The classical method for the design of decoupling control systems involves two steps. The first step is designing a decoupler such that the MIMO plant is decomposed into a series of independent SISO plants. In the second step, a SISO method is used to design controllers for these SISO plants. There are some unsolved problems in this method:

• It is not applicable to NMP plants and plants with RHP poles. • It is difficult to analyze the effect of the decoupler on the closed-loop performance and robustness.