4G and beyond broadband wireless mobile networks are considering deployment of femtocells in order to improve system capacity several folds. However, interference is added to the system due to large number of uncontrolled user deployed femtocells. In this paper, downlink coexistence between macro-cells and co-channel femtocells in a single frequency OFDMA network is investigated with realistic Long Term Evolution (LTE) system parameters. The impact of femtocell density and load variation in femtocells on the performance of macrocell and femtocell UEs is presented. SINR and capacity analysis is used to evaluate the performance of the 3GPP LTE system. An auto-configuration method for femtocells by UE measurements based downlink power control is described. Results show that the interference measurement based power control significantly reduces the downlink interference in a co-channel macrocell-femtocell networks thereby providing method for realizing fem-tocell networks.