04 04 rn31554en10gla0 cell selection and reselection
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Cell Selection and Reselection
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Cell Selection and Reselection
3GRPLS (RN3155) – Section 4
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Cell Selection and Reselection
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At the end of this module, you will be able to:-
• List the motivations for cell selection and reselection
• Explain the meaning of “Equivalent PLMN“
• Name the differences between access classes and cell access
restrictions
• Describe the state transition in the RRC idle mode
• Understand the cell selection criterion, S
• Name the differences, when cell selection is performed with and
without HCS
• Outline the measurement rules for both HCS enabled cells and
cells without priorities• Explain the ranking criterion, R
Objectives
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Four different reasons for camping on a cell in the RRC idle mode were identified in the specifications:-
1. The UE is capable to read the system information, which is broadcasted on the BCCH (P-CCPCH).
Hereby, the UE gains also information of the serving PLMN.
2. If the access to the current cell is not barred, then the UE can use the cell to initiate an RRC
(signalling) connection.
• The access network is accessed by common control channels (CCCH on PRACH) to access
the network.
• 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.
3. If the UE is registered, and there is a mobile terminated call, the serving core network elements
know (in most cases) the location area resp. routing area, in which the UE is camping.
• Paging is done with the common control channel PCH (on one of the S-CCPCHs).
• The UE receives the paging message, because is monitors all paging occasions on thepaging resources of its paging class within the cell.
Cell Selection and Reselection: Motivation
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4. The UE can receive cell broadcast service messages.
In the ETSI specification TS 25.304, the idle mode tasks were subdivided into threeprocesses:
1. PLMN selection and reselection;
2. Cell selection & reselection;
3. Location registration.
Cell Selection and Reselection: Motivation
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CM requestsNAS control radio
measurements
PLMN
selected
PLMN
available
registration
area changes
location
registrationresponse
indication
to user manual mode
location
registrationcell selection
and reselection
PLMN selectionand reselection
automatic mode
RRC tasks:
• PLMN selection &
reselection;
• Cell selection &reselection;
• Location registration LR.
if the UE is in the
RRC idle mode
if the UE is in the
RRC connected mode
states:• CELL_FACH,
• CELL_PCH &
• URA_PCH
Cell Selection and Reselection: Motivation
TS 25.304:Idle mode task
subdivided into 3 processes:
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PLMN search
• The UE shall scan all RF channels in the UTRA bands according to its capabilities to findavailable PLMNs.
• On each carrier, the UE shall search for the strongest cell and read its system information, inorder 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 highquality PLMN (but without the RSCP value), provided that the following high quality criterion isfulfilled:
• For an FDD cell, the measured primary CPICH RSCP value shall be greater than or equal to -95 dBm.
• Found PLMNs that do not satisfy the high quality criterion, but for which the UE has beenable to read the PLMN identities are reported to the NAS together with the CPICH RSCPvalue 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 andoptionally also information on cell parameters, e.g. scrambling codes, from previouslyreceived measurement control information elements.
• For a GSM cell, a PLMN shall be understood to be received with high quality signal if thesignal level is above -85 dBm.
• Once the UE has selected a PLMN, the cell selection procedure shall be performed inorder to select a suitable cell of that PLMN to camp on.
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PLMN search
Search PLMNs
UTRA:
If RSCP > -95 dBm
GSM:
If RSSI > -85 dBm
High qualityPLMNs Other foundPLMNs
PLMN IdPLMN Id
+ RSCP/RSSI
UE AS
UE NAS
TS 25.304 & TS 43.022
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• Operators have observed, that 95% to 98% of their subscribers are served in their home PLMN
(HPLMN). Roaming, i.e. the use of another operator‘s network infrastructure is allowed to many
subscribers. This becomes for instance necessary, if the mobile phone leaves the supply area of the
home operator.• For PLMN selection, two modes of operations were specified:
•Automatic mode
The mobile phone detects available networks. It then utilises a list of PLMNs, which names
PLMNs in a priority order. The PLMN with the highest priority, which is available and which the UE
is allowed to select is chosen for network registration.
•Manual mode
In this mode, the mobile phone scans the radio interface, and indicates to the user to available
PLMNs. The mobile phone attempts to register with the network, the subscriber manually selects.
If the subscriber does not select a (suitable) network, then the UE only offers limited services, i.e.
emergency calls.
• The following data fields are stored on the 3G USIM or the ME and play a major role in the automatic
PLMN selection process:
•IMSI: The MCC and the 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 as preferred by the subscriber (stored on USIM)
• A PLMN list including RATs as preferred by the operator (stored on USIM)
•The list of equivalent PLMNs which were received in last registration (stored on ME)
•HPLMN search period timer (stored on USIM)
PLMN Selection
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• A list of PLMN identifiers associated with an access technology is listed on the USIM in a priorityorder.
•For the PLMN identifier, three bytes memory are required, while for the access technology, two
bytes are necessary. At least one PLMN and associated access technology must be listed.• If there is a PLMN identifier allowing more than one access technology, it is up to the mobile phone
implementation to determine the preferred access technology.
• Not only international roaming is possible, but also national roaming.
•In many countries, the regulation authority enforced national roaming of UMTS subscribers inexisting 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.
•In this case the UE must periodically search for the HPLMN. The “HPLMN Search Period” timeris used for that. Its values range between 6 and 480 minutes with a step size of 6 minutes. Itsdefault value is 60 minutes, which is used, when the “HPLMN Search Period” timer was not set.“HPLMN Search Period” timer can be set in such a way, that no no periodic search attempts areconducted.
PLMN Selection
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Operator controlled
PLMN list
User controlledPLMN list
Last registeredPLMN
Home PLMN
Other high qualityPLMN‘s in random
order
Switch onin automatic PLMN selection operation mode
HPLMN Search Period Timer 6 .. 480 minutes
Default: 60 minutes
PLMN Selection
Other PLMN‘s indecreasing signal
quality order
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• If the operator wants to restrict access to a cell or to set set cell reservations. Two methods can be
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 and re-selection process. But a
mobile phone is not allowed to send an initial access message to the access network.
•This method is normally applied for load control reasons.
•The subscriber‘s access class(es) are determined on cell basis from the IMSI and
parameters, broadcasted on the BCCH. The subscriber‘s access class(es) is (are) stored
on the USIM.
• 1) Cell status and special reservations
•Acceptable cell is a cell, which offers limited services (i.e. the origination of an emergency call)
to the subscriber.
•Suitable cell is a 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 cellselection criterion S, which is covered later on in this chapter.
•Barred cell is a cell, where the mobile phone is not allowed to camp on.
•Reserved cell is a cell, where "ordinary“ mobile phones are not allowed to camp on.
• The IE Cell Access Restriction is used to indicated cell status and cell reservations.
Cell Reservations and Cell Restrictions
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Cell
- acceptable (limited services*)
- suitable (full service set)
- barred
- reserved#
Access Class barred:
(UE access classes 0..9
and 11..15)
BCCH SIB3/4: Cell Access Restriction
Cell status and special reservations
* e.g. Emergency Call (Access Class = 10) in Network w/o Roaming Agreement
# e.g. for operator use
Cell status &
special reservations: Access Control
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• 1) Cell status and special reservations
•The UE must read following three information elements in the IE Cell Access Restriction in order
to learn cell‘s status and cell‘s reservations:
•cell barred•cell reserved for operator use, and
•cell reserved for future extensions.
• Four cases can be distinguished (see next figure):
•Case 1:
•The cell is suitable for selection and re-selection both in the RRC idle mode and RRC
connected mode.
•Case 2:
•The mobile phone behaves as if cell status "barred" is indicated.
•Case 3:
•If a mobile phone is in the HPLMN and assigned to access class 11 or 15, then it is
allowed to select and re-select this sell. If it belongs to another access class, it behaves as
if the cell status is “barred“.
•Case 4:•The UE is not allowed to select and re-select this cell. Not even emergency calls are
allowed. In the IE Cell Access Restriction an IE "Intra-frequency 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.
Cell status and special reservations
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Cell Barred Cell reserved
for operator
Cell reserved
for future extension
Case
1
2
3
4
not barred
not barred
not barred
barred
not reserved not reserved
not reserved reserved
reserved ---
--- ---
suitable for selection
and reselection
suitable for AC 11 or 15 in HPLMN
otherwise “barred”
Cell “barred”
Not suitable – not even for
emergency call
Cell status and special reservations
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• 2) Access Control
• Access Control is normally used to for load reasons. It limits the accesses on the common control channels.• Access class restrictions are broadcasted via the BCCH
•In the cell selection and re-selection process, the 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 themobile phone.
•If the mobile phone is in the RRC idle state, it checks access class restrictions before it sends an RRCCONNECTION REQUEST message.
•If the mobile phone is already in the RRC connected state, there are no access class restrictions valid for the
UE.• Access class restrictions do also not apply, if the UE must enter the RRC connected mode after an Inter-RAT
cell re-selection to UTRAN.
• 3) Emergency Call
• Access class restrictions for emergency calls can be imposed by the operator. The mobile phones are 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 mobilephones 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 accessclass 10 is barred and its own class (11 to 15). Otherwise, emergency calls are allowed for these phones.
Access Control
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UE RRC idle
I ignore all Access
Class related cell
access restrictions
when selecting a cell
to camp on
UE RRC connected
Access Class only
relevant for access(from RRC IDLE)
Access Class
irrelevant
for access
suitable cell
suitable cell
Access Control
Access Control normally
used for load reasons:
Overload Control feature
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• WCEL: CellBarred
•Defines whether the cell is barred or not.
• A barred cell is a cell where a UE is not allowed to camp on.
•This parameter is part of SIB 3.•0 (Barred), 1 (Not barred); default: 1.
• WCEL: TBarred
•If the UE does not select another cell, and the barred cell remains to be the “best” one, the
UE shall after expiry of the time interval Tbarred again check whether the status of the
barred cell has changed.
•This parameter is part of SIB 3.
•[10, 20, 40, 80, 160, 320, 640, and 1280] seconds; default: 40 seconds.
• WCEL: Cell_Reserved
•Defines whether the cell is reserved for operator use.
•This parameter is part of SIB 3.
•0 (Reserved), 1 (Not reserved); default: 1.
• WCEL: IntraFreq_Cell_Reselect_Ind
•Defines whether intra-frequency cell re-selection is allowed when the cell is barred.
•This parameter is part of SIB 3/4.
•0 (Allowed), 1 (Not allowed); default: 0.
NSN Parameters for Cell Reservations and Restrictions
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• Cell Selection Process
•The Non-Access Stratum 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 the cell selection process?
•It provides information about the radio access technology or radio access technologies,
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 two access technologies identify the core network with different
network identifiers. But they can support an inter-RAT handover between the two networks
(which is physically only one).
•It holds a list of forbidden registration areas.
•The Access Stratum manages the interaction between the UE and the RAT. Among others, it is
responsible to find a suitable cell, on which the UE can camp on.
• As soon as the access stratum has found a suitable cell, it commands the UE to camp onthis cell.
•It informs the higher layer non-access stratum protocols about this, so that these can start
the registration procedure. If the UE was successfully registered with the serving core
network elements, the UE enters the internal state Camped normally .
•If the UE is not capable to find a suitable cell to camp on within the selected PLMN, it
enters the internal state Any cell selection.
States and State Transition in the RRC Idle Mode
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Cell Selection Process
NAS controls
AS controls
correspondentprocedures
NAS:
PLMN
selection
• RAT(s) information associated with the selected PLMN
• forbidden registration areas
• list of equivalent PLMNs
UE CN
AS:
attempts
to find
suitable
cells
States and State Transition in the RRC Idle Mode
suitable
cell
found
camped
normally
camped
on any cell
all networkservices available
limited services
e.g. Emergency Call
YES
NO
acceptable cell
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•When the mobile phone listens to the system information, which is broadcasted on the P-CCPCH, it
receives a wide range of PLMN and cell relevant information.
•If initial registration was done successfully, it can also receive paging information on one of the S-
CCPCHs and initiate a mobile originated call 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 and RRC connected
mode.
•In the RRC idle mode, three 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 UEenters the internal state Any cell 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 cell selection state, when the NAS registration procedure failed.
When the UE attempts to access the cell, it leaves the Camped normally state and enters the
Connected mode state.(continued on the next text slide)
States and State Transition in the RRC Idle Mode
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1
I n i t i a l
C e l l S e l e c t io n
C a m p e d
n o r m a l l y
C e l lR e s e l e c t i o n
E v a l u a t io nP r o c e s s
n o c e l l i n f o r m a t i o n
s t o r e d f o r th e P L M N
c e l l i n f o r m a t i o n
s t o r e d f o r th e P L M N
g o h e r e w h e n e v e r
a n e w P L M N
i s s e l e c t e d
n o s u i t a b l e c e l l f o u n d
s u i t a b l e c e l l f o u n d s u i t a b l e c e l l f o u n d
C o n n e c t e d
M o d e
s u i ta b l e
c e l l f o u n d
n o s u i t a b l e
c e l l f o u n d
n o
s u i t a b l e
c e l l
f o u n d
S t o r e d
I n f o r m a t io n
C e l l S e l e c t io n
t r ig g e r
s u i t a b l e
c e l l s e l e c t e d
n o s u i t a b l e
c e l l f o u n d
C e l l S e l e c t io n
w h e n l e a v i n g
c o n n e c t e d
m o d e
l e a v e
i d l e m o d e
r e t u r n t o
i d l e m o d e
N A S
r e g i s t r a t i o n
r e j e c t e d
(adopted from TS 25.304)
States and State Transition in the RRC Idle Mode
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• Any cell selection state
•In this state, the UE tries to find an acceptable cell.
•That means, it attempts to offer to the subscriber at least limited services – the emergency call.
•Consequently, in this state, the cell search is done without regard of the PLMN identity.•The UE also looks for an acceptable cell in every 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.
• Camped on any cell State
•Only limited services – the mobile originated emergency call – 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 RATs it is capable to support.
States and State Transition in the RRC Idle Mode
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Camped on
Any Cell
Any CellSelection
Connected
Mode
(Emergency calls only)
return to
idle mode
go here when no USIM in the UE
USIM inserted1
leave
idle mode
no acceptable cell found
an acceptable cell found
Cell Selection
when leaving
connected
mode
acceptable
cell foundsuitable
cell found
1
CellReselectionEvaluation
Process
trigger
acceptable
cell selected
no acceptable
cell found
(adopted from TS 25.304)
States and State Transition in the RRC Idle Mode
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• Which cells are suitable for (initial) cell selection and reselection, so that the UE can camp on them?
• This is determined by the UE based on the cell selection criterion S.
•It is fulfilled, when•Srxlev > 0 AND Squal > 0 in the FDD mode, and
•Srxlev > 0 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 Qqualmeas of the cell. It is based on CPICH Ec/N0
(dB) for FDD cells. (CPICH Ec/N0 is averaged.)
•The operator determines for each cell the minimum required received level Qqualmin (dB) at the
UE. This value is the broadcasted. Its integer value can range between –24 and 0 dB.
• A cell is not suitable for cell selection and re-selection, if the measured received signal quality
level is below Qqualmin.
• Srxlev stands for the cell selection receive level value (dB).•The UE determines it this way: Srxlev = Qrxlevmeas - Qrxlevmin – Pcompensation
•Qrxlevmeas is the cell RX level measured by the UE, based on the CPICH RSCP for FDD cells
(dBm), and the averaged received signal level for GSM cells (dBm). (All values get averaged!)
•The operator sets the value Qrxlevmin as minimum required RX level in the cell (dBm), which is
sent to the UE via the BCCH. Its integer value can range between –119 dBm and –25 dBm (2 dB
step size).
Cell Selection Criterion S
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Qqualmeas (dB)
(CPICH Ec/N0)
Qrxlevmeas (dBm)
CPICH RSCPQrxlevmin
Srxlev > 0
Pcompensation
Squal > 0S-Criterion
fulfilledSqual >0 AND
Srxlev > 0
suitable
cell?
Cell Selection Criterion S (in the FDD mode)
Squal =
CPICH Ec/No –QqualMin
Srxlev =Qrxlevmeas
– Qrxlevmin
– Pcompensation
QqualMin:-24...0; def.: -18 dB
SIB 11/12
AdjsQrxlevmin:-115...-25; def.:-115 dB
SIB 11/12
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• If the UE determines the cell‘s RX level value Qrxlevmeas 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)
• In order to access a cell, the UE has to use the common channel PRACH.
• The operator determines the maximum 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 – according to the specifications - between –50 dBm and 33
dBm.
On the other hand, there is the UE‘s maximum RF output power, given by P_MAX (dBm).
Cell Selection Criterion S
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Cell Selection Criterion S
Pcompensation
= max(UE_TXPWR_MAX_RACH – P_MAX, 0)
Cell size defining parameters:
• Qrxlevmin
• Qqualmin
I am
outside
I am inside,
but have notenough
power
-50 .. 33 dBm
UEtxPowerMaxPRACH
-50..33 dBm; 21 dBm
SIB3/4
AdjsQrxlevmin:-115...-25; def.: -115 dB
Srxlev = Qrxlevmeas – Qrxlevmin– Pcompensation
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• There exist two 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 mobile phone 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 mobile phone 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
•To speed up the cell selection process – for instance, when the UE is switched on 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 – e.g. the selected PLMN cannot
be found – the UE continues the cell selection process based on the Initial Cell Selectionprocedure.
• 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:
•Srxlev > 0 AND Squal > 0 in the FDD mode, and
•Srxlev > 0 in the TDD mode.
(Initial) Cell Selection Process
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Initial Cell Selection
(scan RF channel)
Stored Information
Cell Selectioncarrier, even Scr.Code may be
stored in UE
or
I have to find
a suitable cell
Squal = Qqualmeas – Qqualmin > 0Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation > 0
Once a suitable cell is found this cell is selected
(Initial) Cell Selection Process
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• 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 – 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.
Cell Selection When Leaving the RRC Connected Mode
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• Active Set cells as candidates for cell
selections; if not suitable, then
• Stored information cell selection Squal > 0Srxlev > 0
Cell Selection When Leaving the RRC Connected Mode
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• WCEL: QrxlevMin
•The minimum required RX level in the cell.
•This parameter is part of SIB 3.
•[-119 ... –25] dBm, step 2 dBm; default: -115 dBm.
• WCEL: QqualMin
•The minimum required quality level in the cell (Ec/No).
•This parameter is part of SIB 3.
•[-24 ... 0] dB, step 1 dB, default: -18 dB.
• WCEL: UEtxPowerMaxPRACH
•This parameter defines the maximum transmission power level a UE can use on
PRACH.
•The value of the parameter also effects the cell selection and reselection
procedures.•The value of the parameter is sent to UE in the Cell selection and re-selection of
SIB 3 and 4 of the serving cell.
[..]
NSN Parameters for Cell Selection
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• As soon as a mobile phone 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 cell 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 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.
Cell Reselection in the RRC Idle and RRC Connected Mode
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S-Criterionfulfilled
cells withhighest HCSPriorities &
H > 0
measurement
thresholds
cells withlower HCS
priority than
in serving cell
If No Cellwith H>0accept all
neighbours
HCS in use: HCS not in use:
S-Criterionfulfilled
measurement
thresholds
H-Criterionfor fast
moving UEsfor slow
moving UEs
R-Criterion
cell ranking
R-Criterion
cell ranking
R-Criterion
cell ranking
Cell Reselection in the RRC Idle and RRC Connected
Mode
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Hierarchical Cell Structures
• 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 higherpriority (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 tocope 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 biggercells on lower priority layers e.g. to macro cells so as to avoid unnecessary cellreselections.
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Hierarchical Cell Structures
3
3
3
33
3
1
2
2
2
Normally, different layers are created using different frequencies
Priority
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Cell reselection without HCS
• Normal re-selection process (without HCS) follows simplified procedure
• Measurement criteria without HCS
• S-criteria
• R-criteria
W i t h o u t H C S
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Cell reselection without HCS
Measuredneighbours
Neighbour list
from BCCH
Best ranked cell
Measurement criteria
S – criteria
Suitableneighbours
R – criteria
Re-selection if not serving cell
W i t h o u t H C S
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• As part of the network planning process, the operator has to determine the threshold values, 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 UE‘s measurements are based on following rules, which can be 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 <= Sintrasearch, it performs intra-frequency measurements.
• Inter-Frequency measurement threshold Sintersearch
•If this parameter is not sent in the serving cell, the UE must always perform inter-frequency
measurements. If it is transmitted and Sx <= Sintersearch, it must perform inter-frequency
measurements, but if Sx > Sintersearch, there is no need to perform this type of measurement.
• Inter-RAT measurement threshold SsearchRAT m
•If this parameter is not sent in the serving cell, the UE must always perform inter-systemmeasurements. If it is transmitted and Sx > SsearchRAT m, it won‘t conduct measurements on cells
of radio access technology “m”. But if Sx <= SsearchRAT m, it has to do these measurements.
• Sintrasearch, Sintersearch, and SsearchRAT m can get integer values ranging from –32 to 20 (step size 2) in
the FDD mode. Negative values are set to 0 by the UE.
Cell Reselection: Measurement RulesW i t h o u t H C S
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SintrasearchSintersearchSsearchRAT m
No need tomeasure
neighbourcells
Intra-frequency
Sx=Squal (in FDD mode)
Intra-frequencyInter-frequency
serving cell
Example: NSNQqualmin = -18 dB,
Sintrasearch = 10dB,
Sintersearch = 8dB,
Ssearch_RAT = 4dB
When to perform
measurements
Intra-frequency
Inter-freqencyInter-RAT
Cell Reselection: Measurement RulesW i t h o u t H C S
-8 dB-10 dB-14 dBEC/N0 =
SinterSearch:0...20; def.: 8 dB
SintraSearch:0...20; def.: 10 dB
Ssearch_RAT:0...20; def.: 4 dB
QqualMin:-24...0; def.: -18 dB
Squal =
CPICH Ec/No
-qQualMin
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• After checking the measurement thresholds, the UE has detected suitable cells to camp on.
• But which of the remaining candidate cells is the best one for cell re-selection?
• For that, a cell-ranking criterion R was specified:•Rs = Qmeas,s + Qhysts (for the serving cell)
•Rn = Qmeas,n - Qoffsets,n (for candidate neighbouring cells for cell 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 re-
selection. It exists in two versions:
•If the quality measure is based on CPICH Ec/No, Qhyst2s is used in (FDD cell only); otherwise
the hysteresis value Qhyst1s is used (if the quality measure for FDD cells is based on CPICH
RSCP, for TDD cells and GSM cells).
•It ranges between 0 and 40 (step size 2).
• The value Qoffset is an offset given for each individual neighbouring cell, which ranges between –50
and 50 dB, with default set to 0.•Qoffset2 when Ec/No is used
•Qoffset1 when RSCP is use
Cell Reselection: R-CriterionW i t h o u t H C S
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• If a neighbouring cell was ranked with the highest value R, will the UE start the cell re-selection?
•If it is a GSM or TDD cell, then indeed the UE performs the cell re-selection process to this cell.
•If it is an FDD cell, it depends on the used quality measure.
•There are two options: CPICH RSCP or CPICH Ec/No.•The UE learns from the system information, which quality measure to use.
•If the quality measure CPICH RSCP is used, the UE perform the cell re-selection.
•If the quality measure Ec/No is used, the UE has to make a second ranking based on the
same measurement quantity.
•The UE performs cell re-selection to the FDD cell, which was ranked best in the
second ranking process.
• Is the cell re-selection initiated immediately after the UE ranks a neighbouring cell to be the best?
•If so, we could face a ping-pong effect – a UE often performing cell reselection between two
neighbouring cells.
•To avoid this, the operator uses the time interval value Treselection, whose value ranges
between 0 and 31 seconds.
•Only when a cell was ranked Treselection seconds better then the serving cell, a cell reselection
to this cell takes place.•In addition to this, a UE must camp at least 1 second on a serving cell, before the next cell re-
selection may take place.
• How often are the cell re-selection criteria evaluated?
•This is done at least once every DRX cycle for cells, for which new measurement results are
available.
Cell Reselection: R-CriterionW i t h o u t H C S
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Rs = Qmeas,s + Qhysts
Rn = Qmeas,n - Qoffsets,n
Qmeas,n
Qmeas,s
Q m e a s
Rs
Rn
Qoffsets,n
Qhysts
Rn > Rs =>“cell reselection“
Treselection
Cell Reselection: R-CriterionW i t h o u t H C S
Qmeas = Ec/No or RSCP
Ec/No Qhyst2 /
RSCP Qhyst1
Rs / Rn: Serving / Neighbouring cell value
Qhyst20.. 40; def.: 2 dB
AdjisQoffset2-50.. 50; def.: 0 dB
FDD cells & Ec/No:
2nd Ranking
Adjg,s,iQoffset1-50.. 50; def.: 0 dB
Qhyst10.. 40; def.: 0 dB
WCDMA & GSM cells; RxLev:
1st Ranking
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• WCEL: UseOfHCS
•This parameter indicates whether the serving cell belongs to a Hierarchical Cell Structure
(HCS).
•This parameter is part of SIB 11/12.•0 (HCS not used), 1 (HCS in use); default: 0.
• WCEL: Sintrasearch
•The threshold for intra-frequency measurements, and for the HCS measurement rules.
•This parameter is part of SIB 3.
•[0 ... 20] dB, step 2 dB, default: 10 dB.
• WCEL: Sintersearch
•The threshold for inter-frequency measurements, and for the HCS measurement rules.
•This parameter is part of SIB 3.
•[0 ... 20] dB, step 2 dB, default: 8 dB.
• WCEL: Ssearch_RAT•The RAT-specific threshold for inter-RAT measurement rules.
•This parameter is part of SIB 3.
•[0 ... 20] dB, step 2 dB, default: 4 dB.
NSN Parameters for Cell Reselection (Part I)
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Cell reselection with HCS
• Normal re-selection process (without HCS) follows procedure
• Measurement criteria without HCS
• S-criteria
• H-criteria
• R-criteria
• HCS specific H-criteria defines whether priorities are applied in re-selection
W i t h H C S
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Cell reselection with HCS
Measuredneighbours
Neighbour listfrom BCCH
Best ranked cell
Measurement criteriawith HCS
S – criteria
Neighboursto rank
R – criteria
Re-selection if not serving cell
W i t h H C S
Suitableneighbours
H – criteria
Prioritized
neighbours
UseOfHCS0 (not used), 1 (used)
SIB 11/12
HCS_PRIO0 (lowest) ..
7(highest); step: 1
SIB 11/12
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Higher HCSpriority
Intra-frequencyInter-frequency
Intra-frequencyInter-frequency
Equal &higher HCS
priorityIntra-
frequencyInter-
frequency
Example: NSN
Sintrasearch = 10 dB,
Sintersearch = 8 dB,
Ssearch_RAT = 4 dB,
SsearchHCS = -1 dB
Cell Reselection: Measurement RulesW i t h H C S
SintrasearchSintersearchSsearchRAT m SlimitsearchRAT m
Sx=Squal (in FDD mode)if Srxlevs > SsearchHCS
BCCH SI: HCS is used
-8 dB-10 dB-14 dBEC/N0 =
Srxlevs ≤ SsearchHCSif SsearchHCS
-1..91; def.: -1 dB*SIB3
AdjsQrxlevmin:-115...-25; def.: -115 dB
SIB 11/12
Srxlev =
Qrxlevmeas – Qrxlevmin
-115 dBmRSCP =
*negative values are set to 0 by UE
Srxlevs ≤ SsearchHCS OR Squal ≤ Sintersearch
Squal =
CPICH Ec/No
- QqualMin
QqualMin:
-24...0; def.: -18 dB
SIB 11/12
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• 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 (Srxlevs <= SHCS,RATm) or (if FDD and Squal <= SSearchRATm)
THEN UE shall measure on all inter-RAT “m“ cells
ELSE
IF (Sx > Slimit,SearchRATm)
THEN UE need not measure neighbouring cells in RAT “m”
ELSE UE shall measure on all neighbouring cells in RAT “m”, which have equal 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)
• SSearchRAT and Slimit,SearchRATm can get integer values ranging from –32 to 20 (step size 2) in the FDDmode. Negativ values are set to 0 by the UE.
• If HCS is used and if SHCS,RATm is not sent for the serving cell, UE performs measurements on all inter-
RATm cells.
Cell Reselection: Measurement RulesW i t h H C S
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Example: NSN
Sintrasearch = 10 dB, Sintersearch = 8 dB,
Ssearch_RAT = 4 dB, Slimit, searchRAT = 2 dB
SsearchHCS = -1dB, SHCS_RAT = -1dB
Cell Reselection: Measurement RulesW i t h H C S
or
SintrasearchS
intersearchSsearchRAT m
Higher HCSpriority
Intra-frequencyInter-frequency
SlimitsearchRAT m
Intra-frequencyInter-frequency
Intra-frequency
Inter-freqencyInter-RAT
Intra-frequency,
Inter-frequency, &
Equal andhigher HCSpriority inter
RAT
Equal &higher HCS
priorityIntra-
frequencyInter-
frequency
Sx=Squal (in FDD mode)
if Srxlevs ≤ SHCS,RATm
measure on all
Inter-RAT cells
if Srxlevs > SHCS,RATm
HCS; Inter-RAT measurements?
-14 dBEC/N0 =
Slimit,SearchRAT0..20; def.: 2 dB
SIB3
-16 dB
SHCS_RAT-1..91; def.:-1 dB*
SIB3
AdjsQrxlevmin:-115...-25; def:. -115 dB
SIB 11/12
Srxlev =
Qrxlevmeas – Qrxlevmin
-115 dBmRSCP ≤
*negative values are set to 0 by UE
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• WCEL: SHCS_RAT
•The RAT specific threshold for inter-RAT measurement rules.
•This parameter is part of SIB 3.
•[-1 ... 91] dB, step 2 dB, default: -1 dB (UE sets this value to 0).
• WCEL: Slimit_SearchRAT
•Threshold for skipping inter-RAT measurement rules in HCS.
• Above this RAT specific threshold in the serving UTRA cell, the UE does not need to
perform any inter-RATm measurements.
•This parameter is part of SIB 3.
•[0 ... 20] dB, step 2 dB, default: 2 dB.
• WCEL: SsearchHCS
•Threshold for intra- and inter-frequency measurement rules in HCS.
•Below this limit in the serving cell, the UE initiates measurements of all intra and inter-
frequency neighbouring cells of the serving cell.
•This parameter is part of SIB 3.•[-1 ... 91] dB, step 2 dB, default: -1 dB (UE sets this value to 0).
NSN Parameters for Cell Reselection (Part II)
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• HCS is introduced, so that slow moving phones 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. For fast moving UEs, following intra-frequency, inter-frequency and inter-RATmeasurement rules apply:
• NCR is a threshold value for the number of cell re-selections.
• TCRmax is a time period.
• If there are more than NCR cell reselections during TCRmax, then a high-mobility has been detected by
the mobile phone.
•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 NCR during the time period
TCRmax?
•Then, for the time period TCrmaxHyst, the high speed cell re-selection rule is applied.
• Afterwards, the UE must apply the normal (low speed) threshold based measurement rules forHCS.
• 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.
Measurement Rules with HCS and fast moving UEs
W i t h H C S
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Measurement Rules with HCS and fast moving UEs
W i t h H C S
NCR = cell reselections threshold (default: 8)
TCRmax = time period
NCR /TCRmax
fast moving UE
time
TCrmaxHyst
measurement rule for fast moving UEs
measures neighbouring cells with
equal or lower HCS prioritypriorizes cells with lower
HCS priority for re-selection
Example: NSNNCr: 1..16; def.: 8; SIB3TCrmax: not used, 30, 60, 120, 180, 240 s;
def.: 60s; SIB3
TCrmaxhyst: not used; 10..70; 10; SIB3
measurement rules for
slow moving UEsmeasurement rules for
slow moving UEs
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• WCEL: NCr
•This parameter defines the maximum number of cell reselections.
•If the number of cell reselections during the TCrmax time period exceeds the value of NCr,
high mobility has been detected.•Not used if TCrmax equals 'not used’.
•This parameter is part of SIB 3.
•[1 ... 16], default: 8.
• WCEL: TCrmax
•The duration for evaluating the allowed amount of cell reselections.
•If the number of cell reselections during the time period defined by Tcrmax exceeds Ncr,
high mobility has been detected.
•This parameter is part of SIB 3.
•[0 (= not used), 30, 60, 120, 180, 240] seconds, default: 60 seconds.
• WCEL: TCrmaxHyst
•Cell reselection hysteresis for reverting from UE high-mobility measurements.•The additional time period before UE reverts to low-mobility measurements.
•This parameter is part of SIB 3.
•[0 (= not used), 10, 20, 30, 40, 50, 60, 70] seconds, default: 0 seconds.
NSN Parameters for Cell Reselection (Part III)
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• HCS is in use. What cells are potential candidates for cell re-selection?
• Here, we use the quality level threshold criterion H.
• It is applied for all neighbouring cells, i.e. intra-frequency cells, inter-frequency cells and inter-RAT
cells. H values are defined for the serving and neighbouring cells:•Hs = Qmeas,s – Qhcss (for the serving cell)
•Hn = Qmeas,n - Qhcsn – TOn * Ln (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 averaged P-CCPCH RSCP.
•For GSM cells, the UE determines it from the averaged received signal level.
• As you can see, 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, if its H-value is equal or larger then zero.
• The value 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.TOn = TEMP_OFFSETn * W(PENALTY_TIMEn – Tn)
0 if HCS_PRIOn = HCS_PRIOs
1 if HCS_PRIOn <> HCS_PRIOs
0 for x < 0
1 for x >= 0
Ln =
W(x) =
(continued)
Cell Reselection: H-Criterion (slow moving UE)W i t h H C S
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• TEMP_OFFSETn introduces an add ional 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 TOn is set to zero.
(continued)
Cell Reselection: H-Criterion (slow moving UE)W i t h H C S
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• According to the specifications, following parameter values are possible:
•HCS_PRIO gives a cells priority.
•Its integer value ranges between 0 and 7, with 7 the highest priority.
•0 is the default value.•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 highest HCS_PRIO, which fulfil the quality level threshold
criterion
H >= 0.•Only these cells are ranked by the UE.
•If no cell fulfills the quality level threshold 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.
Cell Reselection: H-Criterion (slow moving UE)W i t h H C S
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Cell Reselection: H-Criterion (slow moving UE)W i t h H C S
HCS is in use:Candidates
for cell re-selection?
H-Criterion !S-Criterion
fulfilled
only cellswith
highest HCSpriorities &
H > 0
measurementthresholds
cells withlower HCS
priority thanin serving cell
H-Criterion
for fastmoving UEs
for slowmoving UEs
R-Criterion
cell ranking R-Criterion
cell ranking
Hs = Qmeas,s – Qhcss
Hn = Qmeas,n - Qhcsn – TOn * Ln
accept allNeighbours*
cells with
H>0
R-Criterion
cell ranking
no cells withH>0
Qhcs#: -24..0; def.: -24 dB
* id. to HCS not in use
TOn: temporary Offset during Penalty Time;3, 6, 9, 12, 15, 18, 21, 100 dB; Infinity; def: 3
# AdjgHCSthreshold:-110..-37; def.: -110 dB
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Cell Reselection: H-Criterion (slow moving UE) and TOn
Qmeas,n - Qhcsn
0time
TOnTOn
PENALTY_TIMEn PENALTY_TIMEn
TOn = 0 TOn = 0
Start
Tn
Start
Tn
Stop
Tn
Hs = Qmeas,s – Qhcss
Hn = Qmeas,n - Qhcsn – TOn * Ln
Example:HCS_PRIOn <> HCS_PRIOs
=> Ln = 1
TOn = TEMP_OFFSETn * W(PENALTY_TIMEn – Tn)
temp. offset used only
when neighbour has
different priority thanserving cell
TOn: temporary Offset during Penalty Time; Adji/g/sTempOffset1
3, 6, 9, 12, 15, 18, 21, 100 dB; Infinity
Ln = 0, if HCS_Prion = HCS_Prios
= 1, if HCS_Prion ≠ HCS_Prios
Penalty_Timen : 0..60; def.: 0s
H-Criterion:H>0
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• WCEL: HCS_PRIO
•This parameter defines the HCS priority level for a cell.
•Value 0 indicates the lowest HCS priority and value 7 indicates the highest HCS priority.
•[0 ... 7], default: 0.
• WCEL: QHCS
•Quality threshold level for applying prioritised hierarchical cell re-selection for a cell.
•Part of HCS Serving cell information (Qhcss).
•This parameter is part of SIB 3.
•[-24 ... 0] dB, step 0.5 dB, default: -24 dB.
• HOPS: AdjsHCSthreshold
•Defines the threshold level which must be exceeded by the CPICH Ec/No of the intra-
frequency neighbouring cell before the hierarchical cell re-selection becomes possible
(Qhcsn).
•[-24 ... 0] dB, step 0.5 dB, default: -24 dB.
• HOPI: AdjiHCSthreshold:
•[-24 ... 0] dB, step 0.5 dB, default: -24 dB.
• HOPG: AdjgHCSthreshold:
•[-110 ... -37] dBm, step 1 dBm, default: -110 dBm.
NSN Parameters for Cell Reselection (Part IV)
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• HOPS: AdjsTempOffset1
•This parameter is used in the cell ranking between cells when the intra-frequency neighbour
cell has the same HCS priority level as the serving cell.
•[3, 6, 9, 12, 15, 18, 21, 100 (Infinity)] dB, default: 3 dB.•HOPI: AdjiTempOffset1,
•HOPG: AdjgTempOffset1
• HOPS: AdjsTempOffset2
•This parameter is used in the cell ranking between WCDMA cells when the intra-frequency
neighbour cell has the same HCS priority level as the serving cell.
•[2, 3, 4, 6, 8, 10, 12, 100 (Infinity)] dB, default: 2 dB.
•HOPI: AdjiTempOffset2
• HOPS: AdjsPenaltyTime
•Defines the time period during which the Temporary Offset 1/2 is applied in the cell re-
selection procedure for the intra-frequency neighbour cell in question.
•[0 ... 60] seconds, step 10 seconds, default: 0.•HOPI: AdjiPenaltyTime,
•HOPG: AdjgPenaltyTime
NSN Parameters for Cell Reselection (Part IV)
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• After checking the measurement thresholds, 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 specified:
•Rs = Qmeas,s + Qhysts (for the serving cell)
•Rn = Qmeas,n - Qoffsets,n - TOn * (1 – Ls) (for candidate neighbouring cells for cell 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 re-
selection. It exists in two versions:
•If the quality measure is based on CPICH Ec/No, Qhyst2s is used in (FDD cell only); otherwise
the hysteresis value Qhyst1s is used (if the quality measure for FDD cells is based on CPICH
RSCP, for TDD cells and GSM cells).
•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.
Cell Reselection: R-CriterionW i t h H C S
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Qmeas,n – Qof fsets,n
PENALTY_TIMEn PENALTY_TIMEn
TOn = 0 TOn=0
Start
Tn
Start
Tn
Stop
Tn
Rs = Qmeas,s + Qhysts
Rn = Qmeas,n - Qoffsets,n - TOn * (1 – Ls)
Example:HCS_PRIOn = HCS_PRIOs
=> Ln = 0TOn = TEMP_OFFSETn * W(PENALTY_TIMEn – Tn)
Qmeas,s
temp. offset used onlywhen neighbour has
same priority than
serving cell
TOn
TOn
Cell Reselection: R-Criterion and TOnW i t h H C S
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Cell Reselection: R-CriterionW i t h H C S
Rs = Qmeas,s + Qhysts
Rn = Qmeas,n - Qoffsets,n - TOn * (1 – Ls)
Qmeas,n
Qmeas,s
Q m e a s
Rs
Rn
Qoffsets,n
Qhysts
Example: HCS_PRIOn <> HCS_PRIOs => Ln = 1
Treselection: 0..31; default: 2s
Adjg,s,iQoffset
-50.. 50; def.: 0 dB
Qhyst1
0.. 40; def.: 0 dB
Qhyst2
0.. 40; def.: 2 dB Adji&sQoffset2
-50.. 50; def.: 0 dBFDD cells & Ec/No:
2nd Ranking
WCDMA &GSM; RxLev:
1st Ranking
Rn > Rs => “cell reselection“
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• WCEL: Qhyst1 (Qhyst2)
• Qhyst1 is used for TDD and GSM cells, and for FDD cells when cell selection and re-
selection quality measure is set to CPICH RSCP (CPICH Ec/No) (Qhyst1s/ Qhyst2s).
• This parameter is part of SIB 3.• [0 ... 40] dB, step 2 dB, default: Qhyst1 = 0 dB, Qhyst2 = 2 dB.
• HOPS: AdjsQoffset1 (AdjsQoffset2)
• This parameter is used in the cell re-selection and ranking between WCDMA cells.
• The value of this parameter is subtracted from the measured CPICH RSCP (CPICH Ec/No)
of the intra-frequency neighbour cell before the UE compares the quality measure with the
cell re-selection/ranking criteria.
• [-50 ... 50] dB, default: 0 dB.
• HOPS: AdjsQoffset1 (AdjsQoffset2), HOPG: AdjgQoffset1
• WCELL: Treselection
• The UE triggers the reselection of a new cell if the cell reselection criteria are fulfilled during
the time interval Treselection.• This parameter is part of SIB 3.
• [0 ... 31] seconds, default: 2 seconds.
• The remaining parameters were already specified under NSN Parameters for Cell Reselection
(Part IV).
NSN Parameters for Cell Reselection (Part V)
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• How often are measurements conducted for cell re-selection evaluation?
• What happens, if the criterion S is no longer fulfilled for the cell, the UE camps on?
• Serving Cell• At least once every DRX cycle, the UE must
•measure the CPICH Ec/Io and CPICH RSCP level of the serving cell and
•evaluate the S criterion.
•Hereby, the UE filters measurement results.
•It takes at least two measurements, and the time difference between the measurements must
have been at least TmeasureFDD/2.
•If the serving cell is not fulfilling the cell selection criterion S for Nserv consecutive DRX cycles, the
UE has to start measurements on all neighbouring cells.
•The Nserv is known by the UE with the DRX cycle length.
•The UE has to find a suitable cell within 12 seconds.
•If it fails, and the UE is in the RRC idle mode, it initiates the cell selection procedures for
the selected PLMN.
•If it fails to find a suitable cell within 12 seconds, and the UE is in the RRC connectedstates CELL_FACH, CELL_PCH or URA_PCH, it is considered to be “out of service area“.
Cell Reselection: Measurement Requirements
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DRX
cycle
Nserv
S
S
S
Sstart measurement on
all neighbouring cells
12 seconds
if still no suitable cells
UE initiates cell selection
procedures for the
selected PLMN
UE “out of service area” At least one• CPICH Ec/Io
measurement
• CPICH RSCP
measurement
• S criterion evaluation
DRX cycle
if UE RRC idle
if UE in
CELL_FACH, CELL_PCH
or URA_PCH
Cell Reselection: Measurement Requirements – Serving Cell
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• Intra-frequency cells
• At least every TmeasureFDD the UE must measure CPICH Ec/Io and CPICH RSCP for intra-
frequency cells, when measurements are necessary according the the measurement rules.•Hereby, the UE filters measurement results.
•It takes at least two measurements, and the time difference between the measurements must
have been at least TmeasureFDD/2.
•When is the latest possible moment, that the UE has to detect, that a neighbouring intra-
frequency band has become higher ranked than the serving cell?
•We distinguish two cases, depending on the Treselection timer.
•If the Treselection timer is set to zero, and when the neighbouring cell is at least 3 dB
better than the serving cell, than the UE must be capable to rank the neighbouring cell
higher within TevaluateFDD.
•If Treselection is not zero, and the UE must detect, that the neighbouring cell is higher
ranked than the serving cell within TmeasureFDD.
Cell Reselection: Measurement Requirements
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At least one• CPICH Ec/Io measurement, or
CPICH RSCP measurement
TmeasureFDD cycle
3 dB
Serving cellIntra-frequency cell
time
UE is capable of evaluating that an intra-frequency
cell has become better ranked (if Treselection = 0)
within TevaluateFDD
Cell Reselection: Measurement – Intra-frequency Cells
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• Inter-frequency FDD cells
•The number of CPICH Ec/Io and CPICH RSCP measurements depends on the amount Ncarrier of
carriers, where inter-frequency FDD cells can be found according the the neighbourhood list.
•The UE has to carry out these measurements at least every (Ncarrier
-1) * TmeasureFDD
.
•Hereby, the UE filters measurement results.
•It takes at least two measurements, and the time difference between the measurements must
have been at least TmeasureFDD/2.
•The filtering must be done in such a way, that the UE detects a potentially higher ranked cell after
a specified period of time.
•There is now a difference, which type of measurement quantity is used.
•If CPICH Ec/Io is used for cell re-selection evaluation, and the timer Treselection is set to
zero, and when the neighbouring cell is 3 dB better than the serving cell, than a UE must be
capable to rank the neighbouring cell higher than the serving cell within the time period
(Ncarrier -1) * TevaluateFDD.
•If CPICH RSCP is used for cell re-selection evaluation, and the timer Treselection is set to
zero, and when the neighbouring cell is 5 dB better than the serving cell, than a UE must be
capable to rank the neighbouring cell higher than the serving cell within the time period(Ncarrier -1) * TevaluateFDD.
•If the timer Treselection is not zero and a neighbouring inter-frequency cell is ranked better
than the serving cell, the UE evaluates this cell for the time Treselection, whenever this cell
remains higher ranked. If so, it is re-selected by the UE.
Cell Reselection: Measurement Requirements
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Cell Selection and Reselection
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At least one• CPICH Ec/Io measurement or
CPICH RSCP measurement
(Ncarrier -1) * TmeasureFDD cycle
3 dB for CPICH Ec/Io
5 dB for CPICH RSCP
Serving cellInter-frequency cell
time
UE is capable of evaluating that an inter-frequency
cell has become better ranked (if Treselection = 0)
within (Ncarrier -1) * TmeasureFDD
Cell Reselection: Measurement– Inter-frequency Cells
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• Inter-RAT GSM cells
• If inter-GSM measurements are required according the the measurement rules, the UE measures the
RSSI of GSM BCCH carriers.• These measurements are conducted at least every TmeasureGSM.
• A running average of 4 measurements for each GSC BCCH carrier is calculated by the UE.
• In addition to that, the UE must determine the BSIC – which is carried on the SCH of each GSM
BCCH carrier – at least every 30 seconds for the four strongest GSM neighbouring cells.
• Also ranking in accordance to the cell re-selection rules must be done for these cells every 30
seconds.
• But what happens, if the UE detects a BSIC, which was not named in the neighbourhood list of the
serving cell?
• Then, this cell is not considered as candidate cell for cell re-selection.
• And what does the UE with a neighbouring cell, where it is not capable to gain the BSIC value?
•This cell, too, is not regarded as candidate for cell re-selection.
Cell Reselection: Measurement Requirements
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DRX cycle
length [s]
Nserv[number of DRX cycles]
TmeasureFDD [s](number of DRX cycles)
TevaluateFDD [s](number of DRX cycles)
TmeasureGSM[s](number of DRX cycles)
0.08 4 0.64 (8 DRX cycles) 2.56 (32 DRX cycl) 2.56 (32 DRX cycl)
0.16 4 0.64 (4) 2.56 (16) 2.56 (16)
0.32 4 1.28 (4) 5.12 (16) 5.12 (16)
0.64 4 1.28 (2) 5.12 (8) 5.12 (8)
1.28 2 1.28 (1) 6.4 (5) 6.4 (5)
2.56 2 2.56 (1) 7.68 (3) 7.68 (3)
5.12 1 5.12 (1) 10.24 (2) 10.24 (2)
In idle mode, UE supports DRX cycles lengths 0.64, 1.28, 2.56 and 5.12 s
Each GSM neighbour • GSM BCCH carrier RSSI
TmeasureGSM
BSIC at least every 30 seconds for each of
the 4 strongest GSM BCCH carriers
30 seconds
+
(DRX cycle length table source: TS 25.133 V3.11.0 chap. 4.2.2.1)
Cell Reselection: Measurement Requirements + GSM Cells
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• If the UE performs cell re-selection, it won’t be capable to monitor the paging channels for a short period of
time. Thus, it may miss a paging occasion.
•Maximum interruption times were specified, until the UE is capable to monitor the paging channel of the
newly selected cell.
• During the intra-frequency cell re-selection, the interruption time does not exceed 50 ms.
•This can be achieved, because the serving cell and the neighbouring cell are located on the same
carrier frequency.
•The UE monitors the paging channels of the “old” serving cell, until it is capable to receive to downlink
channels of the “new” serving cell.
• Most UMTS phones will only have one receiver.
• For an inter-frequency cell re-selection, the UE synchronises to the new cell, but monitors still the paging
channel of the “old” cell.
• After synchronisation, it must read the system information, transmitted on the BCCH, to gain information
such as the physical channel information of S-CCPCH and PICH, where it is paged.
•TSI is the time, the mobile phone requires to read all relevant system information data.
•The interruption time must not exceed TSI + 50 ms.
• The situation for inter-RAT cell re-selection is pretty much the same as for inter-frequency cell re-selection.•The interruption time must not exceed TBCCH + 50 ms, whereby TBCCH gives the maximum time for the
UE to read the BCCH information.
• These requirements are only valid, where there are sufficient radio conditions and under the condition, that
no errors occurred while decoding the BCCH information.
Cell Reselection: Maximum Interruption in Paging
Reception
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50 ms
50 ms
50 ms
TSI
TBCCH
intra-frequency
inter-frequency
inter-RAT
Cell Reselection: Maximum Interruption in Paging
Reception
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Cell Selection and Reselection
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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
References