Hi,
I posted about this before (High frequency OFDM), but could not get a
reply. I tried my best, but am completely stuck by now and would
accept any wild guesses.
I have two USRPs (TVRX and WBX-TX) connected via cable that transmit
a OFDM signal (courtesy of gnuradio-examples/python/ofdm, default
parameters) in between them. I have manually taken care that the
carrier frequency is centered correctly on the receiver and that the
amplitudes at both the transmitter and receiver are configured
optimally.
Now when I transmit said OFDM signal at around 100MHz, everything is
looking perfectly fine. If I look at the transmitted and received
signal, I see a perfectly sharp edged OFDM block in the spectrum and
get 100% of the data from one USRP to the other. Nice. When I switch
that same configuration into higher areas of the spectrum (400MHz+),
again adjusting carrier-frequency and amplitudes, I also see the same
sharp edged blocks. The SNR is a little worse, but still well beyond
30dB. This is where my problem lies. For some reason, most of the
packets are broken or bursts are missing completely. If I switch to
wider subcarrier distance, the transmission works again - but that
isn't what I am looking for.
1) What happens at higher frequencies that "invisibly" destroys the
OFDM block? Phase noise? Impulsive noise? Something to do with the
sampling rate conversion? Any tips or pointers would be of great
help.
2) How could I model a high frequency transmission (including phase
noise) in GNU Radio, so that I can more easily see the problem?