Question about the bandwidth of OFDM

GNU Radio
1

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

2

Yan Wang <yanwang2 eden.rutgers.edu> writes:

Hi,
I am trying to use benchmark_ofdm_tx.py and benchmark_ofdm_rx.py to test the
…

Am I asking questions at wrong place? If so, where shall I post them?
Can anybody help?
Thanks.

3

On Thu, Jul 09, 2009 at 05:59:57PM +0000, Yan Wang wrote:

Thanks.
This is the right place.

Oftentimes when you don’t get an answer, it indicates that either the
question you asked or how you asked it failed to get anybody’s
attention.

See http://gnuradio.org/trac/wiki/ReportingErrors for suggestions on
how to maximize the chances of getting a useful response on pretty
much any mailing list.

Q: Why would you expect the number of bits in the A/D to have anything
to do with the occupied bandwidth?

Eric

4

Thank you Eric for your replying.
I think I mistook the baseband signal in this system.
So since the benchmark_ofdm_tx.py can only setup size of packets,
fft-length,
occuppied tones, cp-length and interpolation rate. How to calculate the
bandwidth of each sub-carrier or the total bandwidth?

Thanks.