Should We Avoid Nonlinear Effects in a Digital Transmission System?

Nonlinear operations are usually regarded as something undesirable that should be avoided in digital communication systems since they lead to performance degradation, namely spectral widening and decrease in power efficiency, for a given modulations. In this paper we show that the adoption of appropriate nonlinear devices can allow an improvement on the performance of some modulation schemes, either for OQPSK-type schemes (Offset Quadrature Phase Shift Keying) or TC-OQAM (Trellis Coded Offset Quadrature Amplitude Modulation). We present an analytical characterization of these signals after a bandpass memoryless nonlinear device. It is shown that an encoder followed by a nonlinear amplifier can be viewed as a serial concatenated coding scheme and the coding gain of such scheme can be improved relatively to the linear transmission scheme.