In DARPA’s Colosseum, the Combatants are RF Signals

Originally posted on: EETimes

DARPA’s testbed replicates RF-spectrum chaos, taking test and algorithm evaluation beyond basic link testing. You’ve read the history books and seen some of the many movies about the famed Roman Colosseum (the 2000 film “Gladiator” with Russell Crowe is just one of them): in that fabled amphitheater, completed around the year 70 A.D., the combatants included gladiators, slaves, prisoners, and animals.

Times have changed, and among the combatants in the world we now inhabit are the many RF signals fighting each other for slices of the electromagnetic spectrum. Testing how a product works in this RF-laden environment is a major challenge to which almost all design, test, and evaluation engineers can attest.

There are actually are two kinds of RF test. The first assesses if the device meets basic, point-to-point and network requirements, as well as regulatory EMI mandates for unwanted emissions. Those are the relatively easy tests.

The much-harder test scenario is to verify and then optimize performance of the unit in a spectrum swamped with interfering signals (many often stronger than the desired ones), poor SNR, multiple modulation schemes, and worse. It’s the equivalent of driving a car in a traffic zone populated by lots of crazy drivers piloting everything from bicycles and motorcycles to long-haul trucks and hefty construction vehicles, each determined to get where they are going, and get there first or nearly so. Even if the RF-related circuitry is working as intended, the complex algorithms which manage that hardware is severely challenged as it tries to both send out an optimum signal and also extract the desired receive signal.

That’s where DARPA — the Defense Advanced Research Projects Agency — is playing a role. To address this real-world RF test environment, their Colosseum installation is a next-generation emulator of RF sources, and lots of them. It is housed in a modest 20 × 30-foot (6 × 9 m) server room at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. Engineers at APL constructed it with 128 two-antenna, software-defined radio (SDR) units built by National Instruments. The system can emulate tens of thousands of possible interactions among hundreds of wireless communication devices, including cell phones, military radios, IoT devices, and more, all operating at the same time.

The DARPA press release gives some additional facts: “By the numbers, the 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.” Within the Colosseum environment, the RF emulation can be set to represent an open field, a dense city, a suburban shopping mall, a desert, and more.

However, the objective of Colosseum is not just to provide a diverse, fully controllable, RF-intense environment. It is also the testbed for DARPA’s three-year Spectrum Collaboration Challenge (SC2), which competitors will use to hopefully create significant advanced paradigms needed to use and access electromagnetic spectrum in both military and civilian domains; the winner gets $3.75 million in prize money (see more SC2 details are here). SC2 program manager Paul Tilghman noted that “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.” That’s certainly a fairly aggressive test and measurement objective.

How do you evaluate the real-world performance of your RF device? What do you do after the basic anechoic chamber test suite?—Bill Schweber is an engineer and technical writer who writes blogs for EE Times and EDN.com.