Spatial interference management with hierarchical precoding in ultra-dense heterogeneous networks

In this work we focus on ultra-dense heterogeneous networks which are one of the key-concepts to increase the spectral efficiency for future mobile networks. Therefore, macro and small cells are operating on the same frequencies to increase the reuse of the spectrum. With this, interference caused by the macro toward users attached to the small cells becomes the limiting performance factor. Aiming for ultra-dense networks with many small cells, also interference from the small-cells to macro-attached, and other small-cell users has an impact which cannot be neglected. To cope with this challenge we use a novel two-stage hierarchical precoding/beamforming approach to protect small-cell users from macro-interference, while still serving macro-users at the same time. In doing so, Channel State Information (CSI) at the transmitter (CSIT) between the macro and small-cell users is required at the macro cell. We adapted an iterative Interference Alignment (IA) algorithm operating in the common null space of the macro-users to ensure interference decoupling of macro and small cell users. All the processing is done at the macro Base Station (BS) allowing independent transmission of the small-cells. This means no further feedback, information (e.g. precoder) or user data exchange between transmit nodes is required.