Probing Layer 1 – Part 5: DVB-T Signals

Dvt-t-signalVenturing down a bit on the frequency scale I've now also taken a closer look at DVB-T television signals with my DVB-T USB receiver stick and SDR-Sharp. The image on the left shows the right edge of an 8 MHz DVB-T signal that encodes 4 television stations. Like in the LTE signals I wrote about in a previous post, there are also vertical stripes that can be seen in the waterfall diagram. The stripes are much narrower, however, likely due to the 1 or 4 kHz carriers used in the C-OFDM modulation of DVB-T compared to the 15 kHz OFDM carriers used in the LTE downlink. Interesting to note is that parts of the channel on the frequency axis are broadcast with more power than others. Not sure why, my understanding of DVB-T signals is very limited.

I was also quite baffled with how little of the spectrum assigned to terrestrial television is actually used today. In Germany, DVB-T can be broadcast in the 177.5 – 226.5 MHz range (i.e. a total bandwidth of 49 MHz) and between 474 and 786 MHz (i.e. a bandwidth of 312 MHz). That's 361 MHz in total or enough for 45 DVB-T channels. Despite DVB-T being a single frequency technology in which neighboring transmitters can use the same channel, not all can be used simultaneously as different TV stations are broadcast in different parts of the country. But even if only every second channel is used, that spectrum could still hold an impressive 90 TV channels. In practice much less is broadcast today and when scanning through the spectrum in Cologne, most channels were empty.

360 MHz is quite a sizable chunk of spectrum and as the popularity of DVB-T and terrestrial TV broadcasting is on the decline I can see why there are moves to re-assign the 694-790 MHz range for wireless Internet connectivity, i.e. for use with LTE and perhaps other wireless technologies in the future. The Wikipedia article linked above indicates that the additional band could be brought into operation by 2026. This would give network operators access to an additional 96 MHz of spectrum that, with a duplex gap of 11 MHz, would offer 42 MHz for downlink and 42 MHz for uplink data transmission. That's a bit more than the current digital dividend spectrum in the 800 MHz band bundled in LTE band 20. Here, 30 MHz of bandwidth is available in each direction and used by three network operators in Germany today.