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. This modulation produces a signal which is much more tolerant of receiver mis-tunings and frequency drift than MFSK modulation.
Source: Wikipedia — Incremental frequency keying (CC BY-SA 4.0)