Optimum transmit beamforming scheme for underlay cognitive radio networks

Cognitive Radio (CR) is a promising intelligent technology to increase the spectral efficiency by permitting Primary Users (PU) and Secondary Users (SU) to share the radio spectrum. This paper proposes an optimum transmit beamforming scheme that allows multiple SU, stationary and/or moving, to co-exist with a stationary PU. With knowledge of channel information only between the secondary transmitter and stationary users (but not moving SU), the optimization problem is formulated to ensure maximum transmission power towards the desired SU, while constraining the interference to PU. The transmission power is constant in an area of angles when the SU is not static. The interference to PU can be forced to zero. By first converting the problem into convex form, the optimum solution is obtained through second-order cone programming. Simulation results show that the probability of successful transmission between SU is significantly boosted without degrading the communication of the PU. Simulation results also show that the proposed scheme maintains uniform transmission power for moving SU within a desired range of directions, while nulling the interference to PU and other SU.