Decoupling by precompensation while maintaining stabilizability

Plants with two inputs and two outputs are considered. Necessary and sufficient conditions are presented for the existence of a 2*2 precompensator such that the series connection of precompensator and plant is stabilizable and has a diagonal transfer function matrix. The set of all diagonal transfer function matrices achievable by a precompensator of the above form is characterized. The results admit a solution to the decoupling problem by unity output feedback compensation for 2*2 plants.<
>