Before revision:  Efficient coverage optimization requires accurate coverage estimation which relies on geo-located field measurements . These measurements are gathered today during highly expensive drive tests  and will be reported in the near future by users equipments thanks to the 3GPP MDT feature ( still costly in terms of battery consumption and signaling overhead ).
After revision:  Efficient coverage optimization requires accurate coverage estimation which relies on geo-located field measurements which are gathered today during highly expensive drive tests and will be reported in the near future by users equipments thanks to the 3GPP MDT feature ( still costly in terms of battery consumption and signaling overhead ).