Minimum-shift keying (MSK) is a type of continuous phase frequency-shift keying.

Similarly to OQPSK, MSK is encoded with bits alternating between quarternary components, with the Q component delayed by half a bit period. However, instead of square pulses as OQPSK uses, MSK encodes each bit as a half sinusoid. This is implemented by using a raised-cosine filter just before the transmission: this smooths the signal without introducing intersymbol interference.

The resulting signal is represented by the formula

$S(t)\; =\; a\_\{I\}(t)\backslash cos\{(\backslash frac\{\{\backslash pi\}t\}\{2T\})\}\backslash cos\{(2\{\backslash pi\}f\_\{c\}t)\}+a\_\{Q\}(t)\backslash sin\{(\backslash frac\{\{\backslash pi\}t\}\{2T\})\}\backslash sin\{(2\{\backslash pi\}f\_\{c\}t)\}$

where $a\_\{I\}(t)$ and $a\_\{Q\}(t)$ are the square pulses as shown in QPSK.

A similar modulation scheme is Gaussian minimum shift keying, which uses Gaussian instead of sinusoidal pulse shapes.

Radio modulation modes