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EMM Procedure 7. Cell Reselection without TAU
April 23, 2014 | By Netmanias (tech@netmanias.com)
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Page 2 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

 

 

2. Cell Reselection: Required System Information and Criteria

 

Before we learn the detailed cell reselection procedure in Chapter 3, we will provide a brief overview of a cell reselection procedure in Section 2.1 first. Then, we will look into what system information is required for the procedure and the criteria for the cell reselection in Sections 2.2 and 2.3, respectively.

 

2.1 Overview of Cell Reselection Procedure

 

It is UE who is in control of cell reselection. UE obtains information needed for cell reselection (e.g. threshold values used to decide whether to measure the signal strength of neighbor cells or not, parameters used for calculating rank of the serving cell and neighbor cells, etc.) from the system information broadcasted by eNB. In case of intra-frequency cell reselection, required information is delivered through System Information Block (SIB) 3 and SIB 4, which will be discussed further in Section 2.2.

 

 Cell Reselection Triggering

 

Serving Cell Measurement: UE, in Idle state, wakes up at the end of every DRX cycle to measure the signal of its serving cell (Qrxlevmeas) and calculate the received signal level (Srxlev) of the serving cell to decide whether it should stay or move to another cell. Here, the UE’s transmission and reception conditions are reflected in the calculation, for example by applying minimum received signal level Qrxlevmin, allowed maximum TX power level PEMAX, etc., (see Section 2.2 for details).

Cell Reselection Triggering: If the received signal level of the serving cell (Srxlev) is greater than the specified threshold value (s-IntraSearch), the UE stays in the current serving cell. If not, it triggers a cell reselection procedure. The threshold value that works as triggering criterion is delivered through SIB 3, and defined as s-IntraSearch in Release 8 and as s-IntraSearchP and s-IntraSearchQ in Release 9.

 

UE in Idle state wakes up at the end of every DRX cycle to measure the received signal level of its serving cell (Srxlev) when it has stayed in the same location for a while. Let’s assume a UE that has camped on the same serving cell for a while because the received signal level of the cell has remained lower than the set threshold (s-IntraSearch). If it leaves the serving cell, the received signal level of the cell decreases gradually. Finally when the received signal level becomes lower than the threshold (s-IntraSearch), a cell reselection procedure is triggered. Then the UE begins to measure the signal strength of the neighbor cells (i.e. non-serving cells).  

 

 Cell Reselection Criteria

 

Cell-Ranking Criterion: The UE ranks each cell (RsRn) based on the measured signal strength of the serving cell (Qmeas,s) and neighbor cells (Qmeas,n). Parameters required for cell ranking are delivered through SIBs 3 and 4 (see Section 2.2). The serving cell is ranked using the hysteresis (q-Hyst) value stored in SIB 3 while the neighbor cells are ranked based on the offset (q-OffsetCell) value specified for each cell in SIB 4.

Cell Reselection: Once the serving cell and neighbor cells (non-serving cells) are ranked, the UE checks whether the cell reselection criterion is satisfied (Rn > Rs) or not. If there are neighbor cell(s) that satisfy the criterion, the UE selects the best satisfying cell, and then camps there. Cell reselection is performed only when the criterion is satisfied for a certain period of time (t-ReselectionEUTRA).

 

Mobile operators can prevent too frequent cell reselection and make sure reselection is performed in accordance with the cell status by controlling the UE’s dwelling time on the serving cell, based on the hysteresis and cell-specific offset values. In addition, they can control the q-Hyst and t-ReselectionEUTRA values by applying appropriate scaling factor (q-hystSFt-ReselectionEUTRA-SF) depending on the traveling speed of the UE.

 

2.2 System Information

 

System Information (SI) refers to the information broadcasted by eNB and consists of MIB (Master Information Block) and SIBs (System Information Blocks; SIBs 1 ~ 16) [4]. MIB, SIB 1 and SIB 2 are mandatory, but the rest SIBs are optional. All SI is delivered to UE through an RRC message like MIBSIB 1, or SI message4. An SI message consists of a group of SIBs (SIBs 2 ~ 16), excluding MIB and SIB 1.

 

UE performs a cell reselection procedure based on the SI broadcasted by eNB. MIB, SIB 1 and SIB 2 are applied to all the UEs, either in Connected state (EMM-Registered, ECM-Connected, RRC-Connected) or in Idle state (EMM-Registered, ECM-Idle, RRC-Idle). Conversely, SIBs 3 ~ 8 are only used in cell reselection by those in Idle state. Table 1 describes different types of SI and their parameters (see our One-Shot Gallery for more information about for a selected list of SIB information5).

 

Table 1. Cell Reselection-related System Information

 

Let’s assume two mobile operators (A and B) who operate the network as follows:

 

 A:

-

 has a LTE-only nationwide network
   

-

 LTE frequency: only one channel of 10 MHz in 1.8 GHz band (lteFA1)
       

 B:

-

 has nationwide 3G (UMTS) AND LTE networks
   

-

 3G frequency: six channels of 5 MHz in 2.1 GHz band (3gFA 1/2/3/4/5/6)
   

-

 LTE frequency: one channel of 10 MHz in 1.8 GHz band, and another channel of 10 MHz in 850
       MHz band (lteFA1, lteFA2)


In case of Operator A, its eNB needs information relating to intra-frequency for cell reselection (eNB broadcasts SIBs 3 and 4, but not SIBs 5, 6, 7 and 8. On the other hand, Operator B’s eNB needs information relating to all three types of reselection: intra-frequency, inter-frequency, and inter-RAT (3G UTRA) (eNB broadcasts SIBs 3, 4, 5 and 6).

 

The SI broadcasted by eNB is commonly applied to all the UEs, but each UE may receive different type of SI depending on its capacity (e.g. Release 9 UE or Release 11 UE).

 

Figure 1 displays a UE camping on Cell 5 at eNB 2 along with its neighbor eNBs. Figure 2 however shows eNB 2 only, along with its mandatory and cell reselection-related SI.

 

 

Figure 2. Cell Reselection-related SIBs Broadcasted by eNB2 

 

The network used in Figure 1 is an LTE-only network that uses a single frequency (lteFA1) with home PLMN only. This document is about the intra-frequency cell reselection procedure. The SI related to intra-frequency cell reselection is broadcasted through SIBs 3 and 4. Table 2 below lists the parameters used in the two SIBs, but not those related to other frequency, RAT or VPLMN.6

 

Table 2. Cell Reselection Parameters (SIBs 3 and 4)

 

2.3 Cell Reselection Criteria

 

Though not within the scope of this document, below we will review briefly the criteria used in cell reselection after UE is turned on. One of the following criteria is applied (i.e. criterion (1-1) for Release 8 UE and (1-2) for Release 9 or later UE). Table 3 provides a description of parameters used in the criteria.

 

 Cell Selection Criteria

 

 Release 8:

Srxlev > 0

(1-1)

 Release 9:

Srxlev > 0  and  Squal > 0

(1-2)

   

where, Srxlev = Qrxlevmeas - Qrxlevmin - PCompensation [dB]

 
   

  Squal = Qqualmeas – Qqualmin [dB]

 

 

Table 3. Cell Selection Criteria Parameters (TS 36.304 [5])

 

During the initial cell selection, a cell must have the cell RX level (Srxlev) greater than the sum of Qrxlevmin and PCompensation to be a serving cell. The Srxlev must be greater than Qrxlevmin because, in order for UE to correctly receive messages from its serving cell, the measured signal strength must be at least greater than Qrxlevmin. UE’s Tx power (PPowerClass) lower than the value allowed in the cell (PEMAX) will result in greater PCompensation value, making it hard to select the cell. When selecting a cell, UE’s transmission and reception conditions are considered.

 

In case of Release 9, in addition to Srxlev, Squal is added as a cell selection criterion. Qrxlevmeas is the cell’s Reference Signal Received Power (RSRP) while Qqualmeas is the cell’s Reference Signal Received Quality (RSRQ). Compared to RSRP which only indicates the strength of received signal, RSRQ provides more accurate information for radio link quality because it indicates the signal to interference and noise ratio (SINR).

 

 Cell Reselection Triggering

UE camping on the serving cell may continue to camp on there without having to measure other cells if the serving cell’s RX level fulfills the criterion (2-1) for Release 8 UE or (2-2) for Release 9 or later UE.    

 

 Release 8:

Srxlev > SintraSearch

(2-1)

 Release 9:

Srxlev > SintraSearchP  and  Squal > SintraSearchQ

(2-2)

   

where, Srxlev = Qrxlevmeas - Qrxlevmin - PCompensation [dB]

 
   

  Squal = Qqualmeas – Qqualmin [dB]

 

 

In the above inequalities, values of SintraSearch, SintraSearchP and SintraSearchP are given by SIB 3 (see Table 2). If the serving cell does not fulfill either of the foregoing criteria ((2-1) or (2-2)), i.e. if it fulfills the criterion (3-1) in case of Release 8 UE or (3-2) in case of Release 9 or later UE below, the UE begins measuring the neighbor cells for reselection.

 

Cell Reselection Triggering (Neighbor Cell Measurement Triggering)

 

 Release 8:

Srxlev ≤ SintraSearch

(3-1)

 Release 9:

Srxlev ≤ SintraSearchP  or  Squal ≤ SintraSearchQ

(3-2)

 

 Cell Reselection Criteria

Cell Ranking Criterion

If the measured Srxlev of the serving cell fulfills the foregoing criterion ((3-1) or (3-2)), the UE begins to measure the neighbor cell’s RSRP. Then based on the resulting measurements, it ranks all the cells by applying the criteria (4-1) and (4-2) below [5]. Table 4 shows the parameters used in cell ranking.

 

 Serving Cell Rank:

Rs = Qmeas,s + Qhyst

   (4-1)

 Neighbor Cell (non-serving cell) Rank:

Rn = Qmeas,n - Qoffset

   (4-2)

 

Table 4. Cell Reselection Criteria Parameters (TS 36.304 [5])

 

Cell Reselection

If there are multiple neighbor cells that fulfill the criterion (5) below, the UE selects the best cell and camps on it.

 

 

 Rn > Rs

 

 (5)

 

As seen in the foregoing criteria (4-1) and (4-2), Rs and Rn are calculated using different criteria. The greater Qhyst and Qoffset values are, the longer the UE can stay in the serving cell.

 

 

Page 2 of 5
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

Hi,

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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