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Recently collaborative relay systems have been studied to increase the spectral efficiency and extend the range of wireless communications. In this paper, we consider the collaborative relay beamforming (CRBF) for multiuser systems. In the proposed method, we select base station (BS) antennas for each user and optimize their CRBF weights under two different types of power constraints, which are the total relay power constraint and individual relay power constraints, by exploiting the notion of the maximization of the worst-case received signal-to-interference-and-noise ratio (SINR). The optimization problem of the CRBF weights can be efficiently solved using a semidefinite relaxation (SDR) technique.We derive an iterative CRBF scheme to jointly select the BS antennas and optimize the CRBF weights to maximize the worst SINR among all users. Simulation results validate our theoretical analysis and demonstrate the tradeoffs between the number of BS antennas and the number of relays in the CRBF system.