# 3D MIMO Outdoor-to-Indoor Propagation Channel Measurement

## TL;DR

This paper introduces a low-cost 3D MIMO channel sounder and presents measurement results of outdoor-to-indoor propagation, analyzing elevation and azimuth characteristics, phase errors, and channel hardening effects in urban environments.

## Contribution

It develops a novel hybrid array-based channel sounder and provides comprehensive outdoor-to-indoor 3D MIMO channel measurements with phase error correction and analysis.

## Key findings

- Elevation and azimuth angular spreads characterized in urban environments
- Feasibility of user separation in elevation domain demonstrated
- Insights into channel hardening and MRC optimality in Massive MIMO

## Abstract

3-dimensional Multiple-Input Multiple-Output (3D MIMO) systems have received great interest recently because of the spatial diversity advantage and capability for full-dimensional beamforming, making them promising candidates for practical realization of massive MIMO. In this paper, we present a low-cost test equipment (channel sounder) and post-processing algorithms suitable for investigating 3D MIMO channels, as well as the results from a measurement campaign for obtaining elevation and azimuth characteristics in an outdoor-to-indoor (O2I) environment. Due to limitations in available antenna switches, our channel sounder consists of a hybrid switched/virtual cylindrical array with effectively 480 antenna elements at the base station (BS). The virtual setup increased the overall MIMO measurement duration, thereby introducing phase drift errors in the measurements. Using a reference antenna measurements, we estimate and correct for the phase errors during post-processing. We provide the elevation and azimuth angular spreads, for the measurements done in an urban macro-cellular (UMa) and urban micro-cellular (UMi) environments, and study their dependence on the UE height. Based on the measurements done with UE placed on different floors, we study the feasibility of separating users in the elevation domain. The measured channel impulse responses are also used to study the channel hardening aspects of Massive MIMO and the optimality of Maximum Ratio Combining (MRC) receiver.