New open standards drive military software radio architectures
Since the 1994 Perry directive to use COTS [commercial off-the shelf] components, virtually all defense and intelligence organizations have been seeking standards-based solutions for electronic equipment and systems. However, even without that mandate, government customers have come to recognize the commonsense benefits of open standards to take advantage of the latest technology, shorten procurement cycles, and foster competitive pricing. Instead of simply trying to comply with procurement policies, these large government organizations are now actively pursuing and promoting new standards to align with current and future mission requirements. More than simply new standards, these initiatives present a new paradigm of system architectures that can efficiently evolve to accommodate new threats and changing requirements, without starting over from scratch each time.
The Open Group consortium, formed in 1996, promotes the development of vendor-neutral, open technology standards for successful achievement of business objectives. One recent initiative of The Open Group is the Future Airborne Capability Environment (FACE) Consortium, which tasks members from both industry and government to define open standards for avionics systems across all military services. The major goals of FACE: improved interoperability across common components, portability across different deployed platforms, consistent data exchange formats, and a common software environment.
Not only does FACE address the technical approaches and practices, it also promotes standardized methods and formats for the government to issue requirement documents and for vendors to respond. This approach helps ensure the speed and completeness of procurement interactions and promotes innovation and competition within the avionics industry.
Another Open Group initiative spun off from the FACE consortium is the Sensor Open System Architecture (SOSA) effort, which focuses attention on C4ISR [command, control, communications, computers, intelligence, surveillance, and reconnaissance], radar, EW [electronic warfare], electro-optical fusion, and communications systems. Sensors are extremely critical to these systems, often representing some of their toughest technical challenges. The goal of SOSA is development of open standard specifications for cross-service applications to reduce costs, encourage industry competition, and speed up delivery of systems to exploit new signals and respond to new threats.
Two of the most common themes of these initiatives are sharing signals across components within a platform for different functions and sharing data across different platforms. Since all software radio signals in modern equipment are eventually digitized, if equipment digitizes as close to the antenna as possible, sharing can be accomplished through digital links and distribution networks. In 2006, the VITA 49 working group began working on the VRT/VITA Radio Transport protocol to define a standardized format for delivering digitized radio signals.
Figure 1a shows the typical topology of a traditional multiband software radio system, often referred to as “stovepipe” architecture. The red lines show analog signal paths consisting of coaxial cables, connectors, distribution amplifiers, and switches. Each path satisfied the specific signal routing requirements of a particular installation, analogous to fitting a series of different sections of stovepipe between a wood stove and the chimney.
Multiple analog-to-digital (A/D) and digital-to-analog (D/A) converter blocks provided the analog/digital interface between analog intermediate frequency (IF) signals and digital IF samples. Even after digitizing, more stovepiping distributed the digital IF signals to connect the various signal-processing blocks required, as shown in green. And each of these digital links might require unique signal levels, data rates, bit widths, and protocols.
Figure 1b presents the system concept for VRT. The critical RF analog connections to the antennas remain essentially the same, but the transceivers now use the analog RF signal conditioning, translation to IF, and the A/D and D/A functions. These transceivers now send and receive digitized IF signals shown in blue lines as VRT Digital IF links, replacing most of the analog signal distribution shown in Figure 1a. It becomes immediately evident that the new VRT topology is far simpler and much more flexible.
Putting the initiatives to work
The U.S. Army Materiel Command Communications-Electronics Research, Development and Engineering Center (CERDEC) organization is responsible for coordinating new technologies and development efforts for C4ISR and EW systems for electronics systems found in the diverse range of Army vehicles. With many applications having limited space for installation of new equipment, a major imperative became defining new architectures to consolidate existing functions while easing the path for adding future capabilities.
Recognizing the benefits of the new software radio initiatives, CERDEC launched a major initiative of its own, called VICTORY [Vehicular Integration for C4ISR/EW Interoperability]. Based on a network-centric topology to take advantage of high-speed data bus technology (Figure 2), VICTORY dramatically redefines a functional system from being a self-contained dedicated box to an aggregation of shared resources to achieve equivalent or superior operational performance. The VPX standard was selected for VICTORY embedded system components due to its widespread adoption and strong vendor community. VPX also demonstrates a proven track record of rapid evolution to embrace new technology.
In line with VITA 49 VRT, instead of dedicating one antenna to a tactical communications channel, and a second one to a signals-intelligence system, digitized signals from a single antenna can be delivered across network connections to multiple subsystems, each supporting a different function. Therefore, essential cornerstones of VICTORY are not only the reliance on digital networks, but also real-time DSP algorithms running on reconfigurable field-programmable gate arrays (FPGAs) and software programmable processing elements. This sharing of common resources for multiple tasks minimizes redundant hardware and reduces size, weight, and power (SWaP), thereby satisfying another major design objective.
In the first incarnation of VICTORY, analog RF and IF signals from the radio heads are connected via cables through an RF distribution unit to VPX software radio boards containing A/D and D/A converters and FPGAs for front-end processing. In line with the diagram in figure 1b, digital IF signals are then distributed to VPX DSP boards for real-time processing for various functions required by the system. The shared processing unit performs additional processing and supports the display, control and status functions for the operator interface. The VICTORY data bus handles the control and data traffic among these subsystems.
MORA boosts VICTORY performance levels
Following the successful adoption and acceptance of VICTORY, it became apparent that its GbE data bus fell short of handling increasing traffic between subsystems because of wider signal bandwidths for newer data, voice, and video signals. CERDEC addressed this problem by defining the Modular Open Radio Frequency Architecture (MORA). It adds a new MORA high-speed bus, initially proposed to use a 10 GbE switched network.
Figure 3 shows not only the MORA bus, but a major new strategy and benefit it brings: The A/D and D/A converters previously located on VPX modules can now be shifted to remote data-acquisition modules located at the radio heads. Digital IF signals to and from these data converters connect directly through the GbE network links to VPX processing boards, eliminating the need for analog RF cables and connections to the RF distribution unit.
In the ultimate MORA architecture, both the RF distribution unit and the RF cable bus can be eliminated, as analog RF/IF signals are replaced by digital RF/IF signals across the MORA bus. This migration can evolve over time, as budgets and new technology permit, while maintaining the fundamental principles of the VICTORY/MORA architecture.
Other initiatives within DoD including the U.S. Navy Hardware Open Systems Technology (HOST) program, the U.S. Army C4ISR/EW Modular Open Suite of Standards (CMOSS), and others all promote similar architectural objectives and strategies. With so many credible and consistent directives, both suppliers and government customers have a clear mandate to make them happen.
New technologies and standards for software radio
Large markets for business and personal computing, image processing, wireless networks, optical networks, storage servers, and 4G wireless infrastructures fuel innovations and reduce costs through economy of scale. As always, many of these new components and technologies are suitable for software radio embedded systems for military use, but the best path is the definition of open industry standards that define implementation details. This helps ensure compatibility across vendors, products, and upgrades. Listed below are various devices, technologies, and standards:
- High-speed A/D and D/A converters from Texas Instruments, Analog Devices, and others with sample rates of 5 GS/sec and higher to handle wideband digital software radio signals close to the antenna.
- New FPGAs such Xilinx UltraScale and UltraScale+ devices with more than a million logic cells, more than 5,000 DSP engines, and high-speed gigabit serial system and network interfaces.
- JESD204B gigabit serial device interfaces to connect the high-speed data converters to FPGAs, saving significant board real estate.
- VITA 57.4 FMC+ mezzanine specification, which boosts the 10 gigabit serial lanes in the original FMC spec to 24 or 32 lanes using a new high serial pin count (HSPC) connector and supporting additional JES204B links to high-speed data converters.
- VITA 49.2 VRT Radio Transport Protocol extends the original specification by adding support for transmit signals, control and status of software radio equipment, multichannel synchronization for multi-element phased array systems, precision time stamping of received signals, and precision timing of outgoing transmit signals.
- New VITA 66.x optical backplane I/O configurations for multiple optical connector styles, plus an increased number of lanes to support high-speed data links between boards and between chassis, even over long distances.
- New VITA 67.x coaxial RF backplane I/O configurations for up to 12 signals with bandwidths up to 40 GHz simplify analog signal wiring for software radio systems.
- New release of VITA 65.0 and 65.1 OpenVPX specifications separate the base specifications and the numerous profiles into two documents, making it easier to access and understand compliance details. Many new profiles were added including apertures (openings) in the backplane to flexibly support a wide range of optical and RF I/O applications using VITA 66 and 67. A 100 MHz backplane reference clock and a backplane radial clock distribution specification supports synchronization across slots.
Collectively, these achievements offer a wealth of resources to help implement the many initiatives presented earlier.
Putting it all together
To meet the requirements of the MORA VPX software radio modules, the Model 5984 3U VPX Kintex UltraScale FMC+ carrier (Figure 4) leverages many of the new devices and standards listed above. At the front end, an FMC+ module contains the appropriate JESD204B high-speed data converters, capable of sample rates of 3 GS/sec and higher to support signal bandwidths greater than 1 GHz. The JES204B links connect to the FPGA through the new VITA 57.4 FMC+ HSPC connector.
For the MORA system architecture, the Model 5984 can play two different roles. First, it can be installed in the VPX chassis and connect to all four of the power and signal buses shown. In this case, the VITA 66.4 optical connections to the MORA High Speed Bus can join boards within the chassis or connect through external 10 GbE links to other external devices. RF signals connect though the RF Distribution Unit.
But the 5984 is an equally appropriate candidate for the remote data-acquisition modules at the radio heads. Here, the RF signals connect directly to the IF or RF ports of the radio heads. An on-board GPS receiver can supply a 10 MHz frequency reference to the sample clock synthesizer in the Clock and Timing section. It can also deliver a 1PPS signal for the precision time stamping and triggering required by the VITA 49 engine. Control and status flows over the VICTORY GbE bus and digital IF and RF signals flow across the MORA 10 GbE optical bus.
Next up …
Increasingly, government procurement opportunities are incorporating references to these new standards and initiatives. Working closely together, industry vendors, standards organizations, and government customers have demonstrated a proven commitment to advance the technology and performance levels of software radio systems for defense and intelligence applications. Coupled with improved SWaP, lower life cycle costs, and faster deployments, clear evidence of these many undisputed benefits will ensure adoption, continued funding, and active participation in these highly worthwhile initiatives.