This paper presents a new approach to the loop transfer recovery (LTR) in linear multivariable control systems, based on the unknown input reduced-order observer-based controller. Two parametrizations methods for all unknown input observers are presented, the first one is based on the matrix fraction description (MFD) and the second one is developed in RH-infinity. The LTR for both minimum and non-minimum phase systems are considered. An exact recovery, based on the unknown input observer, is provided for minimum phase systems. For non-minimum phase systems, an approximate recovery is obtained by minimizing the H-infinity norm of the recovery matrix.
