In this work, we design a nonlinear state feedback controller based on the State Dependent Riccati Equation (SDRE) technique in HIV/AIDS chemotherapy treatment. A nonlinear model has been utilized to predict the dynamical behavior of disease in the presence of treatment. The model is a system of ODEs with five state variables: healthy CD4+ T cells, infected CD4+ T cells, free HIV viruses, memory CTL precursor cells and memory CTL effector cells. For using the SDRE controller, all state variables must be available for feedback. We suppose that only the number of CD4+ T (sum of healthy and infected CD4+ T cells) and virus cells could be measured. Therefore, we must design a state observer to estimate the unmeasured states from these measurements. The Symmetric Unscented Kalman Filter (SUKF) can be used as a state observer for a nonlinear system and in this work we use the SUKF as a nonlinear state observer. Consequently, we can use the SDRE technique just by measuring the CD4+ T and virus populations. Robustness of controlled system in the presence of measurement noise and uncertainties in the parameters of model is investigated. Numerical simulations are given to illustrate the design procedure and to show the flexibility of the method.
