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I am generating a DQPSK waveform at complex base band with constellation points on the unit circle and when I add AWG noise the constellation shrinks. The noisy constellation "blob" migrate inside the unit circle. My question is why is this happening and is it supposed to happen or am I missing something?

My method of adding noise is:

Xnoise = X + sigma/2 * (randn(N) + 1j*randn(N))

where sigma is the overall standard deviation for the noise. It's being divided by 2 since I'm using two separate randns for the real and imag parts and the two Gaussians added together give me the correct variance.

Ideal DPSK

Noisy DPSK Eb/N0=14

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It looks like your circle got bigger. The constellation is fine.

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  • $\begingroup$ Good call. I didn't even notice that. I am using Matplotlib and just plotting a circle of radius 1, but I must be doing something that is causing it to change. $\endgroup$
    – majorpain1588
    Commented Aug 17, 2015 at 15:54

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