On the Throughput and Energy Efficiency of Cognitive MIMO Transmissions

In this paper, throughput and energy efficiency of cognitive multiple-input-multiple-output (MIMO) systems operating under quality-of-service (QoS) constraints, interference limitations, and imperfect channel sensing are studied. It is assumed that transmission power and covariance of the input signal vectors are varied, depending on the sensed activities of primary users (PUs) in the system. Interference constraints are applied on the transmission power levels of cognitive radios (CRs) to provide protection for the PUs, whose activities are modeled as a Markov chain. Considering the reliability of the transmissions and channel sensing results, a state transition model is provided. Throughput is determined by formulating the effective capacity. The first derivative of the effective capacity is derived in the low-power regime, and the minimum bit energy requirements in the presence of QoS limitations and imperfect sensing results are identified. Minimum energy per bit is shown to be achieved by beamforming in the maximal-eigenvalue eigenspace of certain matrices related to the channel matrix. In a special case, wideband slope is determined for a more refined analysis of energy efficiency. Numerical results are provided for the throughput for various levels of buffer constraints and different number of transmit and receive antennas. The impact of interference constraints and benefits of multiple-antenna transmissions are determined. It is shown that increasing the number of antennas when the interference power constraint is stringent is generally beneficial. On the other hand, it is shown that, under relatively loose interference constraints, increasing the number of antennas beyond a certain level does not lead to much increase in throughput.