Urban Blight

Providing solid, reliable coverage in urban areas requires more finesse than force.

From a business standpoint, cities are ideal places to offer service because the population densities mean a bigger pool of potential customers. But from an RF-engineering standpoint, they're nightmares, where shadowing and multipath confound signals and force calls to drop. Even a bus can wreak havoc.

"You can be holding a handset, and your signal might be marginal," said Russ Arsaga, U.S. Cellular vice president, engineering. "All of a sudden, because the bus is within 20 meters of where you're standing, you get one heck of a reflection pattern, and you'll see it jump up a couple of bars. It's kind of weird."

If there's a bright side, it's that the industry has learned a lot about RF in urban environments over the past two decades.

"In 1982, we didn't know much about RF in Manhattan," said Chris Resavy, former Omnipoint senior director, technology, who built four networks in New York City. "We decided to put them on omni sites and shoot in across the avenues. That didn't work very well. The competitor put them on tall buildings on the rooftops and tried to downtilt. That didn't work. They tried some overlay/underlay with an immense amount of downtilt. That didn't work either. Then they tried to do some microcell filling with a few sites here and there at 30 feet to 50 feet, and they could never get the things to hand off right because of the way AMPS works, you need from three to five seconds to do a handoff. You're moving 1,000 feet or so, depending on the speed, and by that time, you could be past a sector and miss a handoff."

When Resavy designed Omnipoint's network in 1996, he put the Manhattan sites 30 feet to 50 feet above street level because the buildings were so dense. That approach yielded two key benefits: A strong signal at street level, where most people use their phones, and a regular, diamond-shaped signal pattern, which provided more control over the RF.

"We didn't think that the diamond pattern was going to come out as regularly as it did everywhere in the city," Resavy said. "We (also) didn't know that we'd be able to get inside buildings a lot better because of the dense signal strength at the base and upper floors."

RF Engineering Vs. Mechanical Engineering
As a rule, skyscrapers built in the first half of the 20th century tend to be much thicker, particularly in their lower floors, because mechanical engineering wasn't sophisticated enough to build today's svelte towers. In New York City, for example, some older buildings' walls are 18 inches thick on the first floor and lose only about an inch every second floor. Modern buildings can be equally challenging because metallic window tinting can attenuate signals by as much as 60dB.

Even so, good outdoor coverage often translates into good in-building coverage. Case in point: Resavy expected to suffer at least the same attenuation at 1.9GHz as cellular providers did at 800MHz. Not so.

"We thought we were going to lose about 25dB to 30dB in-building penetration," Resavy said. "But instead of losing or being the same as the cellular guys, (who lost) 18dB, we were only at 9dB because the amplitude of the 1900MHz signals was able to squeeze through cracks and moldings that 800MHz (signals) weren't able to. The metallic window tinting didn't bother us much because the separation and cracks were enough to allow the signal to penetrate."

Climate also can determine how well an outdoor site can penetrate a building. In a hot climate, for example, windows might be heavily tinted to block the sun.

"In cold countries, the windows are as big and transparent as possible," said Ofer Ronen, Foxcom Wireless CEO. "In those locations, the interference issues must be dealt with more carefully. On the lower floors, you'll usually have a higher signal from the nearby base stations. As you go up, you have a lower signal from the nearby base station, but you have bigger signals from distant base stations."

Suppose that a caller is on the 10th floor, and like most callers looking for a good signal, he's standing by a window. From roughly 300 feet up, his phone likely can "hear" several sites, all of whose signals probably are barely strong enough to support a call. Worse, because those signals are weak, the phone will increase its power in an effort to improve the chances that the call won't drop. That's a less-than-ideal scenario because indoors or out, a phone shouting at the top of its lungs is interfering with other calls, sapping network capacity and draining its battery faster.

One solution is to increase power at a nearby site to blast a signal inside. But in an urban environment, even if there's a site on a rooftop near that height, chances are that it's already downtilted both to provide better coverage on the street and prevent its signal from sneaking into other areas and causing interference.

A better option is to deploy a microcell inside the building so that the phone has a single, strong signal available and, as a result, runs at lower power because it doesn't have to shout at distant, weaker sites.

"You isolate the building from the outside world," Ronen said. "There's no interference from the outside that (is) strong enough to overcome the inside signal, and the signals inside aren't going outside."

One caveat: When covering buildings without metallic window tinting, make sure that the inside signal doesn't leak out outdoors.

"The balance between the signal level inside the building and outside is more challenging," Ronen said. "You have to change the signal strength as you go up to overcome the outside signal but not be strong enough to leak and interfere with the nearby base stations."

Fiber & Finesse
Tunnels and subway stations also are challenging, but an outdoor site still can provide some coverage. Omnipoint, for example, has sites pointed at both ends of the Holland Tunnel, which itself acts as a waveguide. Unless a bus is between the site and the caller, each site can reach about one-third of the way into the tunnel.

Other tunnel solutions include repeaters, microcells and leaky cables. One example is Foxcom Wireless' RFiber, which transports a nearby site's signal over fiber-optic cable inside a tunnel or subway station, where it's converted back to RF and amplified to provide coverage.

Fiber is handy in urban-coverage solutions because it provides more flexibility in siting: If the base-station equipment can sit in a building's basement and be linked to the antennas via fiber, finding a viable rooftop site no longer means looking for one that can support a 1-ton cabinet.

Another promising urban-coverage tool is superconductor filters, whose greater selectivity can be helpful in areas with more users and thus more interference.

"I'm not sure at this point if it's going to come down to using much of these in an urban environment," said U.S. Cellular's Arsaga. "Most of what I'm looking at it is maybe a suburban environment or rural. But there might be applications that are highly selective and well targeted in some of the urban centers."