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Protection of the primary users (PUs) from interference stemming from secondary user (SU) transmissions is one of the key issues in dynamic cognitive radio systems. Assuming elementary direction of arrival (DOA) or location estimation of PU devices can be carried out in the SU devices, appropriate directional transmission utilizing e.g. antenna arrays and null-steering can then be deployed to avoid interference by steering nulls towards the PUs. In this paper, we study such transmitter digital beamforming and null-steering under practical limitations of the associated radio frequency (RF) circuits, namely the amplitude and phase mismatches between the in-phase and quadrature (I/Q) rails of the parallel up-conversion chains. Closed-form analysis of the available beamforming and null-steering capabilities is first provided, showing that the transmitter null-steering capabilities are heavily degraded due to RF circuit imperfections. Motivated by this, we will then propose and formulate a widely-linear (WL) digital beamforming and null-steering solution which is shown to efficiently suppress the RF circuit imperfection effects from the radiation pattern. Based on the obtained results, the developed solution can provide efficient null-steering and interference suppression characteristics, despite of the imperfections in the RF circuits, and can thus enable, e.g., the use of cost-efficient RF chains in the SU transmitters.

RF-Aware Widely-Linear Beamforming and Null-Steering in Cognitive Radio Transmitters