Limits of the propagation medium with respect to digital signal transmission

The effects of the propagation medium are more pronounced in digital communication systems than in analogue ones, in particular when considering single channel systems. The quality parameters: bit-error-rate and link availability are affected not only by the received signal level but also by multipath propagation which may result in frequency selective fading or just in a too low signal level when changing the location in a mobile system or signal fading when the medium is time variable. While fading decreases the signal to noise ratio of links causing short or long terms outages, frequency selective fading causes intersymbol interference and distortion which results in classical systems in an irreducible error rate. Propagation effects for each frequency region from HF to EHF are discussed individually. For LOS-links the typical multipath geometry is presented and data for coherence bandwidth and fading duration are given. Meteor burst scatter can only be used in digital ARQ-systems. It is shown that present systems are rather power limited than distortion limited. In digital troposcatter systems selective fading due to the mutual differential delay of the signal components is a limiting factor especially in digital systems. Delay power spectra are discussed with respect to geometry and layer structure.