Why Ku-Band for Air-to-Ground
Frequency spectrum available for mobile broadband use is becoming an increasingly scarce commodity. Broadband networks already are overloaded as evidenced by AT&T's announcement in early March that it was ending unlimited data plans. But pay-per-bit surcharges are unlikely to curb consumers' appetites for more data.
To support the growth of mobile devices, the FCC's goal is to make available by 2020 an extra 500 MHz for broadband, including 300 MHz between 225 MHz and 3.7 GHz by 2015. The FCC also has stated that it wants to make 90 MHz of mobile satellite service spectrum available for ground-based systems.
Qualcomm proposes that the FCC repurpose an entirely different 500-MHz swath of spectrum in the 14.0- to 14.5-GHz Ku-band satellite communications band to be licensed on a secondary basis for a proposed next-generation air-to-ground network. Uplinks and downlinks would share the same frequencies, but use time division duplex multiplexing to prevent ground stations or aircraft from “stepping on” each other's transmissions. Each aircraft could be assigned as much as 50 MHz to 100 MHz of bandwidth for downlinks because each ground station would serve no more than four aircraft at the same time. That much bandwidth, along with a much higher frequency, is what makes possible connection speeds 30 times faster than the 3-MHz swath allocated to Gogo in the 800-MHz band.
“This does multiple things,” Jalali says. “The quality of the broadband service improves, while the price for consumers goes way down because of the capacity of the system. We've done the economics, but it's too early to provide specific numbers.”
That chunk of Ku-band also keeps the proposed system well clear of the 1,525- to 1,660-MHz band used by safety of flight L-band satcom and satnav systems, such as Inmarsat's Aero-H/I and GPS, a key consideration. (Just ask Harbinger Capital's Phil Falcone, who invested $3 billion in LightSquared's L-band network only to find that potential interference problems with GPS made the system unworkable.)
Unlike LightSquared, Qualcomm has a long and successful track record in winning approval from the FCC for new wireless broadband uses of spectrum. Getting final approval from the Commission involves working with all stakeholders to assure that any new use of frequency spectrum is compatible with existing uses and with the FCC's announced plans for the future.
“Nobody understands licensing like Qualcomm,” says Gregg Fialcowitz, cofounder and former CEO of Row 44, a Ku-band inflight entertainment satellite broadband services provider. “Its engineers understand the consequences of interference.”
A ground-based Ku-band system operating in the 14.0- to 14.5-GHz sector, however, certainly has the potential to degrade or jam the uplink, or return link, used by Ku-band communications satellites.
So, three months prior to filing its petition for rulemaking with the FCC, Qualcomm reached out to virtually all stakeholders, including Ku-band satellite operators, airlines, avionics firms and even Gogo, to explain its plans.
First, Qualcomm assured the Ku-band satellite broadband service providers, including, Row 44 and Panasonic Avionics, that it has a vested interest in protecting and promoting Ku-band satellite communications. The firm indeed is an “incumbent user” of Ku-band satellite communications since its OmniTRACS Ku-band satellite tracking system is installed on more than 35,000 vehicles and has been operational since 1997.
To prevent interference with geostationary orbit Ku-band satellites, the proposed system is to use fine-tuned RF equipment designs and its ground stations will have directional antennae that only point within 60 deg. of true north. The paths of signals between aircraft and ground stations are well away from the plane of geostationary (GSO) Ku-band satellites over the equator, as shown in the illustration above.
Their relatively close, 134-nm spacing allows the ground stations to operate at low power. Overlapping signal patterns should enable aircraft to be handed off to adjoining stations with seamless broadband connectivity.
Each beam serving one aircraft is to be focused to 2 deg. and steered to keep it aligned with the aircraft. Ground station antennae will concentrate the 2-deg. forward or downlink beams in the horizontal plane. As an aircraft approaches within 32 nm of a ground station and the elevation angle increases above 10 deg., the network will begin a handoff to an adjoining station.
The southernmost row of ground stations, ones sited at 25 to 26 deg. latitude, poses the highest potential risk of interference to GSO Ku-band satellites because each signal beam can point up to 60 deg. off north. At such latitudes, Qualcomm plans to dial down the ground station transmit power if needed to prevent interference with satellites.
Aircraft antennae are mounted on the bottom of the fuselage and they, too, will concentrate return or uplink beams in a shallow -5-deg. cone. Antenna gain falls off rapidly with increase in elevation to prevent the aircraft return links from causing Ku-band satellite interference. The ground stations will use very high gain antennae to enable aircraft to transmit on the uplink with as little as 3 watts within a nominal 2-MHz bandwidth. In addition, it's likely that uplink transmissions will be attenuated or completely stopped while the aircraft is banking to prevent stray signals from reaching Ku-band satellites.
Qualcomm's interference analysis indicates that the combination of directional forward link and return link signal beams, along with low transmit power, will prevent interference with Ku-band satellites. In addition, the company is proposing that the FCC adopt rules for aircraft antenna emissions transmitted into the GSO satellite plane that are performance based to protect Ku-band satellites and yet provide air-to-ground system designers and operators with the flexibility needed to assure the system works as intended.
The company also plans to incorporate functionality to prevent interference with future non-geostationary orbit (NGSO) Ku-band satellites, although no such space vehicles have been announced. The firm wants to use orbit path almanac data to attenuate or shut down air-to-ground beams when a ground station, aircraft and NGSO Ku-band satellite are in alignment.