Threshold Extension in DBS System Design

Application of the threshold-extension technique in a DBS environment is described. For DBS signals comprising analog-video and OQPSK modulated data, computer simulations are used to determine optimum deviations of the RF carrier by video and OQPSK signals. With these deviations, parameters of a second-order phase-locked loop (PLL) threshold extension demodulator are optimized to yield a static-threshold extension of at least 3 dB. For typical video scenes, impulse-threshold conditions are simulated, and threshold degradations due to the OQPSK subcarrier are assessed. Simulation results are provided to permit a subjective evaluation of threshold with a conventional limiter-discriminator and with a PLL. For the OQPSK channel, bit-error probabilities are simulated. Using a video postprocessor, it is shown that an additional threshold extension of at least 1.5 dB is achievable. Furthermore, an adaptive receive front-end IF filter is shown to improve threshold performance by 1-2 dB. This improvement is essential for the OQPSK channel to yield uncoded bit-error probabilities around 10^-3or 10^-4at threshold. From these simulations, a set of DBS system parameters is suggested. With these parameters and a typical link budget, it is shown that acceptable system performance can be established under clear-sky as well as faded conditions.