Understanding and closing the gap between requirements on system and subsystem level

In systems engineering, the increasing complexity of systems is handled by decomposing systems into subsystems. As part of the decomposition typically more abstract system requirements are refined to more detailed subsystem requirements. Refining system requirements to subsystem requirements includes the two steps interface refinement on the system boundaries, and a decomposition of system requirements to subsystem requirements. In order to apply formal analysis and verification techniques on the refinement of requirements, a formal refinement specification is necessary. In this paper we show the results of an exploratory industrial case study provided by Siemens, where we analyzed the refinement from system to subsystem requirements. We show that formal refinement specifications can become very complex, when interface refinement and requirement decompositions are performed in one step. In order to reduce complexity in the formal refinement specification, we introduce a formal restructuring approach for requirements. The main benefits of this restructuring approach are twofold. It enables reuse of requirements and knowledge preservation on the system level when the system architecture changes. Furthermore, quality assurance of the refinement on system level can now be performed independently from the system decomposition.