The FCC's TV/cellular guard bands don't compute

The FCC incentive auction NPRM [1] proposes 6 MHz guard bands between cellular and TV services (actually 6-11 MHz, depending on how the auction works out). The number is arbitrary, and could well have been chosen on political grounds to make room for more unlicensed in the TV bands.

The impact on interference from cellular systems into TV receivers is much the same whether the guard band is 1 or 20 MHz: most receivers will be unaffected, and for the small but significant number that suffer harm (0.5-5%?), only receiver filters will really help. The real question is: who's responsible for buying and installing those filters - the consumer or the cellular companies?

Related posts:

Post-auction cellular interference into TVs? (29 Oct 2012)
TV whitespace vs. cellular power limit anomalies (30 Oct 2012)

Paragraph 125 of the NPRM states that the proposed rules will “increase the quantity of wireless spectrum for unlicensed use by allowing for unlicensed use in our proposed guard bands.” This seems to be a way to work around opposition in Congress to unlicensed use. As the NPRM explains ([1] para 126), the Commission is required under the Spectrum Act to license the spectrum recovered through the TV spectrum reorganization –  with the exception of guard bands, that are constrained by the Act to be “no larger than is technically reasonable to prevent harmful interference between licensed services outside the guard bands.”

The only technical justification I could find for the 6 MHz width of the guard bands is the statement in paragraph 156 that the Commission “has previously found six megahertz of spectrum separation is sufficient to protect digital television receivers against 1 MW DTV transmitters.” Footnote 239 cites the Part 73 rules that place no restriction the placement of DTV stations if they are separated by 6 MHz or more, and concludes that this means that 6 MHz is adequate to protect DTV receivers from high power television transmissions. This may be true for TV – provided that the transmitters are on the same tower, or close to each other. This won’t be the case for cellular deployments.

The 6 MHz guard band works for TV-on-TV interference because TV transmitters in the same areas are usually on the same towers, or close by. That means that the signals from two TV stations are roughly the same strength, no matter where the TV receiver. Under such circumstances, the adjacent channel rejection performance of TV receivers is sufficient to prevent interference.

It’s telling that the European plan for TV/cellular coexistence, cf. the Ofcom technical analysis [2], takes a very different tack. It has a nominal guard band of only 1 MHz, but recognizes that there will be interference from cellular transmissions, and so requires cellular companies to provide filters to TV viewers that encounter problems. Ofcom studied interference from 10 MHz cellular bands A, B and C into TV channels; see Figure 3 in [2]:

Ofcom found that the number of TVs that are affected by cellular don’t really change much as one moves away from the broadcasting band; that is, guard band width doesn't really matter. For example, when one looks at the percentage of TVs affected by each 10 MHz block individually, it doesn’t change much from one to another (my chart, derived from Table 15 in [2]):

This is no surprise; the ability of TV receivers to reject interference from signals outside their tuned channel doesn’t change much as signals are placed further and further from the tuned channel. For example, the US recommended standard for TV performance, ATSC A/74 [3] specifies interference tolerance that only gradually improves as one moves away from the tuned channel (the red line indicates the rejection ration outside 6 MHz guard bands, shown in green):

(The height of the line indicates the maximum interference a TV receiver has to tolerate. At the tuned channel, very little interference can be tolerated, shown by the sinkhole. With increasing frequency, more and more interference is allowed. The dip at the far ends is to allow for "image channel" effects.)

Therefore, there will be interference to a small but significant number of household, say a few percent whether the guard band is 1, 6 or 20 MHz. (See my recent post Post-auction cellular interference into TVs?)

What’s really needed, and what is absent from the FCC NPRM, is a statement of who will be responsible for paying for mitigation. Should cellular companies be responsible for adjusting their transmitters when there are problems? Are they responsible for providing consumers with TV receiver filters, or will the consumers have to bear the cost themselves?

References

[1] FCC (2012). Expanding the Economic and Innovation Opportunities of Spectrum Through Incentive Auctions, Notice of Proposed Rulemaking, docket 12-268, released 2 October 2012 (http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-12-118A1.pdf)

[2] Ofcom (2012). Technical analysis of interference from mobile network base stations in the 800 MHz band to digital terrestrial television: Further modeling, Technical report, 23 February 2012 (http://stakeholders.ofcom.org.uk/binaries/consultations/949731/annexes/DTTCo-existence.pdf)

[3] Advanced Television Systems Committee (2010). ATSC Recommended Practice: Receiver Performance Guidelines, Document A/74:2010 (http://www.atsc.org/cms/standards/a_74-2010.pdf)