peak to average power ratio clarification
"PAPR occurs when, in a multi-carrier environment, the different sub-carriers are out of phase with each other. Thus, at each instant they are offset with respect to each other at different phase values. However, there may come a point when all of them achieve the maximum value simultaneously; this will cause the output envelope to suddenly shoot up. This causes a 'peak' in the output envelope."
So I have a few questions about the definition above:
Is PAPR an issue only in multi carrier technologies like OFDM? Or is it also a problem in single carrier technology like FSK?
Is a communication system that transmits more than 1 bit per symbol considered a multi carrier system? I.E. QPSK, 16QAM, etc.
Also in OFDM the subcarriers are orthogonal (90 degree phase difference) so how does the signal ever achieve the maximum value simultaneously to suddenly shoot up?
Here is a better explanation of the PAR (crest factor):
http://en.wikipedia.org/wiki/Crest_factor
As you see, almost every signal have some kind of peak to average.
For a multi-carrier signal (as OFDM) generally the peak-to-average can be calculated with:
PAR[dB] = 10*LOG(Number-of-Carriers)
For example the PAR of a 12 carriers signal would be 10.7dB
The peak RF level can drive amplifiers into overload (i/p products rise dramatically) and in high power transmitters the high voltages can cause flash overs. While the mean power is the one that can cause over heating, so in an economical transmission system you want to keep the peak to mean powewr as close to unity as possible. Many transmission systems like radar or TV have a big difference between peak and mean (radar 100 :1, analogue TV 5 :1 or so).
Frank
1. PAPR is also an issue in many (but not all) single carrier systems.
2. You can have those types of modulations with single carrier or multicarrier systems. OFDM is a multiplexing technique, each carrier is modulated (BPSK, QPSK, etc.)
3. Each carrier is at a different frequency. As a simple experiment, if you create two sine waves at different frequencies, you will see them add constructively periodically.