Method and apparatus for reporting downlink channel state

A user equipment (UE) configured to report a channel state to a base station supporting downlink 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 downlink channel based on a downlink signal received from the base station, and
• report an index of a codebook for precoding of the downlink signal according to the measurement of the downlink channel,
• wherein the codebook for precoding includes an improved codebook (hereinafter, referred to as an improved 4Tx codebook) having a dual structure composed of a codebook for wideband and a codebook for subband, for 4 transmit (4Tx) antenna ports, and
• wherein the improved 4Tx codebook includes sub-codebooks obtained by sub-sampling the codebook for subband according to whether the base station is configured to use a co-polarization antenna or a cross-polarization antenna for transmitting the downlink signal.

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

• a radio frequency (RF) unit;
• and
• a processor configured to:
• control the RF unit,
• transmit a downlink signal to a user equipment (UE), and
• receive an index of a codebook for precoding of the downlink signal according to a measurement of the downlink channel based on the downlink signal,
• wherein the codebook for precoding includes an improved codebook (hereinafter, referred to as an improved 4Tx codebook) having a dual structure composed of a codebook for wideband and a codebook for subband, for 4 transmit (4Tx) antenna ports, and
• wherein the improved 4Tx codebook includes sub-codebooks obtained by sub-sampling the codebook for subband according to whether the base station is configured to use a co-polarization or across-polarization antenna for transmitting the downlink signal.