[Discuss-gnuradio] USRP noise at higher frequencies

[Joachim Roop]

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?
-- 
GMX DSL: Internet-, Telefon- und Handy-Flat ab 19,99 EUR/mtl.  
Bis zu 150 EUR Startguthaben inklusive! http://portal.gmx.net/de/go/dsl