Performance of altruistic beamforming for mitigation of multiple cross-layer interference sources in heterogeneous networks

In this paper, we study the impact of cross-layer interference in a two-tier Heterogeneous Network (Het-Net), where the assumption of a single dominant interferer is not always valid. We investigate the improvements that can be achieved when Transmit Beamforming (TBF) is utilized to mitigate the interference originating from multiple interference sources. For this purpose, closed-form expressions are presented to model the outage capacity of a heavily interfered Macro User Equipment (MUE), when a variable number of Femto Base Stations (FBSs) substitute their current beamforming vectors that maximize own-user performance (i.e., egoistic TBF) with those that minimize emitted interference (i.e., altruistic TBF). Our analysis reveals that application of altruistic TBF is most efficient when performed in clusters of interferers, ranked from strongest to weakest according to the level of interference that is observed at the MUE. Determination of the optimal number of participating clusters is strongly connected to the type of feedback algorithm used, and the resolution of the feedback message to be exchanged.