Transmission of Digitally Encoded Speech at 1.2 Kbaud for PCN

摘要

The transmission of digitally encoded speech at 1.2 Kbaud for mobile personal communication networks (PCN) is investigated both analytically and by computer simulations. Speech was encoded at 4.8 Kbit/s using a low complexity transform binary pulse excited LPC codec. A 64-level QAM modem was used having 3 sub channels that operated with different BERs. The sensitivity of the encoded speech bits to transmission errors was identified, and the bits classified into 3 groups. Each group was then individually coded by BCH codec of differing power. The output of the BCH coders were grey coded onto 3 QAM channels. By this arrangement the protection given to the speech bits was dependent on their vulnerability. The 6 bit QAM symbols were transmitted at 1.2 Kbaud over Rayleigh fading channels with pedestrian mobiles travelling upto 4 mile/h. For microcells using a propoagation frequency of 1.9 GHz and operating with channel SNRs in excess 26 dB we achieved good communication quality speech when second-order diversity and AGC were used.