This paper presents a framework for a three-dimensional (3D) beam pattern design with a load-balanced cell sectorization strategy. First, characteristics of the 3D beam pattern were observed, and convex optimization was used to provide a solution based on the criteria that describe the observations. In addition, a user equipment (UE) determination problem that arose because of cell sectorization was also addressed. By incorporating a beam pattern design with a proposed UE determination scheme, a framework of a backward compatible system was completed. The network performance regarding throughput for cell sectorization with a dedicated beam pattern design was evaluated and the simulation results show that the proposed system is superior compared to the conventional system and other proposed sectorization schemes.