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Re: [Discuss-gnuradio] QAM Mod / Demod blocks

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] QAM Mod / Demod blocks
Date: Wed, 23 Jul 2014 09:45:13 -0400

On Wed, Jul 23, 2014 at 8:42 AM, Activecat <address@hidden> wrote:
Dear Sir,

I am learning to use the built-in "QAM Mod" and "QAM Demod" blocks.
Refer below flowgraph.

The problem is, at the output of the "QAM Demod" block, I can't get what I fed into the "QAM Mod" block.
What is missing ..?

All modulators take in packed bits and all demodulators output unpacked bits. First, remember that there are filters involved, so there's going to be a delay. You'd need to synchronize the output of the demodulator to some known word to understand which output bit is the correct start of the data stream. Easier would be to unpack the source data. Out of the 'head' block, make another connection to a 'unpack_k_bits' block with an unpacking value of 8. Use this to compare to the demodulated bits.

Also, the vector source should generate values from 0 to 255 to create the packed bits needed as input to the modulator. Otherwise, you'll just be heavily biasing the constellation to just a few points. You can stick a constellation display block on the output of the modulator to see.



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