Eltek ASA : 6 Unique Power Requirements of Small Cells - article by Shannon Wiser

This article was first published inOSP Magazine on March 31, 2015

After several years of fanfare, small cells are now becoming part of the wireless network landscape. They are a much-needed component of today's wireless telecom infrastructure, used to help operators roll out their 4G/LTE networks, extend coverage into rural areas and supplement networks in metropolitan areas where there is exponential growth in mobile data traffic.

For the OSP professional, powering these new mini base stations is a challenge that is different at every site. Not only will power load and availability differ, so will the location of the small cell installation (wall or pole), power metering requirements, and the visual aesthetic. Selecting a DC power system for small cells is not a one-size-fits-all endeavor.

Rectifier mounted on street sign.
2 Types of Small Cell Power Systems As DC power providers have entered this market, they have produced a range of small systems that fall into 2 general camps. The first are cabinet-based power systems that are smaller versions of the systems used to power macrocells. These compact systems offer a modular design and flexible power distribution options, and can support bigger battery installations for longer back-up times. They also typically support controllers that provide remote monitoring of the power system and the back-up batteries.

However, these units typically don't offer the pleasing aesthetic that makes city planning officials want them installed in public areas, such as on a wall or pole next to the small cell.

The second unit developed as a response to that concern. These pole/wall mount category of power systems are all-in-one systems that usually have less configuration flexibility and more limited battery back-up options. That said, they blend well into the background, are light, easy to install and offer a power output range of between 600W and 1.2KW for a variety of base station applications.

Rectifier with small pole clamps.

Rectifier wall mounted.
6 Unique Power Requirements of Small Cells Small cells are mounted on outdoor lampposts, the sides of buildings, or are deployed inside malls, offices, sporting arenas, or other buildings. The initial powering factors that need to be considered include the availability of power to the small cell, the need for an electricity meter, as well as the aesthetics of the DC power system.

There are 6 other considerations providers must evaluate when choosing the right power system for the task at hand.

Requirement #1: The DC power systems themselves must provide a compact, inconspicuous, and environmentally responsible solution. Power density and modularity is an important criterion.

Requirement #2: The cramped quarters within cabinet-based systems often means only fans can be used for cooling rather than air conditioning systems. In pole/wall mount systems, convection cooling can be used to replace the fan and help improve reliability of the system. Fans are notoriously weak points in system reliability, and convection techniques can reduce the need for technicians to climb up to the power system for maintenance.

Requirement #3: As imagined, power supply efficiency has an impact on the overall thermal management of the cabinet. Efficiency is also important for minimizing electricity costs, which can be the largest operating expense at some cell sites. Efficiency measures the amount of power lost in the AC-to-DC conversion process. Standard efficiency ranges are generally found to be 85%-92%, whereas high-efficiency products can range up to 96%. Depending on energy prices at the site, using high-efficiency technology can reduce energy costs by hundreds of dollars annually for each small cell.

Requirement #4: Battery back-up matters as well. In some locales there's a requirement for between 2 to 8 hours of battery back-up, and this can determine which type of DC power system to use. Longer back-up battery life requirements lend themselves to cabinet-mounted DC power systems because there is more room for the batteries.

Requirement #5: Remote control of the DC power system is also a critical requirement because of the large number of small cells that are needed in the network. This requires Internet access for the two-way communications and a system controller in the power system. Cabinet systems offer controller modules that can monitor a wide range of system health and power consumption parameters, and report back on issues.

Requirement #6: Flexible output power level is also an important consideration. Within the category of small cell solutions there are a variety of system types, including picocells, microcells, and distributed antenna systems (DAS). Each has differing power needs. For example, microcells require just under 1KW, and picocells, depending on their configuration, need anywhere from 50W to 300W. Chassis-based DC power systems have a lot of flexibility for output power, with small systems offering output ranging from 250W to 1000W. Pole/wall mount power systems today deliver loads ranging from 300W to 1.2KW.

Given all of these technical considerations, how can the OSP professional select the right power system style for their application? Following are the top 3 small cell applications, along with some specific power system considerations.
Application A: Small Cells in Cities In urban areas where the density of mobile devices is the highest, carriers are deploying both picocells and microcells to enhance coverage in oversubscribed macro cells. Power supplies are most often located next to the small cell. These applications require wall-mount power DC power systems that are compact, yet often they need battery back-up as well. An additional important element for outside networks is a dust- and water-tight IP65 intrusion protection rating.

Application B: Feeding Remote Radio Heads in Feeder-Less Sites
One way of expanding coverage in rural locations is to extend the wireless network coverage of a macrocell base station using a remote radio head. In this scenario, the radio head connects to an antenna and contains radio frequency circuitry plus analog-to-digital/digital-to-analog converters and up/down converters to prepare the signal for processing by the base station. The RRH is connected back to the base station with a fiber optic cable, but must be powered by a separate power source. Most likely, the DC power plant would be located at the base of the radio station connected by a DC cable that has a practical distance of up to 150m.
Application C: Picocells in Rural Areas When the economic rationale for a macro base station can't be made, a picocell, with a 200-meter coverage ratio and reduced power demands, provides a good option for this application. Oftentimes in these applications, usage is so low that operators are under extreme pressure to lower costs to maximize the return on investment. Power is one of the largest ongoing costs in this scenario, so a high efficiency DC power system can help reduce those costs by decreasing the power wasted through DC power conversion. Easy installation is important in this application, as technicians may not be highly trained, which also means that reliability is important. Back-up power capabilities and IP65 protection are also important in these applications.

Just as small cells are a significant part of the wireless network of the future, selecting the right power system is a critical element to the success in deploying them. But, because no 2 installations will be the same, knowing which system to opt for from a selection of cabinet-mounted or pole/wall mount systems gives the OSP professional the opportunity to best match the needs of the varying scenarios out in the field.

Shannon Wiser is Director, Product Management at Eltek. He has over 18 years of industry experience in power solutions. For more information, feel free to contact the author: shannon.wiser@eltek.com

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