LTE Reference Signals Video (Explains RSSI, RSRQ, RSRP)

[swwifty]

I found this recently while doing some more research and learning on reference signals in LTE. I highly recommend watching it, and downloading the spreadsheet in the description and playing with it. This will help you understand what these values mean, and how they related to each other. I've been trying to relate these to Wifi (802.11) signal values, and this helped me understand it a good bit more. One thing I learned that is very interesting is RSRQ indicates cell load, and interference from other cells. If your value is more than about -10.8, then you have interference from another near by cell (on the same band) and if it is at the max of -10.8 then the cell is fully loaded serving users. Enjoy!

https://www.youtube.com/watch?v=FVgLUBVjKu4

[swwifty]

After learning from this video and spreadsheet, I decided to try and see how the reference signal values I see with my 4G connection compare. This required a little bit of a reverse engineering, but I think I figured out a few interesting things.

1. I don't know how much power the tower transmits with, but I did randomly see in several places across the internet that some towers transmit at 44dBM per antenna. I used this value, and the results are very similar to what I see in my real life scenarios. (see results below).

2. I don't know the path loss, so I used a online rf signal estimator (towercoverage in this case) to estimate path loss for various frequencies.

In this example, this was my band 2 test with my 15dbi flat panel antennas. It was at the top of the mountain behind my house (for those of you who are aware in my review post on this antennas) and nearly LOS except for about 3-5 trees.

Screen Shot 2018-10-03 at 11.16.59 AM.png

The at!lteinfo command is very useful as it turns out. Below, you can see I have some interference from intra-frequency bands. This high up on my mountain, I was picking up another cell tower somewhere in the area (not sure where) that is broadcasting on the same frequency and interferring with the signal from the tower I'm connecting to. Hence why you see the RSRQ that is more than -10.8. This means the cell tower was fully loaded (at the time of this command being run) and I was getting interference from another cell tower.

Screen Shot 2018-10-03 at 11.17.12 AM.png

And lastly, I modeled this state in the RSRQ speadsheet, and found it to be fairly close to reality.

Screen Shot 2018-10-11 at 1.32.34 PM.png

This is with a path loss of 110 db (for 1900mhz only) and the tower antenna gain of 17dbi, transmitting at 44dbm (note: in the spreadsheet this was converted to 25 watts).

[swwifty]

I recently went to a really high state park near my house with clear LOS to a lot of cell towers. Out of curosity, I took my PI setup with modem to see what LTE signals I'd pick up. The results were somewhat surprising.

Screen Shot 2018-10-12 at 5.53.35 PM.png

Check out all that interference from all the various cell towers my modem was picking up! You can even see it reflected in the RSRQ, and I've never seen a SNR that is negative! I didn't test my connection, but I'm willing to bet it wouldn't have worked very well.

Screen Shot 2018-10-12 at 5.52.44 PM.png

[swwifty]

This is an interesting paper on Cell Load and RSRQ.

http://tma.ifip.org/2018/wp-content/uploads/sites/3/2018/06/mnm2018_paper9.pdf