Performance of convolutional coded SQAM in hardlimited satellite channels

The hardware implementation and experimental measurements of a differential encoded superposed quadrature amplitude modulation (SQAM) modem combined with 3/4-rate convolutional encoding and threshold decoding for use in bandwidth and power-limited satellite communications are presented. To avoid error propagation caused by differential encoding/decoding, and also to combat bursty channel noise, a data interleave/deinterleave scheme is integrated into the forward error-code (FEC) codec. Specific signal waveshaping is achieved digitally after FEC encoding to produce signals free of intersymbol interference (ISI) and timing jitter while providing a compact power spectrum. Experimentally measured results including signal waveforms, eye diagrams, power spectrum, and bit error rate (BER) performance in linear and hardlimited transmission channels are reported. The BER results show that the power efficiency of the modem is excellent. Spectral regrowth of the modulated signal is also retained at the minimum to ensure minimum interference to adjacent channels and thus maximize the channel utilization efficiency