OFDM USRP question on bandwidth


I am facing a similar problem and I would greatly appreciate your

I am transmitting an OFDM signal with 512 subcarriers, 200 occupied
tones, BPSK, with a sampling rate set to 2MHz. Accordingly, I would
expect to see a bandwidth of 200*(2*10^6)/512 = 781.25KHz of occupied
bandwidth in the corresponding FFT plot. Am I correct?

However, what I observe is a bandwidth of 200KHz. Can someone explain
this or suggest any changes? Since the number of occupied tones is 200,
this means that the bandwidth of each occupied tone is 1KHz.

The above definition is quoted in another post published in here:

“The bandwidth of the signal changes with the interpolation rate. If you
set the interpolation rate such that you get 25 MHz of bandwidth out,
then the OFDM signal will also have a 25 MHz bandwidth. What you will
see over the air is 25e6 * (occupided_tones/fft_length), since the
ratio of the used tones to the number of subcarriers is the amount of
occupied bandwidth. You can also think of it this way. The bandwidth of
a subcarrier is BW/fft_length, where BW is the sample rate out of the

Thanks for your time.