Ofdm transceiver with more than 16QAM

Hi, I'm trying to make an OFDM transmission of a png file (9.3 kB)
with 256QAM with the gnuradio
example of OFDM transmitter / receiver. The USRP I'm using is the
ettus n210 with the cbx daughter board. With the QPSK and 16QAM
modulations it works, but with 64QAM or 256QAM I can´t receive the
file, sometimes it receives some packets but most of the time it
doesn't receive any byte. Can anyone help me with with this problem?

Thanks in advance.
Pedro Viegas
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Pedro Viegas
Signal Processing Engineer
Koala Tech
phone: +351 917 354 070
e-mail: pv@koalatech.pt