Sub-THz vs THz: The Frequency Gap Between 6G and 7G

The Spectrum Landscape: Where 6G Stops and 7G Starts

Every wireless generation has been defined by its spectrum. 4G LTE operated below 6 GHz. 5G extended into millimeter-wave (24–71 GHz). 6G, as defined by the ITU-R IMT-2030 framework published in 2024, targets the sub-terahertz band — specifically the 92–300 GHz range — as its high-capacity frontier.

But above 300 GHz lies a different world. The terahertz band (300 GHz to 3 THz) has been called the "last spectrum frontier" — a massive swath of electromagnetic real estate that remains largely unused for communications. This is the territory that 7G research is targeting for the late 2030s and beyond.

The division at 300 GHz is not arbitrary. It represents a convergence of physical, technological, and regulatory boundaries that make sub-THz and full THz fundamentally different engineering challenges.

Propagation Physics: The Atmosphere as Gatekeeper

The single most important difference between sub-THz and full THz is how the atmosphere treats them. According to ITU-R Recommendation P.676-13, atmospheric attenuation in the 100–300 GHz range is manageable — typically 1–10 dB/km depending on frequency and humidity. Certain windows (notably around 140 GHz and 220 GHz) offer relatively clear propagation paths suitable for outdoor cellular deployment.

Above 300 GHz, the situation changes dramatically. Water vapor absorption lines at 325 GHz, 380 GHz, and 450 GHz create attenuation peaks exceeding 100 dB/km. Even in "windows" between these peaks (notably around 340 GHz and 410 GHz), atmospheric loss remains 10–50 dB/km — making outdoor deployment beyond a few tens of meters impractical without extreme antenna gains or relay architectures.

This is not an engineering problem that can be solved by more transmit power. It is molecular physics. The atmosphere absorbs THz radiation at specific frequencies because water and oxygen molecules resonate at those wavelengths. No amount of silicon innovation changes the rotation spectrum of H₂O.

Semiconductor Reality: The Power Gap

Even if propagation were identical, device physics creates a second barrier. The maximum output power of semiconductor amplifiers drops sharply with frequency. At the 2025 IEEE International Solid-State Circuits Conference (ISSCC), the state of the art for InP heterojunction bipolar transistor (HBT) power amplifiers at 300 GHz was approximately 10 mW — sufficient for short-range links but inadequate for cellular coverage.

At 600 GHz, demonstrated output power drops to the microwatt range. At 1 THz, current devices produce power measurable only in specialized lab setups. The semiconductor community is pursuing multiple paths — III-V compound semiconductors, graphene-based transistors, resonant tunneling diodes, and photonic-electronic hybrid approaches — but none has yet demonstrated the milliwatt-level output at THz frequencies needed for practical communication links.

For comparison, 5G mmWave base stations operate phased arrays with per-element power of 50–200 mW at 28 GHz. The power scaling challenge from 28 GHz to 300 GHz is roughly 100×. From 28 GHz to 1 THz, it exceeds 10,000×.

What 6G Actually Plans to Do with Sub-THz

The 3GPP roadmap for 6G (anticipated in Release 21, circa 2029) focuses on sub-THz as a capacity layer, not a coverage layer. The architecture envisions:

• Dense indoor deployment: Sub-THz cells with 10–50 meter radius in venues, offices, and industrial environments — complementing lower-band macro coverage.
• Massive MIMO at sub-THz: At 140 GHz, a 256-element antenna array fits in approximately 5 cm². This enables extreme beamforming gains (30+ dBi) that partially compensate for path loss.
• Contiguous bandwidth: Unlike the fragmented spectrum below 6 GHz, sub-THz bands offer 10–50 GHz of contiguous spectrum — enabling 100+ Gbps peak rates without complex carrier aggregation.
• Backhaul and fronthaul: Point-to-point sub-THz links replacing fiber in hard-to-wire locations, leveraging high directional gain.

This is a pragmatic approach. Sub-THz extends 5G mmWave principles — dense cells, beamforming, line-of-sight preference — into higher frequencies where more bandwidth exists. It does not require fundamental physics breakthroughs.

What 7G Envisions for Full THz

The 7G vision for terahertz is more radical. Research groups at institutions including the University of Osaka, MIT, and ETH Zurich envision THz communications as enabling capabilities that sub-THz cannot match:

• Terabit-per-second links: With 100+ GHz of instantaneous bandwidth at THz frequencies, individual links could achieve 1 Tbps — 10× beyond 6G peak rates. This enables real-time holographic communication and neural interface data transfer.
• THz sensing fusion: At THz wavelengths, the boundary between communication and imaging blurs. A 7G base station could simultaneously transmit data and generate sub-millimeter resolution images of its environment — enabling joint communication-sensing at unprecedented resolution.
• Nano-networks: THz frequencies match the scale of on-chip and in-body communication. 7G research explores THz links between nanoscale devices for biomedical implants and molecular computing — applications where even sub-THz is too large.

These are genuine capabilities that cannot be achieved at sub-THz frequencies. The question is not whether THz is desirable — it is when the technology stack matures enough to deliver it outside a laboratory.

The Technology Bridge: What Must Change

Bridging the gap from sub-THz (achievable now) to full THz (research frontier) requires advances in at least four domains:

1. Device power. THz power amplifiers must reach at least 1 mW per element for short-range mobile links. Current research paths include GaN-on-diamond thermal management, traveling-wave tube miniaturization, and plasma-wave transistors in graphene.

2. Reconfigurable intelligent surfaces (RIS). Since THz signals cannot penetrate or diffract around obstacles, the environment itself must become part of the network. RIS panels — surfaces that dynamically reflect and focus THz beams — could create artificial propagation paths where natural ones do not exist.

3. Atmospheric adaptation. THz systems must dynamically shift between frequency windows based on real-time atmospheric conditions. A link operating at 340 GHz in dry weather might need to fall back to 220 GHz sub-THz during rain — requiring extreme frequency agility.

4. Antenna-on-chip integration. At THz wavelengths (0.1–1 mm), antennas become small enough to integrate directly into semiconductor dies. This enables massive arrays with thousands of elements — but requires solving thermal, coupling, and packaging challenges that do not exist at lower frequencies.

Timeline: When Does the Gap Close?

Based on current research trajectories and historical wireless generation cycles:

• 2028–2030: 6G standardization and early deployment using sub-THz (100–300 GHz). Point-to-point THz links demonstrated commercially for backhaul.
• 2030–2033: THz device power reaches milliwatt level. Fixed indoor THz access points demonstrated. Research prototypes of mobile THz.
• 2033–2037: 7G standardization begins incorporating THz bands. RIS deployment at scale creates artificial THz propagation environments.
• 2037–2040: Early 7G deployment with THz as capacity layer in controlled environments — smart factories, data centers, specialized venues.

This timeline assumes no fundamental breakthroughs accelerating THz device physics. A materials revolution (such as practical room-temperature graphene transistors or photonic crystal amplifiers) could compress the schedule by 3–5 years.

Why the Gap Matters Commercially

The frequency gap between 6G and 7G is not merely academic. It determines the investment thesis for the next two wireless generations:

6G sub-THz is evolutionary. It extends proven principles (beamforming, dense cells, MIMO) into new spectrum. The engineering is hard but understood. The business case is clear: more capacity for existing applications plus enablement of 100 Gbps fixed wireless access and industrial sensing.

7G THz is revolutionary. It requires new physics (RIS, nano-antennas, atmospheric adaptation), new architectures (environment-as-network), and new applications (holographic telepresence, neural interfaces, molecular sensing). The business case does not yet exist — it must be invented alongside the technology.

For investors, operators, and researchers, the practical implication is clear: 6G sub-THz is a deployment challenge with known solutions. 7G THz is a research challenge with solutions still being discovered. Both are necessary. Neither replaces the other.

Conclusion

The 200 GHz separating sub-THz from full THz — a factor of less than 3× in frequency — represents a gulf in engineering complexity that will take a decade to cross. 6G will bring sub-terahertz communications to market by 2030, delivering 100+ Gbps speeds using extensions of current technology. 7G will bring true terahertz communications in the late 2030s, enabling terabit links and sensing capabilities that require physics we are still developing.

The gap is real. It is not a failure of ambition — it is a reflection of how fundamental physics sets the pace of wireless evolution.