Fibre DAS or off-air repeater? Choosing in-building coverage for a mid-size building

Most indoor coverage projects start with the same question: will a repeater and a few antennas do, or does the building need a distributed antenna system over fibre? The answer follows from a handful of facts about the site. This note goes through them.

Two ways to bring the signal inside

An off-air repeater takes the signal from a donor antenna on the roof, amplifies the selected bands and feeds service antennas inside through coaxial cable, splitters and couplers. It needs a usable outdoor signal at the donor position.

An active DAS carries the signal over fibre from a master unit to remote units placed where coverage is needed. The source can be a donor repeater or a base station provided by the operator. Each remote unit amplifies locally, so the distance from the master unit costs little.

When a repeater is enough

A repeater with a passive antenna network suits a building where a usable outdoor signal exists, the area is moderate and the cable runs are short: one car-park level, one or a few floors, a mid-size office or a hospital wing. Low-power units (13–23 dBm) typically cover one car-park level or one floor; medium-power units (27–33 dBm) a multi-floor office or hospital building, or the feed of a passive DAS.

The limit is the passive network itself. Every metre of coaxial cable and every splitter takes away signal, so the farthest antennas get the least. Past a certain size, adding power at the repeater no longer fixes the far end.

When fibre DAS is the better choice

• Large or spread-out sites: a campus, several buildings, a long tunnel or a large hall.
• Many floors with long risers: where coaxial runs to the far antennas would be long.
• Uneven needs: remote units from 100 mW to 20 W let each area get the power it needs.
• A base station inside: when the operator provides the signal source, a DAS distributes it.

A typical master unit serves several remote units over fibre: ours has eight optical remote ports in a 2U rack.

Several operators, one system

When more than one operator, or TETRA as well as cellular, must share the antennas, a point of interface (POI) combines the sources at the head end. Combiners and passive components in the shared path need low passive intermodulation, otherwise one operator's downlink creates interference in another's uplink.

The questions that decide it

1. Is there a usable outdoor signal? Measure it at the likely donor position, per operator and band.
2. How large is the site, and how long are the cable runs? Floors, area, risers, distances between buildings.
3. Which networks and bands? Operators, cellular bands, TETRA, DMR or P25.
4. Who provides the signal source? A donor antenna or an indoor base station from the operator.
5. What does the link budget say? The right power class and the number of antennas or remote units follow from it.

Our RF link-budget calculator gives a first estimate; floor plans and donor measurements settle it.

Repeaters, active DAS and passive components → · Send your floor plans to an engineer →