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pirelli/{rssi,txcal}: results of the Pirelli to CMU200 experiment
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sat, 16 Feb 2019 18:30:11 +0000 |
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I have been wondering for a long time now how the RSSI bars displayed in end user phone UIs correspond to actual Rx signal levels; Pirelli's RSSI icon on the left side of the home screen can go up to 6 bars, but it was not clear at all what these bars actually correspond to. I just did an experiment to shed some light on this issue: I took a decased Pirelli motherboard and connected it to our CMU200 instrument via the RF test port which Pirelli's mobo has like most phones. Pirelli's RF test port is of the Murata SWD/SWF type, same as Openmoko GTA0x and Motorola C139, and we have a test probe for these ports with precisely known insertion loss numbers, thus combined with the properly calibrated CMU200 instrument and our "main" SMA cable for which we also have precisely known insertion loss numbers (it is the setup we use to calibrate our FCDEV3B modem boards), it is a very trustworthy setup. I varied the downlink signal level put out by the CMU200 and observed the bars displayed by Pirelli's fw on the home screen, and this is what I got: transition from 5 to 6 bars at -75 dBm transition from 6 to 5 bars at -80 dBm transition from 4 to 5 bars at -85 dBm transition from 5 to 4 bars at -90 dBm transition from 3 to 4 bars at -92 dBm transition from 4 to 3 bars at -97 dBm transition from 2 to 3 bars at -97 dBm transition from 3 to 2 bars at -102 dBm transition from 1 to 2 bars at -104 dBm transition from 2 to 1 bars at -105 dBm at -110 dBm it is marginal: sometimes 1 bar, sometimes 0 bars, sometimes no srvc Looking at TI's reference firmware which Pirelli's fw must be based on, the Rx signal strength is passed from L1 to the G23M PS in the form of RxLev numbers that go into GSM 05.08 measurement reports (the standard [0,63] range with 0 corresponding to -110 dBm), ALR does no transformation, and RR passes these RxLev numbers to the Rx driver in the Condat drivers layer, which then converts them to the "bars" numbers for the UI. Pirelli's fw most likely retains this architecture, and their modified version of the Rx driver appears to have the following thresholds or somewhere around there: RxLev >= 35: 6 bars RxLev >= 25: 5 bars RxLev >= 15: 4 bars RxLev >= 10: 3 bars RxLev >= 5 or maybe 6: 2 bars RxLev abysmally low: 1 bar The 5 dB hysteresis seen in my CMU200 experiment is probably caused by the RSSI_MIN_DIFFERENCE logic seen in rr_csf.c in our TCS3-sourced version.