On Fri, 2011-01-28 at 02:09 -0500, Sangho Oh wrote:
Hello, I am transmitting 20Mhz OFDM symbols using UHD code.
When I checked the PSD of the received samples using UHD
(rx_samples_to_file)
I found there is a significant difference in PSD between the
transmitted and received signal.
Reception decimation is 4 and I used re-sampled data (4/5) for OFDM
demodulation.
Here is the figure showing the PDS difference.
http://i1181.photobucket.com/albums/x423/notilas/untitled.jpg
Someone guessed that this is a natural thing related to the low pass
filter in RF parts of Gnuradio.
However, I would like to ask valuable comments from radio experts.
The spectrum looks very strange so I would guess saturation or
something.
However, I have had some experiences I would like to discuss. I have
experimented with OFDM waveforms on USRP1, USRP2 and also another
home-brewed testbed. What I generally find is that an oversampling of a
factor of two is required. Thus with 25MHz sample-rate I can squeeze in
a maximum 12.5MHz signal. I can use more but at the expense of much
worse performance. I think the main problem is nonlinearities. The
nonlinearities causes intermods. If the sampling frequency is less than
twice the bandwidth, 3rd order intermods will fold into the desired
signal.