Tests Show Threat to 900 MHz Unlicensed Band . . . Or Do They?
Licensed provider and unlicensed users disagree on meaning of joint test results.
Most of us rely on multiple unlicensed radio transmitters around our home and office. In fact, most us carry several on our person: Wi-Fi and Bluetooth on our phone, wireless earphones for the music player, the gizmo that unlocks our car from across the street, a building security card . . . All of these, plus the dozens more in most homes and offices, operate under stringent FCC rules. One of those rules says these devices must accept any radio-frequency interference that comes their way. The engineers who design the products know that, and are good at working around other transmitters in the same frequency range.
There is one exception to this rule: one kind of licensed transmitter is required to protect unlicensed devices against interference. These transmitters operate in the “Location and Monitoring Service” (LMS), which uses large parts of the 902-928 MHz band. That same band is also home to a vast array of unlicensed devices that are convenient around the home (like cordless phones and baby monitors), and vital to commerce and industry (like retail inventory systems and remote controls for construction cranes). LMS providers are uniquely required to show through field tests that their equipment will not cause “unacceptable levels of interference” to unlicensed operation. This special rule reflects the FCC’s judgment that unconstrained LMS poses an exceptional threat of interference to unlicensed devices.
Late last year the FCC granted a waiver to an LMS licensee, Progeny LMS, LLC, that eased back two of the LMS rules. As a condition of the waiver, the FCC required Progeny to test for interference into unlicensed devices. It then sought public comment on the test results. Progeny claimed the results showed little or no interference into unlicensed devices, but commercial users of those devices vehemently disagreed.
In response to an informal request from the FCC, Progeny has now conducted additional tests jointly with three major users of unlicensed equipment in the 902-928 MHz band: the Wireless Internet Service Providers Association (WISPA), whose members provide wireless Internet access, and Landis+Gyr and Itron, which both conduct automatic meter reading. (Links in the preceding sentence go to test reports filed with the FCC.)
As often happens in these cases, the parties disagree on how to interpret the data.
Progeny reads the results as showing that its system “will not cause unacceptable levels of interference” to unlicensed users. But the three unlicensed users claim the results predict interference in several respects. They note, for example, that Progeny operates at much higher power than do unlicensed devices in the band; that multiple Progeny transmitters in the same area together occupy frequencies for 90-100% of the time; that the tests understated interference into meter reading equipment; and that Progeny’s signal degraded throughput on wireless Internet systems by 40-50%.
The FCC is now evaluating whether Progeny has met its non-interference obligation. Manufacturers and institutional users of devices in the 902-928 MHz band should take a close look at the test results, and should raise any concerns very promptly with the FCC.