The Ins and Outs of DAS Powering
Powering Different DAS Networks Appropriately
by: Satheesh Hariharan
Over the past few years, Distributed Antenna Systems (DAS) have gained increasing acceptance as a reliable solution for addressing the coverage and capacity shortcomings of macro cell sites. DAS networks support multiple wireless frequencies and technologies for 2 or more wireless service providers, making them ideal solutions for extending the wireless network into indoor venues or outdoor metropolitan areas. DAS networks have a common set of components -- the host unit, remote access units, cabling, splitters, antennas, etc. -- though they vary based on the characteristics of the venue. Likewise, the requirements for a reliable power and battery backup system changes based on the type of DAS equipment, the area served, whether the installation is indoors or outdoors, etc.
This article aims to educate readers on the important factors to consider when selecting a power system for indoor and outdoor DAS applications
In a typical distributed antenna system network, the main interface unit, optical conversion equipment, controller and other electronics would be located in either the Head-end, Intermediate Distribution Frame closets, Telco rooms, or in a BTS Hotel. Fiber optic cabling is used to connect multiple remote access units located throughout a building or venue to the main interface unit as shown in Figure 1. As such, the power requirements for a DAS network are split into 2 segments: Head-end/BTS Hotel and Remote Access Units. The major difference between an indoor distributed antenna system (iDAS) and an outdoor distributed antenna system (oDAS) network is the departure from a controlled environment to a harsh and unpredictable outdoor environment requiring the use of hardened equipment in the network.
As an industry, we are quite familiar with the power solutions available in the market today for powering the Head-end and the BTS Hotel. The -48Vdc power systems with capacity ranging from 18kW to 120kW are similar to those found in a small Central Office or Base Station. There are a number of other factors to consider such as power system efficiency, battery technology, battery backup time, redundant energy source, etc., to ensure there is high reliability of the DC power system. But because these power systems are so familiar, this article primarily focuses on addressing the powering challenges and solutions available for remote access units used in both indoor and outdoor DAS applications.
Remote access units have built-in optical to RF conversion modules as well as RF amplifiers to convert the optical signal received from the main unit to RF, amplify the signal and distribute the signal through antennas located throughout the building. The actual amount of RF energy produced by the amplifier is expressed in dBm or in Watts. For example, 37dBm would be equivalent to 5W of RF signal power. Basically, the higher the output transmit power of the RF amplifier, the greater the signal coverage area of the remote unit. Note that RF power is not the same as DC power, though both are expressed in Watts.
Powering oDAS Remote Nodes
The power requirement of oDAS remotes can vary from 400W to 1800W depending on the number of frequency bands and technologies they are configured to support, such as Single-Input-Single-Output (SISO) or Multiple-Input-Multiple-Output (MIMO). The vast majority of oDAS remotes today are AC powered, most likely due to the convenient availability of AC utility power on a pole. For these AC devices, there are a limited number of power alternatives as shown in Figure 2.
If battery backup is a requirement, then the only solution is an outdoor rated AC UPS housed in an environmentally controlled enclosure. For those installations, there are 7 factors to consider. They include:
Factor 1: Power System
The selection of a proper UPS is dependent on the number of frequency bands that the remote is configured to support. For example, a quad band remote unit can draw up to 1600W of AC power whereas a single band remote unit may only require 400W. Several manufacturers of UPS in the marketplace currently offer a selection of outdoor AC UPS products, anywhere from 300W to 2000W, to meet the power requirements of most oDAS remotes available in the market today. For larger power applications, the UPS systems can be connected in parallel to meet the power requirements. Other considerations in the UPS selection include control and management features, alarm reporting and whether the device meets the requirements for NEBS Level 3.
Factor 2: Battery Backup Time
The amount of battery reserve for oDAS remote nodes can vary from 2 hours to 8 hours depending on the wireless service provider. Other factors to consider include the type of battery technology, number of strings, battery heater mats, cable termination, temperature compensation, etc., all of which needs to be reviewed thoroughly by an experienced applications engineer to properly recommend the right solution.
Factor 3: Aesthetic Considerations for Enclosure Designs
Outdoor DAS installations are more likely to be in continuous view, especially in neighborhoods where unsightly remote nodes and power systems make it very hard to get through the permitting process. As a result, power system solutions providers push to engineer either discreet and/or visually pleasing enclosure solutions that show much more favorably to local civic groups or municipalities. Based on this consideration, smaller is generally better, and this is achieved only by minimizing the amount of power required at the remote node.
Factor 4: Environment
The key elements to consider prior to selecting an outdoor enclosure include: min-max ambient temperature, wind-driven rain/snow/sleet/sand/dust, salt-fog, solar radiation, and seismic activity. The power solution needs to be rugged enough to withstand the harshest elements while maintaining performance and reliability.
Factor 5: Mounting
There are a number of mounting options available for enclosures that include: pedestal mount, pad mount, pole mount, wall mount, ceiling mount, H-frame and stake mount. The installation site will dictate the appropriate mounting structure required on the enclosure.
Factor 6: Thermal Management
The effective application of a thermal management system within an enclosure will maximize the efficiency and lifetime of electrical components. The 3 most commonly utilized HVAC solutions include: forced convection fan cooling, air conditioner, and heat exchanger. For the majority of oDAS remote node applications where a hardened outdoor AC UPS is used in an enclosure, forced convection fan cooling will suffice. Heat exchangers and air conditioners are used when a sealed enclosure is required to keep ambient air contaminants out of the enclosure together with precise temperature control inside the enclosure.
Factor 7: Standards
There are a number of industry standards that apply to electrical enclosures for the North American market. Wireless service providers like AT&T and Verizon have adopted Telcordia’s GR-487-CORE as the standard for their wireless deployments because it ensures a high degree of protection for internal electronics. For non-regulated markets, UL50 and CSA 22.2 standards enforce the ingress protection requirements as well as performance and test criteria’s for electrical enclosures.
Powering iDAS Remote Nodes
The power requirement of in-building Remote Access Units can vary from 80W to 450W depending on the number of frequency bands and technologies they are configured to support, such as SISO or MIMO. The same general set of design factors apply to iDAS, though environment, thermal management and aesthetics can be quite a bit different. And unlike oDAS remote access units, the iDAS counterparts can be powered by AC or DC, with the trend toward DC. Traditional methods of powering iDAS remotes are illustrated in Figure 3.
Like most distributed networks, a major concern with localized powering is the proliferation of batteries. Not only does this result in an increase in operating expenses due to ongoing battery maintenance, but it also complicates the aesthetics issue because of the number of power conversion cabinets that may be mounted in the public eye. A related issue is the need to supply AC utility power at the remote end to power the AC UPS or DC rectifier system. The location of AC power presents a logistical problem in addition to the cost of having an electrician install the connection.
Because of the relatively low power consumption of the iDAS remotes, some service providers and integrators have found a creative solution for dealing with the local power issues. Rather than locate the power conversion and batteries at the RAUs, they use the 48Vdc host site power plant to deliver current over copper cables connected to the remotes. The copper cables can be installed as part of a composite fiber/copper cable, minimizing the incremental cost of installing the copper. To distribute the power to the cables, the 48Vdc host power plant is augmented with a power distribution panel (e.g., GMT fuse panel) as shown in Figure 4.
While this technique can deliver the power, there are other considerations that must be taken into account to ensure technician and public safety. Since the site is considered indoors and beyond the demarcation point between the service provider(s) and end user, it comes under the jurisdiction of the National Electrical Code to ensure safety to the public. Consequently, the cables must either be armored cables or installed in conduit, unless the circuits meet requirements for Class 2 circuits as defined by Article 725 of the NEC, which specifies that the power must be limited to 100 Watts per cable pair (with the voltage not exceeding -60Vdc).
Conduit and armored cable can be prohibitively expensive, so the trend is to use a distribution device that meets NEC Class 2 requirements. For an installation to meet the requirements of Class 2 circuits, a distribution panel equipped with active power limiting circuitry must be used instead of a conventional fuse panel. The NEC and CSA/UL explicitly state that the power limiting feature must be in effect even when the primary protection device is bypassed or, in the case of a fuse panel, when the fuse is shorted. In other words, if a fuse panel is used to distribute the power to the cables feeding the remotes, those cables must be armored or contained in a conduit. So even though power limiting panels are more expensive than fuse panels, the incremental difference is dwarfed by the material and installation cost savings realized with conventional surface-mount cables (i.e., no conduit).
iDAS power limiting panels employ similar circuitry as +/- 190Vdc Line Power systems used for powering Outside Plant devices such as DSLAMs and ONTs. These panels distribute the bulk 48Vdc output of the host plant through the active power limiting circuitry and onto the cables for distribution to the RAUs. A block diagram of this configuration is shown in Figure 5.
The distance between the host and remote sites has 3 main constraints:
- Source voltage
- Power consumption of the RAUs
- Cable characteristics (length and gauge)
To extend the reach, heavier gauge cable is often used. The composite fiber/copper cable may include 14AWG or even 12AWG copper cables to maximize the distance between the host and remotes. With 12AWG connections, a typical line powered iDAS network can support distances of up to 750 feet from the host site to the remotes.
Another way to increase distance is to boost the source voltage. With a conventional 48Vdc power source, circuits must actually be designed at 42Vdc due to cutoff of the battery plants. To overcome these challenges, a DC-DC converter system can be inserted between the -48Vdc power plant and the line power equipment. The DC-DC converter has a wide input voltage range (e.g., -40 to -60Vdc), but provides a constant 57Vdc output voltage increasing the reach by over 35%. Figure 6 shows a DC-DC converter inserted into the host power plant.
In this respect, Line Power is another technique available in the iDAS network designer’s toolkit that warrants a serious consideration when funds present are a finite factor in deployment. Line Powering offers a number of compelling benefits such as providing carrier-grade battery backup protection by using a centralized power source as well as avoiding the cost and environmental concerns of proliferating batteries throughout the network.
In conclusion, the current generation of power products presented in this article offers enough variety to help overcome most powering challenges with iDAS and oDAS remote access units. For iDAS, Line Power (Class 2 circuits) offer the best means of providing carrier-grade battery backup protection from a centralized power source. For oDAS, an outdoor rated AC UPS, in an environmentally controlled enclosure, is the answer until such time DC powered oDAS remotes are made available in the market.
Satheesh Hariharan is a Product Manager for Outdoor Systems at Alpha Technologies, a supplier of integrated AC, DC, and Enclosure Solutions for DAS. He has more than 15 years of experience in the Telecommunications, Cable Broadband, Public Safety & Security, ITS & Industrial Power sectors, having worked extensively in Applications Engineering and Product Management roles. Read more about how Alpha overcomes DAS powering challenges at www.alpha.ca/DAS.