Co- and Adjacent-Channel Interference Issues Loom Large Headed into Incentive Auction, NAB, CTIA Agree
There is a “growing consensus” in favor of a “Down from 51” plan for the 600 MHz band following an incentive auction of broadcast TV spectrum, CTIA told the FCC in reply comments. NAB made similar points. Both major trade groups also highlighted an issue that has gotten less mention to date: problems posed by potential co- and adjacent-channel interference in a variable band plan. NAB asked why the issue has been ignored by FCC staff that is developing rules for the auction. Reply comments were due Friday on a May 17 Wireless Bureau public notice exploring the advantages of a “Down from 51 reversed” band plan as well as on a proposal to use time-division duplexing (TDD) instead of frequency-division duplexing (FCC) (CD June 18 p1).
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"At this point, it seems that the staff will have no choice, even begrudgingly, to put out a further notice on whether variability is feasible. Too many parties have weighed in for them to write it off as a broadcast-only issue,” said a former FCC legal adviser. “Don’t be surprised to see something out from the Wireless Bureau in the next couple of weeks, although even that might take prodding from the 8th floor."
Qualcomm calculates that a distance of more than 300 miles would be necessary to avoid co-channel interference, CTIA said. “The record makes clear that co-channel interference is a major potential problem that must be avoided,” said the reply comments (http://bit.ly/12zHd0v). “This potential interference arises because ’spectrum used for uplinks will be used elsewhere for television broadcasts.’ Extremely large separation distances will be needed to mitigate interference between broadcasters and wireless carriers operating on the same or adjacent channels. Estimates for the separation distance vary, but they are considerable."
"Apart from issues smaller in scope ... only one major problem prevents the Commission from being able to adopt a band plan order: the lack of serious study on the co- and adjacent channel interference challenges inherent in variable plans,” NAB said (http://bit.ly/124Ogdd). It said industry research “suggests that variability is a major concern and likely one that will make impossible certain key features of the proposed auction design (e.g., making licensed blocks fungible), ultimately drive down auction revenues and lead to significant interference between broadband and broadcast providers.” But NAB said in a footnote: “Commission staff has never suggested that it has looked at this issue with any degree of depth."
The record shows a “Down from 51 Reversed” or TDD band plan “would be a poor fit for the 600 MHz band for a number of technical reasons and should not be adopted,” Qualcomm said (http://bit.ly/12h9y8k). “Both the DF51 Reversed band plan and the TDD band plan would jam many higher frequency bands that could be simultaneously operating within the device; this includes critically important global positioning bands, the 2.4 GHz unlicensed Wi-Fi band, and licensed mobile bands such as PCS, WCS, and the [Broadband Radio Service/Educational Broadband Service] bands. Qualcomm has explained in lengthy detail the extensive degree to which harmonics and spurious products from 600 MHz uplink operations below 673 MHz would create crippling intra-device interference and should be avoided."
"Almost all commenters addressing the issue agreed with T-Mobile that the Down from 51 Reversed proposal introduces new inefficiencies not found in other band plan designs and, as a result, delivers too little spectrum for wireless broadband use,” said the carrier (http://bit.ly/18rlUCp). “Commenters urged the Commission to pursue various alternative band plan designs, the most prominent of which was the Down from 51 plan that the Commission originally proposed.”