Digital Beamforming Trends For Satellite Applications

Written by Karl Anderson, republished from Microwave Journal
on September 27, 2021

The increasing number of low earth orbit (LEO) and medium earth orbit (MEO) satellite constellations and a shift toward dedicated ground stations to satellite communications as a service are driving advancements and a change of requirements for both satellite and ground terminal antennas.

Ground terminals are now being specified to flexibly track multiple satellites, while satellite antenna systems are being pushed to the limits to realize ambitious high throughput satellite (HTS) goals. A previously theoretical solution to these challenges has been the use of digital beamforming technologies to enable fully steerable, active antennas to enhance capacity, control, and flexibility.

The emergence and rapid development of active antenna systems for terrestrial communications has also spurred development of highly integrated and capable digital beamforming (DBF) solutions for satellite applications.

What is DBF?

Digital beamforming is a method of changing an antenna array’s radiation pattern using digital signal processing (DSP) techniques that don’t require additional analog hardware to manipulate the phase or amplitude of the signals fed to the individual antenna elements. In this way, the only RF hardware needed in the DBF antenna system is the transmit/receive (T/R) module (TRM) RF front-end (RFFE) and possibly frequency translation hardware, depending on the design.

Using DBF to control the antenna radiation pattern effectively creates a “smart” antenna, whose performance can be improved or adapted to new requirements via software, with all of the intrinsic benefits of beamforming antenna arrays. Moreover, a variety of automated techniques can be employed with DBF systems that can mitigate interference, prioritize beams and throughput to specific users, and can be reprogrammed remotely without the need to retrofit or modify the existing hardware.

DBF is an alternate strategy to analog beamforming or hybrid beamforming methods of controlling antenna arrays. Analog beamforming (ABF) is the traditional method of beamforming, which uses analog/RF phase shifters and amplitude adjustment, done with either variable attenuators or variable gain amplifiers. Hybrid beamforming uses some digital processing components in the signal chain, often some DSP functions on the baseband processing and frequency translation, which is then sent to analog phase shifters and amplitude adjustment hardware.

Digital Beamforming for Satellite Applications Fig 1. A typical analog RF signal chain for a single antenna element

DBF systems remove the need for analog hardware with the exception of RFFE hardware, mainly the low noise amplifiers (LNAs), power amplifiers (PAs), limiters/input protection, antenna filters, impedance matching, circulators/isolators/switches for duplexing, and interconnect. Moreover, advanced DSP techniques can be implemented with a DBF communications link that can “treat” impairments in the RF hardware and enhance performance. Such methods may even be able to extend the usable life or reduce maintenance of communication systems whose RF hardware is beginning to degrade in performance.

Digital Beamforming for Satellite Applications 2 Fig 2. A digital beamforming (DBF) signal chain for a single antenna element

An important justification for using DBF for antenna arrays on satellites is the ability to realize modular electronically-steerable multi-beam array (ESMA) antenna systems. A satellite ESMA essentially consists of networking infrastructure, DBF module, and an integrated TRM. The results of which is an extremely compact communications payload that can be rapidly reconfigured with software to deliver a wide variety of data services to any location within the antenna arrays reach.

Satellite Communications Trends Toward DBF

Remote areas, aircraft, and ships at sea are still underserved in the new Internet age. Communication and data systems for these applications are still limited to tens of megabits-per-second (Mbps) with systems that cost hundreds and thousands of dollars a month. This is compared to terrestrial data and communications systems that can deliver gigabit-per-second (Gbps) speeds and cost around one-hundred dollars a month or less.

Seeing an opportunity to reach a large portion of underserved applications, there are many companies that have moved to creating next-generation HTS systems in LEO and MEO orbits using massive satellite constellations. These satellites are necessarily designed with much more digital and software configurable hardware than typical satellites, are lower cost, more compact, and are deployed in the hundreds, and even thousands. To reach this economy of scale, automotive, industrial, aerospace, military hardware has been adapted to the requirements of these new satellite systems.

However, using non-space grade RF hardware isn’t viable in traditional space applications or mission critical satellites meant to operate for over two decades given the sensitivity of RF hardware to environmental conditions. To overcome the cost and volume limitations typical of space-grade, builders of massive satellite constellations are instead self-qualifying electronics, which is a costly and complex process to perform upfront. However, this approach can provide substantial benefits to return-on-investment (ROI) when the satellite numbers reach a large enough volume threshold.

Either way, RF hardware is often contained in heavy and expensive metallic enclosures, often hermetic, using specialized and ruggedized materials/designs to ensure the hardware can perform in the harsh environments in space/microgravity. Hence, there is a drive toward minimizing the amount of RF hardware in new satellite systems, replacing the RF hardware with digital hardware that is more compact and reprogrammable.

Another aspect is the predicted increase in demand for ground station hardware for consumers and devices. More ubiquitous Internet connectivity and the growing use of data and automated systems to sense, analyze, and control a variety of industrial, robotic, and machine systems, especially mobile systems, is likely to be extremely reliant on the burgeoning HTS satellite systems. This requires ground station hardware that can perform real-time tracking of satellites, even on-the-go.

Real-time satellite tracking for LEO and MEO satellite constellations is an extreme engineering challenge, especially when the goal is to minimize the cost of the user ground station hardware. Hence, DBF systems and antenna arrays for ground station hardware enable near instantaneous tracking without the need for large, expensive, and possibly unreliable mechanical tracking hardware with heavy parabolic antenna dishes.

Though the demands on ground station hardware generally aren’t as stringent as the space hardware, using DBF systems would still reduce the size, weight, and possibly even cost of ground station hardware and provide better user experience.

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