TABLE OF CONTENTS In general, nonlinear amplifier clips the amplitude of input signal. The sudden changes of the input amplitude generate higher spectral components in the power spectrum of the signal, so it results in a spectrum spreading. The spectrum spreading by nonlinear amplification is a cause of adjacent channel interference (ACI).
Let us consider a case where an OFDM signal is amplified by a nonlinear device. The OFDM signal has nonconstant envelope; in other words, the waveform is like that of a narrowband Gaussian noise because of the central limiting theorem. Therefore, even if we try to reduce the spectrum spreading with a large input back-off, we cannot perfectly eliminate sudden inputs of the larger amplitudes. Furthermore, the outband radiation generated from a subcarrier also becomes "intersubcarrier interference" for the neighboring subcarriers. Therefore, the severe intersubcarrier interference drastically deteriorates the BER performance. This sensitivity to nonlinear amplification is also pointed out as a major OFDM disadvantage, as well as the sensitivity to frequency offset discussed in Section 4.8.
When we can deal with an input signal to a nonlinear amplifier as a narrowband Gaussian noise, we can apply Simbo's method for the theoretical analysis of intermodulation [29].
Define the input OFDM signal to a nonlinear amplifier as
| (4.69) |  |
where N c( t) and N s( t) are the real and imaginary components of s i( t), respectively. Furthermore, define the power spectrum of autocorrelation of the input OFDM...
