Self-Optimization of femtocell coverage to minimize the increase in core network mobility signalling

Abstract

Femtocells are low-cost, low-power cellular base stations that are deployed by the end user to supplement macrocellular coverage and provide high data rates in the customer's premises. In femtocell deployments, leakage of the pilot signal to the outside of a house can result in a highly increased signalling load to the core network, as a result of the higher number of mobility events caused by passing users. In this paper, the impact of these mobility events on the core network signalling load is investigated for urban areas. It is shown that without further optimization, the resulting increase in signalling is unacceptably high and as a result prevents the large-scale deployment of femtocells. Different self-optimization methods for residential femtocell coverage are investigated that can significantly reduce the resulting mobility events. First, mobility event based pilot power self-optimization is proposed for femtocells with a single antenna that matches the coverage to the size of the house and thereby can provide both good indoor coverage and a reduced level of mobility events. It is shown that the proposed coverage optimization can significantly outperform simpler methods that aim to achieve a constant cell radius. Then, this concept is further refined by using a low-cost switched pattern multi-element antenna solution. Antenna gain pattern measurements of a prototype with two patch and two inverted F antennas are presented and a corresponding feeder network is discussed. Self-optimization methods are proposed that jointly select an appropriate antenna pattern and optimize the pilot power. Finally, a second multi-element antenna solution is investigated where lobes in different directions, generated by several patch antennas, can be individually attenuated to shape the coverage. Both multi-element antenna solutions allow for a better match of femtocell coverage to the shape of each individual house, and result in a further improvement of both indoor coverage and core network signalling resulting from mobility events.