This paper investigates the well-known separation properties that hold in t
he design of unconstrained output feedback controllers and tries to general
ize them to cope with the design of structurally constrained decentralized
controls. This problem, which until now has been open, presents additional
constraints which destroy the useful properties that lead to separation. Ne
cessary and sufficient conditions for decentralized stability and H-2 norm
minimization are given, providing a framework in which a suitable an hoc se
paration procedure is developed. A numerical algorithm based on cross-decom
position is presented and applied in two situations: first, when separation
Fails to provide a feasible solution; and then, to improve the obtained co
ntroller. An example and a comprehensive benchmark illustrate and validate
the method.