1. An audio signal processing method comprising: performing long term prediction on a time domain using a source audio signal of a preceding frame to generate a temporary prediction gain, a temporary pitch lag, and a temporary residual of a current frame; frequency transforming the temporary residual using the preceding frame and the current frame; inversely frequency transforming the frequency transformed temporary residual to generate a synthesized residual of the preceding frame; performing, in a mobile terminal, long term synthesis using the temporary prediction gain, the temporary pitch lag, and the synthesized residual of the preceding frame to generate a synthesized audio signal of the preceding frame; deciding long term prediction information using the synthesized audio signal of the preceding frame, the long term prediction information including a final prediction gain and a final pitch lag; and generating a bitstream including the final prediction gain, the final pitch lag and the frequency transformed temporary residual, wherein the synthesized residual of the preceding frame is generated by inverse transforming two consecutive and partially overlapping current and preceding frames and then adding the inverse transformed two frames.