Re: [Discuss-gnuradio] question about gr_ofdm_sampler.cc

Hi Tom,
      > Well then, yes, it is modified.
      Since the data are changed, can I say if measure the SNR from the data after FFT,  then they are inaccurate and bias?

Thanks,
Bin

On Thu, Feb 11, 2010 at 2:04 PM, Tom Rondeau <trondeau1122@gmail.com> wrote:

On Thu, Feb 11, 2010 at 6:52 AM, bin zan <zanbin2046@gmail.com> wrote:
> Hi Tom,

>>I'm not sure what you mean by "modified." They have been adjusted by
>>the sync block to fit in the middle of the FFT bins and the CP has
>>been removed. Other than that, it's just the data stream.
> Here "modified", I mean the data which enter into the FFT process are
> different from the data if directly collected from usrp_rx_cfile.py. The
> value of the remaining  data (after sync and CP removed) have been changed?
> If that's true, How can I match the original data ( same data obtained from
> usrp_rx_cfile.py but without CP and unsync'd data) to the FFT bins?
>
> Thank you,
> Bin

Well then, yes, it is modified. The ofdm_sync blocks adjusts the
frequency to make sure the subcarriers are in the center of the
frequency bins for the FFT. If you don't do this, you introduce major
ISI into the received symbol.

Tom