July 1987 | NTIA Technical Report TR-87-221
James A. Hoffmeyer; Martin Nesenbergs
Abstract: Laboratory testing of proposed and new wideband (e. g., spread spectrum) high frequency (HF) systems is currently not possible because wideband HF channel simulators do not exist. Moreover, there are no validated HF channel models for bandwidth on the order of a megahertz on which to base simulator designs with confidence. Enhanced measurement programs over appropriate radio paths are needed to verify the main features of wideband channel models or to propose improvements in the existing narrowband models. This report starts with an elementary review of ionospheric propagation. It summarizes the recent work in spread spectrum technology targeted for the HF radio band. Thereafter follows a short section devoted to additive distortions, namely noise and interference, also in the HF band. The report next presents an assessment of past narrowband HF models: their background, old validation tests, and—to be quite specific—the NTIA/ITS development of the Watterson simulator. That laboratory tool, judged best by many, works in real time and offers accurate representations of HF channel bandwidth up to 10 or 12 kHz. In the present study, an extension to wideband models is attempted. Unfortunately, it suffers from an apparently serious shortage of measured data for the time–varying channel transfer function. A possible wideband model is hypothesized, conjectures are made, and questions are raised. One is left faced with a requirement for an experimental program to ascertain the wideband (1 MHz or more) characteristics of multipath fading for digital radio transmissions in the (2– to 30-MHz) band and over radio propagation paths of interest. Real data on the characteristics of the time*#8211;varying channel transfer function would be invaluable for the ongoing simulator work at several research organizations.
Keywords: spread spectrum; HF propagation; channel simulation; HF channel models; wideband communications
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