  This paper studies the design of a decentralized multiuser multi-antenna
(MIMO) system for spectrum sharing over a fixed narrow band, where the
coexisting users independently update their transmit covariance matrices for
individual transmit-rate maximization via 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
(colored) noise at the receiver, i.e., the conventional single-user decoder
(SUD) is applied. This paper proposes a new decoding method for the
decentralized multiuser MIMO system, whereby each user opportunistically
cancels the co-channel interference from some or all of the other users via
applying multiuser detection techniques, thus termed opportunistic multiuser
detection (OMD). This paper studies the optimal transmit covariance design for
users' iterative maximization of individual transmit rates with the proposed
OMD, and demonstrates the resulting capacity gains in decentralized multiuser
MIMO systems against the conventional SUD.
