Giandomenico Amendola, Daniele Cavallo, Tobias Chaloun, Nicolas Defrance, George Goussetis, Marc Margalef-Rovira, Enrica Martini, Oscar Quevedo-Teruel, Václav Valenta, Nelson J.G. Fonseca, Mauro Ettorre
©SHUTTERSTOCK.COM/NICOEININO
Low-Earth orbit (LEO) constellations are revolutionizing the world of satellite communication (Satcom), providing new opportunities to manufacturers and operators and enabling innovative and attractive services to users. The main advantages of low-orbit satellite systems are
However, the benefits given by LEO satellites come with technical challenges. LEO satellites travel at high speed and can ensure the coverage of targeted areas of Earth’s surface only for a limited time, placing stringent requirements on the antenna for both the space and ground segments. In particular, the antennas should be agile and able to steer their main beam over a large field of view. This field of view of the antenna is related to the number of satellites in the constellations. A tradeoff is therefore required to achieve a reasonable number of satellites and a feasible field of view for robust communications. Furthermore, LEO systems require handovers among satellites to guarantee uninterrupted links, at the cost of a more complex system. The handovers are achieved by radiating several beams and by buffering the information in the antenna terminal.
LEO links alleviate some of the drawbacks of GEO systems but give new technological challenges in the design of antenna systems and RF front ends. The present article attempts to provide a broad overview of the available RF technologies that allow designers to tackle these challenges and help build the future LEO user segment. The article develops in six sections. The “Ku and Ka LEO Systems” section presents a brief account of the current LEO systems at the Ku-/Ka-band by reviewing present and future constellations and providing information relevant to the development of the user segments. The “Characteristics of LEO Terminals” section defines very general requirements corresponding to a Starlink-like LEO system. Also, in this section, we identify the technologies presented in the following parts of the review. In a nutshell, the article follows the structure of an RF front end, starting from the antenna and going down toward the other components. For brevity, we have reduced the scope of this review to antennas, beamforming technologies, low-noise amplifiers (LNAs), and power amplifiers (PAs). Notice that there are fields of research that are equally important and not included in the present work. As an example, the areas of packaging and integration, which is of utmost importance to provide highly integrated solutions, and analog phase shifters will not be treated in this article.
In the “Antennas” section, we review the most recent antenna solutions suited to LEO communications. We present the antennas according to the scanning approach: mechanical and fully electronic. In the “Beamformers” section, we discuss state-of-the-art beamforming technologies. We include analog passive beamformers and silicon (Si) active beamformers, as they are becoming a fundamental building block of electronically steerable antennas. As a final contribution, we account for recent developments in digital beamforming (DBF). While DBF is not identifiable with one single device, it is one of the most promising technologies related to phased arrays and multibeam antennas for future LEO space and ground segments. Finally, the “Semiconductor Technologies” section reports the most relevant performance of LNAs and PAs.
Despite the large investments dedicated to the development of LEO constellations, technical information about the communication technology adopted on board satellites is rather scarce. In this section, we summarize data available in the open literature to constitute a framework into which all the information reported in the following paragraphs can be placed. We consider constellations that are already in operation and in advanced development stages, for example, Starlink, OneWeb, Telesat, and Kuiper [1], [2], [3]. The main constellation parameters are reported in Table 1. The four constellations will be deployed in LEO, with altitudes that may change according to the orbital planes to which the satellites are allocated. Both user and feeder links operate in Ku and/or Ka frequency bands allocated to Satcom communications. Table 1 reports information that can be used in a simplified link budget estimation. The data reported in Table 1 indicate that the onboard resources provided by the constellations are abundant and, in most cases, thanks to the multispot approach and the reduced altitude of the orbits, the design of compact and effective user terminals is possible.
Table 1. The main parameters of the Telesat, OneWeb, Starlink, and Kuiper constellations [1], [2].
The specifications of Satcom terminals vary greatly depending on the application and the data rate. The following section describes some of the characteristics of terminals with bit rates higher than 20 Mb/s, which are typical of broadband Internet access services. Table 2 shows indicative Ku-band link budgets for the downlink and the uplink that consider a download data rate of 100 Mb/s and upload rate of 20 Mb/s [4]. The satellite parameters are the ones of the Starlink constellation reported in [1] and [2], which used U.S. Federal Communications Commission filings as sources. The diameter of the antenna is 0.7 m, with the aperture efficiency set to 30% to include the case of planar phased-array antennas. The receive (Rx) RF front end has a 1-dB interconnection loss and a 3-dB noise figure (NF). The downlink budget reported in Table 2 shows that the link margin at 99.9% availability is more than 6 dB. Note that even considering a receiver with a 6-dB NF and including a scanning loss of 3 dB, the calculated link margin is greater than 2 dB.
Table 2. The downlink and uplink budgets for a Starlink-based terminal.
The uplink budget considers a circular aperture with a diameter of 0.35 m and 5 W of RF input power. This configuration roughly corresponds to a circular array of about 900 elements spaced at half a wavelength, fed with one amplifier per element, with 7.5 dB referenced to 1 mW (dBm) of output power at a 1-dB compression point. The antenna aperture efficiency is 25%, lower than the one used in the Rx case to account for the higher frequency of the transmit (Tx) band. Table 2 shows that the link margin is more than 6.4 dB and leaves room for a 3-dB scan loss.
The evaluations presented in the preceding confirm that link budgets may be closed, considering components with performance that is compatible with the state of the art. However, considerable R&D efforts are still needed to design compact and flexible terminals. The difficulties mainly come from the requirement of scanning at low elevation angles and radiating two simultaneous beams. The difficulties dramatically increase if one must integrate Tx and Rx operations into a single aperture.
The architecture of the RF front end of a ground terminal may change according to the antenna and the beamforming techniques that may combine to adapt to diverse requirements. Even if any attempt at a classification gives a partial view of all possible combinations, we refer to the categorization shown in Figure 1. Since LEO terminals must continuously scan their beam toward moving satellites, the classification is according to the beam scanning techniques. In the subsequent sections, Figure 1 is used as a reference for reviewing antennas, beamforming, and semiconductor technologies for LEO terminals.
Figure 1. The classification of antennas and beamforming techniques.
Because LEO systems require terminals able to follow satellites along their orbits, we classify antennas according to their scanning mechanism, considering two very broad categories: mechanically scanned antennas and electronically scanned antennas. These two approaches present different degrees of maturity, performance, and cost. In the following, the two categories are presented briefly, explaining their principle of operation and describing their performance.
Mechanically scanned antennas offer extended angular coverage with excellent performance in terms of polarization purity, the band of operation, and the antenna gain-to-noise-temperature (G/T) figure. Figure 1 proposes a classification of the available technologies that groups mechanically scanned antennas into four broad classes: based on gimbals, allowing full 2D mechanical pointing, hybrid systems, moving feeds, and rotatable surfaces. The same classification is followed in the subsequent sections to present the characteristics of the antenna systems. Note that this classification is inevitably stretched, and while it covers most of the systems in use, it does not encompass all possible configurations, such as the class of hybrid systems, which covers more than one of the highlighted groups.
Full 2D pointing requires complex mechanical gimbals. Many of these solutions rely on conservative concepts, such as reflector antennas [5], [6]. Such antennas are generally bulky and heavy and may require a gimbal with at least two servomotors to control the pointing direction of the radiated beam and its polarization. Over the years, the profile of mechanically steered antennas has been reduced to ease their integration in moving platforms, such as airplanes. Solutions based on waveguide-based arrays [7] have been adopted because they have a lower profile and better efficiency than reflector antennas. However, they require complex pointing mechanisms that couple azimuthal rotation with movement in elevation.
The complexity of 2D pointing systems is reduced when considering hybrid systems in which only the azimuthal rotation is achieved mechanically. Pointing in elevation is obtained by scanning the beam by using conventional phase shifters, quasi-optical beamformers, and, in the case of lens antennas, adopting multifeed systems and moving feeds. The final two cases are described in the following sections.
Different designs have been proposed in recent years for the Ku-band in which a planar array is integrated with phase shifters to scan the beam in one direction, [8], [9]. In [10], elevation beam steering is obtained through a switched-beam architecture realized in microstrip technology in the form of a Rotman lens. Employing RF switches instead of phase shifters implies lower losses, higher simplicity, and lower cost. Planar quasi-optical beam formers, such as the Rotman lens, also have the advantage of being able to generate simultaneous independent beams, which can be used to realize a make-before-break handover between different satellites. Furthermore, their design generally relies on nonresonant true-time delay structures and is therefore intrinsically broadband. In particular, this implies that Tx and Rx functions can be combined into the same aperture, possibly covering multiple bands. Lens-based concepts in parallel plate waveguide (PPW) technology have been introduced in the Ka-band to preserve the wideband operation of reflector-based solutions and, at the same time, provide a wide coverage (±50° in elevation) [11]. Arrays based on a long slot fed by multimode PPWs, proposed in [12] and [13], show polarization agility, at the cost of a complex design process, a field of view of ±45° in elevation, and a reduced bandwidth (29–32 GHz)
Another class of mechanically scanned antennas suitable for Satcom communications are volumetric lenses using movable feeds and multifeed systems. They are seen as quasi-optical beamformers that may achieve a wide scan range with reduced antenna dimensions compared to reflectors. Volumetric lenses can be classified into two main categories:
Classical designs of graded-index lenses allowing beam scanning, such as the Luneburg lens and the half Maxwell fish eye lens [15], do not meet the low-profile requirement and may not be suitable for Satcom applications, such as Satcom on the move. Arrays of smaller lenses can offer better efficiency than a single lens of a similar aperture, with a lower profile [16]. In recent years, the application of transformation optics concepts [17] was proposed to reduce the profile of the Luneburg lens. Scanning is obtained simply by displacing the feed, at the price of an increased scanning loss and a reduction of the scanning range [18], [19].
A 2D version of the Luneburg lens can provide full azimuthal coverage with a low-profile arrangement, and it can be used as a beamforming network for a radiating aperture. Practical realizations are normally done in PPW technology. Different solutions have been proposed to realize the required effective refractive index variation. This can be achieved by machining a distribution of metal posts between PPW plates [20], photolithographically etching holes into one side of a standard printed circuit board ground plane [21], and loading the parallel plates with electrically small patches printed on a grounded dielectric slab [22]. A similar metasurface-based solution has been used in [23] to implement a half Maxwell fish eye lens. All-metal lenses realized with a bed of nails and by periodic holes in a thick metallic plate have also been presented [24], [25]. An alternative fully metallic solution is obtained in [26], relying on a geodesic approach. This solution has the advantage of increased bandwidth compared to the approaches based on dispersive modulated metasurfaces. Furthermore, it is scalable to high frequencies, due to the lack of small geometric details. However, it has a larger thickness when compared with its fully planar counterparts. Note that in all the 2D lenses reported until now, scanning is achieved by displacing the feed and by implementing a multifeed system.
Low-profile antennas with full 2D scanning may be implemented using rotatable surfaces. The best-performing antenna terminals in the Ku- and Ka-bands in this category are designed by ThinKom. ThinKom’s antennas are based on variable-inclination continuous transverse stub (VICTS) technology, an array of stubs rotating around a single axis for a wide field of view and operating band [27], [28]. The terminal provides interoperability among different constellations, with a large field of view (7.5–90° in elevation) and very appealing G/T performance (>12 dB/K at a 20° elevation). Mechanical solutions do not generally have multiple-beam radiation. Information is, then, buffered to switch among satellites requiring rapid control of the radiated beam.
2D scanning is also achieved with rotatable graded-index lenses placed in front of a radiating aperture. The principle of operation is similar to the so-called Risley prisms. In this solution, a fixed primary radiator with a broadside pencil beam illuminates two parallel planar lenses, providing a linear phase shift. The two lenses can be rotated either synchronously or independently around the axis of the primary radiator to steer the beam in the upper hemisphere. The resulting architecture is significantly simpler and less bulky than the typical azimuth/elevation positioner used for reflector antennas, and its profile remains unchanged while the beam is scanned. The screens needed are passive, and they do not contain reconfigurable elements. The feasibility of the scanning mechanism based on rotatable metalenses was first demonstrated in [29] by using a horn as the primary feed. More recent works have achieved higher aperture efficiency and smaller thickness by using a low-profile primary radiator [30], [31], [32], [33], [34]. The main challenge in the design of this kind of antenna for Satcom applications is the control of the grating lobes during scanning, which is strictly related to the design of the metalenses.
A similar solution is based on translating lenses: in this case, steering in elevation is achieved by the lateral translation of a thin flat lens placed a few wavelengths above a feed antenna, whereas azimuth steering is achieved by rotation of the lens or both the lens and the feed horn [35]. Dual-band operation and a beam steering range up to 50° have been demonstrated [36]. Table 3 summarizes the recent developments for antennas based on mechanical and hybrid scanning for the Ku- and Ka-bands.
Table 3. A comparison of reported Ku- and Ka-band Tx and Rx mechanical and hybrid solutions.
Electronic beam steering antennas are the optimal solution for satellite tracking, especially when reliable and agile operation is desired. Electronic scanning is particularly advantageous for Satcom on-the-move systems (e.g., on aircraft), which require flat antennas that can rapidly repoint to the satellite to compensate for the platform motion. Fully electronic active arrays can also provide various reconfigurability characteristics, radiation pattern shaping, wide scanning capability, multiple-beam generation, and power sharing among beams through distributed amplification.
Despite the extended capabilities offered by phased arrays, the high cost and complexity of such antennas have, for a long time, limited their widespread use in commercial systems. For this reason, several approaches have been proposed to realize cost-effective electronic scanning antennas, including tunable reflectarrays and transmitarrays [36], [37], [38], [39], [41] and liquid crystal-based antennas [42], [43]. However, the typical components used for tuning the elements, such as p-i-n and varactor diodes, microelectromechanical systems, and liquid crystals, are associated with increased dissipation losses.
Recent developments of electronic chipsets with lower cost and higher power (see the “Beamformers” section) have significantly contributed to renewed interest in active array antennas. More specifically, the cost of fully active phased arrays has recently dropped thanks to the advancement of Si multichannel chips to be used for Tx/Rx modules [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], paving the way to the realization of the Satcom phased arrays that are now commercially available.
Recent literature reports the performance of numerous Ku- and Ka-band phased arrays. A comparison between the measured effective isotropic radiated power (EIRP) and G/T for the Tx and Rx cases, respectively, of different prototypes is reported in Table 4. Regarding the scanning capability, the need for terminal antennas able to scan to larger angles is emerging to guarantee agile connections to different satellites. However, conventional planar phased-array antennas exhibit limitations when steering a pencil beam over a large field of view. The first is the deterioration of the antenna active reflection coefficient when scanning at low elevation. The issue is resolved with an extensive optimization process of the radiating elements and the array lattice and with the help of matching layers. However, even when good matching is achieved for wide scan angles, the diminished aperture projection (proportional to $\cos{\theta},$ where ${\theta}$ is the scan angle) translates into a reduced antenna gain. For this reason, a tradeoff is in order among the use of conformal and multipanel arrays [59], curved radomes that increase the scan range [60], [61], [62], [63], and a combination of electronic scanning with a small mechanical tilt.
Table 4. A comparison of reported Ku- and Ka-band Tx and Rx phased arrays.
Another known limitation of current Satcom phased arrays is the narrow bandwidth that forces the use of multiple antennas to cover different frequency bands. In applications that require a terminal mounted on a mobile platform, the space available is often limited, and the presence of two antennas is a major drawback. In this regard, it may be beneficial to use a wideband array simultaneously covering multiple bands [64], [65], [66]. This solution may reduce the volume and costs of the system, including fuel costs created by the weight and drag from the antennas. Note that, in wideband arrays, the isolation between different bands and Tx and Rx channels is a further difficulty that makes the design cumbersome.
Beamforming may be achieved in many ways, with performance that may change considerably if complexity, flexibility, and costs are considered. In this article, we review three main classes of beamforming:
Analog circuit-based beamforming solutions are typically well adapted for array antennas with a moderate size and a limited number of beams, typically a single beam and maybe two for user segment applications requiring a make-before-break handover [67]. These are of interest for LEO constellation systems with medium-gain requirements, such as Internet of Things terminals, where a conventional analog beamforming network may be an acceptable solution in terms of complexity and performance. Over recent years, there has been a resurgence of interest for analog solutions based on orthogonal beamforming networks [68]. Most studies are based on the well-known Butler matrix [69], [70], [71], [72], and some also consider the less-known Nolen matrix [73], [74], [75], [76]. A general form of a parallel orthogonal beamforming network for single-layer implementation has also been recently proposed [77], as beamforming theory is still an active field of research. A comparison of these different beamforming matrices is provided in Table 5. Most papers focus on practical implementations, with particular interest in millimeter-wave designs and, more specifically, 5G terrestrial communications enabling interesting synergies with Satcom in the Ku- and Ka-bands. Solutions based on low-cost substrate-integrated waveguide technology [78] are particularly appealing for applications that require cheap mass-produced terminals, such as maritime transport asset tracking.
Table 5. A comparison of analog beamforming matrices with M input ports and N output ports.
Orthogonal beamforming matrices have the advantage of producing multiple beams while avoiding recombination losses inherent to standard corporate networks [68]. These are generally used in a beam-switching configuration and in simultaneous multiple fixed-beam operation, although some works have explored the capability they offer to combine beam switching and beam steering [79], [80], [81]. Because their complexity generally increases exponentially with the number of beams, most designs reported are limited to 4 × 4 and 8 × 8 matrices, with some rare examples of 16 × 16 Butler matrices [82], [83], [84]. For larger arrays, a hybrid beamforming approach implementing typically analog beamforming at the subarray and tile level and DBF at the array level is considered a promising approach to combine the benefits and mitigate the drawbacks of both technologies [85], [86]. Compared to a fully digital implementation, the hybrid beamforming antenna system also has N radiating elements but only N/Ns RF chains, where Ns is the number of elements per subarray, instead of N on the fully DBF antenna system. The subarrays may be fixed and reconfigurable, possibly including amplitude and phase control in a fully reconfigurable design. The analog part may be further extended to have multiple inputs and multiple outputs, with one RF chain per input port. Using orthogonal beamforming matrices in the analog part enables a more efficient design, at the expense of scanning range restrictions.
Beamforming integrated circuits (ICs) interface the antenna elements and beam-splitting/combining networks. On Tx, their role is to map input signals to multiple outputs with specific gain and phase coefficients that correspond to the specific position of the antenna element. In the Rx direction, the functional role is reversed, and in addition, an adequate low NF is needed to guarantee the required G/T since, in most of the active Satcom arrays, the beamforming IC is interfacing directly with the radiating elements without an external LNA. Today, there is a range of commercial off-the-shelf (COTS) beamforming ICs available in the market, covering X, Ku-, Ka-, and even Q/V bands. They come in various configurations, e.g., in multichannel architectures, where, for instance, four inputs are mapped to four outputs, i.e., containing 16 amplitude and phase control nodes [87]. Other configurations incorporate the signal combining and splitting functions [88] and even the down-converting functions to enable beamforming at the digital level [89]. Unlike 5G beamforming, Satcom active antennas have larger apertures and use a larger communication bandwidth, and hence, beam squint effects need to be considered. For this reason, the beamforming IC should cover the necessary delay to compensate for dispersion across the antenna aperture.
Low dc power, often referred per single beamforming node, is also of very high importance, as beamforming ICs contribute significantly to overall power consumption of the active array. Low-power beamforming ICs available in the market today consume less than 10 mW per beamforming node, but they have a 1 dB compression point (P1dB) of less than 0 dBm. Consequently, an additional output amplification is used to reach the output power level required by Satcom links. The main contribution determining the power consumption comes from the on-chip gain blocks that compensate the splitting losses and losses of amplitude/phase/time delay control blocks. The losses of these elements are minimized through an accurate choice of the beamforming architecture and the integration technologies. As an example, the losses of transmission lines can be reduced by design techniques, such as the thin-film microstrip that shields the line from lossy low-resistivity Si substrates in Si–germanium (SiGe) bipolar CMOS (BiCMOS) implementations.
As far as Si-on-insulator (SOI) technologies are concerned, the inherent physical properties of the insulator will offer higher isolation between channels and low-loss implementation of switching functions and high-Q inductors allowing superior performance. This is evident looking at the NF performance. SOI 22- and 45-nm nodes have recently demonstrated an NF below 1.5 dB, approaching the state-of-art levels obtained in gallium arsenide (GaAs) [90]. On the other hand, considering the P1dB required by Satcom applications, SiGe BiCMOS with higher voltage breakdown levels will outperform RF-SOI amplifiers by several decibels. Furthermore, SiGe performance is achieved without applying transistor stacking techniques, and it avoids the risks associated with the time-dependent dielectric breakdown, which is known to be one of the most important degradation mechanisms affecting the reliability of CMOS devices [91].
The availability of wafer-level packaging solutions for a given technology is another important aspect to consider to facilitate array assembly and allow for proper thermal management, which is a particular challenge for SOI. Suitable wafer-level packaging techniques advance rapidly together with semiconductor processes, and, for instance, the maturity of wafer-level packaging, such as the embedded wafer-level ball grid array, has also been proved at millimeter-wave frequencies [92], [93].
LEO constellations are predicted to deliver high-throughput broadband and ubiquitous services with low latency. The envisioned scenario, however, will necessitate that the user terminal tracks multiple satellites simultaneously in terms of position as well as polarization to provide truly uninterrupted operation. Although analog beamformers offer the unparalleled capability to realize single-beam phased-array antennas with low hardware complexity and power consumption, analog beam steering poses severe challenges when designing point-to-multipoint antenna systems. In particular, for higher operating frequencies in the micro- and millimeter-wave regimes, spatial multiplexing systems based on analog beamformers commonly suffer from inherent space constraints because the amount of analog circuitry is inevitably linked with the number of independent beams. In contrast, multibeam antenna systems using DBF do not show these limitations, as they perform the namesake function solely in the digital back end [94]. This leads to a simplified RF chain per antenna element, basically constituting an amplification and frequency conversion stage for Tx and Rx, respectively. DBF processing on the element-level has the highest degree of flexibility in synthesizing multiple beams simultaneously because the Tx and Rx signals at each antenna element can be arbitrarily manipulated, duplicated, and combined in the digital domain, hence avoiding signal-to-noise ratio (SNR) degradation. Furthermore, unlike their analog counterparts, fully digital beamformer processors can seamlessly apply frequency-dependent amplitude tapering and phase shifting enhanced by the possible compensation of hardware impairments through channel-level equalization, in-phase/quadrature balancing, and static local oscillator phase offsets at each antenna element [95].
Pushed by the ambitious data throughput goals of near-future LEO/medium Earth orbit Satcom, DBF-based antenna terminals are becoming an interesting design approach to cover large instantaneous bandwidths while avoiding the use of analog true-time delay elements [96]. Despite all these advantages, the implementation effort of the signal processing back end strongly scales with the number of antenna elements as well as the system bandwidth [97]. Ahead of their time, the first DBF antenna modules for mobile Satcom terminals at 30 GHz have been presented in [98] and [99]. In these pioneering works, the digital baseband processing unit supports the control of up to 64 radiation elements within a signal bandwidth of a few tens of megahertz. In a situation radically different from 10 years ago, when the use of COTS components ruled active antenna design, the custom development of application-specific ICs (ASICs) for specific commercial sensor and communication applications has become very much mainstream now. An example for the current generation of DBF ASICs for the Satcom market segment is SatixFy’s Prime [100]. The true-time delay DBF chip supports up to 32 antenna elements and can be connected with other chips to span large antenna apertures. The individual signals at each antenna element are translated between the analog and digital domain by high-speed analog-to-digital converters (ADCs) and digital-to-analog converters, respectively.
Apart from high-resolution digital phase shifters and digital delay circuits, the signal processing inside beamformer ASICs may also correct RF front-end imperfections, enabling wideband signal transmission and reception across 1 GHz of bandwidth in single-beam mode. The enormous advances in CMOS semiconductor technologies have also enabled the commercial realization of data converters for direct RF sampling up to the Ka-band [101]. The latest trends show that digital beamformer ASICs enhanced by direct sampling up to the Ka-band are going to be deployed in future satellite payloads and thus circumvent the need for frequency conversion stages [102]. Although this all-in-one DBF technology is not yet competitive to be used on the element level in mobile DBF antenna terminals, the availability of high-performance data converters has paved the way for new code-based beamforming architectures. In [103], more specifically, a code division multiplexing technique has been proposed to aggregate the individual signals from the antenna elements at the analog RF front end into a single ADC. Compared to conventional DBF techniques, a remarkable reduction of the number of required ADCs has been demonstrated, which, in return, must feature a higher sampling rate and analog bandwidth.
The integration level offered by monolithic microwave ICs (MMICs) is a key feature of LEO constellations. Integration processes have advanced considerably, making possible the design of complete systems on chip in the frequency band considered in this review (i.e., the Ku-/Ka-bands). In this scenario, four main semiconductor families appear to be attractive candidates for the aimed-at frequency bands and performance: 1) GaAs, 2) Ga nitride (GaN), 3) SiGe, and 4) Si. The two former technologies belong to the wide-bandgap semiconductor family, which intrinsically leads to greater output power and a reduced NF. On the other hand, Si- and SiGe-based technologies present reduced footprints, higher gains, and the possibility of integrating digital control circuitry in a single die together with RF functions.
Considering power and bandwidth, GaN really emerges as the winner when compared with the competing technologies. In fact, progress in GaN device technology is considered crucial to the viability of power amplification in space and other critical applications in the future. In the past decade, GaN MMICs and discrete GaN devices have significantly improved their efficiency, power density, reliability, and overall output power. In the frequency bands considered in this review, the Ku- and Ka-bands, GaN-based devices and circuits have already demonstrated superior performance in all these figures of merit while simplifying MMIC architectures and minimizing the overall product footprint. Nevertheless, GaN-based MMICs are undergoing further research and development efforts accompanied by considerable investments to improve efficiency, output power, manufacturing processes, and module packaging. Note that prepackaged devices and carriers are still being employed as the most convenient and cost-effective means of building high-power solid-state PAs for lower frequencies, typically below 30 GHz. As far as GaAs is considered, this technology offers the nonnegligible advantage of lower noise and an ability to integrate with CMOS technologies. Also, GaAs allows for the design of complete core chips that integrate into Tx/Rx modules.
The second family, 3) and 4) in the preceding list, is Si based. SiGe offers heterojunction bipolar transistors (HBTs) in addition to MOSFETs [104]. Even though MOSFETs integrated in a 130-nm process can comfortably address the Ku-/Ka-bands with respectable performance, HBTs offer much better performance, usually with the drawback of a more expensive technology. Modern pure Si technologies offer additional features, such as SOI substrates that have effectively pushed forward the performance of these technologies, especially in terms of the NF [105], as reported later in the article.
The choice among the technologies briefly described up to now is usually driven by four principal drivers: the power output, NF, footprint, and cost. The first two drivers determine the performance of a satellite terminal and the quality of the link in terms of the availability and data rate. For this reason, in the succeeding sections, the state of the art of PAs and LNAs will be overviewed in greater detail. The footprint of the device directly affects the level of integration achievable. The area of the devices is a critical parameter affecting the realization of electronically scanned antennas and phased arrays. In fact, phased arrays require that the distance between radiating elements be less than half of a wavelength. The last driver, cost, is probably the most difficult to evaluate, and it will be only marginally considered. In fact, technology selection involves a complex tradeoff among variables that affect cost, such as the technology type and node, volume, expected circuit complexity (∼die area), production wafer size, and process availability (i.e., the wafer fabrication process time).
Costs are critical in terminals that adopt phased-array antennas because of the number of components. In these cases, Si-based technologies are the ones that offer the best level of integration. Si technologies have one principal cost factor in nonrecurring costs, mainly related to the mask set. In the case of RF-SOI technology, nonrecurring costs are absorbed only under a large production volume, while for SiGe, nonrecurring costs could be several times lower than those of advanced SOI nodes (e.g., 130-nm SiGe versus advanced RF-SOI). It is evident that SiGe will make sense in Satcom communications, where lower volume and high-end RF performance (e.g., a higher linearity, higher P1dB, and lower NF) are required. Several foundries have recently improved this tradeoff for SiGe BiCMOS by moving from 200- to 300-mm wafers, enhancing production efficiency and making SiGe suitable even for a large market above the Ku-band. The indicative tradeoff for SiGe and RF-SOI technologies is presented in Table 6.
Table 6. The indicative tradeoff for three selected processes adopted in most beamforming ICs: two RF-SOI technology nodes and SiGe BiCMOS.
The design of RF PAs focuses on increasing the output power and optimizing the dc-to-RF efficiency. Consequently, benchmarking a PA’s saturation power (P sat) and its power-added efficiency (PAE) provides a straightforward way to determine its performance. Figure 2 includes a scatter plot of the current state-of-the-art PAs, comparing their PAE versus their. In applications involving high-data-rate communication using higher-level modulation methods, other factors, such as linearity and the noise–power ratio, intervene to determine an efficient and robust communication link versus their PAE. Figure 2 shows a clear separation between III-V technologies (i.e., GaAs and GaN) and Si technologies, symbolically delimited by the 30-dBm barrier due to the difference in operating voltage between III-V and Si-based technologies. PAs of the III-V group require a voltage around 12–28 V, while Si amplifiers need a voltage between 1 and 4 V.
Figure 2. The PAE versus the Psat of current state-of-the-art GaN [106], [107], [108], [109], [110], [111], [112], GaN [111], [116], Si [117], [118], [119], [120], [121], [122], [123], [124], and SiGe PAs [125], [126], [127], [128], [129] in the Ku/Ka frequency bands.
GaN appears to be a superior candidate for the design of PAs when compared to GaAs and is becoming the technology of choice across frequency bands and markets. GaN amplifiers present a P sat in the 36–46 -dBm range [106], [107], [108], [109], [110], [111], [112], while their GaAs counterparts are within the 34–40-dBm range [111], [112], [113], [114], [115], [116]. In addition, GaN PAs also show a superior PAE, which can surpass 40%, while GaAs PAs present PAEs of the order of 30%. Initially, higher output power and smaller form factors were the focus of GaN product development. However, the resulting thermal constraints at the system level push R&D efforts to achieve a better balance with efficiency to help reduce dissipated power and ease the thermal load at the system level.
In Si-only technologies, output powers of around 23 dBm have been reached [117] in Doherty-based PAs. On the other hand, with single-ended architectures that do not rely on power-combining techniques, most Si-based technologies tend to offer P sat levels in the 14–20-dBm level [118], [119], [120], [121], [122], [123], [124]. In turn, at these frequencies, these technologies tend to report PAE levels in the 20%–45% range [117], [118], [119], [120], [121], [122], [123], [124]. Finally, concerning the linearity of Si-based PAs, they tend to show output 1-dB compression points of around 1 to 1.5 dB below the P sat [117], [118], [119], [120], [121], [122], [123], [124].
On the other hand, SiGe technologies, with their HBT transistors, feature a P sat in the 17–23-dBm range [125], [126], [127], [128], [129] associated with PAE levels of around 30%–43%. Note that, compared to III-V technologies, the differences of the performance of PAs in pure Si and SiGe technologies is not sufficient to determine the choice of a given technology in this band. A clearer difference appears when operating frequencies move to greater ranges, where the performance of SiGe HBTs clearly surpasses that of Si-based MOSFETs.
The LNA is employed in satellite application front-end receivers to amplify the degraded RF signals captured by the antenna to the desired level. The LNA boosts the received signal power by adding minimal noise and distortion to mitigate the impact of noise added by the components of the RF receiver chain. Its effect is to improve the SNR, which is essential for the quality of the radio link. Design requirements combine the minimum NF with the high gain and pose severe challenges to the designer. The complexity of the design further increases if one considers the problems of impedance matching, low power consumption, linearity, and stability.
Figure 3 presents the NF versus the gain of current state-of-the-art GaAs [111], [112], GaN [111], [112], SiGe [131], [132], [133], [134], [135], [136], [137], [138], and Si [130], [139], [140], [141], [142], [143], [144], [145] LNAs in the Ka/Ku frequency bands. LNAs based on GaAs dominate the low-NF part of the plot, followed by GaN devices. Note that low-NF LNAs can also be built with SOI platforms [130]. High-gain LNAs are achievable with SiGe and GaN processes, as well. GaN LNAs ensure good RF performance under low dc power consumption, which is a fundamental requirement for space and ground segments [40]. The latter is often achieved by reducing the nominal bias point to operate GaN HEMTs in low-current-density areas.
Figure 3. The NF versus the gain of current state-of-the-art GaAs [111], [112], GaN [111], [112], SiGe [131], [132], [133], [134], [135], [136], [137], [138], and Si [130], [139], [140], [141], [142], [143], [144], [145] LNAs in the Ku/Ka frequency bands.
Concerning Si- and SiGe-based technologies, SiGe is more suited for the design of high-gain LNAs, while pure Si-based processes offer a better NF thanks to the SOI technology. LNAs with 30-dB-gain LNAs realized with a 0.13-µm BiCMOS process have been reported [131]. SiGe LNAs present an NF in the 2–3-dB range in the Ka-/Ku-bands, with gains that span the 15–30-dB range [131], [132], [133], [134], [135], [136], [137], [138]. Si-based LNAs present more modest gain levels in the 12–20-dB range but show NFs close to those of GaAs (i.e., nearly 1 dB) up to 3.5 dB [130], [139], [140], [141], [142], [143], [144], [145].
LNA performance is not limited only by the intrinsic gain and NF of a given transistor in a technology. The NF is largely impacted by the matching network required to adapt the input/output impedance of the transistor to 50 Ω, typically. Hence, the quality factor of passive devices in a certain technology will greatly limit the performance of an LNA. Moreover, other parameters should be considered. For instance, LNAs are known for being particularly sensitive to high-power RF inputs that can put them into breakdown mode. Among the considered technologies, GaN can withstand the greatest amounts of power without entering a breakdown regime, followed by GaAs and SiGe, in that order. Recently, there were reports of GaN LNAs surviving input power levels over 30 dBm in a continuous wave and nearly 50 dBm in pulse conditions. Moreover, GaN LNAs demonstrated high linearity with third-order output intermodulation points around 40 dBm [146]. Electrostatic discharge (ESD) events can also present a threat for most LNAs. For this reason, ESD must be carefully studied. Note that extra circuitry must be added to protect the LNA against ESD events, which can degrade the performance of the circuit. Hence, choosing a technology that is more resilient against ESD events may not only ease the design of an LNA but also dictate the choice between two technologies. For instance, SiGe LNAs have appeared to be more robust against this kind of event than GaAs LNAs [40].
The present review provided a general outline of the current technologies for satellite user terminals. Even though the technologies reported are common to GEO and LEO applications, the focus was on the former case. Starting from the antenna, the review covered the various components of a user terminal, including the antenna, beamforming network, and LNAs and PAs. Comparative tables were provided for the reader to get a quick overview of the available technologies. The presented material is suitable for researchers seeking an overview of the current state of the art in the field. It is the authors’ hope that the article will prove useful to researchers and engineers working in the field.
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Digital Object Identifier 10.1109/MMM.2022.3217961