Passive, active and hybrid DAS

A distributed antenna system can carry its signals to the antennas in three ways: entirely as radio frequency over coaxial cable (passive), as light over fibre to powered remote units (active), or a mix of the two (hybrid). The right choice depends mostly on the size of the building, the number of services the system has to carry, and how much the building is likely to change.

Three ways to reach the antennas

Every DAS starts with a signal source and a head end, as described in what is a distributed antenna system? The difference between architectures is what happens between the head end and the antennas. Coaxial cable carries the radio signal directly but loses strength along its length, and more so at higher frequencies. Every splitter or tap that divides the signal between antennas also divides its power. Fibre carries the signal as light with very little loss over building distances, but needs powered equipment at the far end to turn it back into radio.

Passive DAS

In a passive system, the signal source feeds coaxial cable directly, and splitters and couplers divide the signal among the antennas. Nothing between the head end and the antennas needs power. That makes passive systems simple, reliable and comparatively inexpensive for small buildings, and the absence of powered components on each floor appeals for public safety systems that must survive a fire.

The limit is loss. Each metre of coax and each split reduces the signal, so antennas far from the source receive less than those near it. The designer balances this with a link budget: a calculation of the signal at every antenna, accounting for cable loss, splitter loss and connector loss. Beyond a certain size, or at higher frequency bands where coax loses more, the budget runs out and the far antennas cannot meet the design target. Uplink suffers too, because a handset's weak signal has to survive the same losses on the way back.

Reading a link budget

A link budget is worth asking for on any passive or hybrid design, because it shows whether the coverage model can be trusted. For each antenna it should list:

1. The power delivered by the source or remote unit for each band.
2. The loss of every length of coax, using the manufacturer's figures for that cable at that frequency.
3. The loss of every splitter, coupler and connector in the path.
4. The resulting power at the antenna, and the antenna's gain.
5. The margin left over the design target, after an allowance for walls and furniture between the antenna and the user.

If the margin at the farthest antenna is thin, small differences during installation, such as an extra connector or a longer route around an obstruction, can push it below target. That is a warning to add margin or consider fibre for the longest runs.

Active DAS

An active system converts the radio signals to light at the head end and sends them over fibre to remote units close to the antennas. Each remote unit converts the light back to radio and amplifies it. Because fibre loss over building distances is very small, every remote unit can deliver a consistent level, whether it is on the second floor or the fortieth, or in another building on a campus.

Active systems scale well and can carry many carriers and bands, and many are managed and monitored remotely. The costs are higher equipment prices, power at every remote unit, more points of failure and, for public safety use, the need to protect and back up power at each powered location. Fibre is also the natural medium for long runs between buildings; see Cat 6 vs Cat 6A vs fibre for how fibre compares with copper in general.

Hybrid DAS

A hybrid system takes the middle path. Fibre runs up the riser to a remote unit on each floor or zone, and short coax runs connect that remote unit to a handful of nearby antennas. The fibre solves the long-distance loss problem, while passive coax on each floor keeps the number of powered devices down. Hybrid designs are common in mid-size and large buildings because they balance cost, performance and reliability.

Comparing the three

How to choose

• Size and shape. Total area, number of floors and the longest run from the head end.
• Services. How many carriers and bands now, and how many are likely later. Higher bands lose more on coax.
• Capacity. Busy spaces may need the signal divided into zones, which active and hybrid systems handle more easily.
• Public safety requirements. Backup power, monitoring and protection at every powered location; see in-building public safety radio coverage.
• Space and power. Room in risers and ceilings for coax, and power and cooling for remote units in telecom rooms.
• Change. Tenant fit-outs and renovations are easier to accommodate when each floor has its own remote unit.

A typical case

A three-storey medical clinic needs public safety coverage in its stairs and basement, and better cellular coverage for one carrier. A passive design works: the building is small, the link budget closes for every antenna, and there are no powered devices outside the protected BDA room. Across town, a twenty-storey office tower wants three carriers on several bands. The coax losses to the upper floors and the number of bands rule out a passive design, so a hybrid system runs fibre to a remote unit in the telecom room on each floor, with short coax runs to the ceiling antennas. When a floor is later renovated, only that floor's coax and antennas need to change.

Whichever architecture is chosen, cable pathways, fire stopping and room space are planned with the rest of the low-voltage work; our guide to cable pathways, conduit and tray covers the general principles.

How ehZee can help

We compare architectures against your building, services and budget, prepare link budgets and coverage models, and design and oversee installation of the system that fits. Design drawings are sealed by a licensed professional engineer where required, and installation is by licensed electrical contractors and qualified installers. See our distributed antenna systems service.

Common questions