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Re: [Discuss-gnuradio] help with QAM demodulation

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] help with QAM demodulation
Date: Mon, 27 Feb 2012 16:57:45 -0500

On Mon, Feb 27, 2012 at 4:51 PM, Jeff Hodges <address@hidden> wrote:
Thanks Ben and Tom, you two have been very helpful.

The code Ben provided does work, and adjusting my GRC program to use the same parameters, I was able to achieve the expected results.

However, it appears there are two issues:

(1) When differential encoding is set to off for both mod/demod blocks, the output data becomes invalid

I'm not sure the differential en/decoder works for QAM. I suggested it before because of the bits that you sent us, but the gray coding was the problem, which makes more sense.
(2) When the samples per symbol is above 10 the output also becomes invalid.

I'm not sure why you would want to run it with that many sps, anyway. I'd probably have to dive back into the code to see what might be the problem there. Theoretically, it should be doable, but since I've never run it with that many, it's probably never been exercised.
Any ideas on what may be causing these discrepancies?

In response to Tom's questions, I am using 16-QAM and the constellation does look very noisy (both phase and amplitude) coming out of the QAM mod and being observed on the WX GUI Constellation Sink. But then again, there a lot of parameters on that signal processing block, so maybe I do not have the proper values set.



The noise might be due to ISI since you're just coming out of a RRC filter at that point. Unless the constellation sink has another RRC in it. Also, that sink tries to do some synchronization designed for PSK signals, so it won't work great and might be causing some of your error. Try just using an oscope and plottng x verus y.


On Mon, Feb 27, 2012 at 12:50 PM, Ben Reynwar <address@hidden> wrote:
On Mon, Feb 27, 2012 at 10:00 AM, Ben Reynwar <address@hidden> wrote:
> On Mon, Feb 27, 2012 at 7:36 AM, Jeff Hodges <address@hidden> wrote:
>> Does anyone know if the QAM demodulator code is working properly?  I would
>> like to get a QAM demodulator working for a symbol rate of 300ksym/s. I
>> don't know whether I am just using the wrong parameters or if the blocks do
>> not work properly, but I am not getting the results I expect.
>> To test the code out I am using the GRC. I have a vector source with a known
>> data sequence running into the qam mod block and then the output of that
>> into a QAM demod block. The output of the QAM demod is running into a Uchar
>> to Float, and I am plotting the results on a WX GUI Scope.  I have the exact
>> same settings on both the QAM mod and demod blocks.  The output I am seeing
>> on the scope looks completely random. (I also tried other vector source
>> data: When the input is 0x0F, repeating, the output I see is 00001010
>> repeating. But when I use 0x0E, the results are random again.)
>> I have also tried demodulating a real QAM signal with a known data sequence,
>> also to no avail.
>> I have been working on this for about a month now and haven't had any
>> success.  I have examined the underlying QAM.py and generic_mod,
>> generic_demod codes, and they seem to be written properly. But I am not
>> getting the expected results.
>> Any help with this problem or advice you can give will be greatly
>> appreciated, and I will return the favor by helping out in any way that I
>> can.
>> Thank you very much in advance,
>> Jeff
>> _______________________________________________
>> Discuss-gnuradio mailing list
>> address@hidden
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> If it's not working it's probably my fault, so I'll do some tests
> myself today and get back to you.
> Cheers,
> Ben

I found a bug in the xml file for the demodulator that would cause
problems if you requested no gray-coding.
It cause gnuradio-companion to pass an incorrect parameter name.
A fix is at https://github.com/benreynwar/gnuradio/commit/e68ab8589ca235563f8c061fbb79d13793d1f21f

In case that wasn't your problem I'll post an example that works for me:

from gnuradio import gr, digital

class qam_mod_demod(gr.top_block):

   def __init__(self):
       super(qam_mod_demod, self).__init__()
       src = "" 1, 1, 0, 1, 0, 0, 1, 1, 0, 1, 0,
0, 0]*1000)
       packer = gr.unpacked_to_packed_bb(1, gr.GR_MSB_FIRST)
       self.snk = gr.vector_sink_b()
       mod = digital.qam.qam_mod(constellation_points=16,
       demod = digital.qam.qam_demod(constellation_points=16,
       unpacker = gr.packed_to_unpacked_bb(8, gr.GR_MSB_FIRST)
       self.connect(src, packer, mod, demod, unpacker, self.snk)

if __name__ == '__main__':
   qmd = qam_mod_demod()
   data = "">    print(data)

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