Measuring the coherence of wideband dispersive channels

Abstract

Ionospheric propagation at HF suffers from dispersion inboth time and frequency. Time dispersion arises from the presence of multipath propagation and limits the bandwidth of the ionospheric channel to a few kilohertz. Frequency dispersion causes phase nonlinearity, which leads to ‘elongation’ of narrow pulses. If multipath propagation can be resolved the coherent band width of the channel will be determined by frequency dispersion. The purpose of the work reported here was to determine the effect of weighting windows, used to reduce the effects of spectral leakage, on the estimation of the coherent bandwidth of wideband HF radio links. A wideband FMCW sounder was used to collect data over a 234 km link in theUnited Kingdom. Analysis of half-megahertz sections of a 5 MHz sweep via the FFT algorithm permitted multipath propagation to be resolved. To estimate pulse distortiondue to frequency dispersion the 6 dB width of the receivedpulse was estimated.