Non-contention vs Contention Access procedure in LTE
Random access procedure is performed for the following five events:
• Initial access from RRC_IDLE;
• RRC Connection Re-establishment procedure;
• DL data arrival during RRC_CONNECTED requiring random access procedure;
• UL data arrival during RRC_CONNECTED requiring random access procedure;
E.g. when UL synchronization status is “non-synchronized” or there are no PUCCH resources for SR available.
Furthermore, the random access procedure takes two distinct forms:
• Contention based (applicable to all five events);
• Non-contention based (applicable to only handover and DL data arrival).
Normal DL/UL transmission can take place after the random access procedure.
CONTENTION BASED RANDOM ACCESS PROCEDURE
In total there are 64 preambles per cell. The 64 preambles are grouped to indicate the length of the needed resource. Furthermore preambles are reserved for contention-free access.
The random access procedure follows the steps:
1. A preamble will be selected by UE and transmitted in the available subframe. Based on correlation the NB may detect the access and furthermore can measure the timing of the UE transmission.
2. The NB answers using the same preamble and at this point a timing advance will be fixed. Information on the scheduled resource will be exchanged and a temporary C-RNTI will be assigned.
3. The UE sends its id. The type of id depends on the state. In case of idle state NAS info has to be provided (IMSI, TMSI) else the C-RNTI is used.
4. The contention resolution is performed, i.e. the NB addresses the UE using the CRNTI.
CONTENTION – FREE RANDOM ACCESS
E.g. during handover a temporary valid preamble will be issued. It is dedicated to this UE. No contention resolution is needed as the preamble shall not be used by UEs which did not get assigned a dedicated preamble, i.e. randomly select one.