Great!! Thanks for your explanation! Guanbo
On Fri, Sep 9, 2011 at 5:41 PM, Tom Rondeau <trondeau1...@gmail.com> wrote: > On Fri, Sep 9, 2011 at 6:36 PM, Guanbo Zheng <gbzh...@gmail.com> wrote: > >> >> >> 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. >> > > 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 > > -- Regards, Guanbo
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