Spectrum sharing in cognitive radio networks using beamforming and two-path successive relaying

In this paper, we propose an effective beamforming scheme for spectrum sharing between the primary and secondary users of a relay-assisted cognitive radio network. In this scheme, two secondary base stations, by employing beamforming and two-path successive relaying (TPSR), transmit their signals to the secondary users, and at the same time, help the primary user to continuously transmit its signal to its destination. We design the beamforming scheme such that the achievable rate of the secondary network is maximized under the constraint that a quality of service (QoS) requirement is guaranteed for the primary network. To solve this optimization problem which is non-convex, we first use the semidefinite programming (SDP) relaxation technique to convert it to a convex problem and then find the solution using the randomization technique. The simulation results show that the performance of the system with increasing the number of secondary users remains satisfactory even when the number of secondary users is greater than the number of antennas employed for beamforming. This observation shows the effectiveness of this beamforming scheme for spectrum sharing.