What's Old Is New Again

PCS boosters like to predict that analog will quickly fade into an historical footnote, but some four years after digital's debut, analog still fleshes out coverage for dual-mode subscribers and has no shortage of its own.

"As of 1998, about 90% of the market was still analog handsets," said John Todd, ADC product manager for wireless-coverage products. "That's on a downward trend. They're not making as many anymore, but you're still going to have people in the rural areas who need 3W bag phones to get back to the base stations."

Well-tuned analog networks also aren't going away. But conventional wisdom holds that optimizing analog is more difficult because a digital network provides volumes more statistics about quality of service (QoS). That's true -- to a point. Carrier-to-interference and signal-strength statistics are the mainstays of assessing analog performance, but they're by no means the only guideposts. Savvy operators know that there are other ways to identify problems. Bell Mobility, for example, mines its switch logs to look for clues to problems throughout the network.

"With that, you could assess if you have interference from a distant cell site or if you have a bad antenna," said Guy Raymond, Bell Mobility director of network operations for the eastern region. "We also use call-fail logs to assess the performance of radios. A bad radio will produce more logs than the others."

So much for the notion that analog operators don't have much to work with.

"There's tons of information," Raymond said. "We don't use 10% of the available information."

CAN YOU HEAR ME?Another type of information that sometimes goes overlooked is the classes of phones operating in the market. Three-watt and 600mW handsets have different uplink requirements, so knowing which types of phones are out there helps optimize coverage to ensure that all can support calls.

"It's pretty important so you can align your handoff boundaries and access thresholds," said Mike McCarthy, Nortel Networks senior portfolio manager, TDMA access. "You want equivalent coverage in a cell for the various different types of mobiles."

Even in networks where most handsets run the same power, path balance remains key to ensuring uniform QoS."A number of the smaller cellular providers that tend to provide rural service are still caught in the contour coverage and different classes of mobiles that are active in the market," said Andy Groome, Safco manager of engineering services. "As a result, they don't have sufficient infrastructure to support those smaller (600mW) units. Therefore, they experience path imbalances to a much greater degree."

One way to ensure that lower-powered handsets have enough uplink power to reach a base station is to tailor coverage with the assumption that all phones are 600mW. Another approach is to treat the 3W phones like 600mW units.

"We force the 3W phone to reduce its power level, and on the reverse link, we have an offset," said Bell Mobility's Raymond. "We've built an offset table to (have) the (3W) phone emulate a (600mW) phone. All the phones look the same, so they hand off at the same location."

One area where both 3W and 600mW phones often have trouble is inside buildings. If an outdoor site is nearby, the temptation is to increase its power to blast a signal inside. But in most cases, the unwelcome byproduct is increased interference elsewhere in the network. Boosting power also doesn't ensure path balance: The handset could receive the signal but still be unable to muster one strong enough to make it outside.

"Now you've got a 600mW handset trying to talk back to the (site) and going through all this free-space loss and attenuation," said ADC's Todd. "Usually carriers find themselves not downlink-limited but uplink-limited."

In both indoor and outdoor applications, simply increasing the signal can do more harm than good. That's a common mistake.

"They rely too much on signal strength," said Greg Evans, Safco senior RF engineer. "They're looking for a lot of RF. For example, for in-building penetrations, they have a tendency to try to put too much signal strength to cover buildings. That causes the network to suffer."

Indoor microcells are popular because they provide a solid signal and improve path balance. The trick is to keep the handset from choosing the wrong signal.

"The handset is going to look for the strongest control-channel signal and lock onto that," Todd said. "If you're blasting in from the outside, it's going to try to lock onto that outside channel and not the inside channel."

One option is to use separate control channels, which also can provide the foundation for applications tailored to a specific user group or environment, such as an office campus.

"There are ways to provide in-building coverage with a certain set of control channels and voice channels at a specific per-minute rate," Todd said. "They know that any calls that are set up on this control channel and these voice channels should be billed at a specific rate for this customer."

In-building coverage in skyscrapers also can be a challenge. On one hand, metallic window tinting, common in commercial construction, can attenuate signals by as much as 60dB. But on the other, the tinting can help indoor microcells do their job by preventing strong outdoor signals from sneaking in.

"There are limitations when you get so high in a building that you start seeing a lot of antennas and a lot of control channels," Todd said. "If the signal is very strong, it's going to be much more difficult to provide in-building coverage unless they can attenuate the outside signals significantly or strategically place a stronger signal inside the building."

OLD DOG, NEW TRICKSOne barometer of analog's viability is the array of optimization tools available. Tweaks such as cell tiering and adaptive channel allocation allow the network to shuffle its resources to accommodate the maximum number of calls.

But the benefits of such tweaks aren't always clear-cut. One example is the practice of setting aside roughly 2% of channels on each site to accommodate handoffs from other sites. Opinions vary about whether handoff reserve is important or mainly a waste of valuable resources, and many operators abandoned it after blocking increased. One variation is an algorithm that monitors traffic loads and channel requirements in real-time and determines how many channels should be reserved for handoffs.

"The reason we developed adaptive handoff reserve was because that was a raging debate," said Nortel's McCarthy. "The operators couldn't find an ideal number of channels to reserve. They would wind up blocking in some instances where they reserved more than they needed to, or they'd wind up dragging calls and dragging the coverage of the cell when they didn't reserve enough."

Given how dynamic traffic patterns and interference levels can be, adaptive solutions likely will become the norm for both analog and digital.

"It's really something that you can't say, 'This is the right number for all different conditions because of all the temporal and geographic (conditions) and changes in traffic,'" McCarthy said. "So the optimal is to have an adaptive algorithm that monitors the traffic."