Worldwide demand for broadband and wireless services is growing at double-digit rates as businesses and consumers rely more and more on high-speed and mobile communications platforms. The networks that service those systems require power - a lot of it.

Verizon has established an objective for its vendors to achieve 20 percent greater efficiency by January 2009, as compared to today’s equipment. France Telecom is planning to reduce the greenhouse emissions per customer by 20 percent between 2006 and 2020. British Telecom claims to have reduced its carbon footprint by 60 percent since 1996, and has an objective to reach 80 percent by 2016.

It should come as no surprise that there are energy inefficiencies to be found within these telecommunications networks. Industry best practices target some of the waste, but most telecom providers and their vendors have limited the discussion to the energy efficiency of individual products. As such, the total impact of deploying higher-efficiency rectifiers or cooling units remains smaller than it could be if considered in the context of the overall network. This often leads to ill-informed investments when service providers overlook real opportunities for reducing energy consumption.

Emerson Network Power analyzed those missed opportunities, along with existing network inefficiencies and available energy-saving actions, and developed 12 strategies* for reducing energy use in these networks: 6 for wireless sites and 6 for the central office. In this article, we will look at the strategies for wireless sites.

Energy Consumption in Telecommunications Networks
Estimates indicate the telecom industry consumed 164 TWh last year, or about 1 percent of the global energy consumption of the planet. That equates to 15 million U.S. homes and matches the CO2 emissions of 29 million cars. In fact, the U.S. Environmental Protection Agency (EPA) estimates a 10 percent reduction in energy use by telcos could save the industry more than $200 million a year and prevent 2 million tons of CO2 emissions.**

But reducing energy consumption is a challenge when consumer demand for telecommunications services is skyrocketing. Broadband subscriptions are growing at a rate of 14 percent annually and require 4 to 8 times more energy than basic telecom service. North American fiber-to-the-home deployments topped 3 million last year - an increase of more than 100 percent since 2007. Internet traffic is increasing by 60 percent annually, due in large part to growing demand for Internet-based VoIP, video streaming, and movie and video downloads (See Telecommunications Industry News report).** On the wireless side, the industry is on its way to 3 billion connected devices, with high-speed data being the ultimate objective. All of these services drive up energy consumption within the network.

Faced with these realities and trends, it is likely only a matter of time until governments start imposing reduction targets - unless the industry takes action on its own first.

Wireless/Wireline Energy Waste
The wireless network can be viewed in two major sections:
1. The operators’ part - includes the Mobile Switching Center (MSC) and Radio Base Station (RBS).
2. The subscribers’ part - normally limited to the handheld device.

Estimates indicate more than 90 percent of wireless network energy consumption comes from the operators. With approximately 4 million installed Base Transceiver Station (BTS) cabinets in the world today and an estimated double-digit growth rate, the impact of any energy savings at this point is significant.

In identifying opportunities to reduce energy consumption at these sites and assessing the impact of various strategies, we used a typical RBS, a 3 sector Omni, as the model. It is the same model analyzed and presented in Ericsson’s August 2007 white paper ”Sustainable energy use in mobile communications” which looked at telecom energy efficiency strategies. In fact, 2 of the strategies presented here come from that paper.

But before discussing the strategies, it is important to understand some characteristics of this RBS. More than 60 percent of the power used by the RBS is consumed by the radio equipment and amplifiers, 11 percent is consumed by the DC power system, and 25 percent by the cooling equipment - an air conditioning unit typical of many such sites. Under these conditions, it takes 10.3 kW of electricity to produce only 120 Watts of transmitted radio signals and to process the incoming signals from the subscriber cell phones. From a system efficiency perspective (output power/input power), this translates into an efficiency of 1.2 percent.

Clearly, there are opportunities for improvement, and they become more obvious when we examine the energy flow inside the RBS. Specifically:
• Ultimately, 120 Watts of RF signals are pushed into the antenna. To deliver this, an additional 120 Watts must be fed to the feeder cable at the base of the tower. That adds up to 50 percent efficiency for the feeder.
• To produce this RF power, the radio equipment consumes 2.1 kW for signal processing and an additional 4 kW for the RF power amplification, with only 6 percent combined modulation and amplification efficiency.
• The power plant feeding this load runs at only 85 percent efficiency, well below its peak level. This is the result of the low utilization of the rectifiers and some system-level losses.
• The air conditioner, another frequently over-engineered component, draws 2.5 kW, or 0.34 W for every 1 W of heat produced by the electronics.

Because of these inefficiencies along the energy path, any Watt saved near the antenna will yield cascading benefits by avoiding the associated losses upstream. That cascade effect maximizes the ultimate energy savings at the source. The benefit of 1 Watt saved at the RF load is multiplied by the system block efficiencies, so the accumulated benefits are much higher than the original 1-Watt reduction.

In our model, saving 1 Watt in the feeder cables saves 17.3 Watts of modulation and amplification losses, 3.3 Watts of rectification losses, and 7.1 Watts of associated cooling energy. In aggregate, this represents a 28X cascading benefit, with smaller benefits also occurring in signal processing and DC power. For these reasons, efforts must start closer to the antenna, where they yield greater benefits and enable reduction in cooling and power requirements.

All the great intentions in the world do not translate into real world sustainability efforts. Read, learn, and live the details behind 6 concrete Energy Logic strategies that translate to substantial energy savings for providers.

Six Energy Logic Strategies

1. Optimize Remote Radio Units
A typical RBS requires 120 Watts of power to push 120 Watts of RF signals to the antenna. By moving the RF converters and power amplifiers (PA) from the base of the station to the top of the tower, close to the antenna, and connecting them via fiber cables, it avoids the power drop inherent in a long feeder cable run. Power is delivered either via a separate feed from the grid or, preferably, via 48V feeds from the base station power system. In either scenario, losses are minimal and the full 120-Watt loss in the feeder cable is eliminated.

This step cuts the power requirements of the PA by half while removing 33 percent of the cooling requirements and 30 percent of the DC power load and losses.

2. Radio Standby Mode
Radio transmitters and receivers can be turned to what is often called ECO mode, which turns the power off when call traffic is low, typically overnight. If a given site isn't equipped already, this capability is available through simple software and hardware upgrades.

Power consumption is fairly stable throughout the day and night, and independent of traffic. In ECO mode, however, power consumption can be reduced by up to 40 percent under low traffic. Overall, this strategy will reduce power consumption between 10 and 20 percent as well as provide associated power conversion and cooling reductions.

3. Passive Cooling
Historically, air conditioners have been the preferred choice for radio sites, but those AC units require power equivalent to 34 percent of the heat load produced inside the RBS. For example, if the RBS produces 1,000 Watts of heat load, the power consumed by the AC will be 0.34 x 1000, or 340 Watts. They also are noisy and maintenance-intensive.

Depending on the geographic location and willingness to trade energy savings for some battery life, other cooling strategies such as free ventilation, forced fan cooling with hydrophobic filtering or heat exchangers will change the energy consumption significantly and often yield a lower total cost of ownership (TCO).

Although it is estimated that passive cooling can provide energy savings of 10 percent or more, not all scenarios favor free cooling. Each RBS should be evaluated independently to identify opportunities to achieve those savings and the overall lower TCO.

4. Advanced Climate Control for Air Conditioners
If an air conditioner remains necessary, energy consumption can be minimized by triggering operation at a higher temperature. The higher set point not only ensures the unit will be turned on less frequently, the higher temperature delta at the air exchange enables improved operational efficiency.

A 10-site trial conducted from May to September 2007 reduced total cooling costs by 14 percent by allowing a wider fluctuation between 31degrees C and 26 degreesC. Of course, raising the internal cabinet temperature has to be weighed against the potential adverse effect on component reliability, but total savings of 3-4 percent can be obtained safely without major availability impacts.

5. DC System to ECO Mode
Rectifiers have a high peak efficiency, which can drop by several percentage points when the load is under 40 percent of the rectifier capacity. Because systems are configured with redundant units, and often sized based on future demand and worst-case assumptions, most remote sites operate well below 40 percent capacity. The strategies outlined above further reduce the load.

An advanced system controller scheme can ensure rectifiers operate at peak efficiency in virtually all conditions. In this energy management control scheme, the controller continuously measures the load current and allows only rectifiers operating at peak efficiency to supply the power. The controller also rotates the rectifiers so they share duty cycles equally over time. In effect, it operates as an ECO mode for the DC system.

The energy savings are small compared to previous steps, but not insignificant, as seen when applied to a system with 11x30 ADC rectifiers, a capacity of 300A, and an actual load of 110A. Without this DC ECO mode, each rectifier is loaded at 33 percent of its capacity, for an approximate efficiency of 89 percent.

With ECO mode, given that 5 rectifiers are in idle mode, the actual load power rectifier increases to 22A, loading the rectifiers 66 percent and providing an operational efficiency above 92 percent. This mode saves 146 Watts of dissipated heat, a 20 percent energy savings. Although this is not negligible, the DC power system is only a small contributor to total RBS energy losses, contributing only 2-3 percent of the entire savings budget.

6. Higher-Efficiency Rectifiers
Until recently, rectifiers were considered an area with little benefit to overall efficiency, and customers overwhelmingly opted for lower initial cost rather than marginal efficiency gains. But this preference may be changing with the advent of higher-efficiency rectifiers.

Higher-efficiency rectifiers are appealing, but it is important to continue to take a holistic, system-wide view in evaluating their overall effectiveness. In the RBS model, the cascaded savings provided by a 4 percent rectifier efficiency gain translate to a 1.8 percent system-level energy savings. But in order to determine whether or not the full savings are realized, it is necessary to determine if the promised efficiency is delivered.

In measuring some of these products, it was determined that they meet the advertised efficiencies, but only at high line voltages. That is not where products typically operate. Beware of misleading information, and demand data at a nominal line voltage across a wide load range (typically 40 to 100 percent load).

Loads often are overstated, and sites take years to reach planned capacity. As a result, rectifiers usually run at a fraction of their capacity, typically around 40 percent. With the AC consumption lower than anticipated and high-efficiency rectifiers being premium-priced, an analysis must justify the financial viability of this option.

Consider the return on investment of replacing a standard 91.5 percent efficient rectifier with a high-efficiency 96.5 percent unit. In the remote site model, over 5 years with an N+1 configuration, the ROI is around 30 percent. When considering ECO mode (i.e., radio standby mode), which reduces the load when traffic is low, the savings and ROI are affected negatively by 5 percent.

When the 91.5 percent rectifier is replaced with a 94 percent efficient model operating in ECO mode, savings are well beyond acceptable levels. This is one reason we believe customers can find more attractive investment options than high-efficiency rectifiers. We believe the preferred choice in today's environment is a 94 percent efficient rectifier, which comes at a minimal price impact versus today's market prices and offers the strongest ROI when operating in ECO mode.

Sensible and Sane Solutions
Providers like Verizon, British Telecom, and France Telecom have all publicized their energy efficiency, power reduction, and carbon footprint reduction objectives. We know others are not far behind.

Using Energy Logic for Telecommunications strategies can generate savings of close to 60 percent in the wireless network. Just in the case of the RBS, providers enrolling the best practices described here could potentially translate into potential global savings of 11.8 TW of demand - or U.S. $10.3 trillion per year.

The key is addressing the issue with a clear and defined approach that optimizes results. Looking at energy consumption at the network level and considering energy-saving actions holistically is the key to successful energy conservation.

*Emerson Network Power's Energy Logic for Telecommunications offers practical roadmap for wireless networks. These strategies are at the heart of Energy Logic for Telecommunications, a comprehensive approach to improving energy efficiency in telecommunications networks. Energy Logic takes a sequential approach to reducing energy costs, applying technologies and best practices that exhibit the most potential in the order in which they have the greatest impact. All of the technologies used in Energy Logic are available today, and many can be phased into the network as part of regular technology upgrades/refreshes, minimizing capital expenditures.

**For more information about:
U.S. Environmental Protection Agency (EPA), www.yosemite.epa.gov.
Telecommunications Industry News, to read the report DSL Providers Seek to Improve Energy Efficiency of Broadband Networks, www.teleclick. 

Paul Misar is Director, OSP Wireless and Alternative Energy, Emerson Network Power. He has more than 13 years experience in the telecom Outside Plant industry in both the wireless and wireline arenas. Paul has held several engineering and product management positions over this time, with a focus on product design and product implementation.  For more information, please visit www.emersonnetworkpower.com. 

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