A method for periodically reporting channel state information, CSI, to a base station based on a linear combination codebook in a wireless communication system, the method comprising: receiving a reference signal from the base station; and reporting, to the base station, CSI calculated based on the reference signal, wherein the channel state information comprises information about a channel quality indicator and a precoding matrix index, wherein the precoding matrix index is selected from a subsampled codebook, the subsampled codebook configured based on a rank calculated by the reference signal being 2 and a number of beams used for the reporting of the CSI being 2, wherein precoding matrices constituting the subsampled codebook comprise a first column vector for a first layer and a second column vector for a second layer, the second column vector being orthogonal to the first column vector, wherein all of the precoding matrices constituting the subsampled codebook are configured based on three co-phase coefficients, the precoding matrices being represented by Equation A, b1 + c1 ∗ p1 ∗ b2 b1 + c1 ∗ p1 ∗ b2 c2 ∗ b1 + c3 ∗ p1 ∗ b2 − c2 ∗ b1 + c3 ∗ p1 ∗ b2 where b1 denotes a leading beam indicator, b2 denotes a second beam indicator, and p1 denotes a value for indicating power of a second beam, wherein c1 is a first co-phase coefficient among the three co-phase coefficients and determined to one of a set of complex values consisting of +1, -1, +j and ―j, where the j denotes an imaginary value, wherein c2 is a second co-phase coefficient among the three co-phase coefficients and determined to one of a set of complex values consisting of +1 and +j, where the j denotes an imaginary value, wherein c3 is a third phase coefficient among the three co-phase coefficients determined to one of a set of integer values consisting of +1 and -1, wherein the information about the precoding matrix index has a 4-bit size and indicates the three co-phase coefficients, and wherein the three co-phase coefficients are used for defining the first and the second column vector. receiving a reference signal from the base station; and reporting, to the base station, CSI calculated based on the reference signal, wherein the channel state information comprises information about a channel quality indicator and a precoding matrix index, wherein the precoding matrix index is selected from a subsampled codebook, the subsampled codebook configured based on a rank calculated by the reference signal being 2 and a number of beams used for the reporting of the CSI being 2, wherein precoding matrices constituting the subsampled codebook comprise a first column vector for a first layer and a second column vector for a second layer, the second column vector being orthogonal to the first column vector, wherein all of the precoding matrices constituting the subsampled codebook are configured based on three co-phase coefficients, the precoding matrices being represented by Equation A, b1 + c1 ∗ p1 ∗ b2 b1 + c1 ∗ p1 ∗ b2 c2 ∗ b1 + c3 ∗ p1 ∗ b2 − c2 ∗ b1 + c3 ∗ p1 ∗ b2 where b1 denotes a leading beam indicator, b2 denotes a second beam indicator, and p1 denotes a value for indicating power of a second beam, wherein c1 is a first co-phase coefficient among the three co-phase coefficients and determined to one of a set of complex values consisting of +1, -1, +j and ―j, where the j denotes an imaginary value, wherein c2 is a second co-phase coefficient among the three co-phase coefficients and determined to one of a set of complex values consisting of +1 and +j, where the j denotes an imaginary value, wherein c3 is a third phase coefficient among the three co-phase coefficients determined to one of a set of integer values consisting of +1 and -1, wherein the information about the precoding matrix index has a 4-bit size and indicates the three co-phase coefficients, and wherein the three co-phase coefficients are used for defining the first and the second column vector.