Bringing Signals Indoors: How In-Building DAS Works

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:

1. A donor antenna brings wireless signals into a building. This approach improves coverage but not capacity (it doesn’t allow more people to connect.)
2. 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.