[Discuss-gnuradio] Question about the bandwidth of OFDM

[Yan Wang]

Hi,
I am trying to use benchmark_ofdm_tx.py and benchmark_ofdm_rx.py to test the 
OFDM communication between two usrp nodes.
While the transmitter is sending data, I use a sprctrum analyzer to observe 
the spectrum, and find the signal bandwidth far beyond my calculation, which 
is 330kHz around with the --fft-length/--occupied-tones=512/256. With my 
calculation, the sampling rate of DAC is 128Msamples/second, and each sample 
has 14bits, so the with the interpolation setting to default(256) the 
bandwidth of signal should be 128M*14/256=7MHz. Is that correct? If not could 
anyone tell me how to calculate in a right way?

Another problem is when I change the occupied-tones to 64, the bandwidth of 
signal will drop to 180kHz. I think the bandwidth of signal should be fixed to 
some bandwidth if you don't change the interpolation, and the occupied-tones 
should only affect the bandwidth of each sub-carrier. Is that a correct 
understanding?

I also tried changing packet size, seemed it also affected the bandwidth of 
ofdm signal. I am really confused.

By the way, the bandwidth I mentioned above with out specified are all null to 
null bandwidth.

Thanks,
Best regards,
Yan