Controlling Satellite Communication System Unwanted Emissions in Congested RF Spectrum Donald Olsen The Aerospace Corporation P.O. Box 92957 Los Angeles, CA 90009-2957 Roger Heymann National Environmental Satellite Service NOAA-NESDIS Building SSMC1, 5th Floor 1335 East-West Highway Silver Spring, MD 20910 ABSTRACT The International Telecommunication Union (ITU), a United Nations (UN) agency, is the agency that, under an international treaty, sets radio spectrum usage regulations among member nations. Within the United States of America (USA), the organization that sets regulations, coordinates an application for use, and provides authorization for federal government/agency use of the radio frequency (RF) spectrum is the National Telecommunications and Information Administration (NTIA). In this regard, the NTIA defines which RF spectrum is available for federal government use in the USA, and how it is to be used. The NTIA is a component of the United States (U.S.) Department of Commerce of the federal government. The significance of ITU regulations is that ITU approval is required for U.S. federal government/agency permission to use the RF spectrum outside of U.S. boundaries. All member nations have signed a treaty to do so. U.S. federal regulations for federal use of the RF spectrum are found in the Manual of Regulations and Procedures for Federal Radio Frequency Management, and extracts of the manual are found in what is known as the Table of Frequency Allocations. Nonfederal government and private sector use of the RF spectrum within the U.S. is regulated by the Federal Communications Commission (FCC). There is a need to control “unwanted emissions” (defined to include out-of-band emissions, which are those immediately adjacent to the necessary and allocated bandwidth, plus spurious emissions) to preclude interference to all other authorized users. This paper discusses the causes, effects, and mitigation of unwanted RF emissions to systems in adjacent spectra. Digital modulations are widely used in today’s satellite communications. Commercial communications sector standards are covered for the most part worldwide by Digital Video Broadcast - Satellite (DVB-S) and digital satellite news gathering (DSNG) evolutions and the second generation of DVB-S (DVB-S2) standard, developed by the European Telecommunications Standards Institute (ETSI). In the USA, the Advanced Television Systems Committee (ATSC) has adopted Europe’s DVB-S and DVB-S2 standards for satellite digital transmission. With today’s digital modulations, RF spectral side lobes can extend out many times the modulating frequency on either side of the carrier at excessive power levels unless filtered. Higher-order digital modulations include quadrature phase shift keying (QPSK), 8 PSK (8-ary phase shift keying), 16 APSK (also called 12-4 APSK (amplitude phase shift keying)), and 16 QAM (quadrature amplitude modulation); they are key for higher spectrum efficiency to enable higher data rate transmissions in limited available bandwidths. Nonlinear high-power amplifiers (HPAs) can regenerate frequency spectral side lobes on input-filtered digital modulations. The paper discusses technologies and techniques for controlling these spectral side lobes, such as the use of square root raised cosine (SRRC) filtering before or during the modulation process, HPA output power back-off (OPBO), and RF filters after the HPA. Spectral mask specifications are a common method of the NTIA and ITU to define spectral occupancy power limits. They are intended to reduce interference among RF spectrum users by limiting excessive radiation at frequencies beyond the regulatory allocated bandwidth. The focus here is on the communication systems of U.S. government satellites used for space research, space operations, Earth exploration satellite services (EESS), meteorological satellite services (METSATS), and other government services. The 8025 to 8400 megahertz (MHz) X band can be used to illustrate the “unwanted emissions” issue. 8025 to 8400 MHz abuts the 8400 to 8450 MHz band allocated by the NTIA and ITU to space research for space-to-Earth transmissions such as receiving very weak Deep Space Network signals. The views and ideas expressed in this paper are those of the authors and do not necessarily reflect those of The Aerospace Corporation or The National Oceanic and Atmospheric Administration (NOAA) and its National Environmental Satellite Service (NESDIS). KEY WORDS: Unwanted emissions, RF spectrum, satellite communications, modulations, communications technology, transmitter linearization INTRODUCTION AND BACKGROUND There is a need for satellite communication systems using the RF spectrum to control “unwanted emissions,” which are defined to include out-of-band emissions (immediately adjacent band), plus spurious emissions to preclude interference to other authorized spectrum users. The ITU, through international treaty, sets standards for member nations and provides authorization for RF spectrum use beyond a nation’s boundaries. Member nations determine their own sub-allocations within the ITU regulations for their own domestic use. In the USA the NTIA regulates, coordinates, and provides authorization for agencies of the federal government to use the RF spectrum as required by federal law. Similarly, the Federal Communications Commission (FCC) regulates the nonfederal use of the radio spectrum. These organizations publish documents defining the permitted levels of both in-band and out- of-band transmitter power. The FCC and the NTIA work together in their respective domains to promote a coordinated use of the various bands within the structure of the ITU limits. The Space Frequency Coordination Group (SFCG), with input from the Consultative Committee for Space Data Systems (CCSDS), plays a significant role in advising the ITU, including proposing spectrum mask definitions to the ITU. Traditionally there has not been a great emphasis by U.S. federal government agencies to use the RF spectrum as efficiently as they might. However, due to the increasing demand for spectrum over the last few years, the spectrum use regulators have greatly increased their emphasis on efficiency. In 2003, the Department of Commerce was directed by the White House to prepare recommendations to improve spectrum management. This became part of a presidential directive on using spectrum more efficiently, titled “Improving Spectrum Management for the 21st Century.” From that paper, President George W. Bush signed a “Presidential Determination: Memorandum for the Heads of Executive Departments and Agencies.” He opens that memo, dated June 5, 2003, by stating: The existing legal and policy framework for spectrum management has not kept pace with the dramatic changes in technology and spectrum use. Under the existing framework, the Federal Government generally reviews every change in spectrum use. This process is often slow and inflexible and can discourage the introduction of new technologies. Some spectrum users, including Government agencies, have argued that the existing spectrum process is insufficiently responsive to the need to protect current critical uses. 1 Later the President wrote, 1 Presidential Memo on Spectrum Policy, Office of the Press Secretary, June 5, 2003. 2 In May 2003, I established the Spectrum Policy Initiative to promote the development and implementation of a U.S. spectrum management policy for the 21st century. This initiative will foster economic growth; promote our national and homeland security; maintain U.S. global leadership in communications technology; and satisfy other vital U.S. needs in areas such as public safety, scientific research, Federal transportation infrastructure, and law enforcement. 2 Then in the same memo he directed: the Secretary of Commerce to prepare recommendations for improving spectrum management. The Secretary of Commerce then established a Federal Government Spectrum Task Force and initiated a series of public meetings to address improvements in policies affecting spectrum use by the Federal Government, State, and local governments, and the private sector. The recommendations resulting from these activities were included in a two-part series of reports released by the Secretary of Commerce in June 2004, under the title Spectrum Policy for the 21st Century - The Presidents Spectrum Policy Initiative (Reports). Within 1 year of the date of this memorandum, the heads of agencies selected by the Secretary of Commerce shall provide agency-specific strategic spectrum plans (agency plans) to the Secretary of Commerce that include: (1) spectrum requirements, including bandwidth and frequency location for future technologies or services; (2) the planned uses of new technologies or expanded services requiring spectrum over a period of time agreed to by the selected agencies; and (3) suggested spectrum efficient approaches to meeting identified spectrum requirements.3 This was followed up by a memo from the Honorable Secretary of Commerce Carlos M. Gutierrez to the Honorable Conrad C. Lautenbacher, Jr., the Under Secretary of Commerce for Oceans & Atmosphere within NOAA, where in part he stated that: As directed by the Executive Memorandum, your agency’s plan must be submitted no later than November 30, 2005, and shall include: (1) spectrum requirements, including bandwidth and frequency location for future technologies or services; (2) the planned uses of new technologies or expanded services requiring spectrum over a period of time agreed to by the selected