On Fri, Sep 9, 2011 at 6:36 PM, Guanbo Zheng <[email protected]> wrote:
> > > On Fri, Sep 9, 2011 at 8:18 AM, Tom Rondeau <[email protected]>wrote: > >> On Thu, Sep 8, 2011 at 6:47 PM, Guanbo Zheng <[email protected]> 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. > I mean exactly that :) > Then how to understand the sampling theory, in which sampling rate is twice > of bandwidth? > Complex signals. Sample rate is the bandwidth. Have a sample for I and Q, so we still have enough information so as not to violate Nyquist. > Thanks, > Guanbo > > > -- > Regards, > Guanbo > Tom
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