I. Introduction
TIME–FREQUENCY (TF) analysis deals with signal representations in the joint TF plane. The simplest TF representation (the short-time Fourier transform) is a straightforward extension of the Fourier transform obtained by introducing a lag window that localizes the spectral content around the considered time instant. The energetic version of this transform is called spectrogram. It belongs to the quadratic TF representations. In order to improve concentration in the TF plane, other quadratic distributions are introduced [11]. The Wigner distribution (WD) originally defined in quantum mechanics is commonly used as a basic (and the best concentrated signal independent [12]) quadratic TF representation. In order to reduce undesired nonlinearity effects, manifesting themselves as the cross-terms, various reduced interference distributions have been introduced [11].