Mason wrote:
Hi list,
So I'm still working on trying to get QAM transmissions between two
USRPs using the following patch:
http://www.cae.wisc.edu/~masond/gr-qam-patch-rev3.diff
The QAM receiver works in simulation but the constellation just spins
and spins when sent over the air.
It apparently is unable to cope well with offsets of frequency between
sender and receiver. Does anyone know either a) better values for the
'magic numbers' of the receiver to make it track better, (or even a
source for the selection of the original values) or b) a better
receiver algorithm to use than the M&M?
--Mason
You'll want to make another receiver. It looks like the qam_receiver is
just the mpsk_receiver. This one calculates the frequency offset by the
instantaneous phase offset in each symbol. For anything not BPSK or
QPSK, it uses the constellation to calculate this by first finding which
point the symbol most likely represents. With QAM, you have both a
sensitivity problem doing this under low SNR. Specifically, all of the
inner points (i.e., the ones with the lowest SNR) are going to bias the
reciever and prevent it from getting lock. You really want to make this
into a QPSK problem where you are only looking at the outter-most
corners and making decisions based on the high SNR symbols.
Tom
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