R: R: Re: R: R: Re: around empty subcarrier in 802.11n implementation

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: [email protected]

Data: 09/10/2013 16.55

A: [email protected], [email protected]

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: [email protected]

Data: 09/10/2013 16.31

A: [email protected]

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

On 10/09/2013 09:55 AM, [email protected] 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: [email protected]

    Data: 09/10/2013 10.52

    A: <[email protected]>

    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: [email protected]

      Data: 08/10/2013 16.22

      A: <[email protected]>

      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, [email protected]
        <[email protected]> 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



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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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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

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