Re: [Discuss-gnuradio] Amplitude settings in uhd_siggen

[Marcus Müller]

Dear Zealdeal,

you should know how OFDM works: An OFDM symbol is nothing more than the
IDFT of a vector of permutated, possibly padded symbols.
No, for the sake of a simple example, assume your symbol vector is
[1,1,1,...,1]; without doubt, you'll notice that the IDFT of that vector
has an entry that exhibits a magnitude much larger than 1, whilst the
other elements of the IDFT have a much smaller magnitude (should be 0!).
This uneven distribution of power in time domain is a problem so common
to all OFDM systems that there is a fixed term coined for that: the PAPR
(peak/average power ratio); I'm pretty sure that you've stumbled across
that in whatever introduced you to OFDM.

Best regards,
Marcus

On 02/17/2015 08:21 AM, zealdeal wrote:
> I'm bit confused here.
>
> You said "In the OFDM example, watch the amplitude of the signal before it
> enters the USRP sink. If it exceeds 1.0, I promise it will clip"
>
> I transmitted OFDM symbols, keeping transmission amplitude as 0.2
>
> Then, on the collected log, I ran gr_plot_ofdm.py tool. It shows the plot of
> transmitted signal in time domain.
>
> I see that the amplitude is much more than 1 in that plot.
>
> Could please tell me where am I going wrong?
>
>
>
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