Simulate Transmission using dsp.DigitalUpConverter and dsp.DigitalDownConverter

I am trying to use the dsp.DigitalUpConverter and dsp.DigitalDownConverter functions to do a simulation of transmitting an MSK signal.

After I run the attached script, I compare the constellation diagrams of the MSK signal and the MSK signal after it goes through the upconversion and downconversion functions.

Why does processing it through the upconversion and downconversion result in two blue "dots" rather than the expected one shown in yellow?

Answer 1

[Jyothis Gireesh]

Here are a few pointers which may be able to help you.

• The baseband signal obtained after decimation contains some spurious signal in the first 5 samples. It may be better to omit those samples and use the “plot_modSigdwn” variable instead to plot the constellation diagram.
• Set the ‘SymbolsToDisplaySource’ property in the cd2 object to ‘Property’ which will allow the maximum number of recent symbols to be displayed to be set by the ‘SymbolstoDisplay’ property.

The constellation diagram after the processing is given below.

Still the points in the constellation diagram may not exactly coincide. This may be due to certain amount of non-linearities created by the use of “dsp.DigitalUpConverter” and “dsp.DigitalDownConverter” objects. The symbols may be decoded by approximating the received symbols to the nearest symbols in the reference constellation diagram.