Measuring PAPR of OFDM signals

Hi there!

I am working on PAPR measurement of OFDM signals connected to a RF
amplifier.I created the OFDM signal from a signal studio. Then the signal
is connected to RF amplifier and with the spectrum generators to see the
input OFDM spectrum and output OFDM spectrum. 

The generated OFDM Spectrum from the Spectrum generator is stationary or
it is changing for a particular input power of RF amplifier? I mean how
can I fix the peak power and the average power of an output OFDM spectrum
for a specific input power of RF amplifier?

Can you help me how to measure the PAPR in such a case?

Waiting to hear from you.......

Regards
Razib 

On Aug 3, 7:27 am, "razib3002" <razib3...@gmail.com> wrote:
> Hi there!
>
> I am working on PAPR measurement of OFDM signals connected to a RF
> amplifier.I created the OFDM signal from a signal studio. Then the signal
> is connected to RF amplifier and with the spectrum generators to see the
> input OFDM spectrum and output OFDM spectrum.
>
> The generated OFDM Spectrum from the Spectrum generator is stationary or
> it is changing for a particular input power of RF amplifier? I mean how
> can I fix the peak power and the average power of an output OFDM spectrum
> for a specific input power of RF amplifier?
>
> Can you help me how to measure the PAPR in such a case?
>
> Waiting to hear from you.......
>
> Regards
> Razib

In IEEE literature the PAPR for OFDM is computed as the magnitude
squared of
the largest sample divided by the average magnitude squared of a
complete
OFDM symbol. In a lab setting the symbol boundaries aren't known, so
what
is typically measured is the peak power divided by the average power
measured over a relatively long time interval. The peak power is not
fixed.

John

Hi Razib,

[snip]
"I mean how
can I fix the peak power and the average power of an output OFDM
spectrum
for a specific input power of RF amplifier? "

Hmm... I do not think that the above statement is correct. For the
baseband waveform xt, the PAPR
(peak-to-average-power ratio) is defined as PAPR_dB =
10*log10(max(xt.conj(xt))/E{xt.conj(xt)}).

You can use your baseband waveform to find out the PAPR. For OFDM
case, theoretically the PAPR is equal
to 10*log10(N), where N is the number of used subcarriers. Typically,
thanks to the scrambler, this maximum value
of PAPR is never reached.

To give margins for the peaks which may occur in the time domain
waveform, the power amplifier needs
to be operated such that the even the peaks in the waveform do not
cause clipping or other non-linearities.

Maybe this url helps: http://www.commsdesign.com/story/OEG20011023S0040

HTH,
Krishna
http://dsplog.blogspot.com

>Hi Razib,
>
>[snip]
>"I mean how
>can I fix the peak power and the average power of an output OFDM
>spectrum
>for a specific input power of RF amplifier? "
>
>Hmm... I do not think that the above statement is correct. For the
>baseband waveform xt, the PAPR
>(peak-to-average-power ratio) is defined as PAPR_dB =
>10*log10(max(xt.conj(xt))/E{xt.conj(xt)}).
>
>You can use your baseband waveform to find out the PAPR. For OFDM
>case, theoretically the PAPR is equal
>to 10*log10(N), where N is the number of used subcarriers. Typically,
>thanks to the scrambler, this maximum value
>of PAPR is never reached.
>
>To give margins for the peaks which may occur in the time domain
>waveform, the power amplifier needs
>to be operated such that the even the peaks in the waveform do not
>cause clipping or other non-linearities.
>
>Maybe this url helps: http://www.commsdesign.com/story/OEG20011023S0040
>
>HTH,
>Krishna
>http://dsplog.blogspot.com
>
>Thanks to John and Krishna.....

I am actually trying to find the PAPR with respect to the Input power of
the RF amplifier that I am using in my experiment. I want to see the
effects of PAPR due to the variaion in RF amplifier power input.

Thats why I need to know how can I measure PAPR for a specific input power
value. Then I want to vary the input power of RF amp. to find the PAPR
again......to eventually create a relationship....

I want to generate the OFDM signal using signal studio and then  Signal
Generator will download the created signal. After that , for every value
of the input power of the RF amplifier, I want to measure peak value and
average value of the OFDM signal to finally calculate PAPR.

My question in this respect is..........should I hold the OFDM signal
using the spectrum analyzer to find the peak value and average value for a
particular RF amplifier input power?

Hope you understood my query...:)

Thanks

 
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