|
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).
|
