DAS meets the need for pervasive connectivity

June 1, 2014
Distributed antenna systems (DAS) continue to grow in popularity as the drivers that create demand for them remain strong.

From the June, 2014 Issue of Cabling Installation & Maintenance Magazine

Exploring not just the "whys" but also the "hows" of outfitting enterprise environments with distributed antenna systems.

by Patrick McLaughlin

Distributed antenna systems (DAS) continue to grow in popularity as the drivers that create demand for them remain strong. Increasing numbers of user organizations are investigating and deploying DAS to support their mobile-connectivity needs.

On April 24, we hosted a web-based seminar titled "Distributed Antenna Systems: Options and Best Practices." The seminar examined the mobile-use trends and facility considerations that make DAS realistic options for enterprise environments, as well as the components that make up a DAS-including supporting cabling infrastructure-and some best practices for accomplishing a DAS deployment as efficiently and effectively as possible. The seminar was presented by Paul Kopera, vice president of emerging technologies and technical services with WESCO CSC (www.gocsc.com); John Spindler, vice president of product management for the wireless business unit of TE Connectivity (www.te.com); and Richard Baldasarre, senior mobility solutions architect with Vision Technologies Inc. (www.visiontechnologiesinc.net).

Growing and changing demand

Delivering a presentation titled "When, Where and Why DAS is a Preferred Solution," WESCO CSC's Kopera explained that macro cellular networks "are challenged by the change in use and dynamics in the marketplace. Ten to fifteen years ago these networks were intended to be voice-only, and the volume on any network was relatively small. In the year 2000 only about 38 percent of the population used wireless devices, and the idea was to be mobile-to have connectivity while moving from one place to another, from home to the office for example."

In this way the macro cell network was intended to be an outdoor expansion of voice coverage. Users did not expect to have that cell signal inside a building, and they reverted to a desktop or landline phone once indoors.

"Fast forward to today and wireless penetration exceeds 100 percent in the United States," Kopera said, with many individuals having multiple wireless devices. In addition to the proliferation of devices over the past decade-and-a-half, Kopera also pointed out that user expectations have changed dramatically in that time as well: "We have an expectation as consumers that we'll have coverage in any building, anywhere, at any time." The change in technical dynamic that arises from this lofty expectation cannot be overstated. Years ago wireless coverage equated to a user being able to maintain a phone conversation from cell to cell while going from one place to another. Today wireless coverage means the user can visit the Internet at will, sometimes while in motion but also often while stationary for any period of time. And, chances are, in close proximity with many other individuals who have the same expectation for wireless coverage. The reality of multiple users in the same place expecting connectivity is why discussions about-and planning for-distributed antenna systems consider not only coverage but also capacity.

Attitudes are also shifting about from whom we expect that coverage and capacity to come. Whereas in the early 2000s, coverage problems were the wireless carriers' problems, today users "tend to think that providing coverage isn't really the problem of the carrier any longer, but is the problem of the venue," Kopera commented. Indeed, with the adoption of either DAS or WiFi by many public venues, users now generally reserve their scorn for locations that do not provide such coverage rather than for their wireless service providers.

Overcoming obstructions

Enterprise-based DAS comes into this picture because, as Kopera also pointed out, macro cell networks are still optimized for outdoor environments. Furthermore, the physical setup of these outdoor networks optimizes them for downward-looking, ground-level communication. "They are not designed to meet the needs of dense urban environments, which include highly efficient buildings and high-rises," he added.

An office building is loaded with elements that degrade a carrier's radio-frequency (RF) signal. For example drywall, a typical construction material, can reduce RF signals by 20 to 25 percent, with each drywall having a multiplicative effect. And a concrete wall can reduce a signal by up to 90 percent. Additionally, materials like low-e glass that are used to make buildings energy-efficient are unfriendly to RF signals, reducing the amount of signal that enters the building at all. And stairwells are notorious dead spots.

A building that includes any of these elements and serves its occupants with a weak RF signal then faces the capacity issue, perhaps best exemplified by an office building's conference room in which everyone expects to have a signal.

These factors conspire to make coverage and capacity a challenge in a typical office building, and several other specific user environments have additional needs that a macro cell network generally cannot meet. Higher-education and medical environments, for example, require pervasive and round-the-clock connectivity.

Project in progress

For these reasons and others, deployment of DAS networks by enterprise user organizations is on the rise. In that vein, Vision Technologies' Baldasarre delivered a presentation titled "Managing a DAS Deployment Project." In it, he emphasized the importance of coordination with the wireless carrier(s) whose signal will be distributed via the DAS. One critical point is that the signal is, in fact, licensed to the carrier-the license was purchased via a Federal Communications Commission auction for a hefty sum. Because the carrier has the license to the signal, it is that carrier rather than the user organization that decides the specifics of the signal's use. Legal agreements between the carrier and user organization are an essential part of a DAS project, Baldasarre noted.

As for the specifics of successfully managing a DAS installation, Baldasarre offered several points that come from a wide range of experience. A fundamental question to ask and answer is: Why is the system being installed? "Is it for mission-critical applications?" he asked. "Public safety? For the convenience of building occupants?" These questions matter because, as Baldasarre noted, when public-safety signal runs on the same system as a carrier's signal, proper filtering must be accounted for to avoid interference. The bands used by these different signal types are close in frequency and therefore prone to such interference.

He also discussed the importance of gaining a thorough understanding of the facility in which the DAS will be installed, and explained that a venue survey is an effective means of gaining such an understanding. A venue survey will cover such nuts-and-bolts installation issues as cable pathways for riser and horizontal areas. "A venue survey will tell you if vertical pathways already exist, or if they will require core drilling," Baldasarre offered as an example. "Are you dealing with firewall penetrations? Understanding the interior of a facility is important. For example, a medical facility can have lead-lined rooms. You'd have to design a DAS around those rooms, because the RF signal will not penetrate."

Also, he explained, "Another key issue is understanding the macro environment, and how much of the macro signal penetrates into the building from outside." Once the enterprise DAS is installed, he pointed out, "carriers will want to have all users inside the building stay on the DAS rather than using the outside network. If you have a strong signal from the macro network coming into the building, you will need to provide an even stronger signal inside the building." A site survey will discover the strength of the macro network's signal inside the building, and will enable the user organization to request a strong-enough signal from the carrier for in-building use.

Baldasarre also brought up the issue of testing an installed DAS, which involves several types of testing and verification procedures. Of specific interest in hybrid (combination of active and passive) DAS networks is the performance characteristic passive intermodulation (PIM). "It's very important to have these [hybrid] systems PIM-tested," he said. "PIM is noise on a system and is very important" to a DAS network's performance, he pointed out.

He also noted that many of the processes he described-surveys, installation procedures, design and testing operations-require tools specialized for these activities. "You may decide it is best to partner with a firm to help through the engineering and design processes, and to make sure those processes are done right," he said.

Piecing it together

TE Connectivity's Spindler, delivering a presentation titled "DAS Components and Architecture," spent time discussing the differences between system types-passive, active and the hybrid type that Baldasarre mentioned.

"You can think of a DAS as similar to a building's sprinkler system," he analogized. "A sprinkler system takes water from a source and distributes it throughout a building using small sprinkler heads, and a DAS takes a cellular or RF signal source, distributing it throughout a building using small antenna points."

Spindler provided big-picture-view differences among passive, hybrid and active DAS. TE Connectivity provides active DAS systems. "Passive DAS is called that –passive-because it doesn't contain active electronics. There is no active amplification of the signal once it enters the system. There is the RF source, then a backbone of coaxial cable-either seven-eighths inch or three-fourths inch. From there, half-inch coaxial cable runs in the horizontal to the passive antenna points." This setup has been used for many years, he explained, and its main drawback is, "Like with a garden hose, the farther you run the coaxial cable, the less signal strength you'll have at the end of it. Coaxial cable tends to be lossy, so there is a limitation on the run length." Passive DAS is not used on a widespread basis in North America any longer, he also pointed out.

"With a hybrid DAS you typically use a hybrid of fiber and coaxial cabling," Spindler said. "From the headend, multimode or singlemode fiber runs to remote units in IDFs [intermediate distribution frames or intermediate crossconnects]." These remote units amplify the signal. "From there, in the horizontal, half-inch coaxial cable goes to small passive antennas in the ceiling. You benefit by not having signal loss between the headend and the remotes," he pointed out, "but there is loss between the remotes and the antennas. And the distance between the closet and the passive antennas could be as much as 100 meters.

"In a purely active DAS there is a main unit or headend fed by the RF source, typically located in a main telco room," he explained. "From there, coaxial cable goes to expansion units, which will be in IDF closets. The difference is, from those expansion units you'll run Category 5e or CATV cable-which is thin coaxial cable in contrast to half-inch coax-to smaller remote units. The passive antenna is close to that remote unit, which gives you effectively a zero-loss system."

In addition to describing the three types of DAS setups, Spindler also explained that the labor cost associated with system installation, as well as the material and labor for the cabling that supports the DAS, are significant considerations. In fact, "As much as 50 percent of the cost of implementing a DAS can be in cabling and installation of the system," he said. "The box cost is not the only consideration for a DAS."

The webinar from which this article was derived can be viewed through October 24, 2014 at www.cablinginstall.com.

Patrick McLaughlin is our chief editor.

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