BIT-STREAM-ARRANGED WEIGHTED MODULATION SCHEME FOR LOW-DELAY SPREAD FREQUENCY-SELECTIVE FADING CHANNELS

This paper proposes a bit-stream-arranged weighted modulation scheme t o improve voice quality in low delay spread frequency selective fading channels. The proposed modulation scheme employs an input bit stream arrangement method that changes the bit stream order for significant b its so that they are not adjacent to each other over time; a mapping m ethod that controls the amplitude of the modulation signals affording to the importance of the bits; and modified differential encoding to p revent the error propagation from insignificant to significant bits. C omputer simulations clarify that the proposed bit-stream-arranged weig hted modulation scheme shows a SM improvement of 8 dB in an S-bit line ar pulse code modulation (PCM) voice signal compared with the conventi onal nan-weighted pi/4-shift quadrature phase shift keying (QPSK) modu lation scheme. The proposed scheme also shows 3.5 dB improvement in a 4-bit adaptive differential pulse code modulation (ADPCM) voice signal . In this case, occurence of 'click noise' in recovered voice signal i s halved. Although the proposed scheme increases the peak power of the modulated signals, the non-linearity of the power amplifier is not fa tal.