Interference rejection in impulsive noise for VLF communications

The use of frequency-shift (FRESH) filtering for the rejection of adjacent channel interference (ACI) in GMSK modulated VLF communications is investigated in the presence of impulsive noise. The effects of such filtering in conjunction with a simple "hole-punching" technique for removing impulses from the signal are characterised. A range of realistic scenarios based on a high powered GMSK interferer (10 to 30 dB greater than the signal of interest (SOI)) in an adjacent channel are described. It is known that in the presence of interference and Gaussian noise a simple FRESH filter, which exploits the cyclostationarity of the interferer, is highly effective at removing the interference. We show that when non-Gaussian impulsive noise is present, the FRESH filter is still effective against interference, even when the two signals have cyclic frequencies in common. However if hole-punching is used to remove the larger impulses of the noise, this adversely affects the performance of the FRESH filter.