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We propose a method that allows opportunistic shared channel utilization in multi-user multiple-input multiple-output (MIMO) systems, without causing any interference towards the primary system. It is shown that with appropriate exploitation of demodulation pilot signaling, the opportunistic system can quantify all the necessary information in order to guarantee complete interference mitigation towards the primary system. Opportunistic signal space utilization is based on the assumption of reciprocal zero-forcing transmitter and receiver design in the primary system and use of time division duplexing (TDD). Linear beamforming is coupled with greedy scheduling for efficient exploitation of multi-user diversity in both the primary system and the cognitive systems.