Development of an FEC combined modem for DS-SSMA communication system

The development of a forward error correcting (FEC) modem for low-bit-rate satellite communication systems using direct-sequence spread-spectrum multiple-access (DS-SSMA) communication techniques is described. Utilizing a digital correlator and a convolutional encoder/Viterbi decoder, the modem achieves rapid acquisition time and low bit error rate performance under very low carrier-to-noise (C/N) ratios. Experimental results indicate a mean acquisition time of 220 ms and bit error rate of less than 1×10^-7 at the received intermediate frequency C/N ratio of -31 dB. The digital matched filter, digital correlator LSI, delay lock loop, and binary phase-shift keying modulator used in the modem are described