| 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 transformed binary pulse excited LPC codec. A 64-level QAM modem was used having three subchannels that operated with different BERs. The sensitivity of the encoded speech bits to transmission errors was identified, and the bits classified into three groups. Each group was then individually coded by BCH codecs of differing power. The output of the BCH coders were Gray coded onto the three QAM channels. By this arrangement the protection given to the speech bits was dependent on their vulnerability. The six-bit QAM symbols were transmitted at 1.2 kbaud over Rayleigh fading channels with pedestrian mobiles travelling up to 4 mile/h. For microcells using a propagation frequency of 1.9 GHz and operating with channel SNRs in excess of 26 dB good communications quality speech was achieved when second-order diversity and AGC were used. |
