Acquiring the PSS & SSS: from a UE perspective

I have been reading LTE materials of late and stumbled upon the process of UE initial procedures to attach to the LTE network.

The procedure starts with searching a suitable cell.
The UE first scans frequencies and then selects an appropriate PLMN, in this case, let it be a 4G LTE PLMN.
In a single PLMN, there can be numerous cells, TAs.
Thus, the UE always tries to camp on the best cell based on measurements.
But, before that, the UE synchronizes with frame timing by decoding the SSS and then to get the cell ID, it decodes PSS.
If I am not wrong, the UE first acquires the Cell ID and Cell group information by decoding the PSS & SSS that comes in a DL frame.

My query is: who sends these PSS & SSS messages via the Downlink?
Only after decoding PSS & SSS, the UE knows the cell identity and subsequently, camps onto it. Before this could happen, from where does the UE get the DL PSS & SSS?


PSS and SSS are physical layer synchronisation channel which will be generated by enode b.

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Thanks for the response. Which eNB generates the PSS & SSS? Only after decoding the PSS & SSS, the UE can find out the cell ID and cell group and shall camp on it. But, at the very first, who sends the PSS and SSS to the UE before it has camped on to any cell? After selecting the PLMN, does it temporarily stay on a random cell and listen to the PSS & SSS and then camp on to cell based on PCI?

As I think, ue does not latch any random cell to acquire PSS & SSS, rather upon power on, ue goes for freq scan , so in terms of power level or by rssi scan, it makes a list of frequencies, once it receives the best earfcn with power level , it goes for PSS & sss reading, then mib & sib & so on…
Inside Uesim all plmn priorities are defined so ue always goes for its own network plmn

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All eNBs generate PSS and SSS. Typically each eNB uses it’s own SSS, and each sector uses one of the three PSS. SSS and PSS are both sequences of values over time-frequency resources, repeated periodically. Sequences are known to UE in advance. UE correlates the known sequences with what it receives, and selects the one with the highest correlation. That is going to be the cell it will camp onto. After that the UE is able to receive MIBs and SIBs.


Noted. Thanks for the response.

Does it mean that the UE beforehand knows the list of PSSs and SSSs of each eNB in a cell? How is the UE informed of the sequences?

Is my understanding correct as mentioned below?

I have read that the UE scans the E-UTRA bands, looking after a carrier frequency (f1), in 100 kHz intervals, by listening to beacon signals. Once it finds the appropriate carrier frequency (f1), it also ensures if this is the strongest carrier frequency among all other cells. The UE captures this strongest frequency and decodes the beacon signal, it listens to, to discover the cell ID and other related information.

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In LTE (within a carrier) all cells typically use same frequency. PSS and SSS are probably what you call the beacon signals. The sequencies are standardized, so the UE knows what sequencies to try for correlation. Please note that this doesn’t yet require any kind of decoding. When UE has determined the PSS and SSS sequencies giving the strongest correlations, it can calculate Cell ID. With cell Cell ID the UE knows where the Reference Symbols are located, and can receive and decode MIB and so on. MIB is the first “real” message to be decoded.

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Yes your understanding is correct. When Power ON, Ue will search all the supported band EARFCN as per its capability and try to filter the list of EARFCN based on RSSI and sorted in ascending order of received RSSI. From the EARFCN Ue can easily calculate centre frequency. Since the time/frequency position of PSS/SSS is fixed by 3GPP(Centre 62 Sub carriers) ,Ue can read the PCI.Then Ue can read MIB in PBCH which was scrambled with PCI.Then Ue comes to know BW,SFN,Antenna config and PHICH.From PHICH,PDCCH config may be obtained.PDCCH will shows the placement of SIB in PDSCH.Then cell selection procedure will be started based on S criteria .(Note preferred PLMN Value list stored in SIM Card and also broadcasted in SIB1).

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