Near-Field Channel Characterization for Midband ELAA Systems: Sounding, Parameter Estimation, and Modeling

6G communication will greatly benefit from using extremely large-scale antenna arrays (ELAAs) and new midband spectrums (7–24 GHz). These techniques require a thorough exploration of the challenges and potentials of the associated near-field (NF) phenomena. It is crucial to develop accurate NF channel models that include spherical-wave propagation and spatial nonstationarity (SnS). However, channel measurement campaigns for midband ELAA systems have rarely been reported in the state of the art. To this end, this work develops a channel sounder dedicated to midband ELAA systems based on a distributed modular vector network analyzer (VNA) incorporating radio over fiber (RoF), phase compensation, and virtual antenna array (VAA) schemes. This novel channel-sounding testbed based on off-the-shelf VNA has the potential to enable large-scale experimentation due to its generic and easily accessible nature. The main challenges and solutions for developing NF channel models for midband ELAA systems are discussed, including channel sounders, multipath parameter estimation algorithms, and channel modeling frameworks. Besides, the study reports a measurement campaign in an indoor scenario using a 720-element virtual uniform circular array (UCA) ELAA operating at 16–20 GHz, highlighting the presence of spherical wavefronts and spatial nonstationary effects. The effectiveness of the proposed NF channel parameter estimator and channel modeling framework is also demonstrated using the measurement data.