Sectorization Savings

Nov 15, 2001 12:00 PM, By Mary Corcoran, Staff Writer

Some sectorization schemes can save you thousands; others can cost you.

As the economy fights to stay positive, carriers are looking at the bottom line with renewed interest. Technologies that can help save money but still improve networks have become popular as carriers look for options other than building new cell sites. Antenna sectorization, which can increase a cell site’s capacity greatly, has become one of the most popular solutions. All sectorization schemes are not created equal, however. If carriers choose poorly, they could end up spending money, not saving it.

Show Me the Savings
Adding a simple 3-sector scheme to a traditional antenna site roughly can triple site capacity. Sectorized antennas cost more, but that is minimal when compared to other options.

“The cost of sectorization is probably going to be about 10% to 20% of the cost of putting in a new site,” said Ray Butler, AT&T Wireless RF engineering director. “It’s definitely cost effective to sectorize rather than build a new site.”

Mark Kelley, Leap Wireless CTO, said that when carriers began to build out their PCS networks, it became clear that sectorization was the way to go because it allowed carriers to use fewer cell sites.

“As far as I know, most people did their full designs with sectorized antennas,” Kelley said, “not because of the notion of reusing capacity but because of antenna gain. The cost trade-off between doing omni and sector was pretty straightforward.”

Sectorization remains an important cost-savings tool even though most carriers now are adding to, not creating, their networks.

Kelley said that Leap, with only 10MHz of spectrum, is conscious of technology that squeezes out more capacity while keeping costs down.

“If there’s a way that I can improve the use of the network elements I’ve already bought by sectorization,” Kelley said, “and I’m able to not add another five cell sites, I’ve saved a significant amount of capital.”

David Tilfton, Til-Tek vice president of operations, said as carriers weigh technologies to see which are worth the money, simple sectorization schemes are coming out ahead.

“We’re seeing a pulling back of roll-out plans because of the slowing economy,” Tilfton said. “The question people are trying to answer is, ‘What is the right amount of capacity for how much money?’”

The answer is up to each carrier, he said. Carriers used to think that more capacity always was worth paying for. Now, some are stepping back.

Simpler Is Better
Although 3- or 4-sector sites can increase capacity without breaking the bank, more complex sectorization schemes usually provide only diminishing returns. It seems as if the math should work like this: Add three sectors, triple the capacity. Add four, quadruple it. Add eight or 12 or 24, get eight or 12 or 24 times the capacity. Too bad it doesn’t actually work that way.

Continuing to subdivide sectors creates diminishing returns, said Paul Polakos, Lucent Mobility Research Group advanced wireless technologies director.

“If you look at networks in general, you’ll see that 98% of the cells are three sectors or less,” he said. “There are some (areas) where they use up to six sectors, but beyond that it becomes impractical.”

Radio signals scatter whenever they hit buildings or other objects in the environment. The smaller the sectors, the more of a problem signal scattering can be. As the signals scatter, they lose their original shape. Problems arise when a base station tries to collect these signals. In some cases, the signal will not be able to fit into the smaller sectors.

“The base station sees a signal coming in with a certain angular distribution,” Polakos said. “That angle spread has to be substantially less than the sector width. Otherwise, there would be no gain in subdividing the sector.” For this reason, he continued, six sectors seem to be the maximum number of sectors that are used commonly.

“And, even with that, there are other issues dealing with 6-sector cells that somewhat diminish the return,” Polakos said.

Those issues include interference and the higher cost of deployment. Going from a 3-sector cell to a 6-sector cell roughly will double the cost for base-station equipment, said Marty Feuerstein, Metawave senior vice president & general manager.

“You basically double the number of radios and antennas,” he said.

Interference in existing 6-sector sites is such a concern that some carriers are retooling them, said Marc Mills, Andrew Broadband Wireless Systems Group RF networks engineering director. Carriers once considering extremely complex sites — from eight to 30 sectors — now are looking into a lower number of sectors, he said.

Sprint PCS is one example. Reza Hosseinmostafa, Sprint PCS RF manager, described a 6-sector site in Kansas City that was turned back into a 3-sector site after it became apparent that it had performed better with three sectors. Six-sector sites are difficult to optimize because they create so much of their own interference, he said. Now Sprint puts in 6-sector sites only after much research.

“You can’t just go anywhere that you have a capacity problem, put in a 6-sector site and expect to double the capacity,” Hosseinmostafa said. “If you’re doing it in an area where you have a lot of clutter or tall buildings, it just causes problems.”

AT&T also sticks with 3-sector sites to meet its capacity needs, occasionally using 4-sector arrays to solve unique coverage problems.

As bad a rap as complex sectorization schemes have today, they may find new life as technology moves forward.

“As we look forward to the 3G technology, we’ll be keeping an eye on higher sectorization,” Butler said, “to see how it might benefit us.”