For example, to horizontally install a large number of antenna elements (e.g., > 8) at the top of a BS
tower operating with the lowest 4G system frequency bands of 700 MHz, eight antenna elements with
0.5λspacing require up to 1.7 m width, where λ is the carrier wavelength.
For the typical 4G system frequency bands of 2.5 GHz, fitting 32 antenna elements with 0.5λ spacing require up to 1.9 m width, which is still not feasible in many BSs that have only limited room on the tower.
This practical limitation has motivated Full-Dimension MIMO (FD-MIMO) cellular communication systems, which place a
large number of active antenna elements in a two dimensional grid at the BSs.