The effect of nonlinearity on a QPSK-OFDM-QAM signal

The effect of nonlinearity on a QPSK-OFDM-QAM signal is analyzed. One of the characteristics of an OFDM-QAM signal is its wide range of envelope fluctuation: its crest factor, defined as the ratio of the peak instantaneous power to the average (rms) power in dB, is typically greater than 10. When an OFDM-QAM signal drives a nonlinear device, nonlinearity will result in signal compression and intermodulation among the sub-carriers at the output. An analytical method is used to predict the input back-off needed to keep the intermodulation below an acceptable level. For sufficient large number of sub-carriers, we show that to keep the intermodulation power 30 dB below the signal power, we need to back-off the input power by 6.5 dB. However, from the BER point of view, the degradation is less than 2 dB at BER=0.01% even for 1 dB input back-off.