After switching on the Modem, need to sync up with both Frame and slot level synchronization. (Will discuss in details about the LTE frame Structure in some other day).
Today let just concentrate regardig Basic RACH and LTE authentication Procedure.
Modem will first read the PSS(Primary Synchronization Symbol) and SSS(Secondaary synchronization Symbol) from Network
for slot level and frame leel synchronization.
a. PSS will have Physical Layer Cell Identity
b. SSS will have Physical Layer Cell Identity Group
From the above two parameter received , Modem will calculate the physical Cell-ID using the below formula
Physical Cell-ID= 3*PLCIG+PLCI
Once Modem calculate the Physical Cell-ID and sync in slot and frame level, It proceed to read MIB first. MIB(Master information Block)contains system bandwidth information and other important parameter(system Frame Number,PLMN-ID,TAC,Radio and core network capability)
After reading MIB, Model will start reading SIB1(Contains information regarding whether or not Modem is allowed to access the LTE cell. It also defines the scheduling of the other SIBs.
carries cell ID, MCC, MNC, TAC, SIB mapping)
and SIB2(Carries common channel as well as shared channel information.
It also carries RRC, uplink power control, preamble power ramping, uplink Cyclic Prefix Length, sub-frame hopping, uplink EARFCN .)
After the above process the Modem is synchronized with the network in the Downlink direction and have read SIB1 and SIB 2. Now, it needs to synchronize in the Uplink direction.
So now for the first time, Modem will send message in uplink direction. This message is called MSG1.
a. Random Access Preamble(MSG1)
This will contain RA-RNTI and Preamble-ID(Modem generate these parameter randomely and with help of SIB2). This MSG1 will be received by eNodeB. eNodeB will reply with
b.Random access Response(MSG2)
This will contain T-C-RNTI, Timing Advance from eNodeB ,Preamble-ID etc. This MSG2 from eNodeB means,eNodeB is asking for MSG3 from Modem
c.RRCConnectionRequest(MSG3)
This is MSG3 and sent using resources given by eNodeB with "Contention Resolution Identity".Which is used to resolve the contention.
Now this is the end for contention less RACH procedure. If Contention Resoltion is taking place then MSG4 will be sent from eNodeB.
Please note if the Modem is getting rebooted then for the first time CRI is not present at MSG3
d.Contwntion Resolution Message(MSG4)
This is sent from eNodeB to Modem side. Need to elaborate this scenario with the help of the picture below.
In contention based, multiple UE's attempt to connect to the network at the same time. The eNB is intelligent enough to tackle this situation because every UE should be unique to the network.
The UE's can always send the same Preamble ID to the network, thereby resulting on collisions. This kind of collision is called "Contention" and is known as "Contention based" RACH Process. The network would go through additional process to resolve these contention and hence this process is called "Contention Resolution" step.
Step 2. The eNB conveys the resources reserved for this UE along with the Timing Advance (TA), Preamble ID and T-CRNTI (a number generated by eNB and asks the UE to send the RRC connection)
Step 3. UE sends the RRC connection Request using resources given by the eNB. It also sends the identifier (CRI) to the eNB which is used to resolve the Contention.
Step 4. The eNB runs an algorithm and generates C-RNTI which will be a permanent ID for the UE till the connection is alive. The eNB sends the UE identifier. In this step, the UE which has received the ID continues while other UE's will back off and try again
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