6G Spectrum Bands: Sub-THz, Terahertz & Optical (What, why, and how)

Every wireless generation has been defined by its spectrum playground.

• 2G/3G lived mostly in sub-2 GHz.
• 4G pushed aggressively into 2–3 GHz for mobile broadband.
• 5G opened up mmWave (24–100 GHz), introducing gigabit speeds but also coverage challenges.

Now, 6G looks beyond. It aims to explore sub-THz (90–300 GHz), Terahertz (>300 GHz), and optical wireless (VLC, LiFi, FSO) bands, alongside continued use of upper-mid frequencies (6–8 GHz). This layered strategy will enable not just faster connections, but new capabilities like joint sensing & communication (ISAC), holographic telepresence, digital twins, and Tbps-class wireless links.

But higher frequencies mean new physics, new hardware challenges, and new regulatory questions. Let’s break down the spectrum layers that will shape 6G.

Spectrum Landscape for 6G

1. Upper-Mid Bands (6–8 GHz): The Coverage Layer
Why it matters: This band balances coverage and capacity. It extends today’s 3.5 GHz mid-band success, but with wider contiguous spectrum (80–200 MHz blocks).

Regulation:

• At WRC-23, 6425–7125 MHz was identified for IMT in Europe, Middle East, Africa, while 7025–7125 MHz was opened in Asia-Pacific.
• The Americas leaned toward keeping this band for Wi-Fi.
• WRC-27 will study 7.125–8.4 GHz and 14.8–15.35 GHz as additional candidates.
• Use cases: Early 6G macro rollouts, suburban & rural coverage, enterprise indoor deployments.

2. Sub-THz Bands (90–300 GHz): The Capacity Layer
Candidate windows:

• W-band (92–114 GHz)
• D-band (130–175 GHz)
• Higher sub-THz “windows” around 220–260 GHz

Opportunities:

• Multi-GHz channels enable 10–100 Gb/s wireless.
• Short-range hotspots, X-haul (backhaul/fronthaul), and wireless fiber replacement.
• High-resolution sensing and localization thanks to narrow wavelengths.

Challenges:

• Free-Space Path Loss (FSPL): increases 6 dB each time frequency doubles.
• Molecular absorption: O₂ peaks (~60 & 118 GHz) and H₂O (~183 GHz) carve usable “windows.”
• Hardware immaturity: Amplifiers, mixers, and ADCs above 100 GHz are still in research stages.
• Mobility: Pencil beams must track users dynamically, requiring hybrid beamforming and AI-assisted control.

Example applications:

• W-band: Wireless fronthaul for urban small cells.
• D-band: Campus/event backhaul links with 25–100 Gb/s capacity.
• 220+ GHz: Room-scale holographic streaming, XR, industrial cobotics.

Reference: NYU Wireless

3. Terahertz (>300 GHz): The Extreme Frontier

• Spectrum status: ITU-R identified 137 GHz (275–450 GHz) for research and experimental use.
• Potential: Tbps-class wireless, real-time holographic telepresence (raw 3D streams need >4 Tbps), and digital twin synchronization.
• Standardization: IEEE 802.15.3d (2017) defined the first wireless standard near 300 GHz for fixed point-to-point ultra-high-speed links.
• Use cases: Intra-device wireless buses (cable-free laptops/servers), data center wireless reconfiguration, short-distance secure transfers, ultra-high-resolution ISAC (radar + communication).

4. Optical Wireless (VLC, LiFi, FSO): The Secure Indoor Layer
Technologies:

• VLC (Visible Light Communication)
• LiFi (Light Fidelity)
• FSO (Free-Space Optics)

Standards:

• IEEE 802.11bb (2023): Interoperable LiFi standard, up to 9.6 Gb/s.
• ITU-T G.9991: Defines high-speed VLC PHY/MAC.
• IEC 62471: Governs optical radiation eye-safety.

Advantages:

• Huge unlicensed bandwidth.
• Immune to RF interference (ideal for hospitals, defense, aircraft).
• High physical security since light doesn’t pass through walls.

Challenges:

• Requires line-of-sight or reflective paths.
• Mobility and handover are active research topics.
• Use cases: Secure enterprise broadband, hospital wards, airplane cabins, defense command centers.

Physics at Higher Frequencies

FSPL Trend:

• At 100 GHz, FSPL over 100 m ≈ 112 dB; at 300 GHz, it rises ≈ 122 dB.
• Beams must be directional and narrow to compensate.

• The plot follows the Friis Free-Space Path Loss (FSPL) equation, increasing with both frequency and distance.
• Doubling frequency adds about 6 dB extra loss, clearly visible in the upward slope.
• At 100 m (0.1 km), FSPL ranges roughly 72–124 dB across 1–400 GHz.
• At 1 km, FSPL is ~20 dB higher (92–144 dB), as expected (10× distance = +20 dB).
• Sub-6 GHz bands (e.g., 3.5 GHz) have manageable losses, while mmWave/sub-THz (>100 GHz) face very high losses (≥130 dB at 1 km).
• This validates why 6G needs beamforming, antenna arrays, and short-range cells to overcome free-space loss at higher bands.

Atmospheric absorption:

• Oxygen absorption: ~60 & 118 GHz.
• Water vapor absorption: ~183 GHz.
• Windows: ~140, 220, and 340 GHz are relatively “cleaner” for 6G links.

• The plot shows specific atmospheric absorption vs frequency (1–400 GHz).
• Sharp absorption peaks appear at ~60 GHz (O₂), ~118 GHz (O₂), ~183 GHz (H₂O), and ~325 GHz (O₂/H₂O cluster).
• Valleys between peaks (“windows”) such as 92–114 GHz (W-band), 130–175 GHz (D-band), and 220–340 GHz are relatively clearer for communication.
• These windows are prime candidates for 6G sub-THz links (high-capacity X-haul, hotspots, Tbps-class short-range).
• Peaks are avoided for long-range links but can be leveraged for short, secure, interference-limited applications.
• Attenuation intensity varies with humidity, temperature, and rain/fog, making hybrid/multi-band strategies essential in 6G.

Weather sensitivity:

• Rain, fog, and blockages hit both sub-THz and optical links hard.
• Solutions: RIS/IRS to bend beams, site diversity, and hybrid RF-optical fallback.

Layered Spectrum Strategy for 6G

Spectrum Layer	Example Bands	Strengths	Challenges	Use Cases
Coverage	6–8 GHz	Good macro coverage + indoor penetration	Wi-Fi coexistence; regional fragmentation	Early 6G rollout, wide-area
Capacity	W-band (92–114), D-band (130–175)	Multi-Gb/s X-haul, hotspots	Beam alignment, rain fade, hardware maturity	Small cell backhaul, XR, cobotics
Extreme	220–340 GHz	Tbps short-range, ISAC	Severe loss, high absorption, device immaturity	Holography, digital twins, intra-device
Optical	VLC/LiFi/FSO	Secure, interference-free, huge bandwidth	LOS reliance, mobility handover	Hospitals, defense, aircraft, enterprise secure zones

Standardization Roadmap

• IMT-2030 Framework (ITU-R, 2023): Defines 6G performance goals.
• Report M.2541 (ITU-R, 2024): Confirms technical feasibility of IMT above 100 GHz.
• WRC-23 Outcomes: Allocated upper-6 GHz portions for IMT (region-dependent).
• WRC-27 Agenda: Studies for 7–8 GHz and 15 GHz as additional candidates.
• Optical Standards: IEEE 802.11bb and ITU-T G.9991 anchor optical wireless.

Real-World Trends & Examples

• Trials: W-band outdoor trials show parity with today’s E-band backhaul. D-band prototypes already demonstrate 25 Gb/s wireless carriers.
• Optical pilots: Enterprise LiFi pilots (802.11bb-compliant) in offices, healthcare, and defense zones.
• Hardware progress: Sub-THz RFICs (amplifiers, beamformers) are advancing, with co-packaged antenna + chip modules emerging for 6G.
• Policy momentum: After WRC-23, regulators are actively consulting on 7–8 GHz and beyond, shaping early 6G allocations.

6G won’t rely on a single spectrum band. Instead, it will build a layered spectrum ecosystem:

• Upper-Mid (6–8 GHz): Coverage and indoor penetration.
• Sub-THz (92–175 GHz): High-capacity hotspots and X-haul.
• Terahertz (>275 GHz): Tbps ultra-short-range + ISAC.
• Optical (VLC, LiFi, FSO): Secure indoor broadband.

This spectrum layering is the only way to satisfy 6G’s ambitious goals—Tbps speeds, sub-ms latency, integrated sensing, and secure connectivity.

As 6G research advances, the real battlefront will be ecosystem readiness: hardware maturity, policy alignment, and hybrid design strategies. If 5G gave us gigabit smartphones, 6G’s spectrum frontier may well give us immersive holographic worlds, tactile networks, and digital twins in real-time.

Reference

• FSPL: ITU-R P.525-4; 3GPP TR 38.900.
• Atmospheric gases: ITU-R P.676-13.
• Rain attenuation: ITU-R P.838-3.
• Fog/cloud attenuation: ITU-R P.840-8.
• 6G above 100 GHz feasibility: ITU-R Report M.2541 (2024).
• 6G framework: ITU-R M.2160 (2023).
• Optical comms: IEEE 802.11bb (2023); ITU-T G.9991.
• WRC outcomes: WRC-23 resolutions on 6–7 GHz IMT use.
• Books: Rappaport Millimeter Wave Wireless Comms; 6G Mobile Wireless Networks (Springer, 2021).
• NYU Wireless