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Irat handover basics


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  • 1. IRAT Handover Basics
  • 2. Introduction IRAT (Inter-Radio Access Technology) handover and cell change allows 3G voice and data services WCDMA RAN to GSM transition to maintain the connection with the UE between coverage areas to prevent dropped calls. The Inter-RAT Handover Evaluation algorithm performs the Inter- RAT (WCDMA RAN to GSM) handover evaluation based on the UE GSM measurement reports. Commercial in confidence 2 2008-11-27
  • 3. UMTS/GSM Coverage UMTS and GSM coverage GSM coverage Commercial in confidence 3 2008-11-27
  • 4. Connection Quality Monitoring The connection quality is monitored in DL based on both CPICH Ec/No and CPICH RSCP and in UL based on UE TX power. The UE informs the RNC of connection quality with 2d, 2f, 6d and 6b events, and RNC continually keeps track of whether the DL Ec/No, the DL Rscp or the UL coverage is good or bad. When at least one of these gets bad, an GSM HO (or IF) attempt will be started, and if the coverage then gets good for all then the IRAT HO (or IF HO) attempt will be aborted. In P6 WCDMA RAN it allows the operator to set different offsets for triggering event 2d for each UeRc Type at RNC level Commercial in confidence 4 2008-11-27
  • 5. Reporting Events 2d, 2f and 3a Start measuring on GSM cells (event 2d): - The quality (CPICH Ec/No) on the UTRAN side below threshold. - The coverage (RSCP) on the UTRAN side below threshold. Stop measuring on GSM cells (event 2f): - The quality on the UTRAN side above threshold. -The coverage on the UTRAN side above threshold. Handover trigger condition (event 3a): - The quality of the UTRAN cell drops below threshold. - Signal Strength (RSSI) of GSM cell is above threshold. usedFreqRelThresh2f utranRelThresh3aEcno Commercial in confidence 5 2008-11-27
  • 6. P6 IRAT ParametersParameter Name Description RangegsmHoAllowed Enables/disables the possibility of 0 = not allowed HO from UMTS to GSM per RAB 1 = allowed type. Default = not allowedfddGsmHoSupp Indicates if the RNC supports inter-RAT HO FALSE/TRUE (Inter Radio Access Technology Handover). Default: FALSEhoType Indicates whether GSM HO, IFHO or no 1 = IFHO_PREFERRED handover shall be attempted when a 2 = GSM_PREFERRED handover from the current WCDMA 3 = None frequency is triggered by the connection quality monitoring, indicating that the Default: GSM_PREFERRED WCDMA quality is low. Commercial in confidence 6 2008-11-27
  • 7. Default P6 IRAT ParametersParameter Name Range DefaultusedFreqThresh2dEcno Valid range for CPICH Ec/No. -12 dB -24 – 0 dBtimeToTrigger2dEcno 0; 10; 20; 40; 60; 80; 100; 120; 160; 200; 240; 320 ms 320; 640; 1280; 2560; 5000 msusedFreqRelThresh2fEcno 0 – 20 dB 1 dBtimeToTrigger2fEcno 0; 10; 20; 40; 60; 80; 100; 120; 160; 200; 240; 320; 1280 ms 640; 1280; 2560; 5000 msutranRelThresh3aEcno Range is valid for CPICH Ec/No. -1 dB -10 – 10 dBtimeToTrigger3a 0; 10; 20; 40; 60; 80; 100; 120; 160; 100 ms 200; 240; 320; 640; 1280; 2560; 5000 msgsmThresh3a Range is valid for GSM carrier RSSI. -102 dBm -115 – 0 dBm Commercial in confidence 7 2008-11-27
  • 8. Default P6 IRAT Parameters (cont)Parameter Name Range DefaultusedFreqThresh2dRscp Valid range for CPICH Rscp. -115 -97 dBm — -25 dBmusedFreqRelThresh2fRscp 0 – 20 dB 3 dButranRelThresh3aRscp Range is valid for CPICH Rscp. -20 -3 dB — 20 dB Commercial in confidence 8 2008-11-27
  • 9. IRAT Handover Sequence Commercial in confidence 9 2008-11-27
  • 10. IRAT Cell Change IRAT Cell Change handles the cell change and cell reselection when a UE is connected to a Packet-Switched service. One difference between Inter-RAT Handover and Inter-RAT Cell Change is that in the IRATCC case there are no resources reserved in the target cell beforehand. Commercial in confidence 10 2008-11-27
  • 11. IRAT Cell Change Sequence Commercial in confidence 11 2008-11-27
  • 12. GSM to WCDMA - Measurement Measurement on WCDMA cells while the UE is on the GSM network is defined by the parameters QSI and QSC The parameters QSI and QSC define thresholds and also indicate whether these measurements shall be performed when signal strength (SS) of the serving cell is below or above the threshold Commercial in confidence 12 2008-11-27
  • 13. IRAT GSM to WCDMA handover IRAT can be performed from GSM to WCDMA even if the radio conditions are good. IRAT 2G to 3G speech handover is triggered due to capacity, radio quality or priority reasons (UTRAN Preferred) Two criteria must be fulfilled to trigger a GSM to WCDMA handover – The percentage of idle TCH in the serving cell ISHOLEV – CPICH Ec/No > MRSL MRSL – GSM to WCDMA handover trigger ISHOLEV – Defines GSM traffic load for enabling GSM to WCDMA handover. Handover evaluations start when the percentage of idle full rate Traffic Channels (TCHs) left in the cell is less than or equal to ISHOLEV. Note – In practise the reliance on TCH can be removed by setting the ISHOLEV low Commercial in confidence 13 2008-11-27
  • 14. GSM to WCDMA Cell Change IRAT Cell Change and Cell Reselection is triggered due to radio quality and priority reasons (UTRAN preferred) Commercial in confidence 14 2008-11-27
  • 15. Conditions for Cell Reselection CPICH E c /N o > FDDQMIN and CPICH RSCP > RLA (serving +neighboring GSM cells) + FDDQOFF CPICH RSCP is a measure of the signal strength for a WCDMA cell. RLA (Received Level Average) is the average of the received signal levels measured in dBm for all monitored GSM frequencies in the BCCH Allocation list. FDDQMIN is the minimum quality of a WCDMA cell for cell reselection. This parameter should not be used to control the cell reselection, but provide a sufficient quality of the candidate WCDMA cell FDDQOFF is the key parameter to control the behavior of the IRATCC. It defines an offset between signal quality of WCDMA and GSM cells. Lower values of the parameter FDDQOFF could be used to off-load the GSM system. Higher values of the parameter FDDQOFF could be used to keep Multi-RAT MSs in the GSM system, for example, if an WCDMA to GSM off-load priority setting is applied in the WCDMA system Cell reselection back to WCDMA shall not occur within 5 seconds after a previous cell reselection from WCDMA. If a cell reselection occurred within the previous 15 seconds, an additional offset to FDDQOFF is increased by 5 dB by the multi RAT MS. Commercial in confidence 15 2008-11-27
  • 16. Directed Retry to GSM Directed Retry is a feature in which excess traffic may be off-loaded to GSM, with the following benefits: – It allows the WCDMA RAN to focus on providing data services. – It makes it possible for the combined system to fully utilize the existing spectrum in GSM Commercial in confidence 16 2008-11-27
  • 17. Directed Retry Directed Retry is a one-way diversion of voice traffic from WCDMA to GSM. Directed retry occurs at RAB establishment, where when the load of the WCDMA network exceeds a certain threshold, the WCDMA RAN will request a “blind” IRAT handover to a pre-configured GSM cell via the core network. Network load is based on downlink transmitted carrier power since this is most often the limiting factor. The power for HSDPA services is not considered in the load measurements It is assumed that the WCDMA RAN is co-sited with the GSM RAN and a 1-to-1 correspondence between WCDMA and GSM cells exists Commercial in confidence 17 2008-11-27
  • 18. Directed Retry Message Flow The load is shared between WCDMA and GSM by sending RAB Assignment Reject Messages with ‘Directed retry’ as the cause for Speech only calls. The CN will redirect the call to the GSM cell. The CN will reinitiate the RAB procedure if the GSM connection fails Commercial in confidence 18 2008-11-27
  • 19. Commercial in confidence 19 2008-11-27