Cell selection and reselection

1. Cell selection and reselection

2. Agenda • Idle Mode Instruction(√) • PLMN Selection • Cell Selection & reselection • Cell Search Procedure • Cell Selection Procedure • Cell Reselection Procedure • Engineering Case

3. Idle mode • In Idle mode, the UE has no connection to the radio network, nor is a Radio Resource Control (RRC) connection established. • However, the UEs must still be able to access the system and be reached by the system with acceptable delays. For this, the following procedures need to be performed: - PLMN selection - Cell selection and reselection - Location Area (LA) and Routing Area (RA) updating - Paging - System information broadcast

4. Idle mode PLMN selection performs registration on a selected Public Land Mobile Network (PLMN). Cell selection looks for a suitable cell in the selected PLMN. Cell reselection ensures that the UE camps on the cell that gives the highest probability for successful monitoring of system and paging information and for successful establishment of a connection.

5. Agenda • Idle Mode Instruction(√) • PLMN Selection (√) • Cell Selection & reselection • Cell Search Procedure • Cell Selection Procedure • Cell Reselection Procedure • Engineering Case

6. PLMN SELECTION A PLMN with at least one acceptable cell is considered available. If it does not find an available PLMN, the UE enters the non-service state, and waits until a new PLMN is available and allowed.

7. Check procedure the UE uses information about the last registered PLMN, such as carrier frequencies or the list of neighboring cells stored in the USIM On each stored carrier frequency, the UE searches first for the strongest signal cell and reads its system information for PLMN identity Finally, the UE attempts registration if the PLMN is allowed. Then the UE decides whether the chosen cell is acceptable

8. Cell search procedure When UE has synchronized and find the scrambling code of the cell, it can decode the system information(BCH) sent on the P-CCPCH.

9. Agenda • Idle Mode Instruction(√) • PLMN Selection (√) • Cell Selection & reselection (√) • Cell Search Procedure (√) • Cell Selection Procedure (√) • Cell Reselection Procedure • Engineering Case

10. Cellselection The UE attempts to make a cell selection in the following cases: • When a UE is switched on • When a UE in Idle mode has had a number of failed RRC connection requests • When a UE returns to Idle mode from the Connected mode on common channels (CELL_FACH state) after a number of failed cell update attempts • When a UE moves from dedicated mode to Idle mode • When a UE returns to Idle mode after an emergency call on any PLMN • When a UE moves from dedicated mode to state CELL_FACH or URA_PCH.

11. Cell Selection and Reselection Procedure Stored information cell selection is used if the UE knows the carrier frequencies that have previously been used. Initial cell selection is used if the UE has no knowledge of the WCDMA radio frequency channels.

12. Cell Camping • A UE obtains limited service when it camps on an acceptable cell. If the cell selection criteria are fulfilled, the cell is considered acceptable. • A UE obtains normal service when it camps on a suitable cell. A cell is suitable when the following criteria are fulfilled: • The cell belongs to the selected PLMN. • The cell selection criteria are fulfilled. • The cell is not a part of a forbidden registration area.

13. Cell Selection Criteria The S-criteria are fulfilled when: Squal > 0 and Srxlev > 0 Squal = Qqualmeas - qQualMin (only for WCDMA cells) Srxlev = Qrxlevmeas-qRxLevMin - Pcompensation (for all cells) Pcompensation = max (maxTxPowerUl - P, 0);

14. Parameters of cell selcetion • Qqualmeas is the quality value for the received signal, expressed in CPICH Ec/No (dB) and measured by the UE. • Cell parameter qQualMin indicates the minimum required quality value in the cell. It is sent in system information, in SIB3 for the serving cell, and in SIB11 for adjacent cells. • Qrxlevmeas is the received signal strength measured by the UE and expressed in CPICH RSCP (dBm). • Cell parameter qRxLevMin indicates the minimum required signal strength in the cell. It is sent in SIB3 for the serving cell and in SIB11 for adjacent cells. • P is the maximum RF output power of the UE according to its class. • Pcompensation is introduced for UEs that cannot transmit at maximum power on the RACH in the cell. • The cell parameter maxTxPowerUl indicates the maximum allowed transmission power when the UE accesses the system on RACH. It is broadcast in SIB3.

15. Parameters setting • After synchronization, the UE reads system information on the BCCH. If all requirements for a suitable cell are fulfilled, the UE selects that cell and tries to register. The recommended setting for qRxLevMin is the lowest possible value. The setting of qRxLevMin is related to the power level on the Primary CPICH

16. Agenda • Idle Mode Instruction(√) • PLMN Selection (√) • Cell Selection & reselection (√) • Cell Search Procedure (√) • Cell Selection Procedure (√) • Cell Reselection Procedure (√) • Engineering Case

17. Cell Reselection • When camping on a cell, the UE shall continuously search for a better cell according to the cell reselection criteria. In order to perform cell ranking, the UE measures the serving cell and neighbor cells listed in SIB11 according to the measurement rules

18. Cell Reselection Procedure • Measurements • Intra-frequency neighbors • Inter-frequency neighbors • GSM neighbors • S-criteria Judgement UE checks whether measured neighbor cells fulfill the cell selection criteria ,and only fulfilled cells are ranked. • Cell Ranking by R-criteria UE shall perform ranking of all measured cells fulfilling S-criteria,and reselects the cell first on the ranked list when the reselection criteria have been fulfilled during a time interval of treSelection.

19. Cell Reselection Procedure The parameter qualMeasQuantity defines what ranking quantity is used. If a GSM cell is highest ranked, no more ranking is done. The recommended setting is qualMeasQuantity = 2 , a second ranking takes place, this time according to CPICH Ec/No, and excluding all GSM neighbors.

20. Measurement rules The UE measures and evaluates the neighboring cells listed in system information (SIB 11).There are three different types of neighboring cells: • Intra-frenquency neighbors measurements Squal<=sIntraSearch • Inter-frenquency neighbors measurements Squal<=sInterSearch • Inter-Rat neighbors measurements Squal<=sRatSearch Srxlev<=sHcsRat

21. Measurement Threshold When this parameter is not sent, the UEs are configured to perform measurements on intra-frequency neighbors continuously. Note:If no inter-frequency neighbors are defined in the cell, the parameter sInterSearch can be set to any value, because it will not be used.

22. Measurements on GSMNeighbors • Note 1: the thresholds sRatSearch are set relatively low to avoid ping-pong behavior between WCDMA and GSM. • Note 2: To activate the CPICH RSCP threshold the parameter sHcsRat should be set to a value > 0. All negative values of sHcsRat are interpreted as 0 by the UE. • Note 3:only UEs supporting 3GPP Rel- 5 CR 130 on TS 25.304 are able to base the start of GSM measurements on both CPICH Ec/N0 and CPICH RSCP (using both sRatSearch and sHcsRat). Other UEs will only use the CPICH Ec/N0 condition (Squal <= sRatSearch) to decide when to start measurements of GSM neighbors. Measurement based on the received quality(Ec/N0): Squal = Qqualmeas - qQualMin<=sRatSearch Qqualmeas <= qQualMin + sRatSearch = -18 + 4 = -14 dB

23. Measurements on GSMNeighbors The figure illustrates when GSM measurements are performed by the UE based on the CPICH Ec/N0 threshold. A similar graph would also be applicable for the corresponding CPICH RSCP threshold.

24. Cell Ranking by R-criteria • The cells are ranked according to the R-criteria: R(serving) = Qmeas(s) + qHyst(s) + Qoffmbms R(neighbor) = Qmeas(n) - qOffset(s,n) + Qoffmbms - TO(n) Note: HCS and MBMS not contained

25. Parameters of Cell Reselection • Qmeas is the quality value of the received signal, which is derived from the average CPICH Ec/N0 or CPICH RSCP level for WCDMA cells and from the average received signal level for GSM cells. (s) and (n) denotes the serving and the neighbor cell values respectively. • qHyst(s) is the hysteresis value that is read in system information of the serving cell. The value is set by the configurable parameters qHyst1 used when the ranking is based on CPICH RSCP and qHyst2 used when the ranking is based on CPICH Ec/N0. • qOffset(s,n) is the offset between the serving cell and the neighbor cell and can be used to move the border between two cells. The value is defined per cell relation and is set by the configurable parameters qOffset1sn used when the ranking is based on CPICH RSCP and qOffset2sn used when the ranking is based on CPICH Ec/N0. • Qoffmbms is an offset applied to cells belonging to the MBMS Preferred layer. • TO(n) is a temporary offset applied to the R criteria only if HCS is used in the serving cell.

26. Parameters setting Ranking of GSM neighbors is always made using the measurement quantity CCPCH RSSI. Note that the parameter qOffset1sn for WCDMA-GSM and WCDMA-WCDMA neighbor relations not directly comparable a too low value, triggering too many reselections in the fading radio environment, and a too high value, that slows down the process.

27. Parameters settingof qOffset1sn • Note that the values of GSM RSSI and WCDMA CPICH RSCP are not directly comparable. Based on measurements of BLER on broadcast channels for the different radio access technologies, a rule of thumb that can be used for initial parameter settings is the following: [CPICH RSCP] + 7 dB is comparable to [GSM RSSI] • For example, a measured RSCP on a WCDMA CPICH of –100 dBm would be comparable to a GSM broadcast channel RSSI of –93 dBm. • A parameter setting of a GSM neighbor to qOffset1sn = +7 will thus make the received signal strength in the WCDMA and the GSM cell comparable. If the value is > 7 dB, the WCDMA cell is prioritized, that is, made larger.

28. WCDMA to GSM Cell Reselection The recommended default values for the offset parameters are the following: • qOffset1sn = 0 dB for WCDMA neighbors • qOffset1sn = 7dB for GSM neighbors • qOffset2sn = 0 dB

29. GSM to WCDMA Cell Reselection When the multi-RAT UE enters an area without WCDMA coverage, it will camp on a GSM cell. In the GSM network, the parameter QSI controls when the UE performs measurements on WCDMA neighboring cells. This parameter is set per cell. • Measurements • Cell Ranking

30. measurements • The parameter QSI can be set in the four following alternative ways, so that WCDMA neighbors are measured: - Always - Never - When signal strength of the GSM cell is above a certain value - When signal strength of GSM cell is below a certain value • To enable cell reselection to WCDMA in areas with good GSM coverage, it is necessary to measure on WCDMA also when GSM coverage is good. The recommendation is that GSM cells that have WCDMA neighbors should have the QSI value set to Always.

31. Cell ranking • The algorithm for cell reselection from GSM back to WCDMA is controlled with the parameters FDDQMIN and FDDQOFF. • The parameter FDDQMIN defines the minimum quality of a WCDMA cell for cell reselection, measured in CPICH Ec/No. This parameter is used to assure a sufficient quality of the candidate WCDMA cell. • The parameter FDDQOFF sets an offset, positive or negative, between signal quality of GSM cells and the defined WCDMA neighbors. The two cell reselection criteria are the following: • CPICH Ec/No > FDDQMIN • CPICH RSCP > RLA + FDDQOFF • RLA is the received signal level average [dBm] for the serving GSM cell and its strongest GSM neighbors. • For a multi-RAT UE just having entered GSM a ping-pong back to WCDMA is prohibited by a timer. The UE must fulfill the two cell reselection criteria above for 5 seconds in order to reselect the WCDMA cell. Furthermore, during a period of 15 seconds after the cell reselection to GSM, an additional offset of 5 dB will be added to the parameter FDDQOFF.

32. Agenda • Idle Mode Instruction(√) • PLMN Selection (√) • Cell Selection & reselection (√) • Cell Search Procedure (√) • Cell Selection Procedure (√) • Cell Reselection Procedure (√) • Engineering Case (√)

33. Engineering Case Following is Taiwan network configuration for 2 Carriers, first carrier is just for R99 service,second Carrier is high priority for HSPA

34. Engineering Case

35. Engineering Case

36. Engineering Case

37. Parameter analysis • If UE camp on f1 cell,f2 cell is it’s neighbour cell. f1: R(serving) = Qmeas(s) + qHyst(s) f2:R(neighbor) = Qmeas(n) - qOffset(s,n)= Qmeas(n) -50 We can see f1 is priority to f2. • If UE camp on f2 cell,f1 cell is it’s neighbour cell. f2: R(serving) = Qmeas(s) + qHyst(s) f1:R(neighbor) = Qmeas(n) - qOffset(s,n)= Qmeas(n) -(-50) = Qmeas(n) +50 We can see f1 is also priority to f2.