Spectrum distortion in wideband OFDM samples

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.

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.

Thanks

On Fri, 2011-01-28 at 02:09 -0500, Sangho Oh wrote:

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.

Another observation is that, given that I have the luxury of two times
oversampling (or more), it is better to use an fft on all the samples
and null (don’t use) some of the subcarriers, than do up/downsampling
after/before the fft. I don’t why, but I guess that it would not be very
hard to prove it with maths.

I wish I had a reference on these issues (conference, journal-paper or
book). Most papers are ignoring all these practical issues …

BR/
Per

On 01/28/2011 02:44 PM, Per Z. wrote:

Here is the figure showing the PDS difference.

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.

What you may be seeing is aliasing, which will be noticeable when you
try to make your signal wider than 75% of the sample rate.

Matt

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 E. wrote:

demodulation.

However, I have had some experiences I would like to discuss. I have
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


From: Matt E. [[email protected]]
Sent: Saturday, January 29, 2011 12:10 AM
To: Per Z.
Cc: Sangho Oh; [email protected]
Subject: Re: [Discuss-gnuradio] Spectrum distortion in wideband OFDM
samples

On 01/28/2011 02:44 PM, Per Z. wrote:

Here is the figure showing the PDS difference.

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.

What you may be seeing is aliasing, which will be noticeable when you
try to make your signal wider than 75% of the sample rate.

Matt

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