Facebook achieves 20Gbps data rate over radio spectrum
Thu 10 Nov 2016
Facebook’s Connectivity Lab has announced that it has achieved data transmission rates of a record-breaking 20Gbps over the millimetre-wave (MMW) section of the radio spectrum; however, the transceiving stations need to be incredibly tightly calibrated to each other, with the team describing the margin for error as equivalent to ‘a baseball pitcher aiming for a strike zone the size of a quarter’.
The announcement on Facebook’s code blog records the ground-breaking transmission as having taken place between two sites in California, one in Malibu and another at Woodland Hills, a distance of 13km (8+ miles).
The equipment used customised components, drawing 105 watts of DC current at both ends, utilising a bandwidth of 2 Ghz, and arriving at a spectral efficiency of 9.8 bits per second per Herz.
‘To put this in perspective,’ reads the post, ‘our demonstrated capacity is enough data to stream almost 1,000 ultra-high-definition videos at the same time.’
The MMW portion of the radio spectrum corresponds to wavelengths in the range 1-10mm – a carrier frequency between 30-300 GHz, approximately 10-100 times higher a frequency than is used for WiFi.
The team employed bespoke MMW power amplifiers to overcome conventional limits on commercial E-band amplifiers, exceeding average output tenfold. RF components expected to operate in long-range and high capacity scenarios have unusually high power requirements, but the need to attain a clean power flow requires lowering the power throughput. Since this makes energy transmission unreliable, the Connectivity Lab team developed ‘post-amplification spectrum multiplexing’, employing multiple smaller power amplifiers with lower efficiency to recreate total output at the required quality.
The system requires use of a 4-foot parabolic antenna which needs to be aimed at an accuracy of 0.07 degrees in order to achieve 20Gbps data rate over such a long distance:
‘This is equivalent to a baseball pitcher aiming for a strike zone smaller than the size of a quarter. Simple GPS coordinate based pointing does not meet the precision pointing requirements of our system. Additionally, all error offsets in the antenna bore-sight and the mount have to be adequately calibrated.’
To resolve the problem the team developed a sun-tracking system which permits the required level of accuracy. Since the research is in aid of Facebook’s efforts to connect outlying or under-served regions of the world, the system is intended to involve UAVs in a variety of climatic situations, so having a reliable Californian sun as a lodestone is clearly not going to be an option in poorer weather conditions involving UAVs; the team is working on further solutions for accurate calibration, and in the Malibu/Woodland experiment also used the old RADAR technique called Conical Scanning, which records the best signal strength from a circular pass and calibrates to that.
There is a long way to go with the MMW research; atmospheric attenuation absorbs electromagnetic waves at MMW’s section of the radio spectrum, meaning that fog, clouds and rain can have a deleterious effect on MMW availability or quality. And the current need for bulky parabolic antennae makes drone design, which is part of the intention of the scheme, problematic.
The Connectivity Lab researchers are currently testing a ground-to-air bidirectional link with a Cessna at 20,000 ft, with the intention of flight-testing a 40Gbps two-way link early in 2017.
