Re: [Discuss-gnuradio] Simple QAM mod/demod

[Fabián Kozynski]

I figured out the problem. There were two things I wasn't taking into account:

• The modulator reads bits in MSB order, which means that I should pack them the same way.
• There is a bit framing problem (can be solved using packet encoder/decoder). Checked converting both sequences to bits (8 per number) and realizing one's a shifted version of the other.

Cheers,

Fabián

On Wed, Oct 3, 2012 at 10:12 AM, Fabián Kozynski <address@hidden> wrote:

It's not just an offset problem. Even though in the output file, there is a delayed sequence, it doesn't have the right bits. As it has exactly the same period it leads me to believe that it's just a problem of phase sync.

Attached is a minimal grc file.

Cheers,

Fabián

On Wed, Oct 3, 2012 at 12:23 AM, Ben Reynwar <address@hidden> wrote:

On Mon, Oct 1, 2012 at 1:27 PM, Fabián Kozynski <address@hidden> wrote:

> As sanity check I'm trying a simple flowgraph in GRC with a QAM mod/demod:

>

> Vector Source ==> Throttle ==> QAM Mod ==> QAM Demod ==> Unpacked to Packed

> ==> File Sink

>

> The parameters in the mod/demod are as default. I also have a File Sink

> before the mod.

>

> In the output I see first a bunch of zeros, then a bunch of random numbers

> and after 20 or so bytes I get a sequence with the same period as the

> original one, but different numbers. I suppose this is a problem of phase

> sync.

>

> What's the right way to simulate a simple mod/demod? How do I add sync

> between mod and demod?

>

> Thanks,

> Fabián

It sounds like you're comparing bytes in the two files.  First thing

that I'd check is that the two files aren't offset from each other by

a few bits so that the bytes don't line up in the same way.

If that's not the problem then email me the grc file and I'll have a look.

Cheers,

Ben

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