Monday, October 29, 2012

Post-auction cellular interference into TVs?

How many TV receivers will be affected by interference from cellular services as a result of the FCC’s "incentive auction" plan?  The FCC’s proposal doesn’t venture an answer; I don't think it even asks the question. Ofcom’s technical analysis in the UK’s rearrangement of the TV bands to accommodate more cellular service suggests that the number will be small, but not negligible. Ofcom therefore decided to require cellular operators to install filters on TV sets where there is a problem; the FCC has not raised this possibility.

In summary, the UK modeling suggests that TV reception will be affected in about 5% of  homes if there's a 11 MHz guard band between TV and cellular channels; the FCC's proposed guard band will be 6 - 11 MHz, depending on auction outcomes. By far the most effective way to mitigate this interference is by installing a TV receiver filter in affected homes.

Related posts:
TV whitespace vs. cellular power limit anomalies (30 Oct 2012)
The FCC's TV/cellular guard bands don't compute (3 Nov 2012)
The FCC Notice of Proposed Rulemaking (NPRM, [1]) mentions impacts on population served in its discussion of moving channels around as a result of the auction (Section V., Repacking), and asks for comments about allowing TV licensees to reduce their coverage area (Section IV. B., Bid Options). However, it does not quantify the loss of coverage due to interference from cellular service into TV receivers.

In principle, there will be interference from cellular transmissions that spill over into a TV channel, and from cellular transmissions in the licensed cellular frequencies that TV receivers don’t filter out well enough. The greater the distance between the two services, the less likely both types of interference are likely to be. The NPRM’s band plan contemplates “guard bands” of at least 6 MHz to create distance between cellular service and TV; depending on how the auction works out in a particular geographical area, these guard bands could be up to 11 MHz wide. Here's Figure 4 from the NPRM:

When the TV signal is weak, a TV set will be adversely affected by cellular transmissions even given a guard band. Since there is no technical analysis in the FCC proposal, I have turned to Ofcom’s analysis [2] of interference from mobile network base stations in the 800 MHz band to digital terrestrial television. European TV deployments differ from those in the US in several ways, and there are differences in many details, such as frequency band (800 MHz vs. 600 MHz), TV channel width (8 MHz vs. 6 MHz) and allowed transmit power (Ofcom assumes base station EIRP of 64 dBm vs. US urban ERP rule of 60 dBm, that is EIRP of 62.15 dBm). However, even given the differences, I’d bet that the order of magnitude in homes affected will be similar.

Before we start, though, let’s consider what might be an acceptable amount of interference. Part 73.616 (e) of the FCC’s rules stipulates that a new TV station will not be accepted if it is predicted to cause interference to more than an additional 0.5% of the population served by another DTV station. Therefore, I'll treat 0.5% as a guide to an acceptable number of homes that may lose service because of interference from new cellular allocations. If the number of homes that suffer TV interference from cellular exceeds 0.5%, the FCC's plan may have a problem.

As the Ofcom study explains, the cross-European band plan contemplates only a 1 MHz guard band (Figure 1 in  [2]).

The Ofcom study considers interference from 10 MHz blocks called A, B and C into TV reception:

The Ofcom findings on interference from blocks B and C approximates the interference from the US band plan, since ignoring A models a guard band in this position. In fact, it’s a conservative under-estimate of interference, since gap between broadcasting and block B is 11 MHz, the largest that the US guard band will ever be.

The Ofcom study examines three categories of affected households: standard domestic installations (12.97 million), communal aerial systems (5.61 million), and domestic installations with amplifiers (9.00 million), for a total of 27.58 million (Tables 8 and 14 in [2]). It finds that for case "BC", that is combined interference from the 20 MHz combination of blocks B and C , 1.71 million or 6.2 % of UK households will be affected if there is no interference mitigation (mitigation case (a), sum of column BC in Tables 15 (a)-(c) in [2]). Even if the Ofcom results are off by a factor of ten when translated to the US situation, which is very unlikely, the result would still be more than the 0.5% threshold for interference in Part 73.616(e)

In addition to the “no mitigation” case, Ofcom considers consumer-based mitigation (putting a filter on the TV receiver), mobile network-based mitigation (filters on the base station transmitter), and their combinations. It finds that filtering the consumer’s TV set is 30x more effective at reducing interference than filtering at the base station, reducing the number of affected households to only 25,159 or 0.1% (mitigation case (b), sum of column BC in Tables 15 (a)-(c) in [2]). The FCC proposal mentions neither mitigation measure.

Added 11/5/2012: The above analysis applies to "over-the-air" TV reception. Homes that receive TV programs via cable and satellite would not be affected. So if, let's say, over-the-air reception would suffer harmful interference in 5% of US homes, but only 15% of those homes used an over-the-air signal for their TV service, then 0.75% of homes would be affected in all.


[1] FCC, Expanding the Economic and Innovation Opportunities of Spectrum Through Incentive Auctions, Notice of Proposed Rulemaking, docket 12-268, released 2 October 2012 (pdf)

[2] Ofcom, 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 (pdf)

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