Adaptive robust beamformers for cognitive radio

Cognitive radio (CR) is an intelligent radio system, which is capable of being aware of the communication environment and consequently adapt to it. Detecting the spectrum pools and locations of licensed users (LU), CR can share the spectrum and coexist with LU by adaptive beamforming, a technique which can display nulls to the known direction of arrival (DOA) of LU. This paper presents three kinds of these adaptive robust beamformers, which direct nulls to the predetermined directions and can be all employed by CR. First two are based on projection method and diagonal loading technique. The other is linear constrained minimum variance (LCMV) beamformer. Performance and Comparisons show that all three methods can steer their main beams towards CR users while placing the nulls at the known DOA of LU as well as other unknown interferences.