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

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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