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In digital modulation, minimum-shift keying (MSK) is a type of continuous-phase frequency-shift keying that was developed in the late 1950s by Collins Radio employees Melvin L. Doelz and Earl T. Heald. [1] Similar to OQPSK, MSK is encoded with bits alternating between quadrature components, with the Q component delayed by half the symbol period.
MSK and GMSK are particular cases of continuous phase modulation. Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one-symbol-time duration (total response signaling).
Minimum frequency-shift keying or minimum-shift keying (MSK) is a particular spectrally efficient form of coherent FSK. In MSK, the difference between the higher and lower frequency is identical to half the bit rate. Consequently, the waveforms that represent a 0 and a 1 bit differ by exactly half a carrier period.
Minimum-shift keying (MSK) is another name for CPM with an excess bandwidth of 1/2 and a linear phase trajectory. Although this linear phase trajectory is continuous, it is not smooth since the derivative of the phase is not continuous. The spectral efficiency of CPM can be further improved by using a smooth phase trajectory.
GMSK is based on two ideas: one is shaping of data pulses with gaussian filter, the other is FSK. MSK uses index that is much lower than indices for FSK. There is no rule that it has to be 0.5 or 0.25. Anything "low enough" is good enough to rename FSK as MSK. Such modulation has very different spectrum so that it may be distinguished from FSK.
The goal of keying is to transmit a digital signal over an analog channel. The name derives from the Morse code key used for telegraph signaling. Modulation is the general technique of shaping a signal to convey information. When a digital message has to be represented as an analog waveform, the technique and term keying (or digital modulation ...
In addition to Gaussian minimum-shift keying (GMSK), EDGE uses higher-order PSK/8 phase-shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase.
Examples of these are quadrature phase shift keying and its generalisation as m-ary quadrature amplitude modulation (m-QAM). Because existing computers and automation systems are based on binary logic most of the modulations have an order which is a power of two: 2, 4, 8, 16, etc.