Three-dimensional fading channel models: A survey of elevation angle research

The explosive increase of mobile traffic demands higher spectrum efficiency for future wireless communications. By properly configuring a 2-dimensional (2D) antenna array at the basestation, the system capacity can be improved with 3 dimensional (3D) multiuser Multiple Input and Multiple Output (MIMO) techniques without modifying the terminal antennas. Therefore, the signal will propagate in a 3D space with angle dispersion in both horizontal and vertical planes. Due to its role in facilitating research and development of 3D MIMO technology, the 3D fading channel model is receiving increasing attention. However, existing fading channel models mostly focus on the azimuth angle characteristics, while neglecting the elevation angle impact. This article provides a state of the art review on 3D fading channel models, emphasizing research related to the elevation angle. We also report some recent field measurements for 3D MIMO and investigate the comprehensive propagation characteristics of the elevation angle.