Feedback Stability of Discrete-Time LTI Systems via Frequency-Wise Scaled Relative Graphs
This paper studies the stability of negative-feedback interconnections of proper, stable discrete-time multivariable linear time-invariant (LTI) systems using scaled relative graphs (SRGs). We propose a frequency-wise condition that compares the SRGs of the two subsystems'frequency-response matrices only at the same frequency along a feedback-scaling homotopy. We prove that this condition guarantees well-posedness along the homotopy and $\ell_2$-stability for both single-input single-output (SISO) and multi-input multi-output (MIMO) subsystems, without requiring the chord property or nonsingular frequency-response matrices. An equivalent form exchanges scaling and geometric inversion. We also derive an operator-level condition from the SRG closures of the convolution operators and show that it implies the frequency-wise condition. Numerical examples show that the operator-level condition is more conservative because it depends on intersections between responses at different frequencies.