Femtocell Coverage Optimization

In femtocell deployments where cells are deployed by the end user in an uncontrolled way, the pilot signal leakage to the outside of its rescribed indoor radiating environment results in patchy outdoor coverage. This can lead to 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 chapter, the impact of such capture effects on the core network signalling load is investigated. 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 considered that can significantly reduce the resulting mobility events. First, mobility event based pilot power self-optimization is examined for femtocells with a single antenna that matches the coverage to the size of the indoor environment 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 outperformsimpler 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 patches 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.