Wideband orthogonal frequency-division multiplexing (OFDM) over extremely large-scale MIMO (XL-MIMO) arrays in the near-field Fresnel regime suffers from a coupled beam-squint and wavefront-curvature effect that renders single-frequency covariance models severely biased: the per-subcarrier compressed covariance diverges from the center-frequency model by 64\% at $B = 100$~MHz and by 177\% at $B = 400$~MHz. We derive the wideband compressed-domain Cramér--Rao bound (CRB) for hybrid analog--digital architectures and decompose the Fisher information gain into a dominant data-diversity term that scales as $10\log_{10}K_s$~dB and a secondary geometric-diversity term arising from frequency-dependent curvature. At 28~GHz with $M = 256$ antennas, $N_\mathrm{RF} = 16$ RF chains, and $K_s = 512$ subcarriers, wideband processing yields $+27.8$~dB of CRB improvement at $B = 400$~MHz, of which $+0.7$~dB is attributable to geometric diversity.

翻译：采用近场菲涅尔区的超大规模MIMO（XL-MIMO）阵列的宽带正交频分复用（OFDM）系统受到波束斜视与波前曲率耦合效应的影响，导致单频协方差模型产生严重偏差：当带宽B=100MHz时，每子载波压缩协方差与中心频率模型偏差达64%，当B=400MHz时偏差达177%。本文推导了混合模数架构下的宽带压缩域克拉美-罗界（CRB），并将Fisher信息增益分解为主数据多样性项（以10log₁₀Kₛ(dB)规律增长）与次要几何多样性项（源自频率相关曲率）。在28GHz频段、M=256天线、N_RF=16条射频链路、Kₛ=512个子载波条件下，宽带处理在B=400MHz时实现+27.8dB的CRB改善，其中+0.7dB归因于几何多样性。