Devil in the Details

Wireless calls are a lot like canaries in the mine shaft: When they start dropping, it's a sure sign there's a serious problem somewhere. But although getting dropped calls under the industry average of 2% means looking in a lot of places, it doesn't necessarily require years of experience with digital networks.

In CDMA, for example, many basic RF principles still can be applied to pilot pollution, which causes calls to drop by upsetting the delicate task of soft handoffs. Multipath makes the situation worse by confusing the handset's RAKE receiver with several strong signals.

"You could have two or three fingers actually listening to one pilot if multipaths are strong enough," said Andrew Singer, Celwave director of technical marketing. "If you're in an area and there are more than three pilots that are dominant -- where there are very similar strong signal levels -- portable phones don't seem to know how to handle that correctly. So it's a real problem for the system."

The solution starts with identifying which areas are afflicted. One important statistic to monitor is frame error rate.

"It's not a 100%-accurate way of looking at voice quality or likelihood of a dropped call," said Usman Goni, LCC senior principal engineer. "But it gives a good indication, probably 75%."

If that rate is high, a drive-around will determine the dominant pilots in the area. One solution is installing a repeater to saturate the area with the pilot that needs to be dominant. It's an approach borrowed from AMPS carriers serving highways along the U.S.-Canada border, where unusual roaming and long-distance charges were traced back to signals sneaking across the border. A network of repeaters kept phones from locking onto a strong signal across the border.

Repeaters and repeater-like devices such as Qualcomm's Pilot Beacon also can help prevent dropped calls in border cells by establishing a crisp boundary between two networks. Some other things to keep in mind:

* More power doesn't necessarily translate into fewer drops. One cell that's too high, too powerful or coveringtoo large an area will wreak havoc. Drops can result when handsets pick up a distant signal that's not strong enough to support a handoff.

* Put the right antennas in the right places. Narrow-sector antennas in the range of 30 can help alleviate interference along long stretches of highways or bridges. Azimuth rotation can reduce the number of signals pointing at a particular area. If mechanically downtilting the antenna doesn't reduce the number of signals in an area, try continuously adjustable electrical-downtilt antennas.

"The electrical tilt not only decreases the signal on the horizon on the bore site of the antenna but also decreases it along the sides," Singer said. "You can get maybe 3dB to 6dB more signal reduction on the horizon on the sides."

* Anticipating is cheaper than correcting. A persistent problem with dropped calls indoors could stem from a flaw in the original network design.

"They go back and look at the plan, and they never designed the network to have in-building coverage in the first place," said Gordon Davidson, Qualcomm QSource CDMA optimization services group senior manager for business development.

* Use remote-monitoring equipment at cell sites to catch anomalies such as large, sudden changes in VSWR and power output.

* Don't assume that manufacturers' default settings are optimum.

"You really need to adapt what you've installed to the surroundings," David-son said. "Often, what we'll find is the default settings are still where they are and haven't been fine-tuned."

GOOD NEIGHBORSOptimization is wasted time and money if it doesn't include updating neighbor lists.

"As you optimize sites and add more sites for coverage and capacity, you continuously have to maintain that neighbor list," Singer said. "You may end up handing off to a site that looks like an appropriate site to hand to for a short time, and then when the portable moves just slightly geographically, all of sudden that becomes a poor site to have handed off to, and it just might drop the call."

Cell splitting can be a double whammy: If neighbor lists aren't updated in the process, calls will be handed off to the wrong cells and possibly dropped.

"It's a lot of hard work," Singer said. "It's not a simple thing to do, but it's something you really have to spend the time to do if you want to ensure a good handoff."

Theoretically, a digital system shouldn't have dropped calls because the handset always can "see" other sites besides the one it's using. In GSM, the BCCH allocation list tells the handset where to listen for neighbor cells, and in CDMA, the search window acts as a set of blinders so the handset sees only the pilots on the neighbor list.

"If it's supposed to be a good candidate, and it's on the neighbor list, if the search window isn't set properly, then you can't have that neighbor included among the possible candidates," said LCC's Goni.

BAD NEIGHBORSAs the airwaves and towers become more crowded, being a good neighbor will remain an important way to avoid dropped calls. But putting several carriers and services on one tower or a couple of towers within a few hundred feet of one another is tricky.

One common mistake is overlooking potential sources of interference. Knowing how much signal spills into your band (or vice versa) is the first step toward determining how much filtering will be needed. Frequency coordination also helps establish guard zones, which prevent signals from slopping over into neighboring bands.

"That's very important before you even launch the system," Goni said.

Bandpass and notch filters are two common ways to minimize interference, but waiting until after everything is deployed means risking dropped calls, shutting down parts of the network to install filters or, worst of all, a costly trip back to the drawing board for redesign. Occasionally, intermodulation simply might be irreconcilable.

"Admit that it is sometimes better to walk away from a co-location than to subject a customer to unresolvable interference issues," said Bentley Alexander, AT&T Wireless vice president of RF engineering.

Successful co-location requires understanding one another's technologies to determine the best antenna orientation and how many wavelengths apart to mount them. A good rule of thumb is the more vertical spacing, the better.

"Vertical spacing provides significantly more isolation between systems than what can be achieved through comparable horizontal spacing," Alexander said.

Another common mistake is allowing two antennas to face each other.

"You've really got to make sure you're looking at the near field and not causing any shadowing," said Qualcomm's Davidson. "Antenna selection and location are probably the key issues."

There's no shortage of engineering consultants with sophisticated software that looks for potential interference in the design, but that doesn't preclude visiting the site.

"You need to go out to the site itself and measure because there are so many aspects to interference that need to be analyzed," said Mats Wallem, Ericsson manager of BSS-product management.

That hands-on approach should continue through the construction stage.

"The engineers do their work, and sometimes they visit the sites, but the contractors, those doing the construction, they're not electrical engineers," Goni said. "Without adequate supervision, they can turn things around."

The best network is only as good as the handsets using it, so preventing dropped calls involves more than diligently optimizing the network. Problem is, handsets are a wild card because carriers usually can't tell which ones are poor performers until subscribers complain.

"Our estimates are that 1% to 2% of all the phones out there are bad in some way," said John Martin, director of product management at Corsair Communications, whose PhoneCheck allows carriers to analyze subscriber handsets over the air.

By looking at information such as frequency deviations, signal level, frame errors and where the handset was used, carriers can catch problems before subscribers start experiencing dropped calls.

Performance also can vary widely not just among manufacturers but also among models manufactured at different times. One way to find out about a particular model is through industry organizations such as CTIA and the GSM MoU Association, which represents 323 GSM carriers worldwide.

The CTIA Certification Program evaluates handsets through tests by independent test labs. The program's web site, at www.wow-com.com/consumer/certification/index.cfm, includes a search engine for reviewing tests by vendor, model, mode and OEM.

The GSM MoU Association will debut a similar certification program in early 1999, and more information is available at www.gsmworld.com.