Concern Over Interference Continues

The saga of the FCC Laboratories Report, “Interference Rejection Thresholds of Consumer Digital Television Receivers Available in 2005 and 2006” continues.
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The saga of the FCC Laboratories Report, “Interference Rejection Thresholds of Consumer Digital Television Receivers Available in 2005 and 2006” continues.

Following up on my column published last month, Table 1 gives the D/U ratios for a single interferer and a pair of interferers on those channel pairs of the form N+K and N+2K where K is an integer from –5 to +5 inclusive, for a received desired signal power of –53 dBm.

(click thumbnail)Table 1: Median DIU thresholds for channel pairs and individual channels
Source: Table A-5 FCC Laboratories Report March 30, 2007

WHAT IT MEANS

These measurements make it clear that where such channel pairs exist, they produce the dominant interference. Either or both undesired signals on these channel pairs may be either a DTV signal or the signal radiated by an unlicensed device operating on a “vacant channel” in the TV broadcast spectrum. While the FCC tested for K between –5 and +5, the results suggest that a wider range of K is justified by the poor RF selectivity of modern DTV receivers. This is evident from the small differences on D/U over the range of channel offsets.

Why is this? A single interferer offset from the desired channel by more than one channel creates no third order intermodulation (IM3) product in the desired channel. However, this single interferer can and does cause cross-modulation of the desired signal and here is the difference; as this column has reported, interferers on a pair of channels of the form N+K and N+2K generate IM3 which falls in the desired channel causing pseudo co-channel interference. Remember that the threshold D/U for co-channel interference is +15.2 dB. There is quite a difference between say –40 dB and +15 dB. There is also some cross-modulation of the desired signal by the signals on the channel pair, but this is negligible at the threshold D/U for the channel pair.

IT GETS WORSE

Is this unique to desired signal powers near –53 dBm? No, it gets worse at D = –68 dBm as shown in the FCC Report, in chapter 16, sheet A-4. How much worse? When the D level drops by 5 dB, so does the U threshold. However, for a drop of 5 dB in the U signal power, there will be a 15 dB drop in the IM3. So, for example, for the N+3/N+6 case the threshold D/U at D = –68 dBm is 43.5 dBm about a 10 dB decrease in the threshold D/U.

The FCC made no measurements involving interferers on other channels and it made no measurements with interferers on three or more channels. They made enough measurements involving interferers on pairs of channels of the form that creates pseudo co-channel interference to prove that IM3 generated in the desired channel by such pairs of signals will be the dominant interference mechanism for DTV-DTV interference, and by implication, also for interference to DTV reception by signals radiated by unlicensed devices on vacant TV channels, in my opinion.

HOW IT MAY BE USED

One might challenge this statement on the basis that these unlicensed devices may not use the same kind of modulation as the ATSC standard, but so what? The bandwidth of such signals will be in the range of 0.5 MHz to 6 MHz. Only modulation systems that are both power efficient—some of the receivers will be battery operated—and spectrum efficient will survive in the marketplace. Most are expected to be used for wireless interconnection between computers and their peripheral devices in both homes and offices, and for wireless access to the Internet. At least that is what I’ve read. Also, as far as I know, there is no reason that anyone could not broadcast DTV signals to his neighbors on his unlicensed device. If this turns out to be true, why can’t broadcasters do the same? I think they could use such unlicensed devices to create low cost gap fillers to provide signals in neighborhoods shadowed from their main transmitter. But I have digressed. I now return to the regularly scheduled program.

So where does all this data lead? First you may recall that there are no protection ratios for DTV-DTV interference in the UHF band. That is right, the UHF taboos only apply to analog TV transmissions, which will end in a few months. The current FCC rules for DTV-DTV interference provide protection ratios for co-channel and adjacent channel DTV-DTV interference only. Why is that?

As the FCC report says, the prototype DTV receiver tested in 1995 measured D/U ratios of about –60 dB for D = –68 dBm. That means the threshold U signals for that prototype receiver was about –8 dBm. DTV signals into receivers just don’t get that strong. So assuming that DTV consumer DTV receiver performance emulates that of the prototype, there would be no need to have UHF taboos. However, as this report shows, the receivers tested all have a single-conversion tuner the output of which is centered at 44 MHz, following commercial practice for analog receivers. The prototype 8-VSB receiver tested in 1995 had a double-conversion tuner designed to eliminate the image frequency interference from channels N+14 and N+15. Interference rejection should be specified and measured with two undesired DTV signals of equal power on channel pairs of the form N+K and N+2K.

As Table 1 shows, the median receiver tested gave results of such tests with D/U ratios at D = –53 dBm at around –35 dB meaning that interference will result when each U signal is received at –53 dBm + 35 dB = –18 dBm within a narrow range (–15 to –19 dBm). Such received signal power levels are not at all uncommon, so this median receiver would fail on some channels at many sites.

The performance of this median receiver is simply not good enough. Some manufacturers will try to increase their market share by improving the interference reject of new designs by 2009. However, in that same year, unlicensed devices will also appear in the marketplace. This is a race between improved receivers and homes having unlicensed devices or neighbors’ homes that will have them. I don’t know who will win, but I believe that manufacturers who do not improve the interference rejection of their products won’t be around in a few years.

THE BIG QUESTION

The problem for my readership is simply stated. Will DTV signals transmitted by broadcasters be around by the time the market for these unlicensed devices begins to saturate? Or will everybody become a cable or DBS subscriber? It will all depend on whether the FCC takes into account their own measurements of interference rejection by consumer receivers, where two or more interferers are present on channels of the form N+K and N+2K, which you’re probably first read about in this column in the past two years or so. Now, concerns over this new and unanticipated form of interference have been justified in this FCC Laboratory Report dated March 30. I urge you to download it from the FCC Web site.

And stay tuned, there is more to report.