Hi Dangerous Mind,
Quote: “How and when does the MS scan the RxLev of the BCCH of the serving cell?”
I don’t understand.. the MS does not scan a RxLev. RxLev is a measurement: the MS measures the received power of the BCCH downlink signal.
Quote: “And also, all the System Information messages are transmitted on the SACCH, so why does the drop call rate increase for a cell if it is experiencing co channel interference? If a cell has 4 TRXs for example, most of the calls would not be established on BCCH TRX, so why does the call drop rate increase?”
No, the SI messages are transmitted on the BCCH timeslot, on the BCCH logical channel. So all MS’s can listen to them.
As soon as a MS gets a SDCCH and/or a TCH there will be a SACCH interleaved with this “dedicated” channel. The SACCH and the TCH are located on the same timeslot. And here, yes, the SI msg are sent on the downlink SACCH. But this SACCH belongs to only one user, nobody else can listen to it.
Now, what’s the relation with cacll drop ? A call drop radio is detected when the BTS or the MS cannot decode the received SACCH. We do the assumption that if the SACCH is not decoded, then the TCH is not either. And that’s correct : they both are on the same timeslot, on the same frequency. So they suffer exactly the same radio conditions.
This assumption however is not working in AMR anymore ; the TCH bursts are highly protected, while the SACCH remains sensitive to interference. Therefore the call drop is detected (no SACCH decoded), while the actual voice (the TCH) could still be decoded by the MS. In this situation the drop occurs, but the subscriber feels like there was no need for a drop.