D2D Operators Tell NTIA: Coexistence With GPS Not a Problem

Many D2D interests argued that MSS operators have used the 1610-1660.5 MHz band for decades, coexisting with GPS devices without a problem. The Mobile Satellite Services Association said it should be similarly possible to offer D2D services in MSS spectrum without posing a risk of harmful interference to adjacent-band GPS operations. Ligado said that years of MSS operations in the L band haven’t raised any NTIA or FCC concerns, thus there’s “no rational basis” for revising the rules now. “Whether the number of MSS devices is one-hundred thousand or an order of magnitude higher is immaterial if all the devices operate consistent with the FCC’s emission requirements,” Ligado said. While the Department of Transportation has raised concerns for years about L-band operations potentially affecting GPS devices, the FCC “has given due consideration to DOT’s perspective on how GPS devices should be protected … and found it wanting," Ligado added.

The FCC’s assumptions underlying the existing out-of-band emissions (OOBE) limits remain valid, and there’s no reason to revisit them, Iridium said. Concerns about D2D use on airplanes are misplaced since L-band-enabled mobile phones will be off or in airplane mode anyway during flights, it noted. The OOBE limits have “worked admirably,” with Iridium terminals coexisting with GPS devices for years, the company said.

The iPhone D2D service, with satellite-enabled emergency service and roadside assistance texting and location sharing, uses satellite connectivity only as a last resort when no terrestrial cellular or Wi-Fi is available, Apple said. It said even if an iPhone isn't in airplane mode during a flight, it won't transmit without first getting a downlink signal. Getting that downlink signal inside an aircraft is “extremely challenging," Apple added.

The OOBE interference that GPS receivers should generally expect from L-band devices in airports and on aircraft will be notably lower than existing OOBE limits allow, Viasat said. “Simply put: D2D applications need not pose a threat to the continued viability of GPS.” Viasat said the right mitigations will allow long-term GPS/D2D coexistence. It said NTIA should look into ways of ensuring more compliance with FCC rules governing use of devices on aircraft, such as requiring device manufacturers to provide more conspicuous notice that use of features involving L-band D2D operations is prohibited on aircraft. Viasat also suggested requiring that device manufacturers set up a network connection denial in MSS spectrum if the user device detects that it is onboard an aircraft.

D2D services and GPS can peacefully coexist in the L band, AST SpaceMobile said, adding that its beam control and filtering precision reduce the risk of interference. Unlike legacy MSS providers and their high-power terminals that could overload GPS receivers, AST's use of low earth orbit satellites eliminates signal saturation concerns. Globalstar said its MSS operations in the 1610-1618.725 MHz and 2483.5-2500 MHz bands, including D2D service, “pose no threat” to GPS receivers. Its products and devices “depend substantially” on GPS, and it will ensure that the GPS L1 signal doesn’t see harmful interference. There’s no evidence that current rules aren’t enough to protect L1, Globalstar said.

Skylo said DOT’s suggested OOBE limit would result in every device Skylo has certified being noncompliant, even though they use an integrated GPS modem. The D2D connectivity provider said it is talking with original equipment manufacturers about letting users easily disable L-band operations, such as via a “satellite mode” setting. It said it's also looking at location-based services that would automatically disable L-band transmissions in specific areas, such as near airports.

GPS Worries

Some parties took pains to contrast their service from Ligado's, whose proposed L-band use was named in a DOT technical analysis accompanying NTIA's request for comments. Apple laid out the differences with its D2D communications, saying the DOT analysis focuses on Ligado handsets in a way that leads to “overbroad conclusions” about the L-band interference environment. Globalstar said its MSS devices aren't a threat “in the way that a terrestrial deployment in Ligado’s MSS L band spectrum might” be.

The expected boom in numbers of MSS devices operating in the L band “requires a fresh, comprehensive assessment” of existing GPS protections, Garmin said. The current safeguards were designed largely to protect aeronautical satellite radio navigation and might not be up to the task of dealing with growing numbers of MSS-enabled devices, it said. Garmin advised NTIA to work with the FCC on a comprehensive study of interference risks in the L band.

Some initial D2D use cases, such as emergency communications, seem consistent with legacy MSS applications, but others could pose challenges to the GPS spectrum environment, GPS Innovation Alliance said. The group called for analysis of the relative signal power of MSS operations and GPS, the proximity of GPS receivers to terrestrial operations, and deployment density of MSS devices. It also warned of “particularly challenging co-existence scenarios” from uplink transmissions by D2D handsets or IoT devices in MSS spectrum adjacent to spectrum in which GPS signals are received. It said that while NTIA is looking at MSS uplink operations in the 1610-1660.5 segment of the L band, MSS operations in the lower L band might also need future consideration.