Tower Trials

As the number of adequate sites for base stations dwindles, carriers are bulking up. Carriers in rural and suburban areas that need smaller and less-costly coverage solutions are relying more on tower-mounted elements and building fewer traditional base stations.

You not only can mount antennas and amplifiers on tower tops and poles, but also entire base stations, power cabinets and RF equipment boxes in outdoor cabinets.

Mounting electronics on towers and poles can enhance coverage and reduce costs. However, there are tradeoffs. You need to consider headaches such as co-location, zoning boards, overloading and maintenance issues.

Tall Challenges According to Paul Sergeant, Nortel senior manager of CDMA product marketing, tower-mounting radio equipment in a rural environment can reduce the number of cell sites by 30% to 40% for a large coverage area or stretch of highway. By tower-mounting the radio elements, you also get a better signal in terms of distance and quality.

"The advantages of having electronics on top of the tower are great," said Marty Cooper, ArrayCom founder & chairman. "(Carriers with electronics) on a 300-foot tower will die for 3dB or 4dB, and that's what you get."

Sergeant said another big advantage of tower mounting is that it avoids the loss inherent in cable.

"The taller the tower, the more loss you have to accept or the more money you have to spend on low-loss cables," he said. "If you can take the radio portions of a cell-site product and mount them on top of the tower, you can avoid all of the cable loss, and that means increasing the coverage and possibly the quality of the radio connection because you're also reducing the interference."

Tower-mounted technology isn't as valuable in metropolitan and high-density environments. But in rural or suburban areas, sometimes it is the only coverage option.

But effective coverage requires more than tacking boxes at the top of a tower. According to Sergeant, the more electronics you put on a tower or pole, the more maintenance it will need.

Maintenance is a problem in itself for some carriers. Cooper said carriers do not like to put equipment on towers because, at least on taller towers, they can't send technicians to the top. In some cases, crews that climb towers are not trained to service anything.

"The problem in dealing with pole-mounted equipment is being able to get people that have both the ability to climb a pole and also the ability to service the more technical equipment mounted at the top of the pole," said David O'Brien, American High Mast Systems vice president of technology.

Because of that concern, manufacturers are making components field replaceable, rather than field repairable. O'Brien said carriers also can sidestep this problem by mounting repeaters, amplifiers and other equipment just above ground level so it can be serviced from a step ladder, rather than putting the equipment where it's designed to be, immediately adjacent to the antenna at the top of the tower.

"That's just the short-term fix, 'we'll get it as close to the antenna as we can while still providing access for our men to service it from near-ground level,'" he said. "Rather than at 120 feet, for example, it's mounted at 10 or maybe six to 20 feet off the ground. That allows them to service it from five or 10 feet from the ground."

According to Sergeant, using fiber connections between radios and digital electronics instead of copper also can ease the tower-mounting process. This option eliminates the need for many cables, Sergeant said. Usually, there are six cables for each sector, and they are thick, signal-carrying cables. Fiber requires only a single, thin, flexible strand and one copper cable to carry power.

Base Stations in the Sky At AT&T Wireless' Spring Run Farms site near Dayton, OH, an entire working base station is mounted at the top of a tower. The tower-mounted items include a PCS minicell and RF equipment, nine antennas and three cabinets, which together weigh 32,000 pounds. The base station was tower-mounted because the site is located on a flood plain. According to FEMA regulations, electronics have to be mounted more than 10 feet above the ground because of the high water hazard posed by the nearby Ohio River.

"When you start building platforms and elevating things 10 feet in the air, it can get quite expensive," said Ray Martin, AT&T Wireless senior project manager in Dayton. AT&T developed a system, using American High Mast System's raise/lower product, to tower-mount electronics in the most cost- and maintenance-effective way. Martin said cost savings, less cable loss, less chance of vandalism and space efficiency were the prime motivators for choosing this option.

"There's new equipment coming that is much smaller and is going to take up less ground space, so why not put (the cell site) with the antennas," Martin said.

In another case, AT&T's equipment was required (because of FEMA regulations) to be 18 feet in the air, which would have cost $140,000 just for the steel for the platform. By comparison, costs with the raise/lower system were less than $50,000, Martin said.

Martin said the tower-mounted cell site hasn't required more maintenance than other towers and has just as many channels as the other sites.

With a raise/lower system, AT&T can keep the line connected on the T1 server during maintenance.

"It has battery backup in it, so the equipment is running on batteries when we bring it down (for maintenance)," Martin explained. "Then we have the ability to hook it back up to electric when it gets down on the ground."

In addition, Martin said the antennas, for maintenance purposes, stop 20 feet above ground. That allows the maintenance crew to still use a ladder to get to the antennas without climbing the tower and keep the equipment live for maintenance purposes.

Equipment Concerns Tower mounting presents challenges for carriers that can't find space for their equipment on existing towers. According to Shubh Agarwal, Nortel GSM product manager, almost 50% of existing towers are co-located, and there may not be space or capacity left on them to install additional elements for more coverage.

To compensate for a lack of space, carriers may reduce the height of tower-top amplifiers by installing them midway up the pole. But that presents the potential problem of overloading a tower. Putting one base station on a pole is equivalent to adding the weight of a refrigerator. To further enhance coverage in an area, you can add tower-top amplifiers as well, which may save cable costs. However, this also can add more burden to towers and poles.

"For example, it's difficult to install tower-top amplifiers on a wooden telephone pole because they cannot take that much weight," Agarwal said.

Besides the weight issue, carriers also must consider the technological concerns related to tower mounting.

"Several (manufacturers) have come out with superconducting filters that very sharply isolate one channel from another and let you put many more antennas on a tower," Cooper said.

According to Cooper, it makes sense to put receivers and the receiver front end, the first element of a receiver, and the power amplifier for the transmitter at the top, and convert everything beyond that to digital form so you send one digital signal from the top of the tower to the bottom.

"That's the way the systems of the future are going to be built," he said. "It's clearly more cost-effective."

Cooper added that the technological challenge for tower mounting is twofold: "extremely reliable and easily fixable," he said. "The two solutions to reliability are making products extremely reliable by adding redundancy or doubling up."

Reliable coverage with less equipment will benefit tower-mounted technology as much as it can help carriers.

Is Tower-Mounting Worth the Trouble? Omnipoint is using tower-mounting to cover the Amtrak railway from Washington to Boston.

"The Omnipoint application along the Amtrak railway is one of the few applications where tower-mounted has been a big boon and allowed an operator to do something they couldn't before," Agarwal said.

Joe Walsh, Omnipoint director of site acquisition, could not say much about the project because of a pending zoning conflict, but he said that the carrier used small, tower-mounted base-station systems to provide coverage. Because of right-of-way requirements, Omnipoint couldn't mount a base station on the ground. That's where the small base station came into play. In these situations, you can hang the base station or the radio portion of it on the pole or the wall and put the tower-top amplifiers up on the tower to provide coverage from the pole. A cement pad or anything else on the ground is not necessary.

Walsh said the biggest challenge that comes with tower mounting electronics is local resistance.

"In New Jersey, we have a prolonged zoning process where we have to appear before the zoning boards and give expert testimony justifying why we need it there," he explained. "Many towns only want you to put it on municipally owned property."

As a result, Omnipoint is not relying on tower-mounted technologies to expand its coverage.

"Of all the sites we put up, about 95% to 97% are not towers," he said. "They're rooftop designs."

In Manhattan, for example, Omnipoint installs patch antennas on the sides of buildings.

For urban environments, a tower is usually the last resort. But Walsh said for the suburban and rural areas Omnipoint serves, towers are sometimes the only choice, in which case the service provider advocates co-location.

If you decide to co-locate, mounting electronics at the top of the tower can give you more freedom in designing your site. Some jurisdictions know they can't stop the tower because that's federally mandated, but they do everything in their power to restrict what you do with the base station and your equipment.

"If the equipment is mounted to the tower, they can't say you have to put bushes around it or you have to put it underground," AT&T's Martin said.

When Omnipoint does use tower-mounting options, Walsh said maintenance is not really a concern. The carrier negotiates maintenance access with the tower owner and uses telemetry for malfunction alerts. In addition, Walsh said all of Omni-point's field-operations personnel are trained to work aloft.

"It's an OSHA-supported program," he said, "and they are not only trained, they are periodically reviewed."

Despite other reports that tower mounting can reduce costs, Walsh said he doesn't see a significant cost savings with tower mounting.

"In some cases, you have two outstanding leases," he said. "If it's an existing pole, you have to pay the person who's managing that pole, and in other instances, you may have to pay the landowner for appeasement rights, so it can be more costly."

However, carriers, including Omnipoint, have found situations when tower mounting electronics is the best alternative.

"It is not a solution I was looking to use forever, for all sites, but it's something to develop and get us into areas we weren't necessarily able to go before," AT&T's Martin said. "I think (tower-mounted solutions are) going to get looked at more and more seriously."

AT&T plans to equip two more towers with cell sites similar to the one in Spring Run Farms. The days of using buildings, Martin said, are quickly disappearing.

"As everything gets to be smaller, at some point nearly ev-erything will be tower-mounted," O'Brien predicted.

At PCS '98, Racal Antennas and Ball Wireless Communications turned heads with their latest tower-mount technologies.

Ball Wireless introduced its eXsite family of PCS base-station antennas for polarization diversity applications. The eXsite base-station antenna is a low-loss, high-performance, high-gain compact antenna that employs dual-slant polarized array technology and provides up to 17dBi gain in a 48-inch package, or up to 14dBi gain in a 24-inch model. The eXcite PCS antenna's electromagnetically coupled RF connections are designed to eliminate passive intermodulation.

Racal Antennas' newest panel antenna design, PCS 1900 VP/65/16, features a single-printed circuit board that incorporates all radiating elements and distribution circuitry. The antenna has only one solder joint where the semi-rigid meets the PCB, which assures low intermodulation. The design uses less than 20 parts. It has a flat back and slim design -- only 1one-half inches deep -- and is lightweight.

t would seem that co-location is the perfect alternative forcarriers to get around zoning boards and regulations, but it is not always that easy. There may be numerous towers already erected across the country, but not all of them are suitable for wireless co-location.

Co-location on AM-radio-station towers always has been a tough sell to broadcasters and carriers alike. Unlike FM and TV towers, which are simply support structures, AM towers are part of the antenna. That design makes co-location expensive and difficult. Worse, wireless and AM barely make good neighbors: The FCC already requires carriers on towers as far away as two miles to pay for any detuning needed to protect the station's signal.

But despite the design difficulties, AM co-location is an attractive option because many stations have directional signal patterns, which require multiple towers, or already are in areas where new towers would not be possible because of costs or zoning.

"A lot of carriers are jumping on this because not only is it economical, but it also gets them to market quicker without having to be concerned about zoning applications," said Mike Britner, Lawrence Behr Associates (LBA) vice president of business development.

Leasing space to wireless carriers can be an attractive new revenue source for AM stations, many of which continue to lose advertisers to FM. Although some broadcasters are concerned about possible changes in their signals, a technology now actually can improve signal quality by rectifying existing problems with impedance mismatch.

This same technology is designed to make co-location at the 4,724 AM stations much easier and, in the process, open up more tower space nationwide.

LBA is one company that offers this opportunity with its CoLoSite product. Launched in February, the technology's users already include AT&T Wireless, Nextel, Omnipoint and Sprint PCS. The technology is available for stations with multiple towers and for single towers. According to LBA, its technology doesn't use iso-couplers, which are vulnerable to lightning and expensive for installations involving PCS or multiple transmission lines. Instead, it grounds the tower and radiates the station's signal from a modified folded unipole. This makes it easier and safer for crews to work on the tower. In addition, an isolation coil keeps the antennas and transmission lines from altering the station's signal pattern.

Typically, installations range from $27,000 to $50,000, depending on a variety of factors such as the station's power and the number of antennas involved.

Could such technologies lead to a detente with AM broadcasters? Early indicators suggest so. According to one RF engineer who used CoLoSite for a PCS installation, some stations actually are approaching carriers.