Abstract: As the key technology of fifth generation (5G), 3-dimensional (3D) massive multi-input and multi-output (MIMO) is expected to be widely used in small cell network (SCN). In this paper, in order to investigated the tradeoff between limited size in SCN and the capacity gain from increasing antenna elements，the spatial performances of 3D massive MIMO based on a MIMO channel measurements at 6 GHz in urban microcell (UMi) scenario are studied. Enormous channel impulse responses (CIR) are collected and reconstructed, which enables us to present comparative results of the capacity and the eigenvalue spread (ES). Furthermore, the impacts of antenna element number and spacing on system performance are investigated, i.e., 32, 64, 128 elements are selected from the 512 transmitter (Tx) array with elevation interval spacing being 0.5, 1 and 2 wavelengths for each. Interestingly, the capacity gap can be obviously observed on the comparison between the 1 and 2 wavelength antenna spacing cases, which implies that correlation cannot be ignored when the antenna spacing is larger than 1 wavelength when massive antennas are equipped. The contrast results show that the capacities are enlarged with the increasing of antenna elements number, and larger antenna spacing will lead to higher channel capacity as expected. However, the capacity gains brought by the increasing of antenna spacing will descend to certain degrees as the antenna number increases. Collectively, these results will provide further insights into 3D massive MIMO utilization.

Key words: massive MIMO, channel capacity, eigenvalue spread, elements spacing, antenna number