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

TL;DR

This work addresses the challenge of dynamic spectrum access and frequency-dependent propagation in FR3 (upper mid-band) for 6G. It combines site-specific ray-tracing evaluations across 7–24 GHz in indoor and outdoor scenarios with a novel fully digital frequency-adaptive MIMO architecture that repurposes ADC/DAC and baseband resources across FR3 subbands via switching. The study shows that while exploiting more spectrum generally yields higher throughput, adaptive resource sharing is especially beneficial under partial spectrum availability and when multiplexing gains concentrate at certain subbands; it also demonstrates substantial gains over purely frequency-integrated designs (e.g., $44.401$ and $41.628$ bits/s/Hz in indoor/outdoor cases, respectively). These findings imply that site-specific, frequency-adaptive designs are crucial for efficient FR3 operation and practical 6G deployments, guiding hardware allocation and real-time spectrum management strategies.

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.

Figures (5)

• Figure 1: FR3 subband division and hardware allocation.
• Figure 2: Fully digital multi-band MIMO architectures considered in this work.
• Figure 3: Per-frequency sum spectral efficiency in function of MIMO dimensions.
• Figure 4: Stacked sum SE for the indoor and outdoor scenarios, for increasing frequency-adaptive ADC/DAC allocation optimum.
• Figure 5: Comparison of architectures in terms of design and performance parameters.