# 6.3 M-ary Frequency Shift Keying (M-ary FSK)

## M-ary FSK application

#### Unlike M-ary ASK and in contrast to many of the discussions to date, M-ary FSK is very much of interest for increasing the noise immunity of the modulation format compared with binary FSK, allowing a designer to achieve reliable data transmission in the presence of high levels of noise.

#### As we shall see shortly, this is only possible by using a set of 'orthogonal symbols', with precisely spaced frequencies which require large amounts of bandwidth. M-ary FSK using orthogonal signalling is one of the few techniques where the modem performance approaches the Shannon bound for minimum Eb/N0 operation of -1.6 dB. |

#### It is also possible to operate M-ary FSK with 'non-orthogonal' symbol frequencies as we saw with binary FSK. By spacing the frequencies very close together it is possible to squeeze four symbols into the space of two symbols, for example, and hence improve the bandwidth efficiency over BFSK. In this case, the noise immunity for the M-ary FSK system decreases compared with the binary system, as the symbol frequencies are no longer orthogonal.