# Around empty subcarrier in 802.11n implementation

**URL:** <https://www.ruby-forum.com/t/around-empty-subcarrier-in-802-11n-implementation/236868>\
**Category:** GNU Radio\
**Created:** [October 8, 2013, 3:53pm UTC](https://www.ruby-forum.com/t/around-empty-subcarrier-in-802-11n-implementation/236868 "2013-10-08T15:53:35Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![alcina](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/a587f6/32.png) [@alcina](https://www.ruby-forum.com/u/alcina)\
**Post date:** [October 8, 2013, 3:53pm UTC](https://www.ruby-forum.com/t/around-empty-subcarrier-in-802-11n-implementation/236868/1 "2013-10-08T15:53:35Z")

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Hi list,

I’m using gnuradio 3.6.0 together with USRPs N210 for implementing OFDM  
802.11  
n communications, just SISO for the moment (in the future it would be  
MIMO). I  
have properly modified parameters such as FFT size, number of occupied  
tones  
and so on. I have also included all preambles (STF, LTF…) and I have  
accordingly modified the “correlate” and “calculate\_equalizer” functions  
in  
ofdm\_frame\_acquisition. FEC is also included in the chain. In the  
ofdm\_frame\_sink file I’ve added a function that computes SNR in a  
per-carrier  
basis.

Everything is working fine, I obtain about 80% of correct packets, but  
I’ve  
noticed from the snr plotting, that the carriers around the central  
empty one  
(3 subcarriers before and 3 subcarriers after) have very poor snr  
values. So  
I’ve investigated about this effect, and I’ve realized that the all  
transmission errors are in those subcarriers and, in most cases, FEC is  
able to  
recover, but I would like to understand why this happens.

I conducted some searches in the web but unfortunately I didn’t find an  
answer.  
Let me know if you need some other information about my implementation.

Thanks for your attention  
francesca

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [October 8, 2013, 4:33pm UTC](https://www.ruby-forum.com/t/around-empty-subcarrier-in-802-11n-implementation/236868/2 "2013-10-08T16:33:22Z")

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On Tue, Oct 8, 2013 at 9:52 AM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Thanks for your attention  
> francesca

The USRPs do a good job at getting rid of the DC carrier, but there  
are still issues around DC that are probably affecting you. The crude  
but easy option is to try to make sure you are perfectly centered on  
DC by some hand-tuning.

Tom

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [March 19, 2014, 8:31am UTC](https://www.ruby-forum.com/t/around-empty-subcarrier-in-802-11n-implementation/236868/3 "2014-03-19T08:31:53Z")

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hey, I’m the beginner of gnu radio with usurp N210.

and, i want to do sth as u to receive 802.11n wifi signal and decode it.

can i ask u about how do u achieve it?

or do u know where can i get more informations?

thank u so much~ !