After having taken a look at GSM and UMTS signals on the physical layer, this post is about how LTE signals are visualized by SDR-Sharp.
In Germany we are in the comfortable position to have three network operators offering LTE services in the 800 and 1800 MHz bands so it is not difficult to find an LTE carrier signal. Like for UMTS, it's not possible to show the full 10 MHz (800 MHz band) or 20 MHz (1800 MHz band) LTE channel as the hardware and software are only capable of showing around 2 MHz at a time. But one can observe the edges of the signal or any particular part in between. The first picture on the left shows part of an LTE signal on the 1800 MHz band. All LTE signals I have observed feature the vertical stripes. While the stripes make sense due to the OFDM modulation using many individual 15 kHz carriers that for a 10 or 20 MHz channel I am not quite sure why there are strips that are marked in yellow while other parts are blue (i.e. a lower signal energy). I first thought that perhaps this might have something to do with the reference signals but they are evenly distributed through the carrier and are not only present in particular spots!? Another particularly interesting thing in the image is the reddish bars in parts which is actual data transmission occurring while I downloaded a web page. In other words, it's pretty easy to see on layer 1 how loaded a cell is.
The second image on the left shows an LTE uplink transmission from my UE. Again, keep in mind that the image only shows a fraction of the total bandwidth used for uplink transmissions. The transmission starts in the lower part of the waterfall diagram and was again recorded while downloading a web page. Once the page was fully loaded one can see very nicely how uplink transmissions become spurious and at some later point, not shown in the image, cease completely. A further detail worth mentioning is the absence of the vertical strips compared to downlink transmissions. This is because in the uplink direction SC-FDMA modulation is used.
Incredible insight gained with a 20 Euro DVB-T receiver and the power of open source SDR-Sharp!
One thought on “Probing Layer 1 – Part 4: LTE Up- and Downlink Observed”
I think the vertical stripes in downlink come from the reference symbols, which are always onair, even if the cell is unloaded. What is the frequency spacing between the stripes, the same separation as the reference symbols?
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