Method and apparatus for transmitting channel state information in wireless communication system

A user equipment for transmitting channel state information (CSI) in a wireless communication system, the user equipment comprising:

• an RF (radio frequency) unit;
• and
• a processor, the processor configured to subsample a first codebook associated with a first PMI (precoding matrix indicator) and a second codebook associated with a second PMI according to a reporting submode for 4 antenna ports, the processor configured to report channel state information based on the subsampled first codebook and the second codebook,
• wherein if a first codebook index for the first PMI corresponds to an add number, a second codebook index for the second PMI has an index belonging to a first index group and wherein if the first codebook index corresponds to an even number, the second codebook index for the second PMI has an index belonging to a second index group.