Charles W. Rhodes /
10.18.2006 12:00 AM
Making Room for Unlicensed Devices
The FCC has issued a timetable to authorize sharing of broadcast spectrum with unlicensed devices in the so-called "white spaces." So whether Congress passes the telecom reform act this year or not, the FCC will complete its rulemaking.
Unlicensed devices will be deployed in broadcast spectrum February 2009 according to this timetable.
That was the bad news. The good news is that the FCC laboratory will test consumer DTV receivers for interference immunities. Presumably, those measurements will provide the commission with the data needed to write its rulemaking to protect broadcasters from harmful interference.
We have already discussed the use of those first-adjacent channels ("Avoid Adjacent Channels For Unlicensed Devices," Aug. 23). Some fear that direct pickup off coax cables in homes fed by community antenna TV systems will also create interference. If so, you've really got a problem. Coax cable is shielded, but how well is it shielded? If it only has a braided outer conductor, it isn't really well shielded at all.
Double braid-and-foil wrap is the best solution, but it costs more.
Now the FCC might have to allow DTV transmitters to use those previously taboo channels. For example, in the Washington, D.C.-Baltimore area, DTV is being broadcast on Channels 33, 34, 35, 36, 38 and 39. This column has shown that strong signals on Channels 33 and 36, for example, can generate third-order intermodulation products in Channels 30 and 39. Channels 34 and 38 can interfere with reception of Channels 32 and 40; Channels 34 and 35 can by this same mechanism, interfere with Channels 33 and 36.
It follows that unlicensed transmitters on certain pairs of channels will create interference to DTV reception at sites near those transmitters. Imagine the problem a broadcaster would encounter trying to track down interference to his signal if it results only when both unlicensed transmitters are simultaneously on-the-air.
Will such interference become widespread? Yes, these unlicensed transmitters can be put to many uses, and they will be mass-produced for a mass market. That is at least one reason why so much political pressure is being exerted on Congress. Another reason is that in rural areas, wireless links would allow people to access the Internet. That will drive the political process.
As this column has already noted, if using one channel of each of these channel pairs were not allowed, such interference would be eliminated. That would leave the other channel of each channel pair to be used by unlicensed transmitters. These channel pairs create interference by generating third-order intermodulation, or IM3 products that fall into the DTV channel thus creating interference in the desired channel.
WORST CHANNEL PAIRS
The worst of these channel pairs is n+5, n+10, as this generates IM3 in the DTV channel (n) and also in n+15, the image channel. This creates a double whammy with up to 3 dB more undesired signal in the intermediate frequency as was described here in my column in the June 28 issue of TV Technology, ("Can Broadcasting Survive Unlicensed Devices?").
There is another interference mechanism yet to be considered: cross-modulation.
In the CATV industry, it is called "triple-beat interference." The DTV signal is amplitude modulated. Any really strong undesired signal that gets to the RF input port of the mixer can amplitude-modulate the desired DTV signal. This is cross-modulation (X-M).
Both intermodulation and X-M result from the same kind of nonlinearity. IM3 may be more potent than X-M because it has the total power of two undesired strong signals behind it. A lot is known about intermodulation and X-M with analog signals, but not much is known about intermodulation versus X-M with digital signals.
I think I am on safe ground to assume that these unlicensed transmitters will not be modulated by analog signals like NTSC, but there is no such prohibition in the FCC's proposed rules. By the time unlicensed transmitters might be in widespread usage, there will be no analog TV broadcasting (after Feb. 17, 2009).
I urge broadcasters to support proper scientific studies with experimental confirmation of findings to compare the interference to DTV reception by one undesired signal a few channels from the desired channel (X-M); versus the interference caused only when certain pairs of channels are in use (intermodulation).
If X-M is deemed an intractable problem, the only white spaces left would be outside the UHF band. The high-VHF band is well-suited to DTV transmission, while the low-VHF band is generally known to be unsuitable for DTV.
There are only 43 DTV channel assignments in the low-VHF band, according to the reported results of the second round of broadcaster choices. I believe the best solution for these unlicensed transmitters will be the creation of a new unlicensed band by reallocating the spectrum from 54-88 MHz.
This would require 43 additional DTV channels in the high-VHF and UHF bands to be found for those broadcasters who have indicated they want a low-VHF DTV channel.
Soon, the results of the third round of the channel selection process will be known. There may then be even fewer broadcasters still requesting a low-VHF channel, and consulting engineers will know exactly which channels will be available in the high-VHF and UHF bands.
But somehow, broadcasters must get answers to these questions I've raised here. Too bad they shut down their Advanced Television Technical Center a few years ago. Consulting engineers can find those needed channels in the high VHF and especially the UHF band, (after NTSC shutdown), but where can the experimental work needed be carried out with FCC oversight and fully documented?