WOODLAND HILLS and MALIBU, CALIF.—The radio frequency folks at Facebook say they’ve achieved a data rate of 20 Gbps over millimeter-wave technology at a distance of more than eight miles. According to Abhishek Tiwari of Facebook Connectivity Labs, the RF team recently hit the 20 Gbps mark over 2 GHz of bandwidth using “custom-built components” and 105 watts of DC power total at the transmitter and receiver.
“To put this in perspective, our demonstrated capacity is enough data to stream almost 1,000 ultra-high-definition videos at the same time,” Tiwari wrote in a Facebook Engineering Blog entry posted Thursday.
MMW technology, designed for use in the extremely high frequency range of 30 to 300 GHz, is the next frontier of broadband and cellular networks. (See, “FCC Opens Higher Frequencies to Phone Companies,” July 14, 2016.) FB is experimenting with millimeter-wave technology “to provide connectivity in areas without traditional infrastructure and reliable power sources,” Tiwari said.
The 20 Gbps data rate was achieved over a millimeter-wave (MMW) link between the rooftop of FB’s lab in Woodland Hills to a Malibu mountaintop 8.2 line-of-sight miles (13.2 kilometers) away. Each location was equipped with E-band gear supporting data rates from 100 Mbps to 3 Gbps, depending on weather conditions.
System equipment included balanced mixers, frequency multipliers, isolators; custom-built amplifiers with “10 times the transmit power” of an E-band commercial-off-the-shelf amplifier, as well as custom-made polarizers, transducers and RF components said to “have more than twice the bandwidth of available off-the-shelf RF communications systems.”
In an effort to keep power consumption in check, the team developed “post-amplification spectrum multiplexing,” using many, smaller, more efficient amplifiers versus one large, low-efficiency version. A two-foot parabolic antenna was used at the transmit end and a four-footer was used at the receive site. The 3 dB beam width of the four-footer was around 0.2 degrees, requiring accuracy within 0.07 degrees to achieve 20 Gbps.
“This is equivalent to a baseball pitcher aiming for a strike zone smaller than the size of a quarter,” Tiwari said.
Calibration was done using the sun, yielding an algorithm that will support installations around the world. Conical scanning was further employed for high-precision aiming.
Ultimately, FB intends to use MMW links to connect a fleet of solar-powered drones designed to provide broadband coverage over a “60-mile-wide area on the ground,” at data rates up more than 30 Gbps.
The proposition requires overcoming significant technological challenges, Tiwari said. Among them, the inverse power-efficiency ratio of MMW amplifiers, the nonaerodynamic nature of parabolic antennas and atmospheric moisture.
“Specifically, rain and humidity could result in very high attenuation for terrestrial MMW links and could severely compromise their availability,” he said.
FB is currently flight-testing it first-generation air-to-ground 20 Gbps link on a Cessna flying at up to 20,000 feet. The next generation of FB’s MMW link will support 40 Gbps and is on deck for flight-testing in early 2017.
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