Television spectrum erosion
The original UHF TV channels 14-83 on 470-890 MHz are the ones that have been affected by the FCC takebacks. First, the FCC took away channel 37 and gave it to the radio astronomers. Then, it took away channels 70-83 and carved that up for cellular and land mobile communications. Then, in 13 metropolitan areas, it took away one to three channels in the channel 14-20 range and gave those to land mobile communications.
More recently, starting in 1997, the FCC has begun taking more of the UHF spectrum from the broadcasters. First, it took TV channels 60-69 (746-806 MHz), then 52-60 (698-746 MHz). There are still broadcasters operating on these channels, and they can stay until their "digital transition" is completed. When they shut down their analog broadcasts, they must return these frequencies.
This takeback was made possible by the improved robustness of digital TV technology, including improvements in TV tuners. Much of the unoccupied UHF TV spectrum was caused by the poor performance of analog TV tuners. (Cable set-top boxes contain tuners that usually employ double conversion technology and are more robust.) TV manufacturers were able to keep the FCC from adopting receiver performance specifications, and were able to continue using tuners that were as inexpensive as possible. In order to protect these cheap tuners from interference, the FCC created a table of TV transmitter spacing requirements that are known as the "UHF taboos." That table is now Section 73.698 of the FCC Rules. There are channel spacing requirements not only for co-channel and adjacent channel stations, but also for stations separated by 2, 3, 4, 5, 7, 8, 14 and 15 channels!
Digital TV tuners will be more robust. For digital TV stations, there will be fewer and simpler spacing requirements. Therefore, the 68 TV channels that were needed for analog can be reduced to 50.
But in every city there will still be unoccupied channels. So now the FCC is proposing to take those channels and allow unlicensed transmitters to use them. There will be two classes of devices with different technical requirements: low-power "personal/portable" and higher power "fixed/access." The fixed/access units, operating with the same one-watt power limit that applies to WiFi and other spread spectrum devices, can be used by commercial Internet Service Providers to support a wireless ISP service. The personal/portable units, operating at one-tenth that power, could be used for local area networks. (WiFi devices, although allowed to use up to one watt, generally operate at much lower power levels.)
The real issue here is interference on the occupied channels. Broadcasters have complained that their viewers will receive interference from portable devices as they move around the country, and that interfering signals can travel long distances. And even the FCC seems to agree that you can't tell whether a channel is unoccupied merely by listening for a signal. The path to your receiver might be blocked, while a viewer next door might be getting a good signal. If your unlicensed transmitter fires up, it could mess up your neighbor's TV reception.
How do you decide when a channel is unoccupied? How does the radio know which frequencies it's allowed to use, particularly when it's in a laptop that you take from one city to another? The FCC has proposed a number of rules and procedures to deal with this. For example, portable devices would not be able to transmit until they first received an off-air control channel that contained a listing of the unoccupied local frequencies.
The FCC has defined the "protected contour" for TV stations (the area where interference is prohibited), and ISPs that use the higher power fixed/access devices would hire a frequency coordinator to calculate whether they are outside a TV station's protected contour. Outside the protected contour, interference is allowed.
So here is an area where cable operators might have an interest in this latest FCC initiative. Do you receive the off-air signals of TV stations? Are your off-air receivers located outside the protected contours of any of the stations? If so, you may have to contend with interfering signals from the fixed access devices.
The FCC's must-carry rules assign the broadcasters the responsibility to deliver a good quality signal to the cable operator. But the rules are written in terms of signal strength of the broadcast signal, not signal-to-noise or signal-to-interference ratio. (This is the case for analog must-carry; maybe we'll see digital must-carry rules one of these days.)
If your receiver is inside the broadcaster's protected contour, and if you can track down the source of interference, you can make that operator stop the interference. If your receiver is outside the protected contour, it isn't clear whether you can make him stop. And in any case, it is not clear whether the cable operator or the broadcaster is responsible for tracking down the interference.
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