Engineers at Facebook announced they were able to demonstrate a record data rate of nearly 20 Gbps over 13 km with millimeter wave (MMW) technology, a feat that was accomplished using a set of custom-built components.
Using custom-built components is pretty standard for MMW experiments given that it’s still early days for the technology. Facebook’s team conducted their research in Southern California using only 105 watts of total direct current (DC) power consumption at the transmitter and receiver – or about as much as a single light bulb. The transmission used a bandwidth of 2 GHz, resulting in an overall spectral efficiency of 9.8 bits per second per Hertz, according to a blog by Facebook Connectivity Lab engineer Abhishek Tiwari.
While that's impressive, he said the technology is applicable to a number of Connectivity Lab’s solutions, including as a terrestrial backhaul network to support access solutions such as OpenCellular, the initiative designed to make it more cost-effective for operators to deploy networks in places where coverage is scarce. Another application for millimeter technology is as a backup to free space optical solutions such as the laser communications gimbal and optical detector in case of fog and clouds.
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But ultimately, the point-to-point MMW radio link is expected to serve as the connection between a ground station and Aquila, Facebook's solar-powered unmanned aerial vehicle (UAV). "However, we still have several connectivity and technical challenges to resolve before the technology is fully ready for deployment," Tiwari wrote.
Still, similar to Google’s ambitious Project Loon, Facebook is making progress with Aquila, which uses lightweight, solar-powered UAV beaming signals to Earth. The plan calls for using free space laser communications as a mechanism for communicating between aircraft in the fleet and E-band technology to beam connectivity from the airplane to receivers on the ground. Facebook shared results last summer of a successful test flight on June 28 in Arizona, where it stayed aloft more than three times longer than originally planned.
Acknowledging the technical challenges associated with millimeter wave, including power consumption, antennas and the effects from rain and fog, Tiwari said the first step in building and testing the system was to characterize the wireless channel between a mountaintop in Malibu, California, and a building rooftop in Woodland Hills, California, separated by a line-of-sight distance of 13.2 km.
“At each location, we installed commercially available E-band equipment that offered data rates from 100 Mbps to 3 Gbps based on link conditions, as well as a weather station,” he wrote. “This allowed us to develop a long-term characterization of the 13-km link under different weather conditions such as clear skies, clouds, fog, high winds and rain. We installed a similar setup at one of our data center locations to characterize losses under heavy rain scenarios. The link characterization helped the team pick the right components for the test.”
The components included balanced mixers, frequency multipliers, isolators, ortho-mode transducers, polarizers, low-noise amplifiers and power amplifiers. The RF components support more than twice the bandwidth of available off-the-shelf RF communication systems, which, combined with the high linearity, helped address the high data rate challenge, he said.
The Facebook team is currently flight-testing its first generation air-to-ground bidirectional link capable of 20 Gbps in each direction. The aerial payload is mounted on a Cessna aircraft and flown at altitudes up to 20,000 feet.
The next generation air-to-ground communication system capable of supporting 40 Gbps each on uplink and downlink between an aircraft and a ground station will be flight-tested in early 2017. “We will continue to push the limits of wireless capacity over long ranges while staying within the tough size, weight and power constraints of Aquila communication payloads,” Tiwari wrote.
For a long time, millimeter wave frequencies were considered ill-suited for mobile applications, but efforts by the likes of NYU Wireless and the University of Texas at Austin changed perceptions. Ted Rappaport, an NYU Tandon professor of electrical engineering and founding director of NYU Wireless, and his students have been conducting pioneering research in MMW technology for years.
Last summer, Rappaport and his team conducted a measurement campaign at his mountain home in Riner, Virginia, a rural town in the southwestern part of the state, using the 73 GHz mmWave frequency band. They discovered that for less than 1 watt of power and using 27 dBi antennas, they were able to achieve a reasonable signal strength. Their work showed that getting millimeter wave technology to work in rural areas might not be as daunting a task as it once appeared.