Emerging trends in satellite communications: High throughput satellites in LEO, MEO, and GEO
In recent years there has been a significant downtrend in the number of GEO satellite orders dropping from between 20-25 on average annually to just 17 in 2016. With nearly half of the operational satellites dedicated to commercial (35%) or military (14%) communications and over 80% of satellite services for consumer applications (Satellite TV, Satellite Radio, and Satellite broadband), there is an emerging market for high-speed and low-latency satellite communications with Ka-band technology due to the available bandwidth. Current communications satellites generally have over a 15 year mission lifetime, in that time several scenarios can occur that require an adjustment in the operational requirements of the payload including changing business and political landscapes, new technologies and applications. Flexible payloads that can reconfigure its frequencies, coverage, and power allocation pose a solution to the rapidly evolving business, political, and technological environment.
The demand for more bandwidth will only increase with Multiple Radio Access Technology (Multi-RAT) planning for 5G that will rely on various heterogeneous networks for over 99% availability such as WiGig, 4G, 100G Ethernet, and satellite networks. High Throughput Satellites (HTS) with nearly 20 times the throughput of fixed service satellites (FSS), present desirable alternative for backhaul with a significant drop in cost-per-bit. This further precipitates the need for highly efficient transmitters with solid-state power amplifiers (SSPA), highly sensitive receivers, and reconfigurable phased array antennas for flexibility.
Flexible payloads
While the SATCOM industry’s general reluctance to embrace new cutting-edge technology over much more established and proven equipment effectively mitigates the risk involved in building and deploying satellites with a 15 year mission lifetime, there is a shift towards technologies that allow for greater flexibility in manufacturing, designing, and launching satellites. The launch of the Highly Adaptable Satellite (HYLAS-1) in 2010 helped to establish the viability of flexible payload technology shifting the payload design challenge from compliance with multiple access, connectivity, and speed requirements to ‘transparent’ payload technologies [3].
Three capabilities are necessary for a reduced cost-per-bit:
- Flexible coverage;
- Flexible power allocation to beams;
- Flexible spectrum allocation to beams.
Flexible coverage
Active electronically scanned arrays (AESA) such as phased-array antennas have already shown a high level of utility in X-band radar [4] and more recently in millimeter-wave 5G and WiGig applications. The shift towards leveraging higher frequency bands (Ku- and Ka-) transponders for uplink and downlink also comes with the benefit of smaller components with higher integration as microwave circuits become smaller at high frequencies. While the cost and power consumption of phased array antennas is high compared to a passive electronically scanned array (PESA) or mechanically scanned antennas, they have a high degree of reconfigurability. The array of radiating elements that individually connect with transmit/receive (T/R) modules containing phase shifters to constructively/destructively add phase in the desired direction yields highly adaptable and directional beams. Furthermore, the size of the antenna elements only become smaller up to the Ka-band allowing for more versatile designs that can fit into application specific integrated circuits (ASIC) – a desirable option for both ground equipment and satellites for the increasing utilization of CubeSats.
According to Northern Sky Research (NSR), flat panel antennas (FPA) are poised reach sales of $9.1 billion by 2026. These ‘smart’ FPAs are not limited to phased array antennas, for instance, switched beam arrays (often used in 77 GHz automotive applications) and PESAs allow for a level of beam adaptability without the power consumption that comes with hundreds of T/R modules (each with power hungry phase-shifters, power amplifiers, low noise amplifiers, etc.). NewSpace companies aiming to serve high broadband, low-latency demands globally through LEO and MEO constellations will require both satellites and ground terminals with high beam agility. OneWeb is one such company looking to launch an initial LEO constellation of 648 satellites with an ambitious goal of providing coverage to over 5 million ground terminals [5], [6]. Boeing, a company known for its capabilities in manufacturing GEO satellites, is considering launching its own LEO broadband constellation [8]. Additionally, SpaceX filed an FCC request last year to launch a 4.425 satellite constellation, providing global internet coverage [19]. The transition from fixed satellite services (FSS) in geostationary orbits (GEO) to high throughput mobile satellite services (MSS) in LEO requires seamless handoff between satellites with high beam agility. This also applies to the integrity of the communication link with the ground terminal where there is potential for interference as the Ku-band is increasingly popular for satellite downlink.
Flexible bandwidth and power allocation
Similar to the flexibility software-defined radio (SDR) platforms have granted Wi-Fi, 4G, 5G, and WiGig research and development, software defined payloads are a significant evolutionary step towards flexibility in satellites. Smart antennas used in concert with software-defined platforms containing analog and digital conversion, as well as digital signal processing (DSP) offer a common RF front-end footprint where the hardware blocks are universal while the complex software can vary. This is a trend is that not limited to the commercial wireless realm or even limited to industrial internet for things (IIoT). For instance, the Space Telecommunications Radio System (STRS) project is aimed to define an open architecture for NASA’s space and ground SDRs [9]. The goal is to provide a general framework that abstracts the applications layer from the physical layer, allowing for more agile reuse of waveforms and services across SDR platforms. Highly custom, vendor specific designs where the applications are highly dependent on hardware is a technological trend fading into obsolescence. The ability to update capabilities with a simple firmware upgrade can significantly extend the lifespan of the satellite; operators are no longer stuck with a fixed coverage area and fixed bandwidth.
Traditionally, satellites are fitted with a number of transponders with fixed bandwidths that are not finely adjustable. Should a customer need more than the purchased transponder bandwidth, they are limited to buying another bandwidth segment while customers that require less, leave the unused excess. Digital payloads instead simulate the analog transponders and can either repeat received signals from ground stations without any modifications, or, ‘regenerate’ uplinked signals through demodulation, decoding, switching, encoding, and modulation. This way, flexible coverage can be established by finely dividing, controlling, and monitoring bandwidth and power allocation. This allows for more agile reuse of spectrum and subsequently drops the price on this valued commodity. Particularly for regenerative payloads, error detection and correction is performed on demodulated data with an On-Board processor allowing for this technology to generally have better link performance than their transparent transponder counterparts [10]. Digital transparent processors (DTP) can divide each incoming channel into subchannels of variable width (a few hundred kHz to a few MHz) without modifying the form of the received signals.
While both transparent and regenerative technologies have primary functions of flexible channelization, frequency conversion, and routing, they form a stepping stone for leveraging advanced digital beamforming (DBF) and beam hopping techniques for multibeam satellites. The benefits for using DBF techniques with smart antennas allows the satellite to allocate more power/bandwidth to beams where traffic demand is high or even illuminate several beams for a predetermined amount of time based on traffic demand [11]. Digital payload technology has also led to advancements in payload testing where Boeing recently patented a Built-in test with all its components constructed within the system under test, allowing digital payloads to loop back certain signals and test themselves limiting the use of external testing equipment and shortening delivery schedules [12].
High throughput satellites in GEO
Currently bandwidth-heavy video streaming for mobility solutions (connecting airplanes, ships, and vehicles) accounts for more than 50% of the satellite telecommunications industry. This is set to change in the coming decade with HTS dropping the price per satellite transponder at an average rate of 2 to 3 percent per year since 2010 [13]. As shown in Figure 2, Northern Sky Research analysts see a plateau in the demand for FSS video while HTS is predicted to rapidly increase with a revenue of approximately $7 billion by 2015.
HTS generally leverage two major advantages over traditional satellite technologies:
- The use of higher Ka-band transponders
- Extensive frequency reuse through the spot beam architecture
Instead of using a singular beam to cover as many users as possible, multiple small beams (spot beams) are implemented such that there is large amounts of frequency reuse – this can be accomplished by altering signal frequencies and polarization. The technology has made significant progress since the first HTS was launched in 2004. For instance, communications company ViaSat have thus far launched four satellites including the Anik-F2 (2004), WildBlue-1 (2006), ViaSat-1 (2011), and ViaSat-2 (2017) with each having a total throughput capacity of 2 Gbps, 7 Gbps, 130 Gbps, and 300 Gbps respectively. ViaSat-3, expecting to be launched in 2019/2020 will have over a 1000 Gbps. According to ViaSat, this exponential increase is accomplished through optimal spectrum reuse in every possible way in order to obtain the maximum bandwidth from the satellite [15]. This includes increasing the number of ground gateways where ViaSat-2 will have twice the number of gateways than ViaSat-1 and hundreds more (10x more than ViaSat-2) installed for ViaSat-3 to accomplish the terabit-per-second throughput. Technological advances in ground equipment allows for this to be comparable in price since the antennas for ViaSat-1 measure 7-meters across while the ViaSat-2 antennas are slightly over 4-meters. The ViaSat-2 gateway will cost less than half of the ViaSat-1 gateway. Finally, the ViaSat-3 gateway antennas are expected to be less than 2 meters across [16].
High throughput satellites in LEO
While large geostationary orbiting HTS have high utility in the decades to come over FSS, smaller high-throughput low-earth orbiting satellites have less upfront cost and, when used in a constellation, can allow for lower latency connectivity. Small LEO satellite constellations have the potential to be built on a production line, driving down the cost per satellite and upfront capital expenditures (CAPEX). Furthermore, the price of launching smallsats has dropped significantly in the past decade and will in the decades to come with ubiquity of reusable rockets (Figure 3).
Even though the rise in CubeSat and smallsat functionality is some of the most talked about developments in the space industry, there are some major hurdles to overcome in order for this technology to offer seamless connectivity. In order to translate the successful imaging smallsat to a communications satellite, significant power density requirements must be accomplished. Communications applications generally require more power than imaging applications due to the need for a higher altitude in order to close a link with two-way ground terminals; this calls for larger battery banks and solar panels [18]. Previous LEO constellations launched in the early 2000s such as Iridium, Globalstar, Skybridge, and Teledesic failed and went into bankruptcy. While thousands of LEO satellites would provide coverage for the entire globe, it can be seen as inefficient considering the amount of time these satellites spend at the poles and relatively deserted areas. Moreover, the technology would require increasingly complex ground terminals with seamless handoff between several satellites – a potential application for phased array antennas. Although smart antenna technology is being developed in parallel due its utility in a plethora of applications, applying this towards satellite technology with high service availability and reliability may take some time due to the need to be used in a wide range of elevations.
Less is more
All trends for satellite technology point toward cutting down costs and optimizing efficiency. The use of all-electric propulsion systems significantly cut down the launch weight of a satellite. The Eutelsat-172B, a HTS built by Airbus, recently broke the record for the fastest satellite in electric orbit raising (EOR). Weighing in at only 3,550 kilograms, the Eutelsat-172B would have weight more than 6,000 kilograms at launch with traditional chemical propulsion [17]. Reuseable rockets further reduce the cost of deploying satellites and may account for the majority of satellite launches in the future. Supplying the ever-increasing data demand – whether it be through large HTS in GEO, or, from smaller, more affordable CubeSats in LEO/MEO – calls for highest use of resources in an extremely power-constrained environment. Advancements in microwave ICs such as Gallium Nitride (GaN) power amplifiers (PA) with low power-added efficiencies and extremely low noise amplifiers (LNA) with GaAs pHEMT/mHEMT technology, allow for high power densities and highly sensitive receivers. Geosynchronous satellites have the benefit of long spacecraft lifetimes without the need to mass produce entire spacecraft systems while low-earth orbiting constellations with CubeSats/smallsats can offer low-latency communications globally.
References
1. https://www.sia.org/wp-content/uploads/2017/07/SIA-SSIR-2017.pdf
2. https://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/Hylas/Generic_Flexible_Payload_technology
3. P. Angeletti, R. D. Gaudenzi, M. Lisi, I. Introduction, S. Division, and C. Scientist, “From Bent Pipes to Software Defined Payloads: Evolution and Trends of Satellite Communications Systems,” 2008.
4. https://www.microwavejournal.com/articles/526-phased-arrays-and-radars-past-present-and-future
5. https://techcrunch.com/2016/04/19/oneweb-will-mass-produce-historic-number-of-satellites-with-new-florida-factory
6. https://interactive.satellitetoday.com/via/may-june-2017/phased-array-antennas-can-they-deliver
7. https://www.satellitetoday.com/technology/2016/04/28/telesat-shares-details-on-leo-constellation-expectations
8. https://www.satellitetoday.com/newspace/2016/09/20/boeing-open-partnerships-leo-broadband-constellation
9. https://strs.grc.nasa.gov
10. https://www.google.com/patents/US20040185775
11. J. Anzalchi et al., “Beam hopping in multi-beam broadband satellite systems: System simulation and performance comparison with non-hopped systems,” 2010 5th Advanced Satellite Multimedia Systems Conference and the 11th Signal Processing for Space Communications Workshop, Cagliari, 2010, pp. 248-255.
12. https://www.google.com/patents/US9720042
13. https://www.spacesymposium.org/sites/default/files/downloads/Belle_Carolyn_The_Industry_High_Tech_At_Last.pdf
14. https://interactive.satellitetoday.com/via/march-2017/5g-the-era-of-convergence
15. https://www.viasat.com/sites/default/files/media/documents/tech_overview_high_cap_sat.pdf
16. https://spacenews.com/viasat-plans-massive-ground-network-of-smaller-gateways-for-viasat-2-and-viasat-3-satellites
17. https://aviationweek.com/space/dawn-all-electric-satellite
18. https://interactive.satellitetoday.com/leo-hts-once-again-a-distraction
19. https://time.com/4638470/spacex-internet-elon-musk
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