A distributed antenna system (DAS) is a network of small antennas spread through a building, connected by cable to a common signal source, that carries cellular or two-way radio signals into spaces the outside network cannot reach. Instead of one strong signal fighting its way through walls, the building gets many weak, well-placed signals from the inside.
Why buildings need one
Cellular networks are designed from outdoor sites. A phone near a window on the tenth floor may see a strong signal, while one in an elevator lobby in the middle of the same floor sees almost nothing. The building envelope is the main reason. Low-emissivity coatings on energy-efficient glazing reflect radio energy as well as heat. Concrete, steel decking and foil-faced insulation absorb or reflect it. Each interior wall, core and floor slab takes a little more, and below-grade levels have the earth itself in the way.
The result is a pattern most occupants know: good service at the perimeter, weak service in the core, and none in stairwells, elevators and parking. Better energy performance tends to make it worse, because the same envelope features that keep heat in also keep radio out.
Two different needs drive most DAS projects. The first is commercial: tenants, residents, patients and visitors expect their phones to work, and leasing teams hear about it when they do not. The second is life safety: firefighters and other responders rely on portable radios, and many fire authorities now require new buildings, and sometimes existing ones, to show that those radios work throughout. That second case is covered in our guide to in-building public safety radio coverage.
How a DAS works
Every DAS has four parts:
- A signal source. Either an off-air donor antenna on the roof, aimed at a nearby carrier or radio site and connected to a bi-directional amplifier (BDA), or base station equipment that a carrier installs in the building and connects to its network directly.
- A head end. Equipment that combines signals from the sources, sets their levels and prepares them for distribution.
- Distribution. Coaxial cable carrying the radio signal itself, fibre carrying it as light to remote units that convert it back, or a mix of both.
- Antennas. Usually small ceiling-mounted antennas, plus directional antennas in places like stairwells, tunnels and parking ramps.
A DAS has to work in both directions. The downlink carries signals from the network to phones and radios; the uplink carries the much weaker signals from handsets back to the network. Uplink is often the harder part of the design, because a handset's transmit power is limited and the system must not add so much noise that it degrades the outdoor network.
A single DAS can carry several carriers and several frequency bands, which is called a neutral-host or multi-operator system. Each additional carrier and band adds equipment at the head end and sometimes changes the antenna and cabling design, so the services to be carried should be settled early. The choice of how the signal is distributed is covered in passive, active and hybrid DAS.
Cellular DAS and public safety DAS
| Cellular DAS | Public safety DAS | |
|---|---|---|
| Who uses it | Tenants, residents, visitors | Firefighters, police, paramedics |
| What it carries | Carrier cellular bands | Emergency responder radio channels |
| Who approves | Each carrier whose signal it carries | The local fire authority |
| Typical extras | Capacity planning for busy areas | Backup power, monitoring, protected enclosures and cabling |
Some buildings combine the two on shared cabling and antennas; others keep them separate so the public safety system can meet its survivability requirements without compromise.
Other options, and when they fit
- Carrier small cells are compact base stations installed by a carrier. They add capacity as well as coverage but usually serve one carrier at a time.
- WiFi calling lets phones place calls over a good WiFi network. It helps where the WiFi is well designed and phones support it, but it does not carry public safety radio and depends on each user's settings.
- Signal boosters amplify an outdoor signal with a simple donor and indoor antenna. In a commercial building, a booster that is not certified, authorised by the carrier and properly installed can interfere with the network and with public safety radio. They are not a substitute for a designed system in a large building.
For many buildings the answer is a combination: a DAS for cellular and public safety, and a well-designed WiFi network, covered by our WiFi design and build service, for the building's own data.
What shapes the design
- The services to carry: which carriers, which bands, and whether public safety radio is included.
- The area and layout: floor plate size, number of floors, stairwells, elevators, parking and any outdoor areas.
- How much capacity busy areas need, such as lobbies, arenas and conference spaces.
- Where the signal source can go, including roof space and line of sight to donor sites.
- Riser and ceiling space for cabling, and rooms for head-end and remote equipment.
- Fire authority requirements for backup power, monitoring and cable protection where public safety coverage is required.
A typical case
A mid-rise office building has good cellular service along the perimeter but complaints from the core, the stairs and two parking levels. A survey confirms usable outdoor signal from several carriers at the roof. The design uses a donor antenna and head end for the carriers that agree to participate, a fibre riser to remote units on alternating floors, and short coax runs to ceiling antennas, with directional antennas down the parking ramps. Because the fire authority has also asked for responder radio coverage, the stairwell antennas and riser are planned so that a public safety system can be added on its own protected equipment.
Questions to ask before you start
- Is the goal tenant cellular service, responder radio coverage or both, and has the fire authority said anything in writing?
- Which carriers matter most to the people in the building, and are they willing to participate?
- Are there renovations or envelope upgrades planned that could change coverage after the system is built?
- Who will own, monitor and maintain the system once it is accepted, and who pays for annual testing?
How ehZee can help
We survey existing coverage, design cellular and public safety systems, coordinate with carriers and fire authorities, and oversee installation and acceptance testing. Design drawings are sealed by a licensed professional engineer where required, and installation is carried out by licensed electrical contractors and qualified installers. Learn more about our distributed antenna systems service.
Common questions
Does a DAS improve data speeds as well as calls?
It can. A DAS improves signal strength and quality, which usually improves data. In very busy areas, capacity depends on how much carrier equipment feeds the system, which is a separate conversation with the carriers.
Is a DAS a health concern?
Indoor DAS antennas operate at low power, and radio frequency exposure from equipment in Canada is governed by federal limits. Because the antennas are close to users, phones can often transmit at lower power than they would to a distant outdoor site.
Can a DAS be added to an existing building?
Yes. Retrofits are common. The main challenges are finding riser and ceiling space for cabling and planning work in occupied areas, both of which are addressed during the survey.