THz Propagation: Viable for Direct Links, Even Bouncing Off Walls

Terahertz waves—sometimes called submillimeter waves—are the part of the electromagnetic spectrum between 100 GHz and 10 THz (corresponding to wavelengths between 3 mm and 30 µm), occupying the zone between what we call microwaves and infrared light. Depending on your perspective, the THz band either bridges the gap between the highest-frequency RF and optical worlds, or it falls into an ambiguous space between them (all subject to Maxell’s equations, of course).

THz waves offer the potential for delivering extreme wideband links at extraordinary data rates and are now being used in some specialized sensing applications (Reference 1 is just one very recent example). However, they also have many challenges due to issues of components, signal creation and capture, interconnections, path loss, and measurement in the spectrum “gap,” which they fill (Reference 2 is a good overview and reality check).

At present, this band is largely unregulated by the FCC and regulatory agencies outside the U.S. It’s being used for some advanced inspection systems as well as line-of-sight (LOS) and specular (reflected) non-line-of-sight (NLOS) links. One the of generally accepted assumptions about THz waves is that they suffer high attenuation passing through solids and reflect poorly from solid surfaces, thus limiting their use. THz-range signals also suffer high levels of power loss and degradation due to atmospheric absorption and free-space path loss.

Now, operating under a temporary experimental license from the FCC, a team at Brown University has literally done “off the wall” tests on the actual performance of THz waves. Both indoor and outside settings using both LOS and reflected links were tested to determine the practical capabilities of this band. The team used center frequencies of 100, 200, 300, and 400 GHz at a data rate of 1 Gb/s and conducted tests at varying humidity levels (a major contributor to attenuation) (Fig. 1).

1. Among the factors studied was the impact of atmospheric attenuation of THz waves at humidity levels from 60% to 100%. (Source: Brown University)

For the indoor tests of reflection, they examined three signal-reflection cases:

• By a painted but otherwise bare cinderblock wall

• By conformal metal foil attached on the wall

• By a smooth metal plate to differentiate between losses due to surface absorption and scattering

Their results, characterized by bit error rate (BER), show that the effect of scattering from the rough surface (i.e., the difference between the black and red curves) is significantly smaller than the effect of absorption (the difference between the red and blue curves). And, as expected, absorption losses increase moderately with frequency, from about 8 dB at 100 GHz to nearly 11 dB at 400 GHz (Fig. 2).

2. Shown is a photo of the 2-m distance link and of the modified wall conditions used in these measurements (a). Given is the log(BER) versus transmitter output power when the signal is reflected by a bare painted cinderblock wall (blue curves), a conformal metal foil attached to the wall (red curves), and a smooth metal plate (black curves), at the frequencies shown (b through e). (Source: Brown University)

They concluded that specular (highly reflective) non-line-of-sight (NLOS) paths are practical for indoor THz links even up to 400 GHz, due to acceptable path losses. They also did tests at different incident/reflection angles to assess the effects of varying this parameter (Fig. 3) and over longer indoor distances, including double reflections from different surfaces on the same link path (Fig. 4).

3. Arrangement of the 2-m distance link with movable rails to allow changing the angle of incidence (a); log(BER) versus transmitter output power for different incident angles, using a carrier frequency of 400 GHz (b). The inset shows the measured (stars) and computed (solid curve) power losses (relative to a smooth metal mirror reflector), and the dashed curve in the inset shows the predicted result when scattering losses are neglected. (Source: Brown University)

4. Shown is a photo of the 30-m link at 200 GHz with a single near-normal-incidence reflection (a) and log(BER) versus (b); transmitter output power when the signal is reflected by the bare wall (black curve) and by a conformal metal foil (26 × 26 cm) attached to the wall (blue curve). The photo of a 35-m link at 200 GHz (c) is shown with two specular reflections: one from the (painted metal) door and a second from the wall. A comparison of the BER performance for single and double reflection (d) is as a function of received power, showing that they’re nearly identical. (Source: Brown University)

For outdoor assessment, they set up an installation to cross a grass surface and a concrete sidewalk (recording temperature, humidity, and wind conditions) with results partially summarized in Figure 5. Among their conclusions was that multipath interference—a well-known occurrence that increases at higher frequencies—indeed occurs in terahertz waves, but it’s less severe over grass as compared to concrete. The presumed reason: Since the grassy surface has more water content than the concrete, and since water absorbs THz waves, the grass-surface multipath signals were more attenuated compared to those of concrete.

5. The photo is the measurement site on the lawn (a) and sidewalk (b). Also shown is the BER performance with respect to link distance on the lawn (c) and sidewalk (d) for carrier frequencies of 100 (black), 200 (blue), 300 (red), and 400 (green) GHz. The inset shows that the square root of path loss versus the product of distance and frequency has the expected linear relation. (Source: Brown University)

Noted Daniel Mittleman, a professor in Brown University’s School of Engineering and senior author of the new research, “I think it’s fair to say that most people in the terahertz field would tell you that there would be too much power loss on those bounces, and so non-line-of-sight links are not going to be feasible in terahertz.” He added, “But our work indicates that the loss is actually quite tolerable in some cases—quite a bit less than many people would have thought.”

Additional details on the THz signal chain and instrumentation, as well as results and analysis, are in their paper “Channel performance for indoor and outdoor terahertz wireless links,” published in APL Photonics. The research was supported by the National Science Foundation and the W.M. Keck Foundation.

References

1. Photonics Spectra, “Terahertz Imaging System on a Chip Offers Speed and Portability”

2. IEEE Spectrum, “The Truth About Terahertz”