Beyond Type-II: Site-Specific Subspace Inference for Efficient CSI Feedback

We propose a site-specific Type-II feedback design for downlink limited-feedback beamforming. The paper first formulates an effective-rate objective that couples channel-state information (CSI) probing, user equipment (UE)-side compression, base station (BS)-side reconstruction, and online overhead. Guided by this objective, the BS uses a low-overhead synchronization signal block (SSB) reference signal received power (RSRP) fingerprint and a learned site-specific propagation prior to infer a UE-dependent dominant beam subspace before explicit CSI acquisition. The UE then estimates and reports only the effective channel coefficients in this subspace, avoiding full-dimensional online subspace search while retaining a Type-II-style multi-beam representation. We analyze the resulting subspace-capture behavior and jointly optimize the probing codebook and subspace inference network for channel-energy preservation. Experiments show Type-II-comparable capture quality with lower online overhead and UE-side complexity, leading to higher effective spectral efficiency.