Absolute priority based cell reselection The cell ranking algorithm (reselection based on thresholds) does n ot allow the network operator to control easily cell reselection between different RATs. RATs. Some parameters can be used to favour reselection to a given RAT, but it gets all the more comple as more RATs are involved and if thresholds!offsets thresholds!offsets vary v ary dynamically. Therefore Release " introduced another degree deg ree of control on the RAT (or fre#uency) to be reselected. This mechanism is called absolute priority based cell reselection. A priority is assigned per $%&'A fre#uency, T T AR*%+ or group of S' AR*%+, which are referred as -layers. The principle of priority based reselection is that the / should always try to camp on the highest priority layer. 0t will measure (and possibly reselect to) lower layer only in case the serving cell #uality becomes too low. This feature is mandatory for multi1RAT /s supporting T. Although the principle of priority based cell reselection is #uite simple, the description is split into several 233 specifications. The purpose of this document is to give an overall description, point to the relevant parts of the 233 specification, specification, describe the basic scenarios and give some hints about abnormal cases in order to crea te good test coverage. Then it illustrates how test scenarios can easily be written with RT&. Although the priority based reselection principle is the same for all RATs, RATs, the parameters are a bit different. Therefore Therefore they are going to be described separately in the following sections.
Absolute priority based cell reselection...................................................... .............................................................. ................ ........... ... 4 References5.................................................... ......................................................................................................... ............................................................. ............ .... 2 3riority characteristics chara cteristics.................................................. .............................................. .... 6 /se case 45 the / is camped on S' .................................................. ........................ 7 3arameters involved in the priority based algorithm....................... ............................... ................ .............. ...... 7 'easurements on o n neighbour RAT RATss.............................................. ........ ................ ................ .......... .. 7 %ell reselection criterion.................................................... ........................................................................ ............................ ................ ........ 8 9lacklisting of /TRA+ cells ................................................ ............................... : 9arred cells.................................................. ........................................................ ........................................................... ... " %ells not suitable..................................................... ........................................................................................ ................................... ........ ............ .... " Abnormal case ...................................................... .................................................................................................... .............................................. .... " *ragmentation of S0#uater for multi RAT /.................................................... ........................................................ .... " /se case ;5 the / is camped on /TRA+ ....................................... ........ ................. ................. .......... .. 4< 3arameters involved in the priority based algorithm..................... ............................. ................. .............. ..... 4< 'easurements 'easure ments on inter1fre#uency, inter1fre#uen cy, inter1RAT cells ..... ........... ............ ........... ........... ........... .......... ......... ........ .... 4< %ell reselection criterion.................................................... ...................................................................... .......................... ................ ........ 4< 'obility states................................................ ........................................ ......... .............. ..... 44 9lacklisting of /TRA+ cells................................................ ........................... ............................. ..44 Abnormal cases................................................ ................................. ........ ................ ........... ... 44 %ells barred, cell access restriction and not suitable cells .................................. .. 44
/se case 25 the / is camped on /TRA+ ............................................... ................. 4; 3arameters involved in the priority based algorithm........................................... 4; 'easurements on intra1fre#uency cells.................................... ........................... 4; 'easurements on inter1fre#uency and inter1RAT cells..................................... ... 4; %ell reselection criterion...................................................................................... 4; 'obility states................................................ ............................................ ..........42
References:
Reference to 233 Release " specification TS 67.<<" &igital cellular telecommunications system (3hase ;=)> Radio subsystem link control (233 TS 67.<<" version ".44.< Release ") TS 66.<4" &igital cellular telecommunications system (3hase ;=)> 'obile radio interface layer 2 specification> Radio Resource %ontrol (RR%) protocol (233 TS 66.<4" version ".4;.< Release ") TS 66.<8< &igital cellular telecommunications system (3hase ;=)> eneral 3acket Radio Service (3RS)> 'obile Station ('S) 1 9ase Station System (9SS) interface> Radio ink %ontrol ! 'edium Access %ontrol (R%!'A%) protocol (233 TS 66.<8< version ".4;.4 Release ") TS ;7.422 /niversal 'obile Telecommunications System (/'TS)> Re#uirements for support of radio resource management (*&&) (233 TS ;7.422 version ".46.< Release ") TS ;7.2<6 /niversal 'obile Telecommunications System (/'TS)> /ser #uipment (/) procedures in idle mode and procedures for cell reselection in connected mode (233 TS ;7.2<6 version ".4;.< Release ") TS ;7.224 /niversal 'obile Telecommunications System (/'TS)> Radio Resource %ontrol (RR%)> 3rotocol specification (233 TS ;7.224 version ".47.< Release ") TS 28.422 T> volved /niversal Terrestrial Radio Access (1/TRA)> Re#uirements for support of radio resource management (233 TS 28.422 version ".46.< Release ") TS 28.2<< T> volved /niversal Terrestrial Radio Access (1/TRA) and volved /niversal Terrestrial Radio Access +etwork (1/TRA+)> ?verall d escription> Stage ; (233 TS 28.2<< version ".@.< Release ") TS 28.2<6 T> volved /niversal Terrestrial Radio Access (1/TRA)> /ser #uipment (/) procedures in idle mode (233 TS 28.2<6 version ".4<.< Release ") TS 28.224 T> volved /niversal Terrestrial Radio Access (1/TRA)> Radio Resource %ontrol (RR%)> 3rotocol specification (233 TS 28.224 version ".46.< Release ")
Priority characteristics
A priority is an integer that ranges between < and :, : being the highest priority. Two different fre#uencies from the same RAT can have the same priority, but two different RATs can never have the same priority. This is in order to avoid ping pong between different RATs. This priority is broadcasted in system information. The layer priority should be consistent5 it should remain the same when broadcasted on different RAT (i.e. if S' has prio 6 in S' system information, it should have priority 6 as well in $%&'A system information). 3riorities can be common or individual. %ommon priorities define the general behaviour for inter1RAT reselection within one cell and apply to /s that do not have individual priorities. They are broadcasted in the system information. 0ndividual priorities override common priorities. They are provided through dedicated signalling (channel release in S', RR%%onnectionRelease in /TRA+ and /TRA+) to a / and are user specific. They could be used for eample to prioritie a layer to provide high data rate for premium users. The Subscriber 3rofile 0& for RAT!*re#uency (S30&) is an inde referring to user information (e.g mobility profile, service usage profile). The information is / specific and applies to all its Radio 9earers. 0t can be used by the network to assign individual priority. See TS 28.2<< for reference S30& values. 0ndividual priorities are only valid until a change of 3'+ is performed, or the / is switched off, or new individual priority is received that override previously assigned ones, o r the validity timer epires (i.e., T2;< in /TRA, T2;; in /TRA and T2;2< in RA+). These timers are optionally provided to invalidate the priority after some time (see TS 66.<4" section 2.;.2.2).
Use case 1: the UE is camped on GSM
0f the / has received S0;#uater containing 1/TRA+ priority information, it shall use priority based cell reselection. ?therwise cell ranking based reselection applies. Parameters involved in the priority based algorithm
The parameters below taken from S0;#uater (see TS 66.<4" Table 4<.7.;.22b.45 S0;#uater Rest ?ctets information element). Serving ell Priority Parameters RA+B3R0?R0TC5 RA+ priority (note that this priority applies to all RA+ cells, unlike /TRA+ and /TRA+ where different fre#uencies can hav e different priorities) TDRSDB3riorityBSearch5Rlev Threshold under which the / shall start monitoring lower priority layers TDRSDBS'Blow5 Threshold where S' should start considering reselection to lower priority layers DB3R0?5 Dysteresis used for reselection to lower priority layers TBReselection5 'inimum duration during which the target cell should be -better than serving in order to perform cell reselection U!RA" Priority Parameters #per U!RA" fre$uency inde%& /TRA+B*RE/+%CB0+&F5 0nde of the fre#uency layer in the 2 fre#uency list /TRA+B3R0?R0TC5 3riority of /TRA+ fre#uency TDRSDB/TRA+Bhigh5 Threshold used when this /TRA+ layer priority is higher than S' serving cell priority TDRSDB/TRA+Blow5 Threshold used when this /TRA+ layer priority is lower than S' serving cell priority /TRA+BERFG'0+5 'inimum RS%3 re#uired on /TRA+. /sed to calculate the Sr parameter in the reselection criterion +ote that it is possible for the network to specify &*A/TB/TRA+B3R0?R0TC, &*A/TBTDRSDB/TRA+ and &*A/TB/TRA+BERFG'0+ that apply to all fre#uencies of the 2 fre#uency list that were not assigned specific parameters. E'U!RA" "eighbour ells parameters #per EAR("& 'easurement 9andwidth5 Specifies the minimum value of the channel bandwidth of all valid 1/TRA+ cells on the specified AR*%+. 0t is defined b y the parameter Transmission 9andwidth %onfiguration, +R9 (see 233 TS 28.4<6). The values indicate the number of resource blocks over which the mobile station could measure. 1/TRA+B3R0?R0TC5 3riority of /TRA+ fre#uency TDRSDB1/TRA+Bhigh5 Threshold used when this /TRA+ layer priority is higher than S' serving cell priority TDRSDB1/TRA+Blow5 Threshold used when this /TRA+ layer priority is lower than S' serving cell
1/TRA+BERFG'0+5 'inimum RSR3 re#uired on /TRA+. /sed to calculate the Sr parameter in the reselection criterion
Measurements on neighbour RA!s
The / measures periodically the signal level on layers that have higher priority than the serving S' cell. The fre#uency of measurements is defined on other RAT is not defined eplicitly but the re#uirements on the reselection time to other RAT are defined accurately in the TS 67.<<" section 8.8.6. The / starts measuring on lower priority layers in case the serving cell Rlev on the serving cell falls below TDRSDB3riorityBSearch.
ell reselection criterion
/ calculates the following values (TS 67.<<" section 8.8.8)5 S/TRA+ H Erlevmeas/TRA+ 1 /TRA+BERFG'0+ S/TRA+ H Erlevmeas/TRA+ 1 1/TRA+BERFG'0+ SS' H %4 The / shall reselect to a higher priority layer in case5 /TRA+ target5 S/TRA+ I TDRSDB/TRA+Bhigh for more than TBReselection • /TRA+ target5 S/TRA+ I TDRSDB1/TRA+Bhigh for more than TBReselection • 0f more than one cell meets the cell reselection criteria above, the / shall reselect the cell with the highest Srlev level among the cells meeting the criteria on the highest absolute priority layer. 0f SS'J TDRSDBS'Blow for more than TBReselection for the serving cell and all S' neighbours, the / shall reselect to /TRA+ target5 S/TRA+ I TDRSDB/TRA+Blow for more than TBReselection • /TRA+ target5 S/TRA+ I TDRSDB1/TRA+Blow for more than TBReselection • 0f no target cell is fulfils the conditions above, then the cells satisfying the condition • S/TRA+!/TRA+ I SS' = D3R0? should be considered in decreasing order of S/TRA+!/TRA+. A /TRA+ *&& cell shall only be reselected if, in addition to the criteria above, its measured c!+o value is e#ual to or greater than *&&BEmin 1 *&&BEminB?ffset (as broadcasted in S0;ter and S0;#uater). A 1/TRA+ cell shall only be considered for reselection if not included in the list of not allowed cells. 0f a /TRA+ or a 1/TRA+ cell has not been assigned a priority, it shall not be considered for reselection. (Since the priority is optional in S0;#uater, it could be that the neighbour cell is ont assigned any priority.) The cell reselection to higher layer and cell reselection to lower layer are illustrated on figures *igure 4 and *igure ;. +ote that there is another scenario where no lower priority cell fulfils
S/TRA+ I TDRSDB1/TRA+Blow for more than TBReselection, an d in that case the / shall reselect a lower priority cell fulfilling S/TRA+ I SS' = D3R0?.
Measured quantity (SEUTRAN)
THRESH_E-UTRAN_high
Trigger reselection to EUTRAN cell
T_reselection
Time
(igure 1: reselection to a higher priority RA! #)ere the serving cell is GSM and the higher priority cell is a EU!RA" cell&
Measured R$le% & THRESH_'riority_Search 2) The low priority (EUTRAN) cell has been above quantity Starts on lower priority layers (EUTRAN) (SEUTRAN or"eas!ring !TE
THRESH_E-UTRAN_low
S"SM or "SM)
THRESH_"SM_lo# THRESH_E-UTRAN_lo#
T_reselection
#) Serving cell an all $S% neighbo!r below Thresh$S%&low
Time
(igure *: reselection to a lo+er priority RA! #)ere the serving cell is GSM and the lo+er priority RA! is EU!RA"&
onditions 1& and *& are fulfilled ,- !rigger a cell reselection to the lo+ priority #EU!RA"& cell.
/lac0listing of EU!RA" cells
The mobile station shall not perform measurements or attempt reselection to the /TRA+ -+ot Allowed %ells sent in the 'easurement information or in the S0;#uater. (see 66.<4" section @.4.76) There are 2 different way to describe a blacklist of /TRA+ cells (see TS 66.<8< section 4;.7:)5 - eplicit list of @ bit long physical cell 0&s - 3%0& bitmap group, a 8 bit long bitmap where each bit stand for a range of "6 cell 0&s (to cover all the 7<2 possible physical cell 0&s). - 3%0& pattern5 a pattern of 4 to @ bits which is applied to the most significant bit of the physical cell 0& of the /TRA+ neighbour cells. &epending on the pattern sense, a match between the pattern and the physical cell 0& will include or eclude the cell.
/arred cells
See TS 67.<<" section 8.8.8. 0n case a target /TRA+ cell is barred, the / shall not attempt any reselection during T barred (value defined on the barred /TRA+ cell). 0n case a target /TRA+ cell is barred, the / shall not attempt any reselection during up to ;< minutes. ells not suitable
See TS 67.<<" section 8.8.8. 0n case the / attempts reselection to a /TRA+ cell which is not suitable (either belonging to the Klist of forbidden As for roamingK or belonging to a 3'+ which is not indicated as being e#uivalent to the registered 3'+), the / may abandon further reselection attempts towards this /TRA+ cell and all other cells on the same fre#uency, for a period of up to ;< min. 0n case the / attempts reselection to a 1/TRA+ c ell which is not suitable due to being part of the Klist of forbidden tracking areas for roamingK (see 233 TS ;6.2<4), and if the 'S has received the 3%0& to TA 'apping information element (see 233 TS 66.<4" and 233 TS 66.<8<) for the fre#uency of the cell, it shall abandon further reselection attempts towards this 1 /TRA+ cell and any 1/TRA+ cell which is known to belong to the same Tracking Area Abnormal case
See TS 67.<<" section 8.8.8. The serving cell has no allocated priority. Then it shall be assigned a -negative priority so that a reselection is triggered as early as possible.
(ragmentation of S$uater for multi RA! UE
The S0;#uater may be #uite long since it contains /TRA+ and /TRA+ neighbour cell information. The maimum sie of a 9%%D message is ;; octets, therefore the S0;#uater is split into several instances. The networks broadcast one instance per multiframe and the ac#uisition by the / can therefore take some time. *or a / that does not support /TRA+ or /TRA+, the START and ST?3 bit may be used to speed up System information reading and several seconds can be saved at cell reselection by not reading the instances containing information that is non relevant for the /. /TRA+ information is not mied with /TRA+ information. They are sent in different S0;#uater instances. 0f instances < to 4 contain /TRA+ information, and instances ; to 6 contain /TRA+ information, then the / that does not support /TRA+ may skip the reading of instances ; to 6.
on*gure mea "SM neigh,our Higher riority l !o#er riority l
no
Measurements do (at least e%ery /s 1 ser%ing cell and all "SM cells Rle% is ,elo# THRES_"SM_lo# durin g T_resele A lo#er layer ul*ls the reselection criterion and ath loss criteria yes reselection3 "o through the higher A higher layer ul*ls t
reselection3+ reselection ro%ided that the target cell is suita,le or normal caming no
"SM neigh
(igure 2: Priority based cell reselection in GSM
Use case *: the UE is camped on U!RA"
0n the S' case, priority based reselection only involves reselection to other RATs, but in the /TRA+ case, it may involve reselection to other /TRA+ fre#uencies. 0t is a bit more comple than S' since both the priority based and cell ranking based reselection can be used at the same time5 priority based reselection can apply to inter1fre#uency layers and cell ranking based reselection to inter1RAT layers and vice1versa. The priority based cell reselection applies for inter1fre#uency layers as soon as S094@ as b een received and assigned at least one inter1fre#uency neighbour cell priority and threshold parameters. Similarly it applies for inter1RAT layers as soon S094@ as been received and assigned at least one inter1RAT neighbour cell priority and threshold parameters. ?therwise if no priority was provided the cell ranking algorithm shall app ly. 0f the priority based reselection applies and one cell has no priority or threshold assigned, it shall not be considered for cell reselection. (see TS ;7.2<6 , 7.;.8.4.6a)
Parameters involved in the priority based algorithm
S#ual and Srlev are defined as5 S#ual H E#ualmeas L E#ualmin Srlev H Erlevmeas L Erlevmin Serving ell Priority Parameters Sprioritysearch45 threshold on the serving cell Srlev used to trigger measurement on lower priority layers Sprioritysearch;5 threshold on the serving cell S#ual used to trigger measurement on lower priority layers threshServingow5 threshold on the serving cell Srlev used for reselection to lower priority layers Treselection5 provided in S092!6. &epend on the state &%D or *A%D. GSM3U!RA"3E'U!RA" Priority Parameters #same& threshFhigh5 threshold on the higher priority target cell Srlev used to trigger reselection threshFlow5 threshold on the lower priority target cell Srlev used to trigger reselection #Rev'inS'5 minimum signal level on S' (used to calculate Srlev for S') #Rev'in*&&5 minimum signal level on /TRA (used to calculate Srlev for /TRA) #Rev'in/TRA5 minimum signal level on /TRA (used to calculate Srlev for /TRA)
Measurements on inter'fre$uency4 inter'RA! cells
The / measures periodically the signal level on layers that have higher priority than the serving /TRA+ cell. The ThigherBpriorityBsearch period is defined as (8< M +layers) seconds, where +layers is the total number of configured higher priority 1/TRA, /TRA *&& and /TRA T&& carrier fre#uencies and is additionally increased by on e if one or more groups of S' fre#uencies is configured as a higher priority.(see ;7.422 section 6.;.;)
Table 6.4 in the same document defines more specifically measurement and evaluation periods depending on the &RF cycle. 0f SrlevServingell JH Sprioritysearch4 or S#ualServingell JH Sprioritysearch;, the / shall perform measurements on lower priority layers. (see ;7.2<6 section 7.;.8.4.;a )
ell reselection criterion
/ calculates the following values for the non serving cells5 Srlev/TRA+ H Erlevmeas/TRA+ 1 Erlev'in/TRA+ Srlev/TRA+ H Erlevmeas/TRA+ 1 Erlev'in/TRA+ SrlevS' H Rlev 1 Erlevmin The ;7.2<6, 7.;.8.4.6a eplains the reselection criterion5 A reselection to a higher priority layer cell is triggered if its Srlev is greater than threshFhigh during a time interval Treselection. 0f SrlevServing%ell J threshServingow or S#ualServing%ell J <, the / shall first try to reselect to a interfre#uency cell with e#ual priority satisfying SrlevnonServing%ell,I threshFlow during a time interval Treselection. 0f not interfre#uency cell satisfies this, it shall reselect to a RAT with lower priority satisfying SrlevnonServing%ell, I threshFlow during a time interval Treselection.
+ote5 new thresholds Thresh,high; and Thresh,low;has been introduced in Release @ that rely only on S#ual for the reselection criteria.
Mobility states
0n /TRA+ are defined mobility states. &epending on the number of reselections that occurred previously, the Treselection value will be scaled to be larger in case of high mobility. This way the number of reselections is reduced. 0n all the above criteria the values of Treselections, Treselections,3%D or Treselections,*A%D apply for Treselection and are scaled according to the / mobility state and target RAT, as specified in TS ;7.2<6 section 7.;.8.4.4a. /lac0listing of EU!RA" cells
The K1/TRA fre#uency and priorityK 0 in S09 4@ can contain a K9lacklisted cells per fre# listK information element. %ells within this list shall not be reconsidered for reselection (see TS ;7.224). Abnormal cases
There is no priority associated with the serving cell. The / shall perform a cell reselection.
ells barred4 cell access restriction and not suitable cells
TS ;7.2<6 section 7.;.8.4.2 describes the / behaviour in case of access restrictions or cell unsuitable for normal camping. 0f the target cell is barred the / shall select another cell and not try to reselect to the barred cell during Tbarred (see section 7.2.4.4). 0f the highest ranked cell or best cell according to absolute priority reselection rules is not suitable due to being part of the Klist of forbidden As for roamingK or belonging to a 3'+ which is not indicated as being e#uivalent to the registered 3'+, the / shall not consider this cell and other cells on the same fre#uency, as candidates for reselection for a maimum of 2<
Use case 2: the UE is camped on EU!RA"
?n /TRA+, only the priority based cell reselection can apply. The cell ranking based reselection does not eist on /TRA+. Parameters involved in the priority based algorithm
S#ual and Srlev are defined as5 S#ual H E#ualmeas L E#ualmin Srlev H Erlevmeas L Erlevmin Serving ell Priority Parameters Sintrasearch5 Threshold below which the / shall perform intra fre#uency measurements Snonintrasearch5 Threshold below which the / shall perform inter fre#uency and inter RAT measurements Threshserving, low5 Threshold below which the / shall consider lower priority cells for cell reselection GSM3U!RA"3E'U!RA" Priority Parameters #same& E#ualmin5 'inimum #uality level on a target cell to be considered for cell reselection Erlevmin5 'inimum signal level on a target cell to be considered for cell reselection Thresh, high5 Threshold to be fulfilled if priority is higher than the serving cell Thresh, low5 Threshold to be fulfilled if priority is lower than the serving cell
TS 28.2<6 section 7.;.6.; describes when measurement are performed. Measurements on intra'fre$uency cells
1 0f Sintrasearch is sent in the serving cell and SServing%ell I Sintrasearch, the / may choose to not perform intra1fre#uency measurements. 1 0f SServing%ell JH Sintrasearch, or Sintrasearch is not sent in the serving cell / shall perform intra1fre#uency measurements
Measurements on inter'fre$uency and inter'RA! cells
The / shall perform measurements on 1/TRA+ inter1fre#uency or inter1RAT fre#uency with a higher reselection priority periodically according to TS 28.422 section 6.;.;.6 and 6.;.;.7. The / shall perform measurements on 1/TRA+ inter1fre#uency with an e#ual or lower priority and on inter1RAT fre#uency with lower priority5 1 0f Snonintrasearch is sent in the serving cell and SServing%ell I Snonintrasearch / may choose not to perform measurements of 1/TRA+ inter1fre#uencies or inter1RAT fre#uency cells of e#ual or lower priority. 1 0f SServing%ell JH Snonintrasearch or Snonintrasearch is not sent in the serving cell the / shall perform measurements of 1/TRA+ inter1fre#uencies or inter1RAT fre#uency cells of e#ual or lower priority according to TS 28.422 section 6.;.;7.
ell reselection criterion
TS 28.2<6 section 7.;.6.7 describes the cell reselection criterion5 %ell reselection to a cell on a higher priority 1/TRA+ fre#uency or inter1RAT fre#uency than serving fre#uency shall be performed if5 - SnonServing%ell, I Thresh, high during a time interval TreselectionRAT - more than 4 second has elapsed since the / camped on the current serving cell. 0f no reselection to higher priority cell or intra1fre#uency cell is possible, cell reselection to a cell on a lower priority 1/TRA+ fre#uency or inter1RAT fre#uency than serving fre#uency shall be performed if5 - SServing%ell J Threshserving, low and the SnonServing%ell, of a cell of a lower priority 1/TRA+ fre#uency or inter1RAT fre#uency is greater than Thresh, low during a time interval TreselectionRAT - more than 4 second has elapsed since the / camped on the current serving cell.
Mobility states
?n /TRA+ are defined mobility states. &epending on the number of reselections that occurred previously, the mobility state will be high, medium or normal and the Treselection value will be scaled to be larger in case of high mobility. This way the number of reselections is reduced. 0n all the above criteria the values of Treselections is scaled according to the / mobility state and target RAT, as specified in TS 28.2<6 section 7.;.6.2.
