Performance of adaptive antennas in the presence of sky-wave multipath propagation

An adaptive antenna array provides an effective means of suppressing interference signals. However, in medium- to long-range HF communications, sky-wave propagation often produces multipath that can hamper effective interference suppression. The differential time delays and elevation separations between paths vary widely depending on which reflecting ionospheric layers are active during the transmission. In mid-range sky-wave propagation (about 1000 km) where single-hop transmissions are dominant, the time-delay differences between paths increase with the elevation-angle differences. When adaptive weights are calculated based on an embedded known reference in the waveform, it is possible to resolve the multiple paths in elevation, based on the differential time delays. This can be achieved by forming composite antenna-pattern nulls pointing towards unwanted-signal paths in the vertical plane. The selection of a desirable signal path is made based on its specific time delay. Nulls are also formed towards all the paths of the interference signals. When the paths are close in elevation, a three-dimensional antenna array arrangement can be used to enhance the vertical resolution. This paper characterizes multipath propagation based on a mid-range sky-wave scenario and investigates, by computer simulation, how antenna-array configuration affects the suppression of unwanted multipath components of the desired signal in the presence of interference.