If a base station in a local network tries to use a directional beam to transmit a signal to a user trying to connect to the network – instead of using a broadcast over a wide area network, as base stations typically do – how does it know in which direction to send the beam?
Researchers at Rice University and Brown University developed a method of discovering links in 2020 using terahertz radiation, with high-frequency waves above 100 gigahertz. For this work, they deferred the question of what would happen if a nearby wall or other reflector creates a line-of-sight (NLOS) path from the base station to the receiver and focused on the simpler situation where the only existing path was along line of sight (LOS).
In APL Photonics, from AIP Publishing, these same researchers address this question by considering two different generic types of transmitters and exploring how their characteristics can be used to determine whether an NLOS path contributes to the signal received by the receiver.
“One type of transmitter sends all frequencies more or less in the same direction,” said Daniel Mittleman, co-author and engineering professor at Brown, “while the other type sends different frequencies in different directions, exhibiting a strong angular dispersion. The situation is quite different in these two different cases. “
Researchers’ work shows that the transmitter that sends different frequencies in different directions has distinct advantages in its ability to detect the NLOS path and distinguish them from the LOS path.
“A well-designed receiver would be able to detect the two frequencies and use their properties to recognize and distinguish the two paths,” Mittleman said.
Many recent reports in the academic literature have focused on various challenges related to the use of terahertz signals for wireless communications. Indeed, the term 6G has become a buzzword to encompass future generations of wireless systems that use these ultra-high frequency signals.
“For terahertz signals to be used for wireless communications, many challenges must be overcome, and one of the biggest is how to detect and exploit NLOS paths,” Mittleman said.
This work is among the first to provide quantitative thinking on how to detect and exploit NLOS paths, as well as a comparison of the behavior of different transmitters in this context.
“For the most realistic indoor scenarios we can envision for a wireless network above 100 gigahertz, the NLOS path issue is certainly going to require careful consideration,” Mittleman said. “We need to know how to exploit these bonding opportunities to maintain connectivity.”
If, for example, the LOS path is blocked by something, an NLOS path can be used to maintain the link between the base station and the receiver.
“Interestingly, with a transmitter creating strong angular dispersion, an NLOS link can sometimes provide even faster connectivity than the LOS link,” said Yasaman Ghasempour, co-author and assistant professor at Rice University. “But you can’t take advantage of such opportunities if you don’t know the NLOS path exists or how to find it.”
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Material provided by American Institute of Physics. Note: Content can be changed for style and length.