A base station which performs Multiple Input Multiple Output (MIMO) transmission. A processor configured to generate reference signals by spreading with four groups of orthogonal code sequences, each group of orthogonal code sequences including four orthogonal sequences, wherein the orthogonal code sequences correspond to transmission layers and each of the orthogonal code sequences has a length of four, and a transmit circuit configured to transmit the reference signals. The four groups include a first group where the orthogonal code sequences are Walsh code sequences, a second group where the orthogonal code sequences are represented by mirroring of the orthogonal code sequences in the first group, a third group where the orthogonal code sequences are represented by cyclic shifts of the orthogonal code sequences in the first group, a fourth group where the orthogonal code sequences are represented by mirroring of the orthogonal code sequences in the third group.