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Compare GMSK and MSK Signals in Simulink

This model compares Gaussian minimum shift keying (GMSK) and minimum shift keying (MSK) modulation schemes.

The Random Integer Generator block provides a source of uniformly distributed random integers in the range [0, M-1], where M is the constellation size of the GMSK or MSK signal. The Unipolar to Bipolar Converter block maps a unipolar input signal to a bipolar output consisting of integers between -(M-1) and +(M-1). The bipolar data is routed to separate paths. The top path applies GMSK modulation by using the GMSK Modulator Baseband block. The bottom path applies MSK modulation by using MSK Modulator Baseband block. Noise is added to both the GMSK and MSK signals by using AWGN Channel blocks. The Eye Diagram Scope blocks are used to visualize eye diagrams of both signals.

The eye diagrams show the similarity between the GMSK and MSK signals when you set the initial pulse length of the GMSK Modulator Baseband block to 1.

Set the initial pulse length in the GMSK modulator to 5 to view the difference that a partial response modulation has on the eye diagram. The increased pulse length results in an increase in the number of paths, showing that the CPM waveform depends on values of the previous symbols as well as the present symbol. Plot the eye diagram of the GMSK signal.

If you change the initial pulse length to an even number, such as 4, you should set initial phase offset of the GMSK modulator to pi/4 and the offset argument of the eye diagram 0 for a better view of the modulated signal. In order to more clearly view the Gaussian pulse shape, you must use scopes that enable you to view the phase of the signal, as described in the CPM Phase Tree example.