Mechanism Analysis and Solutions of Spatial DoF Constraints in Hybrid Beamforming for XL-MIMO-OFDM Systems

As wireless communications evolve toward the 6G era, aiming for terabit-per-second rates and ultra-massive connectivity, Extremely Large-Scale MIMO (XL-MIMO) operating in new mid-band, millimeter-wave (mmWave), and terahertz (THz) frequencies, along with Fluid Antenna Systems (FAS) and Reconfigurable Intelligent Surfaces (RIS), are envisioned as key enablers. However, a fundamental physical conflict arises: the inherent requirement for frequency-selective processing in wideband Orthogonal Frequency Division Multiplexing (OFDM) systems contradicts the intrinsic frequency flatness of mainstream Analog Beamforming (ABF) architectures. This article provides an in-depth analysis of the physics behind this conflict, defining it as "Electromagnetic Degree-of-Freedom (E-DoF) Freezing" from the perspective of Electromagnetic Information Theory (EIT). The direct consequences include the decline in multiuser multiplexing gain and multi-network coexistence issues. Furthermore, we systematically survey crosslayer innovative solutions, ranging from hardware breakthroughs to algorithmic optimization solutions. Finally, the article envisions a 6G paradigm shift from "adapting to constraints" to "breaking constraints," highlighting green architecture, aiming to provide profound insights and guidance for the design and deployment of next-generation wireless systems.