Help: QAM Constellation and PolyPhase Clock

GNU Radio
1

I have simple 16QAM Modulator block and am plotting the constellation
with and without the a polyphase clock block . The constellation
directly out of the modulator is not 16 “points” or even blobs but
“random” looking points. I put it into a QAM Demod block and it
demodulates it correctly. The constellation plot of the polyphase clock
block shows 16 distinct “points” but when demodulated is quite noisy.
Can anyone explain:

  1. Why is the output of the QAM Modulator block not show as 16 points?
  2. Why would the “corrected” QAM Modulation not give better demodulation
    output?

I have attached the ConstellationQuestion.grc file.
Thanks…Tom

2

On Mon, Jul 29, 2013 at 12:14 PM, tom sutherland [email protected]
wrote:

I have attached the ConstellationQuestion.grc file.
Thanks…Tom

It’s what’s called ISI, or inter-symbol interference. The modulator
applies a pulse shaping filter to the transmitted signal, which in
this case is a root raised cosine filter. That has properties that
actually “smear” the symbols in time and make them overlap in this
way. That’s expected and what it’s supposed to do. Also, you have now
increased the sampling rate to 4 samples / symbol, so you’ll get
points in between the actual symbol points now.

The clock recovery block in this case uses another root raised cosine
filter. The convolution of an RRC with an RRC is a raised cosine
filter, which is a Nyquist filter, and so you’ve removed the ISI
you’ve introduced in the first RRC stage. This also finds the correct
symbol points and outputs 1 sample per symbol. The PFB still has a
little bit of noise added due to the quantization of the possible
phases. If you increase nfilts, you’ll see this quantization noise go
down. But the noise using 32 filters is about the same quantization
noise you get for 16 bits of precision, so that limits your situation
there. Besides, you’re going to quickly become dominated by thermal
noise.

Tom