Modulation Index
The modulation index is the ratio of the frequency deviation of the modulated signal to the message signal bandwidth. For FSK modulation, the modulation index h is defined as the ratio of the spacing between consecutive frequencies in the FSK symbol map, to the symbol rate. More precisely,
h=2×fdT/(M-1)
where
M is the modulation format
T is the symbol period
fd is the peak frequency deviation
For example, in 4-FSK, the spacing between consecutive frequencies equals 2fd / 3, hence the modulation index equals (2fd×T) / 3.
For more information about amplitude modulation, refer to modulation depth.