From ptrkrysik at gmail.com Fri Sep 21 08:07:20 2018 From: ptrkrysik at gmail.com (Piotr Krysik) Date: Fri, 21 Sep 2018 10:07:20 +0200 Subject: Questions regarding maintaining synchronization of a mobile station Message-ID: Hi all, Gr-gsm's receiver currently relies on having SCH channel to keep synchronization. I want to remove that requirement so it'll be more usable in an SDR based burst transceiver for OsmocomBB. 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? Thanks in advance, Piotr Krysik From laforge at gnumonks.org Fri Sep 21 09:13:44 2018 From: laforge at gnumonks.org (Harald Welte) Date: Fri, 21 Sep 2018 11:13:44 +0200 Subject: Questions regarding maintaining synchronization of a mobile station In-Reply-To: References: Message-ID: <20180921091344.GN18005@nataraja> 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 -- - Harald Welte http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6) From 246tnt at gmail.com Fri Sep 21 14:18:53 2018 From: 246tnt at gmail.com (Sylvain Munaut) Date: Fri, 21 Sep 2018 07:18:53 -0700 Subject: Questions regarding maintaining synchronization of a mobile station In-Reply-To: References: Message-ID: Hi, > 1. How often mobile station (MS) checks if the synchronization is kept? They listen to the CCCH for their paging slots. The TOA and clock error of that burst will be used to adjust the timing and frequency. Between two CCCH, they just count samples ... their clock sync is good enough to only drift a few samples over several seconds. > 2. What is usually used to check if synchronization is kept, especially > when a MS is on a traffic channel? When it's on a TCH, it just derives the timing and clock from the burst. The TOA allows you to know how well you're aligned to the TDMA. And the average symbol rotation over the burst allows you to do fine frequency tracking. > 3. How MS regains synchronization? Does it always do full FCCH+SCH scan? Well it's not a 'scan', they assume they stayed on the same ARFCN so it will just listen for 12 frames to find FCCH and then SCH. Cheers, Sylvain