RANDOM ACCESS PROCEDURES

LTE sustain two types of random access procedures. Which procedure is used depends on the specific circumstances.

·        Contention – Based Access :

It is the normal access operation, and is used on initial system access when a radio link is lost and during some handover scenarios.

The UE initiates the operation by transmitting a randomly chosen preamble over the RACH, and then waits for a corresponding uplink assignment from the eNB before it can transmit its message.

·        Non – Contention Access :

It is used when the UE is already in communication with the eNB. The network initiates the process by sending an allocated preamble to the UE indicating when it can transmit.

Since the eNB provides a dedicated preamble not used by any UE, there are no collisions with other UEs and latency is significantly reduced.