Robust cooperative beamforming against steering vector uncertainty in cognitive radio networks

Transmit beamforming is one of the promising ways of accessing spectrum in an underlay cognitive radio network (CRN). In this paper, a robust cooperative transmit beamforming scheme is proposed for a mobile (or stationary) secondary receiver in a CRN where steering vector available at the distributed transmitters is error prone. The solution to the beamforming problem is based on stochastic optimization theory. Specifically, the proposed beamforming scheme achieves the following three objectives: i) probability of large-than-a-threshold for received power in the direction of the secondary receiver is maximized; ii) probabilities of less-than-a-threshold for received interference power in the direction of primary receivers are bounded; and iii) transmit power along the directions of the moving secondary receiver is forced to be uniform so as to ensure the same quality of service. Our simulation results demonstrate that the proposed robust cooperative beamformer can achieve noticeable bit-error-rate reductions for primary users as compared to the non-robust approach, where inaccuracy in the estimation of the steering vector is not considered, while ensuring almost the same quality of communication for the secondary user.