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)
Other Cases
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.
During
RACH procedure (Assuming only One UE trying to Access N/w at that
moment)
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 ).
Now
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)
Then
N/W
responds with Msg4 (i.e ContentionResoultion Message) – Some times N/w
piggyBacks RRCConnectionResponse with this ContentionResolution
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.
Now
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.
preambleInfo
numberOfRA-Preambles: n52
preamblesGroupAConfig
sizeOfRA-PreamblesGroupA:
n44
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)
and
45 – 52 preamble set used for Contention Based but for
the UEs who are far from eNodeB