We consider a cellular network with multiantenna base stations (BSs) and single-antenna users, multicell cooperation, imperfect channel state information (CSI), and directional antennas, each with a vertically adjustable beam. We investigate the impact of the elevation angle of the BS antenna pattern, which is denoted as tilt, on the performance of the considered network when employing either a traditional single-cell transmission or a fully cooperative multicell transmission. Using the results of this investigation, we propose a novel hybrid multicell cooperation technique in which the intercell interference is controlled via either  cooperative beamforming   in the horizontal plane or  coordinated beamforming  in the vertical plane of the wireless channel, which is denoted as  adaptive multicell 3-D beamforming . The main idea is to divide the coverage area into two disjoint  vertical regions  and adapt the multicell cooperation strategy at the BSs when serving each region. A fair scheduler is used to share the time slots between the vertical regions. It is shown that the proposed technique can achieve performance comparable with that of a fully cooperative transmission but with significantly lower complexity and signaling requirements. To facilitate computationally efficient simulation and design space exploration, accurate approximations of the user ergodic rate are proposed for different transmission strategies under imperfect CSI.
['Nima Seifi', 'Robert W. Heath', 'Mikael Coldrey', 'Tommy Svensson']
Adaptive Multicell 3-D Beamforming in Multiantenna Cellular Networks