Bit-Stream-Arranged Weighted Modulation Scheme for Low Delay Spread Frequency Selective Fading Channels


IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences   Vol.E81-A    No.7    pp.1452-1458
Publication Date: 1998/07/25
Online ISSN: 
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Fundamentals of Multi-dimensional Mobile Information Network)
weighted modulation,  frequency selective fading,  micro-cell system,  ADPCM,  PCM,  

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This paper proposes a bit-stream-arranged weighted modulation scheme to 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 bits so that they are not adjacent to each other over time; a mapping method that controls the amplitude of the modulation signals according to the importance of the bits; and modified differential encoding to prevent the error propagation from insignificant to significant bits. Computer simulations clarify that the proposed bit-stream-arranged weighted modulation scheme shows a S/N improvement of 8 dB in an 8-bit linear pulse code modulation (PCM) voice signal compared with the conventional non-weighted π/4-shift quadrature phase shift keying (QPSK) modulation 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 is halved. Although the proposed scheme increases the peak power of the modulated signals, the non-linearity of the power amplifier is not fatal.