As the wireless revolution sweeps through the global marketplace, network operators need ever-more-efficient cellular technologies. Radio basestations (RBS) are a crucial element in any wireless network; to ensure maximum basestation performance and availability, operators are now deploying a new and more capable generation of RBS power-supply solutions.
Wireless operators know all too well that it can take just a single lightning strike or wind-damaged limb to cut primary grid power to a remote cell site. When that happens reliable backup power is all that stands between a cellular provider and a serious lapse in service, lost revenue and a cell full of angry subscribers.
Market and competitive pressures, as well as advances in semiconductor technology, are driving down the size of the wireless cell site. Meanwhile, as operators demand more performance from those progressively smaller logic systems, the architecture of basestation power systems must be adapted to meet the new requirements. Many next-generation cell-site cabinets are being replaced with rack-mounted modules that are up to 75 percent smaller. Those smaller, faster cell-site systems require more power and generate far more heat than their larger predecessors.
It is critical that power systems follow the same trend, increasing power density and configurability for site-specific requirements while still being field-upgradable to allow for growth in capacity. By understanding the variety of power requirements for their cellular sites, wireless operators can equip their networks for optimum growth, performance and profitability.
Typically, power systems at traditional cellular sites consist of ac-to-dc power supplies, 24-V batteries, power distribution and protection equipment and some type of control and monitoring gear. The power supplies are designed to output from 22 Vdc to 30 Vdc, from an ac power main running from 170 Vac to 264 Vac (typically, 200-, 208- or 240-V mains). The supply must also charge the standby batteries optimally, depending on temperature and type of battery. The standby batteries provide critical short-term power whenever the primary power source is lost.
The typical levels of power for basestation systems is in the area of 1,500 W to 50,000 W, and that depends on the number of subscriber lines, antennas, repeaters and the like. Also, there is about a 3:1 decrease in power from TDMA to GSM systems as the frequency hopping used in GSM allows more subscribers per unit.
For a basestation antenna putting out 200 W, the power-supply requirement can be an order of magnitude higher. Since most RF amps commonly run from 40 percent to 60 percent efficient, then the range of supply needed for a 200-W antenna amp would be 350 W to 500 W. Adding the call-processing hardware and the battery-charging power, this system would likely average about 3,000 W in total required power.
Most RF transistors used in amplifier design today run optimally around 28 Vdc and the amps require a very tight regulation, typically less than 1 percent at the amplifier. This is a real challenge for the system designer: There are line losses going from the power system to the RF amplifier that must be compensated for because the amplifier performance changes (sometimes dramatically) with input voltage. Also, during temperature changes and after outages, the power-system voltage will vary to charge the batteries to the proper level. That can affect the RF amp, if it is not accounted for during system design.
Almost all new power systems are designed around switching power supplies, as they are smaller, lighter and much more efficient than linear power supplies. Most switching power supplies are running more than 85 percent efficient for the required output range, with many topping 90 percent. The power factor that brings additional efficiency and transient smoothing by orienting rectification to the phase of the ac line is also important. System designers should look for suppliers that offer power-factor-corrected supplies running above 95 percent.
Several techniques are being used to improve the efficiency of switching power supplies. Getting the most attention is synchronous rectification, which can be expected to add 1 percent to 3 percent-possibly a little more-to the efficiency of that type of power supply. The drawback is that it costs more, and since power is a very cost-sensitive item for most system designers, they may not be willing to pay.
Apart from architecture and power-management techniques, fault tolerance is a major concern for network managers selecting RBS power systems. Network reliability has become increasingly important, and fault-tolerant base-station power systems can contribute directly to overall network quality and availability.
A typical cellular basestation might provide 5,000 subscribers with services for which the operator charges an average of 10 cents a minute. So if a power-related breakdown-say, the failure of a single rectifier or battery string-shuts down the basestation for 10 hours, that downtime can cost up to $300,000 in lost revenue, not to mention the cost of the repair and the potential churning of dissatisfied subscribers.
One vital fault-tolerant strategy is to design basestation power supplies to meet both current and future capacity requirements. As the logical elements of a basestation get smaller and more powerful, they also require more robust power supplies. Most systems designers now recommend N+1 redundancy, so if 5,000 W is needed to meet performance requirements, they provide five 1,000-W supplies plus one additional 1,000-W element.
The best power systems are modular, thus allowing operators to quickly and easily plug in additional elements to meet escalating requirements. Truly reliable fault tolerance also includes dual redundant provisioning of input line cords, dc buses and other power-supply elements.
Temperature control is always a crucial issue in base-station power systems. A widely accepted rule of thumb is that for every temperature rise of 10 degrees C the reliability of a system is reduced by half. As advanced components increase heat generated inside a traditional cell site and as more sites are located in extreme temperature environments, manufacturers must develop and deploy ever more innovative thermal designs.
Built-in air conditioners
Highly specialized climate-control systems are available to meet the unique requirements of remote radio base stations, subscriber switches and transmission nodes. Such solutions incorporate small built-in air-conditioners, downstream air distribution, automatic restart protection and conversion kits for base stations in high-temperature environments.
Operators also need convenient and reliable ways to manage remote power supplies. Today's most advanced energy-management systems provide comprehensive monitoring and/or control of the rectifiers, batteries, ambient air temperature, cooling systems, potential faults, error reports and logging systems. And software-driven power-management solutions use distributed computing to provide power-related configuration planning, performance measurements, fault management, alarm reporting and operating data management.
Perhaps the most dramatic change affecting power-supply technologies is the advent of microcells. These compact, miniaturized base stations are fully enclosed and often incorporate high-performance logic components and thus run hotter than traditional Quonset hut base stations. These units are deployed in great numbers, so operators expect them to perform with a high degree of maintenance-free reliability.
Power supplies for the microstation architecture will be smaller by nature, as the typical microcell is a 20-to-40-W system. This type of system requires the highest possible efficiency for the power supply as well as the RF power amp because of the temperature extremes these cells see. Most of the RF amps still run off a 28-Vdc bus and require the same regulation, about 1 percent, as their larger counterparts. The main difference is the lack of cooling available. Because the smaller microcells have no built-in cooling systems and are fully enclosed, designers must push power-related elements such as transistors, rectifiers and other components to the limits of their efficiency to keep heat to a minimum. Thermal management is also important in microcells, and manufacturers now use cast heat sinks, thermally conductive materials and other strategies to move heat quickly from the box to the ambient environment. Even with these advances, it is common to find ambients inside the boxes near 80 degrees C under high heat conditions. Under these extremes, every percent in efficiency can mean several watts of power saved, and every watt saved means the box is several degrees cooler inside.
Fault tolerance is a major challenge in microcell technology, where designers must balance the conflicting demands of economy and the need for maximum reliability. Manufacturers are also working to develop specialized batteries and power-supply components designed to meet the extreme temperature challenges of the microcell environment. Traditional power management is typically not an option in remote microcells because of cost and size constraints-another reason designers must seek to build maximum reliability into these miniaturized power supplies.