  The Best Linear Approximation (BLA) framework has already proven to be a
valuable tool to analyze nonlinear systems and to start the nonlinear modeling
process. The existing BLA framework is limited to systems with additive
(colored) noise at the output. Such a noise framework is a simplified
representation of reality. Process noise can play an important role in many
real-life applications.
  This paper generalizes the Best Linear Approximation framework to account
also for the process noise, both for the open-loop and the closed-loop setting,
and shows that the most important properties of the existing BLA framework
remain valid. The impact of the process noise contributions on the robust BLA
estimation method is also analyzed.
