Common Subspace-Based One-Shot CSI Reporting for MIMO Antenna-Port Adaptation

Abstract

Increasing power consumption of extremely largescale antenna arrays in multiple-input multiple-output (MIMO) deployments motivates antenna-port adaptation for network energy saving (NES), where the next-generation NodeB (gNB) switches among multiple antenna-port sub-configurations (subcfgs) to match traffic variations while maintaining acceptable link performance. In the Third Generation Partnership Project (3GPP) Release-18, NES-oriented multi-CSI reporting can be supported under a Type-I channel state information (CSI) reporting framework by configuring multiple subcfgs. However, to limit feedback overhead, the supported CSI reporting is restricted to low-resolution reporting, which limits the achievable spectral efficiency (SE) in NES operation. For next-generation mobile networks, achieving higher SE in NES operation therefore calls for CSI feedback with higher resolution, but a straightforward per-subcfg extension would make the feedback overhead grow with the number of configured subcfgs and become prohibitive. To address this issue, this paper proposes a one-shot CSI reporting framework based on a common subspace for NES-oriented multi-subcfg operation. We explicitly establish that, for subcfgs corresponding to different port activations, the dominant right-singular subspace of each subcfg channel can be deterministically derived from that of the full-port channel through a mapping combining selection and orthogonalization. Exploiting this property, the user equipment (UE) reports only a single dominant subspace associated with the full-port configuration, while the gNB derives subcfg-specific dominant subspaces and constructs candidate precoders accordingly for efficient subcfg evaluation and selection. Numerical simulations demonstrate that the proposed framework achieves SE nearly identical to the ideal per-subcfg singular value decomposition (SVD)-based scheme and significantly outperforms the Release-18 per-subcfg Type-I CSI reporting baseline, providing approximately $\mathbf{1 0} \boldsymbol{\%}$ higher SE across the evaluated signal-to-noise ratio (SNR) range while avoiding per-subcfg high-resolution CSI reporting.