Spectrum usage examples – FM radio, mobile phones, and television
As part of a recent project, I had a look at differences in spectrum usage for a variety of services, partly to help introduce people to spectrum management and whitespace. Almost everyone should be familiar with broadcast FM radio, GSM mobile phones, and analogue & digital television so these were chosen as examples.
Underneath the hood, each service uses frequency spectrum in significantly different way. The following images show the received power of each service over a range of frequencies and a period of time.
For broadcast FM, the channel bandwidth is 200kHz and the frequency deviation is +/- 75kHz. For an example music radio station, FM104 based in Dublin, the audio content contains a lot of multi-frequency components i.e. music. As a result, the full bandwidth is used for the majority of the time. If you look closely at the image though you can click on the image to see a larger version, you may see a concentration of power in the centre of the band for some of the time. This occurs when the music stops and the DJ or a newsreader speaks (lower range of frequency content and reduced frequency deviation as a result).
This characteristic is more evident in the next image. It is a two-hour view of a music and speech programme by Dublin-based radio station Phantom FM
In a more speech-oriented radio programme, the spectral content is mostly concentrated around the centre frequency. Bursts of wider band activity can be seen also due to jingles, advertisements, and the occasional song.
Moving away from FM radio, the following image is an example of how GSM mobile phones use spectrum in the 900MHz frequency range. This plot shows the received power from Meteor, Vodafone, and O2 mobile phones communicating with basestations in their cellular networks. Unlike FM radio, GSM is a time-slot approach so small spectrum segments are used for a very short period of time in a frequency-hopping manner.
As for television services, we currently have a mix of analogue and digital television in the UHF band. In Oct, 2012, the analogue TV stations will be switched over to digital services (in Ireland, this is called Saorview).
