Forum: GNU Radio R: R: Re: R: R: Re: around empty subcarrier in 802.11n implementation

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on 2013-10-14 17:44
(Received via mailing list)
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.



----Messaggio originale----


Data: 09/10/2013 16.55

A: <>, <>

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:
where freq and offset are passed as parameters. In this way I got no
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

----Messaggio originale----


Data: 09/10/2013 16.31

A: <>

Ogg: Re: [Discuss-gnuradio] R: R: Re: around empty subcarrier in 802.11n

    On 10/09/2013 09:55 AM, 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:

        '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


        ----Messaggio originale----


        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


          ----Messaggio originale----


          Data: 08/10/2013 16.22

          A: <>

          Ogg: Re: [Discuss-gnuradio] around empty subcarrier in 802.11n

          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.

          on Oct 08, 2013,
            <> 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


              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


              Let me know if you need some other information about my

              Thanks for your attention



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    Since you're likely using the Gnu Radio interface that is a wrapper
    for UHD, you'd use the set_center_freq() method.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
Matt Ettus (Guest)
on 2013-10-14 17:59
(Received via mailing list)
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

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