WO2011162422A1

WO2011162422A1 - Transmitter and receiver, method thereof in wireless communication system

Info

Publication number: WO2011162422A1
Application number: PCT/KR2010/004014
Authority: WO
Inventors: Jianjun Li; Kyoungmin Park
Original assignee: Pantech Co., Ltd.
Priority date: 2010-06-21
Filing date: 2010-06-21
Publication date: 2011-12-29

Abstract

The present invention relates to precoding and feedback channel information in wireless communication system.

Description

TRANSMITTER AND RECEIVER, METHOD THEREOF IN WIRELESS COMMUNICATION SYSTEM

There are a number of multi-antenna transmission schemes or transmission such as transit diversity, closed-loop spatial multiplexing or open-loop spatial multiplexing. Closed-loop MIMO(CL-MIMO) relies on more extensive feedback from the mobile terminal.

In accordance with an aspect, there is providedA transmitter comprising; a layer mapper configured to map the data symbols to the layer, a precoder configured to precode the data symbols from the layer mapper by means of two matrices one of which has a block diagonalstructure and the other of which performs a rank adaptation and an antenna array configured to transmit the precoded signal and comprise two polarized groups each of which comprises two sets of antennas where one set is widely spaced with the other set.

In accordance with the other aspect, there is provided a method, the method comprising: method, comprising: mapping the data symbols to the layer, precoding the data symbols from the layer mapper by means of two matrices one of which has a block diagonal structure and the other of which performs a rank adaptation and transmitting a precoded signal by means of an antenna array which comprise two polarized groups each of which comprises two sets of antennas where one set is widely spaced with the other set.

In accordance with another aspect, there is provided a receiver comprising: an estimator configured to estimate a downlink channel from a received signal and feedback two precoding matrices index based on the estimated downlink channel and a post-decoderconfigured to decode the received signal to recover the set of data symbols by means of two matrices one of which has a block diagonal structure and the other of which performs a rank adaptation.

In accordance with another aspect, there is provided a method comprising: estimating a downlink channelfrom a received signal, feedbacking two precoding matrices based on the estimated downlink channel and decoding the received signal to recover the set of data symbols by means of two matrices one of which has a block diagonal structure and the other of which performs a rank adaptation.

FIG.1 is the block diagram of the wireless communication system using closed-loop spatial multiplexing according to one embodiment.

FIG.2 is the block diagram of the wireless communication system using closed-loop spatial multiplexing according to the other embodiment.

FIG 3 is the 8Tx cross-polarized antenna array configuration with equal space according to another embodiment.

FIG.4 is codebook design for the 8Tx antenna array configuration with equal space according to another embodiment.

FIG.5 is the 8Tx cross polarize antenna array configuration with unequal space according to further another embodiment.

FIG.6 is the codebook design for the 8Tx antenna array configuration with unequal space according to further another embodiment.

FIG. 7 is the codeword mapping for the new 8Tx antenna array configuration of the outer precoder FIG.6 is the codebook design for the 8Tx antenna array configuration with unequal space according to further another embodiment.

FIGs. 8 to 10 are the codeword mapping for the new 8Tx antenna array configurations of the outer precoder.

FIG.11 is the flowchart of the CL-MIMO system according to further another embodiment.

It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for purposesof promoting and improving clarity and understanding. Further, where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.

Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

Referring to FIG.1, the communication system may be any type of wireless communication system, including but not limited to a MIMO system, SDMA system, CDMA system, OFDMA system, OFDM system, etc. In the communication system, the wireless communication system 100 using closed-loop spatial multiplexing according to one embodiment comprises a transmitter 110 and a receiver 120. The transmitter 110 may act as a base station, while the receiver 120 may act as a subscriber station, which can be virtually any type of wireless one-way or two-way communication device such as a cellular telephone, wireless equipped computer system, and wireless personal digital assistant. Of course, the receiver/subscriber station 120 can also transmits signals which are received by the transmitter/base station 110. The signals communicated between the transmitter 110 and the receiver 120 can include voice, data, electronic mail, video, and other data, voice, and video signals.

In operation, the transmitter 110 transmits a signal data stream through one or more antennas and over a channel to a receiver 120, which combines the received signal from one or more receive antennas to reconstruct the transmitted data. To transmit the signal, the transmitter 110 prepares a transmission signal represented by the vector for the signal.

The transmitter 110 comprises a layer mapper 130 and a precoder 140.

The layer mapper 130 of the transmitter 110 maps one or two codewords, corresponding to one or two transport, to the layers N_Lwhich may range from a minimum ofone layer up to a maximum number of layers equal to the number of antenna ports.In case of multi-antenna transmission, there can be up to two transport blocks of dynamic size for each TTI(Transmission Time Interval), where each transport block corresponds to one codeword in case of downlink spatial multiplexing. In other words, the block of modulation symbols(one block per each transport block) refers to as a codeword. If there is only one codeword, we call it single codeword(SCW). Otherwise, we call it multiple codeword (MCW).

After layer mapping by the layer mapper 130, a set of N_Lsymbols(one symbol from each layer) is linearly combined and mapped to the N_A antenna port by the precoder 140. This combining/mapping can be described by means of a precoding matrix W of size N_L× N_A.

The precoder 140 has it own codebook, which is accessed to obtain a transmission profile and/or precoding information to be used to process the input data signal to make best use of the existing channel conditions for individual receiver stations. In addition, the receiver 120includes the same codebook for use in efficiently transferring information in either the feedback or feedforward channel, as described herein below.

In various embodiments, the codebook is constructed as a composite product codebook from separable sections, where the codebook index may be used to access the different sections of the codebook. For example, one or more predetermined bits from the codebook index are allocated for accessing the first level matrix, while a second set of predetermined bits from the second level index is allocated to indicate the values for the second level matrix.

In various embodiments, instead of having a single codebook at each of the transmitter 110 and the receiver 120, separate codebooks can be stored so that there is, for example, a codebook for the first level precoding matrix W1, a codebook for the second level matrix W2. In such a case, separate indices may be generated wherein each index points to a codeword in its corresponding codebook, and each of these indices may be transmitted over a feedback channel to the transmitter, so that the transmitter uses these indices to access the corresponding codewords from the corresponding codebooks and determine a transmission profile or precoding information.

To assist the base station in selecting a suitable precoding matrix for transmission by the transmitter 110, the receiver/mobile terminal 120 may report channel information such as a recommended number of layers(expressed as a Rank Indication, RI) or a recommended precoding matrix(Precoding Matrix Index, PMI) corresponding to that number of layers, depending on estimates of the downlink channel conditions.

The receiver 120 may comprise a channel estimator 150 and a post-decoder 160.

The receiver 120 estimates the channel by the channel estimator 150. The transmitter 110 receives PMI feedback for the first level precoding by long term and PMI feedback for the second level precoding by short term. The transmitter 110 precodes the set of symbols by means of the precoding matrix W=W₁W₂ based on the two feedback PMIs as shown in FIG.1.

The receiver 120 recovers the original data symbols by post-decoder 160 with the previous feedback precoding matrices combination. The post-decoder 160 processes the received signal and decodes the precoded symbols.

Referring to FIG.2, in the communication system, the wireless communication system 200 using closed-loop spatial multiplexing according to one embodiment comprises a transmitter 210 and a receiver 220. The transmitter 210 comprises a layer mapper 230 and a precoder 240, an antenna array 246.

The layer mapper 230 is configured to map the data symbols to the layer. After layer mapping by the layer mapper 230, a set of N_L symbols(one symbol from each layer) is linearly combined and mapped to the N_A antenna port by the precoder 240.

The precoder 240is configured to precode the data symbols from the layer mapper by means of two matrices. The precoder 240 comprises two

level precoders

242 and 244 to optimize the performance.

A precoder 240 for a subband performs a function of two matrices W₁ and W₂ and each of these two matrices belongs to separate codebooks C₁ and C₂ respectively.

The antenna array 246 is configured to transmit the precoded signal and comprise two polarized sets. The antenna array 246 may comprise at least two antennas, for example, eight antenna ports which are dually polarized to make two polarized sets. Dual polarized antennas arethe most likely antenna setup for this large number of transmit antennas.

It is well known that "beams" can be achieved for closely spaced antenna arrays by using DFT vectors.

One solution may be the 8Tx antenna configuration of 4 columns, cross-polarized on each column, closely-spaced: X X X X. So block-diagonal with 4-element beamformers taken from 16 DFT based precoders is considered for the first precoder as optimal solution.

Referring to FIG 3, the antenna array configuration for codebook design is closely-spaced (e.g. d=0.5λ to 0.7λ) X-polarized (X-pol) eight antennas. This configuration allows the total antenna width to be reduced by a factor 2 compared to a co-polarized uniform linear array(ULA) with the same inter-element spacing. The / and \polarizations are well decorrelated, which is beneficial for SU-MIMO(Single-User MIMO), while the identically polarized antennas allow directive beams to be formed, which is beneficial for cell-edge performance and MU-MIMO(Multiple-User MIMO). This configuration provides good performance in a compact form factor. Therefore, closely-spaced X-pol antennas are expected to be widely deployed in the wireless communication system such as LTE-Advanced.

Referring FIG.2 again, the overall precoder 240 is thus formed as W=W⁽¹⁾W⁽²⁾ and it is now easy to see HW=HW⁽¹⁾W⁽²⁾=(HW⁽¹⁾)W⁽²⁾=H_effW⁽²⁾.

The first, inner,

precoder matrix W⁽¹⁾ serves to create a new effective and improved

channel matrix H_eff for the outer

precoder W⁽²⁾ to work on. N_T and N_Rare the number of tx and rx antenna ports.

is the number of beams by W⁽¹⁾ or the number of virtual antennas. r is the rank.

In other words, the first precoder 242 may precode a set of symbols from the layer mapper 230 by means of a precoding matrix W⁽¹⁾of size

_. The second precoder 244 may also precode a set of symbols from the first precoder 242 by means of a precoding matrix W⁽²⁾ of size

. As a result, the first and the

second precoder

242 and 244 precode a set of symbols by means of the matrix W=W⁽¹⁾W⁽²⁾.

Referring to FIG.4, in case of an antenna array 246 of closely spaced cross-poles, the antennas can then be divided into two

sets

247 and 248 based on polarization and the corresponding channels are denoted H_/ and H_＼, respectively. Since the correlation is high within each of the

antenna groups

247 and 248, it makes sense to use a grid of beam codebook implemented from 1 by 4 DFT based precoder vectors. This structure is well-suited for efficiently supporting common antenna setups such as closely spaced cross-poles.

This configuration has some disadvantage which is pointed out by some operators.

Now it may be more important to consider the 8Tx antenna array configuration of 4 columns, cross-polarized on each column, 2 widely-spaced sets of closely-spaced columns: X X X X. In this case, the correlation property is different with the previous configuration. The codebook for the previous antenna configuration will not be optimal anymore. So we need consider a new optimized codebook design method for the new antenna configuration to improve the system performance. Now it maybe more important to consider the 8Tx antenna configuration of 4 columns, cross-polarized on each column, 2 widely-spaced sets of closely-spaced columns: X X X X.

In this invention, we proposesan optimized codebook design method for the new antenna configuration. Both beam and codeword to beam design are considered for optimization.

FIG.5 is the 8Tx cross polarized antenna array configuration with unequal space according to further another embodiment. FIG.6 is the codebook design for the 8Tx antenna array configuration with unequal space according to further another embodiment.

Since two closely-spaced X-pol antennas will not be problematic in higher frequencies, 2widely-spaced sets(e.g. d_L=4~10λ) of closely-spaced columns, i.e. X X X X appears to be a useful configuration to consider in addition to those prioritized in each layer.

The number of virtual antennas

is much smaller than the number of antenna ports N _T, offering a considerable dimension reduction and thus requiring a smaller codebook for the outer precoder 242.

This structure is well-suited for efficiently supporting common antenna array configuration. In the new widely-spaced sets of closely-spaced columns, the 8 Tx antennas are divided into 4 groups as shown in FIG.6. Inside each group, the two antennas have high correlation. Different groups have either wide space or different polarization direction, so that the correlation is low among different groups. Therefore the corresponding channels are denoted H_/,1, H_/,2,H_＼,1and H_＼,2 respectively. It is well known that "beams" can achieve good performance for closely spaced antenna array 246 by using DFT vectors.

The codebook for the inner precoder 244 is separately beamforming for each group. For each group, it uses a grid of beam codebook implemented from 1 by 2DFT based precoder vectors. So that there are total 4 beams in the inner precoder 244. The proposed inner precoder W⁽¹⁾ has a block diagonal structure as follows:

[Formula 1]

Where

is a kind of matrix which consists of a part of the inner precoder W⁽¹⁾, in more detail, it's DFT precoder vector and "O" is a kind of zero matrix where all elements of matrix are 0.

The product of the MIMO channel and the overall precoder can then be written as

[Formula 2]

As shown in Formula 2,

separately precodes each group of co-polarized and closely spaced antennas forming a smaller and improved effective channel H_eff as shown in Formula 2. If

corresponds to a beamforming vector, the effective channel would be reduced to 4 virtual antennas, which reduces the needed size of the codebook used for the outer precoder W⁽²⁾ when tracking the instantaneous channel properties.

There are several examples on inner and outer codebook.

[Table 1]

FIG. 7 is the codeword mapping for the new 8Tx antenna array configuration of the outer precoder according to further another embodiment.

Referring to 7, the out precoder 242 may use 3GPP LTE(rel.8) codebook. For rank 3 outer precoder codebook, we can also consider the codeword to beam mapping algorithm as shown in Table 2.The proposed scheme is to make each codeword the maximum beamforming gain and the diversity gain. FIG.7shows this codeword to beam mapping that match our requirement.

In the proposed mapping, the first codeword is mapped to the first layer that cover 2 groups or

beams

246a and 246d, these 2 beams have different polarized direction and wider space. The second codeword is mapped to the 2 layers. These 2 layers are mapped to

different beams

246b and 246c with different polarized direction and wider space.

[Table 2]

Referring to FIG.8, the first codeword is mapped to the 2 layers as shown in Table 3. These 2 layers are mapped to

different beams

246b and 246c with different polarized direction and wider space seperately. The second codeword is mappedto the first layer that cover 2

beams

246a and 246d, these 2 beams have different polarize direction and wider space.

[Table 3]

Referring to FIGs.9 and 10, Table 4 and 5, the first codeword is mapped to the 2 layers. These 2 layers are mapped to different beams with same polarized direction and wider space. The second codeword is mapped to the first layer that cover 2 beams246c and 246d/246a and 246b, these 2 beams have same polarize direction and wider space.

[Table 4]

[Table 5]

In various embodiments, separate codebooks of the transmitter 210 and the receiver 220 may be stored so that there are, for example, a codebook for the first level precoding matrix W⁽¹⁾ and a codebook for the second level matrix W⁽²⁾

. In such a case, separate indices may be generated wherein each index pointsto a codeword in its corresponding codebook, and each of these indices may be transmitted over a feedback channel to the transmitter, so that the transmitter uses these indices to access the corresponding codewords from the corresponding codebooks and determine a transmission profile or precoding information.

The receiver 220 may comprise a channel estimator 250 and a post-decoder 260.

The channel estimator 250 of the receiver 220 estimates the downlink channel condition. The channel estimator 250 feedbacks at least one of RI and PMI to the transmitter 210. The channel estimator 250 may perform many kinds of codebook based PMI feedback.

The receiver 220 estimates the channel by the channel estimator 250. Based on the estimated channel information, then the receiver 220 selects the precoding matrix for each level from the corresponding codebooks. Once the precoding matrix for each level is decided, the receiver/mobile terminal 220 separately feedback the PMIs of both level to the transmitter 210.

There may be codebook based PMI feedback where the receiver/mobile terminal 220 feedbacks the precoding matrix index(PMI) of the favorite matrix in the codebook to the transmitter/base station 210 to support CL-MIMO(closed MIMO) operationin wireless communication system.

The feedback frequency of the receiver 220 is different for different level precoding. The first level precoding is for the rank adaptation. The second level precoding may for the channel adaptation. The first level precoding is by long term feedback and the second one is by short term feedback. So multi level precoding may reduce the feedback overhead.

The transmitter 210 receives PMI feedback for the first level precoding by long term and PMI feedback for the second level precoding by short term. In the other embodiment as shown in FIG.2, the transmitter 210 precodes the set of data symbols by means of the two

level precoders

242 and 244 based on the two feedback PMIs. For example, the first precoder 242 and the second precoder 244 in turn precodes the set of data symbols by means of each of matrices W⁽¹⁾ and W⁽²⁾ based on the long and the short term feedback PMIs.

Then the transmitter 210 transmits the precoded data symbols by different antennas.

The receiver 220 recovers the original data symbols by post-decoder 260 with the previous feedback precoding matrices combination. The post-decoder 260 processes the received signal and decodes the precoded symbols.

Referring to FIG. 11, in the multilevel precoding CL-MIMO system, there may estimate the channel at S810 after receiving Reference signal at S805.

Based on the estimated channel information such as a short term channel information, then there may select the precoding matrix for the inner precoder 244 from the corresponding codebook W⁽¹⁾ and the precoding matrix for the outer precoder 242 from the corresponding codebook W⁽²⁾at S820.

In various embodiments, separate codebooks of the transmitter 210 and the receiver 220 may be stored so that there are, for example, a codebook for the first level precoding matrix W⁽¹⁾ and a codebook for the second level matrix W⁽²⁾. In such a case, separate indices may be generated wherein each index points to a codeword in its corresponding codebook, and each of these indices may be transmitted over a feedback channel to the transmitter, so that the transmitter uses these indices to access the corresponding codewords from the corresponding codebooks and determine a transmission profile or precoding information.

There may feedback the first PMI(Precoding Matrix Indication) for the short term precoding matrix to the inner precoder 244 at S830. The PMI is the index of the selected precodingmatrix in the corresponding codebook.

There may feedback the second PMI(Precoding Matrix Indication) for the long term precoding matrix from the codebook to the outer precoder 242 of the transmitter at S840.

As a result, the receiver/mobile terminal 220 separately feedbacks the PMIs of both level to the transmitter at S830 and S840.

The transmitter 240 receives PMI feedback for the first level precoding by the short term and PMI feedback for the second level precoding by the long term.

The transmitter 240 maps the data symbols to the layer at S850.

After layer mapping at S850, a set of N_L symbols(one symbol from each layer) is linearly combined and mapped to the N_A antenna port.

The transmitter 240 precodes the data symbols from the layer mapper by means of two matrices W₁ and W₂ at S860.

There may use the first PMI to find the precoding matrix for the inner precoder 244. There may also use the second PMI to find the precoding matrix for the outer precoder 242. As mentioned above, the first PMI is the index of the selected precoding matrix from the corresponding codebook as shown in Table 1. The second PMI is the index of the selected precoding matrix from corresponding codebook as shown in Tables 2 to 5.

If the input date symbol is

, the output signal of the transmitter is W⁽¹⁾W⁽²⁾

where W⁽¹⁾ is the precoding matrix for the inner precoder and W⁽²⁾ is the precoding matrix for the outer precoder.

At S870 the transmitter 240 may transmit the precoded data symbols by the 8Tx antenna configuration of 4 columns, cross-polarized on each column, closely-spaced: X X X Xas shown in FIG. 3 and FIG.4. Also the transmitter may transmit the precoded data symbols by the 8Tx antenna array configuration of 4 columns, cross-polarized on each column, 2 widely-spaced sets of closely-spaced columns: X X X X as shown in FIG.5 to 10.

The receiver 220 recovers the original data symbols by decoding the received signal through the downlink channel with the previous feedback precoding matrices combination at S880.

The methods and systems as shown and described herein may be implemented in software stored on a computer-readable medium and executed as a computer program on a general purpose or special purpose computer to perform certain tasks. For a hardware implementation, the elements used to perform various signal processing steps at the transmitter(e.g., coding and modulating the data, precoding the modulated signals, preconditioning the precoded signals, and so on) and/or at the receiver(e.g., recovering the transmitted signals, demodulating and decoding the recovered signals, and so on) may be implemented within one or more application specific integrated circuits(ASICs), digital signal processors(DSPs), digital signal processing devices (DSPDs), programmable logic devices(PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. In addition or in the alternative, a software implementation may be used, whereby some or all of the signal processing steps at each of the transmitter and receiver may be implemented with modules(e.g., procedures, functions, and so on) that perform the functions described herein. It will be appreciated that the separation of functionality into modules is for illustrative purposes, and alternative embodiments may merge the functionality of multiple software modules into a single module or may impose an alternate decomposition of functionality of modules. In any software implementation, the software code may be executed by a processor or controller, with the code and any underlying or processed data being stored in any machine-readable or computer-readable storage medium, such as an on-board or external memory unit.

Although the described exemplary embodiments disclosed herein are directed to various MIMO precoding systems and methods for using same, the present invention is not necessarily limited to the example embodiments illustrate herein. For example, various embodiments of a MIMO precoding system and design methodology disclosed herein may be implemented in connection with various proprietary or wireless communication standards, such as IEEE 802.16e, 3GPP-LTE, DVB and other multi-user MIMO systems. Thus, the particular embodiments disclosed above are illustrative only and should not be taken as limitations upon the present invention, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Accordingly, the foregoing description is not intended to limit the invention to the particular form set forth, but on the contrary, is intended to cover such alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims so that those skilled in the art should understand that they can make various changes, substitutions and alterations without departing from the spirit and scope of the invention in its broadest form.

Benefits, other advantages, and solutionsto problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

Claims

A layer mapper configured to map the data symbols to the layer;

A precoder configured to precode the data symbols from the layer mapper by means of two matrices one of which has a block diagonal structure and the other of which performs a rank adaptation; and

An antenna array configured to transmit the precoded signal and comprise two polarized groups each of which comprises two sets of antennas where one set is widely spaced with the other set.
The transmitter in claim 1, where each of two sets comprises at leas one antenna.
The transmitter in claim 1, where the precoder comprises the outer precoder which precodes the data symbols by means of the other precoding matrix and the inner precoder which precodes the output from the outer precoder by means of one precoding matrix.
The transmitter in claim 2, where the number of the layer is three and the other precoding matrix performs the rank adaptation where one layer covers 2 beams which have different polarized direction and wider space and another two layer cover different beams with different polarizeddirection and wider space.
The transmitter in claim 3, where one precoding matrix is as follows:

Where
is a kind of matrix which consists of a part of the inner precoder W⁽¹⁾and "O" is a kind of zero matrix where all elements of matrix are 0.
The transmitter in claim 3, where the other precoding matrix is one of
or
, ,
where α∈{1, -1, j,-j}.
The transmitter in claim 5, where one precoding matrix is the DFT based vector.
The transmitter in claim 5, where the other precoding matrix is the LTE precoding matrix.
The transmitter in claim 1, where the other precoding matrix performs a rank adaptation for one of rank 3 or 4 which three or four data symbols are transmitted by the antenna array at the same time.
The transmitter in claim 1, where each of one precoding matrix and the other precoding matrix is selected from one of the separated codebooks based on each of two precoding matrix indications feeded back from the receiver.
The transmitter in claim 10, where two precoding matrix indications are feeded back from the receiver with two periods different from each other.
The transmitter in claim 1, where the transmitter is one of a base station or a terminal.
A method, comprising:

mapping the data symbols to the layer

precodingthe data symbols from the layer mapper by means of two matrices one of which has a block diagonal structure and the other of which performs a rank adaptation; and

transmitting a precoded signal by means of an antenna array which comprisestwo polarized groups each of which comprises two sets of antennas where one set is widely spaced with the other set.
The method in claim 13, where each of two sets comprises at leas one antenna.
The transmitter in claim 13, where the precoding precodes the data symbols by means of the other precoding matrix and one precoding matrix.
The method in claim 14, where the number of the layer is three and the other precoding matrix performs the rank adaptation where one layer covers 2 beams which have different polarized direction and wider space and another two layers cover different beam with different polarized direction and wider space.
The method in claim 15, where one precoding matrix is as follows:

Where
is a kind of matrix which consists of a part of the inner precoder W⁽¹⁾ and "O" is a kind of zero matrix where all elements of matrix are 0.
The method in claim 15, where the other precoding matrix is one of
or
, ,
where α∈{1, -1, j,-j}.
The method in claim 17, where one precoding matrix is the DFT based vector.
The method in claim 17, where the other precoding matrix is the LTE precoding matrix.
The method in claim 13, where the other precoding matrix performs a rank adaptation for one of rank 3 or 4 which three or four data symbols are transmitted by the antenna array at the same time.
The method in claim 13, where each of one precoding matrix and the other precoding matrix is selected from one of the separated codebooks based on each of two precoding matrix indications feeded back from the receiver.
The method in claim 13, where two precoding matrix indications are feeded back from the receiver with two periods different from each other.
The method in claim 13, where the transmitter is one of a base station or a terminal.
A receiver comprising:

an estimator configured to estimate a downlink channel from a received signal and feedback two precoding matricesbased on the estimated downlink channel; and

a post-decoder configured to decode the received signal to recover the set of data symbolsby means of two matrices one of which has a block diagonal structure and the other of which performs a rank adaptation.
The receiver in claim 25, where one precoding matrix is as follows:

Where
is a kind of matrix which consists of a part of the inner precoder W⁽¹⁾ and "O" is a kind of zero matrix where all elements of matrix are 0.
The receiver in claim 25, where the other precoding matrix is one of
or
, ,
where α∈{1, -1, j,-j}.
A method comprising:

estimating a downlink channel from a received signal;

feedbacking two precoding matrices index based on the estimated downlink channel; and

decoding the received signal to recover the set of data symbols by means of two matrices one of which has a block diagonal structure and the other of which performs a rank adaptation.
The method in claim 28, where one precoding matrix is as follows:

Where
is a kind of matrix which consists of a part of the inner precoder W⁽¹⁾ and "O" is a kind of zero matrix where all elements of matrix are 0.
The method in claim 28, where the other precoding matrix is one of
or
, ,
where α∈{1, -1, j,-j}.