OFDM problem with USRP and GRC

Hi all

I am trying OFDM transmission and receive test.

I’m using Ubuntu 9.10, gnuradio version 3.3 and two USRP boards with
basic TX and RX and I am using GRC.

In the GRC I make simple test signal flow with usrp

Tx : File source -> OFDM mod -> USRP sink

Rx : USRP source-> OFDM demod ->file sink

As a my test I could not show not only constellation but also any other
signal(scope and file data)

But I could show receive spectrum using fftsink with USRP

However I created simple example like follow, it operate well.

File source-> OFDM mod-> OFDM demod ->file sink

In order to test USRP part I tested gpsk example in the

It operate well. I could show constellation.

How can I operate well OFDMmod / demod examples?

Additionally I wanna confirm constellation of OFDM demod data.

But GRC block of the OFDM demod are encapsulate. So To confirm OFDM
démod constellation, I guess that I sholul split OFDM démod block.

Am I correct?

Can you have a some hint?


Hello all
I am also trying to get OFDM mod and demod blocks already present in
gnuradio distribution to work.
The blocks work when used on the same computer. Once I try to receive on
other USRP, the OFDM demodulator does not demodulate any signal.
Can anyone please let me know what problem might be there.

Someone suggested that there might be synchronaztion problem because of
which the receiver frequency drifts and does not demodulate the

Can someone please help me regarding the same.


손성환 wrote:

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Sent from the GnuRadio mailing list archive at Nabble.com.

On 04/25/2011 09:15 PM, killol wrote:

Can someone please help me regarding the same.


Minor frequency errors are an inevitable part of any radio channel. The
impact of these errors
is more or less severe, depending on the modulation technique in use.

PLL synthesizers are at the mercy of the crystals that are used to
provide a reference clock for them.
The crystal oscillators on the USRP family aren’t top-of-the-line, but
they’re quite good–roughly
+/-20PPM or so. So, what that means is that for every MHz of center
frequency, the actual
frequency could be
off by +/- 20Hz. For every 1GHz of center frequency, it could be off
by as much as +/- 20kHz.

For wideband conventional modulation schemes, an error in center
frequency of up to +/-20kHz
won’t make that much difference, because the error frequency is much
less than the total bandwidth.

But in OFDM, where there a plethora of narrowband frequency bins,
offsets can cause real problems.
So your receiver has to try to frequency track the transmitter, or you
need to use an external
reference clock, which on the USRP1, you can’t do (unless you use a
3rd-party device like the
clocktamer board). I’m not an expert on OFDM, but isn’t the existence
of a “pilot channel” there
to help frequency-track the receiver?

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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