Investigations into the channel characteristics of HF NVIS links and the implications for high data rate transfer

Higher data rates are required over HF (high frequency) radio links in order to meet the ever increasing demands for information transfer. This requirement extends to HF near vertical incidence skywave (NVIS) links which are used when terrain obstructions preclude the use of other radio transmission methods. In order to optimise the design of high data rate modems and systems for HF NVIS, knowledge is required concerning the channel characteristics which are to be encountered. This paper discusses a channel measurement campaign conducted over two HF NVIS links having ground ranges of 30 and 150 km. The analysis is based on comprehensive signal-to-noise and multipath data derived from chirp sounder measurements. Results are presented which focus on the 150 km path during October 1998 and January 1999. Comparisons are drawn with data from propagation prediction. The paper explores the expected availability of high data rate modems over an NVIS link, taking account of modem performance tolerances and transmitter powers. Consideration is also given to the channel requirements in terms of likely range and number of channels needed to optimise link availability, as well as the rate of occurrence of channel changes