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In this paper, a SBC(sub band coding) is proposed to code a speech signal for a digital mobile radio and a robustness of speech quality of the SBC over the Rayleigh fading channel is studied via a computer simulation. Also attempts have been made to compare a performance between the SBC and CVSD(continuously variable slope delta modulation). The simulation consists of two parts : a Rayleigh fading envelope generator, and a 16-ary DPSK(differential phase shift keying) model in which the signal level is varied according to the Rayleigh fading envelope. From the results of computer simulation, it was found that, at higher BER (ie, BER ≥ 5×10-2), the CVSD is clearly better than SBC however, the SBC outperforms the CVSD at BER ≤ 10-2.