Avalanche Communications - A Robust HF Relay Technique for Stressed Channels

One of the most challenging communications tasks facing industry at present is that of providing reliable digital traffic over the HF ionospheric channel. The variability of the channel with time and external conditions which effect the isonosphere and the substantial amount of natural and man made interference have made the HF channel relatively unreliable for analog voice communications. The multipath and rapid phase variations of the channel make it especially difficult for reliable digital communications. Most of the approaches to improving the HF reliability have entered around selection of good frequencies which provide favorable propagation conditions for the particular transmission. Although approaches utilizing good "frequency management" techniques may provide substantial improvements, they usually achieve these gains by involving additional spectral space which is already a very precious commodity in the limited HF spectral band. This paper describes an alternative approach to upgrading HF reliability which uses relay techniques to achieve "path diversity" and does not fundamentally require additional frequency allocation. This approach which is called "Avalanche Communications" is applicable to HF networks in which network timing is available. It also requires the use of digital modems which have the capability of taking advantage of reception from multiple receive paths. It does not require, however, any network status information. The approach is applicable to both long haul strategic networks as well as tactical networks involving ground wave and sky wave propagation modes.