Less Than Stellar

We don't live in a perfect world, but try telling that to the angry subscriber whose call dropped when signal coverage petered out. Black holes are annoying for carriers and subscribers alike, and they're a pox on an industry trying to position itself as an alternative to landline. Today, with wireless solidly in the mainstream, seamless coverage has to go beyond business parks and downtown areas to include malls and suburbs.

"From an operator's point of view, a black hole is lost revenue," said Richard Burke, executive vice president of JMS-MLJ, an engineering-consulting company.

Preventing those losses begins with a solid design. There's no shortage of sophisticated modeling tools fordesigning a network, but gathering data is crucial to tailoring them to the market's topography.

"You can't just take them out of the box and have them give you an accurate picture of what your area is going to look like," said Ben Andrzejewski, US West wireless-network director. "You really should put a transmitter some place and drive that area so you can see how the signal is going to propagate there. Then load the information into your design tools so they give you more accurate data when you're trying to plan."

TIMING IS IMPORTANTThe timing of data collection can affect coverage because networks behave differently at different times of the day and year. One wild card is foliage.

"At PCS frequencies, leaves are like kryptonite," Andrzejewski said.

As a result, data gathered at the wrong time can result in black holes.

"You test the candidate (site) in the fall or winter, and then when the spring or summer comes, you get the additional foliage," said Allen Kozycki, Powertel director of RF engineering. "So you have more attenuation to deal with."

Foliage's effect on coverage also can vary by geographical region.

"In the southeastern United States, where they have a lot of pine trees, those needles can reach a certain length where they become resonant at 800MHz and really wreak havoc with coverage," said Andrew Singer, director of technical marketing at Celwave, a manufacturer of wireless-optimization hardware.

Other design tips include:

* Haste makes waste. Set realistic launch dates that provide a cushion in case zoning or site acquisition doesn't go smoothly.

"Do some kind of pre-screening of the area, and get an understanding of what the time frame is for your targeted area," Andrzejewski said.

Determine which sites are essential and focus on acquiring them first. If you're outsourcing that chore, make sure the site-acquisition company understands your priorities.

* Know the market. Identify the different types of traffic such as in-building, highway and pedestrian.

"That's going to reflect on what kind of cell size you can support," said Alan Post, Nortel TDMA product-line manager. Knowing where the traffic is -- and will be -- also comes from living in the market.

"It's really important to have a design team that has local-market knowledge," Andrzejewski said.

* Study the competition.

"Drive a competitor's system to assess the performance of potential co-location sites as well as to understand better the behavior of RF in particular areas," said Bentley Alexander, AT&T Wireless vice president of RF engineering.

* Create a design that can grow and change with the market.

"Don't design to the limit of the technology," Alexander said. "Leave some margin to allow for site relocation or other RF-attenuating factors."

DRIVE FOR SUCCESSDrive testing remains the most common way to locate black holes, but its effectiveness depends on how well you analyze the collected information.

"There's a lot of great equipment out there that you can use on a regular basis to quantify the performance of your network," said JMS-MLJ's Burke. "It's to the operator's advantage to drive the network and analyze the results regularly. It does help. It's a matter of discipline in doing that work and not just collecting that data but analyzing it."

High frame-error rates, frequent dropped calls, excessive hand-off activity and low signal strength are key statistics to monitor during drive testing, but they can be misinterpreted.

"Although a drive-test tool may accurately report adequate coverage on one street, the lack of coverage one block over may go completely undetected if not also driven with the tool," Alexander said. "So, the more thorough -- and timely and costly -- the drive, the more apt one is to find the coverage holes."

Subscriber complaints are another valuable -- albeit painful -- way to locate black holes because subscribers travel more places more often than any system-performance staff ever could. Many network-management software tools include the ability to merge customer-complaint and engineering databases to identify areas with spotty coverage quickly and efficiently.

"It's alwa ys a good idea to tie customer complaints to system indicators and drive testing because you can waste a lot of time by drive testing the whole network," said Nortel's Post. "But if you get some preliminary indications of problem areas, you can isolate your drive testing."

ONE SIZE DOESN'T FIT ALLThe cheapest way to eliminate black holes is through optimization, which wrings as much coverage as possible out of the existing infrastructure. Repeaters often are the next step.

"Repeaters are good for fill-in coverage and black-hole coverage as long as it's not too large an area," said Celwave's Singer.

Like any solution, a repeater is only as good as its application."There are different flavors of repeaters, so you have to be sure you're applying the right repeater solution to the problem," Burke said. "You have to balance fixing holes with enhancing capacity."

Determining which solution is best starts with asking specific questions about the hole:

* How big is it? Does it encompass a single building, a campus or several blocks?

* Where is it? Urban propagation is different from rural, so a solution that works well in one environment might not perform as well in another.

* What's creating it? Foliage? Terrain? Buildings? No base stations within range?

* Are there site limitations? Are there places to mount antennas and run cables? Are power and interconnection available?

* What are the capacity needs within the hole?

"If you've got the extra capacity, and you have just a small black hole that you want to cover, a repeater should work fine," Singer said. "However, if you need the capacity or you think you might need the capacity someday, you might want to go with a full OEM microcell-type product."

WHAT'S INSIDE COUNTSTraditionally, definitions of black holes haven't included in-building coverage, but that's changing.

"You're building a network to accommodate the highest usage areas, and if it happens to be in-building, then that's a coverage hole," Burke said.

A 1998 survey by the Brookings Institution's Center on Urban and Metropolitan Policy found that the number of people living in downtown areas of major cities is growing steadily. One example is Houston, which expects the number of people living in its downtown to quadruple by 2010. Serving those people means providing seamless coverage not just for their business and personal use but also to meet E-911 obligations.

Plugging indoor holes starts with examining the building materials. Concrete and steel are obvious barriers, but metallic window tinting, common in commercial and residential applications, also attenuates signals.

"We've measured it all the way up to 60dB" of loss, Andrzejewski said.

The use of metallic tinting varies by region.

"Cities in the Pacific Northwest use metallic tinting more than in the Mountain regions and the Midwest, for some reason," Andrzejewski said.

Even the style of construction can affect coverage.

"A large facility with many interior, floor-to-ceiling walls will provide more attenuation than a similarly sized building that uses cubicle panels with an open expanse at ceiling height," said AT&T's Alexander.

One solution is picocells, but a common concern is how to keep the signal from radiating out of the building and creating interference. Ironically, window tinting can be an asset.

"A lot of the buildings have reflective glass material, and that carbon material to create that reflection generally knocks the signal down by 20dB alone," Singer said. "I don't think radiating out of the building is as much of a problem as people thought it was."

The time and expense of stringing cable for in-building solutions might not be a barrier for long, either.

"I think there may be some unique solutions that will be offered in the next year or two where they'll make use of the current cabling systems in a building," Singer said.

That won't be too soon. Those people clustered around office-building exits aren't necessarily smokers sneaking a puff. They might be wireless subscribers who have to step outside to use their phones.