Hi Dimitris,
GNU Radio ships with examples, I think some of the packet examples could
do QAM out of the box, via OFDM. Just swap out the constellation objects
from the PSK they're using to QAM.
Problem with QAM is simply that you need coherent reception, i.e., your
receiver needs to correct the phase of its reception to match the
absolute phase of the transmitter, else the symbols are just logically
impossible to recover.
So, any QAM transceiver system will always encompass a receiver that
does absolute phase recovery, and some transmitter that necessarily
gives structure to the data it transmits, to make that possible. That
typically takes the shape of *framing*.
Furthermore, due to the tight distance between constellation points, and
the fact that complexity-wise, you're often likely to make a design
decision in favor of QAM on a relatively wide channel, you really can't
deal will with the inter-symbol interference that a multipath channel
introduces. You are hence in need of an *equalizer*.
The GNU Radio OFDM infrastructure takes care of equalizing (by doing the
equalization in frequency domain, OFDM!) and framing and phase recovery.
So, look through the examples that were installed with GNU Radio. I
don't know your operating system or how you installed GNU Radio on it,
but if you're on Linux and used your Linux distro's tools (apt, dnf,…)
to install GNU Radio, chances are you'll find these examples under
/usr/share/gnuradio/examples/digital/ofdm.
Best regards,
Marcus
On 2025-03-10 4:27 PM, Dimitris Pliatsios wrote:
Dear all,
I am trying to build a QAM transmitter & receiver that uses a USRP
device as a front-end.
To start with, I am searching for something ready, if it exists, in
order to modify it to my needs.
Is there an official/standard repository for .grc projects?
I searched on GitHub, but many of the projects use previous GNU Radio
versions that contain deprecated blocks.
Thank you in advance!
Kind regards,
Dimitris
