Power to the Site

Fifteen years ago, compact, digital and integrated were not major buzzwords for cellular carriers and probably were not used at all when discussing cell-site equipment. Today, however, no conversation about any product -- especially site equipment -- is complete without them.

The talk is no different for power products. Tim Lowe, Peco II vice president of marketing and international sales, said when wireless companies first started setting up sites, they took a lot of their direction from wire- line. All shelters had enough spare room to allow for more equipment and future growth. Now the trend is smaller sites, he said, with some containing just a cabinet full of equipment, which creates a concern for how much space power equipment takes up and the allocation of that space.

Curt Whitney, BellSouth Cellular director of operations, sees that change, as well. Many PCS carriers build sites with room for only about two car batteries to house the backup power system, whereas cellular carriers used to install much larger systems.

For cellular carriers, this compact trend means finding a balance between living up to the high standards past systems provided and catching the technology wave of the future as power systems develop.

Lowe said cellular carriers approach power systems in different ways, from setting up their systems as they always have to installing completely new designs. The philosophy of the engineers in charge of power provisioning determines the carrier's approach. Some engineers don't like to fix what isn't broken. Others always are looking for the next big thing, especially if it is more efficient or costs less to install and maintain.

MOVING ON UP ... TO DIGITALOne of the biggest changes at cellular sites over the past 15 years has been the drive toward digital. Installing digital equipment at a site that previously only supported analog means change.

According to Ed Pack, Delta Products manager of telecom products, analog has remained viable over the years, and now some carriers must meet the distribution needs of two systems as they add digital. With analog sticking around and more ancillary items added to each site, the need for circuit breakers increases, he said.

Peco II's Lowe said the digital revolution brings other power requirements. If you use a different manufacturer's switch when you move to digital, you must keep in mind that each switch has unique requirements. Many analog systems came in with a 24V requirement, while digital has transitioned to 48V, he said. You also must consider factors such as initial costs, density, packaging, performance and the size of backup power systems.

The way carriers house systems varies significantly. Some providers allow a single rack on the floor; some want the equipment to fit in the racks with other equipment; some stack the power system separately in its own frame.

BellSouth Cellular faced several challenges when it added digital capabilities, according to Whitney. Digital has fewer power requirements per path, but at the same time it requires more capacity per RF transceiver.

"For digital, our amplifiers are less efficient," he said.

Amplifiers for digital systems have become linear and are providing 25% to 50% power efficiency when it comes to RF power output, whereas analog has had 50% to 70% efficiency.

The linear power amplifiers of digital systems do draw more power than their analog counterparts. However, they also are supporting more voice paths. According to Whitney, a typical 50W single-channel amplifier for an analog system draws 8A or 200W, and for TDMA draws 14A or 340W. So the TDMA amplifier draws 70% more power but carries three times more voice paths. Overall, Whitney said, total power requirements have increased with the introduction of digital. Planning for that growth is key when determining a site's power constraints.

FIRST-CLASS RELIABILITYAlong with the introduction of digital, increased competition from PCS carriers and the desire to compete against wireline have caused cellular carriers to address network reliability with increasing vigor.

"Cell sites, especially in rural America, are not replacing wireline yet but are trying to provide the same level of service," Delta Product's Pack said. "If (cellular carriers) want to get as close to wireline reliability as possible, the first place they look is power."

This strategy means increased interest in using generators and a need for greater battery life, he said.

BellSouth has strict backup power requirements for its sites.

"The power plant is part of keeping the system up," BellSouth's Whitney said. "We want two hours of battery backup. That hasn't changed (over the years)," he said.

As part of the reserve system, BellSouth allocates generators to nearly half of its sites, especially critical transmission hubs and sites with high usage.

"Some time, every day, somewhere power goes out," Whitney said. "If it's a tornado, we will have multiple sites, 10 or 15 at a time, go down."

With a network of more than 3,000 sites, it is essential for BellSouth to have such a system in place.

"Our philosophy has been to put generators where needed, especially along the Gulf Coast where hurricanes are likely and widespread outages can occur," he said.

At sites without permanent generators, BellSouth uses portable generators, and some sites have plugs where it can hook up generators easily.

One concern when not using a generator, Whitney said, is the potential loss of air conditioning. When depending solely on battery or dc power, the air conditioning system cannot run, which means the internal temperature gets so high that the system should shut down to protect the equipment from overheating.

MONITORING THE FUTUREMore changes are on the horizon for power systems. Assessing the health of network equipment is essential to assuring its reliability. Monitoring power equipment specifically to determine its operating status and being able to identify its constraints and weak spots is an area that has been tapped only slightly.

Craig Whisman, BellSouth Cellular technical operations manager, said the carrier routinely tests its main and backup power systems to check for problems, but it only does continuous monitoring on a general basis. External alarms are triggered at the network operations center (NOC) if the system detects a power-related problem outside normal thresholds.

BellSouth Cellular conducts fixed-load tests on batteries at all sites annually, Whisman said. The tests determine if power is being distributed equally across the network. If it isn't, which is hazardous to the equipment's life, BellSouth Cellular adjusts the voltage.

Test scenarios such as these cannot be done remotely on an automatic basis, but these types of systems are becoming more prevalent.

"Coming around the corner, we are going to have a large deployment of monitoring equipment," Lowe said.

One of the biggest challenges to implementing remote monitoring for power systems is determining the easiest way to get information from the site to the NOC.

"Everyone understands the importance and the benefits of remote monitoring," Delta Product's Pack said. They just don't know how to access the data.

Pack said carriers are trying to eliminate plain old telephone service lines from their sites if data can be sent over a modem, and they are using T1 lines or overhead channels.

If you are not ready to invest in a continuous connection to the monitoring data, intelligent systems enable you to collect real-time data at all times and analyze it later.

"Sites where they don't have (remote) access, we have technicians download the information when they do radio maintenance," Pack said.

This method enables you to look at the power system over a longer period of time than a 1-shot test allows, he said. You can collect and analyze data such as interruptions to the ac input, the site temperature, battery temperature, how well the batteries are holding their charge and peak loads.

"A discharging test is invasive," Pack said. "It takes something away from the battery in order to compare it year to year. Continual monitoring is a non-invasive way to compare."

For example, he pointed out that active data can help you determine the peak draw easily, whereas scheduled tests show whatever the power draw happened to be when you performed the test, which might not give an accurate idea of peak-power draw conditions.

To make backup power more efficient, Delta Products has worked with Western Wireless to prolong power from batteries and generators at cell sites by coordinating both types of power. Ed Pack, Delta Products manager of telecom products, said when ac power is lost, batteries take over. Once it is time to recharge them, the generator kicks in to supply power to the equipment to recharge the battery. When the battery is recharged, the generator shuts down and the battery powers the site once again. This cycle continues until ac power is restored to the site.

Pack said this innovation keeps the generator from going constantly and running out of gas. It enables one tank of gas to last for days, even weeks, during a prolonged outage.

During smaller power outages, the generator will not kick in, which conserves fuel for longer outages. In situations such as hurricanes where many sites need attention, this cycling system allows technicians to spend time repairing critical network equipment rather than refilling gas tanks.

New Power Products for the Site * BVS Gator transmitters measure signal propagation, position antennas, set power levels or validate coverage. All models are available with Windows 95- and Windows 98-compatible remote-control software, in addition to access via modem or DTMF control. They feature built-in forward and reverse power meter, with VSWR monitoring and digitally controlled output, and are available in 25W Class A or 45W Class A models (10W or 20W Class A for PCS models).

www.bvsystems.com or 732-548-3737

* Chesapeake Microwave Technologies' GaAs FET Class AB linear amplifier provides 18W of CDMA power. Housed in a single assembly, this amplifier serves the forward-channel transmission requirements of a typical PCS system. Using modular design concepts, it meets customer requirements for any uplink or downlink frequency allocations of DCS/PCS bands.

717-235-1655, ext. 112

* MGE UPS Systems' Digital Static Transfer Switch Power-Management Modules (PMM Plus and PMM Ultra) use dual-input power distribution with automatic source selection and feature full power conditioning and distribution for sensitive equipment, distributing power and offering power-quality monitoring. They support 30kVA to 300kVA loads and come with up to six panel boards.

714-557-1636, ext. 7317

* Yuasa has increased the capacity of its industrial front termination PowerLock battery to 150Ah within the same footprint. The new battery is ideal for wireless, where space is limited and high power is essential. It provides a 48V system on a single shelf of a standard 23" rack to maximize capacity and minimize height restrictions. It allows users to configure a system with a 750Ah capacity rating in one 7-foot relay rack.

www.yuasainc.com or 800-538-3627