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I try to demodulate 8dpsk in software. Carrier frequency=1800 Hz, modulation rate=1600 bauds, i.e. itu-t v.27. Demodulator has the following properties:

  1. Input passband hardware filter 50...3600 Hz;
  2. sampling frequency - 9600 Hz;
  3. Matched filter - RRC, Beta=1;
  4. Timing recovery- simply Gardner,first order loop, 2 points per symbol.

Also, demodulator has interpolator to interpolate between matched filter outputs.

Physical line is short and I believe AWGN amount must be relative small. Demodulator works without errors, but constellation diagram looks ugly (see picture). Can anyone tell me how to "improve" constellation diagram?

constellation diagram

SleuthEye
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user8142579
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2 Answers2

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I have placed interpolator before matched filter to minimize ISI and apply equalizer. Now it looks much better.enter image description here

user8142579
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Your question has not general answer, but I want to suggest some solution. At first, it is important what filter do you use in your Gardner algorithm to calculate jitter and mu and how you set the loop filter coefficients. It seems that it will improve your results. Second, the RRC filter roll of factor is very important to get better results. Third, if you revise the frequency offset before timing recovery, you get better results in time to lock and the constellation density. For doing this solution, you should feedback the estimated phase in PLL to the Timing recovery input and compensate phase offset for symbol synchronization. Fourth, the interpolation that you use for symbol extraction from samples is important.

I write these suggestions in order that seems is useful for your constellation.

Mohammad Hassan Shammakhi
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