The viability of very high data rate modems for a specific HF channel type

An application has arisen to provide range-extension into an existing medium quality radio network. This existing network operates in a different frequency band, passing 16 kbit/s digitised voice and data. Telephone channels have the same bandwidth as a standard HF voice channel. An investigation has been carried out to determine whether the equalisation and processing performed by sophisticated telephone modems are capable of being used on HF channels, in particular to determine what parameters are required to pass fast data on a specific HF channel. The current network uses robust error tolerant applications and so the HF modem goal was not quite so ambitious as it initially appeared to be. The two channels which are under consideration have different types of fading. A telephone channel has flat fading whereas a typical HF channel has frequency-selective fading. As the data rates were significantly higher than the bandwidth, the impact of the fading on the signal was expected to be significant. The work explored how the effects of fading changed when the data rate was increased. The paper describes theoretical calculations of signal performance, results obtained by simulation and preliminary indications from practical trials. The tests used an existing DSP-based ionospheric simulator and initially used a commercial V.34 telephone modem, though this will not be the ultimate solution.