I. Introduction
Blind SOURCE SEPARATION (BSS) refers to the recovery of the source signals from their mixtures without any knowledge of the mixing process or the signals [1]–[8]. Based on whether the mixing process is memoryless or not, BSS can be classified as instantaneous or convolutive mixing. The instantaneous mixing process can mathematically be expressed as $${\bf x}[n]={\bf A}{\bf s}[n]+{\bf v}[n], \eqno{\hbox{(1)}}$$where is a vector containing the mixtures, is a vector of source signals, is the mixing matrix, denotes the additive noise and is the discrete time index. In BSS, the objective is to estimate given . When , the problem is well-determined and can be solved using independent component analysis (ICA) [2]. However, when is non-invertible and this renders the underdetermined BSS (UBSS) a challenging problem.