Building Blocks

The bigger, the better. At least that's what architects seem to think when erecting skyscrapers. Not long ago, the United States boasted the world's 10 largest buildings. Now there is only one American building to break the top 100. But all of that is changing as American contractors are gearing up to build a new generation of mega-high buildings.

Yet bigger doesn't always mean better when it comes to wireless communications.

Wireless engineers say super skyscrapers like the World Trade Center or even Chicago's 1,550-foot South Dearborn, planned to be the world's tallest building, are less of an interference problem in today's wireless industry. However, they still are something to watch.

According to Rich Cane, PrimeCo Midwest director of network operations, skyscraper interference can be an engineering headache if not planned for appropriately. All PCS and cellular networks need line-of-sight connections with their antennas. If too many skyscrapers stand in the way, the network could suffer.

But newer technology and a few old tricks almost have eliminated the issue. A downtilt antenna is able to get a lobe to the horizon better, effectively pinpointing the coverage area and decreasing interference. Another tried and true interference solution is building a ring of cell sites around individual areas in a city.

Ben Andrzejewski, US West Wireless executive director of wireless networks, said setting a ring of cell sites away from the targeted area gave his company better urban coverage. Cell-site rings with a 1- or 2-mile radius enable you to serve the corridors of the major urban area as well as cover some of the lower levels of high-rise buildings.

Shrinking coverage like this, while deadly in rural areas, is a necessity in the urban landscape, said Chris Resavy, Omnipoint senior director for engineering operations and facilities.

"When you reduce the amount of coverage out there, you reduce your interference, but you also increase the quality of the coverage closer to the site," Resavy said.

Most engineers agreed that the best way to make skyscrapers all but disappear is by using a highly effective RF propagation tool. Without relocating towers or tweaking antennas, an engineer can save time and money with this simple solution.

Andrzejewski pointed out that big buildings don't come out of nowhere. They are planned months or years in advance. A good engineer uses a propagation tool to tell what kind of attenuation to expect from the offending structure and places sites accordingly.

But careful planning doesn't always work perfectly. Cane explained that his company safeguards itself by insisting that all site leases have provisions in them that allow PrimeCo to terminate a lease for technical reasons such as signal blockage.

OLD PROBLEM: NEW SOLUTIONS Good planning aside, some carriers are not satisfied with using traditional solutions to combat urban interference. GTE Wireless and Omnipoint both employ microcell sites to serve their urban customers. Omnipoint went so far as to design its entire New York City network with micro-cell sites from the beginning. Omnipoint's Resavy explained that microcell-site designs enable you to contain your coverage better than just downtilting or building a ring around the area. A microcell means using more and smaller base stations to send your signal into a very specific location. With coverage this tight, anything that happens to the surrounding landscape will have little effect on the network. Under Resavy's guidance, Omnipoint managed to build more than 500 microcell sites in just nine months.

For carriers not able to start over with an all-microcell design, an overlay or underlay solution can be used in tandem with a few well-placed microcells. For example, say you have a traditional cell that's covering a geographical area. Within that geographical area, there are either some large traffic concentrations or there are some difficult areas to cover because of a dip in a hill or a skyscraper. To boost the coverage a little, you can deploy a microcell for that specific area. The microcell will work in unison with the overlay or underlay cell so that they are orchestrating and handing traffic back and forth between them, even though they share a portion of the same geographical space. GTE Wireless uses this kind of solution in urban areas such as San Francisco and Seattle.

But microcell sites don't just mean better urban coverage. Another bonus is their smaller footprints and lower power requirements.

"A microcell is just that, smaller," said Mark Ripley, GTE Wireless assistant vice president of area networks. "That makes it easier to deploy equipment where space may be prohibitive or power limited."

SMART SITES Using smart antennas to redirect signals better is another interference solution that is still in its developmental stages. Typically, engineers use antennas that are of a fixed pattern. Basically all that can be done to them is tilt them up or down. But smart antennas give engineers software controls that offer more than just up or down options. That way an engineer can put more energy into a trouble spot to serve customers in that particular area better and still easily change it as traffic or interference dictates.

Ripley said though the industry doesn't use smart antennas much right now, they are in GTE Wireless' tool bag of things to use in difficult situations. He also predicted that smart-antenna technology will get even better and become more widespread.

"When we are able to connect the software capability to smart antennas in an overall system that does frequency planning and coordination in real time, then we will have a significant tool to go to the next level to gain more efficiency and capacity out of our networks," he said.

PENETRATION PROBLEMS With interference problems slowly dissolving, the remaining issue facing RF engineers today is urban penetration. Urban-penetration problems arise when signals bounce off tinted glass of highly reflective modern buildings or when ultra-thick concrete of older structures impedes them. Several in-building solutions are available, but most of them are costly and not normally used as general network solutions. For example, placing repeaters, base stations or antennas in a building so customers can use their wireless phones is not cost-effective for any carrier unless a large client commissions it. Pointing neighboring rooftop antennas up to a monstrous skyscraper also isn't a realistic solution, because most network traffic is concentrated down below in the street.

PrimeCo's Cane was able to use his phone at the top of the Sears Tower, not because the network was specifically designed for that kind of service, but because when a user is that high, he has line of sight with several antennas. But line of sight with so many antennas is both good and bad. It's a good thing in that coverage isn't usually an issue. However, interference is. Coverage up high in any building depends on where the customer is located, how many sites the customer is looking at and even where the customer is standing.

"Often when I've made calls from any of these high-rise buildings in town, I can make calls if I'm standing in one particular area, but if I move several feet in one direction or another, I lose my call," Cane said.

For most carriers this kind of coverage in skyscrapers is satisfactory. The market drives how carriers will design their networks. With little demand for high-rise coverage, many carriers have decided not to focus on this issue.

GTE Wireless' Ripley said he didn't see any big changes for high-rise coverage, especially since wireline options are readily available to users. However, in the future there may be even fewer interference problems with increased use of smart-antenna technology and microcell designs.

"It's not widely deployed right now, but it will be," he said.