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#5G #LTE #5GCellSearch
When you first power on the device or your device got into out of coverage and try to detect/search a new cell, UE does not have any idea on which frequency it has to try camp on.
So it may have to do some kind of blind search.
For example,
· UE is off.
· Power On.
· Scan all the frequency.
Here two type of frequency scan: a) SLS ii) DBS
SLS: Storage List Search: Its check and Scan those frequency which was stored in mobile before switch on.
DBS: Deviated Band Search: If after SLS search UE didn’t find any cell then UE will start DBS Search where UE will scan all the frequency for whole band.
For example, let's assume that your device support 5G Band n1, n2, n20. *#0011# or *#2263#
The eNodeB around your UE may use any frequency channel from nr-arfcn range (422000, 434000) FDDBand-n1, nr-arfcn range (158200, 164200) for FDD Band n20 and nr-arfcn range (386000, 398000) for Band n2.
There can be lot of possibilities of frequencies that eNB would use. Then how UE can detect/find the cell (NodeB) it would camp on?
Step-1) UE tune to each and every channel that it supports and measure RSSI. (RSSI is simply a measurement of whatever energy/power it can measure. This measurement does not require any channel coding process. So, at this step, UE does not need to know anything about the network. At this step, UE does not try to decode PCPICH (in WCDMA) or Sync/Reference Signal (in nr-5G) to detect Physical Cell ID. It just measures the power of each channel.) As UE measure RSSI for each channel, it creates a list of each channel numbers with the measured RSSI.
Step-2) Then UE go through the list from Step-1) and figure out all the channels which shows RSSI value greater than the threshold (this threshold is also up to UE/chipset implementation, not determined by 3GPP).
Then the question would be "Any frequency with Passing RSSI value can be the one that UE can camp on?” The answer is "Not Necessarily".
Step-3) This Step is little Complicated so it will take time to understand complete process. UE do synchronization process followed be PSS and SSS and decode reference signal and detect physical cell ID from each candidate from Step-2).
(Some candidate gives successful result but some would not. UE make the list of all the successful tries).
i) Now from Step-2) list, One by One UE will start synchronization process for all frequency.
ii) In above log we can see 1st preferred earfcn is 386670. So UE will scan nr-arfcn 386670 and will try to get suitable SSB.
iii) Actually, for both FDD and TDD network broadcast synchronization signal in downlink direction and this synchronization is primary and secondary synchronization signal. By using PSS and SSS UE would find multiple cells in this process.
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