A method of determining performance of a Radio Access Network (RAN) of a telecommunications network is disclosed in which the RAN covers an area that is divided into a plurality of unit cells. The method includes receiving a measured performance value of the RAN within at least one unit cell of the plurality of unit cells; receiving a simulated performance value of the RAN within each of the plurality of unit cells; identifying a unit cell that is associated with both a measured performance value and a simulated performance value; calculating, for the identified unit cell, a difference function between the simulated performance value and the measured performance value; and applying the difference function to the simulated performance value of at least one adjacent unit cell to the identified unit cell to determine a modified simulated performance value for the at least one adjacent unit cell.