SUPPRESSION OF THE IRREDUCIBLE ERRORS IN A FREQUENCY-SELECTIVE FADINGCHANNEL BY FRACTIONAL-BIT DIFFERENTIAL DETECTION

The irreducible errors in digital mobile communications with high data rate in law-delay-spread channels are mainly caused by time dispersio n (frequency selectivity) of the channel. The errors originate in the transition regions between adjacent symbols when deep fades occur. We show that fractional-bit differential detection with proper timing avo ids these transition regions and decreases the irreducible errors. Ass uming an MSK modulation format and slow Rayleigh fading, we derive a f ormula for calculation of the error probability in which we apply an ' 'effective delay spread.'' A procedure to predict the effective delay spread is proposed. We verify the calculated error probability by a Mo nte Carlo software simulation for three different delay power profiles . The approach increases the tolerable delay spread in an unequalized receiver by a factor of 2 to 5 depending on the details of the delay p ower profile. Filtering the MSK signal tends to smear the transition r egion over the entire symbol duration, reducing the effectiveness of f ractional-bit detection. This explains qualitatively the mismatch betw een theory and experiment in previously published results. We discuss the conditions of filtering and of finite signal-to-noise ratio (SNR) under which fractional-bit detection is effective. For instance, for a delay spread around one-tenth of the symbol length, improvement is ga ined if SNR is above 15 dB.