Propagation Phase Noise. Impact on Future Vsat System Design

The concepts of propagation phase and fm noise are presented comparing oscillator noise with phase fluctuations introduced by the propagation medium. Numerical values of the spectral density and variance are presented. The experimental apparatus used to measure the noise using Italsat’s 40 GHz beacon is surveyed and experimental values of the excess spectral noise observed are presented. The ultimate effect of the propagation phase noise is analysed for its effects in (i) a carrier acquisition system and (ii) the irreducible error rate of a digital communications system using MPSK or MQAM constellations. Several parametric studies are given illustrating the use of the expressions and curves obtained and a way forward in future analysis of optimization of data rate and throughput is presented. One of the conclusions points towards two orders of magnitude of improvement in on board oscillator short–term stability as worth pursuing and permitting to operate with carrier recovery systems of 10 Hz or less. The background of the study is a lap–top type of portable ground station with substantial data throughput.