Comparison of methods for turbulence Doppler frequency shift calculation in Doppler reflectometer

The Doppler reflectometer (DR), a powerful diagnostic for the plasma perpendicular velocity ($u_\perp$) and turbulence measurement has been widely used in various fusion devices. Many efforts have been put into extracting the Doppler shift from the DR signal. There are several commonly used methods for Doppler shift extraction, such as the phase derivative, the center of gravity (COG), and symmetric fitting (SFIT). However, the strong zero-order reflection component around $0 kHz$ may interfere with the calculation of the Doppler shift. To avoid the influence of the zero-frequency peak, the asymmetric fitting (AFIT) method was designed to calculate the Doppler shift. Nevertheless, the AFIT method may lead to an unacceptable error when the Doppler shift is relatively small compared to the half width at half maximum (HWHM). Therefore, an optimized method, which can remove the zero-frequency peak and fit the remaining Doppler peak with a Gaussian function, is devised to extract the Doppler shift. This method can still work reliably whether the HWHM is larger than the Doppler shift or not.