Spatial Degrees of Freedom in Small Cells: Measurements with Large Antenna Arrays

• Published in: 2013 IEEE 77th Vehicular Technology Conference (VTC Spring) pp. 1 - 6
• Main Authors: Jungnickel, V., Brylka, A., Krueger, U., Jaeckel, S., Narandzic, M., Kaeske, M., Landmann, M., Thomae, R.
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01-06-2013
• Subjects: Antenna arrays; Antenna measurements; Azimuth; Base stations; MIMO; Scattering
• ISSN: 1550-2252
• DOI: 10.1109/VTCSpring.2013.6692761

In this paper, we study the spatial structure of multiple-input multiple-output (MIMO) links in small urban macro-cells at 2.6 GHz. We resolve the double-directional structure of the radio channel. Also we compute common metrics used to characterize MIMO links, i.e. the structure of singular values and the resulting capacity. In a line-of-sight (LOS) link, we find that local scattering is not enough to create full-rank MIMO channels. Even behind a sector, we observe a low-rank channel. In a non-line-of-sight (NLOS) scenario, although we resolve several multi-paths with individual delay, direction of departure (DoD) and direction of arrival (DoA), the impact of local scattering is limited. Singular values indicate few more degrees of freedom for NLOS channels, but less than for a random matrix. We can model the spatial degrees of freedom in small cells better by assuming a random but small number of nearly specular paths feeding some local scattering widely spread in azimuth and elevation. Based on our results, we discuss modeling aspects and the value of large antenna arrays in mobile networks.