Passive vs. Active DASs
There are both passive and active DAS types, as well as some hybrid passive/active devices. Active DASs usually transport cellular signals from the RF source across fiber cabling in risers and either fiber or Category 5e Ethernet-grade copper cabling serving each floor. Active DASs are smart, in that they amplify, condition and adapt the signal to deliver overall consistent coverage and performance. In addition to the active RF source from the carrier, active systems make use of expansion hubs on each floor connected to remote access units (RAUs) distributed around the floor, with antennas mounted upon them.
Passive DASs usually distribute signals using “leaky” coaxial cable, whereby notches in the cabling act as distributed antennas. These devices simply pass signals along without any enhancement. Their primary advantage is that they cost less than an active system and there are no active electronics between nodes to break.
But attenuation over distance can degrade the signal and, thus, service.
“In areas where there is high usage, you take some chances with passive systems, because they are designed to support a particular capacity,” explained Dan Shey, principal analyst, business mobility, at ABI Research. In places where there are dense spots of usage, “a passive system won’t adjust for additional capacity and power requirements to provide a good experience, and you’ll suffer dropped calls,” he said.
There is also a category of DAS that is hybrid. It combines active components, installed between the RF source and the floors, and, usually, coaxial cable for passive transmission across the last few feet on each floor. These systems offer some of the signal management and adaptation of the active system and some of the cost savings of the passive system.
Multi-network Support
Historically, DASs could accept input from just one carrier’s micro base station. This left corporate customers with the limited options of supporting just one carrier’s service within their building or installing multiple DASs, which was an expensive proposition.
Now, however, some DASs can accept multiple carrier signal feeds, making the DAS alternative potentially more compelling, particularly for mid-sized organizations that are either sharing the cost of the equipment with the carrier, or paying for it in its entirety (Table 2).
