Robust low bit rate HF data modems

This paper begins by examining the modulation scheme utilized by the US MIL-STD-188-110A 75 bps mode. This orthogonal modulation scheme differs significantly from the higher bit rate schemes of MILSTD-188-110A and of the STANAG 4285 which both use blocks of known symbols interleaved with blocks of unknown symbols. The modulation and demodulation are examined and measured performance for a number of different channels is presented. These conditions include some very severe channels with large multipath and Doppler spreads. The paper next discusses some work by the author in the development of very low bit rate HF data modems. These modems, with user data rates as low as 5 and 10 bps, are extensions of the techniques utilized by the US MIL-STD-188-1I0A 75 bps mode. These very low bit rate modes achieve good performance in the -10 to -20 dB SNR range.