  In this paper, the problem of joint design of Spectrum Sensing (SS) and
receive beamforming (BF), with reference to a Cognitive Radio (CR) system, is
considered. The aim of the proposed design is the maximization of the
achievable average uplink rate of a Secondary User (SU), subject to an
outage-based Quality-of-Service (QoS) constraint for primary communication. A
hybrid CR system approach is studied, according to which, the system either
operates as an interweave (i.e., opportunistic) or as an underlay (i.e.,
spectrum sharing) CR system, based on SS results. A realistic Channel State
Information (CSI) framework is assumed, according to which, the direct channel
links are known by the multiple antenna receivers (RXs), while, merely
statistical (covariance) information is available for the interference links. A
new, closed form approximation is derived for the outage probability of primary
communication, and the problem of rate-optimal selection of SS parameters and
receive beamformers is addressed for hybrid, interweave and underlay CR
systems. It is proven that our proposed system design outperforms both underlay
and interweave CR systems for a range of system scenarios.
