An efficient means of implementing spatial filtering of HF signals propagated over high latitude HF communication paths for the purposes of improving data throughput

The high latitude ionosphere is a dynamic region that is subject to a variety of disturbed conditions, which affect the propagation of HF radio signals. Point-to-point digital communication systems at high latitudes attempting to utilise the narrowband HF ionospheric radio channel can suffer severe performance degradation when the time or Doppler dispersion exceeds certain, system dependent, bounds. The presented results are based around measurements of narrowband pulse compression channel sounding signals, made using a six channel spaced array receiving system, over two high latitude paths. Super resolution direction finding studies of this data revealed significant directional effects in the received signals. Of particular note is an often observed directional structure to received Doppler spread signals. These investigations indicate that spatial filtering, using simple arrays of antennas, can significantly reduce the level of Doppler spread presented at the inputs to modern high speed digital modems. The use of fast solver methods to find the best spatial filtering solutions results in very similar performance at reducing Doppler spread and massively reduced calculation times, thus raising the possibility of implementation of spatial filtering in a real system.