TAU procedure in Connected mode and Idle

Question

What UE is supposed to do when it move from a Tracking Area to another Tracking Area ?

Answer 1

Tracking Area update procedure is used by MME to track the UE at the granularity of tracking area(s). During attach or TAU procedure, MME assigns a list of tracking areas to UE. There are two ways to execute the tracking area update procedure. One way is known as periodic TAU and the other one is known as non-periodic TAU.

Now coming to your question, please find my inputs as following:

1. Tracking Area update procedure is triggered only when UE is in RRC-IDLE state. So it means, when UE has radio connection with eNodeB, there is no need to execute this procedure since MME is already aware about the location of UE at the cell level.

2. In RRC- IDLE state, when UE moves from current cell to other cell and the other cell broadcasting a tracking area which is not present in the TAI list, UE initiates tracking area update procedure.

Hope, now it is clear to you when the tracking area update procedure is executed.

Comments

I think TAU is required in connected mode as well.

---

Hi Faisal, in case of handover, UE triggers TAU procedure if it enters into new tracking area. I agree with you handover is considered as RRC-Connected mode. In other cases when UE goes to RRC-IDLE mode and it detects new tracking area being in RRC-IDLE state, it triggers TAU.

---

But why we need TAU after HO as it is connected mode and eNB and MME already know UE location?

---

After doing handover, it might be possible UE entered into a tracking area which does not match which existing TAI list. In this case UE triggers TAU procedure. Even though MME was already aware about handover procedure but it does not assign new TAI list to UE during handover procedure because handover may fail and UE can go back to old cell.

---

If HO fails and UE goes back to old cell, there is no need for TAU. TAU only happens if HO is succeeded but why?

---

MME may assign new tracking area list to UE based on current tracking area. Since UE detects it entered into new tracking area that's why it informs to MME. It is used to make sync with network.
After receiving tracking area update request, in tracking area accept, MME may include list of tracking areas. In this case UE does not trigger TAU if it finds a new tracking area is already present in tracking area list. It is not necessary in tracking area accept, MME will assign a new list of tracking areas. Network would be interested to provide to list of tracking areas to UE just to minimize the TAU signalling with UE and this way frequency of TAU minimizes and it also helps to save battery life at UE.