A method for converting source data into a channel-modulated signal having a plurality of pairs of in-phase (I) and quadrature (Q) data in a mobile station wherein the mobile station uses at least one channel comprises the steps of: DOLLAR A a) encoding the source data to generate at least a data part and a control part DOLLAR a b) generating at least one spreading code to be allocated to the channel each spreading code is selected on the basis of a data rate of the data part and the control part and the spreading codes are selected such that two successive pairs of the I and Q data corresponding to two points located on a phase domain from the same point or symmetrically with respect to a zero point and DOLLAR a c) spreading said control part and the data part by using the spreading codes so as to to generate the channel-modulated signal. The method is capable of improving a power efficiency of a mobile station by a peak to-average power ratio is reduced in a mobile communication system.