We have proposed a new digital modulation scheme, narrow-band linear modulation scheme with quasi-constant envelope. This scheme cannot only obtain sufficient frequency efficiency but also apply to the system with nonlinear amplifier. The principle of the proposing scheme is that lowering pulse amplitude of data symbol sequence and using full roll-off filter with narrower impulse response for offset-QPSK. As a result, envelope fluctuation of the proposing scheme is restricted within 11%, and the spectrum after nonlinear amplifier is as same as that of GMSK for BT of 0.3. In addition, in both cases of coherent detection and MLSE, the BER performance of the proposing scheme is deteriorated from theory by approximately 1 dB in E{sub}b/N{sub}0 at BER of 10{sup}(-4).
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