Wideband orthogonal frequency-division multiplexing (OFDM) over near-field extremely large-scale MIMO (XL-MIMO) arrays introduces a coupled beam-squint and wavefront-curvature effect that renders single-frequency compressed covariance models severely biased. To the best of our knowledge, no compressed-domain Cramer-Rao bound (CRB) has been reported for this regime under hybrid analog-digital architectures; existing wideband near-field bounds assume full-array observation. We derive the wideband compressed-domain CRB and decompose its Fisher information gain into a dominant data-diversity term scaling as 10 log10(Ks) dB, where Ks denotes the number of independent subcarrier observations, and a secondary geometric-diversity term from frequency-dependent Fresnel curvature. At 28 GHz with bandwidth B = 400 MHz, the total CRB improvement reaches +27.8 dB, comprising +27.1 dB from data diversity and +0.7 dB from geometric diversity; hybrid compression contributes an additional 12.6 dB gap relative to the full-array bound at N_RF = 16 RF chains. Frequency-aware covariance modeling is the dominant requirement; geometric diversity is a secondary but growing benefit as fractional bandwidth approaches 6G regimes.

翻译：近场超大规模MIMO（XL-MIMO）阵列上的宽带正交频分复用（OFDM）会引入耦合的波束斜视与波前曲率效应，使得单频压缩协方差模型严重有偏。据我们所知，现有研究尚未在混合模数架构下针对该场景推导出压缩域克拉美-罗下界（CRB），且已有的宽带近场下界均假设全阵列观测。本文推导了宽带压缩域CRB，并将其Fisher信息增益分解为两项：主导项为数据多样性项，其增益按10 log10(Ks) dB尺度增长（其中Ks表示独立子载波观测数目）；次项为几何多样性项，源于频率相关的菲涅尔曲率。在28 GHz、带宽B=400 MHz条件下，总CRB改善量达+27.8 dB，其中数据多样性贡献+27.1 dB，几何多样性贡献+0.7 dB；与配置N_RF=16条射频链路时的全阵列界相比，混合压缩进一步引入12.6 dB的额外差距。频率感知协方差建模是核心需求；几何多样性虽为次要因素，但随着分数带宽逼近6G体制，其增益持续增长。