Satellite communication systems consist of two main segments, the space segment and the earth or ground station. The ground station system coordinates the communication process with satellites in space. A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth.
Owing to the growing user demand for on-the-move global broadband communications a new generation of satellite terminals have emerged, Earth stations on moving platforms (ESOMPs). ESOMP terminals use small antennas with tracking systems and advanced modulation and coding schemes that allow them to provide two-way, high-speed communications from aircraft, maritime vessels, trains, or land vehicles.
Traditionally satellite terminals onboard vessels (maritime and air) operated over mobile satellite service (MSS) systems at the L-band, these terminals provided modest narrowband services (voice and low data rates). later Very small-aperture terminal (VSAT) systems employing parabolic antennas (1.2–2.4 m) and some type of tracking or stabilizing system were developed. They were designed to provide medium data rates over geostationary orbit (GSO) fixed satellite service (FSS) systems operating at the X-, C-, and Ku-frequency bands.
New technology capabilities, adopted by satellite designers and terminal equipment manufacturers, have allowed the development of more spectrally efficient, ultra-small terminals that can provide broadband communications (near or above 1.5 Mbps) to support voice,
video, high-speed data, and access to the Internet. These ESCOMP are a designed to operate at the X-, C-, Ku-, and Ka-frequency bands and provide on-the-move broadband communication services to land vehicles, aircraft, and ships.
Some of the frequency bands used by communications satellites include L-band (1.6265-
to 1.660-GHz uplink/1.525- to 1.559-GHz downlink); C-band (5.9- to 6.4-GHz uplink/3.7- to 4.2-GHz downlink); X-band (7.9- to 8.4-GHz uplink/7.25- to 7.75-GHz downlink); Ku-band (14.0- to 14.5-GHz uplink/11.7- to 12.2-GHz downlink); and Ka-band (27.5- to 31.0-GHz uplink/17.7- to 21.2-GHz downlink.]
ESOMPs exhibit some technical and operational characteristics that are different from those of fixed (stationary) VSATs. One such characteristic is the small antenna size that is necessary to operate from a moving vehicle, aircraft, or maritime vessel. Another characteristic is the tracking system that is required to maintain accurate pointing to the target satellite at all times.
However, as vehicles or vessels move, there is always a probability that antenna-pointing errors may occur for small fractions of time, thus leading to an increase of interference toward other co-frequency neighboring satellites or other radio systems. This possibility requires that systems be designed and operated rigorously to minimize interference and comply with established regulations. To properly account for the resulting time-varying interference impacts on other systems, it is essential to use statistical methods to analyze performance of these systems.
ESOMPs can be used to enable a wide range of applications, from voice or e-mail to high-definition video; thus, the terminal and network configuration depends on the specific platform used (aerial, ship, or ground vehicle) and the service offered. Typically, an ESOMP network may consist of a large number of terminals deployed over a wide geographical area. These terminals may operate with a range of aperture sizes and may require different transmit power levels, according to the location within the satellite footprint, the weather conditions, the type of modulation and coding used, and the maximum supported data rate. To use network resources
efficiently, these networks may use time division multiple access methods and frequency division multiple access methods
Several Ka-band satellite operators and service providers developing systems that will carry ESOMP traffic over GSO and non-GSO FSS systems. FSS systems are preferred, as opposed to MSS, because FSS systems provide the geographic coverage, capacity, and bandwidth required to support broadband services. Ka-band ESOMP terminals use small, lightweight, high-efficiency antennas such as parabolic, low-profile, or phased-array antennas with equivalent aperture sizes as small as 0.3 m.
ESOMP terminals also include mechanical or electronic tracking systems with servo controllers and positioners to maintain accurate pointing to the target satellite. The tracking systems provide initial signal acquisition and instantaneous reacquisition after a signal loss due to signal obstructions, weather conditions, or antenna mispointing due to sudden turns.
Another important element for the successful deployment of ESOMPs is having appropriate standards and regulations. Service providers, operators, and regulators are beginning to address critical issues such as the use of FSS bands, interference considerations, and licensing procedures, among many others.
For example, in the United States, the Federal Communications Commission (FCC) adopted §25.222 for Earth stations on vessels (ESVs). Similarly, in §25.226 new rules were adopted for vehicle-mounted Earth stations (VMESs) and, more recently, in §25.227 for Earth stations aboard aircraft (ESAAs). According to FCC rules, each of these types of terminals can operate within the United States as a primary application on specified frequencies over GSO FSS systems.
At the regional level, the European Telecommunications Standards Institute (ETSI) has adopted Ku-band standards for ESVs under European Norm (EN) 302 340, for VMESs under EN 302 977, and for aircraft Earth stations (AESs) under EN 302 186. More recently, ETSI has adopted EN 303 978, a new standard for ESOMPs transmitting toward GSO satellites operating in the 27.5-GHz to 30.0-GHz frequency bands.
The ITU granted secondary status to AMSSs, but ESVs and VMESs can only operate in FSS networks under RR No. 4.4. According to this ITU regulation, such stations shall not cause harmful interference to, and shall not claim protection from, interference caused by a station operating in accordance with ITU regulations.
Off-Axis Emission Constraints To limit interference to adjacent GSO satellites, the ITU has established limits on the ESD of a transmit terminal in its off-axis directions. Because of the antenna beam characteristics, terminals with large-aperture antennas are not constrained by the main beam but by the side lobes; hence they can transmit higher ESD levels. However, because the main lobe of small antennas is wide, these terminals can be severely limited by the ESD in the boresight direction (the direction of the maximum gain of the antenna). These off-axis ESD
limits are specified in Rec. ITU-R S.728-1 for Ku-band VSATs and in Rec. ITU-R S.524-9 for Ka-band terminals.
The antenna-pointing error is defined as the angle between the boresight direction of the antenna
and its intended direction, which is the direction toward its target satellite. According to the VMES rules adopted by the FCC (§25.226), the terminals are allowed to operate when they comply with one of the following constraints on the antenna-pointing errors:
(a) Antenna-pointing errors should be less than 0.2°. If they exceed 0.5°, emissions should cease within 100 ms and transmissions shall not resume until they are less than or equal to 0.2°.
(b) Antenna-pointing errors greater than 0.2° are allowed, provided that the peak value of these
errors is declared and the ESD taking into account this peak value complies with the ESD constraints. Moreover, transmissions should cease within 100 ms if the antenna-pointing errors exceed this declared value.
Spectral efficiency Considerations
The spectral efficiency of a communication link, which is the data rate transmitted in the link normalized with respect to the occupied bandwidth of the signal, is a key parameter that can be used to quantify the spectral use of that link. Shannon’s well-known capacity formula demonstrates that the link spectral efficiency is proportional to the signal-to-noise ratio of the received signal.
The off-axis ESD constraints severely limit the transmit power in a given bandwidth for small-aperture terminals. Therefore, it follows that the spectral efficiency realized from a satellite link that uses a small-aperture transmit antenna can be low. Moreover, when a small-aperture
antenna is used at the receiver, the link spectral efficiency can be very low because of the low antenna gain. Finally, the overall link spectral efficiency could be further degraded by adjacent satellite interference.
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