Wideband line-of-sight channel measurements and simulation: Application to digital radio links

The performance of high-capacity digital radio links which is impaired by frequency-selective fading during multipath propagation, even if the signal-to-noise ratio is adequate was examined. The amplitude and group delay distortions were measured in a 40-MHz bandwidth at 4 GHz on a line-of-sight path. It is shown that the complex transfer function H(Omega) of the propagation channel can be well approximated by a two-ray model. The influence of two-path fading can be estimated if the dependence of the notch depth versus the deviation between the notch frequency and the carrier frequency f sub c for a fixed bit-error rate is known. Perfect Nyquist pulse shaping was assumed and the effects of timing and carrier recovery circuits during two-path propagation were considered. The comparison with the measured signature showed a small degradation which is attributed to modem imperfections. A two-channel simulator for two-path fading was developed, where the indirect wave which was delayed by tau = 6.3 ns relative to the direct wave was Rayleigh-distributed. By variation of the parameter-K which describes the power ratio between the indirect and the direct wave, it is possible to simulate different fading processes. The improvement by the use of an inphase diversity combiner is investigated for different values of the correlation coefficient between the two channels.