The Digitization of Satellite RF Systems

Written by Karl Anderson, republished from Microwave Journal
on September 17, 2021

The past several years have seen wide upheaval in the space communication sector, from satellite payloads to ground station technology. This renaissance of space communications has largely been a shift from the five-decade-long approach of highly dedicated satellite communication and ground station technology to increased digitization and virtualization. Ever increasing percentages of the space communication payload and ground station hardware is being converted from analog to digital that are more easily integrated with open standards for computing and data transfer. A key aspect of this transformation is the digitization of satellite RF systems to yield more compact, lower weight, more configurable, and lower cost.

Traditional Satellite RF Systems Overview

A traditional satellite communications payload consisted of the digital data and networking traffic being handled by external systems while the modem, transceiver, and RF Front-end (RFFE) handled analog communication signals from the digital controllers and the antenna system. Much of a satellite modem has long been replaced by digital hardware that processes the modulation and demodulation of communications signals at baseband frequency. Hence, more recent satellite communication systems use modems with Analog-to-digital Converters (ADCs) and Digital-to-analog Converters (DACs) that can operate to baseband frequencies, with the modulation, demodulation, and signal processing being handled in the digital plane by Application Specific Integrated Circuits (ASICs), General Purpose Processors (GPPs) or Field-programmable Gate Arrays (FPGAs).

Digitization of Satellite RF Systems Traditional Satellite Communications Payload

This led to the integration of digital hardware into the transmit and receive chain of satellite communications payload, but only to baseband frequencies. This left RFFE, Intermediate Frequency (IF), and baseband frequency (baseband) analog hardware remaining, even with the move from highly directional, discrete antenna to phased array antenna. This move merely required more RF, IF, and baseband hardware as well as digital controllers and amplitude/phase shifters in the RF section to handle the phase shifting for each antenna element or group.

Digitization of Satellite RF Systems 2 Simplified, traditional RF to baseband satellite communications heterodyne/superheterodyne receive signal chain

Satellite communication systems have typically been in the upper microwave and millimeter-wave range due to throughput and spectrum allocation considerations. With such high RF frequencies, a single frequency conversion stage has largely been untenable, as the limitations of mixers and Local Oscillators (LOs) lead to substantial signal degradation at higher frequency conversion ratios. Therefore, multiple frequency conversion stages have traditionally been necessary, with each frequency conversion stage requiring filters, LO, mixer, and possibly a gain stage to overcome the signal loss associated with each component within the signal chain.

Digitization of Satellite RF Systems 3 Simplified, traditional baseband to RF satellite communications heterodyne/superheterodyne transmit signal chain

Advancements In RF Digitization and Open Standards For Space Communications

Recent advances in direct digital synthesis, direct digital sampling, and digital upconversion/downconversion have led to increasing digitization of the satellite communication signal chain. An enabling factor for this digitization are higher frequency ADCs and DACs that can reach upper microwave and millimeter-wave frequencies and more powerful ASICs, GPPS, DPSs, and FPGAs that can handle the signal processing and data conversion requirements of modern satellite communications protocols have become available.

Digitization of Satellite RF Systems 4 Digitized Satellite Communications Payload

These trends are similar to much of the radar and terrestrial telecommunications industry, where the analog hardware for communications and sensing is being replaced with digital hardware as close to the antenna as possible. Part of this trend is also replacing traditional data transfer technologies with internet technologies, such as IP-based communications and other open communication standards. The root of many of these trends are the change in communications industries from being highly controlled and closed systems, to communications system operators selling their systems as a service. Hence, there is an accelerating push for greater ground station and satellite communications flexibility to be competitive, which also requires lower cost hardware that is highly configurable and is based on widely used industry standards.

Progress In Digitizing RF Systems For Satellite Applications

The result of the advancements in RF digitization is the replacement of the baseband and IF sections of the satellite communication signal chain, leaving only the RFFE and antenna system as analog. This digitization may also include the channelization hardware, and that process can now be done in the digital realm, no longer requiring analog multiplexers or digital switches. Namely, the low noise amplifiers, power amplifiers, circulators/switches, antenna, limiters, front-end filters, pre-amplifiers, and interconnect remain the only analog hardware in this chain for the latest satellites. With digitized modem and transceiver functions, the limiting performance factors for communications is now placed on the ADCs and DACs synthesizing and sampling the RF signals, as well as the RFFE presenting the most pristine signals to and from the antenna.

To further accommodate greater communication system flexibility, RF bandwidth has also increased. This means that wider bandwidth RF components and devices are now needed to pair with the wide bandwidth digital communications hardware. Moreover the new Digital Front-end (DFE) hardware presents its own new design challenges. For instance, the sample rate conversion (SRC) and channelization can have significant impact on the waveform with certain modulations requiring specific sampling rates. Moreover the DFE hardware also introduces its own distortions and noise, which must then be handled by the RF hardware to ensure high signal quality. These factors may result in additional filtering, such as antialiasing filters and channel selection filtering to reduce the burden of channelization on the digital hardware.

Conclusion

Satellite communication systems have now essentially become extremely high throughput software-defined radios (SDRs) coupled with Advanced Antenna Systems (AAS) that use new technologies to deliver the most flexible and highest performance communication link to as many consumers as possible. To meet these requirements a satellite communication payload that may have once consisted of a multitude of analog components, is now built with DACs, ADCS, FPGAs, ASICs, DSPs, and GPPs in highly integrated assemblies.

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