It seems that those ducts—officially dubbed heating, ventilating and air-conditioning (HVAC) ducts—snaking through these old multi-level escalator-using buildings are natural signal boosters. "The HVAC ductwork is an excellent conduit for the radio transmissions because the ducts typically consist of hollow metal pipes," the university's report said. "Those pipes can be used to guide the radio waves, keeping the waves from dispersing, and helping to maintain a strong signal over a greater distance."
The study was originally conducted to study RFID signals and the ducts can certainly be used for that purpose, but many retailers today consider the cell tower signal problem more challenging. Sam's Club, for example, had to offer its customers Wi-Fi partially because they often couldn't use mobile devices any other way, which would have put a crimp in the mobile plans of the Wal-Mart-owned warehouse club. Other chains have struggled with metal roofs that never considered cell signals during construction.
"In some buildings, this could be a very elegant solution," said Dan Stancil, co-author of the study paper, professor and head of NC State's Department of Electrical and Computer Engineering.
Although the stores would still need a microcell repeater to grab and strengthen the signal, the ducts don't merely serve as a convenient, hidden place to house equipment. "You're using the duct itself as a distributed antennae," Stancil said.
When used for Wi-Fi or RFID location-tracking (regardless of whether it's pallets, store associates, item-level products or customer mobile devices being tracked), Stancil stressed that engineers and installers will have to significantly recalculate where transmitters are placed. That's because the ducts create lines of sight that are very different than a straight line so "the signal delays will not be as expected. This wouldn't be the shortest path," he said, but it may be the path with the smallest amount of signal loss.
"The ducts will increase the range in a very different way, meaning that you can cover Wi-Fi with far fewer access points," Stancil said.