Change is inevitable. Nowhere is that more true than when it comes to sites,
especially older sites. In the early 1980s, most sites were geared for broad
coverage and modest capacity goals. Engineers placed antennas high on
towers and pointed them horizontally.
Today, that has changed. As carriers try to drive minutes of use (MoU) ever
higher, they want to make sure their existing sites can handle the high-volume
capacity and still be upgraded easily. The broad-brush approach doesn't
necessarily apply anymore.
Back in 1985, Bell Atlantic Mobile (BAM) launched eight sites. Today, the
company boasts 3,500 sites. As it grew its subscriber base and MoU, it has
had to migrate to digital and now is preparing for its 3G entrance. According
to Tony Melone, BAM vice president of network planning, the company
selected CDMA because it permitted high capacity gains and would allow
BAM to grow its network more efficiently. With CDMA in place, BAM is
better able to handle heavy traffic loads without compromising call quality.
Another advantage to using CDMA, according to Melone, is that upgrading
to 3G will be much smoother than with other technologies.
Industry analysts project that wireless data will surpass wireless voice's
success in airtime usage. If that growth materializes, Melone said he believes
that CDMA's ability to reuse channels and accurately hand off connections
will provide a more secure 3G migration.
Spokespeople from Alltel and Sprint PCS agreed, describing the move from
CDMA to 3G as graceful. Both carriers anticipate upgrades to be easy to
deploy.
“I can't see 3G requiring any re-invention,” said Jeff Chaltas, Sprint PCS
spokesperson. Although the specifics still are being worked out at Sprint PCS,
Chaltas expects 1X3G to be only a matter of replacing base-station circuit
boards, channel cards and possibly some engineering software. For the most
part, antennas, towers and base stations would remain unfazed.
Antenna Manipulation
Regardless of whether your plans call for 3G migration this year or in the next
few, down-tilting and sectorization remain the most popular ways to get better
performance at your sites. Downtilting antennas is the easiest and most
cost-effective way to increase capacity. A downtilted antenna essentially
decreases the size of the cell. Instead of radiating the RF power horizontally,
the downward adjustment increases the signal strength close to the site,
turning a macro site into a micro site. However, the down side to downtilting is
interference. Smart antennas might give the best optimization where
interference is a problem.
In cases where downtilting is not giving you the performance you demand, an
antenna can be moved lower on the tower or replaced with a narrower beam
antenna. A number of vendors offer antenna solutions for this type of
optimization. Choosing a more feature-rich, cutting-edge antenna could be the
change your system needs to win more capacity and help you add more
customers.
For example, Ericsson offers an active antenna, which features an integrated
power amplifier. This antenna can turn a macro site into a micro site with little
reconstruction. Although the antenna is small and can be mounted on a wall
or a pole, it still has the same coverage as a macro site. Ericsson launched two
active antennas last year as part of its optimization solutions. One is designed
to offer very high output power, and the other has less power but more
control for urban areas.
For downtilt optimization, Celwave has a continuous dial-turn electrical
downtilt antenna that allows you to move a dial and fine-tune your system
performance. Downtilting your antenna is the preferred method for coverage
shadow compensation. A common place to downtilt is at a cell site that is on a
tall tower or hill so that your signal coverage area can remain large.
This drive test shows a high frame error rate and the call actually dropping when going through this
intersection.
After downtilt antennas are installed, the frame error rate improves and pilot pollution is virtually
eliminated.
Another widely used optimization technique for digital and analog is
sectorization. By implementing a co-located subcell antenna pattern, you have
greater reuse within cells close together, thereby increasing the overall
capacity of the system. Ed Hanson, Alltel director of network evolution, said
for a network to mature from a start-up, sectorization is fundamental.
In the early days, carriers started with omnidirectional patterns because they
were the easiest and provided the most extensive coverage. Using that
technology, however, meant that you didn't have much interference control.
To combat interference, carriers divided cell sites into smaller segments and
sectorized their coverage. Today sectorization continues to provide capacity
while effectively allowing interference control.
Steven Urban, Motorola senior marketing manager, said sector-ization and
downtilting can be used in a variety of situations and depend on a number of
factors, including how much capacity you need to add. For more capacity,
sectorization is better. However, it is more complex and costly. To stay
competitive, it's recommended you downtilt your antennas after sectorizing
your network. Michael Robinson, Sprint PCS vice president and general
manager of network services, said repositioning an antenna is usually
reserved as a last resort. Robinson said he would rather replace an antenna
than move it on the tower.
“It's not a problem of cost since the antenna can be reused at a different site,”
Robinson said. “But we've found that repositioning takes quite a lot of
fine-tuning and maybe time we don't have,” he said.
Minimizing Downtime
Probably most important when optimizing your cell sites is keeping downtime
to a minimum. Motorola's Urban favors gradual upgrading to avoid
compromising your system capacity.
“Our clients don't want to lose connections,” Urban said. “We have to keep
capacity up, and gradual upgrading is the best way to do that.”
The use of high-grade planning tools is another must. Without a systematic
plan in place, you may be maximizing downtime. Just as before a site is
constructed frequency plans give predictions on its efficiency, a planning tool
can help prevent mistakes.
Other considerations include not waiting until the system is too old. Engineers
insist on being pro-active when optimizing a system. It is always best to plan
optimization before it is needed, which affords you a better time frame to work
in. Also recommended is using equipment and technologies that are easily
upgraded and using a system-solution company to make the switch for you.
Outsourcing your site upgrades can be cost-effective and dramatically reduce
the duration a site is without service.
Add a Channel
To get more capacity without antenna manipulation, you either can build more
sites, which may not be very practical in today's market, or you can simply
add a channel to your base station. Although you may find your subscriber
base does not yet support an added channel, Urban suggests preloading your
system and adding a channel before you actually need it. The best way to
preload a system is by calculating analog phones as digital and preparing for
system loads based on that estimation.
Examples of bandwidth patterns with downtilting and without
For carriers still offering analog, gradually adding digital to your system with
multiple-mode products will allow you to optimize an analog site, while
continuing to serve your analog subscribers. Doing this will displace some
spectrum used for analog. Alltel and BAM both offer analog, but said that
displacing analog is not a crucial problem. About 15% of BAM's analog has
been displaced to make room for digital. BAM's current ratio of analog-only
sites to mixed-mode sites is 1:9. BAM optimized its analog sites by upgrading
them to digital. Alltel, on the other hand, did upgrading similar to BAM, but
also did some retuning of RF beams to aid capacity for analog sites.
Alltel's Hanson has assigned different frequency groups to a sector in order
to control interference. Even though your own sites are the cause of most
interference, you can elude it by retuning. Alltel uses a 21-group scheme to
divide its frequency channels. By having a larger number of groups, you can
increase the capacity dramatically and still avoid any consequential
interference.
Smaller Is Better
System optimization traditionally means smaller and more plentiful sites. This
move from macro sites with far-reaching coverage has prompted Melone to
say that 50% of BAM's sites will be micro within the next five years. To
achieve system optimization, Sprint PCS regularly splits cells. Normally cell
splits are done only when cells in an area are perpetually at 80% or more of
capacity.
According to Anders Akvist, Ericsson product marketing manager, picocells
or minicells are the next wave of optimization. A picocell is the smallest base
station available today and is typically used for indoor coverage at malls and
airports. Akvist said pico sites already are making the move to the great
outdoors to allow better coverage along highways and thoroughfares. He
predicts that as the industry counts down to 3G, and customers embrace fixed
wireless for home and office use, microcells and pico-cells will be the future of
system optimization.
Comments? Write to tarre_beach@intertec.com.
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