UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Characterisation of Narrowband HF Channels in the Mid and Low Latitude Ionosphere

The performance of an HF digital radio system is strongly affected by the response of its various protocols to short-term (sub-hourly) variations in the ionosphere. There is a very limited choice of models of this variation. Channel simulators based on the Watterson 2-path model (ITU-R F.1487) are commonly relied upon for modem testing, system design and modelling. Limited DSTO trials in the past have cast doubt on the validity of the Watterson model in the Australian mid-latitude region. In addition to fading errors, the Watterson model introduces further errors by ignoring variations in the group delay due to diffuse multi-path propagation. These concerns provided the impetus to conduct an extensive and rigorous trial to measure the narrow-band channel impulse response (CIR) at mid latitudes as well as the highly dynamic low latitude region of the ionosphere. A variety of low and mid-latitude narrowband HF channels were probed during a 14 day campaign in April 2006. Channel response measures such as Doppler shift and spread, group-delay spread, and signal fading statistics were used to identify a small set of similar channels by fuzzy clustering the response variables. By including or omitting other factors such as the carrier frequency, time of day, path information, ionospheric electron density profile parameters, and the maximum usable frequency from oblique ionospheric sounding on coincident paths, it is possible to establish various physical dependencies and derive a small selection of typical CIR records that define the cardinal channel types in the geographic region. In this paper the authors describe the trial, the measurements and the analysis methods used to characterise and statistically analyse the short-term properties of individual channel records and to cluster the channels into a manageable set of cardinal channel types. Important analysis results from the first campaign are presented. 1.