Bringing Signals Indoors: How In-Building DAS Works
If you’ve ever tried using your smartphone in a building with lots of low-E glass, concrete or steel, you know that outdoor wireless licensed RF signals (600 MHz to 2700 MHz) don’t travel well through those building materials as they attempt to move indoors. The result: poor coverage, which can make it difficult (if not impossible) for you to use your device. Indoor coverage can also be impacted by large numbers of users and the data they consume.
To improve indoor coverage and make sure signals reach the people and devices inside, wireless outdoor signals need to be conditioned, filtered and combined with other carrier signals. All of this happens through an in-building DAS (distributed antenna system).
What’s an In-Building DAS?
A distributed antenna system allows cellular signals to spread throughout a building. A DAS provides two-way wireless communication between carriers and users. This approach enables owners to extend licensed networks inside their buildings.
Originally, the technology was developed as a two-way radio communications system using radiating cable within tunnels, mines and subways.
Today, in addition to two-way radio communications, DAS is also used in emergency response systems and areas needing licensed wireless access due to high densities of people and high bandwidth levels (venues such as hotels, casinos, arenas, airports, etc.).
Let’s take a closer look at the components of a DAS … and where cable plays a role.
How an In-Building DAS Works
An in-building DAS uses fiber, coax cable and antennae to distribute signals inside a building from either a donor antenna or a direct connection to the carrier network via a base transceiver station (BTS).
Each building floor is equipped with a remote unit (RU) fed from the DAS head-end unit (HEU) with fiber optic cabling. The RU converts the fiber’s light source back to an RF signal and connects to several passive antennae via low-loss coaxial cable.
There are two ways licensed wireless signals can be brought into a building via a DAS:
- A donor antenna brings wireless signals into a building. This approach improves coverage but not capacity (it doesn’t allow more people to connect.)
- A direct connection to the carrier’s core network using a BTS – much like a cell tower’s base station. Each carrier requires a fiber backhaul connection to the BTS, typically through an S1 link. This method improves capacity and
Because of what it can accomplish, the second option is used most often.
The head-end unit (HEU) is the modular rack unit that houses the point of interface (POI), master unit (MU), repeater and power.
The POI conditions the incoming signal from each licensed carrier from either the BTS or donor antenna. Each carrier is combined in the MU into one optical signal for transmission on singlemode fiber (SMF). Up to six carriers can be combined onto one singlemode fiber.
DAS systems are typically powered remotely through a DC power source or a Digital ElectricityTM (DETM) power source. Cabling outside the MU can be singlemode fiber, Next Gen, hybrid fiber, DETM or any combination.
The RU converts the optical signal to radio frequency (RF) in the licensed bands between 600 MHz and 2700 MHz (and vice versa). The remote unit is fed from the MU through SMF and typically powered remotely. The output is a 4.3/10 DIN interface connector attached to an air dielectric coaxial cable that connects to the antennae throughout the building.
The antenna is a passive antenna that wirelessly connects all carriers to the user. The input is a 4.3/10 DIN connector, which is a connector type known for good electrical performance, including low passive intermodulation distortion.
When fiber connects the DAS head-end back to the core network, that licensed network can be extended inside the building however the owner prefers.
In a nutshell, that’s how a DAS works. To evaluate DAS as compared to other options available for boosting wireless coverage, we created a table to help you compare pros and cons.
| DAS | Small Cells | Wi-Fi | BDA/Repeater Systems (Signal Boosters) | |
|---|---|---|---|---|
| How it Works | Network of antennas that process signals on a carrier’s licensed frequency | Act as mini cell towers; often used to boost wireless coverage in specific zones | Converts landline internet connections into wireless signals for computers, phones, TVs, etc. | Capture, amplify and repeat outside wireless signals inside a building |
| Pros |
Reliable
Once installed, remotes and antennas can be added as needed
Supports multiple carriers |
Creates signals indoor even if no outdoor signals are nearby
Easy deployment Can be used as signal source for DAS |
Ubiquitous in commercial spaces
Relatively inexpensive |
No landline connection
Reduces impact of building materials getting in the way of signals |
| Cons |
More costly than other options
Installation can be challenging
Designing, deploying and getting carriers on a DAS takes time |
Competes for bandwidth
Liability to support it or return to carrier
Must have at least one radio node per carrier |
Doesn’t integrate with outside network
Doesn’t support complex range of services
Unreliable
Poor range and speed Vulnerable to cyberattacks |
Requires a re-transmit agreement with carriers
Limited indoor coverage per square footage
Lower performance compared to small cells or DAS |
About the Author
Ron Tellas is a subject-matter expert in RF design and Electromagnetic Propagation and joined Belden in 2016 to help define the roadmap of technology and applications in the Smart Building. He now takes these experiences to incorporate systems, equipment and connectivity into network solutions. Ron represents Belden in several standards organizations, committees of the National Electrical Code and as a board member of the FMP Alliance. He has a master and bachelor degrees in electrical engineering, a master in business administration and the inventor of 17 US patents. Ron is a proud recipient of the 2024 Harry J. Pfister award for his outstanding and impactful contributions to the Telecommunications Industry.