COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
Ref. DTU 100042
Is.Rev. 5.17
Date: Dec.
03, 2021Sept. 30, 2021
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It cannot be duplicated or distributed without expressed written consent.
3.3.1.2.3. GMSK Demodulation
Gaussian Minimum Shift Keying (GMSK) is a constant envelope, Continuous Phase Modulation (CPM). It is
derived from Minimum Shift Keying (MSK) with the addition of a baseband Gaussian filter that further reduces
side lobe levels and spectral bandwidth. The product of the Gaussian filter bandwidth and the coded symbol
period at the input to the modulator, referred to as the BTs factor, is used to differentiate between GMSK
modulations of varying bandwidth efficiencies.
GMSK is inherently a differential Continuous Phase Modulation (i.e., the information is carried in the change of
the phase rather than the phase itself). A differential decoder is needed at the receiver which increases the BER
by approximately a factor of two. By pre-coding the GMSK signal at the transmitter to remove the inherent
differential encoding, the BER can be halved.
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