6G 2025 : À la conquête du fossé des térahertz

Deployment of 5G is barely complete, yet headlines are already focused on 6G.

Usually, the “Next G” is just a marketing number. 4G was faster 3G. 5G was faster 4G. But 6G is distinct; it is a collision with a fundamental barrier in physics known as the Terahertz Gap.

1 Terabit per second (100x faster than 5G) and sub-millisecond latency demands more than cranking up power on existing towers. It requires moving into a frequency range that humanity has never successfully commercialized. If 5G was about connecting phones, 6G is about connecting the fabric of reality itself. However, the physics must be solved first.

The Hook: Why 2025 is the Tipping Point

Right now, in 2025, the “6G Race” has moved from theoretical papers to hardware prototypes. South Korea has successfully tested a holographic call over a proto-6G network. China has launched satellites to test terahertz transmission in vacuum.

Why the rush? Because the bandwidth economy is collapsing. VR/AR headsets (like the Vision Pro), autonomous vehicle swarms, and AI agents communicating server-to-server are eating bandwidth faster than fiber can be built. The increasing “Shannon Limit” of the microwave spectrum is being hit. The only way out is up: up the frequency ladder.

6G Race 2025: The Terahertz Gap & End of Cell Towers

To understand 6G, you have to understand the electromagnetic spectrum.

• Microwaves (4G/WiFi): ~2.4 GHz. Great for range, bad for speed.
• Millimeter Wave (5G): ~30-100 GHz. Fast, but blocked by trees and hands.
• Optics (Fiber/Laser): ~300,000 GHz (Infrared/Visible Light). Insanely fast.

Between “Millimeter Wave” and “Light” sits a dead zone: 0.1 THz to 10 THz. This is the Terahertz Gap.

Why is it empty?

It is a “No Man’s Land” of physics:

1. Too Fast for Electronics: Traditional silicon transistors cannot switch on and off fast enough to generate a 1 THz signal. The electrons physically cannot move through the gate fast enough.
2. Too Slow for Photonics: Lasers work by exciting electrons to jump orbital shells, emitting photons. Terahertz energy is too low to trigger these jumps efficiently using standard optical methods.

For 50 years, this gap has been used only by astronomers (to look at cold dust in space) and airport security scanners. It was impossible to put a Terahertz generator in a phone.

But 6G requires conquering this gap. Why? Because the carrier waves in this frequency are so small (~300 microns) that massive amounts of data can be packed onto them.

The Solution: Indium Phosphide and Graphene

Breakthrough hardware is finally emerging.

• Indium Phosphide (InP): Heterojunction Bipolar Transistors (HBTs) are breaking the speed limit of silicon, pushing into the low THz range.
• Graphene Modulators: Because graphene is 2D, electrons move through it with almost zero resistance (“ballistic transport”), allowing for switching speeds that silicon can only dream of.

The Infrastructure Shift: From Towers to “Smart Surfaces”

The physics of Terahertz waves introduces a massive problem: Propagation Loss.

A 4G signal can travel 10 miles. A 5G signal can travel 1,000 feet. A 6G Terahertz signal? It gets absorbed by water vapor in the air after about 100 meters. It can’t go through walls. It can barely go through rain.

This means the “Cell Tower” model is dead.

To make 6G work, the shift to Mesh Networking and Reconfigurable Intelligent Surfaces (RIS) is mandatory. Instead of one big tower blasting a signal to a phone, the 6G network will be comprised of millions of tiny “femtocells” embedded in streetlights, walls, and even vehicles. Coordinated by AI, these cells beamform the signal around obstacles. Because Terahertz waves are so short, they essentially act like radar. A 6G network won’t just communicate; it will sense.

A 6G router in your room could mathematically analyze the reflections of the waves to detect:

• Gesture Control: Detecting your hand moving specifically without a camera.
• Heartbeat Monitoring: Seeing the micro-vibrations of your chest.
• Spatial Mapping: Building a real-time 3D map of the environment for your AR glasses.

This is the “Joint Communication and Sensing” (JCAS) standard. The network becomes the sensor.

Contextual History: The Cycle of Doubters

• 2000 (3G): Users asked why video was needed on a phone. -> iPhone launches in 2007.
• 2010 (4G): “Who needs 100Mbps? DSL is enough.” -> Streaming economy (Netflix/Uber) explodes.
• 2020 (5G): “Millimeter wave is useless because leaves block it.” -> Fixed Wireless Access becomes the fastest growing broadband sector.
• 2025 (6G): “Terahertz is impossible because air blocks it.”

The pattern is clear. Engineers solve the physics. Then developers fill the pipe.

Forward-Looking Analysis: The Geopolitical Battlefield

The U.S. lost the 5G race to Huawei. Policymakers are concerned about losing 6G.

The Next G Alliance (Apple, Google, Qualcomm) is pushing for North American standards dominance. Meanwhile, China has already prioritized 6G in its 15th Five-Year Plan.

The battlefield isn’t just speed; it’s Space. Because Terahertz works poorly on the ground, 6G includes “Non-Terrestrial Networks” (NTN) as a native feature. Starlink-style LEO satellites will be integrated directly into the 6G protocol stack. Your phone will seamlessly switch from a streetlamp femtocell to a satellite when you drive into the mountains.

The Bottom Line

6G isn’t about downloading movies in 1 second instead of 5 seconds. It is about a world where the distinction between “Online” and “Offline” dissolves.

When the network has sub-millisecond latency, “the cloud” essentially moves onto your device. When the network can “see” the room, your physical reality becomes a digital interface. The Terahertz Gap was the final frontier of the radio spectrum, and it is about to be crossed.