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DARPA’s Colosseum, world-unique wireless testbed shall catalyze the advent of autonomous, intelligent, and collaborative radio technology

In March 2016, DARPA officials launched the Spectrum Collaboration Challenge (SC2), an initiative designed to ensure that the exponentially growing number of military and civilian wireless devices will have full access to the increasingly crowded electromagnetic spectrum particularly  between 9 kHz and 275 GHz,  the range allocated by the Federal Communications Commission (FCC). These networks will be capable of intelligently optimizing the spectrum by collaborating with, and learning from, the other systems that occupy the spectrum with them.


The challenge is expected to both take advantage of recent significant progress in the fields of artificial intelligence and machine learning and also spur new developments in those research domains, with potential applications in other fields where collaborative decision-making is critical.


DARPA’s Colosseum is a path-breaking testbed that can emulate tens of thousands of possible interactions among hundreds of wireless communication devices—including cell phones, military radios, Internet-of-Things devices, and a litany of others—operating simultaneously in a square-kilometer expanse.


Traditional wireless communications systems are defined by a specification—a document that is the product of years of study and debate, and prescribes precisely how a radio system will work and how, if at all, it will get along with other radios. “We are asking SC2 competitors to devise fundamentally new radio systems that can learn from each other in real-time, making the need for arduous radio specifications obsolete,” Tilghman said.


“By contrast, SC2 is asking a group of radios that weren’t designed to work together to learn how to optimize spectrum capacity in real-time, and is relying on artificial intelligence to find and take advantage of ‘gaps’ and other opportunities to increase efficiency. You can’t satisfactorily learn how to solve this puzzle unless you address it at scale, and that’s why the Colosseum is such a critical part of the solution.”


That’s a significant technical challenge in itself, but it carries with it the challenging need to develop a testbed that can put those radios through a set of realistic paces.“Traditional radio systems are designed to ensure they operate only in pre-programmed ways, so it’s perfectly acceptable to test them in relatively small numbers and in simple environments,” Tilghman said.


DARPA’s Colosseum, a next-generation electronic emulator of the invisible electromagnetic world has been opened to competitors vying for $3.75 million in prize money for the Agency’s three-year Spectrum Collaboration Challenge (SC2).


“The Colosseum is the wireless research environment that we hope will catalyze the advent of autonomous, intelligent, and—most importantly, collaborative—radio technology, which will be essential as the population of devices linking wirelessly to each other and to the internet continues to grow exponentially,” said SC2 program manager Paul Tilghman.

DARPA’s Colosseum is a path-breaking testbed

Colosseum is the worlds largest RF emulator designed to support research and development of large-scale, next generation radio network technologies in a repeatable and highly configurable RF environment. It combines 128 Software Defined Radios with a massive digital channel emulator backed by an extensive FPGA routing fabric. Accessible as a cloud-based platform Colosseum also provides other resources to create a real-time, largescale radio employment such as traffic generation, timing, and GPS synthesis.


Though it resides in a mere 30-foot by 20-foot server room on the campus of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD, it is world-unique testbed to create radically new paradigms for using and managing access to the electromagnetic spectrum in both military and civilian domains.


Colosseum testbed is a 256-by-256-channel RF channel emulator, which means it can calculate and simulate in real-time more than 65,000 channel interactions among 256 wireless devices. Each simulated channel behaves as though it has a bandwidth (information content) of 100 MHz, which means the testbed supports 25.6 GHz of bandwidth in any instant.


For each match, we plug in radios so that they can “broadcast” radio-frequency signals straight into Colosseum. This test-bed has enough computing power to calculate how those signals will behave, according to a detailed mathematical model of a given environment. For example, within Colosseum are emulated walls, off which signals “bounce.” There are emulated rainstorms and ponds, within which signals are partly “absorbed.”


DARPA’s Colosseum amounts to an artificial, high-fidelity holodeck that can simulate invisible, fast-as-light communication signals traversing, ricocheting, echoing, and otherwise making their way from transmitters to receivers wherever they might be in a wide variety of simulated environments. Said Tilghman, “The Colosseum can make the radios believe they are operating in an open field, a dense city, a suburban shopping mall, a desert, or any other scenario you can dream up.”


Colosseum occupies 21 server racks, consumes 65 kilowatts, and requires roughly the same amount of cooling as 10 large homes.  There are 64 field-programmable gate arrays that handle the emulation by together performing more than 150 trillion floating-point operations (teraflops).


Moreover, each channel’s transmission and reception frequency is tunable between 10 MHz (as in broadcast FM radio) and 6 GHz (as in WiFi). The amount of digital RF data coursing through the Colosseum each second, more than 52 terabytes, exceeds the estimated amount of information contained in the entire print collection of the Library of Congress.


The emulation provides all the information necessary for the teams’ AIs to make appropriate decisions based on their observations during each emulated scenario. Faced with a cellphone jammer that is flooding a frequency with meaningless noise, for example, an AI might choose to change its frequency to one not affected by the jammer.


In short, said Tilghman, “the Colosseum is a magnificent electronic arena and just what we and the SC2 teams need for testing innovative, collaborative, intelligent radios against one another and, ultimately, to discover how to eke more capacity from the frustratingly finite spectrum.”


Among the toughest challenges of the testbed’s design has been the integration of leading-edge SDR technology with a cloud-computing-like environment, remotely accessible by multiple teams—30 of them for SC2’s first preliminary event this coming December—whose diverse AI and SDR frameworks need to be implemented simultaneously in the same testbed.


NI collaborated with DARPA to supply infrastructure for the world’s largest channel emulator

NI, collaborated with the Defense Advanced Research Projects Agency (DARPA) to supply core infrastructure for a path-breaking channel emulation testbed, called Colosseum, which will play a central role in the DARPA Spectrum Collaboration Challenge.NI will provide USRP software defined radios (SDRs) that support a wide variety of open source and proprietary tool flows including GNU Radio, RFNoC and LabVIEW system design software.


Engineers at APL assembled the Colosseum with 128, two-antenna, software defined radio (SDR) units built by National Instruments (NI). Emulating electromagnetic waves from these radios traversing the physical world is no small task. To tackle this, APL partnered with NI to put 64 field programmable gate arrays (FPGAs) to the task. The FPGAs enable the Colosseum to make the SDRs behave as though they are operating in any of countless environments, each designed like an electromagnetic movie set.


“Putting such a powerful wireless testbed into the cloud is a first-of-its-kind achievement in and of itself,” Tilghman said, “but the most exciting thing will be to see the rapid evolution of how AI solves wireless communications challenges once the competitors have this unique and powerful resource at their disposal.”


The Colosseum channel emulation testbed is based on the USRP X310 software defined radio and NI ATCA-3671 high-performance FPGA processing system, The FPGAs enable the Colosseum to make the SDRs behave as though they are operating in any of countless environments, each designed like an electromagnetic movie set.


“The DARPA Spectrum Collaboration Challenge brings together the greatest minds in cognitive radio and machine learning,” said Matt Ettus, distinguished engineer at NI and founder of Ettus Research. “The Colosseum testbed will allow those researchers to evaluate the performance of their designs to enable more reliable communications in many types of environments including the wireless conditions of a busy city neighborhood or in defense situations.”


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