Hf Modem Design for Extremely High Simultaneous Doppler and Delay Spreads

High Frequency(HF) radio modems are required to operate over a broad range of adverse channel conditions such as severe multi-path, fading, partial-band and carrier-wave interference as well as impulsive noise. The only compensation for the HF modem designer is that the number of digital signal processing(DSP) operations available per bit is usually high. The design of both serial and parallel tone modems has advanced in recent years so that the user can now expect higher data rates and lower bit error rates than ever before over mid-latitude channels [1]. In recent years, HF propagation at high latitudes has been studied through the Doppler and Multi-path Sounding Network(DAMSON) project with results showing that Doppler and delay spreads can occasionally be extremely high. The results suggest that a robust modem should withstand 40 Hz Doppler spread and 12 ms delay spread [2]. Many currently available modems have a robust mode based on applying stronger coding to the basic waveform [3, 4]. The author believes that for extreme Doppler and delay spreads that this is not a good design since both serial and parallel tone modems rely, be it indirectly, on channel estimates in the decoding process. When there are extremely high simultaneous Doppler and delay spreads, channel estimates become outdated during the measurement period. The problem demands a new approach. The author proposes wide-spaced M-ary Frequency Shift Keying(MFSK) in conjunction with a robust receiver. This allows communications to be maintained under extreme conditions at the cost of low data rates. A number of 8-FSK modem designs were tested. Standard 8-FSK with normal spacing is ineffective in the extreme channel conditions. However, wide-spaced Gaussian pulsed 8-FSK in conjunction with a combiner receiver achieves low uncoded error rates. This modem also performs well on a Gaussian channel.