Probing Layer 1 – Part 2: UMTS Layer 1 Visualization With SDR-Sharp

Since introducing SDR-Sharp in a previous post, I've had a lot of fun discovering a lot of stuff on layer 1 all throughout the spectrum. This post shows a couple of screen shots of UMTS carriers in the uplink and the downlink direction.

One limitation of the tracing solution is the maximum tracing bandwidth which is limited to around 2 MHz. While this is good enough to show several 200 kHz GSM carriers on the frequency axis it is by far too narrow to show a full 5 MHz UMTS carrier. But what it can show quite nicely are the signal flanks at either end of the 5 MHz channel or the gap between two 5 MHz adjacent carriers. The later is shown for the downlink direction in the first image on the left. Forget the pseudo signal energy in the middle of the diagrams as this is introduced by the hardware and is not received over the air. Apart from identifying clearly that there are two adjacent carriers on air the image also shows data transmissions on the two carriers. While taking this screenshot my mobile was on the left carrier and I downloaded a mobile web page which left the redder and broader streaks in the middle of the screen. As even this light load can be seen it can be assumed that at that time both carriers were pretty much idle.

The second image shows the same channels in the uplink direction somewhat lower on the frequency axis. At the bottom of the waterfall diagram both uplink channels are unused. Then about 40% into the waterfall I clicked on a link in the web page to start a download. This requires data transmission in the uplink. In this case my mobile transmitted on the carrier on the right. There is some signal energy on the left of the waterfall diagram but this seems to be a reflection of the right carrier, again introduced by the receiver and not really on the air. One can also see quite nicely where actual data was transmitted (the red parts) and where only radio signaling information was exchanged with lower energy (the yellow parts). Also it seems my mobile was redirected as it started uplink communication on the left carrier (the somewhat more solid small yellow line) but the network then took the communication to the second channel.

In case you want to try yourself and wonder where to find UMTS carriers, this UARFCN calculator page gives you the needed details. Have fun!