The power of reduced power
At a national average of $0.128* per kilowatt hour, one of the most expensive costs in operating a land-mobile radio (LMR) system is electrical power consumption for the communications sites and servers. However, with the advent of the electrical smart grid, advanced metering and green technologies such as solar and wind generation, LMR systems now can better manage usage and exchange power with the utilities in order to reduce costs, while using the services and when idle.
Specifically, the LMR sector can consume less electrical power by managing radio frequency link budgets and adjusting operations based on time of day. Electrical power management can start with airflow into the base station sites, in order to lower air-conditioning and heating requirements, and by changing air filters regularly — using high-SEER air conditioning condenser units — and optimizing the placement of air vents.
Alternative electric power sources, such as solar panels and wind turbines, also can be used — as long as there is a secure location to mount these devices. Finally, smart metering and the smart grid will allow for better monitoring of the consumption and, if solar and wind power are leveraged, there may be opportunities to resell excess stored capacity.
While it is true that LMR systems consume a lot of electrical power, the various types of sites lend themselves to the utilization of power control and management systems, which further enable the use of the energy-efficient systems available today. Since most LMR systems are designed with backup power as an integral part of the system, there is an economically sound opportunity to add electrical power management into the system in the initial design of the system, or later as a means to control operating costs.
Base station sites in particular lend themselves to electrical power management in the fact that battery backup already is built into many of the power-supply designs. So, the addition of an alternate power source to charge the batteries is the only added expense to the site. Most sites use 13.8 Vdc for the radio equipment on the LMR portion of the system, and 24 Vdc or 48 Vdc for the microwave or fiber-optic backhaul part of the system.
Meanwhile, every dispatch center today uses computers and servers as part of their operations. The primary power for a dispatch center is 120 Vac. Because many dispatch centers are part of a public-safety network, they must operate 24/7/365. So, they usually are designed to be operating on uninterruptible power systems 100% of the time. This means that they too have battery systems that lend themselves to the use of power-management systems.
Many systems are designed with much more RF power than is needed for good reliable communications. There are 100-watt radio base stations or repeaters that are used to cover a 1-square-block campus of a commercial building or college campus, when a 10-watt radio would provide the same full coverage needed by the building(s). In addition, the walkie-talkie units that the repeaters or base stations communicate with only run at 5 watts, so the base output RF power greatly outtalks the radio input to the system.
Finally, the added sensitivity of the mobile units allows the base station to use less power to achieve the same link budget range. A 100-watt transmitter talking to a receiver 20 miles away with a 0.5 uV (–113 dBm) sensitivity will be exactly the same as a 25-watt transmitter talking to a 0.25 uV (–119 dBm) receiver. Many of the frequency coordinators today are requiring this cutback in transmitter RF power for this very reason.
Because most of the radio transmitters today operate at 30% to 50% efficiency, just dropping the RF output power by a given amount provides a two-fold, or even three-fold, reduction of input electrical power. This same drop in input electrical power also translates to less heat that must be eliminated at the transmitter site. For a transmitter site with 10 to 20 transmitters, this reduction in heat is significant.
Besides the base station radios that are found at a transmitter site, there are quite a few other systems that require electrical power. These include the following:
- Heating and cooling systems
- Lighting systems
- Peripheral equipment (such as data terminals and printers)
- Test sets
- Alarm and entry systems
- Fire protection systems
In some cases, this power draw is significant.
Many base station sites are built to house more transmitters than are operating. Consequently, the air-conditioning systems at these sites have far more capacity than is needed. Often, the air conditioning can run in the fan-only mode, and not in the cooling mode, which requires the compressors to engage. A smart thermostat or remote monitoring-and-control system will pay for itself in a very short time in energy savings.
Meanwhile, a large site can have a substantial lighting requirement. One of the best ways to control energy consumption at such sites is to use motion or proximity sensors to extinguish lights when no one is present. In addition, the use of energy-saving lights, such as florescent or LED lighting, is far superior to the old incandescent lights, with one exception. Never use compact florescent (CF) lights on a tower or rooftop as part of the obstruction-marker or beacon-lighting system. CF lights emit energy in the RF spectrum that definitely will raise the noise floor of a site and will reduce range for the co-located receivers.
Generally, LMR systems also use ancillary equipment — such as data terminals, printers and test sets — that do not need to be powered unless personnel are present at a site. An ancillary benefit of powering these items only when needed is that they will have a longer life because they will avoid most of the electrical power surges caused by lightning or other abnormalities to the primary power coming into the building.
Another way to ensure that a site is energy efficient is to have the proper size generator or UPS to match the load. The batteries should match the time requirement for how long they need to operate in the event of a primary power outage. As part of the battery requirement, the replacement of the batteries on a periodic basis should be considered when selecting battery type and size. Many of the batteries in use today have a 5-year life cycle and they need to be replaced before they fail during a critical period. The last thing that you want during a hurricane or disaster situation is to be out of service because you were trying to get an extra few months on a battery system that is past due for replacement.
At some sites, there is no primary power, so alternate sources are the only option. Other sites can use the alternate power sources to lower the electrical power bill at the site. Some examples of alternative power are:
- Hydro-electric generation
- Fuel cells
As the manufacturers of these alternate power sources have developed improved products in capacity and operation, the prices have been coming down and these items can be utilized at more sites. In addition, if the power requirement at a site can be reduced to match the available alternative source, then that becomes a viable option for using them.
Solar systems today can be installed easily on the roof of the base station shelter or on the roof of the building. The panels also can be installed in the yard or compound of the base station site. There are security risks to verify and mitigate to prevent vandalism and theft, and there must be some form of protection from ice if the site is a radio tower — but the protection, of course, cannot block the direct sunlight.
Current solar technologies and new materials in the near future stemming from nanotechnologies will bring the efficiencies up and the prices down, in turn creating a strong business case to use solar at LMR sites. If enough electrical power can be generated and stored for use of the radio and ancillary equipment, there can be great cost savings — and any excess stored electrical power can be sold back to the electric power company in off-peak times as part of the smart-grid functionality.
Wind turbines, similar to solar panels, are now more readily available to replace or supplement service from the electrical utility. Wind-turbine systems can be installed near the yard or compound of the base station site with underground cabling. Again, there are security risks regarding vandalism and theft that must be mitigated or, ideally, prevented, and there has to be enough land to allow for the turbine to be safely located away from the tower or building. Zoning and permitting also may be an issue, but if these factors can be resolved the wind turbines can generate enough electrical power to drive the site and to sell some of the excess capacity to the electric utility.
Advanced data applications for electrical utilities are replacing SCADA systems as new technologies and standards evolve. The new bundling of services is called the “smart grid,” which allows advanced data applications over the power grid, such as automated distribution — which is a direct replacement for SCADA that adds smart metering.
Smart metering allows LMR operators and the electric utility to better manage power consumption. In addition, it lets LMR operators remotely manage lights and air conditioning over the secured Internet and intranet. The same smart meters also will be used to manage the two-way flow of electric power from the grid. Solar and wind power can be placed at LMR sites and the smart meter will properly manage the flow, as well as billing and reverse billing. Figure 1 offers an end-to-end representation of a typical smart grid.
Now let’s examine “green radio,” which is a term used to define the reduction of the carbon footprint that stems from the use of wireless services, including LMR. This includes lessening the electrical power consumption at base station sites and in server rooms, and using devices that are friendly to the environment when disposed.
Reducing electrical power consumption is just one approach. For example, many wireless system operators now are requiring suppliers to employ cleaner and more efficient air-handling systems, and to lower emissions from generators, if they are used often. In addition, the mobile devices and terminals now required to utilize less materials that harm the environment, starting with the batteries.
The LMR sector should join this effort and — with the deployment of the nationwide 700 MHz LTE network — there is an opportunity to learn from the commercial wireless carriers in terms of energy efficiency, in order to save operating expenses and to be good stewards of our natural resources.
In summary, electrical power consumption is one of the largest operational expenses for LMR system operators. Reducing this consumption will trim costs and result in a positive impact on the environment. Balancing RF link budgets and minimizing the power output of the base stations will reduce consumption. Also, by changing air filters and better managing the air flow and cooling and heating on the base station sites, additional energy savings will follow.
Solar panels, and possibly wind turbines, can save operational costs and pay back capital costs over time. Excess stored electrical power can be sold back to the electric utility via smart meters and the smart grid. This smart grid also allows better remote management of electric power consumption. LMR operators can log into the sites remotely and, by using IEEE Zigbee wireless and machine-to-machine (M2M) technologies, can set levels to best manage consumption.
Next we will focus on base station technologies and ways to continue to save valuable resources. A more in-depth look at current analog and digital technologies, as well as link budgeting and antenna techniques, will enable LMR engineers and operators to design efficient systems.
Ira Wiesenfeld, P.E., is a consulting engineer who has been involved in the radio communications business since 1966. He is a senior member of the IEEE and has been a licensed amateur radio operator since 1963. He can be reached at
Robert C. Shapiro, P.E, is a consulting engineer who has been in land-mobile radio since 1984. He serves on the TIA TR8 committee (TSB-88) as vice chair and is a senior member of the IEEE. He can be reached at [email protected].
LMR 200 Series
Part 1: School’s back in session–LMR in real-world applications
Part 2: Where it all begins–Pros, cons of primary Part 90 spectrum bands
Part 3: Spectrum redux–Part 90 spectrum bands in real-world applications
LMR 100 Series
Part 1: Class is in session: Basic LMR and FCC definitions
Part 2: Start at the beginning: Understanding LMR user needs
Part 3: The devil’s in the details: Conducting a user-needs survey
Part 4: Decisions, decisions: Understanding the LRM procurement process
Part 5: Let’s get started: System engineering begins with RF planning
Part 6: The lynchpin: Receiver planning and noise interference
Part 7: Connecting the dots: How to connect LMR sites
Part 8: The next piece of the puzzle: Understanding dispatch communications
Part 9: Now the real work begins: How to select a suitable LMR site
Part 10: The bane of your existence: How to deal with RF interference
Part 11: Winning the battle: More causes of RF interference
Part 12: Now the fun begins: Installing the LMR system
Part 13: Dotting Is and crossing Ts: Choosing the LMR project, program managers
Part 14: Now you’re done: Maintaining the LMR system