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Heterogeneous cellular networks (HetNets), where low-power base-stations are overlaid with conventional macro base-stations, is a promising technique to improve coverage and capacity of the macro-only networks. To realize the potential benefits of HetNets, it is crucial to jointly optimize the user association, channel assignment, beamforming and power control to ensure that the inter- and intra-cell interference will not overwhelm the cell-splitting gains. This paper presents an iterative algorithm to solve the joint optimization problem with an objective of maximizing the network sum rate and simultaneously guaranteeing the individual user quality-of-service. The proposed algorithm is built on the so-called convex-concave procedure, and the feasibility issue is handled by L1-norm heuristic. Numerical results demonstrate the large gains over currently used methods for cellular networks.