# LMIs in Control/pages/Discrete Time Stabilizability

Discrete-Time Stabilizability

A discrete time system operates on a discrete time signal input and produces a discrete time signal output. They are used in digital signal processing, such as digital filters for images or sound. The class of discrete time systems that are both linear and time invariant, known as discrete time LTI systems.

Discrete-Time LTI systems can be made stable using controller gain K, which can be found using LMI optimization, such that the close loop system is stable.

## The System

Discrete-Time LTI System with state space realization ${\displaystyle (A_{d},B_{d},C_{d},D_{d})}$
{\displaystyle {\begin{aligned}&A_{d}\in {\bf {{R^{n*n}},}}&B_{d}\in {\bf {{R^{n*m}},}}&C_{d}\in {\bf {{R^{p*n}},}}&D_{d}\in {\bf {{R^{p*m}}\;}}\\\end{aligned}}}

## The Data

The matrices: System ${\displaystyle (A_{d},B_{d},C_{d},D_{d}),P,W}$ .

## The Optimization Problem

The following feasibility problem should be optimized:

Maximize P while obeying the LMI constraints.
Then K is found.

## The LMI:

Discrete-Time Stabilizability

The LMI formulation

{\displaystyle {\begin{aligned}P\in {S^{n}};W\in {R^{m*n}}\;\\&P>0\\{\begin{bmatrix}P&A_{d}P+B_{d}W\\*&P\end{bmatrix}}&>0,\\K_{d}=WP^{-1}\end{aligned}}}

## Conclusion:

The system is stabilizable iff there exits a ${\displaystyle P}$ , such that ${\displaystyle P>0}$ . The matrix ${\displaystyle A_{d}+B_{d}K_{d}}$  is Schur with ${\displaystyle K_{d}=WP^{-1}}$

## Implementation

A link to CodeOcean or other online implementation of the LMI
MATLAB Code

## Related LMIs

[1] - Continuous Time Stabilizability