Networking

Distributed Antenna Systems

• By Wendy Chretien
• 11/01/07

DAS Boot!

Move to distributed antenna systems and give your campus a unified system for public safety wireless, cellular, and WiFi coverage.

THE RESIDENTIAL SERVICES group is lobbying to dump its aging hard-wired phone system because students don't use it. The town and campus public safety officials are demanding that their portable two-way radios operate well not only outdoors, but within campus buildings. Your students are now expecting text messaging and WiFi service to work everywhere on campus. Faculty members are grumbling that their chosen wireless provider's coverage is good at home, but poor on campus. And the CFO wants to know why her BlackBerry doesn't work in the tunnel between the main administration building and the annex. Welcome to the wide world of wireless wants and wishes.

Thankfully, it is possible to satisfy such diverse requests for unwired service. The technology required? A distributed antenna system, or DAS.

Was Ist DAS?

In essence, a DAS can connect to a variety of wireless services and then rebroadcast those signals throughout the areas in which the DAS is installed. The sources can include cellular service from multiple wireless carriers, public safety radio frequencies, and WiFi. If your campus has a medical clinic or hospital, the special medical wireless systems it uses also can be supported.

To better grasp how a DAS operates, it helps to know some of the methods by which wireless signals are propagated. Nearly all of us know what a cell tower looks like; well, each of those towers carries antennas for one or more macrocells (multiple carriers when there are “layers” of antenna arrays). But a microcell covering a limited area may have its antenna or antennas placed on a tower or pole, or mounted on or in a building. Wireless carriers use microcells to add capacity in areas with a high density of mobile wireless device users. A picocell has an even smaller coverage area. A WiFi access point can be thought of as one type of picocell. In a DAS setup, any or all of these technologies may come into play.

There are also multiple alternative methods for obtaining the necessary signals from the carriers, including T1s or fiber cabling directly connected to wireless carriers' networks. However, the wireless signal from an existing wireless tower (macrocell) is one of the most common sources of obtaining the service “injection.” The signal of a public safety radio system (e.g., from local or campus police) also would be taken from the air via the nearest repeater for that system.

Now that we know we're “inserting” signals, the next logical step is to think about what we're doing with them. Every DAS has a “head end” into which these source signals are combined for distribution. The signals are amplified and carefully “combed” together as needed, in the electronic equipment and filters at the head end. Intermediate amplifiers (usually referred to as bi-directional amplifiers or BDAs) are added to make up for signal losses due to the physical limitations in how far a cable can carry the signal. Cables then carry the signal out to passive antennas placed where more signal strength or coverage is needed. A DAS might use fiber from the head end to the remote BDAs, plus coaxial or shielded Category 5/6 cable from there. One of the selling points of a DAS is that (when done properly) a system can be designed to support all your wireless traffic: WiFi, cellular, PCS, paging, maintenance, and public safety. Just be aware that each of these technology types uses different radio frequencies, which directly affects the DAS design and the type of antennas to be installed in the buildings. Thus, you must decide in advance which wireless systems the DAS needs to support.

DAS Warning Flags

Distributed antenna systems, like most technologies, have their drawbacks. The three primary downsides to DAS solutions are cost, carrier buy-in, and a lack of standards.

1) Costs for indoor coverage can exceed a dollar or more per square foot; this adds up fast in a large building. Costs to supplement outdoor coverage must be determined on a case-by-case basis, since there are many variables. One way to save on a DAS installation is to prepare the building during construction, which means you will need to develop a schematic design and place necessary sleeves for the routing of DAS cables both horizontally and vertically. This predesign also includes determining where the head end should be placed and selecting cable routes to the roof for antennas.

2) Cooperative wireless carriers. In order to extend or enhance wireless service on campus, the institution must have permission and active support from its service providers. Carriers are responsible for compliance with FCC regulations such as avoiding interference among systems, so if their signals are to be rebroadcast by a third party (such as a higher ed institution), carriers must take an active role to ensure such compliance. Frequently, they aren't interested in getting involved in DAS implementations unless they anticipate a significant increase in the number of minutes used. They also may be reluctant if they believe competition will be too stiff in a multiple-provider scenario. If you want solid coverage from multiple providers, you will need to consider a “neutral host” type of system. Typically, integrators that implement these systems have the contacts and clout to bring multiple cellular providers on board. The integrator's role may also include negotiating revenue-sharing agreements with those carriers, thereby reducing or eliminating ongoing maintenance costs.

3) Standards. On this front, neither the IEEE nor the ITU has developed a standard for DAS. This leaves us in the unfortunate position that networking was, before Ethernet became the accepted standard: with competing proprietary technologies. It's therefore incumbent on those who must select the system to understand the pros and cons of the various manufacturers' offerings and associated methodologies of the various designs. Unbiased third parties (such as independent consultants) can assist you by helping assess the wireless needs of all your campus constituents, and then develop budgetary estimates so that you understand the range of costs you should plan for.

DAS Pioneer: Higher Ed

Don't be daunted by the potential negatives, though, because other universities have successfully gone before you. San Diego State University (CA), the University of Notre Dame (IN), the University of California-Santa Cruz, MIT, American University (DC), and Duke University (NC) all have installed distributed antenna systems in recent years. The first three of these chose NextG Networks to provide and build out those systems. MIT and AU both implemented a MobileAcess solution. The integrator MIT worked with was In-Building Cellular, AU chose BearingPoint, and Duke selected a system by LGC Wireless. In brief, each campus felt the need for ubiquitous wireless and found that a DAS was an effective method to meet that need. The implementations varied according to campus requirements: SDSU limited the extended coverage to outdoor areas, whereas Notre Dame and AU also sought coverage in dorms so they could do away with fixed-line telephone systems in residence halls. In the case of AU, Cisco Systems-brand WiFi access points also comprise part of the DAS. One opportunity in such a situation is to concentrate the wireless access points safely in technology rooms and use only passive antennas placed throughout the buildings to relay the wireless signals.

Additional DAS manufacturers include ADC, Andrew, Cellvine, InnerWireless, Powerwave, and Spotwave Wireless. Other integrators include LComm Global Solutions, InSite Wireless Group, and Nsoro. These lists are not all-inclusive, but they can help you see that there are plenty of fish in the DAS sea. Ultimately, your institution's needs and resources will determine whether a DAS is right for your own campus.

-Wendy Chretien, PMP, is a senior network systems consultant with Elert & Associates, an independent technology consulting firm.