In many practical applications the control performance are defined in the frequency domain (e.g. band- width, robustness margins, 2 or infinity norm of weighted closed-loop transfer functions, etc.). Many types of uncertainty in control systems can be represented in the frequency domain as well. Therefore a design method in the frequency domain is a good alternative for the time-domain approaches. Moreover, rigorous stability and robustness analysis and synthesis can be performed in this domain. The frequency response of a system that can be computed by Fourier transform of a set of time-domain data is a good candidate for the system representation.
