  In this paper, we study structural controllability of a linear time invariant
(LTI) composite system consisting of several subsystems. We assume that the
neighbourhood of each subsystem is unconstrained, i.e., any subsystem can
interact with any other subsystem. The interaction links between subsystems are
referred as interconnections. We assume the composite system to be structurally
controllable if all possible interconnections are present, and our objective is
to identify the minimum set of interconnections required to keep the system
structurally controllable. We consider structurally identical subsystems, i.e.,
the zero/non-zero pattern of the state matrices of the subsystems are the same,
but dynamics can be different. We present a polynomial time optimal algorithm
to identify the minimum cardinality set of interconnections that subsystems
must establish to make the composite system structurally controllable.
