ANR AND TAU

ANR (Automatic Neighbour Relation)

Actually before understand about ANR we have to clear little bit concept on neighbour cell scenario. What happened is every eNB attached the neighbour cell list inside SIBs. This is operator work to configure these details in system Info So There are two ways that network operator put this details inside the various cells.

1.    Manually configuration: Its done manually by the Operator but here are some issue with it:

a.    First operator need to provide special person to investigate it which is cost effective and also neighbour cells are changes time to time by shifting cells on demand

2.    Automatic configuration : This is technique the eNB are used  for automatic configuration of neighbour cell which is known as Automatic neighbour request. Its a easier way of configuration. Procedure is as follows:

a.    First N/W will send Measurement control Req to UE  for performing the measurement of cell around its geographical area and provide list to N/W that has sent Measurement control Req earlier.

b.    N/W will decode the measurement report and update the neighbouring list according to the received report from UE.

TAU(Tracking Area Update)

TAU is a kind of report which sent by UE to MME at a certain time, whenever the TAC(Tracking Area Code) is changedthan TAU is needed not only that time there is some more condition when its needed.
First the question is How UE get to know that TAC is changed? So our answer is this information is present in a SIB1.
Now second question is when TAU report has to send?
Actually we have something called TAI list which is already has provided by MME to UE at the time of registration. For more information Refer TAI.

       Below I have attached the flow of  TAU procedure:

TAU Procedure flow Diagram

TAU trigger conditions:

1.    Normal TAU is due to mobility i.e when UE enters in Tracking Area which is not included within the list of tracking areas with which the UE has registered.

2.    Periodic TAU after T3412 timer Expires.

3.    Reregistering the EPS after CSFB connection has been completed.

4.    MME load balancing. 

Note 1: It is triggered in both in IDLE and Connected mode, but Suppose UE in connected mode and try to to move to new location, In this UE known to the network and easily handed over to new cell but if UE found that its TAC isn’t part of TAI list now than it will triggered TAU procedure to the MME.

 

Note 2: It doesn’t means UE always be in Connected mode but it can be come again in IDLE mode.Than Suppose if N/W want to send some information to the UE than in this case MME need to know the exact location of UE because might be UE was is moving state in IDLE and doing its cell selection and reselection than here also only extreme procedure which we have is TAU procedure. But as in the above flow diagram we have seen that TAU is piggybacked on rrcconnectionsetupcomplete msg so its a connected mode msg so how this stuff work. So The perfect answer is UE can begin the TAU procedure in IDLE mode but UE must be in RRC Connected mode to actually complete the procedure.