Increasing the availability of medium data rates at high latitude HF channels

At high latitudes, HF communications at medium data rates (1200-2400 bps) suffer from low availability because of ionospheric disturbances. The problem is that the channel exhibits large Doppler spread, delay spread and/or auroral absorption, not necessarily at the same time. If one can choose among different waveforms at each data rate, the maximum tolerable Doppler spread and delay spread can be increased (without increasing the maximum tolerable spreading factor). We have used DAMSON measurements to analyse the increase in availability that can be achieved using such a scheme. The largest improvement found is: at 2400(1200) bps the availability is 28(57)% using one waveform, and 35(68)% using 3 waveforms.