When talking about TV White Space (TVWS), it is easy to remember that TVWS devices (WSD) should not cause interference to TV broadcasting, but they are not the only devices to protect. In a regulative context, the radio microphones are either specifically mentioned as an instance of Program Making and Special Events (PMSE) equipment or more generally, they are a part of other radio communication that should not be interfered. The principle is justified and easy to understand, but what does it mean in practice. Which TVWS device power levels cause interference to radio microphones? How far the devices still interfere the radio microphones? The Finnish Tekes WISE project decided to find this out and organized a measurement campaign in the largest theater in Helsinki, in Helsingin kaupungin teatteri. Among other results, the following observations related to a typical WSD use were made: in the audience the WSD interferes radio microphones both on the co-channel and on the adjacent channels. Within a distance of 100 m from the theatre building, the co-channel interference can be observed, but on the adjacent channels the probability of interference is very low. A half a kilometer away neither co-channel nor adjacent channel is a problem.
There are three commonly referred methods for TVWS and cognitive radio interference management: geolocation database, spectrum sensing, and cognitive pilot channel. In all regulations in which the TVWS operation is specified, the geolocation database is chosen alone. In all measurement locations, we were able to detect the transmitting radio microphones with a spectrum analyzer, even if it was not possible to interfere the microphones anymore. This observation raises the discussion of need for spectrum sensing to detect the radio microphones. In many countries, the radio microphones need a licence for radio transmission. In practice, a relatively large portion of radio microphones are used without a licence, and even if the licence exists, the location of use is not specified. For a proper protection of the radio microphones with a geolocation database, either the radio microphone licensing should better cover the real use and the location of the radio microphones should be collected; or the WSD should implement a continuous spectral sensing on the co-channel and a few adjacent channels.
The measurement reports can be found in:
The related Fairspectrum letter (in Finnish) to the Ministry of Transport and Telecommunication can be found in:
Propagating thoughts >