A base station apparatus in a mobile communications system which uses orthogonal frequency division multiplexing (OFDM) for downlink is disclosed. The base station includes a unit which generates a first signal; a unit which generates a second signal; a unit which multiplexes the first signal and the second signal to map the multiplexed signals to a sub-carrier to generate a transmit symbol; and a mapping determining unit which determines a position of mapping the second signal to the sub-carrier, wherein the mapping determining unit determines the mapping position based on a cell ID or cell group ID, and a mapping pattern defined with a sequence having two-stage structure.