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In the
11 months so far that we have been running our long term test and
conducting various other spectrum scans the noise floor has been stable.
Apart
from a weak carrier present on channel 35 which is is from a TV station
40km away and the interference issued mentioned below the spectrum
appears free of other users. Leaving us with 198Mhz of un-occupied
spectrum.
It would appear that one of the few benefits of the heavy
tree coverage in the area is that it leads to a stable low noise floor
of around -103dbm, any signal including un-wanted interference
experiences heavy attenuation.
Our test site is located 2.5km from a high powered cell
phone tower operating in the 1900,800 and 700Mhz bands.
When we point our directional antennas within 30 degrees
of this tower we start to experience signals on channel 39-45 of
typically a few minutes to 20 minutes duration.
At the moment we don't have the equipment to conclusively
track down the source of these signals, current theories are.
1) We are experiencing 3rd order harmonics from equipment
operating in the 1900Mhz band on the cell phone tower, 3rd order
harmonics would fall between 616-663Mhz or channels 38-46
2) We are starting to experience front end overload in
our receiver, the cell phone tower operates in the 700,850 and 1900Mhz
bands, strong signal levels could cause the production of undesirable by
products in our front end, the cure would be additional sharper filters
in the RX front end.
3) We have a bad connection somewhere, possibly in our
antenna or nearby rigging.
Whatever the source of the problem a TVWS operator,
operating close to a cell phone tower needs to be aware of the potential
for problems and plan accordingly.
February 2017
Open further research we have discovered that the problem
of cell phone towers and mobile phones causing problems to television
receivers is not uncommon in some areas, sufficiently so that filters
are now available for television receivers in some markets.
The manufacture of the equipment we are using 6Harmonics
was aware of this potential problem and has now considered it
sufficiently serious that the new version of the radio we are using
incorporates additional filters, we have not yet tested the new radio.
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