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The researchers at the National Institute of Standards and Technology (NIST) found that the optimal height of installation depends on transmission frequency and antenna design.
Attaching equipment at lower heights of around 4m is better for traditional wireless systems with omnidirectional antennas, the team said, whereas higher locations 6-9m up are better for the latest systems such as 5G using higher, millimetre-wave frequencies and narrow-beam antennas.
An international group, the Telecom Infra Project, is promoting the idea of making Wi-Fi available over the unlicensed 60GHz frequency band by installing access points on lampposts.
A technical challenge, however, is that signals in this band, which are higher than traditional cellphone frequencies, are sparse and tend to scatter off rough surfaces. And until now, measurements of 60GHz urban channels have produced limited data.
To tackle this, the team at NIST has developed a channel model for tracking transmissions that recognises the sparse, scattery features of these signals.
This model uses a novel algorithm for analysing the measured paths that extend beyond the usual parameters of signal delays and angles to include receiver locations.
NIST researchers travelled to downtown Boulder, Colorado, to test their model against actual channel measurements. They recorded the measurements at 4, 6 and 9m antenna heights to investigate the trade-offs.
According to the researchers, the model matched real-world measurements very well.
“We verified the model we developed and used measurements from downtown to prove this point further,” said Derek Caudill, an electronics engineer who worked on the project at NIST.
“This work shows that by using our model, someone like a cell provider can account for various advantages and disadvantages of 60GHz access points and signals on light poles in urban environments.”
NIST communications researchers travelled to downtown Boulder, Colorado, to verify their channel model for evaluating high-frequency wireless network designs. Image credit: NIST
The team used custom NIST equipment called a channel sounder, with a stationary transmitter mounted on a mast and a mobile receiver on the roof of a van. The transmitter and receiver are both topped with an array of electronically switched antennas with defined 3D radiation patterns.
According to the researchers, the sounder can also precisely measure many radio channel characteristics and has a unique ability to measure the time dynamics – how the properties of the waves change over time as the receiver moves – of a millimetre-wave channel even when in motion.
The team was especially interested in data on how signals spread across a physical space. Large spreads are considered bad as they indicate multiple received signals and more interference, they said, adding that it is better to have one clear path for communication.
For conventional wireless systems with omnidirectional antennas, the smaller spreads are preferable to avoid interference, which means Wi-Fi equipment should be mounted at lower heights on lampposts, they added.
“However, the next-generation wireless systems will operate at millimetre-wave frequencies and should employ highly directional antennas with very narrow beams, or pencil beams,” Senic said.
“With this configuration, transmitter and receiver will steer their narrow beams in order to find the best possible link; that is, the propagation path that has maximum power.
“Here, a higher angular spread is preferable because it will provide diversity in space; transceivers will have the ability to steer beams in more directions in order to find the best link.”
NIST researchers went a step further and recorded the measurement data on the NIST campus to validate that the new model could be applied to different environments.
The results they gathered on campus were comparable to downtown, substantiating that they can generalise the model to different environments and use cases.
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