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There are two types of in-building wireless solutions that boost signal: passive and active systems. The former achieves stronger coverage by strengthening the existing signal from the nearest cell site (tower). The latter allows for increased capacity because it essentially constructs a miniature cell tower specifically for the site.
A passive system is also referred to as repeater-based. It entails an outdoor antenna pointed toward the nearest cell site. The antenna catches the off-air signal from a tower and sends it via coaxial cable to a bi-directional amplifier (repeater) that strengthens the signal and sends it along to antennas throughout the building.
An active system is also commonly referred to as a distributed antenna system (DAS). A DAS starts with a front-end, central distribution unit.
“This is literally the same equipment that would be at the base of a cell tower,” Townes explains. “The difference is instead of going up cable to the top of the tower and hitting an antenna, the DAS equipment breaks that signal down and sends it through fiber to multiple zones.”
Passive systems are best for areas where the network has available capacity, such as suburban or rural areas, but likely aren’t ideal for densely packed, urban environments.
“They have been very common practice, but we’ve started getting away from those, because they don’t add any capacity to the network. It just strengthens the existing signal from the tower inside the building,” Townes says. “If the network is already overloaded, a passive system puts more stress on it. It robs Peter to pay Paul.”
A DAS solution, on the other hand, is like your own personal tower inside the facility. For this reason, DAS networks are ideal for large airports, convention centers, and even NFL stadiums, says Kelley Carr, president of the custom solutions group at Cellular Specialties, Inc, a national systems integrator for DAS, Wi-Fi, and public safety solutions.
“In large, public venues, there are so many people to accommodate,” he says. “A DAS gives dedicated capacity to the building.”
The Merchandise Mart’s system is a DAS. Its front-end equipment is a neutral host, meaning that other competing carriers like Verizon and Sprint can hook their radios onto it and send out their signal. It requires a 2,000-square-foot space, located on the 18th floor’s northwest turret.
The space needed for the DAS’s location must account for the central distribution unit and allow for multiple carriers to hook their radios to it, and thus the required area can range from 100 to over 4,000 square feet, depending on facility size, Townes says.