What is Lineal Time-Invariant (LTI)

Let us consider a filter that has as input a scaled sum of arbitrary discrete-time signals. If the output of this filter is equal to the scaled sum of the outputs of the filter to every one of these arbitrary signals, the filter is considered linear. If additionally the input signal is delayed in any possible amount and the output signal undergoes the same delay as the input, the filter is also considered time-invariant. A filter is LTI if it has both, linear and time-invariant properties.

Efficient Techniques to Design Low-Complexity Digital Finite Impulse Response (FIR) Filters

David Ernesto Troncoso Romero (National Institute of Astrophysics, Optics and Electronics (INAOE), Mexico) and Gordana Jovanovic Dolecek (National Institute of Astrophysics, Optics and Electronics (INAOE), Mexico)