Rank l : (0,0) (l,0) (2,0) (3,0) (4,0) (5,0) (7,0) (8,0)
(9,0) (,, (ΐ ,, (12,0) (13,0) (14,0) (15,0),
Rank 2 : (0,0) , (1,0) , (2,0) , (3,0) , (4,0) , (5,0) , (6,0) 2 , (7,0) 2
(8.0) , (9,0) 2 , (10,0) 2 , (11,0)2 , (12,0) 2 , (13,0) , (14,0) , (15,0) .
Sub-sampling scheme 2 for rank 1 or 2 codebooks
A 16-point 4-TxDFT for X is selected during design of Wi, and
sixteen 8-TxDFT vectors are generated from i and W2.
The sub-sampled codewords may be indexed as follows.
Rank l : (0,0) (1,1), , (2,2)^ (3,3) (4,0) (5,1), , (6,2) (7,3) (8,0)
(9.1), , (10,2) (11,3) (12,0) (13,1), , ( 2) (15,3^
Rank 2 : (0,0) , (1,1) , (2,o) , (3,1) , (4,o) , (5,1) , (6,0) , (7,1) ,
(8,0) 2 , (9,1)2 , (10,0) 2 , (11,1)2 , (12,0) 2 , (13,1) , (14,0) , (15,l) .
Sub-sampling scheme 3 for rank 1 or 2 codebooks
A 8-point 4-TxDFT for X is selected during design of Wi, and four
8-TxDFT vectors are generated from i and W2.
The first codebook Codebook , = { W,(0) ,W,(2) ,W,(4) ,-, W,(14) } ;
Rank 1: Codebook 2: C2 =
Rank 2 : Codebook 2 : C2 =
Thereby the sub-sampled codewords may be indexed as follows.
Rank l : (0,0) (0,l) (2,o) (2,l) (4,o) (4,l) (6,0) (6,l) (8,0)
(8,1) (10,0) (10,1), , (12,0) (12,1), , (14,0)^ (14,1),
Rank 2 : (0,0) , (0,1) , (2,o) , (2,1) , (4,o) , (4,1) , (6,0) 2 , (6,l) 2 ,
(8,0) 2 , (8,1)2 , (10,0) 2 , (10,1)2 , (12,0) 2 , (12,1) , (14,0) , (14,l)
From the above description, it is apparent that the method and
apparatus for sub-sampling a codebook in the LTE-A system according
to embodiments of the invention is easily to be obtained from the
original codebooks prescribed in the protocol, and may have good
nesting property as well as high spatial resolution. Further, the method
and apparatus for sub-sampling a codebook in the LTE-A system
according to embodiments of the invention is suitable for precoding data
by LTE advanced base station and user equipment (UE).
The foregoing is described with respect to embodiments of the
invention, however, the present invention is not limited to any specific
methods or apparatuses. Various change or modification can be made by
the skilled in the art without departing from the scope of the appended
claims.
What is claimed is:
1. A method for sub-sampling of a codebook in a LTE-A system
where a precoding matrix Wis a product of two matrices i and W2, i.e.
W= WiW2, codebooks for W, i and W2 are denoted as C, Ci and C2
respectively, and r indicates a rank, the method comprising:
sub-sampling the codebook C such that the sub-sampled codebook
C has a size of equal to or less than 4 bits.
2. The method of claim 1, wherein when r = 3 or 4, Ci still has a
size of 2 bits, and C2 is sub-sampled as:
( Y Y 2 ) {(ei,[e ,e ]),(e 2,[e2,e ]),(e 3,[e3,e7]),(e 4,[e4,e
Y {[e ,e ],[e2,e ],[e3,e7],[e4,e ]} ;
wherein indicates a 4x1 selection vector for which the k-t
element has a value of 1 and the other elements are all zero, and the
sub-sampled rank 3 codebook and rank 4 codebook have a nesting
property.
3. The method of claim 1, wherein when r = 1 or 2, Ci still has the
size of 4 bits, and C4 is defined as:
the sub-sampled C2 i C2= r=l, and
the sub-sampled C2 i C = for r=2,
wherein the sub-sampled rank 1 codebook, rank 2 codebook and the
corresponding rank 3 code book and rank 4 codebook all have a nesting
property.
4. The method of claim 1, wherein when r = 1 or 2, CI still has the
size of 4 bits, and for each codeword in CI, only one codeword in C2
corresponds thereto.
5. The method of claim 4, wherein,
when r word w + in C1 =0,1,2,3, Z= 0,1,2,3,
a code in C is extracted to correspond thereto,
wherein = 1, = j , 2 =- 1 and = - j
6. The method of claim 4, wherein,
when r=2, for a codeword W + in Ci, =0,l,-,7 , Z= 0,l ,
a codeword in C is extracted to correspond thereto,
wherein 0 = , = j
7. The method of claim 1, wherein when r = 1 or 2, Ci and C are
sub-sampled respectively that Ci is sub-sampled to 3 bits, and C2 is
sub-sampled to 1 bit.
8. The method of claim 7, wherein
the sampled C is C2
the sampled C is C2 r r = 2 ,
9. The method of claim 1, wherein in the sub-sampling, codewords
evenly distributed in the codebook C are extracted, wherein some or all
of the codewords are in a form of discrete Fourier transform (DFT)
vector to be suitable for evenly linear arrays, and the other codewords
are suitable for cross-polarized linear arrays.
10. An apparatus for sub-sampling of a codebook in a LTE-A
system where a precoding matrix W is a product of two matrices and
W2, i.e. W = WiW 2, codebooks for W, i and W2 are denoted as C, Ci
and C respectively, and r indicates a rank, the apparatus comprising:
sub-sample means for sub-sampling the codebook C such that the
sub-sampled codebook C has a size of equal to or less than 4 bits.
11. The apparatus of claim l , wherein when r = 3 or 4, Ci still has
a size of 2 bits, and C2 is sub-sampled as:
Y e {[ei,e ],[e 2,e ],[e 3,e ],[e 4 ,e ]} ;
wherein indicates a 4x1 selection vector for which the k-t
element has a value of 1 and the other elements are all zero, and the
sub-sampled rank 3 codebook and rank 4 codebook have a nesting
property.
12. The apparatus of claim 10, wherein when r = 1 or 2, Ci still has
the size of 4 bits, and C4 is defined as:
the sub-sampled C2 i C2 = r = 1, and
the sub-sampled C2 i C = for r = 2,
wherein the sub-sampled rank 1 codebook, rank 2 codebook and the
corresponding rank 3 code book and rank 4 codebook have a nesting
property.
13. The apparatus of claim 10, wherein when r = 1 or 2, Ci still has
the size of 4 bits, and for each codeword in Ci, only one codeword in
C2 corresponds thereto.
14. The apparatus of claim 13, wherein
when r = 1, for a codeword w + in C1 =0,1,2,3, Z= 0,1,2,3,
a codeword in C is extracted to correspond thereto,
wherein a =1, =j , =- \ and 3=- j
15. The apparatus of claim 13, wherein
when r=2, for a codeword W + in Ci,
a codeword e e in C is extracted to correspond thereto,
fi - {
wherein = 1, 1=j
16. The apparatus of claim 10, wherein when r = 1 or 2, Ci and C
are sub-sampled respectively that Ci is sub-sampled to 3 bits, and C is
sub-sampled to 1 bit.
18. The apparatus of claim 10, wherein in the sub-sampling,
codewords evenly distributed in the codebook C are extracted, wherein
some or all of the codewords are in a form of discrete Fourier transform
vector to be suitable for evenly linear arrays, and the other codewords
are suitable for cross-polarized linear arrays.