  The design of complex systems is typically uncertain and ambiguous at early
stages. Set-Based Design is a promising approach to complex systems design as
it supports alternative exploration and gradual uncertainty reduction. When
designing a complex system, functional requirements decomposition is a common
and effective approach to progress the design incrementally. However, the
current literature on Set-Based Design lacks formal guidance in functional
requirements decomposition. To bridge the gap, we propose a formal process to
hierarchically decompose the functional requirements for Set-Based Design. A
four-step formal process is proposed to systematically define, reason, and
narrow the sets, and eventually decompose the functional requirement into the
sub-requirements. Such a process can be used by the individual suppliers
working in parallel at multiple levels of abstraction and guarantee that the
resulting system will eventually satisfy the top-level functional requirements.
An example of designing a cruise control system is applied to demonstrate the
feasibility of the proposed process.
