Performance Analysis of Null-Steering Beamformers in Cognitive Radio Systems

We evaluate the performance of a secondary system which is equipped with a multi-antenna transmitter and a single-antenna receiver. The secondary system coexists with a primary system in an underlay cognitive setting where both systems share the same frequency bands simultaneously. A secondary beamforming vector is characterized such that the interference power at each primary receiver is nullified. While constraining the secondary transmitted power, we aim to achieve the maximum received power at the secondary receiver. With perfect channel state information (CSI), we show that the secondary system can achieve a mean received power that grows linearly in the number of secondary transmitting antennas and is directly proportional to the power of the line of sight (LOS) component between the secondary transmitter and the secondary receiver. Furthermore, in the case of imperfect CSI, it is shown that a moderate secondary LOS component can significantly reduce the effect of estimation error on the performance.