LTE femtocell base stations (FBSs) have expanded the coverage of wireless communication systems. The growing use of FBSs has brought a serious issue of inter-FBS interference (also referred to as co-tier interference) due to their easy and convenient installation. This study proposes a systematic approach to reduce FBS co-tier downlink interference. In this study, FBSs are densely deployed in an environment and femtocell user equipment (FUEs) detect interference from FBSs and send an alarm signal to each FBS which interferes with it. To minimize the negative effects of interference, power levels are reduced for FBSs identified as excessive interference sources, which are defined as FBSs that have received a number of alarm signals over a given threshold. The method proposed in this study has been validated through simulation and is found to effectively reduce co-tier downlink interference in shared-spectrum femtocell environments, thereby improving system performance.