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Cellular Wireless Networks in the Upper Mid-Band

Seongjoon Kang, Marco Mezzavilla, Sundeep Rangan, Arjuna Madanayake, Satheesh Bojja Venkatakrishnan, Gregory Hellbourg, Monisha Ghosh, Hamed Rahmani, Aditya Dhananjay

arXiv:2309.03038v6cs.NIeess.SP

TL;DR

The paper examines how cellular systems could use the 7–24 GHz upper mid-band despite incumbent sharing and wideband design challenges. Through multi-band urban simulations, satellite-interference analysis, and compact antenna design, it finds benefits from adaptive frequency selection, identifies potentially substantial interference, and presents an FR3-wide antenna approach.

  • Problem

    The paper addresses how to realize cellular services in the upper mid-band, where incumbent sharing, wide bandwidth, directional transmission, and antenna form-factor constraints require new system designs.

  • Method

    The study combines dense-urban ray-tracing simulations, cellular-satellite interference analysis, interference-nulling evaluation, and compact multi-band antenna-array design.

  • Results

    Adaptive frequency selection improves coverage and data rates, terrestrial interference can be significant, directional nulling can reduce it with some terrestrial capacity loss, and the presented antenna covers the FR3 bands.

  • Takeaways & Limitations

    Wideband FR3 systems can combine lower-frequency coverage and penetration with higher-frequency bandwidth and directionality through dynamic frequency selection.

Abstract

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The upper mid-band - roughly from 7 to 24 GHz - has attracted considerable recent interest for new cellular services. This frequency range has vastly more spectrum than the highly congested bands below 7 GHz while offering more favorable propagation and coverage than the millimeter wave (mmWave) frequencies. The upper mid-band can thus provide a powerful and complementary frequency range to balance coverage and capacity. Realizing the full potential of these bands, however, will require fundamental changes to the design of cellular systems. Most importantly, spectrum will likely need to be shared with incumbents including communication satellites, military RADAR, and radio astronomy. Also, the upper mid-band is simply a vast frequency range. Due to this wide bandwidth, combined with the directional nature of transmission and intermittent occupancy of incumbents, cellular systems will need to be agile to sense and intelligently use large spatial and frequency degrees of freedom. This paper attempts to provide an initial assessment of the feasibility and potential gains of wideband cellular systems operating in the upper mid-band. The study includes: (1) a system study to assess potential gains of multi-band systems in a representative dense urban environment and illustrate the value of wide band system with dynamic frequency selectivity; (2) an evaluation of potential cross interference between satellites and terrestrial cellular services and interference nulling to reduce that interference; and (3) design and evaluation of a compact multi-band antenna array structure. Leveraging these preliminary results, we identify potential future research directions to realize next-generation systems in these frequencies.

I. Introduction

The upper mid-band is presented as a potential cellular range that balances coverage and bandwidth, addressing sub-6 GHz scarcity and mmWave coverage limitations. Realizing its benefits requires adaptive multi-band operation, spectrum sharing, and new sensing and antenna technologies.

  • MmWave systems provide wide bandwidth and high peak rates, but practical coverage is intermittent and penetration indoors is limited by blockage.
  • The 7–24 GHz upper mid-band offers more bandwidth than sub-6 GHz while providing more favorable propagation and penetration than mmWave.
  • Upper mid-band cellular systems must share spectrum with satellite, radar, and radio astronomy incumbents while sensing and adapting across large spatial and frequency dimensions.
  • The paper assesses multi-band coverage and capacity, terrestrial interference with satellite uplinks, and compact wideband antenna designs.

II. Background and Standardization Landscape

Upper mid-band interest follows mmWave’s high capacity but intermittent coverage and mid-band’s practical success. Its development is accompanied by expanding standardization activity and unresolved spectrum-sharing challenges with incumbent services.

  • Background: Mid-band deployments have supported private 5G networks and spectrum expansion in wide-area public networks.Relevant allocations include CBRS at 3.55–3.7 GHz and C-band at 3.7–4.2 GHz.
  • Background: Commercial mmWave networks can deliver high throughput, but outdoor coverage is intermittent and indoor penetration is limited by common obstacles.Ookla reported 1.6 Gbps median downlink throughput in tested mmWave networks, up to seven times higher than C-band systems.
  • Standardization Landscape: The upper mid-band is being considered for cellular use because it may combine data rates near mmWave levels with more uniform coverage.Industry groups have identified these bands, and 3GPP has formally begun studying services from 7 to 24 GHz.
  • Standardization Landscape: Standardization and regulatory activity includes FCC consideration of 12.2–12.7 GHz and 12.7–13.2 GHz bands, although the first band was rejected for cellular services.The FCC TAC surveyed spectrum from 7.125 to 24 GHz and published a related 6G working paper.
  • Spectrum Sharing: Incumbent protection, especially for commercial satellite services, is a central allocation issue because cellular use can create interference with ground satellite units.Spectrum-sharing mechanisms such as SAS, LSA, and more dynamic approaches are being considered for the upper mid-band.
  • Spectrum Sharing: Allocating and sharing this vast frequency range among users is a fundamental future design and policy challenge.The upper mid-band’s value spans numerous services, increasing the importance of sharing arrangements.

III. Coverage and Capacity Gains

Upper-mid-band frequencies trade coverage against capacity: lower frequencies provide more favorable propagation, while higher frequencies offer greater bandwidth and peak rates. Simulations indicate that adaptive multi-band selection can combine these benefits and improve robustness to blockage.

  • III. Coverage and Capacity Gains: Channel studies find that lower upper-mid-band frequencies generally provide more favorable coverage, while higher frequencies offer better bandwidth.Propagation factors vary considerably across the band, and bandwidth allocations generally scale with carrier frequency.
  • III. Coverage and Capacity Gains: Coverage is greatly reduced as frequency increases in the dense-urban ray-tracing scenario.The study models four candidate frequencies—6, 12, 18, and 24 GHz—with rooftop base stations and frequency-scaled antenna apertures.
  • III. Coverage and Capacity Gains: Higher frequencies provide superior data rates despite lower SNRs, with maximum rates of 0.48, 0.96, 1.44, and 1.92 Gbps at 6, 12, 18, and 24 GHz.Cell-edge users experience significantly worse rates at 18 and 24 GHz than at 6 and 12 GHz.
  • III. Coverage and Capacity Gains: Outdoor-to-indoor results indicate that lower frequencies can provide indoor coverage behind concrete, while higher frequencies can offer opportunistic capacity.Standard glass and wood are relatively permeable, whereas infrared-reflecting glass has more than 20 dB loss across RF frequencies.
  • III. Coverage and Capacity Gains: With blockage, 18 and 24 GHz users show significantly worse performance approximately 35% of the time than users at 6 and 12 GHz.The blockage scenarios include no blockage and 3GPP Blockage Model B with K = 4 random human blockers.
  • III. Coverage and Capacity Gains: The best-choice policy achieves high-frequency peak rates together with the improved cell-edge rates of low frequencies.It selects the best base station and frequency for each user, motivating adaptive wideband systems.

C. Penetration Loss

The upper mid-band’s penetration loss depends strongly on exterior building material, with concrete becoming especially restrictive in this frequency range. Indoor-user performance is therefore sensitive to material-specific propagation loss.

  • Model: The 3GPP O2I model expresses path loss using material-dependent constants and frequency.L is path loss in dB, f is frequency in GHz, and a and b are linear constants tied to the carrier frequency.
  • Material effects: Standard glass and wood remain relatively permeable across the modeled range, while infrared-reflecting glass remains relatively impermeable.The reported losses are below 8 dB for standard glass and wood, and above 20 dB for IRR glass.
  • Indoor impact: Indoor-user SNR and rate distributions are reduced at higher frequencies when concrete dominates exterior walls.The simulation attributes this reduction to the higher penetration loss of concrete.
  • Material effects: Concrete undergoes a sharp transition from relatively permeable to impenetrable precisely in the upper mid-band.

D. Multi-frequency Indoor Capacity Gains

Multi-frequency systems can exploit complementary propagation and directional-interference properties across the upper mid-band. Lower frequencies favor penetration and indoor coverage, while higher frequencies can improve interference-limited outdoor rates, subject to blockage.

  • Indoor capacity: Roughly 65% of indoor users achieve higher data rates at lower frequencies than at higher frequencies.The indoor simulation attributes this pattern to increased penetration loss at higher frequencies, especially through concrete.
  • Indoor capacity: Lower frequencies provide valuable indoor coverage behind concrete, while higher frequencies can opportunistically serve outdoor and lightly blocked indoor users.
  • Interference-limited capacity: Higher frequencies experience less interference because directional transmissions produce narrower beamwidths.The comparison concerns full-buffer simultaneous transmissions from 18 base stations to associated outdoor users.
  • Interference-limited capacity: In the no-blocking interference simulation, higher-frequency data rates are almost uniformly better than lower-frequency rates.The authors state that adding blocking would degrade high-frequency performance, so the result is not uniformly expected under blockage.
  • Adaptive selection: Adaptive systems can select bands using path loss, penetration, and directionality rather than relying on one fixed frequency.
  • Satellite coexistence: Terrestrial transmissions can produce substantial satellite-uplink interference, with INR ≥−6 dB for approximately 27% of downlink transmissions at 6 GHz.At this threshold, the satellite SNR degrades by more than 1 dB; terrestrial uplink transmissions exceed −6 dB about 25% of the time at 6 GHz.
  • Satellite coexistence: Satellite interference decreases at higher frequencies, making frequency adaptation potentially useful for protecting satellite services.

C. Reducing Satellite Interference with Nulling

The paper uses regularized transmit beamforming to reduce terrestrial interference at satellites while limiting degradation on the terrestrial link. With sufficiently large regularization, downlink interference is strongly mitigated with minimal terrestrial SNR loss, whereas uplink protection incurs greater degradation.

  • Regularized transmit beamforming penalizes interference toward the satellite while retaining the receiver’s beamforming choice.The regularization term encourages a transmit null along the satellite channel.
  • At λ = 10^8, INR ≥ −6 dB occurs less than 3% of the time under ideal downlink tracking.The evaluation uses the 6 GHz carrier, where interference is higher in the lower band.
  • At λ = 10^8, terrestrial downlink SNR degradation remains below 0.1 dB for 97% of the time.Degradation is defined relative to the optimal terrestrial beamforming gain.
  • For terrestrial uplink transmission, SNR degradation is below 3 dB for 78% of the time at λ = 10^7.The higher cost is attributed to the UE’s smaller antenna count, which makes directional nulling more costly.

D. Tracking Interference Channels to Satellites

The tracking scheme uses satellite ephemeris and antenna-orientation estimates to approximate satellite interference channels, then designs nulls robust to angular uncertainty. Simulations indicate limited sensitivity to angular errors, while the approach relies on LOS-dominated rural channels and negligible time-related errors.

  • Ephemeris data can provide satellite azimuth and zenith angles, avoiding full wideband satellite-channel tracking for the nulling procedure.The transmitter still estimates the relevant channel information across frequencies, while ephemeris-based tracking supplies satellite geometry.
  • The practical channel estimate includes only the LOS interference component because the rural scenario is assumed to be LOS-dominated.The paper states that this assumption is verified in simulation.
  • Orientation calibration uses a high-precision compass for gNBs and magnetometer–IMU systems with filtering for UEs, introducing angular error.The procedure is modified to account for these measurement uncertainties.
  • The robust beamformer averages the interference penalty over channel vectors induced by measured angular errors, creating nulls over angular regions rather than at one estimated angle.This expectation-based regularization modifies the original interference-null formulation.
  • Satellite-location variation within a sub-millisecond 5G TTI and synchronization errors are treated as negligible.The paper cites network synchronization protocols such as PTP in this assessment.
  • With angular errors, downlink and uplink INR performance changes little; UE errors are larger, but their fewer antennas produce less narrow nulls.The downlink curve is described as almost identical, and the uplink case shows no significant INR change.

E. Radio Astronomy

Radio astronomy depends on detecting extremely weak, wideband and spectrally informative emissions with sensitive telescopes and long integrations. Radio-frequency interference can corrupt time–frequency data and reduce observing sensitivity, motivating remote locations and coordination with active services.

  • Astronomical emissions include persistent continuum radiation and narrow spectral lines that reveal source composition, structure, density, temperature, ionization, and fluid dynamics.Spectral-line shifts also provide additional information about astronomical sources.
  • A jansky equals 10^-26 Wm^-2Hz^-1, and a 1 Jy signal at 6 GHz would be received at −267 dBm with an isotropic antenna.The example places the signal more than 90 dB below the noise floor.
  • Detecting such weak emissions requires large collecting areas, low-temperature receivers, wide bandwidths, and integrations lasting seconds to hours.These requirements support high sensitivity in radio telescopes.
  • Radio-frequency interference can corrupt integrated time–frequency resource blocks, which are discarded before astronomical information extraction.The resulting data loss reduces observational sensitivity.
  • Remote observatory locations reduce interference from terrestrial transmitters through propagation loss, while coordination can prevent future nearby deployments.The National Radio Quiet Zone is given as an example of coordinated protection.

V. Wideband Antennas for the Upper Mid-Band

Upper-mid-band antennas must combine wide bandwidth, tunability, reconfigurability, and many elements for directional transmission. The proposed approach combines wideband tunable elements to address performance trade-offs and improve operation across dynamically selected spectrum.

  • Challenges: Upper-mid-band transceivers must support wide bandwidths, high tunability, and many antenna elements, but face size–bandwidth–gain, loss, scanning-SNR, interference, and spectrum-management bottlenecks.These constraints complicate practical wideband directional front-end design.
  • Approach: The proposed architecture combines tunable and reconfigurable wideband elements to mitigate interference and improve SNR across dynamically large upper-mid-band spectrum swaths.The design is intended to address the limitations of existing antenna and RF technologies.
  • Challenges: A single antenna element cannot maintain favorable sensitivity and directional gain across the full band without frequency-dependent trade-offs.The paper therefore calls for innovations in antenna and microwave-circuit design.

B. Compact Wideband Aperture in Aperture Antenna Arrays

The paper presents a compact aperture-in-aperture array using separate antenna designs for the lower, middle, and upper FR3 bands. Simulations evaluate its dimensions, isolation, radiation, VSWR, gain, and efficiency across the wideband.

  • Design attributes: The design is described as scalable, compact, wideband, low-cost, and conformal, with port isolation and radiation patterns evaluated for the tri-band unit cell.Figures report port assignments, isolation, and center-frequency radiation patterns for the three bands.
  • Array architecture: Separate apertures improve beam scanning across the bands compared with a single wideband aperture limited by finite electrical array size.The paper attributes the limitation particularly to lower-frequency operation in finite arrays.
  • Array architecture: The AiA array combines tightly coupled dipoles for 6–12 GHz, a circular monopole for 12–18 GHz, and patches for 18–24 GHz.The three co-located designs provide contiguous coverage across the upper mid-band.
  • Geometry and simulation: The simulated 8-element structure uses 9 mm element spacing and measures 74 mm × 25 mm.The unit-cell simulations use three ports, one for each antenna type.
  • Simulated performance: All three antennas achieve simulated VSWR < 3, with unit-cell gain from −2 to 4 dB across FR3 and average efficiency of 82%.Efficiency reaches a minimum of 67% at the upper band edges.

VI. Open Research Problems

The paper identifies open problems in channel characterization and in validating wideband adaptive-system gains. Key needs are directional measurements, blockage studies, and broader data-driven models beyond a single outdoor urban setting.

  • Channel measurements: Most upper-mid-band measurement campaigns captured omnidirectional path loss, leaving phased-array studies of spatial channel structure needed for MIMO and beamforming models.Blockage dynamics across the band also require investigation.
  • Capacity analyses: The capacity study was limited to one urban area and outdoor users, so statistical models and more measurement-validated data are needed to assess generality.The paper specifically identifies extensions for large-scale statistical dependencies across multiple bands.
  • Capacity analyses: Existing models such as those used by 3GPP do not adequately represent large-scale statistical dependencies between multiple bands.The paper suggests data-driven techniques as one possible research direction.

B. Interference with incumbents

The paper examines upper-mid-band cellular coexistence with incumbents and presents preliminary results on adaptive capacity, satellite interference, and compact antenna design. It concludes that dynamic frequency selection and interference mitigation are promising but require further development.

  • Coexistence: Further coexistence work must address tracking, including NLOS ground-reflection components, and protocols that avoid transmissions whose interference cannot be mitigated.The paper also identifies radio astronomy and other passive sources as requiring additional study.
  • Security and resiliency: The paper identifies security as an unaddressed issue, while spectrally agile systems could support resiliency through sensing and frequency hopping.The authors also note that terrestrial signals can significantly impact satellite services, motivating attack detection and mitigation.
  • Capacity gains: Wideband FR3 systems can improve capacity by assigning lower frequencies to cell-edge and indoor users while using higher frequencies for bandwidth and directional isolation.The paper concludes that dynamic frequency selection across the upper mid-band is valuable.
  • Coexistence: Terrestrial cellular interference may substantially degrade satellite-network performance, motivating interference nulling to reduce that interference.The paper presents nulling as a proposed mitigation for cellular-satellite coexistence.
  • Antenna design: The compact AiA antenna provides contiguous coverage across FR3 using a coupled dipole array, circular monopole, and UWB patches.The current design still requires practical feeding, packaging, coupling compensation, and wideband RF-circuit and switch design.
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