Measuring the Magnitude of Envelope Fluctuations: Should We Use the PAPR?

The PAPR (Peak-to-Average Power Ratio) is widely employed to measure the magnitude of the envelope fluctuations of a given signal. It is particularly used to define the required amplifier back- off for multi-carrier modulations such as OFDM (Orthogonal Frequency Division Multiplexing) schemes. However, the PAPR increases with the block size (i.e., the number of subcarriers, in the OFDM case), but the signal´s distribution converges to a Gaussian distribution for a large number of subcarriers. Therefore, one might ask if the PAPR is indeed the best parameter to define the amplifier´s back-off. In this paper we address this issue. It is shown that the required amplifier´s back-off for a given non-linear distortion level, i.e. referred to the input amplifier saturation power, is almost independent of the number of subcarriers and, therefore, the PAPR is not the most suitable to define it. Therefore, we recommend the use of a parameter related to the instantaneous envelope power distribution to define the amplifier back-off.