Advances in HF parallel tone modem technology

The author describes the HF modem, which contains a 75 to 2400 bps 39-tone waveform, a robust 75-bps direct-spread waveform, and two independent 45-1200-bps binary frequency-shift-keyed (FSK) waveforms. The availability of powerful new microprocessors permits economic implementation of complex signal-processing sizes. Improved signaling techniques and potent error-correcting codes allow greatly improved performance on the HF channel. Measured performance data taken on HF fading simulators and in numerous on-the-air tests are presented. The performance with and without diversity reception is also shown. An implementation that demonstrates the dramatic reduction in modem size, weight, power consumption, and cost realized by embedding the modem in modern digital radios such as the RF-5000 is presented