OFDM spectrum and sidelobes

I would like to know why the OFDM Out of band sidelobes , reduce as the
number of carriers/FFT size increases?What is the reason for this? 
Is this ever a consideration in choosing the FFT size, because if I
consider sidelobes due to PAPR driving PA to saturation, then its the
other way round. More carriers will cause more spectral regrowth.

On Sun, 06 Apr 2008 16:54:38 -0500, "Zeph80"
<surabhi_talwar@hotmail.com> wrote:

>I would like to know why the OFDM Out of band sidelobes , reduce as the
>number of carriers/FFT size increases?What is the reason for this? 

They generally don't, but it can often appear so in plots if the
subcarrier spacing (scaling) isn't kept constant.   In other words,
the sidelobe decay is going to look steeper next to 1024 subcarriers
than it does next to 64 subcarriers, merely because the 1024 take up a
lot more space.

I hope that makes sense, but there's nothing magical about adding
subcarriers that reduces sidelobes.   Strictly speaking the sidelobe
decay rate gets worse as the number of subcarriers increases because
there are more sidelobes adding together.   Since they're high-order
sidelobes this is typically a very small effect, though.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

On Apr 6, 5:54 pm, "Zeph80" <surabhi_tal...@hotmail.com> wrote:
> I would like to know why the OFDM Out of band sidelobes , reduce as the
> number of carriers/FFT size increases?What is the reason for this?
> Is this ever a consideration in choosing the FFT size, because if I
> consider sidelobes due to PAPR driving PA to saturation, then its the
> other way round. More carriers will cause more spectral regrowth.

If you introduce reserved (overhead) tones the PAR can be reduced
quite dramatically, which helps control the regrowth. The technique is
called "Tone Reservation".

John