Maybe, maybe not - Say channel 6 (just pulling one out of thin air for
use as an example), is "empty" in "Town A". That doesn't mean channel 6
isn't being used - It could very well be that it's been deliberately
left "empty" there because in "Town B", there's a station transmitting
on it, and if someone were to fire up a transmitter in "Town A" on
channel 6, it would step on (or be stepped on by) the transmitter in
"Town B".
If the intention is to serve Town A with Program A and nearby Town B
with Program B on the same channel, then the old tradition to use a
single 300 m tower in each town will fail.
According to claims in this threads that even current ATSC 8VSB
receivers would be capable of operating with some kind of single
frequency network (SFN), using lo to medium power transmitters on
medium height poles within the town, will help to concentrate the
signal to the intended reception area and keeping the spill over to
adjacent cells at acceptable level.
In which case, letting Joe-whoever transmit Wi-Fi on it would very
likely cause unacceptable interference to reception in "Town A", and
depending on the power levels involved, could potentially cause trouble
in "Town B" as well.
This is known as the "hidden transmitter problem" as in any CSMA
network, in which a device in an unfavorable location can not hear
that there is some other activity on the channel. Thus the device will
fire up the transmitter thinking that the channel is free.
I don't think that those devices are going to cause problems in
locations, in which the TV signal is strong enough for indoor
antennas, since also the device most likely will hear some activity on
the channel, even if the signal level would be insufficient for TV
reception at that spot.
The situation gets complicated when the TV signal strength is so low
that directional outdoor antennas must be used. A device in the
basement would get no TV signal at all and thinking that the channel
is free, fire up the transmitter and cause interference to TV
reception.
A SFN style TV network would help to keep the TV signal level at
sufficient levels even indoors, preventing the device from
transmitting on that channel.
Also, keep in mind that *THE BROADCASTERS* have *VERY* little control
over which channels they're able to use - Sure, everybody and his dog
could say "I'm using this channel", but there's this entity here in the
USA called the FCC, and they've pretty much been made the "dictator for
life" in almost all aspects of what happens where on the radio spectrum.
Practically every country has such organizations for frequency
coordination. However, at least in US the broadcasters have had a very
strong lobby groups influencing the FCC.
Frequency bands are allocated for various services such as
broadcasting by international agreement within ITU-R (ex CCIR) and the
allocation is more or less the same at least within a continent.
In the US, the demands for setting up your own TV station is so high
that available spectrum would not be sufficient and the FCC tries to
arbitrate between the various players in the broadcast industry. You
enemy is not the FCC but the next door TV station, competing for
frequencies and for income from advertisement.
The relative importance of broadcasting has been reduced with much new
technology used by the majority of people, so for instance cellular
phone lobby groups have much more influence on FCC and similar
organizations worldwide.
The cellular and Wi-Fi industry has traditionally had only very narrow
frequency bands (compared to broadcasting) and hence frequency
efficiency has been very important from the start.
However, in broadcast industry, the frequency efficiency has been
quite bad and the only improvement in frequency efficiency since the
1930's was the digitalization, allowing 4-6 times the number of
channels compared to the analog era. On the network level, the
spectral efficiency is still poor compared to the cellular phone
industry.