Cramming & Jamming

It's a textbook case: The marketing department at one wireless carrier created a promotion that lured new subscribers with the promise of free calls in the evening. It was a roaring success -- except for one thing. As soon as the free-call period began each evening, several key sites couldn't handle the flood of calls. The result: A costly scramble to deploy additional capacity -- and for the carrier's least-profitable subscribers.

Whether capacity shortages sneak up or pop up, they're a black eye. One way to ensure that the right capacity is in the right places is to look for trends in usage patterns. For established carriers migrating to digital, data warehouses of information such as how and where subscribers use their phones can provide guidance.

"Most of our markets are well covered by our existing cellular system," said Ed Hanson, director of RF engineering for Alltel, which is converting many of its analog systems to CDMA. "We collect usage data regularly and analyze it to stay in touch with changing usage patterns and capacity levels. As trigger points are reached, we take action to stay ahead of demand. In 'green-field' deployments, in which a network is built from scratch, we start with demographic data to provision and then closely monitor system load."

Frequent drive testing can identify areas where capacity is nearing its limits before subscribers notice. Excessive handoff activity and a growing number of dropped and blocked calls can indicate areas suffering from shortages.

"Trending data on a sector-by-sector basis can be quite useful," Hanson said. "A robust program of drive testing can keep an operator on the pulse of the network. If there are capacity or performance issues, they will be apparent in a number of different analyses. (Because) we constantly collect and analyze usage data, our trigger points generally keep us safe."

Capacity needs usually vary by time of day. Routinely compiling information such as time and location should help with trending to identify when and where capacity is most needed. Unexpected growth also can sap capacity. For example, a new roaming agreement can produce a sudden surge in subscribers or minutes of use. Working closely with departments such as marketing can help avoid capacity shortages by anticipating where and how subscribers will use their phones.

"Marketing projections are used when budgeting for anticipated capacity," Hanson said. "The impact of special marketing campaigns is considered in each local market. Having such high demand that additional capacity is required is generally a good problem to have and one that we try to anticipate."

Reduce to Increase

Subscribers and interference both sap capacity. The difference is that interference doesn't generate revenues. Downtilting or lowering antennas, reducing power and using narrow horizontal-beamwidth sector antennas all can help keep RF from sneaking into areas where it can cause interference.

"Traditional AMPS cellular always used 105-degree sectors," said Andrew Singer, director of technical marketing at Celwave, a manufacturer of wireless-optimization hardware. "What the digital systems are finding is they generally play much better, depending on the environment, using a combination of 65-degree and 90-degree horizontal-beamwidth sectors. The general rule of thumb is, the higher the site and the less dense the sites, the wider the beamwidth needs to be. In North America, the most popular tends to be 90-degree horizontal beamwidths and 65-degree in the dense-urban areas."

Electrical-downtilt antennas can be adjusted to minimize interference and thus increase the capacity of neighboring sites.

"You want to focus the (RF) energy as well as you can, and that's going to help your capacity," Singer said. "So if you have adjustable electrical-tilt antennas in the first place, as you fine-tune your site, whether it's optimization, adding microcells or cell splitting, you've got that functionality not only to tilt the antenna toward the main bore-site direction but also reduce the energy off to the sides. That reduction in energy can come in very handy later on as you're doing cell splitting or optimization."

During the initial design, anticipating where interference might occur can make it easier to add capacity.

"What most people fail to consider (is) the interference the sites will cause down the road," said John Arpee, whose Scoreboard software determines the interference that each sector causes to other sectors. "If you are anticipating rapid traffic growth, (and) you put in sites that are interfering too much, you have to take them out a couple of years later. Right now, in order to avoid that problem, you have to have someone who has good engineering experience or judgement, or, using the Scoreboard method, you have to have a way of quantifying what the impact of those sites will be."

Adding Capacity

Optimizing the network also can wring out additional capacity. Common solutions include:

* Reducing soft-handoff zones. Soft handoff can be both an asset and a liability. It helps prevent dropped calls, but it also can reduce capacity.

"A lot of operators found themselves in situations where maybe 50% of calls were in the soft-handoff zone," said Singer, who recommends 30%, depending on the application. "One of the negatives of soft handoff is you give up capacity if you have too much of it because you have two channel elements tied up when you're talking to two different sites."

But reducing soft-handoff zones too much can create other problems. For example, CDMA can experience cell breathing, where a site's coverage area shrinks as it nears capacity. If the soft-handoff zone between that cell and a neighbor cell is too small, calls could drop when coverage pulls back.

* Tweaking neighbor lists. Steering calls away from sites that suffer chronic shortages can provide a short-term solution until additional sites can be added. Optimization always should include updating neighbor lists so that calls aren't handed to the wrong sites.

* Cell splitting. Adding sites is expensive, but a design that allows for future cell splitting can help keep costs and interference to a minimum.

"It's time to split cells when you have no other choice," said Alltel's Hanson. "Given the difficulty encountered in siting cells, it pays to consider other approaches. To do it successfully, start early and maintain a positive relationship with the people in the community responsible for zoning."

* Smart antennas. They haven't caught on in the United States, but smart-antenna vendors say the combination of greater capacity, increased gain and better interference rejection will start winning converts. ArrayComm's IntelliCell, for example, costs about the same as a conventional base station, but Martin Cooper, chairman & co-founder, said a network using his technology could deploy three to 10 times fewer base stations. Faced with continued subscriber growth, a shortage of available sites and cutthroat competition, carriers such as Alltel are starting to consider smart antennas as a viable alternative.

"There is no question that five years from now, there will be no personal-wireless-communications base stations built without smart antennas," Cooper said. "It might not be ArrayComm's technology, but it just doesn't make sense given the state of today's digital-signal-processing art and wireless art that anybody would build a base station without smart antennas."