A low complexity digital encoder-modulator for high data rate satellite BISDN applications

The development of a low complexity, flexible digital encoder-modulator (DEM) for medium to high data rate RF communication links is discussed. The DEM is intended for use in data downlinks from communications satellites or near-Earth science platforms. The present configuration supports throughput rates up to 280 Mbps using several pulse shaped quadrature modulation schemes. This includes BPSK and QPSK as well as higher order schemes such as 8PSK and 16QAM. Through software modifications the DEM can also support binary CPM schemes (such as GMSK) and multiboard configurations can support higher order CPM schemes. In addition to higher order modulation schemes, combined modulation and forward-error correction coding is used to improve the efficiency of conveying information through power and bandwidth limited channels that are characteristic of space-borne systems. The baseline configuration supports the B-ISDN OC-3 rate of 155.52 Mbps in 80 MHz of null-to-null bandwidth. Test results show less than a 0.2 dB of degradation from theory in the range of operation. The DEM uses approximately 15 watts of DC power, is constructed on a standard VME card (6U/spl times/160 mm) and has a mass of 0.38 kg.