Communications Blockset

Filtered QPSK vs. MSK

The QPSK vs. MSK demo, qpskvsmsk, enables you to visually compare filtered quadrature phase shift keying (QPSK) and minimum shift keying (MSK) modulation schemes. This document highlights these aspects of the demo:

• Structure of the Demo
• Visible Results of the Demo

Structure of the Demo

This demo uses various Communications and DSP Blockset blocks to model Filtered QPSK and MSK modulation schemes. The demo includes the following blocks:

• Sources of uniformly distributed random integers in the range [0, M-1], where M is the constellation size of the modulation scheme. Two sources are required since QPSK is a quaternary modulation method while MSK is a binary modulation method.
• A Baseband QPSK Modulator block
• A Baseband MSK Modulator block
• FIR Interpolator block that implements raised cosine filtering
• A Unipolar to Bipolar Converter block
• An Additive White Gaussian Noise (AWGN) Channel block
• Eye Diagram blocks

Visible Results of the Demo

The demo include these visual aids to illustrate the difference between the two modulation schemes:

Eye Diagram blocks that show the eye diagrams of Filtered QPSK and MSK signals plus noise. In FQPSK, the value of both the inphase and quadrature components of the signal are permitted to change at any symbol interval. However, for MSK, the symbol interval is half that for QPSK but the inphase and quadrature components change values in alternate symbol epochs. Therefore, the ideal samping time for QPSK is 0.5, 1.5, 2.5, ... while the ideal sampling period for MSK is 0.5, 1.5, 2.5, ... for the inphase signal and 1, 2, 3 ... for the quadrature signal.

GMSK vs. MSK Demo

Rayleigh Fading Channel Demo