WCDMA Cell Selection, Reselection Procedure and Criteria 2009/2/25 Jackson 當UE開機後，首先會透過USIM或RRC database的資訊作Storedy information cell search，若找不到合適cell則會做Initial Cell Search。UE先透過量測RF channels的訊號強度，並嘗試尋找訊號最強的cell，同時讀取其system information來得到 其PLMN identity。並將可用的PLMNs回報給NAS以進行PLMN-Selection。NAS可以透過automatic mode或manually mode來達成。接著RRC會從NAS提供的PLMN service找尋最合適的cell做cell camping。 • This procedure requires no prior knowledge of which RF channels are UTRA carriers. • The UE must scan all RF channels in the UTRA band to find a suitable cell. – On each carrier, the UE searches first for the strongest cell and reads its system information, in order to find out which PLMN the cell belongs to. – If the selected PLMN is found, the search of the rest of carriers may be stopped. – Once the UE has found a suitable cell for the selected PLMN, the UE selects it. Initial Cell Selection Idle mode Cell Selection & Reselection Procedure s no suitable cell Stored Information Cell Selection Initial Cell Selection Cell Reselection Evaluation Process Camped normally Cell Selection when leaving connected mode Connected mode Any Cell Selection & Reselection Process New PLMN is selected suitable cell suitable cell NAS registration rejected leave idle mode return to idle mode no suitable cell suitable cell trigger suitable cell no suitable cell no suitable cell cell info. is stored no cell info. Initial Cell Selection • During the cell search, the UE searches for a cell and determines the downlink scrambling code and frame synchronisation of that cell. The cell search is typically carried out in three steps: – Step 1: slot synchronization to a cell – Step 2: frame synchronization & code group identification – Step 3: scrambling code identification • Scrambling code of a cell needs to be identified during: – Initial cell selection, – Subsequent cell reselection, and – Handover • Hence, scrambling code assignment should be optimised in order to improve the performance of cell selection/reselection and handover – make sure large enough separation between two cells using same scrambling code – avoid conflicts of scrambling codes in the neighbour list including the case in soft handover where two or three neighbour lists are combined – avoid overlapping of SCHs of cells belonging to the same BTS Identify scrambling code from CPICH Acquire slot sync from PSCH …Established Synchronization Identify code group & frame sync from SSCH 3-stage Cell Search Cell Reselection Overview of Idle Mode Operation • When a UE is switched on, a public land mobile network (PLMN) is selected and the UE searches for a suitable cell of this PLMN to camp on. • The UE searches for a suitable cell of the chosen PLMN and chooses that cell to provide available services, and tunes to its control channel. This is known as "camping on the cell". The UE will then register its presence, by means of a NAS registration procedure, in the registration area of the chosen cell, if necessary. • If the UE finds a more suitable cell of the selected PLMN, it reselects onto that cell and camps on it. If the new cell is in a different registration area, location registration is performed. • If necessary, the UE will look for more suitable cells on other PLMNs at regular time intervals, which is referred to as PLMN-reselection. This is described in 3GPP TS22.011: "Service accessibility". • If the UE loses coverage of the registered PLMN, either a new PLMN is selected automatically (automatic mode), or an indication of which PLMNs are available is given to the user, so that a manual selection can be made (manual mode). • Registration is not performed by UEs only capable of services that need no registration. Process – UE shall use one of the following two search procedures: • Initial Cell Selection • Stored Information Cell Selection – Move to state ‘camped normally’ Cell Selection Criteria, S Cell Selection Process & Criteria Squal = Q qualmeas – Qqualmin Srxlev = Q rxlevmeas – Qrxlevmin - Pcompensation The cell selection criteria ‘S’ is fulfilled when Squal > 0 AND Srxlev > 0 Q qualmeas : Measured CPICH Ec/No(dB) for FDD, Q rxlevmeas : Measured CPICH RSCP(dBm) for FDD, P-CCPCH RSCP for TDD and RXLEV for GSM cells, Pcompensation = max(UE_TXPWR_MAX_RACH – P_MAX, 0) (dB) Where, Power On PLMN Selection Cell Selection camped normally Cell Reselection Process with S,H & R criteria Serving cell+measured Neighboring cells HCS in not use: HCS in use: S-criteria OK S-criteria OK Best R Serving cell+measured Neighboring cells Cells with Lower HCS Priority than Serving cell Best R Best R Fast M S Cells with Highest HCS Priorities & H>0 If No Cell with H>0 accept all neighbors Cell Selection camped normally Cell reselection Cell Reselection When serving cell does not belong to a hierarchical cell structure, the UE shall use Squal(CPICH Ec/No) for FDD cells and Srxlev(P-CCPCH RSCP) for TDD for Sx in the following rules. Measurement for cell re-selection - when HCS is not used If Sx > Sintrasearch, UE need not perform intra-frequency measurements. If Sx <= Sintrasearch, perform intra-frequency measurements. If Sintrasearch, is not sent for serving cell, perform intra-frequency measurements. If Sx > Sintersearch, UE need not perform inter-frequency measurements If Sx <= Sintersearch, perform inter-frequency measurements. If Sintersearch, is not sent for serving cell, perform inter-frequency measurements. If Sx > SsearchRAT m, UE need not perform measurements on cells of RAT "m". If Sx <= SsearchRAT m, perform measurements on cells of RAT "m". If SsearchRAT m, is not sent for serving cell, perform measurements on cells of RAT "m". Decide measurement Ranking dBm Time Cell Reselection Criteria • The following 3 types of cell re-selection criteria are used – Cell ranking criterion, R R s = Q meas,s + Qhyst s R n = Q meas,n - Qoffset s,n -TO n *(1 – L n ) where, Q meas : Quality value of the received signal mapped from the average CPICH Ec/No or CPICH RSCP (s: serving cell, n: neighbour cell) TOn = TEMP_OFFSETn * W(PENALTY_TIMEn – Tn) W(x) = 0 for x < 0, W(x) = 1 for x >= 0 TEMP_OFFSETn: The offset applied to the H and R criteria for a neighbouring cell for the duration of PENALTY_TIMEn. PENALTY_TIMEn: The time duration for which the TEMPORARY_OFFSETn is applied for a neighbouring cell. Tn: The timer that is implemented for each neighbouring cell. where, Cell Reselection Decide measurement Ranking SIB3_RSCP OR ECNO Cell reselection hysteresis 1 (Qhyst1)_WCELL – 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. Cell reselection hysteresis 2 (Qhyst2)_WCELL – Qhyst2 is used for FDD cells when cell selection and re- selection quality measure is set to CPICH Ec/No. Cell Reselection Criteria dBm Time Cell Reselection Criteria - Cell ranking criterion R Cell reselection Quality Offset 1 (Qoffset1) – Qoffset1 is used for WCDMA and GSM cells when cell reselection and ranking measure is set to CPICH RSCP – AdjsQoffset1_HOPS – 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 of the neighbour cell before the UE compares the quality measure with the cell re-selection/ranking criteria. Cell reselection Quality Offset 2 (Qoffset2) – Qoffset2 is used for WCDMA cells when cell reselection and ranking measure is set to CPICH Ec/No – AdjsQoffset2 _HOPS – 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 Ec/No of the neighbour cell before the UE compares the quality measure with the cell re-selection/ranking criteria. AdjsEcNoOffset (CPICH Ec/No Offset) Handover Control parameters * CPCIH EcNo < Qqualmin(-18)+Sintrasearch(10)=-8 (default: EcNo < -8 dB) CPCIH EcNo < Qqualmin(-18)+SsearchRAT(4)=-14 (default: EcNo < -14 dB) 3G TO 3G 3G TO 2G QqualMin----The minimum required quality level in the cell (Ec/No). Sintrasearch ---The threshold for intra-frequency measurements, and for the HCS measurement rules. Note: If no threshold is given, MS performs measurements. Ssearch_RAT--- The RAT-specific threshold for inter-RAT measurement rules. Note: If no threshold is given, MS performs measurements. Parameter name Cell reselection triggering time Abbreviated name Treselection 3GPP name Treselection Managed object WCEL Parameter group SystemInfo Description This parameter defines how the UE triggers the reselection of a new cell if the cell reselection criteria are fulfilled during the time interval Treselection. The use of 2s reselection time can avoid too many cell reselections between cells and hence LA/RA updates when crossing LA/RA border. Thus, there are less signaling and less call failures at LA/RA border due to LA/RA update. The reselection time of 0s can be used in areas of high mobility, for example highways. Range 0...31 Step 1 Unit s Formula for getting internal value internal_value = gui_value Default value 2 s Parameter name S intrasearch Abbreviated name Sintrasearch 3GPP name Sintrasearch Managed object WCEL Description This parameter defines the threshold for intra-frequency measurements and for the HCS measurement rules. The parameter is used for cell selection and re-selection in idle mode. Note: If no threshold is given, MS performs measurements. Range 0...20 Step 2 Unit dB Formula for getting internal value internal_value = gui_value / 2 Default value 10 dB Default value notes UE will start the neighboring measurements when CPCIH EcNo < Qqualmin+Sintrasearch (default: EcNo < -8 dB). System information SIB3 Related parameters QqualMin of WCEL - The minimum required quality level in the cell (Ec/No). , UseOfHCS of WCEL - Indicates whether the serving cell belongs to a Hierarchical Cell Structure (HCS), or not. Sintrasearch is needed only if HCS is used. Parameter name CPICH Ec/No Offset Abbreviated name AdjsEcNoOffset 3GPP name Cell individual offset Managed object ADJS Parameter group HCConfiguration Category Radio Resource Utilisation Description The CPICH Ec/No Offset determines an offset value, which the UE adds to the CPICH Ec/No measurement result of the neighbouring cell before it compares the Ec/No value with the reporting criteria. Range -10...10 Step 0.5 Formula for getting internal value internal_value = gui_value * 2 Default value 0 dB Related functions Handover Control, Mobile Device Control System information SIB11, SIB12 Related parameters DropWindow of FMCS - The dB values of the CPICH Ec/No Offset and Drop Window parameters must satisfy the following equation, otherwise the UE triggers off a huge amount of active set updates which leads to unstable behaviour: 0 < DropWindow + AdjsEcNoOffset. RRC and RAB Phases Phases and Counters of RRC & RAB connection The cell is suffering of RRC access failures. Possible scenarios: Call setup in bad coverage area→decrease N312 (def: Idle 2) to speed up synchronisation procedure (N312 should be tuned depending on UE type) UL coverage << DL coverage →tune CPICH Power →UL interference DL coverage << UL coverage →UE does not receive the message (tune SCCPCH power) → tune DL SRB coverage The correct scenario can be analysed only through field testing. RRC Access FailuresL1 Synch •In this case we are sure that the DL Synchronisation has been acquired as well as the UL •This is due to that the UE does not start to transmit ANYTHING before it has acquired DL sync •This problem could be some UL/DL coverage unbalance Dropped Call Category: • RF related: Signal Fading • Analysis: • Initially there were cell SC 226 send RRC request. • Later on, main servers SC226 degraded. • UE kept sending several measurement reports event 1a but there still was no ASU response from RNC then finally the call was fail. Solution / Action proposed: • Try to maintain the link of SC 240 EDT from 4 to 2 degree. Analysis: As the UE moves along this road, missing neighbor was encountered on SC304. UE hanged on the cells of poor RF condition leaded to 3x RRC connection request with no responding. Call setup was successful on the 4th RRC_CONNECTION_REQUEST. However, since the time duration from the 1st RRC Connection Request to the 4th RRC Connection Request was greater than 6.4secs(T300XN300), this would be considered a CAF by Actix. . Solution / Action proposed: SC304 and SC312 change Treselection from 2 to 1 second. Drop Call Category: RF Related – Signal Fading Analysis: As the UE moves along this road, signal fading was encountered on SC351 UE hanged on the poor RF conditions and network leased this call. CPICH power of SC351 is 36dBm causing overshooting and high DL load. Solution / Action proposed: SC351 changes CPICH power from 36 to 34dBm. Drop Call Category: RF Related – Late Cell Reselection Analysis: As the UE moves along this road, signal fading was encountered on SC263 UE sent two times RRC request to 24515, but signal degraded fast and no response from network. Solution / Action proposed: SC263 changes Treselection to 1 second. Drop Call Category: RF Related – Late Cell Reselection Analysis: As the UE moves along this road, signal fading was encountered on SC426. UE sent 3 times RRC request to SC426, but signal degraded fast and no response from network. Solution / Action proposed: SC426 changes Treselection to 1 second. Dropped Call Category: • Network related: High BLER • Analysis: • Initially there was cell (SC 218) in active set. • Have checked stats and alarm were OK. No resource related problem. • RF condition was good but there was high DL BLER at that time. Solution / Action proposed: • None. Drop Call Category: RF Related – Late Cell Reselection Analysis: As the UE moves along this road, signal fading was encountered on SC432. UE hanged on the poor RF conditions, which leaded to no MR response and CAD . Solution / Action proposed: SC432 changes Treselection from 2s to 1 second. Drop Call Category: RF Related – Signal Fading Analysis: As the UE was moving along this road, fast signal fading was encountered on SC427. The signal of SC427 suddenly dropped causing the UE to lose its in- synchronization with the network. UE hanged on the poor RF condition and leaded to no MR responding, then BLER raised and network released this call. Solution / Action proposed: SC427to SC435 adjust AdjsEcNoOffset from 0 to 3dB. AMR-CAD Call Fail Category: RF related – Signal Overshooting Analysis: Due to terrain, SC160 and SC424 can overshoots to this area causing strong RSSI and poor Ec/No. Then, UE hanged on the poor RF condition and leaded to no MR responding, then BLER raised and network released this call. Solution / Action proposed: SC424 adjusts EDT from 2 to 4 degrees. SC160adjusts EDT from 4 to 6 degrees. Drop Call Category: RF Related – No Dominance Analysis: As the UE was moving into this area, poor RSCP and no dominance were encountered. Due terrain many signal can cover this area causing strong RSSI and poor EcNo. This is a serious protest area, it is not easy to add a new site. Solution / Action proposed: Plan a new site. SC429 adjusts EDT from 3 to 4 degrees. SC211adds NBs-a,b,c Drop Call Category: RF Related – No Dominance Analysis: As the UE was moving into this area, poor RSCP and no dominance were encountered. Due terrain many signal can cover this area causing strong RSSI and poor EcNo. This is a serious protest area, it is not easy to add a new site. Solution / Action proposed: Plan a new site. SCxxx adjusts azimuth from 200 to 170 degrees.. Drop Call Category: • RF Related – Signal Fading Analysis: • As the UE was moving along this road, fast signal fading was encountered on SC262. • The signal of SC262 suddenly dropped causing the UE to lose its in- synchronization with the network. • UE hanged on the poor RF condition and leaded to no MR responding, then BLER raised and network released this call. Solution / Action proposed: • SC262to SC254 and SC270adjust AdjsEcNoOffset from 0 to 3dB. Drop Call Category: • RF Related – Signal Fading Analysis: • As the UE was moving along this road, fast signal fading was encountered. • The three AS suddenly dropped causing the UE to lose its in-synchronization with the network. • UE hanged on the poor RF condition and leaded to no MR responding, then BLER raised and network released this call. Solution / Action proposed: • SC427 change FMCS id to 93. • SC381 adjusts EDT 7 to 9. Drop Call Category: • RF Related – Signal Fading Analysis: • As the UE was moving along this road, fast signal fading was encountered on SC353. • The signal of SC353 suddenly dropped causing the UE to lose its in-synchronization with the network. • UE hanged on the poor RF condition and leaded to no MR responding, then BLER raised and network released this call. Solution / Action proposed: • SC353 to SC259 and SC251 adjust AdjsEcNoOffset from 0 to 4dB. Drop Call Category: RF related – Poor coverage Analysis: UE was camped signals them cannot main servers provide enough coverage at this point. Solution / Action proposed: Propose new site or wait new site SCxxx unlock again tuning. Uptilt Enhance cpichpower Drop Call Category: RF related – No dominance Analysis: UE was camped signals them cannot main servers provide enough coverage at this point. Solution / Action proposed: Propose change SC335 the EDT from 2 to 0 degree. THANKS!