Massive MIMO for interference management in heterogeneous networks

In this paper, a spatial interference coordination scheme for a typical macro/small-cell overlay scenarios is proposed which protects small-cell user equipments (UEs) from macro-cell interference and has potential applications in massive multiple-input multiple-output (MIMO) deployments. We assume the macro base stations (BSs) to be equipped with a large number of transmit antennas which are employed to reduce interference towards small-cell users by means of spatial transmit processing. Uncoordinated spatial multiplexing (SMUX) per access point (AP) is considered as a baseline, i.e. the macro BS uses zero-forcing (ZF)-based block diagonalization (BD) [1] to serve their user without taking interference towards small-cell UEs into account. The proposed method considers a joint BD/interference-alignment (IA) - approach to calculate the macro pre-coder and reduces the dimension of the interference sub-space for small-cell UEs without destroying the orthogonality of the macro BD. It results, that the the interference can be reduced even if the number of protected small-cell UEs exceeds the number of free spatial dimensions at the macro BS.