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'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 the symbol period. However, instead of square pulses as OQPSK uses, MSK encodes each bit as a half sinusoid. This results in a constant-modulus signal, which reduces problems caused by non-linear distortion.
The resulting signal is represented by the formula

ight)}cos{(2{pi}f_{c}t)}+a_{Q}(t)sin{left(rac{{pi}t}{2T}
ight)}sin{left(2{pi}f_{c}t
ight)}
where $a\_\{I\}(t)$ and $a\_\{Q\}(t)$ encode the even and odd information respectively with a sequence of square pulses as of duration ''2T''.
Using the trigonometric identity, this can be rewritten in a form where the phase and frequency modulation is more obvious,
$$
s(t) = cos[2 pi f_c t + b_k(t) rac{pi t}{2 T} + phi_k]

where ''b_k(t)'' is +1 when $a\_\{I\}(t)\; =\; a\_\{Q\}(t)$ and -1 if they are of opposite signs, and $phi\_k$ is 0 if $a\_\{I\}(t)$ is 1, and $pi$ otherwise. Therefore, the signal is modulated in frequency and phase, and the phase continuously and linearly changes.
A similar modulation scheme is Gaussian minimum shift keying, which uses Gaussian instead of sinusoidal pulse shapes.
MSK, a proven and widely used technique, should not be confused with the completely unworkable Very minimum-shift keying (VMSK).

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References

References

★ Subbarayan Pasupathy, ''Minimal Shift Keying: A Spectrally Efficient Modulation'', IEEE Communications Magazine, 1979