Facebook is building a wireless Internet service that uses 60GHz WiGig technology to deliver “ubiquitous gigabit citywide coverage” in densely populated urban areas. Facebook said it is testing the technology at its headquarters in Menlo Park, California, and preparing a larger trial for San Jose.

“So far, we have demonstrated 1.05Gbps bidirectional (2.1 Gbps total throughput per distribution node) in P2P mode, up to 250 meters away,” the company said in an announcement yesterday. “This means up to 8.4Gbps of total traffic per installation point assuming 4 sectors, and we think this number can be as high as 12.8Gbps in the future." Facebook also says it will make the technology "open and interoperable" in unlicensed spectrum, just like Wi-Fi.

The project faces technological hurdles related to the use of extremely high frequency spectrum. WiGig technology using 60GHz frequencies is generally aimed at home use, as it is great for high-speed transfers between two devices in a single room but nearly impossible to use in multiple rooms because the airwaves are easily blocked by walls.

But Facebook’s new “Terragraph” project aims to overcome those difficulties with antennas that can route around tall buildings, the company said. Terragraph radios use commercial off-the-shelf components and are based on the WiGig standard but have special modifications implemented by Facebook.

“Given the limited range of the 60GHz signal, these nodes are placed across a city at 200-250 meter intervals,” Facebook project leaders wrote. “Designed to provide street level coverage, Terragraph implements a phase array antenna to retain the highly directional signal required for 60GHz, but makes it steerable to communicate over a wide area. Given the architecture of the network, Terragraph is able to route and steer around interference typically found in dense urban environments, such as tall buildings or internet congestion due to high user traffic.”

There’s a lot more spectrum available in 60GHz than in more widely used Wi-Fi bands, allowing greater throughput, they said.

“Up to 7GHz of bandwidth is available in the 60GHz band, and forward-thinking countries like the United States are seeking input to expand this to a total of 14GHz,” Facebook wrote.

Terragraph also borrows some technology Facebook developed to manage its data centers. This includes “IPv6-only nodes, an SDN-like cloud compute controller, and a new modular routing protocol for fast route convergence and failure detection.” Facebook also said it overhauled the MAC layer to make TCP/IP work better over wireless links.

Facebook said it selected San Jose for its upcoming trial because of its “mix of building types and neighborhoods” and city officials’ commitment to deploying new technologies. Facebook intends to build “large-scale trial networks in multiple markets around the world to demonstrate the potential value and efficiency of the technology.”

Most of Facebook’s efforts to expand Internet connectivity have focused on developing countries instead of the US. But the social-networking giant said it wants to boost Internet access in both developing countries that haven’t progressed beyond 2G speeds and developed countries where Wi-Fi and LTE infrastructure can’t “keep up with the exponential growth in the consumption of photos and video at higher and higher resolutions.”

The blog post also detailed a separate project called ARIES (Antenna Radio Integration for Efficiency in Spectrum), which uses large antenna arrays and a new transmission technology to boost the amount of data that can be transmitted in each slice of spectrum. Facebook said ARIES will also provide substantial improvements in energy efficiency and provided this picture of a prototype antenna array that has 96 transmit antennas:

ARIES is still in the proof-of-concept phase, while Terragraph is further along in development.