No Vacancy

Not only did the number of wireless subscribers increase by more than 25% over the year ending June 1999, but they're also talking more: The average minutes of use (MoU) increased by 39% over the same period, according to the latest CTIA figures. At that rate, ensuring that demand doesn't outstrip capacity will remain a full-time job.

Figuring out where the most significant growth will occur is the tough part. One guide is a market's projected adoption rate.

"That's generally something that my organization gets from our salespeople and marketing," said Michael Robinson, Sprint PCS vice president & general manager of network services. "The forecast articulates how many subscribers they anticipate over the next planning period. We'll translate that to call models that we then size the network on."

Other guides include municipalities' master plans, which show which areas will be residential and which will be commercial, and state and federal transportation agencies' long-term road projects.

"The most useful publicly available data that I've seen is the vehicle counts that usually are available from a county office," said John Arpee, CTO of Scoreboard, a design- and optimization-software company. "A lot of the usage is going indoors with portable phones, but I still think vehicle counts are a pretty good guide to estimating where your hot spots will be."

Population density, household income and home prices are other commonly used, widely available guides, but all must be taken with a grain of salt.

"It's hard to draw direct relationships between those parameters and the actual traffic you see on the site," Arpee said. "Part of the problem is that people don't buy mobile phones to use at their homes. They buy them to use somewhere else."

That's changing. Just a few years ago, few would have dared speculate that rates would be as low as they are. But with cheap buckets of minutes, many people are using wireless at home in lieu of second lines. That trend affects which statistics are useful planning guides.

"If people are using it at home, then household income probably would become a much better indicator of usage patterns," Arpee said. "I think we'll see those trends become more pronounced."

INTERFERENCE ROBS Despite the myriad variables that can affect usage, a good network design still can accommodate growth easily and cost-effectively. One reason is because interference and subscribers are similar: They both use network capacity. The difference is that interference doesn't generate revenue. A good network design doesn't just put RF in the right places; it also keeps it out of the wrong places.

But even the best design can be thwarted by later changes. For example, optimizing one cell to increase its coverage might mean that it now interferes with another cell, which then has less capacity to offer. During both the initial network design and periodic optimization, assessing just how much one site might interfere with another can be tough.

"As the cells get smaller, and you're dealing with dense-urban areas or complicated terrain, the (propagation) predictions really lack credibility when you consider that the standard deviation of error is about 8dB," said Arpee, whose ScoreBoard software shows how much cells interfere with one another. "The end result is that engineers don't trust the model-based tools, and they use their engineering judgement. That works okay if you have a fairly simple network, and the engineers are familiar with it, but it's not a systematic approach, and it's not repeatable because you're depending heavily on the skill of the engineer."

Another way to keep interference to a minimum as the network matures is to think small: High, powerful sites are great for coverage, but an unwelcome side effect is interference. "If you have a high-growth network, you definitely want to minimize the height of the cells going in," Arpee said. Cells with smaller coverage areas and lower power should be less prone to interference and able to accommodate more users.

Even with CDMA, where frequency planning is almost a non-issue, interference still robs capacity.

"Everything is on the same frequency, for the most part, until we go to a subsequent carrier, which is a preselected range anyway," said Sprint PCS' Robinson. "The more challenging issue is if you're going to do a cell split, depending on the initial design, you might have to start lowering antenna heights in order to minimize pilot pollution."

LITTLE VOICES Although MoU are increasing, subscribers aren't necessarily talking more: A 1968 Bell Labs study found that in a typical conversation, pauses can account for more than half of the total time. One capacity-boosting tweak is intermittent transmitting, where the network monitors the call and powers down the transmitter during those pauses.

Another voice-related tweak is enhanced variable-rate coder technology, which provides voice quality comparable to a 13kb/s vocoder but uses only 8kb/s.

"You get a boost in Erlangs, the number of air minutes in a sector, and you get a boost in voice channels per sector," said Jim Smith, Lucent product-marketing director, AMPS/PCS.

Although U.S. growth remains phenomenal, it's lagging compared to some European countries. But if Europe is any indicator, carriers here soon might find it easier to get the additional sites necessary to support more users.

"If you look at the main part of Europe, the penetration rates will vary from about 16% to around 60%," said Norman Korey, Wireless Facilities president of international operations. "The market is more mature, so a lot of the places that you'd normally put sites and RF hardware, the (landlords) have already been approached, so they're a little bit more accustomed to RF infrastructure being co-located in commercial facilities. From a flexibility standpoint, although there are places in Europe that are really tough, I think it's a little easier than some spots in the United States."

WORK IN PROGRESS It's tough enough adding capacity to a mature network, so what happens if your network hasn't even launched? That's what NBTel Mobility faced in September 1999, when a convention brought thousands to Moncton, NB. NBTel Mobility wanted to supply delegates with phones, but its CDMA network was still under construction, and the projected usage in key areas was far more than under normal conditions.

NBTel Mobility's vendor, Nortel Networks, temporarily re-engineered parts of the network to supply enough capacity to hot spots such as hotels and auditoriums. The hottest spot was a courtyard outside the main delegate meeting area.

"During the breaks, it would have 600 to 1,000 people," said Dave Keegstra, Nortel Networks RF-engineering manager, Canada. "At any one time, we could have a significant amount of people online."

A CDMA network's capacity is governed partly by the amount of power each subscriber's phone needs to maintain a call. Less power translates into more room for more users.

One way to reduce power is to focus the RF so that it doesn't sneak into other areas, where it causes interference and saps capacity. Horizontal beamwidths of 60 degrees are common in areas where coverage is the main concern, while in areas where capacity and interference are issues, 33-degree beamwidths are common. Covering the summit's hot spots meant using parabolic dishes, usually reserved for point-to-point microwave applications.

"These had 25dBi gain and very narrow beamwidths: 7-degrees vertical and 9-degrees horizontal," Keegstra said. "We focused them from the macrosite into the particular venue. You don't want too much coverage going outside the venue because then people outside can take up capacity. These parabolic dishes worked so well that we had to place some attenuation because the signal was too hot. We found that for power control in CDMA, you want your signal to be within the power-control range. We had to put in some attenuation so that we could get the full range of power control in the phone. Once we were in the full range, it brought our capacity up to 38 users per sector."

The dishes worked in conjunction with Metro Cell, a Nortel Networks base-station design that spreads heavy traffic loads over multiple sectors.

"This integrated solution allowed us to get about 38 users in a sector," said Tony Grande, Nortel Networks director for wireless engineering, Canada. "The industry average is about 13."