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6G Wireless Communications in 7-24 GHz Band: Opportunities, Techniques, and Challenges

Zhuangzhuang Cui, Peize Zhang, Sofie Pollin

arXiv:2310.06425v2eess.SYcs.SY

TL;DR

FR3 channel behavior and its relation to other frequency ranges remain important open questions, with measurement evidence still limited. This article compares channels using ray tracing and examines FR3 applications and techniques. It reports cross-band channel similarities and higher spectral efficiency at 15 GHz for equal RIS size.

  • Problem

    Comparative evidence on FR3 channel characteristics remains limited, despite the need to understand how its behavior relates to FR1 and FR2.

    Source: §I · p. 1

  • Method

    The article compares 4, 15, and 28 GHz channel characteristics using ray-tracing simulations and evaluates RIS-assisted communication across frequency ranges.

    Source: 1) Potentials · p. 6, §VI · p. 8

  • Results

    Ray tracing indicates FR3 provides more accurate spatial information from sub-6 GHz and more useful direction information for mmWave; at 15 GHz, SE improves by 0.87 and 3.53 bit/s/Hz versus 3.5 and 28 GHz.

    Source: 1) Potentials · p. 6, §VI · p. 8

  • Takeaways & Limitations

    FR3 offers a potential balance across coverage, propagation, and bandwidth, with opportunities in NTN, RIS, and ISAC applications.

    Source: §VI · p. 8

    Measurements in the 7–24 GHz range remain limited, so new measurements are needed for a statistically significant basis for channel models.

    Source: B. Key Aspects of FR3 Channel Models · p. 5

Abstract

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The sixth generation (6G) wireless communication nowadays is seeking a new spectrum to inherit the pros and discard the cons of sub-6 GHz, millimeter-wave (mmWave), and sub-terahertz (THz) bands. To this end, an upper mid-band with a Frequency Range (FR) spanning from 7 GHz to 24 GHz, also known as FR3, has emerged as a focal point in 6G communications. Thus, as an inevitable prerequisite, a comprehensive investigation encompassing spectrum utilization and channel characteristics is the first step to exploiting potential applications and prospects of using FR3 in the 6G ecosystem. In this article, we provide FR3 synergies with emerging technologies including non-terrestrial network (NTN), massive multi-input multi-output (mMIMO), reconfigurable intelligent surface (RIS), and integrated sensing and communications (ISAC). Furthermore, leveraging ray-tracing simulations, our investigation unveils the similarity of channel characteristics in FR3 with other FRs. The analysis of RIS-aided communication shows the insight of higher spectral efficiency achieved in FR3 compared to other FRs when using the same RIS size. Finally, challenges and promising directions are discussed for wireless systems in FR3.

I. INTRODUCTION

FR3, the 7–24 GHz upper mid-band, is attracting interest as 6G seeks spectrum that balances coverage, capacity, and deployment challenges. Despite early regulatory and channel studies, comparative evidence across frequency ranges remains limited.

I. INTRODUCTION

  • FR3 spans 7–24 GHz, and WRC-23 identified 6.425–7.125 GHz and 10–10.5 GHz (with condition) while flagging other ranges for study.The 7.125–8.4 GHz and 14.8–15.35 GHz ranges were potentially identified for study at WRC-27.Source: §I · p. 1
  • About 9 GHz of spectrum is potentially allocated to mobile service on a primary basis across six ranges, while 3GPP Release 19 has initiated FR3 channel-modeling study.The ranges are 7.125–8.4, 10–10.5, 10.7–13.25, 14.8–15.35, 17.7–19.7, and 21.2–23.6 GHz.Source: §I · p. 1
  • Comparative channel study is needed because FR3 lies between FR1 and FR2, while measurements in the 7–24 GHz range remain limited.Prior work includes ray-tracing studies of coverage and penetration loss, and satellite-to-ground measurements at 11 and 14 GHz.Source: §I · p. 1
  • FR3 is presented as a coverage compromise, with 7–14 GHz providing 260–375 m urban coverage from rooftop to indoor, compared with 430 m at 3.5 GHz and 190 m at 28 GHz.The cited study also reports continuous outdoor-to-indoor urban macro connectivity as difficult to achieve in mmWave bands.Source: §I · p. 1
  • FR3’s intermediate frequency offers opportunities for large MIMO arrays and RIS while retaining multipath opportunities.The passage frames this as a source of new research opportunities.Source: §I · p. 1

24 GHz

FR3 opportunities include practical multi-antenna systems and a range of emerging 6G applications. The article outlines these opportunities, then examines propagation, techniques, and implementation challenges.

7 GHz

  • Figure 1 is a spectrum overview for mobile networks.Source: Figure 1 · p. 2
  • FR3’s 15–24 GHz wavelengths of 1.99–1.25 cm allow more antenna elements in space-limited satellite and UAV nodes than sub-6 GHz wavelengths of at least 5 cm.Existing Ku-band phase-shifter work and radar deployments in X, Ku, and K bands provide hardware legacies for multi-antenna systems and ISAC.Source: §7 · p. 2

II. FR3 OPPORTUNITIES

  • The article studies FR3 opportunities, compares radio propagation across frequency ranges using ray tracing, surveys channel parameters, and discusses emerging techniques and implementation challenges.The stated techniques section follows the propagation study.Source: §7 · p. 2

A. Radio Spectrum Overview

The section frames FR3 as an intermediate band for flexible 6G spectrum use. Its balance of coverage and bandwidth supports deployments between the more specialized roles of lower and higher bands.

A. Radio Spectrum Overview

  • FR3 can balance coverage and throughput in multi-band, multi-scenario 6G networks, with medium coverage and more flexible throughput than other frequency ranges.The section describes spectrum scheduling across diverse bands, scenarios, and applications.Source: A. Radio Spectrum Overview · p. 2

B. Advantages of FR3 Band

A. Radio Spectrum Overview

  • Table I is presented as a comparison among different frequency ranges.Source: Table I · p. 3

1) Enhancing 3D Coverage:

  • FR3’s intermediate wavelength supports massive antennas and 3D beamforming for vertical users, while its balance of fading and bandwidth supports continuous outdoor-to-indoor coverage.The 7.125–8.4 GHz band has been identified by the FCC for study use.Source: 1) Enhancing 3D Coverage · p. 3

2) Enabling Flexibility:

FR3 is discussed as a potential middle ground for flexible massive-MIMO deployment and multi-band sensing. The section contrasts its propagation and hardware properties with challenges associated with FR1 and FR2.

2) Enabling Flexibility:

  • FR1’s large wavelengths make extremely large arrays physically larger, while FR2’s sparse scattering and digital beamforming complexity hinder their deployment.The passage also identifies wind loading and weight as FR1 deployment challenges.Source: 2) Enabling Flexibility · p. 3
  • FR3’s centimeter wavelength permits more antenna elements than FR1, while favorable channel conditions compared with mmWave improve channel rank and condition number for spatial multiplexing.The passage also associates larger arrays with more pronounced channel hardening and low-cost linear MIMO processing.Source: 2) Enabling Flexibility · p. 3

3) Empowering Sensing:

  • Figure 2 depicts use cases of FR3 and its coexistence with other frequency ranges in 6G networks.Source: Figure 2 · p. 3
  • FR3 could simplify multi-band sensing because it avoids the extreme frequency-dependent Doppler effects and very different propagation conditions of FR1–FR2 combinations.Large antenna counts, bandwidth, and decent coverage are described as contributing to angle, range, and detection performance.Source: 3) Empowering Sensing · p. 3

C. Use Cases

FR3 is presented as a flexible option for terrestrial, aerial, and space links, balancing coverage and capacity across many use cases. The paper then turns to the radio-channel characterization needed to understand FR3 deployment.

C. Use Cases

  • Space-air-ground applications include satellite links and UAV communications; FR3 is described as addressing FR2 blockage effects for drones and supporting continuous, vertical coverage.Inter-satellite and satellite-ground links are cited at 22.2–23.5 GHz and 10.7–12.7 GHz, respectively.Source: C. Use Cases · p. 3
  • FR3 radar deployments make integrated sensing and communications promising; multi-antenna systems and satellites offer spatial diversity and ubiquitous connectivity for crowd, environment, and remote sensing.Source: C. Use Cases · p. 3
  • FR3 suits medium-coverage urban, factory, and outdoor-to-indoor scenarios, while FR1 is associated with rural or remote use and FR2 with backhaul, vehicular, and hotspot scenarios.The section describes FR3 as spanning a broader subset of use scenarios than either FR1 or FR2.Source: C. Use Cases · p. 4
  • With a 100 m UAV base station, the modeled coverage-capacity balance is strongest from 4.6–18.1 GHz, placing most of FR3 in a range with decent performance on both metrics.The comparison assumes a clean channel, equal transmit power, and bandwidths corresponding to frequency; below 4.6 GHz systems should focus more on coverage and above 18.1 GHz more on capacity.Source: C. Use Cases · p. 4, Figure 3 · p. 4

III. RADIO CHANNEL CHARACTERIZATION AND MODELING IN FR3

  • FR3 channel behavior must be characterized because its more complicated transmission schemes may make observed channels differ from well-explored 5G FR1 and FR2 channels.Source: §III · p. 4

A. Statistical Comparison of Beamforming across Bands

The paper compares spatial-channel similarity across FR1, FR3, and FR2, then identifies measurement and modeling needs for FR3 channels. The comparison finds greater similarity for adjacent frequency pairs than for FR1–FR2.

A. Statistical Comparison of Beamforming across Bands

  • PSP measures total-variation distance between normalized beamformed PAS distributions, while R compares high-frequency beamformed power at desired directions using high- versus lower-frequency channel scans.A low PSP alone does not establish that less useful spatial-channel information transfers across frequencies.Source: A. Statistical Comparison of… · p. 4
  • FR1–FR3 and FR3–FR2 channels show higher similarity than FR1–FR2, and FR3 can use more accurate spatial information from sub-6 GHz to provide useful direction information for mmWave.Ray tracing compares 4, 15, and 28 GHz using over 2,000 airport check-in-area links; Fig. 4 presents CDFs of PSP and power ratio R.Source: A. Statistical Comparison of… · pp. 4–5, Figure 4 · p. 5
  • The comparison uses 4-, 16-, and 28-element half-wavelength-spaced ULAs at 4, 15, and 28 GHz to keep antenna apertures approximately equal; R is at most 0 dB.Source: A. Statistical Comparison of… · p. 5

B. Key Aspects of FR3 Channel Models

  • FR3 channel models need statistically significant 7–24 GHz measurements because existing model parameters mainly come from FR1 and FR2; larger arrays also require near-field effects and additional model components.The passage calls for updating cluster and multipath-component statistics and extending the existing GSCM framework.Source: B. Key Aspects of FR3 Channel Models · p. 5
  • A generalized upper-mid-band model should combine terrestrial and non-terrestrial channels, accounting for elevation-related propagation differences and atmospheric effects in satellite or HIBS links.At 12 and 24 GHz, an additional height-dependent parameter is described as essential for extra diffraction and scattering loss; atmospheric absorption, rain and cloud attenuation, scintillation, and Faraday rotation also matter.Source: B. Key Aspects of FR3 Channel Models · p. 5
  • Existing stochastic models cannot directly generate ISAC channels and environments; the proposed direction is hybrid modeling with mapped target-scattering properties and extensive site-specific measurements and electromagnetic simulations.Source: B. Key Aspects of FR3 Channel Models · p. 5

IV. KEY TECHNIQUES IN FR3

FR3 is considered alongside key 6G techniques, with non-terrestrial networks as a focus because their nodes are size-limited. The section outlines potential FR3 roles in UAV, HIBS, and satellite links.

A. NTN

  • NTN nodes such as UAVs, HIBS, and LEO satellites are size-limited compared with terrestrial stations, shaping FR3 implementation needs.Source: A. NTN · p. 5

1) Potentials:

  • For low-altitude UAVs, FR1 and FR2 beamforming constraints hinder reliable ground-cellular connectivity, while hybrid-beamforming FR3 could support 3D coverage in integrated terrestrial and non-terrestrial networks.Source: 1) Potentials · p. 6
  • FR3 arrays are presented as a size-conscious option for HIBS at 20–50 km and LEO satellites at 300–2,000 km, whose high-altitude links are generally near line of sight.The passage contrasts FR3 with frequencies considered too high for those altitudes and sub-6 GHz arrays constrained by node size and payload.Source: 1) Potentials · p. 6

2) Focuses:

The section identifies unresolved long-range NTN channel and interference issues, then compares RIS-aided efficiency across frequency bands. In the fixed indoor setup, 15 GHz has the highest reported spectral efficiency among the three frequencies.

2) Focuses:

  • Long-range NTN deployment still requires characterizing cumulative atmospheric and rain attenuation, complex multipath consistency across FRs, and interference from clean-environment and multi-beam transmission.Source: 2) Focuses · p. 6

1) Potentials:

  • The RIS case study fixes indoor transmitter, RIS, and receiver locations and compares equal-size arrays across frequencies; a 10 × 10 array at 15 GHz occupies 20 × 20 cm.In the same area, the array has 2 × 2 elements at 3.5 GHz and 18 × 18 at 28 GHz.Source: 1) Potentials · p. 6
  • SE at 15 GHz improves by 0.87 bit/s/Hz over 3.5 GHz and 3.53 bit/s/Hz over 28 GHz; EE is worse than 3.5 GHz but better than 28 GHz for 20 users at 10 dBm.The comparison uses 4, 100, and 324 RIS elements at 3.5, 15, and 28 GHz, respectively; EE is close to 3.5 GHz in the high-SNR regime.Source: Figure 5 · p. 6, 1) Potentials · p. 6

2) Focuses:

FR3’s intermediate position supports multi-band ISAC while presenting hardware and channel-modeling issues for further study. Aggregating frequency ranges can improve delay resolution, but frequency dependence constrains which ranges should be combined.

2) Focuses:

  • FR3 hardware work should model phase-shifter power consumption, near-field sensing benefits, communication-side coupling, and hybrid MIMO trade-offs.Hybrid MIMO balances LoS phased-array beamforming with digital multi-stream processing that exploits multipath.Source: 2) Focuses · p. 6

1) Potentials:

  • Aggregating intra-FR bands creates contiguous multi-band schemes, while aggregating inter-FR bands creates non-contiguous schemes to address limited bandwidth and ADC constraints.For coexistence between 5G NR and future 6G NR, non-contiguous aggregation provides a wider band aperture and higher delay resolution.Source: 1) Potentials · p. 7
  • PSP and R are lower for fL = 4 GHz and fH = 28 GHz than for FR3-based comparisons, indicating greater frequency dependence between 4 and 28 GHz.The paper therefore identifies FR3 as an important bridging band for aggregating FR1 or FR2, rather than aggregating FR1 and FR2 directly.Source: 1) Potentials · p. 7

2) Focuses:

FR3 channel and ISAC implementation require models and design choices that account for diverse propagation conditions, frequency dependence, hardware imperfections, and competing system objectives.

2) Focuses:

  • Multi-band systems need to verify frequency independence and clarify FR3 delay, Doppler, and power-domain multipath, including spatial non-stationarity with extremely large arrays.FR3 and ISAC channel measurements and models are identified as a Release 19 study item for incorporation into the existing 3GPP framework.Source: 2) Focuses · p. 7
  • FR3 ISAC needs optimal trade-offs among arrays, bandwidths, and frames, with hardware imperfections compensated through deployment schemes such as multi-static ones.Energy efficiency must reconcile radar’s preference for more power to increase detection range with communication’s preference for transmitting more bits using less power.Source: 2) Focuses · p. 7

A. Wireless Channel Modeling

  • An integrated FR3 channel model must cover terrestrial and non-terrestrial environments, varied mobility, and frequency dependence; RIS models must include environmental scattering and controlled reflections.A unified multi-band ISAC model is especially challenging because sensing localizes scatterers deterministically while communication uses stochastic channel descriptions such as Rayleigh and Rician distributions.Source: A. Wireless Channel Modeling · p. 7

B. Spectrum Coexistence and Aggregation

FR3’s promise spans spectrum coexistence, adaptive architecture, and application-specific implementation, while the paper reports channel and RIS comparisons and identifies deployment challenges.

B. Spectrum Coexistence and Aggregation

  • FR3 spectrum aggregation can enable high-resolution sensing, but spectrum management must evaluate coverage, throughput, efficiency, implementation cost, and the balance between sensing and communication.A multidimensional management approach should monitor whether end users make the most value from assigned spectrum.Source: B. Spectrum Coexistence and Aggregation · p. 7

C. Adaptive Architecture Design

  • FR3 transceivers need suitable converters, wider-bandwidth ADCs, and power amplifiers with linear output, while hybrid architectures must balance optimization-based and learning-based designs.Hybrid beamforming is presented as a way to balance spatial multiplexing in FR1 with lower beamformer complexity and power consumption in FR2.Source: C. Adaptive Architecture Design · p. 7

D. Realistic Applications Implementation

  • FR3’s lower and higher sub-bands support broad and directional communications, respectively, making application-specific programmable implementations promising but challenging.The paper calls for further categorization of FR3 applications and their specific requirements.Source: D. Realistic Applications Implementation · p. 8

VI. CONCLUSION

  • Ray-tracing simulations show FR3 can obtain more accurate spatial channel information than sub-6 GHz and more useful direction information than mmWave; at 15 GHz, RIS spectral efficiency improves by 0.87 and 3.53 bit/s/Hz versus 3.5 GHz and 28 GHz, respectively.The RIS comparison uses the same RIS size; the conclusion also identifies channel modeling, spectrum management, hardware compatibility, and on-demand implementation as future challenges.Source: §VI · p. 8

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