A method and circuit that gives a sequence pattern that represents directions of positive and negative transitions of the phase that continue over a predetermined number from a certain reference symbol to an adjoining next reference symbol; finds (heuristically) one or more interpolate symbols that meet conditions (such as standards for power spectra) of a predetermined frequency spectrum, i.e., band, and a predetermined (range of) amplitude with reference to the given sequence pattern; and stores the found sequence pattern and a phase value and an amplitude value corresponding to the found one or more interpolate symbols in a memory as a lookup table against the prepared memory area.