GMSK in a nutshell

This report presents a brief overview of the GMSK modulation scheme. 1 What is GMSK ? GMSK stands for Gaussian Minimum Shift Keying. This is a modulation scheme in which the phase of the carrier is instantaneously varied by the \modulating" signal (i.e. the information to transmit). GMSK diiers from MSK (Minimum Shift Keying) in that a Gaussian Filter of an appropriate bandwidth (deened by the BT product) is used before the modulation stage. The time-domain impulse response of the lter is described in Equation 1, where k 1 = p 2 ln 2 and B is the half-power bandwidth. (1) MSK is binary digital FM with a modulation index of 0.5. It has the following important characteristics: constant envelope, relatively narrow bandwidth and coherent detection capability. The most important characteristic of MSK is that it is a constant-envelope variety of modulation. This makes the modulation scheme more immune to noise than the Amplitude Shift Keying (ASK) scheme. However, MSK does not satisfy the requirements with respect to out-of-band radiation for single-channel-per-carrier (SCPC) mobile radio. GMSK uses a pre-modulation Gaussian lter which makes the output power spectrum more compact. The pre-modulation Gaussian lter has narrow bandwidth and sharp cutoo properties which are required to suppress the high-frequency components. Moreover, it has a lower overshoot impulse response which allows to protect against excessive instantaneous deviation.