Researchers find potential path to improve radio communications

Researchers at the National Institute of Standards and Technology have confirmed that underground tunnels and above-ground building corridors can have a frequency sweet spot, at which signals may travel several times farther than at other frequencies.

Researchers discovered that radio signals traveled much farther when sent at the optimal frequency through tunnels and corridors because the passages act like giant waveguides ' structures that guide electromagnetic, light or sound waves. There are different types of waveguides for each type of wave. Waveguides are also built into integrated circuit wafers, antenna feed systems and optical fibers to strengthen radio signals.

Radio transmissions frequently are difficult in tunnels and sometimes in buildings, but the new data may lead to strategies for enhancing rescue communications in subways, mines and other structures.

The research, supported in part by the Justice and Homeland Security departments, is part of a project to improve wireless communications for emergency responders. The findings used extensive new data to confirm models developed in the 1970s. The tunnel studies were performed in 2007 at Black Diamond Mines Regional Park near Antioch, Calif.

The waveguide effect plays a significant role in radio transmissions in tunnels, reducing the losses caused when signals are absorbed or scattered by structural features, researchers said in NIST Technical Note 1546, titled 'Measurements to Support Modulated-Signal Radio Transmissions for the Public-Safety Sector.' The waveguide effect depends on a tunnel's width, height, surface material and roughness, and the flatness of the floor in addition to the signal frequency.

Lead author Kate Remley noted that the results may help in the design of improved first responder wireless systems that could enable better control of search-and-rescue robots in subways. Some handheld radios used by emergency responders for voice communications already operate within the optimal range for a typical subway, between 400 MHz and 800 MHz. Improvement of video transmission would require a bandwidth of at least 1 MHz, and a regulatory change would be needed, said Remley.

The agency plans to use the data to support the development of open standards for design of optimal systems, especially for emergency responders. In the past, companies have designed radios based on proprietary tests.

NIST Technical Note 1546, in addition to a second paper, NIST Technical Note 1545, titled 'Attenuation of Radio Wave Signals Into Twelve Large Building Structures,' will be available at NIST's Metrology for Wireless Systems Web page.The second report describes mapping of radio signals in 12 large structures, including an apartment complex, a hotel, office buildings, sports stadium and shopping mall.

About the Author

Kathleen Hickey is a freelance writer for GCN.

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