Minimum-shift keying

Lore & Background

MSK was developed in the late 1950s by Collins Radio employees Melvin L. Doelz and Earl T. Heald. It is similar to OQPSK in that bits alternate between quadrature components, with the Q component delayed by half the symbol period. However, instead of square pulses, MSK encodes each bit as a half sinusoid, resulting in a constant-modulus signal that reduces problems from non-linear distortion. MSK can also be viewed as a CPFSK signal with a frequency separation of one-half the bit rate, where the difference between the higher and lower frequency is identical to half the bit rate, so the waveforms for 0 and 1 differ by exactly half a carrier period. The modulation index is 0.5, the smallest FSK index that keeps the waveforms orthogonal.

Reader's Guide

MSK's significance lies in its spectral efficiency and reduced inter-channel interference. Its power spectral density shows side-lobes at −23 dB, lower than both BPSK and QPSK (−10 dB), which reduces out-of-band interference. However, the main lobe of the MSK signal is wider, meaning more energy in the null-to-null bandwidth, which can be a disadvantage where extremely narrow bandwidth is required. The minimum symbol distance is the same as in QPSK, allowing a theoretical bit-error ratio bound using the Q-function or complementary error function. A variant, Gaussian minimum-shift keying (GMSK), shapes the data stream with a Gaussian filter before frequency modulation, reducing sideband power and out-of-band interference at the cost of increased intersymbol interference and need for more complex equalization. GMSK is used in GSM, Bluetooth, satellite communications, and the Automatic Identification System (AIS) for maritime navigation.

Did You Know?

• MSK was developed in the late 1950s by Melvin L. Doelz and Earl T. Heald at Collins Radio.
• The modulation index of MSK is 0.5, the smallest FSK index that ensures orthogonal waveforms for 0 and 1.
• MSK's side-lobes are at −23 dB, lower than BPSK and QPSK (−10 dB).
• Gaussian minimum-shift keying (GMSK) is used in GSM, Bluetooth, satellite communications, and AIS.