Rooftop Slights

Rooftop sites offer numerous advantages, but you have to take the bad with the good.

The advantages of rooftop sites have been obvious for some time. In densely populated areas with little land, you can put up these sites without hunting for land for large towers. You typically can deploy these sites faster than towers due to fewer zoning requirements. It is less expensive to put equipment on a roof than to buy a tower. And, rooftop sites are more easily disguised.

In particular, wireless-service providers in metropolitan areas on the East and West coasts, which have denser populations and stricter zoning boards than inland areas, have vast experience with rooftop sites. But through the years they have discovered that despite their good points, rooftop sites are not perfect. Thanks to creative site designs and new technology, these service providers have beaten the challenges that rooftop sites present.

Ashraf Abdu, Nortel Networks product-marketing advisor for GSM, worked closely with Omnipoint when it built out its New York network several years ago. He said Omnipoint found that most rooftops in Manhattan and the Bronx were older and could not support the weight of a base station. Newer buildings were owned by landlords who were uncomfortable with the idea of drilling holes in their roofs, which can create leaks. In such cases, Nortel and Omnipoint put up bracketing systems on the buildings' walls and mounted the base stations there."Walls can bear the load of a base station easi er than a roof," Abdu said. "Mounting it on the wall also saves you from any concerns a landlord or super might have about creating a leak."

The rooftops Omnipoint chose usually had elevator rooms or small buildings already on the roof. Other times, a wall extended above the roofline. The service provider mounted base stations on these walls.

Like Omnipoint, Alltel also takes advantage of mounting base stations to walls if a rooftop can not support the load. Mark Kelso, Alltel vice president of engineering services, said these situations are usually microcell applications that put out less power. If the site needs more capacity and requires a larger site, Alltel will do all it can to strengthen a roof to accommodate its equipment.

"We have structural engineers look at the building in every case for safety reasons, and we have had to lay down major steel plates to bring the roof up to code, get it passed by the zoning board, and have it reviewed by engineers," he said.

Steve Clark, U.S. Cellular vice president of network operations, said that although new technology allows service providers to mount equipment on a pole or a wall, doing so is not always the best answer.

"We will use some (wall-mounted sites), but those typically have a smaller channel capacity than what we put into a regular cell site," he said. "If we are on a roof, it is usually at the core of a metropolitan area, and that is where we need a lot of channels and not just a handful."

Instead, U.S. Cellular will add support to the roof by placing a structure under the cell site to distribute the load over a greater area. If the roof still can't handle the site, U.S. Cellular will go elsewhere.

Coordination Issues
U.S. Cellular's Clark said coordinating among service providers, landlords and tenants is a big challenge to rooftop sites. Some landlords only let service providers access the roof during certain hours. Also, desirable rooftops probably already have several tenants. High RF emissions can endanger engineers who must work on the site.

"If we have to do some maintenance, we will coordinate with the other carriers and not necessarily have them turn their site down but make sure they have someone there in case we need to do something," he said.

The service provider also uses equipment to alert the crew about hazardous RF emissions.

"We try to design the site so our people are not in a direct beam of another carrier, but if we do get into a situation like that, we try to coordinate so nobody is in harm's way," he said.

Abdu added that multiple service providers on one roof creates space issues, as well. Nortel tries to design compact base stations so service providers don't take up a lot of rooftop space. For example, its S8000 Outdoor Base Transceiver Station measures 53 inches by 25 inches, so a 5.5-foot by 2.5-foot platform with an additional 25-inch door clearance is sufficient.

When more than one service provider occupies the same roof, Clark stressed the importance of educating landlords about the civil requirements that emerge when you add more cell sites to a roof.

"You must make sure they understand loading and coordinate that with them as they put up other tenants on the roof," he said.

Performance & Noise Issues
Rooftops also create frustrating system-performance problems, Kelso said. Many times, rooftop sites don't allow service providers to adjust their systems to meet the performance demands of the customer base. For example, if your antennas sit on a penthouse that is 20 feet in from the building's edge, pointing them close to the ground will cause them to transmit into the roof.

"If I adjust the tilt on those antennas, I run the risk of not serving certain areas because the building is blocking the RF from those areas," he said.

Clark agreed that getting a perfect 360-degree radiation is difficult on rooftops. If part of the roof blocks the antenna, a coverage hole is created near the building. U.S. Cellular uses sectored sites to mitigate theproblem. It prefers to hang sectored antennas over the side of the building if l andlords will permit it. In places where it uses omni antennas, it can get 10 to 15 more feet in height if it places them on any part of the building that extends above the roofline.

Nortel's Abdu pointed out that noise is another disadvantage to mounting cell sites on rooftops, especially in residential areas or on office buildings. At night, base stations are quieter because there is less radio activity. The temperature also is cooler, so the fans will run less than during the daytime. But a noisy base station can irritate tenants. Abdu suggested looking for base stations with fan systems that operate at lower speeds for areas where you want less noise.

Rooftop Myths
Two advantages that most people see in rooftop sites are that they face fewer zoning requirements and are cheaper to deploy. That is not always the case. Kelso said that even though you can save money by locating on rooftops instead of building a tower from scratch, rooftop sites still are expensive. Sometimes service providers must place the RF equipment in the building's basement. The service provider will have to run transmission lines through the existing conduit, or drill through the building.

"You are talking about major construction work, and that can delay the cell," he said.

Clark pointed out that zoning boards are not necessarily easier on rooftop sites than on towers. Many times, you won't have to go through land-use zoning boards, but you will run into cities that oppose antennas on buildings for aesthetic reasons.

Rooftops offer many advantages, but be sure to think about their problems and how you will address them before deploying a site. Kelso said the most important thing to remember when designing any site is to be open-minded. Alltel considers each site separately and does not prioritize rooftop sites for any reason.

"Alltel engineers look at what best fits for each particular application or location," he said. "We work with communities and planning commissions in a lot of areas. We go in early and show them the sites we are looking at, and we ask them what their preferences are at that time."

Harter (betsyharter@aol.com) is a freelance writer based in Athens, GA.