Saturday, April 21, 2012

Is 2.4GHz Wi-Fi the next GPS/LightSquared?

No, unlicensed devices in the 2.4 GHz band (2400 – 2483.5 MHz, operation under Part 15.247) probably won’t be the next GPS/LightSquared, where a large installed base of unlicensed devices with significant susceptibility to out-of-band interference was threatened by the deployment of a cellular service in an adjacent band. However, some similar characteristics raise concerns: tens of millions of devices, poor adjacent channel rejection, and a quiet band next door. What would happen if there were a large cellular deployment next door to 2.4 GHz?

There are significant differences to temper concerns: Wi-Fi devices don’t depend on such exquisitely low signal levels as GPS receivers; we’re not talking about safety of life applications; the RF front-ends of Wi-Fi devices are not open many tens of MHz away from the allocated band; and there already is some cellular operation nearby, at least in the US (Clearwire/Sprint’s 4G service in the 2.5 GHz band).

Still, as I’ll argue, the fact that interference has been observed between 4G service in 2.5 GHz and unlicensed devices in 2.4 GHz even with at least 10 MHz of guard band between them suggests that we’ll see interference problems to and/or from unlicensed devices if a cellular service were allocated in the fallow 2360 – 2400 band. That in turn suggests that it could make sense for the FCC to start encouraging or mandating better filtering for unlicensed devices over the 2.4 GHz band now, well before the 2.3 GHz band starts being populated with a potentially interfering service.

Update 4/23/2012: Monisha Ghosh kindly let me know that  2360-2400 MHz has been requested by healthcare device manufacturers (GE, Philips etc.) for Medical Body Area Networks on a secondary basis to Aeronautical Telemetry (OET proceeding 08-59). The June 2009 Notice of Proposed Rulemaking (PDF) provides good background information on the current uses of the 2360-2400 band.

Friday, April 20, 2012

Drones, the RF noise floor and encouraging directional antennas

According to a story in the WSJ, Silicon Valley techies are building drones for $150 to $1,000 each, and some of them provide live video feeds. The story doesn’t specify the wireless channels that are being used, but I’d guess amateur bands and 2.4 GHz or 5 GHz unlicensed.


The impact of broadband unlicensed transmitters up to 400 feet (the current limit for amateur operation) in the air on the RF noise environment is likely to be significant once a significant number start flying. As Carl Martin mentions in the comments to the WSJ article, applications include real estate videos, movie shoots, police monitoring and local small parcel delivery (can you say pizza?). The commercial/professional impact will be even larger once the FAA develops rules for the licensing of commercial drones by 2015 as it has been required to do by Congress. Even now, the there is a surprising range of uses; according to data released by the FAA to the EFF following a Freedom of Information Act request, drones are operated not only by the Customs and Border Protection, DARPA and branches of the military, but also by universities, police departments, and small towns and counties across the United States.

If these live video feeds are transmitted from omni-directional antennas, the signal will cover huge footprint, potentially interfering with many other devices. I wonder whether more should be done to encourage the use of adaptive antennas by unlicensed devices so that transmitters only deliver their signal where it’s needed. As I understand it, EIRP transmit power limits effectively put a ceiling on power delivered in any direction; the rules don’t contemplate allowing transmitters to concentrate their power using antenna gain. However, there could be a benefit to allowing transmitters to deliver a higher amount of power in a small area; yes, the interference there will be greater, but fewer other players will be affected.

There are constraints, of course. The directionality is a function of the antenna array size as well as processing, and even a spot beam could be quite wide at a range of a few hundred meters. There’s also cost; how does one encourage drone wireless links to spend the extra money?

Coda on Privacy.  The EFF FOIA request to the FAA mentioned above seems to have been prompted by concerns about privacy and surveillance. The obvious application is video surveillance, but intercepts of wireless networks are of course possible, too. If Google Street View could amass a trove of passwords, Internet usage history and other highly sensitive personal data by driving around, imagine how easily such an operation could scale with an airborne platform. I wonder, though, if it doesn't get harder from a drone: how well could a receiver disentangle the myriad signals from all the base stations it could see? Not too hard, perhaps, since they're not all operating simultaneously.