Near-Field LoS MIMO With Dual Continuous Apertures (CAPs): Channel Decomposition and EDoF-Optimal Beamforming

This paper proposes a channel decomposition method and a novel beamforming architecture for line-of-sight (LoS) multi-input multi-output (MIMO) systems with dual continuous apertures (CAPs). Specifically, we develop a three-dimensional (3D) dual-CAP geometric model through orthogonal basis projection, establishing a unified electromagnetic analysis framework for arbitrarily oriented non-coplanar CAPs and enabling consistent 3D wave propagation modeling. Building on this model, we derive the closed-form expression for the effective degrees of freedom (EDoF) of the dual-CAP LoS MIMO system by analyzing spherical wave phase differences via sampling theory. This explicitly connects EDoF to wavelength, propagation distance, and angular rotation. Our proposed channel decomposition method decouples dual-CAP LoS MIMO channels into multiple independent SISO subchannels, which enables a novel beamforming architecture. We implement this architecture through a wavefront degrees of freedom orthogonal subchannel decomposition (WDOSD) algorithm to maximize the achievable rate of the dual-CAP LoS MIMO system. Numerical results demonstrate that: i) both component-wise EDoF and total EDoF can be accurately obtained using our proposed closed-form expressions; ii) under varying transmitter CAP sizes with fixed power, our WDOSD algorithm achieves up to three times performance improvement over the minimum mean-squared error (MMSE) algorithm; and iii) under varying communication distance with fixed angle, the WDOSD algorithm achieves up to two times performance improvement over the MMSE algorithm.