In-building coverage is now a fibre problem

The macro network does not survive modern construction. Low-emissivity glass, reinforced concrete and deep floor plates attenuate signal badly enough that a building can sit inside nominal coverage and still be unusable on the third floor. That has been true for years. What has changed is what you should do about it.

Why passive distribution ran out of room

A passive distributed antenna system splits one source across coaxial runs. It is simple, it has no active parts in the ceiling, and it works well in a small venue on one or two bands. The trouble is that coaxial loss scales with distance and with frequency, and every additional band and operator multiplies the combining problem at the head end.

• Cable loss rises with frequency, so the higher bands suffer most
• Multi-operator and multi-band combining gets expensive quickly
• Long runs force more power at the source, not better coverage at the edge
• Changing the design later means pulling cable

What active fibre changes

An active fibre DAS carries the signal optically to remote units placed near the antennas, and converts back to RF locally. Distance stops being the dominant constraint, because the loss budget is no longer being spent on coaxial runs.

That has a design consequence more important than the technology itself: coverage becomes a matter of where you put remote units, not of how much power you can push down a cable. Large and vertically distributed buildings such as hospitals, campuses, towers and transport hubs become tractable.

Digital DAS, and when a repeater is still right

Digital distribution goes further, transporting the signal as data and allowing capacity to be steered between zones. That flexibility matters where load moves through a building across the day: a stadium concourse, a conference floor, a terminal.

None of which makes repeaters obsolete. For a basement, a tunnel or a small venue with a usable donor signal, an off-air repeater remains the proportionate answer, provided donor selection and isolation are planned properly. The failure mode of a badly sited repeater is not poor coverage; it is oscillation that degrades the donor cell for everyone.

Choose the distribution technology after you have surveyed the building, not before. The survey is the design.