th-order minimal asymptotically stable system
, we consider the class
of minimal state-space realizations which minimize the trace of
, where
is the controllability and
the observability Grammian. We show that the optimal structures are defined by
with optimal cost
where
is the sum of singular values. For
, the internally balanced structure belongs to
while if
, the optimal noise structures of Mullis and Roberts [1] belong to
. In particular, both structures are optimal when
and
.