The world can be a cruel place for RF. The problem isn't just that a particular site offers all the coverage of an Ally McBeal skirt. Rather, it's that interference, window-tinting, foliage and a host of other gremlins conspire to prevent mobiles' signals from reaching the site.

Sensitivity degrades as equipment ages and optimization stresses connections. Even moderate variations in weather and temperature can allow moisture to seep into cables and lower the line's conductivity.

Antennas can suffer, too. Bell Mobility does arcing tests on each antenna when it's installed and then once a year. The test involves turning on every transmitter in a given sector at operating power for about one hour.

"If that antenna is susceptible to arcing, you'll see the noise floor going up," said Pierre Caux, Bell Mobility network-support manager.

HEARING AIDS Most optimization tweaks involve trade-offs. Take tower-top low-noise amplifiers (LNAs), which boost the signal to overcome losses in cables and filters. They're popular in Asia, where penetration and population densities are higher and cells often can't be split any further.

"What you'll gain in sensitivity is approximately the loss of the lines going down the tower and the loss of the filters at the bottom," Caux said. "For a 300-foot tower, it's probably 5dB."

Two trade-offs are cost and reliability. A tower-top installation puts the LNA closer to where lightning strikes, and repairs or routine maintenance might mean the added expense of hiring a rigger. Even when they're located on the ground, LNAs aren't a solution for every problem.

"A good example of a limitation would be the trade-off between increasing the gain on a front-end LNA to improve sensitivity and having an increased susceptibility to intermod interference," said Russ Arsaga, U.S. Cellular vice president, engineering.

One variation is the addition of a superconductor.

"Because it operates at cryogenic temperatures, the tower-mounted amplifier (TMA) can have a very, very low noise figure," said Ben Golant, Illinois Superconductor technology vice president. "The best TMAs on the market today at room temperature have noise figures typically of about 1.5dB. We can get down to 0.4dB."

Superconductors also enhance filters to expand coverage areas and lower noise floors.

"The cryofilters we've been deploying have significantly helped," Arsaga said. "Cryos afford us increased sensitivity and still are able to help mitigate interference."

Another option is high-gain antennas. Although they're used mainly along rural interstates and corridors, where coverage is more of a concern than capacity, they can help in urban environments.

"They'll tend to use high-gain antennas in the very densest environments because they want to break the cell into more sectors, each having a smaller coverage angle," said Dean Paschen, Ball Wireless antenna-systems engineer. "That allows you to get more users in a given area and isolate your cell from the other sectors."

The value depends on the environment.

"The trade-off with a high-gain antenna is in the width of the beam," said Ed Hanson, Alltel RF-engineering director. "On occasion, we'll use that for a very tight and very specific coverage requirement."

Then there's cost. "It's (a question of) how many base-station transceivers you want to put in," Paschen said. "You need one per high-gain antenna. In places where you don't have a lot of loading, you'll put in a lower-gain antenna and fewer transceivers."

RISING ABOVE THE DIN Subscriber growth can be a double-edged sword because each user raises the noise floor. Poor site sensitivity can snowball into bigger problems as the network tries to nudge the desired signal above the cacophony of the noise floor. For example, sites tell mobiles when to adjust their power, and particularly in CDMA, power control is a delicate balance that affects everything from battery life to capacity.

"If a site receiver's sensitivity is low, then the system will say, 'Jack up the power,'" said John Richardson, Hewlett-Packard product manager, cellular-base-station test products. "That will have consequences because all these phones are running around with high power."

Accommodating a mix of 3W and 600mW handsets in the same market also can be tricky. One solution is to optimize the network with the assumption that all handsets are 600mW. That approach improves battery life because the 3W handsets run at lower power, which in turn reduces interference.

Another solution is to increase the sites' ranges by raising antenna heights or using repeaters or high-gain antennas. But that's not always desirable because increased coverage usually translates into decreased capacity, and as a network matures, cells typically are split to accommodate additional users rather than expanded.

"The trade-off is you either split the cell and (get) lower power forms that (provide) more capacity and better quality, or you use repeaters and live with the increased interference," said Usman Goni, LCC senior principal engineer.

CDMA is susceptible to a phenomenon called cell breathing, where the site's coverage area shrinks as it struggles to accommodate more users. The unhappy result is that a user near the cell fringe might not be able to make or maintain a call. That's less likely if the network design factors in an estimate of the maximum degradation that could occur.

"I've taken care of the worst-case scenario, so the sensitivity degradation is not to the level that the person at the edge of the cell under that scenario would be dropped," Goni said.

Even the design of the receive path itself can make a difference by limiting the effects of intermod and interference. Receive multicouplers with two LNAs in series, often have "pads" to balance the transmit and receive paths so that the received signal strength at the mobile is the same as at the site.

"To do that, we were padding the receive path, but we were padding after we had boosted the signal through the two LNAs," said Caux of Bell Mobility, which subsequently moved the pads. "The right way to do it is to install the padding between the two LNAs. That's one way to improve the intermod performance of your receiver."

Sites as low as 20 feet above street level are becoming more common in dense-urban areas to improve coverage. Although mobiles usually are more prone to interference because their filtering is less sophisticated than a base station's, in rare cases, street-level sites also can suffer.

"These sites are susceptible to intermod problems generated by mobiles from other carriers that are traveling in front of the antenna," Caux said. "They're kind of saturating the front end. But these situations occur only when the mobile is really close to the antenna. We've found that we don't have many sites where that can occur."