Online Spotlight: Antenna Performance Specifications for Ka-/Ku-Band User Terminals for LEO Satellite Communications
This article discusses critical antenna performance parameters commonly considered in low earth orbit (LEO) system operation and terminal design and reviews the regulations governing them.
LEO constellations consist of multiple satellites operating in a distribution of orbital planes. Each satellite transmits one or more beams, or footprints, in fixed or dynamic shapes and maintains coverage along the designated orbit. The coverage area of each satellite is determined by system parameters, including orbital altitude, target throughputs derived from link budgets and regulations like the International Telecommunications Union’s (ITU) power flux density (PFD) and equivalent power flux density (EPFD) limits.
A user terminal (UT) maintains its connectivity to the LEO system by tracking a designated moving satellite. Terminals must switch (hand over) links from a designated satellite to the next one when the terminal antenna reaches its limit of scanning range or when the satellite footprint moves away. Terminal antenna design and performance is, therefore, a critical parameter for system operation. A terminal’s complexity and cost also significantly impacts the viability of a network’s business model.
ANTENNA PERFORMANCE PARAMETERS
Effective Isotropic Radiated Power (EIRP)
EIRP measures a transmitter's performance by combining the transmitted power emitted with antenna gain.1 The EIRP of an antenna system is defined as:2
Where PT is the transmitter output power, GT is the gain of the transmitting antenna, and LC is signal attenuation in the feed between the transmitter and the antenna.
To achieve a target EIRP, designers can either enlarge the antenna aperture dimension (increase GT) or increase the transmitter output power (enhance PT). These two parameters, however, cause different concerns for the antenna system. A larger antenna aperture equates to higher gain (a more directed main beam) and a larger device form factor. Some mechanically steered flat-panel antennas require more powerful motors to drive the larger apertures; and, at higher frequencies pointing error and pointing loss can be significant. Transmitter pouwer level drives the cost of the power amplifier and the DC power consumption of a terminal. Higher power levels also generate waste heat in the antenna system and reduce G/T performance, which is sensitive to the ambient temperature of electronic components.
Gain to Noise Temperature Ratio (G/T)
G/T is a figure of merit that indicates a receiver’s performance, where G is the receive antenna gain, and T is the noise temperature of the system measured in Kelvin. From a system perspective, G/T dictates the antenna and RF front-end carrier-to-noise ratio, thus impacting the overall terminal throughput.
Two sources of system noise are the antenna noise temperature and the receiver noise temperature. Antenna noise temperature is due to background radiation in the antenna’s environment, i.e., the sky or the ground. Receiver noise temperature comes from components with dissipative losses, electronic noise and reflection losses that generate noise.1
To calculate system noise temperature at an antenna’s output, before any feed loss, the following equation can be used:3
where TA is the antenna noise temperature, TF is the feed operating temperature, TeRX is the effective input noise temperature of the receiver and LFRX is the receiver feed loss, which is inversely related to the gain at the receiver, i.e., GRX = 1/LFRX.
If the system noise temperature is calculated at the input to the receiver, after the antenna and any feed loss, replacing GRX with 1/LFRX, the system noise temperature can be calculated using the following equation:
It is imperative to ensure that the receiver reference stage at which G/T is calculated is clearly specified. In addition, in an array aperture where multiple radiating elements contribute to the gain and directivity, G/T comprises each individual element’s passive gain, each individual channel’s electronic gain, and the associated coherent noise combined through the network. The active electronic gain supporting each passive element effectively manages the noise caused by the passive transmission lines and combiners. This results in the following complex G/T calculation:4
where N is the number of channels, Ge is the element passive gain, η is aperture efficiency, Ti is the antenna sky temperature at the input, T0 is room temperature, Lf and Ld are the front-end and downstream losses respectively, Lk are the differential weightings of each channel, F is the noise figure of the low noise amplifier (LNA), and g is the electronic gain of the LNA. For antennas with power tapering or non-uniform elements, this formula must be extended to consider the summation of each individual channel’s characteristics.
Polarization and Cross Polarization
Polarization is a property of electromagnetic (EM) waves that specifies the geometric orientation of the electric field oscillations perpendicular to the direction of propagation. An EM wave is called unpolarized if the direction of the electric field fluctuates randomly in time.5 Most antennas are either linearly or circularly polarized.
Linearly polarized antennas are polarized in the horizontal or vertical direction. The electric field of a horizontally polarized antenna is parallel to the surface of the earth. Similarly, the electric field of a vertically polarized antenna is perpendicular to the surface of the earth.6
The electric field of a circularly polarized antenna radiates circularly once per wavelength. If the radiation pattern moves in a clockwise direction then the antenna is said to be right hand circularly polarized. Conversely, if the radiation pattern moves in a counterclockwise direction then the antenna is left hand circularly polarized. The polarization of a receiver and a transmitter must be matched for maximum signal integrity at the receiver. Even a slight mismatch in polarization can result in decreased signal strength.
The term co-polarization (Co-pol) refers to the desired polarization component of an antenna while cross polarization (X-pol) is the polarization component orthogonal to the desired polarization component. If an antenna is horizontally polarized, Co-pol is in the horizontal plane while X-pol is in the vertical plane. X-pol is usually unwanted noise.7
Radiation Patterns and Sidelobes
The graphical representation of an antenna’s radiation property, in 2D or 3D space, is its radiation pattern. 2D radiation patterns can be constructed by keeping a fixed azimuthal angle and varying the elevation angle, or by keeping a fixed elevation angle, and varying the azimuthal angle. These patterns are referred to as horizontal patterns and vertical patterns, respectively.8
The distinguishing characteristics of radiation patterns are their lobes. The main lobe is the angular region of maximum radiation, while the side lobes and minor lobes represent radiation in undesired directions. A null lobe represents a zone of zero radiation. Main lobe steering and null lobe steering are important techniques used to direct the beamforming pattern of an antenna.
Tapering is the process of assigning different gains to the various elements within an antenna array, where the center elements are assigned the highest gains, and the outer elements are assigned lower gains. Sharper tapering, i.e. quickly reducing the element gain of elements farther from the center of the array, results in greater suppression of undesired side lobes. Compared to a similar-sized array with uniform gain across every element, however, a tapered array has reduced beam directivity and broader beamwidth.9
An example of tapering is shown in the array factor (radiation pattern) for a 16-element uniformly spaced linear array (ULA) of isotropic radiators at a center frequency of 19 GHz and progressively sharper tapers (see Figure 1). The yellow radiation pattern with sharpest tapering (sidelobe level (SLL) of -40 dB) has the broadest beamwidth while the blue curve with the least amount of tapering (SLL of -20 dB) has the narrowest beamwidth.
A replica antenna pattern caused by the incorrect spacing of radiating elements is defined as a grating lobe. In phased array antennas, the antenna elements spatially sample the incident wavefront. The Nyquist theorem can be extended to the spatial domain if we consider that two samples, or antenna elements per wavelength are required to avoid aliasing. If the element spacing is greater than λ/2, the resulting grating lobes are considered spatial aliasing.10
Figure 2 illustrates why an element spacing of λ/2 is such a common metric in phased arrays. The antenna pattern is for a 64 element ULA antenna constructed with four subarrays. The center frequency is 20 GHz and the main beam is steered to an angle of 40 degrees off boresight. The blue curve shows the array pattern with a normalized element spacing (d) equal to 0.5λ. In red is the array pattern with d = 0.7λ. This results in a reduction in main lobe beamwidth and brings the nulls close together, but it also causes a replica grating lobe to appear at an azimuth angle of –52 degrees.
When a wavefront approaches an array of antenna elements, there is a time delay between the elements based on the wavefront angle θ relative to boresight. For a single frequency, beam steering is performed by replacing the time delay with a phase shift. This works for narrowband waveforms, but for wideband waveforms, where beam steering is produced by a phase shift, the beam can shift direction as a function of frequency. Since time delay is a linear phase shift as a function of frequency, for a given beam direction the phase shift changes as a function of frequency. Conversely, for a given phase shift, the beam direction changes as a function of frequency. This change in beam angle as a function of frequency is called beam squint.10
Due to the time-delay-based nature of phase shifters, the beam is only perfectly steered at the center frequency; understeering is seen at the maximum operating frequency, and oversteering is observed at the minimum operating frequency. Phase shifters are preferred over time delay functions, however, due to the tradeoff between accuracy and circuit size. In fact, phase shifters provide adequate accuracy for almost all active electronically steered array applications, except for very wide instantaneous bandwidth applications or for very large arrays.9
Given that at boresight (θ = 0 degrees) there is no phase shift across the elements and thus no means to produce beam squint. Because beam squint appears and increases in intensity farther away from center frequency, it is a function of scan angle θ normal to the antenna direction as well as the frequency variation. Mathematically, beam squint is expressed as:
Here, Δθ is the beam squint, ƒ0 is the center frequency, ƒ is the frequency at which beam squint is being calculated, and θ0 is the scan angle at center frequency.10 Figure 3 shows the elevation angle and frequency dependency of beam squint for a 64 element, eight subarray Ka-Band ULA antenna with λ/2 element spacing centered at 19 GHz and a modulation bandwidth of 2 GHz.
The reduction in aperture gain when an antenna is steered to wide angles is called scan loss. Scan loss occurs due to a decrease in gain as an antenna’s scan angle deviates from the axis of maximum antenna gain, i.e., antenna boresight.11 Based on physical optics, scan loss can be interpreted as the inclined projection of an aperture and described by a cosine function.
Added to the loss due to optical projection, is the loss due to anisotropic radiation patterns of each array element. To include this, the cosine function is subject to a power term known as the cosine factor, ranging from 1.1 to 1.5, that depends on the characteristics of the radiating elements. The overall scan loss of a flat-panel aperture can be written as:
where θ is the scan angle from the aperture boresight and K is the cosine factor.
Field of View
Field of View (FoV) is the required scanning range for the UT to maintain radio links with satellites from handover to handover. FoV is directly determined by the constellation configurations, including, but not limited to the number of satellites, altitude, footprint and satellite look angle. FoV is also a function of each satellite's beamwidth.
Pointing and Tracking
Antenna pointing aligns the center of the antenna main beam on a target satellite. Tracking ensures antenna positioning to maintain pointing accuracy over time. There are several different tracking methods, which can be used independently or in conjunction with each other. A few of these methods include:
- Autotrack: Continuously aligns the antenna to an RF signal from the satellite in a closed loop system.3 Methods to track the signal include sequential amplitude detection and monopulse tracking.12
- Adaptrack: Creates a predictive model of satellite motion based on data previously collected from step-tracking.12, 13
- Program Track: The antenna follows a predicted trajectory of the satellite based on stored ephemeris data.12, 14
- Steptrack: The antenna searches for a maximum RF signal. It goes step-by-step changing its position around an axis. If the signal increases, it moves in the same direction; if the signal decreases the direction is reversed.12, 15
- Scan: The antenna rotates around an axis and moves according to variation in power from the received signal. This is used for initial satellite acquisition.12, 16
TERMINAL SYSTEM DESIGN CONSIDERATIONS
In addition to meeting technical performance requirements, user terminals of all types (parabolic and flat-panel arrays) must operate within applicable regulatory limits. The ITU is the global regulatory body that oversees spectrum use including allocations and regulatory limits. These regulations are established through a three year regulatory process, known as the World Radio Conference, to manage and limit harmful interference with incumbent and planned systems like Radio Astronomy Services and other existing (or incumbent) Fixed Satellite Service (FSS) networks.
Regulatory bodies, such as the US Federal Communications Commission (FCC) and UK’s Ofcom, function on a national level to ensure such use is aligned with the goals and priorities of individual countries or regions. While it is common for national limits to mimic those of the ITU, it is imperative to confirm designs comply with both international and national regulations.
Pertinent FCC Regulations
Key regulatory limits that pertain to the design of user terminals are specified in the forms of radiation masks; namely gain, EIRP and EIRP spectral density masks. Historically, user terminals, also referred to as earth stations (ES) operating in the FSS, including both non-geostationary (NGSO) and geostationary (GSO) systems, were subject to gain masks defined in Subpart C: Technical Standard of the FCC’s Part 25 Rules, specifically 25.209 Earth Station (ES) Antenna Performance Standards. Several of these masks take the form of either 29 to 25log(θ) or 32 to 25log(θ), where θ is measured in degrees from the boresight of the main beam, with varying levels further off axis to suppress sidelobes that could interfere with the other satellites. Historically, these masks were derived from the radiation pattern of a very small aperture terminal (VSAT), which typically uses an off-axis-fed reflector antenna with a non-scanning beam. Compliance is often challenging for panel antenna designs.
In 2015, the FCC updated their Part 25 rules, and specified that gain masks for Ku-/Ka-Band FSS systems be defined for ESs operating in GSO FSS systems as well as gateway ESs communicating with NGSO FSS satellites in Ku-Band. Gain masks are no longer defined in the Part 25 Rules for NGSO FSS user terminals, but it is important to note that the industry continues to report compliance with these masks on specification sheets.17
Pertinent ITU Regulations
While NGSO gain masks are no longer defined, NGSO FSS user terminals must operate with an EIRP mask that meets the complicated system-level, “equivalent power flux density up (EPFDup)” limits codified in Article 22 of the ITU’s Radio Regulation and in ITU-R S.1503. The EPFDup simulations consider the satellite constellation and UT operation parameters, including orbit geometry, number of satellites, minimum UT look angle and UT EIRP density patterns to compute the aggregated PFD toward the geostationary (GEO) arc. An EIRP spectral density mask is then defined through fundamental and physical analysis of the NGSO constellation’s potential to cause harm to GSO networks.
Other Pertinent Regulations
Other emission-related regulatory requirements include out-of-band and spurious emissions codified in ITU-R Recommendation SM-1539-1 and SM.1541-5. Additionally, European Telecommunications Standards Institute (ETSI) EN 303 980 and EN 303 979, specify the interference limitations into adjacent channels for Ku- and Ka-Band UTs, respectively. These harmonized standards require the use of additional filters and general state machines for UT operations to avoid exceeding unwanted radiation to other communication systems. Many of the requirements within these documents are also based on traditional VSAT-type parabolic dish antennas and are not suitable for phased array apertures that have different beam patterns and sidelobes over scan. As a result, the compliance methodology for flat-panel antennas is not thoroughly defined, and thus proves challenging.
LEO constellations and communication systems must be precisely designed to maintain connectivity, provide optimal performance and meeting regulatory requirements. Therefore, terminal antenna specifications that define these systems must be well understood. This article covers antenna parameters commonly used for LEO system operation and terminal design. Specifically, the definition of these parameters, their implications for system performance, and how they are used to indicate compliance with regulatory limits and required system performance.
Included are parameters that serve as figures of merit such as G/T and EIRP, parameters that describe an antenna’s behavior such as polarization and radiation patterns, parameters that can affect an antenna's performance such as grating lobes, scan loss, beam squint and tapering, as well as parameters that determine connectivity such as FoV, pointing and tracking.
Finally, Regulatory bodies are detailed along with key regulatory limits and requirements.
- T. Milligan, “Properties of Antennas,” Modern Antenna Design, 2nd Ed. Hoboken, NJ, USA: Wiley, 2005.
- Guidelines for Determining the Effective Radiated Power (ERP) and Equivalent Isotropically Radiated Power (EIRP) of an RF Transmitting System, FCC, Washington, DC, USA, 2015.
- G. Maral and M. Bousquet, Satellite Communications Systems: Systems, Techniques and Technologies, 5th ed. Chichester, United Kingdom: Wiley, 2009
- J. J. Lee, “G/T and Noise Figure of Active Array Antennas,” IEEE Transactions on Antennas and Propagation, Vol. 41, No. 2, February 1993, pp. 241-244.
- “Introduction to Polarization,” EdmundOptics. Web. https://www.edmundoptics.com/knowledge-center/application-notes/optics/introduction-to-polarization/.
- “Antenna Polarization,” Electronics-Notes. Web. https://www.electronics-notes.com/articles/antennas-propagation/antenna-theory/polarisation-polarization.php.
- “What is Cross Polarization,” everythingRF. Web. https://www.everythingrf.com/community/what-is-cross-polarization.
- “Antenna Patterns and Their Meaning,” Cisco. Web. https://www.cisco.com/c/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.html.
- I. Gresham and D. Corman, “An AESA Revolution Utilizing the Disruptive Technology of Highly-Integrated Silicon ICs,” Anokiwave Incorporated, 2018. Web. https://www.rellpower.com/wp/wp-content/uploads/2018/09/Anokiwave_AESA_Revolution_White_Paper.pdf.
- P. Delos, B. Broughton and J. Kraft, “Phased Array Antenna Patterns—Part 2: Grating Lobes and Beam Squint,” Analog Devices, 2020. Web. https://www.analog.com/media/en/analog-dialogue/volume-54/number-2/phased-array-antenna-patterns-part-2-grating-lobes-and-beam-squint.pdf.
- D. Corman, “A Comprehensive Guide to Active Antennas,” RF Global Net, 2018. Web. https://www.rfglobalnet.com/doc/a-comprehensive-guide-to-active-antennas-or-beamforming-0001.
- AC4100 Antenna Control Unit, ViaSat, Carlsbad, CA, USA. Web. https://manualzz.com/doc/14465192/ac4100-antenna-control-unit-datasheet.
- Model 8860 Antenna Tracking Controller, ViaSat, Carlsbad, CA. USA. Web. https://manualzz.com/doc/12596140/8860-antenna-tracking-controller-datasheet.
- Model 8861A/8862 Antenna Position Controllers, ViaSat., Carlsbad CA, USA. Web. https://manualzz.com/doc/9408678/8861-antenna-position-controller-datasheet.
- G. J. Hawkins, D. J. Edwards and J. P. McGeehan, “Tracking Systems for Satellite Communications,” IEEE Proceedings, Vol. 135, Pt. F, No. 5, October 1988, pp. 393-407.
- W. Gawronski and E. M. Craparo, “Antenna Scanning Techniques for Estimation of Spacecraft Position,” Proceedings of the IEEE Aerospace Conference, March 2002.
- Code of Federal Regulations, Part 25, Title 47, Federal Communications Commission. Web. https://www.ecfr.gov/cgi-bin/text-idx?SID=06858939fc9387440db08ece79c64479&mc=true&node=pt47.2.25&rgn=div5.
BIOGRAPHIESUtsav Gupta is a fourth-year student at Olin College studying electrical and computer engineering. He currently works as a student researcher at Olin Satellite + Spectrum Technology & Policy (OSSTP) group, where he is designing a real world, apprenticeship style model for project-based learning to address the financial and pedagogical challenges of higher education. Antoinette Tan is a first-year undergraduate student at Olin College majoring in electrical and computer engineering. She currently works as a student researcher in the OSSTP group, where she is building a software suite for link budget and EPFD computations. On campus, Antoinette is part of Olin’s Formula SAE team, learning about PCB design and fabrication. James Liu is an RF and antenna engineer in the satellite communication industry. He has more than 10 years’ experience in developing LEO and GEO satellite systems, user terminals and antenna technologies. James received his Ph.D. in electrical engineering from the University of Texas at Austin and an M.S. in applied mechanics from National Taiwan University. Whitney Lohmeyer is an assistant professor of engineering at Olin College of Engineering and an aeronautics and astronautics research affiliate at MIT. She leads the OSSTP group, manages and contributes to the field of satellite communications systems and works closely with industry to advise on end-to-end system design, antenna systems, RF power amplification, radiation tolerance and spectrum strategy. Whitney is passionate about enabling affordable internet access to generate economic growth and improve healthcare and education. She was the first engineer hired by OneWeb, a company launching hundreds of LEO communications satellites to provide global broadband and bridge the digital divide. While at OneWeb, she held various technical and policy roles. Whitney received her M.S. and Ph.D. in aeronautics and astronautics from MIT and a B.S. in aerospace engineering from North Carolina State University.