WHAT IS THE MEANING OF THE RANDOM-ACCESS BACK-OFF PROCEDURE?

In the incidence of a lot of users sending random access attempts, too many random attempts at the same time can result in Random Access Overload. For the event of Random Access Overload, a Random Access Backoff procedure is supported. This procedure prevents immediate new Random Access attempts.

The UE waits until a back-off timer has expired. The backoff timer is set to a value of 0 msec at the start of the random access procedure, so by default a UE does not have to wait before identifying the next subframe for PRACH transmission.

The eNodeB can increase the value of the back-off timer by attaching and E/T/R/R/BI subheader to a Random Access Response (RAR) message on PDSCH. This is done during periods of congestion.

The backoff timer is signaled as a Backoff Indicator (BI) which has a length of 4 bits. The relationship between BI and actual backoff timer is shown below in the picture.

Relationship between Backoff signal indicator and Backoff timer

Credits for the Above picture goes to LTE in Bullets