The data error and transmission properties will deteriorate in a high speed mobile data transmission channel because of the signal fading and the interference of multi\|channel waves. Time frequency shift keying (TFSK) modulation is presented to improve the mobile channel property. This paper presents a novel model of TFSK, π TFSK. The mathematical model of π TFSK signal and its auto\|correlation function are presented. The spectral density of the TFSK signal has also been calculated in great detail. In...