A Novel Decomposition Analysis of Nonlinear Distortion in OFDM Transmitter Systems

In Orthogonal Frequency Division Multiplexing (OFDM) transmitter systems, nonlinear distortion is a major impairment caused by power amplifier (PA). The distortion is constituted by numerous noise-like inter-modulation products (IMPs). Although the overall behavior of nonlinear IMP distortion has been well studied, in large multi-carrier contexts, such as in OFDM, the knowledge of individual IMPs and their quantitative interrelationships, together with the derivation of behavioural knowledge and the predictability of consequences arising from such interrelationships, is a fairly unexplored topic. This is the subject of this paper. The powers of the very numerous individual IMPs generated are analytically derived based on a Bessel-Fourier PA behavioural model, and it is proven that these powers have only a small number of distinct powers. Proven also is that the ratios of these power values are constants, and that these constants are invariant with the particular PA´s nonlinearity characteristics or operating point. Further, it is proven that specific types of IMPs dominate nonlinear distortion effects. Finally, it is shown that IMP power spectra can be derived from a simple product of two polynomials, which are quadrature functions of frequency and PA operating point, respectively. Application possibilities for findings presented in this paper, such as their potential to contribute efficiency improvements in OFDM system modulation fidelity calculations (over 1100 times faster than conventional methods), are also outlined in the conclusions.