Re: [Discuss-gnuradio] trouble with QAM mod/ WX GUI Constellation Sink

[Bill Eaton]

> ... so I recommend you get an intro book to digital comms and map what
they
> say to the GNU Radio examples.

Yeah. I was afraid it might come to this. Do you have any recommendations?
Most digital comms textbooks are pretty pricey, and I'd hate to spend time
and money on one that might not be a good fit.

There are free options. There's the MIT course and the Lapidoth PDF book. I
suspect I might prefer a real printed book with nice graphics.

Thanks in advance,

Bill Eaton



-----Original Message-----
From: address@hidden
[mailto:address@hidden On
Behalf Of Martin Braun
Sent: Friday, July 17, 2015 3:04 PM
To: address@hidden
Subject: Re: [Discuss-gnuradio] trouble with QAM mod/ WX GUI Constellation
Sink

On 17.07.2015 14:56, Bill Eaton wrote:
>> What I meant was that you plot the result of your timing + phase 
>> recovery
> 
>> into an XY plot, that would take care of those additional points
> you're seeing.
> 
> I have to confess that I don't even know what that means or how to go 
> about accomplishing it. I have to take baby steps because I'm a baby.
> ;-) If you could point me to an example or some other things to try, 
> that would be helpful.

OK, this is starting to get into basics of digital comms and this mailing
list is probably not a good place for that.

In brief, what you're plotting is the output of the QAM Mod -- which is
*not* a QAM constellation, but rather the complex baseband signal that can
be transmitted. This does include the QAM constellation.

> I looked through the examples that ship with GNURadio and didn't see 
> anything obvious. Also, I looked through the GRC block tree to see if 
> I could try something. There's a Synchronizers section. Something from
there?

Try the examples in gr-digital.

> You thought maybe it was oversampled. It occurred to me that maybe the 
> X-Y chart rate should be samp_rate/8, but I tried that and the chart 
> still look largely the same.

That's not quite the right terminology, but if you plotted only a subset of
the samples of your signal, you would see the constellation.

It seems you're fine using GNU Radio itself, so I recommend you get an intro
book to digital comms and map what the say to the GNU Radio examples.

Cheers,
Martin


> 
>  
> 
> mlist_qam_constellation_xy.grc.png
> 
>  
> 
>  
> 
>  
> 
> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden On 
> Behalf Of Martin Braun
> Sent: Friday, July 17, 2015 1:20 PM
> To: address@hidden
> Subject: Re: [Discuss-gnuradio] trouble with QAM mod/ WX GUI 
> Constellation Sink
> 
>  
> 
> On 17.07.2015 12:43, Bill Eaton wrote:
> 
>> From Martin Braun:
> 
>>
> 
>>>My preference is to build the demod/sync chain with it's own blocks,
> 
>>>and then just output the complex symbols
> 
>>
> 
>>> into either an X/Y plot (WX) or the QT Constellation Sink
> 
>>
> 
>> Even an X/Y plot ends up looking funny. I've attached a GRC file for
> 
>> X-Y, and when you turn on persistence, it looks better. But it's very
> 
>> strange. Maybe my bits_per_symbol is wrong. Or maybe the sample rate
> 
>> for the X-Y chart should be different than the random sample rate?
> 
>> Here's the 4 QAM X-Y.
> 
>  
> 
> Your plot looks fine, it's just oversampled (but perfectly 
> time-synchronized, I'm guessing this is post-pulse shaping + noise?).
> 
>  
> 
> What I meant was that you plot the result of your timing + phase 
> recovery into an XY plot, that would take care of those additional 
> points you're seeing.
> 
>  
> 
> M
> 
>  
> 
>  
> 
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> 
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> 
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