Frequency-Adaptive Multi-Band Architecture for Upper Mid-Band MIMO Systems

TL;DR

This paper evaluates the propagation and MIMO characteristics of the upper mid-band spectrum (7-24 GHz) for 6G, and proposes a frequency-adaptive multi-band MIMO architecture to optimize spectrum use and spatial multiplexing.

Contribution

It introduces a novel fully digital, frequency-adaptive multi-band MIMO architecture that dynamically reuses resources across subbands based on availability and channel conditions.

Findings

FR3 exhibits intermediate propagation characteristics supporting spatial multiplexing.

Adaptive resource switching improves spectrum utilization and multiplexing gains.

Exploiting additional spectrum often yields optimal performance.

Abstract

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.