Ionospheric channels can vary unexpectedly, hampering communications with fading, dispersion, Doppler, multipath, and other effects. The HF band is also highly congested; a relatively clear channel with large bandwidth is hard to find. International regulations restrict the channels to 3-KHz slots. Even if a wide bandwidth was available, the ionospheric distortions would not be uniform over the large bandwidth, hindering adaptive equalization and other improved techniques.

We developed and demonstrated a communication system with a high data rate (64 Kbps or above) and using multiple, parallel 3-KHz slots. This system allows the extension of a wireless network with the long haul wireless system, essential to extend communication services in both commercial and military sectors. A data rate of 64 Kbps enables the basic access of one ISDN network. This l allows multimedia, digital video, and other digital connectivities through long-haul HF radio.

Several innovations are used for this performance over the HF channel. The system selects the frequency slots dynamically, first monitoring their propagation and interference conditions, and then predicting their state at a slightly later time (to account for propagation and pocket delays). To ensure maximum throughput, any channel’s communication parameter adapts to match its condition. This adaptive control goes through a DSP-based, pure-software modem. The modem’s receiving section also generates channel performance data.

The DSP-based software modem is the basic building block for each slot. This software modem allows various adaptations of bit-rate, coding, modulation, frame size, interleave depth, and power control.

Ganguly. S et al 1999 MILCOM paper.