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EMM Procedure 7. Cell Reselection without TAU
April 23, 2014 | By Netmanias (tech@netmanias.com)
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Comments (18)
This document will provide the procedure for cell reselection without Tracking Area Update (TAU), as defined as EMM Case 7 in our technical document, “Eleven EMM Cases in an EMM Scenario”. This procedure is performed when a UE, staying in EMM-Registered, ECM-Idle and RRC-Idle after network attach, moves to another TA registered at the network (MME), and reselects a cell. Moving to a TA already registered at the network (MME) does not require a TAU procedure after the reselection procedure. First, this document will explain what system information is needed in cell reselection, and on what criteria cells are reselected. Then, it will describe the cell reselection procedure, with main focus on the criteria for cell reselection triggering and cell ranking. Here in this document, we will only discuss intra-frequency cell reselection (i.e. cell reselection within the same frequency) in a single LTE-only network that operates with a single LTE carrier frequency.
Page 1 of 5

Table of Contents  

1. Introduction
2. Cell Reselection: Required System Information and Criteria

2.1 Overview of Cell Reselection Procedure

2.2 System Information

2.3 Cell Reselection Criteria

3. Cell Reselection without TAU

4. EPS Entity Information: Before/After Cell Reselection without TAU

5. Closing



1. Introduction


Through the three companion documents previously presented[1][2][3], we have looked into the “EMM Case 6. Handover without TAU” procedure for inter-cell handover within a Tracking Area (TA).1 This document provides a description of how UE, in Idle state, moves and reselects a cell.


While handover controls the mobility of UE in Connected (EMM-Registered, ECM-Connected and RRC-Connected) state, cell reselection controls the same while in Idle (EMM-Registered, ECM-Idle and RRC-Idle) state. During a handover, it is the network (MME or source eNB) that decides which cell to handover to. During cell reselection, however, it is UE that decides which cell to camp on2.


A cell reselection procedure can be one of the two types as seen below and in Figure 1. In Figure 1, the UE is camping on Cell 5 that belongs to the TA list of {TA1, TA2} previously assigned by the MME at the time of network attach, and is staying in Idle state.

  • Cell Reselection without TAU (: EMM Case 7): UE moves to a TA that IS registered at MME (i.e. listed in the TAI list of the UE), for example TA2 in Figure 1. Cell reselection is performed but no Track Area Update (TAU) is required.
  • Cell Reselection with TAU (: EMM Case 9): UE moves to a TA that is NOT registered at MME (i.e. not listed in the TAI list), for example TA3 in Figure 1. After cell reselection, a TAU procedure is performed.


Figure 1. Two Cell Reselection Types


UE may reselect a cell i) that uses the same LTE frequency as its current serving cell that it is camping on (intra-frequency), ii) that does not use the same frequency (inter-frequency), or iii) that uses other Radio Access Technology (RAT3) (inter-RAT). In the example cases of cell reselection shown in Figure 1, we only discuss intra-frequency cell reselection in a single LTE network environment that uses only a single LTE frequency.


This document describes cell reselection case only, and the other case () will be covered in our next document “EMM Procedure 8 & 9. Handover & Cell Reselection with TAU”[6].


In Chapter 2 below, we will review the preliminary information required to understand the cell reselection procedure.

In Chapter 3, a detailed description of the cell reselection without TA procedure will be given.








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Ashok 2014-04-23 16:06:51
It a wonderful document.
Can we post some document like Cell search,System Information derivation and then RACH(Contention based and non Contention based).
It will be really great if we can keep some document from Air interface side.
Netmanias 2014-04-24 14:50:45
Hi Ashok,

Thank you for taking interest in our web site. We have sent you an email. Please check your email.
gabileao 2017-09-03 14:44:32

The document is very clearly explained. Thank you.

Can you please send to me the same information you sent to Ashok? I am trying to perform a simulation and more detailed information about how the cell search procedure is made in LTE would be perfect, since I am struggling to understand this. Thank you very much.

Suchitra 2014-07-09 13:41:49

This is a really good website for understanding the LTE technology. It would be good to have some reading materials for procedures like RACH, Paging.

Saurabh Thukral 2014-09-23 21:02:54

Please publish/share articles on RACH, Initial Access

Rahul 2015-01-13 15:06:29

Good Info.

ravi 2015-01-13 15:56:09

Does device have EMM context upon device reselected to UMTS cell from LTE? please answer.

If device reselects back to LTE from UMTS, then does device do attach procedure (considering device already has EMM context prior to cell reselection to UMTS earlier)

Zhou Po 2015-03-26 12:31:31

Do we have the document about Paging procedure?

Netmanias 2015-04-17 04:24:32

Hi Zhou, Sorry but we don't have a document describing Paging procedure.  --;;

PERNEY 2015-04-16 23:48:16

i would like to know the name of tools used to design the flow chart?

Netmanias 2015-04-17 04:30:09

Hi Perney,

We're using Microsoft Visio, and no other special module or script language.

There are some Visio template images that we've made, and most people at our company share them. Hope it was helpful.

PERNEY77 2015-04-22 20:50:29


is it possible to share the visio template that we have made?

Netmanias 2015-04-23 10:10:26

Unfortunately, the templates are parts of our valuable assets, and thus cannot be shared externally.

weiwei 2015-07-22 12:14:52

So good paper!Thanks!

ramakrishna kaagitha 2017-03-04 16:19:11

sir can u please send me the same as TO SEND TO ASHOK(Can we post some document like Cell search,System Information derivation and then RACH(Contention based and non Contention based).
It will be really great if we can keep some document from Air interface side.


Ram 2018-05-06 17:09:19

Hi, can you please share me the document on Cell search,System Information derivation and then RACH(Contention based and non Contention based) .

Thanks in advance


SAMARTH REVANKAR 2018-10-12 16:25:23

Hi netmanias team, good explanation ,very nice documents and easy to understand. Can you please share me the document on Physical cell id acqusition, Cell search procedure ,System Information derivation, then RACH(Contention based and non Contention based) and also documents related to VoLTE ,SIP-Call Flows,IMS registration and IMS related protocols.

Rakesh 2019-04-21 17:38:15

Thank you for such a clear explanation. Could you please send me the docs you have sent to Ashok? 

If you have any documents on HOs like X2/S1, CA please share to my mail id.

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