  This letter studies joint transmit beamforming and antenna selection at a
secondary base station (BS) with multiple primary users (PUs) in an underlay
cognitive radio multiple-input single-output broadcast channel. The objective
is to maximize the sum rate subject to the secondary BS transmit power, minimum
required rates for secondary users, and PUs' interference power constraints.
The utility function of interest is nonconcave and the involved constraints are
nonconvex, so this problem is hard to solve. Nevertheless, we propose a new
iterative algorithm that finds local optima at the least. We use an inner
approximation method to construct and solve a simple convex quadratic program
of moderate dimension at each iteration of the proposed algorithm. Simulation
results indicate that the proposed algorithm converges quickly and outperforms
existing approaches.
