In the United States, responsibility for spectrum management is shared between two federal agencies: FCC, an independent agency, and NTIA, an administration within the Department of Commerce. FCC manages spectrum use for nonfederal users, including commercial, private, and state and local government users. NTIA manages spectrum for federal government users and acts for the President with respect to spectrum management issues. FCC and NTIA, with direction from Congress and the President, jointly determine the amount of spectrum allocated to federal and nonfederal users, including the amount to be shared by federal and nonfederal users. FCC and NTIA manage the radio frequency spectrum in the United States through allocation and assignment: Allocation involves segmenting the radio spectrum into bands of frequencies that are designated for use by particular types of radio services or classes of users. (Fig. 1 illustrates examples of services by frequency band.) In addition, spectrum managers specify service rules, which include the technical and operating characteristics of equipment. Assignment, which occurs after spectrum has been allocated for particular types of services or classes of users, involves providing users, such as commercial entities or government agencies, with a license or authorization to use a specific portion of spectrum. FCC assigns licenses within frequency bands to commercial enterprises, state and local governments, and other entities, while NTIA authorizes spectrum use through frequency assignments to federal agencies. There are several entities that advise FCC and NTIA in their spectrum management activities. FCC’s Technological Advisory Council (TAC) consists of approximately 50 telecommunications experts that provide technical advice to FCC and make recommendations on the issues and questions presented to it by FCC. TAC is a federal advisory committee organized under the Federal Advisory Committee Act. TAC is currently focused on key issues affecting the deployment of new broadband technologies and services, seeking to spur opportunities for innovation, greater efficiencies, and job creation. (CSMAC)—also organized under the Federal Advisory Committee Act—provides advice and recommendations to NTIA on a broad range of issues regarding spectrum management. CSMAC consists of approximately 25 private sector spectrum policy experts that offer insight and perspective on reforms, including long-range spectrum planning. Members are selected based on their technical background and expertise as well as to ensure diversity and balanced viewpoints. NTIA’s IRAC—an interagency advisory committee—comprised of representatives from 19 federal agencies that use spectrum, was established in 1922 to coordinate federal use of spectrum and provide policy advice on spectrum issues. It was originally organized by federal agencies that were seeking a way to resolve issues related to federal spectrum use in a cooperative manner. IRAC and its subcommittees assist NTIA in assigning frequencies for federal spectrum users and developing and executing policies, programs, procedures, and technical criteria pertaining to the allocation, management, and use of spectrum. The purpose of a communications system using spectrum is to relay information—audio, visual, and data—from a transmitter to a receiver (see fig. 2). A variety of technical methods exist to encode information such that it can be transmitted as electromagnetic radiation. Antennas are components of both transmitters and receivers used to emit and admit, respectively, electromagnetic radiation-carrying signals. A variety of factors influence the ability of a receiver to properly capture the transmitted signal and decode the information for use, including the terrain, distance, and atmospheric conditions between the transmitter and the receiver. For instance, buildings, mountains, and foliage can prevent a transmitted signal from being properly received for some types of communications systems. In other instances, the receiver may be located too far away from the transmitter. In addition, communications systems must operate in environments where a variety of natural and man-made electromagnetic radiation is present. Such undesired radiation could impede a communications system’s transmissions from reaching its intended recipients, and such an occurrence is called interference. It is impossible to eliminate all interference from communications systems and not all interference will prevent the proper functioning of a communications system. However, in some cases, the interference is considered harmful, meaning that it “endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service.” Harmful interference can occur when two communications systems use the same or adjacent frequencies in the same geographic area (see fig. 3). In the first case, co-channel interference occurs when two communications systems operate on the same frequency assignment in the same geographic area. In the second case, adjacent band interference occurs between two communication systems operating on different, but adjacent, frequency assignments in the same geographic area. Adjacent band interference, the focus of this report, causes (see fig. 4): Transmitters emit undesired emissions into adjacent frequencies that can cause interference to receivers operating on those assigned frequencies. This is generally known as out-of-band emissions. Public Law 112-96 required GAO to study interference between adjacent spectrum uses, rather than co-channel interference between users of the same spectrum. Receivers admit undesired emissions from transmitters in adjacent frequencies, causing those receivers to experience interference. In other words, the receiver cannot reject this undesired energy, impairing its use. Concerns about harmful interference are a factor in FCC and NTIA spectrum management decisions. To prevent harmful interference, FCC and NTIA have primarily focused on setting emission limits on transmitters and establishing guard bands—spectrum that is left unused between different radio services. For instance, FCC and NTIA often place limits on the out-of-band emissions of transmitters to prevent adjacent band interference. In cases where out-of-band emissions cannot be sufficiently lowered, FCC or NTIA establishes guard bands to separate the assigned frequencies of adjacent communications systems. When a spectrum user experiences interference, it can make a claim of harmful interference to the relevant agency—FCC or NTIA—which will investigate and work to resolve the problem. According to FCC, applying the definition of harmful interference often requires case-by-case consideration, as the definition does not provide objective guidance on what level of service degradation or interruption meets the “harmful” threshold and because several factors, including the type and purpose of the service, must be taken into account. In addition to regulatory actions, well-designed transmitters and receivers can also help prevent harmful interference. Electronic components called filters are used by transmitters and receivers to help ensure that a transmitter only emits electromagnetic signals in its assigned frequency and a receiver only admits electromagnetic signals from its assigned frequency. Ideally, filters would only allow desired signals to pass and block all undesired signals and hence, adjacent band interference would never occur. However, in practice, filters are not perfect, which leads to the potential for harmful adjacent-band interference (see fig. 5). The potential for improving receivers to prevent adjacent-band interference has been receiving increased attention by FCC and NTIA. One of the most recent and high-profile examples, as described earlier, is the potential interference between LightSquared’s proposed wireless broadband network and GPS receivers. There have been a limited number of other cases where interference concerns involved receiver performance to date, including potential interference between mobile communications services and satellite radio, comprised of several proceedings over more than 10 years, and between television receivers and unlicensed devices seeking to use spectrum between channels. However, an FCC official stated in a recent testimony that receiver performance is becoming increasingly important as a limiting factor in repurposing spectrum for new uses and in packing services closer together. FCC, working with NTIA, continues its efforts to repurpose spectrum for new uses and technologies—that is, change its rules to reallocate a band of spectrum from an existing use to a new use—to accommodate the growing demand for spectrum. To improve receiver performance, manufacturers and commercial licensees have developed voluntary standards that are used to design and procure receiver equipment for some services. Manufacturers and commercials licensees have also used private negotiation, improvements to technology, and information sharing between and among spectrum users to improve receiver performance. Some industry-led organizations have adopted voluntary standards for receivers. Standards can help guide receiver design to help prevent interference from adjacent spectrum users and can be either voluntary or mandatory. If voluntary, the specific industry members voluntarily agree to meet them, but are not legally required to do so. If mandatory, the standards would have been imposed by governmental action and industry members would be legally required to meet them. Features of receiver standards often include selectivity, the ability of a receiver to separate the wanted from unwanted signals in the adjacent frequency; sensitivity, the detection limit of the receiver to admit the weakest desired signal level; and dynamic range, the range of desired signal levels from the weakest to the strongest that a receiver can admit and function properly. Several industry representatives told us that voluntary standards for receivers adopted by industry-led organizations are working well to limit interference. Some industry organizations have developed voluntary standards for receivers, including those in the aviation, satellite, and broadcast industries. The processes these organizations use to develop standards are similar and generally include bringing together a diverse group of stakeholders (both commercial and government) for open discussions of specific issues and a transparent, consensus-based decision making process. The Telecommunications Industry Association (TIA) is a trade association representing the global-information and communications- technology industry, including equipment manufacturers and commercial licensees, and is an American National Standards Institute (ANSI) accredited standards developing organization. Within TIA, there are committees that set standards for specific spectrum uses, such as the committee that sets standards for land mobile radios used by, among others, public safety agencies. According to TIA officials, developing a new standard includes the coming together of relevant industry stakeholders who have determined that the creation or amendment of standards is necessary, participation by industry and government stakeholders, and a documented, open process. Standards are typically created or amended within 1 to 3 years. In the aviation industry, RTCA, Inc. develops minimum performance standards for aviation-related receivers.