Method and apparatus for generating and transmitting channel feedback in mobile communication system employing two dimensional antenna array

Abstract

The present disclosure relates to a communication method and system for converging a 5.sup.th-Generation (5G) communication system for supporting higher data rates beyond a 4.sup.th-Generation (4G) system with a technology for Internet of Things (IoT). The present disclosure may be applied to intelligent services based on the 5G communication technology and the IoT-related technology, such as smart home, smart building, smart city, smart car, connected car, health care, digital education, smart retail, security and safety services. Provided are a method and user equipment for sending feedback information to a base station. The method includes receiving a Channel Status Indication Reference Signal (CSI-RS) from the base station; generating feedback information on a basis of the received CSI-RS; and transmitting the generated feedback information to the base station, wherein generating feedback information includes selecting a precoding matrix for each antenna port group of the base station and selecting an additional precoding matrix on a basis of a relationship between the antenna port groups of the base station.

Claims

1. A method of a user equipment to send feedback information to a base station, the method comprising: receiving a channel status indication reference signal (CSI-RS) from a base station; selecting a precoding matrix for each antenna port group of the base station; determining an additional precoding matrix to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes selected respectively for the antenna port groups and the additional precoding matrix; generating feedback information comprising the selected precoding matrix and the additional precoding matrix, on a basis of the received CSI-RS; and transmitting the generated feedback information to the base station.

2. The method of claim 1, wherein transmitting the generated feedback information comprises transmitting the additional precoding matrix via the second channel.

3. A method of a user equipment to send feedback information to a base station, the method comprising: receiving a channel status indication reference signal (CSI-RS) from a base station; selecting a precoding matrix for all antenna port groups of the base station; determining an additional precoding matrix to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes for all of the antenna port groups and the additional precoding matrix; generating feedback information comprising the selected precoding matrix and the additional precoding matrix, on a basis of the received CSI-RS; and transmitting the generated feedback information to the base station.

4. The method of claim 3, wherein transmitting the generated feedback information comprises transmitting the additional precoding matrix via the second channel.

5. The method of claim 1, wherein the precoding matrixes and the additional precoding matrix comprise a first index indicating candidate beamforming vectors selectable for a current channel between the base station and the user equipment, and a second index for selecting a beamforming vector to be used.

6. A method of a user equipment to send feedback information to a base station, the method comprising: receiving feedback configuration information from the base station; receiving a channel status indication reference signal (CSI-RS) from the base station; selecting a precoding matrix for each antenna port group of the base station; determining an additional precoding matrix to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes selected respectively for the antenna port groups and the additional precoding matrix; generating feedback information comprising the selected precoding matrix and the additional precoding matrix, on a basis of the received feedback configuration information and the CSI-RS; and transmitting the generated feedback information to the base station, wherein receiving feedback configuration information comprises receiving feedback configuration information corresponding to antenna port groups of the base station and receiving additional feedback configuration information based on a relationship between the antenna port groups.

7. A method of a user equipment to send feedback information to a base station, the method comprising: transmitting first feedback information generated based on a first channel status indication reference signal (CSI-RS) from the base station; receiving a second CSI-RS beamformed on a basis of the first feedback information from the base station; generating second feedback information on a basis of the received second CSI-RS, to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the first feedback information and the second feedback information; and transmitting the second feedback information to the base station.

8. A method of a base station to receive feedback information from a user equipment, the method comprising: transmitting feedback configuration information to a user equipment (UE); transmitting a channel status indication reference signal (CSI-RS) to the UE; and receiving feedback information generated based on the feedback configuration information and the CSI-RS from the user equipment, wherein the feedback information is generated, by the UE, to comprise a precoding matrix and an additional precoding matrix, the precoding matrix is selected for each antenna port group of the base station, and the additional precoding matrix is determined to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes selected respectively for the antenna port groups and the additional precoding matrix.

9. A method of a base station to receive feedback information from a user equipment (UE), the method comprising: receiving first feedback information from the user equipment; transmitting a channel status indication reference signal (CSI-RS) beamformed on the basis of the first feedback information to the user equipment; and receiving second feedback information generated based on the CSI-RS from the UE, wherein the second feedback information is generated, by the UE, to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the first feedback information and the second feedback information.

10. A user equipment capable of sending feedback information to a base station, comprising: a communicator configured to transmit and receive signals; and a controller configured to control the communicator to receive a channel status indication reference signal (CSI-RS) from a base station, select a precoding matrix for each antenna port group of the base station, determine an additional precoding matrix to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes selected respectively for the antenna port groups and the additional precoding matrix, generate feedback information comprising the selected precoding matrix and the additional precoding matrix, on a basis of the received CSI-RS, and control the communicator to transmit the generated feedback information to the base station.

11. The user equipment of claim 10, wherein the controller is further configured to control the communicator to transmit the additional precoding matrix via the second channel.

12. A user equipment capable of sending feedback information to a base station, comprising: a communicator configured to send and receive signals to and from the base station; and a controller configured to control the communicator to receive a channel status indication reference signal (CSI-RS) from the base station, select a precoding matrix for all antenna port groups of the base station, determine an additional precoding matrix to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes for all of the antenna port groups and the additional precoding matrix, generate feedback information comprising the selected precoding matrix and the additional precoding matrix, on a basis of the received CSI-RS, and control the communicator to transmit the generated feedback information to the base station.

13. The user equipment of claim 12, wherein the controller further configured to control the communicator to transmit the additional precoding matrix via the second channel.

14. The user equipment of claim 10, wherein the precoding matrixes and the additional precoding matrix comprise a first index indicating candidate beamforming vectors selectable for a current channel between the base station and the user equipment, and a second index for selecting a beamforming vector to be used.

15. A user equipment capable of sending feedback information to a base station, comprising: a communicator configured to send and receive signals to and from the base station; and a controller configured to control the communicator to receive feedback configuration information from the base station, receive a channel status indication reference signal (CSI-RS) from the base station, select a precoding matrix for each antenna port group of the base station, determine an additional precoding matrix to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes selected respectively for the antenna port groups and the additional precoding matrix, generate feedback information comprising the selected precoding matrix and the additional precoding matrix, on a basis of the received feedback configuration information and CSI-RS, and control the communicator to transmit the generated feedback information to the base station.

16. A user equipment capable of sending feedback information to a base station, comprising: a communicator configured to send and receive signals to and from the base station; and a controller configured to control the communicator to transmit first feedback information generated based on a first CSI-RS from the base station, receive a second CSI-RS beamformed on a basis of the first feedback information from the base station, generate second feedback information on a basis of the received second CSI-RS, to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the first feedback information and the second feedback information, and control the communicator to transmit the second feedback information to the base station.

17. A base station capable of receiving feedback information from a user equipment (UE), comprising: a communicator configured to send and receive signals to and from the user equipment; and a controller configure to control the communicator to transmit feedback configuration information to the user equipment, transmit a channel status indication reference signal (CSI-RS) to the user equipment, and receive feedback information generated based on the feedback configuration information and the CSI-RS from the user equipment, wherein the feedback information is generated, by the UE, to comprise a precoding matrix and an additional precoding matrix, the precoding matrix is selected for each antenna port group of the base station, and the additional precoding matrix is determined to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the precoding matrixes selected respectively for the antenna port groups and the additional precoding matrix.

18. A base station capable of receiving feedback information from a user equipment (UE), comprising: a communicator configured to send and receive signals to and from the user equipment; and a controller configure to control the communicator to receive first feedback information from the user equipment, transmit a channel status indication reference signal (CSI-RS) beamformed on a basis of the first feedback information to the user equipment, and receive second feedback information generated based on the CSI-RS from the user equipment, wherein the second feedback information is generated, by the UE, to maximize a signal-to-noise ratio (SNR) of a signal, transmitted using a first channel which is based on a second channel corresponding to the first feedback information and the second feedback information.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, features, and advantages of the present invention will be more apparent from the following detailed description, taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 illustrates a communication system to which the present invention is applied;

(3) FIG. 2 illustrates a radio resource with one subframe and one RB serving as a minimum unit for downlink scheduling in an LTE/LTE-A system;

(4) FIGS. 3 to 6 illustrate feedback timings in an LTE/LTE-A system;

(5) FIG. 7 Illustrates CSI-RS transmission according to an embodiment of the present invention;

(6) FIG. 8 illustrates RI, PMI and CQI transmission by a UE for two CSI-RSs;

(7) FIG. 9 is a flowchart depicting a sequence of operations performed by a UE according to an embodiment of the present invention;

(8) FIG. 10 is a flowchart depicting a sequence of operations performed by an eNB according to an embodiment of the present invention;

(9) FIG. 11 is a block diagram of a UE according to an embodiment of the present invention; and

(10) FIG. 12 is a block diagram of an eNB according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

(11) Hereinafter, embodiments of the present invention are described in detail with reference to the accompanying drawings. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. The terms and words used in the following description and claims are not limited to their dictionary meanings and should be construed in accordance with the scope and spirit of the present invention.

(12) The description of embodiments of the present invention is focused on an OFDM-based radio communication system compliant with the 3GPP Evolved Universal Terrestrial Radio Access (EUTRA) standard in particular. However, it should be understood by those skilled in the art that the subject matter of the present invention or variations thereof is applicable to other communication systems having similar technical backgrounds and channel structures without significant modifications departing from the scope and spirit of the present invention.

(13) When an eNB uses a large number of transmit antennas to perform CSI-RS transmission, as in the case of FD-MIMO, to reduce the amount of feedback from a UE, it is possible to divide N antenna ports into G groups for CSI-RS transmission. For example, when transmit antennas of the eNB are arranged in two dimensions as shown in FIG. 1, the eNB may send antenna ports corresponding to each column to the UE by use of separate CSI-RS resources. In this case, the eNB may present the channel of each antenna to the UE by the use of G CSI-RSs.

(14) The MIMO scheme of the present invention, as an advanced version of existing LTE/LTE-A MIMO with 8 transmit antennas, can be applied to a case where 16 or more transmit antennas are used, in particular, to a case where the transmit antennas of the base station are configured as a two dimensional antenna array.

(15) In one embodiment of the present invention, each column in the two dimensional antenna array is operated for one CSI-RS. To make the principle of the present invention applicable, it is not required to divide the antenna ports on a column basis. However, in the following description, it is assumed that the antenna ports are divided on a column basis for CSI-RS operation for ease of description. The antenna ports may also be divided on a row basis or independently of columns or rows to form antenna port groups.

(16) FIG. 7 Illustrates CSI-RS transmission for a two dimensional (2D) antenna array according to an embodiment of the present invention.

(17) In one embodiment of the present invention, an eNB operating a 2D antenna array Includes 64 total antenna ports (e.g. N=64). Among the 64 antenna ports, 32 antenna ports (e.g., A0, . . . , A7, B0, . . . , B7, C0, . . . , C7, D0, . . . , D7) are arranged to form an angle of −45° or 0° with the positive direction of the x-axis, and the remaining 32 antenna ports (e.g., E0, . . . , E7, F0, . . . , F7, G0, . . . , G7, H0, . . . , H7) are arranged to form an angle of +45′ or 90° with the positive direction of the x-axis. The antenna configuration in which every two antenna elements located at the same position make an angle of 90° as above is referred to as a cross Polarization (or XPOL) configuration. The XPOL configuration may be used to obtain a large antenna gain by placing multiple antennas in a small space. FIG. 7 discloses another embodiment of the present Invention, i.e., each CSI-RS resource has a form of a square, as opposed to a form of a bar in the above embodiment.

(18) Unlike the above case, an eNB operating a 2D antenna array may include total 32 antenna ports (e.g. N=32). In this case, 32 antenna ports (e.g., A0, . . . , A7, B0, . . . , B7, C0, . . . , C7, D0, . . . , D7) may be arranged to form an angle of −45° or 0° with the positive direction of the x-axis. The antenna configuration in which all antenna elements are arranged to make the same angle as above is referred to as a Co-Polarization (or Co-POL) configuration.

(19) In the case of Co-Pol, as all antenna ports have the same orientation, when N.sub.RI denotes the number of receive antennas at a UE, N antenna ports are described by a channel matrix H.sub.1 having a size of N.sub.Rx×32 for one antenna group and the UE. In the case of XPOL, as the first antenna group with N/2 members and the second antenna group with N/2 members are arranged at the same location, the radio channels formed by the two antenna groups may have only a phase difference. That is, when N.sub.Rx denotes the number of receive antennas at the UE and the channel matrix with a size of N.sub.Rx×32 for the first antenna group and the UE is H.sub.1, the channel matrix H.sub.2 for the second antenna group and the UE may be represented by a scalar product of H.sub.1 as shown below in Equation (1).
H.sub.2=e.sup.jφH.sub.1  (1)

(20) In this case, the (i,j)-component of H.sub.k indicates the channel value from the j.sup.th transmit antenna in the k.sup.th antenna group to the i.sup.th receive antenna.

(21) In this case, 64 antennas are marked respectively by A0, . . . , A7, B0, . . . , B7, C0, . . . , C7, D0, . . . , D7, E0, . . . , E7, F0, . . . , F7, G0, . . . , G7, H0, . . . , H7. The 64 antenna ports send one CSI-RS for each column of the 2D antenna array.

(22) First, a CSI-RS that causes a measurement of the channel states of each column of the 2D antenna array is composed of a CSI-RS resources 300 each having 8 antenna ports as follows:

(23) CSI-RS resource 0 sends A6, A7, E6, E7, B6, B7, F0, and F7 (or A0, . . . A7 in FIG. 7), respectively, to 8 CSI-RS antenna ports;

(24) CSI-RS resource 1 sends C6, C7, G6, G7, D6, D7, H6, and H7 (or B0, . . . B7 In FIG. 7), respectively, to 8 CSI-RS antenna ports;

(25) CSI-RS resource 2 sends A4, A5, E4, E5, B4, B5, F4, and F5 (or C0, . . . C7 in FIG. 7), respectively, to 8 CSI-RS antenna ports;

(26) CSI-RS resource 3 sends C4, C5, G4, G5, D4, D5, H4, and H5 (or D0, . . . D7 In FIG. 7), respectively, to 8 CSI-RS antenna ports;

(27) CSI-RS resource 4 sends A2, A3, E2, E3, B2, B3, F2, and F3 (or E0, . . . E7 in FIG. 7), respectively, to 8 CSI-RS antenna ports;

(28) CSI-RS resource 5 sends C2, C3, G2, G3, D2, D3, H2, and H3 (or F0, . . . F7 in FIG. 7), respectively, to 8 CSI-RS antenna ports;

(29) CSI-RS resource 6 sends A0, A1, E0, E1, B0, B1, F0, and F1 (or G0, . . . G7 in FIG. 7), respectively, to 8 CSI-RS antenna ports; and

(30) CSI-RS resource 7 sends C0, C1, G0, G1, D0, D1, H0, and, H1 (or H0, . . . H7 in FIG. 7), respectively, to 8 CSI-RS antenna ports.

(31) When multiple antennas are arranged in two dimensions as above (M×N, where M is a vertical direction or column and N is a horizontal direction or row), it is possible to measure FD-MIMO channels by use of N CSI-RSs each having M CSI-RS port resources.

(32) The 64 transmit antennas of the above case perform CSI-RS transmission using 8 CSI-RSs each having 8 CSI-RS ports, enabling the UE to measure radio channels for all antenna ports of the 2D antenna array in the FD-MIMO system. Each CSI-RS causes a measurement of channels for one column with respect to columns of the 2D antenna array. Meanwhile, the UE measures channel states for multiple CSI-RSs sent as in the above case and notifies radio channel states of the FD-MIMO to the eNB by feeding the RI, PMI and CQI generated based on the measurement results back to the eNB.

(33) FIG. 8 illustrates RI, PMI and CQI transmission by a UE for two CSI-RSs.

(34) To feedback PMI information, the UE composes and sends first feedback information (e.g. feedback 1) separately for each CSI-RS by measuring channels associated with one column, and composes and sends second feedback information (e.g. feedback 2) indicating a relationship between the columns. In this case, transmission of the first feedback information includes measuring a CSI-RS configured for one column and sending RI, PMI and CQI information reflecting states of channels associated with the column of the 2D antenna array; and transmission of the second feedback information includes sending information on correlation or interrelation between pieces of the first feedback information composed for individual columns to the eNB. The second feedback information may be indicated in various ways. In the present invention, assuming that virtual channels created by PMIs configured for individual columns correspond to CSI-RS resources for the second feedback information, a description is given of a scheme that feeds back a relationship between the virtual channels by use of PMIs.

(35) In this case, the RI, PMI and CQI are interrelated with one another. That is, the RI in the first feedback information may notify the rank and precoding matrix Indicated by PMIs sent afterwards as the first feedback. The RI and PMI sent as the first feedback may notify the number of channels and precoding matrix indicated by PMIs sent as the second feedback. The RI sent as the second feedback may notify the rank and precoding matrix indicated by PMIs sent as the second feedback. When the eNB transmits data of the rank indicated by the RI of the second feedback, and when the PMI sent as the first feedback is used for individual columns and the PMI sent as the second feedback is applied between columns, the CQI of the second feedback indicates the data rate available to the UE or a value corresponding to the data rate.

(36) In the feedback scheme shown in FIG. 8, the UE may receive feedback allocation for FD-MIMO as follows.

(37) First, the UE receives allocation of at least one CSI-RS resource (CSI-RS-1, . . . , CSI-RS-8). That is, the UE is instructed by the eNB to receive CSI-RS resources distinguished by columns for channel measurement. In this case, the UE may be unaware of the column of the 2D antenna array corresponding to a certain CSI-RS resource.

(38) Thereafter, the UE receives feedback allocation through Radio Resource Control (RRC) information. For example, the RRC information for feedback allocation may be configured as in Table 1 below.

(39) TABLE-US-00001 TABLE 1 Feedback Information (RRC) CSI-RS resource information: CSI-RS-1, . . . , CSI-RS-8 Reporting mode PMI codebook information for first feedback information PMI codebook information for second feedback information Etc . . .

(40) In the RRC information for feedback, the PMI codebook information is information on a set of precoding matrixes available to the corresponding feedback. When no PMI codebook information is contained in the RRC information for feedback, the UE may determine that all precoding matrixes predefined in the standard are usable for feedback. The PMI codebook information may have the same or different information elements for the first feedback and the second feedback. In the feedback information shown in Table 1 above, the “Etc.” information may include information on a period and offset for periodic feedback or information on interference measurement resources.

(41) As shown in FIG. 8, configuring one feedback containing multiple CSI-RSs for multiple transmit antennas of the eNB structured as a 2D antenna array and instructing the UE to report channel state information to the eNB may be an example of a scheme for channel state information reporting for FD-MIMO.

(42) In the scheme for channel state information reporting shown in FIG. 8, it is possible for the UE to generate and report channel state information for a 2D antenna array with fewer CSI-RS resources. However, this scheme may fail to make good use of the performance of the FD-MIMO system.

(43) In the event that configuring one feedback for each of two CSI-RSs (CSI-RS1, CSI-RS2) as shown in FIG. 8 and instructing the UE to measure channel states for one column and one row through some antenna ports and report channel state information for the remaining antenna ports obtained via channel estimation like a Kronecker product (i.e., an operation on two matrixes of arbitrary size resulting in a block matrix) based on the measurement result, as the UE utilizes estimated information without observing all antenna ports, the performance of the FD-MIMO system may not be sufficiently extracted.

(44) This is described in more detail as follows. For example, if

(45) $H_H=\left[\begin{array}{c}{h}_1^{\left(H\right)}\\ \mathrm{.Math.}\\ h_{N_{\mathrm{Rx}}}^{\left(H\right)}\end{array}\right]$
is the N.sub.Rx×N.sub.H channel matrix estimated by the UE through CSI-RS-1 and

(46) $H_V=\left[\begin{array}{c}{h}_1^{\left(V\right)}\\ \mathrm{.Math.}\\ h_{N_{\mathrm{Rx}}}^{\left(V\right)}\end{array}\right]$
is the N.sub.Rx×N.sub.V channel matrix estimated by the UE through CSI-RS-2, the N.sub.Rx×(N.sub.HN.sub.V) channel matrix for N=N.sub.HN.sub.V transmit antennas in two dimensions may be represented as Equation (2) below.

(47) $\begin{array}{cc}{H}_{\mathrm{HV}}=\gamma \left[\begin{array}{c}{h}_1^{\left(H\right)}.Math.h_1^{\left(V\right)}\\ \mathrm{.Math.}\\ h_{N_{\mathrm{Rx}}}^{\left(V\right)}.Math.h_{N_{\mathrm{Rx}}}^{\left(V\right)}\end{array}\right]& \left(2\right)\end{array}$

(48) In Equation (2) above, γ is a scalar value representing the influence of antenna virtualization on a horizontal and vertical antenna basis as a channel value. The value γ may be separately notified by the eNB or precomputed as 1 during CSI-RS channel estimation. In addition, denotes the Kronecker product of matrixes. The Kronecker product of matrixes A and B is represented as Equation (3) below.

(49) $\begin{array}{cc}A.Math.B=\left[\begin{array}{ccc}{a}_{11}B& \mathrm{.Math.}& a_{1n}B\\ \mathrm{.Math.}& \ddots & \mathrm{.Math.}\\ a_{m1}B& \mathrm{.Math.}& a_{\mathrm{mn}}B\end{array}\right],& \left(3\right)\end{array}$

(50) In Equation (3) above,

(51) $A=\left[\begin{array}{ccc}{a}_{11}& \mathrm{.Math.}& a_{1n}\\ \mathrm{.Math.}& \ddots & \mathrm{.Math.}\\ a_{m1}& \mathrm{.Math.}& a_{\mathrm{mn}}\end{array}\right],$

(52) In this case, entries of α.sub.ij only when i=m or j=1 are related to actually measured channels and the remaining entries are channel values generated by the Kronecker product. Such channel estimation errors may cause performance degradation.

(53) In a 2D antenna array system, when the transmit antennas are arranged in two dimensions, both precoding in the vertical direction and precoding in the horizontal direction are applied to signals to be sent to the UE. In this case, when the UE applies precoding corresponding to the PMI.sub.H and PMI.sub.V using some antenna ports only as shown in FIG. 8, the eNB may apply wrong PMIs to the other antenna ports. This may cause degradation of system performance.

(54) Accordingly, in an embodiment of the present invention, the eNB divides antenna ports related with a 2D antenna array into one or more groups and transmits reference signals to the UE by use of one or more reference signal resources. The UE measures these reference signals, generates feedback information on a group basis for all the antenna ports, and reports the feedback information to the eNB. In addition, the UE reports additional feedback information describing a relationship between the groups to the eNB, so that the eNB may perform transmission to the UE in an optimal manner. To this end, a scheme for reporting rank, precoding and CQI information is considered. That is, in the present invention, a feedback scheme suitable for a 2D antenna array structure is designed, and a scheme enabling the UE to generate and report feedback information for FD-MIMO utilizing the feedback scheme is provided. In the description, a set of precoding matrixes defined between the eNB and UE is referred to as a “codebook,” and each precoding matrix in the codebook may be referred to as a “codeword.” The codebook is composed of a set of precoding matrixes with respect to each supportable rank, and selection of a specific precoding matrix corresponds to selection of a specific rank.

(55) In the present invention, antenna ports may be divided into groups in various ways. For example, grouping may be performed so that adjacent or non-adjacent antenna ports belong to the same group. In the following embodiments of the present invention, for ease of description, it is assumed that antenna ports on the same column of the 2D antenna array structure are transmitted by using one CSI-RS resource. In the present invention, grouping may also be formed on the basis of antenna ports on the same row or on the basis of antenna ports randomly selected regardless of columns or rows. In addition, one antenna port may be associated with one or more adjacent or non-adjacent antenna elements.

(56) In an embodiment of the present invention, a UE performs channel estimation for each column of a 2D antenna array on the basis of one or more configured CSI-RSs and selects an optimum precoding matrix for the column from a codebook. Thereafter, the UE selects an optimum precoding matrix from the codebook on the basis of the resulting relationship between the columns and generates and reports an RI, PMI and CQI.

(57) As described above, among 64 total antennas arranged as a 2D antenna array, CSI-RSs differing from each other with respect to a positive direction of an x-axis are configured for individual columns forming 8 antenna groups for channel measurement. In this case, the channel matrix with a size of N.sub.Rx×8 between the first antenna port column and the UE may be represented as H.sub.1, the channel matrix with a size of N.sub.Rx×8 between the second antenna port column and the UE may be represented as H.sub.2, and the channel matrix with a size of N.sub.Rx×8 between the n.sup.th antenna port column and the UE may be represented as H.sub.x.

(58) A description is given below of the selection of an optimum precoding matrix for one column of the channel matrix. A scheme to select a precoding matrix maximizing the Signal-to-Noise Ratio (SNR) may be represented as Equation (4) below.

(59) $\begin{array}{cc}{\hat{P}}_n=\underset{p}{\arg \max }.Math.H_{n}P.Math.& \left(4\right)\end{array}$

(60) In Equation (4) above, P refers to a set of 8×n precoding matrixes. P is a set of 8×n beamforming vectors and Is combined with the channel matrix H.sub.x to form beams of signals propagating in desired directions. In this case, n is a rank of the precoding matrix, and n=1 Indicates a rank-1 precoding matrix and n=2 indicates a rank-2 precoding matrix. In Equation (4) above, precoding matrixes maximizing the SNR are different for different columns and the UE selects N total instances of {circumflex over (p)}.sub.n.

(61) Thereafter, the UE composes virtual ports

(62) $\hspace{1em}\left[\begin{array}{c}{H}_0{\hat{P}}_0\\ \mathrm{.Math.}\\ H_n{\hat{P}}_n\end{array}\right]$
for N CSI-RS resources using {circumflex over (p)}.sub.n selected for 8 CSI-RS resources, and measures virtual channels. In this case, the virtual channels may be represented as a channel matrix with a size of N.sub.Rx×V between virtual ports and the UE, and the number of virtual ports v may be represented as a sum of n for each CSI-RS resource.

(63) The UE selects an optimum precoding matrix specifying the relationship between port groups (e.g. port columns) by using Equation (5) below. In this case, selection of a precoding matrix maximizing the SNR may be represented by Equation (5) as follows.

(64) $\begin{array}{cc}{\hat{p}}^{\prime }=\underset{p}{\arg \max }.Math.\left[\begin{array}{c}{H}_0{\hat{P}}_0\\ \mathrm{.Math.}\\ H_n{\hat{P}}_n\end{array}\right]P.Math.& \left(5\right)\end{array}$

(65) In this case, P is a v×n′ precoding matrix. P is a v×n′ vector between port groups and combines with the virtual channel matrix so that transmission is performed so as to maximize signal reception performance. In this case, n′ is the rank for the precoding matrix, and n′=1 indicates a rank-1 precoding matrix and n′=2 indicates a rank-2 precoding matrix. In Equation (5) above, the precoding matrix maximizing the SNR uses codebooks of different sizes according to the number of virtual ports v, and an optimum {circumflex over (p)}′ is selected accordingly.

(66) Consequently, the UE feeds back N instances of {circumflex over (p)}.sub.n and one instance of {circumflex over (p)}′ for PMI feedback. For the rank, the UE feeds back the rank after {circumflex over (p)}.sub.n is applied for each column and {circumflex over (p)}′ is applied between columns. For CQI feedback, the UE determines the CQI and reports the same to the eNB after {circumflex over (p)}.sub.n selected for each column and {circumflex over (p)}′ selected between columns are applied.

(67) The feedback of the present invention may be reported to the eNB as follows. In this case, it is assumed that {circumflex over (p)}.sub.n is sent as rPMI, {circumflex over (p)}′ is sent as wPMI, and the CQI is sent as the wCQI.

(68) The period for rPMI feedback is N.sub.pd and the offset is N.sub.OFFSET,CQI. The period for wCQI feedback is H.Math.N.sub.pd and the offset is N.sub.OFFSET,CQI, which is identical to that of the rPMI. In this case, H=J.Math.K+1 and K is sent through higher layer signaling, and J is a value determined based on the 2D antenna array configuration or the number of CSI-RS resources. For example, J is set to 8 for 64 antennas. That is, the wCQI is transmitted once at every H rPMI transmissions as a replacement of the rPMI. The period for RI feedback is M.sub.RI.Math.H.Math.N.sub.pd and the offset is N.sub.OFFSET,CQI+N.sub.OFFSETRI. At each rPMI transmission timing, rPMIs for CSI-RS resources may be sent sequentially or simultaneously. In the case of 64 antennas, rPMI transmission may be performed sequentially with J=8 or simultaneously with J=8.

(69) For the rank, the UE may send both n and n′ or send n′ only. When only n′ is sent, n may be set to a fixed value such as 1 or 2. In this case, the UE may use one value notified by the eNB in advance through higher layer signaling or pre-stored in a memory.

(70) In the scheme of an embodiment of the present invention, the UE measures channels for multiple CSI-RS resources and feeds back information regarding the optimum beam selected for each CSI-RS resource and optimum relationship between the beams, so that the eNB utilizing a 2D antenna array may select a beam in an optimum manner. In this scheme, the UE divides up to 64 antenna channels into 8 groups for channel measurement and composes virtual channels for feedback, effectively reducing the amount of feedback and channel reception complexity. The size of a codebook corresponding to the virtual channels may vary according to the sum of ranks selected for individual CSI-RS resources.

(71) In an embodiment of the present invention, a UE performs channel estimation for each column of a 2D antenna array on the basis of one or more configured CSI-RSs and selects an optimum precoding matrix for all the columns from a codebook. Thereafter, the UE selects an optimum precoding matrix from the codebook on the basis of the resulting relationship between the columns and generates and reports the RI, PMI and CQI.

(72) As described above, among 64 total antennas structured as a 2D antenna array, CSI-RSs differing from each other with respect to a positive direction of an x-axis are configured for individual columns forming 8 antenna groups for channel measurement. In this case, the channel matrix with a size of N.sub.Rx×8 between the first antenna port column and the UE may be represented as H.sub.1, the channel matrix with a size of N.sub.Rx×8 between the second antenna port column and the UE may be represented as H.sub.2, and the channel matrix with a size of N.sub.Rx×8 between the n.sup.th antenna port column and the UE may be represented as H.sub.x.

(73) A description is given below of the selection of an optimum precoding matrix for one column of the channel matrix. A scheme to select a precoding matrix maximizing the SNR may be represented as Equation (6) below.

(74) $\begin{array}{cc}\hat{p}=\underset{p}{\arg \max }\underset{n}{.Math.}.Math.H_{n}P.Math.& \left(6\right)\end{array}$

(75) In Equation (6) above, P refers to a set of 8×n precoding matrixes. P is a set of 8×n beamforming vectors and is combined with the channel matrix H.sub.x to form beams of signals propagating toward desired directions. Here, n is the rank for the precoding matrix, and n=1 indicates a rank-1 precoding matrix and n=2 indicates a rank-2 precoding matrix. In Equation (6) above, precoding matrixes maximizing the SNR are the same for individual columns and the UE selects one total instance of {circumflex over (p)}.sub.n.

(76) Thereafter, the UE composes virtual ports

(77) 0$\hspace{1em}\left[\begin{array}{c}{H}_0\hat{P}\\ \mathrm{.Math.}\\ H_n\hat{P}\end{array}\right]$
for N CSI-RS resources using {circumflex over (p)}.sub.n selected for 8 CSI-RS resources, and measures virtual channels. In this case, the virtual channels may be represented as a channel matrix with a size of N.sub.Rx×v between virtual ports and the UE, and the number of virtual ports v may be represented as a sum of rank n of {circumflex over (p)} determined for each CSI-RS resource.

(78) The UE selects an optimum precoding matrix specifying the relationship between port groups (e.g. port columns) by using Equation (7) below. In this case, the selection of a precoding matrix maximizing the SNR may be represented as Equation (7) below.

(79) $\begin{array}{cc}{\hat{p}}^{\prime }=\underset{p}{\arg \max }.Math.\left[\begin{array}{c}{H}_0\hat{P}\\ \mathrm{.Math.}\\ H_n\hat{P}\end{array}\right]P.Math.& \left(7\right)\end{array}$

(80) In this case, P is a v×n′ precoding matrix. P is a v×n′ vector between port groups and combines with the virtual channel matrix so that transmission is performed so as to maximize signal reception performance. Here, n′ is the rank for the precoding matrix, and n′=1 indicates a rank-1 precoding matrix and n′=2 indicates a rank-2 precoding matrix. In Equation (7) above, the precoding matrix maximizing the SNR uses codebooks of different sizes according to the number of virtual ports v, and optimum {circumflex over (p)}′ is selected accordingly.

(81) Consequently, the UE feeds back one instance of {circumflex over (p)} and one instance of {circumflex over (p)}′ for PMI feedback. For the rank, the UE feeds back the rank after {circumflex over (p)} is applied for each column and {circumflex over (p)}′ is applied between columns. For CQI feedback, the UE determines the CQI and reports the same to the eNB after {circumflex over (p)} selected for each column and {circumflex over (p)}′ selected between columns are applied.

(82) The feedback of the present invention may be reported to the eNB as follows. In this case, it is assumed that {circumflex over (p)} is sent as rPMI, {circumflex over (p)}′ is sent as wPMI, and the CQI is sent as the wCQI.

(83) The period for rPMI feedback is N.sub.pd and the offset is N.sub.OFFSET,CQI. The period for wCQI feedback is H.Math.N.sub.pd and the offset is N.sub.OFFSET,CQI, identical to that of the rPMI. In this case, H=J.Math.K+1 and K is sent through higher layer signaling, and J is a value determined based on the 2D antenna array configuration or the number of CSI-RS resources. For example, J is set to 1 for 64 antennas. That is, the wCQI and wPMI are transmitted once at every H rPMI transmissions as a replacement of the rPMI. The period for RI feedback is M.sub.RI.Math.H.Math.N.sub.pd and the offset is N.sub.OFFSET,CQI+N.sub.OFFSETRI.

(84) As another scheme, feedback timing may be determined so that the period for wCQI and wPMI transmission is N.sub.pd with a subframe offset of N.sub.OFFSET,CQI. The period of RI, rPMI transmission is N.sub.pd.Math.M.sub.RI with an offset of N.sub.OFFSET,CQI+N.sub.OFFSET,RI.

(85) In the scheme of an embodiment of the present invention, the UE measures channels for multiple CSI-RS resources and feeds back information regarding the optimum beam selected for each CSI-RS resource and optimum relationship between the beams, so that the eNB utilizing a 2D antenna array may select beams in an optimum manner. In this scheme, the UE divides up to 64 antenna channels into 8 groups for channel measurement, applies one codebook, and composes virtual channels for feedback. Thereby, in the case of high channel correlation between antennas, it is possible to effectively reduce the amount of feedback and channel reception complexity.

(86) In an embodiment of the present invention, the UE performs channel estimation for each column of the 2D antenna array on the basis of one or more configured CSI-RSs and selects an optimum precoding matrix for the column from a codebook. Thereafter, the UE selects an optimum precoding matrix from the codebook on the basis of the resulting relationship between the columns and generates and reports the RI, PMI and CQI.

(87) As described above, among 64 total antennas structured as a 2D antenna array, CSI-RSs differing from each other with respect to a positive direction of an x-axis are configured for individual columns forming 8 antenna groups for channel measurement. In this case, the channel matrix with a size of N.sub.Rx×8 between the first antenna port column and the UE may be represented as H.sub.1, the channel matrix with a size of N.sub.Rx×8 between the second antenna port column and the UE may be represented as H.sub.2, and the channel matrix with a size of N.sub.Rx×8 between the n.sup.th antenna port column and the UE may be represented as H.sub.x.

(88) Precoding matrixes in the codebook may be represented in terms of two indexes as in Equation (8) below.
P(i.sub.1,i.sub.2)=W.sub.1(i.sub.1)W.sub.2(i.sub.2)  (8)

(89) $\mathrm{where}{W}_1\left(i_1\right)=\left[\begin{array}{cc}X\left(i_1\right)& 0\\ 0& X\left(i_1\right)\end{array}\right],X\left(i_1\right)=.Math.p_i^{\left(i_1\right)}\mathrm{.Math.}{p}_M^{\left(i_1\right)}.Math.,p_m^{\left(i_1\right)}\in \left\{c_0,c_1,\mathrm{.Math.},c_{Q-1}\right\},\mathrm{and}$$W_2\left(i_2\right)=\frac{1}{\sqrt{2}}\left[\begin{array}{c}{e}_m\\ a^k{e}_m\end{array}\right],\alpha =e^{-\frac{j2\pi }{K}},i_2=K\left(m-1\right)+k,m=1,2,\mathrm{.Math.},M,k=0,1,\mathrm{.Math.},K-1$

(90) In this case, c.sub.Q refers to

(91) $\frac{N}{2}\times 1$
beamforming vectors for N/2 antennas with the same angle in one antenna group for the XPOL configuration, and it is assumed that Q beamforming vectors are available in Equation (8) above. e.sub.m Indicates a unit vector whose m.sup.th component is 1 and other components are 0, and serves to select the m.sup.th column p.sub.m.sup.(i.sup.1.sup.) of the block diagonal matrix X(i.sub.1)=└p.sub.1.sup.x . . . p.sub.M.sup.(i.sup.1.sup.)┘ as a beamforming vector. That is, after the index (i.sub.1, i.sub.2) is determined, the combined precoding matrix may be represented as Equation (9) below.

(92) $\begin{array}{cc}P\left(i_1,i_2\right)=W_1\left(i_1\right)W_2\left(i_2\right)=\left[\begin{array}{c}{P}_m^{\left(i_1\right)}\\ e^{-\frac{j2\pi }{K}k}{P}_m^{\left(i_1\right)}\end{array}\right],\mathrm{where}{i}_2=K\left(m-1\right)+k& \left(9\right)\end{array}$

(93) In this case, the index (i.sub.1, i.sub.2) determining the precoding matrix may have the following properties.

(94) First, i.sub.1 indicates M candidate beamforming vectors selectable for the current channel among all beamforming vectors of the codebook. i.sub.2 serves to select an optimum beamforming vector suitable for the current channel among the candidate beamforming vectors indicated by i.sub.1 and to adjust the phase between different antenna groups. Hence, when the channel for each column is represented as P(i.sub.1,i.sub.2)=W.sub.1(i.sub.1)W.sub.2(i.sub.2), as i.sub.1 indicates M candidate beamforming vectors selectable for the current channel among all beamforming vectors of the codebook, the same i.sub.1 may be selected. The UE may derive a precoding matrix {circumflex over (p)}.sub.n maximizing the SNR which may be represented as Equation (10) below.

(95) The UE selects i.sub.1 for each column in Equation (10) as follows.

(96) $\begin{array}{cc}{\hat{i}}_1=\underset{i_1}{\arg \max }\Sigma .Math.H_{n}P\left(i_1,i_2\right).Math.& \left(10\right)\end{array}$

(97) The UE selects i.sub.2 for each column on the basis of the selected i.sub.1 in Equation (11) as follows.

(98) $\begin{array}{cc}{\hat{i}}_{2,n}=\underset{i_2}{\arg \max }.Math.H_{n}P\left(i_1,i_2\right).Math.& \left(11\right)\end{array}$

(99) In this case, P(i.sub.1,i.sub.2) is a 8×n precoding matrix. P(i.sub.1,i.sub.2) is a 8×n beamforming vector and combines with the channel matrix H.sub.x so as to form beams of signals propagating in desired directions. In this case, n is the rank for the precoding matrix, and n=1 Indicates a rank-1 precoding matrix and n=2 Indicates a rank-2 precoding matrix. In Equation (11) above, precoding matrixes maximizing the SNR are different for different columns and the UE selects N total instances of P(î.sub.1,î.sub.2,n).

(100) Thereafter, the UE composes virtual ports

(101) $\hspace{1em}\left[\begin{array}{c}{H}_{0}P\left({\hat{i}}_1,{\hat{i}}_{2,0}\right)\\ \mathrm{.Math.}\\ H_{n}P\left({\hat{i}}_1,{\hat{i}}_{2,n}\right)\end{array}\right]$
for N CSI-RS resources using P(î.sub.1,î.sub.2n) selected for 8 CSI-RS resources, and measures virtual channels. In this case, the virtual channels may be represented as a channel matrix with a size of N.sub.Rx×v between virtual ports and the UE, and the number of virtual ports v may be represented as a sum of n for each CSI-RS resource. The UE selects an optimum precoding matrix specifying the relationship between port groups by using Equation (12) below. When P(i.sub.1,i.sub.2)=W.sub.1(i.sub.1)W.sub.2(i.sub.2), i.sub.1 and i.sub.2 may be derived as in Equation (12) below where i.sub.1 relates to virtual CSI-RS resources configured for each column and i.sub.2 relates to beamforming vectors in the codebook.

(102) The UE selects i.sub.1 as in Equation (12) as follows.

(103) $\begin{array}{cc}{\hat{i}}_1^{\prime }=\underset{i_1}{\arg \max }.Math.\left[\begin{array}{c}{H}_{0}P\left({\hat{i}}_1,{\hat{i}}_{2,0}\right)\\ \mathrm{.Math.}\\ H_{n}P\left({\hat{i}}_1,{\hat{i}}_{2,n}\right)\end{array}\right]P\left(i_1,i_2\right).Math.& \left(12\right)\end{array}$

(104) The UE selects i.sub.2 for each column on the basis of selected i.sub.1 as in PE Equation (13) as follows.

(105) $\begin{array}{cc}{\hat{i}}_2^{\prime }=\underset{i_2}{\arg \max }.Math.\left[\begin{array}{c}{H}_{0}P\left({\hat{i}}_1,{\hat{i}}_{2,0}\right)\\ \mathrm{.Math.}\\ H_{n}P\left({\hat{i}}_1,{\hat{i}}_{2,n}\right)\end{array}\right]P\left(i_1,i_2\right).Math.& \left(13\right)\end{array}$

(106) In this case, P(î.sub.1′,î.sub.2′) is a v×n′ precoding matrix. P(î.sub.1′,î.sub.2′) is a v×n′ vector between port groups and is combined with the virtual channel matrix so that transmission is performed so as to maximize signal reception performance. In this case, n′ is the rank for the precoding matrix, and n′=1 indicates a rank-1 precoding matrix and n′=2 indicates a rank-2 precoding matrix. In Equation (13) above, the precoding matrix maximizing the SNR uses codebooks of different sizes according to the number of virtual ports v, and the optimum P(î.sub.1′,î.sub.2′) is selected accordingly.

(107) Consequently, the UE feeds back one instance of î.sub.1 and N Instances of î.sub.2, and one instance of î.sub.1′ and one instance of î.sub.2′ for PMI feedback. For the rank, the UE feeds back the rank after P(î.sub.1,î.sub.2) is applied for each column and P(î.sub.1′,î.sub.2′) is applied between columns. For CQI feedback, the UE determines the CQI and reports the same to the eNB after P(î.sub.1,î.sub.2) selected for each column and P(î.sub.1′,î.sub.2′) selected between columns are applied.

(108) The feedback of the present invention may be reported to the eNB as follows. In this case, it is assumed that P(î.sub.1,î.sub.2) is sent as rPMI, P(î.sub.1′,î.sub.2′) is sent as wPMI, and the CQI is sent as the wCQI. The rPMI and wPMI both indicate that î.sub.1 and î.sub.1′ are sent as the first PMI and î.sub.2 and î.sub.2′ are sent as the second PMI.

(109) The scheme of the present invention may estimate channels for N×M transmit antennas arranged in two dimensions by use of N CSI-RS resources, generate PMI i.sub.1 and PMI i.sub.2 specifying N optimum ranks and associated precoding matrixes, and generate rank, i.sub.1, i.sub.2 and CQI specifying N optimum precoding matrixes. The UE reports the determined rank, i.sub.1, i.sub.2, and CQI at preset timings to the eNB. Then, the eNB may be aware of channel state information of the UE with reference to the predefined codebook and utilize the identified Information to perform data scheduling for the UE. In this case, the rank, i.sub.1, i.sub.2, and CQI may be reported at the same timing together with uplink data or may be reported at different timings via an uplink control channel. In particular, when i.sub.1 and i.sub.2 are reported at different timings, it is preferable that the transmission period of i.sub.1 be greater than that of i.sub.2. That is, i.sub.1 being reported less frequently may remind the eNB of the set of beamforming vectors available, and i.sub.2 being reported more frequently may enable the eNB to select optimum beamforming vectors suitable to actual fading channels and adjust the phase between antenna groups. In this case, i.sub.1 indicates M candidate beamforming vectors selectable for the current channel among all beamforming vectors of the codebook, and i.sub.2 serves to select a beamforming vector for actual use and to adjust the phase between different antenna groups.

(110) The RI is transmitted together with first PMI information (e.g., rPMI and wPMI) and the wCQI is transmitted together with second PMI information. The feedback period for the wCQI and the second PMI is H.Math.N.sub.pd and the offset is N.sub.OFFSET,CQI. In this case, H=J.Math.K+1 and K are sent through higher layer signaling, and J is a value determined based on the 2D antenna array configuration or the number of CSI-RS resources. For example, J is set to 8 for 64 antennas. That is, the wCQI and wPMI are transmitted once at every H second PMI (e.g. rPMI) transmissions as a replacement of the rPMI. The feedback period for the RI and first PMI (e.g., rPMI and wPMI) is M.sub.RI.Math.H.Math.N.sub.pd and the offset is N.sub.OFFSET,CQI+N.sub.OFFSETRI. In this case, if the precoding matrix corresponding to the first PMI is W.sub.1 and the precoding matrix corresponding to the second PMI is W.sub.2, the UE and the eNB share the information indicating that the precoding matrix preferred by the UE is determined as W.sub.1W.sub.2.

(111) As another feedback scheme, the feedback information may further include Precoding Type Indicator (PTI) information. In this case, the PTI is transmitted together with the RI at a period of M.sub.RI.Math.H.Math.N.sub.pd with an offset of N.sub.OFFSET,CQI+N.sub.OFFSET,RI.

(112) For example, for PTI=0, all the first PMI (e.g., rPMI and wPMI), second PMI and wCQI may be fed back. In this case, the wCQI and second PMI (e.g., rPMI and wPMI) are sent together at the same timing at a period of NM with an offset of N.sub.OFFSET,CQI. The first PMI is transmitted at a period of H′.Math.N.sub.pd with an offset of N.sub.OFFSET,CQI. Here, H′ is transmitted via higher layer signaling.

(113) For PTI=1, the PTI and RI are transmitted together. In this case, the wCQI and second PMI (e.g., rPMI and wPMI) are transmitted together, and the sCQI is also transmitted at a separate timing. In this case, the first PMI is not transmitted. The PTI and RI are transmitted at the same period with the same offset as the case of PTI=0. The sCQI is transmitted at a period of N.sub.pd with an offset of N.sub.OFFSET,CQI. The wCQI and second PMI are transmitted at a period of H.Math.N.sub.pd with an offset of N.sub.OFFSET,CQI, and H is set to the same value as the case of 4 CSI-RS antenna ports.

(114) In an embodiment of the present invention, PTI=00 indicates transmission of the first PMI (e.g. rPMI), PTI=01 Indicates transmission of the first PMI (e.g. wPMI), PTI=10 indicates transmission of the second PMI (e.g. rPMI), and PTI=11 indicates transmission of the second PMI (e.g. wPMI).

(115) In an embodiment of the present invention, PTI=00 indicates transmission of the first PMI (e.g. rPMI and wPMI), PTI=01 indicates transmission of the second PMI (e.g. rPMI), PTI=10 indicates transmission of the second PMI (e.g. wPMI), and PTI=11 indicates transmission of all PMIs.

(116) The feedback scheme of the present invention may include the feedback scheme of one of the various embodiments of the present invention or a combination thereof with respect to feedback for each CSI-RS and feedback between CSI-RSs. Such a combined scheme may be utilized when the configuration of the 2D antenna array varies.

(117) In an embodiment of the present invention, the UE performs channel estimation for each of one or more CSI-RS feedback configurations, selects an optimum precoding matrix from the codebook, and generates and reports the RI, PMI and CQI. In addition, the UE selects an optimum precoding matrix from the codebook on the basis of the resulting relationship between the CSI-RS feedback configurations, and generates and reports the RI, PMI and CQI.

(118) The UE identifies one or more feedback configurations based on CSI-RSs. The feedback configuration may be composed of the whole or a portion of RRC information as shown in Table 2 below.

(119) TABLE-US-00002 TABLE 2 Feedback Configuration #1 Channel information: CSI-RS-1 Reporting (feedback) mode PMI codebook information Etc . . . Feedback Configuration #2 Channel information: CSI-RS-2 Reporting (feedback) mode PMI codebook information Etc . . . Feedback Configuration #N Channel information: CSI-RS-N Reporting (feedback) mode PMI codebook information Etc . . .

(120) The UE identifies one piece of feedback configuration information based on CSI-RS feedback configurations. The feedback configuration may be composed of the whole or a portion of RRC information as shown in Table 3 below.

(121) TABLE-US-00003 TABLE 3 Feedback Configuration #N + 1 Channel information: CSI-RS feedback configuration #1, #2, . . . , #N Reporting (feedback) mode PMI codebook information Etc . . .

(122) The UE receives information on CSI-RS feedback configurations differing with each other with respect to the positive direction of the x-axis for individual columns forming 8 antenna groups among 64 total antennas structured as a 2D antenna array, and performs channel measurement for N CSI-RS resources. In this case, the channel matrix with a size of N.sub.Rx×8 between the first CSI-RS feedback configuration and the UE may be represented as H.sub.1, the channel matrix with a size of N.sub.Rx×8 between the second CSI-RS feedback configuration and the UE may be represented as H.sub.2, and the channel matrix with a size of N.sub.Rx×8 between the n.sup.th CSI-RS feedback configuration and the UE may be represented as H.sub.x.

(123) A description is given of selection of an optimum precoding matrix for the channel matrix measured at the CSI-RS feedback configuration. Here, a scheme to select a precoding matrix maximizing the Signal-to-Noise Ratio (SNR) may be represented as Equation (14) below.

(124) 0$\begin{array}{cc}{\hat{p}}_n=\underset{p}{\arg \max }.Math.H_{n}P.Math.& \left(14\right)\end{array}$

(125) In Equation (14) above, P refers to a set of 8×n precoding matrixes. P is a set of 8×n beamforming vectors and is combined with the channel matrix H.sub.n so as to form beams of signals propagating in desired directions. In this case, n is the rank for the precoding matrix, and n=1 indicates a rank-1 precoding matrix and n=2 indicates a rank-2 precoding matrix. In Equation (14) above, precoding matrixes maximizing the SNR are different for different columns and the UE selects N total instances of {circumflex over (p)}.sub.n. Consequently, for PMI feedback, the UE feeds back N instances of {circumflex over (p)}.sub.n for N feedback configurations. For the rank, the UE feeds back the rank after {circumflex over (p)}.sub.n is applied. For CQI feedback, the UE determines the CQI and reports the same to the eNB after the rank and {circumflex over (p)}.sub.n selected for each CSI-RS feedback configuration are applied.

(126) Thereafter, the UE composes virtual ports

(127) $\hspace{1em}\left[\begin{array}{c}{H}_0{\hat{P}}_0\\ \mathrm{.Math.}\\ H_n{\hat{P}}_n\end{array}\right]$
for N CSI-RS resources using {circumflex over (p)}.sub.n selected for 8 CSI-RS feedback configurations, and measures virtual channels for the N+1.sup.th CSI-RS feedback configuration. In this case, the virtual channels may be represented as a channel matrix with a size of N.sub.Rx×v between virtual ports and the UE, and v may be represented as a sum of n for each CSI-RS resource determined at one CSI-RS feedback configuration.

(128) The UE selects an optimum precoding matrix specifying the relationship between the CSI-RS feedback configurations by using Equation (15) below. In this case, selection of a precoding matrix maximizing the SNR may be represented as in Equation (15) as follows.

(129) $\begin{array}{cc}{\hat{p}}^{\prime }=\underset{p}{\arg \max }.Math.\left[\begin{array}{c}{H}_0{\hat{P}}_0\\ \mathrm{.Math.}\\ H_n{\hat{P}}_n\end{array}\right]P.Math.& \left(15\right)\end{array}$

(130) In this case, P is a v×n′ precoding matrix. P is a v×n′ vector between port groups and is combined with the virtual channel matrix so that transmission is performed so as to maximize signal reception performance. In this case, n′ is the rank for the precoding matrix, and n′=1 indicates a rank-1 precoding matrix and n′=2 indicates a rank-2 precoding matrix. In Equation (15) above, the precoding matrix maximizing the SNR uses codebooks of different sizes according to the number of virtual ports v, and the optimum {circumflex over (p)}′ is selected accordingly.

(131) Consequently, the UE feeds back one instance of {circumflex over (p)}′ for PMI feedback. For the rank, the UE feeds back the rank after {circumflex over (p)}′ is applied. For CQI feedback, the UE determines the CQI and reports the same to the eNB after the selected rank and {circumflex over (p)}′ are applied.

(132) The feedback of the present invention may be reported to the eNB as follows. In this case, it is assumed that the PMI for each CSI-RS feedback configuration is sent as wPMI and the CQI is sent as the wCQI. Feedback may be performed by applying the feedback mode described before for each CSI-RS feedback configuration:

(133) reporting mode 1-0 reports RI and wideband CQI (wCQI);

(134) reporting mode 1-1 reports RI, wCQI, and PMI;

(135) reporting mode 2-0 reports RI, wCQI, and subband CQI (sCQI); and

(136) reporting mode 2-1 reports RI, wCQI, sCQI, and PMI.

(137) For the rank, the UE may send both n and n′ or send n′ only. When n′ is sent, n may be set to a fixed value such as 1 or 2. In this case, the UE may use one value notified by the eNB in advance through higher layer signaling or pre-stored in a memory.

(138) In an embodiment of the present invention, the UE measures channels for multiple CSI-RS resources and reports this information to the eNB, feeds back information regarding the optimum beam selected for each CSI-RS resource and optimum relationship between the beams when the reported beams are simultaneously used, so that the eNB utilizing a 2D antenna array may select a beam in an optimum manner. In this scheme, the UE may feed back channel information regardless of antenna configuration Information of the eNB, by dividing multiple antenna channels into 1, 2, 4 or 8 channel measurement resources for channel measurement, composing virtual channels, and transmitting channel feedback to the eNB. The size of the codebook corresponding to the virtual channels may vary according to the sum of ranks selected for individual CSI-RS resources.

(139) In an embodiment of the present invention, the UE performs channel estimation for each of one or more CSI-RS feedback configurations, selects an optimum precoding matrix from the codebook, and generates the RI and PMI. The UE selects an optimum precoding matrix from the codebook on the basis of the relationship between the CSI-RS feedback configurations, and generates and reports the RI and PMI. Thereafter, the eNB send a CSI-RS, which is a result of applying the fed back PMI to the additional CSI-RS configuration on the basis of the received RI and PMI, and the UE measures the corresponding CSI-RS channel and feeds back the CQI.

(140) The UE identifies one or more feedback configurations based on CSI-RSs. The feedback configuration may be composed of the whole or a portion of RRC information as shown in Table 4 below.

(141) TABLE-US-00004 TABLE 4 Feedback Configuration #1 Channel information: CSI-RS-1 Reporting (feedback) mode PMI codebook information Etc . . . Feedback Configuration #2 Channel information: CSI-RS-2 Reporting (feedback) mode PMI codebook information Etc . . . Feedback Configuration #N Channel information: CSI-RS-N Reporting (feedback) mode PMI codebook information Etc . . .

(142) The UE identifies one piece of feedback configuration information based on CSI-RS feedback configurations. The feedback configuration may be composed of the whole or a portion of RRC information as shown in Table 5 below.

(143) TABLE-US-00005 TABLE 5 Feedback Configuration #N + 1 Channel information: CSI-RS feedback configuration #1, #2, . . . , #N Reporting (feedback) mode PMI codebook information Etc . . .

(144) An additional feedback configuration may be signaled to the UE as a resource for measurement of a beamformed CSI-RS channel as shown in Table 6 below.

(145) TABLE-US-00006 TABLE 6 Feedback Configuration #N + 2 Channel information: CSI-RS-N + 1 Reporting (feedback) mode PMI codebock information Etc . . .

(146) The UE receives Information on CSI-RS feedback configurations differing with each other with respect to the positive direction of the x-axis for individual columns forming 8 antenna groups among 64 total antennas structured as a 2D antenna array, and performs channel measurement for N CSI-RS resources. In this case, the channel matrix with a size of N.sub.Rx×8 between the first CSI-RS feedback configuration and the UE may be represented as H.sub.1, the channel matrix with a size of N.sub.Rx×8 between the second CSI-RS feedback configuration and the UE may be represented as H.sub.2, and the channel matrix with a size of N.sub.Rx×8 between the n.sup.th CSI-RS feedback configuration and the UE may be represented as H.sub.x.

(147) A description is given of the selection of an optimum precoding matrix for the channel matrix measured at the CSI-RS feedback configuration. In this case, a scheme to select a precoding matrix maximizing the Signal-to-Noise Ratio (SNR) may be represented as Equation (16) below.

(148) $\begin{array}{cc}{\hat{p}}_n=\underset{p}{\arg \max }.Math.H_{n}P.Math.& \left(16\right)\end{array}$

(149) In Equation (16) above, P refers to a set of 8×n precoding matrixes. P is a set of 8×n beamforming vectors and combines with the channel matrix H.sub.n so as to form beams of signals propagating toward desired directions. Here, n is the rank for the precoding matrix, and n=1 indicates a rank-1 precoding matrix and n=2 indicates a rank-2 precoding matrix. In Equation (16) above, precoding matrixes maximizing the SNR are different for different columns and the UE selects N total instances of {circumflex over (p)}.sub.n. Consequently, for PMI feedback, the UE feeds back N instances of {circumflex over (p)}.sub.n for N feedback configurations. For the rank, the UE feeds back the rank after {circumflex over (p)}.sub.n is applied.

(150) Thereafter, the UE composes virtual ports

(151) $\hspace{1em}\hspace{1em}\left[\begin{array}{c}{H}_0{\hat{P}}_0\\ \mathrm{.Math.}\\ H_n{\hat{P}}_n\end{array}\right]$
for N CSI-RS resources using {circumflex over (p)}.sub.n selected for 8 CSI-RS feedback configurations, and measures virtual channels for the N+1.sup.th CSI-RS feedback configuration. In this case, the virtual channels may be represented as a channel matrix with a size of N.sub.Rx×v between virtual ports and the UE, and the number of virtual ports v may be represented as a sum of n for each CSI-RS resource determined at one CSI-RS feedback configuration.

(152) The UE selects an optimum precoding matrix specifying the relationship between the CSI-RS feedback configurations by using Equation (17) below. In this case, selection of a precoding matrix maximizing the SNR may be represented as Equation (17) as follows.

(153) $\begin{array}{cc}{\hat{p}}^{\prime }=\underset{p}{\arg \max }.Math.\left[\begin{array}{c}{H}_0{\hat{P}}_0\\ \mathrm{.Math.}\\ H_n{\hat{P}}_n\end{array}\right]P.Math.& \left(17\right)\end{array}$

(154) In this case, P is a v×n′ preceding matrix. P is a v×n′ vector between port groups and is combined with the virtual channel matrix so that transmission is performed so as to maximize signal reception performance. In this case, n′ is the rank for the precoding matrix, and n′=1 indicates a rank-1 precoding matrix and n′=2 Indicates a rank-2 precoding matrix. In Equation (17) above, the precoding matrix maximizing the SNR uses codebooks of different sizes according to the number of virtual ports v, and the optimum {circumflex over (p)}′ is selected accordingly.

(155) Consequently, the UE feeds back one instance of {circumflex over (p)}′ for PMI feedback. For the rank, the UE feeds back the rank after {circumflex over (p)}′ is applied.

(156) Thereafter, the UE measures channels for the N+1.sup.th CSI-RS resource and feeds back the CQI for corresponding channels. In this case, as the corresponding CSI-RS is a CSI-RS beamformed with the PMI fed back by the UE, only the CQI is fed back.

(157) FIG. 9 is a flowchart depicting a sequence of operations performed by the UE according to an embodiment of the present invention.

(158) Referring to FIG. 9, in step 910, the UE receives configuration information regarding one or more CSI-RSs to perform channel estimation for each antenna port group. On the basis of the received configuration information, the UE may identify information regarding at least one of the number of ports associated with each CSI-RS, transmission timings and resource positions of CSI-RSs, sequences, and transmit power.

(159) In step 920, the UE receives feedback configuration information based on one or more CSI-RSs.

(160) The feedback configuration information includes information on the reporting mode or feedback mode indicating types of feedback information to be generated and reported by the UE. The feedback scheme may include estimating channels for N transmit antenna groups by using CSI-RS−1−N; generating PMI i.sub.1 and PMI i.sub.2 or CQI specifying optimum ranks and associated precoding matrixes for the estimated channels and reporting the same to the eNB; and generating PMI i.sub.1 and i.sub.2 and CQI specifying optimum rank and precoding matrix between CSI-RSs on the basis of the PMI selected between CSI-RSs and reporting the same to the eNB.

(161) PMI codebook information is information on a set of precoding matrixes usable at the current channel condition in the codebook. When no PMI codebook information is contained in the RRC information for feedback, the UE may determine that all precoding matrixes defined in the codebook are usable for feedback.

(162) In step 930, the UE receives CSI-RSs and estimates channels between N antenna groups of the eNB and N.sub.Rx receive antennas of the UE.

(163) In step 940, the UE generates pieces of feedback information including rank, PMI i.sub.1 and i.sub.2, and CQI on the basis of the estimated channels, virtual channels between CSI-RSs, received feedback configuration, and defined codebook.

(164) In step 950, the UE sends the pieces of feedback information to the eNB at feedback timings determined according to the feedback configuration from the eNB, thereby ending the procedure for generating and reporting channel feedback using a 2D antenna array.

(165) FIG. 10 is a flowchart depicting a sequence of operations performed by an eNB according to an embodiment of the present invention.

(166) Referring to FIG. 10, in step 1010, the eNB sends configuration information regarding one or more CSI-RSs to the UE to perform channel estimation for each antenna port group. The configuration information may include information regarding at least one of the number of ports associated with each CSI-RS, transmission timings and resource positions of CSI-RSs, sequences, and transmit power.

(167) In step 1020, the eNB sends feedback configuration information based on one or more CSI-RSs to the UE. In an embodiment of the present invention, the feedback configuration for two CSI-RSs may be composed of the whole or a portion of the RRC information as shown in Table 2 above.

(168) Thereafter, the eNB sends configured CSI-RSs to the UE. The UE performs channel estimation for each antenna port and performs additional channel estimation for virtual resources on the basis of the estimation results for each antenna port. The UE determines one of the feedbacks described in various embodiments of the present invention, generates corresponding CQI, and sends the CQI to the eNB.

(169) In step 1030, the eNB receives feedback information at preset timings and identifies states of the channels between the UE and eNB on the basis of the received feedback information.

(170) FIG. 11 is a block diagram of a UE according to an embodiment of the present invention.

(171) Referring to FIG. 11, the UE includes a communication unit 1110 and a control unit 1120.

(172) The communication unit 1110 sends and receives data to and from an external entity such as an eNB. In particular, the communication unit 1110 sends feedback information to the eNB under the control of the control unit 1120.

(173) The control unit 1120 controls states and operations of other components of the UE. In particular, the control unit 1120 generates feedback information according to configuration information received from the eNB. The control unit 1120 also controls the communication unit 1110 to feed channel state information back to the eNB according to timings set by the eNB. To this end, the control unit 1120 includes a channel estimator 1130.

(174) The channel estimator 1130 identifies required feedback information according to CSI-RSs and feedback configuration Information received from the eNB, and performs channel estimation on the basis of the received CSI-RSs and the identified feedback Information.

(175) In FIG. 11, the UE is described as including the communication unit 1110 and the control unit 1120. However, the UE may further include other various components according to the features or functions thereof. For example, the UE may further include a display unit to display current states, an input unit to receive an input signal for function execution from a user, and a storage unit to store data generated during operation. In the above description, the channel estimator 1130 is described as being included in the control unit 1120. However, the present invention is not limited thereto or thereby.

(176) The control unit 1120 controls the communication unit 1110 to receive configuration information for each of one or more reference signal resources from the eNB. To measure at least one reference signal and generate corresponding feedback information, the control unit 1120 also controls the communication unit 1110 to receive feedback configuration information from the eNB.

(177) The control unit 1120 measures at least one reference signal received through the communication unit 1110 and generates feedback information according to the feedback configuration information. The control unit 1120 controls the communication unit 1110 to send the generated feedback information to the eNB at feedback timings indicated by the feedback configuration information.

(178) The control unit 1120 receives a CSI-RS, generates feedback information on the basis of the received CSI-RS, and reports the feedback information to the eNB. In this case, the control unit 1120 selects a precoding matrix for each antenna port group of the eNB and selects an additional precoding matrix on the basis of the relationship between the antenna port groups. The control unit 1120 creates a virtual channel on the basis of the precoding matrixes selected for the antenna port groups and sends the additional precoding matrix to the eNB via the virtual channel. The precoding matrixes and the additional precoding matrix include a first index indicating candidate beamforming vectors selectable for the current channel between the eNB and UE, and a second Index for selecting a beamforming vector to be used.

(179) The control unit 1120 receives a CSI-RS, generates feedback information on the basis of the received CSI-RS, and reports the feedback information to the eNB. In this case, the control unit 1120 selects a precoding matrix for all antenna port groups of the eNB and selects an additional precoding matrix on the basis of the relationship between the antenna port groups. The control unit 1120 creates a virtual channel on the basis of the precoding matrix selected for the antenna port groups and sends the additional precoding matrix to the eNB via the virtual channel.

(180) The control unit 1120 receives feedback configuration information from the eNB, receives CSI-RSs, generates feedback information on the basis of the received feedback configuration information and CSI-RSs, and sends the generated feedback information to the eNB. In this case, the control unit 1120 receives feedback configuration information corresponding to each antenna port group of the eNB and receives additional feedback configuration information based on the relationship between the antenna port groups.

(181) The control unit 1120 sends first feedback information generated based on a first CSI-RS from the eNB, receives a second CSI-RS beamformed on the basis of the first feedback information from the eNB, generates second feedback information on the basis of the received second CSI-RS, and sends the second feedback information to the eNB.

(182) FIG. 12 is a block diagram of an eNB according to an embodiment of the present invention.

(183) Referring to FIG. 12, the eNB includes a control unit 1210 and a communication unit 1220.

(184) The control unit 1210 controls overall states and operations of components in the eNB. Specifically, to estimate horizontal and vertical domain channels of the UE, the control unit 1210 allocates CSI-RS resources to the UE and assigns feedback resources and timings to the UE. To this end, the control unit 1210 includes a resource allocator 1230.

(185) To enable the UE to perform channel estimation for each antenna port group, the resource allocator 1230 assigns CSI-RSs to corresponding resources and sends the CSI-RSs through the resources. The resource allocator 1230 allocates feedback configurations and timings so that feedbacks from different UEs do not collide with each other, and receives and analyzes feedback information sent at preset timings.

(186) The communication unit 1220 is configured to send and receive data, reference signals and feedback information to and from a UE. In this case, the communication unit 1220 sends CSI-RSs to the UE through resources allocated under the control of the control unit 1210 and receives feedback information on channel states from the UE.

(187) In the above description, the resource allocator 1230 is depicted as being included in the control unit 1210. However, the present invention is not limited thereto or thereby.

(188) The control unit 1210 controls the communication unit 1220 to send configuration information for one or more reference signals to the UE and generates the reference signals. The control unit 1210 also controls the communication unit 1220 to send feedback configuration information to the UE, so that the UE may generate feedback information corresponding to measurement results.

(189) The control unit 1210 controls the communication unit 1220 to send one or more reference signals to the UE and receives feedback information from the UE at feedback timings set according to the feedback configuration information.

(190) The control unit 1210 sends feedback configuration Information to the UE, sends CSI-RSs to the UE, and receives feedback information generated on the basis of the feedback configuration information and CSI-RSs from the UE. In this case, the control unit 1210 sends feedback configuration information corresponding to each antenna port group of the eNB and sends additional feedback configuration Information based on the relationship between the antenna port groups.

(191) The control unit 1210 receives first feedback information from the UE, sends a CSI-RS beamformed based on the first feedback information to the UE, and receives second feedback Information generated based on the CSI-RS from the UE.

(192) As described hereinabove, embodiments of the present Invention enable the base station having a large number of transmit antennas structured as a two dimensional array to avoid allocating excessive feedback resources for CSI-RS transmission without Increasing channel estimation complexity of a user equipment. The user equipment may measure channels associated with the large number of transmit antennas, compose feedback information based on measurement results, and report the feedback information to the base station in an effective manner.

(193) In a feature of the present invention, the base station having a plurality of transmit antennas structured as a two dimensional array may avoid allocating excessive radio resources to measure channels associated with multiple antenna ports. The user equipment may measure channels associated with multiple antenna ports, compose feedback information based on measurement results, and report the feedback information to the base station in an effective manner.

(194) While the present invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various modifications in form and details may be made therein without departing from the scope and spirit of the present invention as defined by the appended claims and their equivalents.