Wireless Industry Wants FCC to Clarify Adaptive Modulation Rules
The Fixed Wireless Communications Coalition and various companies are asking the FCC to shine a light on whether companies are allowed to reduce data rates for point-to-point transmissions to keep a link in service, without violating commission rules. The stakes are potentially high, since the transmissions involved include wireless public safety backhaul, cellsite backhaul and control of both the electric grid and natural gas and oil pipelines.
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Adaptive modulation is a key focus at the National Spectrum Management Association meeting this week. NSMA Working Group 3 is slated to take on the issue in a meeting Wednesday. During a panel discussion Tuesday, the benefits were hotly debated.
“It’s a new technology that holds great promise and I think people want to see it happen,” Thomas Willis, a technical staff member of AT&T Labs, said in an interview. “But there are concerns about how it’s going to be implemented and how it’s going to affect systems that are existing in those bands and operating today.”
Adaptive modulation raises several issues not yet considered by the commission, Willis said. “One of the issues is that the rules that the FCC wrote about spectral efficiency were before people were using adaptive modulation,” he said. “The rules are sort of not up-to-date with the new technology and these FCC rules need to be looked at again under the light of what does this mean for deploying systems with adaptive modulation.”
In a recent filing, the FWCC was joined by Alcatel- Lucent, Motorola and Ericsson in seeking clarification of commission rules regarding adaptive modulation. The filing addresses Section 101.141(a)(3) of the rules, which imposes capacity and loading requirements on fixed service transmitters operating in the 4, 6, 10, and 11 GHz bands.
“These are the only bands suitable for links in the range of several miles up to tens of miles, and for that reason are critical to the Fixed Service,” the filing said. “We here propose that lower data rates be permitted during brief periods when the link would otherwise be temporarily out of service, such as during short, atmospherically-caused decreases in received signal strength. The data rate will still comply with the minimum during normal operation, and will also comply on average.” The group explained that reducing the data rate maintains a link because receivers can accept a weaker incoming signal when the bit rate falls. This in turn helps maintain system synchronization, avoiding the need for several minutes of down-time for synchronization “reboot.”
Industry companies have generally construed the payload requirements to apply whenever a link is in service, FWCC said. “For example, where the rule specifies a minimum capacity of 134.1 Mbps, the industry has taken this to mean the transmitter must be able to send at least 134.1 million bits during each and every second that the link is in operation and considered available,” the filing said. But “there is no Commission decision that requires this reading.”
Changing the modulation of signals presents risks to other users of spectrum, said Jim Wolfson, president of X- DOT, an engineering and consulting firm. “Spectrum warehousing” would be “very tempting,” unless the commission imposes restrictions, he said. The agency needs to approve rules that are “specific, enforceable and enforced,” he said. “The lower frequency bands are critical and are the only options for longer [communications] paths for many people out there,” Wolfson said.
Mitchell Lazarus, attorney for the FWCC, said the group’s request is simple. FCC rules say “a 30 MHz channel has a minimum payload of 134.1 Mbps,” Lazarus said at the NSMA meeting. “The rule does not say the channel has to carry that many bytes every second. … Our request to the FCC essentially asks them to read the rule as an average requirement.”
Many issues still must be resolved, Wolfson replied. “If I'm standing with one foot in a bucket of boiling water and the other in ice water, that doesn’t make me average.”