Downlink beamforming with broadened and deepened nulls for Cognitive Radio

Cognitive Radio (CR) is an intelligent radio system, which is capable of being aware of the communication environment and consequently adapts 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. In this paper, we present a new CR downlink beamforming method via combining filtering and NB techniques together, which enables CR to broaden and also deepen the nulls towards LU directions. Meanwhile, it also enlarges the width of the main beam to tolerate the DOA ambiguity of CR users. After employing the proposed method, CR directs the main beam towards its own users while putting deeper and wider nulls to the LU in the downlink beamforming. The proposed beamforming scheme causes less interference to LU while sharing the same spectrum.