How 6G will work: Terahertz-to-fiber conversion

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Upcoming 6G wireless, superseding 5G and arriving possibly by 2030, is envisaged to function at hundreds of gigabits per second. Slowly, the technical advances needed are being made.

A hole in the tech development thus far has been at the interface between terahertz spectrum and hard, optical transmission lines. How does one connect terahertz (THz), which is basically through-the-air spectrum found between microwave and infrared, to the transmission lines that will be needed for the longer-distance data sends? The curvature of the Earth, for one thing, limits line of sight, so hard-wiring is necessary for distances. Short distances, too, can be impeded by environmental obstructions: blocking by objects, even rain or fog, becomes more apparent the higher in spectrum one goes, as wavelengths get shorter.

Researchers at the Karlsruhe Institute of Technology (KIT) say they know how to make the fiber link. They say, in a press release, that one must develop modulators that operate on plasmonic nanophotonics, which is nano-scale, light-trapping technology (in this case, made with silicon) that will “directly couple the receiver antenna to a glass fiber.” The radio becomes part of the cable, in other words. It will “enable terahertz connections with very high data rates. Several hundred gigabits per second are feasible,” the researchers say.

In tests, the research team demonstrated a terahertz link that was “seamlessly integrated” into fiber using a link at the terahertz receiver. They performed a terahertz-to-optical transmission rate of 50 gigabits per second. For comparison, current over-air wireless data rates with LTE technology and using radio are often around 20 megabits per second (Mbps)—nowhere near what the team produced. Verizon, now launching millimeter wave 5G in the U.S., says typical speeds, for its fixed 5G service will be around 300 Mbps.

Other 6G challenges

The fiber-terahertz connection in 6G, though, isn’t the only area that must be addressed over the next few years. Spatial multiplexing also needs to be mastered at terahertz to get the kinds of throughputs desired, experts say. Spatial multiplexing is where individual data signals are beamed out in streams. Every bit of the bandwidth thus gets used and reused continually, introducing bandwidth efficiency.

Efficiency gains will also need to be obtained with more advanced MIMO antennas. That’s where antennas take advantage of multipath—signals sent over more than one route.

Penetration loss also needs to be addressed. That’s the difference between the signal strength as it enters a building or structure and air. The loss increases with higher frequencies, as terahertz is; however, the amount of loss is dependent on the material that needs penetrating. Clear glass, for example, has less penetration loss overall than drywall, for example. That means construction materials used for upcoming buildings could be reimagined with new materials science to take advantage of 6G data throughput.

In March, the FCC announced a new category of experimental spectrum licenses for frequencies between 95 GHz and 3 THz. That’s so that telcos and scientists can work on the spectrum.

We think “6G will emerge around 2030,” Ari Pouttu, a professor at the University of Oulu and a 5G system architect, told me when I met him in Finland last year. “It will eventually offer terabits per second,” along with millionth-of-a-second (microsecond) latency.

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