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In-Building Distributed Antenna Systems (DAS) have become a critical part of both carrier cellular networks and enterprise infrastructure. But as the technology has evolved over the last 20 years it has become increasingly complex. The list of acronyms alone can be overwhelming: iDAS, oDAS, eDAS, active DAS, passive DAS, hybrid DAS, off-air DAS, and numerous others. This guide will describe common types of distributed antenna systems and effective implementation strategies.
This guide focuses on cellular DAS. For information about Public Safety DAS and ERRCS, see our other guide: Meeting NFPA and IFC Building Codes with an Emergency Responder Radio Coverage System (ERRCS) .
A DAS is a network of antennas that sends and receives cellular signals on a carrier’s licensed frequencies, thereby improving voice and data connectivity for end users. In its most simplified form, a DAS has two basic components:
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), an on-site BTS (Base Transceiver Station), and finally the newest approach: small cells.
Once received, the cellular signal must be distributed throughout the building. There are four main types of distribution systems: active (using fiber optic or ethernet cable), passive, hybrid, and digital.
A distributed antenna system’s performance depends on the type of technology it uses. To understand what we mean by “performance,” we first need to understand the two main performance metrics: coverage and capacity.
To be able to compare the different technologies, we first need to review the two main performance reasons that DAS solutions are deployed: to provide coverage and capacity.
Some locations experience significantly 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. In such applications, a DAS with high capacity is the primary need.
If there's simply not enough usable signal reaching users, either because the cell tower is too far away or due to building materials such as low-E windows in LEED Buildings blocking cell signal, the primary need is coverage. For example, a newly-built LEED-certified hospital with concrete walls might have no indoor coverage and require a DAS. Highrises often use DAS deployments because the radio frequency noise levels at higher altitudes make the signal unusable.
Identifying one of these needs as the primary requirement of your project is an important first step. Choosing the right DAS technology means making trade-offs between coverage, capacity, and price.
The signal sources for a DAS system are one of the single most important factors in determining both the coverage area and capacity. No matter how well the distribution system performs, a DAS is always limited by the performance of the signal supplying the network. The three main signal sources are off-air; BTS, NodeB or eNodeB; and small cell.
A DAS that uses an off-air signal (sometimes called a repeater) utilizes a donor antenna on the roof to receive and transmit signals from a cell carrier. Off-air signals are the most common signal sources for a DAS. If the signal at the donor antenna is very weak or the nearest tower is quite congested, using an off-air signal isn't typically feasible. But if the donor signal is strong and clear, then an off-air signal is often the easiest and most cost-effective signal source.
A DAS that uses an off-air signal source does not add any extra capacity to the carrier’s network and is primarily used to extend coverage at the edges of the network. These deployments are often the lowest cost option, and are most suitable when the primary reason for deploying a DAS is to extend coverage inside a building.
Choosing an integrator with a strong RF (radio frequency) experience 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.
Base Transceiver Station (BTS), NodeB, and eNodeB refer to the technology used inside cell phone towers to generate a cellular signal. For simplicity, these technologies are often referred to simply as a BTS signal source.
The connection between a cell carrier's BTS and the core network typically require a dedicated fiber connection that is usually installed by the carrier themselves. A distributed antenna system in a large stadium or airport may even connect to multiple BTSes—one for each carrier—to handle the load of tens of thousands of users calling, texting and using data simultaneously.
DAS systems that use BTS signal sources typically take longer to deploy and are more expensive; each carrier must run their own fiber and the BTSes themselves are typically at least $50k+ each.
Small cells are the latest technology used by carriers to provide cellular service inside buildings. There are several variations of small cells, including femtocells, picocells, nanocells, and metrocells. These are all basically the same technology—they create a secure tunnel back to the carrier’s network over a normal Internet connection and generate a high quality wireless signal.
The typical coverage area of a small cell is only about 5,000 to 15,000 square feet, and they are relatively expensive. While covering larger venues with dozens of small cells isn’t cost effective, the coverage area of a small cell can be greatly expanded by using them as a signal source for a distributed antenna system. One limitation of small sell technology is that they require a reliable backhaul Internet connection in order to connect. Each enterprise-grade small cell typically supports around 200 users. We’ve installed a number of DAS projects that use small cells as a signal source, and the results are typically excellent. We expect this will be the fastest-growing new technology in the DAS space.
Depending on the setup of the DAS, it’s 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.
Whichever signal source a system uses, a DAS needs to amplify, distribute and rebroadcast it through the building. There are four main types of signal distribution technology: active, passive, hybrid and digital.
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.
Most DAS systems that we install are passive systems. They are typically simpler than other types of distributed antenna systems, which our customers appreciate. However, we often recommend active or hybrid DAS systems for larger buildings.
An active DAS converts the analog radio frequency transmissions from the signal source to a digital signal for distribution. A master unit performs this analog-to- digital conversion. The master unit may digitize the signal from a single carrier or multiple carriers. Once converted, the DAS transmits the digital signal over fiber optic or Ethernet cables to remote radio units (RRUs) that convert the signal back to an analog signal.
Unlike passive or hybrid systems, active systems do not use coaxial cable to distribute signal. 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.
A hybrid DAS combines characteristics of passive and active systems. The RRUs are separate from the antennas, allowing the system to use both fiber optic cable and coaxial cable to distribute signal throughout a building. Because this configuration requires fewer RRUs, a hybrid DAS normally costs less than an active DAS.
A typical hybrid DAS configuration includes an RRU on each floor that converts from the digital signal to analog RF. The analog RF signal is then connected to multiple antennas on that floor with coaxial cable.
The very latest development in DAS technology is the Common Public Radio Interface (CPRI) specification, which allows a base band unit (BBU, which is a 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.
With three different signal sources and four different distribution systems, there are a total of twelve possible DAS configurations. In practice, however, there are far fewer. Here is an overview of the most common configurations, and the applications for which they are best suited.
This type of DAS costs less than other types and offers the shortest deployment time. This is particularly true when coverage for multiple carriers is necessary.
Most projects up to around 500,000 square feet. Requires suitable outdoor signal and proactive carrier acknowledgement.
If the donor signal quality is poor or nearby towers are congested, combining a small cell with a passive DAS is often a great option. We’re installing an increasing number of small cell-fed DAS systems and often see better results than with off-air systems.
Projects up to 700,000 square feet where a reliable backhaul Internet connection is available.
A hybrid DAS combines the ability of an active DAS to cover very large areas with some of the price advantages of a passive system.
When long cable runs are unavoidable, or the coverage area is very large but sparsely populated.
This option requires carriers to hook into the DAS system you build, which is often time- consuming and bureaucratic.
Best Systems that must serve thousands of users in a single venue and systems where the main goal is high capacity rather than coverage.
If you’re considering a Distributed Antenna System, one of our RF engineers can walk you through the process of figuring out exactly which configurition 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 determine exactly which system will meet your needs for the lowest cost possible.