1. A method for estimating multi-resolution based power spectral density (PSD), the method comprising: computing a first PSD of a signal to determine a first resolution and a first error variance (EV) value associated with the first PSD; computing a second PSD of the signal to determine a second resolution and a second EV value associated with the second PSD; and determining missing frequency variables of the second PSD using the first resolution and the second EV through an expectation maximization technique.