Incoherent scatter radar (ISR) is the most powerful ground-based instrument for the Earth's ionosphere. In the ISR measurement, the echo signal returning to the receiver is the electromagnetic wave signal scattered from the electrons in the ionospheric plasma, which are themselves affected by the positive ions. The radio wave scattering is a zero-mean random process whose amplitude is very weak relative to the transmitted power of radar, but the power spectral density of the scatter signal is not zero but a function of electron temperature (T e ), ion temperature (T i ), electron density (N e ), ion velocity (V i ), etc. The analytical relationship between the spectrum and these parameters