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Which process happens first RACH Process, PSS & SSS Synchronization, PLMN Selection or CELL Selection/Reselection?

+1 vote
976 views

Please provide the order also.

My Doubt
IN ORDER FOR PLMN SELECTION TO HAPPEN UE GETS BROADCASTED INFORMATION AND WILL READ MIB AND SIB1 (CONTAINS PLMN IDS)

FOR PSS AND SSS PROCESS TO HAPPEN, THIS INFORMATION WILL BE IN 1ST AND 11TH SLOT OF FRAME FOR PSS AND PSS PRECEDES SSS. THESE ALL NOTHING MEAN TIME , FRAME SLOT LEVEL SYNCHRONIZATION.

BUT THESE FRAME AND SLOT WORDS COMES IN SFN (WHICH IS IN MIB) WITHOUT READING MIB HOW PSS AND SSS HAPPENS.

AND IF WE SAY SYNCHRONIZATION HAPPENS WHEN UE IS SWITCHED ON AFTER FREQUENCY ACQUISITION. HOW IS THAT AFTER FREQUENCY ACQUISITION, SYNCHRONIZATION IS HAPPENING.

posted May 20, 2015 by Yeshwanth

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3 Answers

+1 vote

LTE Cell Search Procedure used by UE.

A cell search procedure is used by the UE to acquire time and frequency synchronization with a LTE cell and UE detects the physical layer Cell ID (PCI) of that cell. Two cell search procedures in LTE: initial synchronization and detecting neighbor cells in preparation for handover LTE.

Step-1: After being powered on, UE tunes the RF and attempts to measure the wideband received power (RSSI) for specific frequencies (channels as commanded by higher layer) over a set of supported frequency bands one after another and ranks those cells based on signal strength.

Step-2: Then it uses downlink synchronization channels i.e. locally stored P-SS and S-SS to correlate with received one. UE first finds the primary synchronization signal (PSS) which is located in the last OFDM symbol of first time slot of the first and 5th sub-frames This enables UE to be synchronized on sub-frame level. Primary Synchronization Signal helps for Slot Timing Detection and Physical Layer ID (0,1,2) detection.

Step-3:: secondary synchronization symbols are also located in the same sub-frame of P-SS but in the symbol before P-SS. From secondary SS, UE is able to obtain physical layer cell identity group number (0 to 167) It helps for Radio Frame Timing detection, find Physical Layer Cell ID, cyclic prefix length detection, FDD or TDD detection.

Step-4: Once UE knows the PCI for a given cell, it also knows the location of cell Reference signals - which are used for channel estimation, cell selection / reselection and handover procedures. After channel estimation using RS(reference signal), MMSE equalization is performed to remove the effect of channel impairment from the received symbols.

answer May 21, 2015 by Veer Pal Singh Yadav
+1 vote

Its PSS followed by the SSS Synchronization. Check the following diagram which describes the high level process step by step -

LTE Basic Call flow

answer May 21, 2015 by Salil Agrawal
0 votes

For UE to read MIB and SIBs . UE has to be : Frame Synchronized, should know the Time Slot and very important this is that it should know the Physical Cell ID. All these things UE gets from PSS and SSS.

Only After that UE reads MIB and SiBs followed by RACH.

answer Jun 2, 2015 by Anand Yadav
Hi sir, am yaswanth....

As u mentioned.......
[ For UE to read MIB and SIBs . UE has to be : Frame Synchronized, should know the Time Slot and very important this is that it should know the Physical Cell ID. All these things UE gets from PSS and SSS.
Only After that UE reads MIB and SiBs followed by RACH ]

Frame synchronized in the sense are u talking about system frame number.....

My knowledge is that

After getting MIBs and SIB1 only UE can be able to complete  PLMN selection...before PLMN....cell search and frequency acquisition process happens.....

in order for FRAME Synchronized , time slot n PSS and SSS to complete before MIB and SIB1 reading ,
and more to say plmn selection...

will UE try to sync (frame n slot level) with all carrier(eg ; airtel,idea,vodafone,aircel service providers ) bands it can be of capable...and later after plmn selection it goes to that particular carrier.....


PSS info will be sent on 1st and 11th slot of each frame.....so for frame synchronizaton and time to complete without reading MIB.....from where does UE get this information.....


My view is that after reading MIB which contains SFN.....and  SIB1(for selecting particuar network) it completes. PSS and SSS..

Correct me if i was wrong ...i have written wat i thought of.....

correct me briefly.....by analyzing the way i got back to you...

take your own average time to respond back...
Hello Yaswanth,

First UE should be synchronized on the Subframe Level and It should know the Physical Cell ID. Also Without knowing the Physical Cell ID UE can not Read MIB and SIBs. Once it knows the Cell ID it will also come to know the Cell reference Signals which are generally used for channel estimations. so for all this to Happen UE need to know the PSS and SSS.

The SFN which you are talking about is the System Frame Number which is generally used for UL Synchronization.
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