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CHIPS Articles: Obstructions Interfere with Electromagnetic Spectrum

Obstructions Interfere with Electromagnetic Spectrum
By Thomas Kidd - January-March 2021
When discussing obstructions in the context of spectrum management we typically talk about bureaucratic obstructions in the regulatory processes. We could also discuss technological obstructions limiting system capabilities. In this article, we discuss physical spectrum obstructions, objects that block or interfere with electromagnetic waves. When a receiving antenna is blocked from receiving a signal, the system, whether a television or a complex military communications system, cannot function. This type of interference is an ongoing challenge to the Department of the Navy (DON) spectrum community that will increase as spectrum usage increases and evolves.

For example, many Americans receive digital television services from geostationary satellites over the equator. To receive this signal, the customer mounts a small dish on their home pointing up at the southern sky. But the dish doesn’t just need to be pointing south. Installation instructions require the dish to have an unobstructed view of the southern sky. Even trees can significantly degrade reception. If, after years of great television service, a high-rise apartment building is built in line with this satellite dish, it will no longer be able to receive the signal. The owner must either move the dish, change to a cable service, or do without digital television.

Other signals also travel from point to point using radio waves. A university campus may beam its computer network signal from building to building. Businesses extend their computer networks across the street or down the block by beaming the signal between small dishes. The DON uses electromagnetic spectrum for similar functions. For these spectrum-dependent systems to work, whether on a base or on a ship, the transmitted signal must reach its intended receiver. However, there are a number of obstacles that can block these signals, impacting the ability of the system to operate.

As with physical obstructions to digital television reception, trees may grow into a system’s signal path. Certain pine tree needles can actually absorb the radio signal. On a Navy base, signals from satellite dishes mounted on the ground or an antenna on the side of a building might be blocked by new construction unless prior coordination occurs. Windfarms also create radio obstructions because of the way their blades disturb the air. To some types of radio waves, this turbulence acts like a wall blocking the signal. Some energy-efficient building coatings on windows also block radio signals, preventing reception inside the building.

The effects of obstructions vary depending on the signal frequency and the width of the beam. A narrow beam is more susceptible than a wider beam, and higher frequencies (those at the upper end of the electromagnetic spectrum) are more easily blocked than lower frequencies (those at the lower end of the spectrum). As higher frequencies become usable spectrum, obstructions will become a greater concern.

Current U.S. spectrum regulations do not protect authorized licensed spectrum users from obstructions. If a potential obstruction is discovered during the permitting process, builders often do their best to be good neighbors and work with the radio operators. However, generally, they are not obligated to resolve the problem. Current radio rules and regulations offer little protection from obstructions. Balancing the needs of new construction with existing spectrum infrastructure is also a challenge.

The spectrum community can take a few simple steps now that would help reduce the impact of obstructions. The most effective would be registering existing obstructions in spectrum management databases so that engineering software “considers” them. To some degree, this is already possible with software that incorporates high quality three-dimensional city data. High quality electromagnetic compatibility software is capable of considering obstructions as part of terrain modeling. But additional effort is needed.

Two changes need to happen to solve the problem of obstruction to the DON’s spectrum-dependent systems. First, licensed systems need to be afforded some protection from obstructions, for example, building permits could require coordination with local spectrum management personnel. Second, potential obstructions must be required to register key parameters to enable mitigation engineering. Both of these solutions require national rule changes that provide protection from obstruction along the signal path similar to current protections from interference by competing signals.

Working in close partnership with federal spectrum regulators, the DON CIO will be exploring methods to ensure physical obstructions do not impact spectrum-dependent operational capabilities.

Tom Kidd is the director for DON Strategic Spectrum Policy in the Office of the Chief Technology Officer, Department of the Navy Chief Information Officer.

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