In this letter, the performance improvement by the deployment of multiple antennas in cognitive radio systems is studied from a system-level view. The term opportunistic spectrum efficiency (OSE) is defined as the performance metric to evaluate the spectrum opportunities that can actually be exploited by the secondary user (SU). By applying a simple energy combining detector, we show that deploying multiple antennas at the SU transceiver can improve the maximum achievable OSE significantly. Numerical results also reveal that the improvement comes from the reduction of both the detection overhead and the false alarm probability.

The fairness solution without deteriorating the system sum-rate is a challenge under a total energy constraint. One regenerative strategy is proposed to improve the fairness for bi-directional three-node relaying, which is based on decode-and-forward technique with network coding and power optimization. In this letter, the application of network coding decreases the number of transmission phases from traditional four phases to three phases. Moreover, the proposed power optimization algorithm can be applied in practical system, which transforms max-min optimization problem to linear programming (LP) with low complexity. Numerical simulations shows this strategy enhances the minimum of unidirectional transmission rate up to 94% as compared to a four-phase bi-directional strategy, and up to 46% as compared to the three-phase bi-directional strategy with equal-power allocation.