05 - RN31555EN30GLA0 - Cell Selection and Reselection

 

Cell Selection & Reselection

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Module Objectives

 At the end of the module you you will be able to to:: • List the motivations for cell selection & reselection • Explain the meaning of “Equivalent PLMN“ PLMN“  • Name the differences between access classes & cell access restrictions • Understand the cell Selection criterion S • Name the differences between cell selection with & without HCS • Explain the Ranking criterion R

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Cell Selection & Reselection • Motivation; PLMN Search/Selection Search/Selection • Cell Status & Access Control • States & State Transitions in RRC Idle mode • Cell Selection; Selection S- & Ranking R-Criterion •  Cell Selection Criterion S •  HCS •  Cell Reselection without HCS •  Cell Reselection with HCS

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Cell Selection & Reselection: Motivation Four different reasons for camping camp ing on a cell in the RRC idle mode are identified in TS 25.304:

1. 2.

The UE is capa capable ble to rea read d the Syst System em In Inform formatio ation, n, whi which ch is broa broadcas dcasted ted o on n the BCCH (P-CC (P-CCPCH PCH). ). Hereby,, the UE gains also information of the Serving PLMN. Hereby If ac access cess tto o the cu curre rrent nt cel celll is no nott bar barred, red, tthen hen th the e UE ca can n use th the e cell to in initi itiate ate a an n RRC ((sig signal nallin ling) g) connection. • • •

3.

If th the e UE is regi register stered, ed, an and d ther there e is a MT MTC, C, th the e serv serving ing co core re net network work e elem lements ents k know now tthe he Loc Locati ation on  Area and/or Routing Area, in which the UE is is camping. • •

4.

The Access Network is accessed by common control channels (CCCH on PRACH). Initial access is done to register the UE at the network. Other reasons for accessing the network is to request transmission resources for user data transfer.  

Paging is done with the PCH (on S-CCPCH). The UE receives the paging message, because it is monitors all paging occasions on the paging resources of its paging class within the cell.  

Th The e UE can re recei ceive ve c cel elll br broa oadca dcast st s serv ervic ice e CB CBS S me messa ssages ges.. UE camping on a cell in RRC Idle Mode can: Mode can:

UE

•  read S ys tem In Infor format matii on •  initiate R R C conne connect ctions ions •  getting pag ed MTC •  receive Cell Broadcast Service messages 4

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Network

 

Cell Selection & Reselection: Motivation NAS control   indication to user

radio measurements   measurements

CM requests  requests 

manual mode PLMN selected cell selection & reselection

PLMN selection & reselection

Registration  Area changes

Location Registration

PLMN available Location Registration response

automatic mode

RRC tasks: tasks: if the UE is in the  the  

•

RRC idle mode TS 25.304: mode tasks  tasks Idle mode subdivided into 3 processes:

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PLMN selection & reselection

•

Cell selection & reselection

•

Location Registration LR

3G RPLS (RN 3155) / RU30 / AA / 03_2011

if the UE is in the  the  

RRC connected mode states:

• CELL_FACH, • CELL_PCH & • URA_PCH

TS 25.304: UE procedures in idle mode    

Cell Selection & Reselection: Motivation • The UE tries to find a suitable cell to cell to camp on. on. • With a suitable cell, all network services are available. • If it is unable to find such a cell, it attempts to camp on any  – non-barred  – cell. • It enters an internal "lim limite ited d s ervice " state. In this state, only emergency calls can calls can be carried out. The same state is given, when no USIM was inserted in the UE or the Location Registration failed. 

• The UE is also responsible for cell reselection in reselection in the RRC connected mode mode states  states CELL_FACH, CELL_PCH & URA_PCH. URA_PCH. •The purpose of camping on a cell are thethe same asconnection in the RRCalready idle mode with the exception of RRC connection establishment establishment (because  (because RRC exists). •The C ell ell R es esel elect ection ion  procedure is the s ame in the R R C i dle dle & R R C connect connected ed mod ode e.

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PLMN search • The UE shall scan all RF channels in the UTRA bands* to find available PLMNs. • On each carrier, the UE shall search for the strongest cell and read its system information, in order to find out which PLMN the cell belongs to.

• If the UE can read the PLMN identity, the found PLMN shall be reported to the NAS as a high quality PLMN  PLMN  (but without the RSCP value), provided that the following high quality criterion is fulfilled: • For an FDD cell, the measured primary CPICH RSCP RSCP value  value shall be greater than or equal to -95 dBm. dBm.

• Found PLMNs that do not satisfy the high quality criterion, but for which the UE has been able to read the PLMN identities are reported to the NAS together with the CPICH RSCP value for UTRA FDD cells. • The search for PLMNs on the rest of the carriers may be stopped on request of the NAS. The UE may optimise this search by using stored information of carrier frequencies and optionally also information on cell parameters, e.g. scrambling codes, from previously received measurement control information elements. • For a GSM cell cell,, a PLMN shall be understood to be received with high quality signal if the signal level is above -85 dBm. dBm. • Once the UE has selected a PLMN, the cell selection procedure shall be performed in order to select a suitable cell of that PLMN to camp on.

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* according to its capabilities

 

PLMN search

High quality PLMNs

Other found PLMNs

PLMN Id

PLMN Id + RSCP/RSSI

UE NAS UE AS UTRA: If RSCP > -95 dBm

GSM: If RSSI > -85 dBm

Search PLMNs

TS 25.304 & TS 43.022 8

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RSCP: Received Signal Code Power

RSSI: Received Signal Strength Indicator  

PLMN Selection For PLMN selection, 2 modes of operations were specified: mode  •  Automatic mode  The UE detects available networks. It utilises a list of PLMNs, which names PLMNs in a priority order. The PLMN with the highest priority, which is available & which the UE is allowed to select is chosen for network registration.  

•  Manual mode The UE scans the radio interface and indicates to the user available PLMNs. The UE attempts to register with the subscriber selected network. If the subscriber does not select a (suitable) network, then the UE only offers limited services, i.e. emergency calls.

Following data fields are stored on the 3G USIM or ME, playing a major role in the automatic PLMN selection process: • • • • • •

 IMSI: The MCC & MNC of the IMSI identify the HPLMN (stored on USIM)  The last PLMN including RAT to which the UE was registered to (stored on ME)  A PLMN list including RATs RATs as preferred by the subscriber (stored on USIM)  A PLMN list including RATs RATs as preferred by the operator (stored on USIM) USI M)  The list of equivalent PLMNs which were received in last registration (stored on ME)  HPLMN search period timer (stored on USIM)

 A list of PLMN identifiers associated with an access technology is listed on the USIM in a priority order. If there is a PLMN identifier allowing more than one access technology, it is up to the UE implementation to determine the preferred access technology. Not only international roaming is possible, but also national roaming. •  Several national regulation authority enforced national roaming of UMTS subscribers in existing GSM networks in order to grant a higher coverage to UMTS subscribers. •  If national roaming is allowed, the UMTS operator wants to avoid unnecessary roaming fees. Period” timer is used for that. •  In this case the UE must periodically search for the HPLMN. The “ HPLMN Search Period” “HPLMN Search Period” timer can be set in a way, that no periodic search attempts are conducted.  conducted.   9

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PLMN Selection

Switch on

in automatic automatic PLMN  PLMN selection operation mode

Last registered PLMN

Home PLMN other PLMN‘s in decreasing signal quality order

User controlled PLMN list Operator controlled PLMN list HPLMN Search Period Timer   6 .. 480 min; Default: 60 min

other high quality PLMN‘s in random order 10

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Cell Selection & Reselection • Motivation; PLMN Search/Selection Search/Selection • Cell Status & Access Control • States & State Transitions in RRC Idle mode • Cell Selection; Selection S- & Ranking R-Criterion •  Cell Selection Criterion S •  HCS •  Cell Reselection without HCS •  Cell Reselection with HCS

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Cell Reservations and Cell Restrictions • If the operator wants to restrict access to a cell or to set set cell reservations. 2 methods are identified: • 1) Cell status and special reservations: •This method is applied to control the cell selection and reselection process. It informs the UE, if a cell is suitable for cell selection and re-selection.  

• 2) Access control: •In this method, a cell can be selected in the cell selection & re-selection process. But a UE is not allowed to send an initial access message to the access network. •This method is normally applied for load control reasons. •broadcasted The subscriber‘s access class(es) are determined cell basis IMSIonand on the BCCH. The subscriber‘s accesson class(es) is from (are) the stored theparameters, USIM.

• Cell status and special reservations  reservations  • Acceptable cell: cell, which offers limited services (i.e. the origination of an emergency call) to the subscriber. • Suitable cell cell:: cell, where the full service set can be offered to the subscriber. This cell must fulfil a set of requirements, such as being part of the selected PLMN. It must also fulfil the cell selection criterion S, which is covered later on in this chapter. • Barred cell: cell, cell: cell, where the UE is not allowed to camp on. • Reserved cell: cell, where "ordinary“ "ordinary“ UEs are not allowed to camp on. on .

• The IE Cell Access Restriction is used to indicated cell status and cell reservations.

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Cell status & special reservations

BCCH SIB3/4: Cell Access Restriction

Cell status & special reservations:

Access Control

Cell

Access Class barred:

- acceptable (limited services*) suitable  (full service set) - suitable - barred - reserved#

(UE access classes 0..9 and 11..15)

* e.g. Emergency Call (Access Class = 10) in Network w/o Roaming Agreement # e.g. for operator use 13

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Cell status & special reservations reservations  • 1) Cell status and special reservations  •The UE must read following 3 information elements in the IE Cell Access Restriction in Restriction in order to learn cell‘s status & cell‘s reservations: reservations:   • cell barred • cell reserved for operator use, and • cell reserved for future extensions. • 4 cases can be distinguished: 1: The cell is suitable for selection & re-selection both re-selection both in the RRC idle mode & RRC • Case 1: The connected mode. • Case 2: The UE behaves as if cell status "barred" is indicated. • Case 3: If a UE is in the HPLMN & assigned to access class 11 or 15, then it is allowed to select & re-select this sell. If it belongs to another access class, it behaves as if the cell status is “barred“.   “barred“. • Case 4: The UE is not allowed to select & re-select this cell. Not even emergency calls are allowed. In the the  IE Cell Access Access Restriction an IE "Intra-frequ "Intra-frequency ency cell re-selection indicator" can be used to inform the UE, whether is has to re-select a cell on the same or on a different frequency band, if the cell selection and re-selection criteria are fulfilled.

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Cell status & special reservations suitable for selection & reselection

“barred”   Cell “barred” UE reads from IE Cell Access Restriction

Case

Cell Barred

Cell reserved for operator

Cell reserved for future extension

1

not barred

not reserved

not reserved

2

not barred

not reserved

reserved

3

not barred

reserved

---

4

barred

---

---

suitable for AC 11 11 or 15 in HPLMN otherwise “barred”  “barred”  Tbarred Cell Barred WCEL; 0 (Barred), 1 (Not Barred)  Barred)  15

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if cell after Tbarred still best, check again wether barred

WCEL; 10, 20, 40, 40, 80, 160,3G 320, 1280 s / AA / 03_2011 RPLS640, (RN 3155) / RU30

Not suitable – suitable – not  not even for emergency call

x

Cell Reserved for operator use

IntraFreq_Cell_Reselect_Ind

WCEL; 0 (reserved), 1 (Not reserved)  reserved) 

Intra-freq. Cell reselection allowed when cell barred? allowed)   WCEL; 0 (allowed), 1 (Not allowed)

 

Access Control • 2) Access Control • Access Control is normally used to for load reasons. It limits the access on the common control channels. •• Access class restrictions are broadcasted via the  In the cell selection and re-selection process, the BCCH UE ignores cell access restrictions. • A change of the access class while the UE is camping on the cell, is not triggering a cell re-selection by the UE. • If the UE is in the RRC idle state, it checks access class restrictions before it sends an RRC CONNECTION REQUEST message. • If the UE is already in the RRC connected state, there are no access class restrictions valid for the UE. • Access class restrictions do also als o not apply, if the UE must ent enter er the RRC connected connec ted mode after an Inter-RA Inter -RAT T cell re-selection to UTRAN. • 3) Emergency Call • Access class restrictions for emergency calls can be imposed by the operator. The UEs get informed by emergency call access class restrictions, when read on the IE “Access class barred list”:   • Access classes 0 to 9 are barred • This is the case, when access class 10 is barred in the above mentioned IE. This includes also UEs with no USIM (no IMSI). • Access class 11 to 15 are barred • UE of access class 11 to 15 is not allowed to initiated an emergency call, when in the above IE access class 10 is barred and its own class (11 to 15). Otherwise, emergency calls are allowed for these phones.

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Access Control Access Class irrelevant for access suitable cell

suitable cell

UE RRC connected

Access Control normally Control normally used for load reasons:

I ignore all Access Class related Class  related cell access restrictions when selecting a cell

Overload Control feature

to camp on

UE RRC Idle

Access Class only Class only relevant for access (from RRC IDLE)

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Cell Selection & Reselection • Motivation; PLMN Search/Selection Search/Selection • Cell Status & Access Control • States & State Transitions in RRC Idle mode • Cell Selection; Selection S- & Ranking R-Criterion •  Cell Selection Criterion S •  HCS •  Cell Reselection without HCS •  Cell Reselection with HCS

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States and State Transition in the RRC Idle Mode • Cell Selection Process • The NAS protocol layers in the UE are responsible for PLMN selection. • When a PLMN has been selected, the lower layers are responsible to find a cell of this PLMN which is suitable to camp on. • So how far is the Non-Access Stratum controlling Stratum controlling the cell selection process? • It provides information about the RAT(s), which are associated with the selected PLMN. • It holds a list of PLMNs, which are equivalent to the selected PLMN. Please note, that many operators with 2 access technologies identify the CN with different network identifiers. But they can support an inter-RAT inter-RA T Handover between the 2 networks netw orks (which is physically phys ically only one). • It holds a list of forbidden Registration Areas.

•The Access Stratum (AS) manages between etween the UE and the RA RAT T. Among others, (AS) manages the interaction b it is responsible to find a suitable cell, on which the UE can camp on. • As soon as the AS has found a suitable cell, it commands the UE to camp on this cell. • It informs the higher layer NAS protocols about this, so that these can start the registration procedure. If the UE was successfully registered with the serving CN elements, the UE enters the internal state C amped amped nor mally  mally   .. • If the UE is not capable to find a suitable cell to camp on within the selected PLMN, it enters the internal state   A state A ny ccell ell s election electi on. 

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CN: Core Network NAS: Non-Access Stratum

RAT: Radio Access Technolog Technology y  

States and State Transition in the RRC Idle Mode

Cell Selection Process UE NAS:

PLMN selection

CN NAS controls •• RAT(s) RA T(s) information associated with the selected PLMN forbidden registration areas • list of equivalent PLMNs

AS: attempts to find suitable cells

AS controls correspondent procedures

suitable cell found

YES

camped normally

NO

camped on any* cell

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all network services available limited services e.g. Emergency Call

acceptable cell  

States and State Transition in the RRC Idle Mode • The UE receives from the System Information (on the P-CCPCH) a wide range of PLMN and cell relevant information. s uccessfully,, it can also receive pa paging ging information on one of the S• If initial registration was done successfully CCPCHs and initiate a MOC setup on behalf of the NAS. • The AS must inform the NAS about relevant changes in the system information after the cell selection and re-selection process. For instance, the UE may have entered a new routing area after a cell reselection, which enforces the UE‘s NAS to start the Routing Area Update procedure. • Cell re-selection may be based on different measurement methods in the RRC idle & RRC connected mode. • In the RRC idle mode, 3 different UE states can be identified: •Camped normally state • When a UE camps normally on a cell, all network services are available to the phone. • The UE is responsible to perform a Cell reselection evaluation process to determine, whether there is a better cell to camp on. If so, a cell reselection takes place. • It can also happen, that the UE does not find a suitable cell (any more). This is the case, when the subscriber is leaving the mobile communication supply area. If this happens, the UE enters the internal state Any state  Any cell selection, selection , where it attempts to find at least a cell, where limited network services (e.g. emergency calls) are available. • The UE also enters the Any the Any cell selection state, selection  state, when the NAS registration procedure failed. When the UE attempts to access the cell, it leaves the Camped normally state and state and enters the Connected mode state.

(continued on the next text slide) 21

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States and State Transition in the RRC Idle Mode  

go here whenever  a new PLMN is selected

 

 

cell information stored for the PLMN

 

   

no suitable cell found

 

 

 

no cell information stored for the PLMN

1

Stored Information Cell Selection

 

 

cell found  found suitable cell

   

 

 

   

 

suitable cell found cell found

 

return to idle mode

 

leave idle mode

 

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NAS registration rejected

Camped normally

 

rigger 

 

 

 

suitable cell selected cell selected

 

(adopted from TS 25.304) 22

 

Cell Reselection Evaluation Process

 

Connected Mode

 

Initial Cell Selection

suitable cell found cell found

 

Cell Selection when leaving connected mode

 

 

no suitable cell no  cell found  found

 

 

     

no no   suitable cell found cell  found

 

no suitable cell found

       

 

States and State Transition in the RRC Idle Mode • Any cell selection state •• In thismeans, state, the UE triestotooffer find to anthe acceptable cell.  That it attempts subscriber at least limited services – services  – the  the emergency call. • Consequently, in this state, the cell search is done do ne without regard of the th e PLMN identity identity.. • The UE also looks loo ks for an acceptable cell in every RAT RAT,, it supports. • The UE stays in this state till it has finally found an acceptable or suitable cell to camp on. • The UE (AS) informs the NAS about it. • The UE enters also this state, if a NAS registration failed.  failed.  • Camped on any cell State • Only limited services – services – the  the mobile originated emergency call – call  – are  are offered to the subscriber. • The UE is responsible to perform a Cell reselection evaluation process to determine, whether there is a better cell to camp on. If so, a cell reselection takes place. • It also regularly tries to find a suitable cell. • Hereby it evaluates all RAT RATs s it is cap capable able to suppo support. rt.

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States and State Transition in the RRC Idle Mode go here when no USIM USIM in  in the UE  1 

Any Cell Selection

USIM inserted 

cell found  found  no acceptable cell an acceptable cell found cell found  Cell Selection  when leaving  connected  mode  return to 

acceptable  cell found cell  found 

leave  idle mode 

idle mode

 

Connected  Mode  (Emergency  calls only) calls  only) 

(adopted from TS 25.304) 24

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suitable  cell found cell  found 

Camped on  Any Cell 

acceptable  trigger  cell selected

 

Cell Reselection Evaluation Process 

  no acceptable  cell found cell  found 

1 

 

 

Cell Selection & Reselection • Motivation; PLMN Search/Selection Search/Selection • Cell Status & Access Control • States & State Transitions in RRC Idle mode • Cell Selection; Selection S- & Ranking R-Criterion •  Cell Selection Criterion S •  HCS •  Cell Reselection without HCS •  Cell Reselection with HCS

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Cell Selection Criterion S Cells, which are suitable for (initial) cell selection & reselection (so that the UE can camp on them) are determined by the UE based on the cell selection criterion S. S. • It is fulfilled, when Srxlev > 0 AND Squal > 0 Srxlev > 0  0 

in the FDD mode, and in the TDD mode.

• Squal delivers the cell Selection quality value (dB): • The UE determines it according to this formulary: Squal = Qqualmeas  – – Qqualmin • The UE measures the received signal quality Q qualmeas of the cell. It is based on CPICH E c/N0 (dB) for FDD cells. (CPICH Ec/N0 is averaged.) • A cell is not suitable for cell selection & re-selection, if the measured received signal quality level is below Qqualmin.   Qqualmin.

(db)  stands for the  the cell selection receive level value: • Srxlev (db) • UE determines: Srxlev = Qrxlevmeas - Qrxlevmin – Pcompensation • Qrxlevmeas is the cell RX level measured by the UE, based on the CPICH RSCP for FDD cells, and the averaged received signal level for GSM cells. (All values get averaged!) • Qrxlevmin is set by the operator as min. required RX level in the cell, which is sent to the UE via the BCCH. • If the UE determines the cell‘s RX level value Q rxlevmeas and Qrxlevmin calculated the Srxlev accordingly, it may have good RX level which means, that a good DL connection can be established. • But the UE‘s own output power capability has to be taken under consideration. This is done with   • Pcompensation = max(UE_TXPWR_MAX_RACH  – P_MAX, 0) (dB)  (dB)   • In order to access a cell, the UE has to use the common channel PRACH. • The operator determines the max. cell radius by limiting the maximum TX power level, a UE can use on the PRACH. This is the UE_TXPWR_MAX_RACH (dBm). • UE_TXPWR_MAX_RACH can range – range – according  according to the specifications - between – between  –50 50 dBm and 33 dBm. On the other hand, there is the UE‘s max. RF output power, given by P_MAX (dBm). 26

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Cell S Selection election Criterion S (in the FDD mode) mode)

Qqualmeas /N0) (CPICH Ec(dB)

Squal > 0

S-Criterion fulfilled: Squal >0 AND Srxlev > 0

QqualMin -24...0; -18 dB SIB 1; 11/12

Qrxlevmin

Squal CPICH=Ec/No –  QqualMin

Qrxlevmeas (dBm) CPICH RSCP

Pcompensation

Srxlev =

suitable

Qrxlevmeas   – Qrxlevmin  – Pcompensation 27

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cell?

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Srxlev > 0 AdjsQrxlevmin

-1 -115...-25; 15...-25; 2; -115 dBm SIB 11/12

 

Cell Selection Criterion S Cell size defining parameters: • Qrxlevmin • Qqualmin Srxlev = Qrxlevmeas  – – Qrxlevmin – Pcompensation I am outsid e  

UEtxPowerMaxPRACH  

I am inside, but have not enough power

AdjsQrxlevmin : AdjsQrxlevmin:

-50 .. 33 dBm Pcompensation = max(UE_TXPWR_MAX_RACH max(UE_TXPWR_MAX_RACH   – – P_MAX, 0) 28

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WCEL; -50..33 dBm; 0.1;  0.1; 21 dBm SIB3/4

3G RPLS (RN 3155) / RU30 / AA / 03_2011

WCEL; -115...-25; 2; -115 dBm

 

(Initial) Cell Selection Process • There exist two two cell  cell selection procedures: • Initial Cell Selection • The UE has to find a suitable cell of the PLMN, which was selected by the NAS. • To do so, the UE scans all radio frequency carriers of UTRA. Hereby, the UE focuses its cell search to the strongest cell on each carriers. • As soon as the UE has found a suitable cell, it selects it. • In contrast to the initial cell selection procedure “stored information cell selection“, this procedure does not require knowledge in advance, which of the scanned radio frequency bands holds UTRA carriers. • Stored Information Cell Selection  Selection  •To speed up the cell selection process – process – for  for instance, when the UE is switched on again – again –   information about UTRA carriers, even cell parameters such as cell scrambling codes can be stored in the UE. •The UE uses this information to find a suitable cell of the PLMN, which was selected by the NAS. •If the cell selection based on stored information in the UE fails – fails  – e.g.  e.g. the selected PLMN cannot be found – found – the  the UE continues the cell selection process based on the Initial Cell Selection procedure. • Both for Initial Cell Selection and Stored Information Cell Selection, a cell is only suitable for the UE to camp on, if it fulfils the cell selection criterion S: n the FDD mode, and •Srxlev > 0 AND Squal > 0  0 iin •Srxlev > 0  0  in the TDD mode. 29

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(Initial) Cell Selection Process Once a suitable cell is cell is found this cell is selected – Qqualmin > 0 Squal = Qqualmeas  – Srxlev = Qrxlevmeas  – – Qrxlevmin  – Pcompensation > 0

Initial Cell Selection (scan RF channel)

or

Stored Information  Information  Cell Selection carrier, even Scr.Code may be stored in UE  UE 

I have to find a suitable cell 30

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Cell Selection When Leaving the RRC Connected Mode • The subscriber just has terminated a call, i.e. the UE was in the RRC connected mode, dropping back into the RRC idle mode. • Which cell is then a candidate cell for the UE to camp on? • Candidate cells are the ones, the UE was connected immediately before call termination. • But these cells must be suitable cells. • If none of these cells are suitable – suitable – or  or fulfils the criterion S - , the UE starts the Stored Information Cell Selection procedure. • There is a difference, when the UE returns to the RRC idle mode after an emergency call on any PLMN. • Here, candidate cells to be selected to camp on are cells, to which the UE was connected to immediately before it terminated the emergency calls. • These cells have to be only acceptable cells. • But if none of these cells are acceptable, the UE has to start looking for acceptable cells, while it is in the internal state Any cell selection.

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Cell Selection When Leaving the RRC Connected Mode • Active Set cells as cells as candidates for cell selections; if not suitable, then • stored information cell selection

Squal > 0 Srxlev > 0

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Cell Reselection in the RRC Idle and RRC Connected Mode •  As soon as a UE camps on a cell, it must periodically look for a better cell to camp on. • But what is a “better“ cell? • On the BCCH, parameters for cell reselection are broadcast. If a cell meets the cell selection criteria of these parameters, it is a better cell to camp on. The cell re-selection may result in the change of the UTRA carrier or even the RAT.

• If the UE camps normally on a cell, it is responsible to • monitor the BCCH and to obtain the system information; • listen to its paging channel and its paging channel resource; • perform serving cell measurements periodically; • perform neighbouring cell measurements as far as this is required by the cell re-selection evaluation procedure; • conduct the cell re-selection evaluation procedure, when BCCH information has changed or internal threshold measures indicate it.; •execute a cell reselection, if a better cell has been found after the cell re-selection evaluation process; •Move the UE to the internal state Any state  Any cell selection, selection , if no suitable cell can be found.

• The •UE performs its measurements based on the BCCH.  The BCCH also delivers the neighbourhood list to the UE. Given this information, the UE decides, • on which RATs RATs and frequency bands it has to perform its measurements • which cells are candidate cells for measurements within one frequency band and RAT (HCS), and • ranks the cells to determine the best suitable cell for cell reselection.

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Cell Selection & Reselection • Motivation; PLMN Search/Selection Search/Selection • Cell Status & Access Control • States & State Transitions in RRC Idle mode • Cell Selection; Selection S- & Ranking R-Criterion •  Cell Selection Criterion S •  HCS •  Cell Reselection without HCS •  Cell Reselection with HCS

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Cell Re Reselection selection in  in the RRC Idle and RRC Connected Mode HCS in use:

HCS not in use:

measurement thresholds

measurement thresholds

S-Criterion fulfilled

S-Criterion fulfilled

H-Criterion

for fast moving UEs cells with lower HCS priority than in serving cell

R-Criterion cell ranking 35

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for slow moving UEs if no Cell cells with highest HCS with H>0 accept all Priorities & neighbours H>0

R-Criterion  R-Criterion  cell ranking 3G RPLS (RN 3155) / RU30 / AA / 03_2011

R-Criterion cell ranking

 

Hierarchica Hierarchicall Cell Structures HCS • The radio access network may be designed using hierarchical cell structures. • Numbers in the picture describe different layers in the hierarchy. • The highest hierarchical layer, i.e. typically smallest cell size, has the higher priority (number 3 in the figure). • The HCS priority of each cell is given in system information • Normally, different layers are created using different frequencies. • However, in some scenarios, different layers can use the same frequency. • Different frequencies can also be used on the same hierarchical layer e.g. in order to cope with high load in the system.

• In order to cope with UEs travelling fast through smaller cells (e.g. through micro or pico cells), the cell reselection procedure can be performed towards bigger cells on lower priority layers e.g. to macro cells so as to avoid unnecessary cell reselections.

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Hierarchica Hierarchicall Cell Structures HCS

Priority

3 3

2

3 1 2

3 3

3 2

Typical: •  smallest cell  highest HCS priority •  different layer  different frequency 37

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Cell Selection & Reselection • Motivation; PLMN Search/Selection Search/Selection • Cell Status & Access Control • States & State Transitions in RRC Idle mode • Cell Selection; Selection S- & Ranking R-Criterion •  Cell Selection Criterion S •  HCS •  Cell Reselection without HCS •  Cell Reselection with HCS

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Cell reselection without HCS • Normal re-selection process (without HCS) follows simplified procedure • Measurement criteria w/o HCS • S-criteria • R-criteria

Neighbour list from BCCH

Measurement criteria

Measured neighbours S –  – criteria  criteria Suitable R –  – criteria  criteria UseOfHCS

Best ranked cell

WCEL; 0: HCS not used; used; 1: HCS in use  use  SIB 11/12

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neighbours

Cell Re-selection if not serving cell 3G RPLS (RN 3155) / RU30 / AA / 03_2011

 

Cell Reselection (w/o HCS): Measuremen Measurementt Rules •  As part of the network planning process, the operator op erator has to determine the threshold values, va lues, which trigger the cell re-selection process by the UE. • The operator has also to decide, whether to use the HCS. The BCCH is used to inform the UE about the use of HCS. • If HCS is not used, the the UE‘s measurements are based on following rules, which can be found found in the specification TS 25.304 (replace Sx by Squal for FDD cells and by Srxlev for TDD): • Intra-Frequency measurement threshold Sintrasearch  • If this parameter is not sent in the serving cell, the UE must always perform intra-frequency measurements. If it is transmitted and Sx > Sintrasearch, the UE does not perform intra-frequency measurements. If Sx SsearchRAT m, it won‘t conduct measurements on cells of radio access technology “m”. But if Sx  >  Rs  Cell Reselection Rn = Qmeas,n - Qoffsets,n 

WCDMA & GSM cells; cells ;

RxLev RxLev:: 1st Ranking

  s   a   e   m

   Q

 Adjg,s,i  Adj g,s,iQoffset1 Qoffset1

Qmeas,n

Rn

HOPx; -50.. 50; 1; 0 dB

Qmeas,s Qhysts Qhyst1

Treselection:: Treselection

WCEL; 0.. 40; 2; 0 dB

Time Neighbour cell must be ranked better before Reselection

WCELL; 0..31; 1; 2s

Rs Qoffsets,n Treselection Qmeas = Ec/No or RSCP Ec/No  Qhyst2 RSCP  Qhyst1 Rs Rs /  / Rn: Serving Serving /  / Neighbouri Neighbouring ng cell value 44

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FDD cells & Ec/No FDD cells Ec/No:: 2nd Ranking

3G RPLS (RN 3155) / RU30 / AA / 03_2011

AdjisQoffset2  HOPS; -50.. 50; 1; 0 dB

Qhyst2 WCEL; 0.. 40; 2; 2 dB

 

Cell Selection & Reselection • Motivation; PLMN Search/Selection Search/Selection • Cell Status & Access Control • States & State Transitions in RRC Idle mode • Cell Selection; Selection S- & Ranking R-Criterion •  Cell Selection Criterion S •  HCS •  Cell Reselection without HCS •  Cell Reselection with HCS

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Cell reselection with HCS • Normal re-selection process (without HCS) follows procedure • Measurement criteria without HCS • S-criteria

UseOfHCS

WCEL; 0: HCS not used used;; 1: HCS in use  use  SIB 11/12

• H-criteria • R-criteria

• HCS specific H-criteria defines whether priorities are applied in re-selection

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UseOfHCS

Cell reselection with HCS Neighbourr list Neighbou

WCEL; 0: HCS not used; used; 1: HCS in use  use  SIB 11/12

from BCCH Measurement criteria with HCS

Measured neighbours

S – criteria  – criteria

Suitable neighbours H – criteria  – criteria

Prioritized

to rank

neighbours

Best ranked cell Re-selection if not serving cell © Nokia Siemens Networks

3G RPLS (RN 3155) / RU30 / AA / 03_2011

0 (lowest).. (lowest).. 7(highest); step: 1 SIB 11/12

Neighbours R – criteria  – criteria

47

HCS_PRIO*

 Adjg,i,s  Adj g,i,sHCSpriority HCSpriority  

Cell Reselection with HCS: Measurement Rules • The UE gains from the BCCH system information, that HCS is used. Then the UE knows, that it has to perform following threshold based measurements rules (replace Sx by Squal for FDD cells and by Srxlev for TDD cells): • intra-frequency & inter-frequency threshold based measurement rule :

“IF (Srxlev  (Srxlevs  SsearchHCS) OR (if FDD AND Sx  Sintersearch) THEN measure THEN  measure on all intra-frequency & inter-frequency cells ELSE IF IF (  (Sx Sx > Sintrasearch) THEN measure on all intra-frequency & inter-frequency cells, which have higher THEN measure HCS priority level than the serving cell unless measurement rules for fastmoving UEs are triggered ELSE  measure on all intra-frequency and inter-frequency cells, which have equal ELSE  or higher HCS priority level than the serving cell unless measurement rules for fast-moving UEs are triggered ENDIF ENDIF“ 

(cited from TS25.304 V3.11.0, chap 5.2.6.1.2)

• If HCS is used and if S intrasearch or SsearchHCS or Sintersearch (in FDD) are not sent for the serving cell, UE performs measurements on all intra-frequency and inter-frequency cells. SsearchHCS can get integer values ranging from -1 to 91 (step size 2) in the FDD mode. Negative values are set to 0 by the UE.  

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Cell Reselection with HCS: Measurement Rules if Srxlevs > SsearchHCS

BCCH SI: HCS used  used 

(i.e. RSCP > - 115 dBm)

Sx = Squal  Squal (in FDD mode)

Sintersearch

SsearchRAT m EC /N0 =

-14 dB

Sintrasearch  -8 dB

-10 dB

search

Equal & higher HCS priority Intra- & InterFrequency

search ALL

Intra- & Inter- Frequency

search

higher HCS priority Intra- & Inter- Frequency

Srxlev = Srxlev = Qrxlevmeas  – –  Qrxlevmin

Qrxlevmin Qrxlevmin:: WCEL; -119...-25; 2; -115 dBm SIB 11/12

if Srxlevs ≤ SsearchHCS OR OR  Squal ≤ Sintersearch  

if  Srxlevs ≤ SsearchHCS (i.e. RSCP < - 115 dBm): dBm):

search ALL Intra- & Inter-frequency cells 

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Squal = CPICH Ec/No - QqualMin

SsearchHCS WCEL; -1..91; def.: -1 dB dB** SIB3

3G RPLS (RN 3155) / RU30 / AA / 03_2011

Example: NSN Sintrasearch = 10 dB, Sintersearch = 8 dB, Ssearch_RAT  = 4 dB, SsearchHCS = -1 dB

QqualMin: QqualMin: WCEL; -24...0; 1; -18 dB SIB 11/12

negative values are set to 0 by UE  

Cell Reselection with HCS: Measurement Rules • HCS is used in the serving cell, what kind of inter-RAT measurements are conducted? •(replace Sx by Squal for FDD cells and bySrxlev for TDD cells): • Inter-RAT threshold-based measurement rules “IF IF (Srxlev  (Srxlevs  SHCS,RATm (i.e. RSCP > - 115 dBm)

SsearchRAT m EC /N0 =

Sx = Squal (in FDD mode)

Slimit_searchRAT m S intersearch Sintrasearch  -16 dB

-14 dB

search

search ALL

Intra- &  InterFrequency +

Inter-RAT

Intra- & Interfrequency & Equal & higher HCS  HCS  priority InterRAT

search search all

Intra- & InterFrequency

Ssearch_RAT:

Slimit_SearchRAT#

WCEL; 0...20; 2; 4 dB

WCEL; 0..20; 2; 2 dB

Equal & higher HCS priority Intra- & InterFrequency

(i.e. RSCP > - 115 dBm):

higher HCS priority Intra- & InterFrequency

Example: NSN Sintrasearch = 10 dB,

SIB3

if  Srxlevs ≤ SHCS_RATm

search

Sintersearch = 8 dB, Ssearch_RAT = 4 dB,

SHCS_RAT -1..91; 2; -1 dB* dB* SIB3

Slimit, searchRAT = 2 dB, SsearchHCS = -1dB,

search ALL Inter-RAT cells 

SHCS_RAT = -1dB * negative values are set to 0 by UE  UE  

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#

Srxlev = Srxlev = Qrxlevmeas  – –  Qrxlevmin

Qrxlevmin:: Qrxlevmin WCEL; -1 - 119...-25; 2; -115 dBm SIB 11/12

Squal = CPICH Ec/No - QqualMin QqualMin : QqualMin: WCEL; -24...0; 1; -18 dB SIB 11/12

Threshold for skipping inter RAT measurement measurement rules in HCS   

Measurement Rules with HCS & fast moving UEs HCS is introduced, so that slow s low moving UEs are served in cells with high priority, which have often a small radius (micro or pico cells). On the other hand, fast moving UEs should remain on cells with low priority, which often have a large cell radius (macro cells). By doing so, too many re-selection shall be avoided. avo ided. For fast m moving oving UEs, following intra-frequenc intra-frequency y, inter-frequency & inter-RA inter-RAT T measurement rules apply: • NCR is a threshold value for the number of cell re-selections. • TCRmax is a time period. If there are more than N CR cell reselections during TCRmax, then high-mobility has been detected (UE). • In this state, the UE measures on all neighbouring cells, which have equal or lower HCS priority than serving cell. Candidate cells for re-selections fulfil the S-criterion. If there are several candidates for cell re-selection, the UE prioritises re-selection neighbouring cells on lower HCS priority level before neighbouring cells on same HCS priority level.

What happens, if the number of cell re-selections no longer exceeds N CR during the time period TCRmax? • Then, for the time period T , the high speed cell re-selection rule is applied. CrmaxHyst • Afterwards, the UE must apply the normal (low speed) threshold based measurement rules for HCS.  

 According to the specifications, following parameter values are possible: • TCRmax: Enumerated (not used, 30, 60, 120, 180, 240), in seconds. Default value is set to ’60s’.   • NCR : is an integer value ranging from 1 to 16. Default value is 8. • TCrmaxHyst : Enumerated (not used, 10,20, 30, 40,50, 60, 70), in seconds. Default: not used.

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Measurement Rules: HCS & fast moving UEs NCr : WCEL; WCEL;  1..16; 1; 8; SIB3 TCrmax: WCEL; TCrmax: WCEL;  not used, 30, 60, 120, 180, 240 s; s ;  60s 60s;; SIB3 TCrmaxhyst: WCEL;  TCrmaxhyst: WCEL; not used; used; 10..70; 10; SIB3

NCR /TCRmax 

time measurement rules slow moving UEsfor

fast moving UE

TCrmaxHyst

measurement rule for fast moving UEs NCR = cell reselections threshold reselections threshold TCRmax = duration for evaluating the # of cell reselections TCrmaxhyst: TCrmaxhyst : Cell reselection hysteresis for reverting from UE high-mobility measurements

measures neighbouring cells with 53

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equal or 3155) lower priority 3G RPLS (RN / RU30HCS / AA / 03_2011

measurement rules for slow moving UEs

priorizes cells with  with lower   HCS priority  priority for re-selection

 

Reselection: H-Criterion (slow moving UE) Cell Reselection: • HCS is in use. Which cells are potential candidates for cell re-selection? H..  • Here, we use the quality level threshold criterion H • It is applied for all neighbouring cells, i.e. Intra-frequency cells, ce lls, Inter-frequency cells & Inter-RAT cells. H values are defined for the serving & neighbouring cells: •  Hs = Qmeas,s  – – Qhcss  •  Hn = Qmeas,n - Qhcsn –  – TOn * Ln 

(for the serving cell) (for candidate neighbouring cells for cell re-selection)

• Qmeas is the quality of the Rx signal at the UE. • For FDD cells, this value is calculated from the averaged CPICH Ec/No or CPICH RSCP. • For TDD cells, it results from the av averaged eraged P-CCPCH RSCP. RSCP. • For GSM cells, the UE determines it from the averaged received signal level.

• There are two measurement options for FDD cells. The UE derives the measurement quality (Ec/No or RSCP) from UTRAN. •  A cell fulfils the quality level threshold criterion H H,, if its H-value is equal or larger then zero. • The value Qhcs Qhcs  is used to determine the UE “distance“ from the Node B before the cell becomes a candidate for cell re-selection; its value depends on the measurement method. The specification default value is 0.  0.  TOn = TEMP_OFFSETn * W(PENALTY_TIMEn  – – Tn)

Ln = W(x) = 54

0 if HCS_PRIOn = HCS_PRIOs  1 if HCS_PRIOn  HCS_PRIOs 0 for x < 0 1 for x >= 0

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3G RPLS (RN 3155) / RU30 / AA / 03_2011

(continued)  

Cell Reselectio Reselection: n: H-Criterion (slow moving UE) • TEMP_OFFSETn introduces an additional offset to the H (and R) criterion, but only for a certain period of time, which is specified by the parameter PENALTY_TIMEn. • This timer is set for each neighbouring cell. •  A timer is started from value zero, when one of the following conditions is fulfilled: • if HCS_PRIOn  HCS_PRIOs and Qmeas,n > Qhcsn  • if HCS_ PRIOn = HCS_PRIOs and i) for serving FDD and neighbour FDD cells if the quality measure for cell selection and reselection is set to CPICH Ec/No in the serving cell, and: Qmeas,n > Qmeas,s + Qoffset2s,n ii) for serving FDD and neighbour FDD cells if the quality measure for cell selection and reselection is set to CPICH RSCP in the serving cell, and for all other serving and neighbour cells: Qmeas,n > Qmeas,s + Qoffset1s,n“

(from TS25.304 V3.11.0, chap 5.2.6.1.4)

• The timer Tn is only in use, when one of the above conditions is fulfilled. • If a cell-reselection takes place, and the cell, for which this timer was triggered is still a neighbouring cell, and the above conditions are fulfilled with the threshold values of the new cell, then the timer continues to run. Otherwise, it‘s use is stopped. If no condition is fulfilled, the value TO n is set to zero.

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(continued)  

Cell Reselection: Reselection: H-Criterion (slow moving UE) •  According to the specifications, following parameter values are possible: • HCS_PRIO gives a cells priority.

HCS_PRIO* 0 (lowest).. (lowest).. 7(highest); step: 1 SIB 11/12

• TEMP_OFFSET value depends on the measurement method. • If in the FDD mode, CPICH Ec/No is used, then its value are 3, 6, 9, 12, 15, 18, 21, or infinitive; Otherwise, its values are 2, 3, 4, 6, 8, 10, 12, or infinitive. •PENALTY_TIMEn is an integer value of either 0, 10, 20, 30, 40, 50, or 60 seconds. • The default value is 0.   • The H-selection criterion: •The UE has determined all neighbouring cells, which fulfil the S-criterion. •Then, it determines all cells of the th e highest HCS_PRIO, which fulfil the quality level thresho threshold ld criterion H 0. •Only these cells are ranked by the UE. •If no cell fulfils the quality level threshold thresho ld criterion H >=0, then all cells will be ranked by the UE, independent of their HCS priority level. This rule also applies, when no HCS is in use.

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 Adjg,i,s  Adj g,i,sHCSpriority HCSpriority  

Cell Reselectio Reselection: n: H-Criterion (slow moving UE) Qhcs:: Qhcs HCS is in use: Candidates for cell re-selection?  re-selection? 

H-Criterion !

Hs = Qmeas,s –  – Qhcss

measurement thresholds

Hn = Qmeas,n - Qhcsn  – – TOn * Ln

cells with  with  lower HCS priority than priority  than in serving cell

WCEL; -24..0; WCEL;  0.5; -24 dB

TOn: temporary Offset during Penalty Time  next slide

S-Criterion fullfilled

for fast moving UEs

Quality threshold level for HCS

Ln  = 0, 0,  if HCS_Prion = HCS_Prios  = 1, if HCS_Prion ≠ HCS_Prios

H-Criterion cells with

for slow moving UEs

no cells with H>0

H>0 only cells with highest HCS priorities & priorities  &

H>0

accept all Neighbours

(same as HCS not in use)  use) 

R-Criterion cell ranking

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R-Criterion

R-Criterion

cell ranking

cell ranking

3G RPLS (RN 3155) / RU30 / AA / 03_2011

 

Cell Reselection: H-Criterion (slow moving UE) & TOn Adjs,i,gPenalty_Time Adjs,i,gPenalty_Time:: HOPx; 0..60; 10; 0 s 

Hs = Qmeas,s –  – Qhcss

Ln  = 0, 0,  if HCS_Prion = HCS_Prios  = 1, if HCS_Prion ≠ HCS_Prios

Hn = Qmeas,n - Qhcsn –  – TOn * Ln

Adjg,i,sTempOffset;  Adjg,i,sTempOffset; HOPx; 3, 6, 9, 12, 15, 18, 21, Infinity; 3 dB; temp. offset during offset during Penalty Time  Time used when neighbour has different priority than serving cell

TOn = TEMP_OFFSETn * W(PENALTY_TIME W(PENALTY_TIMEn –  – Tn)  PENALTY_TIMEn

Qmeas,n - Qhcsn

PENALTY_TIMEn

0  TOn

TOn

H-Criterion: H>0

TOn = 0

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Example: HCS_PRIOn ≠ HCS_PRIOs  1   Ln = 1 

TOn = 0 Start T n 

Stop Tn 

3G RPLS (RN 3155) / RU30 / AA / 03_2011

time

Start Tn 

 

Cell Reselection with HCS: R-Criterion •  After checking the measurement thresholds, thresho lds, the UE has detected suitable cells to camp on. • If HCS is in use, it has reduced the candidate cells for cell reselection to those with highest priority and H >= 0 (given a slow moving UE). • But which of the remaining candidate cells is the best one for cell re-selection? • For that, a cell-ranking criterion R was R was specified: • Rs = Qmeas,s + Qhysts  (for the serving cell)  cell)  • Rn = Qmeas,n - Qoffsets,n - TOn * (1 – Ls) (for candidate neighbouring cells for cell reselection)  reselection)   • The serving cell and the remaining candidate cells are ranked according to criterion R. • The cell ranked with the highest value R is the best cell for the UE to camp on. • Qhysts gives a hysteresis value to make the serving cell more attractive and thus delay the cell reselection. It exists in two versions: • If the quality measure is based on CPICH Ec/No, Qhyst2 s is used in (FDD cell only); otherwise the hysteresis value Qhyst1  is used (if the quality measure for FDD cells is based on CPICH s RSCP,, for TDD cells and GSM cells). RSCP • It ranges between 0 and 40 (step size 2). • If HCS is applied in the serving cell, TOn is calculated as described on the preceding pages. • If HCS is not applied, then this temporary offset term is not used when calculating Rn.

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Cell Reselection Reselection with HCS: R-Criterion and TOn Rs = Qmeas,s + Qhysts  Rn = Qmeas,n - Qoffsets,n - TOn * (1 – Ls) TOn = TEMP_OFFSETn * W(PENAL W(PENA LTY_TIMEn  – – Tn) 

  s   a   e   m

PENALTY_TIMEn

temp. offset used only when neighbour has same priority than serving cell

PENALTY_TIMEn

   Q

TOn TOn

TOn = 0

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TOn=0 Start

Stop

Start

Tn 

Tn 

Tn 

3G RPLS (RN 3155) / RU30 / AA / 03_2011

Qmeas,s

Example: HCS_PRIOn = HCS_PRIOs   Ln =

0  0 

 

Cell Reselection with HCS: R-Criterion Rs = Qmeas,s + Qhysts  Rn = Qmeas,n - Qoffsets,n - TOn * (1 (1 – Ls) 

WCDMA & GSM; RxLev: 1st Ranking

  s   a   e   m

   Q

Rn > Rs  Cell Reselection  Reselection 

Qhyst1 WCEL; 0.. 40; def.: 0 dB  dB 

Qmeas,n 

Q

HOPx; -50.. 50; def.: 0 dB dB  

 

meas,s

Qhysts 

Rs 

Qoffsets,n  Treselection: Treselection:  WCEL; 0..31; 1; 2s 1; 2s

Example: HCS_PRIOn ≠ HCS_PRIOs  Ln = 1  1  61

Rn 

 Adjg,s,iQoffset1  Adjg,s,i Qoffset1

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3G RPLS (RN 3155) / RU30 / AA / 03_2011

FDD cells & Ec/No FDD cells Ec/No:: 2nd Ranking  Ranking  

 Adji&sQoffset2  Adji&s Qoffset2 HOPx; -50.. 50; 1; 0 dB  dB  Qhyst2 WCEL; 0.. 40; 2; 2 dB  dB 

 

References For this course module, following 3GPP specifications were used: • TS 23.122, Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode • TS 25.133, Requirements for support of radio resource management (FDD) • TS 25 304, User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode • TS 25.331, Radio Resource Control (RRC) protocol specification NSN WCDMA RAN Parameter Dictionary

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