According to the FCC's rules for Distributed Transmission Systems (DTS)—which took effect in February—within the noise-limited service perimeter of the central or main DTV transmitter, additional transmitters will radiate the same program on the same channel as the main transmitter to fill in gaps in their coverage area due to shadowing of the main signal by terrain features or buildings.
All such signals will be synchronous in frequency with each other, but slightly offset in time so that, where more than one of these signals is received, the strongest is demodulated and the others are treated as multipath and filtered out by the DTV receiver's adaptive channel equalizer.
These DTS enabling rules do not allow the broadcaster to operate such transmitters on any channel other than his assigned channel. Although they do allow him to extend coverage beyond his noise-limited DTV coverage, they don't allow him to "cherry pick" to which deeply shadowed neighborhoods he will provide strong signals from his additional transmitters.
But I'm not a lawyer, so please consult the appropriate sources at the FCC if you have specific questions.
My concern with these rules is the interference such signals may cause to reception near the secondary transmitters. The FCC believes that its rules will protect against co-channel interference, but these rules do not, in my opinion, address the well-known interference problems of Adjacent Channel Interference (ACI) or of what I have called Taboo Channel Interfer-ence (TCI). So let us wade into ACI in this column now and TCI next month.
ADJACENT CHANNEL INTERFERENCE
DTV repeaters, like DTV transmitters, will radiate sideband splatter from channel N-1 into channels N and N-2. A repeater on channel N+1 will radiate sideband splatter into channels N and N+2.
The FCC requires these repeaters to comply with the same DTV RF Mask that govern DTV transmitters. This means that the total sideband splatter radiated by these on-channel repeaters is 44.5 dB below the power they radiate on-channel.
As several authors have pointed out, this sideband splatter will be slightly attenuated in receivers, so in effect it can be considered as being 46.5 dB below the received power on N+/-1. That is, the noise power in channel N is 46.5 dB below the repeater's signal power in channel N. If the desired signal on channel N is weak and the undesired signal on either (or both) adjacent channels is strong, reception of the desired signal may be impossible near one of these repeaters.
Let's do the drill. The SNR minimum of 15.2 dB, the desired signal received power is -68 dBm. This results in -83.2 dBm maximum noise in Channel N (4.7864 E - 09 mW). The receiver generated noise is -99.2 dBm (1.2022 E - 10 mW). The maximum additional noise in channel N is then 4.6661 E - 09 mW or 83.2 dBm.
This additional noise in channel N may be due to sideband splatter from a repeater on an adjacent channel.
The received signal power from this repeater cannot exceed -83.3 dBm + 46.5 dBm = 36.8 dBm or reception of the signal on channel N will fail.
However this analysis ignores any in-channel noise, actually third-order intermodulation products, which may be generated in the DTV receiving appliance (DTV receivers and DTV down-convertors).
The FCC limits the desired-to-undesired signals power ratio to about -27 dB for DTV-DTV interference from adjacent channels. We should use this value.
If the desired signal power received is -68, the D/U ratio (ACI) DTV-DTV is -27 dB, our maximum U power at Rx is -41dBm. The sideband splatter below U is 46.5. The noise from radiated splatter in D channel is -87.5 dBm (1.7783 E - 09 mW). The receiver-generated noise is -99.2 dBm (1.2022 E - 10 mW), and the noise from above sources in D channel is -87.2 dBm (1.8985 E - 09). If there is noise from both channels, add 1.7783 E - 09 (1.8985 E - 09 mW). The total noise in D channel from N-1 and N+1 is 84.35 dBm (3.6768 E - 09 mW). The maximum noise in the D channel must not exceed -83.2 dBm.
As the D and U signals will fade independently, I believe these noise margins would not give reliable reception, even if the receiver is perfectly linear at these signal powers.
For example, there are other noise sources that are not taken into account by the FCC. These include second-order distortion products of multiple FM radio signals into DTV channels 7-13, or in the case of DTV channels 14-51, third-order distortion products from soon-to-appear White Space devices (TVBD).
What amount of such distortion products would it take to kill DTV reception? If the maximum in-channel noise at D is -68 dB, 83.2 dBm (4.784 E - 09 mW), and the total noise for one adjacent channel is 1.985 E - 09 mW, the permissible excess noise in D channel is -85.4 dBm (2.8878 E - 09 mW).
This is a very small amount of third-order distortion products, which, if they fall in the desired channel, would jam reception. They will fall in the desired channel (N) if the undesired signals are on channels of the form N+K and N+2K, where K is any integer between -10 and +10 or in some cases an even larger range for K.
With sideband splatter from both adjacent channels, we get a total noise of -84.35 dBm (3.6768 E - 09 mW). If we subtract the maximum in-channel noise -83.2 dBm or 4.7863 E - 09 mW, we get the maximum additional in-channel noise (actually IM3) of 1.1095 E - 09 mW, or -89.55 dBm.
Many combinations of signals in the same TV band can produce more than this amount of IM3 as we will demonstrate next month.
The problem I foresee is one of locating on-channel repeaters so that their signals don't cause interference to nearby DTV reception.
Some repeaters might have to reduce power. But even with reduced repeater power, closer to the noise-limited perimeter, there may be interference.
At D = -81 dBm, the maximum noise in channel N is 13 dB lower, -96.2 dBm or 2.3988 E - 10 mW.
If the receiver-generated noise is unchanged at -99.2 dBm (1.2022 E - 10 mW), the maximum additional noise (splatter) is -99.2 dBm (1.1966 E - 10 mW). The maximum undesired splatter is +46.5 dB and the maximum undesired signal power from the repeater is -52.7 dBm.
Finding sites to meet all these criteria may prove difficult or impossible.
A POSSIBLE SOLUTION
All broadcasters in a given community must agree to adopt DTS using the same sites and transmitting antennas for all repeaters, and to switch over simultaneously. If some broadcasters fail to agree, their signals could be subject to interference. The FCC cannot compel anyone to use this technology, so it must be voluntary and unanimous.
Next month, we will see that this applies to more stations than those on first adjacent channels.
Charlie Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via e-mail email@example.com.
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