A multi-threshold feedback scheme for cognitive radio networks based on opportunistic beamforming

Cognitive radio is a promising technique for efficient spectrum utilization in wireless systems. In multi-user Multiple- Input Multiple-Output (MIMO) system, a large amount of feedback information has to be used to achieve multi-user diversity. In this paper, in order to reduce the feedback amount and hence the wasted energy, we propose a novel scheduling scheme of secondary users (SUs) for an underlay cognitive radio network. Our scheme is based on opportunistic beamforming and employs multiple feedback thresholds. The lowest threshold is chosen to insure a predefined allowed scheduling outage probability. A scheduling outage event occurs if at least one transmit beam has no feedback information. The other thresholds are chosen in order to reduce the number of SUs feeding back their maximum signal to interference plus noise ratio (SINR) (and hence the wasted energy) and the delay due to the number of attempts. We show via simulations that a significant gain in terms of energy is obtained at the price of a reasonable delay.