Backoff indicator in RACH processing

From the LTE MAC spec TS 36.321, section 5 explains about the Random Access procedure which will be used for the UE's UL synchronization.

Backoff Parameter:

If the random access attempt of a UE fails, either because the preamble sent by the UE was not detected by the eNB or the UE lost the contention resolution, the UE has to start the process over again. To avoid contention and overload, the eNB can signal the UEs that they have to wait a certain time before they try to
connect again. The parameter that controls this is called the backoff parameter (B) and is signaled by the eNB in the random access response. The actual time the UE should backoff is chosen uniformly by the UE in the interval [0,B]. As mentioned, the backoff parameter is sent in the RA response, but all RA responses can however be read by all UEs who sent a preamble in step 1 of the random access procedure. This means that also a UE that did not get a random access response. with its own preamble, i.e., was not detected, can receive the backoff parameter and use it.

Backoff:

The eNB can force the UE to wait a certain time before it tries to connect again.  The maximum length of the backoff time is signaled to the UE by the eNB with the backoff parameter B. One possible scenario is that the backoff only is activated when there is an overload in the system. Therefore it would be interesting to study how the observations of AD (Access Delay) are affected by different values on B, during different conditions of the system. If the AD observers cannot be upgraded to accurately estimate an eventual backoff it would mean that the eNB is depending on AD reports from the UEs.
Backoff Indicator :


When it comes to backoff there is only one parameter that affects how long the UEs wait until they can try again. This is the backoff parameter, which is indicated by an index sent from the eNB to the UE. These backoff indicators have been defined below.
 Backoff parameter values as defined
       

Index	Backoff Parameter value (ms)
0	0
1	256
2	512
3	1024
4	2048
5	4096
6	8192
7	16384
8	32768
9	65536
10	131072
11	262144
12	524288
13	Reserved
14	Reserved
15	Reserved

The actual time the UE waits is called the backoff time and is defined as below.

Definition  Backoff Time :

The backoff time is defined as the time a UE waits after a random access attempt has been declared unsuccessful until the UE is free to try again. The backoff time is chosen uniformly by the UE in the interval [0,B].



In Case of NB-IOT, Backoff Parameter values are defined as below.

Index	Backoff Parameter value (ms)
0	0
1	256
2	512
3	1024
4	2048
5	4096
6	8192
7	16384
8	32768
9	65536
10	131072
11	262144
12	524288
13	Reserved
14	Reserved
15	Reserved

Regards
Venu