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The Upper Mid-Band Opportunity

The mmWave bands (FR2, 24–71 GHz) offer enormous bandwidth but suffer from severe propagation challenges: high path loss, blockage sensitivity, and limited outdoor-to-indoor penetration. The sub-6 GHz bands (FR1) provide excellent coverage but are spectrum-starved. The upper mid-band — frequencies in the 7–24 GHz range, now designated FR3 by 3GPP — occupies a sweet spot between these extremes:

Moderate bandwidth: 500 MHz–2 GHz of contiguous spectrum is achievable, compared to $\sim$ 100 MHz at FR1 and $\sim$ 400–800 MHz at FR2.
Reasonable propagation: Path loss exponents are $n \approx 2.0$ – $2.5$ (LOS) and $n \approx 2.8$ – $3.2$ (NLOS), with tolerable outdoor-to-indoor penetration (10–20 dB through standard glass).
Moderate beamforming complexity: Array sizes of 64–256 elements provide 18–24 dBi gain with beamwidths of 5°–15°, balancing directivity with beam management overhead.

The World Radiocommunication Conference 2023 (WRC-23) identified several upper mid-band frequency ranges for International Mobile Telecommunications (IMT), including portions of the 7.125–8.5 GHz and 14.8–15.35 GHz bands, subject to coexistence with incumbent services. This regulatory development has triggered intense interest from operators and vendors planning 6G systems.

Definition:
FR3 Upper Mid-Band

FR3 (Frequency Range 3) is the 3GPP designation for the upper mid-band spectrum spanning approximately 7.125–24 GHz. Key candidate bands under study include:

Band	Frequency range	Bandwidth	WRC-23 status
7–8 GHz	7.125–8.5 GHz	1.375 GHz	Identified for IMT (Region-specific)
10 GHz	10.0–10.5 GHz	500 MHz	Under study
13–15 GHz	13.25–15.35 GHz	2.1 GHz	Partially identified
18 GHz	17.7–19.7 GHz	2 GHz	Shared with satellite

FR3 is positioned to become the capacity layer for 6G networks, providing a balance between the coverage of FR1 and the capacity of FR2. Unlike FR2, FR3 frequencies diffract moderately around obstacles and penetrate standard building glass, enabling macro-cell deployments similar to today's sub-6 GHz networks but with significantly more bandwidth.

The primary technical challenge is coexistence with incumbent services: military and civil radar systems (7–8 GHz), satellite communications (12–18 GHz), fixed wireless backhaul (13–15 GHz), and radio astronomy.

Regulatory Landscape and Coexistence Constraints

The FR3 bands are not greenfield spectrum — they are occupied by incumbent services with established protection requirements. Coexistence studies must demonstrate that IMT systems will not cause harmful interference to these services. Key considerations:

Radar coexistence (7–8 GHz): Military and meteorological radars operate in the 7–8 GHz range. Protection criteria require that the aggregate interference from all IMT base stations at the radar receiver does not exceed $I/N = -6$ dB (interference-to-noise ratio). This translates to protection distances of 30–100 km, depending on terrain, IMT deployment density, and radar characteristics.

Satellite coexistence (12–18 GHz): The Ku-band (12–18 GHz) is extensively used for satellite broadcasting and communications. IMT uplink emissions in adjacent bands can interfere with satellite earth stations. Protection mechanisms include exclusion zones around earth stations (typically 1–5 km radius) and power spectral density limits on IMT transmissions.

Fixed service coexistence (13–15 GHz): Point-to-point microwave links for backhaul operate in the 13–15 GHz band. IMT must coordinate with existing fixed links, potentially requiring dynamic spectrum sharing or geographic exclusion zones.

Practical implications for system design:

Transmit power may be limited to $P_t = 20$ – $30$ dBm EIRP per beam (compared to 43–46 dBm at FR1)
Time-domain blanking may be required when radar pulses are detected
Beamforming toward incumbents must be avoided (spatial nulling)
Database-driven spectrum access (similar to CBRS at 3.5 GHz) is a leading coexistence framework

FR3 Propagation Characteristics

Channel measurements at FR3 frequencies reveal propagation behaviour intermediate between sub-6 GHz and mmWave:

Path loss: The CI model applies with PLEs of:

LOS: $n \approx 2.0$ – $2.2$ (close to free space)
NLOS: $n \approx 2.8$ – $3.3$ (lower than mmWave NLOS)
Shadow fading: $\sigma \approx 4$ – $8$ dB (comparable to sub-6 GHz)

Diffraction: Unlike mmWave, signals at 7–15 GHz retain meaningful diffraction around building corners and over rooftops. The Fresnel zone radius at 10 GHz and 100 m is:

$r_F = \sqrt{\frac{\lambda d}{2}} = \sqrt{\frac{0.03 \times 100}{2}} \approx 1.22\;\text{m}$

This is large enough that building edges and rooftop profiles create significant diffracted components, improving NLOS coverage compared to 28 GHz (where $r_F \approx 0.73$ m at the same distance).

Delay spread and angular spread: RMS delay spreads of 50–200 ns (outdoor urban) and angular spreads of 10°–40° provide sufficient multipath richness for spatial multiplexing while maintaining enough sparsity for effective beamforming.

Outdoor-to-indoor penetration: Standard glass windows attenuate 10 GHz signals by 5–10 dB (versus 30–40 dB for Low-E glass at 28 GHz), and concrete walls add 15–25 dB. This makes outdoor macro-cell coverage of indoor users feasible at FR3.

Example: FR3 System Capacity with Incumbent Protection

Consider an IMT system operating at $f = 12$ GHz with 1 GHz bandwidth. The base station has $N_t = 128$ antennas with 25 dBi beam gain and transmit power limited to $P_t = 23$ dBm per beam due to incumbent protection constraints. The UE has $N_r = 4$ antennas with 6 dBi gain. The noise figure is 5 dB.

(a) Compute the achievable SNR at $d = 200$ m (UMi-LOS, $n = 2.1$ ).

(b) Estimate the per-user throughput assuming 4 MIMO layers.

(c) Compare with an FR2 system at 28 GHz with the same antenna configuration but 400 MHz bandwidth and $P_t = 30$ dBm.

Solution

FR3 path loss

FSPL anchor: $32.4 + 20\log_{10}(12) = 32.4 + 21.6 = 54.0$ dB.

CI path loss: $PL = 54.0 + 10 \times 2.1 \times \log_{10}(200) = 54.0 + 21 \times 2.301 = 54.0 + 48.3 = 102.3$ dB.

FR3 received SNR

Noise power: $N_0 = -174 + 10\log_{10}(10^9) + 5 = -174 + 90 + 5 = -79.0$ dBm.

$\text{SNR} = 23 + 25 + 6 - 102.3 - (-79.0) = 30.7\;\text{dB}$

FR3 per-user throughput

With 4 MIMO layers and Shannon bound:

$R_\text{FR3} = 4 \times 10^9 \times \log_2(1 + 10^{30.7/10}) \approx 4 \times 10^9 \times 10.2 = 40.9\;\text{Gbps}$

With practical coding overhead ( $\sim$ 70% efficiency): $R_\text{FR3}^{\text{prac}} \approx 28.6$ Gbps.

FR2 comparison

At 28 GHz: FSPL anchor $= 32.4 + 28.9 = 61.3$ dB. CI path loss ( $n = 2.0$ ): $PL = 61.3 + 20 \times 2.301 = 107.3$ dB. Noise: $N_0 = -174 + 86 + 5 = -83.0$ dBm.

$\text{SNR}_\text{FR2} = 30 + 25 + 6 - 107.3 - (-83.0) = 36.7\;\text{dB}$

$R_\text{FR2} = 4 \times 400 \times 10^6 \times \log_2(1 + 10^{3.67}) \approx 4 \times 400 \times 12.2 = 19.5\;\text{Gbps}$

The FR3 system achieves $\sim$ 2 $\times$ higher throughput than FR2 despite 7 dBm lower transmit power, primarily due to the 2.5 $\times$ larger bandwidth. $\blacksquare$

FR3 Achievable Rate with Incumbent Protection

Explore how incumbent protection constraints (protection distance, transmit power limits) affect the achievable rate at FR3 frequencies. As the protection distance increases, the allowed EIRP may decrease, reducing the SNR and throughput. Adjust frequency to compare different FR3 candidate bands.

Parameters

Protection distance (km)50

P_\text{tx}

(dBm)23

f

(GHz)12

Quick Check

Which of the following is the primary advantage of FR3 (7–24 GHz) over FR2 (24–71 GHz) for wide-area cellular coverage?

FR3 has more total available bandwidth than FR2

FR3 provides better outdoor-to-indoor penetration, meaningful diffraction, and lower blockage sensitivity, enabling macro-cell coverage similar to sub-6 GHz

FR3 requires fewer antennas for the same beamforming gain

FR3 has no incumbent services and is therefore easier to deploy

Correction:

FR3 provides better outdoor-to-indoor penetration, meaningful diffraction, and lower blockage sensitivity, enabling macro-cell coverage similar to sub-6 GHz

Correct. FR3 frequencies (7–24 GHz) retain meaningful diffraction around obstacles and penetrate standard building glass with moderate loss (5–10 dB), unlike FR2 where Low-E glass causes 30–40 dB loss. This makes FR3 suitable for macro-cell deployments covering indoor users, similar to current sub-6 GHz networks.

FR3 (Frequency Range 3)

The 3GPP designation for upper mid-band spectrum at 7.125–24 GHz, positioned as a capacity layer for 6G with propagation characteristics intermediate between FR1 (sub-6 GHz) and FR2 (24–71 GHz).

Related: Fr1, Fr2, WRC-23, Upper Mid Band

WRC-23

The World Radiocommunication Conference held in 2023, which identified new spectrum bands for IMT including portions of the upper mid-band (7–24 GHz). WRC decisions are binding on ITU member states and shape the global spectrum landscape for the next decade.

FR3 Upper Mid-Band Spectrum