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

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] around empty subcarrier in 802.11n implementation
Date: Tue, 8 Oct 2013 10:31:24 -0400

On Tue, Oct 8, 2013 at 9:52 AM, address@hidden <address@hidden> 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

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.


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