Tilt Angle Optimization in Two-Tier Cellular Networks - A Stochastic Geometry Approach

In this work, we address the antenna tilt optimization problem for a two tier cellular network consisting of macrocells and femtocells, where both tiers share the same spectrum and their positions are modelled via two independent Poisson Point Processes (PPPs). First, we derive the coverage probability for a traditional cellular network consisting only of macrocells and obtain the optimum tilt angle that maximises the overall energy efficiency (EE). Gains of up to 400% in EE were found for a scenario (approximately) equivalent to a hexagonal cell deployment with cell radius of 200 m when the optimum tilt was selected. We then proceed to model the Heterogeneous Network (HetNet) scenario where femtocells are also deployed in the network’s area. We observe that the macro users performance is highly sensitive to the interference emanating from the femtocell tier. In order to circumvent this issue, interference coordination by employing a guard zone for the macrocell user is proposed. Subsequently, we formulate a joint optimization problem where we derive both, the radius of a guard zone protecting the macro user and the tilt angle that maximize the EE of the network.