16 kbps modems for the HF NVIS channel

In the tactical radio environment, VHF communications is the norm. However, there are occasions when VHF links are impracticable, perhaps because of the intervening terrain, and in these cases HF near vertical incidence skywave (NVIS) links are sometimes used. A drawback of these links is that the available data rate with present day equipment is only 2.4 to 4.8 kbps at most, whereas VHF links can offer 16 kbps and above. There is therefore a desire to achieve higher data rates over HF NVIS channels. This paper reports the results of a study into providing a 16 kbps data rate in a 3 kHz HF NVIS channel. The design of the modems is outlined, and both laboratory and field measurements of performance are presented