Move Over SCPAs

As your subscriber base booms, you have to add second- or third-channel coverage in certain areas. Using amplifiers to extend coverage is a no-brainer. But deciding what type of amplifier will best meet your needs isn't.

Cellular providers have used non-linear or single-carrier power amplifiers (SCPA) since early deployments. But the alternative, a linear, multicarrier power amplifier (MCPA), can minimize your operating costs and provide reliable, flexible coverage.

According to Rob Harper, Powerwave director of marketing, multicarrier technology is not new, but it is becoming popular now that more reliable and affordable solutions are available.

When engineers design communications systems to combine RF carriers at lower power levels, signals are fed into the MCPA for linear amplification of the entire channel bank of transmitted signals. MCPAs are designed to boost the power of multiple signals.

The MCPA houses the functionality of a combiner, an amplifier and a filter in a single unit. MCPAs, which support TDMA, GSM and CDMA simultaneously, employ a combining and amplification architecture that provides channel-allocation flexibility and efficient power amplification.

An MCPA offers network operators a different way to combine high-power signals. You could use traditional combining technologies for SCPAs but, typically, they are unreliable, Harper said.

"With SCPAs, you have to have an amp for each carrier and cavity-filter combiners that filter out only that signal," said Joe Veni, Spectrian senior vice president of sales and marketing. "That takes up a lot of room, and you have to have thicker amps than you really need to account for the losses to go through the filtering -- that's what MCPAs do."

Because MCPAs have fewer components, they can be more reliable.

"There's (not) anything inherently more reliable about an MCPA, but you can use two or three in a sector that previously may have required 20 SCPAs and a high-power autotune combiner," Harper said.

MCPAs also offer more power in smaller packages. Harper said when a typical 30W to 40W SPCA goes through standard combining technology, power usually is cut in half. If there's some type of duplexing technology, it will be halved again. The result will be 10W or 15W per carrier maximum.

"There are products today that routinely put out 200W of power for a given sector, so that will accommodate 20 10W transceivers or 13, 14, 15W transceivers," Harper said. "You can get higher power on an MCPA because you don't have to go through that combining loss."

More power means more base-station capacity, especially for digital systems. If you can extend a base station's coverage range, you don't need as many base stations in the network.

For example, BellSouth saved a significant amount of space by replacing SCPAs with MCPAs at several of its base stations last year.

MCPAs ARE THE FUTUREBecause of their many advantages, MCPAs are getting attention, especially from 800MHz carriers. As providers upgrade sites and add digital service, they're installing MCPAs to accommodate analog and digital coverage, Veni said.

"An analog amp cannot pass a CDMA signal, and a CDMA amp cannot pass an analog signal very well, but an MCPA doesn't care about the modulation," Veni said.

MCPAs also have benefits for dynamic channel allocation or changing the frequency channel. Autotune combiners required with SCPAs can't retune as quickly as needed.

MCPAs are easy to install. It's almost as simple as ripping out two racks of equipment and replacing it with one rack of MCPA equipment, Veni said. Sometimes you may need to add transceivers, but you won't have to buy additional amplifiers, and you don't need the troublesome combiner anymore, Harper said.

"We have operators that just take the output, plug them into a MCPA, and that offers them an easy way to grow capacity," he said. "MCPAs are flexible. It's very easy to add transceivers, whereas it's a more significant job when they have to deal with combiner channels and separate amps."

With MCPAs, you can add carriers as you grow.

"We see typical users on a per-sector basis using anywhere from 10 to 24 carriers," Harper said. "A lot of people start with 10 to 14 carriers and add more carrier frequencies along the way."

But there's no limit. Harper said one carrier runs 32 radios through one MCPA instead of using 32 SCPAs.

EXTENDING COVERAGECarriers deploying new cell sites in high-density urban areas are installing more MCPAs than SCPAs because they want more power, more channels and less equipment. They also are adding MCPAs to meet the increasing capacity needs of growing subscriber bases.

However, Veni said you still should use SCPAs in rural areas if you don't need a lot of capacity or carriers, and you want to blast a lot of power.

Because of a largely rural subscriber base, U.S. Cellular uses more SCPAs than MCPAs. Jay Quinlan, U.S. Cellular director of technology and planning, said high-power amplifiers are employed to boom out the signal in rural areas.

"We haven't felt the need that some others have had to go linear," he said.

From a cost standpoint, Harper said, for a rural stretch of highway where you only need to transmit a handful of signals, it may be better to use SCPAs.

But many providers start out with SCPAs and find that MCPAs are more economical for transmitting a lot of carriers. You can use MCPAs to get lower-cost coverage in less-demanding areas.

Veni said MCPAs are helpful for minicell installations or a low-cost system with a small number of carriers in a rural environment because they don't need much real estate.

According to Quinlan, there are certain microcellular applications that have required them to use linear power amplifiers. For example, Quinlan said MCPAs work well in low-output situations such as in-building coverage.

"When we get into a microcellular situation, space becomes a consideration and power isn't," he said. "You usually don't need the high power (amp)."

ADOPTING MCPAsAlthough mostly cellular carriers have deployed MCPAs, Veni predicted PCS providers won't rely on SCPAs forever.

"(Initially), MCPA wasn't really available efficiently or economically at that (PCS) frequency," he said.

For example, typical cellular MCPAs combine 16 or 20 carriers per system. With PCS, MCPA systems only carry three or seven carriers at a time, Veni said. But, he added, the technology is improving quickly.

According to Harper, today's power amplifiers dramatically reduce costs and space, and increase performance. He predicted that in the next year or two, amplifiers with new linearization technology should reduce costs further.

MCPAs currently cost less than a new base station, and sometimes less than SCPAs, depending on the application.

"If you have 16 carriers and the cost to put those in is more than $1,000 apiece, it would cost you $16,000 to put in an SCPA system, whereas maybe you can put in an MCPA for $8,000 or $10,000," Veni said.

Sometimes, MCPA systems are more expensive, but Quinlan said cost isn't always a big factor. When MCPAs are packaged with the base station, it's hard to discern how much of that money was spent on power amplifiers. According to Harper, MCPA costs will continue to fall faster than the price of SCPAs.

"I don't see SCPAs working well from the standpoint that (they) don't really save any money," Harper said. "It's more cumbersome; it's harder; there's a lot of soft costs people have to incur in using a bulky, SCPA technology."

So it's understandable that MCPAs serve engineers' needs best. According to Quinlan, linear amplifiers have their place, but most U.S. Cellular engineers prefer MCPAs. Maybe it's time you moved out SCPAs and moved in MCPAs.

Until recently, coverage and interference issues only could be addressed separately because standard products couldn't resolve them adequately. Improving rural coverage creates distortion problems from interference. Reducing urban interference means sacrificing coverage and call quality. But interference is getting worse, and there is a growing need to solve both problems concurrently.

High-temperature superconducting (HTS) (around -200 degrees) filters used with cooled-receiver amplifiers offer both coverage extension and improved interference rejection at the same time. This allows network managers to improve coverage not only in rural areas where there is little interference, but also in suburban and urban areas where interference is at its worst.

The trend toward smaller, lower-power wireless phones, combined with the launch of TDMA digital services has created an increasing uplink deficit in many coverage areas. In the last two or three years, network managers have begun to look for more creative third-party alternatives for improving uplink coverage. They are increasing their use of tower- and rack-mounted receiver amplifiers, low-noise receiver filters and multicouplers, and other standards-based products.

The tradeoff is that adding gain to improve receiver sensitivity risks desensitization and inter-modulation distortion. Using a low-noise amplifier (LNA) to extend receiver sensitivity and capture more low-power signals means giving up the ability to receive the highest-power signals without distortion. Most distortion sources can be eliminated with more selective filtering. But standard filters degrade sensitivity, partly defeating the purpose of installing the amplifier.

HTS technology offers a practical solution to these tradeoffs. Superconductor-based filter LNAs not only offer more coverage extension than standard technologies, they do so without increasing distortion risk. These filters can be so selective that they pass only the desired signals for amplification. Because the base station controls the in-band signal, there is almost no risk of distortion.

With this new combination of ultra-low noise and superior filter selectivity, you can solve uplink coverage problems. HTS technology is opening a range of options for resolving coverage vs. interference dilemmas.