Minimum-shift keying

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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

S(t) = a_{I}(t)\cos{\left(\frac{{\pi}t}{2T}\right)}\cos{(2{\pi}f_{c}t)}+a_{Q}(t)\sin{\left(\frac{{\pi}t}{2T}\right)}\sin{\left(2{\pi}f_{c}t\right)}

where aI(t) and aQ(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.

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