Fully digital burst modem for satellite multimedia communication systems

This paper presents fully digital high speed (17.6 Mb/s) burst modem for Offset Quadrature Phase Shift Keying (OQPSK) which employs novel digital modem VLSICs. The modulator VLSIC directly generates modulated intermediate frequency (IF) signals in a fully digitalized manner. A newly proposed digital reverse-modulation and pre-filtered carrier filter-limiter scheme realizes low power consumption and stable operation in a low Eb/No condition. The demodulator VLSIC also achieves fast bit-timing acquisition in burst mode. Moreover it supports stable initial burst acquisition by a novel automatic frequency control (AFC) acquisition detector and a digital burst detector. A digital burst automatic gain control (AGC) compensates burst-to-burst level differences without analog circuits. Performance evaluation results show that the new modem achieves satisfactory biterror-rate performance in severe environments. The developed modem has been employed in a commercial portable earth station for ISDN services and reduces the hardware size to one third that of the conventional one.