Today’s cellular networks and Wi-Fi systems rely on microwave radiation to carry data but the demand for more bandwidth is quickly becoming more than microwaves can handle. That has researchers thinking about transmitting data on higher-frequency terahertz waves, which have as much as 100 times the data-carrying capacity of microwaves. But terahertz communication technology is in its infancy — there is much basic research to be done and plenty of challenges to overcome.

In an effort to better understand the architecture needed for future terahertz data networks, researchers investigate how terahertz waves propagate and bounce off of objects both indoors and out. (Mittleman Lab/Brown University)

It’s been assumed that terahertz links would require a direct line of sight between transmitter and receiver. Unlike microwaves, terahertz waves are entirely blocked by most solid objects. The assumption has been that it’s not possible to bounce a terahertz beam off a wall or two to find a clear path around an object, fearing too much power loss on the bounces; however, new work indicates that the loss is quite tolerable in some cases.

Researchers bounced terahertz waves at four different frequencies off a variety of objects — mirrors, metal doors, cinderblock walls, and others — and measured the bit-error rate of the data on the wave after the bounces. Acceptable bit-error rates were achievable with modest increases in signal power.

In one experiment, a beam was bounced off two walls, enabling a successful link when transmitter and receiver were around a corner from each other with no direct line of sight. If a computer is connected to a terahertz router and there is direct line of sight between the two but someone walks in between and blocks the beam and an alternative path can’t be found, the link will be shut down. New research shows that the link could be maintained by searching for a new path that could involve bouncing off a wall somewhere.

The work focuses on specular reflection. When a signal is transmitted over long distances, the waves fan out, forming an ever-widening cone. As a result of that fanning out, a portion of the waves will bounce off the ground before reaching the receiver. That reflected radiation can interfere with the main signal unless a decoder compensates for it. It’s a well-understood phenomenon in microwave transmission that could be characterized in the terahertz range.

This kind of interference occurs in terahertz waves, but occurs to a lesser degree over grass compared to concrete. That’s likely because grass has a large quantity of water, which tends to absorb terahertz waves. Over grass, the reflected beam is absorbed to a greater degree than concrete, leaving less of it to interfere with the main beam. That means that terahertz links over grass can be longer than those over concrete because there’s less interference.

Specular reflection represents another possible path for the signal. If the line-of-site path is blocked, the signal could be bounced off the ground.

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