RACH Procedure in LTE

1. What is full form of RACH?
A. Random access Channel Request.

2. What are the types of RACH?
A. Contention based and Non Contention based.

3. Where to use contention based and where to use non contention based RACH?
A.
Non Contention based:
                                     During handover non contention based RACH to be used as HO is time                                                intolerant and if contention based RACH used then call drop might be                                                    observed.
Contention based:
                                     All Other time contention based RACH can be used.

4. Why do we need RACH?
A. Whenever UE want's to establish a RRC Connection , it needs to perform the RACH procedure.
     This procedure helps UE to get synced with the NW and perform the desired action over the                established RRC Connection.

5. When do we need RACH?  
A. According Spec:

• Initial Access - UE is trying to access the network in RRC idle state.
• During RRC connection re-establishment procedure .
• Handover
• When uplink synchronization is lost :- It means from a certain duration network has not received anything from UE in uplink.
• When UE does not have any PUCCH resources available for SR(Scheduling Request) .
• When timing advance is needed for positioning purpose in RRC connected state for UE.

6. Steps for Random Access Procedure?
A. From 3GPP TS 36.321(MAC) Segment 5.1

1. 5.1.1 Random Access Procedure initialization
2. 5.1.2 Random Access Resource selection 
3. 5.1.3 Random Access Preamble transmission
4. 5.1.4 Random Access Response reception 
5. 5.1.5 Contention Resolution 
6. 5.1.6 Completion of the Random Access procedure  

Reference:

3GPP TS 36.300 segment10.1.5.1

3GPP TS 36.331(RRC) segment 5.3.3.3

3GPP TS 36.321(MAC) segment 5.1.5