Incremental frequency keying

Incremental frequency keying, also known as IFK or IFK+, is a modified type of MFSK modulation where the data to be transmitted is represented by the difference in frequency between the previously received tone and the currently received tone.^[1][2]

This modulation produces a signal which is much more tolerant of receiver mis-tunings and frequency drift than MFSK modulation. Additionally, IFK modulation is more resistant to multipath interference and intersymbol interference caused by multipath propagation than traditional MFSK.^[3][4] This combination of features makes IFK modulation well suited for high frequency communications.

This modulation is used in the amateur radio data-modes DominioEX and THOR.