WO2013015606A2

WO2013015606A2 - Method and apparatus for transmitting control information in wireless communication system

Info

Publication number: WO2013015606A2
Application number: PCT/KR2012/005926
Authority: WO
Inventors: Hyeyoung Choi; Hyunwoo Lee; Hyukmin SON; Seunghee Han; Jinmin Kim
Original assignee: Lg Electronics Inc.
Priority date: 2011-07-25
Filing date: 2012-07-25
Publication date: 2013-01-31
Also published as: WO2013015606A3

Abstract

A method for, at an eNB, transmitting control information to a UE in a wireless communication system includes precoding a codeword including downlink control information using one of precoding matrices included in a candidate precoding matrix set of a codebook, wherein the precoding matrices included in the candidate precoding matrix set are used by the UE for attempting to de-precode for the downlink control information.

Description

[DESCRIPTION]

[invention Title]

METHOD AND APPARATUS FOR TRANSMITTING CONTROL INFORMATION IN WIRELESS COMMUNICATION SYSTEM

[Technical Field]

The present invention relates to a method and apparatus for transmitting control information in a wireless communication system.

[Background Art]

Extensive research has been conducted to provide various types of communication services including voice and data services in wireless communication systems. In general, a wireless communication system is a multiple access system that supports communication with multiple users by sharing available system resources (e.g. bandwidth, transmit power, etc.) among the multiple users. The multiple access system may adopt a multiple access scheme such as Code Division Multiple Access (CDMA) , Frequency Division Multiple Access (FDMA) , Time Division Multiple Access (TDMA) , Orthogonal Frequency Division Multiple Access (OFDMA) , Single Carrier Frequency Division Multiple Access (SC-FDMA) , etc.

[Disclosure]

[Technical Problem] An object of the present invention is to provide a method and apparatus for precoding and transmitting control information .

It will be appreciated by persons skilled in the art that the objects that could be achieved with the present invention are not limited to what has been particularly described hereinabove and the above and other objects that the present invention could achieve will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

[Technical Solution]

In accordance with a first aspect of the present invention, a method for, at an eNB, transmitting control information to a UE in a wireless communication system includes precoding a codeword including downlink control information using one of precoding matrices included in a candidate precoding matrix set of a codebook, wherein the precoding matrices included in the candidate precoding matrix set are used by the UE for attempting to de-precode for the downlink control information.

In accordance with a second aspect of the present invention, a method for, at a UE, receiving control information from an eNB in a wireless communication system include attempting de-precoding using precoding matrices included in a candidate precoding matrix set of a codebook for downlink control information in a predetermined resource region of a subframe .

In accordance with a third aspect of the present invention, an eNB in a wireless communication system includes a transmission module and a processor, wherein the processor precodes a codeword including downlink control information using one of precoding matrices included in a candidate precoding matrix set of a codebook, wherein the precoding matrices included in the candidate precoding matrix set are used by a UE for attempting to de-precode for the downlink control information.

In accordance with a fourth aspect of the present invention, a UE in a wireless communication system includes a reception module and a processor, wherein the processor attempts de-precoding using precoding matrices included in a candidate precoding matrix set of a codebook for downlink control information in a predetermined resource region of a subframe .

The first to fourth aspects of the present invention may include all or some of the following.

The codebook may include a plurality of precoding matrices indicated by combinations of k codebook indexes and 1 layer indexes, and the candidate precoding matrix set may include precoding matrices indicated by at least one of m (m≤ k) codebook indexes and n (n≤l) layer indexes. The candidate precoding matrix set may be transmitted to the UE as a bitmap composed of at least one of bits corresponding to codebook indexes and bits corresponding to layer indexes.

When the eNB transmits control information to a plurality of UEs including the UE, different precoding matrices may be selected from the candidate precoding matrix set for the respective UEs.

The method for transmitting control information may further include mapping the precoded codeword to a resource region of an antenna port and transmitting the codeword through the antenna port, wherein the resource region is one of a resource region corresponding to a control region indicated by a physical control format indicator channel (PCFICH) , a resource region other than the control region in the corresponding subframe, and a resource region other than the control region in the first slot of the subframe.

When two or more codewords are present, different downlink control information formats may be allocated to the two or more codewords .

[Advantageous Effects]

According to embodiments of the present invention, a UE can efficiently perform de-precoding even when a precoding matrix used for control information transmission is not explicitly signaled. It will be appreciated by persons skilled in the art that that the effects that could be achieved with the present invention are not limited to what ^' has been particularly- described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings .

[Description of Drawings]

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment (s) of the invention and together with the description serve to explain the principle of the invention. In the drawings :

FIG. 1 illustrates a radio frame structure;

FIG. 2 illustrates a resource grid in a downlink slot;

FIG. 3 illustrates a downlink subframe structure;

FIG. 4 illustrates an uplink subframe structure;

FIG. 5 illustrates mapping of PUCCH formats to uplink physical resource blocks;

FIG. 6 illustrates an example of determining a PUCCH resource for ACK/NACK;

FIG. 7 is a diagram illustrating a CRS and a DRS;

FIG. 8 is a diagram illustrating carrier aggregation; FIG. 9 is a diagram illustrating cross-carrier scheduling;

FIG. 10 illustrates a heterogeneous network wireless communication system;

FIGS. 11 and 12 illustrate a scheme for alleviating interference through scheduling in different networks;

FIG. 13 illustrates an ePDCCH resource region applicable to embodiments of the present invention;

FIG. 14 illustrates a process of mapping a codeword including DCI to an antenna port according to an embodiment of the present invention;

FIGS. 15 to 17 illustrate candidate precoding matrix sets according to embodiments of the present invention; and

FIG. 18 illustrates configurations of an eNB apparatus and a UE apparatus according to an embodiment of the present invention.

[Best Mode]

Embodiments described hereinbelow are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment.

In the embodiments of the present invention, a description is made, centering on a data transmission and reception relationship between an eNB and a user equipment (UE) . The eNB is a terminal node of a network, which communicates directly with a UE. In some cases, a specific operation described as performed by the eNB may be performed by an upper node of the BS .

Namely, it is apparent that, in a network comprised of a plurality of network nodes including an eNB, various operations performed for communication with a UE may be performed by the eNB, or network nodes other than the eNB. The term 'base station (BS) ' may be replaced with the term 'fixed station', 'Node B' , 'evolved Node B (eNode B or eNB)', 'Access Point (AP) ' , etc. The term 'UE' may be replaced with the term 'terminal', 'Mobile Station (MS)', 'Mobile Subscriber Station (MSS) ' , 'Subscriber Station (SS) ' , etc.

Specific terms used for the embodiments of the present invention are provided to help the understanding of the present invention. These specific terms may be replaced with other terms within the scope and spirit of the present invention. In some cases, to prevent the concept of the present invention from being ambiguous, structures and apparatuses of the known art will be omitted, or will be shown in the form of a block diagram based on main functions of each structure and apparatus. Also, wherever possible, the same reference numbers will be used throughout the drawings and the specification to refer to the same or like parts.

The embodiments of the present invention can be supported by standard documents disclosed for at least one of wireless access systems, Institute of Electrical and Electronics Engineers (IEEE) 802, 3^rd Generation Partnership Project (3GPP) , 3GPP Long Term Evolution (3GPP LTE) , LTE- Advanced (LTE-A) , and 3GPP2. Steps or parts that are not described to clarify the technical features of the present invention can be supported by those documents. Further, all terms as set forth herein can be explained by the standard documents .

Techniques described herein can be used in various wireless access systems such as Code Division Multiple Access (CDMA) , Frequency Division Multiple Access (FDMA) , Time Division Multiple Access (TDMA) , Orthogonal Frequency Division Multiple Access (OFDMA) , Single Carrier-Frequency Division Multiple Access (SC-FDMA) , etc. CDMA may be implemented as a radio technology such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented as a radio technology such as Global System for Mobile communications (GSM) /General Packet Radio Service (GPRS) /Enhanced Data Rates for GSM Evolution (EDGE) . OFDMA may be implemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX) , IEEE 802.20, Evolved-UTRA (E- UTRA) etc. UTRA is a part of Universal Mobile

Telecommunication System (UMTS) . 3GPP LTE is a part of Evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA for downlink and SC-FDMA for uplink. LTE-A is an evolution of 3GPP LTE. WiMAX can be described by the IEEE 802.16e standard (Wireless Metropolitan Area Network (WirelessMAN- OFDMA Reference System) and the IEEE 802.16m standard (WirelessMAN-OFDMA Advanced System) . For clarity, this application focuses on the 3GPP LTE/LTE-A system. However, the technical features of the present invention are not limited thereto.

FIG. 1 illustrates a radio frame structure used in a 3GPP LTE system. Referring to FIG. 1(a), one radio frame may be divided into 10 subframes, each subframe including two slots in the time domain. The transmission time of one subframe is defined as a Transmission Time Interval (TTI) . For example, one subframe may be 1ms long and one slot may be 0.5ms long. One slot may include a plurality of orthogonal frequency division multiplexing (OFDM) symbols in the time domain. Because the 3GPP LTE system uses orthogonal frequency division multiple access (OFDMA) for downlink, an OFDM symbol may represent one symbol period. An OFDM symbol may be regarded as a single carrier- frequency division multiple access (SC-FDMA) symbol or symbol period for uplink. A Resource Block (RB) is a resource allocation unit including a plurality of contiguous subcarriers in one slot. This radio frame structure is exemplary. Accordingly, the number of subframes included in a radio frame, the number of slots included in a subframe, and the number of OFDM symbols included in a slot may vary.

FIG. 1(b) illustrates the type-2 radio frame structure. The type-2 radio frame includes two half frames each having 5 subframes, a downlink pilot time slot (DwPTS) , a guard period (GP) , and an uplink pilot time slot (UpPTS) . Each subframe includes two slots. The DwPTS is used for initial cell search, synchronization, or channel estimation in a UE, whereas the UpPTS is used for channel estimation in an eNB and uplink transmission synchronization in a UE . The GP is a period between downlink and uplink, for eliminating interference with the uplink caused by multi-path delay of a downlink signal.

The aforementioned radio frame structure is purely exemplary and thus the number of subframes included in a radio frame, the number of slots included in a subframe, or the number of symbols included in a slot may vary.

FIG. 2 illustrates a resource grid in a downlink slot. While FIG. 2 shows that a downlink slot includes 7 OFDM symbols in the time domain and each RB has 12 subcarriers in the frequency domain, the present invention is not limited thereto. For example, one slot can include 7 OFDM symbols in a normal cyclic prefix (CP) case whereas one slot can include 6 OFDM symbols in an extended CP case. Each element in the resource grid is referred to as a resource element (RE) . An RB includes 12x7 REs . The number of RBs per downlink slot, N^DL depends on downlink transmission bandwidth An uplink slot structure may correspond to the downlink slot structure.

FIG. 3 illustrates a downlink subframe structure. Referring to FIG. 3, OFDM symbols at the start of a downlink subframe are used for a control region to which a control channel is allocated and the other OFDM symbols of the downlink subframe are used for a data region to which a physical downlink shared channel (PDSCH) is allocated. Downlink control channels used in an LTE system include a physical control format indicator channel (PCFICH) , a physical downlink control channel (PDCCH) , a physical hybrid automatic repeat request indicator channel (PHICH) , etc.

The_. PCFICH is located in the first OFDM symbol of a subframe, carrying information about the number of OFDM symbols used for transmission of control channels in the subframe .

The PHICH delivers a HARQ acknowledgment/negative acknowledgment (ACK/NACK) signal in response to an uplink transmission .

The PDCCH transmits downlink control information (DCI) .

The DCI may include uplink or downlink scheduling information or an uplink transmit power control command for an arbitrary UE group according to format. DCI format

DCI formats 0, 1, 1A, IB, 1C, ID, 2, 2A, 2B, 2C, 3, 3A and 4 are defined in LTE-A (release 10) . DCI formats 0, 1A, 3 and 3A have the same message size to reduce the number of blind decoding operations, which will be described later. The DCI formats may be divided into i) DCI formats 0 and 4 used for uplink scheduling grant, ii) DCI formats 1, 1A, IB, 1C, 2, 2A, 2B and 2C used for downlink scheduling allocation, and iii) DCI formats 3 and 3A for power control commands according to purpose of control information to be transmitted DCI format 0 used for uplink scheduling grant may include a carrier indicator necessary for carrier aggregation which will be described later, an offset (flag for format 0/format 1A differentiation) used to differentiate DCI formats 0 and 1A from each other, a frequency hopping flag that indicates whether frequency hopping is used for uplink PUSCH transmission, information about resource block assignment, used for a UE to transmit a PUSCH, a modulation and coding scheme, a new data indicator used to empty a buffer for initial transmission with respect to an HARQ process, a transmit power control (TPC) command for a scheduled PUSCH, information on a cyclic shift for a demodulation reference signal (DMRS) and OCC index, and an uplink index and channel quality indicator request necessary for a TDD operation, etc. DCI format 0 does not include a redundancy version, differently from DCI formats relating to downlink scheduling allocation, because DCI format 0 uses synchronous HARQ. The carrier indicator is not included in DCI formats when cross-carrier scheduling is not used.

DCI format 4 is newly added to DCI formats in LTE-A release 10 and supports application of spatial multiplexing to uplink transmission in LTE-A. DCI format 4 has a larger message size because it further includes information for spatial multiplexing. DCI format 4 includes additional control information in addition to control information included in DCI format 0. DCI format 4 includes information on a modulation and coding scheme for the second transmission block, precoding information for multi-antenna transmission, and sounding reference signal (SRS) request information. DCI format 4 does not include the offset for format 0/format 1A differentiation because it has a size larger than DCI format 0.

DCI formats 1, 1A, IB, 1C, ID, 2, 2A, 2B and 2C for downlink scheduling allocation may be divided into DCI formats 1, 1A, IB, lC and ID that do not support spatial multiplexing and DCI formats 2, 2A, 2B and 2C that support spatial multiplexing.

DCI format 1C supports only frequency contiguous allocation as compact frequency allocation and does not include the carrier indicator and redundancy version, compared to other formats.

DCI format 1A is for downlink scheduling and random access procedure. DCI format 1A may include a carrier indicator, an indicator that indicates whether downlink distributed transmission is used, PDSCH resource allocation information, a modulation and coding scheme, a redundancy version, a HARQ processor number for indicating a processor used for soft combining, a new data indicator used to empty a buffer for initial transmission with respect to a HARQ process, a TPC command for a PUCCH, an uplink index necessary for a TDD operation, etc.

DCI format 1 includes control information similar to that of DCI format 1A. DCI format 1 supports non-contiguous resource allocation whereas DCI format 1A supports contiguous resource allocation. Accordingly, DCI format 1 further includes a resource allocation header, and thus slightly increases control signaling overhead as a trade-off for an increase in resource allocation flexibility.

Both DCI formats IB and ID further include precoding information, compared to DCI format 1. DCI format IB includes PMI acknowledgement and DCI format ID includes downlink power offset information. Most control information included in DCI formats IB and ID corresponds to that of DCI format 1A.

DCI formats 2, 2A, 2B and 2C include most control information included in DCI format 1A and further include information for spatial multiplexing. The information for spatial multiplexing includes a modulation and coding scheme for the second transmission block, a new data indicator, and a redundancy version.

DCI format 2 supports closed loop spatial multiplexing and DCI format 2A supports open loop spatial multiplexing. Both DCI formats 2 and 2A include precoding information. DCI format 2B supports dual layer spatial multiplexing combined with beamforming and further includes cyclic shift information for a DMRS . DCI format 2C may be regarded as an extended version of DCI format 2B and supports spatial multiplexing for up to 8 layers.

DCI formats 3 and 3A may be used to complement the TPC information included in the aforementioned DCI formats for uplink scheduling grant and downlink scheduling allocation, that is, to support semi-persistent scheduling. A 1-bit command is used per UE in the case of DCI format 3 whereas a 2 -bit command is used per UE in the case of DCI format 3A.

One of the above-mentioned DCI formats is transmitted through a PDCCH, and a plurality of PDCCHs may be transmitted in a control region. A UE can monitor the plurality of PDCCHs .

PDCCH processing

When DCI is transmitted on a PDCCH, a cyclic redundancy check (CRC) is added to the DCI. The CRC is masked by a radio network temporary identifier (RNTI) . Here, different RNTIs may be used according to transmission purpose of the DCI. Specifically, P-RNTI may be used for a paging message relating to network initiated connection establishment, RA- RNTI may be used in a case relating to random access, and SI- RNTI may be used in a case relating to a symbol information block (SIB) . In the case of unicast transmission, C-RNTI, a unique UE identifier, may be used. The DCI with the CRC added thereto is coded into a predetermined code, and then adjusted to correspond to the quantity of resources used for transmission through rate-matching.

In PDCCH transmission, control channel elements (CCEs) , contiguous logical allocation units, are used to map a PDCCH to REs for efficient processing. A CCE includes 36 REs corresponding to 9 resource element groups (REGs) . The number of CCEs necessary for a specific PDCCH depends on a DCI payload corresponding, to a control information size, a cell bandwidth, a channel coding rate, etc. Specifically, the number of CCEs for a specific PDCCH can be defined according to PDCCH format, as shown in Table 1.

[Table 1]

As shown in Table 1, the number of CCEs depends on the

PDCCH format. For example, a transmitter can adaptively use PDCCH formats in such a manner that it uses PDCCH format 0 and changes PDCCH format 0 to PDCCH format 2 when a channel status becomes poor. Blind decoding

While one of the above-mentioned PDCCH formats may be used, this is not signaled to a UE . Accordingly, the UE performs decoding without knowing the PDCCH format, which is referred to as blind decoding. Since operation overhead is generated if a UE decodes all CCEs that can be used for downlink for each PDCCH, a search space is defined in consideration of limitation for a scheduler and the number of decoding attempts.

The search space is a set of candidate PDCCHs composed of CCEs on which a UE needs to attempt to perform decoding at an aggregation level. The aggregation level and the number of candidate PDCCHs can be defined as shown in Table 2.

[Table 2]

As shown Table 2, the UE has a plurality of search spaces at each aggregation level because 4 aggregation levels are present . The search spaces may be divided into a UE-specific search space and a common search space, as shown in Table 2. The UE-specific search space is for a specific UE . Each UE may check an RNTI and CRC which mask a PDCCH by monitoring a UE-specific search space thereof (attempting to decode a PDCCH candidate set according to an available DCI format) and acquire control information when the RNTI and CRC are valid.

The common search space is used for a case in which a plurality of UEs or all UEs need to receive PDCCHs, for system information dynamic scheduling or paging messages, for example. The common search space may be used for a specific UE for resource management. Furthermore, the common search space may overlap with the UE-specific search space. FIG. 4 illustrates an uplink subframe structure. An uplink subframe may be divided into a control region and a data region in the frequency domain. A physical uplink control channel (PUCCH) carrying uplink control information is allocated to the control region and a physical uplink shared channel (PUSCH) carrying user data is allocated to the data region. To maintain a single carrier property, a UE does not transmit a PUSCH and a PUCCH simultaneously. A PUCCH for a UE is allocated to an RB pair in a subframe. The RBs of the RB pair occupy different subcarriers in two slots. Thus it is said that the RB pair allocated to the PUCCH is frequency-hopped over a slot boundary.

Physical uplink control channel (PUCCH)

Uplink control information (UCI) transmitted on a PUCCH may include a scheduling request (SR) , HARQ ACK/NACK information, and downlink channel measurement information.

The HARQ ACK/NACK information may be generated according to whether a downlink data packet on a PDSCH is successfully decoded. In conventional wireless communication systems, 1 bit is transmitted as ACK/NACK information for downlink single codeword transmission and 2 bits are transmitted as the ACK/NACK .information for 2 -codeword downlink transmission The channel measurement information represents feedback information about a multiple input multiple output (MIMO) scheme and may include a channel quality indicator (CQI) , a precoding matrix index (PMI) , and a rank indicator (RI) which may be collectively referred to as a CQI. 20 bits per subframe may be used to transmit the CQI .

A PUCCH can be modulated using binary phase shift keying

(BPSK) and quadrature phase shift keying (QPSK) . Control information of a plurality of UEs can be transmitted through a PUCCH. When code division multiplexing (CDM) is performed in order to distinguish signals of the UEs from one another, a length-12 constant amplitude zero autocorrelation (CAZAC) sequence is used. The CAZAC sequence is suitable to increase coverage by reducing a peak-to-average power ratio (PAPR) of a UE or cubic metric (CM) because it maintains a specific amplitude in the time domain and the frequency domain. ACK/NACK information with respect to downlink data transmitted through a PUCCH is covered using an orthogonal sequence or an orthogonal cover (OC) .

Control information signals transmitted on a PUCCH may be distinguished using cyclically shifted sequences having different cyclic shift (CS) values. A cyclically shifted sequence may be generated by cyclically shifting a base sequence by a specific CS amount. The specific CS amount is indicated by a CS index. The number of available CSs may vary according to channel delay spread. Various types of sequences may be used as the base sequence and the aforementioned CAZAC sequence is an example of the various sequences .

The amount of control information that can be transmitted by a UE through a subframe can be determined according to the number of SC-FDMA symbols (i.e. SC-FDMA symbols other than SC-FDMA symbols used for reference signal (RS) transmission for detection of coherent of a PUCCH) which can be used for control information transmission. PUCCH format 1 is used to transmit an SR only. When the SR is solely transmitted, an unmodulated waveform is applied, which will be described in detail below.

PUCCH format la or lb is used for HARQ ACK/NACK transmission. When HARQ ACK/NACK alone is transmitted in a subframe, PUCCH format la or lb may be used. Furthermore, HARQ ACK/NACK and SR may be transmitted in the same subframe using PUCCH format la or lb.

PUCCH format 2 is used for CQI transmission whereas PUCCH format 2a or 2b is used for transmission of CQI and HARQ ACK/NACK. In the extended CP case, PUCCH format 2 may be used for transmission of CQI and HARQ ACK/NACK.

FIG. 5 illustrates mapping of PUCCH formats to PUCCH

_NUL

regions in uplink physical resource blocks. In FIG. 5, * RB denotes the number of resource blocks on uplink and 0, 1, N^UL 1

i RB— ^A denote physical resource block numbers. PUCCHs are mapped to both edges of uplink frequency blocks basically.

As shown in FIG. 5, PUCCH formats 2/2a/2b are mapped to PUCCH regions indicated by m=0,l, which represents that PUCCH formats 2/2a/2b are mapped to resource blocks located at band-edges. PUCCH formats 2/2a/2b and PUCCH formats 1/la/lb may be mixed and mapped to PUCCH regions indicated by m=2.

PUCCH formats 1/la/lb may be mapped to PUCCH regions indicated by m=3,4,5. The number NR⁽²B of PUCCH RBs can be used by PUCCH formats 2/2a/2b may be signaled to UEs in a cell through broadcast signaling.

Reference signal (RS)

In a wireless communication system, a packet is transmitted on a radio channel . In view of the nature of the radio channel, the packet may be distorted during transmission. To receive the signal successfully, a receiver should compensate for distortion in the received signal using channel information. Generally, to enable the receiver to acquire the channel information, a transmitter transmits a signal known to both the transmitter and the receiver and the receiver acquires knowledge of channel information based on the distortion of the signal received on the radio channel. This signal is called a pilot signal or an RS .

In case of data transmission and reception through multiple antennas, knowledge of channel states between transmit antennas and receive antennas is required for successful signal reception. Accordingly, an RS should exist separately for each transmit antenna.

Downlink RSs include a common reference signal (CRS) shared by all UEs in a cell and a dedicated RS (DRS) for a specific UE only. Information for channel estimation and demodulation may be provided using these RSs. A receiver (UE) may estimate a channel state from a CRS and feed back an indicator relating to channel quality, such as a CQI, PMI and/or RI to a transmitter (eNB) . The CRS may also be called a cell-specific RS . An RS relating to feedback of channel state information (CSI) such as CQI/PMI/RI may be separately defined as a CSI-RS.

The DRS may be transmitted through a corresponding RE when data on a PDSCH needs to be demodulated. A higher layer may signal presence or absence of the DRS to a UE . In addition, it is possible to signal that the DRS is valid only when a corresponding PDSCH is mapped to the UE. The DRS may also be referred to as a UE-specific RS or a demodulation RS (DMRS) .

FIG. 7 illustrates patterns of mapping CRSs and DRSs defined in a 3GPP LTE system (e.g. release-8) to downlink RB pairs. A downlink RB pair as an RS mapping unit may be represented as (one subframe in the time domain) ( 12 subcarriers in the frequency domain) . That is, a RB pair has a length corresponding to 14 OFDM symbols in the time domain in the normal CP case (FIG. 7(a)) and has a length corresponding to 12 OFDM symbols in the extended CP case (FIG. 7(b) ) . reference signal ( SRS) An SRS is used for an eNB to measure channel quality and perform uplink frequency-selective scheduling based on the channel quality measurement. The SRS is not associated with data and/or control information transmission. However, the usages of the SRS are not limited thereto. The SRS may also be used for enhanced power control or for supporting various start-up functions of non-scheduled UEs . The start-up functions may include, for example, an initial modulation and coding scheme ( CS) , initial power control for data transmission, timing advance, and frequency non-selective scheduling (in which a transmitter selectively allocates a frequency resource to the first slot of a subframe and then pseudo-randomly hops to another frequency resource in the second slot of the subframe) . The SRS may be used for measuring downlink channel quality on the assumption of the reciprocity of a radio channel between the downlink and the uplink. This assumption is valid especially in a time division duplex (TDD) system in which downlink and uplink share the same frequency band and are distinguished by time.

A subframe in which a UE within a cell is supposed to transmit an SRS is indicated by cell -specific broadcast signaling. A 4-bit cell-specific parameter ^λ srsSubframeConfiguration' indicates 15 possible configurations for subframes carrying SRSs in each radio frame. These configurations may provide flexibility with which SRS overhead can be adjusted according to network deployment scenarios. The other configuration (a 16^th configuration) represented by the parameter is for switch-off of SRS transmission in a cell, suitable for a cell serving high-speed UEs, for example.

An SRS is always transmitted in the last SC-FDMA symbol of a configured subframe . Therefore, an SRS and a DMRS are positioned in different SC-FDMA symbols. PUSCH data transmission is not allowed in an SC-FDMA symbol designated for SRS transmission. Accordingly, even the highest sounding overhead (in the case where SRS symbols exist in all subframes) does not exceed 7%.

Each SRS symbol is generated for a given time unit and frequency band, using a base sequence (a random sequence or Zadoff -Chu (ZC) -based sequence set) , and all UEs within a cell use the same base sequence. SRS transmissions in the same time unit and the same frequency band from a plurality of UEs within a cell are distinguished orthogonally by different cyclic shifts of the base sequence allocated to the plurality of UEs. Although the SRS sequences of different cells may be distinguished by allocating different base sequences to the cells, orthogonality is not ensured between the different base sequences. Carrier aggregation FIG. 8 is a diagram illustrating carrier aggregation (CA) . The concept of a cell, which is introduced to manage radio resources in LTE-A is described prior to the CA. A cell may be regarded as a combination of downlink resources and uplink resources. The uplink resources are not essential elements, and thus the cell may be composed of the downlink resources only or both the downlink resources and uplink resources. This is defined in LTE-A release 10, and the cell may be composed of the uplink resources only. The downlink resources may be referred to as downlink component carriers and the uplink resources may be referred to as uplink component carriers . A DL CC and a UL CC may be represented by carrier frequencies. A carrier frequency means a center frequency in a cell.

Cells may be divided into a primary cell (PCell) operating at a primary frequency and a secondary cell (SCell) operating at a secondary frequency. The PCell and Scell may be collectively referred to as serving cells. The PCell may be designated during an initial connection establishment, connection re-establishment or handover procedure of a UE . That is, the PCell may be regarded as a main cell relating to control in a CA environment. A UE may be allocated a PUCCH and transmit the PUCCH in the PCell thereof. The SCell may be configured after radio resource control (RRC) connection establishment and used to provide additional radio resources. Serving cells other than the PCell in a CA environment may be regarded as SCells. For a UE in an RRC_connected state for which CA is not established or a UE that does not support CA, only one serving cell composed of the PCell is present. For a UE in the RRC-connected state for which CA is established, one or more serving cells are present and the serving cells include a PCell and SCells. For a UE that supports CA, a network may configure one or more SCells in addition to a PCell initially configured during connection establishment after initial security activation is initiated.

CA is described with reference to FIG. 8. CA is a technology introduced to use a wider band to meet demands for a high transmission rate. CA can be defined as aggregation of two or more component carriers (CCs) having different carrier frequencies. FIG. 8(a) shows a subframe when a conventional LTE system uses a single CC and FIG. 8(b) shows a subframe when CA is used. In FIG. 8(b), 3 CCs each having 20MHz are used to support a bandwidth of 60MHz. The CCs may be contiguous or non-contiguous .

A UE may simultaneously receive and monitor downlink data through a plurality of DL CCs. Linkage between a DL CC and a UL CC may be indicated by system information. DL CC/UL CC linkage may be fixed to a system or semi-statically configured. Even when a system bandwidth is configured of N CCs, a frequency bandwidth that can be monitored/received by a specific UE may be limited to M (<N) CCs . Various parameters for CA may be configured cell -specifically, UE group-specifically, or UE-specifically .

FIG. 9 is a diagram illustrating cross-carrier scheduling. Cross carrier scheduling is a scheme by which a control region of one of DL CCs of a plurality of serving cells includes downlink scheduling allocation information the other DL CCs or a scheme by which a control region of one of DL CCs of a plurality of serving cells includes uplink scheduling grant information about a plurality of UL CCs linked with the DL CC .

A carrier indicator field (CIF) is described first.

The CIF may be included in a DCI format transmitted through a PDCCH or not. When the CIF is included in the DCI format, this represents that cross carrier scheduling is applied. When cross carrier scheduling is not applied, downlink scheduling allocation information is valid on a DL CC currently carrying the downlink scheduling allocation information. Uplink scheduling grant is valid on a UL CC linked with a DL CC carrying downlink scheduling allocation information.

When cross carrier scheduling is applied, the CIF indicates a CC associated with downlink scheduling allocation information transmitted on a DL CC through a PDCCH. For example, referring to FIG. 9, downlink allocation information for DL CC B and DL CC C, that is, information about PDSCH resources is transmitted through a PDCCH in a control region of DL CC A. A UE can recognize PDSCH resource regions and the corresponding CCs through the CIF by monitoring DL CC A.

Whether or not the CIF is included in a PDCCH may be semi-statically set and UE-specifically enabled according to higher layer signaling. When the CIF is disabled, a PDCCH on a specific DL CC may allocate a PDSCH resource on the same DL CC and assign a PUSCH resource on a UL CC linked with the specific DL CC. In this case, the same coding scheme, CCE based resource mapping and DCI formats as those used for the conventional PDCCH structure are applicable.

When the CIF is enabled, a PDCCH on a specific DL CC may allocate a PDSCH/PUSCH resource on a DL/UL CC indicated by the CIF from among aggregated CCs. In this case, the CIF can be additionally defined in existing PDCCH DCI formats. The CIF may be defined as a field having a fixed length of 3 bits, or a CIF position may be fixed irrespective of DCI format size. In this case, the same coding scheme, CCE based resource mapping and DCI formats as those used for the conventional PDCCH structure are applicable.

Even when the CIF is present, an eNB can allocate a DL CC set through which a PDCCH is monitored. Accordingly, blinding decoding overhead of a UE can be reduced. A PDCCH monitoring CC set is part of aggregated DL CCs and a UE can perform PDCCH detection/decoding in the CC set only. That is, the eNB can transmit the PDCCH only on the PDCCH monitoring CC set in order to schedule a PDSCH/PUSCH for the UE . The PDCCH monitoring DL CC set may be configured UE-specifically, UE group-specifically or . cell-specifically. For example, when 3 DL CCs are aggregated as shown in FIG. 9, DL CC A can be configured as a PDCCH monitoring DL CC. When the CIF is disabled, a PDCCH on each DL CC can schedule only the PDSCH on DL CC A. When the CIF is enabled, the PDCCH on DL CC A can schedule PDSCHs in other DL CCs as well as the PDSCH in DL CC A. When DL CC A is set as a PDCCH monitoring CC, DL CC B and DL CC C do not transmit PDSCHs.

In a system to which the aforementioned CA is applied, a UE can receive a plurality of PDSCHs through a plurality of downlink carriers. In this case, the UE should transmit ACK/NACK for data on a UL CC in a subframe . When a plurality of ACK/NACK signals is transmitted in a subframe using PUCCH format la/lb, high transmit power is needed, a PAPR of uplink transmission increases and a transmission distance of the UE from the eNB may decrease due to inefficient use of a transmit power amplifier. To transmit a plurality of ACK/NACK signals through a PUCCH, ACK/NACK bundling or ACK/NACK multiplexing may be employed.

There may be generated a case in which ACK/NACK information for a large amount of downlink data according to application of CA and/or a large amount of downlink data transmitted in a plurality of DL subframes in a TDD system needs to be transmitted through a PUCCH in a subframe. In this case, the ACK/NACK information cannot be successfully transmitted using the above-mentioned ACK/NACK bundling or multiplexing when the number of ACK/NACK bits to be transmitted is greater than the number of ACK/NACK bits that can be supported by ACK/NACK bundling or multiplexing. Heterogeneous deployment

FIG. 10 illustrates a heterogeneous network wireless communication system including a macro eNB (MeNB) and micro eNBs (PeNB or FeNB) . The term 'heterogeneous network' means a network in which an MeNB and a PeNB or FeNB coexist even when they use the same radio access technology (RAT) .

The MeNB is a normal eNB of a wireless communication system having wide coverage and high transmit power. The MeNB may be referred to as a macro cell .

The PeNB or FeNB may be referred to as a micro cell, pico cell, femto cell, home eNB (HeNB) , relay, etc. (the exemplified PeNB or FeNB and MeNB may be collectively referred to as transmission points) . The PeNB or FeNB, a micro version of the MeNB, can independently operate while performing most functions of the MeNB. The PeNB or FeNB is a non-overlay type eNB that may be overlaid in an area covered by the MeNB or in a shadow area that is not covered by the MeNB. The PeNB or FeNB may cover a smaller number of UEs while having a narrower coverage and lower transmit power compared to the MeNB.

A UE (referred to as a macro-UE (MUE) hereinafter) may be directly served by the MeNB or a UE (referred to as a micro-UE (PUE or FUE) hereinafter) may be served by the PeNB or FeNB. In some cases, a PUE present in the coverage of the MeNB may be served by the MeNB .

The PeNB or FeNB may be classified into two types according to whether UE access is limited.

The first type is an open access subscriber group (OSG) or non-closed access subscriber group (CSG) eNB and corresponds to a cell that allows access of the existing MUE or a PUE of a different PeNB. The existing MUE can handover to the OSG type eNB.

The second type is a CSG eNB which does not allow access of the existing MUE or a PUE of a different PeNB. Accordingly, handover to the CSG eNB is impossible.

Inter-cell interference control (ICIC)

In the heterogeneous network environment as described above, interference between neighboring cells may be a problem. To solve this inter-cell interference, inter-cell interference control (ICIC) may be applied. Conventional ICIC can be applied to frequency resources or time resources.

As exemplary ICIC for the frequency resources, 3GPP LTE release- 8 defines a scheme of dividing a given frequency region (e.g. system bandwidth) into one or more sub-regions (e.g. physical resource blocks (PRBs) ) and exchanging an ICIC message for each sub-region between cells. For example, relative narrowband transmission power (RNTP) associated with downlink transmission power, UL interference overhead indication (IOI) and UL high interference indication (HII) associated with uplink interference are defined as information included in the ICIC message for the frequency resources .

The RNTP is information indicating downlink transmission power used by a cell that transmits an ICIC message in a specific frequency sub-region. For example, when an RNTP field for a specific frequency sub-region is set to a first value (e.g. 0), this represents that downlink transmission power of a corresponding cell does not exceed a threshold value in the specific frequency sub-region. When the RNTP field for the specific frequency sub-region is set to a second value (e.g. 1), this represents that the corresponding cell cannot guarantee the downlink transmission power in the specific frequency sub-region. In other words, the downlink transmission power of the cell can be regarded as low when the RNTP field is 0, whereas the downlink transmission power of the cell cannot be regarded as low when the RNTP field is 1.

The UL IOI is information indicating the quantity of uplink interference that a cell transmitting an ICIC message suffers in a specific frequency sub-region. For example, when an IOI field for a specific frequency sub-region is set to a value corresponding to a large amount of interference, this represents that a corresponding cell suffers strong uplink interference in the specific frequency sub-region. A cell receiving an ICIC message can schedule UEs using low uplink transmission power from among UEs thereof in a frequency sub-region corresponding to IOI indicating strong uplink interference. Accordingly, UEs can perform uplink transmission with low transmit power in the frequency sub- region corresponding to the IOI indicating strong uplink interference, and thus uplink interference that a neighboring cell (i.e. cell transmitting the ICIC message) suffers can be alleviated.

The UL HI I is information indicating a degree of interference (or uplink interference sensitivity) that may be generated for the corresponding frequency sub-region according to uplink transmission in the cell transmitting the ICIC message. For example, when an HII field is set to a first value (e.g. 1) for a specific frequency sub-region, this represents that the cell transmitting the ICIC message may schedule UEs having high uplink transmit power for the specific frequency sub-region. On the contrary, when the HII field is set to a second value (e.g. 0) for the specific frequency sub-region, this represents that the cell transmitting the ICIC message may schedule UEs having low uplink transmission power for the specific frequency sub- region. The cell receiving the ICIC message can avoid interference from the cell transmitting the ICIC message by preferentially scheduling UEs to the frequency sub-region to which the HII field is set to the second value (e.g. 0) and scheduling UEs that can successfully operate even in a strong interference environment to the frequency sub-region to which the HII field is set to the first value (e.g. 1) .

As exemplary ICIC for the time resources, 3GPP LTE-A (or

3GPP LTE release- 10) defines a scheme of dividing the entire time domain into one or more time sub-regions (e.g. subframes) in the frequency domain and exchanging information on whether silencing is performed on each time sub-region between cells. The cell transmitting the ICIC message may transmit information indicating that silencing is performed in a specific subframe to neighboring cells and does not schedule a PDSCH or a PUSCH in the specific subframe. The cell receiving the ICIC message may schedule uplink and/or downlink transmission for UEs on the subframe in which silencing is performed in the cell transmitting the ICIC message .

Silencing may represent an operation in which a specific cell does not transmit signals (or transmits zero power or weak power) in a specific subframe on uplink and downlink. As an example of silencing, a specific cell can set a specific subframe as an almost blank subframe (ABS) . There are two types of ABS. Specifically, one type is an ABS in a normal subframe in which a data region is vacant while a CRS is transmitted and the other type is an ABS in an MBSFN subframe in which even a CRS is not transmitted. In the ABS in a normal subframe, interference of the CRS may be present. Accordingly, the ABS in an MBSFN subframe has an advantage in terms of interference. However, use of the ABS in an MBSFN subframe is limited, and thus the two ABS may be used together.

FIG. 11 illustrates a scheme of alleviating interference by allocating PDSCHs to UEs located at the edges of cells in orthogonal frequency regions, which can be used to exchange scheduling information between eNBs . However, a PDCCH is transmitted over the entire downlink bandwidth, as described above, and thus interference due to the PDCCH cannot be mitigated. For example, since a time-frequency region in which a PDCCH is transmitted from eNBl to UE1 and a time- frequency region in which a PDCCH is transmitted from eNB2 to UE2 overlap, PDCCH transmission for UEl and PDCCH transmission for UE2 interfere with each other.

Referring to FIG. 12, a PUCCH or a PUSCH transmitted from UEl may interfere with a PDCCH or a PDSCH received by UE2 adjacent to UEl. Here, if scheduling information is exchanged between eNBl and eNB2 , interference in the PDSCH can be avoided by allocating the UEs to orthogonal frequency regions. However, the PDCCH is affected by the PUCCH or PUSCH transmitted from UEl.

For this reason, introduction of an ePDCCH different from the PDCCH is discussed. A description will be given of a method of transmitting an ePDCCH on part of a PDCCH or on the conventional control region using spatial multiplexing and a method of signaling information about a precoding matrix used for spatial multiplexing to UEs.

FIG. 13 illustrates an ePDCCH resource region applicable to embodiments of the present invention. FIG. 13(a) illustrates transmission of an ePDCCH through the entire time domain of a PDSCH region of a subframe . As shown in FIG. 13(a), the ePDCCH may be multiplexed for UEs UEl, UE2 and UE3 in the frequency domain. FIG. 13(b) illustrates transmission of an ePDCCH (Fast detectable-ePDCCH, FD-ePDCCH) in a PDSCH region of the first slot of a subframe. In this case, the ePDCCH can also be multiplexed for the UEs UEl, UE2 and UE3. A sufficient time for an HARQ process can be secured since the ePDCCH is transmitted on a PDSCH in the first slot. FIG. 13 (c) illustrates transmission of an ePDCCH (beamformed PDCCH) on the conventional control region. The ePDCCHs transmitted in the regions shown in FIGS. 13(a), 13(b) and 13 (c) may be obtained by mapping a codeword including DCI to a layer and precoding the layer. This is described in detail with reference to FIG. 14.

FIG. 14 illustrates a process of mapping a codeword including DCI to an antenna port according to an embodiment of the present invention.

Referring to FIG. 14, a codeword to be mapped to a layer includes DCI. Specifically, two or more codewords may be present. In this case, different DCI formats may be allocated to the codewords. For example, DCI formats 0 and 4 associated with uplink grant can be assigned to codeword #0 and DCI formats 1, 1A, IB, 1C and 2 associated with downlink allocation can be assigned to codeword #1. It the two codewords have different sizes, size matching using zero padding may be employed to adjust the sizes of the codewords to correspond to each other. Scrambling and/or modulation such as BPSK may be applied to the codewords, which is not shown in the figure. Referring to FIG. 14, the codewords (modulated symbols when modulation is applied) are mapped to one or more layers by a layer mapper. The codewords (or modulated symbols) mapped to the layers may be precoded with a precoding matrix of a codebook. The codebook is a set of a plurality of precoding matrices and may be shared by an eNB and a UE . For example, the codebook may include a plurality of precoding matrices indicated by combinations of k codebook indexes and 1 layer indexes, which is shown in Table 3. However, the embodiments of the present invention are not limited to the codebook shown in Table 3 and various codebooks used in LTE/LTE-A systems can be employed.

[Table 3]

W_n = I - 2u

In Table 3, _nuZ/ut ⁿ where I is a 4x4 unit matrix. A total of 64 precoding matrices are indicated by the codebook indexes and the layer indexes. While one of the 64 precoding matrices shown in Table 3 can be used for the codewords (modulated symbols) mapped to the layers, the present invention can set a candidate precoding matrix set and use matrices corresponding to the candidate precoding matrix. Here, the candidate precoding matrix set may include precoding matrices indicated by combinations of m (m≤ k) codebook indexes and n (n≤l) layer indexes. For example, the candidate precoding matrix set can include 6 precoding matrices indicated by code indexes 4, 5 and 6 and layer indexes 2 and 3 in Table 3. In this case, a UE can perform blind de-precoding on the matrices corresponding to the candidate precoding matrix.

The precoded codewords (or modulated symbols) are mapped to resource regions by antenna ports by resource element mappers. Then, the mapped precoded codewords are generated as OFDM symbols and transmitted through the respective antenna ports.

When DCI is transmitted as an ePDCCH based on a DMRS, the UE can perform demodulation by estimating a used precoding matrix through the DMRS. This is because the UE can estimate an equivalent channel matrix obtained by multiplying the precoding matrix by a channel matrix (a radio channel represented in a matrix form, through which an actually transmitted signal is transmitted) since the precoding matrix applied to the DCI is equally applied to the DMRS.

In the case of CRS based ePDCCH transmission, if a legacy PDCCH is used, it is possible to know a precoding matrix used for demodulation through a precoding information field. However, it is necessary to signal information about the precoding matrix to the UE when DCI is not transmitted on the legacy PDCCH, a DCI format including no precoding information field is used even if the DCI is transmitted on the legacy PDCCH, or successful reception cannot be achieved due to interference as described above even if the DCI is transmitted on the legacy PDCCH. Furthermore, when the precoding matrix is selected from the candidate precoding matrix set and the UE performs blind de-precoding on matrices belonging to the candidate precoding matrix set, it is necessary to signal information about the precoding matrix, that is, information about the candidate precoding matrix set to the UE . Methods of signaling the information about the precoding matrix will now be described with reference to FIGS. 15 to 17. In FIGS. 15 to 17, the codebook of Table 3 is used.

Firstly, precoding matrices included in the cookbook can be transmitted in the form of a bitmap to the UE . Specifically, when 64 precoding matrices are included in the codebook, as shown in Table 3, it is possible to generate a 64 -bit bitmap in which bits corresponding to precoding matrices belonging to the candidate precoding matrix set are set to 1 and bits corresponding to the other precoding matrices are set to 0. This bitmap can be signaled to the UE through higher layer signaling (RRC signaling) .

Secondly, the candidate precoding matrix set can be transmitted in a bitmap form corresponding to at least one of the codebook indexes and layer indexes. When the candidate precoding matrix set is limited to the codebook indexes only in Table 4, for example, if the candidate precoding matrix set corresponds to codebook indexes 4 to 6, a bitmap of 0,0,0,0,1,1,1,0,0,0,0,0,0,0,0,0 can be configured. Otherwise, when the candidate precoding matrix set is limited to the layer indexes only, for example, if the candidate precoding matrix set corresponds to layer indexes 2 to 4 , a bitmap of 0,1,1,1 can be configured. If the candidate precoding matrix set is limited to both the codebook indexes and layer indexes, as illustrated in FIG. 15, that is, when the candidate precoding matrix set corresponds to codebook indexes 4 to 6 and layer indexes 2 to 4, a bitmap of 0,0,0,0,1,1,1,0,0,0,0,0,0,0,0,0,0,1,1,1 can be generated.

Thirdly, it is possible to set contiguous precoding matrices in the cookbook as the candidate precoding matrix set and transmit the candidate precoding matrix set in the form of a bitmap to the UE . Referring to FIG. 16, contiguous precoding matrices (shaded parts) are set as the candidate precoding matrix set . candidate precoding matrix index may be defined by Expression 1.

[Expression 1]

\6{v - 1) + codebook index

Here, V denotes the number of layers (layer index) and codebook index represents the codebook index. Indexes of the precoding matrices forming the codebook are defined according to Expression 1, and the candidate precoding matrix set may be indicated using the indexes. To signal the candidate precoding matrix set, bitmaps can be configured using i) the start precoding matrix index and the last precoding matrix index of the candidate precoding matrix set and ii) the start precoding matrix index of the candidate precoding matrix set and information on the number of contiguous precoding matrices. Specifically, the candidate precoding matrix shown in FIG. 16 can be indicated by a bitmap of 0,0,0,0,1,1,0,1,1,1,0,1 using start precoding matrix index 3 and the last precoding matrix index 29 according to i) . According to ii) , the candidate precoding matrix shown in FIG. 16 can be indicated by a bitmap of 0,0,0,0,1,1,0,1,1,0,1,1 using start precoding matrix index 3 and the number of contiguous precoding matrices, 27.

Fourthly, it is possible to set sub sets of the codebook and signal sub sets forming the candidate precoding matrix set to the UE .

FIG. 17 shows 4 sub sets, subset#l to subset#4 of the codebook are set. In FIG. 17, shaded parts indicate the candidate precoding matrix set. Here, a bitmap representing the candidate precoding matrix set may be configured of 2 -bit subset indexes each corresponding to a respective one of 4 layer indexes. That is, the candidate precoding matrix set shown in FIG. 17 can be represented in a bitmap composed of 00010011. If each layer index uses the same subset, the candidate precoding matrix set may be represented using only a 2 -bit subset index.

The above-mentioned bitmap . for indicating the candidate precoding matrix set may be signaled to the UE through , higher layer signaling (RRC signaling) or a medium access control (MAC) message.

Since the aforementioned methods of using the candidate precoding matrix set are signaled through higher layer signaling and the like, a method of using most recently used precoding matrix (or candidate precoding matrix set) may be used along with the above-mentioned methods when the UE has high mobility or instantaneous channel variation is large.

For example, when the UE has low mobility or the instantaneous channel variation is small, the eNB uses one of precoding matrices included in the candidate precoding matrix set, as described above, and the UE performs blind de- precoding using precoding matrices belonging to a long-term candidate precoding matrix set signaled thereto through higher layer signaling. In this case, if UE velocity increases or the channel state rapidly varies, the eNB and the UE may be set to use the most recently used precoding matrix (or candidate precoding matrix set) . The most recently used precoding matrix may be a precoding matrix used for the UE for most recent PDSCH transmission or a precoding matrix indicated by the most recently obtained PDCCH precoding information field. A bitmap for indicating a candidate precoding matrix (or candidate precoding matrix set) may be signaled using an offset value from a long-term candidate precoding matrix (or candidate precoding matrix set) or the most recently used precoding matrix (or candidate precoding matrix set) .

FIG. 18 illustrates configurations of an eNB and a UE according to an embodiment of the present invention.

Referring to FIG. 18, the eNB 1810 may include a reception module 1811, a transmission module 1812, a processor 1813, a memory 1814, and a plurality of antennas 1815. The plurality of antennas 1815 represents that the eNB 1810 supports MIMO transmission/reception . The reception module 1811 may receive signals, data and information on uplink from the UE . The transmission module 1812 may transmit signals, data and information to the UE on downlink. The processor 1813 may control the overall operation of the eNB 1810.

The processor 1813 of the eNB 1810 precodes a codeword including DCI using one of precoding matrices included in a candidate precoding matrix set of the codebook. The UE may need to attempt to de-precode the precoding matrices included in the candidate precoding matrix set for the DCI .

In addition, the processor 1813 of the eNB 1810 may process information received by the eNB 1810, information to be transmitted to the outside, etc. The memory 1814 may store the processed information for a predetermined time and may be replaced by a component such as a buffer (not shown) .

The UE 1820 may include a reception module 1821, a transmission module 1822, a processor 1823, a memory 1824, and a plurality of antennas 1825. The plurality of antennas 1825 represents that the UE 1820 supports MIMO transmission/reception. The reception module 1821 may receive signals^', data and information on downlink from the eNB. The transmission module 1822 may transmit signals, data and information on uplink to the eNB. The processor 1823 may control the overall operation of the UE 1820.

The processor 1823 of the UE 1820 may attempt to perform de-precoding using precoding matrices included in the candidate precoding matrix set of the codebook for DCI in a predetermined resource region of a subframe.

The processor 1823 of the UE 1820 may process information received by the UE 1820, information to be transmitted to the outside, etc. The memory 1824 may store the processed information for a predetermined time and may be replaced by a component such as a buffer (not shown) .

The detailed configurations of the eNB and the UE may be implemented such that the aforementioned embodiments of the present invention can be independently applied thereto or two or more embodiments can be simultaneously applied thereto, description of redundant parts is omitted for clarity.

Description of the eNB 1810 in FIG. 18 may be equally applied to an apparatus as a downlink transmitter or an uplink receiver and description of the UE 1820 may be equally applied to a relay as a downlink receiver or an uplink transmitter .

The embodiments of the present invention may be achieved by various means, for example, hardware, firmware, software, or a combination thereof.

In a hardware configuration, the methods according to the embodiments of the present invention may be achieved by 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, microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the present invention may be implemented in the form of a module, a procedure, a function, etc. For example, software code may be stored in a memory unit and executed by a processor. The memory unit is located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present invention. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims and their legal equivalents, not by the above description, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an exemplary embodiment of the present invention or included as a new claim by a subsequent amendment after the application is filed.

[industrial Applicability]

While the present invention is applied to 3GPP LTE mobile communication system in the above description, the present invention can be used in various mobile communication systems based on the same or equivalent principle.

Claims

[CLAIMS ]

[Claim l]

A method for, at an eNB, transmitting control information to a UE in a wireless communication system, the method comprising:

precoding a codeword including downlink control information using one of precoding matrices included in a candidate precoding matrix set of a codebook,

wherein the precoding matrices included in the candidate precoding matrix set are used by the UE for attempting to de- precode for the downlink control information.

[Claim 2]

The method according to claim 1, wherein the codebook includes a plurality of precoding matrices indicated by combinations of k codebook indexes and 1 layer indexes, and the candidate precoding matrix set includes precoding matrices indicated by at least one of m (m≤ k) codebook indexes and n (n≤l) layer indexes.

[Claim 3]

The method according to claim 2, wherein the candidate precoding matrix set is transmitted to the UE as a bitmap composed of at least one of bits corresponding to codebook indexes and bits corresponding to layer indexes.

[Claim 4]

The method according to claim 1, wherein, when the eNB transmits control information to a plurality of UEs including the UE, different precoding matrices are selected from the candidate precoding matrix set for the respective UEs.

[Claim 5]

The method according to claim 1, further comprising mapping the precoded codeword to a resource region of an antenna port and transmitting the codeword through the antenna port,

wherein the resource region is one of a resource region corresponding to a control region indicated by a physical control format indicator channel (PCFICH) , a resource region other than the control region in the corresponding subframe, and a resource region other than the control region in the first slot of the subframe.

[Claim 6]

The method according to claim 1, wherein, when two or more codewords are present, different downlink control information formats are allocated to the two or more codewords .

[Claim 7]

A method for, at a UE, receiving control information from an eNB in a wireless communication system, the method comprising :

attempting de-precoding using precoding matrices included in a candidate precoding matrix set of a codebook for downlink control information in a predetermined resource region of a subframe.

[Claim 8]

The method according to claim 7, wherein the codebook includes a plurality of precoding matrices indicated by combinations of k codebook indexes and 1 layer indexes, and the candidate precoding matrix set includes precoding matrices indicated by at least one of m (m≤ k) codebook indexes and n (n≤l) layer indexes.

[Claim 9]

The method according to claim 8, wherein the candidate precoding matrix set is transmitted to the UE as a bitmap composed of at least one of bits corresponding to codebook indexes and bits corresponding to layer indexes.

[Claim Ιθ] The method according to claim 7, wherein, when the eNB transmits control information to a plurality of UEs including the UE, different precoding matrices are selected from the candidate precoding matrix set for the respective UEs.

[Claim ll]

The method according to claim 7, wherein the predetermined resource region is one of a resource region corresponding to a control region indicated by a physical control format indicator channel (PCFICH) , a resource region other than the control region in the corresponding subframe, and a resource region other than the control region in the first slot of the subframe . [Claim 12]

An eNB in a wireless communication system, comprising: a transmission module; and

a processor,

wherein the processor precodes a codeword including downlink control information using one of precoding matrices included in a candidate precoding matrix set of a codebook, wherein the precoding matrices included in the candidate precoding matrix set are used by a UE for attempting to de- precode for the downlink control information. [Claim 13]

A UE in a wireless communication system, comprising:

a reception module; and

a processor,

wherein the processor attempts de-precoding using precoding matrices included in a candidate precoding matrix set of a codebook for downlink control information in a predetermined resource region of a subframe.