The Complete Guide to Distributed Antenna Systems (DAS)

Everything you need to know about the technology, pricing, and implementation.

In-Building Distributed Antenna Systems have become a critical part of both carrier’s cellular networks and enterprise infrastructure. But the technology itself has fractured and evolved into so many variants over the course of the last two decades that it’s become opaque to most I.T. experts. The list of acronyms is a little overwhelming: iDAS, oDAS, eDAS, Active DAS, Passive DAS, Hybrid DAS, Off-the-air DAS and over a dozen more.

That’s why we’ve put together this guide–we’ve collected and simplified what we feel are the most important details about the different types of Distributed Antenna System to demystify the field. If you’re considering implementing a DAS, hopefully this guide lays out a basic topography of the industry and helps familiarize you with the associated technologies and key implementation strategies.

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The Basics: How a DAS works

You likely already have an idea of how a DAS works: a network of antennas broadcast cellular signal on the carrier’s licensed frequency bands and improving voice and data connectivity for end users. But it’s important to understand that a DAS, in its most abstract, simplified form, has two basic components:

  1. A signal source

    A Distributed Antenna System, as the name implies, “distributes” signal. But it generally doesn’t generate the cellular signal itself. A DAS needs to be fed signal from somewhere. There are four typical signal sources: off-air (via an antenna on the roof), a fiber feed from the carrier’s network, an on-site BTS (Base Transceiver Station)or from Small Cells. We’ll discuss each of these in more detail below.

  2. Distribution system

    The signal source’s signal needs to be distributed throughout a building. There are different kinds of distribution system, but there are four main types: Active (using fiber optic or ethernet cable), Passive, Hybrid, and Digital.

    Depending on what kind of distribution system is used, a DAS may or may not need “remote units” to convert fiber or Ethernet digital signals back to radio frequency signals. The final RF signal is then broadcast via in-building antennas.

Basic DAS Structure
Basic DAS Structure

Coverage vs Capacity

When choosing to implement a Distributed Antenna System, building owners are typically focused on solving on one of two problems:

  • Problem 1: Extending the edges of coverage

    DAS technology is often used to extend cellular coverage at the edges of existing macro cellular networks. Those edges can be anywhere: underground, inside a building, or between towers where macro network is weak. A typical example might be a hospital with dense building materials where there’s little or no cellular coverage indoors.

  • Problem 2: Increasing capacity in dense usage areas

    Some locations see a lot more cellular data usage than others. Think of a sports stadium hosting the Super Bowl, or a large music venue hosting Justin Timberlake. If the venue relied on a nearby cell tower to provide coverage to all those users, the tower and the local network would quickly become overwhelmed and unstable.

Identifying one of these needs as the primary requirement of your project is an important first step. But choosing the technology for a DAS project is a trade-off between coverage, capacity, and price, and considering both factors is important when making decisions.

Signal Sources

The signal sources used for a DAS system is probably the single most important factor in determining both the coverage area and capacity. No matter how great the distribution system, a DAS is always limited by the quality of the signal fed into the network. Let’s dig into to the three main types of signal source used today:

Off-Air Signal

An off-air DAS (also sometimes called a “repeater”) uses a donor antenna on the roof to receive and transmit signal back to the cell carrier and is the most common signal source for DAS. That means that the quality of the donor signal is limited by the quality of the signal at the donor antenna site. If signal at the donor location is very weak or the nearest tower is quite congested, an off-air DAS sometimes simply isn’t feasible. But if donor signal is strong and clear, an off-air DAS is often the easiest and most cost-effective signal source.

An off-air DAS doesn’t add any extra capacity to the carrier’s network and is primarily used to extend coverage at the edges of the network. Off-air DAS deployments are typically the lowest cost option, and are most suitable when the primary reason for deploying a DAS is extending coverage inside a building.

Advantages

  • Fast deployment
  • Minimal carrier-involvement
  • Lowest cost
  • Can work with multiple carriers
  • Less need to consider "hand-off" zones

Disadvantages

  • Performance depends on donor signal and congestion on macro network
  • If donor signal changes, system performance will change
  • Does not add any capacity – relies on macro cellular network
  • Optimizing signals for multiple carriers can be difficult
  • Retransmit agreements are often required (per carrier) prior to installation

Choosing an integrator with a strong understanding of RF (radio frequency) is critical when implementing an off-air DAS system. The performance of the DAS will depend strongly on proper evaluation and optimization of the donor signal.

Small Cells

Small Cells are the latest technology from carriers for use inside buildings, and there’s a dizzying array of names that all refer to the same thing. Femtocells, Picocells, Nanocells and Metrocells all basically the same technology: they create a secure tunnel back to the carrier’s network over a normal Internet connection and generate fresh wireless signal.

The typical coverage area of a Small Cell is small–about 5,000 to 10,000 square feet–and they’re relatively expensive. So while covering larger venues with dozens of Small Cells isn’t cost effective, their coverage area can be greatly expanded by using them as a signal source for a Distributed Antenna System. One big limitation of Small Cell technology is that they require a reliable backhaul Internet connection in order to connect. Each enterprise-rated Small Cell typically supports around 200 users.

Advantages

  • Create high-quality, fresh signal
  • Relatively low-cost compared to using a BTS
  • Fast to deploy
  • Ideal for buildings with hundreds of users

Disadvantages

  • Hard to scale to provide coverage for thousands of users
  • Relies on a venue-provided Internet connection
  • Requires careful planning of hand-off zones between small cells and the macro network
  • Not all carriers have enterprise Small Cells available

We’ve performed a number of Small Cell-fed DAS integrations, and the results are typically excellent. We expect this will be the fastest-growing new technology in the DAS space.

BTS, NodeB, and eNodeB

Okay, so we said there were lots of acronyms, and this is a perfect example. But it’s really quite simple in this case: BTS, eNodeB, and eNodeB really all mean the same thing – they all refer to the technology used inside cell phone tower base stations to generate cellular signal. For simplicity we’ll call this group just a BTS-fed DAS.

The connection between the ”BTS” and the DAS can happen in one of two ways: there may be a fiber connection back to a nearby tower, or the BTS itself may be installed inside the target building. In larger stadiums and airports it’s not uncommon to have multiple BTS systems for each carrier to handle the load of tens of thousands of users calling, texting and using data simultaneously.

BTS-fed DAS systems are typically slower to deploy and more expensive: each carrier needs to provide either their own on-site BTS or a feed from a nearby cell site.

Advantages

  • Highest performance
  • Can provide as much capacity as needed for venue

Disadvantages

  • Much more expensive than other options
  • Slower to deploy – can often takes months (up to a year) for carrier to integrate their system
  • Require careful planning around “hand-off” zones for users as they enter and leave the building
  • OPEX costs
  • Space and heating requirements

We’ve performed a number of Small Cell-fed DAS integrations, and the results are typically excellent. We expect this will be the fastest-growing new technology in the DAS space.

Mix and Match

Depending on the setup of the DAS, it’s sometimes possible to mix and match the different signal sources listed above in a single venue. For example, one might use a Small Cell signal source for one carrier, and bring the remainder of the carriers from an off-air donor antenna.

Signal Distribution Technologies

Whatever signal source a system uses, a DAS needs to amplify, distribute and rebroadcast it through the building. There are four general types of signal distribution technology: active, passive, hybrid and digital. We dig more into each below.

Passive DAS

A “Passive” DAS uses passive RF components such as coaxial cable, splitters, taps and couplers to distribute signal inside a building. The farther the antenna is from the signal source and any amplifiers, the more attenuation (loss) there will be in the power broadcast from that antenna. Designing a passive DAS correctly requires calculating precise “link budgets” to make sure the outputted power at each antenna is equal.

Advantages

  • Highest performance
  • Can provide as much capacity as needed for venue

Disadvantages

  • Very long runs of cable are harder to achieve because of signal attenuation
  • Plenum-rated coaxial cable, if required, can be expensive
  • Requires precise “link budget” calculations to ensure optimal performance

The majority of projects we install are passive systems, as they’re typically simpler and our customers appreciate the lower associated costs. However, for larger buildings we often recommend Active or Hybrid DAS systems.

Active DAS

An “Active DAS” converts the analog radio frequency transmissions from the signal source to a digital signal for distribution. A “master unit” (also called a “head end”) performs this conversion, taking one or multiple carriers and bands and digitizing the signal. The converted signal is transmitted over either fiber optic or Ethernet cable to “Remote Radio Units” (RRUs) that convert the signal back to regular analog signal.

In an Active system (as opposed to the Hybrid configuration described below) coaxial cable is not utilized–the antennas themselves are the active Remote Radio Units. The fiber optic or Ethernet cable runs straight to the antenna unit, and the conversion back to analog RF is done by circuitry inside the antenna.

Advantages

  • Ethernet or fiber optic cable can be shared with WiFi or public safety infrastructure (often complex in reality)
  • Easily expandable
  • No limits to lengths of cable runs

Disadvantages

  • Considerably more expensive than passive or hybrid systems
  • Remote units are more expensive and require dedicated power

Hybrid DAS

A “Hybrid DAS” is a cross between a Passive and Active systems. The Remote Radio Units are separate to the antennas, allowing the system to utilize fiber optic cable to distribute signal throughout a building, but also utilize coaxial cable to connect antennas. Since fewer Remote Radio Units are needed in this arrangements, costs are typically lower than Active DAS deployments.

A typical arrangement is to have a “remote” unit on each floor that converts from the digital signal back to analog RF, and multiple antennas feeding from a single “remote” covering a defined coverage area.

Advantages

  • Somewhat cheaper than an Active DAS system
  • No limits to length of cables in the digital backbone

Disadvantages

  • More expensive than a Passive system
  • Requires link budgeting on each floor
  • More complicated to install as it requires both fiber and coax

Digital DAS

The very latest development in DAS technology is the Common Public Radio Interface (CPRI) specification, which allows a Base Band Unit (“BBU”–basically a different kind of BTS) to communicate directly with the DAS Master Unit and through to the Remote Units without any conversion to an analog RF interface. A typical arrangement is to have a “remote” unit on each floor that converts from the digital signal back to analog RF, and multiple antennas feeding from a single “remote” covering a defined coverage area.

Advantages

  • Theoretically simpler to deploy
  • Theoretically cheaper to deploy

Disadvantages

  • Competing standards have meant little real-world deployment

Putting it all together

With 3 different signal sources and 4 different distribution systems, there are a total of 12 different possible combinations of source/distribution system. But in reality, there are a more limited number of actual configurations. Below we list the most common, and what they’re best for.

Passive, Off-Air DAS

Passive Off-Air DAS Flow Chart

This is the lowest-cost and generally the quickest kind of Distributed Antenna System deployment, particularly when coverage for multiple carriers in necessary.

Recommended for:

Most projects up to around 500,000 square feet. Requires suitable outdoor signal and proactive carrier acknowledgement.

Small Cell Passive DAS

Small Cell Passive DAS Flow Chart

If donor signal quality is weak or nearby towers are congested, combining Small Cells with a Passive DAS is often a great option. We’re implementing an increasing number of Small Cell-fed DAS systems, and often see better results than with typical Off-Air Passive systems.

Recommended for:

Projects up to 500,000 square feet where a reliable backhaul Internet connection is available.

Off-Air Hybrid DAS

Off-Air Hybrid DAS Flow Chart

A Hybrid DAS system combines the ability of an Active DAS to cover very large areas with some of the price advantages of a Passive system.

Recommended for:

When very long cable runs are unavoidable, or the coverage area is very large but sparsely populated.

Small Cell Passive DAS

Small Cell Passive DAS Flow Chart

If donor signal quality is weak or nearby towers are congested, combining Small Cells with a Passive DAS is often a great option. We’re implementing an increasing number of Small Cell-fed DAS systems, and often see better results than with typical Off-Air Passive systems.

Recommended for:

Projects up to 500,000 square feet where a reliable backhaul Internet connection is available.

Get in touch!

If you’re considering a Distributed Antenna System, get in touch. One of our RF engineers will walk you through the process of figuring out exactly which kind of system is most appropriate.

As a vendor-neutral integrator, we’re not tied to any one particular technology or manufacturer. We’ll work with you to figure out exactly which kind of system will meet your needs for the lowest cost possible.