Adaptive beamforming for Cognitive Radio with Null Broadening

Cognitive Radio (CR) is an intelligent radio system, which is capable of being aware of the communication environment and consequently adapt to it. With Null Broadening (NB) technique, CR can share the spectrum and coexist with licensed users (LU) by displaying nulls to a certain range of Direction of Arrival (DOA) of LU. This paper presents a new NB technique called Virtual Direction Adding (VDA) in downlink beamforming for CR. VDA can be performed by adding two coherent signals to each LU but with different directions. Performance and comparisons show that while keeping the required width of nulls, and even with more LU and CR users, VDA achieves a deeper null than the Covariance Matrix Taper (CMT) method. By introducing VDA, CR can direct its main beam towards its own users while putting broadened nulls to the LUs´ directions in the downlink, which will consequently cause less interference to LUs sharing the same spectrum.