# R: R: Re: around empty subcarrier in 802.11n implementation **URL:** \ **Category:** GNU Radio\ **Created:** [October 9, 2013, 3:55pm UTC](https://www.ruby-forum.com/t/r-r-re-around-empty-subcarrier-in-802-11n-implementation/236889 "2013-10-09T15:55:30Z")\ **Posts on this page:** 2\ **Page:** 1
**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:** [October 9, 2013, 3:55pm UTC](https://www.ruby-forum.com/t/r-r-re-around-empty-subcarrier-in-802-11n-implementation/236889/1 "2013-10-09T15:55:30Z")
Hi guys, I’ve tried to follow the suggested procedure. I guess the problem is inside the uhd driver I’m using (UHD\_003.005.001), since the set\_rx\_freq() gives the following error: AttributeError: ‘uhd\_usrp\_source\_sptr’ object has no attribute ‘set\_rx\_freq’ Or, maybe, the problem is the daughterboard, which is of type XCVR2450. Is there another way to set dc offset? Any hint will be greatly appreciated. Francesca ----Messaggio originale---- Da: removed\_email\_address@domain.invalid Data: 09/10/2013 10.52 A: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Ogg: [Discuss-gnuradio] R: Re: around empty subcarrier in 802.11n implementation Thanks for your answer. I’m a computer scientist, so I’m not so aware of what DC-offset exactly means, but I’m going to try this tuning. I’ll let you know if I solve the problem. Best regards Francesca ----Messaggio originale---- Da: removed\_email\_address@domain.invalid Data: 08/10/2013 16.22 A: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Ogg: Re: [Discuss-gnuradio] around empty subcarrier in 802.11n implementation Likely, if it’s the central tones, you’re looking at DC-offset (or the removal thereof) interfering. You can use offset tuning in the hardware to slide the DC offset outside of your passband. [http://files.ettus.com/uhd\_docs/manual/html/general.html#tuning-notes](http://files.ettus.com/uhd_docs/manual/html/general.html#tuning-notes) on Oct 08, 2013, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: 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 ---
**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:** [January 14, 2014, 11:12pm UTC](https://www.ruby-forum.com/t/r-r-re-around-empty-subcarrier-in-802-11n-implementation/236889/2 "2014-01-14T23:12:27Z")
On 10/09/2013 09:55 AM, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > ‘set\_rx\_freq’ > Francesca > > ``` > Francesca > > ofdm_frame_acquisition. FEC is also included in the chain. > poor snr values. So > implementation. > > ``` > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) > Since you’re likely using the Gnu Radio interface that is a wrapper for > UHD, you’d use the set\_center\_freq() method.