Purpose : This procedure Used to get N/W Access
In General RACH Procedure is triggered in 5 Cases, but Not to confuse with many things the below ONE is the common case
1) To get Initial Access : Which means to register with the network initially this is the first process (After MIB, SIB reading)
2) During Hand Over
3) Radio Link Failure
4) UL Data Arrival and UL is not in Sync
5) DL Data Arival and UL is not in Sync.
In Resource Grid Where Does RACH Sits:
RACH will be present in 6 successive RBs (Resource Blocks) i.e n to n+5
Which means 12 Subcarries of 15 Khz * 6 = 72 SubCarriers
But for PRACH preamble each subcarrier space is 1.25 Khz i.e One 15Khz = 12 PRACH SubCarriers
That makes 12 * 12 * 6 = 864 PRACH SubCarriers
In these 864 SubCarriers 25 Subcarriers are used as Guard Subcarriers (13 + 12) which makes 839 Subcarriers.
This Values makes the Zud-Off Sequence Value.
As we mentioned above RACH occupies n to n+5 RBS. Where Does UE gets this ‘n’ value is from SIB2’s “prach-FreqOffset” value.
UE Selects RA-RNTI value which ranges from 1 to 10 (For FDD – Based on Which SubFrame UE sending RACH) And with a Random Preamble ( < 64 ).
RAR (Random Access Response) will be received from N/W with Same RA-RNTI value.
Once UE Receives RAR
UE responds with Msg3 (i.e RRCConnectionRequest Message)
The above are steps in RACH Procedure.
Now next step is to understand what is ContentionResolution
RACH : Two types of RACH are present
1) Contention Based
2) Contention Free
What is Contention-Based :
The Example Scenario for this is:
Take an example at Airport where all passengers will be switching on UE once they arrive at Airport. Switching-On means everyone is trying to register with Network and we know that in this process RACH procedure will happen. This RACH Procedure is happening in Bursted Mode.
Here The preamble Selection happens in Random by UE. So, there are chances that multiple UEs can send RACH Indication with same Preamble ID. Which Means there is contention for N/W Resources.
As such N/W is not aware of this burst, N/W responds with RAR with Same Preamble ID.
All UE who receives this RAR Messages assumes that RAR is addressed to them and all UE started sending Msg3 .
Msg3 contains ReasonForAttach + Ue Identity (which is independent of UEs)
Then N/w Responds for the Msg3 which it Received only and responds with Contention Resolution Message where it will have all 5 Bytes that are sent by UE.
Now corresponding UE alone responds with Msg5
and other UEs will re-start RACH Procedure.
Contention-Free : Lets suppose there is RadioLinkFailue during Call and it is not wise to allo UE to select Random Value to get Resources from eNodeB. So, each eNodeB preserves few Preambles for this. UEs will come to know these set value from SIB2.
where numberOfRA-Preambles are referring to Contention-Based i.e < 53 and from 53 – 64 , these preambles used by N/W during the scenarios which are mentioned as 2,3,4,5 in first Para. So here 53-64 Preamble values are used only for Contention-Free RACH Procedure.
sizeOfRA-PreamblesGroupA : Contenion Based Preambles Set but who are near to eNodeB ( < 45)
45 – 52 preamble set used for Contention Based but for the UEs who are far from eNodeB