FCC ‘Harmful Interference’ Definition Gives Wireless a Pass

Proposal could relieve wireless operators from eliminating some interference June 8, 2015
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Fig. 1: Graph detailing 11 scenarios for the 600 MHz band according to the FCC Report "Expanding the Economic and Innovation Opportunities of Spectrum Through Incentive Auctions."
Click on the Image to Enlarge

WASHINGTON—In May, the FCC proposed a definition of the phrase “harmful interference” in connection with reception of TV signals in the repacked 600 MHz broadcast spectrum. This harmful interference could be caused by a signal on the frequencies used by the broadcast TV signal—referred to here as “CCI”—or by a signal on an adjacent channel, or “ACI.”

The commission outlined the definition in Appendix D of the document, “Protection of Broadcast Television Service in the 600 MHz Band from Wireless Operations.” According to the document, licensees authorized to operate wireless services in the 600 MHz band “must cause no harmful interference to public reception of the signal of broadcast television stations transmitting co-channel or on the adjacent channel. Such wireless operations must comply with the D/U ratios in Tables 7–13 in OET Bulletin No. 74. If the 600 MHz band licensee causes harmful interference to the public reception of a signal of a broadcast television station that is operating co-channel or on an adjacent channel, that licensee must eliminate the harmful interference.”

As I interpret the definition, both CCI and ACI interferences are prohibited. However, this definition would also relieve wireless operators from responsibility to eliminate interference due to creation of a signal whose frequency relationship to the television signal is neither co-channel nor adjacent channel.

MULTIPLE OPPORTUNITIES FOR INTERFERENCE
Readers of this column and the FCC rules and definitions are aware that there are other causes for interference to DTV reception in this band, including:

· Desensitization of the ATSC receiver by one or more strong signals whose spectrum is more than 6 MHz from the desired television signal.

· Third-order distortion products generated in the ATSC receiver by two or more undesired signals whose spectrum is offset by more than 6 MHz from that of the desired signal.

The worst cases are those where the center frequency of one of these undesired signals is offset by Z MHz from the center frequency of the desired signal and by twice Z MHz, from the center frequency of the second undesired signal relative to the center of the desired signal spectrum. For example, consider the case of ATSC-into-ATSC interference. If the desired channel center frequency is 569 MHz (Channel 30), the worst case is for two undesired ATSC signals whose center frequencies are 569 plus 12 MHz (Channel 32) and 569 plus 24 MHz (Channel 34). (The other worst case is where there are undesired signals on Channels 28 and 26.)

Wireless operators will probably use LTE signals in the 600 MHz band. In other bands, the LTE standard provides multiple blocks of spectrum: 5 MHz wide, 10 MHz wide, and the like. The LTE standard for the 600 MHz band is a work in progress, but I believe that wireless operators expect to see both 5 MHz and 10 MHz blocks of spectrum in that band become available. The FCC intends to auction 5 MHz blocks of spectrum early in 2016. Following that auction, the commission will hold a second auction in which winners of more than one 5 MHz block will bid for their choices of which 5 MHz blocks they prefer from the first 600 MHz auction. Winners in the first auction could also purchase spectrum from other winners. In either case, I believe that most wireless operators will wind up with two contiguous 5 MHz blocks in the 600 MHz band.

Fig. 1 was derived from the FCC Report and Order, “Expanding the Economic and Innovation Opportunities of Spectrum Through Incentive Auctions,”which was adopted May 15, 2014 (p. 453). These are the 11 scenarios for the 600 MHz band. While many in the industry favor a single band plan for the entire country, the FCC seems to believe that this would stifle the growth of wireless communications and result in much less revenue for the U.S. Treasury from the auction’s net proceeds.

Fig. 1 shows the TV spectrum within the 600 MHz band after repacking in green. The 5 MHz blocks to be used for baseband transmissions are blue. The cellphone transmit signal spectrum is shown in yellow. The white spaces are guard bands.

The FCC’s “Scenario 6” would entail clearing 84 MHz of spectrum of TV signals. Only 70 MHz would be available for auction, as 14 MHz is specified for guard bands in this scenario.

AN UNINTENDED (OR UNFORSEEN) CONSEQUENCE
Now suppose that Channel 33 (587 MHz center) is re-allocated to community X. Broadband operations may be set up within this community on any of the seven 5 MHz blocks (blue). Accordingly, Channel 33 (587 MHz center) and Channel 34 (593 MHz) will be free of CCI and ACI according to the FCC. However, if two blocks are in operation: Block A at 619.5 MHz and Block G at 649.5 MHz, then there is opportunity for interference. (Two times 619.5 equals 1,239 MHz; then if you subtract 649.5 MHz from this, the result is 589.5 MHz. The spectrum of third-order products is 590 ±7.5 MHz, so noise is going to spill over into Channels 33 and 34.)

You might say “just co-site these base stations with the TV station and there should be no interference.” However, there will be base stations throughout the coverage area of the TV station. Co-siting works for TV stations that are sharing the same tower.

In examining Fig. 1, you’ll find that in TV channels near the guard band between the green spectrum and the blue blocks, there is a greater probability of third-order distortion products causing interference that was not addressed by the FCC, which wireless operators are not required to eliminate.

Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He may be contacted at [email protected].
 
Want to learn more about how the upcoming spectrum auctions will impact your business? Be sure to attend "Spectrum Auction Preparation: What You Don't Know Will Hurt You," Thursday, July 23 at 2:30 p.m., EST. 

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