Digitized Modem Architecture with Digital IF: Enabling Software-Defined Satellites and Earth Stations

In the fast-evolving realm of satellite communication, innovation is key to meeting the ever-increasing demands for bandwidth, flexibility, and efficient resource utilization. One of the most transformative advancements in this field is the adoption of digitized modem architecture with digital intermediate frequency (IF), which is paving the way for software-defined satellites and earth stations. This blog explores the profound impact of digitized modem architecture with digital IF on satellite communication, highlighting its benefits and potential applications.

Software Defined Satellites and Networks

Where conventional satellites were earlier tailored to comply with single mission requirements, satellite developers are gradually adapting the vision of software-defined satellites which can be reprogrammed and reconfigured, to allow a satellite to take up new applications and expand its performance. Instead of viewing a satellite as a monolithic piece of hardware and software, designed to perform a specific mission, one can see the same satellite as a platform capable of running multiple different missions (defined as software applications) on the same hardware platform.

This transformation is driven by the ongoing New Space revolution which has planned up to 50,000 active satellites to be in orbit over the next 10 years. All these satellites have complex and variegated sets of orbits and waveforms that satellite communication (SATCOM) networks need to support.  This drives the need for SATCOM operators to create flexible and adaptable networks capable of operating on a myriad of different waveforms, orbits, and constellations—while simultaneously maintaining service quality and profitability.

“Software-designed” is defined as any traditionally implemented hardware components replaced by software to configure a function dynamically and programmatically. This definition follows the same approach as other “software-defined” entities, such as “software-defined radio” transceivers that can be reconfigured for a variety of RF tasks or “software-defined networking” appliances that can support a wide range of telecommunications applications.

Digitization enabling Software-defined satellites and networks

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).

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.

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.

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.

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 a 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 filterings, such as antialiasing filters and channel selection filtering to reduce the burden of channelization on the digital hardware.

Digitized Modem Architecture with Digital IF

Traditional satellite communication systems rely on analog IF processing, which presents several limitations, including fixed functionality, complex hardware, and limited scalability. The advent of digitized modem architecture with digital IF has revolutionized this landscape, offering unprecedented flexibility and performance.

The transition to digitized modem architecture has significantly enhanced satellite communication, primarily through the adoption of digital intermediate frequency (IF) technology. This architecture integrates digital modems and RF front ends, also known as edge devices, using the digital IF interface. This IP-based transport protocol facilitates the communication of digital samples and their contexts across data networks, resulting in cost-effective and efficient management of SATCOM networks. This advancement is crucial for maintaining profitability and longevity in the rapidly evolving satellite communication landscape.

Understanding Digital IF

Intermediate Frequency (IF) traditionally involves an RF or 70 MHz analog signal. However, digital IF provides a digitized sample representation of that signal, enabling complete software processing. This digital representation allows for the transport of samples over much longer distances compared to traditional RF or baseband analog signals. A digital IF interface can operate over Ethernet, supporting both local area networks (LAN) and potentially wide area networks (WAN). By replacing a significant portion of hardware and FPGA firmware processing with software, digital IF streamlines the overall architecture.

Key Components of Digitized Modem Architecture:

1. Digital Intermediate Frequency (IF): Unlike analog IF, digital IF converts the signal to a digital format early in the signal chain, allowing for sophisticated digital processing.
2. Software-Defined Radio (SDR): SDR technology enables the modem to perform various signal processing tasks, such as modulation and demodulation, in software rather than hardware.
3. Field-Programmable Gate Arrays (FPGAs): FPGAs provide the reconfigurable hardware platform needed to implement SDR, offering high-speed processing and adaptability.

Advantages of Digital IF over Analog L-band IF

Analog L-band IF systems have notable disadvantages, including cable power loss and distortion from L-band switching systems, which degrade signal-to-noise ratio (SNR). Digital IF addresses these issues by replacing L-band switching equipment with more efficient IP-switched systems. Additionally, since edge devices (RF front ends) are not fixed to the modem, they can be positioned closer to antennas, minimizing L-band IF cable length, reducing transmission power loss, and maximizing SNR. Improved SNR leads to better signal quality and increased SATCOM network throughput.

Enhanced Distance and Data Integrity

Digital IF enables the transport of digitized samples over greater distances than traditional RF or baseband analog signals. This flexibility is crucial for distributed and remote operations. However, there are essential considerations regarding the frequency and resolution of the digitized samples to ensure reliable digital signal processing. For instance, processing a 5 Mbps telemetry downlink with 10 MHz of bandwidth might require 40Msamples/second at 12-bits each, necessitating dedicated network connections due to high throughput and network loading.

Signal Converters and Hardware Requirements

Despite the digitization, there remains a need for hardware that performs analog-to-digital signal conversion for received links and digital-to-analog conversion for transmitted links. Signal converters, which feature RF or IF signals on one side and Ethernet interfaces on the other, fulfill this role. These converters ensure seamless integration between the digital and analog domains.

Cost Efficiency and Network Flexibility

Digitized modem architecture with digital IF offers significant cost advantages. Unlike expensive analog transmission lines and distribution equipment, digital IF transmissions utilize Commercial-Off-the-Shelf (COTS) IP routers and switches, which generally have lower capital and operational costs. Network reconfiguration or migration is simplified, as operators can manage these changes by reassigning digital IF IP addresses or plugging in new digital modems into routers, eliminating the need to disconnect transmission cabling.

Waveform Agnostic Signal Converters

A true software modem incorporates waveform agnostic signal converters, which means they do not require knowledge of the modulation and demodulation types. To these converters, all signals are identical, and any waveform-specific processing is performed in software across the network. This capability allows waveform agnostic signal converters to be deployed in any satellite ground system, supporting both current and future satellite waveforms. This adaptability is essential for maintaining compatibility with evolving communication standards and ensuring future-proof network infrastructure.

The digitized modem architecture with digital IF represents a paradigm shift in satellite communication. By leveraging digital IF and software-defined technologies, this architecture enhances flexibility, scalability, and cost efficiency, making it a cornerstone of modern SATCOM networks. As satellite communication continues to evolve, the adoption of digitized modem architecture with digital IF will be instrumental in meeting the diverse and dynamic needs of global connectivity, ensuring robust and adaptable communication systems for the future.

Advantages of Digitized Modem Architecture

1. Enhanced Flexibility and Scalability: Digital IF and SDR technologies allow modems to be easily reconfigured for different communication standards and frequencies. This flexibility is crucial for adapting to varying satellite communication requirements without the need for extensive hardware changes.

2. Improved Signal Processing Capabilities: Digital signal processing (DSP) techniques offer superior performance compared to analog processing. Enhanced filtering, error correction, and modulation schemes can be implemented, resulting in higher data rates and improved signal quality.

3. Simplified Hardware and Reduced Costs: By consolidating multiple functions into software, digitized modem architecture reduces the need for specialized hardware components. This simplification leads to lower manufacturing and maintenance costs.

4. Remote Upgradability: Software-defined modems can be updated remotely, enabling the deployment of new features and improvements without physical access to the hardware. This capability is especially valuable for satellites in orbit and remote earth stations.

VITA 49.0 and Its Evolution

Innovations in data converter technology, digital signal processing (DSP) devices, system interconnects, processors, software, design tools, and packaging techniques have substantially enhanced the performance of software-defined radio (SDR) systems. These advancements have also reduced the size, weight, and power consumption of such systems. However, the rapid expansion of SDR applications led to the emergence of ad hoc, proprietary interfaces between system elements.

The VMEbus International Trade Association (VITA) plays a critical role in developing standards for real-time, embedded computing systems. One of its significant contributions is the VITA 49 series, particularly vital for digital intermediate frequency (Digital IF) applications. Digital IF technology transforms traditional analog signals into digital formats, enabling more efficient, high-performance signal processing and transport over IP networks. By establishing standardized protocols like VITA 49, VITA ensures interoperability between diverse system components, promotes innovation, and enhances the reliability and scalability of digital communication systems. These standards are crucial for advancing software-defined radios and other digital signal processing applications, making VITA an essential driver of progress in modern communications technology.

VITA 49.0: The First Standard

Approved as an ANSI standard in 2007, VITA 49.0 was the first official standard for the VITA Radio Transport (VRT) protocol. This standard initially defined receiver functions using the VRT IF Data and Context packets. Early adopters demonstrated its utility, leading system designers to seek an extension of its scope to cover more elements of SDR systems. However, VITA 49.0 had several limitations, including the lack of support for transmitters, control and status functions, and signals other than digital IF.

VITA 49.2: Addressing Limitations and Expanding Functionality

To overcome the limitations of VITA 49.0, VITA 49.2 was initiated, introducing new packet classes and expanding the standard’s capabilities.

Signal Data Packets

The original IF Data Packet was replaced with the Signal Data Packet, which supports digitized IF signals, baseband signals, broadband RF signals, and spectral data. Signal Data Packets are backward compatible with IF Data Packets, with new identifier bits specifying the data type.

Signal Data Packets enable bi-directional radio signals, allowing for the transmission of baseband, IF, and RF signals. Time stamps in these packets dictate transmit times, useful for generating precisely-timed radar pulses. Multi-static radar systems, which use one antenna for transmitting pulses and other remotely located antennas for capturing reflections, benefit from GPS synchronization and VRT coordination of transmit and receive signals.

Spectral survey systems can use Signal Data Packets to carry digitized spectral information from scanning receivers, delivering packets to analysts globally. These packets maintain comprehensive context information, including location, conditions, and precise time stamps.

VRT time stamps facilitate beamforming applications by comparing absolute time and phase differences between signals received from multiple antennas, aiding in calculating distance, location, speed, and heading of a transmitter. Similarly, multi-element diversity receivers can use VRT time stamps to create delays and phase shifts in each antenna signal path, optimizing receptivity in a specific direction.

Enhanced Context Packets

VITA 49.2 significantly enhances the original Context Packet, supporting more types of metadata for richer signal data information. Context Packets now allow a VRT resource to respond with a complete set of operational specifications, including:

• Frequency tuning range
• Bandwidth settings
• Range of programmable gain
• Antenna azimuth angle limits
• Range of transmit power levels

Additional characteristics provided by Context Packets include:

• Settling time for switching tuning frequency, bandwidth, or gain
• Angle slew rate for movable dish antennas
• Frequency accuracy and stability
• Time stamp and ephemeris accuracy
• Operating temperature range
• Tolerance limits for shock and vibration
• Effects of temperature drift and aging

A VITA 49.2 System Processor can theoretically connect to a new, unknown SDR resource, automatically discovering its capabilities, controlling and monitoring its operation, and successfully exchanging receive and transmit signals. In practice, Context Packets are most useful when developing new applications on existing platforms and responding to new threats or circumstances during deployed operations.

Command Packets

VITA 49.0’s lack of control over SDR resources was a significant shortcoming. VITA 49.2 addresses this by introducing Command Packets, enabling the VRT System Processor to deliver operating parameters to each element using standardized fields and formats similar to Context Packets. This consistency provides a unified control interface across a wide range of hardware, from antenna positioning systems to transmit power amplifiers.

Command Packets also support status and acknowledgment functions, allowing the VRT processor to verify the operational status of receivers and transmitters, ensuring the successful execution of control commands.

This comprehensive and complementary control/status protocol of VRT is essential for cognitive radio, adaptive spectral management, electronic countermeasures, and other critical applications.

The evolution from VITA 49.0 to VITA 49.2 reflects a significant enhancement in the standard’s scope and capabilities, addressing initial limitations and expanding support for a broader range of SDR system elements. VITA 49.2’s introduction of Signal Data Packets, enhanced Context Packets, and Command Packets provides a robust framework for advanced SDR applications, ensuring

Enabling Software-Defined Satellites and Earth Stations

The integration of digitized modem architecture with digital IF is a key enabler for software-defined satellites (SDS) and software-defined earth stations (SDES). These systems leverage the flexibility and programmability of SDR to adapt to changing operational requirements dynamically.

1. Software-Defined Satellites (SDS): SDS use SDR technology to perform signal processing tasks on board the satellite. This capability allows satellites to support multiple communication standards and frequencies, making them more versatile and efficient. SDS can adapt to different missions and applications, such as broadcasting, broadband internet, and secure communications, by reconfiguring their modems in real-time.

2. Software-Defined Earth Stations (SDES): SDES utilize digitized modems with digital IF to provide flexible and scalable ground segment solutions. These earth stations can quickly adapt to new satellite communication protocols, manage multiple satellite connections, and optimize bandwidth usage. The ability to reconfigure modems remotely also simplifies network management and reduces operational costs.

Waveform-Specific Applications

Many previous-generation modems combined FPGA firmware and software into monolithic configurations, where all features and functions were tightly integrated. This made it difficult to manage and required lengthy regression testing with each new release. True software-defined modems, however, employ separate software applications for each unique waveform, tailored to specific types or families of spacecraft. This approach allows customers to deploy only the waveform applications they need, updating them as necessary. Each application can be started and stopped independently for each contact, enhancing reliability, maturity, and manageability. Moreover, adding a new waveform-specific application does not affect the existing ones, providing a modular and flexible solution.

Reduced Life Cycle Costs

True software-defined modems significantly lower both the initial and ongoing costs associated with satellite telemetry, ranging, and commanding. By hosting the applications on commercial servers, initial costs are reduced compared to custom industrial computers used with traditional modems. Additionally, software applications can be easily migrated to new server platforms, eliminating the need to replace the entire modem system. This reduces vendor production and support costs, as well as the expenses for satellite operators, who only need to purchase specific waveform applications for their satellites.

Increased Network Agility and Capability

Network and terminal agility are increasingly important due to the rapidly evolving space layer. Agile networks and terminals enable seamless migration to new waveforms and constellations, simplify network resource reconfiguration, and modernize the deployment of new capabilities. In a digitized architecture, digital modems are decoupled from edge devices, facilitating easy network reconfiguration. With a standardized digital IF interface, digital modems can replace or back up each other for redundancy through simple IP network configurations.

Additionally, digital IF streams can be digitally duplicated, combined, and separated to provide new capabilities such as diversity gain, beamforming, and amplifier distortion correction. Reliable digital IF transport also allows digital modems to communicate with distant edge devices, enhancing network redundancy and leveraging cloud computing for additional capabilities.

Applications and Use Cases

1. Global Broadband Connectivity: Digitized modem architecture enables the deployment of high-capacity broadband services via satellite, addressing the digital divide in remote and underserved regions. Software-defined modems ensure efficient spectrum utilization and adaptive bandwidth allocation.

2. Secure Military Communications: The flexibility of SDR-based modems allows for the implementation of advanced encryption and anti-jamming techniques, ensuring secure and resilient communication links for military applications.

3. Disaster Recovery and Emergency Response: In disaster-stricken areas, rapidly deployable software-defined earth stations can provide critical communication links. The ability to reconfigure modems on-the-fly ensures reliable connectivity for emergency responders.

4. Internet of Things (IoT) and Machine-to-Machine (M2M) Communication: Digitized modems with digital IF support the growing demand for IoT and M2M communication via satellite. The flexibility to adapt to various IoT protocols and standards makes these modems ideal for a wide range of applications, from environmental monitoring to asset tracking.

Conclusion

The digitized modem architecture with digital IF represents a significant leap forward in satellite communication technology. By harnessing the power of software-defined radio and digital signal processing, this architecture enables the development of software-defined satellites and earth stations, offering unparalleled flexibility, scalability, and performance. As the satellite communication landscape continues to evolve, digitized modem architecture will play a pivotal role in meeting the diverse and dynamic needs of global connectivity. Whether it’s providing broadband internet to remote areas, ensuring secure military communications, or supporting IoT applications, digitized modems with digital IF are at the forefront of the next generation of satellite communication systems.