Method for reporting downlink channel state and apparatus therefor

A user equipment (UE) configured to report a channel state to a base station (BS) supporting downlink (DL) multiple-input multiple-output (MIMO) in a wireless communication system, the UE comprising:

• a radio frequency (RF) unit;
• and
• a processor configured to:
• control the RF unit;
• measure a DL channel based on a DL signal received from the BS;
• and
• report an index of a codebook for precoding of the DL signal according to the measured DL channel,
• wherein the codebook for precoding comprises an integrated codebook having a dual structure comprising a first codebook for a wideband and a second codebook for a subband, and
• wherein the integrated codebook is determined by an index i1 for the first codebook and an index i2 for the second codebook, and the index i1 determines a type of the second codebook to be selected by the index i2.

A base station (BS) supporting downlink (DL) multiple-input multiple-output (MIMO) in a wireless communication system, the BS comprising:

• a radio frequency (RF) unit;
• and
• a processor configured to:
• control the RF unit;
• transmit a downlink (DL) signal to a user equipment (UE);
• and
• receive an index of a codebook for precoding of the DL signal according to a DL channel measured based on the DL signal,
• wherein the codebook for precoding comprises an integrated codebook having a dual structure comprising a first codebook for a wideband and a second codebook for a subband, and
• wherein the integrated codebook is determined by an index i1 for the first codebook and an index i2 for the second codebook, and the index i1 determines a type of the second codebook to be selected by the index i2.