Re: [Discuss-gnuradio] Spectrum distortion in wideband OFDM samples

[Per Zetterberg]

Sorry for my last post on this thread which contained nothing new
(mistake). A new comment on thread from my side at the bottom.


On Fri, 2011-01-28 at 15:10 -0800, Matt Ettus wrote:
> On 01/28/2011 02:44 PM, Per Zetterberg wrote:
> > 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.
> 
> 
> Yes, it does look very saturated.  Remember, OFDM has a very high peak 
> to average ratio.
> 
> 
> >
> > 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.
> 
> Intermod is not affected by sample rate because intermod only happens on 
> analog signals (assuming you don't have digital clipping), and the 
> signals are only converted to/from analog at the maximum sample rate.
> 

I forgot that original the sample-rate is higher. But if the digital
filtering in the FPGA is insufficient (not enough attenuation of
sidebands) the problem persists - because the intermods (which
originates from the analog side) will get folded into the desired at the
final decimation. 


BR/
Per