On Fri, Sep 9, 2011 at 8:18 AM, Tom Rondeau <trondeau1...@gmail.com> wrote:
> On Thu, Sep 8, 2011 at 6:47 PM, Guanbo Zheng <gbzh...@gmail.com> wrote: > >> Hi all >> >> I am currently using OFDM benchmark to generate OFDM signal under the >> setting of FFT len, CP length, occupied-tones and something. >> But I can not find out what is the real bandwidth of signal it generated. >> Because when I changed the Interpolation rate (sampling rate), the >> bandwidth at RX changed as well. >> Ideally we know that setting enough large sampling rate ( In USRP2, the >> max fs = 25MHz), I should observe the constant signal with fixed BW. >> It seems to me that BW of the generated signal is too large. >> >> My question is: how to determine the BW of transmit signal in the codes? >> where I can change it. >> All I found is actual bit rate = (converter_) / xrate / >> samples_per_symbol = 100MHz/4/2. But this one seems not related to the BW of >> signal itself. >> >> Thanks for any suggestions! >> -- >> Regards, >> Guanbo > > > > Guanbo, > 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 USRP. > > Tom > > Hi Tom What you means that, the bandwidth of OFDM signal is actually equal to the sampling rate*occupided_tones/fft_length. Then how to understand the sampling theory, in which sampling rate is twice of bandwidth? Thanks, Guanbo -- Regards, Guanbo
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