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Re: [Discuss-gnuradio] Help: Constellation Receiver


From: Martin Braun (CEL)
Subject: Re: [Discuss-gnuradio] Help: Constellation Receiver
Date: Thu, 8 Aug 2013 09:42:34 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Aug 07, 2013 at 09:17:02AM -0700, tom sutherland wrote:
> I have gotten the "Constellation Receiver" block so it seems to run without
> errors but I don't see much out. I assume I am suppose to be feeding it a
> complex baseband I/Q signal. What do the output "OUT, SYMBOL, Error, Phase
> Freq" represent? I am getting signals out of the Error/phase/freq outputs but
> nothing out of Out or Symbol.

Hi Tom,

the constellation receiver is a combined block that does frequency,
phase and timing sync as well as the symbol demapping. The
error/phase/freq outputs are basically 'debugging' ports. I assume you
know how a Costas loop works? These ports transmit the error signal
which is part of this feedback loop.

> Const Recv. Parameters: (suggestions anyone?)
> Loop bandwidth == 6.28/100
> Min Freq Deviation == 0.6
> Max Freq Deviation == 0.9

Intuitively, I'd say the freq deviation params are off. The loop bw is
OK. To be honest, I keep forgetting how to set these myself, as they
affect the control loop rather than the actual receiver (have you read
these pages?:
http://gnuradio.org/doc/doxygen/classgr_1_1blocks_1_1control__loop.html
http://www.trondeau.com/blog/2011/8/13/control-loop-gain-values.html
)

I don't know the details of the block by heart, but I suggest you do the
following:
In GRC, connect a QPSK source into a channel model into a constellation
receiver. Start with very little noise, no freq offset.
Play around with the params, and see if the SYMBOL output looks like a
noisy QPSK constellation diagram. That gives you an idea of how the
params should look like. Then play around with frequency offset, phase,
timing etc. in the channel model.

Hope this helped,
MB
-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

Kaiserstra├če 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071
www.cel.kit.edu

KIT -- University of the State of Baden-W├╝rttemberg and
National Laboratory of the Helmholtz Association

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