Grassmannian beamforming and null space broadcasting protocols for cognitive radio networks

Abstract

Novel Grassmannian beamforming techniques based on two null space broadcasting protocols are proposed for a cognitive radio network. A secondary user basestation (SBS) with multiple secondary users sharing the primary user spectrum is considered. A protocol for feeding back the channel state information (CSI) under the constraint on the availability of a finite number of bits for feedback is proposed. The SBS informs all the secondary users of the null space of the channel matrices seen between itself and the primary users. The secondary users choose the best beamforming vectors from a Grassmannian codebook as those that are closer to the null space of the primary user channels and those that maximise their projection onto the signal space of the corresponding secondary users. An improvement on the ergodic channel capacity has been observed. The interference leakage to the primary users owing to the quantisation of the CSI has been analytically quantified and verified using simulation results.