An operator's perspective on propagation

In satellite communications, Quality of Service is a key parameter closely related to propagation characteristics of a communications link at a given frequency and for a geographical location. Over the years, a lot of effort has been spent into collecting data and into modeling propagation at various frequency bands of interest, i.e. C-band, Ku-band and Ka-band. In particular, efforts in propagation modeling have been directed towards the requirements of professional communications, giving parameters like link availability over a year or the worst month. With the advent of consumer applications in particular through High Throughput Satellite (HTS) systems, the distribution of statistics becomes an important element. In general, as frequencies increase, the amount of propagation data available is less abundant and consequently less accurate predictions can be expected. As we move up in frequency, Fade Mitigation Techniques (FMT) are of great importance. In particular, if we assume that feeder links of next generation HTS will work in Q/V bands or even beyond, where propagation effects are more severe, we have to think about innovative approaches to site diversity. In these systems, M links are available when N links are required, where M>N. The problem shifts from classical site diversity to system dimensioning where we can see an interesting cross-roads between propagation and networks. Space-time channel characterization and modeling of the propagation channel will certainly help in the prediction and optimization of Quality of Service associated to earth-space communications links.