Licensed vs. Unlicensed Spectrum for Utility Communications

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By Marc Reed

The emerging smart grid will redefine power transmission and distribution. Monitors on transmission equipment will sense trouble conditions and automatically trigger corrective action. Smart meters will communicate power data throughout the day—both power consumed from the grid and returned to the grid by customers who generate alternative energy. In-home devices will inform customers of their utility usage patterns and intelligently shift loads to reduce overall and peak demands.

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New Expectations for Utility Communications

As national energy policy evolves to embrace the smart grid vision, the requirements for utility communications are being fundamentally redefined by the following trends:

• The imperative to use energy more wisely throughout the grid

• The increasing incidence of renewable energy being pushed back into the grid

• Growing number of smart devices at customer endpoints, such as smart meters and controls

• Desire for more granular information, such as more frequent and detailed meter sampling

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Utility networks will have to provide higher performance, range and availability for communications to handle these trends.

The Inevitable Shift to Wireless Communication

Three key communications methods are presently used to enable connectivity between smart grid and advanced metering infrastructure (AMI) elements:

• Power line carrier, using the utility's power lines for connectivity

• Wireless networks based on unlicensed "free" spread radio spectrum

• Wireless networks based on radio spectrum licensed from the U.S. Federal Communications Commission (FCC)

Power line communication offers the advantage that it operates on the utility's existing infrastructure and rights of way. One-way and two-way systems have been successfully used for decades. The disadvantages are slow data rates and the need for many repeaters. The network is also unavailable during power failures; during abnormal substation, feeder or phase switching events; or when protective switch gear has opened the power lines, making the communications path unavailable. Smart grid applications require higher availability and throughput than power line carrier systems can provide.

While unlicensed spread spectrum radio was widely touted as the technology of choice, until recently the cost and difficulty of installing the equipment necessary to run these spectrums prevented utilities from having a viable wireless network approach that could satisfy their business cases.

Utilities and companies are now asking whether smart grid wireless networks would be better run on licensed wireless spectrum in which the airwaves are owned by the customer and protected by federal law.

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Wireless Communications on Unlicensed Radio Spectrum

Unlicensed wireless devices on the smart grid operate in one of the bands set aside by the FCC for industrial, scientific or medical (ISM) applications. While free to obtain, devices on the spectrum must adhere to FCC rules for the ISM bands, most notably requiring that devices transmit 1 watt or less of power, not cause harmful interference and accept any interference received without causing undesired operation.

For messages to be correctly received, the communication signal must be stronger than the noise from incidental interference by a specified amount—an acceptable signal-to-noise ratio—obtainable with the use of a spread spectrum technique, most common being frequency hopping or having the radio hop to a different channel for every message or several times during a message. A message can easily take a half-dozen hops on the way to its destination.

This scenario presents three key problems. First, because the range of a signal is limited, the network requires many intermediate nodes, adding greatly to the cost of deploying, operating and maintaining the network. Second, since every hop is essentially another message, more bandwidth is consumed from a limited supply for each hop made. The additional processing necessary to handle these hops also slows down communication, creating latency issues particularly problematic for real-time communications, such as voice.

As more smart grid services roll out, utilities will need a better business case and more bandwidth than unlicensed spectrum can provide.

Wireless Communications on Licensed Radio Spectrum

Licensed spectrum devices operate within the portion of the radio spectrum designated by the FCC to be reserved for organizations that have been granted licenses. With exclusive rights, a license holder operates without interference or spectrum crowding. The FCC provides legal protection and enforcement to prevent other operators from transmitting over the same frequency in the same geographic area.

Communications in these bands have a tendency to be narrowband in nature, using a single frequency carrier and do not have the power limitations of ISM-band devices. Operating on a dedicated frequency, utilities have clear advantages over unlicensed spectrum for maintaining an excellent signal-to-noise ratio.

Strong signal levels, potentially 10 to 100 times more power than unlicensed in the case of a central collector, and low noise levels resulting from the spectrum's FCC protection, combine to dramatically increase the range, throughput and performance of communications. Instead of a fraction of a mile between endpoints, a wireless network can transmit 20 miles between endpoints—up to 100 times farther than an unlicensed spectrum. Towers and endpoints can communicate directly without intermediate network equipment to buy, install, maintain and repair.

On the downside, available licensed spectrum is a scarce resource, difficult and expensive to acquire. Available bandwidth can be an issue.

The Sensus FlexNet system is currently the only FCC-licensed communications network designed specifically for electric, gas and water utilities. The system addressed these issues by acquiring nationwide protected spectrum that supplies turnkey solutions to its customers. Utilities have access to 325 kHz of bandwidth, plus the highest radio frequency (RF) power in the industry.

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Where Does the Industry go From Here?

Government agencies are taking steps that will influence the future direction of the evolving smart grid. The Utilities Telecom Council is calling for the federal government to make 30 mHz of contiguous bandwidth available directly to utilities. The National Institute of Standards and Technology and the Institute of Electrical and Electronics Engineers are working on interoperability standards to drive universal availability of these devices.


About the author: Marc Reed, director of communication engineering for Sensus, has more than 18 years' experience in systems, software and hardware designs. Since the beginning of his career at Dallas-based defense contractor E-Systems, Reed has specialized in the design and impSlementation of wireless data links. Reed has won several awards for his work in the communications field and has multiple granted patents and patents pending. Sensus leads in innovative and evolving technology solutions that enable intelligent use and conservation of critical energy and water resources. For more information, visit www.sensus.com.

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