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Secondary spectrum usage has the potential to considerably increase spectrum utilization. In this paper, quality-of-service (QoS)-aware spectrum underlay is considered. A multi-antenna access point transmits common information to a set of secondary receivers, by means of appropriate beamforming; the objective is to maximize the multicast beamforming rate (or, equivalently, minimize the signal to noise ratio) for the secondary receivers, while explicitly limiting interference to primary receivers. The formulation incorporates the pragmatic case of inaccurate channel state information (CSI) for both primary and secondary users. Simulation examples illustrate the effectiveness of the proposed approach.