Rural Redux

Rural providers have their work cut out for them as they deploy digital, increase coverage and accommodate low-power handsets.

Walk down New York City's Fifth Avenue, and you'll find almost every other person using a wireless phone. But a revolution also is under way in small towns, where local contractors all have phones on their belts, and the farmer's family doesn't wonder when he'll finish plowing because they just call him on his "tractor phone."

Wireless use in rural communities is exploding, and many smaller providers face myriadchallenges as they expand coverage to maintain quality, add digital to provide competitive services and accommodate usage that they never thought they'd see — all on a limited budget, with limited technical experience and guidance.

Rural providers' technical challenges are quite different from those of urban providers. Urban providers cover small geographic areas with dense populations, while rural providers cover vast geographic areas with fewer users. Rural users, once satisfied with simply having the service in their area, also are demanding improved quality and services on par with those that urban and suburban users enjoy. They want coverage in remote locations, places where minutes of use (MoU) will be low but where having service is important to high MoU users.

Rural providers also are seeing trends that their urban cousins went through years ago. For one, SMR providers' expansion into rural areas is causing intermodulation that most small rural providers are ill-equipped to handle.

Perhaps the biggest challenge, one tied to the need for better coverage, is that rural users are tired of cumbersome bag phones. They want sleek phones that can be latched on belts or tucked in shirt pockets. But these compact phones transmit at a maximum power of only about 600mW and more often lower. That's much less than the bag phones for which most rural networks were originally designed.

Rural providers also are discovering that many digital technologies don't offer coverage equal to those of their AMPS systems. This difference may be due to the ability of an AMPS call to remain up — albeit static-filled — on cell edges, where a digital call might drop. One common problem is many calls handing off from digital to analog as users reach cell edges and often remain on AMPS for the calls' durations.

Another challenge is wireless data. One key measure of wireless-data performance is throughput. In a digital voice call, bit-error rate (BER) determines performance, but intermittently high BER is normal, expected and often even unnoticeable. In data, however, throughput is directly related to BER: If the BER increases during a packet transmission, that packet probably will have to be re-transmitted, and throughput will decrease substantially. As a result, providers deploying data can't tolerate high BER. This requirement is a bigger problem for rural providers that already are stretching their sites' coverage limits just to maintain voice traffic.

E-911 adds another twist. Many location solutions require multiple sites in order to triangulate the caller's location. That's a tall order with low-power handsets and rural sites spread far apart. Mobiles will need to transmit at higher powers — and for reasons of size, battery life and health issues, that's unlikely — or sites will need better sensitivity.

Rocks & Hard Places
Ask a rural provider for a list of its biggest challenges, and chances are that coverage holes will rank near the top. When most rural networks were built, the idea was to put omnisectored antennas as high as possible and crank out maximum power as long as the 2W-or-more phones could get back on the uplink.

But as subscribers are added, this setup develops problems. First, because it's omnisectored and designed to cover large geographic areas, sometimes several small towns, these high-elevation sites quickly experience capacity shortages. Adding sites is a major task because issues such as co-channel interference and coverage gaps must be considered. These sites also usually are limited by the handsets' maximum transmit power, which is decreasing, and they're susceptible to interference, particularly out-of-band interference that can cause intermodulation and lead to dropped and poor-quality calls. Thus, the trend in rural markets was to move from a few, extremely high towers to more and lower towers.

But providers still must cover the same amount of geography. One option is adding more powerful amplifiers on the forward link, but that leaves a reverse-link deficit, particularly in AMPS and TDMA systems. Although they can't increase the handsets' power, providers can improve reception at the sites with tower-mounted amplifiers (TMA), which sit at the top and amplify incoming signals and thus increase the reverse link's range.

Although these fairly inexpensive TMAs can be handy, particularly on very high towers, their drawback is that they amplify undesired signals and noise, too. If an interfering signal is causing intermodulation products in the base station, the TMA generally makes this problem much worse by amplifying the interferer. A common situation is that a competitor's subscriber is far from its base station but right next to yours and transmitting at a high power right into your receiver, which now is plagued by interference.

TMAs also add their own noise. The noise figure, which is the log of the S/N ratio at the input divided by the S/N ratio at the output, is a fundamental aspect of receiver quality. Thus, although the TMA can add sensitivity — the ability to receive weaker signals — it can seriously degrade selectivity, the ability to weed out interferers.

Another coverage booster is repeaters. They're simply low-noise amplifiers that receive and amplify the site's signal while also receiving handset signals and relaying them to the site. Although this approach is a relatively inexpensive and effective way of dramatically increasing coverage compared with adding a site, call quality often isn't as high, and because repeaters don't add channels, they don't add capacity.

Superconductor filters and cooled low-noise amplifiers are another option. Like TMAs, these systems can add gain to a receiver while also providing extreme selectivity and a very low noise figure. The filters have sharp filter skirts that give extreme rejection of interfering signals. Conventional filters normally have high noise figures, but using superconductors results in figures sometimes 3dB or 4dB lower. That means that just by replacing the filter, a base station would have 3dB to 4dB additional sensitivity. These systems also use supercooled low-noise amplifiers, which have very low noise figures.

The result is a much more sensitive receive path that adds little noise while providing extreme selectivity. That's important because in digital systems, the S/N ratio generally determines BER.

Superconductor filters also allow handsets using that site to transmit at lower power, which extends battery life — important as power-hungry wireless data is deployed — with less co-channel interference and lower RF emissions, which are increasingly thought to be a health risk.

In addition to the business-related challenges that today's smaller rural providers face, they also must grapple with the technical challenges of deploying digital, accommodating low-power handsets, increasing capacity and expanding coverage to the entire RSA. Fortunately, there's no shortage of new technologies to help them meet these challenges.

Miceli (amiceli@suptech.com) is Superconductor Technologies regional sales manager and author of Wireless Technician's Handbook.