Crest Factor of OFDM signal

I have been given a OFDM signal of crest factor of 12dB. The crest factor
is the ratio of peak power to the average power of the signal. Does anyone
have any idea how crest factor of 12dB is derived for an OFDM
signal?(assumptions on peak power,average or RMS power etc). I would
highly appreciate any input on this.


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Posted through http://www.DSPRelated.com

On Mon, 02 Nov 2015 10:38:09 -0600, "rihabbanday" <109179@DSPRelated>
wrote:

>I have been given a OFDM signal of crest factor of 12dB. The crest factor
>is the ratio of peak power to the average power of the signal. Does anyone
>have any idea how crest factor of 12dB is derived for an OFDM
>signal?(assumptions on peak power,average or RMS power etc). I would
>highly appreciate any input on this.
>
>
>---------------------------------------
>Posted through http://www.DSPRelated.com

Generally the crest factor has to do with amplitude levels rather than
power.   When dealing with power levels the usual terminology is Peak
to Average Power Ratio (PAPR).

There's nothing special about OFDM as far as determining the crest
factor.  Take the rms value of a sample vector, find the maximum peak
value in the same vector (positive or negative), and take the ratio.

That applies to pretty much all signal types, so OFDM is no different
as far as taking the measurements is concerned.

Just make sure you have it clear whether you're interested in
amplitude values or power values.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

If you have an audio file of the decoded signal, or the modulator signal, you can just scan the file for the largest absolute value peak voltage, and then make a second pass and measure the RMS. The ratio is then taken. Something like this in octave:

[x,fs] = wavread('modem.wav');

x = x(:,1);

rms = sqrt(mean(x.^2));
peak = max(abs(x));
c = 20*log10(peak/rms);

cstr = num2str(c);
rmsstr = num2str(rms);
peakstr = num2str(peak);

disp(['Crest factor = ' cstr ' dB (rms = ' rmsstr ') (peak = ' peakstr ')']);

coupaydeville@gmail.com wrote:

> If you have an audio file of the decoded signal, or the modulator 
> signal, you can just scan the file for the largest absolute value 
> peak voltage, and then make a second pass and measure the RMS. 
> The ratio is then taken. Something like this in octave:

You can do it in one pass, why use two?

I often record 24 bit WAV, then adjust by a bit shift to convert
to 16 bits. I use both the peak and RMS values to decide how much
to shift.  (That is, adjust the level in 6dB increments.)

-- glen