  This article presents a Real-Time Iteration (RTI) scheme for distributed
Nonlinear Model Predictive Control (NMPC). The scheme transfers the well-known
RTI approach, a key enabler for many industrial real-time NMPC implementations,
to the setting of cooperative distributed control. At each sampling instant,
one outer iteration of a bi-level decentralized Sequential Quadratic
Programming (dSQP) method is applied to a centralized optimal control croblem.
This ensures that real-time requirements are met and it facilitates cooperation
between subsystems. Combining novel dSQP convergence results with RTI stability
guarantees, we prove local exponential stability under standard assumptions on
the MPC design with and without terminal constraints. The proposed scheme only
requires neighbor-to-neighbor communication and avoids a central coordinator. A
numerical example with coupled inverted pendulums demonstrates the efficacy of
the approach.
