# GMSK Demod

Posted on Feb 6, 2014

GMSK_Demod is a model of a suboptimal (an optimal receiver would require the Viterbi algorithm) coherent GMSK demodulator (refer to the GMSK_Recovery component). Noncoherent demodulation can be performed with the FM_Demod component. Pin 1 is the input for the GMSK modulated RF signal; pin 2 is the input for a reference RF carrier signal used to de

modulate the GMSK signal and obtain its inphase and quadrature components; pin 3 is the input for a clock baseband signal used to sample the demodulated inphase and quadrature signals. The reference RF carrier signal should be synchronized in frequency and phase to the modulated signal for proper demodulation. The clock signal should have a duty cycle of 50 percent; its frequency should equal half the transmitted data rate; and, its phase should be adjusted so that the positive edge of the clock occurs at the instant when the eye of the demodulated inphase signal is maximum (therefore, the negative edge will occur when the eye of the demodulated quadrature signal is maximum). Coherent GMSK Demodulator Schematic shows the demodulator schematic. To understand the operation of this demodulator, note that GMSK is a digital phase modulation technique where the phase of the carrier G †(t), changes by G‚ ±G /2 radians (approximately) in a symbol period, depending on whether the input is G‚ ±1. Let I(t) and Q(t) be the inphase and quadrature components of the GMSK signal. Let T S be the duration of an data symbol, and let 2 nT S, n=0, 1, 2, . be the instances when I(t) is sampled and (2 n+1)T S, n=0, 1, 2, . be the instances when Q(t) is sampled. The samples of I(t) and Q(t) then provide the following information about the phase of the modulated signal: By following the phase trajectory, the information sequence can be...

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