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We consider a general n×n multiple-input multiple-output (MIMO) system excited by unobservable inputs that are spatially independent, cyclostationary with unknown statistics. Such a MIMO scenario appears in many applications, such as multiuser communications and separation of competing speakers in speech processing. A special case of this problem, i.e., a 2×2 system case, was previously addressed in another work using frequency-domain correlations of the system output. In this paper, we provide a set of conditions under which a general n×n system is uniquely identifiable based on the second-order frequency-domain correlations of the system output. We provide a constructive proof for the uniqueness of the system solution, which could also serve as a basis for a practical algorithm for system identification.