Angular interference suppression in cognitive LTE-A femtocells

We consider a heterogeneous Long Term Evolution Advanced (LTE-A) network where a cognitive femtocell operates in the coverage area of a traditional macrocell sharing the same frequency band. A new method is proposed here to avoid the interference generated by the femtocell on the macrocell downlink communications. In particular, the estimation of the direction of arrival of the primary user signals is used to place nulls in the femtocell transmission paths by means of a suitable beamforming algorithm. The novelty of the proposed approach is that it works on a physical resource basis, taking into account the specific LTE-A access scheme. In this way it is able to manage the interference towards several macrocell users characterized by a high angle dispersion of the multipath components. The obtained numerical results show that the system is able to significantly reduce the interference in different operational conditions, especially when the physical resource blocks belonging to the same macrocell user can be clustered, thus increasing the dimension of the snapshot used for the direction of arrival estimation.