Interference management in two-tier heterogeneous networks using blind interference alignment

The paper presents a new solution for interference management in downlink (DL) heterogenous networks where macrocell base station (MBS) and neighboring femtocell base stations cause interference on femtocell users. A novel scheme is proposed where the i-th macrocell user (MU-i, i = 1, ..., F) and the i-th femtocell (FC-i, i = 1, ..., F) form a pair ([MU-i FC-i] pair), and pairs are orthogonal ([MU-i FC-i] orthogonal to [MU-j FC-j], where i is not equal to j). Without loss of generality, it is assumed that i^th macrocell user and femtocell are chosen. In each femtocell, blind interference alignment (BIA) using staggered antenna switching is utilized and the paired macrocell user´s streams are aligned on the predetermined interference dimensions of the femtocell users set by BIA. For the baseline, it is assumed all femtocells, where BIA is applied in each, share their reserved resources thus causing significant interference on each other, and the macrocell users are orthogonal to all femtocells. It is shown that the degrees of freedom of the proposed and the baseline schemes in the case of 2 femtocells each with 2-user 2 × 1 multiple-input single-output (MISO) broadcast channels (BCs) is 7/3 and 1, respectively. Numerical results also show that the sum-rate of the proposed scheme is higher compared with the baseline where MBS applies zero-forcing precoder with water filling power allocation. The novel proposed scheme can be extended to a general network via judicious selection and orthogonalization approaches.