FR3 ($\approx$7-24 GHz), also referred to as the upper mid-band, has recently emerged as promising spectrum for 6G; however, its propagation and MIMO characteristics vary significantly with frequency and environment, and spectrum availability may be intermittent due to incumbents. Using site-specific ray tracing (Sionna RT) in representative indoor and outdoor scenarios, we evaluate 7, 10, 14, 20, and 24 GHz under SISO and MIMO configurations. The results show that FR3 exhibits propagation characteristics intermediate between sub-6 GHz and mmWave bands while supporting meaningful spatial multiplexing, albeit with strong site dependence. Motivated by these findings, we propose a fully digital frequency-adaptive multi-band MIMO architecture that repurposes ADCs/DACs and baseband processing resources across FR3 subbands via switching, enabling dynamic trade-offs between bandwidth (spectrum gain) and antenna consolidation (MIMO gain) under availability and channel constraints. Simulation results demonstrate that exploiting additional spectrum is often optimal, while adaptive resource repurposing becomes beneficial when subbands are unavailable or when multiplexing gains are concentrated at specific frequencies.

翻译：FR3频段（约7-24 GHz），亦称中高频段，近年来被视为6G的潜力频谱；然而其传播特性与MIMO特性随频率和环境变化显著，且因现有用户占用可能导致频谱可用性呈现间歇性。通过在典型室内外场景中使用站点专用射线追踪（Sionna RT），我们评估了7、10、14、20和24 GHz在SISO与MIMO配置下的性能。结果表明，FR3展现出介于6 GHz以下频段与毫米波频段之间的传播特性，同时支持有效的空间复用，但其性能具有强烈的场景依赖性。基于这些发现，我们提出了一种全数字频率自适应多频段MIMO架构，该架构通过交换机制在FR3子频段间重新配置ADC/DAC与基带处理资源，从而在频谱可用性与信道约束条件下，实现带宽（频谱增益）与天线整合（MIMO增益）之间的动态权衡。仿真结果表明，利用额外频谱通常是最优选择，而当子频段不可用或复用增益集中于特定频率时，自适应资源重配置则显现其优势。