Characterization of time-varying linear systems

Abstract : It is shown that a versatile description of time-varying linear systems is achieved by the use of a family of eight system functions that includes the G and H transfer functions. This approach is extended to multidimensional systems and to ''vector-time'' systems operating on signals that are specified by a function of several variables. The class of time-varying linear systems that is specified implicitly by a set of nth order linear differential equations in r variables is considered. The explicit form of the impulse response of the system is derived in order to bridge the gap between the differential equation and the system-function specifications of a time-varying linear system. The same basic approach for characterizing deterministic systems leads to procedures for the characterization of randomly varying systems subjected to random inputs. The general case where the system and input are nonstationary is considered for both unidimensional and multidimensional systems. (Author)