Spatial-DoF Improvement of MIMO Cognitive Network Through Interference Fusion in Conjunction with Alignment

In a multiple-antennas cognitive radio network (CRN), the rate of the primary link is maximized by water-filling power allocation along the spatial directions associated with the singular values of primary channel matrix. Because of the power constraint of the transmitter even when the primary user (PU) occupies entire spectrum, some spatial opportunities remain that can be utilized by secondary users (SU). This scheme is called opportunistic interference alignment (OIA). This paper extends the OIA by introducing a new approach called OIA and fusion (OIAF). In all the proposed methods that deal with OIA in CRN, SUs avoid the subspace occupied by PU on the SUs receiver and enforce IA on the remaining interference-free subspace. In contrast, OIAF releases this assumption and proposes to precode SU's signals such that the interference from undesired SUs align to this subspace or equivalently to fuse both interference from PU and undesired SUs. Using this technique, more space will be available for the desirable transmitter at the SU's receiver. More free space for singling corresponds to more degrees of freedom (DoF), and in consequence, more achievable sum rate. Also, we provide a processing scheme to perform IA and fusion that is based on alternating minimization technique. Using simulation results, it is shown that the fusion in conjunction with alignment can improve achievable DoF in a CRN that uses interference alignment as an opportunistic accessing technique.