If you’ve been to a sporting event or attended a concert, you may have used your smartphone to text a friend to brag, call your spouse to say you’ll be running late, or upload a video of some amazing scene. Changes are, if you did any of these things, your text, call or file transfer was carried over a Distributed Antenna System, or DAS.

DAS can be found in many large facilities around the country and around the world: stadiums, concert halls, convention centers, airports, malls, you name it. DAS even shows up out-of-doors, covering parks, pedestrian malls, stretches of highways and tunnels, and other locations that may be too difficult or too expensive to serve conventionally. Wherever there are large areas (and often a lot of people) that require improved wireless access, DAS is often the solution of choice.

Fundamentally, a DAS takes the signals from a source (typically a 2G/3G base station or an LTE eNB) and moves it over an intermediate transport network to a number of other remote locations (and back again). In effect, the DAS extends the coverage of the source over a much larger area in a planned and controlled manner, providing users throughout the area significantly better service than would otherwise be possible.

Inside and Out

DAS comes in a wide variety of shapes and sizes and technologies, since the result has to accommodate the specific requirements of the location and the intended audience. One obvious differentiation is where the DAS is located: indoor DAS (i-DAS) installations are deployed within buildings and other structures, while outdoor DAS (o-DAS) provides coverage out in the open air. A variant of i-DAS, the large venue DAS, refers specifically to DAS installations in facilities with either a lot of space, a lot of people, or both.

Yours, Mine and Ours

Ownership of and access to the DAS provides another distinction between systems. When the DAS is owned by a particular service provider or network operator, it’s referred to as an Operator-as-Host installation; although the DAS can be configured to support subscribers from multiple providers, it’s primarily intended for the use of the owner, who is also responsible for managing and maintaining the DAS. Most DAS, however, are owned by a third party (usually the facility owner), who then provides access to the DAS to any operator willing to pay the access charges. This arrangement is referred to as a Neutral Host.

Carrying the Load

The third differentiator between DAS installations is based on the underlying transport technology. There are two main approaches to moving the signals between the source and the remote DAS antennas.

Passive DAS systems take the RF signals transmitted from the source or the antennas and carry them over standard RF coaxial cable (coax). Since the signals are not modified in any way, the components involved (diplexers, duplexers, splitters, combiners, tappers, and so on) are relatively simple and inexpensive, and they do not require power or other connections. On the flip side, however, every length of cable and every component in the path reduces the strength of the radio signal, and the size of the system is limited by the maximum distance the signal can travel before it becomes too weak to be useful. As a consequence, passive DAS is best suited for relatively small indoor applications.

Active systems, on the other hand, convert the radio signals into a digital format that is carried over optical fiber. The optical signals can be carried over essentially unlimited distances, but the components (the master unit, expansion units and remote units) can be expensive and require both power and control interfaces. In addition, the conversion process often adds significant delay to the system, which must be compensated for in the DAS design. Active DAS is used in outdoor deployments and in larger indoor installations.

Active or passive, indoors or outdoors, operator-owned or neutral host, DAS provides the means to connect users to their services and content in locations that would otherwise experience poor performance, or no access whatsoever. By allowing the RF signals to be placed exactly where they’re needed, without the need to install new base stations or eNBs at each spot, DAS can meet the needs of the subscribers more effectively than other, more traditional solutions.