Cell Selection and Reselection

Cell Selection and ReselectionI insert classification level
Cell Selection and Reselection
3GRPLS (RN3155) – Section 4
Soc Classification level
Objectives
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
* © Nokia Siemens Networks 3G RPLS / AA / 10/2008
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Cell Selection and Reselection: Motivation
Four different reasons for camping on a cell in the RRC idle mode were identified in the specifications:-
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.
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.
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 the paging resources of its paging class within the cell.
The UE can receive cell broadcast service messages.
In the ETSI specification TS 25.304, the idle mode tasks were subdivided into three processes:
PLMN selection and reselection;
CM requests
NAS control
if the UE is in the
RRC connected mode
Cell Selection and Reselection: Motivation
The mobile phone tries to find a suitable cell to camp 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 "limited service" state. In this state, only emergency 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 the RRC connected mode states CELL_FACH, CELL_PCH and URA_PCH.
The purpose of camping on a cell are the same as in the RRC idle mode with the exception of RRC connection establishment (because the RRC connection already exists).
The cell reselection procedure is the same in the RRC idle and RRC connected mode.
PLMN search
The UE shall scan all RF channels in the UTRA bands according to its capabilities 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 (but without the RSCP value), provided that the following high quality criterion is fulfilled:
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 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, a PLMN shall be understood to be received with high quality signal if the signal level is above -85 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.
PLMN Selection
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
A list of PLMN identifiers associated with an access technology is listed on the USIM in a priority order.
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 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.
In this case the UE must periodically search for the HPLMN. The “HPLMN Search Period” timer is used for that. Its values range between 6 and 480 minutes with a step size of 6 minutes. Its default 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 are conducted.
HPLMN Search Period Timer
Operator controlled PLMN list
User controlled PLMN list
Cell Reservations and Cell Restrictions
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 cell selection 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
BCCH SIB3/4: Cell Access Restriction
* e.g. Emergency Call (Access Class = 10) in Network w/o Roaming Agreement
# e.g. for operator use
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 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 Barred
Cell reserved
for operator
Cell reserved
suitable for AC 11 or 15 in HPLMN
otherwise “barred”
Cell “barred”

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 the mobile phone.
If the mobile phone is in the RRC idle state, it checks access class restrictions before it sends an RRC CONNECTION 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 mobile phones 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.
UE RRC idle
I ignore all Access Class related cell access restrictions when selecting a cell to camp on
UE RRC connected
(from RRC IDLE)
Access Class irrelevant
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.
[10, 20, 40, 80, 160, 320, 640, and 1280] seconds; default: 40 seconds.
WCEL: Cell_Reserved
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.
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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 on this 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.
Cell Selection Process
forbidden registration areas
States and State Transition in the RRC Idle Mode
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 UE enters 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)
(adopted from TS 25.304)
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.
(adopted from TS 25.304)
USIM inserted
Cell Selection Criterion S
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).
Qqualmeas (dB)
(CPICH Ec/N0)
Qrxlevmeas (dBm)
CPICH RSCP
Cell Selection Criterion S
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).

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 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:
Srxlev > 0 AND Squal > 0 in the FDD mode, and
Srxlev > 0 in the TDD mode.
or
Squal = Qqualmeas – Qqualmin > 0

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 – 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
Active Set cells as candidates for cell selections; if not suitable, then
Stored information cell selection
WCEL: QrxlevMin
[-119 ... –25] dBm, step 2 dBm; default: -115 dBm.
WCEL: QqualMin
[-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.
[..]
Cell Reselection in the RRC Idle and RRC Connected Mode
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
S-Criterion
fulfilled
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 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.
Hierarchical Cell Structures
Priority
Measurement criteria without HCS
Without HCS
Cell Reselection: Measurement Rules
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-system measurements. 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.
Without HCS
Intra-
frequency
Intra-frequency
Inter-frequency
Cell Reselection: R-Criterion
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
Without HCS
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.
Without HCS
Rs = Qmeas,s + Qhysts
Rn = Qmeas,n - Qoffsets,n
Qhyst2
AdjisQoffset2
FDD cells & Ec/No:
Qhyst1
WCDMA & GSM cells; RxLev:
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.
[0 ... 20] dB, step 2 dB, default: 10 dB.
WCEL: Sintersearch
The threshold for inter-frequency measurements, and for the HCS measurement rules.
WCEL: Ssearch_RAT
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Measurement criteria without HCS
HCS specific H-criteria defines whether priorities are applied in re-selection
With HCS
With HCS
SIB 11/12
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 and inter-frequency threshold based measurement rule :
“IF (Srxlevs <= SsearchHCS) OR (if FDD AND Sx <= Sintersearch)
THEN measure on all intra-frequency and inter-frequency cells
ELSE
IF (Sx > Sintrasearch)
THEN measure on all intra-frequency and inter-frequency cells, which have higher HCS priority level than the serving cell unless measurement rules for fast- moving UEs are triggered
ELSE measure on all intra-frequency and inter-frequency cells, 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)
If HCS is used and if Sintrasearch 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.
With HCS
if
-8 dB
-10 dB
-14 dB
Srxlevs ≤ SsearchHCS OR Squal ≤ Sintersearch
Squal =
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
ELSE
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 FDD mode. 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.
With HCS
Ssearch_RAT = 4 dB, Slimit, searchRAT = 2 dB
SsearchHCS = -1dB, SHCS_RAT = -1dB
Equal &
if Srxlevs ≤ SHCS,RATm
measure on all
WCEL: SHCS_RAT
[-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.
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.
[-1 ... 91] dB, step 2 dB, default: -1 dB (UE sets this value to 0).
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Measurement Rules with HCS and fast moving UEs
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-RAT 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 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 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.
With HCS
With HCS
TCRmax = time period
measures neighbouring cells with equal or lower HCS priority
priorizes cells with lower HCS priority for re-selection
Example: NSN
TCrmax: not used, 30, 60, 120, 180, 240 s;
def.: 60s; SIB3
measurement rules for
slow moving UEs
measurement rules for
slow moving UEs
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’.
[1 ... 16], default: 8.
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.
[0 (= not used), 30, 60, 120, 180, 240] seconds, default: 60 seconds.
WCEL: TCrmaxHyst
The additional time period before UE reverts to low-mobility measurements.
[0 (= not used), 10, 20, 30, 40, 50, 60, 70] seconds, default: 0 seconds.
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Cell Reselection: H-Criterion (slow moving UE)
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
Cell Reselection: H-Criterion (slow moving UE)
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)
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 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.
With HCS
With HCS
accept all
* 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
Qmeas,n - Qhcsn
Example:
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
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.
Part of HCS Serving cell information (Qhcss).
[-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:
HOPG: AdjgHCSthreshold:
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
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.
With HCS
Qmeas,n – Qoffsets,n
Example:
Qmeas,n
Qmeas,s
Qmeas
Rs
Rn
Qoffsets,n
Qhysts
Treselection: 0..31; default: 2s
Qhyst1
Qhyst2
Adji&sQoffset2
FDD cells & Ec/No:
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).
[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.
[0 ... 31] seconds, default: 2 seconds.
The remaining parameters were already specified under NSN Parameters for Cell Reselection (Part IV).
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 connected states CELL_FACH, CELL_PCH or URA_PCH, it is considered to be “out of service area“.
DRX
cycle
Nserv
S
UE initiates cell selection
At least one
CPICH Ec/Io measurement
CPICH RSCP measurement
S criterion evaluation
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.
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.
At least one
within TevaluateFDD
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.
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.
At least one
Serving cell
Inter-frequency cell
within (Ncarrier -1) * TmeasureFDD
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 + GSM Cells
In idle mode, UE supports DRX cycles lengths 0.64, 1.28, 2.56 and 5.12 s
Each GSM neighbour
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)
DRX cycle
length [s]
Cell Reselection: Maximum Interruption in Paging Reception
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.
50 ms
50 ms
50 ms
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
Camped on
Any Cell
Any Cell

Documents

Cell Selection and Reselection