WITH CAUTION, FCC BEGINS EXAMINING ‘INTERFERENCE TEMPERATURE’

The concept of an “interference temperature” was among the more controversial ideas proposed in the Spectrum Policy Task Force report to the FCC last fall. The idea is to place a cap on the noise environment in which receivers operate. To the extent that ceiling in a certain swath of spectrum wasn’t reached, a user that operated below that threshold would gain additional operating flexibility. The Commission has described the idea as on a band-by-band basis, defining the maximum harmful interference that an incumbent could expect, potentially allowing underlay technologies such as unlicensed devices to operate in licensed spectrum. Wireless carriers have expressed concern that the concept could limit incumbent licensees’ future network design flexibility.

The proposal seeks feedback on various technical rules that would establish procedures for the idea to move forward. It also would use the interference temperature model “on a limited basis” at 6525-6700 MHz and parts of 12.75-13.25 GHz. “These procedures would enable unlicensed devices to operate in these bands, which are used primarily for satellite uplinks and fixed point-to-point microwave services,” the FCC said. It asked whether the possible introduction of unlicensed operation into those bands would affect the existing services.

The inquiry seeks comment on the potential benefits of implementing interference temperature “as a general approach to spectrum management,” FCC Office of Engineering & Technology senior engineer and attorney Gary Thayer said. The proposal that singled out the 2 bands included possible “first steps” for implementation. The idea represents a possible shift away from fixed interference limits originally imposed regardless of conditions in a given band, Thayer said. Instead, interference temperature limits “would be based on the actual radio frequency environment and would take into account the interactions between the transmitters and the receivers,” he said. It takes into account the cumulative effects of all undesired RF energy seen by a receiver in real time, Thayer said. As a result, it “could potentially provide radio licensees with greater certainty about the levels of harmful interference upon which they would be protected,” he said.

The FCC identified initial steps that could take the concept into use in the near term and provide a testbed for further study before trying to implement interference temperature on a broader basis, Thayer said. The spectrum at 6 GHz and 12-13 GHz was chosen “because we believe the typical operating characteristics of these existing services may allow for implementation of a simplified version of interference temperature,” he said.

While unanimously approving the items, commissioners stressed the difficulty of implementing the idea. “While the interference temperature metric may be a good new way to measure interference, we do not have an adequate way to determine what the right interference temperature is for a given band,” Comr. Copps said. “The only tools we have for this job are the ill-fitting and ill-defined ‘interference’ and ‘harmful interference’ concepts. The inappropriateness and inadequacy of these concepts for the job of prospectively setting interference temperature will make this new metric very hard to use predictably and nonarbitrarily in the real world.” He said the inquiry was a good way to start examining how the FCC could improve the standard it used to determine acceptable levels of interference.

Chmn. Powell said the idea as discussed in the FCC’s Spectrum Policy Task Force report took into consideration the cumulative effects of all undesired radio frequency energy. He said the proposal recommended imposing restrictions on unlicensed devices that would include limiting the transmitter output power and requirements to use transmitter power control and dynamic frequency selection.

Adelstein approved the inquiry but said he could only concur on the rulemaking proposal. He said he supported consideration of applying an interference temperature model to unlicensed operations at 6525-6700 MHz and 12.75-13.25 GHz, but disagreed with covering idea in a proposal instead of a notice of inquiry. “I am not sure what the rush is and am not convinced that moving this discussion to the NOI portion of the item somehow holds back our consideration of the interference temperature approach,” he said.

United Telecom Council Gen. Counsel Jill Lyon, echoing Adelstein, said the idea was interesting but questions remained on how the FCC would find ways of measuring interference under that model and how the model itself would work. “This is one of those areas where we have to come down on the side of ‘please be careful,'” she said. Critical infrastructure users in the band range from mobile links used in railroad operations to utility communications systems to microwave-based backhaul networks used by LECs. “It’s a nice idea but how do you go about it, especially when you are talking about a service that may be incompatible with a fixed wireless use,” Lyon said. “We would hope that they would tread very, very carefully here.”

The FCC decision to choose those 2 bands as a testbed appeared to be aimed at picking bands more resistant to interference, an industry source said. The spectrum at 6 GHz in the proposal covers fixed service terrestrial systems and the spectrum at 12 GHz covers both fixed service and a satellite uplink band. Fixed service receivers tend to be fairly directional and raised off the ground. That could mean that if there was a low-power emitter on the ground it might be less liable to cause interference if it didn’t get up into the receive beams, the source said. On the other hand, those bands are used for critical infrastructure applications, including carrying public safety information. “They are absolutely vital and they must be protected from interference,” the source said.

Washington attorney Mitchell Lazarus, who represents both fixed wireless users and unlicensed technology developers, said existing requirements for unlicensed users required them to avoid creating interference to licensed services. “The last thing that the unlicensed industry wants is to cause interference in a way that would threaten its own operations,” he said. “The lawyers are going to have to back off and the engineers from both industries will have to sit down together and work through the math and try to determine whether this approach make sense. Unless both industries are pretty confident that there won’t be interference, then neither industry is going to want to move forward with this,” he said.

Asked why the 2 bands were chosen to experiment with an interference temperature concept, Office of Engineering & Technology Chief Edmond Thomas cited the “Hippocratic oath” of the FCC, which he said was: “First, do no harm…. We are trying to do it in a band and try it out so that we are very confident that there won’t be any harm.” Bands at both 6 GHz and 12 GHz are primarily satellite uplinks, he said. The antennas are looking at the sky and can be very high power. If the test shows that this could be applicable to other bands, the Commission could try it out in other parts of the spectrum, he said. Thomas said: “Bluntly, we don’t want to foul this up and we don’t want to foul the incumbents up.”