# OFDM TRANSFER and RECEIVE IN GRC

**URL:** <https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358>\
**Category:** GNU Radio\
**Created:** [August 4, 2014, 8:44pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358 "2014-08-04T20:44:01Z")\
**Posts on this page:** 14\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [August 4, 2014, 8:44pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/1 "2014-08-04T20:44:01Z")

</div>

Hi everybody,

I am using USRP N200 and the antenna is VERT2450.  
One system sends the “input” file (in repeat mode)after ofdm  
modulation.The  
other system receives this and demodulates and saves to a “output” file.  
The  
input file can be any text file.

This grc is for sending  
0fdm\_sender.grc  
[http://gnuradio.4.n7.nabble.com/file/n49746/0fdm\_sender.grc](http://gnuradio.4.n7.nabble.com/file/n49746/0fdm_sender.grc)  
This grc is for receiving  
ofdm\_receiver.grc  
[http://gnuradio.4.n7.nabble.com/file/n49746/ofdm\_receiver.grc](http://gnuradio.4.n7.nabble.com/file/n49746/ofdm_receiver.grc)

When I run both these I could be able to receive the file.

Now the problem is that I could able to receive the file for only  
2.45Ghz,  
2.455Ghz ,2.46Ghz center frequencies of source and sink USRP’s but I  
could  
not be able to receive for other frequencies say 2.456Ghz,2.457GHz etc  
(The  
center frequencies are changed in both sender and receiver files ,both  
USRP  
sink and source)

If I use the given example in the source code i.e, the benchmark\_tx.py  
and  
benchmark\_rx.py with the above parameters I could be able to send and  
receive at all frequencies even 2.456Ghz and 2.457Ghz.

So I could not understand why this is happening so.

I checked in the list whether this question is answered earlier or not  
but  
couldn’t find it.So if it is already present ,give that link and I am  
extremely sorry for it.  
Thanks in advance

Regards,  
Chaitanya

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 5, 2014, 8:08pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/2 "2014-08-05T20:08:42Z")

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On 08/04/2014 08:42 PM, chaitanya wrote:

> Hi everybody,
> 
> I am using USRP N200 and the antenna is VERT2450.

Which d’board?

> Now the problem is that I could able to receive the file for only 2.45Ghz,  
> 2.455Ghz ,2.46Ghz center frequencies of source and sink USRP’s but I could  
> not be able to receive for other frequencies say 2.456Ghz,2.457GHz etc (The  
> center frequencies are changed in both sender and receiver files ,both USRP  
> sink and source)

So, at 2.45 GHz it works reliably? And not at all for the rest? I’d  
triple-check you really are setting the freq on both tx and rx (and not  
just changing a variable that isn’t propagated to the devices, for  
example). There’s no frequency-specific code in the OFDM stuff.

And of course make sure there’s no interference on those other  
frequencies.

M

---

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 6, 2014, 3:27pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/3 "2014-08-06T15:27:59Z")

</div>

Thanks for the reply

> Which d’board?

SBX Board (Rev 5.1)

> So, at 2.45 GHz it works reliably? And not at all for the rest?

Yes,this is what exactly I want to say.It is reliable at 2.45GHz ,  
2.455GHz  
and 2.46GHz.  
For other frequencies it is not reliable.

> I’d triple-check you really are setting the freq on both tx and rx (and not  
> just changing a variable that isn’t propagated to the devices, for  
> example).

The variable change is propagated to the device.I used the GUI FFT SINK  
to  
verify that.

> There’s no frequency-specific code in the OFDM stuff.

I did not understand this statement.  
What I am changing is the center frequencies of USRP source and sink in  
grc.  
In the benchmark\_tx and benchmark\_rx , I am changing using -f option.

> And of course make sure there’s no interference on those other frequencies.

I am sure there is no interference

chaitanya

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 7, 2014, 11:11am UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/4 "2014-08-07T11:11:35Z")

</div>

Hi Martin,  
thanks for reply

> Wait, so it does work for more than one frequency? Where exactly does it  
> not work?

Yes,it worked for more than one frequency .They are 2.45GHz ,2.455GHz  
and  
2.46GHz.  
It did not work(that means not reliable) for  
2.451GHz,2.452GHz,2.453GHz,2.454GHz and also between 2.456 to 2.459GHz.

> Put in another way: The OFDM code is agnostic of the actual frequency.  
> There is nothing in the OFDM code that will make it work better at one  
> frequency than another.

Yeah, I also expected that. Thats why i posted this question.It should  
work  
for all frequencies but its not working, I don’t know why.

Please if possible once try running those ofdm\_sender.grc and  
ofdm\_receiver.grc at 2.45Ghz and also at 2.452Ghz.And observe the FFT  
sink.

chaitanya

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 7, 2014, 11:25am UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/5 "2014-08-07T11:25:51Z")

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FFT sink at the receiver or attached to the transmitter GRC?

Your GRCs don’t work for me. With which version of GNU Radio were they  
built?

---

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 7, 2014, 10:30am UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/6 "2014-08-07T10:30:30Z")

</div>

On 08/06/2014 03:26 PM, chaitanya wrote:

> Thanks for the reply
> 
> > Which d’board?
> 
> SBX Board (Rev 5.1)
> 
> > So, at 2.45 GHz it works reliably? And not at all for the rest?
> 
> Yes,this is what exactly I want to say.It is reliable at 2.45GHz , 2.455GHz  
> and 2.46GHz.  
> For other frequencies it is not reliable.

Wait, so it does work for more than one frequency? Where exactly does it  
not work?

> > I’d triple-check you really are setting the freq on both tx and rx (and not  
> > just changing a variable that isn’t propagated to the devices, for  
> > example).
> 
> The variable change is propagated to the device.I used the GUI FFT SINK to  
> verify that.
> 
> > There’s no frequency-specific code in the OFDM stuff.
> 
> I did not understand this statement.

Put in another way: The OFDM code is agnostic of the actual frequency.  
There is nothing in the OFDM code that will make it work better at one  
frequency than another.

M

---

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 7, 2014, 3:39pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/7 "2014-08-07T15:39:29Z")

</div>

Hi Marcus

Thanks for reply

> Your GRCs don’t work for me. With which version of GNU Radio were they  
> built?  
> one is version 3.6.5.1 and the other one is version 3.7.2.1.  
> I think the device addr in usrp sink and source should be changed.And  
> also  
> the file path in file sink and source should be changed according to ur  
> system.

> Thinking about this, that was a bad question from my side.  
> Since the ofdm\_mod doesn’t even _know_ about the center frequency, the  
> signal generated digitally is always the same, so you can only be  
> looking at the rx side’s abs(FFT).

yeah,at rx side only

> Would be nice if you had a description of what happens or a screenshot  
> uploaded somewhere!

These are the screen shots of FFT at 2.45GHz and 2.455GHz center  
frequencies.  
2\_45GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_45GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_45GHz.png)  
2\_455GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_455GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_455GHz.png)

These are the screen shots of FFT at 2.452GHz and 2.454GHz center  
frequencies.

2\_452GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_452GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_452GHz.png)  
2\_454GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_454GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_454GHz.png)

> Ok, you’re seeing frequency selective behaviour. This can happen for a  
> myriad of reasons, among these interference and multipath propagation.  
> You’re excluding interference as a reason on an ISM band, so I presume  
> you’re working with a direct cable between tx and rx or in an anechoic  
> chamber; thus multipath can’t be the reason, either.

No , I am not working with a cable between tx and rx or not in an  
anechoic  
chamber.But I can say there is no interference (as on ISM band) because  
when  
I run the benchmark\_tx.py and benchmark\_rx.py files ,then most of the  
packets are being received (i.e, n\_right value is near to n\_rcvd ) even  
at  
2.452 and 2.454GHz center frequencies.

chaitanya

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 7, 2014, 5:07pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/8 "2014-08-07T17:07:37Z")

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On 08/07/2014 03:32 PM, chaitanya wrote:

> Hi Marcus
> 
> Thanks for reply
> 
> > Your GRCs don’t work for me. With which version of GNU Radio were they  
> > built?  
> > one is version 3.6.5.1 and the other one is version 3.7.2.1.  
> > I think the device addr in usrp sink and source should be changed.And also  
> > the file path in file sink and source should be changed according to ur  
> > system.

Ah, so you’re using the old OFDM components, I guess. Not what I was  
assuming.

What’s your UHD version?

> These are the screen shots of FFT at 2.45GHz and 2.455GHz center  
> frequencies.  
> 2\_45GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_45GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_45GHz.png)  
> 2\_455GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_455GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_455GHz.png)
> 
> These are the screen shots of FFT at 2.452GHz and 2.454GHz center  
> frequencies.
> 
> 2\_452GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_452GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_452GHz.png)  
> 2\_454GHz.png [http://gnuradio.4.n7.nabble.com/file/n49851/2\_454GHz.png](http://gnuradio.4.n7.nabble.com/file/n49851/2_454GHz.png)

Posting this earlier would have helped. Your rx spectrum is pretty  
screwed up at these frequencies. I can’t say why, but I’d like to get  
the version issues sorted out first.

Also, I recommend using the new OFDM subsystem. Refer to the manual for  
details.

Cheers,  
Martin

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 7, 2014, 11:32am UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/9 "2014-08-07T11:32:24Z")

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Thinking about this, that was a bad question from my side.  
Since the ofdm\_mod doesn’t even _know_ about the center frequency, the  
signal generated digitally is always the same, so you can only be  
looking at the rx side’s abs(FFT). Would be nice if you had a  
description of what happens or a screenshot uploaded somewhere!

Ok, you’re seeing frequency selective behaviour. This can happen for a  
myriad of reasons, among these interference and multipath propagation.  
You’re excluding interference as a reason on an ISM band, so I presume  
you’re working with a direct cable between tx and rx or in an anechoic  
chamber; thus multipath can’t be the reason, either.

Greetings,  
Marcus

On 07.08.2014 11:24, Marcus M. wrote:

> > ,2.455GHz and 2.46GHz. It did not work(that means not reliable)

[http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49841.html](http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49841.html)

> >

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 8, 2014, 4:24pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/10 "2014-08-08T16:24:47Z")

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Thanks for reply

> What’s your UHD version?  
> 3.5.3 and 3.7.1

> Ah, so you’re using the old OFDM components, I guess. Not what I was  
> assuming.  
> Also, I recommend using the new OFDM subsystem. Refer to the manual for  
> details.

I did not understand this. Are you asking me to use the latest version  
of  
gnuradio.

Chaitanya

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**Post date:** [August 8, 2014, 4:47pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/11 "2014-08-08T16:47:24Z")

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There are “old style” OFDM blocks in GNU Radio, and “new style”  
blocks, which are cooler, more versatile and cleaner.

On 08.08.2014 16:23, chaitanya wrote:

> version of gnuradio.
> 
> Chaitanya
> 
> – View this message in context:

[http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49873.html](http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49873.html)

> 

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**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 8, 2014, 6:28pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/12 "2014-08-08T18:28:35Z")

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Hi Chaitanya,

in GRC there is a category called OFDM with the new style blocks. Also  
in [pathtognuradiosource]/gr-digital/examples/ofdm configured  
flowgraphs are provided for reference.

Just to make that sure, I’m refering to GNU Radio 3.7.

happy hacking  
Johannes

On 08.08.2014 18:16, chaitanya wrote:

> – View this message in context:

[http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49875.html](http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49875.html)

> 

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<div class="post-metadata">

**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 8, 2014, 6:17pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/13 "2014-08-08T18:17:53Z")

</div>

Thanks for reply

> There are “old style” OFDM blocks in GNU Radio, and “new style”  
> blocks, which are cooler, more versatile and cleaner.

Where can i get that. Can you please name the "new style " OFDM blocks.

chaitanya

–  
View this message in context:  
[http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49875.html](http://gnuradio.4.n7.nabble.com/OFDM-TRANSFER-and-RECEIVE-IN-GRC-tp49746p49875.html)  
Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com).

---

<div class="post-metadata">

**Author:** ![chaitanya](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/848f3c/32.png) [@chaitanya](https://www.ruby-forum.com/u/chaitanya)\
**Post date:** [August 9, 2014, 3:15pm UTC](https://www.ruby-forum.com/t/ofdm-transfer-and-receive-in-grc/231358/14 "2014-08-09T15:15:28Z")

</div>

Also, there’s the manual.

M