Wed Apr 06, 2022 8:30 pm
What basically happens with all mobile technologies (UMTS, LTE, 5G) is that MNO creates two sets of cell identities per cell.
One is sent from cell to mobile device when mobile device is idle. It is used by mobile device to scan for neighbours and to find a better cell to roam to. As long as new cell belongs to same LAC/TAC (Location/Tracking Area Code), mobile device switches to the other cell without any communication with either old or new cell. If the new cell is in different LAC/TAC, then mobile device needs to perform LAU/TAU (Locaton/Tracking Area Update). The whole stuff is needed for network to search (and wake up) mobile terminal in case network has something to send to mobile terminal (e.g. terminating voice call).
Since terminal is idle and has plenty of time to perform measurements on wide range of frequency bands, this list tends to be pretty long and will contain cells from all bands that MNO operates (even if, e.g. they are not valid combinations for CA). It will also contain cells from different technologies (e.g. 3G cell will broadcast the list containing 3G, 4G and 5G cells and 4G cell will broadcast the list containing 4G, 5G and 3G cells). The identification data in this list is pretty definite so mobile terminal can identify cells exactly (i.e. in LTE it can decode MCC, MNC, TAC, eNodeB ID and local cell identity - LCI; together they form CGI - Cell Global Identity).
The other list is sent from cell to mobile terminal when mobile terminal is in connected/dedicated mode. It serves two purposes: mobility (contains a pretty short list of neighbouring cells which according to MNO's expertise are most likely to be used for handovers) and carrier aggregation / 5G (in NSA mode). As mobile terminal is busy transmitting data and vast majority of terminals only have single radio (OK, that's not entirely true for modern multi-CA terminals but similar principles apply), it can only perform limited amount of measurements (and on single frequency band). There are modes where terminal gets opportunity to measure on other frequency bands (in 3G that's compressed mode, in 4G and 5G cell creates gaps in trasmission to give terminal opportunity to tune to another frequency, quickly perform some measurements and switch back). The second list also contains very limited amount of identification data which means terminal can not identify cell whose signal it just measured, that's then up to cell to figure out which cell was actually measured. The limited identification data is there because it takes (too) long time for mobile terminal to receive enough data from measured cell for exact identification (i.e. in LTE terminal can only decode PCI and together with frequency band serving cell should be able to figure out which of neighbouring cells is it ... if MNO engineers or their automagical tool f**ck up, then serving cell might not uniquely determine which cell was measured and handover might fail).