Hi Piotr,

On Fri, Sep 21, 2018 at 10:07:20AM +0200, Piotr Krysik wrote:

Gr-gsm's receiver currently relies on having SCH channel to keep synchronization.

are you referring to carrier clock (oscillator) synchronization, or TDMA multiframe synchronization?

To do this it would be great to know how normal mobile phones maintain synchronization, something I don't know currently. Especially:

1. How often mobile station (MS) checks if the synchronization is kept?
2. What is usually used to check if synchronization is kept, especially

when a MS is on a traffic channel? 3. How MS regains synchronization? Does it always do full FCCH+SCH scan?

For tracking the clock of the currently serving BTS, you have the coarse search window of +/- 1kHz clock difference until you've been to FACCH+SCH sync. From that point on, the clock drift between sender and receiver is tracked by relying on the TCH only. I think e.g. the calypso had something like +/- 50Hz tracking capability while on a dedicated channel, meaning if there's more clock difference, it would no longer lock onto the signal and just receive junk, leading to L2 closing the channel.

In terms of synchronization to the TMDA frame of the other cells: This is part of the neighbor cell measurement process, and I'm quite sure it's specified quite tightly in the relevant specs. IIRC, the MS must at least monitor up to 12 meighbors of which the 6 best are to be reported during the measurement report.

For tracking the neighbors during active use of TCH, the MS uses the IDLE frames in the 26-multiframe. It keeps "sync state" for each of the neighbors, including * carrier clock / oscillator recovered from FACCH on that neighbor * BSIC + frame number on that neighbor

If the BSIC ever changes, the MS knows that the neighbor has changed (different neighbor on same ARFCN) and all higher-layer state must be invalidated.

I'm quite sure pretty much all of this should be in the jolly/handover branch of osmocom-bb.git

Regards, Harald