A method and a transmission device of a mobile communication system are disclosed. The method includes determining a modulation method for data transmission, determining the level of a quadrature amplitude modulation (QAM) according to the determined modulation method, determining the amplitude of a carrier wave and the phase of the carrier wave on the basis of the determined QAM level and the data to be transmitted, and, when the determined modulation method is a first modulation method, determining a sequence length, selecting a sequence form among sequences having the determined length according to the data to be transmitted, and generating a symbol modulating the data to be transmitted, on the basis of the selected sequence, the determined amplitude of the carrier wave, and the determined phase of the carrier wave.