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In this paper, throughput and energy efficiency of cognitive multiple-inputmultiple-output (MIMO) systems operating under quality-of-service (QoS)constraints, interference limitations, and imperfect channel sensing, arestudied. It is assumed that transmission power and covariance of the inputsignal vectors are varied depending on the sensed activities of primary users(PUs) in the system. Interference constraints are applied on the transmissionpower levels of cognitive radios (CRs) to provide protection for the PUs whoseactivities are modeled as a Markov chain. Considering the reliability of thetransmissions and channel sensing results, a state-transition model isprovided. Throughput is determined by formulating the effective capacity. Firstderivative of the effective capacity is derived in the low-power regime and theminimum bit energy requirements in the presence of QoS limitations andimperfect sensing results are identified. Minimum energy per bit is shown to beachieved by beamforming in the maximal-eigenvalue eigenspace of certainmatrices related to the channel matrix. In a special case, wideband slope isdetermined for more refined analysis of energy efficiency. Numerical resultsare provided for the throughput for various levels of buffer constraints anddifferent number of transmit and receive antennas. The impact of interferenceconstraints and benefits of multiple-antenna transmissions are determined. Itis shown that increasing the number of antennas when the interference powerconstraint is stringent is generally beneficial. On the other hand, it is shownthat under relatively loose interference constraints, increasing the number ofantennas beyond a certain level does not lead to much increase in thethroughput.

On the Throughput and Energy Efficiency of Cognitive MIMO Transmissions