Application of frequency-shift filtering to the removal of adjacent channel interference in VLF communications

The use of frequency-shift (FRESH) filtering for the rejection of adjacent channel interference (ACI) in GMSK modulated VLF communications is investigated. A range of scenarios based on a high powered interferer (up to 40 dB greater than the signal of interest (SOI)) in an adjacent channel to the SOI are tested, and the use of a FRESH filter for rejection of the interference is investigated. It is shown that a major improvement in BER performance over a fractionally spaced equaliser (FSE) is possible, and that only one interferer related frequency shift is required. This demonstrates that FRESH filters can be highly effective in ACI removal, with only a moderate increase in complexity over an FSE.