Re: [Discuss-gnuradio] OFDM example from gr-digital with UHD

[Martin Braun]

On 25.06.2015 06:07, Marcus Müller wrote:
> Hi Gabriele,
> 
> have you tried adding more gain on the TX side?

...or less gain?

> Also, you will need to increase sampling rate. Right now, you're using
> 100kHz, which is 1/1000 of the master clock rate of the N200. If I'm not
> mistaken, the maximum decimation for the N200 is 512, and hence the
> minimum rate would be 100MHz/512 = 195.3125kHz. You probably just want
> to use 200kHz or something similarly practical.

Or even more, like 1-2 Msps.

I haven't tried a combo of N-Series specifically with this example, but
I've tried X-Series + B-Series, so I'm confident it's all OK. However,
OFDM is sensitive to distortions, so you need to play around with the
amplitude multiplier before tx'ing as well as the gain for your specific
setup to get a good transmit spectrum.

Since you have a spectrum analyzer, that's pretty useful for getting
those settings right.

You might also try playing with the rolloff length and/or adding more
empty carriers left and right of your tx spectrum while experimenting
with settings.

Cheers,
Martin

> 
> I hope that helps!
> 
> Best regards,
> Marcus
> 
> On 06/25/2015 02:23 PM, Gabriele Galiero wrote:
>> Hello, I was interested to test tx_ofdm.grc and rx_ofdm.grc from
>> gr-digital, at the same time using USRP N200 devices.
>>
>> TX OFDM:
>>
>> The changes that I did were, removing throttle block and adding an UHD
>> Sink instead, with center frequency 430 MHz, which is the frequency band
>> for which my department has license to transmit.
>>
>> RX OFDM:
>>
>> In this case I removed all blocks before throttle (included), and placed
>> there an UHD Source at the same center frequency.
>>
>> Flow graphs attached in the files.
>>
>> I monitored the output signal and the received signal. I also used a
>> spectrum analyzer to check the actual signal that is being transmitted.
>> Pics are attached.
>>
>> In the tx signal measured by a spectrum analyzer, we can see that the
>> spectrum is not that we desire. I would like to ask if I am doing
>> something wrong with the configuration. Before transmission there was no
>> signal at that frequency and tx and rx antennas are suitable for the
>> carrier frequency.
>>
>> In addition I tried to transmit a file instead of just random bits. In
>> reception I was not receiving the file. The config was just adding a
>> file sink at the end of the rx block and a file source at the beginning
>> of the tx block. Could you please help me with some tip?
>>
>> Attachments:
>> http://www.ruby-forum.com/attachment/10886/tx_ofdm.jpg
>> http://www.ruby-forum.com/attachment/10887/rx_ofdm.png
>> http://www.ruby-forum.com/attachment/10888/tx_signal.jpg
>> http://www.ruby-forum.com/attachment/10889/rx_signal.jpg
>> http://www.ruby-forum.com/attachment/10890/tx_sig_spectrum.jpg
>>
>>
> 
> 
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