  3D beamforming is a promising approach for interference coordination in
cellular networks which brings significant improvements in comparison with
conventional 2D beamforming techniques. This paper investigates the problem of
joint beamforming design and tilt angle adaptation of the BS antenna array for
maximizing energy efficiency (EE) in downlink of multi-cell multi-user
coordinated cellular networks. An iterative algorithm based on fractional
programming approach is introduced to solve the resulting non-convex
optimization problem. In each iteration, users are clustered based on their
elevation angle. Then, optimization of the tilt angle is carried out through a
reduced complexity greedy search to find the best tilt angle for a given
placement of the users. Numerical results show that the proposed algorithm
achieves higher EE compared to the 2D beamforming techniques.
