Cooperative beamforming in Cognitive Radio networks

In this paper, the problem of cooperative beamforming is investigated in a heterogeneous underlay cognitive network. A major difference exists between a conventional cooperative beamforming problem and cooperative beamforming in a CR (Cognitive Radio)network due to the interference constraints imposed by primary network. These constraints can greatly increase the complexity of the related optimization problem and make it much more challenging. Here, we have assumed a typical CR network where a set of cognitive relay nodes collaboratively assist the secondary transmitter to relay its message signal to the destination in the presence of a primary transmitter-receiver pair. Each relay employs the Amplify-and-Forward (AF) protocol to retransmit the signal. In fact, the relays comprise a distributed beamformer to beamform the signal towards its destination while maintaining the QoS in the primary user. The objective is designing the beamformer in order to maximize the target SINR in the cognitive network subject to the network power limitations and interference constraints on the primary network. First, the cooperative beamforming problem has been solved and the improved performance in terms of target SINR in the secondary network has been investigated by simulations. Secondly, a parameter has been derived to demonstrate the distance between the optimal beamweights in this problem and the beamweights in a conventional beamforming problem. Simulations show that the target SINR can be dramatically increased by using cooperative beamforming. Moreover, they show that there is a great distinction between the optimal solutions in conventional cooperative beamforming and cooperative beamforming in cognitive radios.