A critique of HF NVIS channel models

A wideband HF NVIS measurement campaign in Southern California collected 7 terabytes of channel sounding across 2-12 MHz from 72 days of data collection between October 2014 and February 2015. This massive dataset demonstrates that the traditional HF NVIS channel models are ill-suited to describe the observed HF NVIS propagation phenomena. In particular, it is observed that the HF NVIS channel is often non-stationary over a few minutes; with taps rapidly shifting in delay, exhibiting abrupt changes in phase characteristics, and periodic variations in magnitude-all having significant implications on modem design, e.g. block length, preamble length and repetition rate, interleaving size, and modulation type. To this end, this study summarizes the empirical findings of the delay and Doppler characteristics, as well as the non-traditional fading modes, of wideband HF NVIS channels.