North Queens TV White Space Trials
Spectrum and Interference
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.
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.