This paper studies the design of decentralized multiuser (MU) multi-antenna/multiple-input-multiple-output (MIMO) systems for wireless spectrum sharing over a fixed frequency band, in which users independently update their transmit covariance matrices for individual transmit-rate maximization in an iterative manner. This design problem was usually investigated in the literature by assuming that each user treats the co-channel interference from all the other users as additional noise at the receiver and, accordingly, the conventional single-user decoder (SUD) is applied. This paper considers a more advanced decoder design approach for decentralized MU-MIMO systems, in which each user opportunistically cancels the co-channel interference from certain subset of coexisting users when their signals are jointly decodable with the desired signal at the receiver. The new decoding scheme is thus termed opportunistic multiuser detection (OMD). This paper derives the optimal transmit covariance matrix for users' iterative maximization of individual transmit rates with the proposed OMD at the receiver, and evaluates the achievable throughput gains for decentralized MU-MIMO systems against the conventional SUD. Promising applications of the OMD for achieving maximum interference mitigation gains in spectrum sharing based wireless systems such as cognitive radio networks and cellular networks are demonstrated.
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