Emerging technologies that use spread spectrum and frequency hopping signals may enable more efficient use of the high-frequency (HF) band. High-powered HF radar is an example of this technology where channels are used that appear to be unoccupied and the frequency hopping signal of the HF radar has been remarkably free from interfering with conventional spectrum occupants. Considering future application of HF systems of this type, it is of interest to examine the availability of free channels in the frequency range between 3 and 30 MHz for bandwidth (BW) availability and time duration. This paper discusses the characteristics of channels that are available for use in the HF spectrum based on spectrum measurements by the Navy's Relocatable Over-the-Horizon Radar (ROTHR) located in the southeast section of Virginia. In particular, the ROTHR data being analyzed are from the spectrum monitor (SM). The SM is a subsystem of the ROTHR which measures the power level in 3-kHz intervals for frequencies between 5 and 28 MHz, the radar's operating frequency range. Measurements of the HF spectrum were analyzed for the fall and winter seasons at the radar's location. The analysis consisted of estimating the number of clear channels of various widths in selected megahertz bands over the ROTHR's operating frequency range. Characteristics of available channels are presented in various graphical forms which include clear channel availability, probability of at least one clear channel of a specified width, and duration of clear channels as a function of frequency. The Weibull distribution is proposed as an analytical model for investigating channel duration. Analysis results indicate which portion of the HF spectrum is most likely to have available bandwidths and what their expected durations are as a function of season and time of day.
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