Urban Renewal

Talk about a Catch-22: Some of wireless' most lucrative subscribers live and work in dense urban areas, where tall buildings can create shadowing by blocking the signal path between the handset and the base station. But with real estate accounting for as much as half of the cost of a typical base station, providing subscribers with seamless coverage often is difficult and costly.

Although the handset-to-base-station uplink is more vulnerable than the stronger downlink, shadowing is tackled mostly at the base station to help keep handset costs low. As a result, a solid network design is key to avoiding both shadowing and costly remedies such as new cell sites. One approach is to set up test transmitters and do drive-arounds to see how signals are propagating.

"That's really the only way to do it because most propagation tools don't take into account shadowing," said Ben Andrzejewski, US West wireless-network director. "So it's very important that in areas where you suspect you're going to have shadowing that you take as much drive-test data as possible before your site selection."

But because of fierce competition for subscribers, the temptation for both established and start-up carriers is to keep overhead low and shorten time to market by cutting back the design process. Even so, short cuts that result in shadowing can come back to haunt carriers.

"Across all the technologies, there's messaging going back and forth to communicate signal strength to the mobile and (manage) power control," said Cedric Taylor, Nortel senior TDMA product-marketing manager. "So there is a need for constant, clear communication. You want to mitigate and avoid shadowing at all times."

Cutting corners in the design can wind up costing more in the long run if extensive optimizing or additional infrastructure are needed to improve coverage.

"This is typically where a lot of people get into trouble," said Gordon Davidson, Qualcomm QSource CDMA optimization services group senior manager for business development. "They work with their link budget, determine the loss-for-shadowing margin and calibrate their models. It determines how much capital they have to spend. They go, 'Oops! This is too much. Let's go back and start tweaking some of our assumptions.' And either they totally forget about the impact of foliage, or they decide they're going to discount that. They end up building a network, and when they're starting to optimize, they find out that they've got problems with shadowing."

The Art of Multipath

Eliminating shadowing doesn't necessarily require a clear line of sight between the base station and the handset. In CDMA, for example, multipath is actually an asset because signals can bounce off buildings into areas where line-of-sight coverage isn't possible.

"That's one of the great things about CDMA: It utilizes multi-path," said Andrzejewski. "The more multipath you have, the better off you are."

When designing coverage for an urban area, Andrzejewski creates a ring of base stations around the downtown to blanket the area. He then determines what shadowed areas still are served by multipath before filling in the remaining gaps with interior sites. The trick is finding the right structures at the interior for siting. Tall buildings aren't necessarily the best choice because downtilting the antenna to cover the streets can result in having the antenna pointed mainly at the rooftop.

"If you've got a 35-story building, you can't go on top of it and downtilt your antenna and expect not to have any problems from that," Andrzejewski said. "You use parking garages. Generally, they're not as tall. It's kind of like a puzzle because you have to know where those buildings are because then you might have to change your ring a bit."

Soft handoff also helps prevent dropped calls because the handset can "see" more than one base station at once.

"The probability that you're being shadowed on all of those links at the same time, plus all the multipath being cancelled at the same time, is pretty low," said Paul Sergeant, Nortel senior CDMA product-marketing manager.

Other technologies have different approaches to shadowing.

"On a narrowband signal like TDMA, the worst concern is usually the mobile signal getting back to the cell-site antenna," said Nortel's Taylor. "One of the things a lot of operators do is use receive diversity. That lowers the probability that the signal coming back to the cell site is going to be in a multipath fade. You've got two separate antennas, and the probability that they'll both be geographically such that they're both in fade is pretty low."

Optimizing 'Til It Hertz

Optimizing usually is the first step toward mitigating shadowing because it tries to wring the most coverage out of the existing infrastructure. Increasing antenna height is one option, but that can create more problems such as pilot pollution in CDMA. Lowering the antenna is anothersolution, but in a dense urban environment, that can create a waveguide effect if the buildings reflect signals.

"Sometimes it's an advantage; sometimes it's a disadvantage," Taylor said. "As the antenna height goes down, the buildings act as a waveguide, and your RF coverage sort of goes along the streets, and it doesn't pan out in a circular fashion."

Another option, particularly in areas where capacity also is a concern, is hierarchical cells, where multiple microcells provide coverage at street level, and a macrocell on a tall building covers the entire area. But choosing the right handoff parameters is key to preventing dropped calls.

"If you have a microcell, and you know that there's a big shadow around the corner like 20dB, you will make your hand-off margin around 20dB," said Shubh Agarwal, Nortel GSM product manager. "Typically, it's 4dB or 5dB, but in this case you would make your handoff margin 20dB between the microcell and the macrocell so that you can hand over and not lose the call."

In CDMA, another wild card is cell breathing, where a cell site's coverage area shrinks as it struggles to accommodate more traffic. If shadowing also occurs along that cell's borders, calls could drop.

"Where shadowing becomes an issue is at the edge of the cell," said Qualcomm's Davidson. "So if you're expecting that you're covering all the way out to the freeway, and then a major building gets constructed there or there's a lot of foliage there that you hadn't taken into account, now all of a sudden you're not going to have coverage of that freeway, and you're going to drop a lot of calls. There's going to be nothing to pick it up because you're at the edge."

Fortunately, combating shadowing might soon become less of a chore. Nortel is developing an automated system for GSM where the network compiles statistics such as signal quality and level. The result, Agarwal said, is that as a new building or structure goes up, "the algorithms will automatically re-assign frequencies based on the shadowing effect and retune the network overnight to provide much better capacity and quality of signal the next day."