# R: R: Re: R: R: Re: around empty subcarrier in 802.11n implementation **URL:** \ **Category:** GNU Radio\ **Created:** [October 14, 2013, 5:44pm UTC](https://www.ruby-forum.com/t/r-r-re-r-r-re-around-empty-subcarrier-in-802-11n-implementation/246436 "2013-10-14T17:44:58Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Rafa\_F](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/958977/32.png) [@Rafa\_F](https://www.ruby-forum.com/u/Rafa_F)\ **Post date:** [October 14, 2013, 5:44pm UTC](https://www.ruby-forum.com/t/r-r-re-r-r-re-around-empty-subcarrier-in-802-11n-implementation/246436/1 "2013-10-14T17:44:58Z")
Hi guys, just for letting you know. No offset correction is needed. The problem was elsewhere in my implementation of 802.11. It was how I was calling the ifft function in the transmitter side. By putting the correct parameters, I obtained a perfect spectrum. Regards Francesca ----Messaggio originale---- Da: removed\_email\_address@domain.invalid Data: 09/10/2013 16.55 A: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Ogg: [Discuss-gnuradio] R: Re: R: R: Re: around empty subcarrier in 802.11n implementation Ok, I was supposing that. So in the python code of the receiver I’ve put: tr=uhd.tune\_request(self.\_freq,self.\_offset)self.uhd\_usrp\_source\_0.set\_center\_freq(tr) where freq and offset are passed as parameters. In this way I got no error. Now, the question is: which values are good for “offset”? Hz? KHz? MHz?I’ve tried small values, like 50 Hz, but nothing changes. Is there a way to find it? I’m sorry, but really I have not ideaThank you again for your support francesca ----Messaggio originale---- Da: removed\_email\_address@domain.invalid Data: 09/10/2013 16.31 A: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Ogg: Re: [Discuss-gnuradio] R: R: Re: around empty subcarrier in 802.11n implementation ``` On 10/09/2013 09:55 AM, removed_email_address@domain.invalid wrote: 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: 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: 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 on Oct 08, 2013, 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 _______________________________________________ Discuss-gnuradio mailing list removed_email_address@domain.invalid https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ``` * * * Discuss-gnuradio mailing list removed\_email\_address@domain.invalid [https://lists.gnu.org/mailman/listinfo/discuss-gnuradio](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. -- ``` Marcus L. Principal Investigator Shirleys Bay Radio Astronomy Consortium > **[Live Draw HK Tercepat | Result HK | HK Pools | Live HK | HK Prize Hari Ini](https://www.sbrac.org/)** > > Temukan live draw hk tercepat melalui halaman hk pools yang resmi dan dapatkan result hk prize melalui live hk secara langsung di situs kami. ---
**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 14, 2013, 5:59pm UTC](https://www.ruby-forum.com/t/r-r-re-r-r-re-around-empty-subcarrier-in-802-11n-implementation/246436/2 "2013-10-14T17:59:00Z")
Thanks for the update! I know that getting the exact right bins in the right place can be hard since every FFT inplementation is slightly different. Matt