This paper proposes a suitable combination of the digital modulation schemes and the coding-rate of forward error correction (FEC) schemes for satellite digital video communication networks. The comparative study is carried out by computer simulation considering non-linearly amplified, narrow bandwidth satellite channels with adjacent channel interference signals. The proposed system employs an offset QPSK modulation scheme supported by the coding-rate of 7/8 convolutional encoding and Viterbi decoding to realize high-quality and compact spectrum characteristics in non-linear channels. By employing a 32Mbps DPCM video codec, the developed prototype system achieves a post demodulated S/N ratio of higher than 52dB. Moreover, it achieves high protection ratio against co-channel interference than conventional analog FM systems. The optimized digital video transmission system makes it possible to transmit high-quality NTSC video signals over non-linearly amplified narrow bandwidth satellite channels, for example 27MHz or 36MHz bandwidth transponders, with high-security digital encryption.

This paper proposes and analyzes experimentally an SSMA (Spread Spectrum Multiple Access) signal transmission over a high speed QPSK (Quadrature Phase Shift Keying) modulated signal to achieve higher transmission efficiency per transponder and to facilitate a lower power transmitter for SSMA signal transmission. The employment of high-coding-gain forward error correction for SSMA-QPSK signals makes it possible to transmit SSMA-QPSK signals over a non-linearly amplified QPSK signal transmission channel. Experimental results show that under the condition of a 20dB less transmission power assignment to SSMA-QPSK signals than QPSK signals, the QPSK signals achieve only 0.5dB Eb/No degradation (at Pe=110-4) by employing coding-rate seven-eighth FEC, and the SSMA-QPSK signals achieve about 7dB Eb/No degradation (at Pe=110-4) by employing coding-rate one-half FEC. The satellite link budget shows that even if the SSMA-QPSK signal transmission generates about 7dB Eb/No degradation, it requires 20 dB less transmission power and it still has an extra margin of 8.5dB compared with the high speed QPSK signals (total relative gain of 28.5dB). Thus, the proposed system makes it possible to realize one-way digital video signal transmission in QPSK mode and both-way digital voice signal transmission over one transponder for business video communication systems.